Handbook of Infra-red Technologies

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Handbook of Infrared Detection Technologies


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Edited by Mohamed Henini and Manijeh Razeghi

ELSEVIER

UK USA JAPAN

Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX 5 1GB, UK Elsevier Science Inc, 360 Park Avenue South, New York. NY 10010-1710, USA Elsevier Science Japan, Tsunashima Building Annex, 3-20-12 Yushima, Bunkyo-ku, Tokyo 113. Japan Copyright ~ 2002 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic. electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without prior permission in writing from the publishers.

British Library Cataloguing in Publication Data Handbook of infrared technologies 1 .Infrared technology I.Henini, Mohamed II.Razeghi, M(Manijeh) technologies 621.3'62

III.Infra-red

ISBN 1 85617 388 7

Library of Congress Cataloging-in-Publication Data Handbook of infrared technologies / edited by Mohamed Henini and Mahijeh Razeghi. p. cm Includes bibliographical references and index. ISBN 1-85617-388-7 (hardcover) 1. Infrared technology-Handbooks, manuals, etc. I. Henini, Mohamed. II. Razeghi, M. TA 1 5 7 0 . H 3 5 8 2002 621.36'2-dc21

2002040767

No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Published by Elsevier Advanced Technology, PO Box 15() Kidlington Oxford OX5 1AS. UK Tel.: +44(0) 1865 8 4 3 0 0 0 Fax: +44(0) 1865 843971 Typeset by Variorum Publishing Limited, Lancaster and Rugby Printed and bound in Great Britain by Biddles Limited, Guildford and King's Lynn

Contents

List of contributors

xiii

Chapter 1

Introduction

Chapter 2

Comparison of photon and thermal detector performance 2.1 Introduction 5 2.2 Fundamental limits to infrared detector performance 6 2.2.1 Photon detectors 9 2.2.2 Thermal detectors 18 2.2.3 Comparison of the fundamental limits of photon and thermal detectors 21 2.3 Focal plane array performance 23 2.4 FPAs of photon detectors 26 2.4.1 InSb photodiodes 30 2.4.2 HgCdTe photodiodes 33 2.4.3 Photoemissive PtSi Schottky-barrier detectors 36 2.4.4 Extrinsic photoconductors 36 2.4.5 GaAs/A1GaAs OWIPs 38 2.4.6 OWIP versus HgCdTe in the LWIR spectral region 39 2.5 Dual-band FPAs 52 2.5.1 Dual-band HgCdTe 53 2.5.2 Dual-band QWIPs 54 2.6 FPAs ofthermal detectors 58 2.6.1 Micromachined silicon bolometers 58 2.6.2 Pyroelectric arrays 61 2.6.3 Thermoelectric arrays 64 2.6.4 Status and trends of uncooled arrays 65 2.7 Conclusions 67 Appendix 70 References 75

vi

Handbookof Infrared Detection Technologies

Chapter 3

GaAs/AIGaAs based quantum well infrared photodetector focal plane arrays

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Chapter 4

Introduction Detectivity D* comparison Effect of nonuniformity 640 • 512 pixel long-wavelength portable OWIP camera 640• 486 long-wavelength dual-band imaging camera 640 • 512 pixel broad-band OWlP imaging camera 640 x 512 spatially separated four-band QWIP focal plane array OWIPs for low background and low temperature operation Summary Acknowledgements References

83 89 91 92 99 102

110 113 115 117 117

GalnAs(P) based QWIPs on GaAs, InP and Si substrates for focal plane arrays 121 4.1 Introduction

4.2

4.3

4.4

4.5

4.6

4.1.1 Overview of infrared detector 4.1.2 Quantum well infrared photodetector 4.1.3 State-of-the-art Fundamentals of OWlP 4.2.1 Intersubband absorption 4.2.2 OWIP parameters 4.2.3 Comparison of n-type and p-type QWIPs 4.2.4 Growth. fabrication and device characterization of a single QWIP device Fabrication of infrared FPA 4.3.1 Infrared FPA fabrication steps 4.3.2 Indium solder bump fabrication steps 4.3.3 ROIC for infrared FPA p-type OWIPs 4.4.1 p-type MWIR QWIPs 4.4.2 p-type LWlR QWIPs n-type OWIPs 4.5.1 n-type LWIR QWIPs 4.5.2 n-type VLWIR OWIPs 4.5.3 Multi-color OWIPs Low cost QWIP FPA integrated with Si substrate 4.6.1 Overview of OWIPs on Si 4.6.2 Growth of GaInAs/InP QWIP-on-Si 4.6.3 Detector performance of GaInAs/InP OWIP-on-Si How to fabricate a monolithic integrated FPA 4.6.4 with Si substrate

121 123 125 126 126 130 133 134 135 135 139 140

141 142 144 144 144 146 147 147 147 148 148 151

Contents

4.7 4.8

New approaches of OWIP Conclusions References

vii

151 153 154

Chapter 5

InAs/(Galn)Sb superlattices: a promising material system for infrared detection 5. ] Introduction 159 5.2 Materials properties 159 5.2.1 BandstructureoflnAs/(GaIn)Sbsuperlattices 159 5.2.2 X-ray characterization 163 5.2.3 Interfaces 165 5.2.4 Sample homogeneity 16 7 5.2.5 Residual doping 16 7 5.3 Superlattice photodiodes 169 5.3.1 Diode structure 169 5.3.2 Diode processing 172 5.3.3 Photo response 172 5.3.4 I-V measurements 173 5.3.5 C-V measurements 181 5.3.6 Noise measurements 183 5.4 Summary and outlook 18 7 References 18 7

Chapter 6

GaSb/InAs supperlattices for infrared FPAs 6.1 Type-II heterostructures 6.1.1 Historical review 6.1.2 Definition oftype-II band alignment 6.1.3 Features oftype-II band alignment and their applications 6.2 Type-II infrared detectors 6.2.1 Principle of operation 6.2.2 Band structure of type-II superlattices 6.2.3 Optical absorption in type-II superlattices 6.2.4 Modeling and simulation of type-II superlattices 6.3 Experimental results from type-II photoconductors 6.3.1 Uncooled type-II photoconductors in the X=8-12 l~m range 6.3.2 Cooled type-II photoconductors for )v > 2013m 6.4 Experimental results from type-II photodiodes 6.4.1 Uncooled type-II photodiodes in the )v=8-12 ~m range 6.4.2 Cooled type-II photodiodes in the )v > 14 l.tm range 6.5 Future work References

191 191 191 192 193 193 193 194 19 7 201 201 208 211 211 215 227 231

viii


Chapter 7

Chapter 8

MCT properties, growth methods and characterization 7.1 Preface 7.2 Introduction 7.2.1 Brief history 7.3 MCT Characteristics and material properties 7.3.1 Composition and crystal structure 7.3.2 Bandgap 7.3.3 Intrinsic carrier concentration 7.3.4 Doping and impurities 7.3.5 Carrier mobility 7.3.6 Carrier lifetime 7.3.7 Defects 7.4 MCT crystal gowth methods 7.4.1 Phase diagrams 7.4.2 Bulk growth 7.4.3 Epitaxial growth 7.5 Material characterization methods 7.5.1 Material composition 7.5.2 Measurements of carrier concentration and mobility 7.6 Summary References HgCdTe 2D arrays- technology and performance limits 8.1 Introduction 8.1.1 Historical perspective 8.2 Applications and sensor design 8.3 Comparison of HgCdTe with other 2D array materials 8.4 Multiplexers for HgCdTe 2D arrays 8.4.1 Photocurrent injection techniques 8.4.2 Scanning architectures 8.4.3 Future trends 8.5 Theoretical foundations for HgCdTe array technology 8.5.1 Thermal diffusion currents in HgCdTe 8.5.2 Leakage currents 8.5.3 Photocurrent and quantum efficiency 8.6 Technology ofHgCdTe photovoltaic devices 8.6.1 Materials growth technology 8.6.2 Junction forming techniques in homojunction arrays 8.6.3 Device structures 8.7 Measurements and figures of merit for 2D arrays 8.7.1 NETD - theoretical calculation 8.7.2 NETD- experimental measurement 8.7.3 Relationship of NETD with other figures of merit

233 233 234 235 235 236 238 239 240 241 245 245 246 247 248 256 256 260 260 260

269 270 271 274 276 277 278 279 279 280 281 282 282 284 287 288 292 293 294 294

Contents

8.8 8.9

8.10 8.11

8.12

HgCdTe HgCdTe 8.9.1 8.9.2

2D arrays for the 3-5 lum (MW) band 2D arrays for the 8-12 ~tm (LW) band Array design issues Introduction to performance limitations in LW arrays 8.9.3 Cause of defective elements in HgCdTe 2D arrays HgCdTe 2D arrays for the 1-3 ~m (SW) band Towards GEN III detectors 8.11.1 Two-colour array technology 8.11.2 Higher operating temperature (HOT) device structures 8.11.3 Retina level processing Conclusion and future trends Acknowledgements References

ix

296 296 296 297 298 299 301 301 303 304 304 305 305

Chapter 9. Status of HgCdTe MBE technology 9.1 Introduction 9.2 HgCdTe MBE equipment and process sensors 9.2.1 Vacuum equipment and sources 9.2.2 HgCdTe MBE process sensors 9.3 HgCdTe MBE growth process 9.3.1 Substrate preparation 9.3.2 Growth conditions 9.3.3 Defects 9.3.4 Doping 9.4 Device applications 9.4.1 Multispectral HgCdTe infrared detectors 9.4.2 Near-infrared avalanche photodiodes High-performance MWIR detectors 9.4.3 9.4.4 Large-format arrays on silicon substrates Acknowledgements References

309 310 310 312 318 318 320 320 322 324 324 331 33 7 341 348 348

Chapter 10 Silicon infrared focal plane arrays 10.1 Introduction 10.2 CooledFPAs 10.2.1 Schottky-barrier FPAs 10.2.2 Heterojunction internal photoemission FPAs 10.3 UncooledFPAs 10.3.1 Silicon On Insulator (SOI) diode FPAs 10.3.2 Si-based resistance bolometer FPAs 10.3.3 Thermopile FPAs 10.4 Summary References

353 354 354 3 71 374 374 379 383 386 387

x


Chapter 11 Infrared silicon/germanium detectors 11.1 Introduction 11.2. Near Infrared detectors 11.2.1 General operation principle 11.2.2 Detector growth and fabrication 11.2.3 Results and discussion 11.3 Mid-and long-wavelength SiGe IR detectors 11.3.1 Introduction 11.3.2 Principle of operation of HIP detectors 11.3.3 Growth and material characterization 11.3.4 Experimental results and discussion 11.3.5 Calculation of optical properties of SiGe HIP detectors 11.3.6 R6sum6 and outlook for SiGe MWIR detectors Acknowledgements References Chapter 12 PolySiGe uncooled microbolometers for thermal IR detection 12.1 Introduction 12.1.1 Uncooled resistive microbolometers 12.1.2 Microbolometer terminology 12.1.3 Microbolometer process options 12.2 Structural, thermal and electrical properties ofpolySiGe 12.2.1 Deposition of polySiGe 12.2.2 Structural properties 12.2.3 Thermal properties 12.2.4 Electrical properties 12.2.5 High-temperature vs. low-temperature polySiGe 12.3 PolySiGe bolometer pixels 12.3.1 Process development 12.3.2 Absorber comparison and trade-offs 12.3.3 Pixel optimization 12.3.4 Vapor HF processing 12.3.5 Stiffness enhancement techniques 12.4 Readout and system development 12.4.1 Introduction 12.4.2 Readout of polySiGe bolometer arrays 12.5 Zero-level vacuum packaging 12.5.1 Introduction 12.5.2 Indent-Reflow Sealing using metal solder 12.5.3 Zero-levelpackagingusingBCB 12.5.4 Hermeticitytestingusingmicrobolometers 12.6 Conclusions and outlook Acknowledgements References

393 398 398 403 408 411 411 412 413 421 438 444 445 445

449 449 451 452 453 453 454 455 455 457 457 457 459 460 462 463 467 467 468 472 472 473 475 475 476 477 477

Contents

xi

Chapter 13 Fundamentals of spin filtering in ferromagnetic metals with application to spin sensors

13.1 13.2

13.3 13.4

13.5

Index

Introduction Theoretical IMFP variation 13.2.1 The simplest m o d e l - mathematical bases of the calculation 13.2.2 Amore complete treatment 13.2.3 An intuitive derivation 13.2.4 Comparison with the Sch6nhense and Siegmann model Experimental study of A c~ Spin precession and spin filters 13.4.1 Density-operator formalism 13.4.2 Electron transmission through ferromagnetic bilayers 13.4.3 The bilayer with collinear magnetizations 13.4.4 The bilayer with perpendicular magnetizations Discussion and conclusion Acknowledgements References

481 483 483 488 492 495 495 498 499 501 502 503 504 506 506 509


List of Contributors

Ian M. BAKER, BAE Systems Infrared Ltd., P.O. Box 217, Southampton, Hampshire SO 15 0EG, UK S. V. BANDARA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91 ] 09, USA L. BURKLE, Fraunhofer Institut f/ir Angewandte Festk6rperphysik, Tullastrasse 72, D-79108 Freiburg, Germany Henri-Jean DROUHIN, Laboratoire de Physique de la Mati~re Condens~e (UMR 7643-CNRS), Ecole Polytechnique, 9 ] 128 Palaiseau cedex, France F. FUCHS, Fraunhofer Institut fiir Angewandte Festk6rperphysik, Tullastrasse 72, D-79108 Freiburg, Germany S.D. GUNAPALA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

M. HENINI, Department of Physics and Astronomy, University of Nottingham, Nottingham, UK Chris van HOOF, IMEC Kapeldreef 75, B-30()1 Haverlee, Belgium and ESATINSYS Department, University of Leuven, Belgium J. E. JENSEN, HRL Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265. USA

J. JIANG, Center for Quantum Devices, Electrical and Computer Engineering Department, Northwestern University, Evanston, Illinois 60208, USA

xiv Handbook of Infrared Detection Technolo,qies Masafumi KIMATA, Senior Technology Department, Advanced Technology, R&D Center, Mitsubishi Electric Corporation, 8-1-1, Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661. Japan Randolph E. LONGSHORE, Raytheon Missile Systems, P.O. Box 1137, MS 8 4 0 / 7 Tuscon, AZ 85734, USA Terry de LYON, HRL Laboratories. 3 O l l Malibu Canyon Road, Malibu, CA 90265, USA H. MOHSENI, Center for Quantum Devices. Electrical and Computer Engineering Department, Northwestern University. Evanston. Illinois 60208. USA Piet de MOOR, IMEC, Kapeldreef 7 5. B- 3()() 1 Heverlee, Belgium Hartmut PRESTING, DaimlerChrysler Research (REM/C), Dep. FT2/H, WilhelmRunge Strasse 11, D-89081 ULM, Germany R. D. RAJAVEL, HRL Laboratories, 3011 Malibu Canyon Road. Malibu, CA 90265, USA

Manijeh RAZEGHI, Center for Quantum Devices. Electrical and Computer Engineering Department, Northwestern University. Evanston. Illinois 6()2()8, USA Antoni ROGALSKI, Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str.. ()()-908 Warsaw. Poland J. A. ROTH, HRL Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265. USA

Chapter 1

Introduction M. R a z e g h i a n d M. H e n i n i

Nature has provided numerous examples of efficient detection systems. Almost all types of life, from bacteria, to plants, to h u m a n beings, have evolved some type of optoelectronic detection system for perceiving the world around them. These systems have had millions of years to develop, and demonstrate a seamless integration of optoelectronics with biological systems. The jewel beetle (Melanophila acuminata) thrives on the remnants of forest fires. Its larva feed on the dead wood, which gives evolutionary incentive for the beetle to find dead wood before other species. Towards this end, the beetles have developed an infrared detection system which allows them to sense a 10 hectare forest fire from up to 12 km away. As shown in Figure 1.1, a pit organ, called a sensilla, is located on either side of the beetle's thorax, which allows both intensity and directional information to be obtained. Absorption of infrared (2.4-4 lxm wavelength) light triggers a mechanical expansion which triggers nerve impulses. Obviously, this system must be small and easy to use. Further, as

Figure 1.1 The jewel beetle and its infrared sensor.

2


a beetle does not have a large built-in power supply or cryogen, the system must be power efficient and be uncooled. Our eyes are also excellent examples. Nature has provided a multi-spectral detection system based on microscopic variation in detector design. These differentiated detector cells add another dimension to the versatility of the eye. With a broadband detector, there is no way to differentiate between the intensity of a source and its emissivity at different wavelengths. This is akin to trying to pick out a matching wardrobe with a black and white camera. Multispectral systems allow separate waveband analysis of objects, which allows faster and more accurate identification to be made. On an evolutionary perspective, this ability allows more efficient target identification, allowing faster response to a potentially hazardous situation. The goal of science is to enhance our senses and better understand the universe around us. Infrared detectors broaden our vision into the realm of heat, allowing remote sensing of an object's temperature. This has had a dramatic impact on how we perceive our environment, and has led to many types of thermal imaging, including night vision, infrared astronomy, medical diagnostics, and failure analysis. These newfound abilities have spurred the development of m a n y new systems, as shown in Figure 1.2. Infrared detectors have seen a remarkable surge in interest over the past several decades. This is thanks in part to the successful development of highperformance devices which have become the core of all the infrared systems listed above. The natural progression of these systems is a multispectral, uncooled, infrared camera, which can, by itself, address most of these applications. As in nature, a good system should be flexible, power efficient, lightweight, and easy to use. While we cannot expect to match the sophistication of natural systems, we can be inspired by them.

Figure 1.2 Examples of mainstream thermal imaging systems.

Introdl~ction

3

One inspiration involves the exploitation of quantum size effects for higher efficiency and added functionality. Most infrared photon detectors have a limited photocarrier lifetime and peak detection wavelength that is fixed by the bandgap of the material. Without changing the chemical composition of the material, patterning on an atomic scale can allow an increase in carrier lifetime and tuning of the peak detection wavelength. This type of effect has already been demonstrated in the form of the type-II InAs/GaSb semiconductor detector. Used in another way, similar to the eye, microscopic alterations can be made to the lateral size of individual detectors to demonstrate multispectral sensitivity in a single focal plane array. The purpose of this book is to present current methods and future directions in infrared detection. By bringing together experts in physics, material science, fabrication technology, and application, we will develop a well-rounded view of how far we have progressed towards the goal of an integrated, versatile, infrared detection system.


Chapter 2

Comparison of photon and thermal detector performance A. Rogalski

2.1 Introduction At present, HgCdTe is the most widely used variable gap semiconductor for infrared (IR) photodetectors. Over the last forty years it has successfully fought off major challenges from extrinsic silicon and lead-tin telluride devices, but despite that it has more competitors today than ever before. These include Schottky barriers on silicon, SiGe heterojunctions, A1GaAs multiple quantum wells, GaInSb strain layer superlattices, high temperature superconductors and especially two types of thermal detectors: pyroelectric detectors and silicon bolometers. It is interesting, however, that none of these competitors can compete in terms of fundamental properties. They may promise to be more manufacturable, but never to provide higher performance or, with the exception of thermal detectors, to operate at higher or even comparable temperatures. The main motivations to replace HgCdTe, are technological problems of this material. One of them is a weak Hg-Te bond, which results in bulk, surface and interface instabilities. Uniformity and yield are still issues. The slow progress in the development of large photovoltaic HgCdTe infrared imaging arrays and the rapid achievements of novel semiconductor heterostructure systems have made it more difficult to predict what types of arrays will be readily available for future systems applications. For spaceborne surveillance systems, low background IR seeker/tracker systems, reliable and affordable sensors with long life are needed which can function effectively at temperatures higher than the 2 0 - 3 0 K currently required by bulk photon detectors. The only alternative to HgCdTe that had been available so far was extrinsic Si, which operates at much lower temperatures where a problematic three-stage cryocooler would be required. Improvement in surveillance sensors and interceptor seekers requires large area size, highly uniform and multicolour (or multispectral) IR focal plane arrays (FPAs) involving long wavelength IR (LWIR) and very long wavelength IR

6 Handbookof Infrared Detection Technologies

(VLWIR) regions. Among the competing technologies are the quantum well infrared p h o t o c o n d u c t o r s (QWIPs) based on lattice matched GaAs/A1GaAs and strained layer InGaAs/A1GaAs material systems. In comparison with photon detectors, thermal detectors have been considerably less exploited in commercial and military systems. The reason for this disparity is that thermal detectors were popularly believed to be rather slow and insensitive in comparison with photon detectors. As a result, the world-wide effort to develop thermal detectors has been extremely small relative to that of photon detectors. In the last ten years, however, it has been shown that extremely good imagery can be obtained from large thermal detector arrays operating uncooled at TV frame rates. The speed of thermal detectors is quite adequate for non-scanned imagers with two-dimensional detectors. At present, uncooled, monolithic FPAs fabricated from thermal detectors, revolutionise the development of low cost thermal imagers. In this paper, we discuss the performance of photon detectors as compared to thermal detectors. In comparative studies, more attention is paid to a wide family of photon detectors, especially to HgCdTe photodiodes and QWIPs. The potential performance of different materials used for photon detectors is examined utilizing the o~/G ratio, where cz is the absorption coefficient and G is the thermal generation. Different types of detectors operated as single element devices, are considered. Also such FPA issues as array size, uniformity, operability, multicolour capability and cost of systems, are discussed.

2.2 Fundamental limits to infrared detector performance Spectral detectivity curves for a number of available IR detectors are shown in Figure 2.1. Interest has centered mainly on the wavelengths of the two atmospheric windows 3-5 ~m [middle w a v e l e n g t h IR (MWIR)] and 8 - 1 4 pm (LWIR region) (atmospheric transmission is the highest in these bands and the emissivity maximum of the objects at T,~3()()K is at the wavelength ;.~10 micron), though in recent years there has been increasing interest in longer wavelengths stimulated by space applications. Depending on the detection mechanism, nature of interaction and material properties, the various types of detectors have their own characteristics. These characteristics result in advantages and disadvantages when the detectors are used in field applications 1-4. Table 2.1 shows a comparison of various IR detectors. Progress in IR detector technology is connected with s e m i c o n d u c t o r IR detectors, which are included in the class of photon detectors. In this class of detectors the radiation is absorbed within the material by interaction with electrons either bound to lattice atoms or to impurity atoms or with free electrons. The observed electrical output signal results from the changed electronic energy distribution. The photon detectors show a selective wavelength dependence of response per unit incident radiation power. They exhibit both perfect signal-to-noise performance and a very fast response. But to

Comparison of photon and thermal detectors performance

7

1012

101,

10'~ o

"D 109

108

1

1.5

2

3

4

5 6 7 8 9 10 Wavelength (lum)

15

20

30

40

Figure 2.1 Comparison of the D* of various infrared detectors when operated at tile indicated temperature. Chopping frequency is 1000 Hz.for all detectors except the thermopile (10 Hz), thermocouple (10 Hz), thermistor bolometer (10 Hz), Golay cell (10 Hz) and p!lroelectric detector (10 Hz). Each detector is assumed to view a hemispherical surround at a temperature of 300 K. Theoretical curves for the backgroundlimited D'for ideal photovoltaic and photoconductive detectors and thermal detectors are also shown.

achieve this, the photon detectors require cryogenic cooling. Photon detectors having long-wavelength limits above about 3 pm are generally cooled. This is necessary to prevent the thermal generation of charge carriers. The thermal transitions compete with the optical ones, making non-cooled devices very noisy. Cooling requirements are the main obstacle to the more widespread use of IR systems based on semiconductor photodetectors, making them bulky, heavy, expensive and inconvenient to use. Depending on the nature of the interaction, the class of photon detectors is further sub-divided into different types as shown in Table 2.1. The most important are: intrinsic detectors, extrinsic detectors, photoemissive detectors (PtSi Schottky barriers), and quantum well detectors. Depending on how the electric or magnetic fields are developed, there are various modes such as photoconductive, photovoltaic, photoelectromagnetic (PEM), and photoemissive ones. Each material system can be used for different modes of operation. In this paper we focus on photodiodes. Photodiodes with their very low power dissipation, easy multiplexing on focal plane silicon chip and less stringent noise requirements for the readout devices and circuits, can be assembled in twodimensional (2D) arrays containing a very large number of elements, limited only by existing technologies. Current cooled IR detector systems use material such as HgCdTe, InSb, PtSi, and doped Si. OWIP is a relatively new technology for IR applications. Among these cooled IR detector systems, PtSi FPAs are highly uniform and

3 Table 2.1

2

Comparison o f infrared detectors

0 ?7

Iletector Type

Advantages

Disadvantages

Thermal (thermopile, bolometers, pyroelectric)

Light, rugged, reliable, a n d low cost Room temperature operation

Low detectivity at high frequency Slow response (ms order)

I'hoton

Easier to prepare More stable materials

Very high thermal expansion coefficient Large permittivity

r.,

Easy bandgap tailoring Well developed theory and cxp. Multicolour detectors

Non-uniformity over large area High cost in growth a n d processing Surface instability

*

(;ood material and dopant.s Advanced technology I'ossiblc monolithic inlcgriition

Heteroepitaxy with large I;itticr mis~iiatcli I.ong wavelength cut-offlimited to 7 p m ( a t 77 K )

Very long wavelength operation Kelativcly simple technology

lligh thermal generation 1:xtremely low temperature operation

I,ow-cost, high yields 1.arge a n d close packed 21) arrays

I.ow q u a n t u m el'liciency [,ow temperature operation

'I'ypc I ((;aAs/Al(;aAs. In(;aAs/Al(;aAsJ)

Matured material growth (;ood uniformity over large area Multicolour detectors

High thermal generation Complicated design and growth

'l'y pe I I (InAs/ln(;aSh. InAs/lnAsShJ

1,ow Auger recombination rate Iksy wavelength control

Complicated design and growth Sensitive to the interfaces

Intrinsic

2

IV-VI (I'bs. I'bSc. I'bSn'l'e)

'5 2 2, u -,

-. 9 ? .

Y

5

0 9. s,

Ill-V (In(;:iAs. InAs. IliSb. InAsShJ

1:rec carriers (I'tSi. l't2Si, IrSi) ()uantum wells

- --

~- --

~

-

-~

~

~

~

-~-.-ppp-p

Comparison of photon and thermal detectors performance 9

manufacturable, but have very low quantum efficiency and can only operate in the MWIR range. The InSb FPA technology is mature with very high sensitivity, but it also can operate in the MWIR spectral range. Doped Si has a wide spectral range from 0.8 to 30 pm and it can only operate at very low temperatures. PtSi, InSb, and doped Si detectors do not have wavelength tunability or multicolour capabilities. Both QWIPs and HgCdTe offer high sensitivity with wavelength flexibility in the MWIR, LWIR and VLWIR regions, as well as multicolour capabilities. HgCdTe can also operate in the short wavelength IR (SWIR) region, while QWIP has to go to a direct band-to-band scheme for SWIR operation. The second class of IR detectors is composed of thermal detectors. In a thermal detector the incident radiation is absorbed to change the temperature of the material, and the resultant change in some physical property is used to generate an electrical output. The detector is suspended on lags, which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, thermal effects are generally wavelength independent; the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. This assumes that the mechanism responsible for the absorption of the radiation is itself wavelength independent, which is not strictly true in most instances. Attention is directed toward three approaches which have found the greatest utility in infrared technology; namely, bolometers, pyroelectric and thermoelectric effects. In pyroelectric detectors a change in the internal electrical polarization is measured, whereas in the case of thermistor bolometers a change in the electrical resistance is measured. In contrast to photon detectors, the thermal detectors are typically operated at room temperature. They are usually characterized by modest sensitivity and slow response (because heating and cooling of a detector element is a relatively slow process), but they are cheap and easy to use. They have found widespread use in low cost applications, which do not require high performance and speed. Being unselective, they are frequently used in IR spectrometers. Uncooled FPAs fabricated currently from thermal detectors revolutionize the development of thermal imagers s'6. 2.2.1 Photon detectors

The photodetector is a slab of homogeneous semiconductor with the actual 'electrical' area, A e, that is coupled to a beam of infrared radiation by its optical area, Ao (Figure 2.2). Usually, the optical and electrical areas of the device are the same or close. The use of optical concentrators can increase the Ao/Ae ratio. The current responsivity of the photodetector is determined by the quantum efficiency, r/, and by the photoelectric gain, g. The q u a n t u m efficiency value describes how well the detector is coupled to the radiation to be detected. It is usually defined as the number of electron-hole pairs generated per incident photon. The idea of photoconductive gain, g, was put forth by Rose 7 as a simplifying concept for the understanding of photoconductive phenomena and is now widely used in the field. The photoelectric gain is the number of carriers passing contacts per one generated pair. This value shows how well the

10 Handbook of Infrared Detection Technologies

Figure 2.2 Model of a photodetector. generated electron-hole pairs are used to generate the current response of a photodetector. Both values are assumed here as constant over the volume of the device. The spectral c u r r e n t responsivity is equal to (1)

R i - -~ccq g

where 2 is the wavelength, h is Planck's constant, c is the light velocity, and q is the electron charge. Assuming that the current gains for p h o t o c u r r e n t and noise c u r r e n t are the same, the current noise due to generation and recombination processes is 7 12 - 2(G + R)AetAfq2g 2

(2)

where G and R are the generation and recombination rates, Af is the frequency band and t is the thickness of the detector. Detectivity D* is the main p a r a m e t e r characterizing normalized signal-tonoise performance of detectors and can be defined as D* --

R , ( A o A f ) 1/2 I,

(3)

According to eqs ( 1 )-(3)s D* -

-

2(A~ 1/2 hc Aee ~[2(G+ R)t] -1/2

(4)

Comparison of photon and thermal detectorsperformance 11 For a given wavelength and operating temperature, the highest performance can be obtained by maximizing q/[t(G+R)] 1/2 which corresponds to the condition of the highest ratio of the sheet optical generation to the square root of sheet thermal generation-recombination. This means that high q u a n t u m efficiency must be obtained with a thin device. For a given wavelength and operating temperature, the performance can be optimized by reducing the total number of generation and recombination acts, which is (G + R)(Aet). In further considerations we put that Ao/Ae = 1. Assuming a single pass of the radiation and negligible frontside and backside reflection coefficients, the quantum efficiency and detectivity are r/= 1 - exp(-c~t)

(5)

D* - hcc "~ (1 - e-at) [2(G + R)t] - 1/2

(6)

where a is the absorption coefficient. The highest detectivity can be obtained for t = l . 2 6 / a for which (1-eat)t -1/2 achieves a maximum value of 0.62a 1/2. This thickness is the best compromise between the requirements of high quantum efficiency and low thermal generation. In this optimum case 17= O. 716 and detectivity is equal 8

D* - 0.4S

2

) 1/2 ot C+R

(7)

To achieve a high performance, the thermal generation must be suppressed to possibly the lowest level. This is usually done by cryogenic cooling of the detector. For practical purposes, the ideal situation occurs when the thermal generation is reduced below the optical generation. At equilibrium, the generation and recombination rates are equal, and we have

d. (or) 1/2

D* - 0.31~c ~

(8)

Considerations carried out in ref. 8 indicate that, for a double pass of radiation, the detectivity of an optimized photodetector, leads to the following expression

~ (~)1/2 D* -- 0"31~cck

(9)

where 1 ~ 20 l~m. 3s Detectors based on silicon and germanium have found the widest application as compared with extrinsic photodetectors on other materials. Si has several advantages over Ge: for example, three orders of

Comparison of photon and thermal detectorsperformance 3 3

possible. The quantum efficiency in InSb photodiodes optimized for this temperature range remains unaffected up to 160 K. InSb photovoltaic detectors are widely used for tactical applications, groundbased infrared astronomy and for applications aboard the Space Infrared Telescope Facility. An array size of 1 0 2 4 • is possible (see Table 2.3) because the InSb detector material is thinned to less than 10 ~tm (after surface passivation and hybridization to a readout chip) which allows it to accommodate the InSb/silicon thermal mismatch. 26 Linear array formats of 64, 128 and 256 elements are also produced with frontside-illuminated detectors for both highbackground and astronomy applications. Element sizes depend on device format and range from 2 0 x 2 0-2 O0 x 2 O0 ~m. The cryogenically cooled InSb and HgCdTe arrays have comparable array size and pixel yield at MWIR spectral band. However, w a v e l e n g t h tunability and high quantum efficiency have made HgCdTe the preferred material. 2.4.2 HgCdTe photodiodes

During the past four decades, HgCdTe has become the most important semiconductor for the middle and long wavelength (~.=3-3()~m) IR photodetectors. The short wavelength region has been dominated by III-V compounds (InGaAs, InAsSb, InGaSb). Spectral cut-off can be tailored by adjusting the HgCdTe alloy composition. Epitaxial techniques are the preferred technique to obtain device-quality HgCdTe material for IR devices. Epitaxial growth of the HgCdTe detector array on a Si substrate, rather than CdZnTe, has emerged as a particularly promising approach to scale up wafer dimensions and achieve cost-effective production. 27 MBE offers unique capabilities in material and device engineering including the lowest growth temperature, superlattice growth and potential for the most sophisticated composition and doping profiles. The growth temperature is less than 200~ for MBE but around 350~ for MOCVD, making it more difficult to control the p-type doping in the MOCVD due to the formation of Hg vacancies. Different HgCdTe photodiode architectures have been fabricated that are compatible with backside and frontside illuminated hybrid FPA technology. 3'2s The fabrication of HgCdTe photodiodes was usually based on the most common n+-p and P+-n double layer heterojunction (DLHJ) structure. In these photodiodes the base p-type layers (or n-type layers) are sandwiched between CdZnTe substrate and high-doped (in n+-p structure) or wider-gap (in P+-n structure) regions. Due to backside illumination (through CdZnTe substrate) and internal electric fields (which are 'blocking' for minority carriers), the influence of surface recombinations on the photodiodes performance is eliminated. Both optical and thermal generations are suppressed in the n+-region due to the Burstein-Moss effect and in the P+-region due to wide gap. The influence of surface recombination is also prevented by the use of suitable passivation. Passivation of HgCdTe has been achieved by several techniques and a comprehensive review was given by Nemirovsky and Bahir. 29 Recently,

34


however, most laboratories have been using CdTe or CdZnTe (deposited by MBE, MOCVD, sputtering and e-beam evaporation) for photodiode passivation. 3o For example, the important element of the DLHJ structure is an In-doped (Na.~.lO is cm -3) HgCdTe absorber layer with a wider bandgap cap layer grown by LPE (MBE or MOCVD), arsenic diffusion (or implantation: N . ~ I O is cm-3), a passivation layer, metal contacts to the diode, and indium bumps for mating to the readout integrated circuit multiplexer. It appears that, for the lowest doping levels achievable in a controllable manner in the base regions of photodiodes (N,,= 5 x l 015 cm- 3 for n+-p structure, and Nd=5• 14cm -3 for P+-n structure), the performance of both types of photodiodes is comparable for a given cut-off wavelength and temperature. 31 1081

\

~\

~ k

~

p-on-n HgCdTe N,=5x 10 TM cm -3 t= 10 ~tm

106

A(35K)

--Q

04

9

9

o

_

Q '~o

(40K) (40K)o

10" " ~ 0

2

rr 1 0 -

~o

N(80K/

o"

(40K)e

"~,

o

50K

~176

+ "

9 0

Zx +

~,

~ x

100

60K

(6OK)

,,.

o

9

o

A

7K

x x

+

o (70K) u .~.

(60oK)

1 0 2L-

8

I

12

]

1

16 kc (pm)

~

1

20

I

24

Figure 2.18 Dependence of the RoA product on the long wavelength cut-off for L WIR p-on-n HgCdTe photodiodes at temperatures

. ~ . , .

c~ CD

:/

n"

10 .2 0

, Si:ln 5

,

,

Si:Ga '.~

.i

10

15

20

25

Wavelength (pm)

30

Figure 2.20 Examples of extrinsic silicon detector spectral response. Shown are Si:In, Si:Ga, and Si:As bulk detectors and a Si:As BIB (after ref. 36).

38 Handbook of Infrared Detection Technologies low, moderate, and high IR backgrounds. 37 The 2 5 6 • format with 30~tm pixels and 2 4 0 • format with 50 ~tm pixels are available for low- and high background applications, respectively. Antimony-doped silicon (Si:Sb) arrays and 128 • 128-pixel Si:Sb hybrid FPAs having response to wavelengths > 40 ~m have been also demonstrated, primarily for use at low and moderate backgrounds. Germanium BIB devices have been developed on an experimental basis, but they have not been reported in large 2D array formats yet. 2.4.5 GaAs/AIGaAs QWlPs

QWIP technology is based on the well-developed A3BS material system, which has a large industrial base with a number of military and commercial applications. OWIP cannot compete with the HgCdTe photodiode as a single device, especially at higher temperature operation ( > 70 K) due to fundamental limitations associated with intersub-band transitions.3S However, the advantage of HgCdTe is less distinct in the temperature range below 50 K due to problems involved in a HgCdTe material (p-type doping, Shockley-Read recombination, trap-assisted tunnelling, surface and interface instabilities). Even though OWIP is a photoconductor, several of its properties such as high impedance, fast response time, long integration time, and low power consumption, comply well with the requirements of large FPAs fabrication. Due to the high material quality at low temperature, OWIP has potential advantages over HgCdTe for VLWIR FPA applications in terms of the array size, uniformity, yield and cost of the systems. A key factor in QWIP FPA performance is the light-coupling scheme. A distinct feature of QWlPs is that the optical absorption strength is proportional to an incident photon's electric-field polarisation component normal to the q u a n t u m wells. For imaging, it is necessary to be able to couple light uniformly to 2D arrays of these detectors, so a diffraction grating or other similar structure is typically fabricated on one side of the detectors to redirect a normally incident photon into propagation angles more favourable for absorption. The pixels of 2D arrays are thinned to about 5 ~tm in thickness. The thinning traps diffracted light inside the illuminated pixels, increasing responsivity and eliminating crosstalk.

Table 2.4

Essential properties of LWIR HgCdTe p h o t o d i o d e s and QWIPs at T=77 K

Parameter

HgCdTe

QWlP (n-type)

IR absorption

Normal incidence

Eoptic, d _Lplane of well required

Quantum efficiency Spectral sensitivity Optical gain Thermal generation lifetime RoA product (2c=10 ~tm) Detectivity (2c= 10 lam,FOV=O)

>~70% Wide-band 1 ~ 1 ~s 300 f2cm2 2 • 1012 cmHz12 W-1

Narrow-band (FWHM~ 1+2 lain) 0.4 ( 30-50 wells) 10 ps 104 ~ c m 2 2 x 1()1o cmHz1,,_,W- 1

Normal incidence: no absorption 99.9 2.4

0.04

0.05

2.3

3 1.6x 10 l~ (55K) 30 (45K)

Pixel pitch (~m) Pixel size (lxm) Optical coupling Peak wavelength (lxm) Cutoffwavelength, 50% (lain) Operability (%) Uncorrected nonuniformity (%) Corrected uniformity. 17-27~ Quantum efficiency (%) D*(cmHz1/2W- 1) NEDTwith f/2 optics (mK)

2,,= 15 ~m

0.03 6.4 2.0x l 0 ll (7OK) 23 {70 K)

6.9

2 . 0 x 1011

33 (70 K)

(7OK) 36 (70 K)


temperature dependence. At temperatures < 70K, the signal-to-noise (SNR) ratio of the system is limited by multiplexer readout noise, and shot noise of the photocurrent. At temperatures > 70 K, temporal noise due to the QWIP's higher dark current becomes the limitation. As mentioned earlier, this dark higher current is due to thermionic emission and thus causes the charge storage capacitors of the readout circuitry to saturate. Comparing the values of NEDT parameters for both types of FPA (see also Tables 2.5 and 2.6), we can see that the performance of LWIR HgCdTe arrays is better. The well charge capacity is the m a x i m u m a m o u n t of charge that can be stored on the storage capacitor of each cell. The size of the unit cell is limited to the dimensions of the detector element in the array (of large LWIR HgCdTe hybrid array, a mismatch in the coefficient of thermal expansion between detector array and the readout can force the cell pitch to 20 pm or less to minimize lateral displacement). However, the development of heteroepitaxial growth techniques for HgCdTe on Si has opened up the possibility of cost-effectively producing significant quantities of large-area arrays t h r o u g h utilization of large-diameter Si substrates. For a 3 0 x 3 0 l~m2 pixel size, the storage capacities are limited to 1 to 5 • 107 electrons. For example, for a 5 • 107 electron storage capacity, the total current density of a detector with a 30• 30 lam a pixel size has to be smaller t h a n 2 7 pA/ cm 2 with a 33 ms integration time. 42 If the total current density is in the 1 m A / c m 2 range, the integration time has to be reduced to 1 ms. For the LWIR HgCdTe FPAs the integration time is usually below 1 O0 ps. Since the noise power bandwidth Af= 1/2t~nt, a small integration time causes extra noise in integration. Usually, LWIR OWIP FPAs using conventional ROIC typically operated at 6 0 65 K. Due to a smaller q u a n t u m efficiency of OWIP, filling the charge capacitor is not a problem at high background application. OWIP allows a longer integration Table 2.6

Performance specifications for LWIR HgCdTe FPAs (after SOFRADIR and SBRC

d a t a sheets) Parameter

SOFRADIR

Format Cut on--cut off (gm) FPA temperature (K) Detector pitch (gin) Fill factor (%) Charge handling capacity Frame rate (Hz) D* peak RMS/T~,,t/pitch (average) (cmHzX/aW-1) Pixel NETD (average) NEI (photons/cmas)(max) Typical FOV Fixed pattern noise Crosstalk (optical and electrical) (%) Array operability (%)

128 x 128 7.7-10.3 < 85 50 > 70 > 118 106 eto 300 1.1 • 1 ()11

SBRC 3 2 0 x 2 56 7.7-9.0 < 90 30

256x256 8.5-11.0 77 (upto 100) 30

12 or 36 to 4 0 0

8 x 106 e- (min) to 120

l O m K for 275 Hz

18

f/2 7% RMS 2 99

f/2

1.52x1012

99

48


50

j/

9.5 pm / 9 Std MC techno. /

40-_

/

/

E,E

Improved

/

//

-

I-

pm

MCT techno. 9.5

,,If /

/

. / o 9 pm

m 20Z

10-

~ !

i

75

I

I

80

85

,

~'1

I

I

*

90 95 100 FPA temperature (K)

105

110

115

Figure 2.25 NEDT of 1 2 8 x 128 HgCdTe FPA (f/2 optics. ~0% well Jill. pitch ~0 l~m) as a fimction of operating temperatures (after ref. f~1 ).

time, which gives a relatively lower NEDT. However, at higher temperatures, the dark current of OWIP is high and fills the charge capacitor very quickly. The current subtraction and switched capacitor noise filtering capabilities of ROICs permit low NEDT at higher operating temperatures. In this case however, the readout circuit is complicated which limits the size of array. A goal of third-generation imagers is to achieve sensitivity improvement corresponding to NEDT of about 1 mK. From equation (2 7) we see that in a 300 K scene in the LWIR region with thermal contrast of 0.04, the required charge storage capacity is above 109 electrons. This high charge-storage density within the small pixel dimensions is probably not possible with standard CMOS capacitors. Norton et al. 52 have suggested using of stacked hybrid structures as at least an interim solution to incorporate the desired charge storage density in detector-readout-capacitor structures. Cost

The cost of a FPA depends strongly on the maturity of the technology and varies with production quantity in different companies. So far, large size LWIR FPAs are developed in R&D laboratories without mass production experience. According to Sofradir, HgCdTe experience, by continuous effort in the domain of industrialization decreased the cost of HgCdTe detectors by a factor of 5-10. 53 The cost of making high performance cooled components can be broken down into three parts of about equal weight: the chip (detector and ROIC): the dewar;

Comparison of photon and tllermal detectors performance

49

50

40~" 30-

a

o 20UJ Z

=

10

Estimated Experiment

_

0

50

I

55

1

60 65 70 Temperature (K)

75

80

Figure 2.26 NEDT of 3 2 0 • QWIP FPA ().c=8.9 I~m. f/2 optics. 50% well fill, pitch 30 I~m) as a function of operating temperature (after ref. 4 7).

integration and tests, s4 In addition, the user must add the cryogenic machine cost that is not negligible compared to those of the component. Even if the detection circuit is free of charge, the total cost would only be reduced by about ] 5-20%. This explains why the cost of PtSi and QWIP detectors is not markedly less than that of q u a n t u m detectors of the same complexity, even though the raw materials (Si or GaAs) are m u c h less than for HgCdTe. Moreover, since PtSi requires a very wide optical aperture to obtain acceptable performance, and since OWIP requires lower operating temperatures than other photon detectors, a possible reduction in the purchase price is counterbalanced by a significant increase in operating costs. HgCdTe detectors have been the centre of a major industry for the last three decades. The technology is relatively mature at MWIR but it does not fold over to LWIR. To make components with more pixels requires reducing the pitch or mastering the thinning operation needed to withstand the thermal cycling (differential thermal expansion between CdZnTe and silicon). In the future, a more advantageous approach would seem to be the use of Si substrates, which offer m a n y well-known advantages relative to bulk CdZnTe substrates (much larger available size at lower cost, a thermal expansion m a t c h to Si readout chips, higher purity, and compatibility with automated wafer processing/ handling methodology due to their superior mechanical strength and flatness). Promising results have been achieved in the SWIR and MWIR spectral region. During last four years the defect density for MWIR layers of HgCdTe grown by MBE on silicon substrates has decreased from 2 0 0 0 cm -2 to below 500 cm-2. s2 Currently MWIR arrays with pixel operability of 98% can be produced from this material. For comparison, CdZnTe material typically has operability of 99% or

50 Handbookof Infrared Detection Technologies better. Defect densities for LWIR material grown on silicon substrates continue to limit performance, but they have been reduced by an order of magnitude in the past decade. ~2.~~ In comparison with HgCdTe FPAs, the industrial experience in OWIP FPAs is lower and improvements can be expected because this technology is at a lower stage of development. The major challenge is changing the device and grating designs to improve the device performance. Because of the maturity of the GaAs growth technology and stability of the material system, no investment is needed for developing OWIP substrates, MBE growth, and processing technology. 42 Development of LWIR and multicolour HgCdTe detectors are extremely difficult, especially for low background applications. It means a lower cost in OWIP technology development and production compared with HgCdTe. Reliability In our discussion the reliability issue has been omitted due to the fact that statistical data on this subject is not available. In several applications, especially military systems high reliability is required to ensure both the success of the mission and minimal risk to the user. Two reliability challenges affect both FPAs; survival in high temperature system storage environments and withstanding repetitive thermal cycles between ambient and cryogenic temperatures. In HgCdTe as well as in OWIP FPAs the indium bumps are used to hybridize both types of detectors with a silicon multiplexer. However, certain problems can be expected in the case of QWIP arrays, since the indium bumps have many known alloys with III-V compounds. Very large FPAs may exceed the limits of hybrid reliability engineered into current cooled structures. Hybrids currently use mechanical constraints to force the contraction of the two components to closely match each other. This approach may have limits, when the stress reaches a point where the chip fractures. Three approaches offer an opportunity to resolve this issue:

9

to eliminate the thick substrate which limits the detector active region from deforming at the slower rate of the silicon readout, 9 to subdivide the array into a plurality of regions, 9 to use silicon as the substrate for growth of the detector material.

A technology for making HgCdTe photodiodes and QWIPs on silicon a substrate would be the ultimate, simply because of the vast existing silicon technology. However, major issues with this approach are: ~2 9 9 9

less area would be available for readout circuitry, microlens arrays would be required to regain the fill factor, material quality may not be adequate for low-leakage detectors, particularly for LWIR HgCdTe photodiodes, 9 silicon integrated circuits are processed on (100) oriented silicon, but e.g. the preferred orientation for HgCdTe growth on silicon is near the (211/ orientation.


51

Less demanding approaches to the elimination of the thick detector substrate is the loophole or high density vertically integrated photodetector device structures already practised by GEC Marconi and DRS, respectively. In this approach, HgCdTe material is glued to the readout and contacts are made through the thin layer (1()-20 ~tm) after the substrate is removed (see Figure 2.16(b)). Another approach is to remove the substrate after hybridization with indium bumps. Substrate removal is standard practice with very large hybrid InSb arrays ( 1 0 2 4 • pixels). This approach has been recently adopted for OWIP arrays. 46'47's6 After epoxy backfilling of the gaps between the array and the readout multiplexer, the substrate is thinned to a very thin membrane ( ~ 1 0 0 0 A). This not only eliminates the thermal mismatch problem between the silicon readout and the GaAs based detector array, but also completely eliminates pixel-to-pixel crosstalk, and finally, significantly enhances an optical coupling of IR radiation into OWIP pixels.

Summary

LWIR QWIP cannot compete with the HgCdTe photodiode as a single device, especially at temperatures above 70 K due to fundamental limitations associated with intersub-band transitions. However, the advantage of HgCdTe is less distinct at temperatures below 50 K. Comparing photovo|taic HgCdTe and OWIP technologies, we arrive at the following conclusions"

9 two major issues that impede the performance of OWIPs should be overcome: optical conversion efficiency and dark current, 9 for HgCdTe, improvement of the array uniformity is necessary, 9 QWIP has more potential to realize VLWIR FPA operation (also with multicolour detection). The main drawbacks of LWIR QWIP FPA technology are the performance limitation for low integration time applications and the low operating temperature. Their main advantages are linked to performance uniformity and to availability of large size arrays. Next, the main drawback of LWIR HgCdTe FPA technology is the unavailability of large size arrays necessary for TV and larger formats. Several properties of OWIP such as high impedance, fast response time, long integration time, and low power consumption, comply well with requirements for fabrication of large FPAs. Due to the high material quality at low temperature, OWIP has potential advantages over HgCdTe for VLWIR FPA applications in terms of the array size, uniformity, yield and cost of the system. Three-band and four-band FPAs will be demonstrated in the near future. State of the art QWIP and HgCdTe FPAs provide similar performance figures of merit, because they are predominantly limited by the readout circuits. The performance is, however, achieved with very different integration times. The very short integration time of LWIR HgCdTe devices of typically below 300 l~s is very useful in order to freeze a scene with rapidly moving objects. OWIP

52

Handbookof Infrared Detection Technolo#ies

devices achieve, due to excellent homogeneity, an even better NEDT, however, the integration time must be 1()-1 ()() times longer for that, and typically 5-20 ms. Choice of the best technology is therefore driven by the specific needs of a system. Observation of the global market over the past several years has indicated that even HgCdTe photodiodes intrinsically exhibit higher performance than the OWIP detectors, the market tendencies for the future are: 9 HgCdTe for small formats (e.g. 128 x128), small pitch, high flame rates and low integration times, 9 OWIP for large formats (e.g. 64()• and larger), low flame rates and large integration time. Despite serious competition from alternative technologies and slower progress than expected, HgCdTe is unlikely to be seriously challenged for highperformance applications and applications requiring multispectral capability and fast response.

