SEMICONDUCTORS and SEMIMETALS V23 Pulsed laser processing of semiconductors

SEMICONDUCTORS AND SEMIMETALS VOLUME 23 Pulsed Laser Processing of Semiconductors Volume Editors R . F. WOOD and C . W...

Author: R. F. Wood | C. W. White | R. T. Young

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SEMICONDUCTORS AND SEMIMETALS VOLUME 23 Pulsed Laser Processing of Semiconductors

Volume Editors R . F. WOOD and C . W. WHITE SOLID STATE DIVISION OAK RIDGE NATIONAL LABORATORY OAK RIDGE, TENNESSEE

R . T. YOUNG ENERGY CONVERSION DEVICES, INC. TROY, MICHIGAN

1984

ACADEMIC PRESS, INC. (Harcourt Brace Jovanovich, Publishers)

Orlando San Diego New York London Toronto Montreal Sydney Tokyo

Academic Press Rapid Manuscript Reproduction

COPYRIGHT @ 1984, BY ACADEMIC PRESS,INC.

ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC.

Orlando, Flonda 32887

United Kingdom Edition published by

ACADEMIC PRESS,

INC. (LONDON) 24/28 Oval Road, London N W l 7DX

LTD.

Library of Congress Cataloging in Publication Data

I S B N 0-12-752123-2 PRINTED IN THE UNITED STATES OF AMERICA

04 85 86 87

9 8 7 6 5 4 3 2 1

65-26058

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin

R. B. JAMES, SANDIA, Division 8341, Livermore, California 94550 (555) G. E. JELLISON, JR., Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (95, 165, 313) D.H . LOWNDES,Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (313, 471) C . W. WHITE, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ( I , 43) R. E WOOD,Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ( I , 165, 251, 625) R. T. YOUNG, Energy Conversion Devices, Inc.. Troy, Michigan 48084 (1, 625) E W. YOUNG,JR., Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (251) D. M . ZEHNER, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (405)

ix

Foreword All of the contributors to this volume received their primary support during the writing of the book and for their own research from the Division of Materials Science of the United States Department of Energy under contract number DEAC05-840R21400 with Martin Marietta Energy Systems, Inc. Valuable additional support for research on the development of laser-processed high-efficiency solar cells was received from the Solar Energy Research Institute under contract number DB-2-02076- 1. The support of these two agencies is gratefully acknowledged. Invaluable assistance was rendered by members of the secretarial staff of the Solid State Division of Oak Ridge National Laboratory not only in taming the frequently recalcitrant word processors and authors, but also in all other aspects of preparing the camera-ready copy. J. T. Luck and V. G. Hendrix bore the heaviest burdens, but the contributions of A. M. Keesee, T. K. Miller, and S. E. Thomas were also indispensable and greatly appreciated. Ms. Hendrix coordinated the entire preparation of the manuscript and a special thanks is her due. Finally, the contributors wish to thank all of their colleagues who allowed illustrative material from published papers to be included for discussion in the book.

xi

Preface This book is concerned with the pulsed laser processing of semiconductors, a field that has emerged as a well-defined area of condensed matter physics and materials science over approximately the last ten years. It is hardly an exaggeration to characterize developments during this period, and particularly during the last five years, as explosive. Moreover, there seems little doubt that the interest and excitement generated by new results of both fundamental and applied significance will continue at a high level for some time. We may also expect laser-related techniques that are continuing to evolve to have a significant impact in a number of areas of semiconductor materials preparation and device applications. Nevertheless, it is apparent that the field has now matured to the point where many of the early misconceptions and controversies, that inevitably arise during a period of rapid growth of a new area of science have been largely resolved. Therefore, although it may still be too early to discern clearly the direction the field will take in the coming years, it does seem particularly appropriate for a book such as this to appear at this time. The authors of the various chapters in the book have in common the fact that they were members of the Solid State Division at the Oak Ridge National Laboratory during the period of very rapid growth of the field of pulsed laser processing of semiconductors. Each of them made significant contributions that led to the recognition of ORNL as a pioneering center for development of the field. All of the chapters were essentially completed while the authors were at ORNL, although R. T. Young and R. B. James have now moved on to other research establishments. In spite of the close interaction of many of the authors, the editors did not insist on extensive cross referencing of the material in the various chapters, so that the individual contributions can generally be read independently of one another. As a consequence, there is some overlap of material in different chapters; on the whole, however, we feel that this overlap has been kept to an acceptable level. We trust that readers will find the book interesting and informative and that it will serve as a useful reference for much of the original work in the field.

...

Xlll

CHAPTER 1 LASER PROCESSING

OF SEMICONDUCTORS:

AN

OVERVIEW

R. F. Wood C. W. White R. T. Young

. . ..

.

I. INTRODUCTION * 11. LASER MACHINING AND LASER PROCESSING * 111. DEVELOPMENT OF LASER ANNEALING 1. Pulsed Laser Annealing 2. T h e o r e t i c a l Modeling o f Pulsed L a s e r Annealing. 3. CW Laser Annealing. IV. OTHER FORMS OF LASER PROCESSI~G 4. Background 5. Laser-Induced D i f f u s i o n of Dopants 6, S i l i c i d e Formation. 7. Ohmic Contacts t o GaAs 8. Laser-Induced E p i t a x i a l Growth o f Deposited S i Films. 9. Laser R e c r y s t a l l i z a t i o n of S i F i l m s on I n s u l a t i n g Substrates. 10. Pulsed Laser Photochemical Processing 11. Excimer Laser L i t h o g r a p h y V. TYPES OF LASERS FOR PULSED LASER PROCESSING 12. Pulsed S o l i d - s t a t e Lasers 13. Pulsed Gas Lasers VI. OTHER SOURCES FOR ENERGY BEAM PROCESSING VII. LASER PROCESSING OF COMPOUND SEMICONDUCTORS, METALS, AND INSULATORS VIII. PLAN OF BOOK REFERENCES

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.. . . . . . . ... ... ... Copyright 01984 by Academic Press, Inc. All nghts of reproduction In any form reserved. ISBN 0-12-752123-2

2

R. F. WOOD E T A L .

I.

Introduction

T h i s i n i t i a l c h a p t e r p r o v i d e s a combined i n t r o d u c t o r y overview and h i s t o r i c a l survey o f t h e development of l a s e r p r o c e s s i n g o f semiconductors.

The h i s t o r i c a l o r c h r o n o l o g i c a l aspects o f t h e

development, i n a d d i t i o n t o t h e i r i n t r i n s i c i n t e r e s t , should serve t o g i v e t h e reader t h e f l a v o r o f t h e e v o l u t i o n o f l a s e r t e c h n i q u e s f o r machining and p r o c e s s i n g m a t e r i a l s , and t o i n d i c a t e t h e e x p l o s i v e growth which t h e f i e l d o f l a s e r p r o c e s s i n g o f semiconductors has undergone.

The overview serves i n p a r t t o i n t r o d u c e and t o

f a m i l i a r i z e t h e reader w i t h some o f t h e t o p i c s which w i l l be d i s cussed i n much g r e a t e r d e t a i l i n l a t e r chapters o f t h e book.

More

i m p o r t a n t l y , however, i t a l l o w s us t o c o n s i d e r several o t h e r t o p i c s which w i l l n o t be covered anywhere e l s e i n t h e book, and t h u s t o g i v e a b e t t e r rounded o v e r a l l view o f t h e s u b j e c t o f l a s e r p r o c e s s i n g o f semiconductors.

F o r example, a b b r e v i a t e d d i s c u s s i o n s

o f s e v e r a l aspects o f cw l a s e r p r o c e s s i n g o f semiconductors were i n c l u d e d i n t h i s c h a p t e r when i t was f e l t t h e y would complement t h e d i s c u s s i o n s o f p u l s e d l a s e r processing.

D e t a i l e d reviews o f t h e

development and c u r r e n t s t a t u s o f cw l a s e r p r o c e s s i n g are c o n t a i n e d i n a companion volume t o t h i s one i n t h e Semiconductor and Semimetals S e r i e s (Vol. 17, e d i t e d by Gibbons).

A d d i t i o n a l examples i n c l u d e

b r i e f s e c t i o n s on v a r i o u s types o f l a s e r s used f o r l a s e r processing, o t h e r energy beam sources f o r processing, and l a s e r p r o c e s s i n g o f m a t e r i a l s o t h e r t h a n semiconductors. 11.

Laser Machining and Laser Processing

A f t e r t h e i n v e n t i o n o f t h e l a s e r i n 1960 i t r a p i d l y came i n t o widespread use f o r a h o s t o f a p p l i c a t i o n s .

Because t h e l a s e r can

s u p p l y monochromatic, coherent 1 i g h t a t extremely h i g h power densities,

i t s p o t e n t i a l as a unique t o o l f o r m a t e r i a l s p r o c e s s i n g

was immediately recognized by m a t e r i a l s s c i e n t i s t s , m e t a l l u r g i s t s , and engineers.

As e a r l y as 1963,

p u b l i s h e d accounts o f l a s e r

w e l d i n g and d r i l l i n g began t o appear and t h e s e were soon f o l l o w e d

1 . LASER PROCESSING OF SEMICONDUCTORS by r e p o r t s of l a s e r c u t t i n g , s c r i b i n g , f r a c t u r e , e t c .

3 These t e c h -

niques were a p p l i e d t o v i r t u a l l y every c l a s s o f m a t e r i a l s i n c l u d i n g many metals, semiconductors, and ceramics. was done w i t h CO,,

ruby, Nd:YAG,

Most o f t h e e a r l y work

and Nd:glass l a s e r s t h a t were of

r e l a t i v e l y low average power by today I s standards.

These a p p l i c a -

t i o n s were o f t e n c h a r a c t e r i z e d by a r a t h e r u n s o p h i s t i c a t e d approach i n which t h e extremely h i g h power d e n s i t i e s o f t i g h t l y focussed beams were used t o m e l t , vaporize, and "explode" t h e m a t e r i a l .

A

review o f t h i s work up t o about 1972 i s g i v e n i n t h e book Lasers i n I n d u s t r y e d i t e d by Charschan (1972).

T h i s book i s s t i l l an

e x c e l l e n t source o f i n f o r m a t i o n on many aspects o f l a s e r physics and l a s e r technology, e s p e c i a l l y as t h e y a r e r e l a t e d t o m a t e r i a l s processing.

Another q u i t e u s e f u l general

r e f e r e n c e on v a r i o u s

aspects o f l a s e r p r o c e s s i n g i s t h e volume by Ready (1971).

Recently

a volume, e d i t e d by Poate and Mayer (1982), on t h e e a r l y phases o f t h e c u r r e n t development o f l a s e r p r o c e s s i n g o f semiconductors has appeared. It was noted i n t h e s e e a r l y a p p l i c a t i o n s t h a t m a t e r i a l s , espec i a l l y metals,

m e l t e d by l a s e r s o f t e n e x h i b i t e d r a t h e r unusual

m e t a l l u r g i c a l c h a r a c t e r i s t i c s and t h i s q u i c k l y l e d t o s t u d i e s which used l a s e r s f o r heat t r e a t i n g , annealing, zone r e f i n i n g , r e c r y s t a l lization,

g r a i n growth, and a v a r i e t y o f o t h e r such a p p l i c a t i o n s .

It was recognized t h a t i r r a d i a t i o n o f m a t e r i a l s w i t h high-powered l a s e r s c o u l d l e a d t o h e a t i n g and c o o l i n g r a t e s s e v e r a l o r d e r s o f magnitude g r e a t e r than those o b t a i n e d by any o t h e r means.

However,

t h e u t i l i z a t i o n o f t h i s aspect o f l a s e r s was i n h i b i t e d by t h e small areas over which h i g h l y u n i f o r m energy d e n s i t i e s c o u l d be o b t a i n e d w i t h l a s e r s a v a i l a b l e a t t h e time.

W i t h t h e gradual improvement i n

l a s e r technology and t h e development o f techniques f o r r a p i d l y and r e p r o d u c i b l y scanning beams over l a r g e areas, t h e i n t e r e s t i n l a s e r s f o r heat t r e a t i n g t o o b t a i n m e t a l l u r g i c a l m o d i f i c a t i o n s o f t h e m i c r o s t r u c t u r e o f m a t e r i a l s has grown r a p i d l y .

A survey o f e a r l y

developments i n t h i s area i s a l s o i n Charschan (1972), and Breinan e t al.

(1976) have given a b r i e f review o f more r e c e n t developments

4

R. F. WOOD ET AL

up t o about 1975 i n t h e area o f what t h e y r e f e r t o as " l a s e r g l a z i n g " . Laser g l a z i n g ,

which u t i l i z e s t h e e x t r e m e l y r a p i d quench r a t e s

c h a r a c t e r i s t i c o f l a s e r processing, has been a p p l i e d p r i m a r i l y t o m e t a l s t o produce a v a r i e t y o f unusual m e t a l l u r g i c a l m i c r o s t r u c t u r e s . T h i s book i s about p u l s e d l a s e r p r o c e s s i n g o f semiconductors. I t i s concerned w i t h t h e remarkably r a p i d e v o l u t i o n and progress o f t h e f i e l d which has t a k e n p l a c e s i n c e about 1976.

An overview

o f t h e developments i n l a s e r a n n e a l i n g o f semiconductors w i l l be given i n t h e next section, but here i t i s useful, i n t h e context o f t h e f o r e g o i n g d i s c u s s i o n , t o d e l i n e a t e what we mean by " l a s e r p r o c e s s i n g " o f semiconductors. any way w i t h micromachining, semiconductors.

The book w i l l n o t be concerned i n scribing,

welding,

or drilling of

These a r e a p p l i c a t i o n s which a r e a l r e a d y we1 1-

developed and i n use; we w i l l c o n s i d e r them t o f a l l i n a category which we can c a l l ' ' l a s e r machining".

The m a t e r i a l s science, metal-

l u r g y , and c r y s t a l l o g r a p h y (and t h e i r a p p l i c a t i o n s ) d e s c r i b e d i n t h i s volume a r e r e l a t e d t o t h e " l a s e r g l a z i n g " phenomena discussed by B r e i n a n e t a l .

(1976).

The i n t e n s e i n t e r e s t and a c t i v i t y i n

t h e area o f science t o be d e s c r i b e d here, has a l r e a d y pushed t h e f r o n t i e r s o f l a s e r p r o c e s s i n g o f semiconductors w e l l beyond t h o s e o f l a s e r processing o f other materials.

Nevertheless, i t must be

emphasized t h a t l a s e r p r o c e s s i n g i s s t i l l i n t h e research and

and t h e i n s t a n c e s o f i t s a d a p t a t i o n t o and i n t e g r a t i o n i n commercial p r o d u c t i o n f a c i l i t i e s a r e s t i l l few.

development stage,

F o r t h i s reason, t h i s book must o f n e c e s s i t y be p r i m a r i l y about t h e fundamentals o f l a s e r p r o c e s s i n g r a t h e r t h a n about i t s demons t r a t e d applications. 111.

1.

Development o f Laser Annealing o f Semiconductors

PULSED LASER ANNEALING

The r e c e n t developments i n l a s e r p r o c e s s i n g o f semiconductors were i n i t i a l l y t i e d c l o s e l y t o t h e problems o f e l e c t r i c a l l y a c t i v a t i n g t h e dopants and removing t h e l a t t i c e damage caused by i o n

5

1. LASER PROCESSING OF SEMICONDUCTORS i m p l a n t a t i o n o f those dopants.

The c o n v e n t i o n a l methods f o r s o l v i n g

t h e s e problems i n v o l v e t h e use o f furnaces t o heat t h e samples t o h i g h temperatures (-lOOO°C)

f o r times s u f f i c i e n t l y long t h a t t h e

l a t t i c e damage i s r e p a i r e d and t h e dopants e l e c t r i c a l l y a c t i v a t e d . U n f o r t u n a t e l y , t h i s high-temperature furnace h e a t i n g o f t h e e n t i r e sample has u n d e s i r a b l e s i d e e f f e c t s f o r d e v i c e f a b r i c a t i o n t h a t

w i l l be discussed i n l a t e r chapters.

One g r e a t advantage o f l a s e r

a n n e a l i n g i s t h a t t h e l a s e r r a d i a t i o n i s h e a v i l y absorbed i n a t h i n s u r f a c e l a y e r a few hundred t o s e v e r a l thousand angstroms deep. T h i s produces t h e very h i g h temperatures (and even m e l t i n g ) i n t h e i m p l a n t e d r e g i o n which a r e necessary f o r

annealing t h e l a t t i c e

damage; y e t t h e absorbed photon energy i s i n s u f f i c i e n t t o r a i s e t h e temperature o f t h e undamaged s u b s t r a t e s i g n i f i c a n t l y above ambient, and hence t h e d e l e t e r i o u s e f f e c t s of h i g h temperatures i n t h i s r e g i o n a r e circumvented.

Although preceded by e a r l i e r e f f o r t s

a t l a s e r p r o c e s s i n g (see Sec. IV.4), t h e r a p i d growth o f i n t e r e s t i n l a s e r p r o c e s s i n g o f semiconductors can be t r a c e d t o t h e work o f S o v i e t s c i e n t i s t s i n t h e p e r i o d 1974-76 on t h e l a s e r i r r a d i a t i o n o f i o n - i m p l a n t e d S i and GaAs.

F o r example, Shtyrkov e t a l . (1976)

observed t h a t p u l s e s from a Nd:YAG l a s e r produced changes i n t h e optical

and e l e c t r i c a l p r o p e r t i e s o f i o n - i m p l a n t e d S i samples.

They r e p o r t e d t h a t t h e l a t t i c e damage caused by t h e i m p l a n t a t i o n process c o u l d be removed and t h e i m p l a n t e d dopants made e l e c t r i c a l l y active.

The S o v i e t s c i e n t i s t s

used t h e t e r m i n o l o g y "laser

annealing" t o d e s c r i b e t h e process and e s t a b l i s h e d many o f i t s most interesting characteristics,

several of which we w i l l now discuss.

The a n n e a l i n g o f l a t t i c e damage by p u l s e d l a s e r i r r a d i a t i o n o f semiconductors S o v i e t work.

has been e x t e n s i v e l y s t u d i e d s i n c e t h e o r i g i n a l Transmission e l e c t r o n microscopy

(TEM) r e v e a l s t h a t ,

a f t e r a s i n g l e p u l s e o f l a s e r r a d i a t i o n of an a p p r o p r i a t e wavel e n g t h and power d e n s i t y , no extended damage remains i n annealed s i l i c o n specimens down t o t h e r e s o l u t i o n of t h e microscopes used, which has been b e t t e r t h a n 10 A (Young e t a1

., 1978).

I n contrast,

a f t e r thermal a n n e a l i n g s i g n i f i c a n t damage u s u a l l y remains i n t h e

6

R. F. WOOD ET AL

f o r m o f d i s l o c a t i o n loops.

This i s i l l u s t r a t e d i n Fig.

1 which

shows a s e r i e s o f micrographs f o r l a s e r - and t h e r m a l l y annealed, i o n - i m p l a n t e d samples,

as d e s c r i b e d i n t h e f i g u r e c a p t i o n .

The

t o t a l lack o f i r r e g u l a r i t i e s i n e l e c t r o n d i f f r a c t i o n patterns from l a s e r - a n n e a l e d samples shows t h a t t h e i m p l a n t e d r e g i o n anneals w i t h t h e same l a t t i c e o r i e n t a t i o n as t h e s u b s t r a t e .

Measurements on

ion-implanted s i l i c o n c r y s t a l s w i t h Rutherford i o n backscatteri n g (RBS) and i o n - c h a n n e l i n g techniques show t h a t t h e long-range c r y s t a l l i n e o r d e r i s r e s t o r e d t o t h e i m p l a n t e d r e g i o n by p u l s e d l a s e r i r r a d i a t i o n , t h u s v e r i f y i n g t h e TEM r e s u l t s .

Such measure-

ments c l e a r l y e s t a b l i s h t h e e f f e c t i v e n e s s o f p u l s e d l a s e r a n n e a l i n g i n removing l a t t i c e damage and r e s t o r i n g c r y s t a l l i n e order. However, t h e r e i s evidence (Mooney e t al.,

1978; K a c h u r i n e t al.,

Benton e t al.,

1983) t h a t small complexes o f

1980; Young e t al.,

vacancies w i t h dimensions l e s s t h a n

- 10 A

1980;

remain o r a r e formed

i n t h e m a t e r i a l a f t e r c e r t a i n t y p e s o f l a s e r annealing.

The e x t e n t

t o which these p o i n t d e f e c t s can be e l i m i n a t e d d u r i n g o r a f t e r l a s e r a n n e a l i n g and t h e i r e f f e c t s on t h e performance o f v a r i o u s devices i s n o t y e t c l e a r (see t h e d i s c u s s i o n s i n Chapters 3 and 10 o f t h i s book ) The e f f e c t i v e n e s s o f p u l s e d l a s e r a n n e a l i n g i n e l e c t r i c a l l y a c t i v a t i n g t h e i m p l a n t e d dopants has been e s t a b l i s h e d by measurements o f t h e sheet c a r r i e r c o n c e n t r a t i o n a f t e r l a s e r a n n e a l i n g o f samples i m p l a n t e d w i t h v a r i o u s dopants o v e r a wide range o f doses. W i t h c o n v e n t i o n a l t h e r m a l a n n e a l i n g i t i s d i f f i c u l t , i f n o t imposs i b l e , t o dope a sample t o c o n c e n t r a t i o n s s u b s t a n t i a l l y above t h e e q u i l i b r i u m s o l u b i l i t y l i m i t ; i t i s remarkable t h a t i n l a s e r anneali n g t h i s l i m i t can be g r e a t l y exceeded.

A s a consequence o f t h i s

d i f f e r e n c e between t h e two t y p e s o f annealing, t h e c a r r i e r concent r a t i o n as a f u n c t i o n o f i m p l a n t e d dose s a t u r a t e s f o r thermal a n n e a l i n g , whereas i n laser-annealed s i l i c o n (Wood and Young, 1980) i t c o n t i n u e s t o i n c r e a s e l i n e a r l y up t o doses t h a t g i v e concentra-

t i o n s w e l l above t h e e q u i l i b r i u m s o l u b i l i t y l i m i t .

This i s i l l u s -

t r a t e d i n F i g . 2, which shows t h e c a r r i e r d e n s i t y as a f u n c t i o n o f

7

1 . LASER PROCESSING OF SEMICONDUCTORS

Fig.

1.

Transmission electron micrographs comparing ( a to c ) laser- and

( d to f ) thermally annealed ion-implanted silicon o f (001 ) orientation.

Implanted

species, energy, dose, projected range, and range straggling were: ( a and d ) I l B (35 keV, 3x1015 cm-2,

1100 A, 420

A ) ; (b and e ) 3 1 P ( 8 0 keV, lx1015

l O O O A , 4 0 0 4 ) ; ( c a n d f ) 75As (lOOkeV, 1x1016cm-*, 560A, 2 0 0 A ) . The boron and phosphorus samples were thermally annealed at llOO°C for 30 cm-2,

minutes and the arsenic sample at 900°C for 30 minutes. Micrographs ( a ) through ( d ) were taken in bright field, and ( e ) and ( f ) in dark field. i s the d i f f r a c t i o n vector.

The symbol g

8

R. F. WOOD ETAL.

i m p l a n t e d dose f o r boron i m p l a n t e d i n t o s i l i c o n a t an energy o f

A dose o f 1 . 5 ~ 1 0 1 6 corresponds t o a c o n c e n t r a t i o n o f

35 keV.

-6x1020/cm3

under t h e l a s e r - a n n e a l i n g c o n d i t i o n s used.

Since t h e

e q u i l i b r i u m s o l u b i l i t y l i m i t o f boron i n s i l i c o n i s -6x10*0/cm3, t h e f i g u r e gives c l e a r evidence t h a t e l e c t r i c a l a c t i v a t i o n can occur w e l l above t h e s o l u b i l i t y l i m i t a t doses o f ~ 3 x 1 0 1 6and h i g h e r . The f o r m a t i o n o f s u p e r s a t u r a t e d s u b s t i t u t i o n a l a l l o y s by l a s e r p r o c e s s i n g techniques

has been demonstrated and s t u d i e d u s i n g

R u t h e r f o r d b a c k s c a t t e r i n g and i o n - c h a n n e l i n g a n a l y s i s (White e t a1 1980, Stuck e t al.,

1980).

.,

A comprehensive i o n - c h a n n e l i n g a n a l y s i s

by White e t a l . (1980) showed t h a t l a s e r a n n e a l i n g o f As-, Ga-, I n - , Sb-, and B i - i m p l a n t e d S i r e s u l t e d i n t h e s u b s t i t u t i o n a l i n c o r p o r a t i o n o f t h e dopants a t c o n c e n t r a t i o n s f a r i n excess o f t h e e q u i l i brium s o l i d s o l u b i l i t y .

T h i s phenomenon, and o t h e r s a s s o c i a t e d w i t h

i t , w i l l be discussed i n d e t a i l i n Chapters 2 and 4.

and co-workers

As K h a i b u l l i n

(1978) recognized, t h e f a c t t h a t equi 1ib r i u m s o l u-

b i l i t y l i m i t s can be exceeded makes i t apparent t h a t t h e phys ica 1

'0l4

Fig.

2.

A

LASER ANNEALING

0

900 "C/30 min

1015 1016 I M P L A N T E D DOSE (crn-')

10'7

C a r r i e r concentration as a function of implanted dose for laser-

and thermally annealed silicon B-implanted

a t an energy o f 35 keV.

9

1. LASER PROCESSING OF SEMICONDUCTORS

processes which t a k e p l a c e d u r i n g p u l s e d l a s e r a n n e a l i n g occur w e l l . away from thermodynamic e q u i l i b r i u m .

The s i g n i f i c a n c e o f these

e f f e c t s f o r device a p p l i c a t i o n s has n o t y e t been e x p l o r e d i n d e t a i l , b u t t h e i r importance f o r i m p r o v i n g our knowledge o f t h e physics of nonequilibrium s o l i d i f i c a t i o n

processes cannot be exaggerated.

Obviously, t h e c a p a b i l i t y o f o b t a i n i n g s u b s t i t u t i o n a l doping conc e n t r a t i o n s which exceed t h e s o l u b i l i t y l i m i t , w h i l e a l s o r e a l i z i n g v i r t u a l l y one hundred p e r c e n t e l e c t r i c a l a c t i v a t i o n ,

provides a

unique t o o l f o r s t u d y i n g heavy-doping e f f e c t s i n semiconductors (Miyao e t a l . ,

1981).

I n t h e e a r l i e s t S o v i e t l i t e r a t u r e on t h e s u b j e c t , i t was noted t h a t l a s e r a n n e a l i n g d i d n o t s i g n i f i c a n t l y reduce t h e m i n o r i t y c a r r i e r l i f e t i m e (MCL) i n t h e s u b s t r a t e .

E x t e n s i v e measurements

by s e v e r a l groups have c o n f i r m e d t h a t values o f t h e MCL i n t h e base r e g i o n b e f o r e and a f t e r l a s e r a n n e a l i n g a r e v e r y n e a r l y equal,

whereas thermal a n n e a l i n g a t 1100°C f o r t h i r t y minutes

reduces t h e MCL by a f a c t o r o f about t e n (Young e t al.,

1978).

On t h e o t h e r hand, t h e r e have been some i n d i c a t i o n s t h a t t h e p-n j u n c t i o n leakage c u r r e n t s i n t h e laser-annealed samples a r e somewhat h i g h and, i f so, t h i s may be r e l a t e d t o r e s i d u a l d e f e c t s l e f t i n t h e laser-annealed l a y e r . An i m p o r t a n t c o n s i d e r a t i o n i n t h e a p p l i c a t i o n o f l a s e r processi n g o f semiconductors

i s t h e f a c t t h a t pulsed l a s e r annealing

u s u a l l y r e s u l t s i n a s u b s t a n t i a l spreading o f t h e c o n c e n t r a t i o n p r o f i l e s o f implanted dopants (Kachurin e t a l . 1978; C e l l e r e t al.,

1978; White e t al.,

, 1976a;

1978).

Young e t a l .

,

This i s i l l u s t r a t e d

i n F i g . 3a, which shows how t h e dopant p r o f i l e s i n B-implanted S i vary w i t h t h e energy d e n s i t y o f i n d i v i d u a l p u l s e s from t h e ruby l a s e r used f o r t h e annealing.

Dopant r e d i s t r i b u t i o n o f t h e magni-

t u d e shown i n Fig. 3a cannot be e x p l a i n e d by any known mechanism o f d i f f u s i o n i n t h e s o l i d f o r t h e t i m e s i n v o l v e d , and i t s t r o n g l y suggests t h a t t h e near-surface annealing.

region melts during pulsed l a s e r

R e d i s t r i b u t i o n o f i m p l a n t e d dopants may be e i t h e r an

advantage o r a disadvantage depending on t h e a p p l i c a t i o n o f l a s e r

Fig. 3. Concentration profiles o f 6 in Si before and a f t e r laser annealing.

Panel ( a )

illustrates :he profile spreading that accompanies annealing w i t h pulses of various energy densities.

Panel ( b ) illustrates the effects of up to three successive pulses of 1 . 1 j / c m 2 .

11

1 . LASER PROCESSING OF SEMICONDUCTORS p r o c e s s i n g t h a t i s b e i n g considered.

F i g u r e 3b shows t h e gradual

f l a t t e n i n g o f dopant p r o f i l e s as a r e s u l t o f t h r e e successive l a s e r pulses.

A f t e r t h e n a t u r e o f t h e p u l s e d l a s e r a n n e a l i n g process i s

d e s c r i b e d i n more d e t a i l below, i t w i l l be apparent t h a t repeated l a s e r pulses can l e a d t o very n e a r l y f l a t p r o f i l e s which t e r m i n a t e a b r u p t l y a t t h e maximum depth o f m e l t i n g o b t a i n e d f o r a g i v e n s e t

o f l a s e r a n n e a l i n g parameters.

T h i s i s another i n d i r e c t i n d i c a t i o n

t h a t m e l t i n g occurs t o a depth determined by t h e l a s e r i r r a d i a t i o n parameters.

2.

THEORETICAL MODELING OF PULSED LASER ANNEALING The r e s u l t s o f mathematical modeling o f t h e p u l s e d l a s e r anneal-

i n g process have been i n v a l u a b l e i n e s t a b l i s h i n g t h e p h y s i c a l mechanisms i n v o l v e d . (Baeri e t a l . , e t al.,

C a l c u l a t i o n s w i t h thermal m e l t i n g models

1978, Wang e t al.,

1979, Wood e t al.,

1978, B a e r i e t al.,

1979a, Surko

1980, Wood and G i l e s , 1981) were c a r r i e d

o u t s h o r t l y a f t e r t h e experimental d a t a began t o accumulate.

The

r e s u l t s gave c o n v i n c i n g evidence t h a t t h e near-surface r e g i o n o f a sample m e l t s d u r i n g p u l s e d l a s e r annealing.

The c a l c u l a t i o n s a l s o

e s t a b l i s h e d t h a t , because d i f f u s i o n c o e f f i c i e n t s i n molten s i l i c o n a r e many o r d e r s o f magnitude h i g h e r t h a n i n t h e s o l i d , t h e spreading o f dopant p r o f i l e s d u r i n g l a s e r a n n e a l i n g was r e a d i l y e x p l a i n e d by t h e m e l t i n g model.

The most s i g n i f i c a n t r e s u l t s o f thermal

transport calculations

(Wood and G i l e s , 1981) a r e i l l u s t r a t e d i n

Fig. 4.

The l e f t - h a n d panel shows c a l c u l a t e d temperature p r o f i l e s

a t v a r i o u s times a f t e r i n i t i a t i o n o f t h e l a s e r pulse.

As discussed

i n Chapter 4, t h e s e and s i m i l a r curves a r e o b t a i n e d from numerical s o l u t i o n s o f t h e one-dimensional heat c o n d u c t i o n equation, generali z e d t o a l l o w f o r t h e p o s s i b i l i t y o f phase changes ( m e l t i n g and v a p o r i z a t i o n ) and f o r temperature-dependent properties.

thermal and o p t i c a l

The break i n each curve a t t h e m e l t i n g temperature

indicates t h e p o s i t i o n o f t h e melt f r o n t a t t h e time a f t e r t h e b e g i n n i n g o f t h e l a s e r p u l s e f o r which t h e c u r v e i s shown.

From

0.8 2400

.-~ I

---- TMnx

2000

-p W

LT

$

\

I

I

= 2220 "C

I

I

'2

Ed= 1.75 J/cm

0.7

I

-

0.6 E i

z 0.5 2 k

1600 'M

v)

0 a 0.4

1200

[L

c

a

0

z

w

0.3

800

5 W I

400

0.2

0 0

Fig. 4. laser pulse.

0

100

200 TIME (nsec)

300

400

L e f t panel: Temperature as a function o f depth at several times t a f t e r beginning o f the Right panel: Melt front position as a function o f time and laser energy density,

i s the pulse duration.

EQ;

13


a s e r i e s of curves such as these, t h e p o s i t i o n o f t h e m e l t f r o n t as a f u n c t i o n o f t i m e can be determined; t y p i c a l r e s u l t s a r e shown i n t h e r i g h t hand panel o f Fig. 4.

F o r Ell

= 1.75 J/cm*,

t h e melt

f r o n t very r a p i d l y p e n e t r a t e s t o a depth o f about 0.7 urn i n t h e s o l i d , b e f o r e r e c e d i n g back t o t h e s u r f a c e w i t h an average v e l o c i t y o f approximately 3-4 m/sec.

While t h i s occurs, a r e g i o n a p p r o x i -

mately 0.4-pm t h i c k remains i n t h e m o l t e n s t a t e f o r t i m e s o f t h e o r d e r of a hundred nanoseconds, d u r i n g which t h e dopants d i f f u s e i n t h e l i q u i d where d i f f u s i o n c o e f f i c i e n t s a r e so much h i g h e r t h a n i n the solid.

Dopant p r o f i l e s f o r v a r i o u s dopants i n s i l i c o n , c a l -

c u l a t e d by assuming t h a t t h e i m p l a n t e d i o n s d i f f u s e i n t h e l i q u i d , a r e i n good agreement w i t h experimental p r o f i l e s ( B a e r i e t al.,

1978;

Wang e t al.,

1980;

Wood e t a1

1978; B a e r i e t al.,

., 1981a).

1979a; K i r k p a t r i c k e t al.,

Based on t h e experimental and t h e o r e t i c a l r e s u l t s d i s c u s s e d thus far,

t h e p u l s e d l a s e r - a n n e a l i n g process i n t h e nanosecond

regime can be p i c t u r e d as f o l l o w s .

The i n c i d e n t l a s e r energy i s

absorbed through e l e c t r o n i c e x c i t a t i o n s and q u i c k l y t r a n s f e r r e d t o t h e l a t t i c e , m e l t i n g t h e c r y s t a l t o a depth g r e a t e r t h a n t h a t o f t h e implanted p r o f i l e and accompanying l a t t i c e damage.

The m e l t e d

r e g i o n t h e n r e c r y s t a l l i z e s from t h e u n d e r l y i n g undamaged s u b s t r a t e by means o f l i q u i d phase e p i t a x i a l regrowth, p e r f e c t s i ngl e-crystal

resulting i n nearly

m a t e r i a l w i t h dopants i n s u b s t i t u t i o n a l

s i t e s i n t h e l a t t i c e . T h i s u l t r a r a p i d m e l t i n g and r e s o l i d i f i c a t i o n sequence has been e x t e n s i v e l y s t u d i e d w i t h a v a r i e t y o f t i m e resolved o p t i c a l

(Auston e t al.,

1978a;

Lowndes, 1982), e l e c t r i c a l ( G a l v i n e t al., e t al.,

Lowndes e t al.,

1981;

1982), and x-ray (Larson

1982) t e c h n i q u e s which w i l l be discussed i n d e t a i l i n

Chapter 6.

D u r i n g t h e t i m e t h e i m p l a n t e d r e g i o n i s molten, dopants

d i f f u s e r a p i d l y i n t h e l i q u i d l a y e r , and hence s u b s t a n t i a l spreading o f dopant p r o f i l e s i s observed.

However, t h e observed dopant d i s t r i -

b u t i o n s are n o t those c h a r a c t e r i s t i c o f r e c r y s t a l l i z a t i o n processes o c c u r r i n g near thermodynamic e q u i l i b r i u m , as we s h a l l now e x p l a i n .

14

R. F. WOOD ETAL.

I n t h e t h e o r y o f c r y s t a l growth o f a d i l u t e b i n a r y a l l o y (see e.g.,

Smith e t a1

., 1955), t h e

i n t e r f a c e segregation c o e f f i c i e n t

k i o f t h e s o l u t e i s d e f i n e d as t h e r a t i o of t h e s o l u t e concentrat i o n i n t h e s o l i d t o t h e solute concentration i n t h e l i q u i d a t t h e l i q u i d - s o l i d interface.

I f k i = 1, t h e s o l u t e i s e n t i r e l y i n c o r -

p o r a t e d i n t o t h e b u l k o f t h e s o l i d and no s e g r e g a t i o n t o t h e s u r f a c e occurs.

When k i d e p a r t s s i g n i f i c a n t l y f r o m u n i t y ,

segregation

e f f e c t s b e g i n t o m a n i f e s t themselves by an accumulation of i m p u r i t i e s i n f r o n t o f t h e advancing l i q u i d - s o l i d i n t e r f a c e ; t h i s w e l l known e f f e c t i s t h e b a s i s f o r f l o a t - z o n e r e f i n i n g .

For c r y s t a l

growth near thermodynamic e q u i l i b r i u m , kq has t h e values ky = 0.80,

0.35, and 0.30 f o r 8, P , and As i n S i , r e s p e c t i v e l y .

I n t h e case

o f p u l s e d l a s e r annealing, we would expect s e g r e g a t i o n t o produce pronounced s p i k e s i n t h e dopant c o n c e n t r a t i o n j u s t a t t h e surface, p r o v i d e d t h e r e i s no l o s s o f dopant.

No such s p i k e s appear i n t h e

p r o f i l e s o f B y P, and As i n l a s e r annealed s i l i c o n and y e t no s i g n i f i c a n t loss o f dopant occurs.

Moreover, when c a l c u l a t i n g t h e

p r o f i l e s o f these dopants, o n l y a value o f k i = 1 g i v e s s a t i s f a c t o r y f i t s f o r t h e l a s e r a n n e a l i n g c o n d i t i o n s used t h u s f a r .

I f recrystal-

l i z a t i o n o c c u r r e d near e q u i l i b r i u m , s e g r e g a t i o n e f f e c t s should have been observed f o r P and As, and hence we a r e f o r c e d t o conclude t h a t t h e c r y s t a l regrowth d u r i n g p u l s e d l a s e r a n n e a l i n g i s a n o n e q u i l i b r i u m process.

Much more d r a m a t i c e f f e c t s have been observed f o r

1979b; White 1980), and these

i m p u r i t i e s w i t h very small values o f k q ( B a e r i e t al., e t al.,

1979; C u l l i s e t a l . ,

1980; White e t a l . ,

w i l l be discussed i n d e t a i l i n Chapter 2.

F u r t h e r evidence f o r

t h e n o n e q u i l i b r i u m n a t u r e of p u l s e d l a s e r a n n e a l i n g comes from t h e c e l l u l a r s t r u c t u r e t h a t i s observed i n t h e d i s t r i b u t i o n o f some dopants a f t e r l a s e r a n n e a l i n g (van Gurp e t al.,

1980; Narayan,

1980).

1979; C u l l i s e t a l . ,

This s t r u c t u r e i s c h a r a c t e r i s t i c o f t h e

breakdown o f a p l a n a r m e l t f r o n t due t o c o n s t i t u t i o n a l s u p e r c o o l i n g and t h e c o n d i t i o n s under which i t appears have been t r e a t e d t h e o r e t i c a l l y by a number o f authors, b u t i n a p a r t i c u l a r l y e l e g a n t manner by M u l l i n s and Sekerka (1964).

The c e l l u l a r f o r m a t i o n which occurs

15

1 . LASER PROCESSING OF SEMICONDUCTORS

d u r i n g p u l s e d l a s e r a n n e a l i n g can be understood w i t h t h e M u l l i n s and Sekerka t h e o r y o n l y i f n o n e q u i l i b r i u m s e g r e g a t i o n e f f e c t s a r e i n c l u d e d (Narayan, 1981; Wood, 1982).

Another remarkable i l l u s -

t r a t i o n o f t h e occurrence o f n o n e q u i l i b r i u m e f f e c t s d u r i n g p u l s e d l a s e r a n n e a l i n g i s t h e o b s e r v a t i o n by s e v e r a l groups o f t h e conv e r s i o n o f molten s i l i c o n t o amorphous s i l i c o n a t very h i g h (15-20 m/sec) regrowth v e l o c i t i e s ( L i u e t al., C u l l i s e t al.,

1982).

1979; Tsu e t al.,

1979a;

T h i s aspect o f l a s e r a n n e a l i n g i s d i s c u s s e d

i n several chapters o f . t h i s book.

3.

CW LASER ANNEALING S h o r t l y a f t e r t h e i n i t i a l work o f S h t y r k o v e t a l .

a p u l s e d Nd:YAG l a s e r , Kachurin e t a l .

(1976) w i t h

(1976b) and Klimenko e t a l .

(1976) e s t a b l i s h e d t h a t cw l a s e r s c o u l d a l s o produce annealing. Annealing w i t h cw l a s e r s d i f f e r s f r o m p u l s e d l a s e r a n n e a l i n g i n t h a t t h e c h a r a c t e r i s t i c t i m e s i n v o l v e d a r e much l o n g e r and m e l t i n g i s u s u a l l y n o t a1 lowed t o occur (Kachurin e t a l .

, 1976b;

1976; Gat and Gibbons, 1978; W i l l i a m s e t al., 1978b).

K1 imenko e t a1

.,

1978; Auston e t al.,

The t y p i c a l d w e l l t i m e o f t h e beam on a g i v e n p o i n t o f t h e

sample d u r i n g cw l a s e r a n n e a l i n g i s o f t h e o r d e r o f msec and t h e s u r f a c e temperature i s h e l d below t h e m e l t i n g p o i n t so t h a t s o l i d phase e p i t a x i a l regrowth occurs.

S i g n i f i c a n t dopant r e d i s t r i b u t i o n

i n t h e regrown l a y e r i s n o t observed, s i n c e regrowth t a k e s p l a c e i n t h e near-surface r e g i o n i n t i m e s t o o s h o r t f o r s o l i d s t a t e d i f f u s i o n . As w i t h p u l s e d l a s e r annealing, complete e l e c t r i c a l a c t i v a t i o n o f

dopants can be achieved and s o l u b i l i t y l i m i t s exceeded ( L i e t o i l a , e t al.,

1979).

However, i n c o n t r a s t t o p u l s e d laser-annealed samples

i n which a d i s l o c a t i o n - f r e e e p i t a x i a l l a y e r can u s u a l l y be obtained, t h e cw laser-annealed

samples n o r m a l l y c o n t a i n some s t r u c t u r a l

d e f e c t s such as m i s f i t d i s l o c a t i o n s , s t a c k i n g f a u l t s , and d i s l o c a t i o n loops.

However, t h e i r d e n s i t y has been shown t o be l e s s t h a n

t h a t i n t h e r m a l l y annealed samples (Gat e t al.,

1978a).

d e f e c t s have been found by b o t h DLTS (Johnson e t al.,

Several 1979) and

16

R. F. WOOD ET AL.

luminescence ( S t r e e t e t al.,

1979; Mizuta, e t a l . ,

1981) s t u d i e s ,

b u t most o f them can be removed by p o s t - i r r a d i a t i o n thermal a n n e a l i n g above 700°C. I n t h e commonly used cw l a s e r a n n e a l i n g systems,

t h e beam i s

focused t o a d e s i r e d s p o t s i z e t h r o u g h a l e n s and t h e a n n e a l i n g can be accomplished e i t h e r by scanning t h e sample under t h e beam on a microprocesser c o n t r o l l e d X-Y t a b l e o r by d e f l e c t i n g t h e beam across t h e sample w i t h an automated X-Y m i r r o r system. s u b s t r a t e h e a t i n g (300-350°C)

Supplemental

i s e s s e n t i a l i n most a p p l i c a t i o n s o f

cw l a s e r a n n e a l i n g t o reduce t h e thermal g r a d i e n t s d u r i n g l o c a l i z e d l a s e r i r r a d i a t i o n so t h a t s u r f a c e s l i p and c r a c k i n g can be prevented and a b e t t e r q u a l i t y o f regrown l a y e r can be o b t a i n e d (Rozgonyi e t al.,

1979).

F o r a p a r t i c u l a r d w e l l t i m e o f t h e l a s e r beam on

an area o f t h e i m p l a n t e d l a y e r , t h e r e i s a minimum s u r f a c e temp e r a t u r e t h a t must be reached f o r f u l l annealing.

Therefore, t h e

c o n t r o l o f s u r f a c e temperature t h r o u g h t h e l a s e r energy d e n s i t y and t h e spot d w e l l t i m e must be p r e c i s e l y m a i n t a i n e d i n o r d e r t o ensure good e p i t a x i a l growth ( H i l l

,

1981).

I n v e s t i g a t i o n s o f cw

l a s e r - i n d u c e d r e c r y s t a l 1i z a t i o n o f i o n - i m p l a n t e d S i by R u t h e r f o r d b a c k s c a t t e r i n g ( W i l l i a m s e t al.,

1978;

C h r i s t o d o n l i d e s e t al.,

1978) show t h a t t h e p h y s i c a l mechanisms o f regrowth are, i n many respects, s i m i l a r t o those o f f u r n a c e annealing.

Several f e a t u r e s

c h a r a c t e r i s t i c o f furnace-annealed samples a r e p r e s e n t i n cw l a s e r annealed samples,

b u t n o t i n p u l s e d l a s e r - a n n e a l e d samples.

For

example, i n s o l i d phase e p i t a x i a l regrowth i n a furnace, t h e growth r a t e and t h e q u a l i t y o f t h e regrown l a y e r a r e dependent on t h e i m p l a n t e d dose,

substrate

orientation,

and i m p l a n t e d species.

Furthermore, t h e p e r f e c t i o n o f t h e regrown l a y e r i s extremely sens i t i v e t o t h e m i c r o s t r u c t u r e a t t h e i n t e r f a c e between t h e damaged r e g i o n and t h e u n d e r l y i n g c r y s t a l 1 i n e s u b s t r a t e .

High-dose o r high-

c u r r e n t i o n i m p l a n t a t i o n may be accompanied by s e l f annealing, which

w i l l p a r t i a l l y d e s t r o y t h e amorphous l a y e r t h a t i s o f t e n produced and cause s e r i o u s problems i n s o l i d phase e p i t a x i a l regrowth.

For

1.

17

LASER PROCESSING OF SEMICONDUCTORS

s i m i a r reasons, o v e r l a p p i n g l a s e r scans can cause p a r t i a l r e c r y s t a l l z a t i o n and e f f e c t t h e q u a l i t y o f t h e e p i t a x i a l regrowth.

All

these phenomena have been observed by W i l l i a m s (1980) i n cw l a s e r annealed S i and c o n f i r m t h e n a t u r e o f solid-phase r e c r y s t a l l i z a t i o n by l a s e r s .

CW l a s e r a n n e a l i n g o f i o n - i m p l a n t e d GaAs has n o t been successful. The problems o f s u r f a c e s l i p and c r a c k i n g d u r i n g l a s e r scanning a r e more s e r i o u s i n GaAs t h a n i n S i . 1980; Olson e t al.,

Several s t u d i e s (Anderson e t al.,

1980a; W i l l i a m s and H a r r i s o n , 1981) have i n d i -

c a t e d t h a t a cw l a s e r power "window" f o r a n n e a l i n g GaAs may n o t e x i s t , i.e.,

a t l a s e r powers j u s t below t h e t h r e s h o l d f o r s u r f a c e

damage, t h e s u r f a c e temperature and t i m e s (< 100 msec) a r e n o t s u f f i c i e n t t o remove t h e l a t t i c e d i s o r d e r i n i o n - i m p l a n t e d GaAs.

IV. 4.

Other Forms o f Laser Processing

BACKGROUND

Even b e f o r e t h e S o v i e t work on l a s e r a n n e a l i n g o f i o n - i m p l a n t e d samples appeared, t h e r e s u l t s o f s e v e r a l a t t e m p t s a t v a r i o u s t y p e s

o f l a s e r p r o c e s s i n g o f semiconductors had been reported.

Rao (1968)

r e p o r t e d t h a t r e s i s t i v i t y changes i n s i l i c o n c o u l d be induced by i r r a d i a t i o n w i t h a ruby l a s e r .

Solomon and M u e l l e r (1968) o b t a i n e d

a p a t e n t on a l a s e r - r e l a t e d method f o r f o r m i n g p-n j u n c t i o n s i n s i l i c o n and GaAs immersed i n a doping atmosphere o f a r s e n i c o r antimony.

F a i r f i e l d and Schwuttky (1968) showed t h a t p-n j u n c t i o n s

c o u l d be formed by d e p o s i t i n g a t h i n f i l m o f phosphorus on s i l i c o n and i r r a d i a t i n g t h e sample w i t h a p u l s e d ruby l a s e r .

Probably

because o f t h e s t a t e o f l a s e r t e c h n o l o g y a t t h a t time, t h e q u a l i t y

o f t h e j u n c t i o n s was n o t high, and t h i s may have caused t h e t e c h n i q u e t o have been overlooked. Pounds e t a l .

(1974) demonstrated t h a t l a s e r s can be used t o

form ohmic c o n t a c t s i n III-V compound semiconductors. L a f f and Hutchings (1974) r e p o r t e d t h a t t h e r a d i a t i o n from a scanned A r - i o n

18

R. F. WOOD ET AL.

l a s e r can induce r e c r y s t a l l i z a t i o n o f f i n e - g r a i n e d p o l y c r y s t a l l i n e s i l i c o n f i l m s d e p o s i t e d on f u s e d s i l i c a s u b s t r a t e s ; c r y s t a l l i t e s as l a r g e as 5 pm were observed.

I n t h i s s e c t i o n , some o f t h e s e

o t h e r forms o f l a s e r p r o c e s s i n g o f semiconductors w i l l be discussed briefly. 5.

LASER-INDUCED DIFFUSION OF DOPANTS

a.

S o l i d Sources

I t has been shown t h a t p-n j u n c t i o n s can be formed i n S i by means o f l a s e r - i n d u c e d d i f f u s i o n o f s u r f a c e - d e p o s i t e d dopant f i l m s ( F a i r f i e l d and Schwuttke, e t al., 1975;

1968; Harper and Cohen,

1978; A f f o l t e r e t al., Young e t a l .

,

1970; Narayan

1978) and GaAs ( P i l i p o v i c h e t al.,

1979b) , w i t h o u t any i o n - i m p l a n t a t i o n and/or

t h e r m a l - d i f f u s i o n steps.

I n t h i s approach, a t h i n dopant f i l m i s

d e p o s i t e d on t h e sample by e-beam e v a p o r a t i o n ,

o r by any o t h e r

technique

which y i e l d s

(painting,

spraying,

reasonably u n i f o r m f i l m .

spin-on,

etc.)

a

A f t e r i r r a d i a t i o n o f the f i l m s with a

p u l s e d l a s e r , t h e dopants a r e i n c o r p o r a t e d i n t o t h e sample s u b s t i t u t i o n a l l y and e l e c t r i c a l l y a c t i v a t e d as a consequence o f l i q u i d phase d i f f u s i o n d u r i n g l a s e r - i n d u c e d s u r f a c e m e l t i n g .

In sili-

con, t h e doped l a y e r s u s u a l l y have about t h e same q u a l i t y as i o n implanted,

laser-annealed layers,

a r e u s u a l l y q u i t e poor.

b u t i n GaAs t h e p-n j u n c t i o n s

Dopant c o n c e n t r a t i o n s may exceed t h e

s o l i d s o l u b i l i t y l i m i t i f h i g h l y c o n c e n t r a t e d dopant sources a r e used (Narayan e t al.,

1978).

p-n j u n c t i o n s i l i c o n s o l a r c e l l s w i t h

e f f i c i e n c i e s approaching t h o s e o f i on-imp1 anted,

1aser-annealed

c e l l s have been f a b r i c a t e d u s i n g t h i s t e c h n i q u e (Young e t a1 Fogarrasy e t al.,

1981).

., 1980;

Laser-induced d i f f u s i o n , e s p e c i a l l y w i t h

a s u i t a b l e l o w - c o s t f i l m d e p o s i t i o n technique, c o u l d be q u i t e u s e f u l f o r t h e large-volume p r o d u c t i o n o f s o l a r c e l l s o r o t h e r b a s i c e l e c t r o n i c s t r u c t u r e s such as p-n j u n c t i o n diodes, ohmic contacts, back surface f i e l d s , etc. a r e needed.

,

s i n c e n e i t h e r masking n o r vacuum t e c h n o l o g y

1. b.

19

LASER PROCESSING OF SEMICONDUCTORS

L i q u i d and Gaseous Sources An obvious e x t e n s i o n o f t h e s t u d i e s o f l a s e r doping from s o l i d

sources i s work on doping from l i q u i d and gaseous sources.

Stuck

e t a l . (1981) have shown t h a t h i g h doping c o n c e n t r a t i o n s and s a t i s f a c t o r y p-n j u n c t i o n s can be o b t a i n e d u s i n g one o r two pulses o f l a s e r r a d i a t i o n i n c i d e n t on a s i l i c o n s u r f a c e i n c o n t a c t w i t h a l i q u i d c o n t a i n i n g t h e d e s i r e d dopant.

Doping d i r e c t l y from t h e

gaseous s t a t e has been demonstrated by Turner e t a l .

(1981).

The

low d e n s i t y o f dopant i o n s i n t h e gaseous s t a t e , even a f t e r phot o l y s i s , would seem t o make t h i s method c o n s i d e r a b l y l e s s a t t r a c t i v e t h a n l a s e r - i n d u c e d d i f f u s i o n from s o l i d and l i q u i d sources. Indeed, Deutsch e t a l . (1979,1981)

found t h e y had t o i r r a d i a t e t h e

same spot on t h e sample w i t h 25 pulses from t h e l a s e r b e f o r e s a t i s f a c t o r y doping l e v e l s c o u l d be obtained.

Increasing t h e pressure

of t h e gas and o t h e r developments may make t h i s method o f doping u s e f u l i n some instances,

b u t c o n s i d e r a b l e research i s necessary

b e f o r e t h e f u t u r e of l a s e r - i n d u c e d gaseous doping can be p r o p e r l y evaluated.

6.

SILICIDE FORMATION Because o f c e r t a i n l i m i t a t i o n s t o s i l i c i d e f o r m a t i o n by conven-

t i o n a l p r o c e s s i n g (see, e t al.,

f o r example,

t h e volume e d i t e d by Poate

1978a), t h e use o f l a s e r r a d i a t i o n t o promote t h e r e a c t i o n

of metal f i l m s w i t h s i l i c o n s u b s t r a t e s i s another p r o m i s i n g area o f l a s e r processing.

Potential applications include the formation

o f gate m a t e r i a l i n MOS t r a n s i s t o r s , device i n t e r c o n n e c t s and ohmic contacts, etc.

As w i t h l a s e r a n n e a l i n g o f i o n - i m p l a n t e d s i l i c o n ,

b o t h pulsed and cw l a s e r s have been used i n t h i s t y p e of process. The mechanism o f s i l i c i d e f o r m a t i o n i n t h e case o f p u l s e d i r r a d i a t i o n i n v o l v e s m e l t i n g and i n t e r d i f f u s i o n o f t h e c o n s t i t u e n t s i n t h e molten phase, f o l l o w e d by r a p i d s o l i d i f i c a t i o n (van Gurp e t al., 1979; Poate e t al., and von Allmen,

1978b; von Allmen and Wittmer,

1979).

1979; Wittmer

S i l i c i d e s w i t h m u l t i p l e phases, many o f

20

R. F. WOOD ET AL.

which a r e thermodynamically metastable,

a r e observed and , as a

consequence o f c o n s t i t u t i o n a l s u p e r c o o l i n g , morphologies o f t h e n e a r - s u r f a c e r e g i o n s e x h i b i t c e l l u l a r s t r u c t u r e s (van Gurp e t a l . 1979; Poate e t al.,

1978b).

,

On t h e o t h e r hand, s i l i c i d e f o r m a t i o n

by cw l a s e r i r r a d i a t i o n i s very s i m i l a r t o t h a t observed w i t h f u r nace h e a t i n g , i n which s o l i d - s t a t e d i f f u s i o n dominates t h e process (Shibata e t al.,

1980; Shibata e t al.,

1981).

With b o t h types o f

l a s e r i r r a d i a t i o n , new m e t a s t a b l e s i l i c i d e phases u n a t t a i n a b l e by thermal a n n e a l i n g can be formed.

Research on t h e l a s e r f o r m a t i o n

o f new s i l i c i d e s w i t h low enough sheet r e s i s t i v i t i e s t o s a t i s f y a new g e n e r a t i o n o f V L S I t e c h n o l o g i e s and f o r o t h e r a p p l i c a t i o n s such as superconducting t h i n f i l m s has been pursued i n s e v e r a l 1a b o r a t o r i e s . 7.

OHMIC CONTACTS TO GaAs The major problems encountered w i t h t h e conventional f a b r i c a t i o n

o f e u t e c t i c c o n t a c t s t o GaAs devices stem from t h e high-temperature t r e a t m e n t o f t h e e n t i r e sample f o r l o n g t i m e s and from f o r m a t i o n of t h e l i q u i d phase.

These problems can be g r e a t l y d i m i n i s h e d when

l o c a l i z e d t r a n s i e n t h e a t i n g by l a s e r s i s u t i l i z e d . The f i r s t s t u d i e s o f t h e use o f p u l s e d l a s e r r a d i a t i o n f o r t h e f o r m a t i o n o f e u t e c t i c c o n t a c t s i n GaAs a t t a i n e d o n l y l i m i t e d success (Pounds e t a l . Margalit e t al.

,

1978).

, 1974;

Subsequently Eckhardt (1980) s t u d i e d i n

more d e t a i l t h e f o r m a t i o n o f AuGe- and InAuGe-based ohmic c o n t a c t s i n n-type GaAs u s i n g p u l s e d CO,, cw A r - i o n l a s e r .

and ruby l a s e r s , and a

The best r e s u l t s were o b t a i n e d by i r r a d i a t i o n w i t h

t h e cw A r - i o n l a s e r . periods,

Nd:YAG,

Because o f t h e l o c a l i z e d h e a t i n g f o r b r i e f

t h e s u r f a c e morphology,

compositional u n i f o r m i t y ,

and

dimensional c o n t r o l were f a r s u p e r i o r t o furnace-annealed contacts. S p e c i f i c c o n t a c t r e s i s t a n c e s as low as 1 x 10’6 ohm-cm* were o b t a i n e d (Eckhardt e t al.,

1980).

refractory metal/epitaxial

The use o f a p u l s e d ruby l a s e r t o form Ge ohmic c o n t a c t s t o n-GaAs has been

s t u d i e d by Anderson e t a l . (1981).

Ta/Ge c o n t a c t s t o 2

x

1017 cm-3

21

1. LASER PROCESSING OF SEMICONDUCTORS doped GaAs w i t h s p e c i f i c c o n t a c t r e s i s t a n c e s as low as 1 ohm-cm2 were obtained;

x

t h i s i s more t h a n an o r d e r o f magnitude

lower t h a n t h e s p e c i f i c r e s i s t a n c e o f t h e same t y p e o f c o n t a c t s formed by t h e thermal a n n e a l i n g process ( 65OoC/5 min).

A1 though

t h e experimental d a t a r e p o r t e d so f a r make i t c l e a r t h a t l a s e r p r o c e s s i n g can be used t o produce ohmic c o n t a c t s w i t h p r o p e r t i e s i n many respects s u p e r i o r t o f u r n a c e annealing, f u r t h e r experiments t o e v a l u a t e c o n t a c t s on completed devices,

especially tests f o r

r e l i a b i l i t y and l i f e t i m e , a r e r e q u i r e d . 8.

LASER-INDUCED EPITAXIAL GROWTH OF DEPOSITED S i FILMS Techniques f o r t h e growth o f h i g h q u a l i t y t h i n e p i t a x i a l f i l m s

on s i n g l e - c r y s t a l

s u b s t r a t e s w i t h l i t t l e o r no dopant r e d i s t r i b u -

t i o n a t t h e i n t e r f a c e have been sought f o r years.

Many e f f o r t s i n

t h e past have been concentrated on t h e study o f solid-phase c r y s t a l l i z a t i o n o f an evaporated amorphous S i f i l m on S i by c o n v e n t i o n a l h e a t i n g a t t h e c r y s t a l 1 i z a t i o n temperature o f 500-600°C e t al.,

1974; Canali e t al.,

Anderson,

1977).

1975; C h r i s t o u e t al.,

(Canali

1977; Roth and

The advantages o f t h i s t e c h n i q u e compared t o

e p i t a x i a l growth by chemical vapor d e p o s i t i o n a r e t h e easy c o n t r o l o f f i l m t h i c k n e s s and low p r o c e s s i n g temperatures t h a t a r e required. However,

t h e growth o f good q u a l i t y s i l i c o n f i l m s by solid-phase

e p i t a x y (SPE) n o r m a l l y r e q u i r e s an u l t r a - h i g h vacuum (UHV) ( < 1 0 - l 0 t o r r ) system because SPE growth i s extremely s e n s i t i v e t o contamina n t s a t t h e growth i n t e r f a c e and t o i m p u r i t i e s t r a p p e d i n t h e f i l m . I n any case, i t seems l i k e l y t h a t t h e combination o f low temperat u r e f i l m d e p o s i t i o n technology w i t h l o c a l i z e d and t r a n s i e n t heat t r e a t m e n t by l a s e r i r r a d i a t i o n can broaden t h e range o f a t t a i n a b l e f i l m p r o p e r t i e s and add f l e x i b i l i t y t o semiconductor device design. CW l a s e r s have been used t o c r y s t a l l i z e e-beam d e p o s i t e d S i

films. that,

Olson e t al.,

(1980b) and Roth e t a l .

as w i t h c o n v e n t i o n a l SPE,

(1981) have found

good q u a l i t y e p i t a x i a l f i l m s can

22

R. F. WOOD ET AL..

be o b t a i n e d o n l y i f t h e e n t i r e process, which i n c l u d e s s u r f a c e cleaning,

f i l m d e p o s i t i o n , and l a s e r c r y s t a l l i z a t i o n , i s c a r r i e d

o u t under UHV c o n d i t i o n s and w i t h o u t b r e a k i n g t h e vacuum between steps.

The presence o f n a t i v e oxides a t t h e i n t e r f a c e o r t h e

a b s o r p t i o n o f i m p u r i t i e s d u r i n g exposure t o t h e a i r w i l l u s u a l l y lead t o t h e formation o f p o l y c r y s t a l l i n e layers.

Because o f t h e

porous n a t u r e o f evaporated f i l m s , t h e y can e a s i l y absorb i m p u r i t i e s from t h e a i r ,

and u n l e s s t h e q u a l i t y o f as-deposited f i l m s

can be improved, l a s e r - i n d u c e d SPE w i l l have t o be performed i n UHV.

Saitoh e t a l .

(1981) r e p o r t e d t h a t i n - s i t u thermal a n n e a l i n g

o f e-beam d e p o s i t e d f i l m s a t temperatures h i g h e r t h a n 200°C can s u b s t a n t i a l l y improve t h e f i l m q u a l i t y .

Whether good q u a l i t y f i l m s

o f t h i s t y p e w i l l improve cw l a s e r induced SPE regrowth i n a i r s t i l l remains t o be e s t a b l i s h e d . The s t r i n g e n t

requirements on t h e vacuum and on i n t e r f a c e

c l e a n i n g procedures a r e n o t so c r i t i c a l f o r f i l m s c r y s t a l l i z e d by p u l s e d l a s e r induced l i q u i d phase e p i t a x y (LPE).

Good q u a l i t y

e p i t a x i a l l a y e r s can be o b t a i n e d s i m p l y by p e r f o r m i n g t h e LPE i n a i r immediately a f t e r f i l m e v a p o r a t i o n i n a vacuum of

torr

and w i t h o u t i n i t i a l l y s p u t t e r c l e a n i n g t h e s u b s t r a t e (Lau e t al., 1978; Revesz, e t a l . ,

1978; Young e t al.,

1979a).

S i n c e t h e den-

s i t y o f t h e evaporated f i l m s i s l e s s t h a n t h a t o f s i n g l e c r y s t a l s i l i c o n , t h e f o r m a t i o n o f s p h e r i c a l v o i d s o r microbubbles i n t h e e p i t a x i a l l a y e r i s o f t e n observed ( C e l l e r e t a l .

, 1981).

However,

t h e s e can be removed by repeated p u l s e s o r by h i g h e r energy pulses. On t h e o t h e r hand, t h i s repeated m e l t i n g o r l o n g e r m e l t d u r a t i o n s o f t h e d e p o s i t e d l a y e r may cause severe dopant r e d i s t r i b u t i o n a t t h e i n t e r f a c e , which may be unacceptable i f a sharp dopant p r o f i l e a t t h e i n t e r f a c e i s desired.

The advantage o f f i l m d e p o s i t i o n by

e-beam e v a p o r a t i o n i s t h a t t h e s u b s t r a t e can be h e l d a t room temperature.

However, as we have seen, t h e p o r o s i t y o f t h e evaporated

f i l m i s t h e major problem i n f i l m c r y s t a l l i z a t i o n by e i t h e r s o l i d

o r l i q u i d phase e p i t a x y .

Methods f o r i n c r e a s i n g t h e evaporated

23

1. LASER PROCESSING OF SEMICONDUCTORS f i l m d e n s i t y ( S a i t o h e t al.,

1981) and a l t e r n a t i v e low temperature

f i l m d e p o s i t i o n techniques, such as low temperature chemical vapor

d e p o s i t i o n (Minagawa e t al.,

1981; van d e r Leeden e t al.,

1982)

and m o l e c u l a r beam e p i t a x y , a r e c u r r e n t l y under i n v e s t i g a t i o n . 9.

LASER RECRYSTALLIZATION

OF S i FILMS ON INSULATING SUBSTRATES

The problems a s s o c i a t e d w i t h t h e c u r r e n t technology o f SOS (silicon-on-sapphire),

dielectric-isolation,

integrated c i r c u i t s

and t h e need f o r h i g h e r p a c k i n g d e n s i t i e s and o p e r a t i n g speeds i n t h e development o f three-dimensional m i c r o e l e c t r o n i c c i r c u i t s make l a s e r p r o c e s s i n g o f p o l y c r y s t a l l i n e S i f i l m s on i n s u l a t i n g substrates quite attractive.

Laser-induced r e c r y s t a l l i z a t i o n of f i n e -

g r a i n e d (300-600 A ) p o l y c r y s t a l l i n e S i f i l m s deposited on a t h i n amorphous d i e l e c t r i c (Si02 o r S i 3 N 4 ) l a y e r on a S i o r g l a s s subs t r a t e o r on glass has been s t u d i e d i n s e v e r a l l a b o r a t o r i e s .

These

f i l m s may be i n t h e f o r m o f u n i f o r m continuous sheets o r t h e y may have t h e form o f i s o l a t e d i s l a n d s t r u c t u r e s .

Laser i r r a d i a t i o n i s

used t o promote g r a i n growth o r t o grow s i n g l e c r y s t a l i s l a n d s , thus improving t h e e l e c t r i c a l properties o f the films.

Both p u l s e d

( C e l l e r e t al.,

1981; Wu and Magee, 1979; Young e t al.,

and Crosthwait,

1981) and cw l a s e r s (Fan and Zeiger, 1975; Gat e t

al.

1978b; Roulet e t a l .

1980; Fastow e t al., i n these studies.

1980; Yaron e t al.,

1980; Gibbons e t al.,

1981; B i e g e l s e n e t al., G e n e r a l l y speaking,

1980; Shah

1981) have been used

t h e f i l m s annealed by cw

l a s e r s have l a r g e r g r a i n s i z e s and t h e e l e c t r i c a l p r o p e r t i e s a r e l e s s s e n s i t i v e t o subsequent thermal treatment.

Fan and Z e i g e r

(1975) demonstrated t h a t a cw Nd:YAG l a s e r can be used t o c r y s t a l l i z e amorphous S i f i l m s up t o 10 pm t h i c k on A1203 substrates. C r y s t a l l i t e s as l a r g e as 25

pm

were observed,

measurements on t h e m a t e r i a l were reported.

b u t no e l e c t r i c a l Gat e t a l .

r e p o r t e d t h a t , w i t h cw A r - i o n l a s e r i r r a d i a t i o n ,

(1978b)

a 0.4 urn t h i c k

boron-implanted f i n e - g r a i n e d p o l y c r y s t a l l i n e S i f i l m c o u l d be conv e r t e d i n t o a f i l m w i t h chevron-shaped g r a i n s -2

pm

wide and -25

pm

24

R. F. WOOD ET AL.

long.

The e l e c t r i c a l p r o p e r t i e s o f t h e s e f i l m s i n terms o f t h e

r e c o v e r y o f c a r r i e r c o n c e n t r a t i o n s and m o b i l i t i e s were e x c e l l e n t . Several a u t h o r s have concluded t h a t t h e improvement o f t h e sheet r e s i s t i v i t y observed i n such f i l m s i s due n o t o n l y t o t h e i n c r e a s e i n grain size,

but a l s o t o t h e laser-induced reduction o f g r a i n

boundary t r a p p i n g s t a t e s (Roulet e t a l .

, 1980;

Yaron e t al.,

1980).

O p t i c a l s t u d i e s i n d i c a t e d t h a t t h e optimum g r a i n growth occurs when t h e l a s e r power i s j u s t h i g h enough t o m e l t t h e e n t i r e f i l m .

Due

t o t h e low thermal c o n d u c t i v i t y o f t h e d i e l e c t r i c f i l m , t h e p o l y c r y s t a l l i n e s i l i c o n f i l m can be m e l t e d w i t h r e l a t i v e l y l o w l a s e r power w i t h o u t m e l t i n g t h e u n d e r l y i n g s u b s t r a t e , t h u s a v o i d i n g f i l m damage and d e v i a t i o n s from s u r f a c e p l a n a r i t y .

To grow o r i e n t e d f i l m s on amorphous s u b s t r a t e s , Geis and coworkers (1979) have used a t e c h n i q u e c a l l e d graphoepitaxy. d e p o s i t e d t h i n S i f i l m s (0.5-2

They

urn) on f u s e d s i l i c a s u b s t r a t e s i n

which a square wave s u r f a c e r e l i e f p a t t e r n had been produced by p h o t o l i t h o g r a p h y and r e a c t i v e i o n e t c h i n g .

A cw l a s e r o r g r a p h i t e

s t r i p h e a t e r was used as t h e heat source f o r f i l m r e c r y s t a l 1i z a t i o n . Large < l o o > - o r i e n t e d g r a i n s (-100 pin) w i t h o n l y small m i s f i t angles were obtained.

C o n t i n u i n g research on t h e improvement o f f i l m

q u a l i t y and t o achieve b e t t e r u n d e r s t a n d i n g of t h e mechanism o f n u c l e a t i o n f o r t h e o r i e n t e d growth i s b e i n g pursued. 10.

PULSED LASER PHOTOCHEMICAL PROCESSING Laser induced photochemical p r o c e s s i n g i s another r a p i d l y grow-

i n g area o f research t h a t may p r o v i d e many a p p l i c a t i o n s i n t h e microelectronics industry.

Deutsch e t a l . (1979) and E h r l i c h e t a l .

(1982) have demonstrated t h a t by u s i n g a focused UV excimer l a s e r , i t i s now p o s s i b l e t o w r i t e submicron metal l i n e s on v a r i o u s semi-

conductors and q u a r t z s u b s t r a t e s .

T h i s t y p e o f processing, which

does n o t r e l y on p h o t o l i t h o g r a p h y b u t i s c u r r e n t l y l i m i t e d by i t s low throughput, may be used i n t h e r e p a i r o f p h o t o l i t h o g r a p h i c masks and f o r f a b r i c a t i o n o f i n t e r c o n n e c t s i n customized programmable

25

1. LASER PROCESSING OF SEMICONDUCTORS l o g i c arrays.

I n addition,

chemical r e a c t i o n s induced by l a s e r

r a d i a t i o n have been used by B i l e n c h i e t a l . e t al.

(1982) and A n d r e a t t a

(1982) t o d e p o s i t semiconductor f i l m s and by Boyer and co-

workers (1982) t o d e p o s i t i n s u l a t o r (Si02, S i 3 N 4 ) f i l m s on subs t r a t e s a t low temperatures. ( E h r l i c h e t al.,

A l s o e t c h i n g (Chuang, 1982) and doping

1981) o f m a t e r i a l s i n h i g h l y l o c a l i z e d r e g i o n s

have been demonstrated.

In t h e s e p r o c e s s i n g steps, t h e chemical

r e a c t i o n s may be d r i v e n by s e l e c t i v e bond breakage i n t h e molecules v i a t h e a b s o r p t i o n o f t h e i n t e n s e UV o r i n f r a r e d l i g h t , by t r a n s i e n t s u r f a c e h e a t i n g , o r even by l a s e r induced plasma formation. The main advantages o f l a s e r chemical p r o c e s s i n g a r e t h e low temp e r a t u r e a t which t h e s u b s t r a t e can be maintained, t h e s u p e r i o r c o n t r o l o f t h e environment which can be r e a l i z e d , and t h e c a p a b i l i t y

o f d i r e c t , maskless e t c h i n g , doping, and w r i t i n g .

We a n t i c i p a t e

t h a t one o r more o f these processes w i l l e v e n t u a l l y be i n t e g r a t e d i n t o m i c r o e l e c t r o n i c f a b r i c a t i o n technology.

11.

EXCIMER LASER LITHOGRAPHY

Laser r a d i a t i o n has l o n g been t h o u g h t t o be i m p r a c t i c a l f o r h i g h r e s o l u t i o n l i t h o g r a p h y because t h e coherent n a t u r e o f t h e 1 i g h t gives r i s e t o c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e r e n c e a t t h e sample s u r f a c e t h a t produces a random p a t t e r n o f f l u c t u a t i n g i n t e n s i t y c a l l e d "speckle."

Very r e c e n t l y , J a i n and co-workers

(1982) demonstrated t h a t h i g h - r e s o l u t i o n , f i n e - l i n e (0.5 wn) photol i t h o g r a p h i c p a t t e r n s can be d e f i n e d w i t h mask exposure by UV excimer l a s e r r a d i a t i o n o f 248 and 308 nm wavelengths. were o f h i g h q u a l i t y and t o t a l l y speckle f r e e .

The images

These f i n d i n g s a r e

g e n e r a l l y regarded as a major advancement i n deep UV l i t h o g r a p h y ; t h e y w i l l be discussed i n some d e t a i l i n Chapter 10.

In p a r a l l e l graphy,

w i t h J a i n ' s work

on UV excimer l a s e r p h o t o l i t h o -

S r i n i v a s a n and Mayne-Banton

(1982) r e c e n t l y developed a

new process f o r t h e c o n t r o l l e d e t c h i n g o f o r g a n i c polymer f i l m s u s i n g an ArF (193 nm) excimer l a s e r .

They demonstrated t h a t t h e

26


193 nm r a d i a t i o n can e t c h o r g a n i c m a t e r i a l i n a p a t t e r n whose r e s o l u t i o n i s determined e n t i r e l y by t h e d i a m e t e r o f t h e l a s e r beam.

The mechanism o f t h i s process, which S r i n i v a s a n r e f e r s t o

as " a b l a t i v e photodecomposition,"

i s b e l i e v e d t o be a b s o r p t i o n o f

UV 1ig h t a t wave1 engths c o r r e s p o n d i n g t o a1 1owed e l e c t r o n i c t ran-

s i t i o n s from bonding t o a n t i - b o n d i n g s t a t e s (>6 eV f o r most o r g a n i c polymers), f o l l o w e d by breakup o f t h e polymer c h a i n s i n t o s m a l l e r fragments and e j e c t i o n o f t h e fragments c o m p l e t e l y o u t o f t h e f i l m , l e a v i n g a v e r t i c a l w a l l d e f i n e d by t h e l i g h t source.

The impor-

t a n t p o i n t i s t h a t t h e e x c i t a t i o n (bond-breaking) energy r e s i d e s e n t i r e l y w i t h i n t h e e j e c t e d fragments, w i t h no evidence o f f l o w o r h e a t i n g o f t h e s u r r o u n d i n g polymer; hence t h e t e r m " a b l a t i v e photodecomposition."

T h i s process appears t o be very a t t r a c t i v e f o r

p h o t o l i t h o g r a p h y s i n c e i t p r o v i d e s b o t h exposure and e t c h i n g i n a s i n g l e step, and t h e c o n v e n t i o n a l wet c h e m i s t r y development process can be e l i m i n a t e d t o t a l l y .

T h i s work w i l l a l s o be d i s c u s s e d f u r t h e r

i n Chapter 10.

V.

Types o f Lasers f o r Pulsed Laser Processing

A v a r i e t y o f l a s e r s can be used f o r l a s e r p r o c e s s i n g and t h e advantages and disadvantages of d i f f e r e n t t y p e s w i l l be d i s c u s s e d here.

F i r s t , however, i t s h o u l d be r e c o g n i z e d t h a t t h e r e a r e essen-

t i a l l y two d i f f e r e n t ways i n which p u l s e d l a s e r s can be used.

In

a scanning mode a l a s e r beam o f small diameter and h i g h p u l s e r e p e t i t i o n r a t e i s r a s t e r scanned o v e r t h e sample and t h e scanning parameters a r e chosen so t h a t s a t i s f a c t o r y a n n e a l i n g i s obtained. The r a s t e r scanning can be arranged e i t h e r by d e f l e c t i n g t h e l a s e r p u l s e s o v e r t h e sample w i t h m i r r o r s o r by t r a n s l a t i n g t h e sample under t h e f i x e d l a s e r beam.

Automated,

microprocesser-control l e d

systems s u i t a b l e f o r e i t h e r t y p e o f scanning a r e now a v a i l a b l e . I n t h e o t h e r method o f o p e r a t i o n , t h e l a s e r system i s designed s o

t h a t one o r two p u l s e s o f t h e r e q u i r e d energy d e n s i t y over l a r g e areas can be used f o r annealing.

27

1. LASER PROCESSING OF SEMICONDUCTORS 12.

PULSED SOLID-STATE LASERS The work r e p o r t e d t o date on p u l s e d l a s e r p r o c e s s i n g has gen-

e r a l l y been c a r r i e d o u t w i t h ruby, Nd:YAG,

and Nd:glass l a s e r s .

The ruby l a s e r operates a t a wavelength X o f 694 nm o r 1.79 eV and t h e Nd:YAG l a s e r has X = 1064 nm o r 1.17 eV i n t h e fundamental i t can be frequency doubled, t r i p l e d ,

and quadrupled.

, but

Since t h e

i n d i r e c t band gap o f s i l i c o n i s -1.1 eV a t room temperature, t h e absorption c o e f f i c i e n t a t

)i

= 1064 nm i s q u i t e small

, and

YAG l a s e r s

a r e o f t e n operated a t t h e frequency-doubled wavelength o f 532 nm, o r i n modes which combine v a r i o u s r a t i o s o f t h e 1064 and 532 nrn radiation.

O f course, frequency d o u b l i n g , t r i p l i n g (353 nm), and

q u a d r u p l i n g (265 nm) can be o b t a i n e d o n l y a t t h e s a c r i f i c e o f efficiency,

and t h e 353 and 265 nm wavelengths a r e l i k e l y t o be

u s e f u l p r i m a r i l y f o r b a s i c s t u d i e s and s p e c i a l i z e d a p p l i c a t i o n s where o n l y very small areas are i n v o l v e d .

The a l e x a n d r i t e l a s e r

(which r e c e n t l y appeared on t h e market), w i t h wavelength t u n a b i l i t y i n t h e range from 680 t o 800 nm, seems s u i t a b l e f o r semiconductor processing, b u t very few r e s u l t s w i t h t h i s l a s e r have been r e p o r t e d

a t t h i s time. A t t h e present time, s o l i d - s t a t e l a s e r s have c e r t a i n l i m i t a t i o n s which make them l e s s t h a n i d e a l f o r l a r g e area l a s e r processing. Foremost among t h e s e l i m i t a t i o n s a r e t h e s p a t i a l inhomogeneities c h a r a c t e r i s t i c o f t h e energy d i s t r i b u t i o n i n t h e pulses and a p u l s e r e p e t i t i o n r a t e l i m i t e d by t h e heat d i s s i p a t i o n o f t h e i n s u l a t i n g crystals.

I f a l a s e r c a v i t y i s c a r e f u l l y tuned and operated i n t h e

TEMoo mode, a n e a r l y gaussian energy p r o f i l e can be obtained, b u t Fraunhofer d i f f r a c t i o n f r o m t h e circumference o f t h e p i n h o l e used t o s e l e c t t h e TEMoo mode and from t h e l a s e r r o d i t s e l f w i l l superimpose i n t e n s i t y modulations on t h i s p r o f i l e i n near and i n t e r mediate f i e l d s .

Under f a r - f i e l d c o n d i t i o n s t h e p r o f i l e assumes an

Airy p a t t e r n i n which over 90% o f t h e i n t e n s i t y i s i n t h e gaussian-

l i k e c e n t r a l peak.

The d i f f i c u l t y w i t h f a r f i e l d c o n d i t i o n s , f o r

l a s e r s o f i n t e r e s t i n l a s e r processing, i s t h a t t h e y a r e g e n e r a l l y

28

R. F. WOOD ET AL

a t t a i n e d o n l y a t very l a r g e distances.

There a r e ways around t h i s

d i f f i c u l t y by u s i n g lenses, s p a t i a l f i l t e r i n g , etc.,

b u t t h e con-

sequences a r e almost always a decrease i n t h e a v a i l a b l e energy d e n s i t y and an i n c r e a s e i n t h e c o m p l e x i t y o f t h e system.

The a t t a i n -

ment o f good beam homogeneity i n r e a l l y l a r g e s o l i d - s t a t e systems, such as some o f t h o s e r e c e n t l y used f o r l a s e r processing, r e q u i r e devices f o r homogenizing t h e beam even when t h e l a s e r i s o p e r a t i n g i n t h e TEMoo mode.

It i s a l s o q u i t e p o s s i b l e o f course t o operate

t h e l a s e r s i n multimode c o n d i t i o n s and t o use beam homogenizers t o smooth o u t t h e s p a t i a l f l u c t u a t i o n s i n t h e beam p r o f i l e s . tunately,

Unfor-

beam homogenization which w i l l be discussed i n g r e a t e r

d e t a i l i n Chapter 10 o f t h i s book, always increases t h e complexity o f t h e p r o c e s s i n g system and i s seldom e n t i r e l y s a t i s f a c t o r y . PULSED GAS LASERS

13.

There are now c l e a r i n d i c a t i o n s t h a t gas l a s e r s a r e i n p r i n c i p l e i n h e r e n t l y s u p e r i o r t o s o l i d - s t a t e l a s e r s f o r l a s e r processing. The e f f i c i e n c i e s o f gas l a s e r s a r e g e n e r a l l y g r e a t e r t h a n t h o s e o f s o l i d - s t a t e l a s e r s , and t h e e l i m i n a t i o n o f l a r g e o p t i c a l l y p e r f e c t c r y s t a l and g l a s s rods which a r e d i f f i c u l t and expensive t o grow, and which are e a s i l y damaged i s an i m p o r t a n t c o n s i d e r a t i o n . power CO,

High-

l a s e r s w i t h n e a r l y 30% e f f i c i e n c y have been designed and

c o n s t r u c t e d , b u t s i n c e t h e coup1 i n g o f t h e long-wavelength r a d i a t i o n (10.6

pm)

t o semiconductors by way o f f r e e c a r r i e r s and phonons i s

n o t very s t r o n g t h e o v e r a l l e f f i c i e n c y o f energy usage i s n o t high. However, because t h e CO,

l a s e r s a r e so e f f i c i e n t ,

it i s clearly

w o r t h w h i l e t o e x p l o r e techniques which w i l l r e s u l t i n b e t t e r coup l i n g between t h e 10.6 urn r a d i a t i o n and t h e more common semiconductors.

Moreover, t h e l a r g e p e n e t r a t f o n depth of t h e CO,

radiation

may be advantageous i n a p p l i c a t i o n s where very deep m e l t i n g i s desirable.

Annealing o f i o n - i m p l a n t e d s i l i c o n w i t h a p u l s e d CO,

l a s e r was r e p o r t e d by Miyao (1979), b u t t h e q u a l i t y o f t h e a n n e a l i n g was n o t t h o r o u g h l y s t u d i e d . More r e c e n t l y , Naukkarinen e t a l . (1982)

29


demonstrated t h a t Cop l a s e r a n n e a l i n g o f h e a v i l y doped s i l i c o n s u b s t r a t e s i m p l a n t e d w i t h antimony c o u l d y i e l d almost complete r e c r y s t a l l i z a t i o n and a c t i v a t i o n of t h e i m p l a n t e d ions.

Good r e -

c r y s t a l l i z a t i o n has a l s o been achieved f o r l i g h t l y doped samples w i t h a dopant c o n c e n t r a t i o n o f 7x1015 CO,

(Blomberg e t al.,

1983).

l a s e r s have been on t h e market f o r a r e l a t i v e l y l o n g time,

b u t r a r e gas h a l i d e excimer l a s e r s o p e r a t i n g i n t h e u l t r a v i o l e t have o n l y r e c e n t l y appeared commercially and a r e s t i 11 undergoing r a p i d development.

An e a r l y r e p o r t o f l a s e r a n n e a l i n g w i t h excimer

l a s e r s was made by Anderson e t a1

., (1980).

Recently, more thorough

s t u d i e s o f t h e q u a l i t y o f t h e a n n e a l i n g o b t a i n e d w i t h XeCl l a s e r s have been p u b l i s h e d by Young e t a l . (1982b) and Lowndes e t a l . (1982), and Young and co-workers

(1983) have demonstrated t h a t s i 1i c o n

s o l a r c e l l s w i t h remarkably h i g h e f f i c i e n c i e s can be f a b r i c a t e d u s i n g low-cost

i o n i m p l a n t a t i o n and XeCl l a s e r annealing.

This

work w i l l be discussed i n d e t a i l i n Chapter 10, and we w i l l o n l y remark here t h a t t h e same c h a r a c t e r i s t i c s o f U V excimer l a s e r s t h a t make them so e f f e c t i v e f o r UV l i t h o g r a p h y discussed above a l s o make them e x c e l l e n t sources f o r a n n e a l i n g r a d i a t i o n . specifically,

More

t h e reduced coherence o f t h e r a d i a t i o n v i r t u a l l y

e l i m i n a t e s d i f f r a c t i o n and i n t e r f e r e n c e e f f e c t s and g i v e s a very u n i f o r m beam t h a t does n o t r e q u i r e t h e use o f beam homogenizers. Recent developments i n excimer l a s e r t e c h n o l o g y ( L i n and L e v a t t e r , 1979; L e v a t t e r and L i n , 1980) suggest t h a t very high-powered excimer l a s e r s w i t h e x c e l l e n t homogeneity o f t h e energy d e n s i t y over l a r g e areas can be constructed.

Such l a s e r s would undoubtedly be o f

great u t i l i t y i n t h e l a s e r processing o f a l l types o f materials.

VI.

Other Sources for Energy Beam Processing

I t should be obvious from t h e d i s c u s s i o n s i n t h e p r e c e d i n g sections o f t h i s chapter t h a t t h e effectiveness o f pulsed l a s e r p r o c e s s i n g depends t o a g r e a t e x t e n t on t h e c a p a b i l i t y o f d e p o s i t i n g r e l a t i v e l y small amounts of energy i n t o r e g i o n s o f small volume

30

R. F. WOOD ETAL.

i n very s h o r t times; t o a l e s s e r e x t e n t cw l a s e r a n n e a l i n g u t i l i z e s t h e same p r i n c i p l e s .

Lasers are used because o f t h e power d e n s i -

t i e s t h e y can d e l i v e r by p o p u l a t i o n i n v e r s i o n , s t i m u l a t e d emission, and Q-switching.

The coherent n a t u r e o f t h e l a s e r r a d i a t i o n i s

n o t o n l y unnecessary,

it is,

because of d i f f r a c t i o n

e f f e c t s caused by a p e r t u r e s , l a s e r rods,

more o f t e n t h a n n o t ,

lenses, d i r t and dust p a r t i c l e s , e t c .

a nuisance

An i n c o h e r e n t l i g h t source

w i t h s u f f i c i e n t power d e n s i t y should be q u i t e e f f e c t i v e f o r energy beam annealing.

S h o r t l y a f t e r t h e advent o f l a s e r annealing,

s e v e r a l r e p o r t e d and u n r e p o r t e d attempts t o use i n c o h e r e n t l i g h t sources such as a r c lamps (Cohen e t al., i n t e n s i t y halogen lamps (Nishiyama e t a l .

1978; Gat 1981), h i g h -

, 1981) , etc.,

f o r anneal-

i n g o f i o n - i m p l a n t a t i o n damage i n semiconductors were made.

The

r e s u l t s were s i m i l a r t o those o b t a i n e d by cw l a s e r a n n e a l i n g because t h e u l t r a r a p i d m e l t i n g and c o o l i n g c h a r a c t e r i s t i c o f p u l s e d l a s e r a n n e a l i n g was n o t achieved.

Moreover, t o o b t a i n a n n e a l i n g i t was

necessary t o h o l d t h e e n t i r e sample a t h i g h temperatures t o prevent wafer d i s t o r t i o n .

T h i s h i g h temperature i s l i k e l y t o degrade t h e

m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h and make t h e samples u n s u i t a b l e f o r some a p p l i c a t i o n s . Another energy source which i s c o m p e t i t i v e even now w i t h l a s e r s i n many areas o f m a t e r i a l s p r o c e s s i n g i s a p u l s e d e l e c t r o n beam generator.

There i s an e x t e n s i v e body o f l i t e r a t u r e on e-beam

p r o c e s s i n g o f m a t e r i a l s ( f o r a d i s c u s s i o n o f e-beam a n n e a l i n g o f s i l i c o n see Greenwald e t al., t o summarize i t here.

1979) and we w i l l n o t even attempt

However, f o r t h e purposes o f t h i s book, i t

i s w o r t h w h i l e emphasizing t w o o f t h e main d i f f e r e n c e s l a s e r s and e l e c t r o n beams as energy sources.

between

The d e p o s i t i o n o f

energy i n t h e sample by a l a s e r i s s t r o n g l y dependent on t h e o p t i c a l p r o p e r t i e s ( r e f l e c t i v i t y and a b s o r p t i o n c o e f f i c i e n t ) o f t h e m a t e r i a l a t t h e wavelength o f t h e l a s e r r a d i a t i o n .

I n contrast,

t h e energy d e p o s i t i o n by e-beams depends on t h e e l e c t r o n energy and t h e s t o p p i n g power o f t h e m a t e r i a l f o r e l e c t r o n s o f t h a t energy,


31

and t h i s i s p r i m a r i l y a f u n c t i o n o f t h e d e n s i t y o f t h e m a t e r i a l . G e n e r a l l y speaking,

100 keV e-beams from commercial e-beam p r o -

cessors d e p o s i t energy i n t h e sample a t s i g n i f i c a n t l y deeper depths t h a n do ruby and YAG l a s e r s , and t h i s may o f f e r advantages i n some a p p l i c a t i o n s and disadvantages i n others.

For example, i t i s n o t

l i k e l y t o be an advantage i n t h e f a b r i c a t i o n o f s h a l l o w - j u n c t i o n s o l a r c e l l s , and indeed e-beam processed s o l a r c e l l s show a r a t h e r poor response i n t h e l o n g wavelength p o r t i o n o f t h e s o l a r spectrum ( K i r k p a t r i c k and Minnucci , 1979).

The o t h e r major d i f f e r e n c e

between e-beam and l a s e r a n n e a l i n g i s t h a t t h e former must be done i n a f a i r l y good vacuum whereas t h e l a t t e r can be done i n a i r .

It

a l s o appears t o be d i f f i c u l t t o achieve u n i f o r m beams over l a r g e areas w i t h present day e-beam sources and t h i s makes i t d i f f i c u l t t o o b t a i n p r e c i s e c o n t r o l o f j u n c t i o n depths,

especially i n t h e

f o r m a t i o n o f abrupt s h a l l o w j u n c t i o n s . Other obvious forms of energy-beam a n n e a l i n g a r e t h o s e which u t i l i z e p a r t i c l e s heavier than electrons.

Reports o f a n n e a l i n g o f

i o n - i m p l a n t e d samples w i t h p r o t o n beams have appeared (Hodgson e t a1

., 1980).

A p u l s e d 200 keV p r o t o n beam can d e p o s i t energy

u n i f o r m l y t o a depth o f 2 um and t h i s should be u s e f u l i n a n n e a l i n g samples w i t h deeply i m p l a n t e d dopants.

I n materials processing o f

semiconductors t h a t i n v o l v e i o n i m p l a n t a t i o n and annealing, t h e q u e s t i o n n a t u r a l l y a r i s e s as t o whether o r n o t i t i s p o s s i b l e t o o b t a i n i m p l a n t a t i o n c o n d i t i o n s which w i l l r e s u l t i n s a t i s f a c t o r y self-annealing.

The experience t o d a t e seems t o i n d i c a t e t h a t t h e

s e l f - a n n e a l i n g t h a t i s known t o occur d u r i n g c e r t a i n i m p l a n t a t i o n c o n d i t i o n s may induce t h e growth o f c l u s t e r t y p e d e f e c t s which a r e subsequently v e r y d i f f i c u l t t o anneal out.

I n s p i t e of t h i s , it

seems t h a t i t may be p o s s i b l e e v e n t u a l l y t o i o n i m p l a n t under energy and c u r r e n t c o n d i t i o n s which r e s u l t i n a power d e n s i t y h i g h enough t o g i v e solid-phase and,

perhaps even l i q u i d - p h a s e regrowth, of

t h e implanted and damaged region.

32

R. F. WOOD ET AL.

VII.

Laser Processing of Compound Semiconductors, Metals, and Insulators

The success o f l a s e r p r o c e s s i n g o f t h e elemental semiconductors s i 1 con and germanium has n a t u r a l l y l e d t o e x t e n s i v e research on t h e a p p l i c a t i o n o f s i m i l a r techniques t o o t h e r m a t e r i a l s . t h e compound semiconductors, bec use industry.

of

i t s potential

Among

GaAs has been o f p a r t i c u l a r i n t e r e s t importance f o r

t h e microelectronics

The thermal p r o p e r t i e s o f GaAs, S i , and Ge a r e roughly

comparable, and i n t h e wavelength range used i n many l a s e r a n n e a l i n g experiments t h e o p t i c a l p r o p e r t i e s o f GaAs and amorphous s i l i c o n are also not grossly d i f f e r e n t .

It i s n o t s u r p r i s i n g t h e n t h a t a

number o f t h e f e a t u r e s o f p u l s e d l a s e r a n n e a l i n g o f GaAs and s i l i con appear t o be q u i t e s i m i l a r (Golovchenko and Venkatesan, Barnes e t al.,

1978).

1978;

The l a t e n t heat o f f u s i o n o f GaAs i s approx-

i m a t e l y one t h i r d o f t h a t of s i l i c o n , which i n d i c a t e s t h a t GaAs s h o u l d r e q u i r e l e s s e n e r g e t i c l a s e r p u l s e s t o o b t a i n comparable melt f r o n t penetration.

That t h i s i s indeed t h e case i s borne o u t

by b o t h experiment and t h e o r y (Auston e t al., 1981b).

1978b; Wood e t a l . ,

Dopant p r o f i l e spreading i s o f about t h e same magnitude

i n GaAs and s i l i c o n ,

and n o n e q u i l i b r i u m s e g r e g a t i o n e f f e c t s have

been observed i n GaAs as i n s i l i c o n (Eisen, 1980; Lowndes e t al., 1981). Important d i f f e r e n c e s between t h e two m a t e r i a l s become apparent when t h e e l e c t r i c a l p r o p e r t i e s are s t u d i e d , as d e s c r i b e d i n Chapter 8.

Pulsed l a s e r a n n e a l i n g has been s u c c e s s f u l i n

a c t i v a t i n g o n l y t h e h i g h e s t f l u e n c e i m p l a n t s i n GaAs, and c a r r i e r m o b i l i t i e s i n t h e c o n d u c t i n g l a y e r s formed i n t h i s way a r e much lower t h a n would be expected i n h i g h - q u a l i t y GaAs, d e s p i t e evidence o f good c r y s t a l l i n i t y i n t h e laser-annealed regions.

Furthermore,

when u n i f o r m l y doped c r y s t a l l i n e GaAs i s p u l s e annealed, h i g h conc e n t r a t i o n s o f compensating d e f e c t s a r e produced near t h e surface. There a r e several apparent problems i n l a s e r a n n e a l i n g compound semiconductors (and p r o b a b l y compounds o f a l l t y p e s ) .

The most

obvious problem concerns t h e v o l a t i l e n a t u r e o f many o f t h e elements

1. LASER PROCESSING OF SEMICONDUCTORS i n these materials.

33

F o r example, i n GaAs a t e l e v a t e d temperatures

(even f a r s h o r t o f t h e m e l t i n g p o i n t ) t h e r e i s r a p i d d e p l e t i o n of a r s e n i c i n t h e n e a r - s u r f a c e r e g i o n (Tsu e t a l e , 1979b); p u l s e d l a s e r m e l t i n g can r e s u l t i n an a g g r e g a t i o n o f g a l l i u m "puddles". A second problem i s t h a t when compound m a t e r i a l s a r e i o n i m p l a n t e d

w i t h o n l y one t y p e o f i o n , a n o n s t o i c h i o m e t r y i s c r e a t e d which i s d i f f i c u l t , though perhaps n o t impossible, t o prevent. C o i m p l a n t a t i o n o f more t h a n one species i s a p o s s i b l e way around t h i s problem, b u t i t complicates t h e i m p l a n t a t i o n process and has n o t been s t u d i e d extensively yet.

However,

recent s t u d i e s i n d i c a t e t h a t t h e most

fundamental d i f f i c u l t i e s i n a p p l y i n g p u l s e d l a s e r p r o c e s s i n g t o compound semiconductors a r e i n h e r e n t consequences of imposing a r a p i d s o l i d i f i c a t i o n process upon t h e more complex p h y s i c s and chemi s t ry o f c r y s t a 11 ine compounds.

U 1t r a r a p i d r e c r y s t a1 1i z a t ion

f r o m t h e l i q u i d phase may n o t g i v e s u f f i c i e n t t i m e f o r t h e v a r i o u s atoms t o f i n d t h e i r p r o p e r l o c a l chemical c o n f i g u r a t i o n s , arrange themselves on t h e c o r r e c t s u b l a t t i c e .

or t o

Thus, d i f f i c u l t i e s

w i t h p u l s e d a n n e a l i n g of GaAs a r e n o t a s s o c i a t e d s i m p l y w i t h t h e ion -i mpl a n t a t ion process

.

Several groups have r e c e n t l y observed h i g h d e n s i t i e s o f compens a t i n g defects

(perhaps

"quenched-in"

concentrations o f mobile

vacancies) i n pulse-annealed c r y s t a l l i n e GaAs.

Unlike the behavior

of s i l i c o n , t h e r e i s now a l s o d i r e c t evidence f o r oxygen i n p u l s e annealed GaAs, b o t h from t h e n a t i v e o x i d e l a y e r and from ambient air.

F i n a l l y , u l t r a r a p i d s o l i d i f i c a t i o n would be expected t o r e s u l t

i n a h i g h c o n c e n t r a t i o n o f a n t i - s i t e d e f e c t s i n compound semiconductors, though t h e y have a p p a r e n t l y n o t been p o s i t i v e l y i d e n t i f i e d yet.

As a r e s u l t , t h e problem of a p p l y i n g pulsed-annealing t e c h -

niques t o compound semiconductors i s now viewed n o t s i m p l y as a problem of removing i m p l a n t a t i o n damage, o r a c t i v a t i n g i m p l a n t e d i o n s (which would be d i f f i c u l t enough), b u t o f l e a r n i n g how t o a v o i d i n t r o d u c i n g new defects t h a t a r e i n h e r e n t t o r a p i d s o l i d i f i c a t i o n . P a r t i c u l a r l y i n t e r e s t i n g i n t h i s r e g a r d a r e t h e use o f s u b s t r a t e

34


h e a t i n g t o reduce t h e regrowth v e l o c i t y and t h e use o f a h i g h p r e s s u r e ambient atmosphere d u r i n g p u l s e d a n n e a l i n g t o c o n t r o l s t o i c h i o m e t r y d u r i n g regrowth.

The d i f f i c u l t i e s encountered and

new techniques developed are discussed i n Chapter 8. I n t e r e s t i n t h e l a s e r p r o c e s s i n g o f m e t a l s , i n s u l a t o r s , ceramics, and glasses i s i n c r e a s i n g r a p i d l y and many s t u d i e s a r e c u r r e n t l y under way.

P r e l i m i n a r y r e s u l t s o f some o f these s t u d i e s have been Here we w i l l o n l y make a few general

reported i n the l i t e r a t u r e .

comments about t h e l a s e r p r o c e s s i n g o f such m a t e r i a l s .

First, it

s h o u l d be recognized t h a t t h e goals o f l a s e r p r o c e s s i n g a r e d i f f e r e n t i n d i f f e r e n t materials.

I n semiconductors, one i s almost

always t r y i n g t o modify t h e e l e c t r i c a l p r o p e r t i e s f o r v a r i o u s device applications.

The e l e c t r i c a l p r o p e r t i e s o f i n t e r e s t a r e u s u a l l y

e x t r e m e l y s e n s i t i v e t o p o i n t and l i n e d e f e c t s and t o v a r i o u s impurities.

I n contrast, w i t h the possible exception o f the modification

o f superconducting f i l m s ,

magnetic bubble devices,

etc.,

laser

p r o c e s s i n g o f metals appears t o be d i r e c t e d toward m o d i f y i n g s u r face properties (usually i n conjunction w i t h ion implantation) for g r e a t e r wear r e s i s t a n c e , l e s s f r i c t i o n , g r e a t e r hardness, s u p e r i o r corrosion resistances, etc. amics,

Laser p r o c e s s i n g o f i n s u l a t o r s , c e r -

and glasses o f t h e t y p e discussed h e r e f o r semiconductors

i s s t i l l i n i t s i n f a n c y and i t i s d i f f i c u l t t o p r e d i c t t h e d i r e c t i o n i t w i l l take.

Obviously by l a s e r p r o c e s s i n g o f t h e s e m a t e r i a l s

we do n o t mean t o i n c l u d e t h e many i n t e r e s t i n g o p t i c a l e f f e c t s such as t h e r u l i n g o f h o l o g r a p h i c g r a t i n g s ,

information storage

by c r e a t i o n o f p o i n t d e f e c t s , e t c .

Plan of Book

VIII.

The p l a n o f t h e book i s f a i r l y obvious from t h e Table o f Contents and from t h e d i s c u s s i o n i n t h i s i n t r o d u c t o r y chapter. 3

However, t h e

i d e a behind t h e arrangement o f t h e chapters i n t h e o r d e r i n which t h e y appear i s t h e f o l l o w i n g .

Chapters 2 and 3 cover mostly e x p e r i -

mental r e s u l t s which a r e obtained a f t e r a sample has been s u b j e c t e d

35


t o v a r i o u s t y p e s of l a s e r p r o c e s s i n g techniques. I n Chapters 4 and 5, v a r i o u s t h e o r e t i c a l developments, p a r t i c u l a r l y i n t h e areas o f

heat flow c a l c u l a t i o n s , dopant r e d i s t r i b u t i o n , and nonequi 1ib r i u m segregation, a r e presented t o r e i n f o r c e t h e v a l i d i t y o f t h e i n t e r p r e t a t i o n s of v a r i o u s experimental r e s u l t s g i v e n i n o t h e r chapters of t h e book.

The r e s u l t s of t i m e - r e s o l v e d measurements and t h e

agreement of t h e s e r e s u l t s w i t h d e t a i l e d c a l c u l a t i o n s based on t h e m e l t i n g model o f p u l s e d l a s e r annealing a r e discussed i n Chapter 6. The i n t e r n a l

c o n s i s t e n c y and remarkable agreement

between t h e

experimental and t h e o r e t i c a l r e s u l t s serve t o e s t a b l i s h t h e b a s i c v a l i d i t y o f t h e m e l t i n g model and t o g i v e c o n f i d e n c e t h a t t h e r e s u l t s o b t a i n e d from i t can be used i n a v a r i e t y o f a p p l i c a t i o n s . and p r o b a b l y more i m p o r t a n t l y i n t h e l o n g run,

Moreover,

the results of

Chapters 2-6 taken t o g e t h e r i n d i c a t e t h a t t o o l s a r e now a v a i l a b l e t o a i d i n t h e development o f our fundamental understanding o f r a p i d m e l t i n g and s o l i d i f i c a t i o n phenomena.

The m a t e r i a l i n Chapter 7

on s u r f a c e s t u d i e s o f p u l s e d l a s e r i r r a d i a t e d m a t e r i a l s i s a l s o of b o t h fundamental and a p p l i e d s i g n i f i c a n c e ,

p a r t i c u l a r l y because

o f t h e prominent r o l e p r e s e n t l y p l a y e d by s u r f a c e sciences i n t h e s o l i d s t a t e and m a t e r i a l s sciences.

Chapter 8 i s devoted t o a

review o f p u l s e d l a s e r p r o c e s s i n g o f GaAs and i n d i c a t e s t h e problems and successes accompanying t h e a p p l i c a t i o n o f l a s e r techniques t o compound semiconductors.

Work on CO,

l a s e r a n n e a l i n g has been

i n c l u d e d as a separate c h a p t e r (Chapter 9 ) because r e c e n t s t u d i e s have i n d i c a t e d t h a t p u l s e d CO,

l a s e r s may have g r e a t e r p o t e n t i a l

f o r semiconductor p r o c e s s i n g t h a n was f o r m e r l y thought. The l a s t c h a p t e r o f t h e book i s devoted t o a p p l i c a t i o n s . Although i t i s s t i l l t o o e a r l y t o p r e d i c t t h e u l t i m a t e impact o f l a s e r p r o c e s s i n g on t h e semiconductor i n d u s t r y , i t was f e l t t h a t a survey o f t h e p r e s e n t s i t u a t i o n i n t h i s r e g a r d would be u s e f u l t o t h e reader.

Single crystal solar c e l l s o f quite high efficiencies

have been f a b r i c a t e d by l a s e r - p r o c e s s i n g techniques; t h e s e techniques and t h e performance o f t h e s o l a r c e l l s r e s u l t i n g from them

36

R. F. WOOD ET AL

a r e d e s c r i b e d i n t h e t h i r d s e c t i o n o f Chapter 10.

Discussions i n

o t h e r s e c t i o n s o f t h e c h a p t e r g i v e b r i e f reviews o f t h e c u r r e n t status o f t h e applications o f l a s e r processing t o t h e f a b r i c a t i o n o f a number of semiconductor devices and t o o t h e r aspects o f d e v i c e re1 a t e d work.

References A f f o l t e r , K., Luthy, W., and von Allmen, M. (1978). Appl. Phys. L e t t . 33, 185. Anderson, C. L., Dunlap, H. L., Hess, L. D., and McFarlane, R. A., Vaidyanathan. (1980). I n "Proc. Laser and E l e c t r o n Beam Processing o f E l e c t r o n i c M a t e r i a l s , " p. 334, Electrochem. SOC., Princeton, New Jersey Anderson, W. T., Jr., C h r i s t o u , A., and G i u l i a n i , J. F. (1981). I E E E E l e c t r o n Dev. L e t t . EDL-2, 115. Andreatta, R. W., Abele, C. C., Osmundsen, J. F., Eden, J. G., Lubben, D., and Greene, J. E. (1982). Appl. Phys. L e t t . 40, 183. Auston, D. H., Surko, C. M., Venkatesan, T. N. C., Slusher, R. E., and Golovchenko, J. A. (1978a). Appl. Phys. L e t t . 33, 437. Auston, D. H., Golovchenko, J. A,, Smith, P. R., Surko, C. M., and Venkatesan, T. N. C. (1978b). Appl. Phys. L e t t . 33, 539. Baeri, P., Campisano, S. U., F o t i , G., and R i m i n i , E. (1978). Appl. Phys. L e t t . 33, 137. B a e r i , P., Campisano, S. U., F o t i , G., and R i m i n i , E. (1979a). J. Appl. Phys. 50, 788. Baeri, P., Campisano, S. V., F o t i , G., and R i m i n i , E. (1979b). Phys. Rev. L e t t . 41, 1246. Leamy, H. J., Poate, J. M., F e r r i s , S. D., and Barnes, P. A., W i l l i a m s , J. S. (1978). Appl. Phys. L e t t . 33, 965. Flamm, D. L., Benton, J. L., Doherty, C. J., F e r r i s , S. D., K i m e r l i n g , L. C. , and Leamy, H. J. (1980). Appl. Phys. L e t t . 36, 670. B a r t e l i n k , D. J., and Moyer, Biegelsen, D. K., Johnson, N. M., M. D. (1981). Mat. Res. SOC. Symp. Proc. 1, 487. B i l e n c h i , R., G i a n i n o n i , I., and Musci, M. (1982). J. Appl. Phys. 53, 6479. Naukkarinen, K., Tuomi, T., A i r a k s i n e n , V.-M., Blomberg, M., Luomajtirvi, M., and Rauhala, E. (1983). J. Appl. Phys. 54, 2327. Boyer, P. K., Roche, G. A., R i t c h i e , W. H., and C o l l i n s , 6. J. (1982). Appl. Phys. L e t t . 40, 183. Breinan, E. M., Kear, B. H., and Banas, C. M. (1976). Physics Today 29, 44. O t t a v i a n i , G., Sigurd, D., and van der C a n a l i , C., Mayer, J. W., Weg, W. F. (1974). Appl. Phys. L e t t . 25, 3. Canali, C., Campisano, S. U., Lau, S. S., Liau, I. L., and Mayer, J. W. (1975). J. Appl. Phys. 46, 2831.

.


37

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38

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Geis, M. W., Flanders, D. C., and Smith, H. I. (1979). Appl. Phys. L e t t . 35, 71. Gibbons, J. F., Lee, K. F., Magee, T. J., Peng, J., and Ormond, R. (1980). Appl. Phys. L e t t . 34, 831. Golovchenko, J. A,, and Venkatesan, T. N. C. (1978). Appl. Phys. L e t t . 32, 147. Greenwald, A. C., K i r k p a t r i c k , A. R., L i t t l e , R. G., and Minnucci, J. A. (1979). J. Appl. Phys. 50, 783. Harper, F. I . , and Cohen, M. I. (1970). S o l i d S t a t e E l e c t r o n . 13, 1103. H i l l , C. (1981). Mat. Res. SOC. Symp. Proc. 1, 361. Hodgson, R. T., B a g l i n , J. E. E., P a l , R., N e r i , J. M., and Hammer, D. A. (1980). Appl. Phys. L e t t . 37, 187. J a i n , K., W i l l s o n , C. G., and L i n , B. J. (1982). I E E E E l e c t . Dev. L e t t . EDL-3, 53. Johnson, N. M., Gold, R. B., and Gibbons, J. F. (1979). Appl. Phys. L e t t . 34, 704. Kachurin, G. A., P r i d a c h i n , N. B., and Smirnou, L. S. (1976a). Sov. Phys.-Semicond. 9 , 946. Kachurin, G. A., Nidaev, E. V., Khodyachikh, A. V., and Kovaleva, L. A. (1976b). Sov. Phys.-Semicond. 10, 1128. Kachurin, G. A., Nidaev, E. V., and Danyushkina, N. V. (1980). Sov. Phys. Semicond. 14, 386. Shtyrkov, E. I . , Zaripov, M. M., Bayazitov, K h a i b u l l i n , I . B., R. M., and Galyautdinov, M. F. (1978). Rad. E f f e c t s 36, 225. Extensive a d d i t i o n a l references t o t h e Soviet l i t e r a t u r e are g i v e n i n t h i s reference. Development o f K i r k p a t r i c k , A. R., and Minnucci, J. A. (1979). Pulsed Processes f o r t h e Manufacture o f S o l a r Cel I s , F i n a l Report, DOE/JPL/954786. K i r k p a t r i c k , J. R . , G i l e s , G. E., Jr., and Wood, R. F. (1980). I n "Heat T r a n s f e r and Thermal Control," Vol. 78, p. 152. A I A A Progress i n A s t r o n a u t i c s and Aeronautics Series, New York. Klimenko, A. G., Klimenko, E. A., and Donin, V. I . (1976). Sov. J. Quantum E l e c t r o n . 5, 1289. Laff, R. A., and Hutchings, G. L. (1974). I E E E Trans. E l e c t . Device ED-21, 743. Larson, B. C., White, C. W., Noggle, T. S., and M i l l s , D. (1982). Phys. Rev. L e t t . 48, 337. Lau, S. S . , Tseng, W. F., N i c o l e t , M.-A., Mayer, J. W., Eckardt, R. C., and Wagner, R. J. (1978). Appl. Phys. L e t t . 33, 130. L e v a t t e r , J. I . , and L i n , S. D. (1980). Appl. Phys. L e t t . 51, 210. L i e t o i l a , A., Gibbons, J. F., Magee, T. J., Peng, J., and Hong, J. D. (1979). Appl. Phys. L e t t . 35, 532. L i n , S. D., and L e v a t t e r , J. I . (1979). Appl. Phys. L e t t . 34, 505. L i u , P. L., Yen, R., Bloembergen, N., and Hodgson, R. T. (1979). Appl. Phys. L e t t . 34, 864. C h r i s t i e , W. H., and Eby, R. E. Lowndes, D. H., Cleland, J. W., (1981). Mat. Res. SOC. Symp. Proc. 1, 223. Lowndes, D. H. (1982) Phys. Rev. L e t t . 48, 267.


39

Lowndes, D. H., Cleland, J. W., C h r i s t i e , W. H., Eby, R. E., J e l l i s o n , G. E., Jr., Narayan, J., Westbrook, R. D., Wood, R. F., N i l s o n , J. A., and Dass, S. C. (1982). Appl. Phys. L e t t . 41, 938. M a r g a l i t , S., Pekete, D., Pepper, D. M., Lee, C. P., and Yariv, A. (1978). Appl. Phys. L e t t . 33, 346. Minagawa, S., Lee, K. F., Gibbons, J. F., Magee, T. J., and Ormond, R. (1981). J. Electrochem. SOC. 128, 848. Miyao, M., Ohyu, K., and Tokuyama, T. (1979). Appl. Phys. L e t t . 35, 227. Miyao, M., Motouka, T., Natsuaki, N., and Tokuyama, T. (1981). S o l i d S t a t e Commun. 37, 605. Mizuta, M., Sheng, N. H., Merz, J. L. (1981). Appl. Phys. L e t t . 38, 453. Mooney, P. M., Young, R. T., Karins, J., Lee, Y. H., and Corbett, J. W. (1978). Phys. Stat. Sol ( a ) 48, K31. M u l l i n s , W. W., and Sekerka, R. F. (1964). J. Appl. Phys. 35, 444. Narayan, J., Young, R. T., and Wood, R. F. (1978). Appl. Phys. L e t t . 33, 338. Narayan, J. (1980). J. M e t a l s 32, 15. Narayan, J. (1981). J. Appl. Phys. 52, 1289. Tuomi, T., Blomberg, M., Luomajzrvi, M., and Naukkarinen, K., Rauhala, E. (1982). J. Appl. Phys. 53, 5634. Jap. J. Appl. Nishiyama, K., A r a i , M., and Watanabe, N. (1980). Phys. L e t t . 19, 256. Olson, G. L., Anderson, C. L., Dunlap, H. L., Hess, L. D., and McFarlane, R. A,, Vaidyanathan, K. V. (1980a). I n "Proc. Laser and E l e c t r o n Beam Processing o f E l e c t r o n i c M a t e r i a l s , " p. 467, Electrochem. SOC. , P r i n c e t o n , New Jersey. Olson, G. L., Kokorowski, S. A,, McFarlane, R. A., and Hess, L. D. (1980b). Appl. Phys. L e t t . 37, 1019. I v l e v , G. D., Morgun, Yu. F., Nechaev, N. V., P i l i p o v i c h , V. A., O s i n s k i i , V. I.,and Peshko, A. Ya. (1975). J. Appl. Spectrosc. 22, 324. Poate, J. M., Tu, K. N., and Mayer, J. W., eds. (1978a). "Thin Films I n t e r d i f f u s i o n and Reactions." Wilev I n t e r s c i e n c e . New York. Poate, J. M., Leamy, H. J., Sheng, T. f., and C e l l e r , G . K. (1 78b). Appl. Phys. L e t t . 33, 918. Poate, J. M. and Mayer, J. W., eds. (1982). "Laser Anneal ng o f Semiconductors. I' Academic Press , New York. Pounds, R. S., S a i f i , M. A., and Hahn, Jr., W. C. (1974). Sol i d S t a t e E l e c t r o n . 17, 245. Rao, D. V. G. L. Narasimha. (1968). J. Appl. Phys. 39, 4853 Ready, J. F. (1971). " E f f e c t s o f High Power Laser Radia i o n . " Academic Press, New York. Revesz, P., Farkas, G., Mezey, G., and Gyulai, J. (1978). A P P l Phvs. L e t t . 33, 431. Rot

1.0

.a

0.8

W

8

3

$ 0.6 z IT 0

8

.

0

0.4

0

0

0.2 0

I 4.5

I 4.0

I 0.5

0

I 0.5

I 1.0 4.5 4.5

I

4.0

I 0.5

I

0

0.5

I 4.0

1 4.5

TILT ANGLE (deg)

Fig. 1 . Angular scans for 2 . 5 MeV He ions across the (110, and channels of lZ1Sb (100 keV, 1 . 6 x 1 0 1 6 / c m 2 ) implanted ( 1 0 0 ) S i after ruby laser annealing. Solid circles refer to scattering from Sb, open circles refer to scattering from Si, and Ax refers to the depth interval from which scattered particles were detected. From White et a l . , 1979a.

2.

SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS

49

r e s u l t s as

Fs = 100 x ( l - ~ ~ ~ " ( S b ) ) / ( l - x ~ ~ ~ ( S3 i ) ) where +n,i

(1)

i s t h e minimum y i e l d d e f i n e d i n t h e usual manner as

the r a t i o o f the scattered p a r t i c l e y i e l d i n the aligned direct i o n t o t h a t i n t h e random d i r e c t i o n . Fig.

For t h e r e s u l t s shown i n

1, t h e s u b s t i t u t i o n a l f r a c t i o n i s 99%.

Corresponding e l e c -

t r i c a l measurements show t h a t a l l t h e implanted Sb i s e l e c t r i c a l l y active

after

laser

annealing

thus

confirming

the

channeling

results. R e s u l t s s i m i l a r t o those i n F i g .

1 have been o b t a i n e d a l s o

f o r t h e case o f implanted As (White e t al., t h a t , even f o r q u i t e h i g h c o n c e n t r a t i o n s ,

1979a), demonstrating

As i s s u b s t i t u t i o n a l i n

t h e l a t t i c e a f t e r l a s e r annealing w i t h a s i m i l a r h i g h degree o f s u b s t i t u t i o n a l i t y (99%).

For dopants such as Ga and B i , however,

t h e i o n channeling r e s u l t s show t h a t t h e dopants a r e d i s p l a c e d s l i g h t l y from s u b s t i t u t i o n a l l a t t i c e s i t e s (White e t a1

., 1980).

T h i s i s demonstrated i n t h e angular scan r e s u l t s o f Fig. t h e case o f Ga i n S i a f t e r l a s e r annealing.

2 for

As shown i n Fig. 2,

t h e y i e l d curves of s c a t t e r i n g from Ga a r e n o t as wide as those f o r Si.

T h i s i m p l i e s t h a t a t l e a s t a p a r t o f t h e implanted Ga i s

d i s p l a c e d s l i g h t l y from a normal

substitutional

l a t t i c e site.

S u b s t i t u t i o n a l f r a c t i o n s , obtained u s i n g Eq. 1 show Ga t o be -98% substitutional

after

l a s e r annealing.

Therefore,

t h e angular

scan r e s u l t s , show t h a t w h i l e Ga i s r e g u l a r l y placed i n t h e l a t t i c e , i t may be d i s p l a c e d s l i g h t l y from a s u b s t i t u t i o n a l s i t e . S i m i l a r r e s u l t s have been obtained a l s o f o r t h e case o f B i i n s i 1icon. I n summary,

i o n channeling (White e t a1

nuclear reaction analysis

., 1979a,

r e s u l t s (Swanson e t al.,

1980) and 1981) show

t h a t Group I 1 1 and V dopants a r e r e g u l a r l y placed i n t h e s i l i c o n l a t t i c e a f t e r pulsed l a s e r annealing.

Dopants such as As, Sb, B,

and P occupy s u b s t i t u t i o n a l l a t t i c e s i t e s , w h i l e Ga, B i and prob a b l y I n are d i s p l a c e d s l i g h t l y from s u b s t i t u t i o n a l l a t t i c e s i t e s .

50

C. W. WHITE

1.6

I

I

-0-

t.4

.

1

I

I

t

Si

Ga

1.2

sw 1.0 >

n A

0.8

a

5

0.6

0.4

0.2

0

0.5

1.0

Fig. 2 .

0.0

0.5

1.0 1.0 TILT ANGLE (deg)

0.5

0.0

0.5

1.0

Angular scans across the and till> axial directions for 69Ga

(100 keV, 3 . 2 x 1 0 1 5 / c m 2 ) implanted (100) Si after ruby laser annealing. From White et a l . , 1980.

I n a1 1 cases, t h e s u b s t i t u t i o n a l f r a c t i o n s a r e c o n s i d e r a b l y b e t t e r t h a n those o b t a i n e d by thermal levels of substitutionality

annealing,

and these very h i g h

can be achieved even when t h e dopant

c o n c e n t r a t i o n s g r e a t l y exceed e q u i l i b r i u m s o l u b i l i t y l i m i t s . 4.

DETERMINATION OF THE INTERFACIAL DISTRIBUTION COEFFICIENT

I n o r d e r t o t r e a t segregation a t t h e l i q u i d - s o l i d i n t e r f a c e , it

is

necessary t o know t h e i n t e r f a c i a l segregation c o e f f i c i e n t

k ' d e f i n e d by k ' = Cs/CL

,

(2 1

2. SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS 51 where Cs and CL a r e c o n c e n t r a t i o n s i n t h e s o l i d and l i q u i d phases a t the interface.

During s o l i d i f i c a t i o n , t h e i n t e r f a c i a l d i s t r i -

b u t i o n c o e f f i c i e n t determines t h e p a r t i t i o n o f dopant between t h e s o l i d and l i q u i d phases a t t h e i n t e r f a c e .

Since d i f f u s i o n coef-

f i c i e n t s i n t h e l i q u i d are several o r d e r s o f magnitude g r e a t e r t h a n those i n t h e s o l i d phase, assume t h a t

t h e dopant

it i s

a good approximation t o

r e d i s t r i b u t i o n i n t h e s o l i d phase i s

n e g l i g i b l e compared t o t h a t i n t h e l i q u i d phase d u r i n g t h e s h o r t T h i s means t h a t Cs i n

t i m e s i n v o l v e d i n pulsed l a s e r annealing. Eq.

2 i s e a s i l y determined from t h e dopant p r o f i l e s measured by A d i r e c t measurement o f CL on t h e o t h e r hand i s g e n e r a l l y d i f f i c u l t i f n o t i m p o s s i b l e t o c a r r y o u t and i t is

RBS or SIMS.

necessary t o r e l y on t h e t h e o r y o f s o l i d i f i c a t i o n processes t o c a l c u l a t e CL i n o r d e r t o determine k'. e q u i l i b r i u m value o f

k',

which

The d e t e r m i n a t i o n o f t h e

we w i l l

denote b y ko,

=4i/

i.e.,

n

L'

(3)

equilibrium

i s s i m p l i f i e d because ko can be r e l a t e d d i r e c t l y t o t h e e q u i l i b r i u m phase diagram.

T h i s i s i l l u s t r a t e d i n Fig. 3 which shows a por-

t i o n o f t h e phase diagram f o r system.

a typical

two-component

alloy

Although n o t s t r i c t l y r e q u i r e d by s o l i d i f i c a t i o n theory,

i t i s customary t o r e q u i r e t h a t Cs and

t i o n s i n solution a t the interface.

CL r e f e r t o t h e concentraWith t h i s r e s t r i c t i o n i n

mind, any d e p a r t u r e s from homogeneous s o l u t i o n s due t o p r e c i p i t a t i o n , c e l l u l a r f o r m a t i o n , etc.

are t o be excluded i n t h e d e f i n i -

t i o n o f k' and ko. A t very l o w growth v e l o c i t i e s ,

s o l i d i f i c a t i o n occurs under

c o n d i t i o n s o f l o c a l e q u i l i b r i u m a t t h e i n t e r f a c e and thus k' = ko. The i n f l u e n c e o f t h e d i s t r i b u t i o n c o e f f i c i e n t on s o l u t e p r o f i l e s i n t h e l i q u i d and s o l i d phases a t several stages d u r i n g s o l i d i f i c a t i o n i s i l l u s t r a t e d i n Fig.

3 f o r t h e case o f ko

f i r s t s o l i d t o f r e e z e w i l l have a s o l u t e c o n c e n t r a t i o n

10 so t h a t nAx

>>

AT =

At/n.

Equation 8 The value o f

m t and D A T / ( A x ) ~was

r e q u i r e d t o be < 0.5 t o i n s u r e t h e convergence o f t h i s numerical method.

Then a t t h e t i m e t = (to+ A t ) t h e m e l t f r o n t advances

t o p o s i t i o n xMel

(Fig.

4d) w i t h t h e corresponding p a r t i t i o n o f

dopant between t h e s o l i d and l i q u i d phase g i v e n by t h e i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t as discussed above.

These calcu-

l a t i o n s are continued u n t i l t h e i n t e r f a c e reaches t o w i t h i n 200 A o f t h e s u r f a c e and t h e dopant remaining i n t h e l i q u i d i s considered t o be segregated t o t h e surface. diffusion coefficient, 1963).

D,

Values f o r t h e l i q u i d phase

a r e taken from t h e l i t e r a t u r e (Kodera,

T h i s model assumes t h a t t h e o n l y mechanism f o r mass t r a n s -

2.


p o r t i s l i q u i d phase d i f f u s i o n ,

57

and regrowth v e l o c i t y i s assumed

t o be constant d u r i n g s o l i d i f i c a t i o n .

The dopant f l u x out o f t h e

s u r f a c e d u r i n g regrowth i s r e q u i r e d t o be zero unless t h e r e was a n e t l o s s o f dopant d u r i n g t h e a n n e a l i n g process as determined by

I f dopant loss occurs, t h i s i s taken

i o n backscattering analysis.

i n t o account by r e q u i r i n g t h a t l o s s occurred from t h e s u r f a c e a t a r a t e p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n a t t h e surface.

I n the

model k l i s assumed t o be a c o n s t a n t , independent o f t h e dopant c o n c e n t r a t i o n , and i s t r e a t e d as a f i t t i n g parameter.

A value of

k ' f o r each dopant was determined by f i t t i n g t h e c a l c u l a t e d profile

in

the solid

t o the

measured p r o f i l e u s i n g l e a s t squares

anal'ysi s.

111. 5.

Dopant Incorporation During Rapid S o l i d i f i c a t i o n

SEGREGATION BEHAVIOR OF B, P, AND As I N SILICON Boron, phosphorus,

used

dopants

in

the

device applications.

and a r s e n i c a r e t h e t h r e e most commonly p r o c e s s i n g of

silicon

for

semiconductor

F i g u r e 5 shows t h e e f f e c t s o f l a s e r annealing

of c r y s t a l s implanted with By P, and As, each a t a dose o f -1016/cm2 (White e t al.,

1978).

The l a s e r a n n e a l i n g c o n d i t i o n s were such

t h a t t h e r e c r y s t a l l i z a t i o n v e l o c i t y was -3 m/sec. and P were measured by SIMS, measured by RBS.

P r o f i l e s for B

w h i l e those f o r As p r o f i l e s were

Concentrations determined by SIMS were estimated

by comparison w i t h r e s u l t s obtained from samples implanted i n t h e dose range lo1'+ t o 1016/cm2.

For each dopant i n F i g . 5, t h e as-

implanted p r o f i l e i s v e r y n e a r l y Gaussian, b u t i n each case l a s e r a n n e a l i n g causes a s i g n i f i c a n t r e d i s t r i b u t i o n o f t h e dopant, b o t h toward t h e s u r f a c e as w e l l

as deeper i n t o t h e c r y s t a l t o t h e

e x t e n t t h a t t h e p r o f i l e i s n e a r l y u n i f o r m i n t h e depth range 1000-2000

k a f t e r l a s e r annealing.

These r e s u l t s demonstrate

t h e r a p i d r e d i s t r i b u t i o n o f t h e dopants which can occur i n t h e l i q u i d phase, due t o t h e very h i g h d i f f u s i v i t i e s i n t h e l i q u i d (DL

- lo4

cm2/sec);

r e d i s t r i b u t i o n over these extended d i s t a n c e s

58

C . W. WHITE

would be i m p o s s i b l e b y s o l i d phase d i f f u s i o n because s o l i d phase d i f f u s i v i t i e s are almost e i g h t o r d e r s o f magnitude lower, and t h e t i m e a v a i l a b l e f o r d i f f u s i o n (a few hundred nanoseconds) i s t o o short. Values o f t h e e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t ko f o r P, and As i n S i a r e 0.80,

1960).

0.35,

and 0.30

B,

r e s p e c t i v e l y (Trumbore,

With these values o f ko pronounced s u r f a c e s e g r e g a t i o n

should have been observed f o r P and As but, as Fig. 5 i n d i c a t e s were not.

The l a c k o f a s u r f a c e s e g r e g a t i o n s p i k e i n t h e t h r e e

p r o f i l e s a f t e r l a s e r a n n e a l i n g i s good evidence t h a t k ' has grown from ko t o n e a r l y u n i t y f o r v

- 3 m/sec.

F i g u r e 6 shows how a comparison o f experimental and c a l c u l a t e d p r o f i l e s f o r As i n s i l i c o n (White e t al., v a l u e o f k ' t o be determined.

1980) a l l o w s t h e

F o l l o w i n g l a s e r annealing,

c h a n n e l i n g r e s u l t s show t h a t As i s

>

95% s u b s t i t u t i o n a l

ion

i n the

l a t t i c e and i s e l e c t r i c a l l y a c t i v e as determined from H a l l e f f e c t measurements.

T h i s h i g h degree of s u b s t i t u t i o n a l i t y i s achieved

even though t h e As c o n c e n t r a t i o n exceeds t h e e q u i l i b r i u m s o l u b i l i t y l i m i t by a f a c t o r o f -4 i n t h e n e a r - s u r f a c e region.

This

demonstrates t h e f o r m a t i o n o f a s u p e r s a t u r a t e d a l l o y as a consequence o f t h e h i g h speed, 1 iquid-phase e p i t a x i a l regrowth process. value

The s o l i d l i n e i n Fig. 6 i s a p r o f i l e c a l c u l a t e d u s i n g a for

the d i s t r i b u t i o n

coefficient

of

k'

=

1.0

and t h e

agreement w i t h t h e experimental p r o f i l e r e s u l t s ( s o l i d c i r c l e s ) i s excellent.

The value determined f o r k ' i s c o n s i d e r a b l y h i g h e r

t h a n t h e e q u i l i b r i u m value distribution coefficient

(ko = 0.3).

The i n c r e a s e i n t h e

r e l a t i v e t o t h e e q u i l i b r i u m value i s a

consequence o f t h e h i g h regrowth v e l o c i t y which causes a depart u r e from c o n d i t i o n s o f l o c a l e q u i l i b r i u m a t t h e i n t e r f a c e d u r i n g sol i d i f ication.

6.

SEGREGATION BEHAVIOR OF OTHER GROUP 111-V DOPANTS I N SILICON As we have j u s t seen,

for

B y P, and As i n

values o f t h e s e g r e g a t i o n c o e f f i c i e n t

S i have a l r e a d y grown from t h e i r ko values t o

2.


59

1022

5

2

102' I

m

1020

5

2

1049

F i g . 6 . Profiles for 75As (100 keV, 6 . 4 ~1 0 l 6 / c m 2 ) in ( 1 0 0 ) S i c o m p a r e d to model calculations. The equilibrium solubility limit is indicated by the horizontal line. From White e t a t . . 1980.

nearly unity for v

- 3 m/sec.

Other Group 111-V dopants have

s u b s t a n t i a l l y s m a l l e r values o f ko than do B, P, and As and more pronounced changes i n t h e s e g r e g a t i o n b e h a v i o r w i t h r e c r y s t a l l i z a t i o n v e l o c i t y can be expected. B i (ko = 0.0007)

and I n (0.0004)

F i g u r e s 7 and 8 show p r o f i l e s f o r i n S i o b t a i n e d w i t h v = 4.5 m/sec.

I n F i g . 7, as a consequence o f l a s e r annealing, a p p r o x i m a t e l y 15% of t h e B i segregates t o t h e s u r f a c e but t h e c o n c e n t r a t i o n remaining

60

C. W. WHITE 102'

5

2 1020

L

z

$

5

z

8 2 1018

5 2 10'7

Fig.

7.

Profiles for

2096i ( 2 5 0 keV,

1.2 x 1015/cm2)

in ( 1 0 0 ) S i

compared to model calculations. The horizontal line indicates the equilibrium From solubility limit and the dashed p r o f i l e i s calculated assuming k' = k,. White et a l . ,

1980.

i n the bulk i s though

this

>

95% s u b s t i t u t i o n a l ( i o n channeling r e s u l t s ) even

concentration

exceeds

the

equilibrium

l i m i t by a p p r o x i m a t e l y two orders o f magnitude. i n Fig.

solubility

The s o l i d l i n e

7 is a p r o f i l e c a l c u l a t e d u s i n g a value f o r k ' = 0.4 and

assuming t h a t t h e l i q u i d phase d i f f u s i v i t y f o r B i i n Si i s

DL

=

2.


61

102’

5

2 1020

2 10’8

5

2

0

0.1 DEPTH ( p m )

0.2

0.3

Fig. 8. Profiles for l i 5 I n ( 1 2 5 keV, 1.2 x lOi5/Cm2) i n Si compared to model calculations. The horizontal line indicates the equilibrium solubility limit and the dashed p r o f i l e i s calculated assuming k l = ko. From White e t a l . , 1980.

1.5 x lo-‘+. Values for DL ( B i i n S i ) have not been reported in the l i t e r a t u r e , b u t the value of 1.5 x gives a satisfactory f i t t o the experimental results and i s in reasonable agreement with the extrapolation o f measured liquid phase diffusivities for

62

C. W. WHITE

lower mass impurities i n liquid s i l i c o n . In Fig. 7 , the calcul a t e d p r o f i l e ( s o l i d l i n e ) i s in good agreement with the experimental p r o f i l e s ( f i l l e d c i r c l e s ) measured a f t e r l a s e r annealing. By c o n t r a s t , a p r o f i l e calculated using the equilibrium value f o r the d i s t r i b u t i o n c o e f f i c i e n t of Bi in Si (ko = 7 x l o m 4 ) , i s shown by the dashed curve in Fig. 7. If s o l i d i f i c a t i o n occurred under conditions of local equilibrium a t t h e i n t e r f a c e , very l i t t l e Bi would remain in t h e b u l k of t h e crystal and almost a l l of the Bi would have zone refined t o t h e surface. Clearly t h i s does not f i t t h e experimental r e s u l t s . Similar r e s u l t s a r e obtained f o r t h e case of In in Si as shown in Fig. 8. As a r e s u l t of l a s e r annealing, approximately 60% of the In i s zone refined t o the surface, b u t the remainder i n the bulk i s highly substitutional and the p r o f i l e can be f i t with reasonable accuracy by using a value f o r k ' = 0.15. This value i s f a r greater than the equilibrium value f o r In in Si If local equilibrium conditions prevailed during (ko = 4 x the s o l i d i f i c a t i o n , very l i t t l e In would have remained in t h e bulk a s indicated by the dotted p r o f i l e in Fig. 8. I t i s i n t e r e s t i n g t o note t h a t t h e experimental p r o f i l e r e s u l t s in Figs. 6, 7 and 8 can be f i t by a s i n g l e value of k ' over t h e e n t i r e range of concentrations. This indicates t h a t the value f o r k ' i s not a strong function of concentration, and is determined, t o f i r s t order, by t h e regrowth velocity. For t h e case of B i i n S i , experiments s i m i l a r t o those i l l u s t r a t e d in Fig. 7 have been c a r r i e d out a t both higher and lower implanted doses (concentrations). In each case the value determined f o r k ' l i e s in t h e range 0.35 t o 0.40 even though the implanted dose was varied by over an order of magnitude. This f u r t h e r reinforces the conclusion t h a t the value f o r k ' i s not a strong function of concentration. Using similar methods, values f o r k ' have been determined f o r a wide variety of Group I11 and V dopants in (100) Si a t t h e very high growth v e l o c i t i e s which can be achieved by pulsed l a s e r

2.


63

Table I Comparison of Distribution Coefficients Under Equilibrium (k,) and Laser Annealed ( k ' ) Regrowth Conditions Dopant B P As Sb Ga In Bi

(a)

(b) k'

0.80 0.35 0.30 0.023 0.008 0.0004

1.0 1.0 1.0

ko

0.0007

0.7 0.2

0.15 0.4

( a ) From Trumbore, 1960. (b) Values f o r k ' were determined a t a growth velocity of 2.7 m/sec f o r B, P, Sb, and a t 4.5 m/sec f o r As, Ga, In, and Bi.

.

anneal i ng (White et a1 , 1980). The values determi ned f o r k ' during pulsed l a s e r annealing a r e summarized i n Table I and compared with the corresponding equi 1ibri um values , ko (Trumbore, 1960). These values f o r k ' were determined a t a growth velocity of 4.5 m/sec except f o r the cases of B, P and Sb. For these t h r e e dopants, a somewhat longer pulse duration time (- 50 x sec) was used f o r annealing, r e s u l t i n g i n a growth velocity of -2.7 m/sec. Values f o r the l i q u i d phase d i f f u s i v i t i e s used t o f i t the experimental p r o f i l e s were taken from the l i t e r a t u r e (Kodera, 1963) except f o r t h e case of Bi where a value of DL = 1.5 x cm2/sec was assumed (see previous discussion). The r e s u l t s presented in Table I show t h a t in every case k ' i s s i g n i f i c a n t l y greater than ko by f a c t o r s t h a t extend up t o -600. The values reported in Table I were the f i r s t determination o f i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t s under conditions of high speed nonequilibrium crystal growth f o r any system. The l a r g e increase in k ' r e l a t i v e t o ko r e f l e c t s the nonequilibrium nature of the l a s e r annealing induced liquid-phase e p i t a x i a l regrowth process. The departure from conditions o f local equilibrium a t

64

C . W. WHITE

the i n t e r f a c e i s brought about by the very high growth v e l o c i t i e s (several meters/sec) which can be achieved by l a s e r annealing. In crystal growth a t low v e l o c i t i e s where local equilibrium cond i t i o n s prevail, s o l u t e atoms exchange many times across the i n t e r f a c e in order t o e s t a b l i s h t h e i r equilibrium concentrations i n the s o l i d and l i q u i d before being permanently incorporated i nto the sol id. During pul sed l a s e r anneal i ng , regrowth vel oci t i e s a r e so high t h a t a new plane of atoms i s being added t o the growing crystal every sec. On t h i s time s c a l e , s o l u t e atoms cannot be exchanged across the i n t e r f a c e a s u f f i c i e n t number of times t o e s t a b l i s h t h e i r equilibrium concentrations before being incorporated i n t o the sol id. Consequently, s o l u t e atoms a r e trapped i n t o the s o l i d a t concentrations t h a t can f a r exceed equilibrium s o l u b i l i t y l i m i t s , a process referred t o as s o l u t e trapping

.

7.

EFFECTS OF REGROWTH VELOCITY AND SUBSTRATE ORIENTATION ON k '

Experiments have shown t h a t the i n t e r f a c i a l d i s t r i b u t i o n coeff i c i e n t is a function of both growth velocity (Cullis e t al., 1980; Baeri et al., 1981) and crystal orientation (Baeri e t al., 1981a). The velocity dependence i s e n t i r e l y expected because as t h e velocity decreases, k ' must approach the equi 1 i bri um Val ue , ko. C u l l i s et a l . (1980) reported t h e f i r s t observations of t h i s expected velocity dependence f o r several d i f f e r e n t Val ues of v f o r the case o f P t in S i where i t was observed t h a t increasing t h e growth velocity resulted in more implanted P t being incorporated i n t o the l a t t i c e during l a s e r annealing. Similar r e s u l t s on velocity dependence a r e shown in Fig. 9 f o r t h e case of Bi in Si (White et a l . , 1981). Substrate temperatures of 650 K, 300 K and 100 K give r i s e t o regrowth velocit i e s of -1.5, 4.5 and 6.0 m/sec f o r the l a s e r conditions used f o r sec, 1.4 J/cm2). A t the low annealing (X = 6943 A , 15 x growth velocity (1.5 m/sec), almost 55% of the implanted B i

d

0

rd C C

tu U 0

L

rd

*-

- +I 2

oa

66

C . W. WHITE

segregates t o the surface as a r e s u l t of l a s e r annealing, while a t the highest growth velocity only 5% i s segregated t o the surface. In each case, the Bi remaining in the bulk of the crystal i s >95% substitutional in the l a t t i c e . Dotted l i n e s in Fig. 9 a r e calculated p r o f i l e s using values f o r k ' = 0.1, 0.35 and 0.45 a t growth v e l o c i t i e s of 1.5, 4.5 and 6.0 m/sec. The agreement between the calculated and experimental p r o f i l e s in Fig. 9 i s excellent and these r e s u l t s demonstrate t h a t k ' and the amount of B i segregated t o the surface a r e strong functions of growth veloci t y , as expected. A similar dependence of k' on regrowth velocity has been reported also f o r the case of In in Si (Baeri et al., 1981), and similar dependencies should be observable f o r a l l Group 111, V species in s i l i c o n . These experiments, i f carried out over a wider velocity range can be expected t o provide fundament a l insight into d e t a i l e d mechanisms of importance t o high speed nonequi 1 i b r i u m crystal growth processes. Baeri e t a l . (1981a) f i r s t demonstrated t h a t in c e r t a i n ranges, t h e value f o r k ' i s a l s o a strong function of crystal o r i e n t a t i o n . An example of t h i s e f f e c t i s shown in Fig. 10 f o r (100) and (111) c r y s t a l s implanted by l151n (125 keV, 1.2 x 1015/cm2) and l a s e r annealed under identical conditions (XeC1 l a s e r , -35 x sec, 1.3 J/cm2). Considerably more In i s trapped i n the b u l k of the (111) crystal implying t h a t the value f o r k ' i s systematically l a r g e r f o r the (111) case. Figure 11 shows the velocity dependence of k ' f o r In in (100) and (111) Si (Poate 1982). For v e l o c i t i e s below -4 m/sec the value f o r k ' in (111) Si i s systematically higher than t h a t f o r (100) Si For identical 1a s e r anneal i ng conditions, t h e regrowth velocity normal t o the surface should be the same since velocity i s determined by heat flow i n t o the underlying substrate. Consequently the anisotropic dependence of k ' on growth velocity must be related t o differences in d e t a i l e d mechanisms of crystal growth f o r (100) and (111) c r y s t a l s . In p a r t i c u l a r , i t has been suggested

.

2. SEGREGATION,SOLUTE TRAPPING,AND SUPERSATURATED ALLOYS 67

I

0

I

I

0.4 DEPTH ( p m )

I

0.2

0

O.!

0.2

DEPTH (,urn 1

Fig. 10. Dopant profiles for 1151n ( 1 2 5 keV, 1.2 x 1 0 1 5 / ~ m 2i)n (100) and ( 1 1 1 ) Si. From White e t al. , 1983.

that a larger interfacial undercooling on the (111) face compared t o the (100) face (Baeri e t al., 1981a; Jackson, 1981) m i g h t explain the differences in dopant incorporation for these two cases. Alternatively the greater ledge velocity which i s expected on the (111) face may be responsible for the increased value for k' (Spaepen and Turnbull , 1982). A dependence of k ' on orientation has been observed for B i , I n , Gay Sn, and Pb i n s i l i con a t velocities of 2-4 m/sec. I n each case the value f o r k' i s greater f o r the (111) case. Impurities for which k ' i s very near t o unity do not show this effect. These include B, P, As, Ge, and Sb.

68

C. W. WHITE

I .o

I

I

I

I

I

I

I

*-----

-,-

A

Fig. 1 1 . i n silicon.

8.

Dependence o f kl on growth velocity and crystal orientation for In From Poate ( 1 9 8 2 ) .

MAXIMUM SUBSTITUTIONAL SOLUBILITIES

White e t a l . (1980) have shown t h a t as the implanted dose of each of the Group 111, V species i s increased, there i s a maximum concentration t h a t can be incorporated s u b s t i t u t i o n a l l y i n t o the Si l a t t i c e as a r e s u l t o f pulsed l a s e r annealing. (See a l s o Stuck et a l . , 1980). This i s shown in Fig. 12 f o r four d i f f e r e n t doses of In (125 keV) in (100) S i , where both the t o t a l dopant concentration and t h e substitutional dopant concentration a f t e r l a s e r annealing are plotted as a function o f depth. These r e s u l t s

Cyx

2.


69

r-7 DOSE = 7.9 x 10'5/cm2

*--+SUBSTITUTIONAL

0

0.! 0.2 DEPTH ( p m )

t ,

0

0.1 0.2 DEPTH ( p m )

Fig. 12. Dose dependence o f solute trapping for I n i n ( 1 0 0 ) Si. Total and substitutional concentration p r o f i l e s a r e shown for each dose a f t e r ruby laser From White e t a l . , 1983. annealing.

were obtained by using Rutherford backscattering and i o n channeling measurements to determine the total dopant concentration and the substitutional concentration as a function of depth. At the two

70

C. W. WHITE

Table I1 S u b s t i t u t i o n a l S o l u b i l t i e s i n S i l i c o n Achieved by R e c r y s t a l l i z a t i o n a t 4.5 m/sec

(100) S i

(111) S i

As

1.5 x 1021

6.0 x 1021

6.0 x 1021

Sb Bi

7.0 x 1019 8.0 x lOl7

2.0 x 1O2l 4.0 x 102O

2.0 x 1O2l 8.6 x 102O

Ge

5.0 x 1022

6.0 x 1021

>1.2 x 1022

Sn Pb

5.5 x loL9

9.8 x 1020 1.0 x 1020

1.4 x 1O2l 3.0 x 102O

B

6.0 x 1020

2.0 x 1021

2.0 x 1021

Ga In T1

4.5 x 1019 8.0 x 1017

4.5 x 1020 1.5 x 1020

7.2 x 1020 4.5 x 1020

---

---

l o w e r doses, tional

---

Thermodynamic Limit C e l l Formation Precipitation C e l l Formation C e l l Formation on (100) C e l l Formation Precipitation C e l l Formation Mechanical Strain C e l l Formation C e l l Formation Coherent Prec ip i t a t ion

---

i n t h e b u l k o f t h e c r y s t a l t h e t o t a l and s u b s t i t u -

c o n c e n t r a t i o n s a r e v i r t u a l l y i d e n t i c a l and t h e p r o f i l e s

s c a l e w i t h implanted dose. For t h e two h i g h e r doses, t h e t o t a l and s u b s t i t u t i o n a l c o n c e n t r a t i o n s a r e n e a r l y t h e same up t o a c o n c e n t r a t i o n o f 1.5-2.0 x 102°/cm3. As t h e t o t a l c o n c e n t r a t i o n increases

above

this

value,

the

substitutional

remains t h e same o r decreases somewhat. d i t i o n s (v = 4.5 m/sec)

t h i s value

maximum c o n c e n t r a t i o n (Cyax)

concentration

For these regrowth con-

o f 1.5-2.0 x 1020/cm3 i s t h e

o f I n which can be i n c o r p o r a t e d i n t o

substitutional l a t t i c e sites. Using

similar

techniques,

values f o r Ca:x

have been d e t e r -

mined f o r n i n e Group 111, I V and V species i n (100) and (111) S i a t a growth v e l o c i t y o f -4.5

m/sec.

These values are l i s t e d i n

Tab1 e I1 and compared t o correspondi ng e q u i l ib r i um s o l u b i l it y

2.


LIQUID

71

+ SOLID

c:

CONC E NT RAT I0N

Fig.

13.

Schematic phase diagram for retrograde alloys.

l i m i t s Ci. With t h e exception of Ge, values f o r C:ax are l a r g e r than those f o r C: by f a c t o r s t h a t range from 4 in the case of As, t o -500 f o r the case of B i . On an equilibrium phase diagram (shown schematically in Fig. 13) most of these dopants exhibit retrograde s o l u b i l i t y in s i l i c o n . T h i s means t h a t the dopant has i t s maximum s o l u b i l i t y C z a t a temperature which i s not simply As shown by Baker and Cahn r e l a t e d t o a e u t e c t i c temperature. (1969), t h e retrograde maximum concentration cannot be exceeded by s o l i d i f i c a t i o n from the l i q u i d unless t h e r e i s a departure from equilibrium a t t h e i n t e r f a c e during s o l i d i f i c a t i o n . In Table 11, t h e large values f o r r e l a t i v e t o C! convincingly demonstrate the nonequil ibrium nature o f the l a s e r anneal ing induced 1 iquid-phase e p i t a x i a l regrowth process.

Cyx

72

C. W. WHITE

Dopant i n c o r p o r a t i o n i n t o t h e l a t t i c e a t these h i g h concent r a t i o n s i s a r e s u l t o f "solute trapping" during s o l i d i f i c a t i o n .

I n t h e s i m p l e s t terms t h i s means t h a t i f t h e t i m e r e q u i r e d t o regrow one o r more monolayers o f atoms d u r i n g s o l i d i f i c a t i o n i s s i g n i f i c a n t l y s h o r t e r t h a n t h e residence t i m e o f t h e i m p u r i t y a t t h e i n t e r f a c e then t h e i m p u r i t y has a h i g h p r o b a b i l i t y o f b e i n g i n c o r p o r a t e d i n t o t h e growing s o l i d .

Theoretical treatments o f

s o l u t e t r a p p i n g are g i v e n i n Baker and Cahn (1969), Cahn e t a l .

(1980), Wood (19801, Jackson e t a l . F o r several Ca:x

of

are l a r g e r i n

the

(1980),

and A z i z (1982).

dopants l i s t e d i n Table 11,

(111) S i compared t o t h e

values f o r

(100) case.

These

species i n c l u d e B i , Ge, Sn, Pb, Ga and I n and f o r these i m p u r i ties

k'

Eg i n i n d i r e c t band gap semiconductors which

have n o t been h e a v i l y damaged o r made amorphous by, i m p l a n t a t i o n a r i s e s f r o m mechanism [d].

e.g.,

ion

The s t r o n g dependence

o f t h e a b s o r p t i o n c o e f f i c i e n t on temperature comes about t h r o u g h

171

4. MELTING MODEL OF PULSED LASER PROCESSING

t h i s mechanism (because o f t h e temperature dependence o f Eg and i n creased phonon p o p u l a t i o n s a t e l e v a t e d temperatures) and n o t t h r o u g h mechanism [b]. Mechanism [ e l i s q u i t e i m p o r t a n t i n l a s e r p r o c e s s i n g o f semiconductors because i o n i m p l a n t a t i o n i s f r e q u e n t l y used.

I n those

near-surface r e g i o n s where t h e i o n i m p l a n t a t i o n c r e a t e s amorphous material,

a may e a s i l y be i n c r e a s e d by an o r d e r o f magnitude o r

more over t h e c r y s t a l l i n e value (Brodsky e t al.,

1970).

I t i s obvious from t h e f o r e g o i n g d i s c u s s i o n t h a t t h e absorpt i o n o f r a d i a t i o n by a semiconductor,

e s p e c i a l l y one l i k e s i l i c o n

w i t h an i n d i r e c t gap, d u r i n g p u l s e d l a s e r i r r a d i a t i o n i s a complex process:

It i s made even more complex because t h e c o n t r i b u t i o n s

o f t h e v a r i o u s mechanisms given above w i l l change d u r i n g t h e l a s e r pulse.

Measurements o f a as a f u n c t i o n o f temperature,

such as

t h o s e d e s c r i b e d i n Chapter 3, are i n v a l u a b l e i n p r o v i d i n g i n f o r m a t i o n f o r use i n m e l t i n g model c a l c u l a t i o n s .

However, s i n c e t h e s e

h i g h l y a c c u r a t e measurements a r e u s u a l l y made o n l y a t low l i g h t intensities,

t h e q u e s t i o n a r i s e s as t o whether o r n o t n o n l i n e a r

a b s o r p t i o n i n v a l i d a t e s t h e r e s u l t s a t t h e very h i g h i n t e n s i t i e s used i n p u l s e d l a s e r processing. (mechanism [c])

The i n d u c e d - m e t a l l i c a b s o r p t i o n

l e a d i n g t o an i n t e n s i t y - d e p e n d e n t a b s o r p t i o n co-

e f f i c i e n t i n s i l i c o n f o r t h e 1.06 pm l i g h t o f t h e fundamental o f t h e Nd:YAG l a s e r has been e x t e n s i v e l y s t u d i e d ( G r i n b e r g e t al., 1967; Gauster and Bushnell , 1970; Svantesson and Nilsson,

1978),

and i t i s c l e a r t h a t i t p l a y s an i m p o r t a n t r o l e in t h i s case. A d e f i n i t i v e answer t o t h i s q u e s t i o n when ruby, frequency-doubled Nd, and some u l t r a v i o l e t l a s e r s a r e used i s g r e a t l y c o m p l i c a t e d by t h e s t r o n g temperature dependence o f mechanism [d],

which can

cause an apparent n o n l i n e a r i t y resembling so c l o s e l y t h e b e h a v i o r expected from mechanism [c] t h e two.

t h a t it i s d i f f i c u l t t o disentangle

( E a r l y c a l c u l a t i o n s by L i e t o i l a and Gibbons (1979) f o r a

ruby l a s e r overestimated t h e r o l e o f induced f r e e - c a r r i e r a b s o r p t i o n because t h e c o r r e c t temperature dependence o f a was n o t t a k e n i n t o I f i n t e n s e p u l s e d l a s e r r a d i a t i o n i s used i n a t t e m p t s account.)

172

R. F. WOOD ET AL.

t o measure a, f u r t h e r c o m p l i c a t i o n s may be caused by t h e l a r g e temperature g r a d i e n t s s e t up o v e r t h e p e n e t r a t i o n depth of t h e radiation.

Although a c o m p l e t e l y s a t i s f a c t o r y s o l u t i o n t o t h e

problem o f d e t e r m i n i n g a under t h e extreme c o n d i t i o n s o f l a s e r a n n e a l i n g i s n o t y e t a v a i l a b l e , t h e r e a r e good reasons t o be o p t i m i s t i c t h a t t h i s d i f f i c u l t y does n o t g r e a t l y e f f e c t t h e modeling, as d i s c u s s e d next. I n Chapter 6 i t i s shown t h a t t h e r e i s good agreement between t h e r e s u l t s o f t r a n s i e n t r e f l e c t i v i t y and t r a n s m i s s i v i t y e x p e r i ments on s i l i c o n and m e l t i n g model c a l c u l a t i o n s employing t h e temperature-dependent o p t i c a l data o f J e l l i s o n and Modine (1982, 1983) and J e l l i s o n and Lowndes (1982).

Therefore, i t appears t h a t

c o n t r i b u t i o n s t o a from mechanism [ c ] a r e n o t l a r g e i n t h e nanosecond p u l s e range,

a l t h o u g h t h e d i s c r e p a n c i e s t h a t do e x i s t may

be due t o t h i s mechanism (Lowndes e t al.,

1983).

Furthermore,

F i g . 2 o f Chapter 3 shows t h a t a f o r photon e n e r g i e s g r e a t e r t h a n t h e d i r e c t band. gap has n e a r l y s a t u r a t e d a t c h a r a c t e r i s t i c o f metals.

-

lo6 cm-l,

a value

The c o n t r i b u t i o n s t o a o f n o n l i n e a r

f r e e - c a r r i e r a b s o r p t i o n i s almost c e r t a i n l y i n s i g n i f i c a n t f o r t h e s e s h o r t wavelengths,

o r more p r e c i s e l y

f u r t h e r i n c r e a s e s a r e unimportant. apparent t h a t t h e use o f u l t r a v i o l e t and KrF (249 nm),

a i s a l r e a d y so l a r g e t h a t I n t h i s connection,

it i s

asers, such as XeCl (308 nm)

i s advantagous i n s i m p l i f y i n g t h e t h e o r e t i c a l

treatment o f laser-induced melting.

l i t h these lasers, a i s v i r -

t u a l l y independent o f b o t h t h e temperature and t h e s t a t e o f t h e m a t e r i a l , i.e., and l i q u i d (9.) 2.

i t i s t h e same f o r c r y s t a l l i n e ( c ) , amorphous ( a ) ,

silicon.

CARRIER-LATTICE INTERACTION The t r a n s i e n t r e f l e c t i v i t y change e x h i b i t e d by semiconductors

d u r i n g i n t e n s e l a s e r i r r a d i a t i o n has been s t u d i e d s i n c e about 1964 (Carmichael and Simpson, 1964; Sooy e t al.,

1964; Birnbaum, 1965).

4.

173

MELTING MODEL OF PULSED LASER PROCESSING

I n some o f these e a r l i e r papers, t h i s r e f l e c t i v i t y change was a t t r i b u t e d t o t h e h i g h d e n s i t y o f photogenerated c a r r i e r s , which p e r s i s t e d a f t e r t h e t e r m i n a t i o n o f t h e l a s e r pulse.

However, B l i n o v

e t a l . (1967) concluded t h a t t h i s e x p l a n a t i o n d i d n o t f i t t h e i r d a t a on t h e a b s o r p t i o n o f long-wavelength r a d i a t i o n d u r i n g i r r a d i a t i o n o f S i and GaAs w i t h p u l s e s f r o m a Q-switched ruby l a s e r .

They

surmized i n s t e a d t h a t t h e r e f l e c t i v i t y change was due t o t h e m e l t i n g o f a t h i n surface layer.

A

c r u c i a l question f o r the a p p l i c a b i l i t y

o f t h e m e l t i n g model, a t l e a s t i n t h e form used here, concerns t h e l i f e t i m e o f electron-hole p a i r s during intense laser i r r a d i a t i o n and t h e t r a n s f e r o f energy from t h e c a r r i e r system t o t h e l a t t i c e . There i s a f a i r l y s u b s t a n t i a l body o f l i t e r a t u r e on t h i s t o p i c and v i r t u a l l y a l l o f t h e experimental data i n d i c a t e t h a t t h i s t r a n s f e r occurs i n t i m e s o f t h e o r d e r o f 1 0 - l 0 sec,

o r less.

I n fact,

Svantesson e t a l . (1971) found t h a t i n s i l i c o n t h e p u l s e w i d t h and shape o f t h e recombination r a d i a t i o n i n t h e r e g i o n around 1.1 eV ( i n d i r e c t band gap o f s i l i c o n ) t r a c k e d t h e 30-nsec e x c i t a t i o n p u l s e almost i d e n t i c a l l y , except f o r a very l o w - i n t e n s i t y ( Eg.

interactions in a semi-

Adapted from von Allmen

178

R. F. WOOD ET A L

i n t i m e s o f t h e o r d e r o f 10-14 sec.

The e l e c t r o n and h o l e popula-

t i o n s a r e brought i n t o thermal e q u i l i b r i u m w i t h one another a t about t h e same t i m e t h r o u g h impact i o n i z a t i o n and Auger recombination.

The schematic i l l u s t r a t i o n o f t h e s e events on t h e f i g u r e

i s meant t o i n d i c a t e t h a t t h e r e i s t r a n s f e r o f energy between t h e e l e c t r o n s and h o l e s w i t h o u t any d i s s i p a t i o n o f t h e energy o f t h e c a r r i e r system.

Phonon emission on t h e t i m e s c a l e o f 10-12 sec

t r a n s f e r s energy t o t h e l a t t i c e and heats i t t o t h e m e l t i n g p o i n t i n a few picoseconds.

Simultaneously,

the density o f carriers

decays (and t h e r e f o r e t h e plasmon frequency decreases) by way o f Auger recombination.

On a much l o n g e r t i m e scale,

a very low

d e n s i t y o f e l e c t r o n s a t t h e bottom o f t h e conduction band can r a d i a t i v e l y recombine w i t h t h e r e m a i n i n g h o l e s i n t h e valence band, p r o d u c i n g a weak, l o n g - l i v e d emission.

This p i c t u r e c l e a r l y

i m p l i e s t h a t melting-model c a l c u l a t i o n s i n which i t i s assumed t h a t t r a n s f e r o f t h e l a s e r energy from t h e e l e c t r o n i c system t o t h e l a t t i c e occurs i n t i m e s comparable t o , o r l e s s than, t h e p u l s e d u r a t i o n s h o u l d remain v a l i d even i n t h e picosecond regime.

However, t h e

e f f e c t i v e a b s o r p t i o n c o e f f i c i e n t may r e q u i r e some m o d i f i c a t i o n i f c a r r i e r d i f f u s i o n o r confinement e f f e c t s become s i g n i f i c a n t , as we now discuss. 3.

CARRIER DIFFUSION AND CARRIER CONFINEMENT

I n c o n n e c t i o n w i t h t h e general problem o f t h e c a r r i e r - l a t t i c e interaction,

Yoffa

(1980a,

1980b) a l s o considered t h e r o l e o f

c a r r i e r d i f f u s i o n i n determining t h e heating o f t h e l a t t i c e . concluded t h a t t h i s r o l e was i m p o r t a n t ,

and even dominant,

d e t e r m i n i n g t h e temperature r i s e o f t h e l a t t i c e . Y o f f a ' s t r e a t m e n t cons id e r a b l y overestimates

carrier diffusion.

She in

It i s l i k e l y t h a t

t h e importance o f

As we have seen, t h e r e i s good evidence t h a t

t h e l a s e r energy i s t r a n s f e r r e d t o t h e l a t t i c e i n times o f t h e o r d e r o f 10-'2-10-'1

sec f o r both nanosecond and picosecond l a s e r pulses,

i n which case l a r g e temperature g r a d i e n t s w i l l be s e t up i n t h e l a t t i c e d u r i n g t h e l a s e r pulse.

The e f f e c t o f these g r a d i e n t s on

4.

179


t h e band s t r u c t u r e a r e such t h a t confinement, r a t h e r than expansion, o f t h e gas o f c a r r i e r s i s expected.

Brown (1980) has drawn a t t e n -

t i o n t o t h i s problem and p o i n t e d o u t t h a t ,

i n t h e l i m i t when t h e

c a r r i e r s and t h e l a t t i c e a r e i n e q u i l i b r i u m , carrier diffusion.

t h e r e i s no e x t r a

The work o f Combescot (1981) and van D r i e l e t a l .

(1982) supports t h i s argument, a t l e a s t a t c a r r i e r d e n s i t i e s l i k e l y t o be encountered i n t h e nanosecond and h i g h picosecond p u l s e durat i o n ranges.

Since t h e experiments o f Shank e t a l . (1983), i n d i c a t e

t h a t Auger recombination i s suppressed and t h a t c a r r i e r d i f f u s i o n becomes i m p o r t a n t on t h e femtosecond t i m e scale,

it i s d i f f i c u l t

t o see how t h e s e e f f e c t s c o u l d have any s i g n i f i c a n t i n f l u e n c e f o r t h e nanosecond 1aser p u l ses g e n e r a l l y employed d u r i n g 1aser p r o cessing.

Moreover, as discussed l a t e r , Wood and G i l e s (1981) have

shown t h a t even a s i g n i f i c a n t amount o f c a r r i e r d i f f u s i o n w i l l n o t g r e a t l y change t h e r e s u l t s o f m e l t i n g model c a l c u l a t i o n s i f t h e a b s o r p t i o n c o e f f i c i e n t i s above values o f

4.

-

3 x

lo4

cm-l.

SUMMARY The p r i n c i p a l a b s o r p t i o n mechanisms f o r i n t e n s e l a s e r r a d i a t i o n

i n semiconductors a r e induced a b s o r p t i o n by photogenerated f r e e c a r r i e r s ( m e t a l l i c mechanism), e l e c t r o n - h o l e e x c i t a t i o n by photons w i t h energies above t h e band gap, breakdown i n c r y s t a l l i n e symmetry.

and processes induced by t h e Once t h e l i g h t has been absorbed

by e l e c t r o n i c e x c i t a t i o n s , t h e c a r r i e r s come t o thermal e q u i l i b r i u m among themselves i n t i m e s o f t h e o r d e r o f densities less than

-

lOZ1

sec.

For c a r r i e r

cm-3, Auger recombination r a p i d l y reduces

t h e d e n s i t y o f e x c i t e d c a r r i e r s w h i l e l e a v i n g t h e energy o f t h e c a r r i e r system v i r t u a l l y unchanged.

The energy i n t h e c a r r i e r

system i s t r a n s f e r r e d t o t h e l a t t i c e by way o f phonon emission p r o cesses i n t i m e s o f t h e o r d e r o f

sec.

On a nanosecond t i m e

s c a l e , normal c a r r i e r d i f f u s i o n i s p r o b a b l y suppressed by t h e l a r g e temperature g r a d i e n t s s e t up i n t h e sample d u r i n g t h e l a s e r pulse, and i n f a c t c a r r i e r confinement may even occur.

R. F. WOOD ET AL.

111. 5.

Formulation o f the Melting Model

HEAT CONDUCTION EQUATIONS AND BOUNDARY CONDITIONS I n a l l o f i t s complexity, t h e general problem o f heat f l o w and

phase change i n a m a t e r i a l i r r a d i a t e d w i t h i n t e n s e l a s e r pulses i s intractable.

F o r t u n a t e l y , however, experience has shown t h a t i t

i s u s u a l l y a good approximation t o t r e a t t h e heat conduction problems encountered i n t h e l a s e r p r o c e s s i n g o f semiconductors as one dimensional.

The diameter o f t h e l a s e r beam i s seldom l e s s t h a n

100 pm, w h i l e i n a m a t e r i a l such as s i l i c o n t h e depth i n which s i g n i f i c a n t temperature g r a d i e n t s occur i s l e s s t h a n 10 pm and m e l t i n g w i l l g e n e r a l l y be l i m i t e d t o a p p r o x i m a t e l y 1 pm.

Spatial

inhomogeneities o f t h e energy d e n s i t y i n t h e l a s e r pulse, t o g e t h e r w i t h i n t e r f e r e n c e and d i f f r a c t i o n e f f e c t s a s s o c i a t e d w i t h t h e co-

h e r e n t n a t u r e o f t h e l i g h t , present troublesome problems which, however, can be overcome with r e f i n e d experimental techniques. Also, t h e t i m e s c a l e a s s o c i a t e d w i t h t h e experiments i s so b r i e f t h a t c o n v e c t i o n i n t h e l i q u i d , which c o u l d d e s t r o y t h e o n e - d i m e n s i o n a l i t y o f t h e problem, i s unimportant.

It i s o n l y when l a s e r i r r a d i a t i o n

o f h e a v i l y doped m a t e r i a l s w i t h low dopant s e g r e g a t i o n c o e f f i c i e n t s a r e s t u d i e d t h a t c l e a r evidence f o r t h e breakdown i n t h e onedimensional c h a r a c t e r o f t h e problem i s found,

as discussed i n

Chapter 5. Although t h e one-dimensional

nature o f t h e heat f l o w provides

a major s i m p l i f i c a t i o n , t h e problem we must deal w i t h remains i n h e r e n t l y n o n l i n e a r and i n v o l v e s a moving boundary between t h e s o l i d and l i q u i d m a t e r i a l .

T h i s problem, f i r s t s t u d i e d e x t e n s i v e l y by

S t e f a n and Newman (Carslaw and Jaeger, 1959), has r e c e i v e d a g r e a t deal o f a t t e n t i o n from p h y s i c i s t s and mathematicians (see t h e volume e d i t e d by Wilson e t al., 1978). The q u a n t i t i e s t o be determined a r e t h e temperature d i s t r i b u t i o n and t h e l o c a t i o n o f t h e p l a n a r l i q u i d - s o l i d i n t e r f a c e (assumed t o be a s u r f a c e ) as a f u n c t i o n of time.

The equations which must be s o l v e d a r e t h e heat c o n d u c t i o n

e q u a t i o n s i n t h e l i q u i d and s o l i d , an e q u a t i o n p r o v i d i n g f o r energy

4.


181

c o n s e r v a t i o n across t h e i n t e r f a c e , and a p p r o p r i a t e space and t i m e boundary c o n d i t i o n s .

F o r our p r e s e n t d i s c u s s i o n , t h e d i f f e r e n t i a l

e q u a t i o n f o r heat conduction can be w r i t t e n i n terms o f t h e temperature d i s t r i b u t i o n T(x,t)

as

The heat g e n e r a t i o n f u n c t i o n P ( x , t ) i s determined by t h e i n t e r a c t i o n o f t h e l a s e r r a d i a t i o n w i t h t h e sample and t h e subsequent t r a n s f e r o f t h e energy t o t h e l a t t i c e ,

as discussed above.

diffusion coefficient, or d i f f u s i v i t y

The thermal

D, i s r e l a t e d t o t h e thermal

c o n d u c t i v i t y K , t h e s p e c i f i c heat c, and t h e d e n s i t y p o f t h e sample m a t e r i a l by t h e e q u a t i o n D = K/cp.

D u r i n g p u l s e d l a s e r annealing,

t h e temperature o f t h e sample may be r a i s e d i n a few nanoseconds ( o r even picoseconds) from ambient t o t h e m e l t i n g p o i n t and even through t h e vaporization point, i f t h e l a s e r pulse i s s u f f i c i e n t l y powerful.

Over t h e s e temperature ranges, t h e thermal c o n d u c t i v i t y

and s p e c i f i c heat a r e n o t constant, as can be seen i n Fig. 2; t h e d a t a on Fig. 2 w i l l be discussed i n more d e t a i l below.

Equation (1)

cannot be used when K and c a r e s t r o n g l y temperature dependent and/or when phase changes occur; t h e n a f o r m u l a t i o n of t h e problem based on f i n i t e d i f f e r e n c e s i s r e q u i r e d .

(1) i s r e p l a c e d by a system o f

I n f i n i t e - d i f f e r e n c e form, Eq.

equations d e r i v e d f r o m a heat balance c o n d i t i o n a t each o f a s e t o f p o i n t s o r nodes a l o n g t h e x axis.

F o r t h e j - t h node ( n o t a t a

f r o n t o r back s u r f a c e ) t h i s c o n d i t i o n i s expressed by

T3m .

i s t h e temperature a t t i m e t n o f t h e j+m node immediately

a d j a c e n t t o t h e j - t h node,

jKj+m i s t h e thermal conductance be-

tween p o i n t s j and j+m, C j i s t h e heat c a p a c i t a n c e o f t h e m a t e r i a l associated

w i t h p o i n t j, and P!

t h e l a t t e r material a t time tn.

is

t h e heat g e n e r a t i o n r a t e i n

182

R. F. WOOD ET AL.

0 0 Q

E

1.6

y

1.2

g

1.0

5 v

2 c

a

2 0

0 1

-----

Si Ge GaAs

i.4

0.8 0.6

a E a 0.4 w

I

c J

0.2 0

Fig.

2.

I

0

I 400

I

I

I

I

I

!200 T, TEMPERATURE ("C)

Temperature-dependent

liquid S i , G e , and GaAs.

I

800

I

1600

2000

thermal conductivity o f crystalline and

A quasi-discontinuity in

K

occurs on melting when

the materials properties transform from those o f a semiconductor to those o f a metal.

S t a t i o n a r y boundary c o n d i t i o n s imposed on Eq. ( 2 ) a r e t h e f i n i t e d i f f e r e n c e e q u i v a l e n t s o f t h e expressions

a p p r o p r i a t e t o Eq.

(1).

The f i r s t e q u a t i o n i m p l i e s t h a t no heat

i s l o s t from t h e f r o n t s u r f a c e w h i l e t h e second r e f l e c t s t h e f a c t t h a t t h e sample i s t h i c k enough t o a c t as a good heat sink. p r a c t i c a l reasons,

t h e f i n i t e - d i f f e r e n c e approach w i l l

For

usually

r e q u i r e t h e second o f these c o n d i t i o n s t o be s a t i s f i e d a t a v a l u e o f x l e s s t h a n t h e t h i c k n e s s of t h e sample.

T r i a l c a l c u l a t on s

i n v a r i a b l y showed both r a d i a t i v e and c o n v e c t i v e heat t r a n s f e r

rom

t h e f r o n t s u r f a c e t o be n e g l i g i b l e because o f t h e s h o r t t i m e s i n volved, t h u s v a l i d a t i n g t h e f i r s t boundary c o n d i t i o n .

C a l c u l a t ons

f o r a wide range o f l a s e r p u l s e energy d e n s i t i e s and d u r a t ons


183

a l s o e s t a b l i s h e d t h a t an e f f e c t i v e sample t h i c k n e s s o f 25-50 vm i s s u f f i c i e n t f o r most cases l i k e l y t o be encountered i n p r a c t i c e . Boundary c o n d i t i o n s a t t h e moving l i q u i d - s o l i d i n t e r f a c e a r e u s u a l l y chosen t o make t h e temperature o f t h e i n t e r f a c e T i equal t o t h e phase change temperature Tc and t o s a t i s f y t h e e q u a t i o n Lvp = K,GQi

(4)

- KsGsi

I n t h i s equation, L i s t h e l a t e n t heat of m e l t i n g , KQ and Ks are r e s p e c t i v e l y t h e thermal c o n d u c t i v i t i e s i n t h e l i q u i d and s o l i d , G Q i and Gsi a r e t h e corresponding temperature g r a d i e n t s a t t h e

Equation ( 4 ) i s a

i n t e r f a c e , and v i s t h e i n t e r f a c e v e l o c i t y .

statement o f t h e requirement t h a t when t h e i n t e r f a c e moves a d i s t a n c e dx i n a t i m e i n t e r v a l d t , t h e l a t e n t heat t h a t i s l i b e r a t e d must be removed by conduction.

Equations (1) o r ( 2 ) and t h e bound-

a r y c o n d i t i o n s discussed above d e f i n e a moving boundary problem f r e q u e n t l y r e f e r r e d t o as t h e S t e f a n problem.

Although t h e r e q u i r e -

ments t h a t T i = Tc and t h a t Eq. ( 4 ) holds a r e q u i t e reasonable under most circumstances,

i t should be recognized t h a t c o n d i t i o n s can

occur where t h e y a r e n o t v a l i d . a t a regrowth v e l o c i t y o f

F o r example,

i t i s thought t h a t

- 15 m/sec i n a d i r e c t i o n o f s i l i c o n ,

t h e i n t e r f a c i a l u n d e r c o o l i n g i s so g r e a t t h a t t h e temperature may be s e v e r a l hundred degrees below Tc;

t h i s w i l l be discussed i n

Chapter 5. To apply t h e f i n i t e - d i f f e r e n c e f o r m u l a t i o n , t h e p h y s i c a l problem i s approximated by i n t r o d u c i n g t h e s e t o f nodes a l o n g t h e x a x i s and a s s o c i a t i n g w i t h each node a small volume o f m a t e r i a l w i t h p r e s c r i b e d thermal and o p t i c a l p r o p e r t i e s . between a d j a c e n t nodes.

Heat flow occurs o n l y

By choosing t h e increments between t h e

nodal p o i n t s and t h e t i m e steps small enough, t h e s o l u t i o n t o t h e system o f equations y i e l d s an a c c u r a t e approximation t o t h e approp r i a t e d i f f e r e n t i a l equation.

S u i t a b l e space and t i m e increments

can be determined f o r each a p p l i c a t i o n by successive r e d u c t i o n i n t h e increments u n t i l t h e r e i s an a c c e p t a b l y small change i n t h e solution.

184

R. F. WOOD ET AL.

Increments on t h e space and t i m e g r i d s can be chosen t o g i v e s a t i s f a c t o r y r e s u l t s f o r a v a r i e t y o f c l o s e l y r e l a t e d problems; however,

major e x t e n s i o n s o f a model t o r a d i c a l l y d i f f e r e n t con-

d i t i o n s may r e q u i r e a r e d e t e r m i n a t i o n of t h e optimum space and t i m e increments.

These choices u s u a l l y i n v o l v e a compromise be-

tween accuracy and computer time.

Typically,

the calculations

r e p o r t e d l a t e r i n t h i s s e c t i o n used a minimum s p a t i a l increment a t t h e s u r f a c e o f 1 x l o m 6 cm, b u t t h i s increment was reduced t o 2 x

cm f o r problems i n which t h e l a s e r r a d i a t i o n was e n t i r e l y

absorbed w i t h i n t h e f i r s t few hundred angstroms o f t h e surface. Deeper i n t h e m a t e r i a l , where t h e temperature i s s l o w l y v a r y i n g , t h e s p a t i a l increment can be i n c r e a s e d s i g n i f i c a n t l y .

The t i m e

increment was v a r i e d w i t h t h e l a s e r power and/or p u l s e d u r a t i o n and ranged between 2 x

and

seconds,

subject t o t h e

convergence c r i t e r i a d i c t a t e d by t h e numerical techniques used.

6.

PHASE

CHANGES

Since t h e temperature a t each node i s c o n s t a n t l y m o n i t o r e d i n finite-difference

calculations,

t h e programs can be designed t o

determine when t h e m a t e r i a l o f a node i s ready t o undergo a phase change.

D u r i n g a phase change, t h e node's temperature can be main-

t a i n e d a t Tc u n t i l t h e n e t heat c o n t e n t a s s o c i a t e d w i t h t h e phase change equals o r exceeds t h e l a t e n t heat o f t h e nodal m a t e r i a l . A f t e r t h e phase change, t h e node's temperature i s again determined by t h e c o n d u c t i v e heat t r a n s f e r equation. The r a t i o o f t h e node's heat c o n t e n t above t h a t r e q u i r e d t o j u s t reach t h e t r a n s i t i o n temperature t o t h e l a t e n t heat r e q u i r e d f o r t h e phase change i s sometimes c a l l e d t h e t r a n s i t i o n r a t i o o r t h e liquid-solid fraction.

The t r a n s i t i o n r a t i o can be i n t e r p r e t e d

e i t h e r as a measure o f t h e f r a c t i o n o f t h e node's m a t e r i a l t h a t has completed t h e phase change o r as t h e e x t e n t t o which a l l o f t h e node's m a t e r i a l has completed t h e phase change. case, t h e

I n t h e former

nodal volume i s comprised o f r e g i o n s o f s o l i d and l i q u i d

185


m a t e r i a l separated by t h e phase i n t e r f a c e , w h i l e i n t h e l a t t e r case, t h e nodal volume c o n t a i n s m a t e r i a l i n a two-phase m i x t u r e r e f e r r e d t o as " s l u s h " o r "mush."

I n a h e a t - t r a n s f e r problem dominated by

conduction from a small r e g i o n i n which heat i s generated o r l a t e n t heat released, t h e former case w i l l e x i s t and t h e t r a n s i t i o n r a t i o can be used as an i n t e r p o l a t i n g parameter t o determine t h e l o c a t i o n o f t h e phase i n t e r f a c e w i t h i n t h e n o d e ' s volume.

This i n t e r p o l a t i o n

t e c h n i q u e i s e a s i l y a p p l i e d t o one-dimensional problems s i n c e t h e t r a n s i t i o n r a t i o i s j u s t equal t o a f r a c t i o n o f t h e node's length. When heat i s generated n e a r l y u n i f o r m l y throughout an extended r e g i o n o f a sample (as w i t h l a s e r r a d i a t i o n o f a low a b s o r p t i o n c o e f f i c i e n t ) i t i s p o s s i b l e f o r t h e m a t e r i a l a t a l l t h e nodes i n t h a t r e g i o n t o reach t h e m e l t i n g temperature a t approximately t h e same t i m e and t o undergo m e l t i n g a t a p p r o x i m a t e l y t h e same r a t e . A d e f i n i t e phase i n t e r f a c e may be d i f f i c u l t t o l o c a t e i n such cases

and t h e e n t i r e r e g i o n can c o n s t i t u t e a t r a n s i t i o n zone. zones are o f t e n observed i n c a l c u l a t i o n s ,

Transition

but w i t h the conditions

u s u a l l y emphasized here t h e y disappear very q u i c k l y a f t e r t h e l a s e r p u l s e has t e r m i n a t e d (see, however, S e c t i o n 1'4.12). 7.

TEMPERATURE-DEPENDENT THERMAL AND OPTICAL PROPERTIES I n l a s e r annealing, t h e heat g e n e r a t i o n r a t e a t each p o i n t i n

t h e sample i s l a r g e l y determined by t h e r e f l e c t i v i t y and t h e e f f e c t i v e optical absorption c o e f f i c i e n t o f t h e material,

t h e energy

t r a n s f e r r a t e t o t h e l a t t i c e , and t h e energy d e n s i t y and p u l s e d u r a t i o n t i m e o f t h e l a s e r pulse.

The f u n c t i o n

Pi

i n Eq. 2 can

be w r i t t e n as n n Py = (1 - R.)F

J

j '

i n which RY and F Y a r e r e f l e c t i v i t y and a b s o r p t i o n f u n c t i o n s r e s p e c t i v e l y f o r t h e j - t h l a y e r a t t i m e tn.

Both

RY and FY

can be

complicated f u n c t i o n s o f t h o s e m a t e r i a l and p h y s i c a l parameters

R . F. WOOD ET AL.

describing t h e j - t h l a y e r a t time tn.

A number o f t h e s e param-

e t e r s show l a r g e changes when t h e m a t e r i a l undergoes a change o f phase.

Thus,

RJ and F!

can change c o n t i n u o u s l y

w i t h t i m e and

d i s t a n c e as t h e m e l t f r o n t advances i n t o t h e sample d u r i n g t h e It w i l l be assumed t h a t c a r r i e r d i f f u s i o n and con-

l a s e r pulse.

finement a r e n o t i m p o r t a n t f a c t o r s ( S e c t i o n 11.3),

and t h e r e f o r e

we w i l l p u t k ( T ) = a(T), where k ( T ) w i l l be r e f e r r e d t o as a h e a t absorption

coefficient

and

a(T)

i s the

temperature-dependent

o p t i c a l absorption coefficient. The i t e r a t i v e procedures used i n most f i n i t e - d i f f e r e n c e programs a l l o w t h e temperature-dependent

o p t i c a l (and t h e r m a l ) p r o p e r t i e s

t o be i n c l u d e d i n a r e l a t i v e l y s t r a i g h t f o r w a r d manner.

A f t e r each

i t e r a t i o n ( o r t i m e s t e p ) an approximate temperature d i s t r i b u t i o n T(x,t)

i s known t h r o u g h o u t t h e sample.

Therefore, u s i n g expressions

such as Eqs. ( 7 ) and (8) o f Chapter 3 , values of k ( T ) and R ( T ) a t any p o i n t i n t h e sample can be determined and used i n t h e next iteration.

To i l l u s t r a t e t h e approach, c o n s i d e r a heat c o n d u c t i o n problem f o r m u l a t e d i n t h e more f a m i l i a r d i f f e r e n t i a l e q u a t i o n form g i v e n by Eq.

(1).

D u r i n g p u l s e d l a s e r annealing,

f u n c t i o n P(x,t)

t h e heat g e n e r a t i o n

d e s c r i b e s how t h e energy o f t h e l a s e r p u l s e i s

d e p o s i t e d i n t h e sample; i t has t h e u n i t s o f W/cm3. t o Eq.

To correspond

( 5 ) , we can w r i t e

P(x,t)

= [l-R(t)]F(~,t)

I [l-R(t)]fl(x,t)f~(t)

.

The r e f l e c t i v i t y R i s a f u n c t i o n o f t i m e because t h e temperature o f t h e s u r f a c e changes w i t h time; i n p r i n c i p l e , i t should a l s o be a f u n c t i o n o f x because t h e temperature g r a d i e n t s may produce i n t e r f e r e n c e e f f e c t s from d i f f e r e n t depths o f t h e n e a r - s u r f a c e r e g i o n . I f i t i s assumed t h a t t h e energy i n c i d e n t on t h e sample a t any

i n s t a n t d u r i n g t h e l a s e r p u l s e i s t r a n s f e r r e d from t h e e l e c t r o n i c system t o t h e l a t t i c e i n t i m e s s h o r t compared t o t h e p u l s e d u r a t i o n , t h e f u n c t i o n f 2 ( t ) should be approximated very w e l l by t h e f u n c t i o n which d e s c r i b e s t h e temporal e v o l u t i o n o f t h e l a s e r pulse.

187

4. MELTING MODEL OF PULSED LASER PROCESSING The f u n c t i o n fl(x,t)

describes t h e s p a t i a l a b s o r p t i o n o f t h e p u l s e

energy; i t may be q u i t e complicated i f l a t t i c e damage, h i g h l y nonuniform concentrations o f impurities, carrier diffusion i s significant.

etc.

a r e present,

or i f

It i s a f u n c t i o n o f t i m e because

o f t h e t i m e dependence o f t h e temperature and phase changes.

Here,

we w i l l t a k e

and w r i t e f o r I ( x , t ) , I ( x , t ) = I, exp(-

t h e l i g h t i n t e n s i t y i n t h e sample, X

1 k(xo,t)dxo) 0

.

The n o r m a l i z a t i o n constant co i s determined from t h e i n t e g r a l F(x,t)dxdt

= co

”

tP

1 [ 1 fl(x,t)dx]f2(t)dt

0

=

0

E,

.

(9)

tp i s t h e t o t a l d u r a t i o n o f t h e p u l s e ( n o t t h e f u l l w i d t h a t h a l f

maximum), a l s o r e c a l l t h a t

E, i s t h e t o t a l p u l s e energy d e n s i t y .