2.5 Dual-band FPAs Multicolour capabilities are highly desirable for advanced IR systems. Systems that gather data in separate IR spectral bands can discriminate both absolute temperature and unique signatures of objects in the scene. By providing this new dimension of contrast, multiband detection also enables advanced colour processing algorithms to further improve sensitivity above that of single-colour devices. Multispectral detection permits rapid and efficient understanding of the scene in a variety of ways. In particular, two-colour IR FPAs can be especially beneficial for threat-warning applications. By using two IR wavebands, spurious information, such as background clutter and sunglint, may be subtracted from an IR image, leaving only the objects of interest. Multispectral IR FPAs can also play many important roles in Earth and planetary remote sensing, astronomy, etc. Thus, the effective signal-to-noise ratio of two-colour IR FPAs greatly exceeds that of single-colour IR FPAs for specific applications. Currently, multispectral systems rely on cumbersome imaging techniques that either disperse the optical signal across multiple IR FPAs or use a filter wheel to discriminate spectrally the image focused on single FPA. These systems contain beam-splitters, lenses, and bandpass filters in the optical path in order to focus the images onto separate FPAs, responding to different IR bands. Also, complex alignment is required to map the multispectral image pixel for pixel. Consequently, these approaches are expensive in terms of size, complexity, and cooling requirements. At present, considerable efforts are directed to fabricate a single FPA with multicolour capability to eliminate the spatial alignment and temporal registration problems that exist whenever separate arrays are used, to simplify optical design, and reduce size. weight, and power consumption. Considerable progress has been recently demonstrated by research groups at Hughes Research


53

Laboratory (Raytheon), 57'58 Lockheed Martin (BAE Systems), s~ DRS Infrared Technology 60 AIM,61 Rockwell ~2 and Leti ~ 3 in multispectral HgCdTe detectors employing mainly MBE (although LPE and MOCVD are also used) for the growth of a variety of devices. Also QWIP's technology demonstrates considerable progress in fabrication of multicolour FPAs. 4 7. s~.-36.64-66 It is, perhaps, the niche in which OWIPs have an intrinsic advantage due to relative ease of growing multi-band structures by MBE with very low defect density. Devices for the sequential and simultaneous detection of two closely spaced sub-bands in the MWIR and LWIR radiation have been demonstrated. 2.5.1 Dual-band HgCdTe In the back-illuminated dual-band detectors, the photodiode with longer cutof wavelength is grown epitaxially on top of the photodiode with the short cut-off wavelength. The shorter cut-off photodiode acts as a long-wavelength pass filter for the longer cut-off photodiode. Both sequential mode and simultaneous mode detectors are fabricated from multi-layer materials. The simplest two-colour HgCdTe detector, and the first to be demonstrated, is the bias-selectable n - p - n back-to-back photodiode shown in Figure 2.2 7(a). The sequential-mode detector has a single indium bump per unit cell that permits sequential bias-selectivity of the spectral bands associated with operating tandem photodiodes. When the polarity of the bias voltage applied to the bump contact is positive, the top (LW) photodiode is reverse biased and the bottom (SW) photodiode is forward biased. The SW photocurrent is shunted by the low impedance of the forward-biased SW photodiode, and the only photocurrent to emerge in the external circuit is the LW photocurrent. When the bias voltage polarity is reversed, the situation reverses: only SW photocurrent is available. Switching times within the detector can be relatively short, on the order of microseconds, so detection of slowly changing targets or imagers can be done by switching rapidly between the MW and LW modes. One bump contact per unit cell. as for single-colour hybrid FPAs, is the big advantage of the bias-selectable detector. It is compatible with existing silicon readout chips. The problems with the bias selectable device are that its construction does not allow independent selection of the optimum bias voltage for each photodiode, and there can be substantial MW crosstalk in the LW detector. Many applications require true simultaneity of detection in the two spectral bands. This has been achieved in a number of ingenious architectures described by Reine. 67 All these simultaneous dual-band detector architectures require an additional electrical contact to an underlying layer in the multijunction structure of both the SW and LW photodiode. The most important distinction is the requirement of a second readout circuit in each unit cell. Integrated two-colour HgCdTe technology has been developed for nearly a decade with a steady progression having a wide variety of pixel size ( 30-6113m), array formats ( 6 4 • up to 32()• and spectral-band sensitivity (MWIR/ MWIR, MWIR/LWIR and LWIR/LWIR). Figure 2.2 8 shows examples of spectral response from different two-colour devices. Note that there is a minimal crosstalk


Figure 2.27 Cross-section views of unit cells for various l~ack-illuminated dual-band HgCdTe detector approaches: (a) bias-selectable n-p-n structure reported b!t Ra!ltheon: ~7 (b) simultaneous n-p-n design reported b!t Raytheon: ~ (c) simultaneous l ~ n - n - p reported b!t BAE S!lstems: ~9 (d) simultaneous n - l ~ p - p n design reported by Leti: 6 ~ and (e) simultaneous structure based on p-on-n junctions reported b!l Rockwell 62 (after ref. 6 7).

between the bands, since the short wavelength band absorbs nearly 1 ()()% of the shorter wavelengths. Test structure indicates that the separate photodiodes in a two-colour detector perform exactly as single-colour detectors in terms of achievable R o A product variation with wavelength at a given temperature. 2.5.2 Dual-band QWIPs

Sanders was first to fabricate two-colour. 2 56 • 2 56 bound-to-miniband OWIP FPAs in each of four important combinations: LWIR/LWIR. MWlR/LWlR, near IR (NIR)/LWIR and MWIR/MWIR-with simultaneous integration, s(~'~s


55

A device capable of simultaneously detecting two separate wavelengths can be fabricated by vertical stacking of the different OWlP layers during epitaxial growth. Separate bias voltages can be applied to each OWIP simultaneously via the doped contact layers that separate the MOW detector heterostructures. Figure 2.29 shows schematically the structure of a two-colour stacked OWIP with contacts to all three ohmic-contact layers. The device epilayers were grown by MBE on a three-inch semi-insulating GaAs substrate. An undoped GaAs layer, called an isolator, was grown between two A1GaAs stop layers, followed by Au/ Ge ohmic contact of a 0.5 ~m thick doped GaAs layer. Next, the two OWIP heterostructures were grown, separated by another ohmic contact. The long wavelength sensitive stack (red OWIP, ;.,= 11.2 pm) is grown above the shorter wavelength sensitive stack (blue QWIP. ,;.,=8.6 lam). Each OWIP is a 20-period GaAs/AlxGal_xAs MQW stack, in which the thickness of the Si-doped GaAs OWs (with typical electron concentration 5 • 1 ()17 cm-~) and the A1 composition of the undoped AlxGal_• barriers (~ 5 5()-6()() A) is adjusted to yield the desired peak position and spectral width. The gaps between FPA detectors and the readout multiplexer were backfilled with epoxy. The epoxy backfilling provides 1.0 0.8 c 0

Q.

4.5 #m utoff

cutoff

14.9 lam\ ~-om0.8-cutoff. l / 0

0.6

Q.

~ 9 0.4

0.2 0.02

0.6-

> 9 0.4-

~ !

m

n"

cutoff

. . . .

3

J

n, 0.2 4

. . . .

Wavelength (pm)

0.0 ' ~ ' ................ 2 3 4 5 6 7 8 910 11121314

5

Wavelength (gm)

1.0-

o t) c 0

0.8

9

~11.4

pm

0.6

=> 9 0.4 rr 0.2

j

0.0 . . . . . . . . . . . . . . . . .

~

...................

2 3 4 5 6 7 8 910 11121314

Wavelength (gm)

Figure 2.28 Spectral response curves for two-colour HgCdTe detectors in various dual-band combinations of MWIR and L WIR spectral bands (after ref. 22 ).

56 Handbook of Infrared Detection Technologies the necessary mechanical strength to the detector array and readout hybrid, prior to the array's thinning process. Most OWIP arrays use 2D grating, which is very wavelength dependent so that the efficiency gets lower when the pixel size gets smaller. Lockheed Martin has used rectangular and rotated rectangular 2D gratings for their two-colour LW/ LW FPAs. Although random reflectors have achieved relatively high quantum efficiencies with a large test device structure, it is not possible to achieve the similar high quantum efficiencies with random reflectors on small FPA pixels due to the reduced width-to-height aspect relations. 47 In addition, it is difficult to fabricate random reflectors for shorter wavelength detectors relative to long wavelength detectors due to the fact that feature sizes of random reflectors are linearly proportional to the peak wavelength of the detectors. The quantum efficiency becomes a more difficult issue for OWIP multicolour FPA than for single colour. The typical operating temperature for QWIP detectors is in the region of 4 0 80K. The bias across each QWIP can be adjusted separately, although it is desirable to apply the same bias to both colours. As shown in Figure 2.30, the responsivity of both OWIPs is around 3()()-35() mA/W. It appears that the complex two-colour processing has not compromised the electrical and optical quality of either colour in the two-colour device since the peak quantum efficiency for each of the 2()-period OWIPs was estimated to be ~1()% in comparison with a normal single-colour OWIP with twice the number of periods which has a quantum efficiency of around 2 ( ) % . A pixel operability for each colour is > 9 7% in comparison with the value of > 99.9% routinely achieved for single-colour OWIPs. The NEDT value was 24 mK for the blue OWIP and 35 mK for the red OWIP. The difference was assigned to the poor transmission properties of the optics in the 11.2 Bm band. An accurate methodology is needed to design

Figure 2.29 Structure o.f two-colour stacked (_)iVIP ( ~(ft('r r(:l: 6 ~ ).


57

the detector structure properly to meet different requirements. In the production process, the fabrication of gratings is still quite involved, and its efficiency is rather uncertain in small pixels and in pixels with thick material layers. To cover the MWIR range a strained layer InGaAs/A1GaAs material system is used. InGaAs in the MWIR stack produces a high in-plane compressive strain which enhances the responsivity. 6~.6~ The MWIR/LWIR FPAs fabricated by Sanders consist of an 8.6 ~tm GaAs/A1GaAs OWIP on top of the 4.7 l.tm strained InGaAs/GaAs/A1GaAs heterostructure. Recently, Gunapala et al. 69"7() have demonstrated the first 8 - 9 and 1 4 - 1 5 l.tm two-colour imaging camera based on a 6 4 0 x 486 dual-band OWIP FPA, which can be processed with dual or triple contacts to access the CMOS readout multiplexer. A single indium bump per pixel is usable only in the case of an interlace readout scheme (i.e., odd rows for one colour and even rows for the other colour) which uses an existing single colour CMOS readout multiplexer. However, the disadvantage is that it does not provide a full fill factor for both wavelength bands. The 6 4 0 • GaAs/A1GaAs gave excellent images with 99.7% of the LWIR pixels and 98% of VLWIR pixels working, demonstrating the high yield of GaAs technology. The estimated NEDT of LWIR and VLWIR detectors at 40 K are 36 and 44 mK, respectively. Due to BLIP, the estimated and experimentally obtained NEDT values of the LWIR detectors do not change significantly at temperatures below 65 K. The experimentally measured values of LWIR NEDT equal to 29 mK are lower than the estimated ones. This improvement is attributed to the 2D periodic grating light coupling efficiency. However, the experimental VLWIR NEDT value is higher t h a n the estimated value. It is probably a result of the inefficient light coupling in the 1 4 - 1 5 Bm region, readout multiplexer noise, and

Figure 2.30 Typical responsivity spectra at 40 K and a comnlon bias of 1.5 V. recorded simultaneously for two QWIPs in the same pixel (after ref. 6:5).

58 Handbookof Infrared Detection Technologies the noise of the proximity electronics. At 40 K the performance of both bands' detector pixels are limited by photocurrent noise and readout noise.

2.6 FPAs of thermal detectors IR semiconductor imagers use cryogenic or thermoelectric coolers, complex IR optics, and expensive sensor materials. Typical costs of cryogenically cooled imagers of around $ 5 0 0 0 0 restrict their installation to critical military applications which allow conduction of operations in complete darkness. Very encouraging results have been obtained with micromachined silicon bolometer arrays s'6'72'73 and pyroelectric detector arrays. 6'74-76 Several countries have demonstrated imagers with NEDT below 1 ()() mK, and the cost of simple systems is sometimes below $10 000. It is expected that high-performance imager system costs will be reduced to less than $1()()(), 7~ and above IR cameras will become widely available in the near future. Although developed for military applications, low-cost IR imagers are used in nonmilitary applications such as: drivers aid, aircraft aid, industrial process monitoring, community services, firefighting, portable mine detection, night vision, border surveillance, law enforcement, search and rescue, etc. 2.6.7 Micromachined silicon bolometers

The most popular thermistor material used in fabrication of the micromachined silicon bolometers is vanadium dioxide, V02. From the point of view of IR imaging application, probably the most important property of V02 is its high negative temperature coefficient of resistance (TCR) at ambient temperature, which exceeds 4% per degree for a single element bolometer and about 2% for FPA. There are two reasons for not using VOx (.x"> 2) with substantially higher temperature coefficient: first, reproducibility of properties suffers in the higher x-value films, second, heating becomes a problem with high resisitivity films. The final microbolometer pixel structure is shown in Figure 2.31. The microbolometer consists of a 0.5 l~m thick bridge of Si 3N4 suspended about 2 l~m above the underlying silicon substrate. The use of a vacuum gap of approximately 2.5 ~tm, together with a quarter wave resonant cavity between the bolometer and the underlying substrate, can produce a reflector for wavelengths near 10 ~tm. The bridge is supported by two narrow legs of Si 3N4. The Si3N4 legs provide the thermal isolation between the microbolometer and the heat-sink readout substrate and support conductive films for electrical connection. A bipolar input amplifier is normally required, and this can be obtained with biCMOS processing technology. Encapsulated in the centre of the Si3N4 bridge is a thin layer (500 A) of polycrystalline VOx. Honeywell has licensed this technology to several companies for the development and production of uncooled FPAs for commercial and military systems. At present, the compact 3 2 0 • microbolometer cameras are produced by Raytheon, Boeing, and Lockheed-Martin in the United States. The


U.S. government allowed these manufactures to sell their devices to foreign countries, but not to divulge manufacturing technologies. In recent years, several countries, including the United Kingdom, Japan. Korea, and France have picked up the ball, determined to develop their own uncooled imaging systems. As a result, although the U.S. has a significant lead, some of the most exciting and promising developments for low-cost uncooled IR systems may come from non-U.S, companies, e.g., microbolometer FPAs with series p-n junction elaborated by Mitsubishi Electric. 77 This approach is unique, based on an allsilicon version of the microbolometer. The 2 4 0 • arrays of 501urn microbolometers are fabricated on industrystandard wafer (4 inch diameter)complete with monolithic readout circuits integrated into underlying silicon. Radford et al. 7~ have reported a 2 4 ( ) x 3 2 0 pixel array with 50 lam square vanadium oxide pixels and thermal time constant of about 40 ms, for which the average NETD (f/1 optics) was 8.6 mK. However, there is a strong system need to reduce the pixel size to achieve several potential benefits. The detection range of m a n y uncooled IR imaging systems is limited by pixel resolution, rather than sensitivity. Therefore, there is a strong system need to reduce the size of the pixels. Because the cost of the optics made of Ge, the standard material, depends approximately upon the square of the diameter, so reducing the pixel size reduces the cost of the optics. These reductions in optics size would have a major benefit in reducing the overall size, weight and cost of manportable IR systems. In addition the reduction in pixel size allows a significantly larger number of FPAs to be fabricated on each wafer. However, the NEDT is inversely proportional to the pixel area, thus, if the pixel size is reduced from 50 • 50 ~tm to 2 5 • 2 5 btm, and everything else remained the same, the NEDT would increase by a factor of four. Improvements in the readout electronics are needed to compensate for this. For future arrays, the f/1 NEDT performance of 25 l.tm pitch microbolometer FPAs is projected to be below 20 mK (see Figure 2.32).79'~() The development of highly sensitive 2 5 ~m microbolometer pixels, however, presents significant challenges in both fabrication process improvements and in pixel design. Microbolometer pixels

Figure 2.31 Bridge structure of Hone!twell microbolometer (after ref. 72).

60 Handbook of Infrared Detection Technologies fabricated with conventional single-level micromachining processes suffer severe performance degradation as the unit cell is reduced below 40 ~m. This problem can be mitigated to some degree if the microbolometer process capability (design rules) is improved dramatically. Table 2.7 summarizes the design and performance parameters for Raytheon's VOx microbolometers. A similar performance has been described by Altman and colleagues at BAE Systems (Lockheed Martin); 82.83 they reported a 6 4 ( ) x 4 8 0 FPA with 2 8 x 2 8 l,tm 2 pixels with NETD (f/1 optics) of below 55 mK. At present, several research programmes are focused towards enhancement of performance level in excess of 109 cmHz 1/2 W -1. It is anticipated that new materials will form the basis of the next generation of semiconductor film bolometers. The most promising material appears to be amorphous silicon (a-Si). The temperature coefficient of resistance values for a-Si, range from - 0 . 0 2 5~ -~ for doped, low resistivity films at room temperature to - 0 . 0 8 ~ - 1 for high resistivity materials. 6 So, although high TC1R values are attainable, they are accompanied by a high level of 1/f noise, s4 Properties of the films depend upon the method of preparation and the type of dopant. Amorphous hydrogenated silicon (a-Si:H) has a metastable state caused by defects arising from prolonged illumination (Staebler and Wronski effect). This is an undesirable feature that requires a specific annealing cycle during preparation (the methodology for reliability enhancement is described in ref. 8 5). If not removed, it adversely affects longterm reliability. Nevertheless, progress in the development of a-Si:H uncooled FPAs has been reported. 320 x 240 arrays with 4 5 lam pitch have been developed with an average NETD of 70 mK (for f/1 aperture and 50Hz imagery

Figure 2.,32 VOw.FPA development (after ref. 79).

Comparison of photon and thermal detectors performance 61 Table 2.7 Performance characteristics of Raytheon's VO~ microbolometers (after refs. 80 and 81)

Performance parameter

Capability (f/1 and 300 K scene)

Array configuration Pixel size (~m 2) Spectral response (~m) Signal responsivity (V/W)

320 x 240 5{}x50 8-14 > 2.5 x 1()' V/W or 50 mK/K......... < 20 < 150 p-p 1.() rms > 40

320 x 240 25x25 8--14 > 2.5xlO'V/W or 20 mK/K...... < 50 < 15() p-p 1.() rms > 1{}{}

or 25 mK/K......

>98 2{}{} 25

>98 15{} 25

>98 390 25

NEDT ~, f/1 {mK) Offset nonuniformity (mV) Output noise (mV) Intrascene dynamic range (~ f/1 (K) Pixel operability (%) Power dissipation (mW) Nominal operating temperature (~

640 x 480 25x25 8--14 > 2.SxlOTV/W

< 15() p-p 0.6 rms > 1{}{}

f r e q u e n c y ) . 85 Using a n i m p r o v e d t e c h n o l o g i c a l stack, fully c o m p a t i b l e w i t h i n d u s t r i a l process, good q u a l i t y 3 2 0 • FPA w i t h 35 ~tm pitch a n d f/1 optics h a s b e e n p r e s e n t e d w i t h NEDT close to 3 5 inK. 86 Because the typical resisitivity of a m o r p h o u s silicon films is several orders of m a g n i t u d e h i g h e r t h a n t h a t of VO• a-Si finds a p p l i c a t i o n in u n c o o l e d a r r a y s in w h i c h bias is c o n t i n u o u s r a t h e r t h a n pulsed w i t h o u t excessive J o u l e a n h e a t i n g of the FPA. ~'

2.6.2 Pyroelectric arrays The i m a g i n g s y s t e m s based on p y r o e l e c t r i c a r r a y s , u s u a l l y n e e d to be o p e r a t e d w i t h optical m o d u l a t o r s , w h i c h c h o p or defocus the i n c o m i n g r a d i a t i o n . This m a y be a n i m p o r t a n t l i m i t a t i o n for m a n y a p p l i c a t i o n s in w h i c h c h o p p e r l e s s o p e r a t i o n is h i g h l y desirable (e.g., guided m u n i t i o n s ) . Most of t h e p y r o e l e c t r i c m a t e r i a l s c o n s i d e r e d for t h e r m a l d e t e c t o r a r r a y s are the l e a d - b a s e d ' p e r o v s k i t e ' oxides s u c h as lead t i t a n a t e [PbTiO3-PT]. These m a t e r i a l s h a v e s t r u c t u r a l similarities w i t h the m i n e r a l p e r o v s k i t e (CaTiO3 }. The basic f o r m u l a is ABO3: w h e r e A is lead, O is o x y g e n a n d B m a y be one, or a m i x t u r e , of c a t i o n s e.g. lead z i r c o n a t e t i t a n a t e [Pb(ZrTi)O~-PZT], b a r i u m s t r o n t i u m t i t a n a t e [BaSrTiO3-BST], lead s c a n d i u m t a n t a l a t e [Pb{Sccj.~Ta{}.~)O3PST] a n d lead m a g n e s i u m n i o b a t e [Pb(Mgl/~Nb2/~)O~-PMN]. Often d o p a n t s are a d d e d to t h e s e basic f o r m u l a t i o n s to e n h a n c e or t u n e the m a t e r i a l properties. A b o v e Curie t e m p e r a t u r e , To, t h e s e m a t e r i a l s form a s y m m e t r i c n o n - p o l a r , cubic s t r u c t u r e . On cooling t h e y u n d e r g o a s t r u c t u r a l p h a s e t r a n s i t i o n to form a polar, ferroelectric phase. The a b o v e m a t e r i a l s c a n be f u r t h e r sub-divided into t w o g r o u p s . The ' c o n v e n t i o n a l ' p y r o e l e c t r i c m a t e r i a l s , s u c h as PT a n d PZT, o p e r a t e at r o o m t e m p e r a t u r e well below their Curie t e m p e r a t u r e w i t h o u t the need for an applied field. R e q u i r e m e n t s for d e t e c t o r t e m p e r a t u r e stabilization is m i n i m a l or c a n be

62 Handbookof Infrared Detection Technologies eliminated since there is little variation in detector performance over quite a large temperature range. It is, however, possible to operate ferroelectrics at or above To, with an applied bias field, in the mode of a 'dielectric bolometer'. This second group of materials (including BST, PST and PMN) have Tc slightly below the detector operating temperature, resulting in minimal pyroelectricity. In this case a constant dc field is applied during operation to induce a polarization and hence regain pyroelectric properties. Barium strontium titanate (BST) ceramic is a relatively well behaved material with a very high permittivity. Texas Instruments (Raython) has improved the performance of pyroelectric FPAs using a bias voltage applied to maintain and optimize the pyroelectric effect near the phase transition. ~7 Figure 2.33 shows details of the completed pyroelectric detector device structure. The 3 2 ( ) x 2 4 0 hybrid arrays with pixels 48.5 IJm are characterized by NEDT as low as 40 mK (array average) and the production average is between 70 mK and 8() mK. A demonstrated sustained production rate in excess of 5()() units per month is a small fraction of factory capacity. For the United Kingdom array programme lead scandium tantalate (PST) material has been chosen, s~ A hybrid pyroelectric/ferroelectric bolometer detector was the first to enter production, and is the most widely used type of thermal detector (in the U.S., the Cadillac Division of General Motors has pioneered this application, selling thermal imagers to the customer for just under $2()()()). 6 Although many applications for this hybrid array technology have been identified, and imagers employing these arrays are in mass production, no hybrid technological advances are foreseen. The reason is that the thermal conductance of the bump bonds is so high that the array NETD (f/1 optics) is limited to about 50 mK. The best NEDT achieved with a hybrid array is about 38 mK, which is consistent with thermal conductance of approximately 4 IJW/K. Pyroelectric array technology, therefore, is moving toward monolithic silicon microstructure technology. The monolithic process should have fewer steps and a shorter cycle time. Most ferroelectrics tend to lose their interesting properties as the thickness is reduced. However, some ferroelectric materials seem to maintain their properties better than others. This seems particularly true for PT and related materials, whereas for BST, the material does not hold its properties well in thin-film form. Thin-film ferroelectric (TFFE) detectors have the performance potential of microbolometers with minimum NEDT of about 1 mK. ~ Figure 2.34 shows NEDT calculated as a function of pixel pitch assuming a constant 30 Hz signal and a thermal time constant of about 15 ms. Reducing the pixel pitch from 50 IJm to 2 5 IJm requires a concomitant increase in thermal isolation by about a factor of four. This seriously impacts the possibility of high performance arrays with small pixels. The TFFE device approach appears remarkably similar to the VOx microbolometer structure developed by Honeywell. However, there are several key futures that distinguish it from that technology. 'ss-~ Since the device is a capacitor rather than a resistor as in a bolometer, the electrodes are located above and below the face of the pixel, are transparent and do not obscure the


63

Figure 2. ~ ~ BST dieh'ctric l~olometer pixel ( ~(fter ref. 8 7).

active optical area. Usually, the electrical resistance of the leads can be quite large without degrading the signal-to-noise ratio, since the detector capacitance is approximately 3 pF. This enables the use of thin, poorly conducting electrode materials to minimize thermal conductance. A key feature of the design is that the ferroelectric film is self-supporting" there is no underlying m e m b r a n e necessary to provide mechanical support. In such a way, with the use of transparent oxide electrodes, the ferroelectric material can dominate thermal conductance. It is well known that absorption of IR radiation is accomplished by means of a resonant optical cavity. In a monolithic bridge structure, the cavity is located within the ferroelectric itself or in the space between ferroelectric and the ROIC. This can be realized in two ways. s9 9

9

The bottom electrode must be highly reflective, the top electrode must be semi-transparent, and the ferrolectric must be approximately 1 btm thick for optimal tuning of the cavity for 1 ()-12 btm radiation. Both electrodes must be s e m i - t r a n s p a r e n t , a reflective mirror must be present on the ROIC under each pixel, and the pixel must be located approximately 2 l.tm above the ROIC.

A key factor to performance of the ceramic thin films is the high temperature processing required in achieving the correct ferroelectric crystal phase. The TFFEs of interest are refractory, and require annealing at elevated temperatures to crystallize and develop good pyroelectric properties. Thermal treatments at temperatures that exceed about 45()~ may lead to an adverse interaction between the silicon and a l u m i n i u m interconnections. Various techniques for the deposition of thin ferroelectric films have been investigated including spin-on metal-organic decomposition, radio frequency m a g n e t r o n sputtering, dual ion

64


102 ,. ,m9 v/'ek 101 v

E

Is UJ Z

10~

,

I I

101 100

10 ~

102 103 Thermal conductance (nW/K)

10 ~

Figure 2.34 The relationship between thermal isolation and performance of uncooh'd IR detectors in the thermal fluctuation noise limit (~ffter r~:L ,~9/.

beam sputtering, sol-gel processing, and laser ablation. Also a number of surface rapid thermal annealing techniques have been investigated to obtain optimum material response whilst leaving the underlying silicon substrate undamaged. ~{I Monolithic arrays produced to date have demonstrated poor sensitivity. Raytheon's group has demonstrated 32()x 24() array with an NEDT of about 400 mK with operability in excess of 9 5 %. Research group from DERA (UK) has developed 'integrated' and 'composite' detector technology. ~'9~1 In the first technology, the detector material was deposited as a thin film onto free standing micro-bridge structure defined on the surface of the silicon ROIC. The 'composite' technology combines elements of hybrid and integrated technologies (see Figure 2.35). Microbridge pixels are fabricated in a similar fashion to the integrated technology and are next formed onto a high density interconnected silicon wafer. The interconnect wafer uses materials that can withstand the intermediate high temperature processing stage during fabrication of thin ferroelectric films and contains a narrow conducting channel via for every pixel, permitting electrical connection to the underside. Finally, the detector wafer is solder-bump bonded to the ROIC as per the established hybrid array process. It is predicted that, using a PST film, an NEDT of 25 mK (50 Hz image rate and f/1 optics)can be achieved. 2.6.3 Thermoelectric arrays

Thermopile detectors, 91 while only of limited use for imaging applications, have a combination of characteristics that make them well suited for some low power applications. They are highly linear, require no optical chopper, and have D* values comparable to resistive bolometers and pyroelectric detectors. They


operate over a broad temperature range with little or no temperature stabilization. They have no electrical bias, leading to negligible 1/f noise and no voltage pedestal in their output signal. However, much less effort has been made in their development. The reason is that their responsivity and noise are orders of magnitude less and thus their applications in thermal imaging systems require very low-noise electronics to realize their potential performance. Thermoelectric detectors found almost no use as matrix arrays in TV flame rate imagers. Instead, they are employed as linear arrays that are mechanically scanned to form an image of stationary or nearly stationary objects. The wide operating-temperature range, lack of temperature stabilization, and radiometric accuracy make thermopiles well situated for same space-based scientific imaging applications. Two-dimensional thermopile arrays have been reported only by two groups. 92'93 In both cases, a desire for low cost and manufacturability led to the use of polysilicon thermoelectric materials, which have relatively low thermoelectric figures of merit. Fote and co-workers ~4'9s have improved the performance of thermopile linear arrays by combining Bi-Te and Bi-Sb-Te thermoelectric materials. Compared with most other thermoelelectic arrays, their D* values are highest, as is shown in Figure 2.3 6. This technology has been developed to improve the performance of 2D arrays using a three-level structure with two sacrificial layers. In such way it is possible to improve fill factor and incorporate a large n u m b e r of thermocouples per pixel. Figure 2.37 shows the thermopile detector structure. The structure allows almost 100% fill factor and the model suggests that optimized detectors will have D* values over 109 c m H z l / 2 / W . Further efforts are continued to fabricate high performance 128 • 128 FPAs. 96 2.6.4 Status a n d trends of u n c o o l e d arrays

The information gathered here follows Kruse. ~ Table 2.8 illustrates the status and trends of uncooled arrays for military and commercial applications. As was

Figzlre 2.35 Schematic cross-section of the "composite" detector arra!l design (after reJ. 90).

66


previously mentioned, microbolometer arrays containing 64()• pixels are under development. The pixel size is being reduced to 2"3 • 5 ~tm (in order to reduce the optics cost): 35 • 3 "3pm is an intermediate objective. The NEDT goal is 10 mK for high performance applications, instead low cost performance applications (e.g. security sensors) use 16()• 120 pixels, 5()• 50 pm, and NEDT of 100 mK. The commercial systems (microbolometer imagers and radiometers and ferroelectric imagers) derive from military systems that are too costly for widespread use. Imaging radiometers employ linear thermoelectric arrays operating in the snapshot mode: they are less costly than the TV-rate imaging radiometers employing microbolometer arrays. ~7~'~ In the large volume production, the cost of commercial systems will inevitably decrease (see Table 2.8). It seems likely that microbolometer FPAs will become dominant in uncooled detector technology in the near future. They do not require a radiation chopper and have very low thermal conductance which results in a high responsivity and a low NEDT. Moreover, changes in the shape of the lags of the monolithic construction allow trade-off between speed of response and NEDT. It is predicted that the performance of monolithic ferroelectric bolometers can be considerably improved, unlike hybrid pyroelectric detectors where only little development effort is under way. The hybrid pyroelelctric FPAs are not commercially available.

10 ~~

......."

9 Bi-Te/Bi-Sb-Te (this work) 9 Constantan/chromel 9 Silicon

~ .109

-

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0 . 0 "

-

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R e s p o n s e time (ms) Figure 2.36 Representative data from literature showing reported I)* values as a fiinction of response timeJor thin film thermopile linear arrays. The dashed line represents the Fote and ]ones results. Its slope indicates D* proportional to the square-root of response time. which is typical for thermopiles or bolometers with different geometries and the same material system (after ref. 9 ~ ).


67

Figure 2.37 Schematic diagram of thermopile detector structure. The top diagram shows two pixels viewed from the top, with part of the left pixel cut awa!l to show the underl!tinq structure. The lower diagram shows a cross-section side view of two pixels (after ref. 9 6 ).

2.7 Conclusions The intention of this paper has been to provide investigations of the performance of both photon and thermal detectors, with emphasis on the material properties, device structure, and their impact on FPA performance, especially in the LWIR and VLWIR spectral regions. At present, HgCdTe is a widely used variable gap semiconductor and has a privileged position both in LWIR as well as VLWIR spectral ranges. Figure 2.38 shows a plot of the thermal detectivity (3()()K, ()~ FOV) versus operating temperature for the most prominent detector technologies. The thermal detectivity is used here to compare the various technologies for equivalent NETD irrespective of wavelength. The thermal D* figure of merit for photon detectors was obtained by equating the NETD of an ideal thermal detector for a given D* to the NETD of an ideal photon detector with a given D~ . The various regions P show the appropriate applications including 'low cost' uncooled therma 1 detectors, 'high performance uncooled' for night vision e n h a n c e m e n t and earth reconnaissance, 'tactical' for most imaging uses, and 'strategic' for various military-type instruments. Strategic sensors generally detect point targets, so the D* must be as high as possible within the constraint that the cooler must not cause overriding size, weight, reliability and cost issues. High performance near infrared has similar performance requirements, but can only provide a m i n i m u m of cooling because cost and weight minimization is critical. The extrinsic silicon

68


Table 2.8

S t a t u s a n d t r e n d s o f c o m m e r c i a l u n c o o l e d arrays (after ref. 6)

Status Feature

Cost ($)

Commercial marketing of military thermal imagers Overspecified for commercial applications 320 x 2 4 0 pixel. 5 0 x 50 ~m bolometer arrays for thermal imagers 3 2 0 x 2 4 0 pixel, 50 x 50 ~tm bolometer arrays for imaging radiometers 120 x 1 pixel. 50 x 5013m thermoelectric arrays for imaging radiometers 3 2 0 x 2 4 0 pixel, 50 x 50 ~m hybrid ferroelectric bolometer a r r a y imagers for drivers' vision e n h a n c e m e n t

1 5 ()()()-2() ()()() 2() ()()()-5() ()()() 1 5 ()()() 2()()()-4()()()

Trends Development of new. low-cost thermal imagers and imaging radiometers Designed for specific commercial applications and meeting commercial requirements 160 • 120 pixel. 50 x 50 lam bolometer arrays for thermal imagers 160 • 120 pixel. 50 • 50 Bm bolometer arrays for imaging radiometers 160 • 120 pixel. 50 • 50 ~m bolometer arrays for driver's vision e n h a n c e m e n t systems 160 x 120 pixel. 50 x 50 Bm bolometer arrays for driver's vision e n h a n c e m e n t systems in extremely large volumes

< 1()()() < 5()()() < 2()()()

1 ()()()

detectors offer very high sensitivity, but at the very low operating temperature which is prohibitive in the most applications. The cryogenically cooled InSb and HgCdTe arrays have comparable array size and pixel yield at MWIR spectral band. However, wavelength tunability and high quantum efficiency have made HgCdTe the preferred material. Thus, the associated cooling and system power requirements can be optimally distributed. The monolithic PtSi Schottky barrier FPAs lead all other technologies with respect to array size (more than 1 ()~' pixels). However, the thermal mismatch barrier in hybrid FPAs has been overcome by developers fabricating InSb and HgCdTe arrays. Detector maturity is a function of accumulated experience and development effort, the complexity of the device required, and the inherent difficulty presented by the material technology. At present, HgCdTe photodiodes and BIB extrinsic silicon detectors are not fully mature. PtSi technology is mature and has reached a plateau. Other two detector technologies such as InSb and silicon bolometers are still evolving significantly as applications for larger array configurations and smaller pixel sizes continue to push the boundaries of technology. Despite serious competition from alternative technologies and slower progress than expected, HgCdTe is unlikely to be seriously challenged for highperformance applications and applications requiring multispectral capability and fast response. The recent successes of competing cryogenically cooled detectors are due to technological, not fundamental issues. The steady progress in epitaxial technology would make HgCdTe devices much more affordable in the near future. The much higher operation temperature of HgCdTe, compared to

Comparison ~f photon and thermal detectors performance

69

Figure 2. $8 Thermal D* versus operating temperature for d(~erent FI'A technologies (~(fter r~:f. 4 ~ ).

Schottky barrier devices and low-dimensional solid devices, may become a decisive argument in this case. The fundamental performance limits of HgCdTe photodiodes have not been reached yet. Continued development of the in situ vapour phase epitaxy methods (MBE and MOCVD) will allow bandgap engineering heterojunction devices of increasing quality and complexity. Also, continued development of epitaxial growth on alternative substrates such as silicon will reduce the cost of 2D arrays. Development of dual-band arrays will continue and three-band detectors will soon be demonstrated. To provide high resolution spectroscopic imaging larger HgCdTe FPAs will be used in Fourier-transform (FT) interferometers. Photodiodes will replace photoresistors for detection out to 15 btm since they are characterized by a more linear response. State of the art QWlP and HgCdTe FPAs provide similar performance figure of merit, since they are predominantly limited by the readout circuits. A low photoconductive gain actually causes a better NEDT of OWIP FPA than that of HgCdTe FPA with a similar storage capacity. The performance is, however, achieved with very different integration times. The integration time of OWIP devices is 1 0 - 1 0 0 times longer than that of HgCdTe arrays, and is typically 5-20 ms. Powerful possibilities of QWIP technology are connected with large LWIR and VLWIR FPA applications and with multicolour detection. Three-band and four-band FPAs will soon be demonstrated in near the future.

70


Thermal detector arrays will increase in size and improve in thermal sensitivity to a level satisfying high performance applications at ambient temperature. It is supposed that the silicon microbolometer arrays and the monolithic ferroelectric arrays will capture the low-cost markets. Current uncooled bolometer FPAs have achieved NEDT of less than 1()mK (with 5()~m pixels and f/1 optics), opening the door to the use of less expensive slower optical systems. It is supposed that sales of IR thermal imaging equipment to the automobile market will begin rapidly to change the relative ratio between military/ government and commercial IR markets. Today only about 1 ()% of the market is commercial. After a decade the commercial market can grow to over 7()% in volume and 40% in value, largely connected with volume production of uncooled imagers for automobile driving.-'-' For large volume production for automobile drivers, the cost of uncooled imaging systems will decrease to below $1000. Of course, these systems will cover other segments of the transportation industry" trucks, trains, ships, barges, buses, and airplanes. For the same applications requiring uncooled detectors, the slow response speed is unacceptable. Recently, a number of concepts (e.g., a non-equilibrium device, 99 multi-junction HgCdTe photodiodes, lc1(I optical immersion) and new materials (InAsSb, InAs/GaSb-based type II superlattices) 1 have been proposed to improve the performance of photon detectors operating at near room temperature. The measurements show the possibility to achieve detectivity of ~ l x l ( ) 9 c m H z l / 2 / W at the 8 - 9 ~m range and potentially, the devices can be assembled in large FPAs. A new IR detector concept is micro electromechanical structures (MEMSs). This technology is a marriage of photolithography and mechanics. FPAs based on MEMS technology and a visible optical readout system may offer lower-cost LWIR imaging systems. ~(~1 Finally, considerable development of signal processing function into FPAs can be anticipated.

Appendix A1 HgCdTe photodiodes The Auger m e c h a n i s m is more likely to impose fundamental limitations to the LWIR HgCdTe detector performance. ~(J2 Assuming a non-degenerate statistic, the Auger generation rate is equal to

GA

.

. . . . 2r~1 + 2rl~7

2ri.tl

n+

(A1)

where n and p are the electron and hole concentrations, r ,41 i and r~t are the intrinsic Auger 1 and Auger 7 recombination times and y = r ,-17 i /r{t 1 is the ratio of Auger 7 and Auger I intrinsic recombination times, r!~1 is given by

Comparison olphoton and thermal detectors performance

.rAli

=

G (1 + bt)

(1 + 2/,)exp

(m,, , /,,,)IF1F212(kT/E,,) ~/2

1 + IJ el kT]

in seconds

71

(A2)

where # - m e ~rot, is the ratio of the conduction to the heavy-hole valence-band effective mass, ~, is the relative static dielectric constant, and F, and F2 are the overlap integrals of the periodic part of the electron wave functions. The value of IF1F21 is taken as a constant equal to ().2. The y term is of high uncertainty. According to Casselman and Petersen ~()2 for Hg~_xCdxTe over the range 0 . 1 6 ~ x ~ ( ) . 4 0 a n d 5()K~ 1, the highest detectivity of Auger limited photodetectors can be achieved with a light p-type doping. The required y l / 2 n i p-type doping is difficult to achieve in practice for lowtemperature photodetectors (the control of hole concentration below the 5 x l O l s c m -3 level is difficult) and the p-type material suffers from some n o n f u n d a m e n t a l limitations such as contacts, surface, and Shockley-Read processes. These are the reasons why the low-temperature detectors are typically produced from lightly doped n-type materials. In contrast, p-type doping is clearly advantageous for LWIR and near-room-temperature detectors.

A2 InSb p h o t o d i o d e s

The standard m a n u f a c t u r i n g technique of InSb photodiodes begins with bulk ntype single crystal wafers with donor concentration about N,t-1 ()is cm-3. The Shockley-Read recombination centres are an intrinsic characteristic of n-type InSb. The electron and hole lifetimes are equal because in n-type material there is a single set of recombination centres. The electron and hole carrier lifetimes of high quality material at temperatures below 1 3()K obey the expression 1() r-

4.4 x 10 8

N,t

in seconds

(A4)

The thermal generation can be described by

Gt,, = n]

N,T

(A5)

72


A3 GaAs/AIGaAs QWlPs The carrier generation rate from the QWs due to optical p h o n o n s may be obtained as 104 Gpl, o. -

Nowns-Z2

(A 6)

T

where Now is the n u m b e r of OWs, n+2 is the two-dimensional (2D) density of carriers originating from the excited sub-band state, and r is the lifetime of electrons in the upper state. It can be shown, that

ns2 - Lp2 (27rm * kT)

exp -

v,,-kT ~+,)

r -- 4hgpAEcoLp

(A7) (A8)

q2EphonI1

In the equations, Lp is the mean well spacing, m* is the electron effective mass, k is the Boltzman constant, Epl,o,, is the p h o n o n energy. I~ is a dimensionless integral close to 2 in the 8 - 1 2 ~m range for a quasibound upper state. Moreover, AEco is the cut-off energy defined as AEa, = hc/;.c = Vt, - El, (Vl, is the barrier energy of the OW, and ;.c is the cut-off wavelength ). 1/Ep = 1 / ~ - 1/~o ( ~ : and Eo is the dielectric permittivity at infinite and zero frequency, respectively), and

Ef. - E1 = n~lh2

(A9)

47rm*

is the Fermi energy of the well. and n~ is the 2D carrier density in the ground state. It should be noted that Gpl, does not depend on Lp. and is therefore a more f u n d a m e n t a l quantity of a OWIP. in contrast to n~e and r. The following parameters have been assumed: tn*=().()67mo. Ej,=63~o, Evl,o,,=36 meV, n~=2• 11 cm-2, andN0~v = 50.

A4 Extrinsic photoconductors At the low t e m p e r a t u r e of operation of impurity p h o t o c o n d u c t o r s (when kT 101~ cm v/Hz/W detectivity is not the relevant figure of merit for FPAs. (Taken from ref. 18).

45 A well of GaAs (doped n - 5 • 1017 c m - ~) and a 500 A barrier of Alo. 3Gao.7As. Stacking m a n y identical q u a n t u m wells (typically 50) together, increases photon absorption. Ground state electrons are provided in the detector by doping the GaAs well layers with Si. This photosensitive MOW structure is sandwiched between 0 . 5 p m GaAs top and bottom contact layers doped n - 5 x l ( ) ]7 cm -3, grown on a semi-insulating GaAs substrate by molecular beam epitaxy (MBE). Then a 0.7 pm thick GaAs cap layer on top of a 300 A AI().3Ga().rAs stop-etch layer was grown in s i t u on top of the device structure to fabricate the light coupling optical cavity. The MBE grown material was tested for absorption efficiency using a Fourier Transform Infrared (FTIR) spectrometer. Figure 3.4 shows the measured absorption q u a n t u m efficiency of this material at room temperature. The epitaxially grow material was processed into 200 pm diameter mesa test structures ( a r e a = 3 . 1 4 • 1() -4 cm 2) using wet chemical etching, and Au/Ge ohmic contacts were evaporated onto the top and bottom contact layers. The detectors were back illuminated t h r o u g h a 45 ~ polished facet s and a responsivity spectrum is shown in Figure 3.5. The responsivity of the detector peaks at 8.5 pm and the peak responsivity (Rp) of the detector is 83 mA/W at bias VB -- --1.1 V. The spectral width and the cutoff wavelength are A 5 . / 5 . - 1()% and ,;.,.- 8 . 9 p m respectively. The measured absolute peak responsivity of the detector is small, up to about V R - - 0 . 5 V. Beyond that it increases nearly linearly with bias reaching Rp - 4 2 0 mA/W at V13 - - 5 V. This type of behavior of responsivity versus bias is typical for a bound-to-quasibound QWIP. The peak q u a n t u m efficiency was 1.4% at bias VR - - 1 . 1 V for a 45 ~ double pass. The lower q u a n t u m efficiency is due to the lower photoconductive gain at lower

94

Handbook of Infrared Detection Technologies 12 10 ILl O

8

e.

.2 L

6

O

"

4

6

7

8

9

10

11

W a v e l e n g t h (~Lm)

Figure ].4 Absorption quantmn e .~cienc!l o.lthe Qi~'II" material at room temperature.

0.1

B i a s = -1.1 V

A

I> cO Z

o

13_ 09

uJ cr

0.2

5

6

7

I

8

9

10

11

12

13

14

15

16

17

WAVELENGTH (#m)

Figure 3.12 Simultaneousl!l measured responsivit!! sl~ectrmn of verticall!! integrated LWIR and VLWIR dualband OWIP detector (Taken from reference 26 ).

significantly at temperatures below 65 K. The estimated NEAT of LWIR and VLWIR detectors at 40K are 36 and 4 4 m K respectively (See Figures 3.13 and 3.14). These estimated NEAT values based on the test detector data agree reasonably well with the experimentally obtained values. The experimental LWIR NEAT value is lower than the estimated NEAT value of 36mK. This improvement is attributed to the 2D periodic grating light coupling efficiency. On the other hand the experimental VLWIR NEAT value is higher than the estimated NEAT value of 44 mK. The authors believe this degradation is due to the inefficient light coupling at the 14-15 l~m region, readout multiplexer noise,

Figure 3.13 The uncorrected noise equivalent temperature d(ffl'rence (NEAT) histogram of 8-9 l~m detector pixels of the 640 • 486 dualband FPA. The mean NEAT is 29 mK.

102


Figure 3.14 The uncorrected noise equivalent temperature tti!ti'rem'e (NEAT) histogram of l 4 - 1 5 ltm detector pixels of the 640 • 4 8 6 dtmlband FPA. The mean .\'EAT is 74 inK.

and noise of the proximity electronics. At 4() K the performance of both LWIR and VLWIR detector pixels of this dualband FPA are limited by photo current noise and readout noise. Video images were taken at a frame rate of 3() Hz. at temperatures as high as T= 74 K, using a ROC capacitor having a charge capacity of 9 • 1 ()~ electrons (the m a x i m u m n u m b e r of photoelectrons and dark electrons that can be counted in the time taken to read each detector pixell. Figure 3.1 5 shows simultaneously acquired 8 - 9 and 14-1 5 micron images using this two-color camera. 2s

3.6 640•

pixel broad-band QWIP imaging camera

A broadband QWIP device structure is designed by repeating a unit of several q u a n t u m wells with slightly different parameters such as well width and barrier height. 27 The positions of ground and excited states of the q u a n t u m well are determined by the q u a n t u m well width (L,,,) and the barrier height, i.e. the AI mole fraction (x) of the barrier 2- Since each single set of parameters for a boundto-quasibound q u a n t u m well 4 corresponds to a spectral band pass of about 1.5 ~m, three different sets of values are sufficient to cover a 1 ()-16 ~m spectral region. As shown in Figure 3.16. the MOW structure consists of m a n y periods of these three-quantum-well units separated by thick barriers. The device structure reported here involved 3 3 repeated layers of GaAs threequantum-well units separated by L t ~ 5 7 5 A thick AlxGa~ xAs barriers (See Figure 3.9). The well thickness of the q u a n t u m wells (see Figure 3.16) of threequantum-well units are designed to respond at peak wavelengths around 1 3, 14, thick and 151um respectively. These wells are separated by Lz, ~ 7 5 A AlxGal_xAs barriers. The AI mole fraction (xl of barriers t h r o u g h o u t the

GaAs/A1GaAs based quantum xvell itll)ared photodetector Jbcal plane arra!ls

1() 3

Figure :~. 15 Both pictures showy (flame-simHltaneousl!l acquired) t~vo-color images ~,i~11the 640 • 486 t~vocolor OWIP camera, hnage on the h'ft is from 14-1 ~ 12m infrared and the image on the right isJ?om 8-9 lxm infrared. Pix'el pitch of the FPA is 2 ~ 12m. The 14-1 ~ micron image is less sharp due to the difJ?action limited spot size being larger than the pix'el pitch of the FPA ( Taken l?om r~:ference 28).

Figure 3.16 Broad-band MOW structz~re is designed b!t repeating a zlnit of several quantum ~vells ~vilh slightl!l different parameters slwh as ~rell ~vidth and barrier height. The excited state energ!l levels broadened due to overlap of the ~vavefunctions associated ~vith excited states ofqlumtum ~vells separated b!l thin barriers.

structure was chosen such that the /.p=l 3 l~m quantum well operates under bound-to-quasibound conditions. ~J The excited state energy level broadening has been further enhanced due to the overlap of the wavefunctions associated with excited states of quantum wells separated by thin barriers. Energy band calculations based on a two-band model show excited state energy levels spreading about 2 8 meV.

104


The sample was grown on a semi-insulating three-inch GaAs substrate by molecular beam epitaxy. It consists of the device structure described above sandwiched between top and bottom contact layers. Transport carriers (electrons) were provided by doping all GaAs wells and contact layers with Si. In order to measure the dark c u r r e n t - t e m p e r a t u r e curve, spectral responsivity (see Figures 3 . 1 7 - 3 . 1 9 ) and noise, 200 ~tm diameter mesas were fabricated using wet chemical etching and Au/Ge ohmic contacts were evaporated onto the top and bottom contact layers. The responsivity spectra of these detectors were measured using a 1()00 K black-body source and a grating monochromator. The detectors were back illuminated through a 45 ~ polished facet to obtain normalized responsivity spectra at different bias voltages. Then the absolute spectral responsivities were obtained by measuring total photocurrent from a calibrated black-body source. In Figure 3.18, responsivity curve at V j ~ = - 2 . 5 V bias voltage shows broadening of the spectral response up to A;. ~ 5.5 Bm, i.e. the full width at half

1 .E-06 "~ 1.E-07

E r (9 L

,-

1.E-08

1.E.09 ~-

O

~" 1.E-10~ r~

1 .E-11 1 .E-12

30

35

40

45

50

55

60

65

70

T e m p e r a t u r e (K)

Figure 3 . 1 7 Dark current verses temperature curve of 1 O- 1 5.4 lira broadband O_WIP at bias \:l~= - 2 . 5 V. Data were taken with a 2 0 0 ~tm diameter test device and normalized to 2 S • 2 S u m 2 pix'el.

0.5 0.4

~o. E

0.3 0.2 0.1

8

10

12

14

16

Wavelength (micron)

Figure 3.18 Responsivity spectrum of a broadband OIVIP test device at temperature T= 5 5 K. The spectral response peak is at 13.5 l~m and the long wavelength cutolJis at 1 ~.4 /2m.

GaAs/AIGaAs based quantum well infrared photodetector focal plane arrays

105

0.6

A

0.4

n

Iz

0.2

0~.