We o b t a i n t

co

=

P

E 11 {I 1 o o

Equations (6-8,

”

1 k(x,t)exp[-

X

1 k ( ~ ~ , t ) d x ~ l d x f ~ ( t ) d t } ~ ~ (10 . 0

10) determine P(x,t)

i n terms o f t h e temperature

dependent h e a t - a b s o r p t i o n c o e f f i c i e n t k ( T ( x,t ) ) , t h e r e f l e c t i v i t y R(T(x=O,t))

and t h e pulse-shape f u n c t i o n f z ( t ) .

I n the f i n i t e -

d i f f e r e n c e f o r m u l a t i o n , t h e choice o f t h e space and t i m e increments

w i l l determine t h e accuracy w i t h which t h e i n t e g r a l s i n Eqs. (8) and (10) can be evaluated.

I n view of t h e l i m i t e d accuracy o f t h e

experimental values o f such q u a n t i t i e s as t h e thermal c o n d u c t i v i t y , s p e c i f i c heat, a b s o r p t i o n c o e f f i c i e n t , etc.,

i t seems l i k e l y t h a t

increments chosen f o r a c c u r a t e s o l u t i o n s o f t h e heat conduction problem w i l l a l s o g i v e e n t i r e l y a c c e p t a b l e approximations t o P(x,t).

188 8.

R. F. WOOD E T A L . INPUT DATA The data on t h e thermal p r o p e r t i e s of t h e sample r e q u i r e d f o r

t h e c a l c u l a t i o n s i n c l u d e l a t e n t heats o f f u s i o n and v a p o r i z a t i o n and t h e corresponding temperatures a t which t h e phase changes occur, thermal c o n d u c t i v i t y , s p e c i f i c heat, d e n s i t y , and t h e i n i t i a l temperature o f the substrate.

R a d i a t i v e and c o n v e c t i v e heat t r a n s f e r

f r o m t h e f r o n t s u r f a c e can be c a l c u l a t e d b u t , as a l r e a d y mentioned, t e s t s i n t h e p a s t have shown t h e s e e f f e c t s t o be e n t i r e l y n e g l i g i b l e f o r pulsed l a s e r heating.

The o p t i c a l d a t a r e q u i r e d c o n s i s t s o f

t h e r e f l e c t i v i t y and a b s o r p t i o n c o e f f i c i e n t o f t h e sample and t h e p u l s e shape, d u r a t i o n , energy d e n s i t y , and wavelength o f t h e l a s e r radiation;

t h e y w i l l be discussed s h o r t l y .

Table I l i s t s t h e

thermal p r o p e r t i e s , t h e symbols and u n i t s used f o r them, and e i t h e r t h e values o r r e f e r e n c e s t o d i s c u s s i o n s i n t h e t e x t .

Data a r e g i v e n

i n t h e t a b l e f o r Si, Ge, and GaAs, a l t h o u g h a l l o f t h e i l l u s t r a t i v e c a l c u l a t i o n s i n t h i s chapter are f o r s i l i c o n ( r e s u l t s o f calculat i o n s on GaAs a r e g i v e n i n Chapter 8).

The p r i m a r y purposes f o r

l i s t i n g and d i s c u s s i n g t h e data on Ge and GaAs a r e t o i n d i c a t e t h e a v a i l a b i l i t y o f t h e data t h a t i s needed f o r c a l c u l a t i o n s on t h e t h r e e semiconductors most o f t e n s t u d i e d i n c o n n e c t i o n w i t h l a s e r p r o c e s s i n g and t o p r o v i d e a comparison of t h e magnitude o f t h e q u a n t i t i e s involved.

G e n e r a l l y speaking, t h e body of d a t a on t h e

thermal p r o p e r t i e s o f Ge i s more e x t e n s i v e t h a n t h a t o f any o t h e r semiconductor, w h i l e i t i s t h e o p t i c a l p r o p e r t i e s o f S i t h a t have been most t h o r o u g h l y i n v e s t i g a t e d .

Although GaAs i s undoubtedly

t h e most f r e q u e n t l y s t u d i e d compound semiconductor, data on i t o f t h e t y p e needed f o r m e l t i n g model c a l c u l a t i o n s i s sparse compared t o t h a t a v a i l a b l e f o r t h e elemental semiconductors (Wood e t al., 1981b). a.

Phase Change Temperatures and L a t e n t Heats The m e l t i n g temperatures o f c r y s t a l l i n e S i , Ge, and GaAs a r e

w e l l e s t a b l i s h e d , b u t v a r i a t i o n s i n t h e r e p o r t e d values o f Lc, Tv, and L v a r e sometimes q u i t e l a r g e .

We b e l i e v e t h a t t h e thermal data


189

Table I Thermodynamic Data f o r S i , Ge, and GaAS Quantity

Symbol

Phase change temperature c rysta1 TC amorphous Ta vaporization TV

Units

Value and Comments Si Ge GaAs

"C

L a t e n t heats c r y s t a1 amorphous v a p o r i z a t i on

L LC La LV

Dens it y

P

g/cm3

Thermal c o n d u c t i v i t y c r y st a1 a m r p h ou s 1i q u i d

K

W/cm deg

KC

Speci f ic heat

c

Ka KQ

1410 1150 3267

937 695 2834

1238

1800 1320 16207

508 355 5116

548 2230

2.33

5.24

5.3

Figure 2 -0.13 Figure 2

---

-0.02 J / g deg

---

see d i s c u s s i o n i n t e x t

f o r s i l i c o n and germanium g i v e n i n H u l t g r e n e t a l . sonably r e l i a b l e .

---

1800

(1973) i s rea-

Since t h e c o m p i l a t i o n by t h e s e authors i s t h e

most r e c e n t l y published, t h e r e i s reason t o b e l i e v e t h a t our knowledge o f t h e thermal p r o p e r t i e s o f semiconductors i s g r a d u a l l y improving.

We a r e c o n f i d e n t t h a t t h e r e s u l t s o f p u l s e d l a s e r -

a n n e a l i n g experiments and c a l c u l a t i o n s w i l l h e l p t o f u r t h e r r e f i n e t h e thermal data, e s p e c i a l l y i n t h e molten phase. The n a t u r e o f t h e phase t r a n s i t i o n and t h e m e l t i n g temperatures and l a t e n t heats o f a-Si and a-Ge c o n t i n u e t o be s u b j e c t s o f l i v e l y debate.

Estimates by Bagley and Chen (1978) and Spaepen and T u r n b u l l

(1978), u s i n g data o f Chen and T u r n b u l l (1969), have i n d i c a t e d t h a t t h e m e l t i n g temperature of a-Ge should be

- 240 K l e s s than t h a t o f

c-Ge and t h a t t h e l a t e n t heat of m e l t i n g s h o u l d be f o r c-Ge.

-

70% o f t h e value

S c a l i n g o f t h e a-Ge r e s u l t s by t h e r a t i o Tc(Si)/Tc(Ge)

suggested t h a t T a ( S i ) should be approximately 300 K l e s s t h a n Tc(Si).

R. F. WOOD ET AL.

Some c o r r o b o r a t i o n o f these r e s u l t s i s given by t h e work o f Fan and Anderson (1981) and Baeri e t a l .

(1980),

although both o f these

s t u d i e s used methods t h a t are subject t o l a r g e e r r o r s .

I n contrast

t o these r e s u l t s , Kokorowski e t a l . (1982) and Olson e t a1

. (1983)

have concluded from t h e i r r e s u l t s on cw l a s e r h e a t i n g o f amorphous l a y e r s t h a t t h e d i f f e r e n c e between Ta and Tc i s no more than about

50 K; Knapp and Picraux (1981) a r r i v e d a t a s i m i l a r conclusion from t h e i r experiments on electron-beam regrowth o f amorphous s i 1 icon. Donovan e t a l . (1983) c a r r i e d o u t d i f f e r e n t i a l scanning c a l o r i m e t r y measurements on a-Si l a y e r s produced by i o n i m p l a n t a t i o n o f argon and xenon.

They concluded t h a t Ta = 1147°C and La = 1319 J/g, which

we have rounded o f f t o 1150°C and 1320 J/g i n Table I. We emphasize t h a t t h e values o f Ta and La shown i n Table I should be considered o n l y as reasonable estimates a t t h i s time.

Complicating t h e prob-

lem o f determining accurate values o f these q u a n t i t i e s a r e u n c e r t a i nt i e s about t h e nature o f t h e amorphous s t a t e produced by various techniques.

It i s c l e a r from many experiments t h a t t h e d e n s i t y

and s t r u c t u r e o f amorphous l a y e r s formed by i o n i m p l a n t a t i o n , glow

discharge,electron-beamevaporation,sputtering,andchemical vapor d e p o s i t i o n are not t h e same (see Chapter 3, Section 111.3 f o r a d i s c u s s i o n o f t h e o p t i c a l p r o p e r t i e s o f ion-implanted s i l i c o n ) . Moreover, t h e r e i s some evidence t h a t w i t h some o f t h e measurement techniques t h e s t r u c t u r e o f t h e amorphous l a y e r s may change d u r i n g t h e measurements, thus f u r t h e r c o m p l i c a t i n g t h e i n t e r p r e t a t i o n o f t h e experiments.

O f p a r t i c u l a r i n t e r e s t i n t h i s connection i s t h e

observation o f Fredrickson e t al.

(1982) t h a t two w e l l - d e f i n e d

o p t i c a l s t a t e s o f a-Si s i l i c o n are produced by i o n i m p l a n t a t i o n ; presumably t h e thermal p r o p e r t i e s may a1 so be somewhat d i f f e r e n t f o r t h e two states. b.

Density, Thermal Conductivity, and S p e c i f i c Heat The d e n s i t i e s o f Si , Ge, and most 111-V semiconductors decrease

s l o w l y w i t h temperature i n t h e c r y s t a l 1 ine phase , increase from

5-13% on m e l t i n g ( S i = lo%, Ge = 5%, GaAs = l l X ) , and then decrease

4.


w i t h i n c r e a s i n g T above Tc (Glasov e t al.,

1969).

191

As i n d i c a t e d

above, t h e d e n s i t y o f an amorphous semiconductor depends s t r o n g l y on t h e method used f o r p r e p a r i n g t h e m a t e r i a l . compaction,

i.e.,

I n a-Si of ideal

w i t h o u t v o i d s and i n c l u s i o n s o f i m p u r i t i e s o r

c - S i , t h e d e n s i t y has been r e p o r t e d t o be from 2-101 l e s s t h a n i t i s f o r c-Si.

Although v a r i a t i o n s i n d e n s i t y from one phase t o

another a r e g e n e r a l l y i m p o r t a n t because o f t h e i n s i g h t t h e y p r o v i d e i n t o changes i n chemical bonding, t h e s e v a r i a t i o n s a r e n o t l a r g e enough t o have a s i g n i f i c a n t impact on t h e agreement between c a l c u l a t i o n s and experiments a t t h i s time.

A s t h e experimental mea-

surements a r e r e f i n e d and o t h e r p h y s i c a l parameters a r e determined more p r e c i s e l y , i t may be necessary t o i n c l u d e d e n s i t y changes i n t h e heat f 1ow c a l c u l a t i o n s . Thermal c o n d u c t i v i t i e s o f semiconductors have n o t been accuratel y measured over t h e extended temperature range ( i n c l u d i n g t h e l i q u i d phase) r e q u i r e d i n m e l t i n g model c a l c u l a t i o n s .

The measurements o f

Glassbrenner and Slack (1964) a r e g e n e r a l l y considered t o have p r o v i d e d t h e most a c c u r a t e data a v a i l a b l e on t h e thermal c o n d u c t i v i t y o f c r y s t a l l i n e S i and Ge up t o t h e i r m e l t i n g p o i n t s .

I n f a c t , small

d i f f e r e n c e s between t h e thermal c o n d u c t i v i t y data of Glassbrenner and Slack and o t h e r s e t s o f data (such as t h o s e o f F u l k e r s o n e t al., 1968) do n o t make l a r g e d i f f e r e n c e s i n most o f t h e c a l c u l a t e d r e s u l t s . However, i n e f f o r t s t o o b t a i n t h e b e s t p o s s i b l e agreement between c a l c u l a t e d and measured d u r a t i o n s o f s u r f a c e m e l t i n g o f s i l i c o n Lowndes e t a l . (1983) found t h a t t h e Glassbrenner and Slack conduct i v i t y data p r o v i d e d somewhat b e t t e r agreement t h a n d i d e a r l i e r d a t a g i v e n i n Goldsmith e t a l . (1961).

Amos and Wolfe (1978) have

measured t h e temperature dependence o f t h e thermal c o n d u c t i v i t y o f GaAs up t o a few hundred degrees below t h e m e l t i n g p o i n t ; here t h e i r d a t a was s t r a i g h t f o r w a r d l y e x t r a p o l a t e d t o Tc. More s e r i o u s d i f f i c u l t i e s a r e encountered i n t h e molten s t a t e where, t o o u r knowledge, n e i t h e r t h e magnitude nor t h e temperature dependence o f K f o r any semiconductor has been measured w i t h s u f f i c i e n t accuracy t o be u s e f u l (see,

however, t h e comnents i n Ho

192

R. F. WOOD ET AL.

e t al.,

1974).

In t h e absence o f t h i s i n f o r m a t i o n , m e l t i n g model

c a l c u l a t i o n s have o f t e n used t h e Wiedemann-Franz (W-F) law t o r e l a t e t h e thermal c o n d u c t i v i t y t o t h e e l e c t r i c a l c o n d u c t i v i t y

U,

which

has been measured i n m o l t e n S i , Ge, GaAs, and o t h e r semiconductors j u s t above t h e i r m e l t i n g temperatures by Glasov e t a l . (1969).

This

usage i s based on t h e f a c t t h a t t h e s e semiconductors behave as good m e t a l s i n t h e molten s t a t e and t h e r e f o r e t h e charge and heat c a r r i e r s a r e expected t o be t h e same.

The temperature dependence

of K i n t h e m o l t e n s t a t e can a l s o be e s t i m a t e d i n t h i s way s i n c e t h e W-F law p r o v i d e s t h e expression K = L*uT,

i n which t h e L o r e n t z

number L* i n good metals i s 2.45 x 10-8 Wn/deg* (see K i t t e l , 1971). The work o f Glasov e t a l .

i n d i c a t e s t h a t u decreases very s l o w l y

w i t h T above t h e m e l t i n g p o i n t i n S i and GaAs, and somewhat more r a p i d l y i n Ge.

The thermal c o n d u c t i v i t i e s o f t h e s e t h r e e semi-

conductors a r e shown on Fig. 2 as a f u n c t i o n o f temperature. The thermal c o n d u c t i v i t y o f amorphous semiconductors p l a y s an i m p o r t a n t r o l e i n d e t e r m i n i n g t h e response o f an amorphous l a y e r t o a l a s e r p u l s e (Chapter 6).

U n f o r t u n a t e l y i t has been d i f f i c u l t

t o o b t a i n r e l i a b l e d a t a f o r Ka. d e p o s i t e d on mica s u b s t r a t e s , Ka

P

For 7000-A t h i c k a-Ge f i l m s

Nath and Chopra (1974) found t h a t

0.13 W/cm deg a t room temperature and i n c r e a s e d o n l y s l i g h t l y

a t h i g h e r temperatures.

From Fig. 2 i t can be seen t h a t t h i s v a l u e

i s r a t h e r c l o s e t o t h a t o f Kc near t h e c r y s t a l l i n e m e l t i n g p o i n t . The s i t u a t i o n i s q u i t e d i f f e r e n t f o r s i l i c o n .

Goldsmid e t a l .

(1983) measured Ka a t room temperature f o r a 1.15-pm

thick a-Si

l a y e r d e p o s i t e d on a s a p p h i r e s u b s t r a t e and found a v a l u e o f 0.026 W/cm deg w i t h a r e p o r t e d p r o b a b l e e r r o r o f e t al.

(1984),

2

15%.

Lowndes

i n work discussed i n Chapter 6 and l a t e r i n t h i s

chapter, found Ka = 0.02 W/cm deg which i s i n s a t i s f a c t o r y agreement w i t h t h e r e s u l t s o f Goldsmid e t a l .

E x t e n s i v e comparisons o f

t h e r e s u l t s o f experiments and c a l c u l a t i o n s on t h e p u l s e d l a s e r m e l t i n g of a-Si l a y e r s by Webber e t a l . (1983) gave an e s t i m a t e o f Ka o f

- 0.01

W/cm deg.

4.

193


The temperature dependence o f t h e s p e c i f i c heat o f S i , Ge, and GaAs i s q u i t e modest a f t e r an i n i t i a l sharp i n c r e a s e a t low temperatures, and w i l l n o t be shown here.

The d a t a on c-Si used i n

t h e c a l c u l a t i o n s were t a k e n f r o m Goldsmith e t a l . (1961) and i s i n reasonably good agreement w i t h o t h e r measurements o f t h e s p e c i f i c h e a t (see, f o r example, Shanks e t al.,

1963.; H u l t g r e n e t al.,

S p e c i f i c heat data f o r Ge can be found i n H u l t g r e n e t al., GaAs i n Amos and Wolfe (1978).

1973). and f o r

We do n o t know of r e l i a b l e data f o r

t h e s p e c i f i c heat o f m o l t e n semiconductors and i n a l l o f t h e c a l c u l a t i o n s discussed h e r e i t was assumed t o remain c o n s t a n t a t i t s v a l u e i n t h e s o l i d a t t h e m e l t i n g temperature o r t o decrease by

- 10% from t h a t

value (Hultgren e t al.,

1973).

Chen and T u r n b u l l

(1969) have measured t h e s p e c i f i c heat o f b o t h c- and a-Ge f i l m s ; t h e d i f f e r e n c e was o n l y about 5% between t h e two forms. c.

Laser-Re1 a t e d Parameters The r e f l e c t i v i t y R and t h e a b s o r p t i o n c o e f f i c i e n t a o f c r y s -

t a l l i n e s i l i c o n a r e discussed a t l e n g t h i n Chapter 3.

The r e s u l t s

g i v e n t h e r e were used as a guide i n t h e c h o i c e o f values f o r t h e c a l c u l a t i o n s d e s c r i b e d below, b u t s p e c i f i c values o f R and a ( o r k )

w i l l be discussed as we proceed.

( R e c a l l t h a t a d i f f e r s from k,

t h e "heat a b s o r p t i o n c o e f f i c i e n t " o n l y i f t h e c a r r i e r s c r e a t e d by t h e i n c i d e n t l i g h t d i f f u s e s i g n i f i c a n t l y b e f o r e g i v i n g t h e i r energy t o t h e l a t t i c e ; h e r e we assume t h a t d i f f u s i o n i s n e g l i g i b l e and t h a t

k = a.)

The r e s u l t s o f Brodsky e t a l . (1970) were used as a guide

t o t h e o p t i c a l p r o p e r t i e s o f amorphous s i l i c o n .

We t a k e n o t e a g a i n

o f t h e f a c t t h a t values o f a measured a t low l i g h t i n t e n s i t i e s may n o t be t h e same as t h e values a p p r o p r i a t e under t h e i n t e n s i t i e s used i n p u l s e d l a s e r annealing.

Nonlinear e f f e c t s i n t h e i n t e n s i t y ( I )

t h a t do occur w i l l presumably always a c t t o i n c r e a s e k over t h e values a p p r o p r i a t e a t low l i g h t l e v e l s .

The remaining l a s e r - r e l a t e d

parameter i s t h e temporal p u l s e shape, f o r t h e i n d i v i d u a l cases considered.

and i t t o o w i l l be given

The l a s e r - r e l a t e d parameters

and t h e i r n o t a t i o n a r e sumnarized i n T a b l e 11.

194

R. F. WOOD ETAL. TABLE I1

O p t i c a l and Laser-Related Parameters f o r S i l i c o n C a l c u l a t i o n s ~

~~~

~

~

Quantity

Symbol

Units

a,k

cm-1

Absorption c o e f f i c i e n t c r y sta1 amorphous 1i q u i d

aC

IV. 9.

x-

T-,

A- T-,

aa

all

R

Ref 1e c t iv i t y c r y st a 1 amo rp hous 1i q u i d

%

RC

Ra Rll

Laser p u l s e d u r a t i o n (FWHM) shape energy d e n s i t y

Comment s

nsec

T,t,

E,

J/cm2

and I-dependent and I-dependent 106

and T-dependent Ra = R c 0.70 f o r most c a l c u l a t i o n s A-

varied discussed i n t e x t varied

Results o f Melting Model Calculations in Silicon

CRYSTALLINE

AND AMORPHOUS MODELS

A p p l i c a t i o n s o f t h e m e l t i n g model d e s c r i b e d i n t h e preceding s e c t i o n a r e complicated by t h e wide v a r i a t i o n s i n t h e p h y s i c a l prope r t i e s o f semiconductors which can be produced by doping, implantation, heating, etc.

ion

For convenience o f d i s c u s s i o n , a major

d i s t i n c t i o n w i l l be made between c r y s t a l l i n e - and amorphous-model calculations.

However, such a c l e a r c u t d i s t i n c t i o n ,

based on t h e

presence o r absence o f long-range c r y s t a l l i n e o r d e r i n a l l o r p a r t o f t h e sample,

does n o t adequately cover a l l t h e cases t h a t can

a r i s e i n p r a c t i c e , as t h e f o l l o w i n g d i s c u s s i o n w i l l make c l e a r . a.

C r y s t a l l i n e Models I n many model c a l c u l a t i o n s r e p o r t e d i n t h e l i t e r a t u r e ,

the

a b s o r p t i o n c o e f f i c i e n t was assumed t o have some average, constant (temperature-independent) v a l u e t h r o u g h o u t t h e sample.

The use of

195


such an approximation o r i g i n a l l y r e f l e c t e d a l a c k o f knowledge o f

1) t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s o f s i l i c o n and o t h e r semiconductors a t e l e v a t e d temperatures, 2) t h e change i n t h e o p t i c a l p r o p e r t i e s due t o moderate l a t t i c e damage (produced f o r example by i o n i m p l a n t a t i o n o f l i g h t i o n s such as boron), 3) t h e effects dopants,

of

various

c o n c e n t r a t i o n s and t y p e s o f s u b s t i t u t i o n a l

and 4) t h e dependence o f t h e a b s o r p t i o n c o e f f i c i e n t on

the l i g h t intensity.

The work o f J e l l i s o n and Modine (1982,1983)

d e s c r i b e d i n Chapter 3 has added g r e a t l y t o o u r knowledge o f t h e temperature-dependent

optical properties o f c r y s t a l l i n e s i l i c o n ;

comparable data f o r o t h e r semiconductors i s n o t y e t a v a i l a b l e .

A

l i m i t e d amount of i n f o r m a t i o n on t h e e f f e c t s o f l a t t i c e damage on t h e o p t i c a l p r o p e r t i e s i s now a v a i l a b l e ,

b u t t h e wide range o f

dopant species and c o n c e n t r a t i o n s t h a t may be p r e s e n t i n samples used f o r s t u d i e s o f l a s e r p r o c e s s i n g make i t i m p r a c t i c a l t o measure t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s f o r each new s i t u a t i o n as i t a r i s e s (see Chapter 3, S e c t i o n 11.3).

I t i s known

t h a t t h e i n t e n s i t y dependence o f t h e a b s o r p t i o n c o e f f i c i e n t f o r wavelengths below o r very near t h e band gap i s an i m p o r t a n t e f f e c t (discussed i n Chapter 9), b u t l i t t l e i s known about i t s importance f o r wavelengths i n t h e r e g i o n above t h e band gap.

O f course, t h e r e

a r e cases when t h e use of a s i n g l e constant v a l u e o f a i s an e n t i r e l y acceptable approximation.

As shown i n Chapter 3, t h e o p t i c a l

p r o p e r t i e s f o r l a s e r s o p e r a t i n g a t wavelengths l e s s than show p r a c t i c a l l y no temperature dependence whatsoever. t h e value o f a i s so h i g h

(2

106 cm-1)

- 360 nm

Moreover,

a t t h e s e wavelengths t h a t

heavy doping, l a t t i c e damage, and even complete amorphization a r e u n l i k e l y t o s i g n i f i c a n t l y change it. Once

re1 i a b l e e s t i m a t e s o f t h e temperature-dependent b e h a v i o r

of r e f l e c t i v i t i e s and a b s o r p t i o n c o e f f i c i e n t s a t many d i f f e r e n t wavelengths became a v a i l a b l e ,

t h e y were i n c l u d e d i n t h e m e l t i n g

model c a l c u l a t i o n s i n t h e manner d e s c r i b e d i n S e c t i o n 111.7.

In

f a c t , t h e disagreement between m e l t i n g model c a l c u l a t i o n s based on a constant a b s o r p t i o n c o e f f i c i e n t and t h e d a t a which was b e g i n n i n g

196

R. F. WOOD ET AL.

t o accumulate on t h e d u r a t i o n o f s u r f a c e m e l t i n g d u r i n g p u l s e d ruby l a s e r i r r a d i a t i o n (see Chapter 6 ) was one o f t h e main m o t i v a t i o n s f o r c a r r y i n g o u t t h e measurements o f temperature-dependent o p t i c a l properties.

The reasonably good agreement between t h e r e s u l t s o f

t i m e - r e s o l v e d r e f l e c t i v i t y and t r a n s m i s s i v i t y experiments and t h e me1t i ng model ca 1cu 1a t ions wh ich i nc 1uded t emperat u re-dependent o p t i c a l p r o p e r t i e s , was a c l e a r i n d i c a t i o n o f t h e need t o t r e a t t h e o p t i c a l p r o p e r t i e s i n a more s a t i s f a c t o r y way t h a n t h a t d e s c r i b e d i n t h e p r e c e d i n g paragraph. t i o n s w i t h c o n s t a n t a and temperature-dependent

For s i m p l i c i t y ,

both t h e c a l c u l a -

R i n t h e s o l i d and t h e c a l c u l a t i o n s w i t h

o p t i c a l p r o p e r t i e s w i l l be r e f e r r e d t o h e r e

as c-model c a l c u l a t i o n s .

A l l such c a l c u l a t i o n s w i l l have i n common

t h e use o f t h e thermal p r o p e r t i e s ( l a t e n t heat, m e l t i n g temperature, thermal c o n d u c t i v i t y ) o f c r y s t a l l i n e s i l i c o n . b.

Amorphous Models C a l c u l a t i o n s on samples w i t h s u f f i c i e n t l y t h i c k amorphous l a y e r s

a r e somewhat s i m p l i f i e d ,

i n so f a r as t h e o p t i c a l p r o p e r t i e s a r e

concerned, because t h e a b s o r p t i o n c o e f f i c i e n t o f t h e amorphous l a y e r i s an o r d e r o f magnitude o r more g r e a t e r t h a n t h e c r y s t a l l i n e value and i s p r o b a b l y l e s s temperature- and intensity-dependent. example, t h e a b s o r p t i o n c o e f f i c i e n t a t X = 0.694 wn i s i n c r y s t a l l i n e and

- 5x104 cm-1

For

- 2 . 5 ~ 1 0 cm-1 ~

i n amorphous s i l i c o n a t 2OOC.

For

a b s o r p t i o n c o e f f i c i e n t s t h i s l a r g e , t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s should n o t p l a y as i m p o r t a n t r o l e as i t does i n c r y s t a l l i n e material.

However, t h e e f f e c t s o f l i g h t i n t e n s i t y

on t h e a b s o r p t i o n o f amorphous m a t e r i a l s may s t i l l be s i g n i f i c a n t . Hence, f o r t h i c k , u n i f o r m l y amorphous l a y e r s , i n which a l l of t h e l i g h t i s absorbed i n t h e amorphous l a y e r , a s i n g l e constant value o f a i s p r o b a b l y again a reasonable approximation. layer i s thin,

I f t h e amorphous

t h e d i f f e r e n t a b s o r p t i o n i n t h e c r y s t a l l i n e and

amorphous m a t e r i a l s should i n p r i n c i p l e be taken i n t o account, b u t

197


modeling o f such a s i t u a t i o n becomes q u i t e complex, e s p e c i a l l y s i n c e changes i n t h e thermal p r o p e r t i e s a t t h e a-c i n t e r f a c e must a l s o be included. The c-model i s used h e r e i n c a l c u l a t i o n s on undoped, c r y s t a l l i n e m a t e r i a l and temperature-dependent c o e f f i c i e n t s a r e o f t e n used.

r e f l e c t i v i t i e s and a b s o r p t i o n

The a-model i s used o n l y when a w e l l -

d e f i n e d amorphous l a y e r i s present, and t h e presence o f such a l a y e r i s i n d i c a t e d much more s t r o n g l y by t h e a l t e r e d thermal p r o p e r t i e s t h a n by t h e o p t i c a l p r o p e r t i e s .

The r e s u l t s discussed i n t h i s

s e c t i o n were chosen p r i m a r i l y t o i l l u s t r a t e s a l i e n t f e a t u r e s o f t h e m e l t i n g model c a l c u l a t i o n s and t h e y do n o t n e c e s s a r i l y r e p r e s e n t t h e most r e c e n t o r most a c c u r a t e f i t s t o t h e r e s u l t s o f p a r t i c u l a r experiments. 10.

CALCULATIONS FOR RUBY AND FREQUENCY-DOUBLED Nd LASERS I n t h i s subsection, we i l l u s t r a t e some of t h e more i m p o r t a n t

r e s u l t s o b t a i n e d from t h e m e l t i n g model by c o n s i d e r i n g c a l c u l a t i o n s f o r c r y s t a l 1ine s i 1 icon ir r a d i a t e d w i t h p u l ses f r o m frequencydoubled YAG ( A = 532 nm) and ruby ( A = 694 nm) l a s e r s .

I n these

wavelength ranges t h e o p t i c a l p r o p e r t i e s a r e known t o be s t r o n g l y temperature-dependent

(Chapter 3) and t o o b t a i n s a t i s f a c t o r y agree-

ment w i t h t i m e - r e s o l v e d r e f l e c t i v i t y and t r a n s m i s s i o n experiments t h i s temperature dependence should be included.

However, t h e pos-

s i b i l i t y t h a t t h e a b s o r p t i o n i s a l s o i n t e n s i t y dependent should be k e p t i n mind. F i g u r e 3 shows t h e temperature as a f u n c t i o n o f d i s t a n c e from t h e f r o n t s u r f a c e o f a sample i r r a d i a t e d w i t h a p u l s e o f energy density

E,

= 1.2

J/cm2 and d u r a t i o n T~ = 18 nsec (FWHM).

The

a b s o r p t i o n c o e f f i c i e n t and r e f l e c t i v i t y i n t h e s o l i d were taken t o be uC = 5x105 cm-1 and R c = Ro + 5x10-5 calculation.

T, r e s p e c t i v e l y f o r t h i s

When t h e s u r f a c e melted, t h e r e f l e c t i v i t y was i n -

creased t o 0.70 and u t o 106 cm-1.

The break i n some o f t h e curves

a t T = 141OOC g i v e s t h e p o s i t i o n o f t h e l i q u i d - s o l i d i n t e r f a c e , o r

198

R. F. WOOD ETAL.

-

I

I

I

- ..

Fig.

3.

I

I

I

I

Xa = 532 nrn 2 El = 4.2 J/crn

4800 2...,

I

................

I

TIME (nsec)

12

-

-

Temperature as a function of depth in silicon during and a f t e r

irradiation with a 18-nsec a frequency-doubled YAG

~

1 .2-J / c m 2 pulse representative o f the pulses from nm) laser.

( A = 532

m e l t f r o n t , a t t h e t i m e f o r which t h e curve i s drawn.

From t h e

r e s u l t s o f a s e r i e s o f c a l c u l a t i o n s l i k e t h o s e represented by Fig. 3, t h e p o s i t i o n o f t h e m e l t f r o n t as a f u n c t i o n o f t i m e f o r a g i v e n p u l s e energy d e n s i t y can be determined and p l o t t e d t o y i e l d t h e curves o f Fig. 4.

Also, t h e d e r i v a t i v e w i t h respect t o t i m e a t any

p o i n t on one o f t h e curves o f F i g . 4 g i v e s t h e v e l o c i t y v o f t h e l i q u i d - s o l i d i n t e r f a c e a t t h a t time. values

2

It can be seen t h a t f o r EQ

0.4 J/cm2 t h e m e l t f r o n t p e n e t r a t e s very r a p i d l y i n t o t h e

s o l i d b e f o r e r e a c h i n g i t s maximum p e n e t r a t i o n .

Near t h e maximum

p e n e t r a t i o n , t h e m e l t - f r o n t v e l o c i t y f i r s t drops s h a r p l y , and t h e n changes s i g n , and t h e m e l t f r o n t recedes back t o t h e s u r f a c e a t s e v e r a l meters p e r second.

These are t h e p r i n c i p a l f e a t u r e s o f t h e

m e l t - f r o n t c a l c u l a t i o n s and t h e i r e s s e n t i a l v a l i d i t y i s confirmed by a v a r i e t y o f experiments discussed i n o t h e r chapters o f t h i s volume.

199


-

E

0.70 0.60

7 -

3.

$

+ Q [r

0.50

-

..............

Al = 532 nm

2

El ( J / c m 2 )

0.4

= 18 nsec

0.6 -----0.8 .-.-.-

-

--_----

4.0 1.2 4.4 f.6

---- 4.8 -

0.40

z

W

a

+ z

0.30

-

0.20

-

0.40

-

5!

lL

'

3 W 5

-

I

0

Fig. 4. Melt-front position as a function of time a f t e r initiation o f an 18-nsec pulse o f various energy densities from a frequency-doubled YAG laser.

Additional information o f i n t e r e s t

in the interpretation o f

p u l s e d l a s e r m e l t i n g i s i l l u s t r a t e d i n Fig. 5, which g i v e s t h e s u r f a c e temperature Ts of t h e sample as a f u n c t i o n o f t h e t i m e a f t e r t h e b e g i n n i n g of pulses of s e v e r a l d i f f e r e n t energy d e n s i t i e s . see t h a t T,

We can

increases very r a p i d l y u n t i l i t reaches t h e m e l t i n g

temperature a t 1 4 1 0 ° C ,

where i t pauses momentarily u n t i l t h e l a t e n t

heat o f m e l t i n g i s absorbed and t h e n begins t o r i s e again ( i f Eg i s g r e a t enough) t o some maximum value. On c o o l i n g , t h e process i s reversed except t h a t T, f o r long periods o f time.

drops q u i c k l y t o 1 4 1 O o C , where i t remains The reason f o r t h i s behavior of Ts i s

t h a t t h e f o r m u l a t i o n o f t h e heat conduction and phase-change problem used here a l l o w s t h e r e l e a s e o f l a t e n t heat o n l y a t t h e m e l t i n g temperature, and r e q u i r e s t h a t a l l of t h e l a t e n t heat i n any part i c u l a r f i n i t e - d i f f e r e n c e c e l l be g i v e n up b e f o r e t h e temperature o f t h a t c e l l can b e g i n t o decrease.

This i m p l i e s t h a t no under-

c o o l i n g o f t h e l i q u i d below t h e c r y s t a l l i z a t i o n temperature i s

200

R. F. WOOD ET AL.

2000

I

I

I

1

I

I

I

I

I

1900 -

-Yw

-

............... 0.6 -

4700 -

-

4000

--.-.-.- 0.0 --- 1.0 4.2

-

-

1500 -

-

1600

(r

3

2 [r

4400 W

4300 4200 -

(400 0

10

20

30

40

50

TIME

Fig. 5. 18-nsec,

60

70

00

90

400

(nsec)

Surface temperature as a function of time a f t e r the initiation o f 532-nm

laser pulses of d i f f e r e n t energy densities.

allowed, b u t t h i s may n o t be a s a t i s f a c t o r y approximation f o r some s i t u a t i o n s which can a r i s e i n l a s e r a n n e a l i n g (see Chapter 5). We now t u r n t o a b r i e f discusson o f a d d i t i o n a l i l l u s t r a t i v e r e s u l t s t a k e n from e a r l i e r c a l c u l a t i o n s r e l a t e d t o t i m e - r e s o l v e d o p t i c a l experiments w i t h a ruby l a s e r ; t h e d e t a i l s o f t h e e x p e r i ments and c a l c u l a t i o n s are g i v e n i n a paper by Lowndes e t a l . (1982a). F i g u r e 6 shows a s e r i e s o f m e l t - f r o n t p r o f i l e s (i.e., t i m e ) which,

when compared t o t h e curves on F i g . 4,

t h a t i r r a d i a t i o n s w i t h 15-nsec p u l s e s a t X p u l s e s a t X = 532 nm g i v e s i m i l a r r e s u l t s . however,

=

p o s i t i o n vs demonstrate

693 nm and 18-nsec It should be noted,

t h a t t h e t h r e s h h o l d f o r m e l t i n g i s s i g n i f i c a n t l y lower

w i t h t h e s h o r t e r wavelength l i g h t , as might be expected from t h e differences i n the absorption coefficients.

F i g u r e s 7a and 7b show

t h e temperature a t v a r i o u s t i m e s a f t e r t h e b e g i n n i n g o f a 1.2-J/cm2 ruby l a s e r p u l s e as a f u n c t i o n o f depth on two d i f f e r e n t depth scales.

We n o t e t h a t f o r depths as s h a l l o w as 5 um t h e temperature


201

0.4

f

I

Y

E

.-P 0.3

E

0

Fig. 6.

40

100

0

20

Melt-front

position as a function of time a f t e r the beginning o f a

15-nsec ruby laser pulse.

60 TIME (nsec)

80

The temperature dependences o f the r e f l e c t i v i t y and

absorption coefficient are indicated by Rc and k c , respectively.

does n o t exceed

- 2OOOC a t any time.

Such low temperatures a t such

s h a l l o w depths i s t h e b a s i s f o r r e f e r r i n g t o l a s e r p r o c e s s i n g as " c o l d processing" s i n c e t h e b u l k o f a 200-pm t h i c k s i l i c o n w a f e r

i s r a i s e d o n l y very s l i g h t l y above t h e ambient temperature. 11.

CALCULATIONS FOR ULTRAVIOLET EXCIMER LASERS As discussed i n Chapter 1, rare-gas h a l i d e (RGH) excimer l a s e r s

a r e b e g i n n i n g t o emerge as e x c e l l e n t r a d i a t i o n sources f o r p u l s e d l a s e r p r o c e s s i n g o f semiconductors.

F o r t h e study o f fundamental

problems a s s o c i a t e d w i t h u l t r a r a p i d me1t i n g and s o l i d i f i c a t i o n , excimer l a s e r s p r o v i d e s e v e r a l d e s i r a b l e f e a t u r e s n o t found i n s o l i d s t a t e lasers. XeCl l a s e r i s

F o r example, a o f c-Si a t t h e 308-nm wavelength o f a

2 lo6

cm-'

and v i r t u a l l y independent o f temperature

(Chapter 3, Table I ) ; a s i m i l a r v a l u e h o l d s f o r b o t h a- and a-Si.

202


? ]'

1400

0

1200

1000

O -

W

a

2 Q

a

800 600

LU

a

-----K$-

L- -------

2 0 0 ~

0

\s_

--*-------

400

0

, 0.2

1

I

I

0.4

0.6

1

, , , 1.0

0.8

1.2

1.4

1400

-

0 0,

w

a

.................

---------.ns --zoo

1000

100 ns 150

800

tn'

?

ns

600 400 200

-

-

-

0 0

F i g . 7.

-

(b)

1200

1

2 3 4 DEPTH (micrometers)

5

6

a . Temperature as a function o f depth a t various times a f t e r the

beginning o f a 15-nsec pulse from a ruby laser. extended depth scale.

b. Same as Fig. 7a but on an

The results show the rapid temperature fall-off

with

distance f r o m the front surface o f the sample.

Moreover, t h e r e f l e c t i v i t y i s s i m i l a r i n t h e c r y s t a l l i n e , amorphous, and m o l t e n s t a t e s ,

a p p a r e n t l y changing by o n l y

- 10% on m e l t i n g .

Therefore, a t t h e wavelength o f t h e XeCl l a s e r , l a r g e e f f e c t s due t o d i f f e r e n c e s i n o p t i c a l p r o p e r t i e s and t h e i r temperature dependences i n t h e v a r i o u s phases a r e n o t i n t r o d u c e d , so t h a t t h e d i f f e r e n c e s

203

4. MELTING MODEL OF PULSED LASER PROCESSlNG

t h a t occur e x p e r i m e n t a l l y can be a t t r i b u t e d almost e n t i r e l y t o t h e d i f f e r i n g thermal p r o p e r t i e s o f t h e phases. I n t h i s subsection, several aspects o f excimer l a s e r p r o c e s s i n g t h a t have become apparent from a combination o f experimental anneali n g s t u d i e s and s u p p o r t i n g model c a l c u l a t i o n s w i l l be discussed. More s p e c i f i c a l l y , i t w i l l f i r s t be shown how such work can p r o v i d e i n f o r m a t i o n about t h e thermal p r o p e r t i e s o f molten s i l i c o n ; t h e n evidence w i l l be g i v e n t h a t m e l t depths o f

>

1 urn may be a t t a i n a b l e

w i t h excimer l a s e r s w i t h o u t p r o d u c i n g s u r f a c e damage; and f i n a l l y i t w i l l be demonstrated t h a t t h e c a p a b i l i t y o f changing t h e p u l s e shape and d u r a t i o n o f an excimer l a s e r (by changing t h e gas m i x t u r e ) p r e s e n t s some i n t e r e s t i n g p o s s i b i l i t i e s f o r c o n t r o l o f s o l i d i f i c a t i o n r a t e s from t h e molten phase.

a.

I n f o r m a t i o n About t h e Thermal P r o p e r t i e s o f L i q u i d S i l i c o n It was mentioned i n S e c t i o n 111.8 t h a t wide v a r i a t i o n s i n some

o f t h e thermal p r o p e r t i e s o f s i l i c o n a r e found i n t h e l i t e r a t u r e .

In

t h e work d e s c r i b e d i n t h e paper by Wood and G i l e s (1981), t h e vapori z a t i o n temperature and l a t e n t heat o f v a p o r i z a t i o n o f s i l i c o n were t a k e n t o be 2315°C and 2535 c a l / g ,

respectively;

b o t h o f these

values are a t t h e l o w e r end o f t h e ranges o f r e p o r t e d values o f T v and Lv.

Also,

i n Wood and G i l e s t h e thermal c o n d u c t i v i t y o f

l i q u i d s i l i c o n was r e l a t e d t o t h e e l e c t r i c a l c o n d u c t i v i t y by t h e Wiedemann-Franz

law

(Sec.

111.8b),

but f o r

simplicity

KQ was

assumed t o be constant r a t h e r t h a n t o vary l i n e a r l y w i t h T as t h e W-F r e l a t i o n s h i p would r e q u i r e .

When t h e s e assumptions about Tv,

L v , and Kp, were used i n m e l t i n g model c a l c u l a t i o n s , s u r f a c e vapori z a t i o n was p r e d i c t e d t o occur a t

- 2 J/crn2 f o r 15-25-nsec

f r o m ruby and frequency-doubled Nd:YAG l a s e r s .

pulses

Since several exper-

iments s t r o n g l y i n d i c a t e d t h a t damage d i d indeed occur a t about t h e s e energy d e n s i t i e s ( a l t h o u g h o t h e r experiments suggested somewhat h i g h e r t h r e s h o l d s ) , t h e assumptions seemed t o be s a t i s f a c t o r y . However, when m e l t i n g model c a l c u l a t i o n s were c a r r i e d o u t i n support o f t h e f i r s t d e t a i l e d experiments on t h e l a s e r a n n e a l i n g o f s i l i c o n

204

R. E WOOD ET AL.

w i t h a XeCl l a s e r (Lowndes, e t al.,

1982b; Young e t al.,

1982), i t

became apparent t h a t t h e c a l c u l a t i o n s were p r e d i c t i n g damage a t much lower energy d e n s i t i e s t h a n those a t which damage was a c t u a l l y By r a i s i n g Tv and L v t o t h e upper values o f t h e i r r e p o r t e d

observed.

KQ,

ranges and by i n c l u d i n g t h e l i n e a r T-dependence o f

i t was pos-

s i b l e t o suppress t h e onset o f v a p o r i z a t i o n t o t h e e x t e n t t h a t reasonably good agreement w i t h experiment c o u l d be obtained. F i g u r e 8 shows m e l t - f r o n t p r o f i l e s f o r a XeCl l a s e r o p e r a t i n g w i t h a p u l s e shape which c o u l d be approximated c l o s e l y by t h e t r a p e z o i d shown on t h e f i g u r e .

Even a t an energy d e n s i t y of 4 J/cm2,

v a p o r i z a t i o n o f t h e s u r f a c e d i d n o t occur, w h i l e a m e l t - f r o n t penetration of

>

1 vm and a m e l t d u r a t i o n o f

>

400 nsec were obtained.

The c a l c u l a t i o n s showed t h a t v a p o r i z a t i o n d i d occur a t an energy d e n s i t y o f 5 J/cm2 so t h a t t h e onset o f s u r f a c e damage should p r e sumably o c c u r between 4 and 5 J/cm2. 4.6 i.4

Both Lowndes e t a1

1

n

I-

. (1982b)

-

0 0

a 0.8 t-

z

0

w 0.6 LL I

0.2 0

Fig. 8.

0

50

400

150

300

350

400

450

Melt-front profiles produced by radiation from a XeCl laser with the

indicated pulse shape and duration. J/cm2,

200 250 TIME (nsecl

Surface damage occurs between 4 and 4 . 5

in good agreement with experiment,

as discussed in the text.

205

4. MELTING MODEL OF PULSED LASER PROCESSING and Young e t a l .

(1982) have r e p o r t e d t h a t t h e onset o f s u r f a c e

damage d i d n o t occur u n t i l used d i f f e r e n t

> 4 J/cmZ.

EQ

Even though t h e two s t u d i e s

l a s e r s w i t h d i f f e r e n t p u l s e shapes and d u r a t i o n s ,

c a l c u l a t e d m e l t - f r o n t curves were n o t g r e a t l y d i f f e r e n t . Although a d e f i n i t i v e s t u d y of t h e onset o f damage produced by u l t r a v i o l e t pulses has n o t y e t been c a r r i e d o u t , t h r e e c o n c l u s i o n s f r o m t h e work t h a t has been r e p o r t e d seem c l e a r : I n f o r m a t i o n about t h e thermal p r o p e r t i e s o f l i q u i d semiconduct o r s can be e x t r a c t e d from a c a r e f u l comparison o f t h e r e s u l t s o f experiments and c a l c u l a t i o n s on excimer l a s e r m e l t i n g once the optical properties (especially

RE) a r e w e l l e s t a b l i s h e d .

The s p a t i a l homogeneity o f excimer l a s e r pulses can be made good enough

t h a t energy d e n s i t i e s

>, 4 J / c d

(depending

on

p u l s e shape and d u r a t i o n ) can be used w i t h o u t p r o d u c i n g s u r f a c e damage;

c a l c u l a t i o n s i n d i c a t e t h a t m e l t i n g t o depths

g r e a t e r t h a n 1 pm can t h e n be achieved. As a c o r o l l a r y t o

Z),

i t now seems t h a t t h e beam homogeneity

problems a s s o c i a t e d w i t h s o l i d s t a t e l a s e r s a r e even more severe than previously realized. Comparison o f Nd:YAG, Ruby, and XeCl M e l t - f r o n t P r o f i l e s Although t h e r e a r e s u b s t a n t i a l d i f f e r e n c e s between t h e o p t i c a l p r o p e r t i e s o f s i l i c o n a t t h e wavelengths o f t h e Nd and ruby l a s e r s and t h o s e o f t h e u l t r a v i o l e t l a s e r s , t h e a n n e a l i n g c h a r a c t e r i s t i c s i n terms o f t h e energy d e n s i t y r e q u i r e d t o m e l t t o a c e r t a i n depth a r e r o u g h l y comparable f o r comparable p u l s e d u r a t i o n s .

This i s

i l l u s t r a t e d by comparing t h e m e l t - f r o n t p r o f i l e s o f Fig. 4 (Nd:YAG, 18 nsec) and Fig. 6 (ruby,

c.

15 nsec) w i t h Fig. 8 (XeC1, 41 nsec).

E f f e c t s o f Pulse Shape and D u r a t i o n The temporal p u l s e shape o f excimer l a s e r s , though q u i t e com-

p l i c a t e d a t times, trapezoids.

can g e n e r a l l y be approximated r a t h e r w e l l by

F i g u r e 9 shows m e l t f r o n t s f o r two d i f f e r e n t excimer

l a s e r p u l s e shapes; t h e approximate p u l s e shapes are a l s o shown on

206

R. E WOOD ET AL.

t h e figure.

I t can be seen from t h e f i g u r e t h a t f o r t h e same energy

density, t h e longer pulse duration r e s u l t s i n a substantially shallower m e l t - f r o n t penetration.

However,

t h e d u r a t i o n o f surface

m e l t i n g f o r t h e two d i f f e r e n t p u l s e shapes a t t h e same energy denT h i s w i l l n o t always be t h e case.

s i t y may be q u i t e comparable.

It should be p o s s i b l e , as o t h e r c a l c u l a t i o n s n o t i l l u s t r a t e d here suggest,

t o arrange combinations o f p u l s e energy,

d u r a t i o n , and

shape such t h a t t h e m e l t d u r a t i o n i s g r e a t l y prolonged by a l o n g Under such c o n d i t i o n s , t h e r e t u r n o f t h e melt

" t a i l " on t h e pulse.

f r o n t t o t h e s u r f a c e can be s e n s i t i v e l y c o n t r o l l e d by t h e i n t e n s i t y i n t h e t a i l o f t h e pulse.

As a r e s u l t , w i t h pulses from an appro-

p r i a t e l y designed excimer l a s e r , i t should be p o s s i b l e t o o b t a i n a wide range o f regrowth v e l o c i t i e s w i t h which t o conduct both fundamental and a p p l i e d s t u d i e s o f l a s e r processing. 0.9

I

0.8

2

-

I

1

I

I

XeCl LASER _ _ _ - -25.5 _ nsec ---70.5 nsec

5 a z

I

-

0.7

I

I T- (nsec) 25.5 25.5 25.5 70.5 70.5 70.5

---i.5

3.

$ fa

I

El (J/crn2) i.0 -------1.5 ---2.0

---2.5

0.6

--ZO

-

0.5 0.4

0

a

5 W z:

0.3 0.2 0.1

0

0

20

40

60

100 I 2 0 TIME (nsec)

80

I40

160

(80

200

Fig. 9. Variation o f melt-front penetration with pulse duration and shape (light lines) for two different pulses from a XeCl laser.

4. 12.

207


EFFECTS OF AMORPHOUS LAYERS Recent t i m e - r e s o l ved r e f l e c t i v i t y measurements , t r a n s m i s s i o n

e l e c t r o n microscopy s t u d i e s ,

and model c a l c u l a t i o n s f o r s i l i c o n

samples c o n t a i n i n g amorphous s u r f a c e l a y e r s (Lowndes e t a l . Wood e t al.,

, 1984;

1984) have p r o v i d e d c o n s i d e r a b l e i n s i g h t i n t o t h e r o l e

p l a y e d by amorphous l a y e r s i n t h e l a s e r - a n n e a l i n g process.

The

work, which i s d e s c r i b e d i n d e t a i l i n Chapter 6, e s t a b l i s h e d t h a t t h e thermal c o n d u c t i v i t y o f a - S i magnitude l e s s t h a n t h a t o f c-Si

i s a p p r o x i m a t e l y an o r d e r of (see a l s o Webber e t al.,

1983).

Furthermore, t h e c a l c u l a t i o n s showed t h a t t h e response o f t h e amorphous l a y e r t o t h e a n n e a l i n g l a s e r p u l s e i s determined p r i m a r i l y by t h i s g r e a t l y reduced thermal c o n d u c t i v i t y and t h a t t h e r e d u c t i o n o f Ta and La from Tc and Lc a r e c o m p a r a t i v e l y unimportant.

However,

s i n c e t h e a - S i m e l t s a t Ta, which may be s e v e r a l hundred degrees l a s e r m e l t i n g o f a-Si

lower t h a n Tc,

l a y e r s on c-Si

substrates

p r o v i d e s a unique method f o r f o r m i n g h i g h l y undercooled molten silicon.

The TEM c a r r i e d o u t i n c o n n e c t i o n w i t h t h e t i m e - r e s o l v e d

o p t i c a l measurements show t h a t ift h e m e l t f r o n t does n o t p e n e t r a t e t h e a-Si l a y e r , a f i n e - g r a i n e d (FG) p o l y c r y s t a l l i n e ( p ) S i l a y e r i s u s u a l l y formed,

f o l l o w e d by a r e g i o n o f l a r g e - g r a i n e d

e x t e n d i n g t o t h e surface.

(LG) p-Si

The presence o f t h e f i n e - g r a i n e d m a t e r i a l

suggests t h a t homogeneous ( o r heterogeneous) b u l k n u c l e a t i o n has

some T

occurred a t

between Ta and Tc.

5 Tn,

t h e b u l k n u c l e a t i o n temperature,

lying

It i s apparent f r o m t h e s e r e s u l t s t h a t t h e

dynamics o f m e l t i n g and r e s o l i d i f i c a t i o n o f amorphous l a y e r s i s a very complex s u b j e c t , and one t h a t we can n o t hope t o cover i n much d e t a i l here.

Therefore,

we w i l l l i m i t o u r s e l v e s t o a d i s c u s s i o n

o f c e r t a i n aspects o f t h e s u b j e c t f o r which m e l t i n g model c a l c u l a t i o n s may p r o v i d e a t l e a s t some u s e f u l i n s i g h t s . F i g u r e 10 shows a schematic i l l u s t r a t i o n o f t h e cases which a r i s e i n t h e l a s e r m e l t i n g o f a w e l l - d e f i n e d a-Si l a y e r on a c-Si substrate;

t h e amorphous l a y e r may have been formed f o r example

by s e l f - i o n i m p l a n t a t i o n o f S i i n t o S i .

The m e l t i n g temperature

208


, a-Si To = 1 1 50°C

C-Si Tc = 1410°C

cose o

cose b

cose c

Fig. 10.

Schematic of the three cases of melt-front

(rnf) penetration that

can arise when amorphous overlayers are present on crystalline substrates.

and thermal c o n d u c t i v i t y i n t h e amorphous and c r y s t a l 1i n e r e g i o n s a r e designated by T and K w i t h a p p r o p r i a t e s u b s c r i p t s . f r o m Table

I,

Ta = 1 1 5 O o C ,

Case a.

F o r example,

F i g . 2, and S e c t i o n I I I . 8 b Tc = 1 4 1 O o C , Kc = K c ( T ) , and Ka = 0.02 W/deg cm.

The p u l s e energy d e n s i t y i s s u f f i c i e n t l y low t h a t t h e

m e l t f r o n t does n o t p e n e t r a t e t h r o u g h t h e a-Si l a y e r .

T h i s means

t h a t a pool o f h i g h l y undercooled molten S i i s formed, separated from t h e c-Si s u b s t r a t e by an a-Si l a y e r o f low thermal c o n d u c t i v i t y .

Case b.

The energy d e n s i t y i s s u f f i c i e n t f o r t h e m e l t f r o n t

t o p e n e t r a t e t o t h e a-c i n t e r f a c e .

The m e l t f r o n t w i l l pause a t

t h e i n t e r f a c e u n t i l t h e heat f l o w adjusts t o t h e d i f f e r e n c e s i n l a t e n t heats,

AL

= Lc-La,

i n t h e a and c regions.

and m e l t i n g temperatures,

AT = Tc-Ta,

Further complicating t h e calculations i s

t h e b e h a v i o r o f t h e thermal c o n d u c t i v i t y which may change by an o r d e r o f magnitude a t t h e a-c i n t e r f a c e .

I n t h i s case,

as i n

case a, a pool of undercooled % - S i w i l l be p r e s e n t i n t h e sample.

209

4. MELTING MODEL OF PULSED LASER PROCESSING Case c.

The m e l t f r o n t p e n e t r a t e s beyond t h e a-c i n t e r f a c e .

I n t h i s case,

s i g n i f i c a n t undercooling o f t h e molten S i i s not

expected f o r any prolonged p e r i o d s , and t h e p h y s i c a l c o n d i t i o n s a r e c l o s e l y s i m i l a r t o t h o s e which e x i s t when c-Si m e l t s and r e solidifies.

For a given

ER, t h e d i f f e r e n c e i n l a t e n t heat between

a- and c-Si a c t s as an a d d i t i o n a l heat source t o i n c r e a s e t h e m e l t f r o n t p e n e t r a t i o n when an amorphous l a y e r i s present. I t appears t h a t m e l t i n g model c a l c u l a t i o n s o f t h e t y p e d e s c r i b e d

i n Sec. I11 can deal reasonably s a t i s f a c t o r i l y w i t h case c, b u t i t i s n o t a t a l l c l e a r how t o best deal w i t h s o l i d i f i c a t i o n f r o m a h i g h l y undercooled m e l t i n which b u l k n u c l e a t i o n may occur.

The

major conceptual d i f f i c u l t y encountered i n addressing t h e problem i s t o f i n d a s a t i s f a c t o r y way t o i n c l u d e t h e e f f e c t s o f b u l k nucleat i o n o f t h e c r y s t a l l i n e phase.

The major computational d i f f i c u l t i e s

a r e t h a t o f i n c l u d i n g s o l i d i f i c a t i o n a t a temperature o t h e r t h a n t h e m e l t i n g temperature and o f i n t r o d u c i n g b u l k n u c l e a t i o n e f f e c t s i n t o an e s s e n t i a l l y one-dimensional c a l c u l a t i o n .

I n the following,

we w i l l d i s c u s s how t h e s e e f f e c t s can be r o u g h l y s i m u l a t e d i n a one-dimensional t r e a t m e n t , b u t i t must be emphasized t h a t t h e c a l c u l a t i o n s do represent o n l y a s i m u l a t i o n and a r e n o t based on a s a t i s f a c t o r i l y rigorous t h e o r e t i c a l formulation.

Nevertheless, t h e

c a l c u l a t i o n s and t h e experiments taken t o g e t h e r g i v e very u s e f u l i n s i g h t s i n t o t h e u n d e r l y i n g p h y s i c a l mechanisms. W i t h t h e f o r e g o i n g i n mind, t h e more standard heat f l o w c a l c u l a t i o n s d e s c r i b e d i n S e c t i o n 111.5 were m o d i f i e d t o i n c l u d e simul a t i o n o f t h e e f f e c t s o f b u l k n u c l e a t i o n as f o l l o w s . While t h e m e l t f r o n t i s s t i l l advancing i n t o t h e a-Si l a y e r , La, Ta, and Kay Kc,

or

K,

a r e used depending on t h e phase o f t h e m a t e r i a l i n a

given f i n i t e - d i f f e r e n c e c e l l .

The l a t e n t heat i s always switched

f r o m La t o Lc as soon as an a-Si c e l l has melted; t h i s means t h a t when c r y s t a l l i z a t i o n occurs due t o b u l k n u c l e a t i o n o r o t h e r w i s e an amount o f energy equal t o Lc-La becomes a v a i l a b l e .

I f t h e melt

f r o n t does n o t p e n e t r a t e t o t h e a-c i n t e r f a c e , a l a y e r o f a-Si w i t h i t s low thermal c o n d u c t i v i t y remains. The R - S i i n a g i v e n c e l l i s

210

R. F. WOOD ET AL.

a l l o w e d t o s o l i d i f y p r o v i d e d i t s temperature i s l e s s t h a n Tn; t h i s c o n d i t i o n s i m u l a t e s b u l k n u c l e a t i o n and a l l o w s t h e i n c r e m e n t a l l a t e n t heat AL t o i n c r e a s e t h e m e l t - f r o n t p e n e t r a t i o n . m o l t e n c e l l s i n which T i s i n i t i a l l y

> Tn

but

Tn.

When t h e m e l t f r o n t i n i t i a l l y c o n t a c t s t h e a-c i n t e r f a c e ,

t h e temperature o f t h e i n t e r f a c e , Tac, w i l l be Ta.

The m e l t f r o n t

cannot p e n e t r a t e f u r t h e r u n t i l Tac r i s e s t o Tc and enough l a t e n t heat i s s u p p l i e d t o b e g i n t o m e l t t h e c-Si region.

I f E,

i s not

g r e a t enough f o r Tac t o reach Tc, c r y s t a l l i z a t i o n w i l l occur a t temperatures between Ta and Tc. formed i f Tac

Tn

F o r 0.3 J/cm2, i n t h e r e g i o n

and a t h i n r e g i o n o f LG m a t e r i a l i s

expected t o grow f r o m t h e u n d e r l y i n g , b u l k - n u c l e a t e d FG m a t e r i a l . As E,

increases, t h e LG r e g i o n grows i n s i z e w h i l e t h e FG r e g i o n J/cm2 o n l y -200 A o f FG p-Si remains; f o r

s h r i n k s u n t i l a t -0.6

E Q > 0.8 J/cm2 t h e m e l t f r o n t p e n e t r a t e s c o m p l e t e l y t h r o u g h t h e

a-Si.

The k i n k a t -0.10

because Ta

kp

t o o b t a i n agreement

between experimental and c a l c u l a t e d p r o f i l e s and t h e f a c t t h a t t h e e q u i l i b r i u m s o l u b i l i t y l i m i t can be g r e a t l y exceeded as a r e s u l t

o f l a s e r a n n e a l i n g show t h a t t h e process i s a h i g h l y n o n e q u i l i b r i u m one. C a l c u l a t i o n s by Wood e t a l . (1981a) o f p r o f i l e s i n B i - and I n i m p l a n t e d S i a r e used here as t h e p r i m a r y i l l u s t r a t i o n s o f t h e r e s u l t s o f t r e a t i n g ki periments (White e t a1 t o a dose of respectively.

- 1.5

as an a d j u s t a b l e parameter.

., 1980),

I n t h e ex-

bismuth and i n d i u m were i m p l a n t e d

1 . 2 ~ 1 0 cmm2 ~ ~ a t energies o f 250 keV and 125 keV, Ruby l a s e r pulses o f

J/cm2 were used f o r annealing.

- 15-nsec

d u r a t i o n (FWHM) and

The experimental r e s u l t s a r e

shown i n Figs. 22a and 22b f o r B i and In, r e s p e c t i v e l y .

When f i t -

t i n g t h e data, t h e experimental p o i n t s i n t h e f i r s t 200 A o f t h e sample where t h e very l a r g e s e g r e g a t i o n s p i k e s occur were excluded. A v a l u e o f t h e d i f f u s i o n c o e f f i c i e n t f o r B i i n S i i s n o t g i v e n by

240

R. F. WOOD ETAL.

Kodera; s e v e r a l values were t e s t e d b u t i n t h e c a l c u l a t i o n s d e s c r i b e d h e r e Dg had t h e v a l u e 2.4~10-4 cm2/sec. a maximum p e n e t r a t i o n depth o f phous l a y e r .

- 0.36

The m e l t - f r o n t p r o f i l e gave pm f o r a 0.18-pm

t h i c k amor-

A value o f approximately 0.35 f o r k i gave t h e best f i t

t o t h e B i p r o f i l e , and t h e r e s u l t s a r e shown on Fig. 22a by t h e s o l i d line.

A l s o shown on t h e f i g u r e i s t h e p r o f i l e f o r k i = 1.0,

i.e.,

w i t h no s e g r e g a t i o n and t h e p r o f i l e u s i n g t h e e q u i l i b r i u m v a l u e o f k y = 0.0007;

the l a t t e r i s clearly

c o m p l e t e l y unable

s a t i s f a c t o r y f i t t o t h e experimental data.

t o give a

An examination o f t h e

amount o f dopant segregated t o t h e s u r f a c e r e g i o n (depth

3

200 A )

g i v e s a u s e f u l check on t h e value o f k i o b t a i n e d f r o m f i t t i n g t h e smooth p a r t o f t h e p r o f i l e p r o v i d e d t h e depth o f t h e dopant d i s t r i b u t i o n i s g r e a t enough f o r t h e i n i t i a l t r a n s i e n t t o have d i e d out.

The experimental data show t h a t a p p r o x i m a t e l y 18% o f t h e

i m p l a n t e d dose i s l o c a t e d w i t h i n t h e f i r s t 200 A o f t h e surface. The c o r r e s p o n d i n g percentages from c a l c u l a t i o n s w i t h t h e f i n i t e d i f f e r e n c e method were 6, 11, 15, and 31 f o r k i values o f 1.0, 0.3,

and 0.1,

respectively.

Thus,

0.5,

b o t h t h e percentage o f dopant

i n t h e s u r f a c e peak and t h e l e a s t - s q u a r e s f i t t i n g o f t h e dopant p r o f i l e (which excludes t h e s u r f a c e peak) i n d i c a t e a v a l u e o f k i i n t h e range 0.25-0.35.

Calculations w i t h t h e modified instan-

taneous a p p r o x i m a t i o n method were a l s o c a r r i e d o u t w i t h r e s u l t s i n When kp = 0.0007 was good agreement w i t h t h o s e j u s t described. used i n t h e c a l c u l a t i o n s , g r e a t e r t h a n 99% o f t h e dopant was segregated t o t h e surface. The r e s u l t s f o r c a l c u l a t i o n s on I n i n S i w i t h D,

= 6.9~10-4

a r e shown i n Fig. 22b. The b e s t f i t t o t h e experimental d a t a was o b t a i n e d w i t h k i = 0.12 which i s q u i t e c l o s e t o t h e v a l u e o f 0.15 found by White e t a l .

(1980).

A c a l c u l a t i o n w i t h k j = 0.12 u s i n g

t h e f i n i t e - d i f f e r e n c e method gave t h e d o t t e d curve on Fig. 22b; t h e calculation

with

k y = 0.0004

a l s o shown on t h e f i g u r e .

u s i n g t h e method o f Sec. V.18d i s

The c a l c u l a t e d amount o f dopant segre-

g a t e d t o t h e s u r f a c e was-61$, i n e x c e l l e n t agreement w i t h t h e v a l u e o f 60% o b t a i n e d e x p e r i m e n t a l l y . When ky = 0.0004 was used i n t h e


241

c a l c u l a t i o n s , n e a r l y 100% o f t h e dopant was segregated t o t h e surface;

c l e a r l y e q u i l i b r i u m values o f t h e segregation c o e f f i c i e n t

g e n e r a l l y cannot be used f o r c a l c u l a t i o n s o f pulsed l a s e r annealing. An a d d i t i o n a l problem i s encountered when dopant p r o f i l e s o f Sb and Ga i n S i are considered, namely, t h a t o f dopant l o s s d u r i n g annealing (White e t al., Wood e t a l .

1980).

I n o r d e r t o study t h i s problem,

(1981a) compared t h e r e s u l t s obtained f o r t h r e e d i f -

f e r e n t assumptions about dopant loss.

I n one s e t o f c a l c u l a t i o n s ,

a l l o f t h e dopant l o s s occurred d u r i n g t h e i n i t i a l p a r t o f t h e surface m e l t i n g when t h e temperatures i n t h e l i q u i d S i were t h e highest.

Equally good f i t s t o t h e experimental data were found f o r

k i values o f 1.0 and 0.8 f o r Sb i n S i .

However, when t h e same

assumption was a p p l i e d t o t h e case o f Ga i n S i , s a t i s f a c t o r y f i t s

A second set o f c a l c u l a t i o n s assumed t h a t

could not be obtained.

dopant loss occurred o n l y when t h e surface concentration exceeded some t h r e s h o l d value.

Good f i t s t o both t h e Sb and Ga experimental

data were r e a l i z e d , b u t t h e f i t f o r Sb r e q u i r e d a value o f k i = 0.4 and a maximum m e l t - f r o n t p e n e t r a t i o n s u b s t a n t i a l l y l e s s than t h a t i n d i c a t e d by t h e heat t r a n s f e r c a l c u l a t i o n s .

The t h i r d s e t o f

c a l c u l a t i o n s assumed t h a t t h e dopant loss i s p r o p o r t i o n a l t o t h e surface concentration, as one would expect from k i n e t i c r a t e theory. I t was found t h a t s a t i s f a c t o r y f i t s t o b o t h Sb and Ga i n S i could

be obtained w i t h t h i s method; t h e r e s u l t s f o r Sb i n S i a r e shown i n Fig. 23. 20.

DISCUSSION AND

CONCLUSIONS

A number o f p o i n t s should be kept i n mind when c o n s i d e r i n g t h e r e s u l t s o f t h i s section.

F i r s t , on t h e experimental side, i t should

be recognized t h a t Rutherford backscattering, though a very powerful technique, y i e l d s f a i r l y s u b s t a n t i a l e r r o r bars on t h e data, unless very l o n g counting times can be used t o reduce t h e s t a t i s t i c a l f l u c tuations.

The c a l c u l a t i o n s discussed above u t i l i z e d RMS f i t t i n g s

t o t h e data as though no e r r o r s were involved.

Had t h e s t a t i s t i c a l

242

R. F. WOOD ET AL

IMPLANTED LASER ANNEALEO ki = t.0

0

-I

0.04 0.08

0

0.12

0.16 0.20 0.24

DEPTH

0.28

(pm)

Fig. 23. Antimony distribution in Sb-implanted Si before and a f t e r laser annealing. There i s some loss of Sb from the sample during laser annealing. The solubility limit of Sb in Si is - 7 ~ 1 0 1 ~ / c m ~ .

e r r o r s been t a k e n i n t o c o n s i d e r a t i o n , i t would have been d i f f i c u l t t o differentiate,

f o r example,

between a ki

o f 1.0 and 0.9 f o r

f i t t i n g t h e As data, o r between k i values o f 0.25-0.45 t h e smooth p a r t o f t h e B i p r o f i l e .

for fitting

Somewhat c o u n t e r b a l a n c i n g t h i s

d i f f i c u l t y a r e t h e p r e c i s e r e s u l t s t h a t b o t h t h e experiments and t h e c a l c u l a t i o n s g i v e f o r t h e amount o f dopant i n t h e s u r f a c e spike. Good agreement between experiment and t h e o r y f o r t h e percentage o f dopant i n t h i s s p i k e i s g e n e r a l l y o b t a i n e d , and t h i s percentage i s q u i t e c o n s i s t e n t w i t h t h e r e s u l t s o f t h e RMS f i t t i n g o f t h e smooth part of the profile.

Nevertheless,

i t s h o u l d be recognized t h a t

t h e e x t r a c t i o n o f k i from t h e e x p e r i m e n t a l data may be s u b j e c t t o some f a i r l y s u b s t a n t i a l e r r o r s . Turning t o t h e calculations,

we see f r o m Eqs.

(20) and ( 2 2 )

t h a t t h e r a t i o o f v/DR i s a fundamental q u a n t i t y i n t h e a n a l y t i c a l s o l u t i o n s f o r t h e case t h a t t h e i n i t i a l dopant p r o f i l e i s c o n s t a n t ;

243


t h i s r a t i o a l s o i s of b a s i c importance i n t h e more complicated case of nonuniform p r o f i l e s .

The heat t r a n s p o r t and m e l t i n g c a l c u l a t i o n s

can be assumed t o g i v e r a t h e r a c c u r a t e values o f v as l o n g as t h e dopant c o n c e n t r a t i o n remains low.

The values o f

D, however a r e

s u b j e c t t o f a i r l y l a r g e u n c e r t a i n t i e s as i n d i c a t e d by t h e e r r o r e s t i m a t e s of Kodera (1965) and t h e d i f f e r e n c e s between Kodera's values and those o f Shashkov and Gurevich (1968).

I n t h i s connec-

t i o n , Wood e t a l . (1981a) c a r r i e d o u t c a l c u l a t i o n s on b o t h B i and I n u s i n g t h e values o f

D,

from Shashkov and Gurevich and found t h e

r e s u l t s t o t a l l y unacceptable u n l e s s t h e m e l t - f r o n t p r o f i l e s were g r e a t l y a l t e r e d f r o m t h o s e d i c t a t e d by t h e heat t r a n s f e r c a l c u l a tions.

T h i s would seem t o be a c l e a r i n d i c a t i o n t h a t Kodera's

values a r e t h e more a c c u r a t e ones.

Apparently a u s e f u l way t o

D,

and k i i s t o employ whatever

proceed i n d e t e r m i n i n g values o f

means a v a i l a b l e t o a s c e r t a i n t h a t t h e c a l c u l a t i o n s o f m e l t - f r o n t p r o f i l e s give r e l i a b l e results. e t al.

Time-resol ved r e f l e c t i v i t y (Lowndes

(1983) and e l e c t r i c a l c o n d u c t i v i t y ( G a l v i n e t a l . ,

1982;

Thompson and G a l v i n , 1983) measurements a r e extremely u s e f u l i n t h i s connection.

Then t h e p r o f i l e s c a l c u l a t e d can be used i n t h e dopant

d i f f u s i o n c a l c u l a t i o n s t o determine r e f i n e d values o f values o f k i .

Thus,

D, and new

systematic e x p e r i m e n t a l s t u d i e s o f l a s e r

annealing, coupled w i t h r e f i n e d c a l c u l a t i o n s o f m e l t - f r o n t p r o f i l e s and f u r t h e r experiments and c a l c u l a t i o n s on dopant r e d i s t r i b u t i o n , can be expected t o l e a d t o improved values o f b o t h l a s t two columns i n Table V show t h e values o f t i o n s o f White e t a l . and from Wood e t a l .

kj

DQ and k i .

The

from t h e c a l c u l a -

When t h e v a r i o u s sources

o f p o s s i b l e e r r o r i n t h e experiments and c a l c u l a t i o n s a r e considered, t h e r e i s e s s e n t i a l l y complete agreement between t h e two s e t s o f c a l c u l a t i o n s which were done c o m p l e t e l y independently. The sketchy experimental r e s u l t s presented t h u s f a r on those cases i n which t h e m e l t f r o n t does n o t p e n e t r a t e e n t i r e l y through t h e implanted p r o f i l e may p o i n t t h e way toward a deeper unders t a n d i n g o f t h e dynamics o f u l t r a r a p i d c r y s t a l l i z a t i o n and b u l k nucleation. It w i l l r e q u i r e h i g h i n s t r u m e n t a l r e s o l u t i o n and a

244


combination o f c a r e f u l e x p e r i m e n t a t i o n and r e a l i s t i c modeling t o d i s e n t a n g l e b u l k n u c l e a t i o n e f f e c t s , t r a n s i e n t e f f e c t s a r i s i n g from s h a l l o w me1t - f r o n t p e n e t r a t i o n , and s t e a d y - s t a t e segregation.

The

c a p a b i l i t y o f t r e a t i n g transients i s already b u i l t i n t o the

QFD

approach, b u t n u c l e a t i o n p r e s e n t s a problem. An e s p e c i a l l y i m p o r t a n t e f f e c t o f t h e t r a n s i e n t s may come i n t o p l a y i n t h e i n t e r p r e t a t i o n o f experiments i n which t h e dependence o f k i on m e l t - f r o n t v e l o c i t y (discussed i n Cfiapter 5) i s studied. shows t h a t f o r k i

1012/sec) and i s f u r t h e r enhanced by a nonlinear dependence on c a r r i e r concentration, via t h e Auger recombination process. Thus, in order t o observe the formation and decay of the high c a r r i e r density photoexcited electron-hole plasma i n s i l i c o n , measurements on t h e picosecond---or even shorter---time scale are required. Such measurements have now been carried out, using pulse-and-probe optical techniques, v i s i b l e and near-infrared probe pulses, and both pico- and femto-second l a s e r s (Liu et a1 , 1982a; Shank e t a1 , 1983a; Lompr6 et a1 , 1984a, b; van Driel e t a1 , 1984). Direct measurements of t h e s t r u c t u r a l change accompanying pulsed l a s e r melting a l s o were c a r r i e d out recently on t h e picosecond time s c a l e , v i a electron d i f f r a c t i o n from aluminum t h i n f i l m s (Mourou and Wi 11 iamson, 1982; Wi 11 iamson e t a1 , 1984), and with 90 femtosecond resolution, via time-resolved second harmonic generation a t a (111) s i l i c o n surface (Shank e t a l . , 1984).

.,

.

.

.

.

.

316

D. H. LOWNDES E T A L

Time-resolved measurements provide a d i r e c t means f o r evaluating our understanding of the physical mechanisms involved i n pulsed 1a s e r anneal i ng of ion-imp1 anted semiconductors , on the nanosecond and longer time s c a l e s ; these measurements provide an especially s t r i n g e n t test of t h e o r e t i c a l models f o r the laser-anneal i n g process. For exampl e , nanosecond resol u t i on time-resol ved measurements of optical r e f l e c t i v i t y have revealed how differences in the i n i t i a l s t a t e of c r y s t a l l i n i t y or amorphization of a semiconductor modify the onset of melting, the duration of melting and

the overall time s c a l e f o r anneal i ng and recrystal 1 i z a t i on (Auston e t a1 , 1979; Lowndes et a1 , 1984a, b; Wood et a1 , 1984). Similar e f f e c t s may r e s u l t from changes in chemical composition in t h e near-surface region (Lowndes and Wood, 1981). Time-resol ved measurements a1 so were used t o moni t o r t h e sol id-phase r e c r y s t a l l izat i o n process r e s u l t i n g from cw l a s e r annealing, though i n t h i s case the c h a r a c t e r i s t i c time scale f o r the measurements was a fact o r of 106-108 longer (Olson et a1 , 1980). Time-resolved measurements have been a t t h e focal point of a controversy regarding t h e physical mechanism f o r pulsed l a s e r annealing, i n connection with "thermal" versus "nonthermal" models f o r t h e annealing process. The thermal melting model assumes t h a t t h e l a s e r energy absorbed by t h e sample i s transferred t o the l a t t i c e i n a time comparable t o or l e s s than t h e typical annealing l a s e r pulse duration (of order 10-100 nsec) and t h a t t h e r e a f t e r normal heat t r a n s f e r and melting occur. Thermal melting calcul at i o n s carried out by several groups (see Chapter 4 ) found e a r l y support in an impressive a r r a y of primarily post-anneal ing experi mental r e s u l t s , including ( a ) the i n t e r p r e t a t i o n of SIMS and RBS measurements of implanted dopant r e d i s t r i b u t i o n p r o f i l e s in both s i l i c o n and GaAs, a f t e r pulsed l a s e r annealing, in terms of liquidphase diffusion (White et al., 1980; Lowndes et al., 1981; Wood e t al., 1981a, b ) and (b) the presence w i t h i n dopant d i s t r i b u t i o n s i n s i l i c o n , a f t e r pulsed l a s e r annealing, of c e l l u l a r s t r u c t u r e ,

.

.

.

.

6.

317

TIME-RESOLVED MEASUREMENTS

which i s a well-known c h a r a c t e r i s t i c o f t h e breakdown o f a p l a n a r liquid-solid

i n t e r f a c e d u r i n g r e c r y s t a l l i z a t i o n (Poate e t a l .

1978; van Gurp e t al.,

1979; Narayan, 1980).

,

E a r l y time-resolved

r e f l e c t i v i t y measurements i n s i l i c o n (Auston e t a1 and i n GaAs (Lowndes and Wood, 1981; Wood e t a1

., 1978,

1979)

., 1981b) were a l s o

i n t e r p r e t e d as g i v i n g a d i r e c t measure o f s u r f a c e me1t d u r a t i o n , v i a t h e d u r a t i o n o f t h e h i g h r e f l e c t i v i t y phase (HRP),

and were

found t o be g e n e r a l l y i n accord w i t h m e l t i n g model c a l c u l a t i o n s . However, t h e thermal me1t i n g model was n o t u n i v e r s a l l y accepted. Van Vechten and co-workers (Van Vechten e t al.,

1979; Van Vechten,

1980; Van Vechten and Compaan, 1981) suggested t h a t annealing occurs by l a t t i c e s o f t e n i n g , n o t m e l t i n g , t h r o u g h promotion o f e l e c t r o n s f r o m bonding t o a n t i - b o n d i n g s t a t e s and accompanied by f o r m a t i o n o f a long-1 i v e d

(-10-100

nsec) , h i g h - d e n s i t y

(-1021-1022/~m3)

e l e c t r o n - h o l e plasma. Several t i m e - r e s o l v e d measurements on s i l i c o n were i n t e r p r e t e d as c o n t r a d i c t i n g t h e m e l t i n g model and p r o v i d i n g support f o r t h e " p l asma anneal ing" model.

Compaan and co-workers

used t h e r a t i o o f Stokes t o a n t i - S t o k e s i n t e n s i t i e s i n attempts t o measure t h e l a t t i c e temperature o f c r y s t a l 1 i n e s i l i c o n ( c - S i ) s h o r t l y a f t e r t h e end o f t h e h i g h r e f l e c t i v t y phase.

These e x p e r i -

ments and s i m i l a r ones by von d e r L i n d e and co-workers,

which

e v e n t u a l l y brought t h e Raman r e s u l t s i n t o agreement w i t h o t h e r temperature measurements, w i l l be discussed i n S e c t i o n 111. Compaan and co-workers a l s o c a r r i e d out a s e r i e s o f t i m e - r e s o l v e d t r a n s m i s s i o n and r e f l e c t i v i t y measurements u s i n g b o t h c-Si and s i l i c o n -

.,

on-sapphire (SOS) samples (Lee e t a1 1981; A y d i n l i e t al., 1981). As discussed i n Sec. 11.1, t h e i r i n i t i a l o b s e r v a t i o n o f f i n i t e

(-25%) t r a n s m i s s i on through c-Si d u r i n g t h e HRP was subsequently shown t o be wrong by Lowndes (1982) o b s e r v a t i o n o f zero t r a n s mission. I n response t o t h e suggestion o f a plasma-annealing mechanism, a v a r i e t y o f a d d i t i o n a l experimental techniques were developed f o r i n d i r e c t l y measuring t h e l a t t i c e temperature o f t h e near-surface

318

D.H. LOWNDES ETAL.

region of a semiconductor during pulsed laser irradiation. Direct temperature measurements are experimentally quite difficult because of the very short times sec) and small laser-beam dimensions used in typical experiments, and the presence of very large ternporal and spatial temperature gradients. A further compl ication i s the need t o select a thermometric parameter, X(T), t h a t can be measured under these constraints b u t whose dependence on T can be calibrated independently (usually under steady-state conditions). Ideally, X should be independent of other variables (e.g., plasma density) t h a t may a1 so change rapidly d u r i n g and imediately after pulsed laser irradiation. During the past several years, timeresolved experimental techniques have been developed f o r re1 i ably estimating l a t t i c e temperature and for placing an upper bound on the el ectron-hole plasma temperature, using as the thermometric parameter (1) the velocity distribution (Stritzker et al., 1981; Pospieszczyk et al., 1983) or (2) total charge (Liu et al., 1982b; 1984) of electrons and positive ions emitted from Malvezzi et a1 1 aser-heated semiconductor surfaces, (3) x-ray diffraction measure1982a,b, 1983a, b ) , (4) ments of l a t t i c e strain (Larson et a1 changes in the complex index o f refraction of silicon films a t visible wavelengths (Murakami et a1 , 1981; Lompr6 et a1 , 1983), and (5) the emission of thermal (blackbody) radiation (Kemmler et a1 1984). Measurements made using a l l of these techniques have now established t h a t surface temperatures do reach, and exceed, the melting p o i n t of silicon d u r i n g the high reflectivity phase t h a t accompanies the pulsed laser-anneal i n g process. I n this chapter we survey the results o f a selection of the time-resolved measurements performed during pulsed laser i r r a d i a t i o n . Particular emphasis has been placed on those experimental results t h a t can be directly compared w i t h predictions o f model calculations, i n order t o provide as complete a picture as possible o f the current state of agreement between experiments and calculations. For this reason the discussion is restricted t o results

.

.,

.

.)

.

6. obtained

319


using s i l i c o n ,

thus avoiding complications t h a t

are

associated w i t h d e v i a ti o n s from s to i c h i o m e t r y i n compound semiconductors (Lowndes and Wood,

1981).

E f f e c t s o f pulsed l a s e r

r a d i a t i o n on GaAs are discussed separately i n Chapters 7 and 8.

11. Experimental Tests and A p p l i c a t i o n s o f t h e Thermal M e l t i n g Model 1.

NANOSECOND TIME-RESOLVED OPTICAL PROPERTIES

a.

Experimental Considerations Time-resol ved measurements o f t h e o p t i c a l transmission

(T) o r

r e f l e c t i v i t y (R) d u ri n g pulsed l a s e r i r r a d i a t i o n are complicated by t h e simul taneous i mp o s i ti o n o f several

requirements;

these

i n c l u d e ( I ) det e c to r response time, (2) s p a t i a l and temporal q u a l i t y o f t he annealing l a s e r beam, (3) s p a t i a l alignment o f low-power cw probe l a s e r beams w i t h the pulsed beam, and ( 4 ) t h e need f o r accur a t e energy den s i ty measurements i n t h e ( f r e q u e n t l y small ) r e g i o n whose o p t i c a l p r o p e r t i e s are monitored by t h e probe beam.

Figure 1

shows schematically two s i m p l i f i e d y e t t y p i c a l experimental setups used i n measurements on t h e nanosecond time scale w i t h v i s i b l e and n e a r-inf rared cw probe beams. The pulsed l a s e r beam must have a smooth temporal and s p a t i a l profile.

Pronounced temporal spikes o r m u l t i p l e pulses are unac-

ceptable because accurate q u a n t i t a t i v e comparisons w i t h model c a l c u l a t i o n s (which f o r convenience g e n e ra l l y assume an i d e a l i z e d temporal pulse) become d i f f i c u l t and because excessive power dens i t i e s may cause damage t o t h e sample. For t h i s reason most experiments have been c a r r i e d out u s i ng pul sed 1asers operated i n t h e TEMoo mode, w i t h attempts also made t o minimize temporal modul a t i o n o f t h e pulse shape (due, t u d i n a l modes).

f o r example, t o m u l t i p l e l o n g i -

Pulses t h a t are n e a r l y Gaussian i n s p a t i a l cross

320

D. H. LOWNDES ETAL.

TIME RESOLVED TRANSMISSION EXPERIMENT OSCILLOSCOPE h, 1 nnr)

-

COLLECTION LENS SYSTEM,

I

THIN SHEET WITH

2 OR 3 m m diam APERTURE PULSED RUBY LASER SILICON

FOCUSING

LENS^ ',/ / '_'

IR HdNe PROBE LASER (1 mW A = 1.152 rm)

m.

0.44-

f"::$p)

-A -

TIME OF ONSET-OF-MELTING MEASUREMENT

THREE h = 0.633 Nrn

PLANAR VACUU

LENS SYSTEM

FOCUSING LENS

(3.4 mW CW. A = 0.633 wrn

Fig. 1. Schematic representation o f experimental setups for time-resolved ( a ) transmission (Lowndes e t al., 198213) and (b) reflectivity (Lowndes and Wood, 1981) measurements.

6.

321


section and s u f f i c i e n t l y smooth temporally t o be e a s i l y approximated i n model calculations a r e obtained with ruby, Nd:YAG, Nd:glass, and excimer lasers. Pulsed excimer l a s e r s possess the very s i g n i f i c a n t advantage of having only low s p a t i a l coherence so t h a t d i f f r a c t i v e modulat i o n of the l a s e r beam by d u s t p a r t i c l e s or apertures is not present t o nearly the extent i t i s w i t h pulsed s o l i d - s t a t e l a s e r s . Local damage t o specimens a t the positions of d i f f r a c t i o n maxima and/or beam inhomogeneity on a f i n e s c a l e can therefore be avoided by using a pulsed excimer l a s e r . Multi-mode pulsed s o l i d - s t a t e l a s e r operation can a l s o be used, provided t h a t s p a t i a l beam inhomogeneities a r e removed by placing t h e sample behind a d i f f u s e r p l a t e or diffusing l i g h t pipe. However, d i f f u s e r plates can cause microscopic damage t o even a s i l i c o n sample surface i f the sample i s too close ( < l o cm) t o the p l a t e , or i f the grinding/etching procedure i s not optimized (Young e t a1 1982). Since t h e pulsed energy density f a l l s off rapidly with distance beyond a d i f f u s e r p l a t e , i t s application is probably r e s t r i c t e d t o multi-mode l a s e r s capable of delivering s u b s t a n t i a l l y more energy per pulse than i s needed. Diffusing l i g h t pipes have the disadvantage in tirneresolved measurements using cw probe beams t h a t t h e sample must be placed very close (_lo0 nsec, use was made of t h e f a c t t h a t any i n i t i a l h i g h ( l a s e r melting-induced) c a r r i e r c o n c e n t r a t i o n

w i l l be r a p i d l y reduced by Auger recombination and, secondarily, by c a r r i e r d i f f u s i o n away from t h e surface.

This r e s u l t s i n a t o t a l

absorption i n t h e c a r r i e r d i f f u s i o n depth L = Lafc = 0.042,

i.e.,

-5% (Lowndes e t a1

-

6 p of order

., 1982b).

I hIlII

This estimate

i s i n unexpectedly good agreement (considering the roughness o f t h e c a l c u l a t i o n ) w i t h t h e experimentally observed " e x t r a " absorpt i o n t10% f o r t > 100 nsec (Fig. 3). I n c o n t r a s t , t h e a d d i t i o n a l f r e e c a r r i e r absorption c a l c u l a t e d assuming only a t h e r m a l l y generated " i n t r i n s i c " c a r r i e r p o p u l a t i o n

7, Chapter 4), i s

100 nsec

a f t e r t h e end o f t h e HRP. b.

Subnanosecond Excite-and-Probe L a t t i c e Temperature Measurements Pulsed l a s e r i r r a d i a t i o n o f a semiconductor such as s i l i c o n

produces changes i n t h e complex d e l e c t r i c constant, T = q + q ) t h a t r e s u l t both from t h e presence o f a l a s e r - e x c i t e d e l e c t r o n -

It i s shown i n Sec.

h o l e plasma and from heating o f t h e l a t t i c e .

IV.9 t h a t : (1) I f excite-and-probe measurements o f t h e o p t i c a l p r o p e r t i e s o f l a s e r - i r r a d i a t e d s i l i c o n are c a r r i e d o u t a t a probe wavelength i n t h e m i d - v i s i b l e region (e.g.,

532 nm), then v a r i a t i o n s i n

E~

are t h e

r e s u l t o f t h e opposing e f f e c t s o f i n c r e a s i n g l a t t i c e temperature and i n c r e a s i n g plasma density, w h i l e c2 i s governed mainly by t h e l a t t i c e tempera(2)

t u r e a t these frequencies. A low-level 532-nm e x c i t a t i o n pulse w i l l produce a plasma t h a t decays i n about 100 ps i n s i l i c o n (see Fig. 21).

Thus, a t l a t e r times t h e o p t i c a l p r o p e r t i e s o f l a s e r - i r r a d i a t e d s i l i c o n are mainly those o f t h e laser-heated l a t t i c e w i t h a minimal p l asma c o n t r i b u t ion. Recognition o f these f a c t s l e d Lompr6 and c o l l a b o r a t o r s (1983) t o c a r r y out l a t t i c e temperature measurements f o l l owing a 532-nm e x c i t a t i o n pulse (using e x c i t a t i o n l e v e l s below t h e m e l t i n g thresho l d ) by probing t h e R and T o f 0.1 and 0.5 pm t h i c k SOS samples, a t a f i x e d time delay o f 200 ps a f t e r t h e e x c i t a t i o n pulse.

The

probe pulse was a l s o a t 532 nm, w i t h special care taken t o avoid spurious effects

caused by t h e i n t e r f e r e n c e o f pulse and probe

358

D. H. LOWNDES ET AL.

beams a t t h e same wavelength.

Because s i l i c o n i s s t r o n g l y absorbing

a t 532 nm (see Table I ) t h e probe pulse fluence was l i m i t e d t o about 0.1% o f t h e e x c i t a t i o n pulse fluence, t o avoid s i g n i f i c a n t h e a t i n g by t h e probe pulse. The v a r i a t i o n s i n n and k t h a t were i n f e r r e d from R and T measurements by Lompr6 and c o l l a b o r a t o r s were i n t e r p r e t e d i n terms o f temperature changes using t h e known i n t r i n s i c v a r i a t i o n s i n n and k ( a r i s i n g from changes i n t h e i n d i r e c t bandgap and i n m a t r i x elements f o r t h e i n d i r e c t t r a n s i t i o n ) ,

using J e l l i s o n and Modine's

(1982a, 1983) measurements o f n and k f o r temperatures up t o 1000 K and an extrapol a t ion o f these r e s u l t s f o r higher temperatures. S l i g h t v a r i a t i o n s i n t h e thickness o f t h e SOS samples were found t o produce l a r g e changes i n R and T across i n d i v i d u a l samples, even w i t h o u t o p t i c a l pumping; t h e independence o f t h e d e r i v e d n and k values from t h e f i l m thickness a c t u a l l y used provided an additional

check

on

experimental

procedures

(Lompr6 e t

a1

.,

1983). I t was found t h a t t h e o p t i c a l absorption c o e f f i c i e n t o f the

SOS samples increased by about an order o f magnitude from room

temperature t o temperatures j u s t be1ow t h e me1t i ng p o i n t (me1t i ng was produced f o r fluences above 0.16 J/cm2). fraction, A

(A

=

1- R

- T),

The absorbed energy

was found t o depend s t r o n g l y on f i l m

t h i c k n e s s a t low fluence, but f o r higher fluences ( s t i l l below t h e m e l t i n g f l u e n c e ) t h e d i f f e r e n c e i n A f o r 0.1-pm and 0 . 5 - p t h i c k specimens was reduced.

If t h e actual temperature p r o f i l e ,

~ ( x ) , does vary s i g n i f i c a n t l y across t h e thickness o f a sample, then the optical density

depends s e n s i t i v e l y on T ( x ) ;

c o r r e l a t i n g R and T then r e q u i r e s

knowledge o f t h e actual p r o f i l e o f a(x) o r T ( X ) (Jacobsson, 1966; Lompre e t al.,

1983).

[The

r i g h t hand side o f Eq.

J e l l i s o n and Modine's form f o r a(x).]

( 4 ) uses

Lompre' and coworkers were

6.


359

unable t o f i t their R and T data by assuming a uniform temperature, even for the 0.1-w thick SOS sample. However, by numerically calculating T(X) (200 ps after excitation pulses of various fluences), taking into account the temperature dependences of optical absorpt i o n , thermal conductivity and heat capacity, and converting these T ( X ) profiles into absorption profiles a ( x ) and then optical density, using Eq. (4), they were able t o f i t their R and T data for the entire range of fluences below the melting transition. Figure 15 shows the resulting surface temperatures, T ~ ,and average temperatures, F, where

SURFACE TEMP Ts 0

AVERAGE TEMPT

0.05

0.10

0.15

El (J/cm2) Fig. 1 5 . Average temperatures and surface temperatures at t = 200 ps vs incident fluence of ~ O - P S , 532-nm heating pulses, for a 0.5-pn thick SOS film (Lompr; et a l . , 1 9 8 3 ) .

360

D. H. LOWNDES E T A L .

versus 532 nm e x c i t a t i o n f l u e n c e f o r a 0.5-IJ~ t h i c k SOS specimen. L a t t i c e temperatures i n excess o f 1500 K are reached a t a f l u e n c e o f 0.15 J/cm2,

i n good agreement w i t h t h e i r observation o f an R

jump i n d i c a t i v e o f m e l t i n g (expected t o occur a t Tc = 1685 K ) f o r fluences i n excess o f 0.16 J/cm2. c.

Time-Resol ved Pyrometry Thermally e m i t t e d r a d i t i o n provides another p o s s i b i l i t y f o r

t h e measurement o f t h e temperature, T, o f t h e near-surface region

of s i l i c o n d u r i n g pulsed l a s e r i r r a d i a t i o n .

According t o Planck's

1aw,

where c1 = 3.7413 x

erg-cm2/sec,

c2 = 1.4388 cm-deg, A i s t h e

wavelength i n cm, and W i s t h e t h e r m a l l y r a d i a t e d energy f l u x per u n i t wavelength.

C l e a r l y , a d e t a i l e d examination o f t h e wavelength

dependence o f emitted r a d i a t i o n can be used t o determine t h e temperature o f t h e near-surface region. I n a recent set o f experiments,

K e m l e r e t al.

(1984) have

measured t h e time-resolved thermal r a d i a t i o n erni t t e d d u r i n g pul sed 1aser heating o f s i 1 icon. They used t h e frequency-doubl ed output ( A = 532 nm) from a p a s s i v e l y Q-switched Nd:YAG l a s e r operated a t 20 Hz, w i t h a pulse w i d t h o f 10 nsec and l e s s than 5%

fluctuation.

rms energy

The t h e r m a l l y e m i t t e d r a d i a t i o n from t h e s i l i c o n

surface was observed i n a backward s c a t t e r i n g geometry w i t h a double monochromator and a p h o t o m u l t i p l i e r tube using a s i n g l e photon counting technique w i t h 400-ps time r e s o l u t i o n .

Measure-

ments were p o s s i b l e from 900 nm ( t h e p h o t o m u l t i p l i e r c u t o f f ) down t o about 500 nm (where t h e signal was l e s s than one photon per 500 l a s e r pulses).

6.

361


Setting the monochromator for specific wavelengths, Kemmler et al. measured the intensity of thermally emitted l i g h t over a 50-ns These observation time t h a t was divided i n t o 2-ns intervals. measurements were reanalyzed (correcting for the measured spectral response of the detection system) and then plotted a t specified times as the logarithm of intensity vs photon energy; the experimental points were found to l i e approximately on a straight line, from which the surface temperature was obtained by f i t t i n g w i t h Planck's Law, Eq. (6). Intensity ratios, a t different measurement times, were also found t o be in accord w i t h Planck's Law. The temperature prof i 1e resulting from such measurments , for a laser Ex = 0.54 J/cm2, i s shown in Fig. 16. The beginning and end of the HRP (marked by the arrows) are seen t o delineate very 1.7 1.6 17

(.r? 1.5

g

U

w 1.4

a

t

3

1.3

a

3I-

1.2 1.1

1.0

0.9

-10

-5

0

5 10 TIME [nsJ

15

20

25

Fig. 16. Solid circles: measured pyrometric temperature vs time. (The solid line i s a guide for the eye.) Dashed curve: laser heating pulse. The arrows mark the beginning and end o f the high r e f l e c t i v i t y phase (Kemmler e t al., 1984).

362


accurately t h e period of time f o r which surface temperatures of 1500-1700 K were measured. The i n i t i a l temperature peak r e s u l t s from overheating of the l i q u i d surface during absorption of the 1a s e r pul se , whi 1e the l a t e r plateau represents the equi 1b r i um temperature of t h e melt. The difference between t h e measured 1500 K and Tc = 1685 K f o r molten s i l i c o n was presumed t o be due t o a systematic e r r o r in the absolute temperature s c a l e , a r i s i n g primarily from uncertainties in the filament temperature of the tungsten lamp t h a t was used t o c a l i b r a t e t h e detection system; t h i s e r r o r was estimated t o be t300 K. Redefining t h e plateau temperature as being Tc = 1685 K r e s u l t s in a maximum l i q u i d s i l i con temperature, near the beginning of the HRP, of nearly 2000 K (Kern1er e t a1 , 1984). Kemmler e t a l . also point out t h a t t h i s type of thermal emission measurement has two important advantages f o r the determi nation of surface temperatures: (1) Spatial averaging of temperatures, in the presence of large temperature gradients normal t o t h e sample surf a c e , i s minimal because of t h e small escape depth of v i s i b l e thermal r a d i a t i o n ; and, ( 2 ) s p a t i a l averaging of temperatures i s f u r t h e r reduced by t h e strong increase i n thermally emitted i n t e n s i t y with temperature, which biases the measurement toward t h e highest temperature region. This l a t t e r behavior i s precisely the opposite of the s i t u a t i o n t h a t occurs i n pulsed Raman experiments, f o r which t h e Raman s c a t t e r i n g efficiency decreases rapidly with increasing l a t t i c e temperature, which may lead t o an underestimate o f surface temperature. Thus , thermal emission measurements seem especially we1 1 -suited f o r the determination of surface temperatures under t h e conditions of l a r g e temporal and s p a t i a l temperature gradients t h a t prevail in typical pulsed l a s e r experiments.

.

111.

Pulsed Raman Scattering Measurements

Much of the controversy regarding the physical mechanism f o r pulsed l a s e r annealing originated in a s e r i e s of time-resolved Raman s c a t t e r i n g measurements t h a t were intended t o d i r e c t l y probe

6.


363

the temperature of silicon during and immediately a f t e r the laserThe i n i t i a l experiments were induced high reflectivity phase. carried out by Lo and Compaan (1980a, 1981) but were subsequently repeated w i t h substantial changes in experimental apparatus (Compaan e t al., 1982a) and at different probe wavelengths (von der Linde and Wartmann, 1982, von der Linde et. a1 1983b). I t i s possible for Raman experiments t o measure l a t t i c e temperature because the Raman interaction involves either the absorption (Stokes, S, process) or the emission (anti-Stokes, AS, process) of a phonon. The population o f phonons in the l a t t i c e i s

.,

= l/Cexp(h~~/kT)-ll

no(wo,T)

(7 1

where wo i s the phonon frequency, T i s the l a t t i c e temperature, and k i s Boltzmann's constant. Since the intensity of the Stokes process will be proportional t o (no + l ) , while the intensity of the anti-Stokes process will be proportional t o no, the Stokes t o antiStokes intensity ratio, R(S/AS) , provides a temperature probe. If R(S/AS) were t o depend only on phonon population, then R(S/AS)

=

F ( n o + l ) / n o = F exp(h%/kT)

(8 1

w i t h the prefactor F = 1. In general, however, F i s a function of temperature and of the optical properties of the material being studied. I t is given for an experiment a t constant temperature by F(T) =

-

7 oana

(91

where w i s the photon frequency, u i s the Raman matrix element, a i s the optical absorption coefficient, n is the index of refraction and R is the reflectivity, for the subscripted processes. The subscript L refers t o the probe laser frequency and the barred If the quantities are the ratios indicated by the notation.

364


sample i s not a t a constant temperature, as i s the case f o r a general laser-annealing experiment, t h e n Eq. (8) cannot be used and a much more complicated formulation must be employed (see J e l l i s o n et a l . , 1983b). [Of course, i f temperature variations a r e not too severe, the use of Eq. (8) may be j u s t i f i e d . ] In this section, we shall b r i e f l y discuss the experiments of e Compaan and co-workers and of von der Linde and co-workers. W s h a l l a l s o discuss several complications involved i n these measurements. I t i s not possible t o give a d e t a i l e d accounting here o f t h e controversy t h a t has resulted from these measurements; instead, we include a comprehensive l i s t of references t o which t h e interested reader i s referred: Lo and Compaan, 1980a, b y 1981; Compaan e t a1 , 1982a, b y 1983a, b; Compaan and Trodahl , 1984; Lee et a1 , 1982; Bhattacharyya et a1 1982; von der Linde and Wartmann, 1982; von der Linde et a l . 1983a, b, 1984; Kemmler e t a1 1984; Wood e t al., 1982a, b y c; J e l l i s o n , et a l . , 1983a, b; J e l l i s o n and Wood, 1984.

.

.,

5.

EXPERIMENTAL RESULTS

a.

Compaan and Co-workers

.

.,

Measurements of the Stokes t o anti-Stokes i n t e n s i t y r a t i o were f i r s t made by Lo and Compaan (1980a), from which they inferred a l a t t i c e temperature of only 3OOOC in c-Si some 10 nsec a f t e r a 1.0 J/cm2 heating pulse, which was s u f f i c i e n t t o produce a t r a n s i t i o n t o t h e HRP, normally interpreted as indicating melting. A twobeam configuration was used f o r these experiments, consisting of an intense heating beam ( h = 485 nm, 90 gm diameter) and a weak probe beam ( h = 405 nm, 50 pm diameter), with the probe beam delayed in time by 10 nsec. The pulse widths of both the heating and probe beams were -7 nsec. The probe beam energy was 0.06 J/cm2, which was s u f f i c i e n t by i t s e l f t o increase the l a t t i c e temperature by -15OOC. Pulse-to-pulse energy v a r i a t i o n s were measured in the

6.

365


center o f each l a s e r beam and were reported t o be tlOX and 27% f o r t h e heating and probe l a s e rs , re s p e c ti v e l y .

A l a t e r refinement o f t h e experiment (Compaan e t al., r e s u l t e d i n improved s p a t i a l and temporal

1982a)

resolution.

These

experiments used a frequency-doubled Nd:YAG l a s e r ( A = 532 nm, 20-nsec pulse, -1 0 0 0 -p diameter) as t h e heating beam, and a l a r g e r diameter probe beam ( A = 405 nm, -200-p diameter, -7-nsec

pulse),

which reduced considerably t h e heating e f f e c t s o f the probe beam. A v a r i a b l e e l e c t r o n i c delay was used t o t r i g g e r t h e probe pulse

a t various times a f t e r th e heating pulse, and t h e sample r e f l e c t i v i t y was monitored w i t h a cw Ar+-ion l a s e r (A = 514 nm).

In

order t o o b t a i n a reasonable signal -to-noise r a t i o , roughly 10,000 l a s e r shots were averaged. I n t h e l a t t e r set o f experiments, no Raman signal was observed f o r t h e f i r s t 80 nsec, corresponding t o t h e d u r a t i o n o f t h e h i g h r e f 1e c t i v i t y phase.

The f i r s t observable Raman signal was observed

w i t h a 110 nsec delay, where t h e r e f l e c t i v i t y had returned t o i t s o r i g i n a l value; using Eq. (8), a temperature o f from R(S/AS).

-4OOOC

was obtained

Compaan e t a l . (1982a) concluded t h a t " t h e f a c t t h a t

t h e l a t t i c e temperature i s so low immediately a f t e r t h e h i g h r e f l e c t i v i t y period shows t h a t t h i s enhanced r e f l e c t i v i t y phase cannot be t he usual 1400°C molten phase o f s i l i c o n unless u n r e a l i s t i c a l l y l a r g e c o o l i n g r a t e s are assumed."

I n a s i m i l a r experiment, Lo and

Compaan (1981) measured R(S/AS) f o r samples implanted w i t h 200 keV As ions a t a dose of lO15/cm2 (a-Si); they r e p o r t e d t h a t t h e highest temperature obtained was 600 k 200°C a t 20 nsec a f t e r t h e end o f t he HRP. Compaan e t a l . r e versal

(1982b) have used a technique i n v o l v i n g time-

invaria n c e t o determine e x p e ri m e n t a l l y t h e c o r r e c t i o n

f a c t o r needed t o re1 a t e the Stokes-to-anti-Stokes Raman i n t e n s i t y r a t i o t o the l a t t i c e temperature. Three measurements were performed: (1) t he Stokes and (2) th e anti-Stokes i n t e n s i t y measurements a t t h e o r i g i n a l probe wavelength, and (3) t h e Stokes i n t e n s i t y

366


measurement a t a probe wavelength equal t o t h e previous anti-Stokes wavelength.

The c o r r e c t i o n f a c t o r i s t h e r a t i o o f t h e i n t e n s i t i e s

i n experiments (1) and (3) i f i t can be assumed t h a t e i t h e r t h e Raman m a t r i x element o f experiment (2) equals t h a t o f experiment (3),

o r i f t h e i n p u t and output channels o f experiment (3) are

reversed from those o f experiments (1) and (2). b.

von der Linde and Co-Workers The measurements o f von der Linde and co-workers (von der Linde

and Wartmann, 1982; von der Linde e t a1 one- and two-beam c o n f i g u r a t i o n s .

., 1983b,

1984) used both

I n t h e two-beam c o n f i g u r a t i o n ,

t h e heating beam was a frequency-doubled Nd:YAG l a s e r ( h = 532 nm, 0.5 mn diameter, -10-nsec

pulse), w h i l e t h e probe beam was s p l i t

o f f from t h e 1.06-w Nd:YAG fundamental and then f r e q u e n c y - t r i p l e d ( h = 355 nm).

beam was used.

I n t h e one-beam experiment, o n l y t h e 532-nm h e a t i n g The Raman-scattered photons were detected w i t h a

f a s t (-1 nsec) p h o t o m u l t i p l i e r , a l l o w i n g measurements o f t h e Stokes and anti-Stokes i n t e n s i t i e s t o be made as a f u n c t i o n o f time.

This

experiment d i f f e r s from those o f Compaan and co-workers i n t h a t Raman s c a t t e r i n g was measured continuously as a f u n c t i o n o f time, b u t o n l y f o r times when t h e l a s e r pulse was on.

The measurements

o f von der Linde and co-workers also r e q u i r e d signal averaging over several thousand laser shots. I n t h e f i r s t s e t o f experiments, von der Linde and Wartmann (1982) adjusted t h e spectrometer bandw i d t h t o 0.5 M t o c o l l e c t most o f t h e Stokes o r anti-Stokes s h i f t e d photons.

I n t h e i r second s e t o f experiments (von der Linde e t al.,

1984), they reduced t h e bandwidth o f t h e i r spectrometer and recorded t h e e n t i r e Raman spectrum. von der Linde and Wartmann i n f e r r e d a maximum temperature o f -800 K using j u s t t h e heating pulse ( s i n g l e beam experiment), but a maximum temperature o f -1600-3000 K from t h e two-beam experiment. These r e s u l t s were obtained using Eq.

( 8 ) , w i t h F = 0.95 f o r t h e

single-beam experiment, and F = 2.5 f o r t h e two-beam experiment,

6.

367


determined from room-temperature measurements of

G

(Renucci et a1

.,

1975) and of a (Jellison and Modine, 1982b). I n a l a t e r revision of these inferred temperatures, von der Linde e t al. (1983a, b ) determined the factor F as a function o f temperature, using constant-temperature measurements, and concluded t h a t the factor F decreased considerably as the temperature was increased for h = 355 nm photons; t h i s resulted in the two-beam experiments yielding a temperature of -600 K. In the experiments of von der Linde et al. (1984), both the time and frequency dependences o f the Raman-scattered photons were recorded. These experiments revealed that i n the experiments of von der Linde and Wartmann not all of the Raman-scattered photons were collected, due t o the spectrometer bandwidth t h a t was used. When the full line was recorded, von der Linde et a1 (1984) found t h a t the R(S/AS) measurements, as well as the Raman line shifts, resulted in temperatures that were consistent w i t h me1 t i ng model calculations, a1 though the associated error bars were very large.

.

6.

COMPLICATIONS I N PULSED RAMAN MEASUREMENTS

Several complications arise in attempting t o infer the l a t t i c e temperature from R(S/AS). a.

Pul se-to-Pul se Fluctuations

All of the pulsed Raman measurements performed t o date involved signal averaging, since the number of photons per laser pulse was general ly small (-0.5-10 photons/pul se) Signal averaging would be acceptable if each laser pulse were exactly the same and if the Stokes and anti-Stokes intensities were reproducible. Unfortunately, t h i s i s not generally the case; the degree of nonreproducibility of the laser pulses depends on the laser being used. However, a1 1 lasers exhibit pul se-to-pul se differences i n energy density, pulse shape, and t i m i n g . In cases of large temperature-time

.

368

D. H. LOWNDES ETAL.

o r temperature-di stance gradients,

signal averaging over these

f l u c t u a t i o n s may not be a good approximation. I n t h e experiments o f Compaan e t a l . (1982b), t h e heating pulse had an

Ex u n c e r t a i n t y o f -108, t h e probe pulse o f -5%.

Therefore,

I n addition,

nsec wide w i t h a time j i t t e r o f -4 nsec.

t h e probe pulse was -7

f o r probe delays very c l o s e t o t h e end o f t h e h i g h

r e f l e c t i v i t y phase, where t h e temperature-time g r a d i e n t i s extremely h i g h (-4 x 1O1O K/sec according t o m e l t i n g model c a l c u l a t i o n s , see Chapter 4), t h e temperature can vary several hundred degrees from p u l s e t o pulse and several hundred degrees from beginning t o end o f t h e probe pulse.

Signal averaging under these circumstances

i s suspect (see J e l l i s o n e t a1 b

.

., 1983b f o r d e t a i l e d c a l c u l a t i o n s ) .

Beam I nhomogenei t ies Heat w i l l d i f f u s e only -1-3

nsec d u r a t i o n o f t h e HRP.

i n s i l i c o n d u r i n g t h e -20-200-

Therefore, any transverse energy f l u c t u a -

t i o n s over distances g r e a t e r than several microns become important. Since it i s v i r t u a l l y impossible t o a s c e r t a i n t h e p r e c i s e energy

of t h e l a s e r pulse as a f u n c t i o n of time, transverse dimensions and pulse number, adequate r e p r e s e n t a t i o n o f these f l u c t u a t i o n s i n t h e general case i s impossible. c.

Temperature-Dependent Parameters I t i s now w e l l e s t a b l i s h e d t h a t t h e o p t i c a l f u n c t i o n s o f silicon

a r e very strong f u n c t i o n s o f temperature, p a r t i c u l a r l y f o r photon energies near t h e d i r e c t band gap (-3.4

eV a t room temperature; see

Chapter 3 and J e l l i s o n and Modine, 1983).

Furthermore, as t h e tem-

p e r a t u r e o f t h e s i l i c o n l a t t i c e i s increased, t h e energy p o s i t i o n o f t h e d i r e c t bandgap moves t o lower photon energies, and many o f t h e s p e c t r a l f e a t u r e s o f t h e o p t i c a l f u n c t i o n s become broadened. The r e f l e c t i v i t y and index o f r e f r a c t i o n can both change s i g n i f i c a n t l y as t h e temperature i s increased, changing t h e value o f t h e p r e f a c t o r F [see Eq.

(9) and Fig.

171 away from 1; however, the

6.


369

1.6 1.4

1.2 I.o

0.8 0.6 3.0

2.5 2.0 1.5

1.o 1

200

I 400

I

I

I

I

600 800 TEMPERATURE (K1

1

I 1000

Fig. 17. Correction factor ratios a, 6, R , and n for the Stokes to anti-Stokes intensity ratio [Eq. (9)] vs temperature for the two probe wavelengths 405 and 355 nm (Jellison et al., 1 9 8 3 a ) .

largest effects are observed i n the optical absorption coefficient (Jellison and Modine, 1982a; see Chapter 3 , Fig. 2) and the Raman matrix element (Jell ison e t a1 , 1983a; Compaan and Trodahl , 1984). I t i s expected theoretically that the Raman matrix element CJ can be represented (Renucci et a l . , 1975) in the resonant region by the expression

.

a(E,T)

=

CD

a2

370


where C i s a numerical f a c t o r , D i s a l i n e a r combination of deformation p o t e n t i a l s , E i s the phonon amplitude [ = 1 / 2 + no], a i s t h e l a t t i c e constant, and E i s the complex d i e l e c t r i c function. versus photon energy, Figure 18 shows a plot of de(T)/dE determined from t h e optical data of J e l l i s o n and Modine (1982a, 1983) ( s e e Chapter 3, Fig. 1). Also shown in Fig. 18 are the data o f Renucci e t a l . (1975), taken a t 300 K. As can be seen, t h e f i t

I

I

I

I

Fig. 18. Raman matrix element squared compared with d e / d E for several temperatures. The arrows a t the bottom o f the figure indicate the positions o f the two probe wavelengths 4 0 5 and 3 5 5 nm, respectively. The data taken from Renucci e t a l . ( 1 9 7 5 ) a r e shown by the closed circles ( 0 ) (jellison et al., 1983a).

371

6. TIME-RESOLVED MEASUREMENTS i s s u r p r i s i n g l y good, except i n t h e r e g i o n near 3.1 eV. because Renucci e t al.

This i s

used t h e o l d e r a data o f Dash and Newman

(1955) i n reducing t h e raw Raman data; i f the more recent a data o f J e l l i s o n and Modine (1982b) i s used i n performing t h e data r e d u c t i o n i n t h i s r e g i o n (shown by t h e open t r i a n g l e s i n Fig. 18), a better fit o f

I de(T

= 300)/dE

I*

w i t h experimental data i s

obtained. I n a d d i t i o n t o t h e temperature dependences o f t h e o p t i c a l prope r t i e s , which g e n e r a l l y a f f e c t o n l y t h e coup1 i n g constants o f t h e Raman i n t e r a c t i o n , t h e r e also i s a temperature dependence t o t h e

r2,-, o p t i c a l phonon t h a t mediates t h e Raman i n t e r a c t i o n , thereby s t r o n g l y changing t h e l i n e p o s i t i o n and l i n e shape (Balkanski e t a1

., 1983).

I n experiments t h a t i n c l u d e a range o f temperatures

extending over several hundred degrees, these e f f e c t s should be in c l uded. d.

Stress E f f e c t s As described i n Section 11.3 o f t h i s chapter, time-resolved x-

r a y d i f f r a c t i o n measurements show t h a t t h e near-surface r e g i o n of t h e s i l i c o n l a t t i c e i s severely s t r a i n e d immediately a f t e r t h e HRP. This s t r a i n i s thought t o be due t o t h e simultaneous normal thermal expansion and l a t e r a l clamping o f t h e l a t t i c e ( t h e l a t t e r due t o t h e cool p a r t o f t h e sample o u t s i d e t h e l a s e r beam), r e s u l t i n g i n a u n i a x i a l s t r e s s w i t h symmetry a x i s perpendicular t o t h e sample surface. This s t r e s s s h i f t s t h e frequency o f t h e rZ5, o p t i c a l phonon and s p l i t s i t from a 3 - f o l d degenerate l i n e i n t o a s i n g l e t and a doublet.

Furthermore, t h e a p p l i e d s t r e s s reduces t h e symnetry

o f t h e near-surface r e g i o n from Oh ( f o r unstressed m a t e r i a l ) t o D2d ( f o r m a t e r i a l stressed along t h e d i r e c t i o n ) o r t o C3,, ( f o r m a t e r i a l stressed along t h e d i r e c t i o n ) .

One r e s u l t o f

s t r e s s i s t h a t t h e band s t r u c t u r e no l o n g e r possesses t h e unstressed symmetry, making t h e conduction band v a l l e y s i n e q u i v a l e n t , s p l i t t i n g t h e normally degenerate bands a t t h e

r

p o i n t o f the B r i l l o u i n zone,

372

D.H. LOWNDES E T A L .

and a s y m e t r i c a l l y s h i f t i n g t h e bands a t a general p o i n t i n t h e zone.

Other m a n i f e s t a t i o n s o f an a p p l i e d s t r e s s are t h a t t h e Raman

s c a t t e r i n g tensor, which i s normally s y m e t r i c f o r cubic, unstressed

m a t e r i a l , becomes r i g o r o u s l y nonsymmetric , and t h e o p t i c a l propert i e s are changed (Gobeli and Kane, 1965; Chandrasekhar e t al., 1978).

A1 though stress-induced b i r e f r i n g e n c e i s measurable a t a1 1

v i s i b l e wavelengths,

t h e e f f e c t becomes even l a r g e r f o r photon

energies near t h e d i r e c t bandgap. For pulsed l a s e r annealing experiments,

t h e stress-re1 ated

change i n t h e phonon frequency i s p o s i t i v e and can be as l a r g e as 15 an-1 f o r s i l i c o n a t t h e m e l t i n g p o i n t ( J e l l i s o n and Wood, 1984). I f s t r e s s i s not included (as was t h e case w i t h Compaan e t al.,

1983b) , then t h e temperature i n f e r r e d from l i n e - p o s i t i o n measurements w i l l be too low. e.

E f f e c t s o f Excess Electron-Hole P a i r s Large numbers o f e l e c t r o n - h o l e p a i r s are created d u r i n g pulsed

1 aser i r r a d i a t i o n of semiconductors using above-bandgap r a d i a t i o n . However, i n s i l i c o n Auger recombination very q u i c k l y reduces t h e e l ectron-hol e pai r concentrat i o n t o -1019 e-h pai rs/cm3 (see Sec. IV.9).

It i s known t h a t very h e a v i l y p-doped s i l i c o n e x h i b i t s a

Fano s h i f t (Fano, 1961) due t o t h e i n t e r a c t i o n between excess holes i n t h e continuous band s t a t e s and t h e d i s c r e t e o p t i c mode. Although i t i s known t h a t t h e combined e f f e c t s of u n i a x i a l s t r e s s and heavy

doping can a f f e c t t h e Raman lineshape and i n t e n s i t y i n s i l i c o n (Cerdeira e t al.,

1973), e f f e c t s s p e c i f i c t o t h e case o f pulsed

l a s e r annealing are not known a t t h i s time. f.

Resonant E f f e c t s I f Raman s c a t t e r i n g experiments are performed a t photon ener-

g i e s near c r i t i c a l p o i n t s i n t h e B r i l l o u i n zone, t h e n t h e Raman s c a t t e r i n g i n t e n s i t i e s are enhanced by resonant e f f e c t s .

The Raman

s c a t t e r i n g m a t r i x element becomes very l a r g e i n t h i s region, but

i t a1 so becomes c r i t i c a l l y dependent upon wave1 ength , temperature ,

6.


373

.,

and stress (Jellison e t a1 1983a; Compaan and Trodahl , 1984). In silicon, t h i s resonant enhancement occurs near the direct band-

gap (-3.4 eV or 370 nm). When pulsed Raman experiments are performed w i t h probe photon energies in the resonant Raman regime, extreme care must be taken t o include all these effects. Unfortunately, much o f the required data i s not available so t h a t only guesses can be made concerning these effects. g.

Temperature-Depth Profile Effects

If the photon energy of the probe beam used in pulsed Raman measurements i s much smaller t h a n the direct bandgap, then reson a n t Raman effects are not very important. However, the probe light t h e n penetrates much deeper i n t o the material and can conceivably sample materi a1 whose temperature varies by hundreds of degrees. This i s the case for the single-beam experiments of von der Linde e t al. (1983b, 1984), for which both the heating and probe beam are at 532 nm. In t h i s case, the probe beam penetrates -2 pm a t room temperature, b u t only -0.4 pn at 700°C (Jellison and Modine, 1982a). Since thermal diffusion will n o t distribute heat from the front surface region t o depths greater than -0.5 p during the duration of the -10-nsec laser pulses used in Raman scattering experiments, large spatial temperature gradients are set up (see Chapter 4, Fig. 7 ) . These gradients are extremely important for probe-beam photon energies t h a t sample deep into the material (e.g., 532 nm), b u t are not nearly as important for highly absorbed probe wavelengths (such as 405 n m ) , for times a f t e r the HRP. 7.

SUMMARY:

PULSED RAMAN MEASUREMENTS

We have described recent pul sed Raman temperature measurements performed during pulsed laser irradiation of silicon. These measurements can be generally classified by the probe wavelength used: (1) Resonant experiments, where the probe laser wavelength i s near or above the direct band edge and ( 2 ) nonresonant experiments,

374


where the probe laser wavelength i s well below the direct band edge. Fluctuations of the pulse-to-pulse energy density, beam inhomogeneities, and stress (Secs. III.6aY b y and d ) complicate both types of measurements; in addition, the resonant experiments are complicated mainly by the temperature dependence of quantities entering i n t o R(S/AS) (Secs. 111.6~ and f ) , while the nonresonant experiments, because of the probe wavelength, must take the extremely large spatial temperature gradients (Sec. 111.6s) i n t o account. Therefore, there does not appear t o be an ideal probe wave1 ength for time-resol ved Raman temperature measurements i n pulsed laser-irradiated silicon. I t is our belief t h a t these complications are so numerous, and so difficult t o treat, t h a t the l a t t i c e temperatures inferred from all R(S/AS) measurements t o date are subject t o large errors. I n particular, apparent low l a t t i c e temperatures should n o t be believed in the face of the overwhelming evidence of other experiments (see Secs. I1 and IV) t h a t indicate t h a t silicon does i n fact melt if laser energy sufficient t o produce the HRP i s incident upon the silicon surface. The HRP can therefore be identified w i t h the normal no1 ten phase of si 1 icon. IV.

Energy Transfer f r o m Optically Excited Carriers t o the Crystal Lattice

The central assumption of the thermal melting model calculations described i n Chapter 4 is t h a t the transfer of energy from laser-excited carriers t o the crystal l a t t i c e takes place i n a time t h a t is comparable t o or less t h a n the nanosecond or longer d u r a t i o n light pulses used for laser annealing of semiconductors. However, the time scale for establ ishment of this local thermodynami c equi 1 ib r i urn, between 1aser-exci ted carriers and the phonon system, is itself of fundamental interest; this time scale also determines the ultimate limit of applicability of a thermal melting model.

6.

375