-v

0

1

2

3

4

Bias Voltage (V) Figure 3.19 Peak responsivit!t as a function of bias voltage at temperature T-- :~ ~ K.

m a x i m u m from 1 0 . 5 - 1 6 gm. This b r o a d e n i n g A2/2p ,-~42% is about a 4 0 0 % increase compared to a typical b o u n d - t o - q u a s i b o u n d QWIP. 4 Unlike n a r r o w - b a n d OWIPs, these detectors show spectral peak shifts from ,~ - 11.5 jJm to ,~. - 15.1 gm as negative bias voltage increased from VR - - 1 V to VB - - 5 V and similar behavior (~. - 11.5 IJm to ,~. - 14.7 IJm for VR - + 1 V to VB - + 5 V) was observed u n d e r positive bias voltages as well. This suggests that there is no substantial carrier depletion due to the applied electric field within the t h r e e - q u a n t u m - u n i t s because the direction of peak shift r e m a i n s the same u n d e r both positive and negative biases. The responsivity of the detector peaks at 13.5 l~m and the peak responsivity (Rp)of the detector is 2 5 0 m A / W at bias VR - 2 . 5 V. The bias dependent peak responsivity of the detector is s h o w n in Figure 3.19. The m e a s u r e d absolute peak responsivity of the detector is small, up to about VB--0.5 V. Beyond t h a t it increases nearly linearly with bias r e a c h i n g Re - 580 m A / W at V B = - 3 . 5 V. This type of behavior ofresponsivity versus bias is typical for a b o u n d - t o - q u a s i b o u n d OWIP. The peak q u a n t u m efficiency was 11% at bias V~--2.5 V for a 45 ~ double pass. The lower q u a n t u m efficiency is due to the lower well doping density (2 x 10 ~7 c m - 3) as it is necessary to suppress the dark c u r r e n t at the highest possible operating t e m p e r a t u r e . A peak q u a n t u m efficiency as high as 25% has already been achieved with regular well doping density (i.e., 1• cm-3). Due to lower readout multiplexer well depth (i.e., 11 • electrons) a lower dark c u r r e n t is m a n d a t o r y to achieve a higher operating t e m p e r a t u r e . In this case, the highest operating t e m p e r a t u r e of 45 K was determined by the well depth of the readout multiplexer. The dark c u r r e n t noise i,, of the device was m e a s u r e d using a s p e c t r u m analyzer at f - 55 K as a function of bias voltage. The noise gain g,, can n o w be obtained using the g - r noise calculated based on s t a n d a r d noise expression" i,, = x/4eI~g, AB w h e r e In is the dark c u r r e n t and AB is the b a n d w i d t h . Using experimental m e a s u r e m e n t s of noise and responsivity, one can n o w calculate specific detectivity D* form D * - R~/-A~/i,,, w h e r e A is area of the detector. Calculated D* value for the present device (,~. - 15.4 l~m) at, f - 55 K, and VR = 2.5 V is 3 x 10 l~ c m v ~ z / W . Even with broader response, this D* is c o m p a r a b l e to

106 Handbook of Infrared Detection Technoloqies previously reported D* of OWIPs with narrow spectral response. Figures 3.20 and 3.21 show the detectivity D* and the noise equivalent temperature difference (NEAT) as a function of the operating temperature of the device. It is well known that QWIPs do not absorb radiation incident normal to the surface unless the IR radiation has an electric field component normal to the layers of superlattice (growth direction) s. As we have discussed before s m a n y more passes of IR light inside the detector structure can be obtained by incorporating a randomly roughened reflecting surface on top of the detectors which also removes the light coupling limitations and makes two-dimensional OWIP imaging arrays feasible. This random structure was fabricated on the detectors by using standard photolithography and CCI,F, selective dry etching. After the r a n d o m reflector array was defined by the lithography and dry etching, the photoconductive OWIPs of the 64()• 512 FPAs were fabricated by dry etching through the photosensitive GaAs/AlxGa~_xAs m u l t i - q u a n t u m well layers into the 0.5 l,tm thick doped GaAs bottom contact layer. The pitch of the FPA is 2 5 l.tm and the actual pixel size is 2 3 x 2 3 lain2. The random reflectors on

1.E+12

A

1.E+11 N

-IE

~ 1.E+10

1 .E+09 30

40

50

60

70

T e m p e r a t u r e (K)

Figure 3.20 Detectivity as a fimction o.f temperatures at bias volta#e V~=-2. :~V.

1.E+04 1 .E+03 E a

uJ z

1.E+02 1.E+01 1.E+00 I 30

, 40

, 50

, 60

70

Temperature (K)

Figure 3.21 Noise equivalent temperature difference as a fi~nction o[temperatures at bias volta#e VB=-2.

~ V.

CaAs/A1GaAs based quantum well infrared photodetector focal plane arralls

1()7

top of the detectors were then covered with Au/Ge and Au for Ohmic contact and reflection. Figure 3.7 shows twelve processed OWIP FPAs on a three-inch GaAs wafer. Indium bumps were then evaporated on top of the detectors for Si readout circuit (ROC) hybridization. A single OWIP FPA was chosen and hybridized (via indium bump-bonding process) to a 64()x512 CMOS multiplexer (Indigo Systems 9803) and biased at VR = - 2 . 5 V (see Figure 3.22). At temperatures below 48 K, the signal-to-noise ratio of the system is limited by array nonuniformity, multiplexer readout noise, and photo current (photon flux) noise (see Figure 3.23). At temperatures above 48 K, temporal noise due to the OWIP's higher dark current becomes the limitation. As mentioned earlier this higher dark current is due to thermionic emission and thus causes the charge storage capacitors of the readout circuitry to saturate. Since the OWIP is a high impedance device, it should yield a very high charge injection coupling efficiency into the integration capacitor of the multiplexer. In fact Bethea et al. ~ have demonstrated charge injection efficiencies approaching 90%.

Figure 3.22 .4 size comparison of the 640 • ~ 12 long-wavelength QWIP FPA to a quarter.

30K

1.0E-07 -

35K

Q. E

"'Jr- 40K

,=I: 1.0E-09 -

Y.

c

o 'o L_

] OE-]] -

45K 50K

.

- e - - 55K

1.0E-]g 1.5

2

2.5

3

3.5

4

photo

Bias Voltage (V) Figure 3.23 Dark-current and the photo-current of 10-15.412m broadband QWIP as a fi~nction of bias voltage.

lO8 Handbookof InfraredDetectionTechnoloqies Charge injection efficiency can be obtained from equation (2 5 ), where g,,, is the transconductance of the MOSFET and is given by g,,, - eli)et/kT. The differential resistance RDet of the pixels at - 2 V bias is 5 . 4 • l~ ohms at T - 4 5 K and detector capacitance CDet is 1.4 • 1 O-14 F. The detector dark current IDet - 8 pA under the same operating conditions. According to equation (1) the charge injection efficiency rli,,i-99.5% at a frame rate of 30 Hz. The FPA was backilluminated t h r o u g h the flat thinned substrate membrane (thickness ~ 1 3 0 0 A). This thinned GaAs FPA membrane has completely eliminated the thermal mismatch between the silicon CMOS readout multiplexer and the GaAs based QWIP FPA. Basically, the thinned GaAs based QWIP FPA membrane adapts to the thermal expansion and contraction coefficients of the silicon readout multiplexer. Thus, thinning has played an extremely important role in the fabrication of large area FPA hybrids. In addition, this thinning has completely eliminated the pixel-to-pixel optical cross-talk of the FPA. This initial array gave very good images with 99.9% of the pixels working, demonstrating the high yield of GaAs technology. The operability was defined as the percentage of pixels having noise equivalent differential temperature less than lOOmK at 3OOK background with f/2 optics and in this case operability happens to be equal to the pixel yield. We have used equation (26) to calculate the NEAT of the FPA. where Dt*3 is the blackbody detectivity, dPR/dT is the derivative of the integrated blackbody power with respect to temperature, and 0 is the field of view angle (i.e., sine(0/ 2)-(4f2+1) -1, where f is the f number of the optical system). The background temperature T B - 30OK, the area of the pixel A=(231.tm) 2, the f number of the optical system is 2, and the frame rate is 30 Hz. Figure 3.24 shows the experimentally measured NEAT histogram of the FPA at an operating temperature of T -- 35 K, bias VR - - 2 . 5 V at 300 K background with f/2 optics and the mean value is 55 mK. This agrees reasonably well with our estimated value of 25 mK based on test structure data. The read noise of the multiplexer is 500 electrons. The factor of two short-fall of NEAT is mostly attributed to decrease in bias voltage across the detectors during charge accumulation (common in m a n y direct injection type readout multiplexers) and read noise of the readout multiplexer. The experimentally measured peak q u a n t u m efficiency of the FPA was 9.5% which agrees well with the 11% q u a n t u m efficiency estimated from the single element detector data. A 6 4 0 • 512 OWIP FPA hybrid was mounted onto an 84-pin lead-less chip carrier and installed into a laboratory dewar which is cooled by liquid neon, to demonstrate a LWlR imaging camera (FPA was cooled to 35 K). The other element of the camera is a 100 mm focal length AR coated germanium lens, which gives a 9.2~ 6.9 ~ field of view. It is designed to be transparent in the 8 12 ~tm wavelength range (which is not fully compatible with the 1 0 - 1 5 ~tm broadband OWIP array). An image processing station was used to obtain clock signals for the readout multiplexer and to perform digital data acquisition and non-uniformity corrections. The digital data acquisition resolution of the camera is 14-bits, which determines the instantaneous dynamic range of the camera (i.e., 16 384), however, the dynamic range of QWIP is 85 decibels.

GaAs/A1GaAs based quantum well infrared photodetector focal plane arrays

109

14000 12000 C

o

10000

O

8000

=

6000

r

4000

O

2000

0 -0.2

-0.1

0

0.1

0.2

0.3

NEAT (K) Figure 3.24 Noise equivalent temperature difference (NEAT) histogram of the 327 680 pix'els of the 6 4 0 x 5 1 2 array showing a high un(formit!t of the t:PA. The uncorrected non-uniformit!l (=standard deviation/mean) of this unoptimized FPA is onl!l 6.3% includin~l 1% non-uniformity of ROC and 1.4% nonuniformit# due to the cold-stop not being abh' to ~3ivethe sameJield of vicar to all the pixels in the FPA.

The measured mean NEAT of the OWIP camera is :35 mK at an operating temperature of f - 35 K and bias Vt~- - 2 . 5 V at 3 0 0 K background with f/2 optics (see Figure 3.24). This is in good agreement with expected focal plane array sensitivity due to the practical limitations on charge handling capacity of the multiplexer, read noise, bias voltage and operating temperature. The uncorrected NEAT non-uniformity (which includes a 1% non-uniformity of the ROC and a 1.4% non-uniformity due to the cold-stop in front of the FPA not yielding the same field of view to all the pixels) of the 327 680 pixels of the 6 4 0 x 5 1 2 FPA is about 6.3% ( - s i g m a / m e a n ) . The non-uniformity after twopoint (17 ~ and 27 ~ Celsius) correction improves to an impressive ().1%. As mentioned earlier, this high yield is due to the excellent GaAs growth uniformity and the mature GaAs processing technology. Video images were taken at a frame rate of 15 Hz at temperatures as high as T - 35 K using a ROC capacitor having a charge capacity of 11 x 1 ()~ electrons (the m a x i m u m number of photoelectrons and dark electrons that can be counted in the integration time of each detector pixel). Figure 3.2 5 shows a flame of video image taken with this 1 0 - 1 5 . 4 ~m 64()• 512 broadband OWIP imager. These high resolution images comparable to standard TV, demonstrate the high operability (i.e., 99.9%) and the stability (i.e., lower residual uniformity and l f f noise) of the 6 4 0 • long-wavelength OWIP staring array camera. It should be noted that these initial unoptimized FPA results are far from optimum. The light coupling gratings were not optimized (as described earlier) for maximum

1 ]0

Handbook of Infrared Detection Technolo~lies

Figure 3.25 This picture sho~vs one J?ame of video clip taken ~vith the 640• 512 pixel I 0 - I 5.41~m broadband QWIP focal plane arra!l. This ima~le sho~vs the liquid h, vel of a soda can and some.fingerprints on the C~II.

light coupling efficiency, no anti-reflection coatings were used on the backside of the FPA,

3.7 640• array

pixel spatially separated four-band QWIP focal plane

One unique feature of this spatially separated four-band focal plane array is that the four infrared bands are independently readable on a single imaging array. This feature leads to a reduction in instrument size, weight, mechanical complexity, optical complexity and power requirements, since no moving parts are needed. Furthermore, a single optical train can be employed, and the focal plane can operate at a single temperature. This four-band device structure was achieved by the growth of multi-stack QWIP structures separated by heavily doped contact layers, on a GaAs substrate. Device parameters of each QWIP stack were designed to respond in different wavelength bands. Figure 3.26 shows the schematic device structure of a fourcolor OWIP imager. A typical OWIP stack consists of a MOW structure of GaAs quantum wells separated by AlxGal_• barriers. The actual device structure consists of a 15 period stack of 3-5 pm QWIP structure, a 2 5 period stack of 8.5]Opm QWIP structure, a 25 period stack of ]()-12 ~tm OWIP structure and a 30 period stack of 13-15.:3 jam OWIP structure. Each photosensitive MOW stack was separated by a heavily doped (thickness ().2-().813m) intermediate GaAs contact layer (see Figure 3.26). Since the dark current of this device structure is dominated by the longest wavelength portion of the device structure, the VLWIR QWIP structure has been designed to have a bound-to-quasibound intersubband

GaAs/A1GaAs based quantmn well infrared photodetector focal plane arrags

111

Figure .3.26 Lager structure diagram ql four-band ()~'II ~ device and the 2I) periodic grating structure. Each pixel represents a 640 x 128 pixel area of the four-balld focal plane arra!t.

absorption peak at 14.0 ~m. Other OWIP device structures have been designed to have a bound-to-continuum intersubband absorption process, because the photo current and dark current of these devices are relatively small compared to the VLWIR device. This whole four-band OWIP device structure was then sandwiched between 0.513m GaAs top and bottom contact layers doped with n - 5 • 1017 cm- 3 and was grown on a semi-insulating GaAs substrate by MBE. The individual pixels were defined by photolithographic processing techniques (masking, etching, chemical vapor deposition, metal deposition, etc.). Four separate detector bands were defined by a deep trench etch process and the unwanted spectral bands were eliminated by a detector short-circuiting process. The unwanted top detectors were electrically shorted by gold coated reflective 2D etch gratings as shown in Figure 3.26. In addition to shorting, these gratings serve as light couplers for active QWIP stack in each detector pixel. The design and optimization of these 2D gratings to maximize QWIP light coupling were extensively discussed in ref. 5. The unwanted bottom detectors were electrically shorted at the end of each detector pixel row. Typically, in single-band QWIP FPAs. quarter wavelength deep (h -- 2p/4nGaAs) grating grooves are used for efficient light coupling. However, in the four-band FPA, the thickness of the quarter wavelength deep grating grooves are not deep enough to short-circuit the top three MQW QWIP stacks (e.g., three top OWIP stacks on 14-15.5 gm OWIP in Figure 3.26). Thus, three-quarter wavelength groove depth 2D gratings (h - 3;.p/4n~;,~.,) were used to short the top unwanted detectors over the 1 ()-12 and 14-15.5 microns bands. This technique optimized the light coupling to each OWIP stack at corresponding bands while keeping the pixel (or mesa) height at the same level, which is essential for the indium bump-bonding process used for detector array and readout multiplexer hybridization. Figure 3.27 shows the normalized spectral responsivities of all four spectral bands of this four-band focal plane array.

112


Figure 3.2 7 Normalized spectral respo~lse of the lbltr-hand OWIt~.lbcal plane arrajt.

Few OWIP FPAs were chosen and hybridized to a 64()x 512 CMOS multiplexer (ISC 9803) and biased at V~ - - 1 . 1 V. At temperatures below 83 K, the signalto-noise ratio of the 4 - 5 l.tm spectral band is limited by array non-uniformity, multiplexer readout noise, and photo current (photon flux) noise. At temperatures above 45 K, temporal noise due to the 14-15.5/.tm QWIP's higher dark current becomes the limitation. The 8-1() and 1 0 - 1 2 ~m spectral bands have shown BLIP performance at temperatures between 45 and 83 K. The FPAs were back-illuminated through the fiat thinned substrate m e m b r a n e (thickness ~ 1 3 0 0 A). This initial array gave excellent images with 99.9% of the pixels working (number of dead pixels ~ 2 5()), demonstrating the high yield of GaAs technology. The operability was defined as the percentage of pixels having noise equivalent differential temperature less than 1 ()()mK at 30()K background and in this case operability happens to be equal to the pixel yield. A 640• pixel four-band OWIP FPA hybrid was mounted onto an 84-pin leadless chip carrier and installed into a laboratory dewar which is cooled by liquid helium to demonstrate a four-band simultaneous imaging camera. The FPA was cooled to 45 K and the temperature was stabilized by regulating the pressure of gaseous helium. The other element of the camera is a 1()() m m focal length AR coated g e r m a n i u m lens, which gives a 9 . 2 ~ ~ field of view. It is designed to be transparent in the 8 - 1 2 l.tm wavelength range. The SEIR | image processing station was used to obtain clock signals for readout multiplexer and to perform digital data acquisition and non-uniformity corrections. The digital data acquisition resolution of the camera is 14-bits, which determines the instantaneous dynamic range of the camera (i.e., 16 384), however, the dynamic range of OWIP is 85 decibels. Video images were taken at a frame rate of 30 Hz at temperatures as high as T - 45 K, using a ROC capacitor having a charge capacity of 11 • 1 ()r electrons {the m a x i m u m n u m b e r of photoelectrons

GaAs/A1GaAs based quantum well itffrared photodetector focal plane arra!ls

113

and dark electrons that can be counted in the time taken to read each detector pixel). Figure 3.28 shows one flame of a video image taken with a four-band 640 x 512 pixel OWIP camera.

3.8 QWIPs for low background and low temperature operation Although QWIP performs exceptionally well under high background levels, the detector exhibits anomalous behavior when it operates under stringent low irradiance and low temperature conditions {i.e. at extremely low photocurrent). Singh et al. have modeled and explained some of the irregular behaviors by treating OWIP as a resistance in series with a capacitance. 32'33 Arrington et al. 34 have shown experimentally the non-fiat frequency response curves of OWIP at low background and low operating temperature conditions. Under such conditions, responsivity decreases as the detector operating frequency increases. The roll-off frequency is completely determined by the device design and operational conditions. ~4 This behavior is empirically similar to dielectric relaxation effects observed in bulk extrinsic silicon and germanium photoconductors under similar operational conditions. ~s. ~, In principle, QWIP operates very similar to extrinsic bulk photoconductors. Electrons in the subbands of the isolated quantum wells can be visualized as electrons attached to impurity states in bulk photoconductors. As photogenerated electrons depart the active doped quantum well regions, they leave behind a space-charge buildup. Thus, in order to operate QWIP steadily, requires a sufficient dark or background photo current to replenish the depleted quantum wells. This is not an issue for QWIP detectors operating at high background conditions or high temperatures, because high photocurrent or high dark current can easily provide carriers to re-fill the space-charge buildup. However,

Figure 3.28 One frame of video image taken ~vith the 4-1 ~. ~ 12m cutoff four-band 640• ~ 12 pixel O.WIP camera. The image is barely visible in the 14-1 ~. ~ 12m spectral band due to the poor optical transmission of the anti-reflection layer coated germanium lens.

114

Handbookof Infrared Detection Technoloqies

at low operating temperatures for low-background irradiance levels, high resistivity of thick barriers in the active region impedes electrons from entering the detector from the opposite electrode. This could lead to a delay in refilling the space-charge buildup, and result in a lower responsitivity at high optical modulation frequencies. In order to overcome this problem we have designed a new detector structure, the Blocked Intersubband Detector (BID) with separate active quantum well region and blocking barrier. 37 As shown the Figure 3.29, the active region of BID consists of quantum wells separated by thin barriers, which creates sub-minibands due to large overlap of sub-level wave functions. Thus, space-charge buildup will get quickly refilled by electrons via sequential resonant tunneling along the ground state miniband from the emitter contact layer. A thick, impurity free blocking barrier is placed between the active region and collector to suppress the dark current of the device. We have fabricated a BID device consisting of 50 period MOW photosensitive regions and a 1 l~m thick A1.2Ga l_.2As blocking barrier layer. Each quantum well period contains a 60 A GaAs well and 1 ()()A A1.2Gal_.2As barrier. The whole device structure is sandwiched between two GaAs contact layers (Figure 3.29). The sample was grown on a semi-insulating GaAs substrate by molecular beam epitaxy. Transport carriers (electrons)were provided by doping all GaAs wells and contact layers with Si to a density of n=4 • 1017 cm-3. Mesas 20()13m in diameter were fabricated using wet chemical etching in order to measure dark current-voltage curves, spectral responsivity and noise. The responsivity spectra of these detectors were measured using a 10()0 K black-body source and a grating monochromator. To obtain normalized responsivity spectra at different bias voltages, detectors were back illuminated through a 45 ~ polished facet. Then the absolute spectral responsivities were calculated by measuring total photocurrent due to a calibrated black-body source. Figure 3.30 shows responsivity curves at different bias voltages from VI~ = -(). :3 to l. :3 V. Unlike in typical OWIPs, the spectral response of BID shows extra broadening because the photoexcitation occurs between sub-minibands instead of localized sub-levels. Figure 3.30 also illustrates the absorption quantum efficiency spectrum of the BID, measured using a 45 ~ wave guide geometry. Although the BID shows comparable absorption quantum efficiency, it shows much smaller responsivity than thick barrier OWIP devices. Therefore, the photoconductive gain of the BID (g = 0.06 at V = - 1 V bias voltage) is a few

Figure 3.29 Energy band diagram of GaAs/Al(;aAs Blocked Intersubband Detector (BID).

GaAs/A1GaAs based quantum well infrared photodetector focal plane arra~ts 115

Figure 3.30 Responsivity and absorption quantum efficienc!l spectra of the BID measured using a 4 ~~ wave guide geometry. Shown responsivity curves are for different bias voltages from VB=--O.5 to 1.5 V with 0.25 Vsteps.

times smaller (g ~ 0.2 - 0.5 at V = - 3 V bias voltage). This can be attributed to the lower electric field across the active region of the BID. Due to higher resistivity of the blocking layer, compared to the active superlattice, much of the applied bias voltage drops across the blocking layer. Thus, the majority of the photoexcited electrons relax within the superlattice before reaching the blocking layer. In contrast, the typical QWIP has uniform resistivity along the active layer, which allows a substantial uniform electric field across the device. In order to improve the carrier collection efficiency, we have designed a BID with a thinner blocking barrier. Although, this device operates at a lower bias voltage, the electric field distribution across the active layers is much higher than in the previous devices. Figure 3.31 shows the comparison of optical gains for two different devices obtained by measuring absorption quantum efficiency and responsivity. Figure 3.32 shows dark currents measured at V = - 1 V bias voltage and at different operating temperatures. The estimated detectivity D* at different operating temperatures, for a low background, with photon flux 4)= 4 • 108 photons/sec/pixel is also illustrated in Figure 3.32. This flux simulates 120 K space background withf/2-optics aperture.

3.9 Summary In summary, we have discussed the importance of FPA uniformity in NEAT, the general figure of merit that describes the performance of large imaging arrays. It is important to note that when D*>~IO l(~ cmv/Hz/W, the performance is uniformity limited and thus essentially independent of the detectivity, i.e., D* is not the relevant figure of merit. 2 Furthermore, we have demonstrated the 6 4 0 • pixel LWIR portable OWIP camera based on bound-to-quasibound device structure, the first 640• 512 pixel four-band FPA, 640• 486 pixel dual-band imaging camera, and

116


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Effective Electric Field (kVIpm) Figure 3.31 Optical gain obtained for two d(fferent B/D devices b!l measuring absorption quantum eSficienc!t and responsivity, hnprovement in optical gain is due to efficient carrier collection in the newest design.

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Figure 3.32 BID dark current and peak detectivit!! D* against operating temperature. The dark current was measured with 200 I~m diameter mesa and normalized to a 30 • 30 i~m 2 pix'el. D* is calculated based on responsivit# and dark currents for a background with photon flux 4)=4 • 10 '~photons/s/pixel which simulates 120 K space background and f/2-optics.

640• pixel broad-band imaging camera. Furthermore, we have demonstrated that OWIP is able to operate at low temperatures, with a higher sensitivity under high background irradiance levels. The anomalous behavior of QWIP w h e n operating under stringent low irradiance levels at low temperatures was addressed. A new blocked intersubband detector (BID) structure, with active superlattice and a thick blocking barrier, has been proposed to overcome this problem. Experimental m e a s u r e m e n t s show similar absorption q u a n t u m efficiency, with a lower photoconductive gain. This can be attributed to a lower voltage drop across the active superlattice of the BID. The real advantage of the BID detector is that its infrared sensing photoemitter is a GaAs/A1GaAs based

GaAs/A1GaAsbasedquantum well infraredphotodetectorfocal plane arrays l 17 superlattice, thus its cutoff wavelength can be easily tuned by band gap engineering (i.e., tailorable cutoff wavelengths). Thus, if one seeks shorter wavelength operation such as 1() or 15 microns cutoff, the MOW based BID detectors will operate at much higher cryogenic temperatures such as 30K which can be achieved passively at space low background conditions.

Acknowledgements The research described in this paper was performed by the Jet Propulsion Laboratory, California Institute of Technology, and was jointly sponsored by the Breakthrough Sensors and Instrument Component Technology Thrust of NASA Cross Enterprise Technology Development Program, Air Force Research Laboratory, and the Ballistic Missile Defense Organization.

References 1. C. Weisbuch, Semicon. Semimet. 24, 1 - 1 3 3 (1987). 2. B. F. Levine, J. Appl. Phys. 74, R1 (1993 ). 3. S. D. Gunapala, B. F. Levine, L. Pfeiffer, and K. West, Dependence of the Performance of GaAs/A1GaAs Q u a n t u m Well Infrared Photodetectors on Doping and Bias, J. Appl. Phys. 69, 6517 ( 1991 ). 4. S. D. Gunapala, J. K. Liu, J. S. Park, T. L. Lin, and M. Sundaram, Infrared Radiation Detecting Device, US Patent No. 6,211.529. 5. S. D. Gunapala and S. V. Bandara, Q u a n t u m Well Infrared Photodetector (OWIP) Focal Plane Arrays, Setniconductors and Semimetals 62, 1 9 7 - 2 8 2 , Academic Press (1999). 6. J. Y. Andersson, L. Lundqvist, and Z. F. Paska, J. Appl. Phys. 71, 3600 (1991). 7. G. Sarusi, B. F. Levine, S. J. Pearton, S. V. Bandara. and R. E. Leibenguth, ]. Appl. Phys. 76, 4 9 8 9 (1994). 8. C. G. Bethea, B. F. Levine, V. O. Shen, R. R. Abbott, and S. J. Hseih, IEEE Trans. Electron Devices 3 8, 1118 ( 1991 ). 9. C. G. Bethea and B. F. Levine, B. F. In Proceedings of SPIE International Symposium on Optical Applied Science and Engineering, San Diego, CA ( 1992 ). 10. C. G. Bethea, B. F. Levine, M. T. Asom, R. E. Leibenguth, J. W. Stayt, K. G. Glogovsky, R. A. Morgan, J. Blackwell, and W. Parish, IEEE Trans. Electron Devices 40, 1957 ( 1993 ). 11. W. A. Beck, T. S. Faska, J. W. Little, J. Albritton, and M. Sensiper, Proceedings of the Second International Symposium on 2-20 l~m Wavelength Infrared Detectors and Arrays: Physics and Applications, Miami Beach, Florida (1994). 12. S. D. Gunapala, J. S. Park, G. Sarusi, True-Lon Lin, J. K. Liu, P. D. Maker, R. Muller, C. A. Shott, and T. Hoelter, 15 Bm 128 x 128 GaAs/AIGaAs Q u a n t u m Well Infrared Photodetector Focal Plane Array Camera, IEEE Trans. Electron Devices 44, 4 5 - 5 0 (1997).

118 Handbookof Infrared Detection Technologies 13. S. D. Gunapala, J. K. Liu, J. S. Park. M. Sundaram, C. A. Shott, T. Hoelter, True-Lon Lin, S. T. Massie, P. D. Maker, R. E. Muller, and G. Sarusi, 9 ~m Cutoff 2 5 6 x 2 5 6 GaAs/AlxGal_xAs Q u a n t u m Well Infrared Photodetector Hand-Held Camera, IEEE Trans. Electron Devices 44, 5 1 - 5 7 (1997). 14. S. D. Gunapala, S. V. Bandara, J. K. Liu, W. Hong, M. Sundaram, P. D. Maker, R. E. Muller, C. A. Shott, and R. Carralejo, Long-Wavelength 6 4 0 x 486 GaAs/AlxGal_xAs Q u a n t u m Well Infrared Photodetector Snap-shot Camera, IEEE Trans. Electron Devices 45, 1890 ( 1998 ). 15. J. Y. Andersson, J. Alverbro, J. Borglind, P. Helander, H. Martijn, and M. Ostlund, 3 2 0 x 2 4 0 Pixels Q u a n t u m Well Infrared Photodetector (QWIP) Array for Thermal Imaging: Fabrication and Evaluation. SPIT 3 0 6 1 , 7 4 0 - 7 4 8 (199 7). 16 K. K. Choi, A. C. Goldberg, N. C. Das, M. D. Jhabvala, R. B. Bailey, and K. Vural, S P I E 3 2 8 7 , 1 1 8 - 1 2 7 (1998). 17. R. Breiter, W. Cabanski, 1R. Koch. W. Rode. and J. Ziegler, SPIT 33 79, 423 (1998). 18. R. H. Kingston, Detection of Optical and Infrared Radiation Springer, Berlin (1978). 19. A. Zussman, B. F. Levine, J. M. Kuo, and J. de Jong, 1. Appl. Phys. 70, 5101

(1991). 20. F. D. Shepherd, In Infrared Detector and Arrays, SPIT 9 3 0 (SPIT Orlando, FL), p. 2 (1988). 21. J. M. Mooney, F. D. Shepherd. W. S. Twins. J. E. Murgia, and J. Silverman, Opt. Eng. ZS, 1151 (1989). 22. Grave and A. Yariv, Intersubband Transitions in Ouantum Wells, edited by E. Rosencher, B. Vinter, and B. Levine, Cargese. France (Plenum, New York), p. 15 (1992). 23. B. F. Levine, C. G. Bethea, K. G. Glogovsky, J. W. Stayt, and R. E. Leibenguth, Semicond. Sci. Technol. 6, C114 ( 1991 ). 24. M. T. Asom, C. G. Bethea, M. W. Focht, T. R. Fullowan, W. A. Gault, K. G. Glogovsky, G. Guth, R. E. Leibenguth, B. F. Levine, G. Lievscu, L. C. Luther, J. W. Stayt, Jr., V. Swaminathan, Y. M. Wong, and A. Zussman. Proceedings of the IRIS Specialty Group on Infrared Detectors I. 13 ( 1991 ). 25. V. Swaminathan, J. W. Stayt, Jr., J. L. Zilko, K. D. C. Trapp, L. E. Smith, S. Nakahara, L. C. Luther, G. Livescu, B. F. Levine, R. E. Leibenguth, K. G. Glogovsky, W. A. Gault, M. W. Focht, C. Buiocchi, and M. T. Asom, Proceedings of the IRIS Specialty Group on Infrared Detectors, Moffet Field, CA ( 1992). 26. D. Duston, BMDO's IS&T faces new hi-tech priorities, BMD Monitor, 1 8 0 183, May 19 (1995). 2 7. M. T. Chahine, Sensor requirements for Earth and Planetary Observations, Proceedings of Innovative Long Wavelength Infrared Detector Workshop, Pasadena, California, pp. 3-31, April 24-26 (1990). 28. S. D. Gunapala, S. V. Bandara, A. Singh, J. K. Liu, S. B. Rafol, E. M. Luong, J. M. Mumolo, N. 0. Tran, J. D. Vincent, C. A. Shott, J. Long, and P. D. LeVan, 6 4 0 x 4 8 6 Long-wavelength Two-color GaAs/A1GaAs Quantum Well Infrared Photodetector (OWIP) Focal Plane Array Camera IEEE Trans. Electron Devices

4 7 , 9 6 3 - 9 7 1 (2ooo).

GaAs/A1GaAs basedquantum well infraredphotodetectorfocal plane arrays 119 29. S. D. Gunapala, S. V. Bandara, ]. K. Liu, S. B. Rafol, C. A. Shott, R. ]ones, S. Laband, ]. Woolaway II, ]. M. Fastenau, and A. K. Liu, 9 IJm Cutoff 6 4 0 x 512 Pixel GaAs/Al• Quantum Well Infrared Photodetector Hand-held Camera, to be published in SPIE Proceedings 4 7 2 1 (2002). 30. S. V. Bandara, S. D. Gunapala, ]. K. Liu, E. M. Luong, ]. M. Mumolo, W. Hong, D. K. Sengupta, and M. ]. McKelvey, 10-16 IJm Broadband Quantum Well Infrared Photodetector, Appl. Phys. Lett. 72, 2427 (1998). 31. S. V. Bandara, S. D. Gunapala, ]. K. Liu, S. B. Rafol, ]. M. Mumolo, and D. Z. Ting, Array of OWIPs With Spatial Separation of Multiple Colors, NASA Tech Briefs 26 No. 5, 8a (2002). 32. A. Singh and D. A. Cardimona, SPIE, 2 9 9 9 , 46 (1997). 33. A. Singh and D. A. Cardimona, Opt. Eng. 38, 1424 (1999). 34. D. C. Arrington, ]. E. Hubbs, M. E. Gramer, Gary A. Dole, SPIE 4 0 2 8 , 288 (2000). 35. M. Ershov, S. Satou, and Y. Ikebe, 1. Appl. Phzds. 86, 6442 (1999) 36. N. M. Haegel, C. R. Brennan, and A. M. White, 1. Appl. Phys. 80, 1510 (1996). 37. S. Gunapala, S. Bandara, ]. Bock, M. Ressler, ]. Liu, ]. Mumolo, Sir Rafol, D. Ting and M. Werner, Large Format Long-wavelength GaAs/A1GaAs Multiquantum Well Infrared Detector Arrays for Astronomy SPIE 4 2 8 8 , 2 7 8 - 2 8 5 (2001).


Chapter 4

GalnAs(P) based QWIPs on GaAs, lnP, and Si substrates for focal plane arrays J. Jiang and M. Razeghi

4.1 Introduction 4.7.1 Overview of infrared detector

Every object emits infrared light. The hotter an object is, the more radiation it will emit. Detection and processing of infrared radiation can provide a wealth of information about an object that is not available in other regions of the spectrum. Infrared detectors are very important in both civilian and military applications and have been extensively studied over the past century. In general, infrared detectors can be categorized as either thermal detectors or photon detectors (as shown in Figure 4.1). Photon absorption in thermal detectors leads to an increase in temperature, resulting in a measurable change in certain material properties. The operation of the photon detectors, on the other hand, is based on the measurement of an electrical photocurrent generated by photon absorption in a semiconductor. A comparison between thermal and photon detectors is given in Table 4.1. The atmosphere has a few important transmission windows in the infrared region (as shown in Figure 4.2). These atmospheric windows are important for almost all infrared detector applications, allowing detection of objects at long distances. The short wavelength infrared (SWIR) window extends from the visible region up to 2.5 pm, the middle wavelength infrared (MWIR) window lies between 3 and 5 l~m, and the long wavelength infrared (LWIR) window ranges from 8 to 14 pm. The location of the LWIR window is fortunate since room temperature objects have a peak wavelength of almost 101~m. Detection of wavelengths larger than 141um, so called very long wavelength infrared (VLWIR), is mainly used for outer space application.


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Almost all the real-world applications of the infrared detector are based on thermal imaging using the infrared focal plane array (FPA) camera. An FPA is an optical sensor placed at the focal plane of an optical system such as a camera, spectrometer or telescope. The infrared FPA is composed of an infrared detector array, which can be designed and manufactured to be sensitive to wavelength range from SWIR to VLWIR based on both thermal and photon detectors. Infrared FPAs are combined with a read out integrated circuit (ROIC) or multiplexer which allows the electronic access of every pixel in the array.

GalnAs(P) based OWIPs on GaAs. InP. and Si substrates for focal plane arrays Table 4.1

123

Pros and c o n s o f thermal and p h o t o n infrared detectors

Detectors

Advantages

Disadvantages

Thermal

9 Relatively low cost imaging possible 9 Sensitive over a wide infrared r a n g e 9 Light, rugged, reliable, and convenient to use 9 A d e q u a t e response time for some imaging applications 9 Easy to tailor the b a n d g a p of the alloys to cover the entire infrared region 9 High detectivity 9 Well developed theoretical and experimental results 9 Good material properties 9 Fast response

9 Slow response {ms o r d e r ) d u e to reliance on h e a t i n g of atomic s t r u c t u r e 9 Relatively low detectivity (D*~ 1 ()s cmHz 1,2/W )

Photon

9 Difficulty in device processing 9 High cost in g r o w t h and device processing

4.1.2 Quantum well infrared photodetector What is a Q WIP?

The q u a n t u m well infrared photodetector (OWIP) is a semiconductor infrared photon detector relying on intersubband absorption within either the conduction band (n-type) or the valence band (p-type). The idea of utilizing a q u a n t u m well for infrared detection was first presented by Esaki and Sakaka in 19 7 7 and can be explained by using the basic principles of quantum mechanics. 1 The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent SchrSdinger equation. The solutions to this problem are the eigenvalues that describe energy levels inside the q u a n t u m well in which the particle is allowed to exist. OWIPs are built from quantum wells of wide-bandgap materials that in bulk do not absorb in the MWIR or LWIR. However, electron (hole) excitation may occur between the ground state and excited states in a conduction (valence) band quantum well (see Figure 4.3), making intersubband absorption possible in the MWIR or LWIR regions. The quantum well structure is designed so that these photoexcited carriers can escape from the quantum well and can be collected as photocurrent. These detectors afford greater flexibility because the peak and cutoff wavelength can be continuously tailored within a range by varying layer thickness (quantum well width) and barrier composition (barrier height). With appropriate choice of the well and barrier material, the detection wavelength of a QWIP can be tailored to any wavelength from MWIR to VLWIR. Why QWIP?

So far, the 'king' of infrared detectors has been the HgCdTe intrinsic photodetector (MCT). HgCdTe is the most studied semiconductor material for IR detectors and it is the standard against which all of other IR photon detectors are matched against. 2'3'4 It has very high quantum efficiency and detectivity: at

124


Figure 4.3 Schematic of intersubband absorption, which takes place entirely within the valence band (for ptype doping, from H~ to H2) or conduction band (for n-type doping, from E1 to E2) of a quantum well. respectively. 7 7 K, a reported q u a n t u m efficiency exceeding 70%, and a detectivity exceeding 1012 cmHzl/2W-1. Focal plane arrays (FPAs) as large as 6 4 0 x 4 8 0 and dualcolor 1 2 8 x 1 2 8 FPAs have been demonstrated, s'6 However, the future of this material is uncertain because of the difficulties associated with its growth, processing, and device stability (see Table 4.2). Yields as low as 1% for 128 • 128 arrays produce costs per array as high as $10 0 0 0 - - 6 0 000. 7 On the other hand, QWIPs use the III-V material system, which is very mature due to the previous development of GaAs- and InP-based lasers, LEDs, and microwave circuitry. The growth and fabrication of QWIPs can be borrowed directly from these technologies. The advantages of QWIPs are also shown in Table 4.2, including their use in high frequency applications, s Although most QWIPs must be operated at cryogenic temperature, advances in closed cycle cryo-coolers have reduced both the size and cost of such systems. QWIP FPAs show better or comparable array performance to HgCdTe at a much lower cost. 9 The first n-type OWIP device based on intersubband absorption was demonstrated by Levine et al. on GaAs/AIGaAs material system in 1987.1~ Its peak spectral response occurs at l O.8~tm. The first p-type QWIP was demonstrated by Levine et al. in 1991.11 Also in 1991, only four years after the first demonstration of the QWIP, Bethea et al. obtained the first infrared image using a ten-element linear scanning OWIP array. 12 W h y Al-ffee?

GaAs/A1GaAs historically has been the most studied III-V material system because of its simplicity. Today, most of the reported FPAs were based on GaAs/ A1GaAs OWIP structures. Al-free III-V materials, on the other hand, have some inherent advantages, like reduced surface recombination velocity, less stringent

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arrays Table 4.2

125

C o m p a r i s o n b e t w e e n i n f r a r e d HgCdTe a n d QWIP d e t e c t o r .

Detector type

Advantages

Disadvantages

HgCdTe

9Excellent quantum efficiency 9 Very high detectivity

9 Poor array operability and uniformity 9 Material defects result in low RoA for many VLWIRpixels. Ro non-uniform 9 Unstable alloy and difficult material growth control 9 Radiation-hard arrays are difficult due to narrow bandgap and defects in material 9 Low yield and high costs for large-area arrays 9 Reproducibility is poor due to large sensitivity of bandgap to composition

9 Bandgap can be adjusted to vary detection wavelength

QwIP

Mature III-V growth technology 9Lower quantum efficiency than intrinsic HgCdTe 9 Wide-bandgap material is better for radiation-hard applications. 9Excellent array uniformity 9 High RoA allows long integration time 9 Narrow photoresponse spectrum 9Multi-color arrays demonstrated 9Very fast response for ultra-high frequency applications

9 Requires lower sensor temperature than intrinsic detectors for ). < 12 Hm 9 Require light coupling for n-type QWIPs

processing r e q u i r e m e n t s , a n d a b s e n c e of DX centers. To take a d v a n t a g e of these properties, people started to g r o w OWIPs w i t h o t h e r m a t e r i a l systems, like I n G a P / G a A s , G a l n A s P / I n P , G a I n A s P / G a A s , et a/. 13'14'1s Table 4.3 gives a c o m p a r i s o n b e t w e e n Al-free a n d Al-based s e m i c o n d u c t o r materials. W i t h the a d v a n t a g e s of Al-free m a t e r i a l s given in Table 4.3, high-reliability a n d low-cost infrared OWIP FPAs based on these m a t e r i a l s y s t e m are h i g h l y expected. In addition, the Al-free OWIP g r o w n on silicon s u b s t r a t e also s h o w e d better d e t e c t o r p e r f o r m a n c e t h a n t h a t of Al-based OWIP on silicon (see Section 4.6), w h i c h t r a n s l a t e s to better m o n o l i t h i c i n t e g r a t i o n infrared FPA based on Al-free QWIPs. As a result, the Al-free GalnAs(P) m a t e r i a l system, as a n i m p o r t a n t a l t e r n a t i v e to A1GaAs m a t e r i a l s y s t e m for OWIP s t r u c t u r e , has been receiving m o r e a n d m o r e a t t e n t i o n in r e c e n t years. In the rest of this c h a p t e r , we will focus on the GaInAs(P) OWIP s t r u c t u r e g r o w n on v a r i o u s substrates, s u c h as GaAs, InP, a n d silicon.

4.1.3 S t a t e - o f - t h e - a r t

The detectivity (D*) is the p r i m a r y figure of merit used in e v a l u a t i n g the single e l e m e n t p h o t o d e t e c t o r p e r f o r m a n c e . Today. the D* for 4 pm a n d 9 pm

126


Table 4.3

C o m p a r i s o n b e t w e e n Ai-free and Al-based s e m i c o n d u c t o r m a t e r i a l s 16't7

Al-free material

Al-based materials

9 No oxidation, which results in higher device reliability 9 Simple device processing

9 Passivation needed for device fabrication

9 Low material growth temperature 9 Reduced surface recombination 9 DX-defect centers associated with AI are avoided 9 Potential for growth of monolithic integration with Si based device.

9 Higher material growth temperature which results in interdiffusion problem 9 Poor device reliability

OWIPs measure about 1.5 • 1012 cm-Hzl/2/W and 2x 1()~ cm-Hzl/2/W at 77 K, respectively. 1s.19 In addition, thanks to the narrow absorption peak exhibited by QWlPs, multicolor operation is possible by simply stacking different types of q u a n t u m wells. Tidrow et al. reported a four-color OWIP detector based on stacked InGaAs/ A1GaAs and GaAs/A1GaAs m u l t i - q u a n t u m wells (MQWs). 2~ OWIP FPAs using a different material system have been demonstrated by m a n y groups. 9'21'22 These FPAs have shown excellent imagery in the LWlR atmospheric window. Large format LWlR OWIP FPAs up to 6 4 0 x 4 8 0 pixels have been commercialized by the Jet Propulsion Laboratory (JPL). 23.24 JPL also demonstrated a 15 pm VLWlR OWIP FPA. 2s For the portable market, a handheld LWIR infrared camera based on a 2 5 6 x 2 5 6 GaAs/AIGaAs FPA was realized. 26 For multi-color FPAs, two-color LWIR/VLWlRFPAs were demonstrated. This FPA gave excellent images with 99.7% of the LWIR pixels and 98% ofVLWIR pixels working. 27 As an example, Figure 4.4 shows a thermal image taken by a 2 5 6 • infrared OWlP FPA camera. The handprint on the book can be seen very clearly.

4.2 Fundamentals of QWIP 4.2.1 Intersubband absorption Traditional interband optical absorption involves photoexciting carriers across the band gap E~, i.e., promoting an electron from the valence-band ground state to the conduction-band excited state (see Figure 4.5). An intrinsic semiconductor photon detector utilizing the interband absorption is only sensitive to the photons whose energy is larger than the semiconductor's energy gap. For the detection of MWIR and LWIR regions of spectrum, a narrow E~ material must be used. It is well known that these low band gap materials are more difficult to grow and process than large band gap semiconductors such as

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arrays

127

Figure 4.4 Thermal image taken by a 256 • ~60WIPfocal plane array camera. The handprint on the book can be seen clearly. (Courtesy of Dr. Murzy ]habvala ).

conduction band absorbed photon

~

-

valence band

Figure 4.5 intrinsic semiconductor detectors, an excited electron must cross the entire bandgap to reach the conduction band.

GaAs. To take advantage of the superior growth technology that exists with the wide bandgap semiconductor material, intersubband absorption inside the quantum wells is employed for detection of MWIP, or LWIR. For intersubband absorption, photon absorption takes place from ground state to excited states in quantum wells, as shown in Figure 4.3. The energy separation between the two bound states, E1 and E2 or H1 and H2, is much smaller than the bandgap of the well or barrier material,which translate to a much longer intersubband absorption wavelength. Thus, by controlling the location of the confined energy levels, the absorbed infrared wavelength can be

128 Handbookof Infrared Detection Technologies controlled. For a one-dimensional periodic q u a n t u m well structure, the position of confined energy levels can be computed by the Kronig-Penney model. 2s'29' 3o The relationship between the energy states and the well and barrier characteristics is given by:

1(

cosxL - coskwcoshabb + -~ rl - 1 )

sinkwsinha,,b

(1)

where X is a solution parameter, L is the period of superlattice thickness (well plus barrier width), k = v / 2m,,,E/~-. m;,,is the effective mass of the well material, E is 9 , the energy, /i is the reduced Planck's constant, w is the well thickness, ab = v/2m*b(Vo- E)ffi2, m~ is the effective mass of the barrier material, V(, is the conduction band offset, b is the barrier thickness, and 71 - m,*,,ab/m~,k. The values of allowed energy levels E can be obtained by numerical solution of equation ( 1 ). From (1), we can see that the absorbed infrared wavelength can be tailored by the bandgap and thickness of well and barrier material. For n-type QWIPs, which utilize an intersubband transition between F-valley derived states, the oscillator strength of a m u l t i - q u a n t u m well structure is given by:

f12 ~ I(FllT"-p[F2}[ 2

2

where F1 and F2 are the electron envelope functions, -K is the polarization vector for the incident infrared light, and - f the m o m e n t u m operator. If we assume the q u a n t u m well growth direction is along the z-axis, then the envelope functions should depend on z only, with F1-Fl(Z) and F2-F2(z). It is seen readily that for normal incidence of infrared radiation, for which 7=(e,.. e.,/, 0), results in zero absorption. One remarkable note to this q u a n t u m selection rule is that it only applies to n-type doped q u a n t u m wells. For p-type q u a n t u m wells, there is strong mixing among the heavy holes and lights holes at non-zero wave vector in the valence band which makes absorption of normal incidence infrared light possible. 31.32 One requirement for detection using q u a n t u m wells is that the well material must be doped. Because the energy of an absorbed photon is less than the bandgap, it cannot produce an excited photocarrier by itself. With the addition of carriers in the well, excitation only needs to happen from the first bound state to the next bound state. For n-type doping, excitation occurs entirely within the conduction band" for p-type doping, excitation occurs entirely within the valence band. Photocurrent is produced w h e n an excited photocarrier is able to escape the well whereupon an externally applied bias can sweep it out to be collected by one of the contacts. For n-type QWIPs, depending on the position of the first excited state in the q u a n t u m well 33 and barrier layer structure (see Figure 4.6), intersubband transitions in OWlPs can be classified to four types" bound-to-bound state (BB), 1~ bound-to-quasibound state (BQB), 34 b o u n d - t o - c o n t i n u u m state (B-C), 3~ and bound-to-miniband (B-M). 36 These structures all exhibit different degrees of

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arrays

129

Figure 4.6 Schematic of energ!t band diagram for B-B. B-C. BOB. and B-M OWIPs.

electron confinement within the q u a n t u m well. In general, less confined upper states exhibit a larger photoresponse bandwidth and weaker peak detectivity. The B-B OWIPs have intersubband absorption occurring between two bound states contained within the q u a n t u m well. However, the carriers excited to the higher bound state still need a way to escape from the well and get swept out. The two possible ways are tunneling through the barrier and thermionic emission. This sort of OWIP has a narrow absorption spectra and large dark current. The large dark current is related to the thin barriers used to facilitate the tunneling of photocarriers out of the q u a n t u m well. In a B-C QWIP, the intersubband absorption is based on the bound-toc o n t i n u u m states transition. The peak absorption is relatively small because the wave functions of the upper excited states spread over the barrier region. The absorption spectrum is expected to be wider due to the broad final states band above the barrier. In a BQB QWIP, the wave function of the upper excited states is aligned with the top of barrier. Since the barrier height also limits the cut-off wavelength, there is a sharp cut-off for this type of detector. In a B-M QWIP, the transition is from the localized bound ground state in enlarged wells to a resonant-coupled miniband of superlattice barriers. Since the well width in a B-M QWIP is m u c h wider than that of B-C and BOB QWIPs, there is a large overlap of wave functions between the initial and final states, and the interaction is strong. A blue shift of the peak detection wavelength is obtained by

130


aligning the excited state to the top of the miniband, while a maximum bandwidth is achieved when the excited state is lined up in the middle of the miniband. Thus, B-M QWIPs offer more flexibility and fine tuning of the detection wavelength and spectral bandwidth than other types of OWIPs. For p-type QWlPs, the absorption and responsivity are much lower than those of n-type OWIP due to the large hole effective mass and low hole mobility. Reduced absorption stems from the absorption coefficient, which is inversely proportional to the effective mass of holes. On the other hand, a key benefit, as mentioned before, is the ability to absorb normal incidence radiation. To improve the intersubband absorption of the p-type QWlP, tensile strained-layer (TSL) or compressive strained-layer (CSL) quantum wells have been utilized. 37-4~J The strong mixing between light and heavy hole states substantially increases the normal incidence absorption in the strained-layer p-type OWIPs. In TSL p-type QWIPs, the intersubband absorption occurs between the ground light-hole (LH1) state to heavy-hole continuum states. Since the ground states are occupied by the light holes with smaller effective mass, the linear optical absorption is greatly enhanced over the unstrained case. In CSL p-type QWIPs, the intersubband absorption occurs between the ground (HH1) heavy-hole state to excited heavy-hole states (e.g. HH 3). Apart from the reduction of hole effective mass by compressive strain in the quantum well, the conducting holes behave like light holes with higher hole mobility due to the resonance between heavyhole and light-hole states. The energy band diagram for TSL and CSL p-type OWIP is shown in Figure 4.7. 4.2.2 Q W i P p a r a m e t e r s

Single OWIP detectors are usually rated by four figures of merit" absorption, dark current, responsivity, and detectivity. The photocurrent I t, generated in a QWIP can be written as Ip - npqv

(3)

Figure 4.7 Schematic energy band diagram for p-t!tpe tensile strained-la!ler (TSL) and compressively strained-layer ( CSL ) QWIP.

GalnAs(P) based O_WIPs on (;aAs. InP. and Si substrates for focal plane arra!ts

1] 1

where np is the volume density of photogenerated carriers, q the electron charge, and v is the transport velocity through the OWIP. For example in n-type OWIPs, np is given by Hp

z

Pcos0c~ Pe l'I, hid

where P is the incident optical power on the detector, p,, is the probability of escape from the q u a n t u m well, 0 is the angle between the incident light and the direction perpendicular to the q u a n t u m wells, and rL is the recapture lifetime. From this information we can calculate the detector's responsivity, which is defined as the ratio of the electrical current generated by the detector to the total optical power absorbed. Expressed mathematically, the peak responsivity Rp m the m a x i m u m value of responsivity as a function of photon energy - - is Rp =

Ip

Pcos0

=

q O.p,,g hv

(4)

where 77,, is double-pass absorption q u a n t u m efficiency and g is the photoconductive gain. Given the length of the m u l t i - q u a n t u m well region as 1. r/,, and g are defined as -

1 - exp(-2cd) 2

(s)

and VrL = _ra = _L

g-

1

(6)

1

rT

where r r = l / v is the transit time and L is the hot electron mean free path. The overall q u a n t u m efficiency 17is calculated as 77 - rl,,pe -

Rp h 1) qg

(7)

This gives an indication of how many carriers are being generated, which may contribute to the photocurrent, versus how many photons are absorbed. Another very important type of current in a OWIP. the dark current, can be calculated from I,(v)

- ,*(V)qv(V)A

(8)

where n*(V) is the effective number of electrons excited from the well into the c o n t i n u u m state, v ( V ) is the average transport velocity, and A is the detector area. n * ( V ) i s expressed as: 41

132


~ f(E)T(E, V)dE n* ( V) - ~ 2m*Lp J E~

(9)

where Lp is the superlattice period. T(E. V) is the tunneling current transmission factor for a single barrier, andf(E)is the Fermi factor, given by

f(E) --

1 + [ e x p ( E - E1 - E,:)/kT]

where E is relative to the bottom of the q u a n t u m well, Et: is the Fermi level, k is Boltzmann's constant, and T is the temperature. The average transport velocity v(V) is given as

p.F -

V/1 + (#F/v,) 2 where # is the mobility, F is the average electric field, and vs is the saturated drift velocity. Related to the dark current is the noise current, which ultimately becomes the limiting factor for the sensitivity of a detector: the signal (responsivity) of a detector can be externally amplified, but the noise current would be amplified as well, leaving the signal-to-noise ratio unchanged. Several kinds of noise have been identified in photodetectors, though there are just two that can dominate the noise current in a QWIP. 42 The first is called generation-recombination noise, or GR noise. It is composed of r a n d o m thermal excitation and decay of carriers in the q u a n t u m wells, leading to fluctuations in the n u m b e r of carriers. The second is Johnson noise, which is a strictly thermal noise, and causes fluctuations in the velocity of carriers. Since noise is a r a n d o m process, the addition of noises obeys the root-mean-square (rms) system, that is, the square of the total noise equals the sum of the squares of the component noises. The total noise current i,, in a OWIP, then, is

In 29

_

i2R 27 llohnson .2

The GR noise current and the Johnson noise current are given by

icR -- v/4qlagAf and

~

( 1 O)

T

where Af is the m e a s u r e m e n t bandwidth and R is the series resistance of the QWIP.

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arra!ts 13 3

Lastly, the specific detectivity gives the signal-to-noise ratio of the detector. It is an indication of how small a signal a detector can pick up. The specific detectivity D* is given by D* = Rp x/-AAf 9 In

(12)

It is a standardized figure of merit for both photovoltaic and photoconductive detectors and is independent of detector area or peak absorption wavelength. The general figure of merit of a 0WIP FPA is the noise equivalent temperature difference (NEAT). NEAT is the m i n i m u m temperature difference across the camera target that would create a signal-to-noise ratio of unity. It's given by 4 ~ NEAT =

v~Af

(13)

D*(dPB/dT)

where D* is the detector detectivity and P I~ is the background photon power. When D* is larger than 101() cm-Hzl/2/W, the NEAT is limited by the uniformity of FPA. 44 4.2.3 Comparison of n-type and p-type Q WIPs

Today, almost all the OWIP FPAs are made of n-type doped OWIP detectors (see Section 4.1.3). The main advantages for n-type 0WIPs are higher responsivity and detectivity due to the higher electron mobility, higher optical absorption and photoconductive gain related with the smaller electron effective mass compared with p-type OWIPs. Today, the values of detectivity for n-type OWIPs are generally higher than that of p-type QWIPs at the same peak wavelength. The obvious drawback in n-type OWIPs is the zero normal incidence absorption due to the q u a n t u m mechanical selection rules. Different light coupling schemes (such as gratings) are thus needed for the n-type QWIPs to obtain better detector performance. This usually complicates the fabrication process of 0WIP FPAs and also limits the pixel size. The normal incidence absorption feature of p-type QWIPs allows the fabrication of grating-less FPAs. Another advantage of p-type 0WIPs is the Fermi Table 4.4

n-type

p-type

Comparison b e t w e e n n-type and p-type QWIP detector

Advantages

Disadvantages

9Higher responsivity and detectivitv due to the higher electron mobility and low electron effective mass 9Detectionrange from SWIRto VLWIR 9Normal incidence absorption 9Grating-less FPA 9Lowerdark current

9Light-couplingrequired 9ComplicatedFPA processing and limited pixel size 9Lowerresponsivity and detectivity 9Difficultyfor VLWIRdetection

134 Handbook of Infrared Detection Technologies level pinning effect at higher doping density, in which the Fermi level above the ground bound state in the OW is nearly independent of the doping density in the OW. Thus, it is possible to dope the OW at a higher doping density to increase the linear absorption coefficient and the responsivity without increasing the dark current in the detector. 4.2.4 Growth, fabrication and device characterization of a single QWIP device

The most widely used growth technologies for OWIP devices are molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). MBE is an ultra-high vacuum thin film technology developed in the early 1970s. 4s Both gas source (GSMBE) and solid source (SSMBE) can be used for material growth. The advantages of this growth technology are an ill sitll controllable growth process, extremely precise control over layer thickness and doping profile, and high uniformity ( > 3 inch). Its main disadvantage is the high expense. MOCVD has established itself as a unique and important epitaxial crystal growth technique which yields high quality low dimensional structures for fundamental semiconductor physics research and devices. 4~' This growth technique is attractive in its ability to grow uniform layers, sharp interfaces, and for its commercial applications. The details of MBE and MOCVD technology have been described in many publications. 4 s-4 r In general, a OWIP structure is grown on a semi-insulating substrate (e.g. InP, GaAs) and consists of 2 0 - 5 0 periods of multiple q u a n t u m well (MOW) sandwiched between a top and bottom contact layer. The q u a n t u m well is doped n or p-type, as desired. Once a OWIP structure has been grown either by MBE or MOCVD, it must undergo a fabrication process to create a working detector. The basic fabrication procedures are: 1. Cleaning of sample in the order of trichloroethylene (TCE), acetone, methanol, and DI water. 2. Positive photolithography to define the detector pixel and light coupling structure (if needed). 3. Etching of the exposed semiconductor, either by dry etching {e.g. electron cyclotron resonance-enhanced reactive ion etch (ECR-RIE)) or wet chemical etching. 4. Negative photolithography for the metal contact definition. 5. Metal contact evaporation. 6. Lift-off process and alloying. Z Device packaging (die-bonding and wire-bonding). A finished GalnAs/InP OWIP mesa detector is shown in Figure 4.8. As discussed in Section 4.2.1 for n-type QWIPs, since there is no absorption for normal incidence, a light coupling scheme needs to be applied on the OWIP device for detection of the desirable normal incidence illumination. For a single n-type OWIP detector, as one step of device fabrication, a 45 ~ facet is usually polished on the substrate for light coupling. Different coupling schemes,

GalnAs(P) based QWIPs on (;aAs. InP. and Si substrates for focal plane arra!ls

13 5

Figure 4.8 A schematic diagram of the GalnAs/InP QWIP on InP substrate. This is how the device appears after fabrication, with the mesas etched and the metal contacts deposited.

for mull-element detectors, like diffraction gratings, 4~ random scattering reflectors, 49 microlenses, ~(~ and corrugated structures ~1 have also been developed. Several device characteristics, like the absorption spectra, relative spectral response, blackbody responsivity, noise current, and dark current, are usually measured to evaluate the performance of OWlP devices. The packaged OWIP device is usually tested inside a liquid nitrogen cryostat. The absorption coefficient given by an absorption spectra m e a s u r e m e n t can be used to calculate the room temperature (T=300K) and low temperature ( T = 7 7 K ) absorption q u a n t u m efficiency using r/~.~2 The relative spectral response gives information about a detector's peak absorption wavelength, cut-off wavelength, and spectral linewidth. Both the absorption spectra and relative spectral response can be performed with a Fourier Transform Infrared (FTIR) spectrometer. The absolute magnitude of the responsivity can be accurately determined by measuring the photocurrent, Ip, with a calibrated blackbody source that is at a specific temperature. Noise current m e a s u r e m e n t can be performed on a fast fourier transform (FFT) spectrum analyzer. The dark current of the OWlP can be measured with a semiconductor parameter analyzer and a dark shield placed around the OWIP inside the cryostat. Once the noise current and dark current are known, the overall q u a n t u m efficiency, photoconductive gain, and specific detectivity of a detector can be calculated by the formulas given in Section 4.2.2.

4.3 Fabrication of infrared FPA 4.3.1 Infrared FPA fabrication steps

As we mentioned in Section 4.1.1, most of the infrared detector applications are based on FPA cameras. So, how to make an operational infrared FPA also becomes an important issue for OWIP researchers. In this section, we will give a

136


Figzire 4.9 Schematic qf ir(lrared FPA fabrication process.

GalnAs(P) based O WIPs on (;aAs. InP. and Si substrates for focal plane arrays

137

general description of the infrared FPA fabrication process. A schematic of infrared FPA fabrication is shown in Figure 4.9. Many technologies and much equipment are involved in this complicated fabrication process. The details of this process are described below step by step. 1. Growth of an infrared detector structure on the appropriate substrate. The growth is performed by MOCVD or MBE technology. This step basically determines the final FPA~sperformance. Different characterization will be performed to ensure that the detector's properties meet the requirements. 2. Fabrication of detector pixels. This includes the UV-photolithography for the sample patterning and the etching (like ECR-RIE dry etching or wet chemical etching) for the pattern transfer. For large format FPAs, the pitch size (array spacing) is usually between 30 and 50 l~m. The detector pixel size is even smaller. 3. Detector pixel passivation. Si02 or Si3N4 are common choices for the passivation material. The deposition of Si02 and Si 3N4 can be performed by plasma enhanced chemical vapor deposition (PECVD). The passivation layer will neutralize the surface states and insulate the covered areas. 4. Patterning and etching of the passivation layer. The purpose of this step is to make openings for the metal connection to each detector pixel. The opening size depends on the pixel size. 5. Fabrication of indium solder bumps. This is the most complicated step during the FPA fabrication and will be described in more detail in the next section. Indium bumps will be used as the interconnection between detector and ROIC pixels. 6. Flip-chip bonding of the detector substrate with a Si-based ROIC. The schematic of the alignment procedure is shown in Figure 4.10. An optical head will be brought between the upper and lower chips. The surface image of both upper chip and lower chip will be displayed on a monitor with the help of two video

Figure 4.10 Schematic offlip-chip bonding procedure.

138

Handbook of Infrared Detection Technoloqies

Figure 4.11 An indimn joint ~(fter reflow process.

cameras. The alignment can be done either manually or automatically. After fine alignment and parallelism, the optical head will be removed and the upper and lower chip will be pressed together with preset pressure and temperature. The indium joint needs to be refiowed at the preset temperature within an inert atmosphere. A refiowed indium joint is shown in Figure 4.11. 7. Underfill dispensing between FPA and ROIC. Underfill is usually based on an epoxy system. 53 The epoxy underfill provides the necessary mechanical strength to the detector array and readout hybrid, prior to the thinning process. It also reduces the effect of global thermal expansion mismatch between the detector array and readout chip and protects the FPA hybrid, from moisture, ionic contaminants, and hostile operating environments such as shock and vibration. 8. Detector substrate thinning process. This is usually performed by abrasive polishing or wet chemical etching. In some cases, the substrate is completely removed. The purpose of this step is to further reduce or completely eliminate (for substrate removal) the thermal expansion mismatch between detector array and readout chip. This basically to allow the OWlP FPAs to go through an unlimited number of temperature cycles without any indium bump breakage or delamination. Furthermore, the substrate thinning process also eliminates the pixel-to-pixel optical cross-talk and significantly enhances the optical coupling of infrared radiation into the OWIP pixels. 54 9. Packaging of the FPA hybrid. The hybrid will be loaded onto a lead-flee ceramic chip carrier (LCCC) by the die-bonding process and input/output metal pads on the readout chip will be connected to the LCCC pins by a wire-bonding process. 10. The final step of OWIP FPA fabrication is the test and evaluation of the FPA hybrid's performance. The test system is usually composed of a liquid-nitrogen Dewar, a cold clamp assembly, a camera head, digital electronics, and computerbased testing software.

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arra!ls

139

4.3.2 Indium solder bump fabrication steps

Indium has become the most important mating material for infrared FPAs due to the fact that it stays ductile at liquid helium temperature, is easy to work with, and forms a good bond at low temperature. Highly-uniform and tall indium bumps are required for the infrared FPA fabrication. It has been shown that taller indium bumps can increase the FPA hybrid's reliability and thus its operating life. ~~ An indium solder bump is chiefly fabricated by two technologies: evaporation and electroplating. The comparison between the two technologies is shown in Table 4.5. In this section, the low-cost electroplated indium bump technology will be described. Figure 4.12 shows the schematic representation of the fabrication process for an electroplated indium solder bump. ~6-~s The details of an indium solder bump fabrication process are given below step by step. 1. Passivation of detector pixels. This is same as step three in the FPA fabrication described above. 2. Deposition of a Ti shortcut layer. This layer serves as a current path for the later electroplating process. 3. Thick photoresist ( > 10 ~m) lithography. The opening on the thick photoresist provides the mold for the indium electroplating process. Very strict control of photoresist spinning and UV-exposure is required to assure a good mold profile, s9 4. Electroplating of Under-Bump-Metallurgy (UBM) layers. A high reliability solder bump interconnect is comprised of two features: the UBM, also known as the Ball Limiting Metallurgy (BLM), and the solder ball itself. UBM is usually composed of several different metal layers and it provides the following features: good adhesion to the detector pixel metal pad, an effective solder diffusion barrier, and a solder wettable metal of appropriate thickness. 6~ For indium solder bumps, Ti/Au can be used as UBM layers. 5. Electroplating of indium solder bumps. This is performed in an appropriate indium plating bath (e.g. indium sulfamate) under a controlled current density. The plated indium bumps usually have a mushroom-shaped profile before reflow (see Figure 4.13 (a)). Table 4.5

Comparison between evaporated and electroplated solder bumping technology Advantages

Disadvantages

Evaporated solder bumping technology

9 Highly uniform bump 9 Excellent metallurgical control of bump

Electroplated solder bumping technology

9 Lower equipment costs 9 Simple fabrication process

9 High cost due to expensive evaporation equipment 9 Limited to material with high vapor pressure 9 Complicated fabrication process 9 More pollution to environment. Why? 9 Low t h r o u g h p u t 9 Non-uniform bump height 9 Induced subsurface wafer stress 9 Poor alloy control


Figure 4.12 Schematic of the indium solder bump fabrication process.

6. Removal of the thick photoresist. This can be done with Acetone or photoresist stripper. 7. Reflow of the indium solder bumps. The purposes of reflow are to increase bump height by reshaping the indium into a sphere (see Figure 4.13(b)) and to facilitate the flip-chip bonding alignment. Reflowed indium bumps have also been shown to have higher reliability. 61 Reflow is usually performed in a rapid thermal process oven (RTP) under N2 atmosphere. Some novel reflow methods utilizing ECR-RIE and low-pressureMOCVD were also demonstrated. 62'123 These methods can effectively remove the indium oxide on the surface of indium bumps to ensure the high quality reflow. 8. Removal of Ti shortcut layer. This can be done by wet chemical etching with dilute HF (2.5 %). 4.3.3 ROIC for infrared FPA

As we described in Section 4.3.1. infrared FPAs are mated via indium bump technology to silicon based readout integrated circuits (ROIC) which allow electronic access to every single detector pixel in the array. The ROIC serves as an electrical interface between the infrared FPA and off-focal plane signal

GalnAs( P) based OWIPs on GaAs. InP. and Si substrates for focal plane arrays

141

Figure 4.13 (a) Mushroom-shaped indium sohh'r bump before reJlo~v. (b) Indium solder ball after reflow process.

processing electronics. A ROIC is usually a hybrid between a multiplexer and an array of low power circuits which, at a minimum, provide a precise bias voltage to the detectors and extract the photocurrent from them with the least possible noise. During the FPA operation, the integration capacitor in the ROIC unit cell stores the charges provided by an individual detector photocurrent within a fixed integration time. The resulting signal is multiplexed to a serial stream of sampleand-hold signals within the fixed readout time. Additional focal plane circuitry is usually required to improve the uniformity, signal-to-noise, and versatility of the FPA. In most ROICs, the signal is pre-amplified at the front-end stage to avoid further corruption by other noise sources. A number of readout preamplifiers, such as self-integrator (SI), source-follower detector (SFD), direct injection (DI), source-follower direct injection (SFDI), and capacitor feedback trans-impedance amplifier (CTIA), have been utilized for infrared FPA applications. (~3 Figure 4.14 shows a ROIC unit cell with the most popular DI type preamplifier. 64 The detector bias voltage is controlled by the Vbias_adj gate and the signal is multiplexed out from the sample and hold capacitor during the readout period. The anti-bloom gate keeps the input circuit from saturating. Today, most ROICs are built by advanced sub-micron cmOS technology which has the advantages of higher operation frequency and capability for random access. 65 Many types of large format ROICs are currently commercially available. 66'67'126 The general requirements of ROICs for infrared FPAs are: stable detector bias; small input resistance: low nonlinearity: low power consumption; and large dynamic range. (~ For a specific type of infrared detector, the ROIC has to be tailored to the detector characteristics.

4.4 p-type QWIPs Hole intersubband absorption is attractive because the strong mixing of heavy and light holes in the valence band (at k~:()) allows the desirable normal incidence illumination geometry to be used. To fully utilize the advantage of


Figure 4.14 Simplified schenmtic of a ROICmlit cell based on a direct injection input circuit.

p-type intersubband transition, p-type OWIPs based on different material systems have been studied by m a n y groups. In this section, we will give some examples of p-type OWlP for detection of different wavelength range. All of the OWIP structures reported here are based on Al-free GalnAs(P) material systems. 4.4.1 p-type MWIR QWIPs p - t y p e GaAs/GalnP

J. Hoff et al. reported MWIR p-type OWIP based on a lattice-matched (no strain) GaAs/GaInP MQW structure. 69 Three p-type OWIP structures were grown on semi-insulating GaAs substrate by a low-pressure MOCVD reactor. Each structure contained 50 GaAs q u a n t u m wells separated by nominally 280 A wide Gao.51 Ino.49 P barriers. These superlattices were sandwiched between thick GaAs layers used for top (0.5 l~m) and bottom (1.() l~m) contacts. All GaAs layers were doped with Zn to a net acceptor concentration of 1 x 1 ()lg cm-3. Different GaAs well widths (22A, 32A, and 55A) were used for the three different structures. 4 0 0 x 4 0 0 l~m mesa detectors were fabricated using photolithography and wet chemical etching, and 1 5 0 x l S ( ) l ~ m square Au/AuZn electrodes were evaporated and alloyed. The devices were front-illuminated under normal incidence. The spectral response shifts towards longer wavelength as the well width is reduced, which is expected as the hole ground state is pushed up towards the top of the barrier. Dark current was measured as a function of bias for the 22 A well at a different temperature (see Figure 2 in reference 69). At low bias, tunneling is negligible and the dark current originates from thermionic emission above the top of the barrier. The activation energy of holes in the q u a n t u m well can be calculated directly from the dark current data. Since dark current is dominated by thermionic emission from the q u a n t u m well at temperatures greater than 4 5 K, there is an Arrhenius dependence of dark current with temperature.

GalnAs( P ) based OWIPs on GaAs, InP, and Si substrates for focal plane arrays

l n ( I D / T ) oc - E A / k T

14 3

(14)

Where EA is the activation energy out of the quantum well. The EA revealed from this method for the three samples are 2 3 5 , 3 0 0 . and 330 meV for the 22, 32, and 55 A wide well QWIP respectively. These activation energies correspond to wavelength of 5.28, 4.13, and 3.75 l~m, respectively. Those values closely match to observed cut-off wavelength in photoresponse curves (see Figure 1 in reference 69). The detectivity of 8 x l O ~ cmHzl/2W-] at 2.5 l~m and T=77K was obtained for the 22 A well QWIP. This low detectivity is related to its small gain and absorption quantum efficiency (,-, 1.4 x 1 ()- 3) at 2.5 Bm. Each of these devices was found to be background limited at liquid nitrogen temperatures ( 77 K) or higher. Typical p-type GaAs/A1GaAs OWIPs peak within 3 0 - 5 0 meV above the cut-off wavelength. 7~ The magnitude of the photoresponse decreases with decreasing wavelength thereafter. However. the peak response in these GaAs/ GaInP QWIPs is typically 2 0 0 m e V or more above the cut-off wavelength. Moreover, the 22 A wide well sample appears to have two peaks to its spectral response. These unique spectral shapes were proven, through the device modeling by a Kane Hamiltonian, to be caused by the influence of the spin splitoff band. Therefore, since there is significant coupling between the heavy hole and the split-off and between the heavy hole and the light hole, the observed spectral shapes result from the dual influence of split-off extended state absorption with light-hole extended state absorption. The lower energy peak seen in the 22 A wide sample corresponds to the appearance of the second lighthole state in the continuum of the standard q u a n t u m well. The 32 A and 55 A wide well samples are wide enough for the second light-hole state to be buried in the standard quantum well so it cannot contribute to photoresponse. p-type GaAs/GalnAsP QWIP

To increase the cut-off wavelength, a lattice-matched GaAs/ Gao.71Ino.29Aso.39Po.61 OWIP was grown by LP-MOCVD with 50 periods of 30 A wide GaAs quantum wells separated by 280 A wide GaInAsP (Eg=l.8 eV at T= 300 K) barriers. 72 The effect of the quaternary alloy was to reduce the valence band offset relative to lattice matched GaAs and thus increase the cut-off wavelength. A photovoltaic effect is observed from the photoresponse curve (see Figure 1 in reference 72). This arises from an asymmetric quantum well potential profile. Such asymmetry could be related to either a structural difference in the two q u a n t u m well interfaces or to dopant migration during the material growth. The photoresponse has a peak around 4 lum with broad maxima. Compared with the GaAs/GaInP OWIP reported in the last section, the peak wavelength increased slightly. This OWIP remains background limited up to a detector temperature of 100 K for biases between - 7 . 5 V and +2.5 V. The activation energies derived from Arrhenius plots of clark current, 170, 204. and 226 meV for bias voltages +9, - 9 , and 0.1 V respectively, agree with the cut-off wavelengths observed in the photoresponse curve. 72

144 Handbookof Infrared Detection Technologies p-type GalnAs/InP QWIP Sengupta et al. reported a lattice-matched GaInAs/InP p-type QWIP. 7~ The

QWIP structure was grown by GSMBE on InP substrate and consisted of 30 periods 10 A uniformly Be-doped (p= 3 x 1 () 1~ cm- 3) Ga(~.47In(}.s ~As wells and 500 A thick, uniformly Be-doped (1 x 1 ()l 7 cm- ~) InP barriers. 200 ~m diameter mesa was fabricated by wet chemical etching. The 4 5 ~ incidence responsivity at 80 K was measured as a function of wavelength at different bias voltage (see Figure 2 in reference 73). The peak photoresponse shifts slightly with bias and an increase in magnitude of response was observed with positive bias voltage. They attributed the increase in response with magnitude of the bias voltage to the increased photoexcited carrier escape and the increased carrier drift velocity. 4.4.2 p-type LWIR Q WIPs p-type GalnAsP/GalnAsP QWlP

To achieve LWIR detection, J. Hoff et al. proposed a fully quaternary ptype O W I P . 74'7~ The result, according to theory, should be a QWIP with a cut-off wavelength of approximately 1() ~m. p-type lattice-matched Gao.87Ino.13Aso.74Po.26/Ga(~.~,2In(~.3sAs(j.22P(j.7~ QWIP with 30 A wells and 2 8 0 A barriers was grown by LP-MOCVD on GaAs substrate. One remarkable feature of this q u a t e r n a r y - q u a t e r n a r y QWIP is that the photoresponse is extremely broad, from 2.5 lam to 1 () btm. 74 This unique spectral shape occurs because of the dual influence of the light-hole extended state and the split-off extended state. It also indicates that high quality quaternaryquaternary superlattices should be inherently capable of multi-color photoresponse.

4.5 n-type QWIPs Compared to p-type QWIPs, it is easier to realize longer infrared wavelength detection with n-type OWIPs due to the smaller conduction band discontinuity. Also, due to the smaller electron effective mass and higher mobility, n-type OWIPs have shown excellent detector performance. In this section, we give some n-type QWIP examples that cover the wavelength range from MWIR to VLWIR. In addition, a multi-color QWIP example is also given at the end. 4.5.1 n-type t WIR Q WIPs

LWIR is the most important atmospheric window since room temperature objects have a 10 l.tm peak wavelength. LWIR QWlPs based on m a n y different material systems have been demonstrated. In this section, detector performance of three n-type GaInAs/InP OWlP samples will be described.

GalnAs(P) based OWIPs on GaAs, InP. and Si substrates for focal plane arra!ts

145

n - t y p e GalnAs/InP Q W I P

GalnAs/InP OWIP was firstly fabricated by Gunapala et al. in 1991. 76 Compared to the most popular GaAs/AlxGal_xAs OWIPs, the Al-free GaInAs/InP QWIPs have the following advantages" 1. more straightforward device fabrication process since no passivation is required; 2. higher electron mobility due to the lower effective electron mass: 3. binary InP barrier layers have an inherently lower defect density which results in lower dark current. Samples (A, B, and C) were grown on semi-insulating InP subsrate by a LPMOCVD. 77 Each QWIP structure consists of 20 periods of 60 A GaInAs wells, separated by 500 A InP barriers. Only the middle 50 A of each well was doped with Si and doping densities are 1.7• 5• and 1.7x1018 cm -3 for sample A, B, and C, respectively. 4 0 0 x 4 0 0 Bm OWIP mesa detectors were fabricated by photolithography and ECR-RIE dry-etching. Since each mesa has 4 5 ~ sidewall after dry-etching (with BCI ~/C12/Ar etch chemistry), the OWIPs were measured with normal incidence with front-illumination geometry. The dark current for the three OWIPs was measured as a function of bias voltage (see Figure 3 in reference 77).The large increase of dark current with the doping density in the well is due to the decrease in activation energy. The peak detection wavelength was measured at 9.013m for all three samples (see Figure ] in reference 77). One noteworthy aspect to point out is the exponential dependence of the responsivity with applied bias (see Figure 2 in reference 77). '

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Figure 4.15 Peak responsivit!l of a corrugated (;alnAs/InP C)iVII~with corrugation period 10 I~m. T= 77 K.

146 Handbookof Infrared Detection Technologies This behavior is very different from what is usually observed in GaAs/AIGaAs QWIPs, whose responsivities increase linearly with bias then saturate at higher bias. A likely cause is avalanche gain, which is due to impact ionization of electrons excited out of the quantum wells. 7s Sample B yields a maximum detectivity of 3 . 5 x l O l ~ at an operating bias 0.75V. Further, by changing the doping density by a factor of 10, the detectivity changes by over a factor of 20 (see Figure 6 in reference 77). With a more effective light coupling scheme, the detector performance of GaInAs/InP OWIPs can be improved dramatically. Figure 4.15 shows a peak responsivity curve for a GaInAs/InP OWIP which is covered by corrugation on its detector mesa. 79 This QWlP has the identical structure parameters as sample B, and the corrugation period is 1()~m. The corrugation is created with dryetching by a ECR-RIE and has approximately 60 ~ sidewall. Compared with a peak responsivity of 33.2 A/W at a bias voltage of + 5 V for sample B, 77 a record high responsivity of 218 A/W was obtained at bias of + 5 V (mesa top positive) for this corrugated GaInAs/InP QWIP. 4.5.2 n-type VLWIR Q WIPs

n-type InGaAsP/InP Due to the smaller conduction band offset between GaInAsP and InP, peak photoresponse wavelengths ranging from 8-2()IJm and beyond are possible by changing the composition of the quaternary, g() Two lattice-matched GaInAsP/ InP QWIPs (A and B: E,=O.82eV) and one GaInAs/InP (C, the same structure parameter as sample A from Section 4.4.1) was grown by LP-MOCVD. gl The GaInAsP/InP OWIP structure consists of 20 periods of 65 A GaInAsP wells, separated by 500 A InP barriers. The bandgap of well material for sample A and B are 0.95 eV (2=1.3 ~tm) and 0.80 eV (,;.=1.55 lam). The n doping level in the well is 1.7 x 10 ~7 c m - 3. The relative response curves were measured by a FTIR at 80 K (see Figure 4 in reference 81 ). For a bias of - 1 V (mesa top negative), the 50% cut-off wavelengths for the three samples are 9.3, 10.7, and 14.2 ~m for A, B, and C, respectively. The FWHM (as Ak/)~) of the three samples are 36%, 10%, and 5.5% for A, B, and C, respectively. The switch from a bound-to-bound to a bound-to-continuum transition between sample B and A is evident. The measured peak detectivity for the three samples were 1.1 x l 0 s cmHzl/2W -1 (T=3OK), 2x l 0 s cmHzl/2W-~ (T=80 K), and 8x 1 0 9 cmHzl/2W -1 (T=80 K) for sample A, B, and C, respectively. n-type InGaAs/GaAs QWIP The longest detection wavelength for OWIP was reported by Perera et al. on a InGaAs/GaAs QWIP with cut-off wavelength at 35 ~tm. 82 The OWIP structure consisted of 20 periods of 9 8 A In().()sTGa().gj3As wells (Si doped to n=4xlOl~ -2) and undoped 4 1 8 A GaAs barriers. This QWIP structure showed a bound-to-quasibound intersubband transition. 240 • 240 ~m detector mesas were fabricated by wet chemical etching. The peak photoresponse was

GalnAs(P) based QWIPs on GaAs. InP, and Si substrates for focal plane arrags

14 7

measured about 35~tm at T=4.2K and bias O.15V. The measured peak at responsivity and detectivity were 0.3 A / W and l.O• T=4.2 K. 4.5.3 Multi-color QWIPs

Compared to QWIPs based on GaAs substrates, a major advantage of InP-based OWIPs is that both 3-5 ~m and 8-20/3m spectral bands can be realized with lattice-matched n-type devices (see Figure 9 in reference 83 ), which is not true for GaAs-based QWIPs. A two-color n-type QWIP was grown on semi-insulating InP by GSMBE.s4 The MWIR MOW consisted of 20 periods of 3 5 0 A Ino.s2Alo.48As barriers and 35A Int).s3Gat~.47As (n-doped to 5 x 1 0 1 7 c m -3) quantum wells. The LWIR MOW consisted of 20 period 500 A InP barriers and 55A Ino.s3Gao.47As wells (n-doped to 5xl()lTcm-~). 4 ( ) 0 x 4 0 0 p m detector mesas were fabricated by ECR-RIE dry etching. Since no middle metal contact was made, this QWIP operated as a voltage tunable two-color detector. At T= 7 7 K, for bias less than 7 V, only the 8.5 Jam peak was observed and for bias larger than 7 V, the 4 lum peak was observed.

4.6 Low cost QWIP FPA integrated with Si substrate 4.6.7 Overview of QWIPs on Si

Currently, most QWIPs are grown on GaAs (GaAs/A1GaAs heterostructure) and InP (GaInAs/InP h e t e r o s t r u c t u r e ) s u b s t r a t e s . Unfortunately, the readout integrated circuits (ROICs) for FPAs are silicon-based. As seen in Section 4.3, complicated techniques, like flip-chip bonding, are necessary to hybridize the FPAs with the silicon ROIC. GaAs/A1GaAs OWIPs directly grown on Si have been reported, ss-87 OWIPs directly grown on Si can make use of large-area substrates, with higher thermal conductivity and mechanical strength. Most importantly, this arrangement makes monolithic integration with the ROIC possible. The major obstacle to this goal is the large lattice mismatch between silicon and III-V materials (4.1% between GaAs and Si and 8.1% between InP and Si), which creates very high densities of threading dislocation in the epitaxial layer. These dislocations dramatically deteriorate the III-V material's optical and electrical qualities. To reduce the threading dislocation densities, many growth techniques, such as ex situ or in situ thermal annealing, low temperature nucleation, lateral epitaxial overgrowth, and insertion of a very thick, graded, or superlattice buffer, have been developed to improve the quality of GaAs and InP grown on Si. ss-91 Thermal annealing effectively reduces threading dislocation densities both in InP and GaAs epitaxial layers grown on Si. Annihilation and coalesence of dislocations are caused by dislocation movement under the thermal stress induced by annealing. 9~ Due to the larger immobile In atom in InP, stress induced

148 Handbook of Infrared Detection Technologies by thermal annealing can reduce threading dislocation density in InP-on-Si with less interdiffusion problems than in GaAs. 4.6.2 Growth of GalnAsllnP QWlP-on-Si

In this section, growth process of a OWIP-on-Si will be described in detail. Two 20 period GaInAs/InP OWIP structures were grown on Si substrate and on InP substrate by LP-MOCVD, respectively. 1~ The Si substrates used are GaAs-on-Si manufactured by Kopin Inc. with a GaAs coating thickness of 1 ~m. The {1 ()0) Si substrate has a 4 ~ miscut toward the [()11] direction. This misorientation is necessary to prevent the formation of anti-phase domains in the growth of III-V materials o n Si. 46 The growth of this QWIP-on-Si starts with substrate deoxydation and deposition of two different thin buffer layers. The substrate is heated up to 550~ under AsH3 flow, followed by growth of a thin GaAs buffer layer at 510C. After that, the substrate is cooled to 4()()~ and a 5()0 A thick InP nucleation layer is grown at growth rate of 3() A/min. Next, a thick buffer is grown, followed by a thermal annealing cycle. The temperature is raised to 500~ and a 6 ~m InP thick buffer layer is grown at growth rate of 180 A/rain. After every 1 ~m of InP. the growth is interrupted and an in situ thermal cyclic annealing (TCA) is performed. From room temperature, every annealing cycle consists of a temperature increase to 55()~ for 3 rain, followed by a decrease to room temperature. This procedure is repeated 3 times for every TCA. After TCA, the temperature is raised to 500~ and the next 1 ~m InP layer is grown. PHi/He is flown during the whole TCA process. Threading dislocation densities and rms surface roughness, for a 2 0 x 2 0 l.tm field estimated by atomic force microscopy (AFM) image lc~4 for thick InP buffer layer on Si, are 2 • 10 5 c m - 2 and 42 A respectively. After the growth of the entire 6 ~m InP thick buffer layer, the temperature is decreased to 480~ and a GaInAs/InP is grown. The details of the GaInAs/InP OWIP structure was described elsewhere. 77 X-ray diffraction spectra for QWIPon-Si and OWIP-on-InP are shown in Figure 4. ] 6. Satellite peaks generated by the periodicity of the MOW structure were observed. Satellite peaks were observed to the ] 7 th and 8 ~h order for ()WIP-on-InP and QWIP-on-Si samples respectively. More satellite peaks correspond to better interface quality and layer thickness uniformity. 4.6.3 Detector performance of GalnAsllnP QWIP-on-Si

Figure 4.17 shows the dark current Id measured as a function of bias voltage for a 20 period GaInAs/InP QWIP-on-Si sample. 4()()• detector mesas are fabricated with the process described in Section 4.2.4. For comparison, the dark current of a GaInAs/InP OWIP-on-InP with an identical structure is drawn on the same figure. At a bias voltage of - 1 .()V, the Ia of the QWIP-on-Si sample is 2.039 l.tA at T = 7 7 K . With the temperature change from 77K to 120K, the I,lOf the OWIP-on-Si increases only one order of magnitude. This behavior is very

GalnAs(P) based OWIPs on (;aAs. InP. and Si substrates for focal plane arrays

149

Figure 4.16 The measured (002) X-ray d(ffraction spectra.for ()WIP-on-Si and OWIP-on-InP samples.

different compared to the QWlP-on-InP. which changes by three orders of magnitude from 7 7 K to 120 K. This temperature-insensitivity can be attributed to the threading dislocations present in the QWlP-on-Si. The threading dislocations contribute a large temperature-insensitive leakage current under bias. Figure 4.18 shows a comparison of I,l at a bias of - 1 V for both the QWlPon-Si and the QWIP-on-InP as a function of temperature. For the purpose of comparison, a leakage current of 2.0 ~A was subtracted from the Ia for the QWIP-on-Si sample. Figure 4.18 shows almost the same I,t versus T relationship within the whole temperature range for the two samples. ~ s The GaInAs/InP QWIP-on-Si detector has a peak response at 7.80 ~tm, while the peak response of the GaInAs/InP OWIP-on-InP is at 8.15 ~tm. The small blueshift (0.35 ~tm) is due to the presence of some residual strain in the epitaxial layers which leads to a small change of q u a n t u m well parameters. This blueshift was also observed in a GaAs/A1GaAs OWIP on Si. ~~'~ 7 The 77 K peak responsivity for the QWIP-on-Si and QWIP-on-InP as a function of bias is shown in Figure 4.19. The two detectors have identical peak responsivities up to a bias voltage of (). 75 V. Beyond that. the responsivity of the QWIP-on-Si starts to trail off. At a bias voltage of 4.0 V, the peak reponsivity of 29.96 A/W for QWIP-on-InP is about 18 times higher than the 1.64 A/W responsivity for the QWIP-on-Si. The large increase of responsivity at higher bias voltage is attributed to an avalanche mechanism in the QWIP detectors. 78 Due to the presence of threading dislocations, the efficiency of the impact ionization

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Figure 4.181 d at bias - 1 V versus Tfor QWIP-on-InP sample (solid squares) and QWIP-on-Si sample (open circles, subtract 2.0 i~A for Id).

process in QWIP-on-Si seems lower than that of the OWIP-on-InP. Similar tendencies are also observed for M-based GaAs/AIGaAs OWIPs-on-Si, although the InP-based OWIPs-on-Si have three times higher responsivitiy. Lastly, the GaInAs/InP QWIP-on-Si detector showed a high detectivity of 2.3 • 109 cmHzl/2/W at a field of 0.89 kV/cm at T=77 K. This detectivity is one

GalnAs(P) based QWIPs on GaAs. InP. and Si substrates for focal plane arrays i

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order of magnitude lower than that of the QWIP-on-InP, which is 3 . 3 • 10 l~ cmHzl/2/W. 4.6.4 How to fabricate a monolithic integrated FPA with Si substrate

With the successful growth of a GaInAs/InP OWIP on Si substrate, a monolithically integrated infrared focal plane array becomes feasible. In Figure 4.20, we show a proposed fabrication process based on n-type GaInAs/InP OWIPs. 1. Fabrication of ROIC on Si wafer. 2. Passivation of ROIC surface. 3. Thinning and polishing the backside of Si ROIC wafer. 4. Growth of n-type GaInAs/InP OWIP structure on the backside of Si substrate. 5. Fabrication of detector array by standard lithography and etching process. 6. Passivation of surface of detector array. 7. Lithography and etch through the Si substrate. 8. Deposition of connection metal between every detector pixel and its corresponding ROIC.

4.7 New approaches of QWIP N e w characterization techniques of QWIPs

Despite the rapid development of the quantum well (OW) infrared technology, the intrinsic properties of the QWIPs have not been directly measured under the operating conditions of the detector. However, these details would be extremely useful for detector optimization. New techniques need to be explored for better QWIP characterization. K.K. Choi et al. utilized the surface corrugation to probe

152


Figure 4.20 Schematic of the proposed monolithic FPA fi~brication process.

the absorption coefficient and the photoconductive gain of a OWIP under different operating conditions. 9s.~, With a series of varied corrugation period QWIPs, by measuring the detector responsivity as function of period of corrugation, absorption coefficient ~ and gain g can be deduced by curve fitting accurately. Novel QWlP devices

For most applications of n-type QWIPs, effective light-coupling schemes are very important. Based on a corrugated structure. N.C. Das et al. incorporated dielectric or metal coverage on the top to improve performance of the corrugated quantum well infrared photodetectors (C-QWlP) in two wavelength regimes. 97 With non-absorbing Si3N4 coverage, the responsivity of the C-OWIP was improved 3.3 times in the 8 Ftm range and 1.8 times in the 14 ~m range. K.K. Choi et al. improved the C-OWIP detector performance by adding a center vertical trench in each of the corrugations. 9s Ting et al. also proposed a submonolayer QWIP structure for improved n-type normal absorption. ~8 With the insertion of a submonolayer of large lattice mismatched material in the quantum well, the wave function within the quantum well will be modified and therefore make the normal incidence possible. Development of two-color or multi-color OWIP detectors is highly desirable for future high performance IR systems. Without the stacking of different MOW, a postgrowth bandgap engineering technique was recently utilized to tune the wavelength of OWIPs and to achieve multi-color detection. L. Fu et al. used single

GalnAs(P) based QWlPs on GaAs. InP. and Si substrates for focal plane arrays

15 3

high-energy proton implantation and rapid thermal annealing to tune the spectral response ofOWIPs. A redshift as large as 1.8 l~m was observed. 99 With integration of OWIPs with other semiconductor devices, m a n y novel applications are made possible. Liu et al. reported a OWIP-LED for pixelless largearea imaging applications. 1~176OWIP (MWIR or LWIR) and LED (near-IR emission) structures are grown on top of each other. Under forward bias, the decrease of serial resistance of OWIP with the detection of MWIR or LWIR radiation increases the bias voltage on the LED, thus increasing its near-IR (NIP,) emission. This device is actually a MWIR and FWIR to NIR converter. The resulting NIR emission from theLED can be imaged easily using the well developed Si-CCD. Down to nanoscale

It is well know that dipole selection rules forbid absorption of photons polarized in the plane of the wells. Different light coupling schemes have been developed. To solve this problem fundamentally, people start to create the lateral electron confinement with different nanoscale structures, like q u a n t u m dot, q u a n t u m wire, and q u a n t u m grid, in addition to the usual vertical confinement by the material layers. S. Kim et al. demonstrated an InGaAs/InGaP q u a n t u m dot detector for MWIR detection. 1~176 The InGaAs q u a n t u m dots were formed by intrinsic strain due to a lattice mismatch. Normal incidence photoconductivity was observed at a peak wavelength of 5.5 ~m. X.O. Liu et al. demonstrated a 9 l~m A1GaAs/GaAs q u a n t u m wire infrared detector which was based on a V-grooved substrate. 1~ L. P. Rokhinson et al. demonstrated a q u a n t u m grid infrared detector, l~ The grid pattern, with a very narrow linewidth in the OWIP active region, was fabricated by electron-beam lithography and reactive ion beam etching. The coupling efficiency increase at very narrow linewidth, gave evidence for lateral q u a n t u m confinement.

4.8 Conclusions In this chapter, we have given a brief review of current Al-free GalnAs(P) QWIPs grown on InP, GaAs, and Si substrates. Device physics, fabrication, and m e a s u r e m e n t have been described in detail. Fabrication processes for infrared FPAs and indium solder bumps are also described in detail. After more t h a n 10 years of research and development, infrared OWIP detectors have moved from the laboratory to the thermal imaging market. Today, OWIP research is still very active and requires more development to reach the QWIP's full potential. Higher operating temperature and multi-spectral OWlP cameras are the most straightforward goals. With new processing technology, a m u c h smaller and larger format infrared FPA based on Al-free OWIP structures is likely to be available on the market soon. Even more promising are the same FPAs monolithically integrated on Si, which, as discussed (or should have been), can lead to a simpler process and a significant reduction in camera costs.

154 Handbookof InfraredDetectionTechnologies

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44. B. F. Levine, J. Appl. Phys. 74, R 1( 1993 ). 45. A. Y. Cho, The Technology and Physics of Molecular Beam Epitaxy. New York: Plenum Press ( 1985). 46. M. Razeghi, The MOCVD Challenge Vol 1. Bristol: Adam Hilger (1989). 4 7. M. Razeghi, The MOCVD Challenge Vol 2. Bristol: Adam Hilger (1995). 48. G. Hasnain, B. F. Levine, C. G. Bethea, R. A. Logan, J. Walker, and R. J. Malik, Appl. Phys. Lett. 54, 2515 (1989). 49. G. Sarusi, B. F. Levine, S. ]. Pearton, K. M. S. Bandara, and R. E. Leibenguth, Appl. Phys. Lett. 6 4 , 9 6 0 ( 1994 ). 50. N. T. Gordon, SemicondSci. Technol. 6, C106 (1991 ).

15 6 Handbookof InfraredDetection Technologies 51. C. J. Chen, K. K. Choi, M. Z. Tidrow, and D. C. Tsui, Appl. Phys. Lett. 68, 1446(1996). 52. G. Hasnain, B. F. Levine, C. G. Bethea, R. R. Abbott, and S. J. Hsieh, I. Appl. Phys. 67, 4361 (1990). 53. J. H. Lau, Low Cost Flip-Chip Technologies for DCA, WLCSP, and PBGA Assemblies, New York: McGraw-Hill (2000). 54 .S.D. Gunapala, S. V. Bandara, A. Singh, J. K. Liu, S. B. Rafol, E. M. Luong, J. M. Mumolo, N. O. Tran, J. D. Vincent, C. A. Shott, J. Long, and P. D. LeVan, IEEE Transactions on Electron Devices, 47, 963 (2000). 55. M. ]. Varnau, S. Yeh, Flip Chip/BGA Workshop, Binghamton, New York (October 1996 ). 56. T. Flynn, C. W. Argento, and ]. O'Brien, Proc. SPIE 3 9 0 6 , 8 (1999). 57. A. B. Frazier and M. G. Allen, ]. Microelectromech. Syst. :2, 87 ( 1993). 58. E. K. Yung and I. Turlik, IEEE Trans. Components, Hybrids, and Manufacturing Technology 14, 549 ( 1991 ). 59. W. W. Flack, S. White, and B. Todd, Proc. SPIE 3 6 7 8 , 49 (1999). 60. J. H. Lau (Ed.) Flip Chip Technologies. New York: McGraw-Hill (1996). 61. Y.-H. Kim, J.-H. Choi, K.-S. Choi, H. C. Lee, and C.-K. Kim, Proc. SPIE ] 0 6 1 , 60 (1997). 62. J. Jiang and M. Razeghi (unpublished). 63. H. H. Martijn, U. Halldin, P. Helander, ]. Alverbro, and ]. Y. Anderson, Proc. SPIE 3698, 789 (1999). 64. N. Y. Aziz, R. F. Cannata, G. T. Kincaid, R. J. Hansen, ]. L. Heath, W. J. Parrish, S. M. Petronio, and ]. T. Woo]away, Proc. SPIE 3698, 766 (1999). 65. F. Bertrand, ]. L. Tissot, and G. Destefanis, Proc. SPIE 3316, 713 (1998). 66. R. F. Cannata, R. ]. Hansen, A. N. Costello, and W. ]. Parrish, Proc. SPIE 3698,756(1999). 67. N. Y. Aziz, G. T. Kincaid, W. J. Parrish, J. T. Woolaway, J. L. Heath, Proc. SPIE 3 3 6 0 , 80 (1998). 68. H. Kulah and T. Akin, Proc. SPIE 3 6 9 8 , 778 (1999). 69. J. Hoff, X. He, M. Erdtmann, E. Bigan, and M. Razeghi, ]. Appl. Phys. 78, 2126(1995). 70. F. Szmulowiz and G. Brown, Appl. Phys. Lett. 66, 1659 (1995). 71.F. SzmulowizandG. Brown, Phys. Rev. B 52, 13203 (1995). 72. ]. Hoff, S. Kim, M. Erdtmann, R. Williams, I. Piotrowski, E. Bigan, and M. Razeghi, Appl. Phys. Lett. 67, 22 ( 1995). 73. D. K. Sengupta, S. L. Jackson, D. Ahmari, H. C. Kuo, ]. I. Malin, S. Thomas, M. Feng, G. E. Stillman, Y. C. Chang, L. Li, and H. C. Liu, Appl. Phys. Lett. 69, 3209(1996). 74. J. Hoff, C. Jelen, S. Slivken, G. ]. Brown, and M. Razeghi, Proc. SPIE 2 6 8 5 , 62(1996). 75. I. R. Hoff, M. Razeghi, and G. Brown, Phys. Rev. B 54, 10773 (1996). 76. S .D. Gunapala, B. F. Levine, D. Ritter, R. Harem, and M. B. Panish, Appl. Phys. Lett. 58, 2 0 2 4 (1991 ). 77. M. Erdtmann, A. Matlis, C. Jelen, M. Razeghi, and G. Brown, Proc. SPIE 3948,220(2000).

GalnAs(P) basedOWIPs on GaAs. InP. and Si substratesfor focal plane arrays 15 7 78. B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik, App1. Phys. Lett. 5 1 , 9 3 4 (1987). 79. J. Jiang and M. Razeghi, (unpublished). 80. S. D. Gunapala, B. F. Levine, D. Ritter, R. A. Hamm, and M. B. Panish, App1. Phys. Lett. 6 0 636 (1992). 81. M. Erdtmann, J. Jiang, A. Matlis, A. Tahraoui, C. Jelen, M. Razeghi, and G. Brown, Proc. SPIE 3 9 4 8 , 227 (2000). 82. A. G. U. Perera, S. G. Matsik, H. C. Liu, M. Gao, M. Buchanan, W. J. Schaff, and W. Yeo, Infra. Phys. and Technol. 42, 157 (2001 ). 83. M. Razeghi, M. Erdtmann, C. Jelen, F. Guastavinos, G. J. Brown, and Y. S. Park, Infra. Phys. and Technol. V42, 135 (2001 ). 84. C. Jelen, Gaxln~_.,.AsyP~_,j-Based n-type Long Wavelength Quantum Well Infrared Photodetectors: Growth, Characterization, and Fabrication. NU Doctoral Thesis, Evanston (1998). 85. D. K. Sengupta, W. Fang, J. I. Malin, J. Li, T. Horton, A. P. Curtis, K. C. Hsieh, S. L. Chuang, H. Chen, M. Feng, G. E. Stillman, L. Li, H. C. Liu, K. M. S. V. Bandara, S. D. Gunapala, and W. I. Wang, AppI. Phys. Lett. 71, 78 (1997). 86. E. R. Brown, F. W. Smith, G. W. Turner. K. A. Mclntosh, and M. J. Manfra, Proc. SPIE 173 5 , 228 ( 1992 ). 87. D. K. Sengupta, M. B. Weisman, M. Feng, S. L. Chuang, Y. C. Chang, L. Cooper, I. Adesida, I. Bloom, K. C. Hsieh, W. Fang, J. I. Malin, A. P. Curtis, T. Horton, G. E. Stillman, S. D. Gunapala, S. V. Bandara, F. Pool, J. K. Liu, M. Mckelvey, E. Luong, W. Hong, J. Mumolo, H. C. Liu, and W. I. Wang, J. Electron. Mater. 2 7 , 8 5 8 (1998). 88. T. E. Crumbaker, H. Y. Lee, M. J. Hafich, and G. Y. Robinson, ]. Vac. Sic. Technol. BS, 261 ( 1990). 89. Y. Ababou, P. Desjardins, A. Chennouf, R. Leonelli, D. Hetherington, A. Yelon, G. L'EspOrance, and R. A. Masut, J. App1. Phys. 8 0 . 4 9 9 7 (1996). 90. M. Yamaguchi, A. Yamamoto, M. Tachikawa, Y. Itoh, and M. Sugo, App1. Phys. Lett. 53, 2293 (1988). 91. S. Naritsuka, T. Nishinaga, M. Tachikawa, and H. Mori, Jpn. J. Appl. Phys. 34,1432(1995). 92. M. Razeghi, J. Jiang, C. Jelen, G. J. Brown, submitted for Proceeding of SPIE Photonic West 2 0 0 2 Symposium for Photodetectors Materials and Devices VII Conference, San Jose, CA (Jan 2002). 93. M. Erdtmann, GalnAs/InP quantum well infrared photodetectors on Si substratefor low-costfocalplane arrays. NU Doctoral Thesis, Evanston (2001). 94. J. Jiang, C. Jelen, M. Razeghi, and G. J. Brown, IEEE Photonic Tech. Lett. 14, 372(2002). 95. K. K. Choi, C. J. Chen, and D. C. Tsui, ]. Appl. Phys. 88, 1612 (2000). 96. C. J. Chen, K. K. Choi, L. Rokhinson, W. H. Chang, and D. C. Tsui, Appl. Phys. Lett. 75, 3210 ( 1999). 9 7. N. C. Das and K. K. Choi, IEEE Trans. Electron. Dev. 47, 653 (2000). 98. D. Z.-Y. Ting, S. V. Bandara, S. D. Gunapala, J. K. Liu, S. B. Rafol, and J. M. Mumolo, InfraPhys. and Technol. V 4 2 , 2 0 5 (2001 ).