A d e t a i l e d understanding o f t h i s thermal i z a t i o n process a c t u a l l y r e q u i r e s t r e a t i n g two processes t h a t occur simul taneously:

(1) The

dynamics o f a hot e l e c t r o n - h o l e plasma whose d e n s i t y may be varying r a p i d l y i n time and (2) t h e proce5s o f energy t r a n s f e r from these e x c i t e d c a r r i e r s t o t h e l a t t i c e v i a t h e electron-phonon i n t e r a c t i o n . Bloembergen e t al. (1982) and Yoffa (1980) have discussed t h e carr i e r generation and r e l a x a t i o n processes t h a t are i n v o l v e d and have estimated t h e i r c h a r a c t e r i s t i c t i m e scales (see Chapter 4). B r i e f l y , these are as f o l l o w s :

E l e c t r o n - h o l e p a i r s are produced by t h e

absorption o f l a s e r l i g h t i n d i r e c t o r i n d i r e c t t r a n s i t i o n s across t h e energy bandgap; f r e e - c a r r i e r i n c r e a s i n g c a r r i e r density.

absorption a1 so increases w i t h

The hot c a r r i e r s r e s u l t i n g from both

processes then thermalize, w i t h o t h e r c a r r i e r s and w i t h t h e l a t t i c e .

A common c a r r i e r temperature f o r e l e c t r o n s and holes i s expected t o be e s t a b l i s h e d i n w (imaginary np and high r e f l e c t i v i t y ) a t high excitation. Shank e t al. (1983a) used the expression for wP and the magnitude of the reflectivity change t o estimate N = 5 x 1021/cm3 for an excitation of 0.63Eth, assuming m* equal t o the free electron mass. [Other estimates of m* indicate t h a t a value 2-4 times smaller, and a corresponding reduction

6. TIME-RESOLVED MEASUREMENTS

379

I

I

0

-03 -

Fig. 19. Transient r e f l e c t i v i t y data at three probe wavelengths following femtosecond excitation. The solid lines a t 0.63 Eth are calculated assuming c a r r i e r diffusion into the bulk; f o r E > Eth, the calculations use the thin-film melting model. The decrease in r e f l e c t i v i t y a t the highest excitation i s due t o surface damage; the dashed curve i s a guide t o the eye (Shank e t al. , 1983a).

380 in al.


N, i s more n e a r l y c o r r e c t ; see van O r i e l (1984) and Lomprg e t (1984b)l.

Shank e t al.

(1983a) a l s o c a l c u l a t e d t h e decay o f

t h e r e f l e c t i v i t y a t 0.63 Eth f o r models based on c a r r i e r d i f f u s i o n and on Auger recombination; t h e y a t t r i b u t e d t h e observed slow decay (Fig. 19a) t o c a r r i e r d i f f u s i o n and concluded t h a t t h e i r measurements provide evidence f o r t h e s a t u r a t i o n o f Auger recombination a t h i g h c a r r i e r d e n s i t i e s , as described by Yoffa (1980).

This

conclusion has been challenged by Combescot and Bok (1983) who used a lower value o f

N and Auger recombination t o o b t a i n good

agreement w i t h t h e r e f l e c t i v i t y decay observed by Shank e t a1

.

The increase o f t h e r e f l e c t i v i t y t o a plateau value a f t e r a

.

few picoseconds (Fig. 19a) i s explained by Shank e t a1 as r e s u l t i n g from a t h i n l a y e r o f molten s i l i c o n forming on t h e sample surface and expanding i n t o i t s b u l k w i t h a v e l o c i t y

V i a

By t a k i n g t h e

o p t i c a l constants o f t h i s expanding f i l m t o be those o f molten silicon,

and using

Vi

and t h e plateau value o f r e f l e c t i v i t y as

f r e e parameters, Shank e t a l . (1983a) were able t o simultaneously f i t t h e t h r e e sets o f r e f l e c t i v i t y data i n Fig. 19, f o r each e x c i -

t a t i o n i n t e n s i t y , using a s i n g l e o f 6.2

x

lo3,

9 x

lo3,

and 2.5

Vi

value.

They obtained

Vi

values

x lo4 m/s f o r e x c i t a t i o n s o f 1.0

Eth, 1.26 Eth, and 2.5 Eth, r e s p e c t i v e l y .

The phenomenon of a m e l t -

i n v e l o c i t y p r o p o r t i o n a l t o e x c i t a t i o n i n t e n s i t y i s w e l l known from laser-anneal i n g experiments and model c a l c u l a t i o n s on t h e nanosecond time scale, though a t much lower v e l o c i t i e s (see Chapter 4,

Fig.

9 and Thompson e t al.,

value obtained by Shank e t al.

1983a).

However, t h e lowest

V i

i s already near t h e v e l o c i t y o f

sound i n c r y s t a l l i n e s i l i c o n ; they speculate t h a t a g r a d i e n t i n m e l t i n g r a t e vs depth may e x i s t .

A d i r e c t measurement o f t h e time scale f o r t h e disappearance o f c r y s t a l l i n e s t r u c t u r e was also made by Shank and c o l l a b o r a t o r s (1983b), who used a 90-fs, 620-nm o p t i c a l pulse t o e x c i t e a (111) c r y s t a l l i n e s i l i c o n surface and then measured t h e second harmonic r a d i a t i o n generated by a second much weaker and time-delayed probe

6. pul se.

381


S i nce s i 1i c o n has i n v e r s i o n symmetry , second harmonic gen-

e r a t i o n r e q u i r e s higher-order terms i n i t s nonlocal o p t i c a l susc e p t i b i l i t y , i n c l u d i n g both i s o t r o p i c and a n i s o t r o p i c second harmonic c o n t r i b u t i o n s ; it i s t h e l a t t e r , a n i s o t r o p i c ,

terms which

c a r r y i n f o r m a t i on regarding t h e exi stence o f c r y s t a l 1 ine symnetry (Bloembergen e t a1

., 1968;

Shank e t a1

., 1983b).

Although t h e

exact o r i g i n o f t h e second harmonic r a d i a t i o n i s s t i l l a subject f o r study, it i s known t o be generated i n t h e experiments o f Shank and co-workers w i t h i n a near-surface l a y e r t h a t i s no m r e than

70 A deep,

corresponding t o t h e escape depth f o r t h e i r second

harmonic l i g h t . Figure 20 shows t h e observed second harmonic r a d i a t i o n as a f u n c t i o n o f time and o f t h e angle o f r o t a t i o n 4 about t h e axis;

t h r e e - f o l d symmetry,

i n t e r v a l s o f 120 degrees,

w i t h maxima and minima repeating a t i s apparent a t a low e x c i t a t i o n o f 0.5

Eth (Fig. 20a) where Eth = 0.1 J/cm2 i s t h e t h r e s h o l d f o r formation

o f an amorphous surface l a y e r , i n d i c a t i v e o f melting.

However,

a t a higher e x c i t a t i o n of 2.0 Eth, t h e second harmonic r a d i a t i o n

measured a t t h e maximum ( 6 = 120O) d r a s t i c a l l y decreases w h i l e t h a t a t t h e minimum p o s i t i o n ( 4 = 60')

increases s l i g h t l y b u t decreases

again w i t h i n about 500 fs (Fig. 20c).

As shown i n Fig. 20b, t h e

surface has l o s t considerable order a f t e r 240 fs; t h e second harmonic r a d i a t i o n becomes completely i s o t r o p i c w i t h i n 1 ps.

Shank

e t a l . (1983b) speculate t h a t since t h e second harmonic energy (4.0 eV) i s very c l o s e t o a sharp absorption peak o f s i l icon, t h e l a r g e decrease i n i n t e n s i t y i s probably due t o t h e l o s s o f resonant enhancement o f second harmonic generation t h a t would be expected t o accompany m e l t i n g . I n conclusion, t h e measurements o f Shank and c o l l a b o r a t o r s demonstrate a loss of c r y s t a l l i n e order on t h e (111) s i l i c o n surf a c e t h a t i s c o n s i s t e n t w i t h m e l t i n g o c c u r r i n g on a time scale of s u b s t a n t i a l l y l e s s than one picosecond. T h e i r r e f l e c t i v i t y measurements show t h a t t h e m e l t i n g process i s apparently i n i t i a t e d a t

382


a1

120.

1.2

r

I

-1

0

I

I

I

1

I

2

3

4

J

5

t (PSI

Fig. 20. Polar plot of second harmonic intensity vs angle 41, at several times, for excitation energies o f ( a ) 0.5 Eth and ( b ) 2.0 Eth. ( c ) Normalized second harmonic intensity vs time for an excitation energy o f 2.0 Eth. Upper curve: 4 = 120° (maximum). Lower curve: 4 = 60° (minimum) (Shank et a l . ,

1983b).

6.


383

the surface, b u t is preceded by formation of a dense (>1021/cm3) electron-hole plasma in the near-surface region. 9.

OBSERVATIONS OF THE OPTICALLY EXCITED ELECTRON-HOLE PLASMA IN SILICON

From Eq. (11) and the expression for the plasma frequency, 9, i t follows t h a t a plasma-induced high reflectivity signal is expected only for optical probing frequencies w < 9.For E = 11.8 (the high-frequency dielectric constant of silicon), m* = 0.3 m, (mo = free electron mass) and w i t h N (=Ne 'Nh) = 5 x 1020/cm3being the electron-hole pair density, this cutoff frequency for the observation of plasma effects corresponds t o photon energies less t h a n 1.55 ev or t o probing wavelengths longer t h a n about 800 nm. Furthermore, since N varies both during creation and decay of the plasma, optical probing a t near-i nfrared wave1 engths should reveal two reflectivity minima, on either side ( i n time) of a plasmainduced reflectivity maximum (Lietoila, 1981). This effect provides a characteristic "signature" for plasma formation and decay and a sort of "holy grail" for experimentalists, w i t h which t o unambiguously distinguish between plasma formation and melting i n timeresolved reflectivity measurements. van Driel e t al. (1984) have pointed out that although a pl asma-i nduced ref1 ectivi t y mi nimum has been observed i n several semiconductors, the expected plasmon-resonance reflectivity maximum has not been observed in all cases. (However, see Gallant and van Driel (1982) for such observations in germanium.) For silicon, the plasma-induced reflectivity minimum was not observed until 1982, apparently because of the limited time resolution of e a r l i e r measurements. Figure 21 shows results of time-resolved reflectivity ( R ) and transmission ( T ) measurements carried o u t by von der Linde and Fabricius (1982) using 25-ps, 532-nm pulses for excitation of (100) silicon, and using weaker, time-delayed probe pulses at 1.06 pm, with the probe pulses focused t o a small central part of the

384 1.0 L

I

I I

-

0.6‘k

a4

-

0.2

-

0

.-

I

-

0.8

I I

It

.

&

:

c-Si

o.xi~m-2

. -. - - -‘I’. *

1

-150 -100

-50

50

0

100

150

200

DELAY TIME [ps]

I

1.0 0.8

I I I I I

-

ol

r-Si

I

I

-150

-100

-50

I

0

I

50

100

0.11 Jcm-2

150

200

DELAY TIME [ p s ]

Fig. 21. Reflectivity and transmission vs time delay for excitations of ( t o p ) 0.35 J / c m 2 and (bottom) 0.11 J / c m 2 (von der Linde and Fabricius, 1982).

excited area of the sample surface. von der Linde and Fabricius found t h a t there was a d i s t i n c t energy density threshold a t E t h = 0.21 (+ 0.01) J/cm2, separating two entirely different types of R and T behavior. For E l > E t h (Fig. 21, t o p ) the i n i t i a l 32%reflect i v i t y drops t o 28% about 20 ps before the peak of the excitation pulse, then rises t o a flat-topped plateau value of 76%, equal t o the r e f l e c t i v i t y of molten silicon a t 1.06 pm; simultaneous with the l a t t e r transition, the T signal drops t o about 5%, uncorrected f o r photoluminescence (see Fig. 4) or for l i g h t scattered from

6. evaporating material


(Liu

385

et al., 1982b), both of which may be

recorded along with transmitted probe light. I n contrast, for ER E t h (Fig. 21, bottom) a distinct R minimum was observed about 10 ps after the excitation pulse, with no l a t e r transition t o a

E t h can only be due t o melting: Attributing i t t o the plasma would require a three-orders-of-magnitude increase i n the plasma lifetime over only a 10%range in ER (von der Linde and Fabricius, 1982). Similar R results t o those shown in Fig. 21 were obtained for bulk sil icon by L i u e t a1 (1982b), who also carried out R and T measurements using SOS specimens. They point

.

o u t that the t h i n ( 4 . 5 pn) Si film provided by SOS samples is useful in t h a t i t minimizes carrier diffusion effects while promoting uniform temperatures and plasma profiles within the penetration depth of the excitation pulse; multiple interferences of the probing pulse at the air-sil icon and silicon-sapphire interfaces also result i n h i g h sensitivity t o time-dependent plasmainduced changes i n the complex index of refraction of silicon. However, the interpretation of T measurements i s a1 so made slightly less direct by the presence of these multiple reflections from the f r o n t and back surfaces of specimens (see also Lowndes e t al., 1982b). More quantitative information about the behavior of optically induced electron-hole plasmas in silicon can also be obtained from exci te-and-probe experiments by varying the frequency of the probing pulse, as demonstrated by Lompr6, Liu, Kurz, Bloembergen 1984a, b; van Driel e t al., 1984). and van Driel (Lompr6 e t a1

.,

386


I n p a r t i c u l a r , changes i n o p t i c a l p r o p e r t i e s due t o v a r i a t i o n s i n l a t t i c e temperature (Lomprb e t al.,

1983) can be separated from

changes due t o v a r i a t i o n s i n c a r r i e r d e n s i t y (Lomprb e t al., 1984b; van O r i e l e t a1

., 1984).

Assumi ng a Drude treatment of t h e e l e c t r o n - h o l e p l asma, t h e r e a l and imaginary p a r t s o f t h e complex d i e l e c t r i c constant ( T = €1 + i

~ may ~ be w r) i t t e n as

- k[

E~

= nL

E~

= 2nL k L

L

- aN/w2 ,

(12)

+ bN/w3 ,

(13)

where

and where R = n + i k , t h e absorption c o e f f i c i e n t a = 4nk?/, t h e d e n s i t y of e l e c t r o n - h o l e p a i r s , me (mh) and t h e e l e c t r o n ( h o l e ) mass with

m* =

(m,'

+ mi1)-'

N is

()

are

and mean s c a t t e r i n g time, r e s p e c t i v e l y , and

the subscript

frequency l a t t i c e p r o p e r t i e s of

L refers

t o the high

(unexcited) c r y s t a l l i n e s i l i c o n

i n t h e absence o f a laser-induced plasma. From Eqs. (12)-(15) i t f o l l o w s t h a t (1) EL increases w i t h l a t t i c e temperature (dn/dT > 0) b u t decreases w i t h N, w h i l e (2) and N.

E~

increases w i t h both temperature

These dependences can be separated e x p e r i m e n t a l l y by

c a r r y i n g out measurements b o t h a t s h o r t probe wavelengths (e.g., 532 nm),

f o r which plasma c o n t r i b u t i o n s become small,

and f o r

l o n g e r wavelengths (2-3 p ) , f o r which plasma e f f e c t s dominate. Thus, time-resolved n e a r - i n f r a r e d r e f l e c t i v i t y and transmission measurements a c t u a l l y provide a means f o r studying t h e time evolut i o n o f t h e c a r r i e r d e n s i t y i n l a s e r - e x c i t e d e l e c t r o n - h o l e plasmas, and are not r e s t r i c t e d t o simply d i s t i n g u i s h i n g between plasma f o r m a t i o n and melting.

However, o p t i c a l measurements are not able

6.

387


t o determine N and m* s e p a r a t e l y , b u t o n l y t h e r a t i o N/m*, t h e s c a t t e r i n g t i m e s ( < T ~ > , ) a l s o e n t e r Eqs.

since

(12)-(15)

as

unknown parameters. van D r i e l e t a l .

(1984) r e c e n t l y used t i m e - r e s o l v e d R and

T

measurements t o observe t h e plasmon resonance i n s i l i c o n and i n germanium, pm.

v i a measurements a t probe wavelengths o f 1.9

The plasmas were

pul ses,

excited

and 2.8

u s i n g

t

,

\

1

I

I

I

I

I

BULK SILICON WAFER

Ib)

0.04J/cm* REFLECTIVITY A TRANSMISSION

at 2.8pm

-

_---*------J -a

Fig. 22. Reflectivity and transmission o f bulk silicon as functions o f probe t i m e delay, for probe wavelengths of ( a ) 1.9 km and ( b ) 2.8 pm and excitation pulse ( 5 3 2 n m ) energy densities of 0.015 and 0.04 J / c m 2 .

6.

389


t i m e s c a l e o f t h e i r experiments.

The f i r s t p u l s e (20 ps, 530 nm)

was used t o c r e a t e a plasma; a second p u l s e (30 ps, 1.06 pm) a t a f i x e d t i m e d e l a y o f 100 ps was used t o add energy t o t h e plasma by f r e e c a r r i e r a b s o r p t i o n ; a t h i r d p u l s e t h e n probed t h e r e s u l t i n g changes i n R and T as a f u n c t i o n o f i t s own t i m e delay.

The d a t a

r e s u l t e d i n changes i n R and T t h a t c o u l d be e x p l a i n e d e n t i r e l y by l a t t i c e h e a t i n g (see Sec. i n c r e a s e o f N.

11.4.b),

w i t h no evidence o f an

T h i s r e s u l t enabled Lompr6 e t a1

. (1984)

t o cal-

c u l a t e t h e h i g h e s t p o s s i b l e plasma d e n s i t y t h a t can be reached, assuming instantaneous thermal i z a t i o n o f a 20-ps, 0.1-J/cm2, e x c i t a t i o n p u l s e and no impact i o n i z a t i o n .

530-nm

The peak c a r r i e r den-

s i t y c a l c u l a t e d a t t h e s i l i c o n s u r f a c e was s l i g h t l y l e s s t h a n 8 x lO2O/crn3 ( i n good agreement with t h e o t h e r e s t i m a t e s above f o r lower e x c i t a t i o n levels).

V. 10.

S o l i d i f i c a t i o n o f H i g h l y Undercooled L i q u i d S i l i c o n

LIQUID-TO-AMORPHOUS PHASE TRANSFORMATION

I t i s now w e l l known t h a t t h i n l a y e r s o f amorphous s i l i c o n can be formed by u l t r a r a p i d s o l i d i f i c a t i o n from a s h a l l o w pool o f m o l t e n s i l i c o n t h a t i s produced by pulsed l a s e r i r r a d i a t i o n o f a c-Si surface; e i t h e r nanosecond pulses o f u l t r a v i o l e t l i g h t o r picosecond pulses o f v i s i b l e and n e a r - i n f r a r e d l i g h t may be used ( L i u e t al., 1979, 1981; Tsu e t a1 1979; C u l l i s e t a1 1982a, b; Boyd e t al., 1984). The amorphous phase forms as a r e s u l t o f u l t r a r a p i d

.,

.,

c o o l i n g o f a v e r y t h i n m o l t e n l a y e r by conduction t o t h e u n d e r l y i n g c r y s t a l l i n e substrate.

The c r i t i c a l i n f l u e n c e o f c o o l i n g r a t e on

amorphous phase f o r m a t i o n i s i l l u s t r a t e d by t h e f a c t t h a t t h e amorphous l a y e r can be formed o n l y i f t h e pulsed l a s e r f l u e n c e l i e s w i t h i n a narrow Ea window, j u s t above t h e m e l t i n g t h r e s h o l d fluence. Higher fluence r e s u l t s

i n m e l t i n g t o a s u f f i c i e n t depth t h a t

thermal energy s t o r e d i n t h e l i q u i d l a y e r prolongs t h e d u r a t i o n

390


o f m e l t ng, w i t h c o o l i n g and s o l i d i f i c a t i o n t h e n o c c u r r i n g s u f f i c i e n t l y s l o w l y t h a t e p i t a x i a1 regrowth from t h e c r y s t a l 1 ine substrate

e s u l t s ( L i u e t al.,

1979, 1981; Yen e t al.,

1982).

Thus,

amorphous phase f o r m a t i o n i s f a v o r e d by t h e use o f a pulsed l a s e r source having a s h o r t a b s o r p t i o n l e n g t h and s h o r t p u l s e d u r a t i o n , i n o r d e r t o minimize thermal d i f f u s i o n and produce t h e h i g h e s t p o s s i b l e temperature g r a d i e n t s .

Amorphous l a y e r s a r e a1 so formed

more e a s i l y on a (111) s i l i c o n s u r f a c e t h a n on (100).

However, a

q u a n t i t a t i v e understanding o f t h e d i f f e r e n c e s observed on (100) and (111) s u r f a c e s has n o t been achieved so f a r ,

because o f ( 1 )

u n c e r t a i n t i e s r e g a r d i n g t h e e x t e n t o f undercool i n g i n t h e me1t near t h e growing i n t e r f a c e (Jackson; see Poate, 1982) and ( 2 ) because o f fundamental d i f f i c u l t i e s i n m i c r o s c o p i c a l l y modeling i m p u r i t y s e g r e g a t i o n and c r y s t a l growth phenomena w i t h i n t h e growi ng i n t e r f a c e i t s e l f (Wood, 1982, 1983; Poate, 1982). Large me1t undercool ings (below Tc = 1685 K, t h e normal f r e e z i n g temperature o f c-Si ) a r e expected t o accompany t h e l a r g e temperature g r a d i e n t s and c o o l i n g r a t e s t h a t r e s u l t i n amorphous phase f o r m a t i o n , b u t t h e p r e c i s e r o l e o f m e l t u n d e r c o o l i n g i s not understood (see Chapter 5,

Sec.

Thompson e t a1

11).

., 1983a)

One model

(Spaepen and T u r n b u l l ,

1982;

d e s c r i b e s t h e process o f amorphous phase

f o r m a t i o n i n e s s e n t i a l l y thermodynamic terms,

and assumes t h a t

amorphous phase f o r m a t i o n can occur o n l y when t h e undercooled m e l t temperature fa1 1 s we1 1 below t h e me1t i ng temperature o f a-Si , Ta, which i s e s t i m a t e d t o l i e some 200-250 K below Tc (see t h e f o l l o w i n g s e c t i o n o f t h i s c h a p t e r and Chapter 4.12).

An a l t e r n a t i v e model

(Wood, 1983) emphasizes k i n e t i c , r a t h e r t h a n p u r e l y thermodynamic, effects.

Using k i n e t i c r a t e t h e o r y and t h e concept o f a r a t e con-

s t a n t w i t h an a c t i v a t i o n energy dependent on regrowth v e l o c i t y , t h i s model a t t r i b u t e s amorphization t o t h e " t r a p p i n g " o f macroscopic c o n c e n t r a t i o n s o f d e f e c t s a t a r a p i d l y moving i n t e r f a c e , when t h e r e g r o w t h v e l o c i t y exceeds some c r i t i c a l value; t h i s c r i t i c a l velocit y does n o t necessari l y correspond t o undercool ing t o be1 ow Ta.

6.

391


Thermal m e l t i n g model c a l c u l a t i o n s have been used t o estimate t h a t t h e liquid-to-amorphous phase t r a n s i t i o n occurs f o r melt-sol i d i n t e r f a c e v e l o c i t i e s g r e a t e r than about 18 m/s ( C u l l i s e t al., 1982b);

however,

undercooling.

these c a l c u l a t i o n s neglect t h e e f f e c t o f me1t K i n e t i c models f o r c r y s t a l growth,

on t h e o t h e r

hand, d i r e c t l y r e l a t e t h e regrowth v e l o c i t y t o t h e free energy d i f f e r e n c e between t h e l i q u i d phase and regrowing s o l i d phase and, w i t h some approximations, t o m e l t undercooling (see Chapter 5 and a l s o Spaepen and Turnbull,

1982, f o r discussions o f t h i s p o i n t ) .

Thus, d i r e c t measurements o f t h e m e l t - s o l i d i n t e r f a c e v e l o c i t y , under c o n d i t i o n s l e a d i n g t o amorphous regrowth , can i n p r i n c i p l e be used t o i n f e r t h e amount o f undercooling, i f t h e form o f t h e equation connect ing v e l o c i t y and undercool ing i s known. Thompson e t al.

(1983a) have r e c e n t l y a p p l i e d t h e t r a n s i e n t

conductance technique

(see Sec.

11.2)

t o d i r e c t l y measure t h e

v e l o c i t y o f t h e s o l i d i f y i n g m e l t - s o l i d i n t e r f a c e under c o n d i t i o n s such t h a t t h e liquid-to-amorphous Using 2.5-ns,

t r a n s i t i o n o f s i l i c o n occurs.

347-nm l a s e r pulses, amorphous regrowth was observed

o n l y f o r El between 0.2 and 0.3 J/cm2 (compare w i t h Fig. 9), w i t h a maximum depth o f a-Si o f 14 nm (measured by RBS and TEM) being obtained f o r El

=

0.27 J/cm2.

F i g u r e 23 (Thompson e t a1

., 1983a)

shows t h e regrowth v e l o c i t y p l o t t e d as a f u n c t i o n o f maximum melt depth.

By v i s u a l l y observing t h e r e f l e c t i v i t y change character-

i s t i c o f formation o f a-Si,

a f t e r t h e t r a n s i e n t conductance mea-

surements, Thompson e t al. were able t o associate t h e occurrence as

o f amorphization w i t h a p a r t i c u l a r regrowth v e l o c i t y range;

shown i n Fig. 23, amorphization o f a (100) SOS surface occurs f o r regrowth v e l o c i t i e s exceeding 15 m/s.

I f t h e measurement o f a c r i t i c a l amorphization v e l o c i t y o f 15 m/s i s i n t e r p r e t e d s t r i c t l y w i t h i n t h e l i m i t s o f t h e thermodynamic model f o r a-Si formation, 200-250

K (i.e.,

Thompson e t a1

then a melt undercooling o f a t l e a s t

t o Ta) i s i m p l i e d a t t h i s v e l o c i t y .

. (1983a)

However,

p o i n t out t h a t t h e i r measurements may a1 so

392


01

I

0

50

1

I

I50 Melt depth (nm) 100

I

200

250

Fig. 23. Regrowth velocity vs melt depth for pulsed uv-irradiated SOS. The hatched area and solid data points represent the regime in which a-Si forms from the melt (Thompson et al., 1 9 8 3 a ) .

p r o v i d e i n d i r e c t support f o r t h e idea t h a t t h e r e a r e s t r o n g k i n e t i c

1i m i t a t i o n s on n u c l e a t i o n o f t h e amorphous phase: Considerably more Si was found t o melt (40-50 nrn) and be subjected t o h i g h regrowth v e l o c i t i e s than t h e maximum amorphous l a y e r thickness o f 14 nm t h a t t h e y observed.

One e x p l a n a t i o n f o r t h i s i s given by TEM micro-

graphs t h a t revealed undulations i n amorphous l a y e r thickness and i s l a n d s of a-Si,

suggesting t h a t n u c l e a t i o n o f a-Si may be i n i -

t i a t e d by i n t e r f a c e breakdown a t h i g h regrowth v e l o c i t i e s , a k i n e t i c limitation.

A l t e r n a t i v e l y , t h e discrepancy between maximum

m o l t e n and amorphous l a y e r thicknesses could be explained p u r e l y thermodynamically

i f t h e r e were a s u b s t a n t i a l

delay t i m e [-30

nm/(15 m/s) = 2 ns] r e q u i r e d t o undercool t h e l i q u i d t o Ta (Thompson e t a1

., 1983a).

6.

393


Closely r e l a t e d measurements o f m e l t - s o l i d i n t e r f a c e v e l o c i t y were also c a r r i e d out by Bucksbaum and Bokor (1984), who used a 248-nm, 15-psec m e l t i n g l a s e r pulse and a delayed 1.64-pm i n f r a r e d probe pulse t o determine t h e thickness o f t h e molten s i l i c o n f i l m on t h e picosecond time scale, using both r e f l e c t i v i t y and t r a n s mission measurements.

P r o f i l e s of m e l t depth vs time were obtained

f o r both (100) and (111) c-Si surfaces f o r fluences o f 0.03-0.11 J/cm2, corresponding t o maximum m e l t depths o f 2-55 nm, melt durat i o n s from -200 ps t o >2 ns, and r e s u l t i n g i n a n e a r l y constant s o l i d i f i c a t i o n v e l o c i t y o f about 25 m/s over about 80% o f t h e d u r a t i o n o f amorphous regrowth.

I n these experiments,

liquid

f i l m s up t o 40-nm t h i c k were found t o be f u l l y amorphized upon resolidification.

Bucksbaum and Bokor a l s o found t h a t they were

unable t o f i t t h e i r set o f measured melt depth vs time p r o f i l e s ( f o r v a r i ous 1aser f l uences) u s i ng a conventional thermal me1t ing model c a l c u l a t i o n t h a t assumed t h a t s o l i d i f i c a t i o n occurred a t

Tc = 1685 K.

However, by modifying t h e i r c a l c u l a t i o n t o i n c l u d e

a c o n s t r a i n t re1 a t i ng me1t - s o l i d i n t e r f a c e v e l o c i t y t o me1t undercooling, w i t h separate c o n s t r a i n t s f o r c r y s t a l l i n e s o l i d i f i c a t i o n ( f o r v e l o c i t i e s below 15 m/s) and amorphous s o l i d i f i c a t i o n (above t h e c r i t i c a l v e l o c i t y o f 15 m/s) they were able t o f i t t h e i r data. The undercool i n g a t t h e crossover p o i n t between c r y s t a l 1 ine and amorphous s o l i d i f i c a t i o n , i n t h i s f i t t e d c a l c u l a t i o n , was found t o be AT = 365-415

K.

However, t h i s c a l c u l a t i o n appears not t o

have taken i n t o account t h e very low thermal c o n d u c t i v i t y o f t h e growing a-Si l a y e r . Other c a l c u l a t i o n s , using a thermal conduct i v i t y value of 0.02 W/cm-K f o r a-Si (see Sec. II.l.d),

were a b l e

t o reproduce Bucksbaum and Bokor's experimental r e s u l t s w i t h much s m a l l e r m e l t undercooling

(R. F. Wood, p r i v a t e communication).

Bucksbaum and Bokor (1984) have discussed a number o f discrepancies between t h e i r experiments and model c a l c u l a t i o n s , but t h e p r i n c i p a l experimental conclusions appear t o be t h a t o p t i c a l measurements can be used t o i n f e r t h e p o s i t i o n o f t h e m e l t - s o l i d i n t e r f a c e w i t h

394

D. H. LOWNDES ET AL

-20 psec resolution, and t h a t t h e regrowth velocity s a t u r a t e s a t

about 25 m/s under conditions leading t o regrowth of r e l a t i v e l y thick amorphous films. 11.

NUCLEATION AND GROWTH OF POLYCRYSTALLINE SILICON FROM UNDERCOOLED LIQUID SILICON

.

Recent experiments by Thompson et a1 (1984) appear t o confirm t h e e a r l i e r estimate by Donovan and co-workers (1983) t h a t the melting temperature, Ta, of a-Si l i e s some 200-300 K below Tc = 1685 K f o r c-Si. The average thermal conductivity of a-Si has a l s o been found t o be Ka 0.02 W/cm-K, between one and two orders of magnitude l e s s than Kc f o r c-Si (see Sec. 1I.l.d). Lowndes and Wood have recently shown t h a t these differences in the properties o f the a- and c-phases of Si provide t h e basis f o r a pulsed l a s e r method f o r transforming an ion-implanted a-Si layer t o a highly undercooled l i q u i d (1)phase, and f o r studying the subsequent rapid s o l i d i f i c a t i o n process in a time-resolved way (Lowndes e t a1 1984a, b y c; Wood e t a1 1984). According t o these authors, i f a low fluence nanosecond l a s e r pulse i s used t o melt only p a r t i a l l y through an implanted a-Si layer, t h e molten Si t h a t i s produced remains a t a r e l a t i v e l y low temperature (T >, Ta)y corresponding t o undercooling of the e n t i r e pool of ,t-Si; the undercooled melt a1 so remains thermally and physically isolated from t h e c-Si beneath by t h e remaining, unmelted, low-Ka a-Si b a r r i e r layer. The absence of a c r y s t a l l i n e (or polycrystalline) s u b s t r a t e in contact with the melt prevents e p i t a x i a l regrowth, making i t possible t o observe what they i n t e r p r e t as bulk (volume) nucleation and crystal growth, d i r e c t l y from t h e highly undercooled melt. (This volume nucleation process i s not necessarily identical with homogeneous nucleation, since e i t h e r chemical impurities or small c r y s t a l l i t e s may be present in t h e i n i t i a l ion-implanted a-Si layer.) Time-resol ved r e f l e c t i v i t y ( R ) measurements showed t h a t the flat-topped h i g h - r e f l e c t i v i t y phase t h a t characterizes pulsed l a s e r melting o f c-Si was not observed following melting of r e l a t i v e l y

-

.,

.,

395

6 . TIME-RESOLVED MEASUREMENTS

t h i c k a-Si l a y e r s a t low Ea (near and above t h e a-Si m e l t i n g threshold).

Instead, w i t h i n a few nanoseconds o f reaching t h e maximum

R, t h e R s i g n a l began t o decay continuously t o lower values, behavi o r t h a t Lowndes e t a l . ( 1 9 8 4 ~ )a t t r i b u t e d t o t h e bulk n u c l e a t i o n and growth o f f i n e c r y s t a l l i t e s o c c u r r i n g w i t h i n t h e depth o f t h e

a-Si t h a t was o p t i c a l l y accessible t o t h e i r cw probe l a s e r beam. The importance of t h e thermal i s o l a t i o n provided by t h e unmelted a-Si was i l l u s t r a t e d by R measurements o f surface melt duration, T ~ ,vs

Ell f o r a-Si l a y e r s o f several thicknesses.

range o f

El,

T~

Over a l i m i t e d

was observed t o decrease w i t h i n c r e a s i n g E l ( f o r

an a-Si thickness o f -200 nm), o r t o e x h i b i t a l o n g plateau vs EJ ( f o r thinner a-layers)

,

behavior t h a t model c a l c u l a t i o n s showed

r e s u l t e d from s u b s t a n t i a l changes i n t h e r a t e o f heat l o s s t o t h e s u b s t r a t e when t h e 1 - S i had n e a r l y melted through t h e a-Si l a y e r t o t h e c-Si s u b s t r a t e beneath (Lowndes e t al., e t a1

., 1984).

Cross-sectional

1984a, b y c; Wood

TEM micrographs revealed t h a t a t low l a s e r Ex

t h e r e s o l i d i f i e d near-surface r e g i o n was composed o f very small (-100 A diam), equiaxed and randomly o r i e n t e d g r a i n s o f polycryst a l l i n e S i (Narayan and White, 1984; Lowndes e t a1

., 1984b).

At

h i g h e r Ex, t h i s f i n e - g r a i n e d m a t e r i a l was found o n l y deeper w i t h i n t h e r e s o l i d i f i e d region, w i t h large, columnar g r a i n s o f polycryst a l l i n e s i l i c o n (p-Si) extending back t o t h e surface.

The f i n e -

grained p-Si i s b e l i e v e d t o r e s u l t from t h e bulk n u c l e a t i o n process,

-

o c c u r r i n g a t a temperature Tn Ta + 50 K. The t h e o r e t i c a l model used by Wood e t a1 (1984) t o simulate bulk n u c l e a t i o n and growth

.

i n undercooled S i , and t o account f o r t h e two d i f f e r e n t types o f p-Si

regrowth,

i s summarized i n Sec.

IV.12,

Chapter 4 o f t h i s

book. An a l t e r n a t i v e t o b u l k n u c l e a t i o n as t h e explanation f o r t h e f o r m a t i o n o f f i n e - g r a i n e d p-Si , d u r i n g s o l i d i f i c a t i o n of l a s e r melted a-Si layers, has been given by Thompson e t a l . (1984). They p o s t u l a t e t h a t (1) t h e i n c i d e n t pulsed l a s e r energy produces an

396

D. H. LOWNDES ETAL.

i n i t i a l l i q u i d layer t h a t s o l i d i f i e s and forms large-grained p-Si b u t t h a t (2) the deeper-lying fine-grained p-Si i s formed by an explosive c r y s t a l l i z a t i o n process when a t h i n , self-propagating l a y e r t r a v e l s through the remaining unmelted a-Si a t a velocity estimated t o l i e i n the 10-20-m/s range. As evidence o f the existence of t h i s t h i n , propagating l a y e r , they c i t e t r a n s i e n t conductance measurements which, a t l e a s t f o r very low l a s e r E l , e x h i b i t two peaks in the plot of melt depth vs time. The second peak occurs a f t e r the surface has r e s o l i d i f i e d ( a s determined by separate monitoring of the surface r e f l e c t i v i t y ) and i s believed t o represent this t h i n , propagating molten layer. The energy t o d r i v e a nearly s e l f - s u s t a i n i n g molten layer i s expected t o be provided by the release of l a t e n t heat of fusion when c r y s t a l l i z a t i o n occurs (see Leamy e t a1 , 1981). However, Wood et a1 (1984) had e a r l i e r pointed out t h a t mu1 t i p l e simultaneously propagating phase f r o n t s , including buried molten layers driven by the release of 1a t e n t heat, a r e present in t h e i r cal cul a t i ons simul a t i ng bul k nucleation (Chapter 4, Sec. IV.12). T h u s , t h e suggestion of Thompson e t a1 (1984) may be a special case of t h e more general modeling of Wood e t al. Lowndes e t a l . (1984b) have a l s o stressed t h e f a c t t h a t TEM micrographs of the fine-grained p-Si region show no evidence of the l i n e a r f e a t u r e s t h a t might indicate explosive regrowth or t h e existence o f a pl anar regrowi ng i n t e r f a c e during i t s formation. Instead, only randomly oriented, and apparently equiaxed, grains are observed, suggesting t h a t a planar i n t e r f a c e did not e x i s t during formation of t h i s region. If a planar recryst a l l i z a t i o n front i s not present, then the a t t r a c t i v e simplicity of i n t e r p r e t i n g t r a n s i e n t conductance changes in terms of an equivalent melt d e p t h may be l o s t , especially so i f a mixture of nucleating undercooled 1iquid and growing grains i s present.

.

.

.

397

6. TIME-RESOLVED MEASUREMENTS VI.

Concluding Observations

There now e x i s t s a l a r g e body o f remarkably p r e c i s e experimental data,

t o g e t h e r w i t h model c a l c u l a t i o n s ,

which e s t a b l i s h

t h a t t h e mechanism f o r pulsed l a s e r annealing o f s i l i c o n (and o t h e r semiconductors) i s thermal m e l t i n g f o l l o w e d by r a p i d s o l i d i f i c a tion.

D e t a i l e d comparisons between experiments and model calcu-

l a t i o n s demonstrate t h a t c a l c u l a t i o n s using t h e known o p t i c a l and thermal parameters o f s i 1 icon, augmented by f r e e - c a r r i e r absorption effects,

account very w e l l

f o r t h e observed time o f onset o f

melting, f o r m e l t i n g t h r e s h o l d l a s e r fluences, and f o r t h e d u r a t i o n

o f me1t ing , i n experiments using both nanosecond and picosecond l a s e r pulses.

This agreement i s now s u f f i c i e n t l y good t h a t com-

parisons between experiments and c a l c u l a t i o n s on t h e nanosecond t i m e scale a c t u a l l y - have been used t o i n f e r p r o p e r t i e s o f t h e s o l i d phase, p r i o r t o m e l t i n g (e.g.,

t h e thermal c o n d u c t i v i t y o f

As a r e s u l t o f t h i s success, f u t u r e d e t a i 1ed comparisons between me1t ing model c a l c u l a t i ons and timean amorphous s i l i c o n l a y e r ) .

resolved measurements seem l i k e l y t o focus on o b t a i n i n g a d e t a i l e d understanding o f t h e r a p i d m e l t i n g and s o l i d i f i c a t i o n phase t r a n s i t i o n s themselves.

Overheating i n t h e s o l i d phase, p r i o r t o melting,

has not been observed t o date; t h e r e l a t i o n s h i p between melt underc o o l i n g and t h e v e l o c i t y o f t h e regrowing m e l t - s o l i d i n t e r f a c e has n o t been established; t h e r e l a t i v e r o l e s and importance o f k i n e t i c vs thermodynamic e f f e c t s , a t t h e h i g h i n t e r f a c e v e l o c i t i e s r e s u l t i n g i n amorphous regrowth, are not understood; an experimental technique f o r measuring m e l t undercooling

( o r s o l i d overheating)

a t the

growing ( o r m e l t i n g ) i n t e r f a c e i s needed. The widespread

fundamental

interest

i n the p o s s i b i l i t y o f

p l asma-i nduced , r a t h e r than thermal l y induced, phase t r a n s i t i o n s a l s o s t i m u l a t e d t h e development o f a wide v a r i e t y o f time-resolved experimental techniques f o r d i f f e r e n t i a t i n g between t h e two, and has helped t o push t h e time scale f o r o p t i c a l measurements down i n t o t h e 10-100 femtosecond range.

I n i t i a l a p p l i c a t i o n s o f these

398


t e c h n i q u e s were l i m i t e d almost e x c l u s i v e l y t o s i l i c o n .

Similarly,

much o f t h e e f f o r t t o d a t e focused on t h e s i n g l e problem o f making t i m e - r e s o l v e d l a t t i c e and/or c a r r i e r temperature measurements, i n o r d e r t o d i r e c t l y v e r i f y t h a t thermal m e l t i n g o f t h e l a t t i c e , r a t h e r t h a n k o l d " p l asma anneal ing , occurred, and t o e s t a b l ish whether c a r r i e r s and t h e l a t t i c e were a t s i m i l a r temperatures. T h i s narrow focus was u s e f u l , s i n c e i t r e s u l t e d i n t h e c o m p e t i t i v e e v a l u a t i o n o f a v a r i e t y o f d i f f e r e n t experimental techniques, t h e b e s t o f which are now capable o f making i n d i r e c t l a t t i c e temperat u r e measurements on a t l e a s t t h e nanosecond t i m e scale. Recent t i m e - r e s o l v e d o b s e r v a t i o n s o f t h e l o s s o f c r y s t a l 1 i n e s t r u c t u r e accompanying me1t i n g demonstrated t h a t t h e t r a n s f e r o f energy from pulsed l a s e r - e x c i t e d c a r r i e r s t o t h e l a t t i c e ( r e s u l t i n g i n m e l t i n g ) a c t u a l l y occurs i n l e s s t h a n con.

The f i r s t o b s e r v a t i o n s o f v e r y h i g h

one picosecond i n s i l i density

(N >, 1021/cm3)

s o l i d s t a t e plasmas, p r i o r t o t h e occurrence o f m e l t i n g , have a l s o been c a r r i e d out.

I t i s c l e a r t h a t a f e r t i l e area f o r f u t u r e

r e s e a r c h i s t h e study o f t h e p r o p e r t i e s and t i m e e v o l u t i o n o f such h i gh-densi t y plasmas,

and o f el ectron-phonon

coup1 i n g and t h e

e l e c t r o n - l a t t i c e energy r e l a x a t i o n processes t h a t r e s u l t i n l a t t i c e melting.

The study o f energy and momentum r e l a x a t i o n processes

o c c u r r i n g e n t i r e l y w i t h i n an i s o l a t e d , h i g h - d e n s i t y , l a s e r - i n d u c e d plasma w i l l a l s o soon become p o s s i b l e , as t i m e - r e s o l v e d techniques a r e pushed t o t h e low end o f t h e femtosecond range. Finally, some developments beyond t h e Drude model w i l l be needed i n t h e i n t e r p r e t a t i o n o f these experiments, i n o r d e r t o a c c u r a t e l y r e l a t e plasma p r o p e r t i e s t o t h e semiconductors i n which t h e y a r e produced.

Acknowledgments We would especi a1 l y 1ike t o acknowledge t h e c o n t r i b u t i o n s , t o o numerous t o number and e x t e n d i n g over several years, r e s u l t i n g from many e n j o y a b l e c o n v e r s a t i o n s and c o l l a b o r a t i o n s w i t h R. F. Wood.

6.


399

We a l s o f e e l a s p e c i a l debt o f g r a t i t u d e t o J u l i a Luck f o r h e r seemingly boundless patience w i t h r e v i s i o n s , attention t o detail,

and h e r continued

throughout t h e t y p i n g o f t h i s manuscript.

F i n a l l y , we would l i k e t o acknowledge h e l p f u l conversations w i t h

D. H. Auston, A. A y d i n l i , M. J. Aziz, P. B a e r i , A. Compaan, A. G. C u l l i s , G. J. Galvin, H. Kurz, B. C. Larson, D. von der Linde, C. L. Marquardt, S. C. Moss, G. L. Olson, M. 0. B. R. Appleton,

Thompson, and J. A. Van Vechten, and t h e h e l p o f a number of c o l leagues

who provided f i g u r e s f o r use i n t h i s chapter.

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Shank, C. V., Yen, R., and H i r l i m a n n , C. (1983a). Phys. Rev. L e t t . 50, 454. Shank, C. V., Yen, R., and H i r l i m a n n , C. (1983b). Phys. Rev. L e t t . 51, 900. Yen, R., and H i r l i m a n n , C. (1984). Mat. Res. SOC. Shank, C. V., Symp. Proc. 23, 53. Shvarev, K. M. , Baum, B. A., and Gel 'd, P. V. (1977). High Temp. 15, 548. G e l l e r , M. , and B o r t f e l d , D. P. (1964). Appl. Phys. Sooy, W. R., L e t t . 5, 54. Spaepen, F., and T u r n b u l l , D. (1979). A I P Conf. Proc. 50, 73 ( A I P , New York. Spaepen, F., and T u r n b u l l , D. (1982). I n "Laser Annealing o f Semiconductors" (J. M. Poate and J. W. Mayer, eds.), Chapter 2. Academic Press, New York. and Tagle, J. A. (1981). Phys. S t r i t z k e r , B., Pospieszczyk, A., Rev. L e t t . 47, 356. Thompson, M. 0. , G a l v i n , G. J. , Mayer, J. W. , Hammond, R. B. , P a u l t e r , N., and Peercy, P. S. (1982). Mat. Res. SOC. Symp. Proc. 4, 209. Thompson , M. 0. , Mayer , J. W. , C u l l i s , A. G. , Webber , H. C. , Chew, N. G., Poate, J. M., and Jacobson, D. C. (1983a). Phys. Rev. L e t t . 50, 896. Peercy, P. S., and Thompson, M. O., G a l v i n , G. J., Mayer, J. W., Hammond, R. 6. (1983b). Appl. Phys. L e t t . 42, 445. Thompson, M. O., Galvin, G. J., Mayer, J. W., Peercy, P. S., Poate, J. M., Jacobson, D. C., C u l l i s , A. G., and Chew, N. G. (1984). Phys. Rev. L e t t . 52, 2360. and G a l v i n , G. J. (1983). Mat. Res. SOC. Symp. Thompson, M. O., Proc. 13, 57. Thurmond, C. D. (1975). J. Electrochem. SOC. 122, 1133. Tsu, R. , Hodgson, R. T. , Tan , T. Y., and B a g l i n , J. E. E. (1979). Phys. Rev. L e t t . 42, 1356. van D r i e l , H., Lompr6, L.-A,, and Bloembergen, N. (1984). Appl. Phys. L e t t . 44, 285. van D r i e l , H. M. (1984). Appl Phys. L e t t . 44, 617. Van Gurp, G. J., Eggermont, G. E., Tamminga, Y., Stacy, W. J., and G i j s b e r s , J. R. M. (1979). Appl. Phys. L e t t . 35, 273. Van Vechten, J. A. Tsu, R., and S a r i s , F. (1979). Phys. L e t t . A 74, 422. Van Vechten, J . A. (1980). J. Phys. 41, C4-15. Van Vechten, J. A., and Compaan, A. (1981). S o l i d S t a t e Commun. 39, 867. von der Linde, D. , and Wartmann , G. (1982). Appl Phys. L e t t . 41, 700. von der Linde , D. , and Fabricus , N. (1982). Appl Phys. L e t t . 4 1 , 991. von der Linde, D. , Wartmann, G., and Compaan, A. (1983a). J. Appl Phys. 43, 613. von der Linde, D., Wartmann, G., and 0207s~A. (1983b). Mat. Res. SOC. Symp. Proc. 13, 17.

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von der Linde, D., Wartmann, G., Kemmler, M., and Zhu, Z . 4 . (1984). Mat. Res. SOC. Symp. Proc. 23, 123. Wang, J. C. , Wood, R. F. , and Pronko, P. P. (1978). Appl Phys. L e t t . 33, 455. White, C. W., and Peercy, P. S., eds. (1980). "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s." Academic Press, New York. White, C. W., Wilson, S. R., Appleton, B. R., and Young, F. W., Jr. (1980). J. Appl. Phys. 51, 738. Williamson, S., Mourou, G., and L i , J. C. M. (1984). Phys. Rev. L e t t . 52, 2364. Wood, R. F., and G i l e s , G. E. (1981). Phys. Rev. B 23, 2923. Wood, K. F., K i r k p a t r i c k , J. R., and G i l e s , G. E. (1981a). Phys. Rev. B 23, 5555. Wood, R. F. , Lowndes, D. H., and C h r i s t i e , W. H. (1981b). Mat. Res. SOC. Symp. Proc. 1, 231. Wood, R. F. (1982). Phys. Rev. B 25, 2786. Wood, R. F., Lowndes, D. H., J e l l i s o n , G. E., Jr., and Modine, F. A. (1982a). Appl. Phys. L e t t . 41, 287. Wood, K. F. , Lowndes , D. H. , and G i l e s , G. E. (1982b). Mat. Res. SOC. Symp. Proc. 4, 67. Wood, R. F., Rasolt, M., and J e l l i s o n , G. E., Jr. ( 1 9 8 2 ~ ) . Mat. Res. SOC. Symp. Proc. 4, 61. Wood, R. F. (1983). Mat. Res. SOC. Symp. Proc. 13, 83. Wood, R. F., Lowndes, D. H., and Narayan, J. (1984). Appl. Phys. L e t t . 44, 770. Yen, R., L i u , J. M., Kurz, H., and Bloembergen, N. (1982). Appl. Phys. A 27, 153. Yoffa, E. J. (1980). Phys. Rev. B 21, 2415. Young, R. T. , Wood, R. F. , and C h r i s t i e , W. H. (1982). J. Appl. Phys. 53, 1178.

.

CHAPTER 7 SURFACE STUDIES SEMICONDUCTORS

OF

PULSED LASER IRRADIATED

D. M. Zehner

. . . . .. .. .. .. .. .. .. .. .. .. .. . . . . . .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. ... ... ........

I. INTRODUCTION. 11. EXPERIMENTAL APPROACH 1. Sampl e P r e p a r a t i o n 2. C h a r a c t e r i z a t i o n Techniques. 111. PRODUCTION OF ATOMICALLY CLEAN SURFACES 3. S i l i c o n . 4. Germanium. 5. Group 111-V Compounds. GEOMETRIC SURFACE STRUCTURE IV. 6. Ordered Surfaces 7. Metastable Surfaces. 8. V i c i n a l Surfaces 9. Defects. SURFACE AND SUB-SURFACE STUDIES OF V. ION-IMPLANTED SILICON 10. S u b s t i t u t i o n a l Implants. 11. I n t e r s t it i a1 Imp1a n t s APPLICATIONS. VI. CONCLUSIONS VII. REFERENCES.

. . . .. .. .. .. .. .. .. .. . . .. .. .. .. .. .. .. .. ..

............... . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . ................ ................ I.

Introduction

The process of pulsed l a s e r annealing p r o v i d e s a way o f very r a p i d l y t r e a t i n g t h e near-surface

r e g i o n o f semiconductors.

t h i s a d i a b a t i c mode o f thermal processing, 1 iquid-phase

epitaxial

In

m e l t i n g f o l l o w e d by

regrowth from t h e s u b s t r a t e occurs w i t h

growth v e l o c i t i e s o f t h e o r d e r o f meters/s.

Thus t h e h e a t i n g and

c o o l i n g r a t e s achieved by t h i s form o f processing are o r d e r s o f

405

Capyright 01984 b i Academic Press, Inc. All rights of reproduction in any form reserved.

ISBN 0-12-752123.2

406

D. M. ZEHNER

magnitude f a s t e r t h a n t h o s e achieved by more c o n v e n t i o n a l t r e a t ments.

i t has been shown t h a t

With proper annealing conditions,

r e g i o n s f r e e o f extended d e f e c t s can be formed and s u b s t i t u t i o n a l i m p u r i t i e s can be i n c o r p o r a t e d i n t o t h e l a t t i c e f a r i n excess o f t h e e q u i l i b r i u m s o l u b i l i t y l i m i t s (see Chapters 1-4).

The f i n a l

a c t o f s o l i d i f i c a t i o n i s t h e f r e e z i n g o f t h e surface.

I n view o f

the

region,

i t may be

(impurities,

geometric

results

expected

that

structure, ductors

obtained f o r the

the

surface

near-surface

properties

e l e c t r o n i c energy l e v e l s ) o f l a s e r - a n n e a l e d semicon-

can

be

significantly

altered

with

respect

to

those

o b t a i n e d by c o n v e n t i o n a l h e a t i n g treatments. R e s u l t s o f experiments d i s c u s s e d i n t h i s c h a p t e r show t h a t p u l s e d l a s e r a n n e a l i n g can be used t o produce a t o m i c a l l y c l e a n s u r f a c e s , remove damage i n t h e outermost s u r f a c e l a y e r s , and a l t e r t h e e l e c t r o n i c p r o p e r t i e s i n t h e s u r f a c e region.

D e t a i l s con-

cerned w i t h p r o c e s s i n g o f t h e s u r f a c e i n o r d e r t o achieve these conditions

and t h e measurement

discussed i n Section

11.

of

the surface properties are

R e s u l t s which show t h a t

unwanted i m p u r i t i e s i n semiconductors (0, C, etc.) to

near

the

practical

detection

limits

of

levels of

can be reduced

surface

sensitive

I n Section I V the

s p e c t r o s c o p i e s a r e presented i n S e c t i o n 111.

s u b j e c t o f o r d e r i n t h e outermost l a y e r s f o r b o t h f l a t and v i c i n a l s u r f a c e s i s discussed, and

changes

in

along w i t h the production o f defects

stoichiometry

for

compound

semiconductors.

S e c t i o n V d e a l s w i t h t h e changes i n b o t h geometric and e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n which occur when i o n i m p l a n t a t i o n

is

combined w i t h

laser

annealing.

Finally

in

Section

YI,

examples o f how t h e unique s u r f a c e p r o p e r t i e s achieved w i t h l a s e r a n n e a l i n g can discussed.

be a p p l i e d t o o t h e r s u r f a c e

investigations

are

7.

407

PULSED LASER IRRADIATED SEMICONDUCTORS

11.

Experimental Approach

For most i n v e s t i g a t i o n s concerned w i t h l a s e r p r o c e s s i n g o f semiconductors,

t h e i r r a d i a t i o n o f t h e sample has been performed

i n a standard atmospheric environment.

It i s g e n e r a l l y assumed

t h a t t h e m o d i f i c a t i o n o f t h e subsurface p r o p e r t i e s i s u n a f f e c t e d by i n t e r a c t i o n s a t t h e g a s - s o l i d i n t e r f a c e .

However, when one i s

concerned w i t h o b t a i n i n g i n f o r m a t i o n about t h e p r o p e r t i e s o f t h e s u r f a c e r e g i o n (1-20 A ) and t h e changes which occur due t o l a s e r annealing,

t h e i r r a d i a t i o n o f t h e sample and subsequent a n a l y s i s

must t a k e p l a c e i n an u l t r a h i g h vacuum (UHV) environment Torr).

I n t h i s s e c t i o n , t h e experimental d e t a i l s concerned w i t h

b o t h l a s e r annealing o f semiconductors i n UHV and t h e subsequent s u r f a c e c h a r a c t e r i z a t i o n a r e presented.

1.

SAMPLE PREPARATION

A v a r i e t y o f l a s e r s has been used i n i n v e s t i g a t i o n s concerned w i t h surface s t u d i e s o f laser-annealed semiconductors.

The most

f r e q u e n t l y used l a s e r s are e i t h e r p u l s e d ruby o r p u l s e d Nd:YAG, a l t h o u g h UV excimer and t u n a b l e dye l a s e r s have a l s o been employed. The procedures f o l l o w e d i n p e r f o r m i n g t h e l a s e r a n n e a l i n g i n a

UHV environment are very s i m i l a r i n a l l i n v e s t i g a t i o n s and w i l l be i l l u s t r a t e d by d i s c u s s i n g t h e approach used w i t h p u l s e d ruby l a s e r s (Zehner e t al.,

1980a,b).

A f t e r bakeout,

t h e background

p r e s s u r e i n t h e chamber which c o n t a i n e d t h e sample was t y p i c a l l y l e s s than 2 x 10-10 Torr.

The l i g h t from a Q-switched ruby l a s e r

( A = 694 nm,

FWHM), t r a n s m i t t e d i n t o t h e UHV system

T

= 15 nsec,

t h r o u g h a glass window, vacuum environment.

was used t o i r r a d i a t e t h e sample i n t h e

The samples were p o s i t i o n e d so t h a t any evap-

o r a t e d m a t e r i a l o r s c a t t e r e d l i g h t was c o n t a i n e d i n an enclosure which s h i e l d e d a1 1 s u r f a c e a n a l y s i s

instruments.

i r r a d i a t e d u s i n g t h e single-mode (TEMoo)

Samples were

o u t p u t o f t h e ruby l a s e r

a t energy d e n s i t i e s t h a t c o u l d be v a r i e d between -0.2 J/cm2.

The beam diameter was t y p i c a l l y between 3.0

and -4.0

and 6.0 mn.

Energy d e n s i t i e s , which have been c o r r e c t e d f o r t h e r e f l e c t i v i t y

408

D. M. ZEHNER

( - 10%) o f t h e g l a s s window,

were determined by measuring t h e

photon energy d e l i v e r e d through an a p e r t u r e o f known diameter positioned i n f r o n t o f a calorimeter. was

measured

with

an

in-1 i n e

against the calorimeter.

The energy o f each p u l s e

photodi ode

assembly

c a l ib r a t e d

Implanted samples were prepared i n a

separate i o n i m p l a n t a t i o n f a c i 1it y which was a1 so equipped f o r making R u t h e r f o r d b a c k s c a t t e r i n g (RBS) measurements.

T h i s tech-

n i q u e was used t o determine t h e i m p l a n t p r o f i l e and t o charact e r i z e t h e changes i n t h e subsurface r e g i o n t h a t occurred w i t h An e x t e n s i v e d i s c u s s i o n o f these r e s u l t s i s


c o n t a i n e d i n Chapter 2.

2.

CHARACTERIZATION TECHNIQUES Many o f t h e s u r f a c e s e n s i t i v e s p e c t r o s c o p i c techniques used t o

i n v e s t i g a t e t h e s u r f a c e r e g i o n o f laser-annealed semiconductors employ e i t h e r e l e c t r o n s o r photons as t h e i n c i d e n t probe. t h e s e cases t h e d e t e c t e d p a r t i c l e i s an e l e c t r o n .

In

As a con-

sequence o f t h e s h o r t mean f r e e path o f e l e c t r o n s w i t h energies between 20 and 1000 eV, o n l y t h e outermost s u r f a c e region, -20 A , i s probed. Auger

Several d i f f e r e n t techniques have been used. electron

spectroscopy

(AES)

was

used t o m o n i t o r t h e

l e v e l s of b o t h i m p u r i t i e s and implanted species i n t h e s u r f a c e r e g i o n of t h e sample. elements w i t h Z > 3. in

terms

of

the

T h i s technique i s capable o f d e t e c t i n g a l l L e v e l s o f i m p u r i t y c o n t a m i n a t i o n a r e quoted

ratios

of

the

peak-to-peak

signals

of

the

i m p u r i t y Auger t r a n s i t i o n s t o a p r i n c i p a l Auger t r a n s i t i o n o f t h e substrate. this

Although one must be c a r e f u l

technique

to

make

quantitative

i n a t t e m p t i n g t o use

measurements,

in

many

s i t u a t i o n s reasonable estimates o f t h e upper l i m i t o f t h e amount o f a p a r t i c u l a r species present

i n t h e s u r f a c e r e g i o n can be

made. Low-energy e l e c t r o n d i f f r a c t i o n (LEED) was employed t o d e t e r mine geometric o r d e r i n t h e s u r f a c e r e g i o n o f t h e sample.

By

examining t h e p o s i t i o n s o f t h e r e f 1e c t e d beams ( s p o t p a t t e r n s ) ,

7.

409


t h e symmetry and s i z e ( i n t r a - a t o m i c spacing) o f t h e s u r f a c e u n i t c e l l can be determined. with

Thus, any changes i n these spot p a t t e r n s are a

surface modification

r e f l e c t i o n o f changes

i n the

geometric arrangement o f atoms i n t h e outermost s u r f a c e l a y e r s . P h o t o e l e c t r o n spectroscopy

(PES)

was

used t o o b t a i n i n f o r -

m a t i o n about t h e e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n o f t h e sample.

I n f o r m a t i o n about t h e valence and conduction bands o f

t h e s o l i d can be obtained

by employing photons w i t h energies

t y p i c a l l y l e s s than 50 eV.

With t h e use o f angle-resolved tech-

niques, i t i s p o s s i b l e t o map o u t bands and c h a r a c t e r i z e t h e symmetry o f s u r f a c e s t a t e s .

By employing photons o f h i g h e r energies

i t i s p o s s i b l e t o measure c o r e - l e v e l

b i n d i n g energies f o r b o t h

s u r f a c e and subsurface atoms.

111.

P r o d u c t i o n o f A t o m i c a l l y Clean Surfaces

The c r e a t i o n o f an a t o m i c a l l y c l e a n s u r f a c e i s one o f t h e obvious b u t f r e q u e n t l y d i f f i c u l t t a s k s t h a t must be performed p r i o r t o conducting experiments i n t h e f i e l d o f s u r f a c e science. I n v e s t i g a t i o n s concerned w i t h examining t h e p h y s i c a l and chemical properties

o f s u r f a c e s i n o r d e r t o understand s u r f a c e - r e l a t e d

phenomena r e q u i r e t h a t t h e l e v e l o f unwanted contaminants i n t h e f i r s t few monolayers be 250 eV) and t h a t t h e y c o n t a i n e d an i n t e n s e backround r e s u l t i n g from d i f f u s e s c a t t e r i n g . This

o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e presence o f a n a t i v e

o x i d e l a y e r c o n t a i n i n g 0 and C as determined by AES and shown i n F i g . 1.

The LEED p a t t e r n s shown i n Fig. 5 i l l u s t r a t e t h e e f f e c t s

o f m u l t i p l e - p u l s e i r r a d i a t i o n on a S i ( 100) sample. d i a t i o n w i t h one l a s e r p u l s e o f -2.0

J/cm2,

A f t e r irra-

a (2x1) LEED p a t t e r n

w i t h moderate background i n t e n s i t y due t o d i f f u s e s c a t t e r i n g was obtained,

as shown a t t h e t o p o f Fig.

5.

Improvements i n t h e

q u a l i t y o f t h e d i f f r a c t i o n p a t t e r n occurred w i t h subsequent l a s e r pulses.

After

diffraction observed,

f i v e pulses,

reflections

a LEED p a t t e r n e x h i b i t i n g

and very

as shown i n F i g .

5.

sharp

low background i n t e n s i t y was

The f a c t t h a t w e l l - d e f i n e d LEED

p a t t e r n s can be obtained i n d i c a t e s t h a t c r y s t a l l i n e o r d e r extends to

t h e outermost monolayers

regrowth process.

after

the

1iquid-phase

epitaxial

No d e t e c t a b l e change i n t h e LEED p a t t e r n s was

observed w i t h a d d i t i o n a l pulses, as can be seen by comparing t h e patterns

for

five

and t e n pulses

shown i n Fig.

5.

Similar

r e s u l t s were o b t a i n e d from samples t h a t were i n i t i a l l y s p u t t e r cleaned by A r + bombardment.

Although t h e LEED p a t t e r n o b t a i n e d

subsequent t o one l a s e r p u l s e on a s p u t t e r e d s u r f a c e was

of

7.


h i g h e r q u a l i t y t h a n t h a t shown i n Fig.

5,

425

m u l t i p l e pulses were

a l s o r e q u i r e d t o o b t a i n t h e sharpest d i f f r a c t i o n p a t t e r n s . The LEED p a t t e r n s obtained from t h e t h r e e low-index o r i e n t a t i o n s o f S i subsequent t o l a s e r i r r a d i a t i o n w i t h f i v e pulses a r e shown i n F i g . 6.

A l l p a t t e r n s show sharp d i f f r a c t i o n r e f l e c t i o n s accompanied

by low background i n t e n s i t y , and i n a l l cases t h e q u a l i t y o f t h e p a t t e r n obtained improved w i t h m u l t i p l e - p u l s e f i v e shots.

i r r a d i a t i o n up t o

The (2x1) and ( 1 x 2 ) LEED p a t t e r n s o b t a i n e d from t h e

(100) and (110) surfaces,

respectively,

are s i m i l a r t o those ob-

t a i n e d u s i n g conventional thermal t r e a t m e n t s and show t h e presence o f r e c o n s t r u c t e d surfaces.

These o b s e r v a t i o n s i n d i c a t e t h a t t h e

atoms i n t h e outermost l a y e r s have enough t i m e a t a temperature, under t h e l a s e r annealing c o n d i t i o n s used, t o r e o r g a n i z e i n t o t h e r e c o n s t r u c t e d arrangements f r o m w h i c h t h e LEED p a t t e r n s a r e obtained. T h i s i s c o n s i s t e n t w i t h t h e proposed s u r f a c e s t r u c t u r e models f o r t h e (100) s u r f a c e t h a t i n v o l v e o n l y small l a t e r a l and v e r t i c a l d i s placements o f t h e atoms i n t h e f i l l e d outermost monolayers.

Laser

a n n e a l i n g o f e i t h e r (100) o r (110) S i samples cooled t o 100 K (Zehner e t a1

., 1980d)

produced surfaces from which LEED p a t t e r n s

i d e n t i c a l t o those shown i n Fig. 6 were obtained. The LEED p a t t e r n o b t a i n e d from t h e (111) s u r f a c e suggests t h a t as a r e s u l t o f l a s e r annealing t h e normal s u r f a c e s t r u c t u r e ( t r u n c a t i o n o f t h e b u l k ) i s obtained,

and t h e r e i s no evidence o f any

ordered l a t e r a l r e c o n s t r u c t i o n (Zehner e t a1

., 1 9 8 0 ~ ) . T h i s p o i n t

w i l l be discussed i n more d e t a i l l a t e r . Although a ( 2 x 1 ) LEED p a t t e r n can be obtained from a cleaved S i ( l l 1 ) surface, t h e (7x7) p a t t e r n shown i n Fig. 7 i s always observed on a clean, t h e r m a l l y annealed c r y s t a l surface.

A f t e r i r r a d i a t i o n w i t h t h e l a s e r and structure,

t h e S i sample was

t h e r m a l l y annealed a t e l e v a t e d temperatures.

The o b s e r v a t i o n o f

production o f the (1x1)

surface

1/7-order d i f f r a c t i o n spots, i n d i c a t i v e o f t h e r e c o n s t r u c t e d surface, occurred a f t e r annealing a t temperatures g r e a t e r t h a n -800 K. By h e a t i n g f o r a s u f f i c i e n t t i m e (>30 rnin) a t these temperatures,

a well-defined

( 7 x 7 ) p a t t e r n s i m i l a r t o t h a t shown i n Fig. 7 was

426

Fig. 6.

D. M. ZEHNER

LEED patterns from clean ( a )

(loo),

( b ) ( 1 1 0 ) , and ( c ) ( 1 1 1 ) Si

surfaces a t primary beam energies o f ( a ) 4 9 , ( b ) 9 2 , and ( c ) 47 eV. are shown subsequent to laser annealing at -2.0

Patterns

J / c m 2 for 5 pulses.

7.

Fig. 7.


427

LEED pattern from a clean thermally annealed ( 1 1 1 ) Si surface at a

primary beam energy o f 1 1 1 eV.

observed.

Subsequent i r r a d i a t i o n a t room temperature w i t h t h e

l a s e r resulted i n a (1x1) surface structure, possible t o cycle Moreover,

back and f o r t h between t h e two s t r u c t u r e s .

i t was determined t h a t t h e sample c o u l d be h e l d a t a

temperature between -100

and 700 K,

and a f t e r i r r a d i a t i o n w i t h a

l a s e r p u l s e t h e (1x1) s t r u c t u r e was observed.

LEED

defined

showing t h a t i t i s

patterns

single-crystal (Zehner e t a1

were

obtained

from

s u r f a c e s o f Ge subsequent

., 1 9 8 0 ~ ) .

As w i t h S i , w e l l low-index-oriented

t o laser irradiation

Mechanisms o f energy a b s o r p t i o n and r e d i s t r i b u t i o n i n t h e s u r f a c e r e g i o n , as w e l l as t h e development o f a comprehensive unders t a n d i n g o f t h e s t a t e o f t h e s u r f a c e under l a s e r annealing conditions,

have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n i n r e c e n t years.

The

f a c t t h a t ordered surfaces can be produced w i t h l a s e r annealing, as j u s t discussed,

suggests t h a t a n a t u r a l probe o f t h e s u r f a c e

r e g i o n which would y i e l d s t r u c t u r a l i n f o r m a t i o n on t h e f i r s t few atomic l a y e r s i s t i m e - r e s o l v e d LEED.

By measuring t h e i n t e n s i t y

o f a d i f f r a c t e d beam d u r i n g t h e l a s e r a n n e a l i n g process, i n f o r m a t i o n about t h e s t a t e o f t h e s u r f a c e can be obtained.

Such measurements

in conj u n c t i on w i t h time- r e s o l ved opt ic a l r e f 1 e c t iv i t y measurements have r e c e n t l y been made u s i n g a 'Ge (111) sample (Becker e t al., 1984a,b).

The LEED i n t e n s i t y was measured i n a temporal window

428

D.M. ZEHNER

e x t e n d i n g from a few nanoseconds b e f o r e t h e l a s e r p u l s e t o 1000 ns a f t e r t h e l a s e r pulse.

These i n t e n s i t i e s were then compared

t o those o b t a i n e d from a sample r a i s e d t o successive s t e a d y - s t a t e temperatures by r a d i a t i v e h e a t i n g from a f i l a m e n t - t y p e Results

show an e x t i n c t i o n

o f the diffracted

sistent

with

increase

the

observed

in

heater.

intensity,

optical

con-

reflectivity,

c l e a r l y i n d i c a t i n g t h a t t h e Ge s u r f a c e i s n o n c r y s t a l l i n e d u r i n g t h e l a s e r a n n e a l i n g process.

These changes i n i n t e n s i t y can be

c o m p l e t e l y accounted f o r i n t h e m e l t i n g model. To i l l u s t r a t e t h e a p p l i c a t i o n o f l a s e r a n n e a l i n g i n producing o r d e r e d s u r f a c e s t r u c t u r e s on c r y s t a l faces o f compound semicond u c t o r s , e s p e c i a l l y those i n which one o f t h e components i s v o l a tile,

r e s u l t s o b t a i n e d from t h e low-index faces o f GaAs c r y s t a l s

w i l l be presented (Zehner e t a1

., 1982).

A l l r e s u l t s t o be d i s -

cussed were o b t a i n e d from surfaces t h a t were i n i t i a l l y s p u t t e r e d i n o r d e r t o remove t h e C and 0 i m p u r i t i e s ,

s i n c e t h i s procedure

p e r m i t t e d t h e use o f r e l a t i v e l y low l a s e r p u l s e energy d e n s i t i e s i n o r d e r t o o b t a i n c l e a n surfaces. the

(loo),

The LEED p a t t e r n s obtained f o r

(110), and (111) o r i e n t a t i o n s o f GaAs c r y s t a l s f o l l o w i n g

i r r a d i a t i o n a t an energy d e n s i t y o f -0.3

J/cm2 a r e shown i n Fig. 8.

The q u a l i t y o f t h e s u r f a c e s t r u c t u r e r e s u l t i n g from l a s e r annealing, as r e f l e c t e d i n these p a t t e r n s , d i f f e r e d s i g n i f i c a n t l y from t h a t o b t a i n e d from elemental semiconductors.

The h i g h e s t qua1 i t y p a t -

t e r n s were o b t a i n e d from t h e (110) o r i e n t a t i o n ,

and reasonable

q u a l i t y p a t t e r n s were o b t a i n e d from both A- and B-type (111) o r i e n t a tions.

Very poor q u a l i t y p a t t e r n s were o b t a i n e d from t h e (100)

orientations.

A l l LEED p a t t e r n s were b a s i c a l l y (1x1)

,

suggesting

no long-range ordered r e c o n s t r u c t i o n as n o r m a l l y observed a f t e r c o n v e n t i o n a l thermal annealing.

I n a l l cases t h e o b s e r v a t i o n o f

d i f f u s e background i n t e n s i t y and/or s t r e a k i n g i n d i c a t e d t h e presence

o f d i s o r d e r i n t h e s u r f a c e region.

These o b s e r v a t i o n s a r e consis-

t e n t w i t h b o t h AES and RBS r e s u l t s , which i n d i c a t e t h e e x i s t e n c e o f excess Ga i n l o c a l r e g i o n s which a r e n o n s t o i c h i o m e t r i c i n t h e n e a r - s u r f a c e region.

Although a range o f energy d e n s i t i e s and a

7.

Fig. 8.


LEED patterns from clean laser-annealed

429

( a ) ( l o o ) , ( b ) 1 1 0 ) , and

( c ) ( 1 1 1 ) GaAs surfaces a t primary beam energies o f ( a ) 1 1 3 eV, ( b ) 1 2 3 eV, and ( c ) 95 eV.

D. M. ZEHNER

v a r i a t i o n o f t h e number o f pulses were t r i e d , i t was n o t p o s s i b l e t o produce surfaces from which b e t t e r q u a l i t y LEED p a t t e r n s c o u l d be observed.

Similar

r e s u l t s have been o b t a i n e d from t h e InP

( 100) s u r f a c e (Moison e t a1 7.

., 1982).

METASTABLE SURFACES The o b s e r v a t i o n t h a t a (1x1) LEED p a t t e r n i s o b t a i n e d from t h e

( 1 1 1 ) s u r f a c e o f S i a f t e r l a s e r i r r a d i a t i o n i n a UHV environment and t h a t t h e s u r f a c e i s a t o m i c a l l y c l e a n a f t e r such t r e a t m e n t suggests t h a t t h i s s u r f a c e p r o v i d e s t h e o p p o r t u n i t y f o r i n v e s t i g a t i n g a c l e a n semiconductor s u r f a c e t h a t e x h i b i t s no ordered l a t e r a l reconstruction.

The understanding o f t h i s s t r u c t u r e i s o f v i t a l

importance i n view o f t h e o r e t i c a l d e s c r i p t i o n s o f t h e S i (111) surface.

I f t h e s u r f a c e i s t r u l y b u l k - l i k e except f o r s u r f a c e r e -

l a x a t i o n , i t should d i f f e r from t h e d i s o r d e r e d high-temperature (1x1)

., 1981) and i m p u r i t y - s t a b i l i z e d (1x1) (Eastman e t a1 ., F l o r i o e t a1 ., 1971) surfaces. Furthermore, o t h e r i n v e s t i -

( B e n n e t t e t a1 1980a,b;

g a t i o n s o f t h e (111) s u r f a c e subsequent t o i r r a d i a t i o n w i t h l a s e r p u l s e s have i n d i c a t e d t h a t some degree o f d i s o r d e r i s present. T h i s s u b j e c t w i l l be discussed i n d e t a i l l a t e r i n t h i s s e c t i o n . W h i l e i n f o r m a t i o n about t h e symmetry and s i z e o f t h e twodimensional

unit

cell

diffraction

patterns,

on

the

surface

information

about

can

be

obtained

surface

from

relaxations

r e q u i r e s t h e measurement o f t h e i n t e n s i t i e s o f t h e d i f f r a c t e d e l e c t r o n beams as a f u n c t i o n o f i n c i d e n t e l e c t r o n energy ( I - V profile).

The e x p e r i m e n t a l l y measured p r o f i l e s must t h e n be com-

pared w i t h r e s u l t s o b t a i n e d from f u l l y converged dynamical LEEU c a l c u l a t i o n s assuming v a r i o u s s t r u c t u r a l models f o r t h e geometric arrangement i n t h e outermost l a y e r s . between t h e experimental

A measure o f t h e agreement

r e s u l t s and t h e p r e d i c t i o n o f model

c a l c u l a t i o n s i s p r o v i d e d by t h e R f a c t o r ( t h e lower t h e R f a c t o r value, t h e

b e t t e r t h e agreement).

A d e t a i l e d LEED a n a l y s i s f o r

l a s e r - a n n e a l e d (111)-( 1x1) s u r f a c e s o f S i has been performed, and t h e r e s u l t s are discussed below (Zehner e t al.,

1981a).

7.

431


A S i (111) s u r f a c e t h a t had been i r r a d i a t e d w i t h t h e o u t p u t o f t h e l a s e r a t an energy d e n s i t y o f -2.0 investigations.

J/cmz was used i n these

The i n t e n s i t i e s o f t h e d i f f r a c t e d beams were

measured as a f u n c t i o n o f e l e c t r o n energy u s i n g a Faraday cup operated as a r e t a r d i n g f i e l d analyzer.

Data were obtained f o r

a l l o f t h e { l o } , {01}, {20}, and {02} beams and f o r t h r e e each o f t h e { l l } and {21} beams.

Based on o b s e r v a t i o n s and c o n c l u s i o n s

drawn from p r e v i o u s s t u d i e s , s y m m e t r i c a l l y e q u i v a l e n t beams were averaged t o p r o v i d e a data base c o n t a i n i n g s i x average p r o f i l e s . The experimental data base has been compared w i t h t h e r e s u l t s o b t a i n e d from f u l l y converged dynamical LEED c a l c u l a t i o n s .

Details

o f these c a l c u l a t i o n s can be found elsewhere, and o n l y t h e r e s u l t s

w i l l be summarized here. t h e dynamical

Comparison o f p r o f i l e s o b t a i n e d from

LEED c a l c u l a t i o n s t o t h e measured I - V

suggests t h a t t h e f i r s t i n t e r l a y e r spacing, d,

profiles

i s c o n t r a c t e d by

25.5 a 2.5% w i t h respect t o t h e b u l k value and t h a t t h e second i n t e r l a y e r spacing, d,, b u l k value.

i s expanded 3.2 r 1%w i t h respect t o t h e

P r o f i l e s c a l c u l a t e d u s i n g these values a r e shown i n

Fig. 9, which a l s o c o n t a i n s t h e corresponding experimental p r o f i l e s and single-beam r e l i a b i l i t y f a c t o r s ( R ) determined f o r each comparison.

The six-beam R f a c t o r corresponding t o Fig. 9 i s 0.115.

T h i s value i n d i c a t e s a very good agreement between c a l c u l a t e d and experimental p r o f i l e s i n a conventional LEED a n a l y s i s and suggests t h a t t h e proposed s t r u c t u r a l model i s h i g h l y probable.

Furthermore,

t h i s R value i s s i g n i f i c a n t l y lower than any r e p o r t e d value o b t a i n e d i n a LEED a n a l y s i s o f any semiconductor surface.

The changes i n

i n t e r l a y e r spacings determined from t h i s a n a l y s i s correspond t o n e a r e s t - n e i g h b o r bond l e n g t h changes o f -0.058 and t0.075 A.

These

r e s u l t s are c o n s i s t e n t w i t h a t o t a l energy c a l c u l a t i o n f o r such a s u r f a c e which g i v e s an inward r e l a x a t i o n o f t h e outermost l a y e r ( N o r t h r u p e t al.,

1981).

I n a separate

investigation

of

a Si

(111) s u r f a c e l a s e r

annealed w i t h pulses from a doubled Nd:YAG l a s e r ( A = 530 nm), a V i d i c o n camera was used t o scan t h e LEED p a t t e r n recorded on

432

D. M. ZEHNER

(40)BEAM R = 0.466

( 0 2 ) BEAM R = 0.095

I

I

(24) BEAM R = 0.088

-

CALCULATED AVERAGE EXPERIMENTAL

4 20

00

00

(60

420

460

200

ENERGY (eV)

Fig. 9 .

A comparison o f the averaged experimental I-V p r o f i l e s with calcu= -25.5%

lated results for Adl2

and Ad23 = 3.2%.

P o l a r o i d f i l m i n o r d e r t o o b t a i n t o o b t a i n angular i n t e n s i t y prof i l e s (Chabal e t al., weak peak [-0.02 ha1 f - o r d e r

1981a).

I n these measureriients, a broad and

t i m e s t h e (11) i n t e n s i t y ]

position,

characteristic

was p r e s e n t a t t h e

o f a (2x1)

reconstruction.

From these data i t was concluded t h a t no long-range o r d e r e x i s t s b u t t h a t d i s o r d e r e d domains w i t h a buckled ( 2 x 1 ) - l i k e r e c o n s t r u c t i o n are present.

The absence o f such o b s e r v a t i o n s i n t h e pre-

v i o u s l y discussed LEED a n a l y s i s suggests t h a t surfaces prepared w i t h d i f f e r e n t l a s e r a n n e a l i n g parameters may d i s p l a y d i f f e r e n c e s i n t h e d e t a i l s o f s u r f a c e order. To examine t h e q u e s t i o n o f s u r f a c e order, scattering

medium energy i o n

combined w i t h channel i n g and b l o c k i n g ,

a technique

which is a l s o s e n s i t i v e t o geometrical s t r u c t u r e i n t h e s u r f a c e r e g i o n , has been used t o i n v e s t i g a t e t h e S i ( l l 1 ) s u r f a c e (Tromp

433

7 . PULSED LASER IRRADIATED SEMICONDUCTORS e t a1

., 1982).

I n t h i s study,

data were obtained b o t h from a

s u r f a c e e x h i b i t i n y a sharp (7x7) LEED p a t t e r n ,

prepared by con-

v e n t i o n a l procedures, and from a s u r f a c e e x h i b i t i n g a sharp (1x1) LEED p a t t e r n , prepared by i r r a d i a t i n g t h e sample w i t h a s i n g l e p u l s e from a ruby l a s e r .

From an a n a l y s i s o f t h e data i t was concluded

t h a t t h e atomic displacements on both s u r f a c e s a r e r e s t r i c t e d t o two monolayers,

probably t h e f i r s t double l a y e r o f t h e c r y s t a l .

T h i s c o n c l u s i o n i s c o n s i s t e n t w i t h t h e r e s u l t s o f t h e LEE0 a n a l y s i s . However, i n t h i s model t h e atoms i n t h e f i r s t two monolayers occupy w e l l - d e f i n e d p o s i t i o n s and should g i v e r i s e t o a s t r o n g b l o c k i n g effect.

This blocking e f f e c t

i s n o t reproduced i n t h e data,

suggesting t h a t t h e atoms may occupy d i f f e r e n t l a t e r a l p o s i t i o n s and g i v e r i s e t o less e f f i c i e n t and smeared-out b l o c k i n g .

Thus, t h e

r e s u l t s are i n c o n s i s t e n t w i t h a simple r e l a x a t i o n model and i n d i c a t e some degree o f d i s o r d e r i n t h e s u r f a c e region.

A s i m i l a r LEED a n a l y s i s has been performed on a laser-annealed Ge (111) s u r f a c e (Zehner e t a1

., 1981b).

As w i t h S i , t h e b e s t agree-

ment i s o b t a i n e d f o r a s t r u c t u r a l model i n which atoms i n t h e o u t e r most l a y e r are d i s p l a c e d inward and t h o s e i n t h e second l a y e r a r e d i s p l a c e d outward r e l a t i v e t o t h e i r b u l k p o s i t i o n s , r e s p e c t i v e l y . The corresponding nearest-nei ghbor bond l e n g t h changes are -0.037 and +0.066 A . An examination o f t h e e l e c t r o n i c s t r u c t u r e i n t h e s u r f a c e r e g i o n o f t h e laser-annealed S i (111) and Ge (111) s u r f a c e s i s o f i n t e r e s t i n view o f t h e r e s u l t s o f both t h e LEED analyses and i o n s c a t t e r i n g r e s u l t s j u s t discussed.

P h o t o e l e c t r o n spectroscopy

d i r e c t l y y i e l d s i n f o r m a t i o n about t h e l o c a l bonding b u t i s l e s s s e n s i t i v e t o t h e long-range o r d e r than LEED.

Therefore,

resolved

studies

and

anyle-i ntegrated

photoemi s s i o n

valence band s u r f a c e s t a t e s and s u r f a c e c o r e - l e v e l been performed f o r t h e f o l l o w i n g s u r f a c e s :

of

angleboth

s h i f t s have

( 1 ) laser-annealed S i

and Ge (111)-(1x1)

surfaces prepared as f o r t h e LEED s t u d i e s and

(2) S i (lll)-(7x7)

and Ge ( l l l ) - ( 2 x 8 )

s u r f a c e s prepared by t h e r -

m a l l y annealing t h e (1x1) surfaces (Himpsel e t al.,

1981).

The

434

D. M. ZEHNER

measurements were made u s i n g t h e d i s p l a y - t y p e spectrometer a t t h e s y n c h r o t r o n r a d i a t i o n source, Tantalus I. I n Fig.

10, a n g l e - i n t e g r a t e d photoemission s p e c t r a a r e pre-

sented f o r laser-annealed

(1x1) surfaces ( f u l l

curves) and f o r

t h e t h e r m a l l y annealed surfaces (dashed curves) o f Ge (111) and Si

The d o t t e d l i n e s show t h e s p e c t r a o b t a i n e d a f t e r a

(111).

hydrogen exposure, which r e s u l t s i n about a s a t u r a t i o n monolayer coverage o f hydrogen.

Below -4 eV, hydrogen induces e x t r a s t a t e s

t h a t are w e l l understood b u t n o t i m p o r t a n t i n t h i s c o n t e x t . difference

between

the

solid

(dashed)

curves

The

and t h e d o t t e d

A l l four

curves above -3 eV r e p r e s e n t s s u r f a c e - s t a t e emission.

s u r f a c e s have a d o u b l e t o f s t a t e s near t h e t o p o f t h e

clean

valence band which i s quenched by hydrogen exposure.

Relative t o

t h e t o p o f t h e valence band, these s t a t e s l i e a t -0.4

and -1.3

f o r t h e annealed S i (111) s u r f a c e s and a t -0.7 annealed Ge (111) surfaces. dependent

photoelectron

and -1.3

By u s i n g a n g l e - r e s o l v e d p o l a r i z a t i o n -

spectroscopy,

the

surface

states

determined t o have d i s t r i b u t i o n s i n momentum (Ell)-space

i n Fig.

are

and sym-

m e t r i e s which are s i m i l a r f o r a l l f o u r annealed surfaces. results

eV

f o r the

These

a r e summarized r e l a t i v e t o t h e hexagonal B r i l l o u i n zone

11.

I t is remarkable t h a t t h e predominant s u r f a c e s t a t e s f o r t h e

t h e r m a l l y annealed Ge (111) and S i (111) surfaces match t h e (1x1) s u r f a c e B r i l l o u i n zone and show no i n d i c a t i o n o f t h e small r e c i p r o cal

(2x8)

that

or

(7x7)

photoemission

unit

cells.

(This

can indeed sense

by t h e l a r g e r (1x1) u n i t c e l l i n b,, space.) t i o n f o r the S i ( l l l ) - ( 7 x 7 ) appears near t h e Fermi l e v e l

surface:

, which

observation

confirms

t h e short-range o r d e r given There i s one excep-

a weak t h i r d s u r f a c e s t a t e

makes t h i s s u r f a c e m e t a l l i c , i n

c o n t r a s t t o t h e o t h e r t h r e e surfaces.

This exception i s consistent

w i t h a band p i c t u r e , wherein t h e S i ( l l l ) - ( 7 x 7 )

s u r f a c e has t o be

m e t a l l i c because t h e r e i s an odd number o f e l e c t r o n s i n t h e ( 7 x 7 ) unit cell. tially

Each band holds two e l e c t r o n s ,

filled

band.

The

extra

surface

which leaves a parstate

for

the

Si

7.

F i g . 10.


435

Angle-integrated photoelectron spectra f o r the annealed G e ( 11 1 )

and S i ( l l 1 ) surfaces showing emission from two surface states near the top o f the valence band which i s quenched by hydrogen exposure (dotted l i n e s ) . denotes the valence-band

maximum.

Ev

436

D. M. ZEHNER

LOWER STATE

UPPER STATE

n

EXTRA STATE

EF

AT

F OR

Si (111)- (7x7)

Fig. 1 1 . Characteristic locations (dashed areas) of different surface states in the ( 1 x 1 ) surface Briliouin zone (hexagon) for the annealed G e ( l l 1 ) and S i ( l l 1 ) surfaces. At the zone center, the lower surface state has A 3 ( P ~ , ~character ) and the upper state has A, ( s , p z ) character.

(lll)-(7x7)

i s c o n c e n t r a t e d near t h e m i d d l e o f t h e edges o f a

( 7 x 7 ) surface B r i l l o u i n zone as shown i n Fig.

11, and i t s i n t e n -

s i t y i s s e n s i t i v e t o t h e long-range (7x7) order. Additional

information

about

the

surface

geometry

can be

o b t a i n e d by measuring t h e s h i f t s i n energy and i n t e n s i t y of c o r e l e v e l s f o r s p e c i f i c surface

atoms.

The s u r f a c e - s e n s i t i v e angle-

i n t e g r a t e d photoemission s p e c t r a f o r Ge(3d) and S i ( 2p) core l e v e l s ( w i t h experimental mean-free paths o f 5.9 S i , r e s p e c t i v e l y ) a r e shown i n Fig.

12.

and 5.4

A f o r Ge and

By comparing s p e c t r a f o r

c l e a n ( f u l l l i n e s ) and hydrogen-covered ( d o t t e d l i n e s ) surfaces, i t i s c l e a r t h a t t h e r e are c o r e l e v e l s a t lower b i n d i n g energies which are c h a r a c t e r i s t i c o f t h e c l e a n s u r f a c e (marked by arrows

7.


Si(111) hv=120 eV

Ge(ll1) h v = 7 0 eV

7x7

2x8

-1.0

Fig.

12.

437

- j.0 0 1 .O 0 1.0 I N I T I A L STATE ENERGY RELATIVE TO BULK (eV)

Surface-sensitive

core-level

spectra f o r the, annealed Ge( 1 1 1 )

and S i ( l l 1 ) surfaces showing s h i f t e d c o r e levels f o r special surface atoms. The Ge data consits o f spin-orbit-split

3 d 3 / 2 and 3 d g / 2 levels, whereas in

t h e S i data the 2 p 1 / 2 levels have been removed by spin-orbit D o t t e d lines are f o r hydrogen-covered Si(ll1)

-

( 2 x 1 ) + H, r e s p e c t i v e l y ] ,

l o w e r binding energies are removed.

deconvolution. ( 1 x 1 ) + H and wherein the surface core levels at

surfaces [ G e ( l l l )

-

438

D. M. ZEHNER

i n Fig.

The r e s u l t s o f a l e a s t - s q u a r e s f i t t o t h e data a r e

12).

g i v e n i n Table 1 and can be summarized as f o l l o w s :

t h e annealed

Ge (111) and S i (111) s u r f a c e s have r o u g h l y 1/4 o f a monolayer o f s u r f a c e atoms,

w i t h l a r g e core-level

l o w e r b i n d i n g energy.

s h i f t s (0.6-0.8

L i t t l e difference

eV) t o w a r d

i s observed between

t h e r m a l l y annealed and l a s e r - a n n e a l e d surfaces. TABLE I S p e c i a l S u r f a c e Atoms f o r t h e Annealed Ge( 111) and S i ( 111) Surfaces Core-level s h i f t (towards lower binding energy, M.1 e v ) (ev)

Number o f atoms in v o l ved (20.05 l a y e r ) ( 1dyer)

Ge( 111)-( 2x8)

0.75 0.35

0.28 >O. 25

Ge( 111)-( 1x1)

0.60

0.37

S i ( 111)-(7 x 7 )

0.70

0.16

S i ( 111)-( 1x1)

0.80

0.23

The s t r o n g s i m i l a r i t y o f t h e valence band s u r f a c e s t a t e s and surface core-level s p e c t r a f o r b o t h t h e l a s e r - a n n e a l e d and t h e r m a l l y annealed S i and Ge s u r f a c e s i n d i c a t e s t h a t these s u r f a c e s have very s i m i l a r l o c a l bonding geometries and d i f f e r m a i n l y i n long-range o r d e r i n v o l v i n g g e o m e t r i c a l arrangements t h a t a r e o n l y a p e r t u r b a t i o n o f t h e average l o c a l bonding geometry.

An i n t e r -

e s t i n g q u e s t i o n t h e n i n v o l v e s t h e LEED analyses (Zehner e t al., 1981a; Zehner e t al.,

1 9 8 l b ) , which g i v e such good agreement w i t h

d a t a u s i n g a model ( 1 x 1 ) geometry t h a t appears t o be d i f f e r e n t f r o m t h a t needed t o d e s c r i b e t h e s u r f a c e e l e c t r o n i c s t r u c t u r e . One p o s s i b l e e x p l a n a t i o n i s t h a t LEED i s n o t p a r t i c u l a r l y s e n i t i v e t o long-range d i s o r d e r i f i t i s p r e s e n t on t h e ( 1 x 1 ) surface. Thus,

t h e i n t e r l a y e r displacements determined may be considered

439

7 . PULSED LASER IRRADIATED SEMICONDUCTORS

t o be averages over t h e coherence l e n g t h o f t h e e l e c t r o n beam. Another relaxed,

explanation

is

that

photoemission

can

rule

out

the

ordered ( 1 x 1 ) geometry o n l y i f t h e s u r f a c e s t a t e s a r e

band-1 ike as assumed i n one-el e c t r o n band c a l c u l a t i ons [ Pandy e t 1974; S c h l i i t e r e t a1

al.,

calculations

predict

., 1975;

that

such

C i r a c i e t al.,

1975).

These

a s u r f a c e would be m e t a l l i c ,

w i t h a h a l f - f i l l e d band o f d a n g l i n g bond s t a t e s a t t h e Fermi energy, EF, and t h i s i s i n c o n s i s t e n t w i t h t h e data, which show no emission near EF f o r t h e (1x1) surfaces.

However,

correlation

e f f e c t s might be very i m p o r t a n t f o r these narrow s u r f a c e l e v e l s .

A number o f researchers (Duke e t al., al.,

1981; Lannoo e t al.,

f o r t h theoretical

1981,

1982; L o u i s e t al.,

proposals

1982; Del Sole e t 1982, 1983) have p u t

t h a t would make t h e photoemission

d a t a from t h e laser-annealed S i (111) s u r f a c e c o n s i s t e n t w i t h t h e u n r e c o n s t r u c t e d r e l a x e d s u r f a c e p r e d i c t e d by t h e LEED a n a l y s i s . I n t h e s e models i t i s assumed t h a t s t r o n g c o r r e l a t i o n s dominate the

surface

state

band

structure,

and

they

predict

a

low-

temperature a n t i f e r r o m a g n e t i c ground s t a t e and downward d i s p e r s i o n o f t h e d a n g l i n g bond s t a t e s along r-J. have n o t been t e s t e d e x p e r i m e n t a l l y .

These p r e d i c t i o n s

Nevertheless,

e f f e c t s cannot e x p l a i n t h e s i m i l a r i t y i n c o r e - l e v e l

correlation shifts for

b o t h laser-annealed and t h e r m a l l y annealed surfaces. B o t h a n g l e - i n t e g r a t e d (McKinley e t a l . (Chabal e t al.,

, 1981) and angle-resolved

1981a) photoemission data have been obtained from

laser-annealed S i (111) s u r f a c e s u s i n g d i f f e r e n t annealing conditions.

I n agreement w i t h t h e r e s u l t s j u s t discussed and w i t h

r e s u l t s o b t a i n e d i n an independent i n v e s t i g a t i o n u s i n g a ruby l a s e r (Dernuth e t al.,

1984), no occupied s t a t e s a t EF are observed.

However, t h e energies of t h e s u r f a c e s t a t e s and t h e i r d i s p e r s i o n , o b t a i n e d a f t e r i r r a d i a t i o n w i t h e i t h e r a XeCl o r frequency-doubled Nd:YAG d i f f e r somewhat from t h e r e s u l t s presented u s i n g a ruby laser.

I n fact,

i t i s argued t h a t t h e laser-annealed

surface

examined i n these s t u d i e s i s buckled w i t h no long-range o r d e r b u t w i t h a short-range ( 2 x 1 ) r e c o n s t r u c t i o n .

From these r e s u l t s and

440

D. M. ZEHNER

t h o s e o b t a i n e d from t h e r m a l l y

quenched S i

(111)

surfaces,

it

appears t h a t d i f f e r e n t l a s e r a n n e a l i n g c o n d i t i o n s (depth o f m e l t , r e g r o w t h v e l o c i t y ) can r e s u l t i n d i f f e r e n t l o c a l bonding arrangements.

8.

VICINAL SURFACES The

chemical

influence o f

steps

reactivity of

on t h e e l e c t r o n i c

properties

semiconductor s u r f a c e s a r e well

and

known.

Stepped ( v i c i n a l ) s u r f a c e s can be prepared by i n s i t u c l e a n i n g o r i o n etching,

but t h e control o f step density,

step height,

and

ease o f r e p r o d u c i b i l i t y has proved d i f f i c u l t u s i n g t h e s e convent i o n a l procedures.

The r a p i d m e l t i n g and regrowth achieved w i t h

l a s e r a n n e a l i n g suggest t h a t t h i s procedure can be used w i t h vicinal

surfaces t o produce s u r f a c e s c o n t a i n i n g monatomic steps

and u n i f o r m t e r r a c e widths. To demonstrate t h a t such s u r f a c e s can be produced, o b t a i n e d from a S i ( l l 1 ) f r o m a (111) plane (Zehner e t a1

c r y s t a l whose s u r f a c e was c u t a t 4.3'

toward t h e

., 1980b).

results

[ i i 2 ] d i r e c t i o n w i l l be discussed

F o r t h i s d i r e c t i o n , t h e edge atoms have

o n l y two n e a r e s t neighbors.

The w e l l - d e f i n e d (1x1)

LEED p a t t e r n

o b t a i n e d from t h e c l e a n s u r f a c e and shown i n Fig. 13 ( a ) was observed after

i r r a d i a t i n g t h e s u r f a c e with f i v e p u l s e s a t -2.0

J/cm2.

The p a t t e r n i n d i c a t e s t h e e x i s t e n c e o f a stepped s u r f a c e which can be indexed [ 1 4 ( 1 1 1 ) x ( i i 2 ) ] . energy, and [ O l ]

By v a r y i n g t h e p r i m a r y e l e c t r o n

t h e t h r e e f o l d spot s p l i t t i n g a l t e r n a t e s between t h e r e f l e c t i o n s a t s p e c i f i c e l e c t r o n energies.

[lo]

The energies

a t which a g i v e n r e f l e c t i o n i s s p l i t or n o n s p l i t g i v e s p e c i f i c i n f o r m a t i o n on t h e s t e p h e i g h t , and t h e angular s e p a r a t i o n between s p l i t spots provides i n f o r m a t i o n on t h e t e r r a c e width.

An a n a l y s i s

o f t h e spot s p l i t t i n g s i n t h i s p a t t e r n u s i n g o n l y a k i n e m a t i c t r e a t m e n t o f s i n g l e s c a t t e r i n g from t h e t o p l a y e r (Henzler, 1970) i n d i c a t e s t h a t t h e s u r f a c e c o n s i s t s o f monatomic s t e p s w i t h an average s t e p h e i g h t o f one double l a y e r (3.14 w i d t h s -45 A as i l l u s t r a t e d i n Fig. 14.

A)

with terrace

The absence o f f r a c t i o n a l

7.

Fig.

13.

441


LEED

patterns from clean vicinal

beam energies o f ( a ) 40 and ( b ) 68 eV.

Si(ll1 )

surfaces a t primary

( a ) Laser annealed, (b) thermally

annealed.

LASER ANNEALED

-

( 4 x 4 ) WITH SPLIT SPOTS

4;3"

THERMALLY ANNEALED

-

(7 x 7 )

4.30

Fig.

14.

Schematic

the (710) plane.

illustration o f the vicinal

Top view i s for the laser-annealed

surface projected into surface.

Bottom view

illustrates a possible configuration obtained with thermal annealing.

442

D. M. ZEHNER

order r e f l e c t i o n s ,

i n d i c a t i v e of

reconstruction,

suggests t h a t

t h e l o c a l atomic arrangement produced by t h i s a n n e a l i n g procedure may be s i m i l a r t o t h a t produced on f l a t (111) surfaces. I n o r d e r t o achieve such a h i g h step d e n s i t y c o n f i g u r a t i o n , a l a r g e amount o f atom motion has t o t a k e place.

T h i s movement can

be accomplished e i t h e r by e v a p o r a t i o n o f s u r f a c e atoms o r by d i f f u s i o n i n t h e molten phase.

R e s u l t s o f r e c e n t experiments w i t h

stepped s u r f a c e s (Osakabe e t al., evaporation 1475

K.

1980, 1981) show t h a t some

t a k e s p l a c e a t step edges a t temperatures as low as

Assuming m e l t i n g occurs d u r i n g t h e l a s e r a n n e a l i n g con-

d i t i o n s used, about

lo9

atoms/cm2 evaporate i n a 10-ns p u l s e f o r

an e v a p o r a t i o n r a t e o f 1017 atoms/cmzs [vapor p r e s s u r e 5 x 10-3 T o r r (Chabel e t al., monolayer,

T h i s corresponds t o o n l y 10-6 o f a

1982)].

which i s n o t enough t o account f o r t h e l a r g e atomic

rearrangements over hundreds o f angstroms. mechanism must dominate, be e s t i m a t e d D

2

Thus,

the diffusion

and a s u r f a c e d i f f u s i o n c o e f f i c i e n t can’

(100 A ) 2 / ( 10 ns) -loe4 cm2/s.

This high d i f -

f u s i o n c o e f f i c i e n t would be q u i t e i n c o m p a t i b l e w i t h a nonthermal model

of

l a s e r a n n e a l i n g b u t i s c o n s i s t e n t w i t h experimental

measurements ( N i shizawa e t a1 the melting

point,

the

., 1972).

surface

A t temperatures c l o s e t o

arrangement

i s dominated

by

e n t r o p y , which i s r e s p o n s i b l e f o r a s t e p - s t e p r e p u l s i o n (Gruber et al., disorder

1967).

is

As t h e c r y s t a l c o o l s down and t h e e n t r o p y - d r i v e n

reduced,

the

surface d i f f u s i o n

decreases

t o the

e x t e n t t h a t t h e steps cannot recombine; t h e y remain f r o z e n i n t h e high-temperature c o n f i g u r a t i o n . The s t a b i l i t y o f t h e r e g u l a r a r r a y o f steps was i n v e s t i g a t e d by s u b j e c t i n g t h e laser-annealed s u r f a c e t o a s e r i e s o f thermala n n e a l i n g t r e a t m e n t s a t h i g h e r and h i g h e r temperatures. f o r f l a t (111) S i surfaces,

As observed

thermal annealing o f t h e c r y s t a l t o

temperatures g r e a t e r than -800 K r e s u l t e d i n a s u r f a c e from which t h e ( 7 x 7 ) d i f f r a c t i o n p a t t e r n shown i n Fig. 13 ( b ) was o b t a i n e d i n accord w i t h p r e v i o u s o b s e r v a t i o n s (Olshanetsky e t a l .

, 1979).

The

absence o f s p l i t t i n g of i n d i v i d u a l spots i n d i c a t e s t h e e l i m i n a t i o n

7.

443


o f t h e r e g u l a r a r r a y o f monatomic steps, and t h e sharpness o f t h e integral-order

reflections

i s c o n s i s t e n t w i t h a s u r f a c e having

t e r r a c e s wider than -200

A.

macroscopic

inclination,

multilayer

illustrated

i n Fig.

14.

I n o r d e r t o m a i n t a i n t h e average steps must be present

as

A s u r f a c e c o n t a i n i n g monatomic steps

c o u l d be regenerated by i r r a d i a t i n g t h e t h e r m a l l y annealed surface with the laser.

These o b s e r v a t i o n s i n d i c a t e t h a t i t i s

p o s s i b l e t o produce r e p e a t e d l y a p a r t i c u l a r s t e p arrangement by i n i t i a l l y c u t t i n g the crystal t o the desired orientation. I n v e s t i g a t i o n s o f v i c i n a l S i (111) s u r f a c e s c u t a l o n g t h e [ i i 2 ] d i r e c t i o n have produced r e s u l t s very s i m i l a r t o those discussed above (Chabal e t al.,

1981b).

Steps along t h i s d i r e c t i o n c o n t a i n

edge atoms t h a t have t h r e e nearest neighbors.

Although d e t a i l e d

s t u d i e s on t h e angular p r o f i l e s show t h e step h e i g h t t o be 3.06 A i n t h i s d i r e c t i o n , somewhat l e s s than t h e d o u b l e - l a y e r separation, t h e o v e r a l l behavior f o r laser-annealed v i c i n a l surfaces i s t h e same f o r b o t h types o f steps.

9.

DEFECTS As a consequece o f m e l t i n g d u r i n g t h e l a s e r annealing process,

atoms a r e evaporated from t h e s u r f a c e region.

I n f a c t , measure-

ment o f S i p a r t i c l e emission d u r i n g e v a p o r a t i o n u s i n g a c l a s s i c a l t i m e - o f - f l i g h t technique has been used t o determine t h e l a t t i c e temperature and t o demonstrate t h a t me1 t i n g occurs ( S t r i t z k e r e t al.,

1981).

I n a d d i t i o n t o n e u t r a l p a r t i c l e emission, b o t h i o n

and e l e c t r o n e j e c t i o n s have been d e t e c t e d (Moison, e t a1

., 1982).

The t h r e s h o l d f l u e n c e r e q u i r e d f o r d e t e c t i o n o f such p a r t i c l e emission bas been determined f o r a number o f m a t e r i a l s (Moison e t al.,

1983).

R e s u l t s o b t a i n e d f o r InP and GaAs s i n g l e c r y s t a l s

a r e c o n s i s t e n t w i t h AES and RBS observations, erential Si,

l o s s o f t h e more v o l a t i l e component.

indicating a prefI n t h e case o f

t h e amount o f m a t t e r removed was observed t o be orders o f

magnitude l e s s .

444

D. M. ZEHNER

The o b s e r v a t i o n t h a t e v a p o r a t i o n occurs d u r i n g l a s e r a n n e a l i n g i n d i c a t e s t h a t t h e c r e a t i o n o f d e f e c t s i s p o s s i b l e and t h a t d u r i n g t h e quenching p e r i o d a c o m p e t i t i o n t a k e s p l a c e between t h e e l i m i n a t i o n o f d e f e c t s c r e a t e d a t t h e m e l t i n g temperature and t h e growth o f an ordered s u r f a c e

region.

T h i s p o s s i b i l i t y may be par-

t i c u l a r l y i m p o r t a n t i n t h e case o f r e c o n s t r u c t e d s u r f a c e l a y e r s . I f t h e d e f e c t s a r e n o t e l i m i n a t e d f a s t enough,

t h e y may impede

growth o f t h e s u p e r s t r u c t u r e by v a r i o u s mechanisms.

Thus,

the

r e g r o w t h v e l o c i t y o f t h e m e l t f r o n t may p l a y an i m p o r t a n t r o l e i n the d e t a i l s o f the

f i n a l geometric o r d e r i n g .

been suggested (Chabel e t al.,

I n fact,

i t has

1982) t h a t t h e d i f f e r i n g photo-

emission r e s u l t s o b t a i n e d i n independent laser-anneal i n g s t u d i e s can be i n t e r p r e t e d as a consequence o f d i f f e r e n t f i n a l

state

geometric o r d e r i n g due t o d i f f e r e n t regrowth v e l o c i t i e s . I n order

t o e x p l o r e t h e dependence on regrowth v e l o c i t y ,

measurements have been made subsequent t o annealing w i t h a pulsed XeCl excimer l a s e r ( A = 308 nm) (Zehner e t a1

., 1984a).

Measurements

were made a f t e r l a s e r a n n e a l i n g t h e c r y s t a l w i t h an energy d e n s i t y i n t h e range 1.0-4.0 (Wood and G i l e s ,

J/cm2.

Standard heat f l o w

calculations

1981) have been used t o e s t a b l i s h t h a t a v a r i a -

t i o n i n regrowth v e l o c i t y from 1 m/s a t 4.0 J / c d t o 4.5 1.0

J/cm2

m/s a t

can be o b t a i n e d w i t h t h e excimer l a s e r used i n t h i s

experiment (see Chapter 4).

R e s u l t s o b t a i n e d from p h o t o e l e c t r o n

spectroscopy

determine

were

used

to

s t r u c t u r e o f v a r i o u s laser-annealed annealed S i ( l l 1 )

-

(7x7),

the

surface

-

S i ( 111)

and cleaved S i ( l l 1 )

-

electronic

(1x1)

,

thermally

( 2 x 1 ) surfaces.

The s u r f a c e s t a t e s near t h e t o p o f t h e band are i m p o r t a n t s i n c e t h e y have c h a r a c t e r i s t i c energies and angular d i s t r i b u t i o n s t h a t have been s t u d i e d p r e v i o u s l y (Zehner e t al.,

1 9 8 1 ~ ) . I n Fig.

15

t h e energy d i s t r f b u t i o n s o f s u r f a c e s t a t e s near t h e t o p o f t h e valence band a r e shown f o r v a r i o u s S i ( l l 1 ) surfaces.

As evidenced

by t h e s e n s i t i v i t y t o hydrogen exposure ( n o t shown), s u r f a c e s e x h i b i t t h r e e dominant Fig.

the (7x7)

s u r f a c e s t a t e s l a b e l e d 1-3 i n

15 and t h e ( 2 x 1 ) s u r f a c e i s dominated by two s u r f a c e s t a t e s

7.

445


-6

-5

-3

4

-2

-1

0

1

INITIAL ENERGY ( RELATIVE TO VALENCE BAND MAXIMUM )

Fig.

15.

Angle-integrated

spectra from freshly cleaved S i ( 1 1 1 )-( 2x1 )

,

UV (308 nm XeCI) laser-annealed Si( 1 1 1 )-( 1x1 ) produced with 1, 2, 3 , and 4 J / c m 2 pulses, ruby (694 n m ) laser-annealed Si( 1 1 1 )(lxl) produced with a 2 J /cm2 pulse, and

Si (1 1 1 )-(7x7 )

obtained by thermal annealing.

446

D. M. ZEHNER

l a b e l e d 4 and 5.

For t h e v a r i o u s laser-annealed surfaces,

two

s u r f a c e s t a t e s which c l o s e l y resemble s t a t e s 1 and 2 on t h e ( 7 x 7 ) s u r f a c e and a r e t o t a l l y d i f f e r e n t from those observed on t h e c l e a v e d (2x1) s u r f a c e were i d e n t i f i e d . change

of

variation

this of

surface

state

regrowth v e l o c i t y

Moreover, no s i g n i f i c a n t

structure from

was

1-4.5

observed over

m/s,

apart

a

from a

weakening o f t h e s u r f a c e s t a t e s f o r t h e f a s t e s t regrowth velocity.

T h i s weakening c o u l d be due t o t h e onset o f d i s o r d e r when

t h e energy d e n s i t y employed approaches t h e me1t t h r e s h o l d . I t i s known t h a t t h e S i ( l l 1 ) s u r f a c e undergoes a s t r u c t u r a l

change from (7x7) ( B e n n e t t e t al.,

t o (1x1) a t a c r y s t a l temperature o f 1150 K 1981).

For very low c o o l i n g r a t e s t h e s t r u c -

t u r a l t r a n s i t i o n i s reversible,

b u t i f quenching r a t e s exceed

approximately

lo2

irreversible.

Consequently, t h e quenching r a t e a t t h e t r a n s i t i o n

K / s (Hagstrum e t al.,

1973), t h e t r a n s i t i o n i s

temperature, subsequent t o l a s e r i r r a d i a t i o n , may be i m p o r t a n t i n When S i ( 111) i s quenched

d e t e r m i n i n g t h e s u r f a c e s t a t e spectra.

t h r o u g h t h e t r a n s i t i o n temperature a t 102 K/s, s u r f a c e s t a t e s p e c t r a s i m i l a r t o those f o r t h e laser-annealed surfaces shown i n Fig. 15 a r e observed (Eastman e t al.,

1980b).

Heat f l o w c a l c u l a t i o n s f o r

l a s e r a n n e a l i n g a t 1.0 J/cm2 p r e d i c t a quenching r a t e o f 1010 K / s a t 1150 K.

Thus,

s i m i l a r s u r f a c e s t a t e s p e c t r a are observed f o r

quenching r a t e s between 102 and 1010 K/s.

I f t h e quenching r a t e

a t t h e t r a n s i t i o n temperature i s o f importance i n d e t e r m i n i n g t h e surface

state

spectra,

rates

i n excess

of

1010 K / s

will

be

necessary t o produce s u r f a c e s t a t e f e a t u r e s s i m i l a r t o those o f t h e (2x1) surface.

Furthermore, f o r regrowth v e l o c i t i e s g r e a t e r

t h a n 15 m/s, where an amorphous l a y e r i s formed ( C u l l i s e t a l . , 1982),

one would expect t o see s u b s t a n t i a l

differences i n the

s u r f a c e s t a t e spectra. Additional

i n f o r m a t i o n about b o t h t h e s i m i l a r i t i e s and d i f -

ferences i n geometric s u r f a c e s t r u c t u r e f o r s u r f a c e s prepared by d i f f e r e n t t r e a t m e n t s can be obtained by i n v e s t i g a t i n g a d s o r p t i o n phenomena.

The technique o f h i g h - r e s o l u t i o n i n f r a r e d spectroscopy

7.

447


(Chabel , 1983) has been used t o study t h e v i b r a t i o n a l spectrum o f hydrogen chemisorbed on S i ( l l l ) - ( 7 x 7 ) prepared by thermal a n n e a l i n g and S i ( l l 1 ) - ( 1 x 1 )

prepared by l a s e r annealing.

T h i s technique

g i v e s d i r e c t i n f o r m a t i o n on t h e number, p o s i t i o n , and p o i a r i z a t i o n o f d a n g l i n g bonds, which a r e present a t t h e s u r f a c e o f a semiconductor.

For coverages as low as 1.5% o f a monolayer o f hydrogen

on t h e S i ( l l l ) - ( 7 x 7 ) observed.

surface,

two d i s i i n c t a d s o r p t i o n peaks are

Each peak corresponds t o a S i - H s t r e t c h i n g v i b r a t i o n

f o r hydrogen chemisorbed a t d i f f e r e n t s i t e s .

By i n v e s t i y a t i n g

t h e change i n i n t e n s i t y and energy o f these v i b r a t i o n s i t i s concluded t h a t a unique chemisorption s i t e e x i s t s on t h i s s u r f a c e and i s recessed from t h e outermost plane.

R e s u l t s o b t a i n e d from

t h e laser-annealed S i ( 1 1 1 ) - ( 1x1) s u r f a c e show o n l y one a d s o r p t i o n peak.

The peak a s s o c i a t e d w i t h t h e unique a d s o r p t i o n s i t e i s

absent.

This

observation

strongly

suggests

that

the

unique

a d s o r p t i o n s i t e on t h e (7x7) s u r f a c e i s a r e s u l t o f long-range rearrangement

which

i s absent

on t h e

1aser-anneal ed surface.

Since b o t h a (1x1) u n r e c o n s t r u c t e d b u t r e l a x e d s u r f a c e as d e t e r mined i n t h e LEED a n a l y s i s and a m o s t l y d i s o r d e r e d s u r f a c e as determined by PES would n o t c o n t a i n such a w e l l d e f i n e d hole, t h e s e r e s u l t s cannot be used t o d i s c r i m i n a t e between t h e proposed structures. Rare gas t i t r a t i o n i s another technique used t o i n v e s t i g a t e geometric s t r u c t u r e .

The approach employed i s based on t h e concept

t h a t d i f f e r e n t geometric a d s o r p t i o n s i t e s f o r r a r e gas atoms can have d i f f e r e n t l o c a l work f u n c t i o n s .

Such l o c a l work f u n c t i o n

d i f f e r e n c e s produce d i f f e r e n t e l e c t r o n b i n d i n g energies re1a t i v e t o EF f o r these adsorbed atoms,

which a l l o w t h e d e l i n e a t i o n o f

v a r i o u s s i t e s as w e l l as t h e d e t e r m i n a t i o n o f t h e i r r e l a t i v e conc e n t r a t i o n s when examined w i t h PES. Si(lll)-(7x7) show

Recent i n v e s t i g a t i o n s o f t h e

s u r f a c e f o r xenon a d s o r p t i o n (Demuth e t a1

coverage-dependent

changes

i n t h e measured

., 1984)

PES b i n d i n g

e n e r y i e s a t b o t h h i g h and low coverages i n e i t h e r a d s o r p t i o n (as l o n g as near e q u i l i b r i u m a d s o r p t i o n c o n d i t i o n s a r e maintained) or

448

D. M. ZEHNER

d e s o r p t i o n experiments.

The sequence and number o f a d s o r p t i o n

s i t e s found f o r t h i s s u r f a c e are c o n s i s t e n t w i t h ( 1 ) a s p e c i a l h i g h b i n d i n g energy s i t e a t low coverages,

(2) a majority o f

nearly

equivalent

sites

over

surface

higher

coverages

where

rare-gas

most o f

the

adatom

(including

interactions

become

and ( 3 ) another t y p e o f m i n o r i t y s i t e p r i o r t o f o r -

important),

m a t i o n o f condensed o r m u l t i l a y e r s .

These r e s u l t s are c o n s i s t e n t

w i t h proposed s t r u c t u r a l models f o r t h e (7x7) s u r f a c e which have adatoms. (ruby

S i m i l a r measurements have been made on a laser-annealed S i ( 111)-(1x1)

laser)

surface.

The s i m i l a r i t i e s

i n the

r e s u l t s o b t a i n e d from t h i s s u r f a c e and those from t h e (7x7) surf a c e suggest t h e e x i s t e n c e o f adatoms.

This conclusion i s i n

c o n t r a s t t o t h e LEED r e s u l t s s u p p o r t i n g a f l a t ,

compressed s u r -

face.

A s t e p can be t r e a t e d as a d e f e c t and ordered a r r a y s o f such d e f e c t s produced by l a s e r a n n e a l i n g have been considered i n t h e discussion o f v i c i n a l

It i s w e l l

surfaces.

known t h a t l a s e r -

annealed s u r f a c e s have a r i p p l e d topography when examined on a macroscopic s c a l e (Leamy e t al.,

1978).

T h i s i m p l i e s t h a t steps,

randomly d i s t r i b u t e d , must e x i s t on such surfaces.

The p o s s i b i l -

i t y t h a t t h e S i ( 111)-(1x1) s u r f a c e s t r u c t u r e observed a f t e r l a s e r

annealing

can

be

associated

(Haneman,

1982; Moisum e t al.,

m i n i m i z e i t s f r e e energy.

with

steps

1983).

has

been

A surface reconstructs t o

The l o w e r i n g i n f r e e energy achieved

by r e c o n s t r u c t i o n can be e s t i m a t e d t h e o r e t i c a l l y , g r e a t accuracy, entropy.

considered

but not w i t h

due t o d i f f i c u l t i e s w i t h c o r r e l a t i o n e f f e c t s and

The presence o f s t r a i n w i l l t e n d t o oppose t h i s e f f e c t .

Based on

results

suggested (Haneman,

from a v a r i e t y

o f experiments

it

has been

1982) t h a t a s t r a i n e d r e g i o n a t t h e base o f

s t e p s on laser-annealed ( 111) surfaces causes s u r f a c e r e c o n s t r u c t i o n t o be i n h i b i t e d ,

r e s u l t i n g i n a (1x1)

surface structure.

Furthermore, i t i s suggested t h a t t h e behavior o f (100) surfaces, where t h e laser-annealed s t r u c t u r e i s t h e same as t h a t produced by thermal

annealing,

i s then

not

unexpected s i n c e t h e s t e p

7.

449


s t r u c t u r e s are o f d i f f e r e n t c r y s t a l l o g r a p h y and t h e r e i s no s i m i l a r evidence f o r step-associated s t r a i n .

V.

Surface and Subsurface S t u d i e s o f Ion-Implanted S i l i c o n

P r e v i o u s i n v e s t i y a t i o n s (White e t al.,

1980b) have shown t h a t

group I11 o r V i m p l a n t s occupy s u b s t i t u t i o n a l s i t e s subsequent t o l a s e r annealing and t h a t , as a consequence o f b o t h t h e h i g h l i q u i d phase d i f f u s i v i t i e s and t h e h i g h values o f d i s t r i b u t i o n c o e f f i c i e n t s , t h e y are a b l e t o d i f f u s e i n t o t h e c r y s t a l d u r i n g t h e regrowth I n c o n t r a s t , i t has been shown (White

process a f t e r i r r a d i a t i o n . e t al.,

1980c) t h a t those i m p l a n t s which do n o t form c o v a l e n t

bonds e x h i b i t , dependiny on t h e i m p l a n t dose, s e g r e g a t i o n t o t h e s u r f a c e as w e l l as t h e f o r m a t i o n o f a c e l l s t r u c t u r e subsequent t o l a s e r annealing.

The RBS and secondary i o n mass spectroscopy

( S I M S ) techniques employed i n these i n v e s t i g a t i o n s p r o v i d e d e t a i l e d

i n f o r m a t i o n about t h e d i s t r i b u t i o n w i t h respect t o depth b u t prov i d e no i n f o r m a t i o n about t h e c o n c e n t r a t i o n i n t h e s u r f a c e r e g i o n (15) o f pulses where RBS r e s u l t s i n d i c a t e uniform concentration liquid-solid

interface.

from t h e subsurface

r e y i o n down t o t h e

I t i s d i f f i c u l t t o q u a n t i f y t h e AES

r e s u l t s t o t h e same degree as can be done w i t h t h e RBS data. Thus, a l t h o u g h i t can be concluded t h a t a r e d u c t i o n i n concentrat i o n occurs w i t h m u l t i p l e - p u l s e i r r a d i a t i o n , t h e r e c o u l d s t i l l be a p o s s i b l e chanye i n c o n c e n t r a t i o n i n going from t h e s u r f a c e t o subsurface r e g i o n t h a t

i s a consequense o f t h e surface-vacuum

451


5

2

5

Fig.

16.

in S i ( 1 0 0 )

E f f e c t o f laser annealing on dopant profiles for As implanted as determined by RBS.

Profile

results are

implanted condition and subsequent to laser annealing at -2.0

shown

for

J/cm2.

as-

452

D. M. ZEHNER

gJ O

2

I

I

I

I

I

I

I

4 6 0 10 12 NUMBER OF PULSES (E0-2.1 J/cm2)

14

Fig. 17. Plot of the ratio of the As M W ( 3 1 e V ) to Si L W ( 9 1 eV) 4,5 283 Auger transition intensities as a function of the number of laser pulses.

interface. v a r i e t y of

Similar

Auger

results

have

been

obtained

for

a

i m p l a n t e d doses o f s u b s t i t u t i o n a l dopants i n S i ( l 0 U )

and (111) c r y s t a l s .

To examine t h e e f f e c t o f t h e i m p l a n t e d s p e c i e s on s u r f a c e o r d e r , LEED p a t t e r n s have been o b t a i n e d from t h e same 7%-implanted S i ( 1 0 0 ) sample.

Only a very weak,

was observed a f t e r

one l a s e r pulse.

p o o r l y d e f i n e d LEED p a t t e r n F o l l o w i n g two p u l s e s o f

i r r a d i a t i o n , t h e p a t t e r n shown a t t h e t o p o f F i g . 18 was obtained. I n t e g r a l o r d e r beams a r e observed, as w e l l as weak s t r e a k s between them.

With a d d i t i o n a l l a s e r pulses t h e s t r e a k s b e g i n t o coalesce


Fig. 18.

LEED patterns from an As-implated Si( 100) surface a t a primary

beam energy of 49 eV.

-2.0

453

Patterns are shown subsequent to laser annealing at

J / m 2for ( a ) 2 , ( b ) 5 , and ( c ) 10 pulses.

454

D. M.ZEHNER

i n t o half-order reflections, surface structure.

i n d i c a t i n g t h e f o r m a t i o n o f a (2x1)

They c o n t i n u e t o become sharper and more

i n t e n s e w i t h a d d i t i o n a l pulses, as shown i n t h e f i g u r e .

However,

t h e p a t t e r n observed a f t e r t e n l a s e r pulses i s n o t as good as t h a t o b t a i n e d from a v i r g i n Si(100) c r y s t a l as shown i n Fig. 5 and t h u s i n d i c a t e s t h e presence o f d i s o r d e r i n t h e s u r f a c e region. Nevertheless,

i t i s i n t e r e s t i n g t o note t h a t t h e (2x1) LEE0 pat-

t e r n shows t h e e x i s t e n c e o f t h e r e c o n s t r u c t e d surface, s i m i l a r t o that

obtained

results

have

from a been

virgin

obtained

Si(100) for

a

crystal.

variety

Similar

of

LEED

substitutional

dopants i n Si(100) w i t h t h e q u a l i t y o f t h e LEED p a t t e r n o b t a i n e d f o r a s p e c i f i c l a s e r annealing c o n d i t i o n decreasing w i t h i n c r e a s i n g i m p l a n t dose.

I n c o n t r a s t t o these o b s e r v a t i o n s , (1x1) LEED p a t t e r n s were o b t a i n e d from S i ( l l 1 ) c r y s t a l s i m p l a n t e d w i t h a group I11 o r V dopant and then l a s e r annealed.

The p a t t e r n s are o f much h i g h e r

q u a l i t y a f t e r a g i v e n number o f l a s e r pulses when compared w i t h t h o s e obtained from t h e (100) surfaces, and t h e y show no evidence o f ordered l a t e r a l r e c o n s t r u c t i o n . The o b s e r v a t i o n t h a t l a s e r anneal i n y can be combined w i t h i o n i m p l a n t a t i o n t o p r o v i d e semiconductor s u r f a c e r e g i o n s c o n t a i n i n g n o v e l doping c o n c e n t r a t i o n s ( s u p e r s a t u r a t e d a l l o y s ) suggests t h a t t h e s e t e c h n i q e s may be used t o a l t e r o r t a i l o r t h e e l e c t r o n i c s t r u c t u r e i n t h i s region.

To examine t h i s p o s s i b i l i t y , photoemis-

s i o n techniques have been used t o i n v e s t i y a t e h i g h l y degenerate n-type S i ( l l 1 )

-

(1x1) surfaces as a f u n c t i o n o f As c o n c e n t r a t i o n

up t o - 5 x lO21/cm3 (-10 a t . %) and degenerate p-type S i ( l l 1 )

-

( 1 x 1 ) surfaces as a f u n c t i o n o f B c o n c e n t r a t i o n up t o -1 x 1021/cm3

( - 2 at.

% ) (Eastman e t a1

centrations electrically

are

about

., 1981).

10 and

These maximum doping con-

3 times

the concentrations o f

a c t i v e As and B a c h i e v a b l e by c o n v e n t i o n a l t e c h -

niques, r e s p e c t i v e l y . Angl e - i n t e g r a t e d photoemi s s i on s p e c t r a f o r t h e valence bands a r e presented i n F i g . 19 f o r i n t r i n s i c S i ( l l 1 )

-

( l x l ) , degenerate

7.

455


I

I

I

I

h u = 21 eV s / p POL. ANGLE-INTEG.

I

I

I

1 I

A.R.

A

7% AS 1.1 eV

INTRINSIC^

X- POCKEl

I

-I!

‘\ \“‘z”

(EF-EvIs =0.5

1 -8

-6

-2

-4

ENERGY (eV) Fig. valence

19.

Photoemission spectra ( p a r t i a l density o f states PDOS) for the

bands

of

highly doped Si. states.

laser-annealed

( 1 11 )-( 1 x 1 )

The levels near -0.4

ES, ES, and E denote the v c F

band minimum, and Fermi-level

surfaces o f

and -1 .3

valence-band

intrinsic

and

eV are due to surface maximum,

positions at the surface.

conduction-

456

D. M.ZEHNER

n-type As-doped ( 4 and 7 a t . %) S i ( l l 1 ) ( 1 a t . %) S i ( l l 1 )

p-type B-doped are

normalized

to

constant

-

-

( l x l ) , and degenerate

(1x1) surfaces.

total

emission

The s p e c t r a

within

5

eV

of

EF, and e n e r y i e s are g i v e n r e l a t i v e t o t h e valence-band maximum a t the

surface

(E:).

EF i s seen t o eV above E:

from 0.25

(i.e.,

shift

for the

markedly w i t h

doping

B-doped sample t o t h e con-

d u c t i o n band minimum Ec = 1.1 eV f o r t h e 7% As-doped sample). Relative t o i n t r i n s i c Si, doping,

f o r h i g h l y degenerate ( 1 a t .

%) B

t h e two s u r f a c e s t a t e s are u n a l t e r e d , and t h e p r i n c i p a l

changes a r e t h a t EF moves down by 0.25 eV and t h e s u r f a c e becomes metallic. at.

More dramatic e f f e c t s are seen w i t h As doping.

At 4

% As doping,

t h e s u r f a c e s t a t e s have become s i g n i f i c a n t l y

EF

has i n c r e a s e d by 0.1 eV r e l a t i v e t o t h e i n t r i n -

altered, while sic Si.

That i s ,

t h e upper "sp,-like"

d a n g l i n g bond s t a t e has

become much weaker and s h i f t e d upward i n energy by -0.3 l o w e r -1.4

eV; t h e

eV s t a t e has i n c r e a s e d s i g n i f i c a n t l y i n i n t e n s i t y , b u t

i t i s u n s h i f t e d i n energy;

w i t h new s t a t e s near

EF.

and t h e s u r f a c e has become m e t a l l i c

As t h e dopiny i s f u r t h e r i n c r e a s e d from

4 t o 7 a t . %, EF r a p i d l y s h i f t s and becomes pinned a t t h e conduct i o n band minimum Ec.

Also,

t h e upper sp,-like

surface s t a t e

c o n t i n u e s t o d i m i n i s h i n i n t e n s i t y so as t o be n e a r l y impercept i b l e by 7 a t .

% doping,

extremely intense. become occupied,

and t h e lower s u r f a c e s t a t e becomes

The conduction-band minima

( A min)

near X

and emission from these minima i s observed as

i n t e n s e e l l i p t i c a l lobes i n angle-resolved photoemission s p e c t r a ( d o t t e d l i n e l a b e l e d "AR"

i n Fig. 19).

By d e p o s i t i n g a t h i n Au

f i l m on t h i s s u r f a c e i t was p o s s i b l e t o show v i a S i 2p c o r e - l e v e l

measurements t h a t EF remained unchanged ( w i t h i n -50 meV). a "zero-barrier-height" e l e c t r i c a l purposes,

Thus,

Schottky b a r r i e r was formed , a l t h o u g h f o r

t h e Au-Si

i n t e r f a c e i s undoubtedly shorted

because o f t h e extreme degenerate n-type doping.

457

7. PULSED LASER IRRADIATED SEMICONDUCTORS 11.

INTERSTITIAL IMPLANTS I n o r d e r t o determine t h e e f f e c t s o f i n t e r s t i t i a l i m p l a n t s on

surface properties,

i n v e s t i g a t i o n s o f t h e segregation and zone

r e f i n i n g o f i m p u r i t i e s t o t h e s u r f a c e r e g i o n f o l l o w i n g pulsed 1 aser anneal ing have been performed. a c q u i r e d i n these s t u d i e s , implanted w i t h lOl5/cm2,

To i11u s t r a t e t h e r e s u l t s

data o b t a i n e d u s i n g S i ( l l 1 ) samples

Fe t o doses o f 1.13

x 1015 atoms/cm2,

x

6.0

and 1.8 x 10’6 atoms/cm2 and w i t h Cu t o a dose o f 6.9 x

101s atoms/cm2 and l a s e r annealed a t -2.0

J/cm2 (Zehner e t a1

.,

1984b) w i 11 be discussed.

As mentioned p r e v i o u s l y , examination w i t h AES showed t h a t a l l samples were covered w i t h insertion

i n t h e UHV

large quantities

system,

as

shown f o r

i m p l a n t e d w i t h Fe a t t h e t o p o f Fig. 20.

o f 0 and C a f t e r a Si(ll1)

sample

(Compare w i t h s i m i l a r

o b s e r v a t i o n s f o r v i r g i n S i c r y s t a l s as shown i n Fig.

1.)

The

s u r f a c e s were then s p u t t e r e d with 1000 eV Ar+ ions, which r e s u l t e d i n t h e removal o f most o f t h e 0 and C s u r f a c e contaminants as shown i n Fig. 20.

Auger s i g n a l s from t h e implanted species c o u l d

n o t be detected a f t e r t h i s t r e a t m e n t .

Following i r r a d i a t i o n w i t h

one l a s e r pulse, AES s p e c t r a showed t h e i m p l a n t e d species t o be p r e s e n t i n t h e s u r f a c e region.

T h i s i s i l l u s t r a t e d i n Fig. 20,

where Fe Auger s i g n a l s a t 46 and 703 eV are r e a d i l y detected. F o r t h e low dose case, l i t t l e i n c r e a s e i s observed i n t h e i n t e n s i t y o f t h e Fe Auger s i g n a l o b t a i n e d from a s u r f a c e i r r a d i a t e d w i t h a d d i t i o n a l pulses.

A1 though a t i n t e r m e d i a t e doses several

p u l s e s (two o r t h r e e ) are s u f f i c i e n t t o produce t h e s u r f a c e conc e n t r a t i o n t h a t r e s u l t s i n t h e maximum Fe Auger s i g n a l i n t e n s i t y , i n t h e h i g h dose case a t l e a s t f i v e pulses are r e q u i r e d t o produce t h e same r e s u l t . w i t h multiple-pulse 20.

An example o f t h e i n c r e a s e t h a t occurs

i r r a d i a t i o n i s shown a t t h e bottom o f Fig.

These observations are c o n s i s t e n t w i t h p r e v i o u s KBS r e s u l t s ,

showing a dependence o f t h e s e g r e g a t i o n t o t h e s u r f a c e t h a t i s a f u n c t i o n o f t h e i m p l a n t dose and number o f l a s e r pulses used f o r a n n e a l i n g (White e t a1

., 1 9 8 0 ~ ) .

458

D. M.ZEHNER

-

Jt-

-

AFTER Ar" SPUTTERING

AES 56Fe (150 keV. 6 X 1015/cm 2) IN (111) Si PRIMARY BEAM: 2 keV, 5 p A MODULATION: 2 Vp-p

w

e z U

r/l

-

-

z

+

1 PULSE

5 PULSES

+ +

Fe Si

Fig.

+

a +

Ar

1

I

I

0

100

200

20.

Auger

+

0

C

1

I

I

500 ELECTRON ENERGY (eV)

300

electron spectra

400

Fe I

600

I 700

from an uncleaned S i ( l l 1 ) surface

implanted with 56Fe ( 1 5 0 KeV, 6 ~ 1 0 ~ ~ / c m af~ t e )r , sputtering and a f t e r pulsed laser annealing at -2.0 J / c m 2 .

7.

459


The e f f e c t o f segregation on s u r f a c e o r d e r was determined by LEE0 observations. Fe-implanted

The LEED p a t t e r n s o b t a i n e d from each o f t h e

samples

subsequent

l a s e r pulses are shown i n Fig. 21.

t o the

irradiation with

five

For purposes o f comparison, a

LEED p a t t e r n o b t a i n e d from a v i r g i n S i ( l l 1 ) c r y s t a l f o r t h e same i n c i d e n t e l e c t r o n energy i s a l s o shown i n t h i s f i g u r e .

Although

( 1 x 1 ) LEE0 p a t t e r n s were obtained a f t e r one p u l s e o f i r r a d i a t i o n on each sample, a h i g h e r backyround i n t e n s i t y was always observed r e l a t i v e t o t h a t obtained from t h e v i r g i n c r y s t a l . increased

segregation,

at

intermediate

and

The e f f e c t o f

h i g h doses,

with

m u l t i p l e l a s e r pulses was t o degrade t h e q u a l i t y o f t h e LEED patterns.

I n yeneral

i n Fig.

21,

, the

background i n t e n s i t y increased , as shown

although t h e symmetry o f t h e p a t t e r n observed was

s t i l l (1x1). I n c o n t r a s t t o t h e r e s u l t s o b t a i n e d from Fe-implanted samples, t h e LEED p a t t e r n o b t a i n e d from t h e Cu-implanted sample a f t e r one l a s e r p u l s e was a (1x1) w i t h h e x a g o n a l - l i k e

r i n g s around each

i n t e g r a l o r d e r r e f l e c t i o n as shown i n Fig. 22.

T h i s i s t o be com-

pared w i t h t h e

(5x5)

pattern,

shown a t t h e t o p o f F i g .

22,

o b t a i n e d from a t h e r m a l l y annealed (111) s u r f a c e which contained Cu e i t h e r due t o s e g r e g a t i o n from t h e b u l k o r as a r e s u l t o f beam deposition.

The r i n g s around t h e i n t e g r a l

became more i n t e n s e and sharp w i t h a d d i t i o n a l shown a t t h e bottom o f Fig.

22,

order

reflections

l a s e r pulses,

although a well-defined

as

(5x5)

LEED p a t t e r n was never obtained. T h i s sugyests t h a t t h e domains c o n t a i n i n g Cu on t h e laser-annealed s u r f a c e a r e n e i t h e r as w e l l o r d e r e d nor as l a r g e as those on t h e t h e r m a l l y annealed surface. Subsequent

examination o f t h e ion-imp1 anted laser-anneal ed

c r y s t a l s w i t h RBS (2.5-meV f o l l o w i n g features:

He+ i o n b a c k s c a t t e r i n g )

( 1 ) For t h e Cu-implanted c r y s t a l

showed t h e

, one

pulse

o f l a s e r r a d i a t i o n caused t h e t r a n s p o r t o f a l l Cu t o t h e near-

s u r f a c e region, and ( 2 ) f o r t h e low dose Fe-implanted c r y s t a l , one p u l s e i s s u f f i c i e n t t o cause t h e complete t r a n s p o r t o f Fe t o t h e near s u r f a c e region. A t i n t e r m e d i a t e doses, s u b s t a n t i a l segregation

460

Fig. 21. (a) Si(ll1 ) S6Feat ( b ) ( Patterns are j lcrn2.

D. M. ZEHNER

LEED patterns, at primary beam energy of 110 eV, from a surface and from ( 1 1 1 ) surfaces of crystals implanted with 1 . 3 ~ 1 0 ~ ~ / c, m ( c ~) () 6 . 0 ~ 1 O ~ ~ / c and m ~ ()d,) ( 1 . 8 x 1 0 1 6 / c m 2 ) . shown subsequent to five pulses o f laser annealing at -2.0

7.

Fig. 22.


461

LEED patterns, a t a primary beam energy o f 71 eV, from ( a ) a

thermally annealed S i ( l l 1 ) surface a f t e r -1

-

monolayer deposition o f Cu and

from a ( 1 1 1 ) surface o f a crystal implanted with 6 . 9 ~ 1 0 ~ ~ / and c m laser ~ annealed with ( b ) 1 and ( c ) 5 pulses at

2.0 J / c m 2 .

462

D. M. ZEHNER

t o t h e surface occurs d u r i n g t h e f i r s t pulse,

b u t two pulses

a r e r e q u i r e d t o c o m p l e t e l y segreyate t h e Fe t o t h e near-surface region.

F i n a l l y a t h i g h doses,

even a f t e r f i v e l a s e r pulses,

s u b s t a n t i a l q u a n t i t i e s o f Fe remain i n t h e f i r s t 1000 A o f t h e c r y s t a l a t an averaye c o n c e n t r a t i o n o f -2 x 1021/cm3.

Furthermore,

c h a n n e l i n g s t u d i e s showed t h a t Fe i n t h e b u l k o f t h e c r y s t a l i s not i n s o l i d solution. From t r a n s m i s s i o n e l e c t r o n microscopy s t u d i e s that,

i t i s known

i n t h e case o f a high-dose Fe-implanted c r y s t a l ,

a well-

d e f i n e d c e l l s t r u c t u r e (see Chapters 1 and 4) i s observed i n t h e n e a r - s u r f a c e r e y i on subsequent t o l a s e r anneal ing (White e t a1 1980~).

The i n t e r i o r o f each c e l l i s an e p i t a x i a l

.,

column of

s i l i c o n e x t e n d i n g t o t h e s u r f a c e (average c e l l diameter -250 A ) . Surrounding each column o f s i l i c o n i s a c e l l w a l l and e x t e n d i n g t o a depth o f -1000 A,

,

650 A t h i c k

c o n t a i n i n g massive quan-

t i t i e s o f segreyated Fey p o s s i b l y i n t h e form o f Fe s i l i c i d e s . These r e s u l t s show t h a t subsequent t o l a s e r a n n e a l i n g t h e Fe (and Cu) i s n o t u n i f o r m l y d i s t r i b u t e d i n t h e plane o f t h e near-surface reyion but instead i s h i g h l y concentrated i n the w a l l s o f t h e c e l l structure. Fe-implanted

Thus,

t h e (1x1) LEE0 p a t t e r n s observed f o r t h e

samples a r i s e from t h e b u l k t e r m i n a t i o n o f (111)

planes i n t h e columns o f s i l i c o n a t t h e surface.

The absence o f

any o t h e r w e l l - d e f i n e d d i f f r a c t i o n f e a t u r e s from t h e Fe-implanted r e g i o n shows t h a t no long-range o r d e r e x i s t s i n t h e t e r m i n a t i o n o f t h e c e l l w a l l s a t t h e surface. rings

i n t h e Cu-implanted

The presence o f h e x a g o n a l - l i k e

crystals

order e x i s t s i n those c e l l walls, scale.

The

high

background

o b t a i n e d from t h e v i r g i n c r y s t a l

shows t h a t ,

i t i s on an w+xxmel_y small

intensities,

,

i f long-range

relative

to

that

observed f o r a l l i m p l a n t con-

d i t i o n s f o r Fe and Cu i n d i c a t e t h e presence o f d i s o r d e r ( p o s s i b l y s t r a i n i n t h e r e y i o n o f t h e c e l l w a l l boundaries) i n t h e o u t e r most l a y e r s , which increases w i t h i n c r e a s i n g i m p l a n t dose. F o r t h e s e samples , s p u t t e r i n g f o l 1owi ng 1aser

ir r a d i a t i o n

r e s u l t e d i n t h e removal o f some o f t h e i m p l a n t from t h e s u r f a c e

7. region.

463


However,

subsequent i r r a d i a t i o n w i t h t h e l a s e r again

r e s u l t e d i n t h e segregation o f l a r g e q u a n t i t i e s o f t h e i m p l a n t t o t h e s u r f a c e region.

Furthermore, f o r samples i n which i n t e r s t i -

t i a l species such as Cu a r e present i n t h e b u l k as a r e s u l t o f t h e growth process, l a s e r i r r a d i a t i o n can be used t o zone r e f i n e t h e s e species t o t h e s u r f a c e r e g i o n from a depth e q u i v a l e n t t o t h e maximum m e l t

penetration.

These i m p u r i t i e s can t h e n be

removed from t h e s u r f a c e w i t h l i g h t i o n S p u t t e r i n g ,

l e a v i n g an

i m p u r i t y - f r e e subsurface r e g i o n ( t o a depth determined by t h e melt

front

penetration),


which

remains

such a f t e r

subsequent

I n many d e v i c e a p p l i c a t i o n s i n v o l v i n g s i l i c o n ,

Cu and Fe i m p u r i t i e s a c t as very e f f i c i e n t recombination c e n t e r s and adversely a f f e c t m i n o r i t y - c a r r i e r l i f e t i m e .

The above obser-

v a t i o n s show t h a t l a s e r annealing combined w i t h s p u t t e r i n g can be used as a r a p i d p u r i f i c a t i o n t r e a t m e n t i n o r d e r t o produce an i m p u r i t y - f r e e s u r f a c e region.

VI.

Applications

I n v e s t i g a t i o n s discussed i n S e c t i o n I11 and I V concentrated on examining s p e c i f i c s u r f a c e p r o p e r t i e s a s s o c i a t e d w i t h l a s e r annealing changes

while in

S e c t i o n V was p r i n c i p a l l y

these

properties

that

i m p l a n t a t i o n w i t h l a s e r annealing.

occurred

concerned w i t h t h e by

combining

ion

I n t h i s section the u t i l i z a -

t i o n o f laser-annealed surfaces i s discussed.

The most p r o m i s i n g

a p p l i c a t i o n o f t h e l a s e r annealing t e c h n i q u e f o r producing atomic a l l y c l e a n surfaces i n d e v i c e processing appears t o be i n preparing

surfaces

application,

for

molecular

beam e p i t a x y

(MBE).

In this

t h e high-temperature t r a n s i e n t induced by t h e l a s e r

o f f e r s a very a t t r a c t i v e and e f f i c i e n t a l t e r n a t i v e t o t h e present prolonged preheat t r e a t m e n t a t t h e moderate temperature r e q u i r e d t o c l e a n t h e semiconductor s u r f a c e t o t h e h i g h standard e s s e n t i a l f o r good q u a l i t y e p i t a x y . a p r o d u c t i o n technique.

T h i s b r i n g s MBE a s t e p nearer t o being

464

D. K.ZEHNER

I n a r e c e n t i n v e s t i g a t i o n (de J o n j e t al.,

1983) LEED was

used t o study t h e i n i t i a l stages o f e p i t a x i a l growth o f s i l i c o n on s i l i c o n .

Both thermal a n n e a l i n g and l a s e r i r r a d i a t i o n were

used f o r s u r f a c e p r e p a r a t i o n , 1-10 nm.

and S i d e p o s i t i o n s were t y p i c a l l y

Using LEEO p a t t e r n s , t h e e p i t a x i a l growth temperature

was d e f i n e d as t h a t p a r t i c u l a r s u b s t r a t e temperature a t which an e p i t a x i a l overlayer, same q u a l i t y

grown on t h e c l e a n s u b s t r a t e ,

of diffraction

p a t t e r n as t h e s u b s t r a t e

R e s u l t s o b t a i n e d from 1aser-anneal ed vicinal

exhibits the

(loo),

itself.

(110) , ( 111) , and

(111) S i o r i e n t a t i o n s showed t h a t e p i t a x i a l growth can

t a k e p l a c e on surfaces prepared by t h i s procedure.

The growth

temperature f o r t h e (100) s u r f a c e was i d e n t i c a l t o t h a t o b t a i n e d u s i n g t h e r m a l l y prepared surfaces.

For t h e (111)

surface the

growth temperature determined f o r t h e thermal l y annealed s u r f a c e was h i g h e r t h a t t h a t determined f o r t h e l a s e r - a n n e a l e d s u r f a c e and a l s o f o r generally

the

laser-annealed

vicinal

surface.

accepted growth mechanism i n Si:MBE

growth by s t e p f l o w ,

the

r e s u l t s obtained f o r

Since t h e

above 870 K i s (111)

surfaces

sugyest t h e presence o f steps on t h e laser-annealed surface. Using an approach s i m i l a r t o t h a t j u s t described, t h e growth o f epitaxial

m u l t i l a y e r f i l m s o f v a r y i n g t h i c k n e s s on s i l i c o n

s u r f a c e s has been i n v e s t i y a t e d (de Jong e t a l .

Laser-

(loo), ( l l o ) ,

(111) and v i c i n a l (111)

A f t e r preparation,

s i l i c o n f i l m s were de-

annealed and t h u s c l e a n S i s u r f a c e s were used.

, 1982b,c).

p o s i t e d and subsequently l a s e r annealed a t i n c r e a s i n y energy dens i t i e s i n o r d e r t o determine t h e t h r e s h o l d f o r growth. determined by LEEU t o be -0.9 these

experiments.

After

T h i s was

J/cm2 f o r t h e ruby l a s e r used i n

determining t h e threshold,

silicon

l a y e r s were s e q u e n t i a l l y d e p o s i t e d and l a s e r annealed on a l l s u r faces.

I n t h i s way e p i t a x i a l

l a y e r s up t o 800 nm were yrown,

b u i l t up out o f 1 t o 20 sublayers.

The reappearance o f a LEEU

p a t t e r n a l l over t h e annealed area a f t e r each i r r a d i a t i o n i n d i cated e p i t a x i a l

regrowth o f a l a y e r .

o r i e n t e d samples,

I n particular,

on S i ( l l 1 )

annealed d e p o s i t e d l a y e r s e x h i b i t e d a (1x1)

7.

465


p a t t e r n which

i n t h e case o f t h e v i c i n a l

s u r f a c e had charac-

t e r i s t i c spot s p l i t t i n y i n t h e same c r y s t a l l o g r a p h i c d i r e c t i o n and t o t h e same amount as a nondeposited sample.

T h i s means t h a t

t h e steps i n t h e s u r f a c e are preserved by d e p o s i t i o n and pulsed Spectra obtained w i t h RBS show t h e e p i t a x i a l l y


grown r e g i o n s t o be o f good q u a l i t y . e x t r a r o u t e t o three-dimensional

T h i s method may p r o v i d e an

s i l i c o n structures.

By combininy t h e i o n i m p l a n t a t i o n , l a s e r annealing techniques discussed i n S e c t i o n V with m o l e c u l a r beam e p i t a x y , i t i s p o s s i b l e t o produce b u r i e d doped l a y e r s .

T h i s approach has been f o l l o w e d

i n a recent i n v e s t i g a t i o n ( S m i t e t a1 was f i r s t implanted w i t h As.

., 1982).

A Si(100) wafer

A f t e r subsequent i n s e r t i o n i n t o a

UHV system,

t h e sample was i r r a d i a t e d w i t h f i v e pulses from a

ruby l a s e r .

I n a d d i t i o n t o producing a clean, ordered surface, as

determined by LEED, As was r e d i s t r i b u t e d i n depth, as p r e v i o u s l y i l l u s t r a t e d i n Fig.

16.

The sample was t h e n heated ( t y p i c a l l y

K), and s i l i c o n was deposited a t a r a t e on t h e o r d e r o f 0.1

-800 nm/s.

A t y p i c a l l a y e r t h i c k n e s s was 100 nm.

The samples were

R e s u l t s showed (1) good e p i t a x y w i t h i n

t h e n examined w i t h RBS.

t h e d e p o s i t e d r e g i o n and ( 2 ) t h e e x i s t e n c e o f a b u r i e d As l a y e r w i t h an abrupt doped-undoped substrate-epitaxy with

specific

interface.

dopant

i n t e r f a c e ( 0.8 J/cm2, Lowndes and Wood (1981) noted t h a t t h e R s i g n a t u r e f o r GaAs changes both qua1 i t a t i v e l y and

time-resolved

quantitatively.

The t r a i l i n g edge o f t h e R s i g n a l f i r s t becomes

rounded and then, a t Ex

>

0.9 J/cm2, t h e d u r a t i o n o f Rl

d r a m a t i c a l l y ; almost as soon t o a value

decreases

as t h e GaAs becomes molten, R drops

below R g and RF, which

was i n t e r p r e t e d as

signaling

t h e onset o f s u r f ace damage accompanied by s i g n i f ic a n t v a p o r i z a t ion.

-

[A s i m i l a r e f f e c t i s observed w i t h s i l i c o n f o r E l 3.2 J/cm2 (Auston e t a1 1979).] That t h i s sudden drop i n R does s i g n a l a

.,

damage t h r e s h o l d has been confirmed by post-anneal ing measurements : I n s p e c t i o n w i t h an o p t i c a l microscope r e v e a l s s u r f a c e d i s c o l o r a t i o n and damage; t h e H a l l m o b i l i t y o f c a r r i e r s i n i o n - i m p l a n t e d and laser-annealed l a y e r s i s found t o decrease f o r El

>

0.8 J/cm2 (see

s e c t i o n 111); and, a sudden onset o f oxygen uptake f r o m t h e ambient atmosphere occurs f o r El

-

1 J/crn2 (see s e c t i o n V).

These p o s t -

a n n e a l i n g o b s e r v a t i o n s are a l s o i n good agreement w i t h t h e f l a t t e n i n g o f t h e t o p s o f t h e c a l c u l a t e d m e l t - f r o n t p r o f i l e s f o r 0.8 and 1.0 J/cm2 Fig. 3), which s i g n a l s t h e onset o f v a p o r i z a t i o n i n t h e model c a l c u l a t i o n s . As shown i n Fig. 2, t h e r e i s good agreement between t h e c a l c u l a t e d d u r a t i o n o f s u r f a c e m e l t i n g o f c-GaAs and t h e measured durat i o n o f t h e h i g h - r e f l e c t i v i t y phase, f o r a l l E l between t h e m e l t i n g and damage t h r e s h o l d s . The p o s s i b i l i t y t h a t t h e very h i g h v e l o c i t y o f e p i t a x i a l r e g r o w t h , f o l 1owi ng pul sed 1a s e r me1t ing , p l ays a s i g n i f ic a n t r o l e i n c o n n e c t i o n with l a s e r - i n d u c e d

defects

(such as quenched-in

484

D. H. LOWNDES

vacancies o r a n t i - s i t e d e f e c t s ) i s discussed i n s e c t i o n s I11 and Here we simply note t h a t t h e v e l o c i t y w i t h which t h e r e c r y s -

IV.

t a l l i z i n g i n t e r f a c e approaches t h e s u r f a c e may be e s t i m a t e d from t h e s l o p e o f t h e m e l t - f r o n t p r o f i l e s i n Fig. 3, and i s -3.5 m/sec a t En = 0.4

(0.8)

(-1.8)

J/cm2.

b. Ion-Imp1 anted GaAs As Fig. 2 shows, near t h e

t h r e s h o l d (En

- 0.2

J/cm2) f o r a t -

t a i n i n g RYax t h e measured m e l t d u r a t i o n s f o r Se- and Te-implanted GaAs samples are i n good agreement w i t h each o t h e r and a l s o d i f f e r s u b s t a n t i a l l y from m e l t d u r a t i o n s f o r c-GaAs.

The m e l t i n g t h r e s h -

o l d f o r these i m p l a n t e d samples i s a l s o lowered by 4 . 0 4 below t h e t h r e s h o l d f o r c-GaAs, Wood,

1981).

A t t h e time,

i.e.,

J/cm*

by about 20% (Lowndes and

i t was suggested t h a t these e f f e c t s

m i g h t r e s u l t from t h e h i g h e r f r e e energy o f t h e amorphous phase (i.e.,

energy s t o r e d i n t h e i o n - i m p l a n t e d r e g i o n ) , o r f r o m e i t h e r

a lower l a t e n t heat o f f u s i o n , La, o r l o w e r m e l t i n g temperature, Ta, f o r a-GaAs t h a n f o r c-GaAs. t h e l a t t e r suggestions,

I n o r d e r t o q u a n t i t a t i v e l y check

m e l t i n g m d e l c a l c u l a t i o n s were c a r r i e d

o u t by Wood e t a l . (1981a) u s i n g t h e a-model w i t h an i m p l a n t a t i o n damaged l a y e r assumed t o be 220 nm deep.

W i t h i n t h e amorphous

l a y e r , t h e l a t e n t heat and m e l t i n g temperature were b o t h assumed t o be reduced from t h e i r c r y s t a l l i n e values. It was found t h a t t h i s d i d n o t g r e a t l y modify e i t h e r m e l t - f r o n t p r o f i l e s ( s i m i l a r t o Fig. 3) o r s u r f a c e m e l t d u r a t i o n s except near t h e l o w e s t ,El where a reduced l a t e n t heat o r m e l t i n g temperature f o r t h e amorphous l a y e r d i d play a r o l e i n prolonging surface m e l t duration.

A more

r e c e n t d e t a i l e d comparison o f t h e dynamical b e h a v i o r o f c - and as i l i c o n d u r i n g pulsed l a s e r m e l t i n g a l s o demonstrated t h a t La and Ta have o n l y a small e f f e c t on t h e n e a r - t h r e s h o l d m e l t i n g b e h a v i o r , which i s dominated i n s t e a d by t h e d r a s t i c a l l y reduced thermal cond u c t i v i t y o f t h e amorphous phase o f s i l i c o n (Lowndes e t a1

., 1984;

see Chap. 6); a s i m i l a r c o n c l u s i o n r e g a r d i n g t h e importance o f t h e l o w thermal c o n d u c t i v i t y o f t h e a-phase may a l s o h o l d f o r GaAs.

8. However,

PULSED BEAM PROCESSING OF GALLIUM ARSENIDE

485

i t appears t h a t S i may be an unusual case i n t h a t t h e

d i f f e r e n c e i n K f o r a- and c-Si i s e s p e c i a l l y l a r g e ; i n Ge, and probably i n GaAs, t h i s d i f f e r e n c e i s much smaller. F u r t h e r support f o r t h e v a l i d i t y o f u s i n g t h e c-GaAs m e l t - f r o n t p r o f i l e s o f Fig. 3 t o d e s c r i b e p u l s e d l a s e r m e l t i n g o f i o n - i m p l a n t e d GaAs, a t h i g h e r 1981a).

.

El, comes f r o m TEM measurements ( F l e t c h e r e t a1 ,

As shown i n F i g u r e 4, as-implanted samples d i s p l a y heavy

l a t t i c e damage t o a depth o f 180 r u b y l a s e r i r r a d i a t i o n (Fig.

Fig. 4.

rm; TEM measurements a f t e r pulsed

4) show t h a t a minimum El

TEM views of 1 6 0 keV, 5 x 1 0 1 5 / c m 2 Se-implanted

implanted; ( b ) E a z O . 2 5 J / c m 2 ; ( c ) El = 0 . 3 6 J / c m 2 ; p a t t e r n corresponding t o ( c )

(Lowndes e t a l . ,

of 0.4

G A S : ( a ) as-

( d ) electrondiffraction

1981b).

486

D. H. LOWNDES

J/cm2 i s needed t o anneal i m p l a n t a t i o n damage and r e s u l t s i n good regrowth,

epit a x i a

i n e x c e l l e n t agreement w i t h t h e m e l t depth

c a l c u l a t d f o r c-GaAs a t El

= 0.4 J / c d (Fig.

3).

These r e s u l t s

a r e f u r t h e r supported by t h e minimum E,

o f 0.4

needed t o e l e c t r i c a l l y

i m p l a n t s i n GaAs (see

activate similar

J/cm2 t h a t was

S e c t i o n 111). I n t h e i n t e r m e d i a t e El

range (-0.5

J/cm2),

T

f o r t h e Se- and

Te-implanted samples d i f f e r s by a f a c t o r o f 2 (Fig.

2).

A con-

t r i b u t i n g f a c t o r i n t h i s d i f f e r e n c e might be d i f f e r i n g degrees o f a m o r p h i z a t i o n produced by t h e i m p l a n t a t i o n c o n d i t i o n s , r e s u l t i n g i n d i f f e r e n t values f o r t h e thermal c o n d u c t i v i t y i n t h e amorphous r e g i o n (Lowndes e t al.,

However, i f o n l y t h i s e f f e c t were

1984).

p r e s e n t , t h e n t h e m e l t d u r a t i o n s should have become n e a r l y equal again a t higher

En.

Model c a l c u l a t i o n s u s i n g reasonable combina-

t i o n s o f t h e o t h e r thermal and o p t i c a l parameters o f a-GaAs were a l s o unable t o reproduce t h e wide range o f i m p l a n t e d samples i n Fig. 2.

T

shown f o r t h e i o n -

To c l a r i f y t h i s r e s u l t , Lowndes and

Wood (1981) conducted an a d d i t i o n a l s e t o f experiments: Using E,

0.4-0.5

J/cm2,

Se-implanted

=

samples were s u b j e c t e d t o repeated

(up t o 5) l a s e r i r r a d i a t i o n s , and

T

was measured each time. Although

t h e r e was some v a r i a t i o n i n T f r o m p u l s e t o pulse, t h e d u r a t i o n s f o r a g i v e n sample never decreased t o t h e s u b s t a n t i a l l y l o w e r v a l u e f o r c-GaAs, even though TEM showed t h a t these i n i t i a l l y amorphous samples e p i t a x i a l l y r e c r y s t a l l i z e d a f t e r a s i n g l e i r r a d i a t i o n a t

EL > 0.36 J/cm2.

The d i f f e r e n c e s i n T f o r a l l t h r e e s e t s o f sam-

ples, f o r t h e higher

El, were t h e r e f o r e i n t e r p r e t e d as due p r i m a r i l y

t o chemical e f f e c t s a r i s i n g from s u b s t a n t i a l d i f f e r e n c e s i n t h e doping o f t h e near-surface r e g i o n . ( S I M S ) measurements (Wood e t a1

Secondary i o n mass spectroscopy

., 1981a)

demonstrated t h a t Se does

segregate toward t h e sample s u r f a c e as a r e s u l t o f pulsed l a s e r annealing.

F o l l o w i n g a s i n g l e 0.5

J/cm2 i r r a d i a t i o n ,

a mean Se

c o n c e n t r a t i o n > 2 x 1020 cm3 was found i n t h e f i r s t 0.1 pn below t h e surface.

Compared w i t h t h e GaAs atomic d e n s i t y o f 4.4 x 1022

8.


487

atoms/cm3, these f i g u r e s demonstrate h i g h l y degenerate doping o f at. %.The higher-dose,

-0.5

e t al.

shallower Te implant used by Auston

(1979) should have r e s u l t e d i n a s t i l l higher dopant con-

centration.

The corresponding d i f f e r e n c e s i n

T

t h a t were observed

support t h e idea t h a t melt duration, i n t h e intermediate Ex range, i s c o n t r o l l e d p r i m a r i l y by the chemical composition o f t h e nearsurface region. Dopant r e d i s t r i b u t i o n p r o f i l e s r e s u l t i n g from d i f f u s i o n o f Mg and Zn implants i n l i q u i d GaAs were a l s o c a l c u l a t e d as f u n c t i o n s of

El, using c a l c u l a t e d melt d u r a t i o n s t h a t d i f f e r e d o n l y s l i g h t l y

from those f o r c-GaAs,

and found t o be i n good agreement w i t h

experimental SIMS p r o f i l e s (Wood e t al.,

1981a; see s e c t i o n 11.4).

This r e s u l t a l s o supports t h e conclusion t h a t the longer melt d u r a t i o n s f o r Se- and Te-implanted GaAs are more c h a r a c t e r i s t i c o f departures from stoichiometry, and o f t h e formation o f lower m e l t i n g p o i n t l i q u i d s very near t h e surface, than o f s t o i c h i o m e t r i c GaAs.

I n s p e c t i o n o f t h e Ga-Se (Ga-Te) phase diagram does reveal

a number o f intermediate a l l o y s and compounds w i t h m e l t i n g p o i n t s around 1000°C ( S O O O C ) ,

i n a d d i t i o n t o pure Ga ( 3 O O C ) .

I n summary, time-resolved r e f l e c t i v i t y measurements f o r GaAs show t h a t surface m e l t d u r a t i o n s are dramatical l y d i f f e r e n t f o r c-GaAs and for a-GaAs produced by Se and Te implantation. c-GaAs,

For

t h e r e i s good agreement between measured and c a l c u l a t e d

values o f t h e m e l t i n g threshold, t h e t h r e s h o l d f o r damage due t o v a p o r i z a t i o n , and t h e d u r a t i o n s o f m e l t i n g a t i n t e r m e d i a t e energy densities. The experiments and c a l c u l a t i o n s by Lowndes and Wood suggest t h a t the longer me1 t d u r a t i o n s observed f o r ion-imp1 anted GaAs are t h e r e s u l t o f s u b s t a n t i a l d e v i a t i o n s from s t o i c h i o m e t r y and t h e formation of lower m e l t i n g - p o i n t m a t e r i a l a t t h e surface o f the sarnpl es.

488 4.

D. H. LOWNDES

DOPANT REDISTRIBUTION DURING PULSED LASER MELTING

The e f f e c t of pulsed melting and subsequent rapid r e s o l i d i f i c a t i o n upon dopant solid s o l u b i l i t y l i m i t s , and upon dopant redist r i b u t i o n and segregation, has not been as extensively investigated f o r GaAs as f o r Si. However, following some i n i t i a l uncertainty, i t has recently become c l e a r t h a t nonequil ibrium i n t e r f a c e phenomena s i m i l a r t o those occuring f o r Si a l s o occur in GaAs. Early RBS and channeling studies of 50 keV, 1016/cm2 Te-implanted GaAs revealed b e t t e r than 90% s u b s t i t u t i o n a l i t y (Te atoms residing on e i t h e r a Ga or an As s i t e ) following pulsed l a s e r annealing, corresponding t o a s u b s t i t u t i o n a l s o l i d s o l u b i l i t y of more than 1021/cm3, which exceeds the equilibrium s o l i d s o l u b i l i t y value by more than an order of magnitude (Barnes et a1 , 1979). However, t h e r e was d i s agreement i n e a r l y s t u d i e s regarding the extent of implanted dopant r e d i s t r i b u t i o n following pulsed annealing: I n s i g n i f i c a n t dopant r e d i s t r i b u t i o n was observed in some studies (Golovchenko and Venkatesan, 1978; Campisano e t al., 1978) while others found subs t a n t i a l r e d i s t r i b u t i o n (Barnes e t al., 1979; Sealy e t a l . , 1979). There was a l s o the observation t h a t some implanted impurities apparently diffused appreciably, whereas others did not, during 30 nsec pulsed ruby l a s e r annealing (Sealy et a l . , 1979). Williams (1983b) has pointed out t h a t pulsed-anneal ing studies in GaAs have often used l a s e r conditions just s u f f i c i e n t t o remove implantation damage in order t o avoid laser-induced damage and defects (see s e c t i o n s 111, I V , and V ) a t higher El. Less dopant r e d i s t r i b u t i o n i s t o be expected under such conditions. More importantly, redist r i b u t i o n i s probably then s e n s i t i v e l y dependent upon both the l a s e r parameters and upon the type of damage e x i s t i n g i n the implanted region (e.g., f u l l y amorphized vs defective c r y s t a l l i n e ) , which can influence the melting threshold and melt duration, as was shown above. Wood e t a l . (1981a) more recently reported the r e s u l t s of q u a n t i t a t i v e SIMS measurements and model calculations of dopant

.

8.

489


r e d i s t r i b u t i o n (see Chapter 4) f o l l o w i n g l a s e r annealing o f ionimplanted GaAs.

One r e s u l t o f t h e i r study was t o e s t a b l i s h t h a t

t h e measured d o p a n t - r e d i s t r i b u t i o n p r o f i l e s could be i n t e r p r e t e d as r e s u l t i n g from l i q u i d - p h a s e d i f f u s i o n o f implanted i o n s d u r i n g m e l t i n g , by using surface m e l t d u r a t i o n s t h a t were c o n s i s t e n t w i t h those o f Fig. 2.

An e q u a l l y important r e s u l t o f t h i s study was t o

determine, apparently f o r t h e f i r s t time, values f o r liquid-phase d i f f u s i o n c o e f f i c i e n t s , ,O,

and n o n e q u i l i b r i u m i n t e r f a c e segrega-

t i o n c o e f f i c i e n t s , k i , f o r Zn and Mg ions i n molten GaAs d u r i n g rapid solidification.

I n these studies, unencapsulated GaAs

Sam-

p l e s implanted w i t h 150 keV Zn o r w i t h 35 keV Mg ions t o a dose o f 5 x l O l 5 ions/cm2 were i r r a d i a t e d a t room temperature w i t h s i n g l e pulses (FWHM d u r a t i o n = 15-25 nsec) from a ruby l a s e r , w i t h t h e l a s e r beam s p a t i a l l y homogenized using a bent d i f f u s i n g l i g h t p i p e ( C u l l i s e t al.,

1979).

Dopant l o s s d u r i n g annealing was estimated

from t h e i n t e g r a t e d number o f secondary i o n counts.

Essentially

no losses were observed f o r t h e Mg-implanted samples, but t h e Znimplanted samples showed losses monotonically i n c r e a s i n g from -2% t o -20% f o r 0.5 Wood e t a1

G

Ex < 1.0 J/cm2.

. (1981b)

have described several methods f o r calcu-

l a t i n g dopant d i f f u s i o n d u r i n g pulsed l a s e r melting.

A method

(described i n Chapter 4) designed t o reduce t h e computer time r e q u i r e d by f i n i t e - d i f f e r e n c e c a l c u l a t i o n s , w h i l e s t i l l m a i n t a i n i n g acceptable accuracy, was used t o c a l c u l a t e 1iquid-phase dopantd i f f u s i o n p r o f i l e s ; values o f Da f o r dopant ions i n l i q u i d GaAs were not known, b u t were assumed t o be of t h e same magnitude as DA i n Si. The experimental and c a l c u l a t e d dopant r e d i s t r i b u t i o n p r o f i l e s are shown i n Figs. 5 (Mg) and 6 (Zn). An i n i t i a l attempt t o f i t t h e Mg SIMS data w i t h 01 = 5 x

lo-'+ cm2/sec

gave r e s u l t s

incompatible w i t h t h e c a l c u l a t e d and measured m e l t - d u r a t i o n times (Figs.

2 and 3).

E v e n t u a l l y i t was found t h a t DA = 2.5

x 10-4

cm2/sec gave good f i t s t o t h e experimental curves, as shown i n Fig.

5.

Since t h e Mg p r o f i l e s show c l e a r evidence o f surface

490

D. H.LOWNDES 1022 5

Mg-IMPLANTED GOAS AS-IMPLANTED EXP. E1(J/cm21 CAL. 0.51 ---

: -

loz'

5

2

2

2

1020

-

2

z

-----

0.62 0.81

R

-

1.03

z 0 II-

z

z

u 0 5

2 1019

0

0.05

0.10

0.15 0.20 DEPTH ( p m l

0.25

0.35

0.30

Fig. 5. Experimental (SIMS) and calculated Mg redistribution profiles in G a A s (Lowndes et al., 1981a). 1022

I

I

I

I

I

Zn-IMPLANTED GaAs AS- IMPLANTED E1(J/crn2) EXP.

0.84

I

=4

CAL.

-

I

g L

c K

w z

P 1020

0

40'9

0

0.05

0.40

0.45 0.20 DEPTH ( p n )

0.25

0.30

0.3!

Fig. 6. Experimental (SIMS)and calculated Z n redistribution profiles in G a A s (Lowndes et al., 1981a).

8. segregation,


491

i t was also necessary t o determine t h e i n t e r f a c e

segregation c o e f f i c i e n t , k i .

Wood e t a l .

(1981a) p o i n t out t h a t

t h e experimental data make a very d e f i n i t e statement about t h e amount o f dopant segregated t o t h e surface:

The necessity t o f i t

both t h e surface segregation "spike" and t h e p r o f i l e f a r below t h e surface severely c o n s t r a i n s k i ; a value k i = 0.6 the f i t t i n g .

r e s u l t e d from

TEM measurements on Mg-implanted GaAs samples revealed

a h i g h d e n s i t y o f very small d e f e c t s present a f t e r l a s e r annealing, b u t confined t o a narrow band a t t h e sample surface.

These defects

were presumed t o be associated w i t h t h e surface segregation of Mg, s i n c e they were not observed f o r o t h e r samples ( F l e t c h e r e t al., 1981a,b). The t h e o r e t i c a l f i t s t o t h e Zn data (Fig. 6 ) were somewhat l e s s s a t i s f a c t o r y than t o t h e Mg data but r e s u l t e d i n determination o f a Zn d i f f u s i o n c o e f f i c i e n t o f 3.0 x c o e f f i c i e n t k i = 1.0

(no segregation).

cm2/sec and a segregation These authors p o i n t out

t h a t although t h e i r computer programs allowed f o r dopant l o s s ( i n t h e case o f Zn), such losses complicated t h e f i t t i n g procedure because o f inherent d i f f i c u l t i e s w i t h SIMS measurements i n t h e c l o s e v i c i n i t y o f the surface:

The sharp drop i n t h e experimental

p r o f i l e s j u s t a t t h e "surface"

i s an u n r e a l i s t i c experimental

a r t i f a c t and i m p l i e s t h e need t o determine t h e p r e c i s e p o s i t i o n o f t h e surface i n t h e experimental data w i t h m r e accuracy.

Despite

t h i s d i f f i c u l t y w i t h t h e Zn data, t h e f i t between experiments and c a l c u l a t i o n s (Fig. 6) was s a t i s f a c t o r y . I t should be emphasized t h a t t h e k i values o f 0.6 (Mg) and 1.0 (Zn) found by Wood e t a1

. (1981a)

are not a p p r o p r i a t e f o r c r y s t a l -

l i z a t i o n under e q u i l i b r i u m conditions, but are c h a r a c t e r i s t i c o f nonequil i b r i u m segregation a t t h e growing i n t e r f a c e d u r i n g r a p i d solidification.

(See Chapters 2 and 4 f o r a discussion o f t h e o r e t -

i c a l models and t h e much more extensive experimental r e s u l t s now a v a i l a b l e f o r s i l i c o n . ) For comparison, W i l l a r d s o n and A l l r e d (1967) measured n e a r - e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t s o f 0.1 f o r Mg

492

D. H.LOWNDES

and 0.40 f o r Zn, f o r c r y s t a l s pulled from t h e melt by t h e Czochralski technique. The s t r i k i n g deviation of t h e ki values derived from Figs. 5 and 6, from these equilibrium values, confirms t h e highly nonequilibrium nature o f t h e s o l i d i f i c a t i o n process in pulse annealed GaAs. Surface segregation of Se, b u t not of S i , following pulsed l a s e r melting was a l s o reported by Wood et a1 (1981a), using SIMS measurements. Harrison and Wil l i ams (1980) reported hi gh-resol uti on channeling spectra f o r a 60 keV, 2 x lO15/cm* In' implant in GaAs. Clear evidence was found f o r In segregation a t t h e GaAs surface following pulsed ruby l a s e r annealing a t 0.3 J/cm2 (which was suff i c i e n t t o remove implantation damage), and the In remaining in the bulk GaAs was found t o be s u b s t i t u t i o n a l . Finally, a substant i a l e f f e c t of oxygen in the ambient atmosphere, and of atmospheric pressure, on k i f o r Si dopant ions has been observed during pulsed l a s e r melting of c-GaAs (Sato e t a l . , 1982; see section V). However, more extensive and d e t a i l e d studies of high i n t e r f a c e veloci t y zone-refining e f f e c t s , such as have been documented f o r lows o l u b i l i t y impurities in s i l i c o n (see Chapter 2 ) , have not been c a r r i e d out f o r GaAs. As a r e s u l t , l i t t l e accurate information i s a v a i l a b l e f o r GaAs regarding s o l i d s o l u b i l i t y l i m i t s or the occurrence of c e l l ul a r s t r u c t u r e s (anal agous t o those seen i n s i 1 i con) , and thei r dependence upon recrystal 1i z a t i on f r o n t velocity (Chapters 2 and 5 ) .

.

111.

E l e c t r i c a l Activation o f Implanted Ions

E l e c t r i c a l a c t i v a t i o n o f ions implanted in GaAs requires annealing the implanted layer. Several reviews o f furnace and t r a n s i e n t annealing techniques f o r GaAs (Anderson, 1982; Williams, 1983a, b; Wi 11 i ams and Harri son , 1981 ) have appeared. Conventional thermal annealing of GaAs involves extended (>5 min) exposure t o elevated ( >6OO0C) temperatures , with r e s u l t a n t arsenic l o s s and degradation of the GaAs surface, unless specimens are encapsulated. Anderson

493

8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE

(1982) has p o i n t e d out t h a t , d e s p i t e t h e inconvenience o f encapsul a t i o n , several o r g a n i z a t i o n s have now developed complex b u t successful

annealing

technologies

annealing o f i o n - i m p l a n t e d GaAs.

based on encapsulated

thermal

D o n n e l l y (1977) has s t r e s s e d t h e

importance o f d e v e l o p i n g r e p r o d u c i b l e e n c a p s u l a t i o n techniques t o avoid inconsistent results.

This d i f f i c u l t y , together with e f f e c t s

o f encapsulants on deep-level

i m p u r i t i e s and t h e i n a b i l i t y t o

achieve h i g h - c a r r i e r c o n c e n t r a t i o n s (Anderson, 1982), have p r o v i d e d t h e p r i n c i p a l a p p l i c a t i o n s - o r i e n t e d m o t i v a t i o n t o develop a l t e r n a t i v e annealing techniques. Thermal annealing and cw beam annealing ( u s i n g both l a s e r and electron

beams)

have

( d O l 3 / c m 2 ) implants.

been

successful

i n activating

low-dose

However, pulsed anneal i n g becomes competi -

t i v e i n e l e c t r i c a l a c t i v a t i o n f o r h i g h e r (>1014/cm2) doses, and i s c l e a r l y s u p e r i o r t o f u r n a c e a n n e a l i n g f o r t h e h i g h e s t (>1015/cm2) doses.

Experience t o d a t e i n o b t a i n i n g e l e c t r i c a l a c t i v a t i o n v i a

pulsed annealing o f i o n - i m p l a n t e d GaAs can be sumnarized i n a few sentences:

High doses o f b o t h n- and p-type i m p l a n t s can be p u l s e

annealed t o produce c a r r i e r c o n c e n t r a t i o n s i n t h e 10l9/crn3 ( n - ) t o low lO20/cm3 (p-) annealing.

However,

range, f a r h i g h e r t h a n r e s u l t from f u r n a c e t h e f r a c t i o n o f implanted i o n s t h a t a r e

e l e c t r i c a l l y a c t i v e i s s u b s t a n t i a l l y l e s s t h a n 1002, and i s espec i a l l y low f o r n-type i m p l a n t s , d e s p i t e e x c e l l e n t s u b s t i t u t i o n a l i t y o f dopant i o n s on l a t t i c e s i t e s .

These pulse-annealed l a y e r s a l s o

have lower m o b i l i t i e s , t y p i c a l l y by a f a c t o r o f 2-5, t h a n those expected f o r a given c a r r i e r c o n c e n t r a t i o n i n high-qua1 i t y GaAs (see, f o r exampl e, Sze and I r v i n , 1965). l a y e r s a r e a l s o t h e r m a l l y unstable,

Pul se-anneal ed , n-type

e x h i b i t i n g n e a r l y an o r d e r -

of-magnitude decrease i n c a r r i e r c o n c e n t r a t i o n f o l l o w i n g o n l y lowtemperature 1981).

(-300°C)

thermal t r e a t m e n t ( P i a n e t t a e t a1

.,

1980,

F i n a l l y , low-dose (

-E

$200-

too

kev s e . 5 . 1 0 ' % m ~ ~A 8 5 keVSe,5~(G'~/crn~

----0t60

keVS1.t atG'%m2 keVSe.4 xlO'%m'

-080

----a5

A I

I "-TYPE

Ex

0

I

I

I

I

I

Fig. 8. ( a ) Percent electrical activation and (b) H a l l mobility, vs high-dose n-type implants (Lowndes e t at., 1 9 8 1 a ) .

Ex

for

D. H. LOWNDES

c r y s t a l l i n e GaAs,

and a sudden onset o f oxygen uptake from t h e

atmosphere i s a l s o observed i n t h i s El range (see s e c t i o n V ) .

Thus,

t h e r e i s a w e l l - d e f i n e d E l window w i t h i n which e l e c t r i c a l a c t i v a t i o n o f s h a l l o w i o n i m p l a n t s i n GaAs can occur,

0.4

< El

6 0.8

J/cm2, w i t h t h e upper bound determined by t h e onset o f c a t a s t r o p h i c damage due t o v a p o r i z a t i o n and t h e lower bound governed by t h e n e c e s s i t y t o m e l t c o m p l e t e l y t h r o u g h t h e h e a v i l y damaged i o n i m p l a n t e d region, so t h a t e p i t a x i a l r e g r o w t h f r o m t h e s i n g l e - c r y s t a l s u b s t r a t e beneath can occur. between 0.4

The i n c r e a s e i n c a r r i e r m o b i l i t y

and 0.8 J / c d i s not a s s o c i a t e d w i t h f u r t h e r damage

removal, b u t occurs because t h e me1 t depth, and t h e r e f o r e t h e depth o f dopant d i f f u s i o n ,

i n c r e a s e s w i t h i n c r e a s i n g .El

Thus,

the

average dopant c o n c e n t r a t i o n i n t h e h e a v i l y doped near-surface l a y e r decreases w i t h i n c r e a s i n g increase o f c a r r i e r mobility.

El, r e s u l t i n g i n t h e observed

Lowndes e t a l . (1981a) have p o i n t e d

o u t t h a t t h e h o l e m o b i l i t i e s i l l u s t r a t e d i n Fig. t o be low i n r e l a t i o n t o

t h e h i g h (-1020/cm3)

r i e r c o n c e n t r a t i o n t h a t i s present,

7 do n o t appear

uncompensated car-

i f one e x t r a p o l a t e s Sze and

I r v i n ' s (1968) data t o s i m i l a r values o f c a r r i e r d e n s i t y . annealed n-type l a y e r s , by f a c t o r s o f 2-5,

however,

I n pulse-

t h e m o b i l i t y i s low ( t y p i c a l l y

i n t h i s and o t h e r work) i n r e l a t i o n t o t h e

e l e c t r o n d e n s i t i e s present. I t should a l s o be noted t h a t t h e h i g h e l e c t r i c a l a c t i v a t i o n o b t a i n e d f o r high-dose Zn imp1 ants, t o g e t h e r w i t h t h e Zn concent r a t i o n p r o f i l e s i n Fig. 6, i m p l y a Zn s u b s t i t u t i o n a l s o l i d solub i l i t y i n excess o f 1020/cm3 f o r pulsed annealing, about an o r d e r

o f magnitude g r e a t e r than i n c o n v e n t i o n a l near-equi 1 ib r i um c r y s t a l growth. I n summary, t h e s e r e s u l t s o f e l e c t r i c a l p r o p e r t i e s measurements

a r e i n e x c e l l e n t agreement with m e l t i n g model c a l c u l a t i o n s o f m e l t depth vs E l (Fig. 3 ) and w i t h TEM micrographs o f t h e l a s e r - a n n e a l e d n e a r - s u r f a c e r e g i o n (Fig. 4) f o r several values o f E.l c a t e a t h r e s h o l d El

Both i n d i -

= 0.4 J/crn2 t o m e l t e n t i r e l y t h r o u g h and e p i -

t a x i a l l y r e c r y s t a l l i z e t h e implantation-damaged region.

For h i g h e r

8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE

499

El, c a r r i e r mobility i s a l i n e a r l y increasing function of El (Figs. 7b and 8 b ) , but drops abruptly with t h e onset o f vaporization, surface damage, and oxygen uptake (El > 0.8 J/cm2). 6.

THERMAL STABILITY OF ACTIVATED CARRIERS

Pianetta and co-workers (1980, 1981) have shown t h a t t h e high n-type c a r r i e r concentrations produced i n GaAs by Se or Te implantat i o n and pulsed annealing a r e only metastable and can be d r a s t i c a l l y reduced by thermal annealing. Figure 9 i l l u s t r a t e s t h e e f f e c t of subsequent isochronal thermal anneal ing (15 seconds a t a given temperature) on the c a r r i e r concentration of samples implanted Te+/cm2 and annealed w i t h a 0.8 J/cm2 ruby with 250 keV, 5 x l a s e r pulse. A two-stage loss of c a r r i e r s was found, the i n i t i a l I

5 x 1OY5

250 keV Te ++ GaAs

ISOCHRONAL ANNEAL AFTER LASER ANNEALING

4

1013;

I

200

I

400

1

600

I

800

TEMPERATURE ("C) Fig.

9.

Loss o f sheet c a r r i e r concentration caused b y isochronal heating

see t e x t ) following pulsed laser annealing (Pianetta et a l . ,

1980).

so0

D. H. LOWNDES

stage b e g i n n i n g a t about 200°C and t h e second stage a t about 600°C. The same behavior was observed f o r samples p u l s e annealed w i t h e i t h e r a ruby l a s e r o r an e l e c t r o n beam.

Differential electrical

measurements a1 so r e v e a l e d t h a t t h e major l o s s o f c a r r i e r s occurred w i t h i n 0.1 pm o f t h e surface, and t h a t c a r r i e r p r o f i l e s below 0.2 pm were i d e n t i c a l b e f o r e and a f t e r thermal annealing.

The s i g n i f -

icance o f t h i s i n s t a b i l i t y ,

reliability

i n c r e a t i n g long-term

problems f o r pul se-annealed devices and i n causing d i f f i c u l t i e s

i n c o n t a c t i n g t h e i r s u r f a c e i s apparent.

P i a n n e t t a e t a l . (1981)

have a l s o r e p o r t e d r e s u l t s o f d e t a i l e d channeling s t u d i e s c a r r i e d o u t on Te-imp1 anted, laser-annealed GaAs, i n which t h e y searched f o r s t r u c t u r a l changes accompanying t h e two-stage c a r r i e r concentration.

reduction i n

B a c k s c a t t e r i n g s p e c t r a and h i g h - r e s o l u t i o n

a n g u l a r scans o f Te-imp1 anted samples s u b j e c t e d t o thermal t r e a t m e n t a t 45OOC a f t e r l a s e r annealing r e v e a l e d no change i n t h e minimum y i e l d o f GaAs, i n t h e f r a c t i o n o f n o n s u b s t i t u t i o n a l Te, o r i n t h e c h a n n e l i n g h a l f a n g l e o f Te; i.e.,

t h e m a j o r i t y o f Te atoms remained

on s u b s t i t u t i o n a l l a t t i c e s i t e s f o l l o w i n g 450°C thermal annealing, even though t h e e l e c t r i c a l l y a c t i v e f r a c t i o n o f Te decreased from

20% t o 6%.

P i a n e t t a e t a l . (1981) have suggested t h a t t h i s b e h a v i o r

i s c o n s i s t e n t w i t h thermal m i g r a t i o n o f As o r Ga vacancies, which a r e known t o be m o b i l e a t l o w temperatures, and t h a t t h e i n i t i a l stage o f r e d u c t i o n i n t h e e l e c t r o n c o n c e n t r a t i o n may be due t o t h e f o r m a t i o n o f Te-vacancy complexes. These complexes c o u l d r e s u l t f r o m subsequent thermal annealing, i f a l a r g e number o f vacancies a r e f r o z e n i n i n i t i a l l y by r a p i d s o l i d i f i c a t i o n from t h e melt. Such a model a l s o seems t o f i t i n w i t h o t h e r m i c r o s t r u c t u r a l and

i n t h a t good s u b s t i t u t i o n a l i t y o f h o s t and dopant atoms on l a t t i c e s i t e s i s n o r m a l l y found i n r a p i d l y

d e f e c t data (see s e c t i o n I V ) ,

s o l i d i f i e d GaAs; thus, a h i g h d e n s i t y of vacancies c o u l d account f o r b o t h t h e low c a r r i e r c o n c e n t r a t i o n s t h a t a r e found i n t h e nears u r f a c e r e g i o n f o l l o w i n g pulsed annealing and f o r t h e general f a i 1 u r e o f pul sed anneal ing t o a c t i v a t e 1ow-dose imp1 ants. However,

8.


501

i n t h e absence o f d i r e c t evidence f o r a l a r g e number o f mobile quenched-in vacancies, t h i s suggestion must s t i l l be regarded as conjecture.

IV.

Defects and Damage Induced by High-Intens t y Laser Pulses The nature and depth d i s t r i b u t i o n o f l a s e r - nduced damage and

d e f ect s have been studied r e c e n t l y i n both cr: j t a l l i n e and ionimplanted GaAs.

The techniques used i n c l u d e o p t i c a l and e l e c t r o n

microscopy, photo1 uminescence, b a c k s c a tt e r i n g spectra, DLTS, and I - V and C-V measurements on both Schottky b a r r i e r s and p-n junc-

tions. As an a i d t o t h i n k i n g about t h e problem o f defects and damage i n GaAs f o l l o w i n g pulsed annealing, i t i s useful t o separate the problem i n t o two p a rts :

F i r s t , t h e problem o f r e p a i r i n g l a t t i c e

damage t h a t i s caused by i o n i m p l a n t a t i o n and, second, t h e problem o f damage t h a t i s i n h e re n t t o the pulsed m e l t i n g and r a p i d s o l i d i f i c a t i o n process i t s e l f (found,

f o r example,

f o l l o w i n g pulsed

i r r a d i a t i o n o f even c-GaAs). The most obvious d i f f e r e n c e between t h e anneal i n g requi rements f o r elemental and compound semiconductors i s t h a t f o r an elemental semiconductor a l l t h a t i s re q u i re d t o r e s t o r e a c r y s t a l l i n e s t r u c t u r e i s t h a t bo th host and dopant atoms should occupy s u b s t i t u t i o n a l l a t t i c e sites.

For a compound semiconductor, t h e r e i s t h e

a d d i t i o n a l requirement o f stoichiometry, both l o c a l l y and a t l o n g range: Ga and As atoms should be coordinated w i t h neighbors o f opposite type, and should each be found o n l y on the c o r r e c t subl a t t i c e . Dopant atoms a l s o need t o occupy t h e c o r r e c t s u b l a t t i c e , i f compensation i s t o be avoided and f u l l e l e c t r i c a l a c t i v a t i o n

obtained.

Thus, even though channel i n g measurements do normally

demonstrate t he occurrence of h i gh-qua1 it y epi t a x i a1 regrowth f o l l o w i n g pulsed annealing o f GaAs, i n t h e sense t h a t both host and dopant atoms occupy o n l y s u b s t i t u t i o n a l l a t t i c e s i t e s , t h i s does not imply t h a t pulse-annealed GaAs i s d e f e c t free.

502

D. H.LOWNDES

For example, i t may be t h a t t h e u l t r a r a p i d s o l i d i f i c a t i o n from t h e m e l t d u r i n g pulsed annealing i s i n h e r e n t l y n o n s t o i c h i o m e t r i c , i n t h a t l a r g e concentrations o f a n t i s i t e ( A s G ~ or GaAs) defects, as w e l l as dopant atoms "trapped" on t h e wrong s u b l a t t i c e , may r e s u l t from t h e h i g h v e l o c i t y o f t h e r e c r y s t a l 1 iz i ng f r o n t d u r i n g e p i t a x i a1 regrowth. Deviations from s t o i c h i o m e t r y can a l s o occur i n a second way f o r implanted compound semiconductors:

I f o n l y one type o f i o n i s

implanted, an i n h e r e n t nonstoichiometry i s created. Co-implantation o f two species provides a s o l u t i o n i n p r i n c i p l e t o t h i s problem, b u t has not been e x t e n s i v e l y studied. However, t h e most obvious o r i g i n o f d e v i a t i o n s from s t o i c h i o m e t r y i n pulsed (and o t h e r ) annealing o f compound semiconductors i s t h e h i g h vapor pressure o f t h e column V c o n s t i t u e n t r e l a t i v e t o t h e column I 1 1 c o n s t i t u e n t .

I n i t i a l s t u d i e s o f pulsed annealing o f

implanted GaAs were m t i v a t e d i n p a r t by t h e hope t h a t t h e s h o r t m e l t d u r a t i o n (-100 nsec) would minimize loss o f v o l a t i l e As and would make i t p o s s i b l e t o anneal i m p l a n t a t i o n damage i n GaAs w i t h o u t encapsulation.

However , t h e equi 1 ibrium vapor pressure o f As a t

t h e m e l t i n g p o i n t o f GaAs i s -1 bar, which corresponds t o an As f l u x a t t h e l i q u i d surface o f about 1014/cm2, o r about one monolayer i n f i v e nanoseconds. D i r e c t evidence o f p r e f e r e n t i a l As loss, d u r i n g t h e time t h a t the GaAs surface i s molten, and o f l o s s o f s t o i c h i o m e t r y i n t h e near-surface region, i s provided by channeling and TEM observations o f a Ga-rich surface residue f o l l o w i n g pulsed annealing.

It has a l s o been suggested, by various authors, t h a t

a r s e n i c l o s s g r e a t l y enhances quenched-in vacancy formation, w i t h t h e vacancies then a c t i n g as compensating defects. There i s now extensive evidence from sheet and d i f f e r e n t i a l e l e c t r i c a l p r o p e r t i e s measurements t h a t pulsed l a s e r n l e l t i n g o f ionimp1anted o r c-GaAs , under c o n d i t i o n s t h a t have normally been used , r e s u l t s i n q u i t e h i g h d e n s i t i e s o f quenched-in, e l e c t r i c a l l y a c t i v e p o i n t defects o r d e f e c t complexes.

The f a i l u r e t o a c t i v a t e low-dose

8.

503


impl ants , t h e low mobi 1i t i es observed f o r hi gher-dose impl ants , and observations of both low conductivity and semi-insulating l a y e r s , just below the surface, following pulsed annealing, a l l t e s t i f y t o t h e presence of these defects. Since a l l of these problems become more severe with increasing El, i t i s apparent t h a t defect-free annealing of deep implants may be impossible, and t h a t even f o r shallow implants the Ex "window" f o r successful annealing may be narrow. 7.

CRYSTALLINITY FOLLOWING PULSED LASER MELTING

A number of authors have noted t h e excellent c r y s t a l l i n i t y t h a t i s obtained following pulsed l a s e r melting and rapid s o l i d i f i c a t i o n of ion-implantation amorphized GaAs (Barnes et a1 1978; Campisano

.,

.,

.,

e t a1 1978; Golovchenko and Venkatesan, 1978; Sealy e t a1 1979). 1980; Williams and Williams and co-workers (Williams et a1 Harrison, 1981) used channeling spectra t o compare the effectiveness of cw argon-i on 1a s e r anneal i ng , furnace anneal i ng , and pul sed 1a s e r annealing f o r removal of implantation l a t t i c e damage, and concluded t h a t pulsed l a s e r s are f a r superior t o regrowth in t h e s o l i d phase. In Figure 10, backscattering spectra are used t o i l l u s t r a t e the conversion of an i n i t i a l l y amorphous GaAs layer t o a nearly perfect c r y s t a l 1 ine s t r u c t u r e . Such high-qua1 i t y crystal 1 i ne regrowth i s t y p i c a l l y obtained when the E, i s high enough f o r t h e melt front t o penetrate e n t i r e l y through the implanted l a y e r , so t h a t highvelocity, liquid-phase e p i t a x i a l regrowth from the crystal1 ine s u b s t r a t e can occur. However, i t has been shown t h a t i f El i s less than the value needed t o melt through the implanted region, then t h e quality of c r y s t a l l i n e regrowth a l s o depends upon the type of damage in the implanted region. Penetration of t h e melt f r o n t only p a r t i a l l y through a f u l l y amorphized implanted layer r e s u l t s in polycrystall i ne regrowth (Campi sano et a1 , 1980). However , i f t h e impl anted 1ayer i s not e n t i r e l y amorphous , b u t a1 so contai ns a highly defective c r y s t a l l i n e region t o which the melt f r o n t

.,

.

504

D. H. LOWNDES

2

0 4.6

9.4

1.8

2.0

2.2

ENERGY ( M e V )

Fig. 10. Backscattering spectra of 2.5-MeV He' ions incident in a random direction o f GaAs samples implanted with 400-KeVTe 1015 direction in the cm-2, after pulsed ruby laser irradiation o f energy ( a ) 0.2 to 0.8 J / c m 2 , ( b ) 0.9 j/crn2, ( c ) 1 .O-1.4 )/crn2. Curve ( d ) i s obtained from the unimplanted GaAs sample (Compisano e t at. , 1980).

penetrates , then poor-qua1 i t y c r y s t a l 1 ine regrowth wi 11 occur , with d e f e c t s propagated back t o t h e surface. This defective crystal1 ine type of regrowth i s i l l u s t r a t e d in F i g . 4 (Fletcher et al., 1981a, b ) ; i n t h i s case the implanted samples (Table 11, section 111) were general l y polycrystall i ne r a t h e r than amorphous , as a r e s u l t of beam heating during the high-dose b u t shallow room-temperature implantation. A polycrystall ine region extends from the surface t o a 40 nm depth (Fig. 4 a ) ; the region from 40-100 nm (near the peak of the implanted dopant p r o f i l e ) was r e l a t i v e l y defect f r e e , containing only small loops and defect c l u s t e r s ; but, the region

8.

505


from 100-180 nm c o n t a i n e d a h i g h d e n s i t y o f loops, d i s l o c a t i o n t a n g l e s and small t w i n s (-30 nm across). (Fig.

I r r a d i a t i o n a t 0.25 J / c d

4b) r e s u l t e d i n a m e l t - f r o n t p e n e t r a t i o n o f s l i g h t l y more

t h a n 100 nm, j u s t i n t o t h e deeper damage l a y e r ; e p i t a x i a l regrowth r e s u l t e d i n a h i g h d e n s i t y o f d i s l o c a t i o n s and f a u l t e d r e g i o n s i n t h e regrown l a y e r ,

b u t p o l y c r y s t a l l ine r i n g s were absent from

s e l e c t e d area d i f f r a c t i o n p a t t e r n s and t w i n spots were more e a s i l y distinguished.

For E,

= 0.36

J/cm2,

(Figs.

4c and d ) , t h e m e l t

f r o n t p e n e t r a t e d t o a depth o f 175 nm, almost t o t h e narrow band o f small loops t h a t mark t h e end o f i m p l a n t a t i o n damage.

Epitaxial

r e g r o w t h r e s u l t e d o n l y i n a low d e n s i t y o f d i s l o c a t i o n p a i r s propa g a t i n g i n t o t h e laser-regrown

r e g i o n , o r i g i n a t i n g from r e g i o n s

where t h e m e l t f r o n t o n l y p a r t i a l l y p e n e t r a t e d t h i s band o f small loops.

But f o r E,

>

0.36 J/cm2, t h e m e l t f r o n t p e n e t r a t e d beyond

t h e implantation-damaged r e g i o n , and t h i s i n i t i a l l y h i g h l y defect i v e c r y s t a l l i n e r e g i o n was observed t o e p i t a x i a l l y regrow w i t h no evidence o f imp1 a n t a t i on damage remai n i ng w i t h i n t h e

- 10-8, reso-

l u t i o n o f these TEM s t u d i e s . 8.

NEAR-SURFACE LOSS OF STOICHIOMETRY:

Ga-RICH RESIDUES

Although c h a n n e l i n g s p e c t r a and TEM micrographs c o n f i r m a h i g h l y substitutional

s t r u c t u r e f o l l o w i n g pulsed annealing,

these same

techniques a t h i g h e r r e s o l u t i o n r e v e a l a s i g n i f i c a n t d e v i a t i o n f r o m s t o i c h i o m e t r y i n t h e near-surface r e g i o n , i n t h e f o r m o f As l o s s and a Ga-rich s u r f a c e residue.

F i g u r e 11 (Barnes e t al.,

1978)

shows random and a1 igned channel ing s p e c t r a f o r Te-imp1 anted GaAs b e f o r e and a f t e r p u l s e d l a s e r annealing.

Over 90% o f t h e i m p l a n t e d

Te r e s i d e s on s u b s t i t u t i o n a l s i t e s , b u t t h e w e l l - r e s o l v e d Ga and As s u r f a c e peaks i n d i c a t e an excess o f s u r f a c e Ga. E t c h i n g w i t h warm HC1 removes most o f t h e excess Ga, r e s u l t i n g i n t h e r e t u r n t o a 1:l Ga:As r a t i o i n subsequent channeling measurements.

506

D. H. LOWNDES

I

+ Nd: YAG + Nd: YAG

-IMPLANTED A-IMPLANTED

HCI ETCH

ANNEAL ANNEAL

+

\

n

AS - IMPLANTED

I 01

-I

300

VIRGIN

--

1

I

I

1

1

350 CHANNEL NUMBER

Fig.

11.

Random and < l o o > channeling spectra for 50 keV, 1OI6 Te/crn2

implanted GaAs before and a f t e r Nd:YAG laser annealing (1.06 p ~1,2 5 nsec FWHM 20 M W / c r n 2 ) , and before and a f t e r removal o f surface Ga residue (Barnes

,

et al.,

1978).

Optical and transmission-electron micrographs have also been used t o study the formation and growth of these Ga-rich surface deposits w i t h increasing pulsed l a s e r El (Lowndes e t a l . , 1981; Fletcher et a1 1981a). These studies c l e a r l y demonstrate t h a t even though El 2 0.4 J/cm* i s s u f f i c i e n t t o remove implantation l a t t i c e damage and obtain high e l e c t r i c a l a c t i v a t i o n of p-type implants, degradation of t h e near-surface region occurs f o r a1 1

.,

l a s e r energy d e n s i t i e s above the melting threshold (Ex -0.2 J/cm*).

8.


507

Figure 1 2 shows plan view TEM micrographs of the surface of Znimplanted GaAs a f t e r pulsed ruby l a s e r i r r a d i a t i o n . The darker regions, present only a f t e r l a s e r annealing, were determined by x-ray analysis t o be gallium rich. These regions are small b u t numerous a t low El; t h e i r s i z e increases with increasing El while t h e i r number density decreases. For El = 0.81 J/cm2, these Ga-rich regions a r e 1inked together t o form a network or "cell -type" struct u r e with a c e l l s i z e o f 1-2 pm. Samples annealed a t 0.81 J/cm2 or higher energy d e n s i t i e s a l s o displayed a much f i n e r network s t r u c t u r e of Ga-rich regions (on a scale of t h e order of 100 nm c e l l s i z e ) , some o f which c e l l s were associated with very shallow d i s l o c a t i o n s , extending i n t o only t h e top 50 nm of the samples; these represent another form of laser-induced damage f o r El > 0.8

Fig. 12. Plan-view electron micrographs showing Ga-rich regions (dark) after pulsed ruby laser annealing of Zn-implanted GaAs. ( a ) 0.25 J / c m 2 ; ( b ) 0.36 J / c m 2 ; ( c ) 0.49 J / c m 2 ; ( d ) 0.81 J / c m 2 (Lowndes e t al., 1 9 8 1 b ) .

508

D. H. LOWNDES

J/cm2. Thus, there i s good overall agreement between TEM observat i o n s and e l e c t r i c a l activation measurements: Both show t h a t Ex 0.4 J / c d i s required t o completely remove the shallow ionimplantation damage considered here and t o a c t i v a t e implanted dopant ions. However, TEM measurements a l s o make i t c l e a r t h a t t h e "onset" of " e l e c t r i c a l " damage a t about 0.8 J/cm2 i s r e a l l y just the culmination of damage processes t h a t r e s u l t in a loss of stoichiometry and occur e s s e n t i a l l y continuously f o r a l l Ex above the melting threshold, when samples are annealed in a i r . SIMS depth p r o f i l e s of normalized arsenic counts, in both c r y s t a l l i n e and ion-implanted GaAs a f t e r l a s e r annealing, a l s o reveal t h a t the arsenic loss r e s u l t i n g from pulsed l a s e r i r r a d i a t i o n i s much m r e serious f o r implanted GaAs than f o r c-GaAs (Lowndes et a l . , 1981). This l a s t r e s u l t i s consistent with photo1 uminescence s t u d i e s , which showed a gradual decrease i n photoluminescent i n t e n s i t y with increasing E l f o r c-GaAs, b u t no photoluminescence from ion-implanted GaAs, regardless of the laser-annealing conditions (Lowndes and Fel dman , 1982; Fel dman and Lowndes , 1982). Davies e t a1 (1981, 1982a) have compared the surfaces of samples pulse annealed with and without a deposited f i l m of As2Se,. (The As2Se, f i l m was used as a dopant source f o r pulsed diffusion of Se dopant ions.) They reported t h a t microscopic measurements showed Ga globules only on surfaces annealed without As2Se3, while high-resolution channeling measurements revealed no excess of the Ga surface peak over t h e As surface peak. Apparently As i s cont r i b u t e d from t h e diffusion source t o produce a more nearly s t o i chi omet r i c surface. Rose and co-workers (1983) have recently reported a method f o r accurately measuring As and Ga losses from GaAs during annealing. Their method uti 1i zes quartz "catcher" sl ides t h a t are located just above, b u t not touching, t h e GaAs samples during annealing; t h e Ga and As deposits are then subjected t o neutron activation analysis, followed by gamma-ray counting f o r q u a n t i t a t i v e determination of

-

.

8.


deposited As and Ga c o n c e n t r a t i o n s .

509

For comparison, RBS measure-

ments were a l s o c a r r i e d o u t on some o f t h e s l i d e s .

Rose e t a1

.

p o i n t out t h a t a l t h o u g h RBS has a s e n s i t i v i t y o f 1013/cm2 f o r comb i n e d As and Ga, o v e r l a p o f t h e As and Ga peaks makes i t d i f f i c u l t t o a c c u r a t e l y measure t h e amounts of

As o r Ga i n d i v i d u a l l y ; f o r

t o t a l As and Ga l o s s e s g r e a t e r t h a n 5 x 1015/cm2, t h e Ga and As peaks cannot be separated a t a l l .

I n contrast,

t h e y found t h a t

n e u t r o n - a c t i v a t i o n a n a l y s i s had a c o n s e r v a t i v e l y e s t i m a t e d sensi t i v i t y o f 1 x 1013/cm2 f o r As and Ga i n d i v i d u a l l y , and no degradat i o n o f s e n s i t i v i t y with i n c r e a s i n g q u a n t i t y .

Combined As and Ga

l o s s e s measured by t h e two methods were found t o be i n good agreement.

Rose e t a l . compared t h e As and Ga l o s s e s from v i r g i n (100)

GaAs c r y s t a l s f o r t h r e e d i f f e r e n t annealing methods:

p u l s e d ruby

1aser anneal i ng, anneal ing w i t h a q u a r t z ha1ogen 1amp, and anneal ing w i t h a v i t r e o u s carbon s t r i p heater.

(25 nsec pulse,

i n air,

For ruby l a s e r annealing

with a q u a r t z beam homogenizer) t h e i r

p r i n c i p a l r e s u l t was t h a t As l o s s e s s u b s t a n t i a l l y exceeded Ga losses, t y p i c a l l y by a f a c t o r o f 2 t o 3, f o r 0.3 > El

> 1.1 J/cm2,

w i t h t h e l o s s e s t e n d i n g toward e q u a l i t y a t t h e h i g h e r

-

El b u t

El 1.4 J/cm2. Arsenic l o s s e s ranged from about 1 x 1015/cm2 a t El = 0.4 J/cm2 t o 4 0 x lO15/cm2 a p p a r e n t l y o n l y becoming equal f o r a t 1.1 J/cm2.

(For comparison,

t h e r e a r e about 6 x lOl4/cm2 As

atoms i n a s i n g l e (100) l a t t i c e plane.) Only a few measurements were made by Rose e t a l . u s i n g i o n i m p l a n t e d GaAs samples, so i t i s n o t p o s s i b l e t o determine from t h e i r r e s u l t s whether As l o s s d u r i n g l a s e r annealing i s enhanced b y imp1 a n t a t i o n .

However, many e a r l i e r thermal-anneal i n g s t u d i e s

(Lou and Somorjai, 1971; Picraux, 1973) have shown t h a t t h e e f f e c t of i m p l a n t a t i o n i s t o enhance t h e t o t a l r e l e a s e of As a t temperat u r e s below 500'C

by as much as a f a c t o r o f e i g h t , and t o lower

t h e temperature a t which s i g n i f i c a n t As r e l e a s e begins from >600°C t o about 200°C.

Lou and Somorjai (1971) concluded t h a t t h e r a t e -

l i m i t i n g step i n t h e v a p o r i z a t i o n o f As from c r y s t a l l i n e GaAs i s

510

D. H. LOWNDES

Fig. 13. Optical Nomarski micrographs of pulsed ruby laser annealed GaAs ( a ) 0.49J/cm2; (b) 0.62J/cm2; ( c ) 0.81 j/cm2; (d) 0 . 9 8 j / c m 2 (Fletcheret a l . , 1981a).

t h e a v a i l a b i l i t y ( i .e.y formation and d i f f u s i o n ) of e i t h e r vacancies or divacancies a t the GaAs surface. Picraux (1973) has argued t h a t the e f f e c t of implantation i n enhancing As vaporization can t h u s be understood in terms of implantation providing s i g n i f i c a n t conc e n t r a t i o n s of vacancies (as well as other d e f e c t s ) close t o the surface. 9.

LASER DAMAGE AND BEAM 'HOMOGENIZERS

Optical Nomarski interference micrographs of the surface of c-GaAs specimens a1 so reveal large-scale surface r i p p l e s t h a t become more pronounced with increasing pulsed ruby l a s e r El (Fig. 13). For the higher El, a high density o f f i n e r background structure a1 so appears, and occasional l a r g e r vaporization " c r a t e r s "

8.


511

Fig. 14. Q t i c a l micrographs showing inhomogeneous i r r a d i a t i o n o f a &As surface under d i f f e r e n t pulsed ruby laser beam conditions. The samples were f i r s t coated with -100 A Sn, i n order t o visibly enhance the e f f e c t o f local variations i n energy density, for polarized light photography. A l l pictures taken following a single pulse a t 0.5 J / c m 2 (scale: long axis = 1 mm). ( a ) Microcraters resulting from the use o f a diffuser plate. ( b ) D i f f r a c t i o n e f f e c t s from the bare single-mode beam. (c)

Homogeneous irradiation obtained with a bent, d i f f u s i n g light pipe.

512

D. H. LOWNDES

a r e also present. I t should be noted t h a t the r i p p l e patterns in Fig. 13 a l l resulted from i r r a d i a t i o n s using a beam-homogenizing l i g h t pipe. Even larger-scale surface degradation can r e s u l t from i r r a d i a t i n g a polished GaAs surface with a bare single-mode ruby l a s e r beam (Figure 1 4 ( b ) ) or by homogenizing the beam with a d i f f u s e r p l a t e such as i s used in pulsed ruby l a s e r annealing of s i l i c o n (Figure 1 4 ( a ) ) . In the l a t t e r case "microfocusing" by p i t s i n the d i f f u s e r p l a t e r e s u l t s i n c r a t e r s i n the GaAs surface. Figures 13 and 14 emphasize the limited usefulness of pulsed solids t a t e lasers and beam homogenizers f o r pulsed annealing. The recent demonstration of high homogeneity annealing of ion-implanted s i l i c o n using pulsed-excimer l a s e r s without the need f o r d i f f u s e r p l a t e s or other beam homogenizers (Lowndes e t a l . , 1982, 1983; Young e t a l . , 1983) suggests t h a t excimer l a s e r s should be espec i a l l y useful f o r GaAs, which i s f a r more susceptible t o surface damage than i s s i l i c o n . 10.

PHOTOLUMINESCENCE STUDIES

Lowndes and Fel dman (1982) have studied the photo1 umi nescence ( P L ) spectra o f both c- and implanted-GaAs following pulsed ruby l a s e r annealing. Samples were i r r a d i a t e d without encapsulation, i n a i r , mounted on s u b s t r a t e s a t room temperature, using single pulses (with E l u p t o 0.6 J/cm2) from a ruby l a s e r t h a t was spat i a l l y homogenized by a f u s e d - s i l i c a l i g h t pipe. Their study was motivated by the f a c t t h a t previous experiments using s i l i c o n had shown t h a t PL measurements following l a s e r annealing are a quite s e n s i t i v e indicator of the degree o f successful l a t t i c e regrowth (Mizuta et a l . , 1981; Skolnick e t al., 1981; Uebbing e t a l . , 1980) and of the introduction of new defects by e i t h e r cw ( S t r e e t e t a1 1979) or pulsed (Skolnick et al., 1981) l a s e r annealing. Figure 15 i l l u s t r a t e s t h e decrease i n PL i n t e n s i t y t h a t occurred A a f t e r i r r a d i a t i n g e i t h e r p- or n-type GaAs w i t h increasing El. substantial reduction of PL i n t e n s i t y was found even a t El = 0.4

.,

513

8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE 50

-efe

GoAs: Si 4.2 K

I

u) .c

40

._

-g 30 m

5I-

$

-

a Ep=O.O J/crn2 b E p = 0.2 J/cm2 c Ed=0.4 J/cm2 d El =0.6 J/cm2

20

L)

z

w 0 m

z W

5, J 1.42 4.44 1.46 1.48 1.50 1.52 ENERGY ( eV I

40

0

0.8

1.0 4.2 ENERGY ( e V )

4.4

Fig. 1 5 . Photoluminescence spectra for ( a ) p-type and ( b ) n-type c-GaAs a t 4 . 2 K (Lowndes and Feldman, 1 9 8 2 ) .

J/cm*, the lowest En value t h a t produced good e l e c t r i c a l activat i o n of high-dose, p-type implants (Fig. 7), corresponding t o a calculated maximum melt depth in c-GaAs of 180 nm (Fig. 3 ) . By considering the l / e absorption length a t t h e i r photoexcitation wavelength of 514.5 nm, and by varying t h i s wavelength, Lowndes and Feldman were able t o show t h a t the observed f a l l o f f in PL i n t e n s i t y was representative of bulk (not surface) recombination processes for photoexcited el ectron-hole pairs created w i t h i n an increasingly thick layer of material t h a t was melted and recryst a l l i z e d by the pulsed l a s e r . They concluded t h a t PL measurements show t h a t pulsed melting of c-GaAs always r e s u l t s in creation o f more nonradiative defect s i t e s than a r e eliminated. Some differences were found in the El dependence of PL intens i t y f o r n- and p-type GaAs, a t low En. The integrated PL intens i t y peaked near the melting threshold f o r n-GaAs ( a t both 77 K and 300 K ) but dropped off rapidly w i t h increasing El i n p-GaAs

D. H. LOWNDES

( a t a l l temperatures).

This d i f f e r e n c e i n PL behavior was a t t r i -

buted by Lowndes and Feldman t o d i f f e r e n c e s i n t h e nature o f t h e t r a n s i t i o n s being observed, i.e.

, r a d i a t i v e t r a n s i t i o n s predomi-

n a n t l y v i a near-band edge l e v e l s f o r t h e p-type m a t e r i a l vs r a d i a t i v e t r a n s i t i o n s through deep l e v e l s f o r t h e n-type m a t e r i a l .

One

e x p l a n a t i o n i s t h a t pulsed l a s e r i r r a d i a t i o n creates t r a p s a t i n t e r m e d i a t e l e v e l s , and t h a t these can be reached by t u n n e l i n g o r p e r c o l a t i o n from near t h e band edge.

These traps, i n t u r n , can

populate deeper l e v e l s , a1 so by tunnel ing and percol ation.

Thus,

PLA o f c-GaAs would be expected t o quench PL from t r a p l e v e l s very near t h e band edge (as observed i n p-type GaAs and i n n-type GaAs a t 300

K), w h i l e i n i t i a l l y not much a f f e c t i n g ( o r even s l i g h t l y

enhancing) recombination through much deeper l e v e l s (as observed i n n-type GaAs a t 77 K and 300 K).

DLTS measurements provide

independent evidence o f both t h e c r e a t i o n and removal o f e l e c t r o n i c deep l e v e l s i n GaAs v i a pulsed l a s e r i r r a d i a t i o n ; 1V.ll.c.

see s e c t i o n

Lowndes and Feldman also observed t h a t decreasing t h e i r

p h o t o e x c i t a t i o n wavelength always r e s u l t e d i n a decrease o f PL i n t e n s i t y i n PLA GaAs , apparently because e l ectron-hol e pai r s are t h e n created c l o s e r t o t h e sample surface, where t h e highest d e f e c t d e n s i t y i n r e c r y s t a l l i z e d m a t e r i a l i s also found. The p r i n c i p a l r e s u l t o f Lowndes and Feldman’s study o f PL from

I 1 GaAs was t h a t no PL was observed i n GaAs subjected t o high-dose ions/cm2) i o n i m p l a n t a t i o n , e i t h e r w i t h o r w i t h o u t subsequent pulsed l a s e r annealing. Thus, PL provides more evidence t h a t pulsed l a s e r annealing o f I 1 GaAs, though i t produces e p i t a x i a l regrowth, does not remove d e f e c t s t h a t act as n o n r a d i a t i v e recomb i n a t i o n centers.

Since PL i s observed i n c-GaAs subjected t o

s i m i l a r pulsed l a s e r energy d e n s i t i e s , though w i t h reduced i n t e n s i t y , these measurements suggest t h a t II/PLA GaAs always contains higher near-surface r e g i o n d e f e c t concentrations than GaAs subjected t o PLA alone, regardless o f t h e l a s e r El

used.

8.


515

The lack of PL from high-dose II/PLA GaAs i s in agreement with r e s u l t s of a depth-resol ved cathodol umi nescence study by Norri s and m *ions a t 200 Peercy (1981), u s i n g GaAs implanted with 2 ~ 1 0 ~ ~ / c Cd keV (projected ranged -53 nm). No "range zone" PL (from the top 300 nm of the samples) was detected following pulsed ruby l a s e r annealing, except f o r En = 0.15 2 0.05 J/cm*--below the Ell thresholds f o r melting and f o r e p i t a x i a l regrowth--in which case PL intens i t y recovered t o about 10% of the i n t e n s i t y observed f o r c-GaAs. However, f u r t h e r exposure of such an "optimally" annealed sample t o a second, higher-energy pulse a t El = 0.25 J/cm2 ( t o promote e p i t a x i a l regrowth) removed t h e PL recovery. 11.