158 Handbookof Infrared Detection Technologies 99. L. Fu, H. H. Tan, C. Jagadish, Na. Li, N. Li, X. Liu, W. Lu, and S. C. Chen, Appl. Phys. Lett. 78, 10 (200]). 100. H. C. Liu, Proc. SPIE 3 9 7 5 , 35 (20()()) 101. S. Kim, H. Mohseni, M. Erdtmann, E. Michel, C. Jelen, and M. Razeghi, Appl. Phys. Lett. 7 3 , 9 6 3 (1998) 102. S. Kim, M. Erdtmann, and M. Razeghi, J. Korean Phys. Soc. 35, 303 (1999). 103. S. Kim, M. Erdtmann, and M. Razeghi, Proc. SPIE 3 6 2 9 , 371 (1999) 104. X. O. Liu, N. Li, Z. F. Li, W. Lu, S. C. Shen, Y. Fu, M. Willander, H. H. Tan, C. Jagadish, and J. Zou, lpn. I. Appl. Ph!ls. 39, "3124 (20()()). 105. L. K. Rokhinson, C. J. Chen, D. C. Tsui, G. A. Vawter, K. K. Choi, Appl. Phys. Lett. 7 4 , 7 5 9 ( 1 9 9 9 ) . 106. M. J. Varnau and S. Yeh, Flip Chip/BGA Workshop, Binghamton, New York (October 1996). 107. T. Flynn, C. W. Argento, and J. O'Brien, Proc. SPIE 3 9 0 6 , 8 (1999). 108. A. B. Frazier and M. G. Allen, J. Microelectromech. Syst. 2, 87 ( 1993 ). 109. E. K. Yung and I. Turlik, IEEE Trans. Components. Hybrids, and Manufacturing Technology 14, 549 (1991 ). 110. W. W. Flack, S. White, B. Todd, Proc. SPIE 3 6 7 8 . 4 9 (1999). 111. J. H. Lau (Ed.), Flip Chip Technologies. New York: McGraw-Hill (1996). 112. Y.-H. Kim, J.-H. Choi, K.-S. Choi, H. C. Lee, and C.-K. Kim, Proc. SPIE 3061,60(1997). 113. J. Jiang and M. Razeghi (unpublished). 114. H. H. Martijn, U. Halldin, P. Helander, J. Alverbro, J. Y. Anderson, Proc. SPIE 3 6 9 8 , 789 (1999). 115. N. Y. Aziz, R. F. Cannata, G. T. Kincaid, R. J. Hansen, J. L. Heath, W. J. Parrish, S. M. Petronio, and J. T. Woolaway, Proc. SPIE 3 6 9 8 , 766 (1999). 116. F. Bertrand, J. L. Tissot, and G. Destefanis. Proc. SPIE 3 3 1 6 , 7 1 3 (1998). 117. R. F. Cannata, R. J. Hansen, A. N. Costello, and W. J. Parrish, Proc. SPIE 3698,756(1999). 118. N. Y. Aziz, G. T. Kincaid. W. J. Parrish, J. T. Woolaway, and J. L. Heath, Proc. SPIE 3 3 6 0 , 8 0 ( 1 9 9 8 ) . 119. H. Kulah and T. Akin, Proc. SPIE 3 6 9 8 , 778 ( 1999 ).

Chapter 5

InAs/(Gain)Sb superlattices: a

promising material system for infrared detection L. BOrkle and F. Fuchs

5.1 Introduction InAs and GaSb constitute a nearly lattice matched material system offering great flexibility in the design of optoelectronic devices. Because of the negative band overlap and similarities to the HgTe/CdTe material system, InAs/GaSb superlattices have been considered by Sakaki and Esaki as early as 1978.1 In 198 7 the material system was proposed for infrared detection devices by Smith and Mailhiot. 2 Intense work focussing on mid infrared laser devices started in the early 90s, 3 followed by first reports on detection devices with promising electrooptical properties comparable to the established Mercury-Cadmium-Telluride (MCT) material system. 4's However, for the practical use of InAs/(GaIn)Sb superlattices in infrared imaging devices, stable and reproducible growth conditions and device processing technology have to be established. Most demanding is the development of a stable and robust passivation. Regarding the latter, up to now only one report has been available."

5.2 Materials properties 5.2.1 Bandstructure of InAs/(Galn)Sb

superlattices

Because of their small difference in lattice constant, InAs and (GaIn)Sb form an ideal material system for the growth of semiconductor heterostructures. E.g., Gal_xInxSb with an indium concentration of ] 5% grows compressively strained on a GaSb substrate with a lattice mismatch of A a / a - 0.94%, while InAs is

160 Handbook of Infrared Detection Technologies under tensile strain with a lattice mismatch of A a / a = - 0 . 6 2 % . In an InAs/ (GaIn)Sb superlattice the compressively strained Gal_xInxSb layers can compensate for the tensile strain in the InAs layers. The average lattice mismatch of an InAs/(GaIn)Sb superlattice is given by ( Aa~

\aj

_ 2_

~2dsL

1

(1)

nMLaO

where ao is the lattice constant of the GaSb substrate, dsL is the superlattice period, and nML designates the number of atomic monolayers in a superlattice period. For a certain layer thickness ratio and indium mole fraction x, it is possible to achieve strain compensation on a GaSb substrate. InAs is a low band gap semiconductor with a direct energy gap of (). 35 eV, while GaSb has a direct gap of ().74eV at room temperature. 7 The relative alignment of conduction and valence bands is also of utmost importance. InAs and (GaIn)Sb establish a broken gap type II system, where the conduction band edge of InAs is lower in energy than the (GaIn)Sb valence band edge. The relative alignment of the band edges is shown in Figure 5. l(a). The band gap energy of the ternary Gal_xInxSb alloy depends on the In molar fraction x, and is well described by 8

E~(x) -

813.3 - 991.3x + 413x 2

(2)

[meV]

while the valence band offset of Gal_xlnxSb relative to the binary GaSb compound is given by

800 813.3

f

+67.4 - - - -

7

712.1

673.9

400 ...................

r(9 UJ

o

t

231.4

-T

J

T

GaSb InAs

40.3

NN~ LH

-28.8

349.5

410 -400

+29.2

Gao 8~lno1rSb

(a) unstrained

InSb

LH +31 7 ~.~-.-.-.-..---, -17.7 HH

InAs

Gao 851no~Sb

(b) strained

Figure 5.1 Band alignment between the constituents of an InAs/(Galn)Sb superlattice in: (a) unstrained condition: and (b) coherently strained to the GaSb substrate. The conduction band edge of unstrained InAs was chosen as zero energy.

InAs/(Galn)Sb superlattices: a promising material s!lstemfor infrared detection 161

AEv(x) = 234.7x-

78.7x 2

[meV]

(3)

In the case of the binary compound GaSb the band overlap with InAs is about 140 meV. 9 Because of the type II band alignment, the effective band gap of InAs/ (GaIn)Sb superlattices is smaller than the band gaps of its constituents. The band gap of an InAs/(GaIn)Sb superlattice can be continuously adjusted in a range between 0 and about O. 3 eV. -~ In particular, in the case of thick individual layers the superlattice can become semi-metallic with a vanishing band gap energy. Since electrons are mainly located in the InAs layers of the superlattice, whereas holes are confined to the (GaIn)Sb layers, optical transitions occur spatially indirectly and, thus, the optical matrix element for such transitions is relatively small. In addition, the wave function overlap strongly decreases with increasing superlattice period, which is inconvenient for detection applications in the far infrared. In order to compensate for the spatial separation of electrons and holes, the superlattices are commonly grown with ternary (GaIn)Sb layers instead of binary GaSb. By introducing indium into the binary GaSb material, these layers become compressively strained. The important influence of strain on the band alignment in the superlattice is shown in Figure 5.1 (b). Biaxial strain can be decomposed into a hydrostatic and a uniaxial component. The compressive hydrostatic strain component in the (Gain)Sb layers raises the band edge energies, while the uniaxial component leads to a splitting of heavy and light hole states, such that the heavy hole state becomes the valence band ground state. On the other hand, the tensile strain in the InAs layers leads to a similar effect for InAs but with opposite signs, thus leading to an overall reduction of the superlattice band gap. The band structure of an InAs/(GaIn)Sb superlattice is shown schematically in Figure 5.2. The band gap of the superlattice is determined by the energy difference between the electron miniband E1 and the first heavy hole state HH 1 at the Brillouin zone centre and can be varied continuously in a range between 0 and about 0.3 eV by adjusting the individual layer thicknesses and the indium molar fraction in the ternary (GaIn)Sb layers. 2 The dispersion of the heavy hole states exhibits a strong anisotropy, while the other subbands are nearly isotropic. The electronic structure of InAs/(GaInSb) superlattices can be calculated with satisfactory accuracy by using a three-band envelope-function approximation (EFA) formalism. 1~ Table 5.1 shows the results of such a calculation for a superlattice comprising 13 monolayers InAs and 8 monolayers Ga~.~sln~.~sSb. The superlattice is coherently strained to the lattice constant of the GaSb substrate and shows a residual net strain of Aa/a = - 0 . 8 9 • 1()-~. The zero of the energy scale is set to the value of the unstrained InAs conduction band minimum. Values in parentheses show the deviation to the results of a full EFA 8 • 8k.p EFA calculation. ~ For the electron miniband E~ and the first heavyhole band HH1, which determine the superlattice band gap, the values of the three-band EFA calculation are only some millielectronvolts larger than those of the 8 • 8 band calculation. Hence, the deviation for the fundamental band gap is

162

Handbook of Infrared Detection Technolo#ies

Figure 5.2 Band structure of a superlattice consisting of 1 ~ monola!lers InAs and 8 monolayers Gao.~ slno. 1sSb.

Table 5.1 R e s u l t s o f a t h r e e - b a n d EFA c a l c u l a t i o n for a s u p e r l a t t i c e c o m p r i s i n g 15 m o n o layers I n A s ($.995 n m layer t h i c k n e s s ) a n d 8 m o n o l a y e r s Gao.sslno.lsSb layers ( 2 . 4 6 2 n m layer t h i c k n e s s ) . The s u p e r l a t t i c e is c o h e r e n t l y s t r a i n e d to a GaSb s u b s t r a t e a n d s h o w s a r e s i d u a l n e t s t r a i n o f A a / a = - O . 8 9 x l O --3. T h e zero o f Energy is c h o s e n at t h e c o n d u c t i o n b a n d m i n i m u m o f u n s t r a i n e d InAs. Values in p a r e n t h e s e s s h o w t h e d e v i a t i o n to t h e r e s u l t s o f a full 8 xSk.p EFA c a l c u l a t i o n ll Band

EL HH1 LH1 HH2

Zone center q = () [meV] 242.3 9"3.6 -67.2 -205.3

{+3t (+5) (+42) (+18)

Zone edge q: = 7l'/ds-l. [meV] 386.1 9-5.6 -174.6 -203.7

1+7t {+51 I+36) i+17)

Miniband width AE [meV] 143.8 ().(14 1()7.4 1.6

1+41 1-().()1 ) (-16) t-l)

Effective mass m-, 0.026 -96.5 -().()32 2.4

(-().(1(12 t (+16.71 [ +().()()2) (+().61)

only 2 meV, which corresponds to a relative error of about 1%. However, for higher subbands the deviation from the results of a full 8 • 8 band calculation drastically increases. For superlattices with a band gap energy in the mid and far infrared, the individual layer thicknesses are typically in a range between :3 and 10 monolayers, leading to a strong coupling between electron wave functions of adjacent potential wells. An example for the distribution of the electron and hole wave functions is shown in Figure "3.3 for a superlattice consisting of 13 monolayers InAs and 8 monolayers Ga~.ssIn~.l sSb. The probability density of the electrons is concentrated in the InAs layers. However, due to the strong coupling in the electronic system there is a significant delocalization of the electron wave function over the superlattice. Typical values of the electron miniband width easily exceed 1 ()()meV. On the other hand, the heavy holes are strongly localized in the (GaIn)Sb layers. Thus. as a consequence of the type II

InAs/(Galn)Sb superlattices: a promising material S!lstem for infrared detection 9

I

'

9

InAs

0.6

I

9

(Galn)Sb

I

9

I

InAs

9

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112

Position z [nm] Figure 5. ] Probability densit!t distribution o.f the first electron miniband El and the first heavy hole state HH1 in a 13 ML InAs/8 ML Gao.~cslno. 1~Sb superlattice.

band alignment of the material system, electrons and holes are spatially separated. This is particularly disadvantageous for optical absorption, where a significant overlap of electron and hole wave functions is needed. The delocalization of electrons and, consequently, the optical absorption in an InAs/ (GaIn)Sb superlattice is promoted by a reduction in the electronic confinement. This can be achieved either by growing thinner (Galn)Sb barriers or by introducing more indium into the (GaIn)Sb layers, causing a reduction in the energy of the barrier material. ~(~ On the other hand, the effective masses in an InAs/(GaIn)Sb superlattice are not directly dependent on the band gap energy, as it is the case in a bulk semiconductor, j2 Compared to the electron effective mass of HgCdTe, the electron effective mass (m*,~O.()2 in an InAs/(GaIn)Sb superlattice is larger, leading to a reduction in tunnelling contributions in the leakage currents of an IR photodiode. Also related to the larger effective mass in the superlattice is a higher combined density of states, which in turn compensates for the smaller optical matrix element of the superlattice. Thus, despite the type II character of the optical transitions in an InAs/(Galn)Sb superlattice, the optical absorption is comparable to that of HgCdTe. In addition, larger minority carrier lifetimes are expected because of smaller Auger recombination rates compared to MCT. 13

5.2.2 X-ray characterization High-Resolution X-ray diffraction is an important tool for the structural characterization of the MBE grown superlattices. In Figure 5.4 the X-ray diffraction pattern of an InAs/(GaIn )Sb superlattice, close to the (004) diffraction order of the GaSb substrate, is shown. The superlattice peaks can be observed up to the seventh order, indicating an excellent materials quality. From the angular

1 6 4


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32

34

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36

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o~ [~

Figure 5.4 High-resolution X-raft diffraction pattern of an InAs/((;ahl)Sb superlattice close to the (004) reflection of the GaSh substrate. The superlattice has a period of 4.56 nm and a residual lattice mismatch of + 0 . 8 8 x 1 0 -~

spacing Am of adjacent peaks the superlattice period dsI. can directly be calculated using 2

dsL = 2 Ao)cos|

(4)

where 2 is the wavelength of the x-ray beam and O designates the average diffraction angle between the peaks considered for evaluation. The net strain in the superlattice (-~) •

-- sina)6aSb sine.OSL

(5)

can be determined from the relative position of the zero-order diffraction peak of the superlattice a)sL with respect to the position of the (004) peak a)G~Sb of the GaSb substrate. E.g., in Figure 5.4, the zero-order diffraction peak of the superlattice is found at a smaller angle than the peak of the GaSb substrate (coSL < a)caSb). Thus, the average lattice constant of the superlattice in growth direction is larger than the lattice constant of the GaSb substrate and the net strain of the superlattice is compressive with a lattice mismatch of +0.88 • 1()- 3. For a residual mismatch below 3 • 1 ()- 3, strain relaxation is not observed in the growth of a 1 l.tm thick superlattice stack. In the inset of Figure 5.4 the diffraction pattern close to the substrate reflection peak is shown at an enlarged scale. The oscillations that can be observed between the diffraction peaks are the so called 'pendellasung-fringes', which can be considered as the X-ray analogue to optical

InAs/( GaIn)Sb superlattices: a promising material s!!stemfor infrared detection 16 5

interference fringes. Such X-ray fringes can only be observed in very homogenous samples. Strain or thickness inhomogeneities in the order of A a / a > 10 -5 and A t / t > 3 x 10-~ lead to a significant reduction in the visibility of the interference fringes. 14 The oscillation period of the fringe pattern can be used to determine the overall thickness of the superlattice stack. For a full structural characterization of the superlattice the individual thicknesses of the InAs and Gal_xlnxSb layers as well as the In molar fraction x have to be determined. Information about the individual layer thicknesses in a superlattice can usually be gained by evaluating the envelope of the X-ray diffraction pattern. However, because of the very short periodicity of our IRsuperlattices, the superlattice diffraction pattern is spread out over a very wide angular range, and therefore not enough diffraction peaks can be observed to provide significant information on the individual layer thicknesses. The error of such an evaluation in the case of short period InAs/(GaIn)Sb superlattices remains too large. However, the missing structural parameters can be obtained by making use of photoluminescence measurements, providing the fundamental band gap of the superlattice system. 1s The fundamental band gap is very sensitive to both the InAs layer thickness and the In molar fraction in the ternary (GaIn)Sb layers. The EFA band calculation described in the previous section can be used to extract the individual layer thicknesses as well as the In molar fraction with satisfactory accuracy from the superlattice band gap determined from IR photoluminescence measurements and the superlattice period d~L and net strain zXa/a obtained from X-ray characterization. 5.2.3 Interfaces

InAs/(GaIn)Sb superlattices can be considered as a pseudo-quaternary system. At the transition from a Gal_xlnxSb to an InAs layer both the group II! and the group V elements change. Thus, depending on the shutter sequence during the MBE growth two interface types with either Ga-As or In-Sb interface bonds can be established (Figure 5.5), which differ strongly in interface bond length and strain introduced into the superlattice stack. The type and quality of interface bonds in a binary InAs/GaSb superlattice can be examined using R a m a n spectroscopy. In a R a m a n spectrum both InSb- and GaAs-bonds show characteristic modes by which the different interface types can be identified. 17 Because of their larger lattice constant with respect to GaSb, In-Sb interface bonds introduce compressive strain into the superlattice, whereas Ga-As interfaces induce tensile strain. In order to clarify the influence of different interface types on superlattice period and net strain, Figure 5.6 compares the Xray diffraction patterns of two InAs/GaSb superlattices, both having identical layer thicknesses. The diffraction pattern shown in the upper half of the figure, stems from a sample grown with InSb-like interfaces only. The superlattice has a period of 5.8 nm and is compressively strained with a strong lattice mismatch of A a / a = +4.1 • 10-3. Thus, in this sample the tensile strain of the InAs layers is overcompensated by the compressive strain of the InSb interfaces. By replacing

166


Figure 5.5 Illustration of the possible interface t!lpes at the transition between a GaSh and an InAs la!ler.

105 ~ 100Periods: i E_

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29

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3a/a = -0.4x10 .3 dS L = 11 1 nm

+1

,

"

31

i :-

-

32

Incidence Angle o [o] Figure 5.6 X-ray diffraction pattern of two InAs/(;aSb superlattices. B!! replacing ever!t one out of four InSblike interfaces with a GaAs-like interface, the lattice mismatch is reduced from +4.1 x 1 O- ~ to - 0 . 4 x 1 O-

every one out of four InSb-like interfaces with a GaAs-like interface, the lattice mismatch of the superlattice is reduced to - ( ) . 4 • 1()-~ (Figure 5.6 bottom). In addition, by exchanging only every fourth interface bond type, the periodicity of the superlattice stack is doubled and. as a consequence, the angular spacing between adjacent superlattice peaks in the diffraction pattern is reduced by a factor of 2.

InAs/( Galn)Sb superlattices: a promising material s!tstem for infrared detection

16 7

5.2.4 Sample homogeneity An important issue for the realization of large area focal-plane arrays (FPA), like, e.g., a 2 5 6 x 2 5 6 FPA with an active area of around 1 cm 2, is the homogeneity of the epitaxial layer across the wafer. Figure 5.7 shows the band gap variation of an InAs/(GaIn)Sb superlattice across a two-inch wafer as determined from photoluminescence spectroscopy measurements. The structural parameters of the superlattice exhibit an excellent homogeneity with a decrease in superlattice period by only 3% and a change in the lattice mismatch from - 1 . O to - 1.3 • 10 -3 when going from the center to the edge of the wafer. The change in the structural parameters is accompanied by an increase in the superlattice band gap, which can be attributed to an increase in confinement in the individual superlattice layers. Within the diagonal size of a 2 5 6 x 2 5 6 FPA, which is roughly 15 mm, the superlattice band gap increases by about 2 meV. This corresponds to a relative change in the band gap energy of less than 2%.

5.2.5 Residual doping For the design optimization of IR photodiodes, the residual doping of InAs/ (GaIn)Sb superlattices is a crucial parameter. The influence of the growth temperature on the residual doping of InAs/(GaIn)Sb superlattices was investigated, with a series of samples grown at substrate temperatures ranging from 360 to 440 ~C. 16 The dependence of the residual doping on growth temperature, as determined from magnetotransport measurements, is shown in Figure 5.8. Superlattices grown at substrate temperatures of 360 and 380~ exhibit residual n-type doping with donor concentrations of 2 x 1 ()~ 6 and 6 x 101 s c m - 3, respectively. At a growth temperature of 400~ the superlattices change to residual p-type 136 Diagonal size of a 256 x 256 FPA 134

E E J

132 O0

130

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5

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1'0

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15

,

20

Distance from Center [mm] Figure 5.7 Band gap variation of an InAs/ ( GaIn )Sb superlattice across a two-inch wafer.

168

Handbook of Infrared Detection Technologies I

4X1016 all-"

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Figure 5.8 Dependence of the residual doping of lnAs/( (;aln )Sb superlattices on growth temperature.

doping with an acceptor concentration of 6 • 1 ()1 5 cm- 3, and growth temperatures exceeding 400~ result in a further increase in the acceptor concentration. In addition, a reduction of the in-plane electron mobility from 15 000 down to about 4 0 0 0 c m 2 / V s can be observed when the growth temperature for the superlattices is increased from 360 to 44()~ It is well known that the in-plane electron mobility in InAs/(GaIn)Sb superlattice is limited by interface roughness scattering and, thus, the electron mobility/~,, ~x A -2 is conversely proportional to the square of the height fluctuations, A, at the superlattice interfaces. 1s The reduction in the electron mobility by a factor of 5 with the increase in growth temperature can consequently be attributed to roughly a doubling of the height fluctuations A caused by increased interface roughness. 16 The conversion of the InAs/(GaIn)Sb superlattices from residual n- to p-type doping can be traced by photoluminescence (PL) measurements. For superlattices grown at temperatures below 4()0~ a strong increase in PL intensity is observed when the growth temperature is increased, whereas at temperatures above 400~ the PL intensity remains almost constant. As shown in Figure 5.9, for n-type samples, the PL intensity is a direct measure for the background donor concentration. An increase in the n-type background leads to a significant reduction in the PL intensity of the InAs/(GaIn)Sb superlattice. In contrast, for samples with p-type background the PL intensity is almost independent of acceptor concentration, as long as the latter does not exceed ~1 x 1017 c m - 3 . 1 6 Thus, the optimum growth temperature for low gap photodiodes can be found by using the PL intensity as a monitor. The optimum growth temperature for InAs/(GaIn)Sb superlattices is found at the lowest temperature which leads to a

InAs/(Galn)Sb superlattices: a promising material sflstem for infrared detection

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Doping Concentration [cm-3] Figure 5.9 Photoluminescence intensity of lnAs/ ( GaIn )sb superlattices as a flmction of doping concentration. The samples with doping concentrations below 1 • l()~; cm -~ are nominall!l undoped, all others are intentionally doped.

PL intensity close to the maximum. Using this optimization procedure, a residual background in the mid 1015cm -3 range can be obtained within a growth window of + 10~

5.3 Superlattice photodiodes 5.3.1 Diode structure

Figure 5.10 shows the cross-section of a typical InAs/(GaIn)Sb superlattice diode mesa. The epitaxial layers were grown by molecular beam epitaxy (MBE) at substrate temperatures around 400~ on undoped (001) oriented two-inch GaSb substrates. The group V to group III beam-equivalent-pressure (BEP) ratio was set between 2 and 4.5 for the growth of (GaIn)Sb and close to 6 for InAs. 14 Overall strain compensation of the superlattice stack was achieved by appropriate interface engineering. For growth on GaSb substrates, the smaller lattice constant of InAs is compensated by the larger lattice parameter of Gal_xInxSb (0.1 < x < 0.3), with the detailed strain balance depending on the InAs to (GaIn)Sb layer-thickness ratio. However, the interfaces across which both the group-III and group-V atoms change, have to be dealt with separately. For InAs/(GaIn)Sb superlattices, two types of interfaces with either Ga-As or In-Sb interface bonds can be realized, which differ strongly in interface bond length and strain introduced into the superlattice stack. Strain compensation was achieved by growing the layers with


Figure 5.10 Cross-section of an InAs/( Galn )Sb superlattice diode mesa.

alternating GaAs-like and InSb-like interfaces, terminating each individual layer with its group-V element and starting the following layer with a monolayer of the respective group-III element. ~r. ~9.2~ For a band gap energy in the 8 - 1 2 l.tm atmospheric window the period of the superlattices is typically in a range between 5 and 7 nm corresponding to 1 5 - 2 0 monolayers, and the indium molar fraction in the ternary (GaIn)Sb layers was set close to 20%. A 5 0 0 n m thick, 3 x 1() is cm -3, Be-doped, GaSb layer forms the p-contact of the diode. The following InAs/(GaIn)Sb superlattice comprizes 150 periods and is approximately 7 5 0 n m thick. The first 60 periods of the superlattice are intentionally p-doped with an acceptor concentration of NA "-, 5.7 x 1016 cm -3. The p-n junction of the diode is located between the 30 low p-doped (Na ~-, 2.3 x 101~ cm -3) and the following 40 nominally undoped, but residually n-type (ND "-~ 1 x 1016 cm -3) periods. The final 20 periods of the superlattice are highly n-doped with a donor concentration of ND '~' 1.8 x 1017 cm -3. The superlattice is capped with a 20 nm thick, Si-doped (N/) ~-, 2 x 1018 cm -3) InAs layer which forms the n-contact of the device. Be and Si are used as p- and n-type dopants for the superlattice. In the n-doped superlattice regions the InAs-layers are doped with Si, while the (GaIn)Sb-layers remain undoped. The p-doping of the superlattice is achieved by doping the (GaIn)Sb superlattice layers with Be and leaving the InAs layers undoped. Both contact metallizations of the diode are 500 nm thick. In the case of the pcontact it consists of a series of Ti/Pt/Au-layers, whereas for the n-contact, a Ti/ A u / P t / A u - c o n t a c t metallization is used. In order to protect the device from ambient influences, the superlattice diode mesas are coated with a SiN-based passivation. Figure 5.1 l(a) shows the band alignment simulated at thermal equilibrium for the above device at 77 K. The Fermi level is indicated by the dashed line. In the simulation, the one-dimensional Poisson equation was numerically solved along

InAs/( Galn )Sb superlattices: a promising material s!tstem for infrared detection

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Position z [IJm] Figure 5.11 Simulation of a InAs/(Galn)Sb superlattice diode with a band gap of 145 meV in thermal equilibrium at 77 K. The Figure shows: (a) the band alignment: (b) the electric field: and (c) the carrier concentration as afunction of position in the diode.

the growth direction, treating the InAs/(GaIn)Sb superlattice as a bulk semiconductor. The superlattice properties such as relative band alignment and effective masses were determined from a three-band EFA calculation. The strength of the electric field as a function of position in the diode is shown in Figure 5.1 ] (b). Apart from the built-in electric field at the p-n junction, which is indicated by the dashed line, electric fields also appear at the intersection between different doping levels. The m a x i m u m electric field at the p-n junction at thermal equilibrium is Fo= 13 kV/cm and the built-in potential is Vb~= 124 mV. Assuming a triangular-shaped field dependence, the depletion layer width of the diode is given by d = 2 V~i/Fo = 191 nm. Figure 5.1J(a) also clearly shows that both the n- and the p-contact layers can serve as diffusion barriers for the minority carriers in the diode. Because of the long diffusion length of electrons in the superlattice (which is on the order of several microns and, hence, is beyond the dimensions of the diode) minority electrons in the p-doped superlattice region are reflected at the large band discontinuity in the conduction band between the InAs/(GaIn)Sb superlattice and the p-GaSb contact layer, leading to an increase in the dynamic impedance RoA of the superlattice diode. 21

172


5.3.2 Diode processing

The processing of the mesa photodiodes requires four lithographic steps. These steps are illustrated in Figure 5.12. In the first processing step, the top contact of the diodes is defined by a lift-off technique (Figure 5.12(a)), followed by wet chemical etching of mesa structures using citric acid in the second step (Figure 5.12(b)). Since citric acid etches the InAs/(GaIn)Sb superlattice with a high selectivity with respect to GaSb, the etching process stops when the p-GaSb contact layer is reached. Figure 5.13 shows an electron microscopy image of a wet chemically etched mesa sidewall. Following the definition of the bottom pcontact metallization (Figure 5.12(c)) a SiN based passivation layer is deposited, which protects the diode mesa from ambient influences (Figure 5.12(d)). In the final processing step the passivation layer is opened in order to enable contact between the diode n- and p-contacts (Figure 5.12(e)). Figure 5.14 shows a microscope image of a processed InAs/(GaIn)Sb superlattice diode. In the figure, the diode mesa and the two contact pads of the diode can be clearly distinguished. For front side illumination of the device, a square area is left open in the top contact metallization. 5.3.3 Photo response

The cut-off wavelengths of InAs/(GaIn)Sb superlattice photodiodes can be tuned to cover the whole mid-IR spectral range (3-2OBm). Figure 5.15 shows the (a) Top Contact

(d) Passivation

(b) Defenition of Mesa

(e) Etching of Contact Holes

(c) Bottom Contact

Figure 5.12 Schematic of the individzlal steps in the processing of lnAs/( GaIn )Sb sz~perlattice diode mesa.

InAs/( GaIn)Sb superlattices: a promising material system for infrared detection

17 3

Figure 5.13 Electron microscope image of a wet chemicall!t etched mesa sidewall.

Figure 5.14 Optical microscope image of a processed InAs/ ( GaIn )Sb superlattice photodiode.

spectral responsivity of different photodiodes with superlattice band gaps ranging from 0.1 to 0.2 7 eV and having an active layer thickness of 0.8 l~m. All photodiodes exhibit high responsivities at normal incidence with peak values typically exceeding 2 A/W. 5.3.4 I-V measurements Current-voltage characteristics

The current mechanisms in a p-n junction can be differentiated by their characteristic temperature dependence. While the diffusion current changes with temperature as n2, the generation-recombination current is proportional to ni. Therefore, at high temperatures, the dark current in a diode is limited by diffusion currents, while generation-recombination currents can dominate at lower temperatures.


Figure 5.15 Spectrally resolved photo response of several InAs/( (;aln)Sb superlattice diodes with band gaps rangingfromO. 1 to0.27 eV.

Figure 5.16 shows the current-voltage characteristics of an InAs/(GaIn)Sb superlattice diode with a band gap of 143 meV in a temperature range between 20 and 200 K. At 7 7 K, the forward branch of the diode I--V curve exhibits an ideality of/~ = 1.34, while at voltages above ,~(). 13 V the diode current is limited by the diodes series resistance of Rs-= 2.26 x 10 -3 f2cm 2. Towards higher sample temperatures the diffusion current more and more prevails against the generation-recombination current and thus, the ideality of the diode continues to decrease with increasing temperature and reaches a value o f / ~ - - 1.18 at 150 K. On the other hand, at lower temperatures the g-r current dominates significantly, leading to an ideality of/~ = 2 at 20 K. The reverse bias branch of the diode I - V curves can be subdivided into two voltage regimes. For small reverse biases up to approximately - 0 . 4 V the diode leakage current shows a strong dependence upon temperature. When the temperature is increased from 20 to 200 K the diode current density increases by more than nine orders of magnitude. While at high temperatures, diffusion currents dominate the leakage of the superlattice diode, which is obvious from the almost flat current-voltage characteristic at small reverse biases. Below 1 0 0 K the generation-recombination currents increasingly prevail, and the characteristic flat current-voltage characteristic of diffusion currents with a saturation current density Jo is replaced by the I - V characteristics of g-r currents, which is slowly increasing with reverse bias. At temperatures below 50 K, defect assisted tunnelling currents gain an increasing influence on diode leakage. At voltages below - 0 . 4 V the current-voltage characteristics of the superlattice diode only exhibit a weak dependence upon sample temperature,

InAs/( GaIn )Sb szIperlattices: a promisiug material s!istem for infrared detection

175

Figure 5.16 Current-voltage characteristics of an hlAs/((;ah~)Sh sllperlattice diode with a band gap of 143 meV in a temperature range between 20 and 2OO K.

and a strong increase of the diode current with increasing reverse bias can be observed. This behaviour is caused by electrons tunnelling from the valence band in the p-region across the superlattice band gap into the conduction band in the n-region of the diode and is known as Zener tunnelling. 22

Dependence ot the dynamic impedance R()A on temperature and band gap Further insight in the leakage current mechanisms in an InAs/(Galn)Sb super]attice diode can be gained by investigating the temperature dependence of the dynamic impedance at zero bias

dl

R(,A - ~ 1 : - o

(6)

as shown in Figure 5.17 for a diode with a band gap of 14 3 meV. The diode shows diffusion limited performance according to R~jA :x exp(E~/kT) down to 120 K. However, below about 75 K the dynamic impedance exhibits a temperature dependence according to RoA :x exp(E,t/2kT ), suggesting that the electrical performance of the diode is limited by g-r currents. The deviation of the measured data from g-r limitation at temperatures below 5() K can likely be attributed to an increasing influence of defect-assisted tunnelling currents at lower temperatures. The effective suppression of such tunnelling currents can be attributed to both a low density of defect states in the superlattice band gap, as well as a large out-of-plane reduced effective mass of ().02 6 m~)in the superlattice as compared with bulk semiconductors with the same band gap. The solid curve in Figure 5. ] 7 shows the result of a fit of the total dynamic impedance

176


Figure 5.17 Temperature dependence of the d!!namic impedance at zero bias R()A Jor an InAs/((;aln)Sb superlattice diode having a band gap of 14 ~ me ~'.

(R(,A)

+(R(,A)-'(,R

(7)

to the experimental R()A values, where the generation-recombination lifetime rcR in the depletion region, as well as the minority carrier lifetimes r,, and rh were used as fitting parameters. The dashed lines in the plot represent the contributions of the generation-recombination currents 2

rcR vl,~ (RoA)c,R-- edni

(8)

and the diffusion currents at electrically reflecting contact layers-'

(R()A)dif-

kT ( w 1,

+

w,, ) - 1

(9)

to the total dynamic impedance of the diode, respectively. For simplicity, equal minority carrier lifetimes were assumed for electrons and holes in the fitting procedure. The superlattice band gap was set to Eq-- 143 meV as determined from a measurement of the spectral responsivity of the diode. From the leastsquares fit a minority carrier lifetime of r , , - - r n - - l ( ) n s and a generationrecombination lifetime of r ( ; ~ - 2 8 n s were determined. For comparison, it should be noted in this context that for an InAs/Ga().7sln().2sSb superlattice with a band gap of 1 4 0 m e V Grein et al. theoretically predicted an Auger limited minority electron lifetime of r,, - 700 ns. 2 ~ This discrepancy can be explained by the fact that g-r processes seem to dominate the minority carrier lifetime of the

InAs/(Galn)Sb superlattices: a promising material s!!stem for infrared detection

177

superlattice. In addition, the contact layers may not be perfectly electrically reflecting, leading to an underestimation of the determined carrier lifetimes in the fitting process. Figure 5.18 plots the dynamic impedance R~A of several InAs/(GaIn)Sb superlattice diodes at 77 K as a function of their band gap. For comparison, the gray shaded area indicates the dynamic impedance range achieved with state-ofthe-art HgCdTe photodiodes. 2s The band gap energies of the superlattice diodes cover the whole range of the atmospheric transmission window between 8 and 12 ~m. The solid line in the figure shows the dynamic impedance as calculated with (8) and (9) based on the lifetimes determined previously. The dashed lines in the figure give the contributions of the diffusion and g-r currents to the total dynamic impedance of the diodes, respectively. Both contributions depend exponentially on the superlattice band gap. The transition between diffusion and g-r limited behaviour takes place for superlattice band gaps between 75 and lOOmeV at 7 7 K . While the dynamic impedance of diodes with a superlattice band gap below 75 meV is diffusion limited at 77 K. the electrical performance of diodes with a band gap above l()()meV is limited by g-r currents. A superlattice diode with a band gap of 143 meV (corresponding to a cut-off wavelength of 8.7~tm) exhibits a dynamic impedance of R o A = 1.Sk~2cm 2, while a diode with a band gap of 1 ()() meV (corresponding to a cut-off wavelength of 12.4 jam) reaches an R ~ A of 1 ()f2 cm 2. Promising data at even longer cut-off wavelengths were reported by Mohseni et al. 24 Hence, the dynamic impedances of InAs/(Galn)Sb superlattice diodes are comparable to the ones of state-of-theart HgCdTe photodiodes. 2 s.2~

Figure 5.18 D!tnamic impedance R~A ~[several InAs/( (;aln)Nb superlattice diodes at 77 K as a fllnction ~[ their band gap. The band gap energies ~I the diodes cover the ~vhoh' range of the atmospheric transmission window between 8 and 12 IJm. The gra!j shaded area indicates the d!lnamic impedance range achieved ~vilh state-of-the-art HgCdTe photodiodes.2 ~.2~,

178


Surface leakage currents A major aspect in the development of photovoltaic detectors for the mid and far infrared is the development of a suitable passivation process. The function of an ideal passivation is to protect the photodiode against ambient influences without degrading its electrical and optical properties. Since the contribution of surface leakage currents to the total diode current, increases with decreasing size of the devices, a passivation becomes especially important for the very small photodiodes in a focal plane array (FPA). Under the assumption that the surface leakage current is proportional to the diode perimeter p the total diode current is given by

1(v) - ?(v) + j (V)p

(lo)

where the proportionality constant jp denotes the surface leakage current density per unit length and I a is the bulk contribution to the diode current, which is proportional to the diode cross-sectional area A. For the dynamic impedance at zero bias this yields 6

Ro---A ~(RoA) =

+ ~"

(11)

where the first term defines the bulk and the second term the surface contribution to the diode dynamic impedance. The parameter -1

d-

\av

(12) V=O

is the dynamic impedance of the surface leakage current per unit length jP at zero bias and, therefore, describes surface leakage behaviour of the diode. The value of r{') depends, among other things, on the passivation of the diode, band gap and temperature. The larger r Pll the smaller is the contribution of surface leakage currents to the overall diode leakage current. Since the second term in (11) is reciprocally dependent on the size of a diode mesa, surface leakage currents particularly dominate in small diodes. In the course of process development, a multitude of different coatings were tested for their suitability as a passivation of the mesa sidewalls in InAs/(GaIn)Sb superlattice diodes. In order to assess the influence of surface leakage on the superlattice diodes, current-voltage measurements on series of diodes with different perimeter-to-area ratios were performed. The series included diodes of different sizes having perimeter-to-area ratios in a range between ().011 and 0.048 ~m -a. Measurements on diodes with an even larger p/A value are difficult, since devices with a mesa size smaller than 8 0 • pm 2 cannot be contacted with a ball bonder. Figure 5.19 shows a graph of the reciprocal dynamic impedance (RoA) -1 at 77 K as a function of the perimeter-to-area ratio E/A for a series of differently passivated diodes having a 143 meV band gap. The different passivations are

InAs/(GaIn)Sb sziperlattices: a promising material s!!stem for infrared detection

179

Figure 5.19 Reciprocal dynamic impedance (R~A)-l at 77 K plotted as a fllnction of the perimeter-to-area ratio p/A for a series of differentl!l passivated InAs/( GaIn )Sb szlperlattice diodes having a 14 3 meV band gap.

denoted with the letters A, B, and C. Using ( 1 1 ) the surface leakage currents can be separated from the bulk dynamic impedance of the diodes. The bulk contribution to the total dynamic impedance is obtained from the intercept of the linear fit with the y axis, while the slope determines the contribution of the surface leakage currents. For the bulk dynamic impedance a value of (RoA)a= 1 4 6 0 f a c m 2 is obtained in all three cases, indicating that the bulk dynamic impedance is actually dependent only on the bulk properties of the diodes. On the other hand, the slopes of the graphs, which determine the surface contribution, differ significantly for the different passivations. Diodes with passivation A ) exhibit the worst leakage current behaviour with an 4j value of 104 kf2 cm, whereas the least degradation of the electrical performance of the superlattice diodes by surface leakage (~') = 843 kf2 cm) is introduced by passivation C. The influence of a passivation on surface leakage in even smaller diodes, e.g. like the ones in a FPA, can be estimated by extrapolating the graphs of the linear fits in Figure 5.19 to higher p/A values. The size of the individual photodiodes in the 2 5 6 • FPA (Figure 5.20) developed at the IAF is 38• 2, which corresponds to a perimeter-to-area ratio of ().1051.tm -~. Thus, for the photodiodes in a FPA it can be expected that with passivation A the bulk dynamic impedance (RoA) A is reduced by more than an order of magnitude by surface leakage currents, whereas with passivation C the bulk dynamic impedance is only degraded by a factor of three. In order to experimentally investigate the surface leakage currents in FPA pixels, a 2 5 6 x 2 5 6 FPA was hybridized to a silicon fan-out structure by a flipchip technique. The electrical contacts between the photodiodes and the silicon fan-out chip were realized by indium bumps. Figure 5.21 shows the results of measurements on a fan-out sample having a superlattice band gap of 120 meV. At 77K, the bulk contribution to the total dynamic impedance of the


Figure 5.20 InAs/(Galn)Sb photodiodes in a 256 x 2 5 6 FPA. The FPA pitch is 40 I~m and the size of the individual diodes is 38 x 38 l~hi2 co rresponding to a perimeter- to-area ratio of O. 105 t2 m - i

'

0.04 -

I

9

-E]-77K -o-67

I

9

I

'

I

'

I

'

(RoA)A = 5 7 ~ c m 2

K (RoA)A = 549 ~cm 2

30

0.03 40 "i" v

0.02 ~ ~ ~ ~ : ~ ~

256x 256 FPA 60

Eg= 120 meV

loo

0.01

I

0.00

,__N

80

0.02

0.04

0.106 '

p/A Figure 5.21 Results of measurements on a 120meV.

0/08

0.10

200 500 0.12

[pm-1]

256x256 fan-out samph' with a superlattice band gap of

InAs/(Galn)Sb superlattice diodes is (R()A)4- 57 ~ c m 2. By extrapolating the linear fit, one would expect a dynamic impedance of R()A -- 3 3 ~ cm 2 for a pixel in a 2 5 6 • FPA. The actually measured dynamic impedance of 27 ~ c m 2 lies slightly below that value. However, it is still only a factor of 2 below the bulk dynamic impedance ( R o A ) A. With respect to 77 K, the bulk dynamic impedance at 67 K has increased by one order of m a g n i t u d e and reaches a value of (RoA)4= 549 f2 cm 2. The dynamic impedance of a FPA pixel is R()A - 108 f2 cm 2 and, hence, is degraded by a factor

InAs/(Galn)Sb superlattices: a promising material system for infrared detection

181

of 5 with respect to the bulk value. Thus, by increasing the diode temperature from 6 7 K to 77 K the bulk leakage current increases by an order of magnitude, whereas the surface leakage currents only double. At 67 K the contribution of the surface leakage currents to the overall diode current is therefore larger than at 77K. In order to reduce free carrier absorption and thermal stress in the 2 56 x 2:36 FPA hybrid the GaSb substrate was thinned to 30 ~m. Figure :3.22 shows the measured spectral responsivity of a FPA pixel under backside illumination through the thinned substrate. The fringe pattern that is superimposed on top of the spectral response of the photodiode is due to interference of the light reflected by the front and backside surfaces of the thinned GaSb substrate. 5.3.5 C - V measurements

The capacitance of a reverse-biased p-n junction is determined by the capacitance of the space charge in the depletion layer. The space charge capacitance of a diode is given by 27

c.(v)

-

-d(V)

(1])

where ~ is the dielectric constant, and d(V) is the width of the depletion layer. Bias-dependent capacity measurements can therefore be used to determine the doping level in a diode. Such measurements were performed on InAs/(Galn)Sb 5

15 10 I rrT"

I

Wavelength [Iam]

'1

~ ~

r

. ...,,

=3

2

I

256 x 256 FPA-Hybride Thinnedto 30 Iam Anti-reflectioncoated

. .....,

>

o0 r

8.

T=77K O'.1

0.2 I

O'.3 0'.4 0'.5 Photon Energy [eV] ,

|

i

,

0.6 I

,,

Figure 5.22 Spectral responsivit!l of a 2 56 • 2 56 t:PA pi.rel under backside illumination. The (;aSb substrate is thinned to 30 I~tn and tire photodiode is illuminated through the thinned substrate in order to reduce free carrier absorption and thermal stress.

182


superlattice diodes using an HP 4192A impedance analyser. Figure 5.2 3 shows the measured reciprocal quadratic capacity of an InAs/(GaIn)Sb superlattice diode with a band gap of 143 meV as a function of applied bias. Two voltage regimes can be distinguished in the plot. At reverse biases below - 0 . 2 5 V a linear fit of the data yields a reduced doping level of Nred- 1.33 • ] 016 cm- 3 and a builtin voltage of Vbi--O.041 V. The deviation of the measured data from the linear behaviour around zero bias can be attributed to an increasing influence of the diffusion capacitance on the overall measured diode capacitance in that voltage regime. At reverse biases above - 0 . 2 5 V the slope of the curve in Figure 5.23 is smaller, thus leading to a higher reduced doping of Nred= 1.92 x 1016 cm- 3. The occurrence of these two voltage regimes can be explained by the doping profile of the device. The diode n region consists of a weakly doped region near the p-n junction (20 superlattice periods) and a heavily doped region towards the n contact (20 superlattice periods), whereas the p region of the device is relatively wide (70 superlattice periods) and intrinsically p-doped. At small reverse biases, the depletion layer only extends into the weakly doped n region. Because of the low doping level, the depletion layer expands rapidly into the weakly doped n region with increasing reverse bias, which according to equation (l 3) corresponds to a large slope in Figure 5.23. At a voltage of approximately - 0 . 2 5 V the depletion layer reaches the heavily-doped n region, and from then on expands more slowly with increasing reverse bias. The slower expansion of the depletion region results in the smaller slope in Figure 5.23. Because of the high n-doping level ND>>NA, the acceptor concentration approximately equals the reduced doping level NA~Nred in this voltage regime. In order to determine the exact doping levels of the diode, one has to take into account that in the n-doped regions of the device the nominal n-doping ~T~omis partly compensated by the residual p-doping NA of the superlattice, and therefore N D - Np)~ N3. In addition, in the investigated diode the nominal n-doping '

I

4x106

% (3 ,
,

10~

-

"5 0 ~

io ~

I

,

5

I

,

10

ll5

,

I

Inverse Temperature

!

,

20

25

IO00/T [K-~]

Figure 5.27 Detectivity of an InAs/(Galn)Sb superlattice photodiode with a cut-off at 8.3 I~m as a function of temperature.

Wavelength 11 10

12 I

~"

1012

'

I

9

Xc [pm] 9

I

8

I

T= 77K

'

QO

oO

R = 2A/W

9

1(~

::3

3

E .o.

8 .~ >

o

10~

o'SS~

...... O

_ . - - - - - -

. . . . . b.-- --~ . . . . . . . . . . . . .

o

9

I

1oo

,

I

125 Band g a p E

,

I

150

[meV]

Figure 5.28 Detectivity of lnAs/( Galn )Sb superlattice photodiodes having different band gap energies.

atmospheric transmission window between 8 and 12 pm exhibit background limited performance in photovoltaic operation. The dynamic impedance of such devices is comparable to those of state-of-the-art HgCdTe diodes with the same band gap energies. It can be expected that superlattice diodes having comparable q u a n t u m efficiencies to HgCdTe photodiodes will be realized in the future by developing devices with thicker superlattice stacks.

InAs/( Galn)Sb superlattices:a promising materialSZlStemfor infrareddetection 18 7

5.4 Summary and outlook It has been shown that the performance of infrared detectors comprising InAs/ (GaIn)Sb superlattices in the active layer can be comparable to M(TT photodiodes. Large-sized IR photodiodes with cut-off wavelengths in the third atmospheric window (8-12 ~tm wavelength) with background-limited performance were fabricated. The development of processing technology is still in progress, showing first successful results on passivated focal plane arrays hybridized on silicon fan-out structures. The present technology is based on two-inch GaSb substrates. However, three-inch substrates are already commercially available. From a fundamental point of view, the antimonide-based materials system exhibits advantages regarding tunnelling currents because of a higher effective electron mass which undergoes only a weak dependence on the effective band gap. In addition, advantages are expected because of smaller Auger recombination rates compared to MCT. Especially for applications at very long wavelengths and under low background conditions, InAs/(GaIn)Sb superlattice photodiodes are expected to outperform M(TT based imagers.

References 1. H. Sakaki, L. L. Chang, G. A. Sai-Halasz, (7. A. Chang, and L. Esaki, Twodimensional electronic structure in InAs-GaSb superlattices, Solid State Commun. 26 (9), 589 (1978). 2. D. L. Smith and (7. Mailhiot, Proposal for strained type II superlattice infrared detectors, J. Appl. Phys. 62 (6), 2545 (1987). 3. D. H. Chow, R. H. Miles, T. (7. Hasenberg, A. R. Kost, Y. H. Zhang, H. L. Dunlap, and L. West, Mid-wave infrared diode lasers based on GaInSb/InAs and InAs/A1Sb superlattices, Appl. Phys. Lett. 67 (2 5), 3700 (199:3). 4. J. L. Johnson, L. A. Samoska, A. (7. Gossard, J. L. Merz, M. D. Jack, G. R. Chapman, B. A. Baumgratz, K. Kosai, and S. M. Johnson, Electrical and optical properties of infrared photodiodes using the InAs/Gal_xInxSb superlattice in heterojunctions with GaSb, J. Appl. Phys. 80 (2), 1116 ( 1996). 5. F. Fuchs, U. Weimar, W. Pletschen, J. Schmitz, E. Ahlswede, M.Walther, J.Wagner, and P. Koidl, High performance InAs/Gal_xInxSb superlattice infraredphotodiodes, Appl. Phys. Lett. 71 (22), 3251 (1997). 6. L. Biirkle, F. Fuchs, R. Kiefer, W. Pletschen, R. E. Sah, and J. Schmitz, Electrical characterization of InAs/(GaIn)Sb infrared superlattice photodiodes for the 8 to ]2 l.tm range, Mat. Res. Soc. Syrup. Proc. 607, 77 (2000). 7. O. Madelung (ed.), Intrinsic Properties of Group IV Elements and III-V, II-VI, and 1-VII Compounds, Vol. III/22a, Landolt-B~rnstein New Series, SpringerVerlag, Berlin ( 198 7). 8. (7. G. van de Walle, Band lineups and deformation potentials in the modelsolid theory, Phys. Rev. B 3 9 (3), 18 71 ( 1988 ). 9. I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, Band parameters for III-V compound semiconductors and their alloys, ]. Appl. Phys. 89 ( 11 ), 5815 (2001).

188 Handbookof InfraredDetectionTechnologies 10. F. Szmulowicz, E. Heller, K. Fisher, and F. Madarasz, Optimization of Absorption in InAs/InxGal_xSb superlattices for long-wavelength infrared detection, Superlattices and Microstructures 17 (4), 373 ( 1995). 11. F. Szmulowicz, Private communications. 12. G. Bastard, Wave Mechanics Applied To Semiconductor Heterostructures, Les Editions de Physique, Les Ulis C6dex ( 1986). 13. C. H. Grein, P. M. Young, M. E. Flatt6, and H. Ehrenreich, Long wavelength InAs/InGaSb infrared detectors: Optimization of carrier lifetimes, J. Appl. Phys. 78 (12), 7143 (1995). 14. F. Fuchs, L. B/irkle, W. Pletschen, ]. Schmitz, M. Walther, H. G611ich, N. Herres, and S. Mfiller, InAs/Gal_xInxSb infrared superlattice diodes: Correlation between surface morphology and electrical performance, Proc. SPIE 3794, 41 (1999). 15. F. Fuchs, L. B/irkle, R. Hamid, W. Pletschen, E. Sah, R. Kiefer, and J. Schmitz, Optoelectronic properties of photodiodes for the mid- and far-infrared based on the InAs/GaSb/A1Sb materials family, Proc. SPIE 4288, 171 (2001 ). 16. L. B/irkle, F. Fuchs, J. Schmitz, and W. Pletschen, Control of the residual doping of InAs/(Galn)Sb infrared superlattices, Appl. Phys. Lett. 77 (11), 1659 (2000). 17. N. Herres, F. Fuchs, J. Schmitz, K. M. Parlor, J. Wagner, J. D. Ralston, and P. Koidl, Effect of interracial bonding on the structural and vibrational properties oflnAs/GaSbsuper]attices, Phys. Rev. B 53 (23), 15688 (1996). 18. C. A. Hoffman, J. R. Meyer, E. R. Youngdale, F. J. Bartoli, and R. H. Miles, Interface roughness scattering in semiconducting and semimetallic InAsOa~_xlnxSb superlattices, Appl. Phys. Lett. 63 (16), 2210 ( 1993 ). 19. J. Wagner, J. Schmitz, F. Fuchs, U. Weimar, N. Herres, G. Trhnkle, and P. Koid], Structural characterization of InAs-(Galn)Sb superlattices for IR optoe]ectronics, Mat. Res. Soc. Syrup. Proc. 421, 39 (1996). 20. J. Wagner, J. Schmitz, N. Herres, F. Fuchs, and M. Walther, Spectroscopic e]lipsometry for characterization of InAs/Gal_xInxSb superlattices, I. Appl. Phys. 83 (10), 5452 (1998). 21. M. B. Reine, A. Sood, and T. J. Tredwell, Photovo]taic Infrared Detectors, in R. K. Willardson and A. C. Beer (ed.), Mercury Cadmium Telluride, Vol. 18 of Semiconductors and Semimetals, chapter 6, pp. 201, Academic Press, New York (1981). 22. U. Weimar, F. Fuchs, E. Ahlswede, J. Schmitz, W. P]etschen, N. Herres, and M. Wa]ther, Tunneling effects in InAs/GaInSb superlattice infrared photodiodes, Mat. Res. Soc. Symp. Proc. 4 8 4 , 1 2 3 (1998). 23. C. H. Grein, P. M. Young, H. Ehrenreich, and T. McGill, Auger Lifetimes in Ideal InGaSb/InAs Superlattices, J. Electron. Mat. 22 (8), 1093 (1993). 24. H. Mohseni, M. Razeghi, G. J. Brown, and Y. S. Park, High-performance InAs/GaSb superlattice photodiodes for the very long wavelength infrared range, Appl. Phys. Lett. 78 (15), 2107 (2001). 25. A. Rogalski, Heterostructure infrared photovoltaic detectors, Infrared Physics and Technology 41 (4), 213 (2000).

InAs/(Galn)Sb superlattices: a promising material system for infrared detection

189

26. J. L. Johnson, The InAs/GaInSb strained layer superlattice as an infrared detector material: An overview, Proc. SPIE 3 9 4 8 , 118 (2000). 27. S. M. Sze, Physics of Semiconductor Devices, 2nd edition, John Wiley & Sons, New York ( 1981 ). 28. S. P. Tobin, S. Iwasa, and T. J. Tredwell, ] / f noise in (Hg,Cd)Te photodiodes, IEEE Trans. Electron Devices ED-2 7 ( 1 ), 43 ( 1980).


Chapter 6

GaSb/InAs supperlattices for infrared FPAs M. Razeghi and H. Mohseni

6.1 Type-II heterostructures 6.1.1 Historical review

In 19 77, Sai-Halasz and Esaki suggested type-II band alignment and some of its interesting physical behavior. ~ Soon after that they reported the optical absorption of type-II superlattices, 2 and later the semimetal behavior of the superlattice. 3 The applications of such a superlattice were proposed only several years after. 4 The flexibility of the material to cover a huge infrared range (2 to > 50~tm) and the possibility of a reduced Auger recombination rate s captured the attention of m a n y groups. Type-II heterojunctions have been found in m a n y applications of electronic devices such as resonant tunneling diodes (RTD) and hot electron transistors. However. perhaps the most important applications have been in optoelectronics, and m a n y significant results have been achieved from type-II modulators, ~ detectors, 7"s9'~(~ and laser diodes. 11.12 6.1.2 Definition of type-II band alignment

The band alignment of any heterojunction can be categorized as type-I, type-II staggered or type-II misaligned. In type-I heterojunctions, one material has lower energy for electrons and the holes and therefore both carriers are confined in that layer. In type-II heterojunctions, however, the electrons are confined in one material and the holes in the other. In the extreme case, which is called typeII misaligned, the energy of the conduction band of one material is less than the valence band of the other one.

192


Figure 6.1 Type-I, type-II staggered, and t!lpe-II misaligned heterojunction and some of the material with these band alignments.

6.1.3 Features of type-II band alignment and their applications The special band alignment of the type-II heterojunctions provides three important features that are shown in Figure 6.2. These features are used in m a n y devices to improve the overall performance of the device. The first feature is that a superlattice with the type-II band structure can have a lower effective bandgap than the bandgap of each layer. This is an important issue for the applications in the mid and long infrared wavelength range, since one can generate an artificial material (the superlattice) with a constant lattice parameter but different bandgap. Recently, very successful detectors 7 and lasers 12 have been implemented in the 2 - 1 5 B m wavelength range with InAs/GaInSb superlattices lattice-matched to GaSb substrates. The second feature is the spatial separation of the electrons and holes in a typeII heterojunction. This p h e n o m e n o n is a unique feature of this band alignment and is due to the separation of the electron and hole potential wells. As a result of such spatial separation, a huge internal electrical field exists in the junction without any doping or hydrostatic pressure. High performance optical modulators have been implemented based on this feature. 6 The third feature is the zener-type tunneling in a type-II misaligned heterojuntion. Electrons can easily tunnel from the conduction band of one layer to the valence band of the other layer, since the energy of the conduction band of the former layer is less than the energy of the valence band of the later layer. Unlike a zener tunneling junction which requires heavily doped layers, no doping is necessary for such a junction. Therefore, even a semimetal layer can be implemented with very high electron and hole mobilities since the impurity and ion scattering are very low. This feature of type-II heterojunctions has been successfully used for resonant tunneling diodes (RTDs) and recently for the implementation of type-II q u a n t u m cascade lasers. 1

(;a~b/InAs supperlattices for infrared FPAs

19 3

Figure 6.2 Unique features of t!lpe-II heterojunctions and superlattices.

6.2 Type-II infrared detectors 6.2.1 Principle of operation The active layers of photovoltaic and photoconductive type-II detectors are made from superlattices with a type-II band alignment. Similar to a type-I superlattice, the allowed energy states form the 'minibands', due to the coupling of electrons and holes in adjacent wells. However, unlike type-I superlattices, one can adjust the bandgap of type-II superlattices from a finite value to virtually zero. These superlattices resemble a direct gap semiconductor, since the minimum of the miniband in m o m e n t u m space is located at zero. Knowing the band structure and the optical absorption process in type-II superlattices, one can practically use the conventional photovoltaic and photoconductive structures to realize high performance type-II detectors. In the following sections, we shall explain the band structure of the type-II superlattices as well as their optical absorption process.

6.2.2 Band structure of type-II superlattices Let us study the minibands of a type-II s u p e r l a t t i c e in a more detail. Figure 6.3 shows an example of the miniband energy profile versus the superlattices period. Note that the effective mini bandgap of the superlattice, that is the energy between the minimum of E1 and maximum of HH1 decreases as the period of the superlattice increases. This feature provides a wide tunability range for the design of IR detectors. The mini gap energy shrinks to zero at a period of about 150 A which represents the transition between semiconductor and semimetal in the superlattice. The miniband width for electrons and holes decreases as the period of the superlattice increases, since the coupling between the adjacent well decreases. This means that for a photovoltaic device, where the charge transport is perpendicular to the superlattice interfaces, the mobility decreases for longer period superlattices. Another important fact is that a set of superlattices can be designed with different layer thickness and a constant energy gap by reducing

194

Handbook of Infrared Detection Technolo#ies ]

l'-

.........

~'

1'

i

r"

ga2

-0,2

H3

H2

-O.4

..

0

SO

100

d c ,l

150

200

_

.

250

Figure 6.3 The miniband energy profile of t!lpe-II superlattices with equal InAs and GaSb la!ters versus the period of the superlattice.

the electron well width and increasing the hole well width appropriately. Although the energy gaps of the members of the above set are identical, their band structures are totally different. We used this property to en#ineer the band structure of the type-II superlattices such that devices based on these superlattices can operate at higher temperatures.

6.2.3 Optical absorption in type-II superlattices Selection rules Assuming an in-plane effective mass to the electrons and holes and retaining the decoupling of mi=_+3/2 and m j = + l / 2 states, the dispersion relation of electron, light-holes, and heavy-holes can be written as: -)

EHH.n

(k•

=

-Eg

-

HHn

ELH,n

(k_,_) =

-Eg

-

LH.

)

)

h-k~_ Ec.n(kt_) = En 4 - ~ 2me

h2k~_ 2M,1 /i2k2 2m.

(1)

(;aSbllnAs suplmrlatticesJbr infrared FPAs

195

w h e r e HH~, LHn and E, are the n th e n e r g y level in the well and Mr,, m~, and mc are the effective mass of each particle. The e n e r g y reference is the b o t t o m of the c o n d u c t i o n band. Using this method, the polarization selection rule of the i n t e r b a n d t r a n s i t i o n in type-II and type-I q u a n t u m wells will be as the following table w h e r e i

I-I -

-

(S p.,.lX

>-
14 pm range

Motivation

The application of infrared detectors in the VLWIR range as well as the physical limitations of currently available devices were detailed previously. Obviously an interband detector is the best candidate for high q u a n t u m efficiency and higher operating temperature, if a good uniformity is available. The experimental results of the previous chapter suggest that type-II superlattices have superior uniformity compared with the available interband detectors in the VLWIR range. However, realizing that the material quality cannot be enhanced on a lattice mismatch substrate0 we started the work on GaSb substrates. Photovoltaic devices were the natural choice due to the high conduction of the undoped substrate as well as the benefits of the photovoltaic devices for imaging applications. E x p e r i m e n t a l results

The material is grown by an Intevac Modular Gen II molecular beam epitaxy (MBE) equipped with As and Sb valved cracker sources on p-type GaSb substrates. The photodiode structures were grown at 395~ according to a calibrated pyrometer. First, a 1 pm GaSb b u f f e r ~ c o n t a c t layer doped with Be ( p ~ l x l O 18cm-3) was deposited. Then a 0.5 pm thick InAs/GaSb:Be (p~1• is cm -3 to 3 • -~) superlattice was grown followed by a 2 pm thick nominally undoped superlattice. Finally. a 0.5 pm thick InAs:Si/GaSb (n~l• -3) superlattice was grown and capped with a IOOA thick InAs:Si ( n , ~ 2 • top contact layer. The growth rate was 0.5

216


Table 6.2 The performance of uncooled type-ll superlattice photodiodes compared with the uncooled HgCdTe and microbolometer at 8 ~ m Parameters

Type-II SL

HgCdTe

Detectivity

~ 108 cmHz 1'2/W

~ 1 ()s cmHz I

Microbolometer

2/W

,~ 10 ~ cmHz l'

)~=8 ~tm; T = 3 0 0 K RoA kc=8 ~tm: T = 3 0 0 K

1 . 4 x 10 -2

~ 1 x 1 ( ) 4 fZcm 2

_

Uniformity

Very good

Poor

Very good

Zero bias o p e r a t i o n R e s p o n s e time

Possible ~1() - s s

Possible ~1 ()-s s

Not possible ~1()-~ s

f2cm 2

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monolayer/s for InAs layers and 0.8 monolayer/s for GaSb layers. The V/III beam-equivalent pressure (BEP) ratio was about 4 for InAs layers and about 1.2 for GaSb layers. The cracker temperature for As and Sb cells was 800~ The selected thickness of InAs and GaSb layers were determined for specific cutoff wavelengths using a four-band superlattice k.p model. For devices with a cutof wavelength of nearly 16 ~m, the thickness of InAs layers was 54 A and the thickness of the GaSb layers was 40 A. Structural quality of the epitaxial layers was assessed using high resolution Xray diffraction. Figure 6.20 shows the typical X-ray diffraction pattern of the photodiode structures. The mismatch between the average lattice constant of the superlattice and the GaSb substrate was below 0.06%, while the full width at half maximum (FWHM) of the satellite peaks was below 70 arcsec for the grown devices. The surface morphology of the samples was also studied with an atomic force microscopy (AFM). Theoretical calculations show that surface and interface roughness lead to defect-like energy states inside the superlattice energy gap and broadening of the band edges. 34 Moreover, experimental results show a strong correlation between the surface roughness and electrical performance of InAs/ Gal_xInxSb superlattice photodiodes. Figure 6.21 shows the gray-scale surface morphology of a sample. Wide atomic steps are visible which is an indication of excellent surface smoothness. We could routinely grow samples with a root mean square (rms) surface roughness below 4 A over a 2 0 B m x 2 0 ~ m area which is among the best reported values for this material system. The transmission electron microscope (TEM) images of the material provided by Wright Patterson Air Force Base (WPAFB) also showed very good structural quality of the material. Figure 6.22(a) shows the bright field TEM image of the undoped area of a superlattice, while Figure 6.22(b) shows the dark field TEM image of the p-type layers of the same sample. Standard photolithography, wet etching, lift-off, and metallization were used to fabricate 400 l~m• lam mesas with 150 ~m• 150 ~m top contacts and 300 ~m • 150 l~m bottom contacts. Similar to the uncooled devices, very smooth side walls with a nearly 60 ~ angles could be etched routinely (see Figure 6.2 3). The processed samples were indium bonded to a copper heatsink and attached to the cold finger of a helium refrigerator equipped with KRS- 5 infrared windows. Since the refrigerator is a high-power device, it can easily induce a substantial

GaSb/InAs supperlattices for infrared FPAs

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Figure 6.20 High resolution X-ray diffraction of an InAs/GaSb ( ~4 A / 4 0 A) superlattice grown on a GaSb substrate. F W H M of the satellite peaks is below 60 arcsec, and the mismatch to the substrate is below O. 06 %.

a m o u n t of noise to the measurements. Proper grounding was used to reduce the noise level, however the average noise was considerably higher than a liquid nitrogen cryostat. Figure 6.24 shows the measured and modeled current densities versus the applied bias for devices with a cutoff wavelength of )~c= 16 ~m. The calculated current density, which consists of tunneling, generation-recombination, and diffusion current densities, shows good agreement to the measured values for forward and reverse biases. We assumed an effective mass of m,=O.O 3 mo for electrons and mh=0.4 mo for holes based on previous theoretical calculations 34 and experimental results. Based on the experimental measurements on similar devices, 3s we also assumed an electron mobility parallel to the growth direction Of~e-lO00 cm2/Vs. The mobility of the holes is not significant in the diffusion current, since the device has a n+-p junction. The fitting parameters for the model were carrier lifetime ~e=~h=220 ns, unintentional background doping level p=2.1 • 10 is cm -3, and generation-recombination lifetime in the depleted layer "~GR=0.6 ns. In contrast to HgCdTe, tunneling is not significant even at high values of reverse bias due to the higher effective mass of the electrons in type-II superlattices. However, generation-recombination current is the dominant source of dark current for these devices at T=8()K, and hence further improvement of the growth should increase RoA and detectivity. Also, we could observe the W a n n i e r - S t a r k oscillation in the Zener tunneling current of these devices. Figure 6.25 shows the differential resistance of the device versus reverse voltage bias. An oscillation with a peak-to-peak separation

218


Figure 6.21 AFM image of the surface of the device. Atomic steps with several micron width are clearly visible.

Figure 6 . 2 2 (a) bright and (b) dark field TEM images of a superlattice photodiode.

(;aSb/InAs supperlattices for infrared FPAs

219

Figure 6.23 SEM image of a mesa cross-section produced by wet etching. The sidewalls are very smooth with an angle of about 60 ~

of about 29.9 mV is clearly visible. Using the same formalism as previously, we could extract the thickness of the depleted layer to be dp=120nm, and the background doping level to be p=2.1 • 1015 cm-3. This value shows an excellent agreement to the calculated background doping level based on the dark current modeling. This agreement and the fact that the background calculation based on the Wannier-Stark oscillation can be calculated without the involvement of unknown parameters, indicates that the above value must be very close to the real background level. Absolute spectral responsivity was calculated from the measured spectral response of the device, using a Fourier transform infrared (FTIR) spectroscopy system, and its photoresponse to a calibrated blackbody setup. Figure 6.26 shows the typical spectral responsivity of the detectors with kc =16 Hm. The absorption features of CO2 and H20 are due to the small difference in the optical path length of the background measurement and the detector measurement. The peak responsivity for the sample is about 3.5 A/W which leads to a quantum efficiency of ~35% at 12~tm. Adding anti-reflection coating can reduce the 30% surface reflection of the device and the q u a n t u m efficiency will reach to nearly 50%. The 90% to 10% cutoff energy of the responsivity is only about 13 meV at T=80 K which is close to the thermal limit of 2kT= 12 meV. As it will be detailed later, such a sharp cutoff is a unique characteristic of our superlattices compared to the published results of other groups working on type-II superlattice detectors.

220


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Some of the samples with longer cutoff wavelengths were sent to the Wright Patterson Laboratory, since their FTIR system is purged with nitrogen and a very accurate spectral response can be measured. The low temperature m e a s u r e m e n t of these samples showed a clear set of features in the spectral response of these devices. In order to explain such behavior, we modeled the energy dispersion of the superlattices with a simple four-band k . p model. Figure 6.27 shows the results of such modeling for a sample with a 24 pm cutoff wavelength. The large superlattice period has led to strongly localized energy states evidenced by the small energy change of the bands over the whole m o m e n t u m space. Consequently, the density of states is approaching a two-dimensional configuration. In this situation, the optical absorption increases at the beginning of a transition between two bands and decreases at the edge of the Brillouin zone. The calculated energies of such transitions as well as the measured photoresponse of the detector at T= ] () K is shown in Figure 6.28. It is clear that the photoresponse increases at the onset of a transition and decreases at the edge of the Brillouin zone. Theoretical calculations predicts that unlike type-I q u a n t u m wells, the selection rule in type-II q u a n t u m wells do not require that the s u m m a t i o n of the q u a n t u m numbers between the relative states must be an even number. Figure 6.29 shows that there are strong transitions between the C1 and HH2 which is in agreement with the above.

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222 Handbook of Infrared Detection Technolo~lies 9

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Comparison with state o[ the art

In this part, we shall compare the performance of the devices developed in this work for the VLWIR range operation with the other high performance type-II superlattices as well as HgCdTe and extrinsic silicon detectors. Although several groups are actively working on type-II photodiodes, we could not find any published data about the absolute optical responsivity and detectivity of their devices in the VLWlR range. However, the relative spectral response of the devices reported by other groups was compared with the spectral response of the devices developed in this work. Unlike most of the other groups, we used only binary materials in our superlattices. We believe that the use of binary layers in these superlattices has significantly enhanced the uniformity and reproducibility of the energy gap. The 90%-10% cutoff energy width of devices with a wide range of cutoff wavelengths is only about 2kT which is at least four times smaller compared to similar devices based on InAs/ Gal_xIn• ( 0 . 2 5 < x < 0 . 3 5 ) superlattices. 7'~'3~'~7"3~ Figure 6.29 shows the spectral response of devices with cutoff wavelengths from 8 to 20 l~m versus the energy of the photons. The slope of the cutoff edge is nearly independent of the cutoff wavelength and is nearly 14 meV for these detectors at T=80 K. This value is very close to the thermal broadening limit or 2 k T - 12 meV at 80 K, considering the Fermi distribution of the energy of the electrons in the conduction and valence minibands. These measurements were confirmed with similar measurements at the Wright Patterson Laboratory. For example, their results

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Energy (meV) Figure 6.28 The features of the measured photoresponse of a device with a cutoff wavelength of 24 i~m at T= 9.5 K show good agreement with the calculated transition energies at critical momentum values.

show that one of our detectors with Xc=24 l~m has a 9()%-10% cutoff energy of about 15 meV at T=9.5 K. The study of the cutoff wavelength of the InAs/Gal_xInxSb superlattices shows that the higher the value of x, the more sensitive the cutoff wavelength is to both the value ofx and the thickness of the layers. Figure 6.30 shows the cutoff wavelength of InAs/Gal_xInxSb superlattices for a 7 ML (monolayer) thick Gal_xInxSb layer and 7-15 ML thick InAs layers versus different In mole fractions x calculated by our four-band k.p model. For example, this graph shows that for a superlattice with a 1()lam cutoff wavelength, the slope of the curves or dX/dx increases at higher values of x. Similarly, the cutoff wavelength is more sensitive to the InAs thickness at higher values of x. For example, at x=0.4, if the thickness of the InAs layer changes from 13 ML to 15 ML, the cutoff wavelength changes by ~ 6 ~m. While at x=0.3, similar change leads to a 2.5 l.tm change in the cutoff wavelength of the superlattice. Indeed, the study of the results of the other groups shows considerable correlation between the 90% and 1()% cutoff energy and the indium mole fraction independent of the superlattices ~cutoff wavelengths (see Figure 6.31 ). The available data on HgCdTe devices in the VLWIR range is limited since most of the high performance devices have military applications. Therefore, it is not possible to have a conclusive comparison between type-II devices and the best HgCdTe devices available. Although the proper design of type-II superlattices with Xc=161~m has improved the value of the RoA product significantly, it is still about one order of


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magnitude less than the R~A product of HgCdTe with a similar cutoff wavelength. This is mainly due to the higher background carrier concentration in type-II superlattices. The improvement of the growth has decreased this value from ~ 5 x l O 1 6 c m -3 to the current value of ~ 2 x l ( ) l S c m -3 over the past five years. Therefore, it is foreseeable that the current value can be decreased further to the ~ 5 x 1014 cm -3 range which is close to the background level of current HgCdTe material. Detectivity is proportional to the square-root of R~A, and consequently type-II devices show about three times lower detectivity compared with HgCdTe devices at ~= 16 lam. Most of the available data indicate that the uniformity of HgCdTe over a large area degrades rapidly as the cutoff wavelength increases above 1413m. Some of the most recent data provided by Lockheed Martin show a variation of more than 16meV in the energy gap over a 40cm• area for HgCdTe with a cutoff wavelength of about 15 l.tm. 39 In comparison, we demonstrated only a 5 meV variation in the energy gap over a 70cm• area of type-II superlattices with a cutoff wavelength of about 19 l~m. 3~ Extrinsic silicon detectors are the most commonly used detectors in the VLWIR range. Some of the reasons for their popularity are: excellent uniformity over large areas; high q u a n t u m efficiency: and high detectivity. However, they have

GaSb/InAs supperlattices for infrared FPAs

225

Figure 6.30 Cutoff wavelength of lnAs/Gal _.,.In.,.Sb superlattices for 7 ML thick Gal _,.In,.Sb and 7-15 ML thick InAs layers versus different In mole fractions, x.

two major drawbacks that are especially significant for space base applications. These are very low operating temperature and very low speed at a low background photon flux. In comparison with extrinsic silicon, type-I! superlattices show a m u c h higher operating temperature with similar q u a n t u m efficiency. This is an inherent advantage of the intrinsic detectors to extrinsic detectors and is explained briefly here. Under a low excitation condition, the detectivity of an intrinsic detector can be simplified as 4~ D . * - r l ) v ( T i n ~ 1/2 2hc k,tinni/ m

- -

(31)

where ~:in is the intrinsic carrier lifetime, ti, is the intrinsic detector thickness, ni is the intrinsic carrier concentration, r I is the q u a n t u m efficiency, h is the Planck constant, c is the speed of light, and )v is the wavelength. Similarly, the detectivity of an extrinsic detector can be calculated as

Dex

2hc \texnth

(3 2)

where 17exis the extrinsic carrier lifetime, t~x is the extrinsic detector thickness, and nth is the concentration of thermally generated carriers. The ratio of the detectivities is then


Figure 6.31 The 90% to 10% cutoff energll of devices with different indiunl nlole fraction, versus their cutoff wavelengths.

D i n _ ('qntexnth) 1/2 D~x k Textinni

(33)

To obtain high q u a n t u m efficiency, the thickness of the detectors must be on the order of their inverse optical absorption coefficient :x, therefore , ( ,~1/2 Din _ .Tin0~innt___h Dex \ "CexO~exni,/

(34)

where (~in is the optical absorption coefficient of intrinsic material and ~ex is the optical absorption coefficient of the extrinsic material. The optical absorption coefficient of extrinsic detectors is nearly two orders of magnitude lower than intrinsic detectors. For example, ~ex "-' 15 cm- 1 for Si:Ga versus ~in " " 1800 c m - 1 for type-II superlattices with k c - 1 6 l.tm. This fact combined with nth/ni~,lO0, results in nearly two orders of magnitude higher detectivity for intrinsic detectors at a given temperature. For example, the detectivity of Si:Ga at 80 K is about ,-~108 cmHzl/2/W (extrapolated data), 41 while it is 1.5 • 1() l~ cmHzl/e/W for type-II superlattices with a similar cutoff wavelength and temperature. Another disadvantage of extrinsic silicon detectors is their low speed due to a long dielectric relaxation time. Since extrinsic detectors need to be cooled to very low temperatures, the carrier concentration in the silicon can be as low as 1012 cm -3. This means that the material has such a high resistivity that the internal resistor-capacitor (RC) time constant, or the so-called dielectric


227

relaxation time constant Zrel~,x~t~,,n,is very large. The optically generated carriers in an extrinsic detector remain excited nearly the same length of time as Trelaxation, during which the detector is practically saturated. The value of this time constant can be calculated as 41 Trelaxation --

gtex qrl~sTexla

(3 5)

where ~ is the permittivity of the material, t~x is the device thickness, ~ is the background photon flux, ~ex is the carrier lifetime, and ~t is the carrier mobility. The typical parameters for extrinsic silicon are r1=30%, t~x-().O5cm, andg=8• At a low radiation of ~ - 1 ( ) 12 photons/cm2s, which is the case for most space based applications, the value of ~r~l~x~tion is nearly 0 . 0 8 4 s leading to a cutoff frequency of about 12 Hz! Even with a 300 K background, the cutoff frequency of the extrinsic silicon detectors is a few kHz. In comparison, type-II and other intrinsic detectors have cutoff frequencies in the tens of megahertz range, independent of the background photon flux.

6.5 Future work The technology developed during this work has led to the demonstration of typeII detectors with some of the 'best' and 'first' reported results. However, these detectors are still far from their optimal performance. Although we have decreased the background carrier concentration by one order of magnitude to its current record level of 2 • 10 ~5 c m - 3, this value is still considerably higher than the best HgCdTe material. Therefore, an even better control of the interfaces, stability of the growth parameters during the growth, and optimization of the MBE shutter sequences is required. At the current value of the background doping level, the major source of the dark current is due to diffusion and not the surface leakage, hence the developed processing techniques seem to be adequate. However, once the material quality is improved, the device processing and passivation is an important technical issue that must be addressed. Although the wet and dry etching techniques that are developed provide very smooth surfaces, the chemical property of the finished surface has not yet been studied. This characteristic determines the surface leakage current, which could be a considerable source of the device dark current and noise. Passivation is a step that can affect the surface chemistry significantly, and hence can alleviate some of the above problems. Another method that can increase the detectivity and the impedance of the devices is the e n h a n c e m e n t of the optical-to-electrical area ratio using microlenses. Theoretically, this can increase the detectivity by about one order of magnitude without any negative side effects. Mass transfer and binary mask methods can be used to realize arrays of microlenses on the back of the substrates. The major difficulty is the alignment of the lenses and the detectors, since the substrates are opaque and conventional mask aligners cannot be used.

228


Although the Auger recombination rate could be decreased by engineering the band structure of the type-II superlattices, the Auger process has not been completely eliminated yet. Figure 6.32 shows the energy dispersion of the minibands in the different m o m e n t u m spaces. It is clear that the Auger recombination is not possible at kx=() and kv-(). However, because of the inplane dispersion, the in-plane m o m e n t u m of the electrons is not necessarily zero and hence Auger recombination is still possible. By inducing q u a n t u m confinement in the x and y directions, the dispersion in these directions can be quantized. This can eliminate the Auger recombination and hence the carrier lifetime will be limited by other mechanisms such as radiative and non-radiative recombination, which are several orders of magnitude slower than the Auger recombination at room temperature. 1 ~total

=

1

t ~

1

+

1

"lTAuger 17radiative Tnon-radiative

(36)

Since the responsivity of the detector is proportional to the carrier lifetime, the responsivity will also increase dramatically which, in the absence of any noise e n h a n c e m e n t , will lead to a m u c h higher detectivity. Although infrared detectors based on self-assembled q u a n t u m dots have already been demonstrated, 42 the non-uniformity of the dot size leads to linewidth broadening and reduction of the signal. Here a method is proposed which can produce high quality and highly uniform q u a n t u m dot structures. The starting material is a superlattice. Stacks of high quality q u a n t u m dots can be developed by etching pillars and consequent dielectric coating. In order to achieve q u a n t u m size effect, one needs to confine the electrons within tens of nanometers. However. surface leakage current will be a severe problem for such a small device, since the ratio of the surface to the volume increases dramatically. In order to circumvent this problem, one can use a gated device shown in Figure 6.33. Although the diameter of the device is 5 0 - 2 0 0 nm, the effective confinement diameter can be adjusted by the gate voltage to a m u c h smaller

Figure 6.32 The energy dispersion of the minibands in the different momentum spaces.


229

value. Moreover, the allowed energy states of the electrons can be changed by the gate voltage and hence the cutoff wavelength of the device. The sensitive area of such a q u a n t u m pillar is very small and not useful for m a n y applications. Figure 6.34 shows a realization scheme that can provide a large detector area containing a large number of closely packed pillars. Below, the feasibility of this approach is demonstrated. Low energy e-beam lithography was used to produce top metal contacts. High quality metal contacts with diameter in the range of 1()()()-1()()nm were successfully defined on the surface of the samples as shown in Figure 6.35. Reactive ion etching (RIE) was used to produce uniform anisotropic etching of the pillars through the GaSb test material. The dry etching parameters was

Figure 6.33 Schematic diagram of the gated pillars ~vith an adjustable lateral confinement.

Figure 6. J4 Realization of the pillars with advanced metalization and passivation techniques.


Figure 6.3 5 Top metal contacts defined b!l e-beam lithograph!t on the samples: (a ) 500 nm diameter contacts: and (b) 1 O0 nm contacts.

Figure 6.36 (a) Reactive ion etching was usedjbr the formation of un(form two-dimensional arra!ls of pillars. (b) SEM image of the cleaved edge of a mesa covered b!l a z~nfform la!ler of Si ~N4 with a thickness of about 5Onto.

similar to V.G.3. Figure 6.36(a) shows the SEM image of an array of 5()()nm pillars produced by the dry etching. The vertical to horizontal etching ratio is in excess of five. We were able to produce two-dimensional arrays of pillars with excellent uniformity over thousands of square microns. Finally, passivation of the material was studied. Passivation is one of the most important steps for the realization of these q u a n t u m devices. The essential issues are the uniformity of the dielectric and the coverage of the device surface. Plasma enhanced chemical vapor deposition (PECVD) was used to form uniform layers of silicon nitride on these structures. Figure 6.36 shows SEM image of a cleaved edge of a mesa covered with a 50 nm Si 3N4 layer by this method.


2 31

References 1. G. A. Sai-Halasz, R. Tsu, and L. Esaki APL 3 0 , 6 5 1 (1977). 2. G. A. Sai-Halasz, L. L. Chang, J. M. Welter and L. Esaki, Solid State Commun. 27,935(1978). 3. G. A. Sai-Halasz and L. Esaki, Phys. Rex,. B 18, 2812 ( 19 78). 4. D. L. Smith and C. Mailhiot, J. Appl. Ph!ls. 62, 2 545 (1987). 5. E. R. Youngdale, J. R. Meyer. C. A. Hoffman and F. 1. Bartoli, Appl. Phys. Lett. 64,3160(1994). 6. H. X i e a n d W . I. Wang, J. Appl. Ph!ls. 76, 92 (1994). 7. ]. L. Johnson, L. A. Samoska, A. C. Gossard, J. Merz, M. D. Jack, G. R. Chapman, B. A. Baumgratz, K. Kosai and S. M. Johnson, I. Appl. Phys. 80, 1116 (1996). 8. F. Fuchs, U. Weimer, W. Pletschen, J. Schmitz, E. Ahlswede, M. Walther, J. WagnerandP. Koidl, Appl. Phys. Lett. 71, 32-51 (1997). 9. H. Mohseni and M. Razeghi, Photon. Technol. Lett. 13, 517 (2001 ). 10. H. Mohseni, M. Razeghi, G. Brown and Y. S. Park, Appl. Phys. Lett. 78, 2107(2001). 11. B. H. Yang, D. Zhang, Rui Q. Yang, C.-H. Lin, S. J. Murry and S. S. Pei, Appl. Phys, Lett. 72, 2 2 2 0 (1998). 12. C. L. Felix, J. R. Meyer, I. Vurgaflman, C. H. Lin, S. J. Murry, D. Zhang and S.S. Pei, Phot. Tech. Lett. 9 , 7 3 4 ( 1 9 9 7 ) . 13. C. H. Lin, R. Q. Young, D. Zhang, S.J. Murry, S. S. Pei, A. A. Allerman and S. R. Kurtz, Elec. Lett. 3 3 , 5 9 8 (1997). 14. C. G. Van de Walle and R. M. Martin, Phys. Rev. B 3 5, 8154 ( 198 7). 15.C.G. VandeWalle, Phys. Rev. B39, 1871 (1989). 16. C. H. Grein, P. M. Young, H. Ehrenreich and T.C. McGill, ]. Electronic Material 22, 1093 (1993). 17.H. Mohseni, V.I. Litvinov and M. Razeghi, Ph!ls. Rex,. B 58, 378 (1998). 18. D. L. Smith, T. C. McGill and J. N. Schulman, Appl. Ph!!s. Lett. 43, 180 (1983). 19. C. Mailhiot and D. L. Smith, J. Vat'. Sci. Technol. A7(2 ), 44 -5 (1989). 20. N. Herres, F. Fuchs, J. Schmitz, K. Pavlov, J. Wagner, J. Ralston, P. Koidl, C. Gadaleta and G. Scamarcio, Phys. Rex,. B 5 3 . 1 - 5 6 8 8 ( 1 9 9 6 ) . 21. G. Zegrya and A. Andreev, Appl. Phys. Lett. 6 7 , 2 6 8 1 (199-5). 22. J. Piotrowski, W. Galus and M. Grudzien, Infrared Phys. 31, 1 (1990). 23. D. Wang, G. Bosman, Y. Wang and S. Li, J. Appl. Phys. 77, 1107 (199-5). 24. S. Slivken, A. Matlis, C. Jelen, A. Rybaltowski, J. Diaz and M. Razeghi, Appl. Phys. Lett. 74, 173 (1999). 25. H. Mohseni, J. Wojkowski and M. Razeghi, IEEE J. Of Quantum Elect. 3 5, 1041 (1999). 26. H. Mohseni andM. Razeghi, Proceedin#s 1999-ISDRS, -563 (1999). 2 7. W. Radford, D. Murphy, A. Finch. K. Hay, A. Kennedy, M. Ray, A. Sayed, J. Wyles, R. Wyles and J. Varesi, SPIE Proceeditl#s 3 6 9 8 , 119 (1999). 28. A. Rogalski, Infrared Ph!lsics and Technolog!140, 2 79 ( 1999 ).

232 Handbookof Infrared Detection Technoloqies 29. D. Chow, R. Miles, J. Schulman, D. Collins and T. McGill, Semicond. Sci. Technol. 6, C4 7 (1991 ). 30. H. Mohseni, A. Tahraoui, J. Wojkowski, M. Razeghi, G. J. Brown, W. C. Mitchel and Y. S. Park, App1. Phys. Lett. 77, 1:572 (2000). 31. C. Elliott, N. Gordon, D. Wilson, C. Jones, C. Maxey, N. Metclafe and A. Best, J. of Modern Opt. 45, 1601 (1998). 32. C. Elliott, N. Gordon, R. Hall, T. Phillips, C. Jones and A. Best, J. Electronic. Materials 26, 643 (1997). 33. S. Slivken, A. Matlis, C. Jelen, A. Rybaltowski, J. Diaz and M. Razeghi, Appl. Phys. Lett. 74, 173 (1999). 34. G. Bastard, Phys. Rev. B 25, 7584 (1982). 35. L. Burkle, F. Fuchs, R. Kiefer, W. Pletschen. R. E. Sah and J. Schmitz, Mat. Res. Soc. Syrup. Proc. 607, 77 (20()0). 36. M. Young, D. Chow, A, Hunter and R. Miles, App1. Surface Sci. 1 2 3 / 1 2 4 , 395(1998). 37. F. Fuchs, U. Weimar, E. Ahlswede, W. Pletschen, J. Schmitz and M. Walther, Proc. SPIE 3 2 8 7 , 14 (1998). 38. R. Miles andJ. Wilson, SPIE 2 8 1 6 , 76 (1996). 39. S. Tobin, M. Hutchins and P. Norton, J. Electronic. Materials 29, 781

(2000).

40. A. Rogalski, Infrared Photon Detectors, (SPIE Bellingham WA, 1995), p. 40. 41. D. Schroder, Charge-coupled Devices, (edited by D. Barbe, Springer-Verlag, Heidelberg, 1980), p. 57. 42. S. Kim, H. Mohseni, M. Erdtmann, E. Michel, C. Jelen and M. Razeghi, Appl. Phys. Lett. 7 3 , 9 6 3 (1998).

Chapter 7

MCT properties, growth methods and characterization Randolph E. Longshore

7.1 Preface Mercury Cadmium Telluride (MCT) is a very important semiconducting material developed over the past 44 years. MCT is used to fabricate infrared (IR) detectors for military and scientific applications. Military applications include the use of IR detector focal plane arrays (FPA's) in thermal imaging systems and in ground and space surveillance IR imaging systems. Scientific applications include IR detector arrays in telescope sensors for astronomy and IR detectors for spectrometers and radiometers. This chapter will describe MCT characteristics, MCT material growth techniques and MCT material characterization methods. The focus of this chapter will be on recent developments and emerging technologies for MCT. For example, Molecule Beam Epitaxy (MBE) thin layer MCT growth methods are being developed which will allow the design and fabrication of new devices. These devices include Avalanche Photodiodes (APDs), High Operating Temperature (HOT) IR detectors, very long wavelength IR (VLWIR) detectors using strain-layer super lattice (SLSL) designs, and Multi-spectral Focalplane Arrays (MFPAs).

7.2 Introduction MCT is very useful as an IR detector material, primarily because of two of its features compared to other IR detector materials: 1. MCT is a direct bandgap semiconductor and 2. the bandgap of MCT can be adjusted by varying the ratio of the amount of the HgTe compound to the amount of CdTe in the particular MCT composition.

234 Handbookof Infrared Detection Technologies Because of the direct bandgap, MCT has a high adsorption coefficient for the IR, resulting in high q u a n t u m efficiency in a relatively thin IR detector structure (approximately 10 ~tm thick). The direct bandgap allows operation of the IR detector at a higher temperature than other long wavelength IR detectors, such as extrinsic silicon photoconductors. The variable bandgap feature allows the tuning of a detector's cutoff wavelength in the wavelength region from 0.7 micrometers (lam) to slightly greater than 25 ~m by adjusting the MCT composition. Requirements on MCT materials are driven by the desire to produce IR detectors with optimum performance at a low cost. Optimum performance means high operability (i.e. few dead pixels), high quantum efficiency, uniform responsivity, low noise and detector stability. For the production of low cost, large area IR FPAs, large, uniform, defect flee, MCT wafers are needed. Three basic MCT material requirements are: 1. large wafer size, 2. small variations in composition and doping across a large wafer, and 3. very few defects across a large wafer. Other material requirements are determined from the specific IR device that is to be fabricated with the MCT material. For example, heterojunction IR arrays and multispectral IR detector arrays require multilayer material structures with uniformity in layer parameters, a sharp interface between each layer (or, depending on the device design, a controlled doping profile and/or controlled composition profile across layers) and stability of the layers. In addition, low cost production requires reproducibility of material parameters over many growth runs and rapid, non-destructive material evaluation methods. 7.2.1

Brief history

Research and development of MCT materials and devices have been documented in several books covering the periods from approximately 1970 to the present. 1-7 Developments of MCT before 1970 were not published in detail because of the potential military applications. Most of the early research was performed at military laboratories or commercial laboratories under government supervision. In the early 1980s, universities became more involved in the research and development of MCT. W. D. Lawson and co-workers at the Royal Radar Establishment in England did the initial studies on mixtures of HgTe and CdTe in 1958. s Others involved in the early development of MCT included French, United States, Polish and former Soviet Union, investigators. 1'2 A review of the state of the art for MCT for the mid 1980s is given by Herman and Pessa. 9 A history timeline for MCT material and IR detector development through the 1990s is given by Norton. 1~ Several journals and proceedings give information on the development and status of MCT technology. The Proceedings of the US Workshop on the Physics and Chemistry of Mercury Cadmium Telluride was published in the Journal of

MCT properties, growth methods and characterization

235

Vacuum Science and Technology during the period from 1981 to 1993. Currently, the workshop is published in the Journal of Electronic Materials. The workshop topics have been broadened to cover all II-VI materials. The Proceedings of the International Conference on II-VI Compounds is published in the Journal of Crystal Growth. Each year there are several proceedings published by SPIE that address MCT IR detector materials and MCT IR detectors and sensors.

7.3 MCT characteristics and material properties 7.3.1 Composition~crystal

structure

MCT is a ternary alloy consisting of a solid solution of the two compounds, HgTe and CdTe. A MCT composition is typically denoted as Hg 1_xCdxTe where the x (or x-value) is the mole fraction of CdTe and thus ( l - x ) is the mole fraction of HgTe. The Hgl_xCdxTe alloy system has a zincblende crystal structure which is in the form of two interpenetrating face-centered cubic lattices. In this crystal structure, each lattice site is surrounded by four nearest neighbors in which the Te-ion forms a sublattice with four nearest neighbors of the other sublattice. The other sublattice consists of randomly distributed Cd-ions and Hg-ions. (see Figure 7.1) There are five possible selections of the Cd and Hg as nearest neighbors to the Te-ion: 1. four Cd-ions, 2. four Hg-ions,

Figure 7.1 MCT zincblende crystal structure and ( 1 1 1 ) crystal surfaces.

236 Handbook of Infrared Detection Technologies 3. two Cd-ion and two Hg-ions, 4. three Cd-ions and one Hg-ion, or 5. three Hg-ions and one Cd-ion. These five structures occur randomly t h r o u g h o u t the MCT material in a perfect crystal. For the (111 / crystal plane, MCT has a B type surface (Te-rich) and an A type surface (Cd-rich) as shown in Figure 7.1. The lattice constant, ao, for MCT depends only slightly on the x-value, < O. 31% between x=O to x = l varying nonlinearly by approximately 2 0 x l O - l ~ m . 11 Hence, the lattice constant for MCT does not follow Vegard's law where the lattice constant varies linearly with composition. Lattice constant (in Angstroms) as a function of composition is given by the following equation: 12 ao -

6.4614 + (8.4x + 11.68x 2 - 5.7x3)1() -3

/1)

7.3.2 Bandgap CdTe is a direct bandgap semiconductor, whereas HgTe is a semimetal with zero bandgap. For x greater than approximately 0.15, the MCT alloy is a direct bandgap semiconductor. A simplified schematic of the MCT bandgap, (Eq), is shown in Figure 7.2. The band structure shown at the F point at the center of the Brillouin zone for a zincblende crystal structure consists of the conduction band, (F6), a light hole

l~g

F8 ]

Figure 7.2 MCT simplified band structure.

,\ICT properties, grou,th nlethods and characterization

2 37

band, (F8), a heavy hole band, (F8) and spin-orbit split-off band. (F7). Detailed band structure calculations for MCT can be found in 'The properties and applications of the Hgl_xCdxTe alloy system'. 13 The band structures for the conduction band, the heavy hole valence band and the light hole valance band are described by the following equations: 14

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3( 4 l a m ) c a n be readily measured by subjecting the film to a wet chemical etch that preferentially decorates threading segments intersecting the surface of the film through the formation of pits. The etched surface can be observed under an optical microscope to measure the density of these pits. 4~ Threading dislocation densities < 5• 1() ~ cm -2 are desirable for the satisfactory operation of LWIR diodes at 7 7 K. The dependence of diode performance on the dislocation density has been reported by Johnson et al. 42 The performance of MWIR diodes appears to be less sensitive to the dislocation density as attested by the performance of MWIR p-n diodes grown on Si substrates for which the dislocation density is generally in excess of l x l() ~' c m - 2 . 4~ The dislocation density in a device structure is primarily dependent on the degree of lattice-matching with the underlying substrate. Figure 9.6 shows the near-surface dislocation densities measured for a series of LWIR device structures.

Status of HgCdTe MBE technolog!l

~21

Figure 9.6 Dislocation densit!l measured for series of I.i'~'lR devices grown in our lahoratory. A vast majoritzl of the device structures have near-surfi~ce dislocation densities of about ~ • 10 s cm 2.

Typical dislocation densities for LWIR and MWIR device structures grown on CdZnTe substrates are 3 • 1 () 5 c m - 2 and 5 • 1 () ~ c m - 2, respectively. One of the most challenging aspects of HgCdTe device growth lies in controlling the macroscopic defect density. For a chosen Hg/Te flux ratio, the substrate temperature must be controlled within a range of approximately 5 ~ to realize the optimum quality (minimum defect density) in the film. At temperatures higher t h a n the optimum growth temperature, or when the Hg flux is deficient, a large density of morphological defects, known as voids, forms in the film. The term 'void' is used in the literature to describe a polycrystalline defect whose diameter ranges between 2 and 1() microns and which has an irregular shape. Voids can be seen when the surface of the HgCdTe film is observed under Nomarski contrast optical microscopy. This defect, illustrated in Figure 9.7, is a Te-rich polycrystalline region in the film. Cross-sectional microscopy and TEM studies have shown this defect to have a conical profile, resembling an ice c r e a m c o n e . 4 4 - 4 6 Void defects have been shown to cause shorted diodes, resulting in inoperable pixels. 47 The density of voids decreases as the growth temperature is lowered and approaches the optimum growth temperature for the chosen Hg/Te flux ratio. However, as the growth temperature is reduced below the optimum growth temperature, a different population of defect, the microvoid, is generated. These defects are smaller in diameter than conventional void defects and have a more regular shape. Microvoids are thought to form when the Hg flux is higher than the optimum value, or the substrate temperature is lower than the optimum growth temperature. Piquette et al. have reported 4s in detail the dependence of

322


Figure 9.7 Cross-sectional TEM view of a HgCdTe.fihn showing the "conical'prqfile of a void defect. The image on the right illustrates the pol!tcr!lstalline region contained in the defect.

the density of both voids and m i c r o v o i d s on growth conditions. In our laboratory we find that, under optimal growth conditions, the total density of both voids and micro-voids is typically between 5()() and 1 5()() cm-2 for n-p-n heterostructures, such as dual-band detectors, whose thickness is 2() gm.

9.3.4 Doping The growth of n-type HgCdTe layers with the desired levels of In concentration and structural perfection is relatively straightforward. Typical mobilities for MWIR and LWIR HgCdTe alloys doped with In at 1-3 • 1 ()is cm-3 measured at 7 7 K for films grown in our laboratory are 50()()() and 80 ()()() cm2/Vs, respectively. Data on the reproducible growth of n-type HgCdTe films have also been reported by Rockwell. 4~ In contrast with the relative simplicity of n-type doping with In, the low sticking coefficient of As and the tendency for compensation of acceptor impurities due to the presence of Hg vacancies, has raised challenging issues for in situ p-type doping, s~ The ability to achieve good structural perfection with desired levels of hole concentration in in situ doped p-type layers is a prerequisite for the growth of high-performance n-p-n two-color device structures. Our laboratory has developed a unique process that enables growth of in situ doped ptype layers with excellent structural and electrical properties. 51 To assess the electrical properties of these in situ doped p-type layers, individual 4 - 6 l.tm thick MWIR-HgCdTe:As films were deposited on (211)B CdZnTe substrates. The samples were subjected to a post-growth Hg-anneal to remove any Hg-vacancies in the films, which are deep level acceptor impurities. Electrical contacts were made with indium and variable temperature Hall effect and resistivity

Status of HgCdTe ,\IBE technolog!!

32 3

m e a s u r e m e n t s were performed. The in s i t u doped MWIR-HgCdTe:As films exhibit classic p-type conductivity for hole concentrations from 5xl()l~' to 2 x l O ~scm -3. As reported previously, the electrical activity in these in s i t u arsenic-doped films is in the range of (~()-1()()% as determined from SIMS and Hall effect measurements. An activation energy of 7 meV was measured for the As acceptor from variable t e m p e r a t u r e Hall m e a s u r e m e n t s for HgCdTe films with x=0.30. The dependence of hole mobility on the hole concentration for As-doped MWIR HgCdTe layers is illustrated in Figure 9.8. As expected, the hole mobilities exhibit a trend of decreasing values with increasing hole c o n c e n t r a t i o n due to increased ionized and neutral impurity scattering, similar to that observed for n-type alloys. 82 The hole mobility is also sensitive to the band gap of the HgCdTe alloy, with mobility increasing with a decrease in the band gap. For this reason there is some scatter in the data s h o w n in Figure 9.8, since the alloy composition of the As-doped samples extends over the range x = 0 . 2 8 - 0 . 3 8 . Nevertheless, the trend of decreasing mobility with increasing hole concentrations is clear, as s h o w n by the dotted line. The r u n - t o - r u n variability in dopant incorporation for As and In are contrasted in Figure 9.9, which summarizes dopant concentrations as determined from SIMS m e a s u r e m e n t s for a set of LWIR two-color detectors. Typical doping c o n c e n t r a t i o n for the n-type absorbing layer is 1- 3 • 1 ()) s c m - 3, and that for the p-type layer is 1-1 5 x 1 ()~ - c m - 3. The c o n c e n t r a t i o n of As that is

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324


Growth Run Index Figure 9.9 Control over As and In dopant concentrations, a s measured b!t SI,\IS. jor a series of LWIR multispectral device structures. T/re horizontal lines indicate t/re acceptable range for the dopant concentrations.

incorporated in the films shows more scatter than the In concentration due to the strong substrate temperature dependence of As incorporation.