ELECTRICALLY ACTIVE DEFECTS

Pribat (1982) has noted t h a t t h e experimental methods t h a t were most useful f o r characterizing dopant i o n - l a t t i c e locations and residual near-surface l a t t i c e damage (e.g. , RBS in t h e channeling mode), following pulsed l a s e r annealing of high-dose ion implants i n GaAs and s i l i c o n , cannot provide useful information f o r low(20 A t h i c k and i s composed p r i m a r i l y o f As203 and Ga203, though i n p r o p o r t i o n s t h a t depend upon t h e c o n d i t i o n s

., 1980; Breeze e t a1 ., 1980). A s i n g l e (Matsuure e t a1 ., 1981) had i n d i c a t e d t h a t oxygen

o f growth (Thurmond e t a1 p r i o r experiment

t r a p p i n g can occur d u r i n g nanosecond PLA o f GaAs i n a 3-4 atm. oxygen ambient;

p r i o r s t u d i e s o f oxygen i n c o r p o r a t i o n d u r i n g PLA

o f s i l i c o n suggested l i t t l e oxygen i n c o r p o r a t i o n f o r v i s i b l e nanosecond pulses, b u t t h a t t h e process i s complex and s t r o n g l y dependent on parameters t h a t i n c l u d e t h e pulsed l a s e r wavelength ( v i s i b l e vs u v ) ,

pulse duration

p r e s s u r e ( L i u e t a1 1980; C u l l i s e t a1

(nanosecond vs picosecond), and ambient

., 1979,

., 1982).

1981; Tsu e t a1

., 1979;

Hoh e t al.,

The p r i n c i p a l issues addressed by BBCDJPS are: ( a ) The r e l a t i o n s h i p between t h e t h r e s h o l d energy d e n s i t y f o r p u l s e d l a s e r m e l t i n g o f GaAs and t h e ( d i f f e r e n t ) t h r e s h o l d energy d e n s i t i e s f o r oxygen i n c o r p o r a t i o n from a n a t i v e o x i d e o r from t h e ambient ;

8 . PULSED BEAM PROCESSING OF GALLIUM ARSENIDE

531

( b ) elucidation of t h e role of the native oxide layer, e i t h e r a s a source of oxygen or as a possible b a r r i e r t o incorporation of oxygen from the ambient;

( c ) the composition (stoichiometry) of pulsed laser-formed oxides on GaAs and the mechanism f o r t h e i r formation; and ( d ) changes i n the composition of the native oxide, and i n the depth p r o f i l e in GaAs of oxygen derived from t h e native oxide, a t low En. In order t o distinguish between 0 originating from a native oxide layer and 0 from the ambient atmosphere, BBCDJPS formed -10 nm thick l80-enriched oxide layers by anodic oxidation in an l80enriched solution. A 15-nsec duration ruby l a s e r and a beam The l60 and l 8 O homogenizer p l a t e were used f o r i r r a d i a t i o n s . contents in a near-surface region o f about 1-2 pm thickness (i.e., somewhat thicker than t h e maximum me1 t depth) were determined using nuclear reactions with deuteron and proton beams, respectively. Depth p r o f i l e s of the l 8 O content were a l s o obtained via a f i t t i n g procedure applied t o the resonant l80 ( p , a ) 15N reaction a t 629 keV, with a d e p t h resolution of about 20 nm (Cohen e t a l . , 1983, 1984). a.

Oxygen from the Ambient Atmosphere

Measurements by BBCDJPS under an 0 pressure of 4 atm. on native oxide-covered samples revealed no change i n the t o t a l near-surface 0 content (No 20 x lOl5/cm2) f o r En < 1 J/cm*. However, a large uptake of oxygen was observed f o r E, 1.1 J/cm2, w i t h No approachi n g 150 x 1015/cm2 a t E, = 1.7 J/cm2. A t t h i s point s a t u r a t i o n of t h e t o t a l 0 content occurs, apparently due t o competition between 0 uptake and nearly simultaneous evaporation a t the high temperat u r e s reached by the GaAs melt. Studies of t o t a l 0 content a t a fixed El = 1.5 J/cm? a l s o revealed t h a t the 0 uptake i s proport i o n a l t o 0 overpressure, over t h e 0-4 atm pressure range (Bentini 1982). BBCDJPS i n t e r p r e t these r e s u l t s as showing t h a t e t a1 neither diffusion of 0 atoms in the l i q u i d nor t h e i r r e a c t i v i t y

-

.,

532

D. H. LOWNDES

are limiting steps i n 0 uptake; they also calculate that about 10% of the 0 atoms striking the liquid surface are trapped i n a surface oxide layer (Bentini et al., 1982). Time-resolved reflectivity measurements and me1 t i n g model calcul a t i ons show that me1 t i ng of GaAs occurs under these conditions for El >, 0.2 J/cm2 (Fig. 2), yet significant 0 uptake from ambient a i r (po2 = 0.2 atm) was not observed by BBCDJPS until El > 1.1 J/cm2. T h i s result suggested t o BBCDJPS t h a t the presence o f native oxide on a GaAs substrate could hinder 0 uptake and that there i s an energy threshold t o remove native oxide by evaporation, before 0 uptake from the ambient can occur. Figure 21 shows results of measurements of l6O, l s O , and total 0 content vs Ex, following i rradi a t ion of GaAs sampl es covered by -10 nm thick 180-enri ched

q-

X

z

W

U

>-

X 0

0

0.4

0.8

1.0

1.4

ENERGY DENSITY ( J I c m ' l Fig. 21. El-dependence of l60incorporation from a Po = 4 atm. ambient and 2 of l80loss from a surface oxide for GaAs. The full line shows total 0 content (Bentini et al., 1982).

8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE n a t i v e oxides i n a p = 4 atm. 1 6 0 ambient.

The r e s u l t s demonstrate

1019~ m - ~ )t h, e l i n e a r f r e e - c a r r i e r a b s o r p t i o n c o e f f i c i e n t a t room temperature i s about 103 cm-1 o r larger.

For an absorption c o e f f i c i e n t o f 103 cm-1, t h e l i g h t pene-

t r a t i o n depth i s several microns, which i s somewhat g r e a t e r than t h a t a t t a i n a b l e using a l a s e r w i t h a photon energy above t h e bandgap. Although t h e p e n e t r a t i o n depth o f CO, l a s e r r a d i a t i o n is r a t h e r deep, one expects t h a t a t h i g h i n t e n s i t i e s t h e energy d e p o s i t i o n r a t e w i l l be s u f f i c i e n t t o m e l t t h e near-surface r e g i o n without n e c e s s a r i l y i n v o l v i n g an intensity-dependent absorption mechanism.

9.

561

PULSED COz LASER ANNEALING

Table I 1 Values f o r t h e l i n e a r f r e e - h o l e a b s o r p t i o n c r o s s s e c t i o n q, f o r l i g h t w i t h a wavelength o f 10.6 p and l i g h t w i t h a wavelength o f 9.6 pm. A l l values a r e f o r a l a t t i c e temperature o f 300 K, except f o r InSb, which i s f o r a temperature o f 20 K.

Si Ge GaAs GaSb InAs InSb A1 Sb

7 x 6.1 4.2 3.2

7 x 5.7 3.9 3.1 8.7 2.4 3.2

10-17 a x x 10-16 C x 10-l6 8.6 x 10-16 e 2.5 x 10-15 f,g 4.0 x 10-16 C

10-17 a 10-16 10-16 10-l6 10-16 10-15 10-l6

x x x x x x

b c

d

e f,g C

a Hara and N i s h i (1966). K a i s e r e t a l . (1953). B r a u n s t e i n and Kane (1962). Becker e t a l . (1961). Matossi and S t e r n (1958). K u r n i c k and Powell (1959). g Jamison and Nurmikko (1979). I n moderately o r l i g h t l y doped m a t e r i a l , t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t i s much s m a l l e r and,

consequently,

the penetration

depth o f t h e l i g h t i s much l a r g e r .

I n o r d e r t o achieve successful

a n n e a l i n g o f moderately o r 1 i g h t l y doped samples, t h e f r e e - c a r r i e r d e n s i t y must be increased i n o r d e r t o i n c r e a s e t h e c o u p l i n g o f t h e C02 l a s e r l i g h t t o t h e s u b s t r a t e . The most s t r a i g h t f o r w a r d way t o i n c r e a s e t h e c a r r i e r d e n s i t y i s by h e a t i n g t h e l a t t i c e , where t h e i n c r e a s e i n t h e e l e c t r o n and h o l e d e n s i t i e s r e s u l t s from t h e temperature dependence o f t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n . A l t e r n a t i v e l y , one can i n c r e a s e t h e c a r r i e r d e n s i t y by s i m u l t a neously i r r a d i a t i n g t h e m a t e r i a l w i t h a l a s e r having a photon energy g r e a t e r t h a n t h e bandgap (Miyao e t a1

., 1979).

E l e c t r o n - h o l e p a i r f o r m a t i o n can a l s o be achieved by a m u l t i photon a b s o r p t i o n mechanism (Yuen e t a1

.,

1980).

Multiphoton

a b s o r p t i o n has been used by Hasselbeck and Kwok (1982) t o anneal

562

R . B. JAMES

InSb w i t h a CO, laser

l a s e r by a two-photon process.

annealing

by

mu1 t i p h o t o n

and

Possibilities for

subsequent

free-carrier

a b s o r p t i o n should a l s o e x i s t f o r o t h e r narrow bandgap semiconductors,

such as InAs and a l l o y s o f Hgl-xCdxTe

( B a h i r and K a l i s h ,

1981). For s u f f i c i e n t l y h i g h CO,

laser intensities,

nonequil ib r i u m

f r e e c a r r i e r s can a l s o be generated by impact i o n i z a t i o n processes. Here, t h e l a s e r heats t h e f r e e - c a r r i e r d i s t r i b u t i o n t o t h e e x t e n t t h a t a s i g n i f i c a n t f r a c t i o n o f t h e c a r r i e r d e n s i t y has an energy g r e a t e r t h a n t h e bandgap (as measured from t h e conduction band minima f o r e l e c t r o n s and valence band maximum f o r holes).

These

h i g h l y e n e r g e t i c f r e e c a r r i e r s can r e l a x by c r e a t i n g e l e c t r o n - h o l e p a i r s through i n v e r s e Auger events.

Electron-hole p a i r formation

by impact i o n i z a t i o n has been proposed t o e x p l a i n t h e n o n l i n e a r a b s o r p t i o n o f CO,

l a s e r l i g h t i n n-type InAs, InSb, Hg0.77Cd0.23Te

(Jamison and Nurmikko,

1979),

and Ge (James and Smith,

1982a).

Approximate values f o r t h e i n t e n s i t i e s a t which c a r r i e r m u l t i p l i c a tion

by

impact

ionization

becomes

important

have

also

r e p o r t e d f o r several o t h e r semiconductors (James, 1983).

been

The Sen-

e r a t i o n o f n o n e q u i l i b r i u m e l e c t r o n - h o l e p a i r s by i m p a c t - i o n i z a t i o n events suggests t h e p o s s i b i l i t y o f l a s e r - p r o c e s s i n g free-carrier

samples v i a

a b s o r p t i o n w i t h p r a c t i c a l l y any below-bandgap

laser

r a d i a t i o n , p r o v i d e d t h e i n t e n s i t y o f t h e pulsed e x c i t a t i o n source i s s u f f i c i e n t l y g r e a t t o c r e a t e an e l e c t r o n - h o l e plasma.

111.

PULSED CO, LASER ANNEALING OF SILICON

I n t h e f a b r i c a t i o n of many e l e c t r o n i c devices, one i s r e q u i r e d t o grow both doped and s t r u c t u r e d t h i n l a y e r s o f s i l i c o n and s i l i c o n dioxide.

The a p p l i c a t i o n o f l a s e r - p r o c e s s i n g techniques t o t h e

m a n u f a c t u r i n g o f s i l i c o n devices has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n due t o t h e p o s s i b i l i t i e s o f i n c r e a s e d c o n t r o l o f j u n c t i o n depths, c a r r i e r c o n c e n t r a t i o n s i n doped l a y e r s , and c a r r i e r l i f e t i m e s .

The

c a p a b i l i t y o f c o n t r o l l i n g these c r i t i c a l parameters should r e s u l t

9.

i n improved d e v i c e performance.

I n t h i s section,

r e s u l t s are presented on t h e pulsed CO, i s organized as f o l l o w s : o f h i g h - i n t e n s i t y CO, next,

experimental

l a s e r annealing o f i o n -

i m p l a n t e d s i l i c o n t o o b t a i n device-grade m a t e r i a l .

of silicon;

563

PULSED C 0 2 LASER ANNEALING

The s e c t i o n

F i r s t , r e s u l t s a r e given f o r t h e e f f e c t

l a s e r r a d i a t i o n on t h e o p t i c a l p r o p e r t i e s

the r e c r y s t a l l i z a t i o n o f ion-implanted layers

t h a t have been melted by a CO,

l a s e r i s discussed; and f i n a l l y ,

r e s u l t s are presented f o r t h e r e d i s t r i b u t i o n o f imp1 anted dopants i n t h e near-surface region.

1.

OPTICAL PROPERTIES Time-resolved r e f l e c t i v i t y and transmi s s i v i t y measurements have

have been performed on s i l i c o n t o determine t h e onset o f n o n l i n e a r a b s o r p t i o n , t h e t h r e s h o l d f o r l a s e r - i n d u c e d m e l t i n g , and t h e m e l t durations.

R e s u l t s w i l l f i r s t be presented f o r t h e t i m e - r e s o l ved

r e f l e c t i v i t y measurements,

f o l l o w e d by t h e r e s u l t s f o r t h e time-

r e s o l v e d t r a n s m i s s i v i t y measurements. I n t h e experiments o f Naukkarinen e t a l . (1982) , a t r a n s v e r s e l y e x c i t e d atmospheric (TEA) CO,

l a s e r was used as an e x c ' i t a t i o n source.

The pulses had a d u r a t i o n o f 100 ns and maximum energy o f 2 J.

The

r e f l e c t i v i t y measurements shown i n Fig. 1 were performed as a funct i o n o f t h e l i g h t i n t e n s i t y on a l i g h t l y doped sample (5 x 1015 boron atom/cm3) and a h e a v i l y doped sample ( 5 x 1019 boron atoms/cm3).

A

photon drag d e t e c t o r and f a s t o s c i l l o s c o p e were used t o measure t h e i n t e n s i t y o f t h e r e f l e c t e d pulses.

The r e f l e c t a n c e o f t h e l i g h t l y

doped sample stayed constant f o r i n t e n s i t i e s i n t h e range o f 0.1 t o 100 MW/cm2.

I n t h e experiments, a i r breakdown would o c c a s i o n a l l y

occur near t h e focus.

I f t h e a i r breakdown occurred between t h e

sample and t h e d e t e c t o r i n t h e r e f l e c t a n c e measurements, no r e f l e c t e d p u l s e was observed.

I n t h e h e a v i l y doped sample, t h e r e f l e c t a n c e

remained constant up t o i n t e n s i t i e s i n t h e range o f 20-35 MW/cm2, t h e n i t began t o i n c r e a s e r a p i d l y from about 40-45% t o 80-100% as t h e i n t e n s i t y was f u r t h e r increased.

564

R. B. JAMES

R

0

:I , 2

4 0

0.4

1

d-\,

40 20 40 100200 I(MWcm-2)

Fig. 1 . Reflectance for S i doped with ( a ) 5 x 1 0 1 5 B a t o m s / c m 3 , ( b ) 5 x 1019

B atoms/cm3 and transmittance for Si doped with ( c ) 5 x 1015 B atoms/cm3, ( d ) l o z o B atoms/cm3 down to a 3.5-pm depth as a function o f the incident laser intensity a t 1 0 . 6 pm.

Time-resolved

[ A f t e r Naukkarinen et al.

(1982).]

r e f l e c t i v i t y measurements were

a l s o made by

Naukkarinen e t a l . (1982) f o r a sample i n which 1020 boron atoms/cm3 were d i f f u s e d t o a depth o f 3.5 crystal.

Dm i n t o a l i g h t l y doped s i l i c o n

The onset o f m e l t i n g m a n i f e s t s i t s e l f by a change i n

t h e slope o f the r e f l e c t i v i t y , since the r e f l e c t i v i t y o f molten s i l i c o n a t 10.6 MW/crn2,

prn i s c l o s e t o u n i t y .

F o r an i n t e n s i t y o f 60

the duration of the high-reflectivity

phase i s measured

t o be about 500 ns, a l t h o u g h t h e u n c e r t a i n t y i s f a i r l y l a r g e i n comparing t h e r a t i o s o f t h e i n c i d e n t and r e f l e c t e d pulses.

9.

565


A b e t t e r way t o m o n i t o r t h e h i g h - r e f l e c t i v i t y phase i s by u s i n g a probe l a s e r and measuring t h e t i m e - r e s o l v e d r e f l e c t i v i t y o f t h e probe (Auston e t al.,

1978).

The t r a n s i e n t r e f l e c t i v i t y o f a cw

633-nm HeNe l a s e r i n t h e presence o f a h i g h - i n t e n s i t y

CO, l a s e r

p u l s e has been s t u d i e d by two groups (Hasselbeck and Kwok, 1983 and James e t al.,

1984a).

R e f l e c t i o n o f t h e probe beam from t h e

s i l i c o n s u r f a c e was measured as a f u n c t i o n o f t h e pump i n t e n s i t y . The experiments by Hassel beck and Kwok (1983) were performed w i t h a s i n g l e l o n g i t u d i n a l mode TEA CO,

laser.

The c r y s t a l l i n e

samples used i n t h e s t u d i e s were u n i f o r m l y doped w i t h antimony t o a r e s i s t i v i t y o f 0.02 a-cm, which corresponds t o an e l e c t r o n conc e n t r a t i o n o f about 1.5 x l O l *

Both t h e CO,

l a s e r and t h e

u n p o l a r i z e d cw HeNe probe l a s e r were focused o n t o t h e sample. The angle o f i n c i d e n c e o f t h e probe beam was 45" and t h e pump beam was s e t a t normal incidence.

The HeNe l a s e r was focused o n t o t h e

sample and a s i l i c o n photodiode w i t h a 3-ns response t i m e was used t o monitor t h e probe beam. i n Fig.

Fig. 2.

The t i m e - r e s o l v e d r e f l e c t i v i t y i s shown

2 f o r a 100-ns l a s e r p u l s e a t an i n t e n s i t y o f 78 MW/cmZ.

Transient reflectivity o f HeNe probe laser as induced by a 100-nsec

C 0 2 laser pulse with an intensity of 78 MW/crn2. division.

Vertical scale:

Kwok ( 1 9 8 3 ) .]

Horizontal scale: 0.5 us/ 15% reflectivity/division. [ A f t e r Hasselbeck and

566

.

R. B. JAMES

The r e f l e c t i v i t y increases r a p i d l y t o a constant value where i t remains f o r an extended p e r i o d before decreasing t o i t s o r i g i n a l value.

The f l a t t o p i s measured t o have a r e f l e c t i v i t y o f 70 t

5%, which agrees w i t h t h e r e f l e c t i v i t y value f o r molten s i l i c o n a t 633 nm (Lowndes e t al.,

1982 and Kwok e t al.,

1981).

The

d u r a t i o n o f t h e h i g h - r e f l e c t i v i t y phase i s found t o decrease a t lower pump i n t e n s i t i e s . I n t h e experiments by James e t al. w i t h a pulse d u r a t i o n o f 70 ns was used.

(1984a), a TEA CO,

laser

The wavelength was con-

t r o l l e d by an i n t e r n a l g r a t i n g t h a t allowed a tunable output over t h e 9- t o ll-pm region.

For a low n i t r o g e n mix, t h e output pulse

had a maximum energy o f 5 J and was r e p r o d u c i b l e t o w i t h i n 2% from p u l s e t o pulse.

The l a s e r pulses, a f t e r impingement on a C02 l a s e r

beam i n t e g r a t o r , had a s i z e o f 12 x 12 m i n t h e t a r g e t plane and a u n i f o r m i t y o f +_lo%,according t o t h e s p e c i f i c a t i o n s o f t h e i n t e grator.

The energy d e n s i t y i n t h e t a r g e t plane o f the i n t e g r a t o r

was a d j u s t a b l e up t o about 3.1 J/cm2 f o r a wavelength o f 10.6 mn. When higher energy d e n s i t i e s were required, a germanium lens w i t h

a f o c a l l e n g t h o f 100 mn was used a t t h e t a r g e t plane o f t h e i n t e g r a t o r . The samples were u n i f o r m l y doped w i t h boron and, p r i o r t o i o n implantation, had a r e s i s t i v i t y of 0.0073 62-cm a t room temperature,

which corresponds t o a h o l e concentration o f about 2-3 x

loi9

The near-surface l a y e r o f each sample was amorphized by i m p l a n t a t i o n o f As ions a t an energy o f 180 KeV t o a dose o f 10l6

cw2.

The r e f l e c t a n c e o f t h e C02 pulse on t h e s i l i c o n sample was

measured t o be about 0.40

a t low pump i n t e n s i t i e s .

A t energy

d e n s i t i e s greater than about 2.9 J/cm2, t h e r e f l e c t a n c e o f t h e C02 r a d i a t i o n from t h e ion-implanted surface was observed t o increase i n a manner s i m i l a r t o t h a t reported by Naukkarinen e t a l . (1982) f o r h e a v i l y doped c r y s t a l l i n e s i l i c o n . Time-resolved r e f l e c t i v i t y measurements were a l s o c a r r i e d out on t h e ion-implanted samples by James e t a1

. (1984a)

t o determine t h e d u r a t i o n o f t h e h i g h - r e f l e c t i v i t y phase as a f u n c t i o n o f t h e energy d e n s i t y o f t h e Cop-laser pulse.

A cw 633-nm HeNe l a s e r was used t o measure t h e time-resolved

9.

567

PULSED CO2 LASER ANNEALING

o p t i c a l r e f l e c t i v i t y of t h e implanted surface d u r i n g and immediately a f t e r i r r a d i a t i o n w i t h a CO, l a s e r pulse. The angle o f incidence o f t h e unfocused HeNe probe l a s e r was 30" from t h e surface normal, and t h e CO, l a s e r beam impinged on t h e sample a t normal incidence.

A

s i 1i c o n avalanche photodiode and a f a s t o s c i l l oscope were used t o monitor t h e r e f l e c t i v i t y o f t h e probe beam.

Narrow-band-pass HeNe

f i l t e r s were placed d i r e c t l y i n f r o n t o f t h e photodiode t o attenuate s c a t t e r e d r a d i a t i o n from t h e s i l i c o n surface.

The observed t r a n -

s i e n t r e f l e c t i v i t y s i g n a l s were s i m i l a r i n shape t o those r e p o r t e d by Auston e t al. (1978).

For energy d e n s i t i e s exceeding t h e m e l t

threshold, t h e s i g n a l s consisted o f a f l a t t o p w i t h a d u r a t i o n t m and a decaying t a i l , f o l l o w i n g t h e h i g h - r e f l e c t i v i t y phase.

The

r e f l e c t i v i t y d u r i n g t h e h i g h - r e f l e c t i v i t y phase was approximately two times i t s i n i t i a l value ( R o ) ,

corresponding t o a r e f l e c t i v i t y

o f about 70%, which i s c o n s i s t e n t w i t h t h e measured r e f l e c t i v i t y o f molten s i l i c o n .

The r e f l e c t i v i t y was c a l i b r a t e d by using a

chopper i n t h e path o f t h e HeNe l a s e r and equating t h e signal w i t h out t h e CO, l a s e r w i t h t h e known r e f l e c t a n c e o f amorphous s i l i c o n . Values f o r t h e m e l t d u r a t i o n s were taken by adding t h e times, t m and tf, where tf i s t h e t i m e r e q u i r e d f o r t h e r e f l e c t i v i t y R t o f a l l t o a value a t which (R-Ro)

= 0.5 Ro.

The values f o r t h e m e l t

d u r a t i o n s as a f u n c t i o n o f t h e energy d e n s i t y o f t h e C02 l a s e r pulse a r e shown i n Fig. 3.

( I n t h e present case, approximate i n t e n s i t i e s

can be deduced from measurements o f t h e energy d e n s i t y by assuming a r e c t a n g u l a r pulse o f 70 ns. The pulses, however, are c l o s e r t o Gaussian w i t h about 80% of t h e energy i n a spike ( f o r a low N2 m i x ) w i t h a FWHM o f 70 ns, and t h e remainder i n a long t a i l which l a s t s f o r several hundred nanoseconds.)

The values f o r t h e m e l t dura-

t i o n s o f t h e h e a v i l y doped, ion-implanted samples are found t o be as long as 1 p s , which i s considerably l o n g e r than t h e observed m e l t d u r a t i o n s using l a s e r s w i t h a photon energy g r e a t e r than t h e bandgap (see, f o r example, Lowndes e t a1

., 1984 and Chapter 6).

These m e l t

d u r a t i o n s are s t i l l somewhat smaller than t h e melt d u r a t i o n s o f c r y s t a l 1i n e s i 1 i c o n reported by Hassel beck and Kwok (1983).

568

R. B. JAMES

1000

I

I

I

I

I

a -

- C02 CO? LASER ENERGY DENSITY

1

g

-SURFACE MELT DURATION VS

-7

i -

F & M OF PULSE = 70 nS 800 t-FWHM h = 10.6pm

-

600 8

400

2o i

rn

THRESHOLD

01.o

3.0

5.0

7.0

9.0

ENERGY DENSITY (J/cm2)

Fig. 3. Measured duration of the high-reflectivity phase versus the energy density o f the pulsed CO2 laser. The wafers were doped with boron and had a resistivity o f 0.0073 8-cm a t room temperature prior to implantation. The nearsurface region of the samples was amorphized by implantation o f 75As ions a t an energy of 180 KeV to a dose of 10l6 crne2.

The transmittance o f l i g h t l y doped p-type s i l i c o n samples has been measured as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y .

I n the

experiments by James e t al. (1982b), a TEA l a s e r o p e r a t i n g i n t h e TEMoo mode was used. The output pulse had about 50% o f t h e energy i n t h e form o f a spike w i t h a FWHM o f 40 ns. The remainder o f t h e energy was i n a l o n g t a i l which l a s t s about 0.4 ps (depending on t h e N2 mix).

An i r i s was used t o i s o l a t e t h e c e n t r a l p o r t i o n of

t h e Gaussian beam i n order t o minimize t h e s p a t i a l v a r i a t i o n o f t h e beam.

The power d e n s i t y was c o n t r o l l e d by using c a l i b r a t e d

CaF2 attenuators, ZnSe o p t i c a l l e n s f o r focusing, and by a d j u s t i n g t h e high-voltage power supply.

The samples were coated f o r broad-

band a n t i r e f l e c t i o n a t a wavelength o f 10 pm and normal incidence. I n t h i s way, transmission data could be obtained as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y w i t h approximately u n i f o r m s p a t i a l v a r i a t i o n

9.

569

PULSED CO;! LASER ANNEALING

and high energy r e p r o d u c i b i l i t y (1-2%) from shot t o shot.

The

maximum peak i n t e n s i t y used i n t h e experiment was i n t h e range o f 50 t o 100 MJ/cm2, a t which p o i n t a spark appears a t t h e surface o f

t h e s i l i c o n and damages t h e a n t i r e f l e c t i o n coatings.

The t r a n s -

mittance was observed t o increase s l i g h t l y f o r i n t e n s i t i e s up t o about 40 MW/cm2, which was a t t r i b u t e d t o a s t a t e - f i l l i n g e f f e c t i n t h e intervalence-band t r a n s i t i o n s (James and Smith, 1981).

For

i n t e n s i t i e s i n t h e range o f about 40 t o 70 MW/cm2, t h e t r a n s m i t tance remained a t a constant value.

A t i n t e n s i t i e s h i g h enough

f o r a spark t o appear a t t h e surface o f t h e s i l i c o n , t h e r e was a drop i n the transmittance, which was a t t r i b u t e d t o damage o f t h e a n t i r e f l e c t a n c e coatings and t o t h e generation o f an increased number o f f r e e c a r r i e r s by t h e v i s i b l e f l a s h o f l i g h t .

Similar

measurements on an n-type sample which was l i g h t l y doped w i t h antimony (8 x lOl5 antimony atoms/cm3) showed no n o t i c e a b l e change i n t h e transmittance f o r C02 l a s e r i n t e n s i t i e s l e s s than t h e damage t h r e s h o l d o f t h e a n t i r e f l e c t i o n coatings. The t r a n s m i t t a n c e o f l i g h t l y boron-doped s i l i c o n (5 x 1015 has also been i n v e s t i g a t e d by Naukkarinen e t a1

. (1982).

The t r a n s -

m i t t a n c e o f a 100-ns pulse w i t h a wavelength o f 10.6 pm was found t o remain almost constant f o r measured i n t e n s i t i e s up t o 100 MW/cm2 (Fig.

1).

When a l i g h t f l a s h would occur a t t h e surface, a drop

i n t h e transmission was observed, i n agreement w i t h t h e observation o f James e t a1

. (1982b).

.

Transmission measurements were a1 so made by Naukkari nen e t a1 t h i c k h e a v i l y boron-doped l a y e r (-102O cm-3) formed by d i f f u s i o n . For l a s e r i n t e n s i t i e s l e s s than (1982) on samples w i t h a 3 . 5 - p

about 20 W/cm2, t h e transmittance was constant, but, i n t h e range of

about 20 t o 60 MW/cm2,

(Fig.

Id).

t h e transmittance decreased r a p i d l y

The drop i n t h e t r a n s m i t t a n c e a t i n t e n s i t i e s g r e a t e r

than 20 MW/cmz i s due t o t h e heating o f t h e surface region, which causes an increase i n both t h e r e f l e c t a n c e and t h e absorption c o e f f i c i e n t o f the diffused layer.

570

R. B. JAMES

100

a

I ( R E LATlVE UNITS)

0 5 IT

0 4 IT

0 1

L

IT

0

500

0 t

Fig.

4.

Time dependence of

(ns)

( a ) the incident

laser pulse and o f the

transmitted intensity I T ( b ) with I = 8 MW/cm2, ( c ) I = 21 MW/cm2, and ( d ) I = 60 MW/cm2. The sample was doped with 1020 boron a t o m s / c m 3 t o a depth o f 3 . 5 pm. [ A f t e r Naukkarinen e t a l . ( 1 9 8 2 ) . ]

Time-resolved t r a n s m i s s i o n measurements were a1 so made same samples by Naukkarinen e t al. (1982).

o f t h e i n c i d e n t p u l s e i s shown i n Fig. 4a.

on t h e

The temporal dependence The p u l s e has a l a r g e

peak w i t h a FWHM o f approximately 100 ns f o l l o w e d by a l o n g t a i l which l a s t s about 1 p s .

The p u l s e t r a n s m i s s i o n f o r an i n c i d e n t

l a s e r i n t e n s i t y o f 8 MW/cm2 i s shown i n Fig. intensity,

4b.

A t t h i s laser

t h e t r a n s m i t t e d p u l s e has about t h e same shape as t h e

9.

571


i n c i d e n t pulse, and t h e a b s o r p t i o n appears t o be l i n e a r .

For an

i n c i d e n t l a s e r i n t e n s i t y o f 21 MW/cm2, t h e t r a n s m i t t e d p u l s e w i d t h begins t o s h o r t e n (Fig. 4c).

The onset o f t h e decrease i n t h e w i d t h

o f t h e peak i n t h e p u l s e shape i s c o n s i s t e n t w i t h t h e measured i n c r e a s e i n t h e r e f l e c t a n c e , as shown i n Fig. 1.

The t r a n s m i s s i o n

a t an i n c i d e n t i n t e n s i t y o f 60 MW/cm2 i s shown i n Fig.

4d,

in

which case t h e r e i s a s i g n i f i c a n t decrease i n t h e d u r a t i o n o f t h e Gaussi an-1 ike peak. I n view o f t h e o p t i c a l measurements discussed above and t h e ripple-like

f e a t u r e s t h a t appear on t h e s u r f a c e a t h i g h l i g h t

i n t e n s i t i e s (James e t al.,

1984a), t h e occurrence o f sample h e a t i n g

and subsequent me1t i n g seems t h e obvious e x p l a n a t i o n o f t h e l a s e r induced changes.

From t h e t i m e - r e s o l v e d r e f l e c t i v i t y measurements

o f James e t a l .

(1984a),

i t i s found t h a t t h e onset o f m e l t i n g

depends on t h e i n i t i a l f r e e - c a r r i e r d e n s i t y i n t h e m a t e r i a l .

A

t h r e s h o l d v a l u e a t which thermal m e l t i n g occurs i s about 3 J/cm2 f o r t h e h e a v i l y doped samples used i n t h e experiment.

The r e l a -

t i v e l y l o n g m e l t d u r a t i o n s , which have been measured by Naukkarinen e t a1

. (1982),

Hassel beck and Kwok (1983) , and James e t a1

. (1984a) ,

s t r o n g l y suggest t h a t t h e r e c r y s t a l 1 i z a t i o n times are c o n s i d e r a b l y 1 onger t h a n those observed i n laser-anneal ing experiments t h a t use l a s e r r a d i a t i o n i n t h e v i s i b l e r e g i o n o f t h e spectrum.

The measure-

ments o f Hasselbeck and Kwok (1983) on c r y s t a l l i n e s i l i c o n i r r a d i a t e d w i t h a CO, l a s e r showed a m e l t d u r a t i o n l a s t i n g almost a f a c t o r o f t e n l o n g e r than t h e d u r a t i o n s a s s o c i a t e d w i t h ruby and Nd:YAG l a s e r s . These l o n g e r r e c r y s t a l l i z a t i o n t i m e s f u r t h e r suggest t h a t t h e m e l t depths a r e somewhat g r e a t e r than had been p r e v i o u s l y achieved u s i n g a l a s e r which has a photon energy g r e a t e r t h a n t h e i n t r i n s i c a b s o r p t i o n edge. 2.

RECRYSTALLIZATION For pulsed CO,

l a s e r annealing t o be u s e f u l i n t h e processing

of e l e c t r o n i c devices, t h e removal o f damage a s s o c i a t e d with t h e i o n i m p l a n t a t i o n and an a p p r e c i a b l e a c t i v a t i o n o f t h e implanted

572

R. B. JAMES

species must occur. pulsed

Naukkarinen e t a l . (1984a),

The r e c r y s t a l l i z a t i o n o f m e l t e d l a y e r s by

CO, l a s e r s has been i n v e s t i g a t e d by C e l l e r e t a l . (1978), (1982), Blomberg e t a l .

and Narayan e t a l .

backscattering spectra (TEM),

(1984a)

,

(RBS)

(1983), James e t a l .

by t h e use o f R u t h e r f o r d

transmission

and t r a n s m i s s i o n s y n c h r o t r o n x - r a y

e l e c t r o n microscopy topographs.

Results

w i l l f i r s t be presented f o r t h e a n n e a l i n g o f l i g h t l y doped s i l i c o n samples, f o l l o w e d by t h e r e s u l t s f o r h e a v i l y doped samples. Attempts t o anneal a l i g h t l y doped sample w i t h 5 x l o i 5 boron atoms/cm3 by Naukkarinen e t a l .

(1982) were o n l y p a r t l y s u c c e s s f u l .

When t h e sample was n o t preheated, t h e b e s t c r y s t a l l i z a t i o n a t t a i n e d i s shown by c u r v e f i n Fig.

575 C,

5.

By p r e h e a t i n g t h e s u b s t r a t e t o

good r e c r y s t a l l i z a t i o n o f a l i g h t l y doped sample ( 7 x 1015

(3111-3) was achieved by Blomberg e t a l .

(1983).

The c h a n n e l i n g

3000

-

I

P

c

0

0

2000

J W

-

>

(3

5

LL W

c

+ a

. -

1000

*-.

I

CJ

ln Y V

d

a

0

m

b

80

400

120 440 C H A N N E L NUMBER

160

4 80

Fig. 5. 4He-ion backscattering spectra o f Si doped with 5 x 1019 B atoms/cm3: ( a ) random, (b) -aligned incidence on a 7.5 x 15N+ ions/cm2 implanted sample, ( c ) a f t e r annealing with a 21-MW/cm2 pulse, ( d ) a f t e r a 40-MW/cm2 pulse , and ( e ) the -aligned spectrum o f the unimplanted crystal. The -aligned spectrum o f Si doped with 5 x 1015 B atoms/crn3 and implanted with 7.5 x 1015 15N ions/cm2 a f t e r a 150-MW/cm2 laser pulse i s given i n ( f ) . [ A f t e r Naukkarinen e t al. ( 1 9 8 2 ) . ]

9.

573


e

2E 3500 3

0 0

u

a

0

cn ..

Y 0

a

m

50

4 00 150 C H A N N E L NUMBER

200

Fig. 6. 4He+-ion backscattering spectra o f Si doped with 7 x 1 0 1 5 atoms/cm3 and implanted with 5 x 1015 50 keV 15N+ ions/cm2: ( a ) random, ( b ) shows the -aligned spectrum before, and ( c ) a f t e r a C02 laser pulse The -aligned o f 70 MW/cm2 w i t h the sample preheated t o 575OC. [ A f t e r Blomberg e t al. spectrum o f the unimplanted crystal i s given i n (d). (1983).]

s p e c t r a i s shown i n Fig. 6 f o r an i n c i d e n t i n t e n s i t y o f 70 MW/cm2. The sample was i m p l a n t e d a t room temperature w i t h 5 x 1015 n i t r o g e n atoms/cm2 a t an energy o f 50 KeV.

The p r e h e a t i n g o f t h e s u b s t r a t e

was found t o be necessary t o a v o i d damaging t h e c r y s t a l s .

When

t h e p r e h e a t i n g was decreased and t h e p u l s e i n t e n s i t y increased i n o r d e r t o m e l t t h e sample, t h e sample was damaged. sequence o f t h e thermal

"run-away"

T h i s i s a con-

i n t h e a b s o r p t i o n which

is

e s p e c i a l l y d e t r i m e n t a l t o t h e a n n e a l i n g o f samples having a small l i n e a r absorption c o e f f i c i e n t .

The p r e h e a t i n g o f t h e l i g h t l y doped

sample s i g n i f i c a n t l y increases t h e d e n s i t y o f e l e c t r o n and h o l e carriers,

and t h e r e b y s i g n i f i c a n t l y increases t h e l i n e a r f r e e -

c a r r i e r absorption.

574

R. B. JAMES

Similar measurements were performed by Narayan e t al. (1984a) and James et a l . (1984a) on l i g h t l y doped s i l i c o n (111) samples which were heated t o 610°C f o r 5 minutes. The samples were implanted w i t h llB ions a t several d i f f e r e n t ion energies in order t o induce implantation damage up t o a d e p t h of about 0.9 p. The d i s l o c a t i o n loops induced by the boron implantation a r e stable against the thermal treatment a t 610°C (Gyulai and Revesz, 1979). The preheating of the ion-implanted samples increases the absorption c o e f f i c i e n t by e l e c t r i c a l l y a c t i v a t i n g a f r a c t i o n of t h e boron implants and by increasing the temperature-dependent i n t r i n s i c c a r r i e r concentration. The samples were i r r a d i a t e d with a s i n g l e pulse (A = 10.6 i.m and FWHM = 70 ns) from a COP l a s e r , and crosssection transmission electron microscopy was used t o i n v e s t i g a t e t h e melt depth i n the material and the defects in the r e c r y s t a l l i z e d layer. Figure 7 shows a cross-section micrograph of a sample i r r a d i a t e d with an energy density of 7.3 J/cm2. The complete removal o f dislocation loops in the annealed region and the abrupt

Fig. 7.

Cross-section TEM micrograph showing defect-free recrystallization

o f boron-implanted

silicon a f t e r C 0 2 laser irradiation.

lightly doped and multiply implanted with llB+ t o induce deep implantation damage.

The sample was

a t several d i f f e r e n t ion energies

The sample was heated at 6 1 0 * C for five

minutes and then irradiated with a single pulse having an energy density o f

7.3 J / c m Z .

9.

575


change i n d e n s i t y o f loops between t h e annealed and unannealed r e g i o n s a r e c o n s i s t e n t w i t h t h e m e l t i n g o f t h e near-surface l a y e r . M e l t depths r a n g i n g up t o about 8000 A were observed i n t h e l i g h t l y doped samples which were preheated t o i n c r e a s e t h e c o u p l i n g o f t h e CO,

laser l i g h t t o the material. Attempts t o anneal l a r g e areas o f l i g h t l y doped samples w i t h o u t

p r e h e a t i n g t h e s u b s t r a t e by James e t a l . t o be unsuccessful.

(1984a) were a l s o found

For t h e l i g h t l y doped samples,

irradiation

a t room temperature produced l o c a l i z e d s i t e s o f m e l t i n g and damage. The regions o u t s i d e o f t h e m i c r o s i t e s o f damage were n o t melted by t h e CO,

l a s e r pulse.

These small s i t e s o f l a s e r - i n d u c e d m e l t i n g

occur because o f t h e a b r u p t i n c r e a s e i n t h e a b s o r p t i o n c o e f f i c i e n t o f t h e m a t e r i a l w i t h i n c r e a s i n g temperature, which makes even small spatial

inhomogeneities i n t h e beam d e t r i m e n t a l t o t h e u n i f o r m

a n n e a l i n g o f l a r g e areas o f t h e wafer. Complete l a s e r - i n d u c e d r e c r y s t a l 1 i z a t i o n o f amorphous l a y e r s produced by i o n i m p l a n t a t i o n has been r e p o r t e d f o r l i g h t l y doped s i l i c o n w i t h o u t p r e h e a t i n g o f t h e s u b s t r a t e ( C e l l e r e t al.,

1979).

The RBS spectrum i n d i c a t e d good e p i t a x i a l regrowth o f t h e s u r f a c e l a y e r from t h e c r y s t a l l i n e s u b s t r a t e .

There appears t o be some d i s -

crepancy between t h e r e s u l t s o f C e l l e r e t a l . (1979) and t h e r e s u l t s o f Blomberg e t a l . (1983), Naukkarinen e t a l . a1

.

(1982), and James e t

(1984a) r e g a r d i n g t h e annealing o f l i g h t l y doped s i 1 i c o n w i t h o u t

t h e preheating o f the substrate. used by C e l l e r e t a l .

Unless t h e beam o f t h e CO,

laser

(1979) i s c o n s i d e r a b l y more s p a t i a l l y homo-

geneous than t h a t used by others, o r unless some a n n e a l i n g o f t h e i r samples occurred d u r i n g i m p l a n t a t i o n , a d d i t i o n a l experimental i n f o r m a t i o n w i l l be r e q u i r e d t o r e s o l v e t h i s discrepancy. I n a d d i t i o n t o removing i o n - i m p l a n t a t i o n damage, i t i s advantageous t h a t pulsed CO,

l a s e r annealing can be used t o e l e c t r i c a l l y

a c t i v a t e implanted dopants.

Van der Pauw measurements have been

performed by James e t a l . (1984a) t o determine t h e sheet r e s i s t i v i t y and s h e e t - c a r r i e r l a y e r s a f t e r CO,

c o n c e n t r a t i o n o f boronl a s e r annealing.

and a r s e n i c - i m p l a n t e d

The t h r e s h o l d energy d e n s i t y o f

R. B. JAMES

t h e l a s e r f o r complete annealing was found t o be dependent on t h e energy, species, and dose o f t h e implants. Van der Pauw measurements were made on n-type s i l i c o n wafers, 1-5 Q-Cm,

which were implanted w i t h l l B a t an energy o f 100 KeV

t o a dose o f 1 x 10l6 cm-2.

The samples were preheated t o 590°C

f o r f i v e minutes and i r r a d i a t e d w i t h a s i n g l e CO, The energy d e n s i t y o f t h e l a s e r was varied,

l a s e r pulse.

and t h e e l e c t r i c a l

a c t i v a t i o n was measured as a f u n c t i o n o f t h e energy density.

The

r e s u l t s o f t h e measurements showed t h a t f o r energy d e n s i t i e s exceeding t h e m e l t threshold, one could achieve up t o 100% a c t i v a t i o n o f t h e boron implants by pulsed CO, l a s e r annealing.

The H a l l

m o b i l i t y o f t h e samples w i t h 90-100% a c t i v a t i o n was measured t o be 30 cm2/v-s.

S i m i l a r l i g h t l y doped samples were implanted w i t h

I l B a t an energy o f 35 KeV t o a dose o f 2 x

1015 cm-2.

a c t i v a t i o n was also achieved i n these samples by pulsed anneal ing.

Complete

CO, l a s e r

The Hal 1 mobi 1 it y o f t h e samples w i t h g r e a t e r t h a n 90%

a c t i v a t i o n was measured t o be 34 cm2/v-s. P-type s i l i c o n wafers w i t h a r e s i s t i v i t y o f 5-11 62-cm were implanted w i t h 75As i o n s a t an energy o f 100 keV t o a dose o f 1 x

1016 c r 2 .

E l e c t r i c a l a c t i v a t i o n o f up t o 90% o f t h e implanted

a r s e n i c was observed i n some o f these samples, although o n l y about 75-85% o f t h e implanted arsenic was a c t i v a t e d i n most o f t h e l a s e r annealed wafers.

It i s u n c e r t a i n whether t h e s l i g h t l y lower a c t i v a t i o n o f t h e implanted arsenic i s due t o an incomplete annealing, a loss o f arsenic from t h e surface, t h e presence o f deep t r a p s which compensate t h e laser-annealed region, and/or a f r a c t i o n o f t h e arsenic atoms i n t h e r e c r y s t a l l i z e d l a y e r occupying nonsubstitutional sites.

For s i m i l a r l i g h t l y doped samples which were

implanted a t a lower dose (100 keV,

1 x 1015 cm-*),

complete

a c t i v a t i o n was achieved as a r e s u l t o f i r r a d i a t i o n w i t h l a s e r energy d e n s i t i e s g r e a t e r than about 4 J/cm*. R e s u l t s w i l l now be discussed f o r t h e annealing o f h e a v i l y doped s i l i c o n using a pulsed CO, l a s e r .

Aligned b a c k s c a t t e r i n g spectra

were taken by Naukkarinen e t a l . (1982) on samples doped w i t h boron

9.

577


t o a c o n c e n t r a t i o n of 5 x 1019 cm-3 and implanted w i t h n i t r o g e n t o a dose o f 7.5 x 1015/cm2.

The r e s u l t s f o r t h e s p e c t r a u s i n g 1.0-MeV

4He i o n s i n c i d e n t on t h e s i l i c o n wafers a r e shown i n Fig. an i n c i d e n t i n t e n s i t y o f 21 MW/cm2,

5.

At

partial recrystallization i s

observed (curve c ) , and a t an i n t e n s i t y o f 40 MW/cm2 ( c u r v e d ) , t h e amorphized l a y e r i s almost c o m p l e t e l y r e c r y s t a l l i z e d .

It was

a l s o found t h a t near t h e s u r f a c e t h e laser-annealed r e g i o n had essentially perfect crystallinity,

b u t deeper i n t h e sample t h e

dechannel ing 1eve1 i n c r e a s e d somewhat.

The same sampl es were

i m p l a n t e d t o a dose o f 1015-1016 antimony atoms/cm*, complete r e c r y s t a l l i z a t i o n was

and almost

again observed a f t e r annealing

(Fig. 8).

3000

-

Po

ORNL-DWG84-44t05

e?

In

c 3 E

- 2000 0

J

w

F

U

4000

u

u, r

i

p-k%?+P"

t

4

I-

0

U

m 0

60

90

4 20

450

480

270

300

CHANNEL N U M B E R

Fig. 8. 4He-ion backscattering spectra o f Si doped with 5 x 1019 B atoms/cm3: ( a ) random, ( b ) -alignedincidenceona 5 x 1 0 1 5 S b i o n s / c m 2 implanted sample, and ( c ) a f t e r annealing with a 4 0 - M W / c m 2 pulse. The (001 >aligned spectrum o f the unimplanted crystal i s given in ( d ) . [After Naukkarinen e t al. (1982).]

578

R. B. JAMES

Fig. 9 .

Transmission synchrotron x-ray topographs o f Si implanted with 7 . 5 x

1 0 1 5 N i o n s / c m 2 a f t e r laser annealing: ( a ) and ( b ) d o p e d w i t h 1 0 2 0 P a t o m s / c m 3 ,

{lrl}and { 3 i 1 } reflections, { 301 }

reflection.

respectively; ( c ) doped w i t h 5 ~ 1 O ~ ~ B a t o m s / c m ~ ,

[ A f t e r Naukkar inen et al

Transmission s y n c h r o t r o n x - r a y

.

.

(1982) ]

topographs o f

h e a v i l y doped

s i l i c o n samples which were i r r a d i a t e d w i t h a p u l s e d CO, shown i n Fig. 9 (Naukkarinen e t al.,

1982).

l a s e r are

F i g u r e s 9a and 9b a r e

f o r a sample doped w i t h phosphorous t o a c o n c e n t r a t i o n o f 1 0 2 0 ~ m - ~ , and Fig. 9c i s f o r a sample doped w i t h boron t o a c o n c e n t r a t i o n o f

5 x 1 0 1 9 cm-3.

The t r a c e o f t h e l a s e r p u l s e i s marked by A and t h e

p e r f e c t s i n g l e c r y s t a l i s marked by C.

The r e g i o n near t h e c e n t e r

o f t h e l a s e r spot i s almost i d e n t i c a l t o t h e p e r f e c t s i n g l e c r y s t a l . I t i s found t h a t t h e l a s e r - a n n e a l e d area i s a t a h e i g h t d i f f e r e n t

from t h e s u r r o u n d i n g i m p l a n t e d region.

Some c o n t r a c t i o n o f t h e

l a t t i c e i s expected, s i n c e t h e volume occupied by t h e i m p l a n t e d n i t r o g e n atoms i n s u b s t i t u t i o n a l s i t e s i s s m a l l e r t h a n t h e volume occupied by s i l i c o n atoms i n the l a t t i c e .

F i g u r e 9c shows a {301}

topograph o f a l a s e r - a n n e a l e d sample which was h e a v i l y doped w i t h boron.

Except f o r t h e s t r i a t i o n s

defect-free,

, the

1aser-anneal ed r e g i o n 1ooks

i n d i c a t i n g t h a t t h e r e c r y s t a l l i z e d l a y e r does n o t

c o n t a i n many a d d i t i o n a l d e f e c t s .

The KBS spectrum o f t h i s area i s

comparable t o t h a t o f t h e s u r r o u n d i n g b u l k s i l i c o n . marked by "He" i s damage caused by t h e h e l i u m i o n beam.

The r e g i o n

9.

579


Studies o f t h e r e c r y s t a l l i z a t i o n o f h e a v i l y doped s i l i c o n were a l s o performed by James e t a l . (1984a).

The samples were u n i f o r m l y

doped w i t h phosphorus and had a r e s i s t i v i t y a t room temperature o f 0.0026 n-cm.

The near-surface r e g i o n was i m p l a n t e d w i t h I l l 3 a t an

energy o f 185 KeV t o a dose o f 1.5

x 1016

i m p l a n t e d r e g i o n remains c r y s t a l l i n e ,

Although t h e

e l e c t r i c a l measurements on

t h e as-implanted samples show t h a t most o f t h e f r e e c a r r i e r s near t h e s u r f a c e are trapped by t h e i m p l a n t a t i o n - i n d u c e d defects. one expects t h a t much o f t h e energy i n t h e C0,

Thus,

l a s e r p u l s e w i l l be

d e p o s i t e d i n t h e l o w - r e s i s t i v i t y m a t e r i a l below t h e damaged l a y e r . The samples were annealed w i t h a pulsed CO,

l a s e r ( A = 10.6 urn,

FWHM = 70 ns) over a range o f energy d e n s i t i e s t o determine t h e

m e l t t h r e s h o l d , m e l t depth, and r e d i s t r i b u t i o n o f boron ions.

The

r e s u l t s o f t h e c r o s s - s e c t i o n TEM measurements on t h e h e a v i l y doped samples show t h a t one can m e l t r e g i o n s near t h e h i g h - t o - l o w r e s i s t i v i t y i n t e r f a c e w i t h o u t m e l t i n g t h e h i g h e r - r e s i s t i v i t y l a y e r which encapsulates t h e molten r e g i o n (Fig.

10).

Melting of the material

Fig. 10. Cross-section TEM micrograph o f a heavily doped silicon wafer which has been implanted with cm-2.

llB+ at an energy o f 185 KeV to a dose of 1 . 5 x 10l6

The sample has been irradiated a t room temperature with a pulse having

an energy density o f 6.1 J / c m 2 . i n the figure.

Some o f these samples tend to crack as shown

580

R. B. JAMES

which i s embedded i n t h e sample r e s u l t s from t h e l a r g e a b s o r p t i o n o f t h e CO, l a s e r l i g h t i n t h e l o w - r e s i s t i v i t y s u b s t r a t e , as compared t o t h e weaker a b s o r p t i o n i n t h e i m p l a n t e d r e g i o n . As t h e i n t e r f a c e r e g i o n between t h e damaged and undamaged l a y e r s m e l t s , t h e conduct i o n o f heat d r i v e s t h e m e l t f r o n t i n b o t h t h e d i r e c t i o n s o f t h e i m p l a n t e d l a y e r and t h e unimplanted s u b s t r a t e . h i g h energy d e n s i t i e s o f t h e CO,

For s u f f i c i e n t l y

l a s e r , t h e conduction o f heat

m e l t s t h e e n t i r e i m p l a n t e d region.

Since t h e a b s o r p t i o n o f t h e

l a s e r energy i s c o m p a r a t i v e l y l a r g e i n t h e undamaged s u b s t r a t e , t h e dynamics o f r e c r y s t a l l i z a t i o n may be somewhat d i f f e r e n t t h a n t h e r e c r y s t a l l i z a t i o n o f a s i m i l a r sample which has been m e l t e d w i t h a v i s i b l e laser.

I n a d d i t i o n , t h e m e l t i n g o f embedded r e g i o n s

f u r t h e r demonstrates how one can use f r e e - c a r r i e r a b s o r p t i o n t o control m a t e r i a1

t h e energy d e p o s i t i o n o f t h e l a s e r r a d i a t i o n i n t h e

.

I n many o f t h e h e a v i l y doped samples, cracks appear as a r e s u l t o f p u l s e d CO,

l a s e r a n n e a l i n g (Fig.

10).

Since t h e d e n s i t y of

b o t h heated and l i q u i d s i l i c o n i s l e s s t h a n c r y s t a l l i n e s i l i c o n a t room temperature,

t h e presence o f r a p i d h e a t i n g and subsequent

m e l t i n g o f t h e embedded l a y e r causes l a r g e s t r e s s i n t h e m a t e r i a l . The f r a c t u r i n g o f t h e sample i s a r e s u l t o f t h e l a s e r - i n d u c e d h e a t i n g o f t h e embedded l a y e r and t h e l a c k o f a f r e e s u r f a c e t o r e 1 i e v e t h e thermal s t r e s s . I n a d d i t i o n t o t h e l a s e r - i n d u c e d m e l t i n g o f t h e subsurface

l a y e r , we see from Fig. 10 t h a t m e l t i n g o f t h e s u r f a c e l a y e r a l s o occurs.

Channeling measurements by Tsien e t a l .

(1982) i n d i c a t e

t h a t t h e i m p l a n t a t i o n of boron a t 185 KeV produces much l e s s damage t o t h e t h i n c r y s t a l l i n e l a y e r a t t h e surface than t o the underlying region.

Thus,

t h e c o u p l i n g o f t h e C02 l a s e r l i g h t i s probably

somewhat l a r g e r near t h e s u r f a c e t h a n i n t h e more h e a v i l y damaged l a y e r i n t h e subsurface. If the coupling o f the l i g h t t o the i m p l a n t e d r e g i o n i s l a r g e s t near t h e surface, t h e n t h e temperature would i n c r e a s e a t a f a s t e r r a t e a t t h e s u r f a c e d u r i n g t h e t i m e which t h e l a s e r p u l s e propagates t h r o u g h t h e i m p l a n t e d r e g i o n .

As

9.

581


t h e temperature i n t h e surface l a y e r increases by t h e absorption o f t h e l a s e r pulse, t h e f r e e - c a r r i e r d e n s i t y also increases due t o a p a r t i a l a c t i v a t i o n o f t h e implanted dopant, which leads t o a f u r t h e r increase i n t h e absorption c o e f f i c i e n t .

An enhancement i n

t h e heating o f t h e surface, as compared t o t h e underlying region, can a l s o r e s u l t from t h e absorption o f t h e l a s e r l i g h t by t h e t h i n oxide l a y e r on t h e surface.

(For l i g h t w i t h a wavelength i n t h e

9-11-pm region, t h e l a t t i c e absorption i n s i l i c o n d i o x i d e can be as l a r g e as lo5 cm-l.)

More i n v e s t i g a t i o n s are needed t o study

t h e dynamics o f t h e energy d e p o s i t i o n and r e c r y s t a l l i z a t i o n i n both t h e melted r e g i o n a t t h e surface o f t h e sample and t h e melted r e g i o n which i s encapsulated on both sides by s o l i d m a t e r i a l .

It

should be noted t h a t t h i s me1 t i n g phenomenon o f ion-implanted s i l i c o n i s probably not p o s s i b l e t o o b t a i n w i t h a v i s i b l e o r u l t r a v i o l e t 1aser.

A s i m i l a r m e l t i n g phenomenon l i k e l y occurs f o r a h e a v i l y doped sample where t h e surface has been amorphized by i o n implantation. Since t h e absorption o f CO,

l a s e r l i g h t i n amorphous s i l i c o n i s

r e l a t i v e l y small compared t o t h e absorption i n t h e h e a v i l y doped c r y s t a l l i n e s u b s t r a t e (Brodsky e t al.,

1970), much o f t h e l a s e r

energy w i l l be deposited near t h e amorphous-crystal1 i n e i n t e r f a c e . Thus,

i n principle,

one can m e l t t h e embedded r e g i o n near t h e

i n t e r f a c e without m e l t i n g t h e e n t i r e amorphous l a y e r .

The observa-

t i o n of such m e l t i n g phenomena may r e q u i r e f a i r l y t h i c k amorphous l a y e r s due t o t h e reduced m e l t i n g temperature o f amorphous s i l i 1984 and con compared t o c r y s t a l l i n e s i l i c o n (Lowndes e t a1

.,

Wood e t al.,

1984) and t h e d i f f i c u l t y i n m a i n t a i n i n g a thermal

g r a d i e n t so t h a t t h e temperature o f t h e surface does not exceed t h e m e l t i n g temperature o f amorphous S i . c o u p l i n g o f t h e CO,

(This r e l a t i v e l y weak

l a s e r l i g h t i n t o t h e amorphous l a y e r i s i n

c o n t r a s t t o t h e strong c o u p l i n g o f v i s i b l e and u l t r a v i o l e t lasers, where one can m e l t much o f t h e amorphous l a y e r without m e l t i n g any o f t h e u n d e r l y i n g c r y s t a l l i n e substrate.)

R. B. JAMES

3.

REDISTRIBUTION

OF IMPLANTED DOPANTS

I n many device a p p l i c a t i o n s ,

one would l i k e t o c o n t r o l t h e

dopant l e v e l and p r o f i l e t o optimize t h e device properties.

Here,

we present experimental data on t h e r e d i s t r i b u t i o n o f ion-implanted species and show how dopant p r o f i l e s can be c o n t r o l l e d by varying t h e l a s e r energy density, i m p l a n t a t i o n energy, and substrate temp e r a t u r e d u r i n g l a s e r annealing.

The r e s u l t s o f SIMS measurements

w i l l f i r s t be shown f o r l i g h t l y doped s i l i c o n where the s u b s t r a t e must be heated above room temperature f o r COP l a s e r annealing, and f o l l o w e d by t h e r e s u l t s f o r h e a v i l y doped s i l i c o n where heating o f t h e s u b s t r a t e i s not r e q u i r e d f o r successful annealing. N-type

(loo ),

2-4 61-cm,

s i l i c o n samples were implanted w i t h

I 2 l S b a t an energy of 150 KeV t o a dose o f 2 x 1015 cm-*. samples were annealed w i t h a pulsed C02 l a s e r

(A

=

The

10.6 pm, FWHM

= 70 ns), and SIMS measurements were performed t o measure t h e re-

d i s t r i b u t i o n o f t h e antimony atoms (James e t a l

., 1984a).

As shown

i n Fig. 11, annealing w i t h d i f f e r e n t energy d e n s i t i e s o f t h e pulsed l a s e r provides c o n t r o l o f t h e d i f f u s i o n o f t h e antimony atoms. The f o u r curves show t h e depth p r o f i l e s o f antimony f o r : unannealed sample; d e n s i t y , EL,

(1) an

(2) a sample annealed a t a COP l a s e r energy

of 4.4 J/cm2;

(3) a laser-annealed sample a t EL = 5.9

J/cm2; (4) and a sample annealed a t EL = 8.9 J/cm2. Each wafer was heated t o 660°C f o r f i v e minutes p r i o r t o i r r a d i a t i o n w i t h t h e C02 l a s e r and was removed from t h e s u b s t r a t e heater immediately a f t e r l a s e r annealing.

As can be seen i n t h e f i g u r e , t h e antimony

atoms can r e d i s t r i b u t e t o a depth o f well over 6000 A by v a r y i n g t h e laser-energy density, thereby p r o v i d i n g a degree o f v a r i a t i o n i n t h e j u n c t i o n depth.

I n addition,

we see t h a t some o f t h e

antimony atoms segregate t o t h e surface o f t h e s i l i c o n .

Similar

segregation o f antimony has been observed i n RBS measurements o f C02 laser-annealed s i 1 i c o n by Cell e r e t a1 e t a l . (1982). o f 0.023

. (1978)

and Naukkari nen

This segregation behavior i s c o n s i s t e n t w i t h a value

f o r t h e d i s t r i b u t i o n c o e f f i c i e n t o f Sb a t t h e m e l t i n g

p o i n t o f S i ( C e l l e r e t al.,

1978; Trumbore,

1960).

Channeling

9.

583


I

I

1

ANTIMONY-121 ATOMS IN SILICON AFTER Cop LASER ANNEALING

A 0

AS IMPLANTED EL = 4.4 Jlcm2

+ EL = 5.9 J/cm2

x EL = 8.9 J/cm 2

1

3

I

0.2

‘t I

I

0.4

1

0.6

DEPTH ( p m ) Fig. 1 1 . Concentration profiles of 1 2 % b in Si before and a f t e r C02 laser annealing a t various energy densities. The four curves show the Sb profiles f o r the following samples: A, an unannealed sample; 0 , a sample annealed a t E ~ = 4 . 4 J / c m ~ ; a sampleannealedat E ~ = 5 . 9 J / c m ~a ;n d x , a sampleannealed a t 4 = 8 . 9 J / c m 2 . Each o f the samples was heated t o 66OOC for five minutes prior t o irradiation.

+,

R. B. JAMES measurements by C e l l e r e t al. (1979) on l i g h t l y doped, Sb-implanted S i showed t h a t a f t e r l a s e r i r r a d i a t i o n w i t h i n t e n s i t i e s o f 100 t o 150 MW/cm2, t h e C1001-aligned y i e l d m i n dropped t o about 5?? of t h e random value, i n d i c a t i n g e p i t a x i a l regrowth o f t h e amorphous layer.

The Sb atoms i n t h e r e c r y s t a l l i z e d l a y e r were found t o be

about 95% s u b s t i t u t i o n a l . L i g h t l y doped, p-type S i samples were a l s o implanted w i t h llB (100 keV t o 1 x 1 0 l 6 c w 2 ) ,

and concentration p r o f i l e s o f t h e

implanted boron were measured as a f u n c t i o n o f t h e energy d e n s i t y o f t h e COP laser.

Each o f t h e wafers was preheated f o r f i v e

minutes a t a temperature o f between 650 and 690°C t o increase t h e a b s o r p t i o n c o e f f i c i e n t o f t h e C02 l a s e r l i g h t i n t h e m a t e r i a l . Both t h e boron and arsenic implants were found t o d i f f u s e t o approximately u n i f o r m concentrations a f t e r l a s e r annealing and no segregation behavior was observed. 9.2 J/cm2,

For a l a s e r energy d e n s i t y o f

t h e arsenic d i f f u s e d t o a depth o f 7000 A, and boron

d i f f u s e d t o a depth o f about 1.0 p. Measurements o f dopant p r o f i l e s have also been made by Hauck e t al.

(1981) on t h e r e d i s t r i b u t i o n o f phosphorus implants i n

l i g h t l y doped s i l i c o n annealed w i t h a C02 l a s e r ( h = 10.6 p and pulse d u r a t i o n = 400 ns).

The phosphorus ions were implanted a t

an energy o f 175 KeV t o a dose o f 5 x 1015 cm-* i n a boron-doped sample w i t h a r e s i s t i v i t y o f 8 t o 10 62-cm.

As a r e s u l t o f l a s e r

annealing, t h e phosphorus ions move deeper i n t o t h e sample as shown i n Fig. 12.

Also shown i n t h e f i g u r e i s a c o n t r o l wafer which was

t h e r m a l l y annealed a t 1000°C f o r 30 minutes i n N,. Moderately doped s i l i c o n samples have a1 so been s t u d i e d by James e t al.

(1984a).

implanted w i t h llB

Antimony-doped,

0.018

Q-cm [lll] S i was

a t an energy o f 35 KeV t o a dose o f 1 x 1 0 l 6

cm-,. The r e s u l t s o f SIMS measurements f o r t h e boron p r o f i l e s are shown i n Fig. 13. The f i v e curves i n t h e f i g u r e are f o r an asimplanted sample, a sample i r r a d i a t e d a t room temperature w i t h EL = 5.8 J/cm2, a sample i r r a d i a t e d a t room temperature w i t h EL = 8.4

9.

585


4O 2'

c'

40"

0

Fig. 12.

0.2

0.6 DEPTH ( p m 1 0.4

Calculated !!as implanted!! impurity profile

impurity profiles a f t e r laser annealing Hauck et al.

(0)

1 .o

0.8

(0)

and the measured

and thermal annealing ( 0 ) . [ A f t e r

(1981 ) .]

J/cm2, a sample i r r a d i a t e d a t 6.3 J/cm2 w i t h t h e s u b s t r a t e heated t o 6 9 O O C f o r 5 minutes, and a sample i r r a d i a t e d w i t h f i v e shots a t 4.2

J/cm2 w i t h t h e substrate heated t o 690°C f o r 5 minutes.

The boron d i f f u s e s up t o a depth o f about 4000 A i n t h e samples due t o l a s e r annealing, and t h e r e i s no i n d i c a t i o n o f segregation o f t h e boron t o t h e surface.

The e f f e c t o f preheating t h e substrate

on t h e r e d i s t r i b u t i o n o f boron atoms can be seen from Fig.

13.

For EL = 5.8 J/cm2 and no preheating, t h e boron r e d i s t r i b u t e s up t o a depth of about 500 A , b u t when t h e substrate i s heated t o

6 9 O O C f o r f i v e minutes, t h e increased absorption causes deeper m e l t i n g t o occur,

and t h e boron r e d i s t r i b u t e s up t o a depth o f

about 3000 A a t t h e same energy d e n s i t y o f t h e l a s e r . S i m i l a r measurements were

performed on t h e antimony-doped

samples w i t h boron implanted a t an energy of 150 KeV and doses i n t h e range o f 5 x 1014 cm-2 t o 1 x 1OI6 cm-2.

The boron was found

586

R. B. JAMES

0.0

0.1

0.2

0.3

0.4

DEPTH (prn)

Fig. 13. Concentration p r o f i l e s o f llB in S i before and a f t e r CO laser 2 annealing. The five curves show the boron profiles for the following samples: + an as-implanted sample; 0 , a sample annealed a t EL = 5.8 J / c m 2 ; x, a sample annealed at EL = 8.4 J / c m 2 ; a, a sample annealed a t EL = 6.3 J / c m 2 and To = 690OC; and A, a sample annealed by five laser shots a t 4 = 4.2 J / c m 2 and the substrate heated to 690OC.

,

9.


587

t o d i f f u s e i n t h e laser-annealed samples a t approximately a u n i f o r m concentration,

where t h e maximum depth of boron d i f f u s i o n was a

f u n c t i o n o f t h e energy d e n s i t y o f t h e l a s e r .

The dopant p r o f i l e s

f o r t h e d i f f e r e n t imp1 a n t doses and l a s e r w e r g y d e n s i t i e s showed t h a t dopant p r o f i l e s can be obtained w i t h n e a r l y equal j u n c t i o n depths b u t w i t h a l a r g e range o f dopant c o n c e n t r a t i o n s by t h e use o f i o n i m p l a n t a t i o n and C02 l a s e r annealing. Depth p r o f i l e s o f i m p l a n t e d i o n s have a l s o been measured by James e t a l . (1984a) and Naukkarinen e t a l . (1982) i n s i l i c o n wafers which were h e a v i l y doped w i t h boron. I n t h e experiment o f James e t a l . (1984a), T5As i o n s were implanted a t an energy o f 180 keV t o a dose o f 1 x 1 0 l 6 r e s i s t i v i t y o f 0.0073

61-cm.

i n t o a boron-doped sample w i t h a The samples were annealed w i t h a

p u l s e d C02 l a s e r and SIMS measurements were performed t o examine t h e r e d i s t r i b u t i o n o f t h e a r s e n i c atoms.

The r e s u l t s o f t h e m a -

surements are shown i n Fig. 14 f o r several energy d e n s i t i e s o f t h e p u l s e d C02 l a s e r .

From t i m e - r e s o l v e d r e f l e c t i v i t y measurements,

t h e l a s e r t h r e s h o l d f o r m e l t i n g t h e s u r f a c e o f these samples was found t o be about 3.0 J/cm2 f o r a s u b s t r a t e temperature o f 20°C. From t h e f i g u r e we see t h a t f o r energy d e n s i t i e s up t o 8.1 J/cm2 t h e a r s e n i c i o n s d i f f u s e t o a depth o f 7000 A, and t h e r e i s no i n d i c a t i o n o f segregation o f t h e arsenic.

Some o f t h e wafers were l a s e r

annealed w i t h t h e s u b s t r a t e heated t o 690°C f o r 5 minutes.

Imne-

d i a t e l y a f t e r t h e i r r a d i a t i o n s w i t h t h e C02 l a s e r , t h e samples were removed from t h e s u b s t r a t e heater. Van der Pauw measurements on t h e as-implanted samples showed t h a t s i g n i f i c a n t thermal a c t i v a t i o n o f t h e a r s e n i c i m p l a n t s occurred d u r i n g t h e f i v e minutes i n which t h e s u b s t r a t e was a t a temperature o f 690°C.

The r e s u l t s o f t h e

SIMS measurements on t h e samples w i t h s u b s t r a t e h e a t i n g are shown i n Fig. 15.

The curves i n t h e f i g u r e show t h e As c o n c e n t r a t i o n

p r o f i l e s f o r an as-implanted wafer, a sample which has been i r r a diated at =

EL

4.1 J/cm*,

= 2.0 J/cm2, a sample which has been i r r a d i a t e d a t EL

a sample which has been i r r a d i a t e d w i t h f i v e pulses

588

R. B. JAMES

I

I

I

I

ARSENIC-75 ATOMS IN SILICON AFTER CO2 LASER ANNEALING

DEPTH ( p m ) Fig. 14. Concentration profiles o f 75As i n silicon before and a f t e r C 0 2 laser annealing at several different energy densities. Each o f the samples were heavily doped with boron and had a room temperature resistivity of 0.0073 Q-cm before implantation. The five curves show the arsenic concentrations f o r t h e following samples: 0 , an as-implanted sample; x, a sample annealed a t EL = 4.2 J /cm2; m, a sample annealed a t EL = 6.0 J /cm2; A, a sample annealed a t EL = 7.2 J / c m 2 , and a sample annealed a t EL = 8.1 J/cm2.

+,

9. ,

PULSED CO:, LASER ANNEALING

I

589

I

ARSENIC-75 ATOMS IN SILICON AFTER C02 LASER ANNEALING

Fig. 15. Concentration p r o f i l e s o f 75As i n silicon before and a f t e r Cog laser annealing. Each o f the wafers was heavily doped with boron and had a The room temperature resistivity o f 0.0073 61-cm prior t o implantation. samples were heated t o 690°C for five minutes p r i o r t o laser annealing. The five curves show the arsenic concentration which r e s u l t s from the following excitation conditions: x, an as-implanted sample; 0 , a sample annealed a t EL = 2.0 J / c m 2 ; a sample annealed a t EL = 4.1 J/cm2; A, a sample annealed w i t h five shots a t EL = 4.1 J / c m 2 ; and m, a sample annealed a t EL = 8.8 J / c m 2 .

e,

R. B. JAMES a t EL = 4.1 J/cm2,

and a sample which has been i r r a d i a t e d w i t h a

s i n g l e p u l s e a t EL = 8.8 J/cm2.

The i n c r e a s e d a b s o r p t i o n due t o

p a r t i a l a c t i v a t i o n o f t h e a r s e n i c i m p l a n t s lowers t h e m e l t t h r e s h o l d so t h a t m e l t i n g occurs a t energy d e n s i t i e s below 2.0 J/cm2.

As a

r e s u l t , one has some degree o f c o n t r o l o f t h e m e l t t h r e s h o l d o f i o n - i m p l a n t e d samples by p a r t i a l l y a c t i v a t i n g t h e imp1 anted dopants, and t h e r e b y g r e a t l y i n c r e a s i n g t h e f r e e - c a r r i e r a b s o r p t i o n i n t h e i m p l a n t e d region.

( P a r t i a l a c t i v a t i o n can be achieved by h e a t i n g

t h e s u b s t r a t e t o a p o i n t where solid-phase e p i t a x y can occur o r by a l l o w i n g some s e l f - a n n e a l i n g d u r i n g t h e i o n i m p l a n t a t i o n ) . addition,

In

we see t h a t t h e e f f e c t o f m u l t i p l e shots from t h e CO,

l a s e r on t h e r e d i s t r i b u t i o n o f a r s e n i c can be s i g n i f i c a n t (Fig. 15). The e f f e c t o f m u l t i p l e l a s e r shots on t h e maximum m e l t depth r e s u l t s f r o m t h e change i n t h e o p t i c a l p r o p e r t i e s o f t h e near-surface r e g i o n

with t h e subsequent l a s e r shots.

Once t h e near-surface r e g i o n

has m e l t e d and r e s o l i d i f i e d by t h e a b s o r p t i o n o f t h e f i r s t pulse, t h e number o f e l e c t r i c a l l y a c t i v e a r s e n i c atoms f u r t h e r increases,

As t h e c o n c e n t r a t i o n p r o f i l e o f e l e c t r i c a l l y a c t i v e i o n s changes from shot

which causes t h e f r e e - c a r r i e r a b s o r p t i o n t o a l s o increase.

t o shot, t h e r e i s a corresponding change i n b o t h t h e p e n e t r a t i o n depth and r e f l e c t a n c e o f t h e C02 l a s e r l i g h t . Depth d i s t r i b u t i o n s o f 1 5 N i n pulsed C02 l a s e r - a n n e a l e d s i l i con have been c a l c u l a t e d from t h e measured broadening o f t h e Ep = 429 KeV resonance-yield c u r v e o f t h e l 5 N (p, reaction (Naukkarinen e t al.,

1982).

The samples were u n i f o r m l y doped w i t h

boron a t a c o n c e n t r a t i o n o f 5 x 1019 B/cm3.

The annealing was done

i n an argon atmosphere w i t h a s i n g l e l a s e r p u l s e ( d u r a t i o n = 100 ns) a t an i n t e n s i t y o f about 150 MW/cm2. A f t e r l a s e r annealing t h e n i t r o g e n tends t o m i g r a t e t o t h e s u r f a c e and move o u t o f t h e sample. As t h e i n t e n s i t y i s f u r t h e r increased, t h e tendency f o r t h e n i t r o g e n t o m i g r a t e t o t h e s u r f a c e increases.

This type o f r e d i s t r i b u t i o n

o f t h e n i t r o g e n i m p l a n t s i n t h e laser-annealed r e g i o n s was a l s o observed by Blomberg e t a l . (1983) i n a l i g h t l y doped s i l i c o n wafer, which was preheated t o 575OC p r i o r t o i r r a d i a t i o n .

9. IV.

591


Model Calculation of Sample Heating

The exact n a t u r e o f t h e process which occurs d u r i n g pulsed l a s e r annealing

( m e l t i n g versus plasma f o r m a t i o n ) has r e c e n t l y

been a m a t t e r o f debate.

I n t h e plasma-annealing model

(van

Vechten, 1980), t h e annealing r e s u l t s f r o m t h e presence o f a dense e l e c t r o n - h o l e plasma which p e r s i s t s i n a w e l l - l o c a l i z e d r e g i o n f o r t i m e s on t h e o r d e r o f hundreds o f nanoseconds.

This high density

o f e l e c t r o n - h o l e p a i r s (-1022/~m3), which i s formed by i n t e r b a n d t r a n s i t i o n s between t h e valence and conduction bands, i s assumed t o cause t h e l a t t i c e t o become f l u i d - l i k e w i t h o u t s i g n i f i c a n t h e a t i n g o f t h e sample.

Although t h e r e appears t o be good agreement

between p r a c t i c a l l y a1 1 t h e

laser-anneal i n g

experiments

and

a

m e l t i n g model, t h e debate has c o n t i n u e d f o r several y e a r s due t o t h e d i f f i c u l t y i n q u a n t i f y i n g t h e plasma-annealing model, and t h e general acceptance t hat a dense e l e c t r o n - h o l e

plasma i s

ormed

d u r i n g t h e a b s o r p t i o n o f t h e h i g h - i n t e n s i t y l a s e r l i g h t by

nter-

band t r a n s i t i o n s .

However,

i n t h e case o f pulsed

COP 1a s e r

annealing, a dense e l e c t r o n - h o l e plasma i s n o t formed, s i n c e s n g l ephoton i n t e r b a n d t r a n s i t i o n s are not e n e r g e t i c a l l y allowed.

Even

a1 1owing f o r nonequil i b r i u m c a r r i e r s t o be generated by t h e absorpt i o n o f C02 l a s e r l i g h t , t h e d e n s i t y would be c o n s i d e r a b l y less t h a n t h e d e n s i t i e s r e q u i r e d i n a plasma-annealing model.

It i s n o t my

i n t e n t i o n t o present f u r t h e r arguments f o r a m e l t i n g - o r plasmaannealing process, b u t s u f f i c e i t t o say t h a t a plasma-annealing model seems i n a p p r o p r i a t e i n d e s c r i b i n g t h e anneal i n g o f s i 1 i c o n by a pulsed l a s e r having a wavelength w e l l below t h e i n t r i n s i c a b s o r p t i o n edge.

Furthermore, a1 1 o f t h e present experimental

o b s e r v a t i o n s on t h e annealing o f s i l i c o n wafers with a C02 l a s e r support a thermal model as t h e e x p l a n a t i o n o f t h e observed recrystal1ization.

As a consequence, t h e model c a l c u l a t i o n presented

i n t h i s s e c t i o n assumes t h a t thermal m e l t i n g i s t h e process which occurs i n t h e pulsed C02 l a s e r annealing o f s i l i c o n .

592

R. B. JAMES

I n an attempt t o understand t h e c o u p l i n g between t h e C02 l a s e r r a d i a t i o n and t h e semiconductor, i t i s o f i n t e r e s t t o d e v i s e a model f r o m which t h e computed values on o p t i c a l h e a t i n g can be compared w i t h experiment (see, f o r example, Wang e t a1.(1978), (1980a),

o r Wood and G i l e s (1981)).

c a r r i e r concentration,

n(z,t),

and t h e l i g h t i n t e n s i t y , I ( z , t ) ,

Meyer e t a l .

The equations governing t h e

t h e l a t t i c e temperature,

T(z,t),

i n t h e b u l k are g i v e n by

and

Here, a u n i f o r m l a s e r i r r a d i a t i o n i s assumed, so t h a t f o r a semii n f i n i t e sample thickness, sional.

I n Eqs. (1-3),

t h e r e l e v a n t equations a r e one dimen-

DA i s t h e ambipolar d i f f u s i o n c o e f f i c i e n t ,

g i s t h e r a t e o f e l e c t r o n - h o l e p a i r generation, n i i s t h e tempera-

ture-dependent

c a r r i e r concentration,

T ,

i s the bulk c a r r i e r

l i f e t i m e , K i s t h e thermal c o n d u c t i v i t y , p i s t h e m a t e r i a l d e n s i t y ,

C i s t h e s p e c i f i c heat, G i s t h e r a t e o f heat g e n e r a t i o n i n t h e sample, aL i s t h e a b s o r p t i o n c o e f f i c i e n t due t o t h e g e n e r a t i o n o f phonons, anI("'1) i s t h e a b s o r p t i o n c o e f f i c i e n t due t o an n-photon a b s o r p t i o n mechanism, ae i s t h e f r e e - e l e c t r o n a b s o r p t i o n c r o s s s e c t i o n , and ah i s t h e f r e e - h o l e a b s o r p t i o n cross s e c t i o n . Most o f t h e o p t i c a l and t r a n s p o r t p r o p e r t i e s depend on b o t h t h e c a r r i e r d e n s i t y and temperature and should g e n e r a l l y be taken i n t o account f o r an a c c u r a t e treatment. Equation ( 1 ) w i l l be d i f f e r e n t f o r

9.

593


e l e c t r o n and h o l e c a r r i e r s , except i n i n t r i n s i c m a t e r i a l where t h e e l e c t r o n and h o l e d e n s i t i e s i n e q u i l i b r i u m are t h e same. n e g l i g i b l e r a d i a t i o n losses t h e r e l e v a n t

Assuming

boundary c o n d i t i o n s

in

s o l v i n g t h e above equations f o r i n t r i n s i c m a t e r i a l a r e

K aT t G (Z=O) = 0 az s

-

n(z,t=O)

= n(z+-,t)

I(z=O,t)

= Io(t)

= "(To)

,

(4c 1

(4e)

y

and I(z +

m,t)

=

0.

(4f)

Here, To i s t h e i n i t i a l sample temperature, Gs i s t h e r a t e o f heat g e n e r a t i o n o f t h e surface, vs i s t h e s u r f a c e recombination v e l o c i t y , and I, i s t h e l i g h t i n t e n s i t y a t t h e surface. An a n a l y t i c a l s o l u t i o n t o t h e coupled s e t o f equations i s n o t possible,

and f u r t h e r

simplifications

are

required

t o obtain

reasonable estimates o f t h e o p t i c a l h e a t i n g by t h e a b s o r p t i o n o f

COE laser light.

The maximum temperature a t t h e s u r f a c e o f t h e

sample can be c a l c u l a t e d f o l l o w i n g t h e approximations made by Meyer e t a l .

(1980a).

T h i s approach c o n s i s t s o f f i r s t o b t a i n i n g

s o l u t i o n s f o r s h o r t pulses which do n o t account f o r t h e conduction o f heat.

It i s f u r t h e r assumed t h a t t h e Auger and r a d i a t i v e re-

combination times are much s h o r t e r than t h e p u l s e d u r a t i o n s , t h a t a t a p a r t i c u l a r depth,

so

t h e c a r r i e r d e n s i t y i s a t a steady

s t a t e value f o r a g i v e n l a t t i c e temperature.

Unless t h e c a r r i e r

594

R. B. JAMES

d e n s i t y i s q u i t e high, t h e recombination t i m e s i n m a t e r i a l s such as s i l i c o n and germanium can be l a r g e r t h a n t h e p u l s e d u r a t i o n i n many experiments, which would i n v a l i d a t e t h e c a l c u l a t i o n a l approach. However, i f one f u r t h e r assumes t h a t t h e a b s o r p t i o n process does n o t i n v o l v e t h e c r e a t i o n o f e l e c t r o n - h o l e p a i r s by m u l t i p h o t o n o r impact-ionization

processes,

t h e n t h e approach i s s t i l l v a l i d .

(Note t h a t t h e g e n e r a t i o n o f nonequil i b r i u m e l e c t r o n - h o l e p a i r s by t h e a b s o r p t i o n o f C02 l a s e r l i g h t has been observed i n germanium by Yuen e t a1

.,

1980 and may have been observed i n s i l i c o n by

Hasselbeck and Kwok, 1983). = 0 and T ( z ) = T(z=O),

I n i t i a l l y i t i s assumed t h a t K = an/at

so t h a t t h e equations decouple, and one

can s o l v e f o r t h e c a r r i e r d e n s i t y a t t h e s u r f a c e n(z=O,t) v a l u e o f T(z=O).

f o r each

Incorporating the effects o f c a r r i e r d i f f u s i o n

and s u r f a c e recombination i n a phenomenological way, n(z=O,t)

is

approximated by

where a i s t h e f r e e - c a r r i e r

absorption coefficient,

LA i s t h e

ambipolar d i f f u s i o n l e n g t h , and z i s t h e c a r r i e r l i f e t i m e due t o b o t h b u l k and s u r f a c e recombination (r-l = q-l Having obtained an e x p r e s s i o n f o r n(z=O,T),

+ vS/L~). Eq.

(2) can be

s o l v e d f o r a g i v e n p u l s e shape. Assuming t h a t t h e p u l s e shape i s r e c t a n g u l a r w i t h d u r a t i o n tp, one o b t a i n s from e q u a t i o n ( 2 ) (Meyer e t al.,

1980a)

where Tf i s t h e f i n a l s u r f a c e temperature a t t h e end o f t h e l a s e r pulse. The heat g e n e r a t i o n r a t e i n t h e near-surface r e g i o n can be w r i t t e n as G(z

- 0)

=

(1 -

R) Ioa ,

(7)

9.

where R i s t h e r e f l e c t i o n c o e f f i c i e n t o f t h e surface.

(7),

595


one can i n v e r t Eq.

Using Eq.

( 6 ) t o o b t a i n t h e power d e n s i t y (Io)

r e q u i r e d t o i n c r e a s e t h e s u r f a c e temperature f r o m To t o Tf as a f u n c t i o n of t h e p u l s e d u r a t i o n tp. One f i n d s

where

=v 1 -R(To)

LH

Tf

CV(T)C1

The s o l u t i o n can be g e n e r a l i z e d t o i n c o r p o r a t e thermal conduction by t h e phenomenological arguments presented by Meyer e t a l . (1980a). When f r e e - c a r r i e r a b s o r p t i o n domi nates t h e heat g e n e r a t i o n r a t e , one o b t a i n s

where LT i s t h e thermal d i f f u s i o n l e n g t h and has t h e approximate form LT(T) =

d/* [K(T)tp]’’

Equations ( 8 ) ,

(lo),

(T - To)/AT

.

(11 1

and (11) were solved by Naukkarinen e t a l .

(1982) assuming t h a t (1) t h e a b s o r p t i o n process does n o t i n v o l v e t h e c r e a t i o n o f e l e c t r o n - h o l e p a i r s and (2) t h e f r e e - c a r r i e r absorption c o e f f i c i e n t i s 6 x 0.45,

lo3

cm-I and t h e r e f l e c t i v i t y i s

where b o t h are independent o f t h e l a t t i c e temperature.

596

R. B. JAMES

4 500

MELTING POINT

444OOC

500

0

40

4

20

30 40

400

I (MWcrn-') Fig.

16.

Surface temperature ifas a function o f the incident laser

intensity I calculated for two laser pulse lengths for a sample doped with 5 x 1019 B atoms/cm3.

[ A f t e r Naukkarinen et al.

(1982).]

These values f o r the absorption c o e f f i c i e n t and r e f l e c t i v i t y a r e taken from measurements on s i l i c o n c r y s t a l s doped with about 1020 phosphorous atoms/cm3 t o a d e p t h of 3.5 @. K(T) and C(T) a r e taken from empirical expressions by Meyer e t a l . (1980b). Calculated values f o r the surface temperature ( T f ) a r e shown i n Fig. 16 f o r two rectangular pulses with durations of 50 ns and 100 ns. The c a l c u l a t i o n predicts t h a t melting of the surface l a y e r should occur a t i n t e n s i t i e s of 20 t o 30 MW/cm2, which i s in agreement with experiment. For smaller c a r r i e r d e n s i t i e s , the absorption c o e f f i c i e n t i s reduced considerably, and higher i n t e n s i t i e s are required t o melt t h e material. The surface temperature has been calculated f o r samples over a range of doping d e n s i t i e s using the method described 1982). For l i g h t l y or moderately doped above (Naukkarinen e t a1 samples, one must include t h e temperature dependences of the f r e e c a r r i e r absorption and r e f l e c t i v i t y , and Eq. (8) takes the form

.,

9.


As t h e l a t t i c e temperature increases, t h e increase i n t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i s given by (Meyer e t al., ni(T)

=

2.01 x 1020 (T/300

K)ls5

exp(-7020 K/T)

1980b)

.

(13)

Since t h e p r o b a b i l i t y f o r absorption o f a photon by a f r e e e l e c t r o n depends on t h e d e n s i t y o f f i n a l s t a t e s which t h e e l e c t r o n can occupy and t h e cooperation o f another p a r t i c l e t o conserve c r y s t a l momentum, t h e f r e e - c a r r i e r absorption cross s e c t i o n a1 so depends on t h e temperature.

The cross s e c t i o n i s assumed t o increase w i t h

temperature as T 3 l 2 (Smith, 1978), and i t s room temperature value has been chosen t o f i t t h e data by S p i t z e r and Fan (1957).

For

f r e e - c a r r i e r d e n s i t i e s l e s s than about 1 0 l 6 ~ m - ~l a, t t i c e absorption should also be included i n t h e expression f o r t h e absorption coefficient.

The room temperature value o f t h e multiphonon absorption

c o e f f i c i e n t a t 10.6 pm has been measured by Johnson (1959) t o be about 2 cm-'.

The l a t t i c e absorption increases w i t h temperature,

b u t since t h e temperature dependence i s considerably weaker than t h e temperature dependence o f t h e f r e e - c a r r i e r absorption, i t i s assumed t o be independent o f temperature.

Calculated values f o r

t h e surface temperature Tf are given i n Fig. 17 a t several c a r r i e r concentrations as a f u n c t i o n of t h e C02 l a s e r i n t e n s i t y f o r pulses w i t h a 100-ns d u r a t i o n (Naukkarinen e t al., 1982). As can be seen i n t h e f i g u r e , t h e r e e x i s t s a p a r t i c u l a r i n t e n s i t y a t which t h e surface temperature increases abruptly.

This thermal run away occurs

when t h e temperature i s reached a t which t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n exceeds t h e doping density.

When t h i s temperature

t h r e s h o l d i s reached, t h e c a r r i e r concentration s t r o n g l y increases with intensity.

The abrupt increase i n t h e f r e e - c a r r i e r d e n s i t y

leads t o a l a r g e increase i n t h e f r e e - c a r r i e r

absorption and,

598

R. B. JAMES

I

++----

4500 141OOC

I

I

I

I

-

I

1

I

I

4000

500

i

I

I

I

I

I

I

I

2

5

10

20

50

400

200

500 4000 2000

I ( M W err-')

Fig. 17.

Surface temperature Tf as a function o f the incident laser inten-

sity I for d i f f e r e n t doping concentrations.

Laser pulse length tp = 100 ns.

The broken lines give T f ( l ) for n = 5 x 1015 ~ r n -and ~ n = 5 x l O l 9 ~ m - ~when , the sample i s preheated to 300OC.

consequently,

[ A f t e r Naukkarinen et a l .

t o f u r t h e r h e a t i n g o f t h e sample.

(1982).]

T h i s run-away

phenomenon i n t h e a b s o r p t i o n a l l o w s f o r m e l t i n g o f t h e s u r f a c e l a y e r f o r samples w i t h r e l a t i v e l y low doping d e n s i t i e s by o n l y small increments i n t h e energy d e n s i t y o f t h e l a s e r . F o r l i g h t l y doped samples, a c o n s i d e r a b l e i n t e n s i t y (-1 GW/cm2)

i s r e q u i r e d t o m e l t t h e s u r f a c e l a y e r (Fig. 17). v e r y a b r u p t l y as t h e i n t e n s i t y o f t h e

COP

The m e l t i n g occurs

l a s e r p u l s e i s increased.

Since i t i s d i f f i c u l t t o p r e c i s e l y c o n t r o l t h e p u l s e i n t e n s i t y , t h i s thermal run-away phenomenon i s d e t r i m e n t a l t o a t t a i n i n g complete r e c r y s t a l l i z a t i o n o f l a r g e areas w i t h o u t damaging t h e c r y s t a l s . One would l i k e t o be a b l e t o anneal l i g h t l y doped samples and a t t h e same t i m e a v o i d t h e thermal run-away i n t h e absorption.

This

can be accomplished by p r e h e a t i n g t h e sample t o a p o i n t where t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n exceeds t h e doping d e n s i t y ,

so

9.

599

PULSED Cop LASER ANNEALING

to4

-

to3

'E 0

U

4 O2

40'

' 40

I

400

800

4200

4600

T (K) Fig. 1 8 .

Absorption coefficient

a o f Si as a function o f the temperature T [ A f t e r Blomberg et al. ( 1 9 8 3 ) .]

f o r different dopant concentrations n.

t h a t the increase i n the absorption c o e f f i c i e n t w i t h increasing l i g h t i n t e n s i t y i s n o t so abrupt.

The e f f e c t o f t h e l a t t i c e tem-

p e r a t u r e on t h e a b s o r p t i o n c o e f f i c i e n t i s shown i n Fig. 18 f o r seve r a l d i f f e r e n t doping d e n s i t i e s (Blomberg e t al., a(T) = 1.9 x 1020[cm2

K-3/21

1983).

x T 3 i 2 [ n + ni(T)]

+

9

Using Y

(14)

t h e peak s u r f a c e temperature has been c a l c u l a t e d as a f u n c t i o n o f t h e peak i n t e n s i t y by t h e method discussed above f o r a sample w i t h n = 5 x 1015 ~ m - ~ .The r e s u l t s o f t h e c a l c u l a t i o n are shown i n Fig.

19 f o r i n i t i a l s u b s t r a t e temperatures o f 300, 600, 900,

and 1200 K.

As seen i n t h e f i g u r e , p r e h e a t i n g o f t h e sample t o a

temperature o f 900 K o r l a r g e r s i g n i f i c a n t l y decreases t h e runaway b e h a v i o r i n t h e a b s o r p t i o n and a l s o decreases t h e i n t e n s i t y r e q u i r e d t o m e l t t h e surface, t h e r e b y making it e a s i e r t o achieve good r e c r y s t a l l i z a t i o n o f l i g h t l y doped s i l i c o n by t h e a b s o r p t i o n o f C02 l a s e r l i g h t .

600

R. B . JAMES

t 600

- to00 Y

Y

I-

800

600K

/

400

1

2

5

40

20

50

100 200

500 1000 2000

I (MW ern-')

Fig. 19.

Surface temperature Tf o f Si with a dopant concentration n = 5 x

1015 ~ r n - as ~ a function o f the incident laser intensity for d i f f e r e n t initial substrate temperatures To. [ A f t e r Blomberg et at. ( 1 9 8 3 ) .]

V.

Interaction o f High-Intensity Pulsed CO, Laser Radiation w i t h ether Semiconductors A t t h i s point,

t h e focus o f a t t e n t i o n w i l l be p l a c e d on t h e

i n t e r a c t i o n of h i g h - i n t e n s i t y p u l s e d C O P l a s e r l i g h t w i t h o t h e r Group I V and 111-V semiconductors.

I n s t e a d o f a t t e m p t i n g t o present

a complete study f o r several semiconductors,

the discussion w i l l

c o n c e n t r a t e on t h e a v a i l a b l e experimental

results f o r gallium

arsenide, indium antimonide, and germanium.

1.

GALLIUM ARSENIDE James e t a l . (1984b) have s t u d i e d t h e n o n l i n e a r a b s o r p t i o n and

o p t i c a l h e a t i n g o f zinc-doped g a l l i u m arsenide by p u l s e d C O P l a s e r radiation.

The dominant a b s o r p t i o n mechanism i s d i r e c t f r e e - h o l e

9.

601


t r a n s i t i o n s between s t a t e s i n t h e heavy- and l i g h t - h o l e bands. E x p e r i m e n t a l l y , it has been observed t h a t t h e intervalence-band a b s o r p t i o n by f r e e holes decreases with i n c r e a s i n g i n t e n s i t y due t o a s t a t e - f i l l i n g e f f e c t i n t h e resonant r e g i o n (Gibson e t al., 1972 and James e t al., i n k-space,

1984b).

Since t h e t r a n s i t i o n s a r e d i r e c t

b o t h energy and wave v e c t o r a r e conserved i n t h e

i n t e r v a l e n c e - b a n d o p t i c a l absorption.

Thus, o n l y holes i n a narrow

r e g i o n o f t h e heavy-hole band can d i r e c t l y p a r t i c i p a t e i n t h e absorption,

and t h e a b s o r p t i o n c o e f f i c i e n t i s governed by t h e

p o p u l a t i o n o f these h o l e s t a t e s .

A t low i n t e n s i t i e s , t h e popula-

t i o n o f heavy-hole s t a t e s i n t h e resonant r e g i o n i s maintained c l o s e t o t h e e q u i l i b r i u m v a l u e by t h e v a r i o u s s c a t t e r i n g processes. However, as t h e i n t e n s i t y becomes l a r g e , s c a t t e r i n g cannot m a i n t a i n t h e e q u i l i b r i u m p o p u l a t i o n o f t h e resonant heavy-hole s t a t e s , and t h e y become depleted.

As a r e s u l t , t h e a b s o r p t i o n o f C02 l a s e r

r a d i a t i o n i n p-type GaAs s a t u r a t e s a t h i g h i n t e n s i t i e s .

A theory

which has been successful i n e x p l a i n i n g most o f t h e measurements has been g i v e n by James and Smith (1980a). Transmission measurements were performed as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y f o r a GaAs:Zn c r y s t a l with a h o l e d e n s i t y o f

1 x 1017

(James e t a l .

, 1984b). The decrease i n t h e a b s o r p t i o n

c o e f f i c i e n t w i t h i n c r e a s i n g i n t e n s i t y was found t o be reasonably w e l l s a t i s f i e d by

where a o ( w ) i s t h e a b s o r p t i o n c o e f f i c i e n t a t low i n t e n s i t y . I S ( w ) i s t h e s a t u r a t i o n i n t e n s i t y and has a v a l u e o f 20 MW/cm2 f o r l i g h t w i t h a wavelength o f 10.6 prn and room temperature c o n d i t i o n s . S i m i l a r t r a n s m i s s i o n measurements were performed on a GaAs:Zn c r y s t a l with a h o l e d e n s i t y o f 4 x l o L 7 ~ m ' ~ . Due t o l a r g e r f r e e h o l e a b s o r p t i o n i n these samples, i t was r e q u i r e d t h a t t h e wafers

602

R. B. JAMES

- 2,

where L i s the thickness of the sample. I t was found t h a t as the doping density was increased, higher i n t e n s i t i e s were required t o s a t u r a t e t h e resonant t r a n s i t i o n s . In addition, t h e onset of surface damage occurred a t lower i n t e n s i t i e s , and t h e samples often fractured due t o a shock wave r e s u l t i n g from the l a s e r radiation impinging on the samples. The decreased threshold f o r surface damage and the f r a c t u r e o f the samples g r e a t l y increased the d i f f i c u l t y in making a n accurate measurement of Is f o r t h e more heavily doped wafers. For GaAs:Zn c r y s t a l s w i t h a hole density of about 10l8 (3111-3 and l a r g e r , i t was found t h a t l a r g e areas of the surface could be melted by t h e absorption of t h e l a s e r r a d i a t i o n , i n c o n t r a s t t o the be mechanically t h i n n e d t o about 120 pm so t h a t q L

Fig. 20. SEM photograph of irradiated region where melting and thermal stresses have produced fissures in the material. The longest bar in the lower right-hand corner of the figure has a length o f 100 pn.

9.

603


r e s u l t s f o r moderately doped samples where small s i t e s o f damage would appear.

The s u r f a c e topography was s t u d i e d w i t h scanning

e l e c t r o n and Normarski i n t e r f e r e n c e microscopes.

Examination o f

t h e s u r f a c e a t t h e i n t e r a c t i o n r e g i o n showed s t r o n g evidence o f m e l t i n g and l a r g e thermal stresses.

Smooth p e r i o d i c r i p p l e s on t h e

s u r f a c e r e s u l t i n g from t h e process o f m e l t i n g and r e s o l i d i f y i n g were v i s i b l e .

A t h i g h e r energy d e n s i t i e s ,

f i s s u r e s develop on

t h e surface, which a r e o r i e n t e d along t h e c r y s t a l planes o f t h e s u b s t r a t e (Fig.

a). The

f o r m a t i o n o f t h e f i s s u r e s i s most l i k e l y

due t o t h e r e l i e f o f thermal s t r e s s e s i n t h e i n t e r a c t i o n r e g i o n f o l l o w i n g t h e a b s o r p t i o n o f t h e C02 l a s e r r a d i a t i o n . The f a c t t h a t GaAs i s a compound semiconductor a f f e c t s t h e requirements f o r successful annealing i n a v a r i e t y o f ways.

One

o f t h e most obvious i s t h a t pulsed l a s e r annealing can cause a l o s s o f s t o i c h i o m e t r y due t o t h e h i g h vapor pressure o f a r s e n i c r e l a t i v e t o gallium.

(See,

f o r example,

Badawi e t al.,

1980.)

The pulsed l a s e r annealing experiment by James e t a l . (1984b) was c a r r i e d out i n a i r a t room temperature w i t h o u t encapsulation.

A

C02 l a s e r beam i n t e g r a t o r was used i n an attempt t o remove s p a t i a l inhomogeneities i n t h e beam and t o o b t a i n as u n i f o r m annealing as possible.

The wafers were i r r a d i a t e d i n a i r , and x-ray f l u o r e s c e n c e

d a t a were taken on t h e annealed samples t o study t h e r a t i o o f Ga t o As emissions.

The r a t i o s were taken on samples which were

annealed under a v a r i e t y o f e x c i t a t i o n c o n d i t i o n s , and t h e r e s u l t s were compared t o t h e r a t i o i n t h e unannealed GaAs c r y s t a l .

The

energy o f t h e e l e c t r o n s was v a r i e d between 2, 5, 10, 15, and 30 KeV i n o r d e r t o c o n t r o l t h e e l e c t r o n p e n e t r a t i o n and, thereby, d i f f e r e n t depths i n t h e n e a r - s u r f a c e region.

probe


measurements showed t h a t a r s e n i c l o s s does r e s u l t from t h e m e l t i n g o f t h e s u r f a c e w i t h a C02 l a s e r p u l s e ( A = 10.6 urn, FWHM = 70 ns). The l o s s o f s t o i c h i o m e t r y i s g r e a t e s t w i t h i n a depth o f about 600 A, which i s t h e approximate p e n e t r a t i o n o f t h e 2 KeV e l e c t r o n s .

For

30 KeV e l e c t r o n s , t h e r a t i o s o f g a l l i u m t o a r s e n i c x-ray counts i n

t h e L and K s e r i e s f o r t h e annealed samples a r e w i t h i n 1-2%o f t h e

604

R. B. JAMES 4

X-RAY FLUORESCENCE DATA FROM Ga AND As ATOMS Ga

3-

As

h

t

0

.-

Y

-

v)

I - 2

z

3

0 0

1-

0

0

0.5

1

1.5

PHOTON ENERGY (KeV)

Fig. 21. Results o f x-ray fluorescence data f o r 2 keV electrons i n GaAs crystals before and a f t e r laser irradiation. The three curves show the fluorescence from the gallium and arsenic atoms in the following samples: curve ( a ) , an unirradiated sample; curve (b), a sample irradiated with a single shot a t an energy density o f 4.5 J / c m 2 ; and curve ( c ) , a sample irradiated with ten shots a t an energy density o f 5.9 J/cm2.

respective r a t i o s in the unirradiated GaAs wafers. T h e r e s u l t s o f the x-ray fluorescence data f o r 2 keV electrons are shown in Fig. 21 f o r a GaAs:Zn crystal with a hole concentration of 5.1 x 10l8 c w 3 . The three curves show the L-series x-ray fluorescence from gallium and arsenic atoms i n an unirradiated sample, a sample i r r a d i a t e d w i t h a single shot a t an energy density of 4.5 J/cm2, and a sample i r r a d i a t e d with ten shots a t an energy density of 5.9 J/cm2. The r a t i o s o f the counts corresponding t o gallium and a r s e n i c emissions a r e 1.05, 1.10, and 1.49, respectively.

9.

605

PULSED COZ LASER ANNEALING

As noted i n t h e f i g u r e , t h e s e v e r i t y o f t h e a r s e n i c l o s s depends on t h e energy d e n s i t y o f l a s e r pulse. Thus, t h e occurrence o f s u r f a c e e v a p o r a t i o n o f a r s e n i c s e t s an upper l i m i t on t h e energy d e n s i t y o f t h e C02 l a s e r f o r o p t i m a l , d e f e c t - f r e e r e c r y s t a l l i z a t i o n . T h i s ''window'' f o r pulsed C02 l a s e r annealing may be q u i t e narrow due t o t h e r e l a t i v e l y deep p e n e t r a t i o n o f COP l a s e r l i g h t i n t o t h e GaAs s u b s t r a t e , as compared t o t h e p e n e t r a t i o n o f l i g h t when t h e photon energy exceeds t h e i n t r i n s i c a b s o r p t i o n edge.

For near-

s u r f a c e r e g i o n s which have a l a r g e a b s o r p t i o n c o e f f i c i e n t a t C02 l a s e r wavelengths (i.e.,

f o r h e a v i l y doped s u b s t r a t e s and s h a l l o w

i m p l a n t s o r s h a l l o w i m p l a n t s which have been p a r t i a l l y a c t i v a t e d ) , an energy d e n s i t y "window" may e x i s t which r e s u l t s i n h i g h e l e c t r i c a l a c t i v a t i o n o f t h e i m p l a n t e d i o n s w h i l e remaining below t h e damage t h r e s h o l d due t o v a p o r i z a t i on. The v a p o r i z a t i o n o f a r s e n i c d u r i n g t h e molten phase causes a h i g h c o n c e n t r a t i o n o f a r s e n i c vacancies and/or p e n e t r a t i o n o f o t h e r atoms i n t o t h e a r s e n i c s i t e s . Experiments by James e t a l . (1984b) on GaAs c r y s t a l s i r r a d i a t e d i n a i r were designed t o study t h e p e n e t r a t i o n o f oxygen i n t o t h e sample d u r i n g t h e r a p i d s o l i d i f i c a t i o n immediately a f t e r C02 l a s e r m e l t i n g .

The r e s u l t s o f SIMS

measurements o f t h e depth p r o f i l e s o f l60 i n t h e near-surface r e g i o n a r e shown i n Fig. 22.

The GaAs:Zn c r y s t a l s used i n t h e experiment

have a f r e e - h o l e d e n s i t y o f 5.1 x

lo1*

c w 3 , and a l i n e a r a b s o r p t i o n

c o e f f i c i e n t a t room temperature o f about 2 x l o 3 cm-1.

The f o u r

curves i n t h e f i g u r e a r e f o r an u n i r r a d i a t e d GaAs c r y s t a l , a samp l e i r r a d i a t e d w i t h t e n shots a t an energy d e n s i t y o f 5.3 J/cm2 i n an a i r pressure o f 10-4-10-5 bar, a sample i r r a d i a t e d w i t h one s h o t a t an energy d e n s i t y o f 6.0 J/cm2 i n an a i r pressure o f 1 bar, and a sample i r r a d i a t e d w i t h t e n shots a t an energy d e n s i t y o f 6.7 J/cm* i n an a i r pressure of 1 bar.

I n t h e samples i r r a d i a t e d i n

a i r , oxygen i s i n c o r p o r a t e d i n t o t h e l a t t i c e t o depths comparable t o t h e depths a t which most o f t h e a r s e n i c l o s s occurs.

I n the

sample t h a t had t e n shots i n a i r , t h e oxygen p e n e t r a t e d t o a depth o f about 2000 A,

which i s c o n s i d e r a b l y deeper t h a n t h e oxygen

606

R. B . JAMES

0.1

0.0

Fig. 22.

0.2

(

SlMS measurements o f the depth p r o f i l e s o f l60i n the near-surface

region i n samples before and a f t e r pulsed C02 laser irradiation. curves shown in the figure are for the following samples: sample;

*,

0,

The four

an unirradiated

a sample irradiated with ten shots a t an energy density o f 5.3 j / c m 2

in an air pressure o f

-

bar;