9.4 Device applications 9.4.1 Multispectral HgCdTe infrared detectors The capabilities of infrared thermal imaging systems can be significantly enhanced through the use of multispectral sensors in place of traditional singlewavelength infrared sensors. By providing sensitivity to wavelength-dependent variations in emissivity and reflectance, detection in multiple spectral bands can aid in scene interpretation and signature recognition. Multispectral FPAs can also provide additional flexibility through the selection of a particular spectral band in which scene contrast is optimized. The next generation of multispectral devices will be based on a structure that utilizes stacked p-n junctions grown by MBE. This approach enables significant savings in weight, power, and optical system complexity compared with multispectral system approaches that depend either on dispersion of the optical signal across multiple detector arrays, or the use of a filter wheel to perform spectral decomposition in tandem with a single FPA. The stacked junction approach also guarantees image

Ntatus of HflCdTe AIBE technolo#!i

32

alignment at the level of the individual pixels from the various spectral bands, which can significantly simplify video image processing by eliminating the need to accurately align separate FPA fields-of-view. The standard device architecture for two-color detection grown by MBE in our laboratory was developed at Raytheon Vision Systems s~-ss in the early 1990s. The architecture is based on an n-p-n device structure, as illustrated in Figure 9.10. The structure, which consists of two back-to-back p+-n HgCdTe photodiodes sharing a common p-type layer, enables both detectors to be physically positioned in the same pixel location. The spectral sensitivities of the two bands, labelled 'Band 1' and 'Band 2' in Figure 9.1 (). are determined by the alloy compositions of the two n-type HgCdTe absorbing layers, which can be precisely controlled in the MBE growth process. Because the structure is designed for backside illumination through the CdZnTe substrate, the alloy composition of the Band 1 absorber layer is higher (and corresponding detector cutoff wavelength shorter) than that of the overlying Band 2 absorber. An essential precursor technology for demonstration of these N-p-n architecture devices was a process for in sitzi p-type doping with As. Development of this process was hampered by several materials growth issues raised by introduction of As into the crystal growth process. Control of substrate temperature, in particular, has been a significant issue with respect to control of p-type doping, due to the sensitivity of As incorporation to substrate temperature, s~''s7 In addition, the e l e c t r i c a l a c t i v i t y of As has been shownS'~ to diminish at concentrations above a p p r o x i m a t e l y 2 • 1() ~'~ cm-~. Increasing the atomic concentration of As to l()2~cm-~ has also been observed to have a deleterious effect on dislocation density in the p-layer and topmost n-layer ofn-p-n devices. 59 For these reasons, reproducible control of As dopant c o n c e n t r a t i o n in the 1 - 1 0 x 1 0 1 7 c m -~ range has been desirable. The ABES n o n - c o n t a c t temperature sensor, discussed in Section 9.2.2, has enabled us to achieved sufficiently tight control over substrate temperature, as manifested in good runto-run reproducibility of As concentration during the growth of two-color device structures. ~'~)A typical depth profile, obtained with oxygen bombardment SIMS analysis, of the dopant species in a multispectral device structure is presented in

Figure 9.10 n-p-n device architecture developed b!l Ra!ltheo~l i'ision Sjlstelns for IR detection in two spectral bands. The HgCdTe allo!l composition of the Band 1 and Band 2 ahsorbin~l la!lers determines the spectral bands sensed b!l the detector.

326

Handbook of Infrared Detection Tecilnolo#ie,,;

Figure 9.11. The In and As dopant c o n c e n t r a t i o n profiles are observed to be wellbehaved, w i t h o u t any evidence of segregation or m e m o r y effects. The b a c k g r o u n d dopant levels of In and As are m e a s u r e d to be at the SIMS detection limits of 5 • ~cm-~ and 5 • under oxygen and cesium b o m b a r d m e n t , respectively. HgCdTe epitaxial wafers intended for multispectral device applications can be processed into either sequential-mode or s i m u l t a n e o u s - m o d e detector arrays, as illustrated in Figure 9.12. The fabrication sequence for such multispectral arrays utilizes process steps that are identical to those used in the m a n u f a c t u r e of singlecolor arrays, with the exception of the mesa delineation. This is performed with reactive ion etching r a t h e r t h a n wet chemical etching in order to create nearly vertical sidewalls on the mesas, for m a x i m u m optical fill factor in the top junction. In the sequential-mode detectors illustrated in Figures 9.12 (a) and 9.12 (c), electrical contacts are provided to the top and bottom n-type photon-absorbing layers, while the middle p-type layer is left floating, resulting in a format that has

1019

!

l,,sl A !

E 0

Z

0 m

i O. 6. Insertion of APDs in place of unity-gain PIN detectors can significantly improve the signal-to-noise performance of laser ranging systems, which translates into either improved range or decreased laser power requirements. 63 The noise characteristics of the APD are governed by two distinct sources: (1) shot noise associated with both the diode dark current and the opticallygenerated current, and (2) excess noise deriving from the statistical nature of the multiplication process. The shot noise component arising from the detector dark current is controlled by extrinsic sources such as electrically active semiconductor crystal defects, either in the bulk portion of the detector or at passivated surfaces of the device. The excess noise is determined by an intrinsic property of a particular semiconductor system, the ratio of the hole impact ionization coefficient (13) to the electron impact ionization coefficient (~), k= 13/0~. In comparison with HgCdTe-based APDs, commercially available APDs constructed in Si, Ge, or III-V materials such as GaInAsP have significant drawbacks when applied to the detection of eye-safe laser wavelengths near 1.5 ~tm. Si has near-ideal noise properties due to its non-unity k-value, but does not offer significant optical absorption at 1.5 lam. InGaAs, Ge and InP offer inferior noise performance at moderate frequencies (tens to hundreds of MHz) and limited gain-bandwidth product at higher frequencies (GHz and above) due to their near-unity k values. 63 HgCdTe alloys, because of their unique energy band structure, can be designed to provide low-excess-noise carrier multiplication. The advantage of HgCdTe alloys for low-excess-noise avalanche multiplication derives from a 'resonance' in hole impact ionization coefficient resulting from the material's band structure at a particular x-value around X - - ( ) . 7 3 . 64"6s At this alloy composition, the semiconductor's energy bandgap (E.q) and valence-band spin-orbit splitting (Ao) are equal, which induces a large ratio of hole-to-electron ionization coefficients, an effect which has also been previously observed in the GaAISb material system. 66 In order to take advantage of the flexibility of the HgCdTe alloy system to provide both efficient optical absorption of 1.5 5 pm radiation at x,~().6 and hole-dominant avalanche multiplication at x=().73, a separate absorption and multiplication-APD (SAM-APD) architecture has been adopted. With the addition of ohmic contact layers, the alloy composition of epitaxial layers in a single HgCdTe SAM-APD device structure can span a range as wide as

Status of HqCdTe ,\IBE technoloq!t

333

x = 0 . 2 - 0 . 8 . Because this range is considerably broader than typically encountered during the growth of conventional passive MWIR and LWIR devices, it has been necessary to utilize SE to perform in situ monitoring of alloy composition during the growth of HgCdTe APD device structures. Recent experiments 2~' indicate that SE has excellent sensitivity and run-to-run repeatability when applied as a sensor for surface HgCdTe composition during the growth of near-IR APD device structures. Figure 9.1 7 gives an indication of the noise floor of the SE-based composition sensing during monitoring of a wide bandgap, x = 0 . 5 9 6 layer, for which the standard deviation in derived x'-value is 0 . 0 0 0 1 7 during the course of the m e a s u r e m e n t in which the surface was held static under a stabilizing Hg flux. A deviation in .x'-value of this magnitude during epitaxial growth would correspond to a variation in CdTe flux of only ().()3%, which is substantially below what can be sensed by any other available technique. Precise regulation of HgCdTe alloy composition in the APD device structure is most critical in the avalanche gain layer. Although the range of x'-value over which hole impact ionization resonance occurs is not precisely known for HgCdTe, prior m e a s u r e m e n t s in the GaAISb system suggest that the range of A/Eq over which resonance occurs might be as small as _+().()2. This range would correspond to an .~*-value variation of _+().()1 in HgCdTe. The extent to which SE can be used to achieve this level of precision in run-to-run control of HgCdTe alloy composition has been assessed during the course of a campaign of more

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2-1962 STATIC SURFACE

0.600

rUJ 1.:::)1.1 0.598 ,_1 > 2 x 1 (I: cm 2) or low ( < 2 x 10 s cm 21 dislocation densit!l. The diode size in each case is SO lira • ~0 l~m. Also displa!led is a spec([ication of dark current for commerciall!t availal~h, III- ~' (;ah~As l ~API) devices. ~'"

2• 105 cm -2 as compared to epitaxial material with dislocation density greater than 2 • 107 cm -2. It is also important to note that the median array dark current for these recent HgCdTe APDs is comparable to that specified for commerciallyavailable GaInAsP APDs. r which has enabled us to demonstrate APDs with noise performance comparable to that of GaInAsP APDs. The MBE-grown HgCdTe diode arrays are characterized by dark currents (normalized to gain) of less than 10 nA and noise-equivalent power (NEP) levels less than 2 n W . ~'2 MBE growth technology, besides allowing for the integration of sensors that can be used to precisely control the process, also offers the added benefit of excellent lateral uniformity in HgCdTe epitaxial material quality. The lateral uniformity of source and dopant fluxes over large substrate areas translates into excellent uniformity of device electrical characteristics. In Figure 9.21. we plot a histogram of the voltage required to attain a gain of 13 in a 1 () • 1 () mini-array of MBE-grown HgCdTe APDs. The diode voltages are distributed in a n a r r o w +_0.25 V range about 91.4V. Such a n a r r o w range of required bias across a photodetector array considerably simplifies the design of bias circuitry in associated readout chip electronics. The demonstration of excellent device uniformity and low NEP device performance are important milestones for MBEgrown APD epitaxial materials and offer encouragement for continued future investigation.

Status of HgCdTe ,\1BE technolog!l

337

Figure 9.21 APD arrays fabricatedJ)'on~ ,\IBt:,-~lrown epitaxial materials exhibit excellent un(formit!l of diode gain characteristics. The histogram.lot a 10 • 10 element arra!l of ~,01~m diodes illustrates the unilbrmit!l ~[ diode bias required to achieve a gain of 1 3.

9.4.3 High-performance MWIR detectors

Infrared FPAs designed for operation in space-based surveillance applications must currently be operated at temperatures below 12()K to attain adequate noise performance. 6~ Cooling to such low temperatures requires large cryoradiators. By boosting the FPA operating temperature above 145K, significant reductions in cooling subsystem size, weight, power and cost, can be realized. In order to address this need for an increase in FPA operating temperature, an approach based on reduction of the active volume of the detector and reduction of the concentration of defects contributing to dark current generation has been pursued. ~ By reducing the active volume of the detector material, either through reduction of the device thickness or crosssectional area, the deleterious effect of current leakage mechanisms that fundamentally limit device performance can be mitigated. The improvement in detector performance that can be achieved by reduction in absorbing layer thickness can be seen by examining the dependence of diode R o A product, which is commonly employed as a device figure-of-merit, on material parameters. The thermal ]ohnson-Nyquist noise associated with an If{ detector is inversely related to the diode R.A product. In a simple onedimensional analysis of diode diffusion currents, neglecting non-ideal currents arising from effects such as surface recombination and bulk generationrecombination, the RoA product is given by ~')

338 Handbookof Infrared Detection Technolo#ies

R,,A-

9

j*

(2)

where Na is the donor doping density on the n-side of the p-n junction, n~ is the intrinsic carrier concentration, ~p is the minority carrier (hole) lifetime, q is the electron charge, k is Boltzmann's constant, T is the detector operating temperature, and d is the absorbing layer thickness. The formula indicates that diode performance should theoretically scale inversely with d. The demands placed on the crystal growth process to achieve this ultimate performance limit include: (1) the ability to reproducibly control layer thicknesses, (2) the capability for growth of backside interfaces with low recombination velocity, and (3) the ability to control absorber alloy composition so that ni is minimized for a given detector cutoff wavelength specification. By virtue of its ability to construct complex doping and composition profiles, MBE is especially well-suited to fabrication of detector structures whose performance should closely approach the ideal limit expressed in equation (2). Theoretical calculations of detector performance as a function of absorbing layer thickness indicated that absorbing layer thicknesses in the 2-5 ~tm range, with no compositional grading, would likely prove optimum. Growth of such thin, ungraded HgCdTe epitaxial films by more established techniques, such as liquidphase epitaxy, would not be possible. To establish the utility of MBE for fabrication of thin, complex multiplayer MWIR detectors, the device structures illustrated in Figure 9.22 were grown with a 140 K cutoff wavelength target of 3.8 ~tm. The impact of variations in the absorbing layer thickness over the range of 3-7 lam have been examined, along with the effect of including a wide bandgap (x=0.45) buffer layer between substrate and absorbing layer. The absorbing layer doping ([In]=2• and cap layer doping ( [ A s ] = l • -3) were held fixed for all device structures. HgCdTe epitaxial wafers were processed

Figure 9.22 High-performance MWIR device structures: (A) no wide bandgap buffer layer/71~m-thick absorbing layer, (B) wide bandgap buffer la!ler/ 5 t~m-thick absorbing layer, (C) wide bandgap buffer layer/ 3 ~m-thick absorbing layer.

Status of HgCdTe MBE technolo~,l!l

] ]9

into mesa-architecture discrete diodes and diode mini-arrays using standard wet chemical etch processes and CdTe surface passivation. The electrical performance of the three device structures is summarized in Figure 9.23, which contains plots of the temperature dependence of the R o A product. All three device structures exhibit diffusion-limited performance down to 140K; below this temperature, test equipment impedance limitations dominate the measured data. Current transport mechanisms related to generation-recombination centers and trap-assisted tunneling do not appear to be significant above 140 K, attesting to the high crystalline quality of the MBEgrown HgCdTe. To assess the impact of structural variations on diode performance, m e a s u r e m e n t s of median 140 K R(~A values were taken on a total of 6() diodes populating two mini-arrays on each device wafer. The median RoA values are summarized in Table 9.1. It is evident from the values in the table that the insertion of a high x'-value buffer layer in Structures B and C to electrically isolate the substrate from the 10 9

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340


Table 9.1 S u m m a r y of a r r a y - m e d i a n RoA product for Structure A, B, a n d C wafers. The RoA product for Structure A, corrected to 3.75 ~tm c u t o f f wavelength, has been e s t i m a t e d by assuming diffusion-limited performance, for w h i c h RoA xl/n~ z Structure

14() K cutoff

Median RoA ( 140 K )

A B

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absorbing layer appears to offer a substantial performance improvement relative to Structure A, which has no buffer. The improved performance associated with the buffer layer derives from a reduction in back-surface recombination velocity, which effectively increases the minority carrier lifetime. Inclusion of such performance-enhancing buffer layers can be readily achieved with MBE. In examining the results in Table 9.1, it is also evident that further improvement in the diode RoA product has been achieved through reduction in the absorbing layer thickness, from 5 pm in Structure B to 3 pm in Structure C. This performance enhancement is consistent with the diffusion-limited performance characteristic expressed in equation (2). Variation of the detector absorbing layer thickness, besides affecting the diode RoA product, can also significantly modify the shape of the detector spectral response characteristic. Figure 9.24 compares theoretical calculations of single-pass absorption and experimental diode spectral response data for absorbing layer thicknesses of 3, 5

I. 0 0.8
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Wavelength (l~m) Figure 10.3 Spectral response of l)ollt-illunlinawd PtSi Schollk!l-barrier detector (1,~'. reproduced b!l permission of Plenum Publishin~l Corporation).

360 Handbookof Infrared Detection Technologies photoemission is the only valid photodetection m e c h a n i s m in the spectral region beyond the wavelength corresponding to the band gap energy of Si. FPA architectures

PtSi is formed on Si substrates on which we can fabricate readout circuits with sufficient performance for large format FPAs. PtSi as a contact material for Si LSIs has been investigated for a long time, and it has been confirmed that introduction of PtSi into the Si LSI process has no detrimental effects on readout circuits. Though PtSi has a limited endurance to high-temperature heat treatments, we are able to design the process flow so as to form the PtSi film after all high-temperature treatments in the fabrication process of readout circuits have been completed. Therefore, monolithic PtSi SB FPAs can be manufactured by using processes fully compatible with the planar Si LSI technology used in current Si LSI fabrication plants. In addition to the monolithic structure, the process compatibility with Si LSIs offers m a n y other advantages such as low cost, high reliability, and stable mass production. Figure 10.4 shows a typical design of a PtSi SB FPA pixel along with potential diagrams, which explain its operation.-' The pixel shown in Figure 10.4 uses a CCD readout. The components in a pixel (a PtSi SB detector, a transfer gate, and a part of a vertical CCD (VCCD)) are monolithically integrated on an Si surface. Infrared rays reach the PtSi electrode through the Si substrate. In this back-illuminated structure, the Si substrate serves as a refraction-indexmatching layer for the silicide film, and thus reduces the reflection loss. Furthermore, absorption in the PtSi layer is increased by incorporating an optical cavity, which consists of an a l u m i n u m reflector and a dielectric film (Si02) between the reflector and PtSi film. The effect of the optical cavity is optimized by adjusting the thickness of the dielectric film to the wavelength range of interest. The example shown in Figure 1().4 is a self-integrator pixel, in which the signal charge is accumulated on the stray capacitance of the detector. In order to obtain practical sensitivities with low OE SB detectors, we have to operate SB FPAs in a full-flame integration mode. PtSi SB detectors on moderately-doped ptype Si have sufficiently high breakdown voltages and low dark current at around 80 K, which allow the self-integrator pixel to accumulate the signal and the dark charges generated during the integration time compatible with the standard TV flame rate. A typical design gives a m a x i m u m accumulation capacity of around 106 electrons for 2() to 3()/Jm square pixels at several volts of reverse bias. Although the example shows a pixel with CCD readout, the selfintegrator pixel can also be constructed with MOS switch readout. In the operation of the self-integrator pixel, the detector is reset to a reversebiased state by pulsing the surface-channel transfer gate at the beginning of integration (Figure ] 0 . 4 (c-l)), and then it is isolated from the readout circuit during integration (Figure 10.4 (c-2)). The photoemission of holes from PtSi into Si leaves excess electrons at the PtSi electrode, thus reducing the reverse bias on the SB detector. The accumulated excess signal electrons are read out to the VCCD channel at the next reset process.

Silicon infrared focal plane arra!ls

361

Figure 10.4 Pixel design and operation of PtSi Schottk!l-barrier infrared FPA. which uses CCD readout. (a) and (b) show a plane view and a cross-section of the A-A line. respectivel!t. (c-1) and (c-2) are the potential diagrams during the readout (reset) and integration states, respectivel!t (2. reproduced by permission of Kluwer Academic Publishers).

Self-integrator SB pixels have a built-in blooming control capability. Blooming is a signal charge which spreads to neighboring pixels when excess light is illuminated. This phenomenon is caused by diffusion of excess minority carriers generated in the substrate or signal charge overflow in CCDs, and is found in FPAs with intrinsic detectors, unless appropriate suppression methods are employed. In the self-integrator SB pixel, a strong illumination forward biases the detector and no further electrons are accumulated at the detector. This forward bias at the SB detector is too small for the pn-junction of the guard ring

362


to inject electrons into the Si substrate. Therefore. blooming is perfectly suppressed in SB FPAs as long as the vertical CCD has a sufficient charge handling capacity. The most typical readout architecture used for PtSi SB FPAs is the interline transfer CCD (IL-CCD). l~ The IL-CCI) is constructed by arranging the selfintegrator pixels in a two-dimensional m a n n e r and then placing a horizontal CCD at the bottom of the pixel array. The signal charges in the vertical CCDs are transferred to the horizontal CCD row by row in each horizontal blanking period, and a charge-voltage conversion amplifier called a floating diffusion amplifier generates the output signal from the horizontal CCD. The IL-CCD readout architecture is widely used in commercial visible FPAs. This type of FPAs, however, needs a substantial area for the readout CCD and relevant components, leaving a limited area for the PtSi SB detector. Therefore it is important for low OE SB FPAs to enlarge the fill factor as much as possible. As a response to this problem, considerable efforts have been made to break t h r o u g h the fill factor limitation in the course of the development of the PtSi SB FPA. The Charge Sweep Device (CSD) 2~ is one of the most successful readout architectures that has been developed in order to improve the fill factor. Figure 10.5 shows the construction and operation of the CSD FPA. In the IL-CCD, signal charges accumulated in all pixels are simultaneously read out to vertical CCD and are transferred t h r o u g h the vertical CCD as in a bucket relay. In the CSD FPA, on the other hand, signal charges in a row of pixels are read out to vertical charge transfer devices in a horizontal period. The figure illustrates the timing when the third row is selected. The vertical charge transfer device called CSD can

Figure 10. 5 Construction and operation of Charge Sweep Device ( CSD) FPA. 2~

Silicon in fraredfi~calplane arra!ts

36

be compared to a drain with a large capacity. The signal charge is transferred through the CSD by pushing the potential barrier downward and it is then collected under the storage gates within the horizontal period. This operation allows the signal charge to spread over the vertical charge transfer device. Since the whole vertical channel of the CSD is used to store and transfer the signal charge from a single pixel, the charge handling capacity of the CSD is much larger than the charge storage capacity of the detector. Thus, by adopting highcapacity structures for detectors, the saturation levels can be enlarged even with very narrow vertical charge transfer devices. This large saturation level feature is especially important for infrared imaging because terrestrial infrared imaging is done under high background conditions. The operation of the CSD is based on the charge-coupled concept and inherently low noise. Using the conventional 0.8 lam Si LSI design rules, the CSD readout architecture enables us to design 20 lam square pixels with fill factors of around 7()% and saturation levels larger than 2 x 1 ()6 electrons.2 l Other pixel designs, shown in Figure 1 ().6. have also been proposed in order to improve the fill factor. One is the hybrid structure, which is generally used in compound semiconductor infrared FPAs. Although there are some difficulties in making the hybrid structure, it has the attractive advantage of providing independent optimization of the detector and multiplexer, as well as a large fill factor. In the hybrid detector structure shown in Figure 10.6 (a), individual Schottky electrodes are fabricated so close to each other that their depletion regions merge, and diode isolation is achieved only by a 2 lam oxide gap between the silicide electrodes without the guard ring (the self-guarded detector). 22 This structure further enlarges the effective detector area. A fill factor of 8()% was obtained for a 20 lam square pixel by employing this structure. 23 The other pixel design shown in Figure 1().6 (b)is that for the Direct Schottky Injection (DSI) FPA 24 proposed by Kosonocky et al. This structure substantially provides a 100% fill factor. The DSI focal plane array consists of a continuous silicide SB electrode (DSI surface) formed on one surface of a thinned (10-2 5 lam) Si substrate with the CCD readout registers formed on the other side. During operation, the Si substrate is depleted between the DSI surface and charge collecting elements of the readout structure. Hot holes injected from the DSI surface by internal photoemission drift along the electric field line toward the collecting elements. A cross talk of less than 2()% was reported with a 50 lam square DSI pixel. 24 High-resolution PtSi FPAs

The first two-dimensional PtSi SB FPA with 2 5 x 5 0 pixels was reported in 1978. 2~ Since then, the array size has been doubled every 18 months on average, as shown in Figure ] 0.7. This rate of evolution is the same as that of dynamic random access memory (DRAM), which is the most typical product using Si. State-of-the-art technology has reached a level that makes it possible to manufacture high-resolution PtSi SB FPAs with over one million pixels. During the evolution of the PtSi SB FPA, pixel size has been reduced from 1 6 0 x 80 lam 2 to 17 x 17 ILtm2. In spite of this 50-fold reduction in the pixel size, noise equivalent

364


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temperature difference (NETD) of about 1 K in earlier PtSi SB SB FPAs has been improved to below 0.1 K in current advanced PtSi SB FPAs. Table 10.1 summarizes the specifications and performances of typical highresolution PtSi SB FPAs that have full TV resolution. Current PtSi SB FPAs are manufactured on 150 mm Si wafers using process technologies with around 1 lam design rules. Except for a 6 4 0 • 480 element hybrid FPA, all other FPAs in the table have a monolithic structure.

Silicon infrared focal plane arraw

365

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Table 10.1 High-resolution PtSi Schottky-barrier infrared FPAs Array size

Readout

Pixel size.

F.F.

Saturation

NETI) ( F / # )

Ref.

512x512 512x488 512x 512 640x486 640 x48() 640x488 64()x48() l()40x 1()4() 512x512 656x492 811 x5()8 801x512 537x5()5 1968 x 1968

CSD ILCCD LACA ILCCD MOS ILCCD HB/MOS CSD CSD ILCCD ILCCD CSD ILCCD ILCCD

26x2() pro-' 3 1 . 5 x 2 5 Bin-' 3()x 3()pro 2 2 5 x 2 5 lJm-' 24 x 24 ~un 2 21 x21 ~Ill 2 2()x2()~m 2 17x17~m 2 26x2()Mm 2 26.Sx26.5~m 2 18x21 ~m-' 1 7x2() lain-' 1 5 . 2 x 1 1 . 8 l.tm~ 3()x 3() l.tm-'

39% 36'Ii, 54% 54% 38% 4()% 8()% 53% 71% 46% 38% 61% 32% -

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().()7 K (1 ().()7 K (1 ().I()K (1 ().I()K (2 ().()6 K ( 1 ().I()K (1 ().I()K (2 ().I()K (1 ().()3K(1 ().()6 K ( 1 ().()6 K (1 ().()4K ( 1 ().13K(1

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low

366

Handbook of Infrared Detection Techtmlogies

Compared with IL-CCDs, the CSD FPAs included in the table have larger fill factors and larger saturation levels despite their smaller pixel sizes. The earliest 512 • 512 element CSD FPA, developed in 198 7. was made with design rules of 2 Bm and has a fill factor of 39% for a 2 6 • 2() IAm-' pixel. 2~J The merit of the CSD readout architecture becomes more prominent as the design rules are reduced, and in 1992 the fill factor for the same pixel size was improved to 71% by using a 1.2 btm fine pattern process. ~-'. Figure 1 ().8 shows photographs of the chip and pixel of the 71% fill factor FPA. The width of the A1 vertical scanning line is 1.2 Bm and the channel width of the CSD is 2 ~tm. The current CSD process using 0.8 l.tm design rules (the effective width of the guard ring is determined by the accuracy of mask alignment and is much smaller than the design rules) makes it possible to manufacture a high sensitivity FPA with a 78% fill factor. Figure 10.9 (a) demonstrates the high uniformity of the PtSi SB FPA. This is an uncorrected thermal image with an 8()1 • element PtSi SB FPA. ~s The measured responsivity non-uniformity was ().3% rms. A two-point corrected image with the same FPA is also shown in Figure 1().9 (b). The residual fixed pattern noise in the corrected image is less than the random shot noise. Figure 10. ] O is the first infrared image with a mega-pixel PtSi FPA. ~1 This FPA has four output ports, each of which operates at a 1 () MHz pixel rate. The image from this mega-pixel FPA is displayed on an HDTV monitor at a frame rate of 30 Hz. Several PtSi linear FPAs have been developed for applications such as spaceborne radiometers and spectrometers. Table 1().2 shows typical linear FPAs developed for the spaceborne SWIR remote-sensing application using PtSi SB technology. The 4 ( ) 9 6 - e l e m e n t x 4 - b a n d FPA ~s was developed for the Short Wavelength Infrared Band Optical Sensors (OPS-SWIR) of the Japanese Earth Resources Satellite-1 (JERS-1), which was launched in 1992, and the 2 1 0 0 - e l e m e n t x 6 - b a n d FPA ~ was developed for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)of the Earth Observing

Figure 10.S Chip (a) and pixel (b) photogral~h.~ of:, 12 • ~ 12 element l)tSi Schottk!l-barrier CSI) FPA with 71%fill/actor.

Silicon infrared focalphlne arra!ls

367

Figure 10. 9 Exanlples of thernlal inlages with gO 1 • ~ 12 eh'nlent PtNi Scilottk!l-l~arrier CSI) t:I)A. ( a ) is an Imcorrected inlage, and ( b ) is a two-point ('orre('ted image.

System-AM1 (EOS-AM1). which was launched in 1999. These spaceborne FPAs integrate multiple bands on a single chip. The multiband chip design reduces the size and power consumption of Stirling cycle coolers, which cools the FPAs down to around 80K, and simplifies the optical design. If we did not make use of mature Si LSI manufacturing technology, multiband integration would be impossible because otherwise multiband integration would give us a poor production yield. The QE of these PtSi linear FPAs is around 4% and is almost constant from 1.6 to 2.4 btm. Responsivity non-uniformity obtained from a broad spectral band measurement {without the filter) was 4.5% p-p for a single band of the 34 mm long ASTER FPA. The CCD used in these FPAs is optimized for lowtemperature and low-signal level operation. The transfer inefficiency of the CCD of ASTER was below 2.5• l()-s per transfer at 77K in a signal range down to 4 electrons. The measured non-linearity was less than 1 % . In a Co ~'cJ 1• gamma-ray irradiation test, no detectable degradations were observed up to 5 x 103 rad, and it was proved that the radiation hardness of the FPAs is sufficient for low-altitude satellite applications. A special packaging t e c h n o l o g y , shown in Figure 1(). 11, has also been developed to ensure high planarity of the focal plane at 80 K and heat cycle endurance between 6():C and 8() K. Other silicide detectors and cutoff extension techniques Besides PtSi. there are four silicides that have been used for SB infrared detectors so far' Pd,Si 4().411rSi 42-44 Co,Si 4s and Nigi. 4~' The first solid-state SB infrared FPA was demonstrated with Pd,Si detectors on p - t y p e Si. 4{) The barrier height of the Pd2Si detector is ().34 eV and the cutoff wavelength is 3.5 l.tm. The PdeSi detector cannot be used for thermal imaging because of its poor thermal sensitivity. The main interest of this detector is for applications of spaceborne remote-sensing in the 1-3 lam spectral band. 41 The advantage of the PdeSi detector is its higher operating temperature. The operating temperature is around 12()K, which is compatible with the current satellite passive-cooling technology. CoSi2 and NiSi detectors have also been .

,

,

368


Figure 10.10 Thermal image with 104(I • 1040 element PtSi Schottk!l-barrier CSD FPA.

developed for remote-sensing applications in the short wavelength infrared band. Optical barrier heights of CoSi2 and NiSi detectors on p-type Si were reported to be 0.44 eV and 0.4() eV. respectively. 4 s.4~, By 1980, IrSi had been found to be the silicide with the highest electrical barrier on n-type Sl, 47 and it was expected to extend the cutoff wavelength for IrSi detectors on p-type Si. Since Ir can be deposited by e-beam deposition or by sputtering, and the reaction temperature is between 350 and 600~ IrSi detectors can be readily integrated with m i n i m u m modifications into the PtSi SB FPA fabrication process. In 1982, Pellegrini et al. reported the first IrSi SB infrared detectors with a cutoff wavelength of 8.2 btm. 42 A longer cutoff wavelength of 10.7 ~m has been reported for IrSi SB detectors as a result of using oxygen-free in situ v a c u u m annealing. 44

Silicon infrared focal phme arra!ls

369

Table 10.2 PtSi Schottky-barrier infrared FPAs for spaceborne remote-sensing applications {69 reproduced by permission of SPIE)

Satellite/Radiometer

]ERS-1/()PS-SWIR

E()S-A~I1/ASTER

Photograph

Number of Detectors Number of Bands

Detector Size Pixel Interval Chip Size Readout Process Package

21 ()() pixels/band 6 bands/chip ( 1.600-1.7()() lain, 2.145-2.185 lain, 2.185-2.225 lam, 2.2 ~5-2.285 gm, 2.295-2.365 jam. 2.36()-2.4 3() l.tml 2{) t-tinICT)• 1 7 ~amtAT) 1() lam iCT)x 1() l.tmI,;\T~ 16.5 btm {CTIx 3 3 tam {AT) 1() jam (CT)x 2() lain I:VI'I 48.() turn x 1(). 5 mm 49.4 mm• 7.()mm 4-phase BCCI), 2 CCI)s band, 2 outputs/band 3 t.tmdesign rules. 2 Polv/2 AI SiC-Ale() ~-SiC SiC-AIN-SiC

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A l t h o u g h infrared i m a g i n g with large f o r m a t IrSi SB FPAs has a l r e a d y been d e m o n s t r a t e d , 4s'49 t h e r e are two serious difficulties in IrSi f o r m a t i o n . 43 One is related to the r e a c t i o n kinetics of the Ir-Si system. Unlike PtSi f o r m a t i o n , Si is the m a j o r diffusing species d u r i n g the w h o l e r e a c t i o n process, and the c o n t a m i n a n t s at the original Si surface r e m a i n on the IrSi/Si interface and t h u s d e g r a d e diode c h a r a c t e r i s t i c s . The o t h e r difficulty is related to the p h a s e control. Generally, at least t h r e e phases, IrSi, IrSi• I x - 1 . 5 - 1 . 7 5 t. and IrSi 3, w e r e detected in r e a c t e d films and it is difficult to obtain a s i n g l e - p h a s e film using c o n v e n t i o n a l silicide f o r m a t i o n processes. In order to o v e r c o m e these difficulties in the IrSi f o r m a t i o n , Pt-Ir silicidesS() s~ and MBE codepositionS2s~ h a v e been proposed. While T s a u r et al. formed Pt-Ir silicide by s e q u e n t i a l e - b e a m deposition of Pt and Ir followed by f u r n a c e a n n e a l i n g , L a h n o r et al. insisted t h a t the most successful process for Ir-Si silicide f o r m a t i o n was Ir presilicidation followed by Pt silicidation d u r i n g deposition. H i g h e r OE coefficients w e r e observed in both these Ir-Si silicide SB detectors w h e n c o m p a r e d w i t h those of Ir-only SB detectors. A possible s t r u c t u r e for these Ir-Si silicide detectors m i g h t be a t w o - p h a s e m i x t u r e of Pt and Ir silicides forming a parallel SB. By using MBE codeposition t e c h n o l o g y , a c c u r a t e p h a s e control is possible, a n d a s i n g l e - p h a s e IrSi~, w h i c h is a potential p h a s e for LWIR application, c a n be formed at relatively low t e m p e r a t u r e s (6()()-8()()~ A l t h o u g h the b a r r i e r h e i g h t is p r i m a r i l y d e t e r m i n e d by the c o m b i n a t i o n of the m e t a l electrode a n d s e m i c o n d u c t o r , b a r r i e r l o w e r i n g c a u s e d by a h i g h e r e x t e r n a l electric field n e a r the interface is expected from the Schottky effect. I n c r e a s i n g the i m p u r i t y c o n c e n t r a t i o n n e a r the interface e n h a n c e s the Schottky effect by r e d u c i n g the depletion layer width w i t h o u t i n c r e a s i n g the reverse-bias voltage. Early w o r k to h a r n e s s the Schottky effect to extend the cutoff w a v e l e n g t h used

370


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low-energy implantation of thallium (T1)s4 and boron (B) ss in PtSi SB detectors. The ion-implantation technique can easily be incorporated in the standard PtSi SB FPA fabrication process. This technique, however, has a limitation caused by the existence of a narrow potential spike that substantially reduces the collection of photoexcited carriers. Further ion-implantation beyond the limitation only raises the dark current, and is no longer effective in lowering the optical barrier height. Lin et al. found that this detrimental potential spike is caused by the broad depth profile of the implanted impurities, and that a very narrow doping spike layer eliminates this potential spike, s~ Their estimation shows that an nm-order doping spike with an impurity concentration level of 1()2~ cm-3 may make it possible to create PtSi SB detectors with cutoff wavelengths suitable for LWIR imaging. The critical parameter in this doping spike structure is the width of the doping spike layer. To suppress the diffusion of the dopant, Lin et al. employed a low-temperature ( < 500~ Si molecular beam epitaxy (MBE) technique with an elementary B dopant source, and achieved a cutoff wavelength of 22 ~tm for a PtSi SB by incorporating a 1 nm p+ doping spike with a concentration of 2 x 10 2o c m - 3. s6 A recent development in MBE technology has also made it possible to grow high-quality strained SiGe films on Si substrates, which has given us another option for extending the cutoff wavelength. The band gap energy of the strained

Silicon infrared_focalplane arra!ts

3 71

SiGe is smaller than that of Si, and can be tailored by changing the composition. SB detectors on epitaxially-grown p-type SiGe have lower barrier heights than those on p-type Si substrate, even if the same metal electrodes are used, because the major band offset between SiGe and Si appears at the valence band. Kanaya et al. reported the first PtSi/p-SiGe SB detector using MBE technology, s7 Xiao et al. employed rapid thermal CVD as a SiGe epitaxy technique, and obtained a cutoff wavelength of 8.3 l~m for a PtSi/p-Si(~.~sGe(~.l s de tector.s~ They reported that the reaction of Pt with Ge prevents diodes from reducing the barrier height. Therefore, they inserted a thin sacrifice Si layer between the SiGe and deposited Pt layers, which is consumed during the PtSi formation process. Since even a very thin excess Si layer of nm-order thickness is sufficient to form an energy barrier and impede photoemission, the thickness of the sacrifice layer must be rigorously controlled in PtSi/p-SiGe SB detectors. Figure 10.12 compares the spectral response of detectors having extended cutoff wavelengths with that of a PtSi SB detector. 2

10.2.2 Heterojunction internal p h o t o e m i s s i o n FPAs Operation of heterojunction internal photoemission (HIP) detector The valence band discontinuity between SiGe and Si can also be used as an energy barrier for internal photoemission. The idea of utilizing the internal photoemission of heterojunction diodes for infrared detection was proposed by 10-1

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Shepherd et al. in 1971. s9 Although this technology was very attractive because it could be used to continuously control the barrier height and improve the q u a n t u m efficiency, it took quite a long time to demonstrate the operation of the first HIP infrared detector because of poor epitaxy technology. Developments in MBE technology made it possible to grow high quality SiGe thin films onto Si substrates and fabricate SiGe HIP infrared detectors. SiGe HIP detectors are constructed by replacing the silicide electrode of SB detectors with degenerately doped SiGe. Figure 1 (). 1 3 illustrates the structure and energy band diagram of the SiGe HIP detector. Since major band offset between SiGe and Si appears at the valence band. an energy barrier with barrier height ~t, is established when we choose p-type materials. Photon absorption mechanisms in the SiGe layer are free-carrier absorption and intra-valence band transitions. Assuming that the density of the state of the valence band is proportional to the square-root of the energy, the internal QE can be expressed as

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The micromachining process for the SOI diode uncooled FPA detector is as follows. After completion of CMOS circuits and temperature-sensing diodes, etching holes are formed so as to access the bulk Si layer (Figure 10.20 (a)). Then, an amorphous Si sacrificial layer is deposited and patterned (Figure 10.20 (b)). In the next step, the multi|ayer infrared absorbing structure is formed (Figure 10.20 (c)). Finally, the amorphous Si layer and bulk Si under the temperature-sensing diodes are removed by a single etching process (Figure 10.20 (d)). The buried oxide of the SOl substrate plays the role of an etching stopper during the final etching process. Although the final release process is not a standard Si process, all other processes can be performed on a conventional SiLSI production line. The temperature-sensing diodes are active when they are forward-biased in the SOI diode FPA. The forward-biased design makes it possible to use the temperature-sensing diodes as switching devices and eliminates selection MOS switches in the pixel. Figure 10.21 shows a block diagram of the SOI diode uncooled FPA. The pixel diodes are biased row by row during a horizontal period,

378

Handbook of Infrared Detection Technoloqies

(a) Figure 10.19 Pixel structure of SOI diode uncooled FPA. ( a ) shows a cross-section of the whole structure, and (b) shows the detailed structure of an SOl diode temperature sensor (r,~. reproduced by permission of SPIE ).

and signal amplification and bandwidth reduction are performed in column gate modulation integrators. A single diode driven in the constant current mode generates a differential output voltage of 1-2 mV/K. Since this output voltage is not sufficient for FPA readout circuits, it is increased by connecting several diodes serially. The serial connection also improves the production yield because the pixel is defective only when all diodes in a pixel have poor reverse-biased characteristics. A 3 2 0 x 2 4 0 element uncooled infrared FPA has been developed using SOI diode technology. The pixel size is 40• 40 lam 2. and the chip measures 17 x 17 mm 2. Figure 10.22 shows the chip and the pixel photographs. The width of the slit between adjacent infrared absorbing structures is 2 Ftm, and the resulting fill factor is 90%. The measured thermal conductance of 1.1 • 1 0 - 7 W/K is the same level as those of advanced VOx microbolometer FPAs.

5;ilicon inlraredJbcal plane arra!ls

379

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Eight-series diodes yield a temperature coefficient of 9.7 mV/K. High uniformity is one of the most important features of the SOl diode uncooled FPA. Non-uniformity of forward-bias voltage and its temperature coefficient across an 200 mm wafer are only 0.13% and 0.62%, respectively. Reflecting the high uniformity of the diode characteristics, a low responsivity non-uniformity of less than 2% has been achieved with 3 2 0 • element FPAs. The total FPA noise and the NETD with f/1 optics are reported to be ll01uVrms and 0.12K, respectively. Table 10.3 summarizes the specifications and performances of the 3 2 0 x 2 4 0 element SOI diode uncooled infrared FPA. 6~ Figure 10.23 shows examples of thermal images with the SO! diode uncooled FPA. 70.3.2 Si-based resistance bolometer FPAs

The results of the VOx resistance bolometer uncooled FPA reported in 19927(~ are so impressive that most of the companies that entered the uncooled FPA business later on have chosen VOx. However, it is not the only material that can be used for resistance bolometers. Si-based resistance bolometers including amorphous Si (a-Si) and single-crystal Si are being studied for use in uncooled FPAs. Early work on a-Si was done by a group at the Defense Science and Technology Organisation in Australia. 71 One of the issues in applying a-Si to uncooled FPAs

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is large 1/f noise. They prepared a-Si with sputtering and plasma-enhanced chemical vapor deposition (PECVD), and compared the noise characteristics of films with a similar resistivity. The level of 1/f noise is highly dependent on the film preparation method, and a-Si deposited with PECVD exhibited a very high value of 1/f noise, as shown in Figure 10.2 4. They deduced that the origin of the higher noise in PECVD a-Si is the higher hydrogen content, which results in an increase of metastable Si-H bonds. In addition, they also found that a-Si prepared by PECVD commonly has a random telegraph signal (RTS) noise. The RTS noise is a characteristic of current transported along a narrow filament through a barrier of wide-band-gap material. When thermal or UV loads are applied to a-Si, it exhibits instability behavior, which is caused by a high impurity level in the material. 72 Tissott et al. overcame this problem by mastering the kinetic cooling rate to ensure thermodynamic equilibrium below equilibrium temperature. Their films have an equilibrium temperature of 180~ which is much higher than those of standard a-Si films (around 90~ and they exhibited a high stability during a 1000 hour high temperature 12 5~ storage test. 72

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Amorphous silicon has temperature coefficient of resistances (TCRs) from 1.8%/K to 5.5%/K, depending on the growth techniques. Figure 10.25 shows an example of the relationship between the TCR and resistivity. 7 ~ Since it is difficult to control quality a-Si films with higher resistivities, highly-doped a-Si films with TCRs around 2.5%/K are generally used as bolometer materials. Successful demonstrations of a-Si resistance bolometer uncooled FPAs were given by LET172-74 in France and Raytheon' ~" ~'in the United States. Figure 1 0 . 2 6 shows the structure of the LETI pixel. 7~ Their microbridge comprises a thin layer (0.1 ~m) of doped a-Si with no extra supporting layer or membrane. Thin TiN electrodes on the a-Si bolometer layer and an aluminum reflector on the readout circuit constitute an interference infrared absorbing

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structure. In order to obtain the m a x i m u m absorption at 1 () l.tm. the height of the microbridge from the reflector and the sheet resistance of the electrodes are adjusted to be 2.5 l.tm and 3 77 f2/square, respectively. The LETI group employs a micromachining process with a thick polyimide sacrificial layer. 74 Thanks to the very low thermal mass of this thin microbridge structure, a higher flame rate operation than the standard TV frame rate is possible with a thermal time constant of a few ms. After they confirmed the feasibility of their technology with a 50 pm pixel pitch 2 5 6 • element FPA. :~ they developed 32()• element FPAs with pixel pitches of 45 l.tm 72 and 35 ~ m . 74 The 35 pm pixel FPA has

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achieved an NETD of 36 mK (f/l) by using an enhanced technology, in which 60% reduction in the 1/f noise has been accomplished by a thicker a-Si film and the thermal resistance has been increased to 4.2 • 1 ()" K/W. Raytheon reported a 16()x ] 2() element a-Si resistance bolometer uncooled infrared FPA with a pixel size of 46.8~tm square.' ~" r The high a-Si pixel resistance of 30 Mr2 of this FPA allows the use of a DC detector bias. The high pixel resistance and the switched capacitor integrating amplifier in each pixel make it possible to realize full-frame integration operation. Although the single-crystal Si has limited TCRs of (). 5-(). 7%/K, it is attractive because it has the same features as those of the SOl diode uncooled FPA. A group at Middle East Technical University in Turkey reported some results on a singlecrystal Si resistive microbolometer uncooled infrared F P A . ' " ' s Figure 1().27 shows the pixel structure of their FPA. The whole structure is fabricated using a standard CMOS process technology in an Si foundry, and the only additional process is the final release of the freestanding structure. Anisotropic etching of the bulk Si is done with Tetramethyl A m m o n i u m Hydroxide (TMAH) solution. 10.3.3 Thermopile FPAs The thermocouple is a temperature sensor that consists of two dissimilar metals or semiconductors connected in series. As the temperature of this junction varies, the electromotive force developed at the output terminal varies. Although single thermocouples have poor sensitivity, the sensitivity can be improved by

384


Figure 1 0 . 2 6 Pi.vel structure ofLETI a-Si re.~istive l~ololm'ter t:PA ( - ; reproduced b!l permission of SI~IE ).

connecting thermocouples serially. Serially-connected thermocouples are called a thermopile. There are many combinations of materials for thermopiles, including materials that are used in current Si LSI technology. In a thermopile FPA pixel, the temperature difference between the hot and cold junctions is detected. The cold junctions are located on the heat sink (the substrate) and are used as a temperature reference. Therefore, the temperature drift of the FPA is automatically compensated without temperature stabilizer. The serial connecting design of multi-thermocouples, however, needs many interconnections between the freestanding structure and substrate, limiting the thermal isolation in the pixel. This results in lower sensitivity, compared with other types of uncooled FPAs. Because of these features of thermopile FPAs, a major effort is focused on developing low-cost moderate-sensitivity uncooled FPAs. Kanno et al. reported a 128• 128 element thermopile uncooled infrared FPA with a CCD readout. 7~ Their thermopile consists of 32 pairs of p-type and n-type polysilicon thin films deposited on SiO2 diaphragms. An NETD of 0.5 K was achieved with f/1 optics. Other efforts to develop low-cost thermopile uncooled FPAs have been made at the Physical Electronics Laboratory of ETH Zurich. They have developed the

Silicon infrared focal plane arra!ts

38 5

Figure 10.2 7 Microbolometer pixel using n-t!lpe sin~lle-cr!lstal Si (77. reproduced b!l permission of SPIE ).

Figure 10.28 Cross-sectional structure of polJisilicon/aluminum thermopile uncooled FPA (m~, reproduced by permission of Editions Frontieres ).

pixel structure shown in Figure 1().28. s~ Thermal isolation is performed by a bulk micromachining process from the backside of the wafer. All pixels on the FPA are located on a single membrane that is made of dielectric films. Neighboring pixels are separated by electroplated gold lines formed using a process service for tape-automated bonding. They chose a combination of n-type polysilicon and a l u m i n u m for their thermopile, which is available from a standard CMOS foundry service. They have developed two uncooled FPAs with array sizes of 10• 10 ~ and 16• 16. ~ Figure 10.29 shows an example of a thermal image using the 16 • 16 element thermopile uncooled infrared FPA.

386


Figure 10.29 Example of thermal image with 16 • 16 element thermopile uncooled FPA (~I. reproduced b!t permission of lEEE ).

A group at the University of Michigan have also been developing thermopile uncooled infrared FPAs. They employed a combined front-undercut and backetching micromachining process technology. ~2 They demonstrated the operation of a 3 2 • element FPA with 32 n-type and p-type polysilicon thermocouples in each pixel.

10.4 Summary In this chapter, we have discussed Si-based infrared FPA technology. Although Si-based infrared FPA technology does not offer the highest sensitivity, it has many attractive features such as low cost, high productivity, and large-scale integration. A wide spectral range from SWIR to LWIR can be covered with Si-based photoemissive detectors. PtSi SB FPAs have led to the commercialization of cooled infrared cameras, which are being used in many commercial applications. High quality LWIR images have already been demonstrated with a full TV resolution GeSi HIP FPA. Uncooled FPAs are expected to be used for low-end applications to which cooled infrared systems cannot be applied. Such applications require low-cost high-volume production of infrared FPAs, and Si-based technology is the best choice for manufacturing such uncooled FPAs. Si-based uncooled infrared technologies have already reached a level suitable for many commercial applications, and will open up a huge new infrared market.

Silicon infraredjbcalplane arra!ls 387

References 1. F. D. Shepherd and A. C. Yang, Silicon Schottky retinas for infrared imaging, Tech. Digest IEDM, 31 ()- 31 3 ( 19 73 ). 2. M. Kimata, Metal silicide Schottky infrared detector arrays, in Infrared Detectors and Emitters: Materals atut Devices (ed. P. Capper and C. T. Elliott), Chap. 4, pp. 77-98, Kluwer Academic Publishers, Dordrecht {2(}(}1 ). 3. J. Cohen, J. Vilms and R. J. Archer, Investigation of semiconductor Schottky-barriers for optical detection and cathodic emission, Final Report, AFCRL-68-0651, Air Force Cambridge Research Lab., Boston ( 1968 ). 4. M. Kimata, M. Denda, T. Fukumoto, N. Tsubouchi, S. Uematsu, H. Shibata, T. Higuchi, T. Saeki, R. Tsunoda and T. Kanno, Platinum silicide Schottkybarrier IR-CCD image sensors, Jim. ]. Appl. Ph!ls., 21 {Suppl. 21-1 ), 2 3 1 - 2 3 5 (1982). 5. J. M. Mooney and J. Silverman, The theory of hot-electron photoemission in Schottky-barrier IR detectors, IEEE Tratls. Electron Devices, ED-32, 3 3 - 3 9 (1985}. 6. S. M. Sze, Metal-semiconductor devices, in Physics of Setniconductor Devices. Chap. 6, pp. 3 6 3 - 4 2 4 . John Wiley & Sons, Inc., New York {1969 ). 7. J. P. Gambino and E. G. Colgan, Silicides, in Encyclopedia of Applied Physics, Update 2, pp. 2 0 1 - 2 4 8 , Wiley-VCH, Weinheim {1999 }. 8. M.-A. Nicolet and S. S. Lau, Formation and characterization of transitionmetal silicides, in VLSI Electronics Microstrz,'ture Science, 6, (ed. N. G. Einspruch and G. B. Larrabee), Chap. 6, pp. 3 2 9 - 4 6 4 , Academic Press, New York {1983). 9. A. Prabhakar, T. C. McGill and M.-A. Nicolet, Platinum diffusion into silicon from PtSi, Appl. Phys. Lett. 43, 1118-112{) ( 1983 ). 10. C. A. Crider, J. M. Poate, J. E. Rowe and T. T. Sheng, Platinum silcide formation under ultrahigh v a c u u m and controlled impurity ambients, ]. App1. Phys. ~2, 2 8 6 0 - 2 8 6 8 {1981). 11. K. N. Tu and J. W. Mayer, Depth profiling techniques, in Thin Film Interdiffusion and Reactions, (ed. J. M. Poate, K. N. Tu and J. W. Mayer}, Chap. 6, pp. 1 1 9 - 1 6 0 , John Wiley & Sons, New York {19 78 }. 12. R. Pretorius, C. L. Ramiller and M.-A. Nicolet, Marker studies of silicide formation, silicon self-diffusion and silicon epitaxy using radioactive silicon and Rutherford backscattering, Nuclear Instruments and Methods 149, 6 2 9 - 6 3 3 (1978). 13. K. Affolter, X.-A. Zhao and M.-A. Nicolet, Transition-metal silicides formed by ion mixing and by thermal annealing: Which species moves? J. Appl. Phys. 58, 3 0 8 7 - 3 0 9 3 (1985). 14. J. E. McLeod, M. A. E. Wandt. R. Pretorius and C. M. Comrie, Marker and radioactive silicon tracer studies of PtSi formation. ]. Appl. Phys. 72, 22 3 2 - 2 2 4 1 (1992). 15. R. J. Blattner, C. A. Evans, S.S. Lau, J. W. Mayer and B. M. Ullrich, Effect of oxidizing ambients on platinum silicide formation, ]. Electrochem. Soc.: Solid-State Science and Technology 122, 1 7 3 2 - 1 7 3 6 ( 19 75 }.