A,

a sample irradiated with one shot at

an energy density o f 6.0 J / c m 2 i n an air pressure of 1 bar; and m, a sample irradiated with ten shots at an energy density o f 6.7 j / c m 2 in an air pressure

o f 1 bar.

i n c o r p o r a t i o n t h a t r e s u l t s from a s i n g l e shot i n a i r .

An i n c r e a s e

i n t h e oxygen counts i s observed t o a depth o f about 800 A i n t h e w a f e r which was i r r a d i a t e d w i t h 10 shots i n an a i r pressure o f 10-'+-10-5

bar, a l t h o u g h t h e c o n c e n t r a t i o n o f oxygen i n t h e f i r s t

200 A i s much l e s s t h a n i t was f o r t h e wafer i r r a d i a t e d w i t h a

s i n g l e shot i n one bar o f a i r . tion,

T h i s evidence o f oxygen i n c o r p o r a -

r e s u l t i n g from p u l s e d l a s e r annealing o f GaAs, p o i n t s con-

c l u s i v e l y t o t h e importance o f t h e immediate envi ronment d u r i n g t h e high-temperature c y c l e ( t h i s i s n o t t h e case f o r S i ) .

9.

607


It i s n o t c l e a r a t t h i s t i m e whether t h e d i f f u s i o n o f oxygen

i n t o t h e near-surface r e g i o n i s enhanced by t h e a r s e n i c vacancies t h a t are present, o r whether t h e p e n e t r a t i o n o f oxygen enhances t h e a r s e n i c loss by f o r c i n g t h e a r s e n i c toward t h e surface.

In addi-

t i on , t h e bondi ng c o n f ig u r a t i on o f t h e oxygen impuri t ies i s pres e n t l y unknown.

If t h e l o c a l i z e d o x i d e l a y e r s can be formed i n a

c o n t r o l l a b l e way, t h i s beam p r o c e s s i n g technique may be u s e f u l i n t h e f o r m a t i o n o f t h i n i n s u l a t i n g f i l m s on GaAs.

S I M S measurements were a l s o made o f t h e depth p r o f i l e s o f z i n c i n t h e COP l a s e r - i r r a d i a t e d samples. The z i n c was u n i f o r m l y doped i n t h e wafers, and a c o n s t a n t count r a t e f o r z i n c atoms was observed prior t o irradiation.

A f t e r i r r a d i a t i o n , t h e count r a t e f o r z i n c

was found t o decrease near t h e surface.

For a C02 l a s e r energy

d e n s i t y of 6.2 J/cm2, a n o t i c e a b l e decrease i n t h e z i n c counts was observed t o a depth o f about 5000 A, although t h e g r e a t e s t l o s s occurred a t depths o f 150 t o 3000 A f r o m t h e surface, where t h e z i n c counts were o n l y 6 5 7 0 % o f t h e counts i n t h e u n i r r a d i a t e d

wafer.

A t depths g r e a t e r than 5000 A, t h e count r a t e f o r z i n c atoms was uniform, as observed i n t h e wafer p r i o r t o l a s e r annealing. P r e s e n t l y , more work i s needed on t h e C02 l a s e r annealing o f GaAs t o determine i f a s u i t a b l e energy d e n s i t y "window" e x i s t s . Samples i r r a d i a t e d i n a i r are found t o have a d e f i c i e n c y o f As and marked i n c o r p o r a t i o n of oxygen,

b o t h o f which e f f e c t s cause

d e t e r i o r a t i o n o f t h e e l e c t r i c a l p r o p e r t i e s o f t h e wafers.

5.

INDIUM ANTIMONIDE Several experiments on t h e t r a n s m i s s i o n o f h i g h - i n t e n s i t y C02

l a s e r r a d i a t i o n t h r o u g h InSb have been r e p o r t e d f o r pulses o f nanosecond (Fossum e t al.,

1973b; Gibson e t a l .

, 1976;

Nee e t a1

., 1978;

and Jamison and Nurmikko, 1979) and picosecond (Schwartz e t al., 1980 and Hassel beck and Kwok, 1982) d u r a t i o n s .

These i n v e s t i g a t i o n s

have confirmed t h a t h e a t i n g and subsequent m e l t i n g o f t h e c r y s t a l can occur by f r e e - c a r r i e r absorption. The r e s u l t s o f t h e e x p e r i ments w i t h nanosecond pulses will be discussed f i r s t .

608

R. B. JAMES

n

9

Io5 L

- lo4

c u) .C 3

-

?!

Y

/o'-o-O-o

% c .c 5 lo= C

0

v)

/:

/ooO

-

.E loo t e + -

bpo /

0)

c

u)

a

lo'

0

/O

0

s

0

7 :/ -

0 0

- 0 0

Ts20K T = 77K

-0

-

1

Fig. lattice

23.

I 1111l1l

I

1

I

1

Illustration o f high-intensity

temperatures

of

20

and

77 K.

I

I

1

1

transmission [After

I

1

limit

in lnSb at

Jamison and Nurmikko

( 1 9 7 9 ) .]

E x p e r i m e n t a l l y , i t has been observed t h a t InSb e x h i b i t s a d i s t i n c t h i g h - i n t e n s i t y t r a n s m i s s i o n l i m i t f o r CO,

l a s e r l i g h t beyond

whichthetransmittancestronglydecreaseswithincreasing intensity. The onset o f t h i s l a s e r - i n d u c e d o p a c i t y occurs w e l l below t h e damage threshold o f the material.

The r e s u l t s f o r t h e t r a n s m i s s i o n o f l - n s

p u l s e s w i t h a wavelength of 10.6 urn are shown i n Fig. 23 f o r l a t t i c e temperatures o f 77 K and about 20 K (Jamison and Nurmikko, 1979). F o r a l a t t i c e temperature o f 20 K, t h e t r a n s m i t t a n c e a b r u p t l y decreases a t an i n c i d e n t i n t e n s i t y o f 2 MW/cm2.

A t h i g h e r tempera-

t u r e , t h e r e i s a much more gradual decrease i n t h e t r a n s m i t t a n c e ,

9.

609


and t h e onset of t h e n o n l i n e a r absorptTon occurs a t somewhat lower intensities.

Abrupt t r a n s m i s s i o n l i m i t s were a l s o measured by

Jamison and Nurmikko (1979) f o r Hg0.77Cd0,23Te

a t 20 K and InAs

a t 300 K f o r 10.6 pm l i g h t . S i m i l a r o b s e r v a t i o n s o f n o n l i n e a r a b s o r p t i o n i n InSb have been r e p o r t e d a t 4 K by Nee e t a l . i n t e n s i t i e s above 2 MW/cm2,

(1978) f o r 40-ns pulses and

by Fossum e t a l .

(1973) a t 77 K f o r

100-ns pulses and i n t e n s i t i e s above 1 MW/cm2, and Gibson e t a l . (1976) a t 300 MW/cm*.

K f o r 50-ns pulses and i n t e n s i t i e s g r e a t e r t h a n 1

The observed decrease i n t h e t r a n s m i t t a n c e i s a s s o c i a t e d

w i t h t h e g e n e r a t i o n o f nonequil i b r i u m e l e c t r o n - h o l e subsequent

free-carrier

absorption.

p a i r s and

The proposed mechanisms

r e s p o n s i b l e f o r t h e g e n e r a t i o n o f nonequil i b r i um c a r r i e r s have been based on t h e occurrence o f two-photon a b s o r p t i o n (Gibson e t a1

., 1976)

and impact i o n i z a t i o n events (Jamison and Nurmikko,

1980 and James,

1983), which a r e b o t h f u n c t i o n s o f t h e l a t t i c e

temperature. Picosecond pulses were used t o reduce t h e e f f e c t s o f sample h e a t i n g i n t h e t r a n s m i s s i o n measurements.

Pulses o f c o n t i n u o u s l y

v a r i a b l e d u r a t i o n between 5 and 60 ps and c o n s t a n t a m p l i t u d e were i n c i d e n t on a sample o f InSb (ne = 5 x lOI3 cm-3 a t 77 K) having a t h i c k n e s s o f 350 p (Schwartz e t a1

., 1980).


t r a n s m i s s i o n experiments are shown i n Fig. 24 f o r l a t t i c e temperat u r e s a t 20, 88, and 295 K.

For pulses w i t h a d u r a t i o n of l e s s

t h a n 12 ps and an i n t e n s i t y o f l e s s t h a n 30 MW/cm2, no n o n l i n e a r t r a n s m i t t a n c e was observed a t 20 and 88 K. As t h e p u l s e d u r a t i o n was i n c r e a s e d w h i l e m a i n t a i n i n g a f i x e d p u l s e i n t e n s i t y , a reduced t r a n s m i s s i o n was observed which shows t h a t t h e nonl i n e a r absorpt i o n occurs a t lower i n t e n s i t i e s f o r pulses o f l o n g e r d u r a t i o n . A t room temperature, t h e onset o f n o n l i n e a r t r a n s m i s s i o n occurred f o r pulses as s h o r t as 5 ps a t an i n t e n s i t y o f 20 MW/cm2 (Fig. 24). The experimental r e s u l t s a t 20 K were i n t e r p r e t e d i n terms o f t h e g e n e r a t i o n o f excess e l e c t r o n - h o l e events

and

subsequent

strong

p a i r s by impact i o n i z a t i o n

a b s o r p t i o n by

intervalence-band

610

R. B. JAMES

100

80

60 40

-.4-03

c

3

20

0

80

60 40

20 0

a

I-

Y I T = 295K

8oj

100

/

//

60

/

/

/

0

10

20

30 40

50

60

INCIDENT PULSE DURATION (psec)

Fig. 24. High-intensity 10.6-bm transmission o f picosecond pulses i n lnSb a t d i f f e r e n t lattice temperatures. The intensities I1 are shown in units of MW/cm2. The dashed lines depict the transmission o f 30-MW/cm2 pulses i n the absence o f nonlinearities. [ A f t e r Schwartz e t al. (1980). 1

9.

611


A t h i g h e r l a t t i c e temperatures, t h e two-photon a b s o r p t i o n process i s much more s i g n i f i c a n t due t o t h e decrease i n t h e bandgap o f InSb w i t h i n c r e a s i n g temperature.

transitions.

Ref1 e c t i v i t y measurements have been performed by Hassel beck and Kwok (1982) w i t h 75-ps COP l a s e r pulses on a sample o f uncoma t 300 K).

pensated i n t r i n s i c InSb (n = 1.2 x 10l6

The r e -

f l e c t a n c e i s shown i n Fig. 25 as a f u n c t i o n o f the. peak i n t e n s i t y incident

on t h e c r y s t a l

B r e w s t e r ' s angle.

surface,

which was

oriented a t

the

The i n t e n s i t i e s were determined by t h e f o c u s i n g

geometry o f t h e l a s e r .

A t i n t e n s i t i e s l e s s than about 0.4 GW/cm2,

t h e r e f l e c t i v i t y remains constant a t 4%.

A t an i n t e n s i t y g r e a t e r

t h a n 0.4 GW/cm2, t h e r e f l e c t i v i t y increases t o 20% and t h e n drops t o 10% a t about 1.3 GW/cm2.

The damage t h r e s h o l d o f t h e sample

was measured t o be 4.3 GW/cmz, which i s c o n s i d e r a b l y g r e a t e r t h a n t h e damage t h r e s h o l d o f 40 MW/cm* measured by Kruer e t a l . (1977)

25 20 15 0

10

5 -0 0

i

o%o

0

o.oo

0 0

I

I

I

Fig. 25. integrated reflectivity o f 7 5 9 s laser pulses a s a function o f peak intensity. A nonlinear plasma generation threshold and a melting threshold can be identified. [After Haasselbeck and Kwok ( 1 9 8 2 ) . ]

612

R. B. JAMES

f o r p u l s e s with 170-ns d u r a t i o n .

The experiment o f Hasselbeck and

Kwok (1982) was repeated several times by v a r y i n g t h e i n t e n s i t y f r o m weak t o s t r o n g t o v e r i f y t h a t no permanent damage occurred t o t h e sample d u r i n g t h e measurement. The f o l l o w i n g i n t e r p r e t a t i o n has been proposed by Hassel beck and Kwok (1982) t o d e s c r i b e t h e measured r e s u l t s .

A t an i n t e n s i t y

o f 0.4 GW/cm2, a s u f f i c i e n t l y dense e l e c t r o n - h o l e plasma was gene r a t e d i n t h e c r y s t a l t o s i g n i f i c a n t l y modify t h e r e f l e c t i v i t y o f t h e sample.

A t 1.3 GW/cm* t h e dense plasma (-10l8 f r e e c a r r i e r s / c m 3 )

absorbs enough energy t o m e l t t h e c r y s t a l , r e s u l t i n g i n a l a r g e

At

i n c r e a s e i n t h e a b s o r p t i o n and decrease i n t h e r e f l e c t i v i t y .

4.0 GW/cm2, t h e s u r f a c e i s so h o t t h a t v a p o r i z a t i o n and e l e c t r o n i o n emission occur a t t h e surface.

4.3

GW/cm2,

A t i n t e n s i t i e s g r e a t e r than

t h e s u r f a c e plasma i s s u f f i c i e n t l y dense t h a t t h e

a b s o r p t i o n o f t h e l a s e r r a d i a t i o n by t h e plasma i s dominant.

A

spark i s formed and t h e r e s u l t i n g shock wave forms a c r a t e r on t h e s u r f a c e o f t h e m o l t e n semiconductor. One o f t h e most s i g n i f i c a n t r e s u l t s o f these measurements i s t h a t t h e r e e x i s t s a range o f i n t e n s i t i e s where m e l t i n g o f t h e surf a c e l a y e r occurs w i t h o u t permanent s u r f a c e damage. The r a t i o o f t h e damage t h r e s h o l d t o t h e m e l t i n g t h r e s h o l d i s approximately t h r e e (Hasselbeck and Kwok, 1982), which i s a c o m f o r t a b l e margin f o r semiconductor processing. A t present, o n l y scanning e l e c t r o n microscopy (SEM) photographs have been used t o examine t h e r e c r y s t a l l i z e d surfaces.

Since

a r s e n i c loss i s known t o occur i n laser-annealed GaAs and t h e a n t i mony vapor pressure i s h i g h compared t o t h a t o f indium, f u r t h e r s t u d i e s should be made o f t h e p o s s i b l e departures from s t o i c h i ometry i n InSb as a r e s u l t o f pulsed l a s e r annealing. 6.

GERMANIUM The a b s o r p t i o n o f h i g h - i n t e n s i t y C02 l a s e r r a d i a t i o n by f r e e

c a r r i e r s i n Ge should be d i v i d e d i n t o two c a t e g o r i e s :

t h e absorp-

t i o n a s s o c i a t e d w i t h f r e e - h o l e c a r r i e r s and t h a t a s s o c i a t e d w i t h

9.

613


free-electron carriers.

E x p e r i m e n t a l l y , i t has been observed t h a t

t h e a b s o r p t i o n by f r e e h o l e s decreases with i n c r e a s i n g i n t e n s i t y (Gibson e t al.,

1972), and t h a t t h e a b s o r p t i o n by f r e e e l e c t r o n s

i n c r e a s e s w i t h i n c r e a s i n g i n t e n s i t y (Yuen e t al.,

1979).

Both o f

t h e s e n o n l i n e a r o p t i c a l p r o p e r t i e s have p r a c t i c a l uses t h a t have s t i m u l a t e d much o f t h e research on t h e i n t e r a c t i o n o f h i g h - i n t e n s i t y pulsed C02 l a s e r r a d i a t i o n w i t h germanium (James and Smith, 1982a).

In p-type germanium, d i r e c t f r e e - h o l e t r a n s i t i o n s between t h e heavy- and l i g h t - h o l e

bands a r e p r i m a r i l y r e s p o n s i b l e f o r t h e

a b s o r p t i o n o f l i g h t i n t h e 6- t o 25-pm r e g i o n (Kahn,

1955).

For

C02 l a s e r i n t e n s i t i e s g r e a t e r t h a n about 1 bM/cm2, t h e a b s o r p t i o n c o e f f i c i e n t due t o i n t e r v a l e n c e - b a n d t r a n s i t i o n s decrease.

i s found t o

T h i s n o n l i n e a r i t y i n t h e a b s o r p t i o n i s found t o be w e l l

2.0 -

PHIPPS AND THOMAS CARLSON et 01. B KEILMANN JAMES et ai. 0

-

A

+

Fig. 26.

295K.

Saturation intensity as a function o f the photon energy for p-Ge at

The calculated values are shown by the solid curve, and the experimental

results are from Phipps and Thomas ( 1 9 7 7 ) , Carlson et al.

( 1 9 7 6 ) , and James et al.

(1982b).

( 1 9 7 7 ) , Keilrnann

[ A f t e r James and Smith (1982b3.1

614

R. B. JAMES

s a t i s f i e d by Eq.

(15).

The measured values o f t h e s a t u r a t i o n

i n t e n s i t y I S ( w ) a r e shown i n Fig.

26 as a f u n c t i o n o f t h e photon

energy f o r doping d e n s i t i e s l e s s t h a n about 1 0 l 6 cm’3 1976; Phipps and Thomas, al.,

1982b).

1977; Carlson e t al.,

(Keilmann,

1977; and James e t

The s o l i d c u r v e on t h e f i g u r e shows t h e t h e o r e t i c a l

values o f I S ( w ) c a l c u l a t e d by James and Smith (1979).

A t i n t e n s i t i e s much g r e a t e r than I s i n l i g h t l y o r moderately doped m a t e r i a l

,

t h e t r a n s m i s s i o n model which produces Eq.

b e g i n s t o break down.

For a 1.3-ns

(15)

pulse, t h i s breakdown occurs

f o r averaged i n t e n s i t i e s g r e a t e r t h a n 200 MW/cm2 (Phi pps and Thomas, 1977).

F o r i n t e n s i t i e s g r e a t e r t h a n t h e breakdown t h r e s h o l d , t h e

c a r r i e r d e n s i t y increases a b r u p t l y . free-carrier

density

This abrupt increase i n the

has been a t t r i b u t e d t o impact i o n i z a t i o n

events by James and Smith (1982a) and t o r n u l t i p h o t o n t r a n s i t i o n s

.

by Yuen e t a1 ( 1980).

Time-resolved p h o t o c o n d u c t i v i t y measurements

have been made on n-type samples by Yuen e t a l .

(1980) and on

i n t r i n s i c samples by James ( 1 9 8 4 ~ ) . I n t h e experiments, t h e photoresponse was measured as a f u n c t i o n o f t h e l i g h t i n t e n s i t y f o r 10.6-vm r a d i a t i o n .

The r e s u l t s o f these measurements have confirmed

t h a t t h e generated c a r r i e r s a r e n o t i n e q u i l i b r i u m w i t h t h e l a t t i c e and t h e r e f o r e a r e n o t due s o l e l y t o o p t i c a l heating. Measurements o f t h e photovol t a g e were made by James ( 1 9 8 4 ~ )on i n t r i n s i c germanium c r y s t a l s w i t h a t h i c k n e s s o f 0.4 nun.

For a

70-ns pulse, t h e onset o f a p h o t o c o n d u c t i v i t y s i g n a l was found t o occur a t an energy d e n s i t y o f 1.3 Jjcmz, a f t e r t a k i n g i n t o account t h e r e f l e c t i o n l o s s a t t h e f r o n t surface. be d e t e c t e d a t lower energy d e n s i t i e s ,

Some p h o t o v o l t a g e c o u l d b u t t h e s i g n a l was much

smaller,

and t h e t r a n s m i s s i o n o f t h e l a s e r r a d i a t i o n was s t i l l

linear.

The photoresponse o f t h e c r y s t a l s was found t o decay w i t h

a t i m e c o n s t a n t o f about 40 us, which i s c o n s i s t e n t w i t h t h e e l e c t r o n - h o l e recombination r a t e i n a sample o f i n t r i n s i c germanium w i t h t h i s thickness.

I n a d d i t i o n t o t h e l a r g e peak w i t h a 4 0 - v ~

decay time, t h e r e was a l o n g t a i l o f much s m a l l e r magnitude i n t h e p h o t o v o l t a g e which l a s t e d f o r more t h a n 20 ms.

The l o n g t a i l i s

9.

615


probably due t o thermal e f f e c t s , i n which some f r e e c a r r i e r s are t h e r m a l l y generated i n t h e sample by t h e energy d e p o s i t e d by t h e l a s e r pulse.

As t h e sample s l o w l y c o o l s down, t h e r e i s a decrease

i n t h e e q u i l i b r i u m c a r r i e r c o n c e n t r a t i o n , which g i v e s r i s e t o t h e l o n g t a i l on t h e p h o t o v o l t a g e s i g n a l .

The experiments were a l s o

performed w i t h a s t r o b e l i g h t as an e x c i t a t i o n source, same e l e c t r o n - h o l e recombination r a t e was measured.

and t h e

The measured

p h o t o v o l t a g e due t o t h e s t r o b e showed no l o n g t a i l , which would be expected s i n c e t h e s t r o b e l i g h t d i d n o t heat t h e sample. When t h i s i n t e n s i t y t h r e s h o l d f o r e l e c t r o n - h o l e plasma f o r m a t i o n i s exceeded, t h e a b s o r p t i o n c o e f f i c i e n t can g r e a t l y i n c r e a s e due t o t h e subsequent f r e e - c a r r i e r t r a n s i t i o n s , and thermal energy deposit i o n near t h e s u r f a c e becomes l a r g e enough t o m e l t t h e c r y s t a l . P e r i o d i c r i p p l e f o r m a t i o n s are e a s i l y observed f o r i n t e n s i t i e s approximately t w i c e as l a r g e as t h e plasma f o r m a t i o n t h r e s h o l d . I n n-type

germanium,

t h e dominant

intraband f r e e - e l e c t r o n absorption,

a b s o r p t i o n mechanism i s

where an e l e c t r o n absorbs a

photon and i s e x c i t e d t o a s t a t e i n t h e same band.

The cross sec-

t i o n f o r t h i s process i s much s m a l l e r t h a n f o r i n t e r v a l e n c e - b a n d free-hole

transitions

(Fan e t al.,

1956), since f o r intraband

absorption, t h e c o n s e r v a t i o n o f energy and c r y s t a l momentum cannot b o t h be s a t i s f i e d w i t h o u t i n v o l v i n g a t h i r d p a r t i c l e . measurements w i t h 90-ns pulses o f 9 . 6 - p

Transmission

r a d i a t i o n have been made

on t h i c k n-type germanium c r y s t a l s by Yuen e t a l .

(1980).

Figure

27 shows t h e t r a n s m i t t e d energy E t t h r o u g h c r y s t a l s o f v a r i o u s l e n g t h s L, h e l d a t room temperature, as a f u n c t i o n o f t h e i n c i d e n t energy d e n s i t y E i o f t h e pulses. The samples were l i g h t l y doped w i t h antimony t o a r e s i s t i v i t y a t room temperature o f about 10 9cm and had an a b s o r p t i o n c o e f f i c i e n t o f about 0.02 cm’l laser radiation.

f o r C02

The energies shown i n t h e f i g u r e a r e energies

i n s i d e t h e c r y s t a l , with r e f l e c t i o n losses a l r e a d y t a k e n i n t o account.

For t h e t h i c k e r samples, t h e t r a n s m i s s i o n e x h i b i t s a

sudden t r a n s i t i o n from l i n e a r t o n o n l i n e a r absorption, q u i c k l y reaches a maximum.

For t h e 2.5-cm

and E t

c r y s t a l , t h i s maximum

616

R. B. JAMES

!.5

I

I 0 L= A L = 0 L = L=

0.6 cm 2.5 cm

0

0

0

i0 crn i5crn

0

h

N

>5

4.0

O0

-

0

u

>

o

(3 (L

AAA

A

W

z

A

A A

A

O O

0

w

0

w

c

+ -

5 z

0.5

a

OOOO

U

0

I-

IQ 0

fn

I

I

u O

0 0

I

INCIDENT ENERGY (J/crn2) Fig. 27. Transmitted 9.6-p~n CO 2 laser energy as a function of incident laser energy for n-type germanium crystals o f various lengths. [ A f t e r Yuen et al. (1980). ]

value o f t h e t r a n s m i t t e d energy d e n s i t y occurs a t E i = 1.1 J/cm2 (corresponding t o an averaged i n t e n s i t y of about 12 MW/cm2). maximum value o f E t i s higher i n t h e t h i n n e r c r y s t a l s . 0.6-cm

sample,

The

For t h e

t h e maximum transmission was not reached a t t h e

h i g h e s t i n c i d e n t energy d e n s i t y a t t a i n e d w i t h t h e experimental setup. P h o t o c o n d u c t i v i t y measurements were made by Yuen e t al. (1979) on a n-type sample doped w i t h antimony t o a r e s i s t i v i t y o f 10 Qcm a t 300

K.

For a 80-ns pulse a t a wavelength o f 9.6 pm, a photo-

v o l t a g e i s observed for i n t e n s i t i e s g r e a t e r than about 10 MW/cm2 (Fig.

28).

The p h o t o c o n d u c t i v i t y signal i s found t o be d i r e c t l y

9. I

PULSED C02 LASER ANNEALING

I

I

I

I

I

I

1.5

c c

.c

=-,

e

L

-wg 1.0 +

0

a

5 0

>

8a 0.5 I

0 0

Fig. 28. f o r a 10-cm

10 20 INCIDENT INTENSITY

30

Photovoltage as a function of incident 9.6-pm long germanium crystal.

function of time.

[ A f t e r Yuen et al.

40

I;(MW em-2) C02 laser intensity

The insert shows the photovoltage as a (1979).]

proportional t o the incident i n t e n s i t y and has a decay time of about 100 p s . The overshoot f o r times greater than 180 p s was interpreted t o r e s u l t from a decrease in t h e electron mobility due t o heating effects. The overshoot disappears a f t e r approximately 2.5 ms f o r the 10-cm long c r y s t a l s . Similar transmission measurements were a1 so performed on thinner c r y s t a l s t o i n v e s t i g a t e the onset of the nonlinear absorption and the maximum transmitted energy density a t t a i n a b l e (James, 1 9 8 4 ~ ) . The c r y s t a l s were ultrapure germani um w i t h a room temperature r e s i s t i v i t y o f 43 66-cm and a thickness of 0.4 mn. The l a s e r pulses were multimode, with 80% o f the energy in the form of a peak of 70 ns (FWHM) and the remaining 20% in a long t a i l which l a s t s f o r hundreds o f nanoseconds. An i n t e g r a t o r was used in an attempt

618

R. B. JAMES

t o s p a t i a l l y homogenize t h e beam. The onset o f t h e nonlinear transmission occurs a t an incident energy density of 1.4 J/cm2 inside the c r y s t a l s and i s more gradual than the nonlinear transmission observed by Yuen e t al. (1980) i n thick germanium c r y s t a l s . The transmitted energy density remains almost constant f o r incident energy d e n s i t i e s between 2.0 and 2.8 J/cm2. For incident energy d e n s i t i e s greater than 3.0 J/cm2, t h e r e i s a spark which appears a t the germanium surface and a sudden drop in t h e transmission. A l a r g e increase i n t h e peak of the time-resolved photoconductivity response i s also observed when a f l a s h of v i s i b l e l i g h t appears a t t h e germanium surface. R i pple-1 i ke f e a t u r e s have been observed a t s l i g h t l y higher energy d e n s i t i e s with Normarski optical and A t incident energy d e n s i t i e s scanning electron microscopes. g r e a t e r than about 9 J/cm2, cracks appear a t t h e surface which a r e oriented along crystal planes. Similar f e a t u r e s of l a s e r damage i n germanium have been observed by Willis and Emmony (1975). For heavily doped germanium c r y s t a l s , t h e l i n e a r absorption i s l a r g e enough t o d i r e c t l y heat t h e s u b s t r a t e without invoking the occurrence of c a r r i e r mu1 t i pl i c a t i on and subsequent free-carri er absorption. These intensity-dependent n o n l i n e a r i t i e s i n t h e absorpt i o n may be present, b u t t h e measurements of Yuen et a l . (1980) on n-type samples and James e t a l . (1982b) on p-type samples indic a t e t h a t t h e nonlinear absorption f o r a fixed l a s e r i n t e n s i t y is less important as the doping density i s increased. Due t o t h e s i m i l a r i t i e s of germanium with s i l i c o n , one expects t h a t t h e surface layer of heavily doped germanium can be melted w i t h a pulsed C02 l a s e r without damage t o t h e samples. Following i r r a d i a t i o n with a pulsed C02 l a s e r , r i p p l e - l i k e features appear over the interaction region a t s u f f i c i e n t l y h i g h energy d e n s i t i e s , which strongly suggests t h a t melting does occur. The threshold f o r the formation of surface r i p p l e s i s found t o be lower in samples with lower r e s i s t i v i t i e s (James, 1 9 8 4 ~ ) . This i s expected since the energy deposition i s dominated by f r e e - c a r r i e r t r a n s i t i o n s . Additional experiments should be performed on t h e pulsed C02 l a s e r

9.

619


annealing o f i o n - i m p l a n t e d germanium samples w i t h d i f f e r e n t f r e e c a r r i e r c o n c e n t r a t i o n s t o determine t h e energy d e n s i t i e s r e q u i r e d f o r t h e removal o f

i m p l a n t a t i o n damage and a c t i v a t i o n o f t h e

i m p l a n t e d species.

VI. The a b s o r p t i o n o f CO

Summary and Conclusions

2

l a s e r r a d i a t i o n i n most doped semiconduc-

t o r s i s dominated by f r e e - c a r r i e r t r a n s i t i o n s . high l i g h t intensities,

For s u f f i c i e n t l y

t h e energy d e p o s i t e d by t h e l a s e r p u l s e

can m e l t t h e near-surface region.

H e a v i l y doped s i l i c o n c r y s t a l s

amorphized by i o n i m p l a n t a t i o n a r e observed t o r e c r y s t a l l i z e almost c o m p l e t e l y a f t e r annealing w i t h a 100-ns p u l s e a t an i n t e n s i t y o f about 40 MW/cm2.

R e c r y s t a l l i z a t i o n o f amorphous l a y e r s has a l s o

been achieved w i t h l i g h t l y doped s i l i c o n a t comparable i n t e n s i t i e s by p r e h e a t i n g o f t h e s u b s t r a t e t o i n c r e a s e t h e a b s o r p t i o n c o e f ficient.

Channeling measurements have confirmed t h a t t h e regrowth

o f t h e amorphous l a y e r i s e p i t a x i a l t o t h e s u b s t r a t e , t h e r e i s h i g h s u b s t i t u t i o n a l i t y o f 8, As, results of

and t h a t

and Sb implants.

The

SIMS measurements show t h a t these dopants can d i f f u s e

t o depths as g r e a t as -8000 A a f t e r i r r a d i a t i o n o f t h e samples w i t h a pulsed

C O P l a s e r , which i s comparable t o o r deeper t h a n t h e depth

one can achieve w i t h a v i s i b l e o r u l t r a v i o l e t l a s e r .

TEM s t u d i e s

o f boron-implanted c r y s t a l l i n e s i l i c o n show t h a t m e l t depths i n excess o f 8000 A a r e o b t a i n a b l e w i t h o u t l a s e r - i n d u c e d d e f e c t s i n t h e annealed region.

Furthermore, by c o n t r o l 1 i n g t h e a b s o r p t i o n coef-

f i c i e n t i n d i f f e r e n t r e g i o n s near t h e surface, one can p r e f e r e n t i a l l y d e p o s i t t h e COP l a s e r energy, and thereby m e l t r e g i o n s which a r e embedded i n t h e n e a r - s u r f a c e region.

It i s u n l i k e l y t h a t t h i s

t y p e o f m e l t i n g can be o b t a i n e d u s i n g a l a s e r wavelength a t which t h e a b s o r p t i o n i s dominated by an i n t r i n s i c a b s o r p t i o n process. M e l t i n g w i t h a pulsed C02 l a s e r i s a l s o observed i n Ge and i n compound semiconductors,

such as GaAs and InSb.

For Ge and InSb,

t i m e - r e s o l v e d p h o t o c o n d u c t i v i t y measurements show t h a t n o n e q u i l i b rium electron-hole

p a i r s a r e generated p r i o r t o t h e onset o f

R. B . JAMES m e l t i n g . The mechanism r e s p o n s i b l e f o r t h e n o n l i n e a r a b s o r p t i o n i s impact i o n i z a t i o n and/or mu1t i photon events. C a r r i e r mu1t i p l i c a t i o n processes and subsequent f r e e - c a r r i e r a b s o r p t i o n suggest t h e p o s s i b i l i t y of l a s e r annealing samples which are o p t i c a l l y t h i n a t low l i g h t i n t e n s i t i e s w i t h a l a s e r having a photon energy w e l l below t h e bandgap o f t h e m a t e r i a1 , a1 though homogeneity requirements may be more s t r i n g e n t under these e x c i t a t i o n c o n d i t i o n s . I n conclusion,

one f i n d s t h a t i n t r i n s i c a b s o r p t i o n processes

a r e n o t r e q u i r e d i n t h e l a s e r a n n e a l i n g o f semiconductors.

In fact,

t h e r e may be d i s t i n c t advantages i n u s i n g a l a s e r f o r which t h e a b s o r p t i o n i s dominated by f r e e - c a r r i e r

transitions.

The two

g r e a t e s t advantages a r e t h a t one can c o n t r o l t h e a b s o r p t i o n c o e f f i c i e n t i n such a way as t o a l l o w f o r a deeper p e n e t r a t i o n o f t h e l i g h t t h a n i s a c h i e v a b l e w i t h a v i s i b l e o r u l t r a v i o l e t l a s e r , and one can p r e f e r e n t i a l l y d e p o s i t t h e l a s e r energy i n c e r t a i n l a y e r s which may be embedded i n m a t e r i a l t h a t i s r e l a t i v e l y t r a n s p a r e n t t o the laser light. free-carrier implanting.

T h i s t y p e o f h e a t i n g i s p o s s i b l e by v a r y i n g t h e

density

i n a g i v e n r e g i o n t h r o u g h doping o r i o n

F u r t h e r s t u d i e s are r e q u i r e d t o m r e f u l l y understand

t h e c r y s t a l l i n i t y o f t h e m e l t e d amorphous l a y e r s , t h e energy dens i t y "windows" f o r successful annealing, t h e s t o i c h i o m e t r y prob-

1ems t h a t occur w i t h t h e compound semiconductors , and homogeneity r e q u i r e m e n t s o f t h e CO, l a s e r beam in a c h i e v i n g u n i f o r m j u n c t i o n depths over reasonable areas.

Acknowledgments I t is a p l e a s u r e f o r me t o thank R. Jr.,

M.

I. Baskes, M. S.

Daw, and G.

commenting on t h i s manuscript. Luck,

T. K. M i l l e r , and

a r a t i o n o f t h i s chapter.

F. Wood, G. E. J e l l i s o n , J. Thomas f o r r e a d i n g and

I would a l s o l i k e t o thank J. T.

C. J. P r i c e f o r a s s i s t a n c e w i t h t h e prep-

9.


621

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CHAPTER 10

APPLICATIONS

OF

PULSED LASER PROCESSING

R. T. Young R. F. Wood

. . . . . . . . .

I. INTRODUCTION 11. EXCIMER LASERS AND EXCIMER LASER PROCESSING 1. Excimer Lasers 2. Comparison o f Annealing C h a r a c t e r i s t i c s o f Excimer and S o l i d - s t a t e Lasers 3. E f f e c t o f Pulse D u r a t i o n on Annealing 111. PHOTOVOLTAIC APPLICATIONS 4. Laser Processing and High E f f i c i e n c y Solar Cells. 5. F a b r i c a t i o n o f S o l a r C e l l s by Beam-Processing Techniques 6. I n f l u e n c e o f Dopant P r o f i l e on S u r f a c e Recombination 7. Laser-Induced Dopant D i f f u s i o n 8. Laser Damage G e t t e r i n g . 9. G r a i n Boundary Studies. 10. Summary. IV. OTHER D E V I C E APPLICATIONS 11. I m p a t t Diodes. 12. Silicon-on-Sapphire. 13. I n t e g r a t e d C i r c u i t s . V. LASER PHOTOCHEMICAL PROCESSING VI. SUBMICRON OPTICAL LITHOGRAPHY. V I I . SUMMARY AND CONCLUDING REMARKS REFERENCES

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625

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.. . .-. . . .. .. *

Copyright 01984 by Academic Press, Inc. All righrs of reproduction in any form reserved.

ISBN 0-12-752123-2

626

R. T. YOUNG E T A L .

I.

Introduction

The s t u d y o f p u l s e d l a s e r p r o c e s s i n g o f semiconductors has developed s i n c e 1977 as one o f t h e most dynamic areas o f d e v i c e r e l a t e d research. twofold:

The d r i v i n g f o r c e s u n d e r l y i n g t h i s research a r e

(1) The u l t r a r a p i d m e l t i n g and s o l i d f i c a t i o n induced by

high-intensity

l a s e r pulses provide a unique t o o l t o s o l i d s t a t e

p h y s i c i s t s and m a t e r i a l s c i e n t i s t s f o r t h e s t u d y o f nonequi l i b r i u m c r y s t a l growth and t h e a s s o c i a t e d s u r f a c e m o d i f i c a t i o n phenomena. ( 2 ) The new c a p a b i l i t y o f a c h i e v i n g s u r f a c e p r o c e s s i n g w h i l e r e s t r i c -

i n g h i g h temperatures t o a r e g i o n w i t h i n a few microns o f t h e s u r face,

and f o r very b r i e f t i m e s

sec), provides s i g n i f i c a n t

advantages over c o n v e n t i o n a l p r o c e s s i n g steps i n t h e f a b r i c a t i o n o f semiconductor devices; e s p e c i a l l y i n t h e search f o r novel approaches t o t h e f o r m a t i o n o f submicron and t h r e e - d i m e n s i o n a l structures.

integrated

The fundamentals o f t h e i n t e r a c t i o n o f p u l s e d l a s e r

r a d i a t i o n w i t h semiconducting m t e r i a l s, p a r t i c u l a r l y s i 1i c o n , have been i n v e s t i g a t e d i n t e n s i v e l y ,

b o t h t h e o r e t i c a l l y and experimen-

t a l l y ; t h e s e i n v e s t i g a t i o n s and t h e i r r e s u l t s have been discussed i n d e t a i l i n t h e p r e c e d i n g c h a p t e r s o f t h i s book.

I n t h i s chapter,

we r e v i e w and d e s c r i b e t h e p o t e n t i a l a p p l i c a t i o n s o f p u l s e d l a s e r s i n t h e p r o c e s s i n g o f semiconductor devices and i n t e g r a t e d c i r c u i t s . Since space does n o t p e r m i t an e x h a u s t i v e r e v i e w o f a l l o f t h e d e v i c e - r e l a t e d work, we have chosen t o p l a c e p a r t i c u l a r emphasis on t h e most r e c e n t r e s u l t s r e l a t i n g t o t h e f a b r i c a t i o n o f p-n j u n c t i o n s o l a r c e l I s , f o r which beam-processing t e c h n i q u e s have proved t o be o u t s t a n d i n g l y s u c c e s s f u l .

Our more a b b r e v i a t e d d i s c u s s i o n s

o f o t h e r areas a r e i n t e n d e d p r i m a r i l y t o c o v e r t h e broad o u t l i n e s o f developments i n t h o s e areas.

To complement t h e s e d i s c u s s i o n s ,

t h e i n t e r e s t e d r e a d e r s h o u l d c o n s u l t r e c e n t reviews by Hess e t a l . (1983) and H i l l (1983) on t h e a p p l i c a t i o n o f beam p r o c e s s i n g t o i n t e g r a t e d c i r c u i t t e c h n o l o g y and t h e volume e d i t e d by Osgood e t a l . (1983),

which c o n t a i n s

processing.

numerous papers on l a s e r photochemical

627

10. APPLICATIONS OF PULSED LASER PROCESSING

I n i t i a l i n t e r e s t has been i n t h e areas o f l a s e r a n n e a l i n g o f i o n - i m p l a n t e d l a y e r s and t h e use o f l a s e r r a d i a t i o n t o 1) induce dopant d i f f u s i o n from s o l i d , l i q u i d , and gaseous sources; 2 ) d i s s o l v e second-phase p r e c i p i t a t e s ; 3 ) produce s u p e r s a t u r a t e d a1 1oys;

4 ) form metal s i l i c i d e s ; 5 ) reduce c o n t a c t r e s i s t a n c e ; 6) promote g r a i n growth; silicon;

7 ) reduce sheet r e s i s t i v i t i e s i n p o l y c r y s t a l l i n e

8) c r y s t a l l i z e d e p o s i t e d f i l m s ;

p r o p e r t i e s i n SOS ( s i l i c o n - o n - s a p p h i r e ) back-surface damage g e t t e r i n g .

9) improve i n t e r f a c e

d e v i c e s ; and 10) induce

I r r e s p e c t i v e o f so many p o t e n t i a l

a p p l i c a t i o n s , t h e d e v i c e t h a t has been f a b r i c a t e d most s u c c e s s f u l l y by p u l s e d l a s e r a n n e a l i n g i s t h e s i l i c o n s o l a r c e l l , which i s a l a r g e - a r e a d e v i c e t h a t does n o t have a complex s t r u c t u r e .

Good

r e s u l t s have a l s o been o b t a i n e d f o r high-frequency s i l i c o n IMPATT (impact avalanche and t r a n s i t t i m e ) diodes.

For a p p l i c a t i o n s t o

t h e f a b r i c a t i o n o f more complex devices, such as MOS (metal-oxidesemiconductor) o r b i p o l a r t r a n s i s t o r s i n VLSI ( v e r y l a r g e s c a l e i n t e g r a t i o n ) and VHSIC ( v e r y h i g h speed i n t e g r a t e d c i r c u i t ) t e c h nologies, l a s e r annealing i s s t i l l i n i t s infancy. I n t h i s connection, we would l i k e t o emphasize t h a t t h e success of l a s e r p r o c e s s i n g o f devices o f t e n depends t o a g r e a t e x t e n t on t h e c h a r a c t e r i s t i c s and performance o f t h e l a s e r which i s chosen f o r the p a r t i c u l a r application.

The l a s e r s used i n d e v i c e work i n

t h e past were m o s t l y s o l i d - s t a t e l a s e r s (ruby, YAG, etc.).

These

l a s e r s have c e r t a i n drawbacks and 1 i m i t a t i o n s f o r s o p h i s t i c a t e d d e v i c e p r o c e s s i n g steps.

Foremost among t h e s e l i m i t a t i o n s i s t h e

i n h e r e n t s p a t i a l inhomogeneity o f t h e energy d e n s i t y i n t h e pulses. Also,

a diffraction-related

s t r u c t u r e i s f r e q u e n t l y observed on

s u r f a c e s due t o t h e coherent r a d i a t i o n o f s o l i d - s t a t e l a s e r s .

In

o r d e r t o reduce t h e p u l s e inhomogeneities t o a c c e p t a b l e l e v e l s , i t i s necessary t o t r a n s m i t t h e beam t h r o u g h a beam homogenizer.

The

beam homogenization techniques u s u a l l y produce e i t h e r h i g h t r a n s m i s s i o n losses o r i n t e r f e r e n c e f r i n g e s from t h e o v e r l a p p i n g beams. Furthermore, t h e homogenizers can o n l y p a r t i a l l y reduce t h e inhomog e n e i t i e s , and t h e y add t o t h e c o m p l e x i t y o f t h e processing.

628


Other drawbacks of s o l i d - s t a t e l a s e r s i n c l u d e low p u l s e r e p e t i t i o n r a t e s f o r systems w i t h l a r g e diameter rods (because o f t h e heat d i s s i p a t i o n problem i n t h e i n s u l a t i n g c r y s t a l s ) and low o v e r a l l energy c o n v e r s i o n e f f i c i e n c y .

These d i f f i c u l t i e s appear t o p u t

unacceptable l i m i t a t i o n s on d e v i c e t h r o u g h p u t r a t e and c o s t f o r many a p p l i c a t i o n s . Gas l a s e r s have few o f t h e drawbacks o f s o l i d - s t a t e l a s e r s . The r e c e n t l y developed rare-gas h a l i d e excimer l a s e r s have many o f t h e c h a r a c t e r i s t i c s needed f o r e f f i c i e n t l a s e r p r o c e s s i n g o f semiconductors.

However, excimer l a s e r s w i t h s u f f i c i e n t power (e.g.

,

1.5 J / p u l s e a t 0.5 Hz) and beam u n i f o r m i t y f o r p r o c e s s i n g o f l a r g e areas, were n o t a v a i l a b l e commercially u n t i l 1981, and l a s e r s which m i g h t be viewed as t h e predecessors o f t r u e p r o d u c t i o n - t y p e l a s e r s

(1 J / p u l s e ,

100 Hz) a r e o n l y now appearing on t h e market.

Excimer

l a s e r s , i n a d d i t i o n t o t h e u n i f o r m beam and h i g h average power, can p r o v i d e many wavelengths r a n g i n g f r o m u l t r a v i o l e t ( U V ) t o vacuum U V ; t h e y a l s o have t h e advantages o f r e l a t i v e l y poor s p a t i a l and temporal coherency and good e f f i c i e n c i e s .

This type o f l a s e r not

o n l y o f f e r s t h e advantage o f b e t t e r a n n e a l i n g o f devices, b u t a l s o makes new areas o f research i n photochemical p r o c e s s i n g and i n h i g h r e s o l u t i o n o p t i c a l 1i t h o g r a p h y more promising. The remainder o f t h i s c h a p t e r i s d i v i d e d i n t o s i x sections. I n S e c t i o n 11,

t h e p r e s e n t l e v e l o f development o f UV excimer

l a s e r s , as i t r e l a t e s t o semiconductor processing, and t h e advantages o f t h e s e l a s e r s f o r such p r o c e s s i n g a r e reviewed.

I n Section

I11 , t h e l a s e r - p r o c e s s i n g technology developed f o r t h e f a b r i c a t i o n o f s i l i c o n s o l a r c e l l s i s described.

The a p p l i c a t i o n o f p u l s e d

l a s e r p r o c e s s i n g t o o t h e r devices such as IMPATT diodes, SOS s t r u c tures,

and i n t e g r a t e d c i r c u i t s i s d e s c r i b e d i n S e c t i o n I V .

The

use o f U V and I R photon-induced photochemical processes f o r f i l m d e p o s i t i o n , e t c h i n g , and doping a r e b r i e f l y discussed i n S e c t i o n V. The u t i l i z a t i o n o f t h e s h o r t wavelength and i n c o h e r e n t n a t u r e of t h e excimer l a s e r r a d i a t i o n f o r submicron o p t i c a l l i t h o g r a p h y i s i l l u s t r a t e d i n Section V I .

I n t h e l a s t s e c t i o n , we summarize t h e

629


c h a p t e r and p r o v i d e a few c o n c l u d i n g comments on t h e f u t u r e p r o s p e c t s o f p u l s e d l a s e r s i n semiconductor d e v i c e a p p l i c a t i o n s .

11. Rare-gas

Excimer Lasers and Excimer Laser Processing h a l i d e (RGH) excimer l a s e r s f o r m a c l a s s o f newly

developed l a s e r s which a r e capable o f e f f i c i e n t l y g e n e r a t i n g h i g h powered pulses o f r a d i a t i o n a t u l t r a v i o l e t wavelengths.

The r a p i d

advancement o f excimer l a s e r technology and t h e many unique charact e r i s t i c s a s s o c i a t e d w i t h these l a s e r s have made them very a t t r a c t i v e f o r many aspects o f semiconductor d e v i c e f a b r i c a t i o n . I n t h i s section, a b r i e f discussion o f the present s t a t e o f t h e i r development, as i t r e l a t e s t o semiconductor processing, w i l l be g i v e n and a comparison o f t h e c h a r a c t e r i s t i c s o f excimer l a s e r s w i t h those o f t h e s o l i d - s t a t e l a s e r s most commonly used i n semiconductor p r o c e s s i n g w i l l be discussed.

1.

EXCIMER LASERS The t e r m excimer was a p p a r e n t l y o r i g i n a l l y i n t r o d u c e d by Stevens

and Hutton (1960) t o r e f e r t o “ e x c i t e d dimers,” m o l e c u l a r species,

such as Xe2, A r 2 ,

which a r e c e r t a i n

and Hg2 t h a t e x i s t o n l y i n

t h e upper o r e x c i t e d s t a t e and have a r e p u l s i v e and, t h e r e f o r e , d i s s o c i a t i v e lower s t a t e .

I t was subsequently found ( B i r k , 1975;

Beens and Weller,

1975) t h a t c e r t a i n m o l e c u l a r complexes such as

KrF*, XeOH*, etc.,

a l s o e x h i b i t e d t h e same c h a r a c t e r i s t i c s and t h e s e

were r e f e r r e d t o as exciplexes.

These two classes o f molecules

provide a nearly ideal s i t u a t i o n f o r creating t h e nonequilibrium population inversion required f o r l a s e r action.

I n practice, the

d i s t i n c t i o n between e x c i t e d dimers and e x c i p l e x e s i s f r e q u e n t l y i g n o r e d and t h e y a r e commonly r e f e r r e d t o as excimers.

From t h e

mechanism and k i n e t i c s o f excimer f o r m a t i o n , i t i s p r e d i c t e d t h a t among a l l t h e excimers t h e RGH excimer l a s e r s o f f e r t h e advantages o f h i g h average power and h i g h e f f i c i e n c y ; t h e y a r e t h e most commonl y discussed excimer l a s e r s .

Since t h e demonstration o f t h e f i r s t

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R. T. YOUNG ET AL

e-beam-pumped

RGH excimer l a s e r i n 1975,

(Bran and Ewing,

1975;

S e a r l e s and H a r t , 1975) t h e p r e s e n t technology has advanced t o t h e p o i n t t h a t e-beam-pumped l a s e r s w i t h k i l o j o u l e o u t p u t energies have been c o n s t r u c t e d .

Although e-beam-pumped l a s e r s can be s c a l e d t o

h i g h p u l s e energies, t h e y a r e n o t r e l i a b l e because a t h i n , mechani c a l l y fragile,

f o i l window between t h e high-vacuum e l e c t r o n gun

chamber and t h e high-pressure gas discharge chamber i s i n v o l v e d . The downtime r e s u l t i n g from a f o i l f a i l u r e i s unacceptable f o r a p r a c t i c a l system o f h i g h average power.

On t h e o t h e r hand, excimer

l a s e r s e x c i t e d by s e l f - s u s t a i n e d e l e c t r i c discharges (Burnham e t a l . , 1976) have been s u c c e s s f u l l y developed r e c e n t l y i n t o l o n g - l i f e t i m e systems o f h i g h average power. The p h y s i c s and e n g i n e e r i n g requirements f o r making these l a s e r s re1 i a b l e , and t h e necessary c o n d i t i o n s f o r t h e homogeneous f o r m a t i o n of

p u l s e d avalanche discharges a t h i g h gas pressures have been

examined by L i n and L e v a t t e r (1979) and L e v a t t e r and L i n (1980). Based on these s t u d i e s , a 100-watt (1 J / p u l s e a t 100 Hz) RGH l a s e r w i t h e x c e l l e n t beam u n i f o r m i t y (5% v a r i a t i o n over an area 3 cm by

3 cm) and p u l s e - t o - p u l s e r e p r o d u c i b i l i t y i s now s a i d t o be a v a i l a b l e commercially. U n l i k e more c o n v e n t i o n a l poor s p a t i a l coherence.

lasers,

excimer l a s e r s have very

T h i s i s due t o t h e f a c t t h a t t h e beam i s

e x t r e m e l y rnultimode, which i s a consequence o f t h e l a r g e discharge volume and t h e s u p e r - r a d i a n t n a t u r e o f t h e l a s e r emission. result,

As a

i n t e r f e r e n c e e f f e c t s due t o l i g h t s c a t t e r i n g f r o m d u s t

p a r t i c l e s , s u r f ace i m p e r f e c t ions,

o r m a t e r i a1 inhomogenei t i e s i n

t h e n e a r - s u r f a c e r e g i o n o f t h e sample can be n e a r l y e l i m i n a t e d . The RGH l a s e r s can be operated w i t h a number o f d i f f e r e n t gas mixt u r e s , r e s u l t i n g i n d i f f e r e n t o u t p u t wavelengths.

Some o f t h e most

commonly used gases and t h e r e s u l t i n g wavelengths a r e : ArF (193 nm), K r C l (222 nm), KrF (249 nm), XeCl (308 nm), and XeF (350 nrn).

The

a v a i l a b l e wavelength range can be extended i n t o t h e v i s i b l e and i n t o t h e vacuum u l t r a v i o l e t by employing t h e RGH l a s e r as a pump

631

10. APPLICATIONS OF PULSED LASER PROCESSING l a s e r f o r n o n l i n e a r frequency c o n v e r s i o n schemes.

These charac-

t e r i s t i c s have been demonstrated r e c e n t l y t o be advantageous f o r a n n e a l i n g o f i o n - i m p l a n t a t i o n damage (Young e t a l . Lowndes e t a l .

,

, 1982a, 1983a;

1982), photochemical p r o c e s s i n g (see, e.g.,

volume e d i t e d by Osgood e t al., l i t h o g r a p h y ( J a i n e t al.,

1982).

the

1983), and high-reso1utio.n o p t i c a l

I n a d d i t i o n t o these applications,

excimer l a s e r s a r e a l s o a t t r a c t i v e t o r e s e a r c h e r s i n many areas o f fundamental s t u d i e s .

F o r example, because t h e o p t i c a l p r o p e r t i e s

of s i l i c o n a t UV wavelengths a r e v i r t u a l l y c o n s t a n t f o r t h e c r y s t a l l i n e , amorphous, and molten phases, c a l c u l a t i o n s o f energy absorption,

h e a t f l o w , and m e l t i n g i n v o l v e fewer parameters, and t h i s

makes comparisons between c a l c u l a t e d and experimental r e s u l t s more straightforward.

More s p e c i f i c a l l y , as discussed i n Chapter 3, t h e

o p t i c a l a b s o r p t i o n c o e f f i c i e n t , a, o f s i l i c o n i n e i t h e r t h e c r y s t a l l i n e o r t h e amorphous s t a t e a t UV wavelengths i s -106 cm’l

and t h e

r e f l e c t i v i t y R i s -70%; n e i t h e r q u a n t i t y depends s t r o n g l y on t h e s o l i d - l i q u i d phase change or on temperature.

I n contrast,

the

o p t i c a l p r o p e r t i e s o f s i l i c o n a t v i s i b l e wavelengths a r e s t r o n g l y temperature dependent and change d i s c o n t i n u o u s l y on m e l t i n g . 2.

COMPARISON OF ANNEALING CHARACTERISTICS

OF EXCIMER AND SOLID

STATE LASERS A comparative study of t h e a n n e a l i n g c h a r a c t e r i s t i c s o f XeCl excimer and ruby l a s e r s i n terms o f s u r f a c e morphology, dopant prof i l e r e d i s t r i b u t i o n s , and r e s i d u a l d e f e c t s has been made by Young The r e s u l t s a r e summarized i n t h e f o l l o w i n g . e t a l . (1983).

Surface morphology. An i m p o r t a n t concern i n t h e use o f p u l s e d l a s e r s i n semiconductor p r o c e s s i n g i s t h e s u r f a c e morphology a f t e r l a s e r treatment.

The p r e s e r v a t i o n o f a f l a t , f e a t u r e l e s s s u r f a c e

i s extremely i m p o r t a n t i n d e v i c e s i f m u l t i - s t e p required.

processing i s

A p a r t f r o m h o t spots and d i f f r a c t i o n - i n d u c e d l o c a l i z e d

s u r f a c e damage, (Leamy e t al.,

a p e r i o d i c s u r f a c e s t r u c t u r e has been observed 1978) f r e q u e n t l y i n ruby and Nd:YAG laser-annealed

632

R. T. YOUNG ETAL.

samples.

T h i s p e r i o d i c p a t t e r n i s t h o u g h t t o be due t o h e a t i n g

and m e l t i n g by a s t a n d i n g wave r e s u l t i n g from t h e i n t e r f e r e n c e o f t h e i n c i d e n t and t h e s c a t t e r e d wave (Oron and Sorenson,

1979).

F i g u r e s l a and l b show t y p i c a l s u r f a c e s t r u c t u r e s observed a f t e r ruby l a s e r annealing.

A beam homogenizer such as a ground g l a s s

d i f f u s e r p l a t e can e f f e c t i v e l y remove t h e major i n t e n s i t y v a r i a t i o n s . However, t h e d i f f u s e r p l a t e may produce f o c u s i n g e f f e c t s on a f i n e scale.

T h i s m i c r o f o c u s i n g can c r e a t e randomly d i s t r i b u t e d surface

r i p p l e s , as shown i n Fig. l c .

Although t h e s e r i p p l e s can be e l i m -

i n a t e d by p l a c i n g t h e sample f a r t h e r from t h e d i f f u s e r p l a t e , t h e

Fig.

1.

Surface morphology o f laser-annealed silicon surfaces.

( a ) EBlC

image showing d i f f r a c t i o n pattern and hot spots produced by a direct multimode beam o f a ruby laser (EQ = 1.6 J/crn2) ; ( b ) optical micrograph showing periodic ripple structures f r o m a direct multimode beam of a ruby laser (Er = 1.8 J /cm2) ; ( c ) optical micrograph showing randomly distributed surface ripples produced by a multimode beam o f a ruby laser transmitted through a diffuser plate

(EL = 1.8

/ c m 2 ) ; ( d ) optical micrograph showing the smooth surface a f t e r irradiation with a multimode beam o f a XeCl laser (EL = 3.5 J / c m 2 , T = 5 5 nsec).


633

a v a i l a b l e energy d e n s i t y f o r f e a t u r e l e s s s u r f a c e a n n e a l i n g i s r e duced t o -1.6

J/cm2.

On t h e o t h e r hand, t h e s u r f a c e morphology o f

t h e samples a f t e r XeCl excimer l a s e r a n n e a l i n g a t energy d e n s i t i e s up t o 4-5 J/cm2 (depending on t h e p u l s e d u r a t i o n t i m e ) i s smooth and f l a t (Fig. I d ) .

No unusual s u r f a c e f e a t u r e s caused by h o t spots,

d i f f r a c t i o n p a t t e r n s , o r o t h e r i n t e r f e r e n c e e f f e c t s a r e observed. To e v a l u a t e t h e u n i f o r m i t y o f t h e l a s e r beam on a m i c r o s c o p i c scale,

t h e i n t e r f a c e s between annealed and unannealed r e g i o n s i n s i l i c o n samples i m p l a n t e d w i t h boron (200 kV) were examined by t r a n s m i s s i o n e l e c t r o n microscopy (TEM).

F i g u r e 2 shows cross s e c t i o n micrographs

o f samples annealed w i t h t h e ruby l a s e r a t 2.5 J/cm* and w i t h t h e XeCl l a s e r a t 2.0 J / c d (Young e t al.,

Fig.

2.

1983a).

It i s c l e a r l y seen

TEM micrographs showing the interfaces between annealed and

unannealed regions in B-implanted

(200 k V ) Si.

634

R. T. YOUNG ETAL.

102'

AS IMPLANT 1020

"B+(35kV, l x iO'6 1019

IN Si

XeCl LASER ANNEALING 1.5 J/cm2 A

2.0 J/cm2

RUBY LASER ANNEALING

0

300

200

100

400

DEPTH (nm)

F i g . 3.

Comparison of boron implanted dopant profiles a f t e r annealing a t two

d i f f e r e n t laser energy densities with ruby and XeCl lasers.

(From Young e t a l . ,

1983a)

t h a t t h e XeCl l a s e r a n n e a l i n g r e s u l t s i n an i n t e r f a c e a t a much more u n i f o r m depth.

I n c o n t r a s t , a v a r i a t i o n i n m e l t depth as l a r g e as

-25% o v e r a 2-urn wide r e g i o n i s observed i n ruby l a s e r - a n n e a l e d

s amp 1es

.

Dopant profiles. A comparison o f l a s e r - i n d u c e d s u r f a c e m e l t i n g and dopant d i f f u s i o n i n ruby and XeCl l a s e r - a n n e a l e d samples o f llB+

(35 k V , 1 ~ 1 0 1cm-2) ~ implanted

Si was made by secondary i o n mass

spectroscopy (SIMS) p r o f i l i n g (Young e t a l . , had p u l s e d u r a t i o n t i m e s o f -25

1983a).

Both lasers

nsec, a l t h o u g h t h e p u l s e shapes

(excimer l a s e r : a p p r o x i m a t e l y t r a p e z o i d a l ; ruby l a s e r : m a t e l y Gaussian) were q u i t e d i f f e r e n t .

approxi -

F i g u r e 3 shows t h e dopant

r e d i s t r i b u t i o n i n samples annealed w i t h t h e two l a s e r s a t energy d e n s i t i e s o f 1.5 J / c d and 2.0 J/cm2.

I t i s i n t e r e s t i n g t o see t h a t

10.

635

APPLICATIONS OF PULSED LASER PROCESSING

a t t h e same energy d e n s i t y t h e r e s u l t i n g dopant p r o f i l e s a r i s i n g from t h e two l a s e r s a r e almost i d e n t i c a l .

These r e s u l t s s t r o n g l y

suggest t h a t r e g a r d l e s s o f t h e l a r g e d i f f e r e n c e s i n t h e o p t i c a l p r o p e r t i e s o f S i a t UV and v i s i b l e wavelengths,

the efficiency o f

usage o f t h e i n c i d e n t energy f o r m e l t i n g S i s u r f a c e regions t o comparable depths i s approximately t h e same f o r t h e two l a s e r s w i t h The q u a l i t y o f t h e a n n e a l i n g o f these

s i m i l a r pulse durations.

samples was subsequently examined by TEM and by van der Pauw mea-

I n a l l cases, a d i s l o c a t i o n - f r e e ,

surements.

a c t i v a t e d laser-regrown l a y e r was observed.

fully electrically Because o f t h e wide

energy window f o r excimer l a s e r annealing, deep j u n c t i o n p r o f i l e s can be r e a d i l y o b t a i n e d w i t h m u l t i p l e p u l s e s o f l a s e r r a d i a t i o n ,

4.

as i l l u s t r a t e d i n Fig.

These r e s u l t s show t h a t i n a s i l i c o n

sample a j u n c t i o n depth c l o s e t o 0.9

can be achieved w i t h 10

l a s e r p u l s e s a t 3.5 J/cm2 p e r p u l s e w i t h o u t any n o t i c e a b l e s u r f a c e 1021

I

I

I

I

I

I

I

I

600

700

000

B ( 1 0 0 k V , 1 X 1016crn-2 ) XeCl LASER ANNEALING o AS IMPLANTED

3.5 J/crn2, A

10'8

0

Fig. 4.

1 PULSE

3.5 J/cm2, 10PULSES

100

200

300

400 500 DEPTH ( n m )

900

SlMS profiles for boron in silicon a f t e r XeCl laser annealing at 3 . 5

J /cm2 with 1 and 10 pulses.

(From Young et a l . ,

1983a)

636


damage.

Comparable j u n c t i o n depths c o u l d be o b t a i n e d w i t h s o l i d -

s t a t e l a s e r pulses, b u t t h e p r e v e n t i o n o f s u r f a c e damage would be e x t r e m e l y d i f f i c u l t , i f n o t impossible.

Electrically active defects.

The laser-annealed r e g i o n s a r e

d i s l o c a t i o n - f r e e under TEM o b s e r v a t i o n and have good e l e c t r i c a l p r o p e r t i e s (sheet r e s i s t i v i t y and m o b i l i t y ) under van d e r Pauw examination, b u t i t has been r e p o r t e d ( K i m e r l i n g and Benton, 1980; Mooney e t al.,

1981) t h a t h i g h c o n c e n t r a t i o n s (1013-1015 cm-3) o f

e l e c t r i c a l l y a c t i v e d e f e c t s were d e t e c t e d by DLTS (deep l e v e l t r a n s i e n t spectroscopy) i n samples i r r a d i a t e d w i t h p u l s e d ruby and Nd:YAG l a s e r s .

These d e f e c t s were thought t o be f r o z e n i n d u r i n g

t h e r a p i d quenching process.

The e x i s t e n c e of t h e s e d e f e c t s may

have a l a r g e i n f l u e n c e on d e v i c e performance. laser-annealed

samples,

i t was found

I n t h e study o f XeCl

that electrically

active

d e f e c t s a r e present a t c o n c e n t r a t i o n s much lower t h a n t h o s e r e p o r t e d f o r samples annealed w i t h s o l i d s t a t e l a s e r s (Young e t a l . ,

1983a).

F i g u r e 5 shows a t y p i c a l DLTS spectrum from S c h o t t k y diodes made on I

I

E,

I

+ 0.38eV

~~

100

150

250

200 TEMPERATURE

300

(K)

Fig. 5 . DLTS spectrum of Si-implanted ( 1 0 k V , 5 ~ 1 0 crn-2) ~ ’ p-type Si after eC\ laser annealing at 2.0 ~ / c r n 2 .

637

10. APPLICATIONS OF PULSED LASER PROCESSING S i - i m p l a n t e d (10 kV, 5x1015 cm-2),

B-doped s i l i c o n samples a f t e r

XeCl l a s e r a n n e a l i n g w i t h a 2-J/cm2,

25-nsec pulse.

A single defect

l e v e l l o c a t e d a t 0.38 eV above t h e energy Ev o f t h e valence band i s observed.

The c o n c e n t r a t i o n o f t h i s d e f e c t i s - 5 ~ 1 0 1 1 cm-3.

These r e s u l t s s t r o n g l y suggest t h a t t h e e x i s t e n c e o f e l e c t r i c a l l y a c t i v e d e f e c t s i n laser-regrown r e g i o n s i s n o t r e l a t e d s o l e l y t o t h e r a p i d quenching produced by t h e very h i g h regrowth v e l o c i t y v, as was speculated i n t h e past,

s i n c e m e l t i n g model c a l c u l a t i o n s

(see Chapter 4 ) o f t h e l a s e r - a n n e a l i n g process show t h a t t h e values o f v f o r t h e ruby and XeCl l a s e r s w i t h comparable p u l s e d u r a t i o n times are not g r e a t l y d i f f e r e n t .

The mechanism o f defect f o r m a t i o n

i n samples annealed w i t h s o l i d - s t a t e l a s e r s a p p a r e n t l y needs cons i d e r a b l e f u r t h e r study. 3.

EFFECT OF PULSE DURATION ON ANNEALING Advances i n excimer l a s e r technology i n d i c a t e t h a t , i n a d d i t i o n

t o t h e c a p a b i l i t y f o r s c a l i n g t o h i g h e r power, a l a s e r system can be designed so t h a t t h e p u l s e d u r a t i o n t i m e T~ ( o r s i m p l y

T)

can be

a d j u s t e d over a range from t e n t o several hundred nanoseconds simply Variation o f

by changing t h e r a t i o o f gas mixtures.

range w i t h s o l i d - s t a t e l a s e r s i s d i f f i c u l t ,

over t h i s

i f n o t impossible.

I n t h i s subsection, we discuss t h e e f f e c t o f t h e p u l s e d u r a t i o n on t h e a n n e a l i n g o f i o n - i m p l a n t e d s i l i c o n by comparing t h e m e l t i n g depth, c r y s t a l p e r f e c t i o n , dopant p r o f i l e s , and e l e c t r i c a l propert i e s o f samples annealed w i t h a XeCl l a s e r w i t h energy d e n s i t y E, i n t h e range o f 0.5-3.0 e t al.,

1983b).

J/cm2 and f o r

T~

o f 25 and 70 nsec, (Young

As i n d i c a t e d f r o m model c a l c u l a t i o n s (Wood and

G i l e s , 1981; Chapter 4), i t i s expected t h a t 25-nsec pulses should be more energy e f f i c i e n t i n a n n e a l i n g i o n - i m p l a n t i o n damage t h a n a r e 70-nsec pulses.

The c a l c u l a t e d r e s u l t s a r e shown i n Fig. 6.

The m e l t i n g depth as a f u n c t i o n o f l a s e r energy d e n s i t y f o r t h e two l a s e r p u l s e s as determined from TEM i s p l o t t e d i n Fig. 7. t h e o r e t i c a l and experimental

Both

d a t a show t h a t a t t h e same energy

638

R . T. YOUNG E T A L .

0.9 0.8

-

I

I

-

I

I

Fig. 6.

1

9 fnsec) 25.5 25.5 25.5 70.5 70.5 70.5

---2.0 --.-i.5 ---2.0 ---2.5

0.7 -

0.6

I

-------!.5

3.

$ -

I E~ ( J / c m 2 ) 1.0

XeCl L A S E R _ _ _ - - 25.5 - nsec --- 70.5 nsec

-

-

Calculated melt-front profiles for pulses o f various energy densities

and two different values o f the pulse duration, as indicated by the trapezoidal pulse shapes.

-

a 10,000

z

5 W 5

0 I

I

I

I

XeCl ( X = 0 . 3 0 8 p m ) o ~ = 7 0 n s o ~ = 2 5 n s

0

LL

1

5000 -

I-

n W

x’

O W

/-*

-

Fig. 7. Melting depth as a function o f laser energy density for 25- and 70-nsec laser pulses as determined from TEM.


639

d e n s i t y , c o n s i d e r a b l y deeper m e l t i n g i s achieved w i t h 25-nsec pulses F i g u r e 8 shows t h e e f f e c t o f

t h a n w i t h 70-nsec pulses, as expected.

p u l s e d u r a t i o n on t h e dopant p r o f i l e r e d i s t r i b u t i o n o f B-implanted (100 k V ) S i annealed w i t h EQ = 2.5

and 3.0 J/cm2;

these r e s u l t s

demonstrate t h a t s h o r t e r 1aser pulses p r o v i d e deeper dopant spreadi n g , as would be expected from t h e m e l t d u r a t i o n s . However, i t i s i n t e r e s t i n g t o see t h a t a very a b r u p t dopant p r o f i l e was o b t a i n e d on t h e sample t h a t was annealed w i t h 70-nsec pulses a t an energy d e n s i t y j u s t above t h e t h r e s h o l d f o r complete a n n e a l i n g (i.e.,

2.5 J/cm2 i n t h i s case).

observed i n a r s e n i c-imp1 anted samples.

S i m i l a r r e s u l t s were a l s o

T h i s phenomenon has n o t been

seen i n ruby o r s h o r t p u l s e (25-nsec) X e C l laser-annealed samples. The q u a l i t y of annealing, i n terms o f c r y s t a l l i n e p e r f e c t i o n o f t h e

iI

(02'

s8 u

I

I

I

I

I

I

1

B ( l 0 0 kV. I X40'6cm-Z) EXCIMER LASER ANNEALING

1O2O

t

S-IMPLANTED

toq3 0

lo'8

Fig. 8.

0

25 n see,, 2.5 J/cm'

I00

200

400 DEPTH (nm)

300

500

600

700

Comparison o f concentration profiles o f B in Si a f t e r XeCl laser

annealing a t 2 . 5 and 3.0 J / c m 2 with 25-

and 70-nsec

pulses.

640


regrown l a y e r (by TEM), j u n c t i o n c h a r a c t e r i s t i c s (by dark I - V measurements), and r e s i d u a l d e f e c t s (by DLTS), i s very s i m i l a r f o r t h e two p u l s e d u r a t i o n s . From t h e s e r e s u l t s , i t can be concluded t h a t f o r a d e v i c e i n which a j u n c t i o n depth deeper t h a n 1000 A i s d e s i r e d , a l a s e r w i t h s h o r t e r p u l s e d u r a t i o n i s more energy e f f i c i e n t f o r annealing. However, l o n g e r p u l s e d u r a t i o n s may have t h e advantage o f b e t t e r c o n t r o l l i n g shallow s u r f a c e m e l t i n g (200-500 h ) and may t h e r e f o r e p r o v i d e more abrupt dopant p r o f i l e s .

Such p r o f i l e s a r e e s p e c i a l l y

c r i t i c a l f o r h i g h s w i t c h i n g speed devices t h a t r e q u i r e sharp doping changes on t h e s c a l e o f a few hundred angstroms.

I 11.

Photovol t a i c Applications

One o f t h e f i r s t , and perhaps s t i l l t h e most s u c c e s s f u l , a p p l i c a t i o n s o f p u l s e d l a s e r p r o c e s s i n g t o d a t e has been i n t h e f a b r i c a t i o n o f s i l i c o n s o l a r c e l l s (Young e t al.,

1978, 1980, 1982b).

This i s not p a r t i c u l a r l y s u r p r i s i n g since a photovoltaic c e l l i s a p-n j u n c t i o n d e v i c e t h a t does n o t have t h e complex s t r u c t u r e r e q u i r e d by most m i c r o - e l e c t r o n i c devices.

I n t h i s section, t h e

v a r i o u s l a s e r - r e l a t e d techniques which have been developed f o r s o l a r c e l l a p p l i c a t i o n s a r e discussed.

To s e t t h e stage f o r t h i s d i s c u s -

sion, we f i r s t r e v i e w some o f t h e f a c t o r s t h a t make p u l s e d l a s e r p r o c e s s i n g so s u i t a b l e f o r t h e f a b r i c a t i o n o f s o l a r c e l l s , and s k e t c h how t h e t e c h n i q u e s have e v o l v e d t o t h e e x t e n t t h a t h i g h e f f i c i e n c y s i l i c o n s o l a r c e l l s can be e a s i l y and s i m p l y f a b r i c a t e d . We t h e n p r e s e n t experimental data t o show t h a t t h e h i g h dopant conc e n t r a t i o n s achieved by i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g p r o v i d e an e f f e c t i v e ' ' i n s i t u " s u r f a c e p a s s i v a t i o n t h a t suppresses s u r f a c e r e c o m b i n a t i o n and minimizes t h e e m i t t e r recombination c u r r e n t .

The

m e l t i n g o f t h e n e a r - s u r f a c e r e g i o n produced by p u l s e d l a s e r i r r a d i a t i o n o f s i l i c o n has made p o s s i b l e t h e development o f s e v e r a l

641

10. APPLICATIONS OF PULSED LASER PROCESSING p o t e n t i a l l y low-cost techniques f o r j u n c t i o n formation.

Laser-

induced s u r f a c e v a p o r i z a t i o n has been demonstrated t o be an e f f e c t i v e method o f p r o d u c i n g c o n t r o l l e d damage on t h e backside o f a c e l l blank f o r

impurity gettering.

The u l t r a r a p i d m e l t i n g and

r e c r y s t a l l i z a t i o n c h a r a c t e r i s t i c o f p u l s e d l a s e r p r o c e s s i n g have a l s o proved t o be o f c o n s i d e r a b l e i n t e r e s t i n connection w i t h fundamental s t u d i e s and m o d i f i c a t i o n s o f g r a i n boundaries, and f o r t h e f a b r i c a t i o n o f solar c e l l s from p o l y c r y s t a l l i n e silicon.

These

t o p i c s w i l l a l s o be discussed i n t h i s s e c t i o n .

4.

LASER PROCESSING AND HIGH-EFFICIENCY SOLAR CELLS It i s i n t e r e s t i n g t o c o n s i d e r b r i e f l y some o f t h e reasons why

PU

sed l a s e r a n n e a l i n g i s so s u i t a b l e f o r t h e f a b r i c a t i o n o f s o l a r

ce 1s.

These reasons become apparent ifwe compare t h e s t r u c t u r e

o f h i g h - e f f i c i e n c y c e l l s made by c o n v e n t i o n a l c e l l technology w i t h t h e s t r u c t u r e o f t h e c u r r e n t g e n e r a t i o n of h i g h - e f f i c i e n c y , l a s e r processed c e l l s .

The s o - c a l l e d " v i o l e t " c e l l technology developed

by Lindmayer and co-workers

(1972; see a l s o Hovel,

1975) uses a

low-temperature (8OO0C) was found

very e f f e c t i v e i n r e d u c i n g s u r f a c e recombination and i t i s now r o u t i n e l y used i n t h e f a b r i c a t i o n o f l a b o r a t o r y research-type c e l l structures.

T h i s t y p e o f s u r f a c e p a s s i v a t i o n , even though e f f e c -

t i v e , adds c o n s i d e r a b l e c o m p l e x i t y t o t h e p r o c e s s i n g because photol i t h o g r a p h y has t o be used t o c u t t h r o u g h t h e o x i d e l a y e r and d e f i n e t h e g r i d contacts.

A r e c e n t study by Cuevas e t a l .

(1984) i n d i -

cates t h a t t h e very h i g h s u r f a c e dopant c o n c e n t r a t i o n s t h a t can be achieved w i t h beam p r o c e s s i n g ( i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g ) techniques p r o v i d e an " i n s i t u " s u r f a c e p a s s i v a t i o n t h a t suppresses recombination a t t h e s u r f a c e and t h u s reduces t h e e m i t t e r recombin a t i o n c u r r e n t ; t h i s w i l l be discussed next. E m i t t e r recombination c u r r e n t s were measured f o r c e l l s made from 0.25 a-cm,

n-type F Z - s i l i c o n wafers i m p l a n t e d w i t h 5 k V boron t o

doses from 2x1014 t o 1x1016 cm-2 and annealed by a ruby l a s e r .

The

r e s u l t i n g s u r f a c e c o n c e n t r a t i o n s ranged f r o m 2x1019 t o 1x1021 cm-3

652

R. T. YOUNG E T A L . I

I

I

I

1

102’

z

0

n

m 0

500

4000

4500

DEPTH

Fig.

13.

Boron concentration

2500

(density) as a function o f depth f o r five

experimental p+n silicon solar cells.

The p r o f i l e s were measured by secondary

(Cuevas e t a l . ,

ion mass spectroscopy.

2000

(s)

1984)

and t h e c o n c e n t r a t i o n p r o f i l e s were o f t h e form shown on F i g . 13. Since t h e a n n e a l i n g process does not a l t e r t h e p r o p e r t i e s o f t h e s u b s t r a t e , values o f t h e m i n o r i t y c a r r i e r ( h o l e s ) d i f f u s i o n l e n g t h Lp i n t h e base r e g i o n a r e t h e same (-200 pm) f o r d i f f e r e n t samples. The t o t a l s a t u r a t i o n c u r r e n t d e n s i t y Jo has two components, Jeo and Jbo,

r e p r e s e n t i n g t h e recombination c u r r e n t from t h e e m i t t e r and

base regions, r e s p e c t i v e l y . p l e s was about 400

pm

The base r e g i o n t h i c k n e s s o f t h e sam-

and s i n c e t h i s was g r e a t e r t h a n t h e measured

Lp, t h e long-base diode t h e o r y can be used t o c a l c u l a t e Jbo from t h e equation

653


I n t h i s equation, n i (= 1.25~1010 cm-3 a t 25OC) i s t h e i n t r i n s i c c a r r i e r d e n s i t y , Dp (=11 cmn/sec) i s t h e h o l e d i f f u s i o n c o e f f i c i e n t and Ndb (= 1.5~1016 Jbo = 5x10'13

(3111-3)

A/cm2.

i s t h e base donor c o n c e n t r a t i o n ; hence,

The e m i t t e r s a t u r a t i o n c u r r e n t d e n s i t y Jeo

can be o b t a i n e d by s u b t r a c t i n g Jbo from t h e measured t o t a l s a t u r a t i o n c u r r e n t d e n s i t y Jo; t h e r e s u l t s f o r p'n

j u n c t i o n s formed by

t h e f i v e i m p l a n t a t i o n doses used i n t h e s t u d y a r e given i n Table 11. The r e s u l t s i n d i c a t e t h a t Jeo decreases w i t h i n c r e a s i n g s u r f a c e dopant c o n c e n t r a t i o n N,

and s a t u r a t e s t o a low l i m i t o f about

5x10'13 A/cm* i n t h e two more h e a v i l y doped e m i t t e r s .

These r e s u l t s

a r e c o n s i s t e n t w i t h t h e measured i n c r e a s e o f Voc w i t h p r e v i o u s l y observed i n s i m i l a r p'n (Young e t al., e t al.

1982b).

Ns t h a t was

c e l l s made on 10 n-cm s u b s t r a t e s

An a n a l y t i c a l model was developed by Cuevas

(1984) t o d e s c r i b e m i n o r i t y c a r r i e r t r a n s p o r t i n shallow,

h e a v i l y doped e m i t t e r s .

Two p o s s i b l e mechanisms were suggested t o

e x p l a i n t h e b e h a v i o r o f Jeo w i t h doping d e n s i t y ; ( 1 ) a s t r o n g b u i l t i n r e t a r d i n g e l e c t r i c f i e l d i n t h e h e a v i l y doped s u r f a c e r e g i o n Table I1 Measured and c a l c u l a t e d e m i t t e r recombination c u r r e n t s a s a f u n c t i o n o f s u r f a c e dopant c o n c e n t r a t i o n . Re i s t h e sheet r e s i s t i v i t y o f t h e e m i t t e r . 2B 1

282

2B3

2B4

0 (cm-2)

2x1014

6x1014

2x1015

6x1015

1x1016

N, (cm-3)

2x1019

6x1019

2x1020

6x1020

1x1021

Re ( ~ / o )

580

191

63

24

16

5

3

2.5

1.6

1.5

7

3

1.5

0.5

0.5

SAMPLE

J eo (10-12 A/cm2) calculated

measured

28 5

654


keeps t h e m i n o r i t y c a r r i e r s away from t h e s u r f a c e , a r e g i o n o f h i g h r e c o m b i n a t i o n v e l o c i t y , c o n f i n i n g them t o a moderately doped r e g i o n h a v i n g a r e l a t i v e l y l o n g l i f e t i m e and t h e r e b y p r o d u c i n g low values o f Jeo; ( 2 ) t h e m i n o r i t y c a r r i e r m o b i l i t y i s e x c e p t i o n a l l y low i n t h e h e a v i l y doped s u r f a c e l a y e r (Neugroschel and Lindholm, 1983) and hence Jeo i s suppressed.

The Jeo c a l c u l a t e d u s i n g these two

assumptions a r e a l s o g i v e n i n Table 11.

The r a t h e r l a r g e d i s c r e -

pancies between t h e c a l c u l a t e d and measured values i n t h e two most h e a v i l y doped samples may be due t o u n c e r t a i n t i e s concerning t h e e f f e c t s o f band-gap n a r r o w i n g and m i n o r i t y c a r r i e r m o b i l i t y . From t h e l i m i t e d e x p e r i m e n t a l data, i t seems t h a t t h e s u r f a c e p a s s i v a t i o n induced by heavy-doping e f f e c t s may n o t be as e f f e c t i v e as t h a t induced by a l a y e r o f t h e r m a l l y grown SiO,;

however, t h e

s i m p l i c i t y o f " i n s i t u " p a s s i v a t i o n shows promise f o r f a c i l i t a t i n g t h e p r o c e s s i n g procedures f o r t h e f a b r i c a t i o n o f h i g h - e f f i c i e n c y c e l Is.

7.

LASER-INDUCED DOPANT DIFFUSION A s a l r e a d y mentioned above, t h e n o r m a l l y s t r i n g e n t requirements

on t h e p u r i t y o f dopant sources can be s i g n i f i c a n t l y r e l a x e d when p u l s e d l a s e r s a r e used f o r j u n c t i o n f o r m a t i o n .

This very useful

c h a r a c t e r i s t i c r e s u l t s from t h e c o n d i t i o n s t h a t 1) t h e m e l t f r o n t p e n e t r a t e s o n l y a few t e n t h s o f a micron,

and 2) most o f t h e

m a t e r i a l i n t h e s u b s t r a t e r e g i o n remains a t t h e ambient temperature. A s a consequence, any source contaminants a r e unable t o d i f f u s e o u t o f t h e j u n c t i o n r e g i o n and degrade t h e m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h (MCDL) i n t h e s u b s t r a t e .

I n contrast, thermal annealing

and c o n v e n t i o n a l thermal d i f f u s i o n w i l l a1 low f a s t d i f f u s i n g i m p u r i t i e s t o m i g r a t e deep i n t o t h e s u b s t r a t e . tages,

Because o f t h e s e advan-

p u l s e d l a s e r p r o c e s s i n g can be used t o f o r m p-n j u n c t i o n s

i n a v a r i e t y o f ways o t h e r t h a n i o n i m p l a n t a t i o n , d iscus s

.

as we w i l l now

655

10. APPLICATIONS OF PULSED LASER PROCESSING a.

S o l i d Sources As discussed i n Chapter 1, p-n j u n c t i o n s can be formed i n S i by

l a s e r - i n d u c e d d i f f u s i o n o f dopant f i l m s d e p o s i t e d on t h e s u r f a c e without using ion-implantation

or t h e r m a l - d i f f u s i o n steps.

I n this

approach, a t h i n (50-100 A ) dopant f i l m i s d e p o s i t e d on t h e sample by e-beam evaporation, o r by any o t h e r t e c h n i q u e ( p a i n t i n g , sprayon, spin-on,

etc.)

t h a t y i e l d s a reasonably u n i f o r m f i l m .

After

i r r a d i a t i o n o f t h e f i l m s w i t h a p u l s e d l a s e r , source dopants a r e i n c o r p o r a t e d i n t o t h e sample and e l e c t r i c a l l y a c t i v a t e d as a consequence o f l i q u i d - p h a s e d i f f u s i o n d u r i n g l a s e r - i n d u c e d s u r f a c e melting.

I n t h i s case dopant c o n c e n t r a t i o n s may exceed t h e s o l i d

s o l u b i l i t y l i m i t i f h i g h l y c o n c e n t r a t e d dopant sources a r e used (Narayan e t al.,

1978).

Experimental r e s u l t s have shown t h a t p-n

j u n c t i o n s i l i c o n s o l a r c e l l s w i t h e f f i c i e n c i e s comparable t o i o n implanted, laser-annealed c e l l s can be f a b r i c a t e d u s i n g t h i s t e c h n i q u e (Young e t al.,

1980; Fogarassy e t al.,

1981).

Laser-induced

diffusion,

especially w i t h a s u i t a b l e low-cost f i l m deposition

technique,

c o u l d be q u i t e u s e f u l f o r t h e large-volume p r o d u c t i o n

o f s o l a r c e l l s o r o t h e r b a s i c e l e c t r o n i c s t r u c t u r e s such as j u n c t i o n s i n b i p o l a r t r a n s i s t o r s , ohmic c o n t a c t s , back s u r f a c e f i e l d s , etc.,

b.

s i n c e n e i t h e r masking n o r vacuum t e c h n o l o g y i s needed. L i q u i d and Gaseous Sources An obvious e x t e n s i o n o f t h e s t u d i e s o f l a s e r doping f r o m s o l i d

sources i s work on doping from l i q u i d and gaseous sources.

Stuck

e t a l . (1981) have shown t h a t h i g h doping c o n c e n t r a t i o n s and s a t i s f a c t o r y p-n j u n c t i o n s can be o b t a i n e d u s i n g one o r two p u l s e s o f l a s e r r a d i a t i o n i n c i d e n t on a s i l i c o n s u r f a c e i n c o n t a c t w i t h a l i q u i d c o n t a i n i n g t h e d e s i r e d dopant.

Doping d i r e c t l y f r o m t h e

gaseous s t a t e has been demonstrated by Turner e t a l .

(1981).

The

low d e n s i t y o f dopant i o n s a t t h e g a s - s o l i d i n t e r f a c e seems t o make t h i s method c o n s i d e r a b l y l e s s e f f e c t i v e t h a n l a s e r - i n d u c e d d i f f u s i o n from s o l i d and l i q u i d sources.

Indeed, Deutsch e t a l .

(1979, 1981) found t h a t t h e y had t o i r r a d i a t e t h e same s p o t on t h e

656

R. T. YOUNG ET AL

sample w i t h 25 pulses from t h e l a s e r b e f o r e s a t i s f a c t o r y doping l e v e l s c o u l d be obtained.

I n c r e a s i n g t h e p r e s s u r e o f t h e dopant

gas and t h e use o f UV l a s e r s may improve t h e doping e f f i c i e n c y and may make t h i s method of doping u s e f u l i n some instances. 8.

LASER DAMAGE GETTERING The m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h i s t h e key f a c t o r i n

d e t e r m i n i n g t h e e f f e c t s o f back s u r f a c e f i e l d s on t h e e f f i c i e n c i e s o f silicon solar cells.

Laser p r o c e s s i n g has t h e advantage o f

p r e s e r v i n g t h e MCDL i n t h e base r e g i o n o f c e l l s b u t i t does n o t improve i t .

I t i s g e n e r a l l y b e l i e v e d t h a t t h e MCDL i n s i n g l e -

crystal o r large-grain polycrystalline s i l i c o n i s l i m i t e d p r i m a r i l y by t h e presence o f t r a n s i t i o n metals and/or p o i n t d e f e c t s i n t h e as-grown m a t e r i a l (see,

f o r example, Katz e t al.,

induced damage g e t t e r i n g u s i n g a Nd:YAG, Ar-ion

l a s e r (Sandow,

1980;

ruby l a s e r (Young e t al.,

(Katz e t al.,

Hawkins and E r i k s o n ,

1982b),

1981).

Laser-

1981), an

1984),

and a

has been demonstrated t o be an

e f f e c t i v e method f o r i m p r o v i n g t h e MCDL by e l i m i n a t i n g t h e d e t r i mental e f f e c t s o f t h e s e heavy metals.

I n t h i s method, extended

d e f e c t s o f a w e l l - d e f i n e d and c o n t r o l l e d t y p e a r e c r e a t e d on t h e back s u r f a c e o f t h e sample by i n t e n s e l a s e r r a d i a t i o n , a f t e r which t h e sample i s s u b j e c t e d t o a high-temperature heat t r e a t m e n t t o generate d i s l o c a t i o n s .

These d i s l o c a t i o n s a c t as e f f e c t i v e g e t t e r -

i n g s i t e s f o r t h e heavy metal i m p u r i t i e s and, p o s s i b l y , f o r p o i n t d e f e c t s d u r i n g t h e high-temperature t r e a t m e n t .

S t r u c t u r a l charac-

t e r i s t i c s o f these extended d e f e c t s were s t u d i e d q u i t e e x t e n s i v e l y by TEM (Eggermont e t al.,

1982).

The r e s u l t s i n d i c a t e d t h a t t h e

d i s l o c a t i o n s generated by l a s e r damage a r e more s t a b l e a g a i n s t t h e r m a l t r e a t m e n t t h a n a r e t h o s e generated by mechanical damage.

An example o f t h e e f f e c t i v e n e s s o f l a s e r g e t t e r i n g as a f u n c t i o n o f l a s e r energy d e n s i t y f o r a Nd:YAG l a s e r i s demonstrated by t h e data i n Table I 1 1 which i s taken from Eggermont e t a l . (1983).

For

comparison, data on t h e g e t t e r i n g e f f i c i e n c y o f A r - i o n i m p l a n t a t i o n

657

10. APPLICATIONS OF PULSED LASER PROCESSING Table I 1 1 M i n o r i t y c a r r i e r l i f e t i m e (MCL) b e f o r e and a f t e r l a s e r , mechanical, and i o n - i m p l a n t a t i o n damage g e t t e r i n g . E Q i s t h e l a s e r energy d e n s i t y from a Nd:YAG l a s e r o p e r a t i n g a t a wavelength o f 1.06 um and a p u l s e d u r a t i o n o f 150 nsec. Laser1

El ( J/cm2

35

1

MCL~ (msec) ~

~

2.0

30

26

1.1

0.9

Mechanical

Imp1a n t a t i on2

Undamaged

0.5

2.0

0.4

~~

1) Spot s i z e 65 urn, spot spacing 150 pin, row spacing 250 pm 2 ) 7x101s argon ions/cm2 a t 140 keV 3 ) Average MCL based on c - t measurements on 40 MOS c a p a c i t o r s and mechanically damaged samples a r e a l s o shown i n t h e t a b l e .

The

r e s u l t s show t h a t t h e g e t t e r i n g e f f e c t i v e n e s s i s q u i t e s e n s i t i v e t o t h e l a s e r energy d e n s i t y and t o t h e t y p e o f damage i n t r o d u c e d i n t o t h e wafer.

The non-contact n a t u r e o f l a s e r damage g e t t e r i n g ,

t h e b e t t e r c o n t r o l o f t h e depth and amount o f damage, and t h e h i g h p r o c e s s i n g throughput (i.e.,

300 3" wafers p e r hour) have made t h e

use o f l a s e r s f o r g e t t e r i n g very a t t r a c t i v e t o t h e i n t e g r a t e d c i r c u i t industry.

T h i s same t e c h n i q u e should be u s e f u l i n t h e

f a b r i c a t i o n o f h i g h - e f f i c i e n c y c e l l s from low-cost r i b b o n and c a s t p o l y c r y s t a l l i n e s i l i c o n materials. 9.

G R A I N BOUNDARY STUDIES

The p o t e n t i a l use o f p o l y c r y s t a l l i n e

s i l i c o n f o r low-cost

t e r r e s t r i a l p h o t o v o l t a i c devices has s t i m u l a t e d numerous s t u d i e s o f g r a i n boundaries and t h e i r e l e c t r i c a l p r o p e r t i e s .

I n t h i s sub-

s e c t i o n , we w i l l d i s c u s s a few examples t h a t i l l u s t r a t e how l a s e r p r o c e s s i n g techniques can be used i n t h e s t u d y o f these p r o p e r t i e s (Wood e t a1

., 1980).

658

R. T. YOUNG ETAL.

A g r a i n boundary i s t h e i n t e r f a c e a l o n g which two c r y s t a l s o f d i f f e r e n t o r i e n t a t i o n are j o i n e d together.

This interface usually

c o n s i s t s o f o n l y a few atomic l a y e r s o f d i s o r d e r e d atoms, b u t e l a s t i c s t r a i n and e l e c t r i c f i e l d s due t o t r a p p e d charges may extend t h e g r a i n boundary e f f e c t s t o g r e a t e r d i s t a n c e s . m i s o r i e n t a t i o n angle

(e), d e n s i t y o f c o i n c i d e n c e s i t e s , and t y p e s

o f d i s l o c a t i o n s (such as edge d i s l o c a t i o n s , etc.)

Depending on t h e

screw d i s l o c a t i o n s ,

i n t h e d i s o r d e r e d region, g r a i n boundaries can be c l a s s i f i e d

as l o w - a n g l e ( 0

l o o ) , twin, tilt, t w i s t ,

The e l e c t r i c a l p r o p e r t i e s , i n terms o f t h e

d e n s i t y o f d e f e c t s t a t e s and t h e e f f e c t s o f t h e d e f e c t s i t e s on c a r r i e r recombination, as w e l l as t h e tendency f o r i m p u r i t y segreg a t i o n a t t h e boundaries a r e expected t o be d i f f e r e n t a t d i f f e r e n t t y p e s o f boundaries.

W e l l - d e f i n e d " c l e a n " boundaries seldom e x i s t

i n p o l y c r y s t a l l i n e s i l i c o n grown by c o n v e n t i o n a l c r y s t a l growth techniques.

C u r r e n t research s t u d i e s e x p l o r e t h e e x t e n t t o which

v a r i o u s t y p e s o f grain-boundary r e c o m b i n a t i o n mechanisms reduce t h e photogenerated c u r r e n t and whether o r n o t an e f f e c t i v e method can be found t o " p a s s i v a t e " t h e boundaries.

To answer t h e s e q u e s t i o n s ,

s t u d i e s o f r e c o m b i n a t i o n e f f e c t s by electron-beam-induced

current

(EBIC) and scanning l a s e r spot (SLS) t e c h n i q u e s a r e f r e q u e n t l y used. These t e c h n i q u e s r e q u i r e t h e f o r m a t i o n o f a p-n j u n c t i o n and l a s e r " c o l d " p r o c e s s i n g appears t o p r o v i d e t h e i d e a l method f o r j u n c t i o n f o r m a t i o n i n t h e s e t y p e s o f s t u d i e s s i n c e contaminants a r e n o t i n t r o d u c e d i n t o t h e b u l k o f t h e samples. measurements,

By u s i n g EBIC and SLS

t h r e e t y p e s o f g r a i n boundary e f f e c t s have been

observed by Young e t a l .

( 1 9 8 2 ~ )i n l a r g e - g r a i n e d p o l y c r y s t a l l i n e

silicon.

These a r e i l l u s t r a t e d i n Fig. 14 and may be d e s c r i b e d as

follows:

(1) some boundaries a c t as r e c o m b i n a t i o n s i t e s r e d u c i n g

t h e photogenerated c u r r e n t ; ( 2 ) some show no r e c o m b i n a t i o n and do n o t a f f e c t t h e photoresponse; and ( 3 ) o t h e r s a c t u a l l y show an enhancement o f t h e photogenerated c u r r e n t .

However, i t i s reasonably

w e l l e s t a b l i s h e d t h a t i n samples which r e c e i v e a h i g h - t e m p e r a t u r e heat t r e a t m e n t , as f o r example when t h e j u n c t i o n i s formed by

10.

F i g . 14.


659

Various types of grain boundaries observed in single-pass, float-

zone, large-grained polycrystalline Si.

a ) grain boundaries show recombination;

b ) grain boundaries show enhancement of the photogenerated current; c ) grain boundaries show no recombination i n i t i a l l y ,

but; d ) they are converted to

recombination sites a f t e r heat treatment.

thermal d i f f u s i o n , t h e boundaries i n c a t e g o r i e s 2 and 3 w i l l be converted t o category 1 and become e l e c t r i c a l l y a c t i v e recombination sites.

Apparently, t h e s e phenomena a r e n o t dependent on t h e mis-

o r i e n t a t i o n angle o f t h e boundary.

S i m i l a r r e s u l t s were observed

by Turner e t a l . (1982) by u s i n g t h e SLS technique.

They r e p o r t e d

t h a t t h e enhancement e f f e c t o f t h e g r a i n boundaries was m o s t l y found f o r samples c u t from t h e m i d d l e o f a b l o c k o f Wacker c a s t polycrystalline silicon.

Since a g r a i n boundary i s i n some ways

analogous t o a surface, t h e e l e c t r i c a l p r o p e r t i e s o f a g r a i n bounda r y may be d e s c r i b e d i n terms o f an i n t e r f a c e w i t h an accumulated, depleted,

or inverted layer.

The t h r e e d i f f e r e n t t y p e s o f g r a i n

660

R. T. YOUNG ETAL.

boundary e f f e c t s c o u l d t h e n be explained, b u t t h i s can o n l y be cons i d e r e d an h y p o t h e s i s a t t h i s p o i n t . s e v e r a l q u e s t i o n s a r e needed.

Further in-depth studies o f

For i n s t a n c e , what i s t h e d r i v i n g

f o r c e t o f o r m t h e accumulated o r i n v e r t e d l a y e r , and i s t h e mechanism c h e m i c a l l y o r s t r u c t u r a l l y r e l a t e d ?

I n any case,

it i s

i m p o r t a n t t o recognize t h a t t h e r e a r e g r a i n boundaries i n t h e asgrown m a t e r i a l which do n o t a c t as recombination s i t e s , some cases, may even a c t as c u r r e n t c o l l e c t o r s .

and, i n

The l a t t e r phe-

nomenon may a r i s e from a mechanism s i m i l a r t o t h a t suggested by D i s t e f a n o and Cuomo (1977),

i.e.,

a t h i n layer o f impurity o f

o p p o s i t e t y p e t o t h e b u l k doping i s segregated t o t h e boundary i n t e r f a c e d u r i n g c r y s t a l growth.

A thorough understanding o f g r a i n

boundary s e g r e g a t i o n mechanisms and t h e d i s c o v e r y o f a way t o cont r o l t h e development o f g r a i n boundary p r o p e r t i e s d u r i n g c r y s t a l growth may p r o v i d e t h e s o l u t i o n f o r t e r r e s t r i a l p h o t o v o l t a i c a p p l i cations o f polycrystalline silicon. Pulsed l a s e r r a d i a t i o n , i n a d d i t i o n t o b e i n g used t o form j u n c t i o n s , can a l s o be used t o modify t h e m i c r o s t r u c t u r e o f t h e g r a i n boundaries, e.g.,

i n c o h e r e n t boundaries can be converted i n t o co-

h e r e n t ones by l a s e r - i n d u c e d s u r f a c e m e l t i n g (Young e t a l

., 19824).

Examples o f t h e t y p e o f r e s u l t s o b t a i n e d a r e g i v e n i n Fig.

15.

Panels 15a and 15b show a comparison o f SEM images i n t h e secondary e l e c t r o n and EBIC modes of a s e l e c t e d area o f a p o l y c r y s t a l l i n e s i l i c o n sample, a f t e r l a s e r i r r a d i a t i o n . electrically inactive

The d i f f e r e n c e s between

(B') and e l e c t r i c a l l y a c t i v e

have been examined by TEM.

( A ' ) boundaries

F i g u r e 15d i s a b r i g h t - f i e l d t r a n s -

m i s s i o n e l e c t r o n micrograph o f t h e e l e c t r i c a l l y i n a c t i v e boundary

(8'); t h e s t r u c t u r e seen i n t h e micrograph i s t y p i c a l o f a coherent boundary w i t h an a / 6 t w i n vector.

On t h e o t h e r hand, t h e

e l e c t r i c a l l y a c t i v e boundary ( A ' ) d e v i a t e s from t h e i d e a l coherent morphology,

as i n d i c a t e d by t h e presence o f networks o f p a r t i a l

d i s l o c a t i o n s i n t h e TEM micrographs.

F i g u r e 15c i s a dark f i e l d

TEM micrograph ( t i l t 40") o f boundary A ' . a r e i n d i c a t e d i n t h i s micrograph.

Two d i s t i n c t r e g i o n s

The bottom r e g i o n

B, which i s

10.

661


Fig. 15. ( a ) Image o f a portion of a laser-annealed polycrystalline silicon solar c e l l taken with an electron microscope operating i n the secondary electron emission mode; ( b ) image o f the same area on the sample taken with the microscope operating i n the EBlC mode t o show the electrical response; ( c ) dark f i e l d TEM micrograph o f boundary A ' ; ( d ) bright f i e l d TEM micrograph o f

B' , the

electri-

cally inactive boundary.

seen t o c o n t a i n d i s l o c a t i o n s , i s t h o u g h t t o be r e s p o n s i b l e f o r t h e e l e c t r i c a l a c t i v i t y o f t h e boundary.

The t o p r e g i o n T, shows t h e

c h a r a c t e r i s t i c s o f a coherent boundary. the

T and

The d i f f e r e n c e between

B r e g i o n s was produced by l a s e r m e l t i n g , which e v i d e n t l y

eliminated t h e twinning dislocations.

However, t h e l a s e r r a d i a t i o n

d i d n o t a l t e r t h e m i c r o s t r u c t u r e o f the coherent t w i n boundary, as shown i n Fig. 15d.

The r e s u l t t h a t l a s e r - i n d u c e d s u r f a c e m e l t i n g

c o n v e r t s i n c o h e r e n t boundaries t o coherent ones may suggest t h a t t h e regrowth o f t h e boundary i n t h e l a s e r - m e l t e d r e g i o n has a h i g h tendency t o occur i n t h e d i r e c t i o n o f t h e low energy boundary, i.e.

,

662


t h e coherent boundary.

These phenomena have been observed i n t h e

case o f regrowth o f d i s l o c a t i o n s by l a s e r m e l t i n g (Narayan e t al.,

1984). 10.

SUMMARY

Experimental r e s u l t s have demonstrated t h a t h i g h - e f f i c i e n c y p-n j u n c t i o n s o l a r c e l l s can be f a b r i c a t e d by t h e use o f l a s e r p u l s e s t o anneal i o n - i m p l a n t a t i o n damage i n s i n g l e c r y s t a l s u b s t r a t e s . Other low-cost, non-vacuum, j u n c t i o n f o r m a t i o n techniques have a l s o been developed and good r e s u l t s obtained.

Due t o t h e s i m p l i c i t y

and l o w c o s t o f t h i s t y p e o f processing, i t i s a n t i c i p a t e d t h a t t h e methods discussed here, a f t e r f u r t h e r e v o l u t i o n , may c o n t r i b u t e t o t h e development o f automated p r o c e s s i n g f o r volume p r o d u c t i o n o f solar cells.

I n a d d i t i o n t o t h e t e c h n o l o g i c a l advantages,

laser

p r o c e s s i n g has been found t o be u s e f u l i n many fundamental s t u d i e s , such as t h o s e concerned w i t h t h e e f f e c t s o f heavy doping on d e v i c e performance and t h e e l e c t r i c a l p r o p e r t i e s o f g r a i n boundaries. It was found t h a t t h e v e r y h i g h dopant d e n s i t i e s a t t h e s u r f a c e

achieved w i t h i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g p r o v i d e an " i n s i t u " s u r f a c e p a s s i v a t i o n t h a t can ease t h e f a b r i c a t i o n requirements f o r s h a l l o w p-n j u n c t i o n c e l l s . The r e s u l t s from g r a i n boundary s t u d i e s i n d i c a t e t h a t t h e use o f laser r a d i a t i o n f o r junction formation i n p o l y c r y s t a l l i n e s i l i c o n

can p r o v i d e t h e f o l l o w i n g advantages: 1) C o n t r o l o f enhanced dopant d i f f u s i o n a l o n g g r a i n boundaries, which f r e q u e n t l y causes problems o f s h o r t i n g i n t h e device.

2)

P r e v e n t i o n o f g r a i n boundary t r a p p i n g o f contaminants, which

i s b e l i e v e d t o be p a r t i a l l y r e s p o n s i b l e f o r t h e development o f e l e c t r i c a l a c t i v i t y a t t h e s e boundaries.

3)

M o d i f i c a t i o n o f g r a i n boundary m i c r o s t r u c t u r e s , w i t h t h e r e s u l t t h a t e l e c t r i c a l l y a c t i v e boundaries a r e t r a n s f o r m e d i n t o e l e c t r i c a l l y i n a c t i v e ones, and/or r e d u c t i o n i n g r a i n boundary t r a p p i n g s t a t e s by 1aser-induced n e a r - s u r f a c e me1t i ng.

10.

663


These f i n d i n g s suggest t h a t l a s e r processing may have important a p p l i c a t i o n s i n t h e f a b r i c a t i o n o f s o l a r c e l l s from p o l y c r y s t a l l i n e s i l i c o n made by a v a r i e t y o f low-cost techniques.

IV.

Other Device Applications

IMPATT DIODES

10.

The IMPATT diode i s one o f t h e most powerful s o l i d - s t a t e sources o f microwave power today.

The power output and e f f i c i e n c y o f t h i s

k i n d o f device c u r r e n t l y are l i m i t e d by t h e m a t e r i a l p r e p a r a t i o n methods i n v o l v e d i n t h e device f a b r i c a t i o n .

For example, i n t h e

f a b r i c a t i o n o f high-frequency (100 GHz) p'pnn'

IMPATT diodes, pre-

c i s e c o n t r o l o f t h e f i l m thickness and dopant concentrations i n t h e two d r i f t regions (i.e.,

t h e p and n region), minimum i m p u r i t y

r e d i s t r i b u t i o n between t h e p'p

and n'n

j u n c t i o n , and a low contact

r e s i s t a n c e are r e q u i r e d i n order t o minimize t h e p a r a s i t i c r e s i s t i v e losses.

A schematic o f an i d e a l i z e d d o u b l e - d r i f t IMPATT s t r u c t u r e

and t h e associated e l e c t r i c f i e l d d i s t r i b u t i o n i s shown i n Fig. 16.

2

I

0 l-

a

a k

z 0 z

w

n

P

P+

HOLE DRIFT REGION

+ A

IND - N A I

n+

ELECTRON DRIFT

REGION

n 1

0

w

U

W

U

% D

a

E

0 l-

U w -I w I

1

Fig. 16. Schematic diagram o f an idealized double-drift IMPATT diode and the associated electric field. (Hess et a l . , 1980)

664

R. T.YOUNG E T A L .

P r e s e n t l y , t h e d r i f t r e g i o n s a r e formed e i t h e r by two-sided e p i t a x y o r s i n g l e epitaxy i n conjunction w i t h i o n implantation.

The e p i -

t a x i a l process i n c u r r e n t d e v i c e f a b r i c a t i o n i s based p r i m a r i l y on chemical vapor d e p o s i t i o n o f d i c h l o r o s i l a n e o r s i l a n e a t 1000-llOO°C under atmospheric pressure. sing,

e x t e n s i v e dopant

u s u a l l y occurs.

D u r i n g t h i s h i gh-temperature proces-

r e d i s t r i b u t i o n a t t h e growth i n t e r f a c e

Furthermore, due t o t h e r e l a t i v e l y h i g h d e p o s i t i o n

r a t e s a t t h e s e temperatures, i t i s very d i f f i c u l t t o r e p r o d u c i b l y control t h e f i l m thickness o f t h e t h i n a c t i v e layer required f o r high-frequency o p e r a t i o n ( f o r example, t h e n o r p r e g i o n i n 100 GHz IMPATT diodes i s 0.3

pm).

On t h e o t h e r hand,

progress i n t h e

development o f i o n i m p l a n t a t i o n i n t o t h i c k e p i t a x i a l l a y e r s has been hampered by t h e i n c o m p l e t e thermal annealing, w i t h t h e r e s u l t t h a t r e s i d u a l d e f e c t s remain i n t h e avalanche as w e l l as i n t h e d r i f t region.

I n t h e f i n a l d e v i c e f a b r i c a t i o n step, t h e c o n t a c t

r e s i s t a n c e o f c u r r e n t l y a v a i l a b l e devices i s l i m i t e d by t h e s o l i d s o l u b i l i t y l i m i t i n b o t h t h e n+ and p+ region. Laser-processing techniques may very we1 1 p r o v i d e an a t t r a c t i v e method o f c o m p l e t e l y o r a t l e a s t p a r t i a l l y s o l v i n g t h e aforement i o n e d problems i n so f a r as l a s e r s can be used t o (1) grow h i g h q u a l i t y e p i t a x i a l l a y e r s a t l o w s u b s t r a t e temperatures (

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