388 Handbookof Infrared Detection Technolo#ies 16. P. W. Pellegrini, A. Golubovic and C. E. Ludington, A comparison of iridium silicide and platinum silicide photodiodes, Proc. SPIE 782, 9 3 - 9 8 (1987). 17. J. M. Mooney, Excess low-frequency noise in PtSi on p-type Si Schottky diodes, IEEE Trans. Electron Devices 38, 16()-166 ( 1991 ). 18. M. Kimata, M. Denda, S. Iwade, N. Yutani and N. Tsubouchi, A wide spectral band photodetector with PtSi/p-Si Schottky-barrier, Int. J. Infrared and M M Waves 6, 1 0 3 1 - 1 0 4 1 ( 1985 ). 19. A. J. P. Theuwissen, Solid-state imaging at a glance, in Solid-State Imaging with Charge-Coupled Devices, Chap. 4, pp. 1()9-13(), Kluwer Academic Publishers, Dordrecht (1995). 20. M. Kimata, M. Denda, N. Yutani, S. Iwade and N. Tsubouchi, A 512 x ,312 element PtSi Schottky-barrier infrared image sensor, IEEE JSSC SC-22, 1 1 2 4 1129(1987). 21. M. Kimata T. Ozeki N. Tsubouchi and S. Ito, PtSi Schottky-barrier infrared focal plane arrays, Proc. SPIE, 3 505, 2 - 1 2 ( 1998 ). 22. F. D. Shepherd, Recent advances in platinum silicide infrared focal plane arrays, Tech. Digest IEDM 3 7 0 - 3 7 3 (1984). 23. J. L. Gates, W. G. Connelly, T. D. Franklin, R. E. Mills, F. W. Price and T. Y. Wittwer, 488 x 640-element hybrid platinum silicide Schottky focal plane array, Proc. gPIE 1 ~40, 2 6 2 - 2 73 (1991). 24. W. F. Kosonocky, T. S. Villani, F. V. Shallcross, G. M. Meray and J. J. O'Neil, A Schottky-barrier image sensor with 1()()% fill factor, Proc. SPIE 1 3 0 8 , 7()-80 (1990). 25. B. Capone, L. Skolnik, R. Taylor, F. Shepherd, S. Roosild, W. Ewing, W. Kosonocky and E. Kohn, Evaluation of a Schottky IRCCD staring mosaic focal plane, Proc. SPIE 1 5 6 , 1 2 0 - 1 3 1 (1978). 26. H. Elabd, Y. Abedini, W. Shieh, J. Kim, M. Shih, J. Chiu, F. Nicol, W. Petro, J. Lehan, M. Duron, M. Manderson. S. Otto, C. Diaz, S. Lam, H. Balopole, P. Coyle, P. Cheng and R. Marin, 4 8 8 x 5 1 2 - and 2 4 4 x 2 5 6 - e l e m e n t monolithic PtSi Schottky IR focal plane array, presented at SPIE S!ll~lp., No. 1107-29 (1989). 2 7. W.-L. Wang, R. Winzenread, B. Nguyen and J. J. Murrin, High fill factor 5 1 2 x 5 1 2 PtSi focal plane array, Proc. SPIE 1 1 6 1 , 7 9 - 9 5 (1989). 28. E. T. Nelson, K. Y. Wong, S. Yoshizumi, D. Rockafellow, W. DesJadin, M. Elzinga, J. P. Lavine, T. J. Tredwell, R. P. Khosla, P. Sorie, B. Howe, S. Brickman and S. Refermat, Wide field of view PtSi infrared focal plane array, Proc. SPIE 1 3 0 8 , 3 6 - 4 3 (1990). 29. D. J. Sauer, F. L. Hsueh, F. V. Shallcross, G. M. Meray and T. S. Villani, A 640• PtSi IR sensor with low-noise MOS X-Y addressable multiplexer, Proc. SPIE 1 3 0 8 , 8 1 - 8 7 (1990). 30. K. Konuma, S. Tohyama, A. Tanabe, K. Masubuchi, N. Teranishi, T. Saito and T. Muramatsu, A 6 4 8 x 4 8 7 pixel Schottky-barrier infrared CCD image sensor, Tech. Digest ISSCC 2 1 6 - 2 1 7 ( 1991 ). 31. N. Yutani, H. Yagi, M. Kimata, J. Nakanishi, S. Nagayoshi and N. Tsubouchi, 1 0 4 0 x 1040 element PtSi Schottky-barrier IR image sensor, Tech. Digest IEDM 1 7 5 - 1 7 8 (1991 ).

Silicon infrared focalphme arraJls 389

32. H. Yagi, N. Yutani, S. Nagayoshi. J. Nakanishi, M. Kimata and N. Tsubouchi, Improved 512 • 512 IRCSD with large fill factor and high saturation level, Proc. SPIE 1 6 8 5 , 3 7 - 4 7 (1992). 33. Y. S. Abedini, O. R. Barrett, J. S. Kim. D. D. Wen and S. S. Yeung, 656 • 492-element platinum silicide infrared charge-coupled-device focal-plane array, Proc. SPIE 2 0 2 0 , 36-4() ( 1993 ). 34. M. Shoda, K. Akagawa and T. Kazama, A 41 ()K pixel PtSi Schottky-barrier infrared CCD image sensor, Proc. SPIE 2 7 4 4 , 2 3-32 (1996). 35. H. Yagi, T. Shiraishi, K. Endo, Y. Kosasayama, M. Kimata and T. Ozeki, High-performance 801 • 512-element PtSi Schottky-barrier infrared image sensosr, Tech. Digest 1 5 th Sellsor S!lzllnp. (it1 ]apall ), 155-16() ( 1997 ). 36. M. Shoda, K. Akagawa and A. Komai. New high-performance PtSi IRCCD and its electrical shutter operation, Proc. SPIE 3 1 2 2 , 3 9 9 - 4 0 8 ( 1 9 9 7 ) . 37. C. Kauffman, Emergence of tactical, framing infrared reconnaissance, Proc, SPIE 3 4 3 1 , 1 3 0 - 1 4 2 ( 1998 ). 38. M. Denda, M. Kimata, S. Iwade, N. Yutani, T. Kondo and N. Tsubouchi, Schottky-barrier infrared linear image sensor with 4-band • 4()96-element, IEEE Trans. Electron Devices 3 8, 1 1 4 5 - 1 1 5 1 ( 1991 ). 39. M. Ueno, T. Shiraishi, M. Kawai, Y. Yonada, M. Kimata and M. Nunoshita, PtSi Schottky-barrier infrared focal plane array for ASTER/SWIR, Proc. SPIE 2 5 5 3 , 5 6 - 6 5 (1995). 40. E. S. Kohn, S. A. Roosild, F. D. Shepherd and A. C. Yang, Infrared imaging with monolithic CCD-addressed Schottky-barrier detector arrays: Theoretical and experimental results, Proc. hit. Application of CCD's, 5 9 - 6 9 (1975). 41. H. Elabd, T. Villani and W. Kosonocky, Palladium-silicide Schottky-barrier IR-CCD for SWIR applications at intermediate temperatures, IEEE Electron Device Lett. EDL-3, 8 9 - 9 0 ( 1982 ). 42. P. W. Pellegrini, A. Golubovic, C. E. Ludington and M. M. Weeks, IrSi Schottky barrier diodes for infrared detection, Tech. Digest IEDM 157-16() (1982). 43. P. W. Pellegrini, A. Golubovic and C. E. Ludington, A comparison of iridium silicide and platinum silicide photodiodes. Proc. SPIE 782. 9 3 - 9 8 (1987). 44. B.-Y. Tsaur, M. M. Weeks, R. Trubiano. P. W. Pellegrini and T.-R. Yew, IrSi Schottky-barrier infrared detectors with 1 ()-lam cutoff wavelength, IEEE Electrotl Device Lett. 9, 6 5 0 - 6 5 3 ( 1988 ). 45. J. Kurianski, J. Vermeiren, C. Claeys. W. Stessens, K. Maex and R. De Keersmaecker, Development and evaluation of CoSi2 Schottky barrier infrared detectors, Proc. SPIE 1 1 5 7, 1 4 5 - 1 5 2 ( 1 9 8 9 ). 46. J. Kurianski, J. Van Dammer, J. Vermeiren. M. Maex and C. Claeys, Nickel silicide Schottky barrier detectors for short wavelength infrared applications, Proc. SPIE 1 3 0 8 , 2 7 - 3 4 (199() ). 47. I. Ohdomari. K. N. Tu, F. M. d'Heurle, T. S. Kuan, and S. Petersson, Schottky-barrier height of iridium silicide. Appl. Ph!ls. Lett. 33, 1()28-1()3()

(1978).

390 Handbookof Infrared Detection Technologies 48. N. Yutani, M. Kimata, M. Denda. S. Iwade and N. Tsubouchi, IrSi Schottky-barrier infrared image sensor, Tech. Digest IED,V1124-127 (1987). 49. B.-Y. Tsaur, M. J. McNutt, R. A. Bredthaue and IR. B. Mattson, 128x 128element IrSi Schottky-barrier focal plane arrays for long-wavelength infrared imaging, IEEE ElectronDevice Lett. 1 0 . 3 6 1 - 3 6 3 (1989). 50. B.-Y. Tsaur, M. M. Weeks and P. W. Pellegrini. Pt-Ir silicide Schottkybarrier IR detectors, IEEE Electron Device Lett. 9, 1 ()()-102 (1988). 51. P. Lahnor, D. Worle and M. Schulz. Tailoring of the Schottky barrier height of Pt-Ir mixed silicide infrared detectors. Appl. Ph!ls. A A64, 1 0 1 - 1 0 8 (1997). 52. T. L. Lin and J. M. Iannelli, Fabrication of IrSi3/p-Si Schottky-barrier diodes by a molecular beam epitaxy technique, Appl. Ph!ls. Lett. 56, 2013-2() 15 (1990). 53. D. A. Lange, G. A. Gibson and C. M. Falco. MBE-codeposited iridium silicide films on Si(100) and Si(111 ), Proc. SPIE 2 0 2 1 , 6 7 - 7 7 ( 1993 ). 54. P. Pellegrini, M. Weeks and C. Ludington, New 6.'5 lam photodiodes for Schottky barrier array applications, Proc. SPIE 31 1 , 2 4 - 2 9 ( 1981 ). 55. N. Tsubouchi, M. Kimata, M. Denda, M. Yamawaki, N. Yutani and S. Uematsu, Photoresponse improvement of PtSi-Si Schottky-barrier infrared detectors by ion-implantation, Tech. Digest 12th European Solid-State Device Research Conference, 1 6 9 - 1 7 1 (1982). 56. T. L. Lin, J. P. Park, S. D. Gunapala, E. W. Jones and H. M. del. Castillo, Long-wavelength infrared doping-spike PtSi detectors fabricated by molecular beam epitaxy, Proc. SPIE 2 0 2 0 , 3()-35 ( 199 3 ). 57. H. Kanaya, F. Hasegawa, E. Yamaka, T. Moriyama and M. Nakajima, Reduction of barrier height of silicide/p-Sil_xGex contact for application in an infrared image sensor, ]pn. J. Appl. Ph!ls. 28.1,544-L546 (1989). 58. X. Xiao, J. C. Sturm, S. R. Parihar. S. A. Lyon. D. Meyerhofer and S. Palfrey, Silicide/Sil_xGe• Schottky-barrier long-wavelength infrared detectors, Tech. DigestlEDM, 1 2 5 - 1 2 8 (1992). 59. F. D. Shepherd, V. E. Vickers and A. C. Yang, Schottky barrier photodiode with a degenerate semiconductor active region. Urlited States Patent. 3,6()3,847 (1971). 60. B.-Y. Tsaur, C. K. Chen and S. A. Marino, Long-wavelength GexSil_x/Si heterojunction infrared detectors and focal plane array, Proc. SPIE 1 540, 58()595(1991). 61. T. L Lin, J. S. Park, S. D. Gunapala, E. W Jones and H. M. De Castillo, Photoresponse model fo Sil_xGex/Si heterojunction internal photoemission infrared detector, IEEE Electroll Device Lett. 1 5.1 () 3-1 ()5 (1994). 62. T. L. Lin, A. Ksendzov, E. W. Dejewski, 12,.W. Fathauer, T. N. Krabach and J. Maserjian, A novel Si-based LWIR detector: The SiGe/Si heterojunction internal photoemission detector, Tech. Digest oflEDM 6 4 1 - 6 4 4 (199()). 63. T. L. Lin, T. George, E. W. Jones. A. Ksendzov and M. L. Huberman, Elemental boron-doped p+-SiGe layers grown by molecular beam epitaxy for infrared detector applications, Appl. Phlls. Lett. 60, 3 8 0 - 3 8 2 ( 1992 ).

Silicon infraredfocal plane arra!lS 391

64. J. S. Park, T. L. Lin, E. W. Jones, H. M. del. Castillo, T. George and D. Gunapala, Stacked long-wavelength heterojunction internal photoemission infrared detector using multiple Si~_xGex/Si layers, Proc. SPIE 2 0 2 0 , 12-21 (1993). 65. H. Wada, M. Nagashima, K. Hayashi, J. Nakanishi, M. Kimata, N. Kumada and S. Ito, 512 • 512 element GeSi/Si heterojunction infrared focal plane array, Opto-Electronics Review 7.3() 5- 311 ( 1999 ). 66. S. M. Sze, p-n junction diodes in Ph!lsics of Selniconductor Devices, Chap. 3, pp. 7 7 - 1 4 9 , John Wiley & Soms Inc., New York, (1969). 67. T. Ishikawa, M. Ueno, K. Endo, Y. Nakaki, H. Hata, T. Sone. M. Kimata and T. Ozeki, Low-cost 32()• uncooled IRFPA using conventional silicon IC process, Proc. SPIE 3 6 9 8 , 5 5 6 - 5 6 4 ( 1999 ). 68. T. Ishikawa, M. Ueno, Y. Nakaki, K. Endo, Y. Ohta, J. Nakanishi, Y. Kosasayama, H. Yagi, T. Sone and M. Kimata, Performance of 32()x24() uncooled IRFPA with SOI diode detectors. Proc. SPIE 4 1 3 0 , 1 5 2 - 1 5 9 (2()()()). 69. M. Kimata, H. Yagi, M. [Jeno. J. Nakanishi. T. Ishikawa, Y. Nakaki, M. Kawai, K. Endo, Y. Kosasayama, Y. ()hota, T. Shugino and T. Sone, Silicon infrared focal plane arrays, Proc. SPIE 4 2 8 8 , 2 8 6 - 2 9 7 (2()() 1 ). 70. R. A. Wood, C. J. Han and P. W. Kruse, Integrated uncooled infrared detector imaging arrays, Tech. Digest Solid-State Sensor and Actuator Workshop, pp. 1 3 2 - 1 3 5 (1992). 71. M. H. Unewisse, B. I. Craig, R. J. Watson, O. Reinholed and K. C. Liddiard, The growth and properties of semiconductor bolometers for infrared detection, Proc. SPIE 2~ 54, 4 3 - 5 4 ( 19951. 72. J.-L. Tissott, J.-L. Martin, E. Mottin. M. Vilain, J.-J. Yon and J.-p. Chatard, 3 2 0 x 2 4 0 microbolometer uncooled IRFPA development, Proc. SPIE 4 1 1 0 , 4 7 3 - 4 7 9 (2000 ). 73. J.-L. Tissott, F. Rothan, C. Vedel, M. Vilain and J.-J. Yon, LETI/LIR's uncooled microbolometer development, Proc. SPIE 3 4 3 6 , 6() 5-61 () ( 1998 ). 74. E. Mottin, J.-L. Martin, J.-L. ()uvrier-Buffet, M. Vilain, A. Bain, J.-J. Yon, J.L. Tissott and J-P. Chatard, Enhanced amorphous silicon technology of 3 2 0 x 2 4 ( ) microbolometer array with a pitch of 35 Bm, Proc. SPIE 4 3 6 9 . 2 5 0 526 (2001). 75. J. Brady S. Scimert, D. Ratcliff R. Gooch, B. Ritchey P. McCardel, K. Rachels, S. Ropson, M. Wand, M. Weinstein and J. Wynn, Advances in amorphous silicon uncooled IR systems, Proc. SPIE 3 6 9 8 , 1 6 1 - 1 6 7 ( 1999 ). 76. G. L. Francisco, Amorphous silicon bolometer for fire/rescue, Proc, SPIE 4 3 6 0 , 1 3 8 - 1 4 8 (2001). 77. D. S. Tezcan, S. Eminoglu, (). S. Akae and T. Akin, An uncooled microbolometer infrared focal plane array in standard CMOS, Proc. SPIE 4:288, 1 1 2 - 1 2 1 (2001). 78. S. Eminoglu, D. S. Tezcan and T. Akin, A CMOS n-well microbolometer FPA with temperature coefficient enhancement circuitry, Proc. SPIE 4 3 6 9 , 2 4 0 - 2 4 9 (20() 1 ). 79. T. Kanno, M. Suga, S. Matsumoto, M. Uchida, N. Tsukamoto, A. Tanaka, S. Itoh, A. Nakazato, T. Endoh. S. Tohyama, Y. Yamamoto, S. Murashima, N.

392 Handbookof Infrared Detection Technoloqies

Fujimoto and N. Teranishi, Uncooled infrared focal plane array having 128 • 128 thermopile detector elements, Proc. SPIE 1 1 6 9 . 4 5 0 - 4 5 9 (1994). 80. O. Paul, N. Schneeberger, U. Mfinch, M. W~ilti, A. Schufelbfihl, H. Baltes, C. Menolfi, Q. Huang, E. Doering, K. Mfiller and M. Loepfe, Thermoelectric infared imaging microsystem by commercial CMOS technology, Proc. 28th European Soild-State Device Research Conference, pp. 52-55 (1998). 81. A. Schaufelbuehl, U. M/inch, C. Menolfi, O. Brand, O. Paul, O. Huang and H. Baltes, 256-pixel CMOS-integrated thermoelectric infrared sensor array, Tech. Digest IEEE MEMS, pp. 2 0 0 - 2 0 3 (2001 ). 82. A. D. Oliver and K. D. Wise, A l()24-element bulk-micromachined thermopile infrared imaging array, Sensors and Actuators 7 3 , 2 2 2 - 2 3 1 (1999).

Chapter 11

Infrared silicon/germanium detectors Hartmut Presting

11.1 Introduction Generally, the infrared (IR) spectrum can be divided into the short wave or near IR (NIR, 1-3 gm), medium wave IR (MWIR, 3-5 [am), long wave IR (LWIR, 8 - 2 0 lam), and very long w a v e IR (VLWIR, > 20 gm) regime. We report here about silicon/silicon-germanium (Si/SiGe) detectors grown on Si substrate for the SWIR (~1.3/am), the MWIR and the LWIR regime which are envisaged for different application fields such as fibre optical c o m m u n i c a t i o n as well as inter (and intra) chip-to-chip optical c o m m u n i c a t i o n on Si IC chips, and thermal imaging detectors for civil and military applications. Sil_xGex alloy grown on Si for that purpose is an ideal material because, due to its 100% complete miscibility (O~<x~< 1) the SiGe bandgap can be continuously tuned from the Si (1.1 eV) down to the Ge bandgap (0.66eV). Both Si and Ge crystallize in the diamond lattice having an fcc (face-centered cubic) crystallographic unit cell w i t h a cubic point group (Oh, 4 / m 3 2/m). Since Ge has a 4.17% larger lattice constant, the lattice mismatch between Si and Ge leads to strained hetero-epitaxy which means that for a given Ge-content x, a Sil_xGex alloy can only be deposited on Si up to a critical thickness t~ without the formation of dislocations. 1 The strain also leads to a substantial change in the SiGe b a n d s t r u c t u r e (especially a shift of the bandgap). It lifts the six-fold degeneracy of the c o n d u c t i o n band m i n i m u m in the (100) direction (A-direction) of the Brillouin zone into a 2-fold degenerate set of electronic states extending in the (001) growth direction and a set of 4-fold degenerate states which extend in the x-y plane perpendicular to growth direction. The bandgap curve of a (strained and unstrained) Sil_xGex alloy as a function of x is illustrated in Figure 11.1. One can see the big influence of the strain to the bandgap when one compares the unstrained curve (dashed) with the strained one (solid lines). The sharp drop of the bandgap for x > 0.85 for the unstrained alloy, indicates the transition point to the Ge-like bandstructure for

394

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this high Ge-content where the conduction band minimum changes from the Apoint to the L-Point (111 ) of the Brillouin zone. In addition, the height of the valence band offset at the Si/SiGe interfacerelevant for intraband transitions from the SiGe well to the Si boundaries in the MWIR detectors- can be continuously adjusted by changing the Ge-content of the alloy. Thus SiGe detectors offer sensitivity in the whole IR (SWIR, MWIR and LWIR) wavelength regime 2's'6 as schematically depicted in Figure 11.2. In this figure the sensitivity range of different IR detectors, such as silicide detectors (IrSi, PtSi) as well as III-V and II-VI semiconductor detectors (InGaAs, HgCdTe) is also shown. On the bottom of this figure the spectrum of the transmission of the atmosphere is shown, which defines the technological important wavelength regime of the atmospheric transmission window between 1.5 and 2 pm, 3 pm and 5 Jam and 8 l.tm and 12 pm. Furthermore, one of the key advantages of the SiGe material system is that the detector devices such as large area focal plane arrays (FPA) can be fabricated using the cost effective and mature Si technology and can then be monolithically integrated with commercially available Si readout circuits. The ubiquitous silicon technology which governs todays and tomorrows microelectronic market (99% of world semiconductor market in year 2 0 0 1 , total volume 320 bill. US $ t) leads to the effort of fabricating Si based devices for Future H o r i z o n s (2001).

Infrared silicon~germanium detectors

IR detector technologies NOVEL Si IR TECHNO LOGY

395

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IR DETECTOR TECHNO LOGIES

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Figure 11.2 Comparison of different IR detector technolo[lies and their sensitivit!t spectrum. Si(;e is the on~!~ material from which one can fabricate detectors for the whole infrared regime (near. mid and far IR : 1 l~m30 l~m ). On the bottom the atmospheric transmission is depicted showing the technologically important three transmission windows between 1.5 I~m and 212m. 3 l~m and ~ lain and 8 lira and 12 ~m.

fibre-optical communication. In essence the hope is that the material inherent disadvantage of Si as optoelectronic material - its indirect bandgap - can in some respect be overcome by the introduction of the Si/Ge heterosystem which allows application specific tailoring of the electronic and optical material properties up to a certain extent. The realisation of integrated optical circuits on a Si wafer requires Si based emitter and receiver device functions which can be monolithically integrated on a Si IC chip with their corresponding electronic driver circuits and digital signal processing units as schematically depicted in Figure 11.3(a). A scheme of possible realisation of interchip and intrachip coupling via Si/Ge optical devices can be seen in Figure 11.3(b). A great impact to this field has been given by the realization of 1.3 ~m SiGe waveguide photodetectors with external efficiencies of r1~12% and response times of 4 0 0 ns from strained Si/SiGe q u a n t u m well layers. 6 In addition, about the same time short-period SimGen strained layer superlattices (SLS) 7 have been grown with strong optical transitions at 1.3 ~m up to room temperature. Other passive optical device functions such as modulators and interferometers with SiGe waveguides on Si substrate became available thanks to the progress of silicon-on-insulator (SOI) technology and the excellent work of the group at the Technical University of Berlin 6'~ as well as work from a group at the University of California, Los Angeles. 9 This opened the way of a monolithic integration of Sibased optical devices integrated with Si electronic driver and signal processing circuits as schematically depicted in Figure 11.3(b) also gives an estimation of the total loss (propagation and coupling losses) for an optical free space interchip coupling between two Si IC chips. Besides the big effort to enter the market of


Figure 11.3 (a) Concept of Si optoelectronics with SiGe photodetectors and waveguides and the electronic driver circuit on board. The external clock rate is provided b!t an external optical signal. (b) Concept of optical transmission between two Si IC chips with active SiGe optoelectronic components such as laser, LED, photodetector and modulator together with the electronic driver and signal processing unit. The optical signal losses due to coupling and propagation are estimated.

fibre optical communication devices with SiGe, there have not yet been any published efforts to fabricate Si/SiGe imaging detectors in the NIR regime for the so-called third transmission window, i.e. in the 1.5 ~m-2 ~m regime. In the MWlR regime, however, imaging systems of infrared detectors with FPA technology have important applications in both commercial and military areas. Important commercial applications are medical diagnosis, fire and combustion

Infr~tred silicon/,qermanium detectors

397

control, surveillance and driver's vision e n h a n c e m e n t . The military applications include night vision, rifle sight, military surveillance, guidance and tracking for missiles as well as for interceptors. For exo-atmospheric, space-based surveillance sensors, where cool targets with low background irradiance levels are often present, LWIR and VLWIR are the appropriate wavelength bands. For tactical military applications as well as for the major part of commercial applications, the important wavelength bands are determined by the atmospheric transmission windows in the NIR. MWIR and LWIR regime. Therefore high resolution, large area IR FPAs in the two atmospheric windows (3-5 IJm and 8 - 1 2 ILtm) with high sensitivity, high uniformity and stability are demanded. For the 3-5 IJm window large area FPAs are fabricated in the well developed and cost effective PtSi technology. However, for the 8 - 1 2 IJm regime there is no completely satisfactory detector material available for fabricating large area staring FPAs. HgCdTe offers, for example, excellent detector performance, but large substrates are not available and costs are high. Due to the lack of large substrates one often has to adopt the line-scanning technique, which is more expensive and less reliable because of fast moving mechanical parts in the system. In addition, bonding of the III-V detector array to the Si readout circuit is highly challenging, especially for larger arrays, lc~'ll For nA1GaAs/GaAs q u a n t u m well infrared detectors (QWIP's) coupler gratings have to be added due to their very low sensitivity to normally incident radiation. ~2, ~3 Consequently, there has been a recent interest in fabricating MWIR detector arrays using the Sil _xGe• alloys grown on Si. 14 These SiGe detectors operate on the principle of hetero-internal photoemission (HIP) within the valence band (intraband transition) from a highly p-doped Sil_• well into an undoped Si layer where the cutoff wavelength is determined by the valence band offset at the Si/SiGe interface (i.e. by the distance of the Fermi energy in the SiGe well to the top of the Si valence band). This type of internal photoemission detector which we refer to as a HIP detector consists of a degenerately p-doped semiconductor electrode in place of the metallic silicide contact in the well established silicide detectors, such as PtSi and IrSi. This variant to the Schottky barrier detector was first proposed by Shepherd et al. in 19 71.1 s The concept was then for the first time implemented in 1990 by a group at the Jet Propulsion Laboratory. ~ They fabricated Si based heterojunction detectors which consisted of a heavily p+ doped GexSil_x epitaxial layer (with a Ge-content of x~().2-(). 3) on a p-doped Si substrate. These pioneering HIP devices were sensitive up to 1 ()IJm but suffered from a large dark current due to an imperfect Si/SiGe heterojunction which made it necessary to operate them at very low temperatures. Apparently, it is now more than 10 years since the first single SiGe/Si HIP detectors were realized and in 1994 the first large area FPA of SiGe HIP detectors with up to 4()()• 4()() pixels was demonstrated by B.Y. Tsaur. 1, We report here about state-of-the-art near, mid and long wavelength infrared SiGe detectors from the material growth, fabrication and performance point of view. We will concentrate on near and mid-infrared devices for imaging systems since SiGe has a clear advantage in the use of the cost effective and m a t u r e Si technology which has the main advantage over competing devices.

398


11.2 Near infrared detectors 11.2.1

General operation principle

Most of the near IR (NIR) Si/SiGe detectors are designed for the fibre optical communication wavelength which means that they should have their m a x i m u m sensitivity at the m i n i m u m dispersion wavelength (1.3 lJm: ().95eV) and the m i n i m u m absorption wavelength ( 1.5 5 ~m: ().8 eV) of fibre optical propagation. The SiGe approach for integrating NIR detectors on Si has been thoroughly discussed in the pioneering work of Lury et al. 1~ The key challenge is the lattice mismatch between Ge (or SiGe) and Si. When Ge or SiGe alloys are deposited directly on Si, misfit dislocations which form due to lattice mismatch relaxation occur and cause residual threading dislocations in the SiGe epilayers which are known to affect negatively the detector performance. Many different methods have been proposed to overcome this problem. Among them has been the growth of pseudomorphic q u a n t u m wells (OW) 2 and strained layer superlattices. 7'2{J'21 The usage of a thick ( ~> ] ~m), compositionally graded, fully relaxed buffer layer to avoid threading dislocations in the active layers and for strain adjustment of the subsequent strained layer superlattice, has been also demonstrated with s u c c e s s . 22 Although the up to several microns thick, fully relaxed S i l _ y b G e y b buffer layer is able to solve the lattice mismatch problem between the Si-substrate and the active SiGe/Ge layers, its growth requires deposition times m u c h longer than for the active absorption layer, which makes it rather troublesome and expensive. The direct deposition of Ge photodetectors on Si substrate, both as polycrystalline Ge and as thick, relaxed Ge grown on a Ge-buffer layer, has also been demonstrated.-' ~.24 However, the huge n u m b e r of misfit dislocations at the Si/Ge interface leads to device unacceptable high dark currents (jd > ,--,1 m A / c m 2 at RT, 1 V). A reasonable compromise between performance and costs might be the use of polycrystalline Ge which can be deposited at rather low temperatures (~ 3 5()~ even on a readily processed IC chip for the fabrication ofa Si optoelectronic integrated circuit (OEIC). 2s The two types of NIR SiGe detector structures which we want to focus more closely on are the GeiSi2(~Gei (j= 2 . . . 4 ) double OW and the short-period SimGe~ SLS structure. Both structures work as interband detectors, i.e. they are based on the bandgap transition (interband) from the valence to the conduction band. In the latter case the transition is spatially localized at the Si/SiGe interface. Due to a change in the Ge-composition of the structure (Ge-ML j resp. n/(n+m)) the bandgap can be adjusted roughly from (). 7 eV to 1 eV. The GeiSieoGe j double OW (DQW) structure just described is, for practical applications, a very promising structure for 1.3 13m light detection, since it is pseudomorphically grown on Si substrate and the electronic transition originates from quantized electronic valence band states (hh, lh) in the Ge well to the Si-conduction band. Figure 1 1.4 shows a typical band alignment of a DOW GeiSieoGe i structure deposited on Si. The corresponding optical transitions which occur around 0.9 eV to 1 eV (~ 1.3 btm) are schematically depicted. The Ge

lnfi'ared.,;ilicon/germanium detectors

399

Figure 11.4 Band alignment of GejSi2~)(;e i double QI~' structure ( j = 2 - 4 ) used.for near-infrared detection. Tire hole wavefunction in tire valence band is located near the (;e wells, whereas the electron wavelilnction is located in the 20 ML wide Si well which results in a strong excitonic intertnmd transition around 1. J ltm (0.95 eV). The SiGeo. 1~ allo!t cladding la!lers improve the localization of the electrons and hoh's which allo~vs fl~nctioning of this device up to room temperature.

double wells which are j atomic monolayers (ML. j = 2 - 4 ) wide, are separated by 20 ML of Si, the OW layers are embedded by two Sil_•215 alloy cladding layers. It is well known that for fully strained Ge on Si (1 ()()) substrates a large conduction band offset occurs only for the conduction band states with k-space minima in the growth direction. These 2-fold degenerate states lie m u c h higher in the strained Ge wells than in the unstrained bulk Si, giving rise to confinement of the electronic states in the Si well in the growth direction. 26 The two thin Ge wells together with the Si layer in between and the band bending effect ensure, in an ideal material (no defects, sharp interfaces etc.), that the electron state confined in the central Si well constitutes the lowest electron level in the system. In addition the discrete states of the Ge wells are the topmost valence band states. The sharp Si/Ge interface provides the symmetry breaking potential which leads to the spatially localized strong excitonic transition as schematically indicated in Figure 11.4. The Sil_• cladding layers provide the strong localization of the electron and hole wavefunction which makes the structure suitable for a room temperature device. Photoluminescence {PL)27 and electroluminescence (EL)2 have been measured in these structures up to room temperature which is shown in Figure 11.5(a) and (b). Figure 1 1.5{a) shows the temperature dependent PL spectra of 2 GejSi2oGej OW structures from ref. 2 7. At lower temperatures the no phonon (NP) and the transverse optical (TOt phonon replica lines are well separated in the PL spectrum. At room temperature, there is a broad peak present

400


Figztre 11.5 (a) Photolmninescence ( PL ) spectra.l)'om the dozihh' (.)~" (;eiSi2o(;e i strm'tzlres for d(flerent well width (left plot) and measzired at d(fferent temperatz~res (right plot) Zll~to room temperatz~re (taken from Ref. 27). The total PL intensitH is onl!l slightl!t decreasing at room temperature, tlze sz~bstrate related PL lines (at 1.06 and 1.08 eV) disappear above 50 K. (b) EL spectra at room temperature from d(ff. erent Si(;e I)C)W mesa diodes taken from ref. 2. A red shift o.fabozlt A/, ,~0.21~m with increasing (;e well widthJ?om j=2 to j=4 can be seen. This occurs because of tire correspolrdiprg shift of the valence band groz~nd state (*denotes samples with unsHmmetrical SiGex cladding la!ler). (c) EL spectra from a Si6(;e4 SLS mesa diode grown on a thick. compositionall!l graded and fldl!! relax'ed Si ~ -,t,(;e,i, buff. er at various temperatures up to room temperature. 7

In h'ared,,;ilicon/germanimn detectors

401

Figure 1 1. ~ (continued).

at 0.95 (1.35 IJm) eV with an integral intensity which is only a factor of 2 less compared to lower temperatures. Figure 11.5(b) shows the EL spectra at room temperature from different double QW structures with the parameter j (Ge-well width) changing. A clear red shift in the transition energy with increasing well width can be seen in Figure 11.5(b) due to lowering of the electronic ground state in the Ge-well with increasing size (AE--,(). 16 eV (().2 13m) from j=2 to j=4 1. Please note that, because of band filling effects due to a higher excitation density. there is a shift in the transition energy between the EL and the PL experiment. 2 Short-period SimGen SLS consists of an alternating sequence of m monolayers (ML) of Si and n ML of Ge stacked on top of each other and repeated N times (N ~> 100). By introducing a new super-period of Psl.s (ps~.s-n+m) in the Brillouin zone, a folding of the bandstructure in k-space occurs, which leads according to the calculations of Gnutzmann and Klausecker 2s for appropriate values of PsLs to a direct bandgap semiconductor in one dimension parallel to the growth direction (see Figure 11.6). The lattice mismatch between Si and Ge can be accommodated either by strain or by formation of misfit dislocations. According to the equilibrium theory of V. der Merwe 2'~ Ge deposited on Si is fully compressively strained up to the critical thickness t,. above which misfit dislocations form, which partly relax the strain. However, the experimentally found critical thickness is higher than predicted by equilibrium theory, esp. at low growth temperatures ~r which defines a metastable regime of pseudomorphic growth. To achieve a device meaningful thickness of the SLS ( > 200 nm) and to provide sufficient stability of the superlattice layer as a whole, one has to adopt

402


Figure 11.6 Brillouin zone folding of a Si-like bandstrlwture introduced b!l a superlattice with a period L= 5ao (ao lattice constant of Si ). The bandstructure is.folded into the first minizone (bold) which has the extension of 2Kst, =2(rc/5 ao). By this folding process the original bandstructure with the indirect bandgap transforms into a direct bandgap structure.

the strain symmetrization of the SLS which requires that the average strain within one period adds up to zero. Under the approximation of equal elastic constants of Si and Ge 31 this leads to ESLS _SLS __ 0 Si -~ ~Ge

with asi _SLS --rice~L,

ESIeS - -ftsi/L.

(1)

and f=2(ace-asi)/(asi+a(,,,) being the lattice mismatch between Si and Ge t(;,,=na• and L=tsi+t(,,,, being the individual layer (f - 0.0417), tsi=m a• and total thickness of one period of the SLS (aj is the perpendicular lattice constant in growth direction (z), all is the in-plane lattice constant in the interface plane (x-y)). 8si SLS ,8(; eSI,S are the magnitude of the strain of the Si and Ge layers of the SLS. For the most important case of equally thick layers (tsi-tce=L/2) equation ( 1 ) reduces to _SLS ESiS L- - S--~Ge

-f/2

(2)

The symmetrical strain is of special importance because it marks the lowest energy state of the SLS. It is only in this strain situation that the superlattice has its lowest energy content (the SLS produces zero average strain) and can be


4()3

stable grown up to infinite thickness. However. to fulfil this condition for arbitrary superlattices one needs to grow an intermediate relaxed Sil_ybGeyb alloy buffer layer between substrate and superlattice ! which provides the average in-plane lattice constant of the subsequent superlattice allt3~'a s~.s Figure 11.7 depicts schematically the strain relation (compressive or tensile) of the single layers in a SimGen SLS grown on a strain adjusting buffer layer. If the sum of the Si and Ge ML in an SLS is a multiple of 10 {(m+n)=s• 10, s = 1 , 2 . . . } strong interband transitions have been predicted by theory ~2 and experimentally found. ~~ Further on. electroluminescence up to room temperature has been observed from fabricated mesa d i o d e s - schematically shown in Figure 1 1 . 9 ( c ) - from a strain symmetrized Sir SLS for different temperatures from 159 K up to room temperature. 7 The EL has been measured with a constant injection current density (3 A/mm 2) up to room temperature. One can see a strong peak indicating an interband transition at 0.78 eV (1.5 l~m) from the SLS, which roughly agrees with the calculation. ~2 11.2.2 Detector

growth and fabrication

All structures reported here are grown on a 1()() mm diameter (4 inch), (100) oriented silicon substrate by molecular beam epitaxy (MBE). This UHV epitaxy technique which operates at a base pressure of 1() -l{~ mbar far from thermodynamic equilibrium, with slow deposition rates, is the method of choice for the growth of the QW and SLS structures with dimensions of atomic monolayers. 35

Figure 11.7 Strain symmetrization b!! a virtual substrate ( Si+Si~ _~1(;e,t bz(ffer) in a Si,,(;e,, SLS. Dots depict the natural lattice spacing, arro~vs the lattice spacing distortion b!t strain. Ill the strain s!immetrized state the average strain of the SLS over one period is zero.


GemSi2oGem double quantum well detectors The QW samples with Ge wells of several atomic MLs are grown as pn-doped structures on a p+-doped (1()()) Si substrate by molecular-beam epitaxy (MBE). Figure 11.9(a) schematically depict the grown OW structures. Details of the growth process (MBE machine, growth parameters) and the substrate preparation which can be found in refs. 34 and 35. After growth of a 10()nm Si start layer, one ML of Sb has been deposited for surfactant mediated growth 3~,. Subsequently, the active Ge/Si/Ge QW layer stack, together with the surrounding SiGeo.ls cladding layers, is deposited with a p-doping level of 5 x 101 s cm-3. After growth of a 50 nm Si barrier layer, the whole layer sequence is repeated ] 0 times to enhance the absorption of the active layers and provide device relevant thickness. On top of the p-doped OW layer stack, a 400 nm thick, n-doped SiGeo.o25 alloy layer has been grown as a waveguiding layer to improve the coupling of an optical beam from the side facet of the detector and to enable its waveguide propagation in the detector. The structure is terminated by a 10 nm n+-doped Si contact layer deposited on top of the waveguide layer. Figure 11.9(a) shows this structure schematically. As indicated, a pre-deposited ML of Sb is used as surfactant during growth to enhance the two-dimensional layer growth and the Si/Ge interface sharpness by hampering the segregation of Ge on the Si surface during growth. ~' The process temperature, growth rate and boron flux must be carefully chosen in this structure to overcompensate the n-type background level from spontaneous incorporation of the pre-deposited antimony. Figure l 1.8(a) shows a high resolution cross-sectional transmission electron micrograph (TEM) of the OWs in the (110) direction. Very sharp interfaces and fiat layers indicating excellent growth quality can be seen. SimGen short-period superlattice and SiGe quantum well detectors The short-period SimGen SLS samples are grown by MBE on highly n-doped substrates as pn diodes (n + substrate for SimGen SLS). After deposition of the 100 nm thick Si start layer and one ML of Sb acting as surfactant, a partially or fully relaxed Sil_ybGeyb buffer layer has to be grown for strain adjustment of the highly strained SLS. 37 To provide the strain adjustment of the subsequent SLS a rather thick (0.65 ~m), fully relaxed step-graded Sil_y~,~tGeyiz~ buffer layer with a slow stepwise increase of the Ge-content from y=() at the bottom to yf-O. 39 at the top buffer interface in 13 steps has been deposited (rate Ay/Az=().03/ 50 nm). The slow increase of the Ge-content provides a slow build-up of the strain in the buffer which drives the misfit dislocation segments - nucleating at the Si/SiGe interface since the buffer thickness exceeds the critical thickness to the edge of the sample which leaves large parts in the centre of the sample with a low density of threading dislocations. As schematically depicted in Figure 11.9(b) the graded Sil_ybGeyblz~buffer which is almost fully relaxed, is followed by a 5()0 nm thick, constant composition Sil_yfGeyf buffer layer which stabilizes the mean in-plane lattice constant required for the strain symmetrization of the subsequent SLS (yf,~(n/(n+m)). On top of the buffer the 14 5 periods of the Si6Ge4

Infrared silicon/germanium detectors

4()5

Figure 1 1.8 High-resolution cross-sectional transmission elect ton micro~lraph of: ( a ) Si~,(;e4 SLS gro~vn on a compositionally graded and constant composition, full!l relaxed Si l ~(;e,~ allo!! bz((fer la!ler. Ver!l ~,lood Si/(;e interface sharpness and layer qualit!t can be seen lor the ,\IBE gro~vn SL$. (b) (;e4Si2~(;e4 double quantztm ~vell ( DOW) detector structure surrounded b!l a one-sided ~,'i(;ec~.2claddin~t la!ler.

406

Handbook of Infrared Detection Techllologies

GejSi~Gej double QW structure on Si (B2817, B2818, B2975, B3185 )

2. p=l 10 ~" B/cm .3 , j=2, 20ML Si (B2975) 3. p=l 10 ~7 Blcm -3 , J=4, 30ML Si (B3185) (a) Strain symmetrized Si~Ge 4 SLS on step-graded buffer with waveguide layer (B2805)

m•6

p-n junction

! 0 " Nh/cm ~ (=1MI.) Nb--surfactant

! OOn

n + Si I Si T, =

600C

substrate

65Ohm

~=o.03Si~-~iG~'%~n~s~ ~,=0.39a 600C-520C

step-graded buffer

(at N=130 periods)

50Ohm

Si~176r~)OC

constant buffer

200 n m

2_~0 n m

Sio.(Geo.,

145 x (SisGe , }

n-

P 500(."

SLS

I0 n m

P

Si

P*

500("

waveguide

cap

y(z)

(b) Figure 11.9 Layer sequence of MBE grown (;(,jSi2t~(;('i l)(3it ~(a) with unS!lmnletrical (a 1 ) and symmetrical (a2) SiGex alloy cladding layers and Si(,(;e4 SLS structure (b) tjrown on aridly relaxed. Ce-~lraded Si l~jt:~Ge,l,(:~ buffer layer. In (c) a processed mesa detector is schematically depicted, which is also used for EL experiments.


407

Figure 1 1.9 (continm'd).

SLS follow, which result in a total SLS thickness of 2()() nm. Buffer and SLS layers are n-doped due to incorporation from the residual Sb adlayer which has been used as surfactant and has been deposited prior to growth and co-evaporated during growth to e n h a n c e the Si/Ge interface sharpness in the SLS. On top of the SLS, a 4 0 0 n m , p-doped Si~.6Ge~.4 alloy layer is deposited to enable 1.3 l~m waveguide propagation from a into the side facet of the crystal coupled in optical beam. Finally the SLS sample is terminated by a 1 () nm thin, p+-doped Si contact layer. Figure 11.8(b) illustrates a cross-sectional high resolution TEM micrograph of a Si6Ge4 SLS showing an excellent layer quality with good interface sharpness. The diodes are fabricated by standard semiconductor processing techniques, a detailed description of this fabrication process can be found in ref. 2 ]. Circular mesa and waveguide structures with different diameters ranging from 1 ()() ~tm to 1500 Bm and waveguide ridge lengths up to 4 mm have been fabricated. Figure 11.9(c) depicts schematically such a processed mesa diode. A very promising device structure for a Si based NIP, detector, from the integration aspect, is a ridge type waveguide/detector combination first realized by A. Splett. 6 The schematic layout of this device is shown in Figure 11.1 (). On a SIMOX (Separation by IMplantation of OXygen) substrate with a 38()nm Si02 layer u n d e r n e a t h the surface, a 11 ~m thick Si layer is grown by chemical vapour deposition (CVD). After a pre-epitaxial wafer cleaning, the growth is continued by MBE with a 1 ()()nm thick, p-doped bottom contact layer on top of which the active OW layer stack is deposited. On top of the OW an n-doped Si top contact layer is deposited and the structure is terminated by a 20 nm thick n + contact layer. The fabrication technology is done as follows. In the first mask


Figure 1 1.10 Waveguide/detector combination device fabricated on a SIM()Xsubstrate. The waveguide layer has been grown by CVD, the active la!lers of the detector are ,\IBE grown NiGe OWs (shown is GeiSi2oGe i DO W structure).

layer the detector mesa is dry etched by an SF~/O2 plasma using a photoresist mask. Then the whole structure is covered by 1 l~m SiO2 deposited by plasma enhanced chemical vapour deposition (PECVD). The oxide serves as etch mask for the subsequent wet chemical waveguide etch using potassium hydroxide (KOH). In the second mask step, the Si waveguide layer is defined and subsequently etched to a depth of 5 ~m to 6 btm. The anisotropic etch of the KOH creates the (111 / oblique side walls of the ridge spanning an angle of 54 ~ to the (()()1 / face which is in favour of the propagation of an optical mode. In the last step the ridge structure is metallized by 3()()nm aluminum, bottom and the top contact pads are formed by lift off. The processed device with its layer thickness and waveguide dimensions can be seen in Figure 11.1 (). The from a single mode fibre into the Si waveguide coupled in optical mode travels in the waveguide. It will then be attracted by the higher refractive index and coupled into the active detector layers where it will be absorbed and detected. The ridge length of the detector is optimized for maximum coupling efficiency between waveguide and detector at the operating wavelength at 1.3 l.tm. 11.2.3 Results and discussion

Table 11.1 lists the up-to-now realized Si/SiGe NIR detectors giving the reference, the device structure and the figure of merits in terms of external

Table 11.1 Si/SiGe photodetectors for the NIR range Keference

Structure

Efficiency at operating wavelength

(;c p-i-n Si(;e/ Si MOW/ S1.S (PC')

qt,,l=40'%, irr 1 .45 prn qV,!=iY'%,crr I . 3 11m 11,,,~= 300'%,(11 I . 3 pni l l , . x l z ~ O (11 ' x ~I. 3 h1r11

--

-

1,ury ('I trl. 1984 Ic' 'I'emkin 1.1 ttl. I 9 8 h

''

'l'ernkin 1.1 ttl. I 98f1"" tluangct trl. 1 '1')i"' Iluangc,f (11. 199f>." Splelt 1.1 (11. 199 4" I3.Schiippcrt 1.1 nl. I 99 3 'l'ashiro 1.1 trl. 1997'' S:rmavedam t,t trl. 1998"" Shaoc,tctl. 19C)X4i C'ol:rcer~tt~l.1999'"'

'

S i ( k MOW/ S1.S ( p i n ) wavcguidc Si(;e MOW, Al'l) wavcguidc Si(;c MOW Si(;eC' S i ( k MOW pin waveguide Si(;c SI, pin I'urc ( k - pn diotle Si(;cC' pin MSM (;e

~ 1 ~ , , ~ = 4 0I I I0 1. ' %3 ~11111 1 (XI 111 I . 3 111n 11,.,,= I 'XI ( ( 1 I . 3 pr11 11,,,~=I 1 'XI (11 I. 3 prn 11,.,~=40'%, (11 1 . 3 ~1111 I ~ < . , ~5'%, = Lcrr O.C)X11n1 q c y= l I 2 .O'%,(11 I . 3 ptn

.-

Dark current density - ~ - - ~

Remarks ----

-

--

-

-

- --

i 0 niA/cmL at K'I'

dem. LOO Mh/s li)r L 5 krn

7 rnA/cm2

mi2/crn2 7 1111\/c111~ I mA/cm2

f l = 1 (;lIz. I:WIIM= 300 ps dcm. 8 0 0 Mb/s f o r 1 5 km

(10

50 ~cA/crnL 0 . l 5 mA/c&

f , = L . i (;IIz

410

Handbook of Infrared Detection Technolo~ties

q u a n t u m efficiency rlext ("~Pel/P{~) and dark current density as well as the system application of the used detector. Even though this list does not lay claim to completeness it tries to give an historical overview about some milestone works on Si/SiGe detectors. The pioneering work of Lury et al. lu from 1984 dealt with an MBE grown, ll, tm thick Ge pin diode deposited on a Si substrate where an 0.18btm intermediate Sil_xGex graded alloy layer has been deposited for strain adjustment and to getter the threading dislocations. Even though some of the dislocations have been kept in the graded alloy, there have been a substantial n u m b e r of threading dislocations r u n n i n g through the active Ge-diode to the top, thereby deteriorating the detector's performance. They lead to large dark currents and hence lower detectivity of the detector. The achieved external q u a n t u m efficiency (QE) has been 40% at )v= 1.4 5 l,tm. In addition, a substantial shift of the m a x i m u m responsivity wavelength has been found with wavelength. The work of H. Temkin et al. from 1986~s'394t~ concentrated on Sil_xGex/ Si superlattice detectors at 1 . 3 p m set up as photoconductor with a pseudomorphically deposited Sil_xGex/Si superlattice (SL) on Si-substrate. The SL consisted of 20 periods with the Ge-fraction varying from x=0.2 to x-().8 and a corresponding decrease in well thickness from 1()0 nm to 1.5 nm to meet the critical thickness limit. A m a x i m u m responsivity of 0.6 A/W at a bias of 7 V has been obtained which corresponded to a m a x i m u m butt coupled efficiency of 59% from a single mode fibre at 1.3 pm which - taking into account a coupling efficiency from the fibre to the semiconductor device of no more than 2 0 % results in an internal QE of rlint,-~300%. Several months later the same group realized an avalanche photodiode at 1.3 l,tm with the above described SL stack of 20 Geo.6Sio.4/Si layers. A rib waveguide with 12 ~tm width and 1.5 l,tm height which consisted of the active layers and a p-type layer on top, has been fabricated. Light from a single mode fibre was coupled into the side facet of the 500 lam long, cleaved devices which resulted in a m a x i m u m responsivity of 4 A/ W at 1.3 l,tm or an external QE of rlext,~,4()()%. Much larger DC gains in the order of 1 0 0 - 2 0 0 have been reported by the authors. The work of the group of K.L. W a n g at UCLA (University of California, Los Angeles, USA) concentrated on pin photodiodes from SiGe().5/Si superlattices ( l O x 1 0 / 4 0 nm) pseudomorphically deposited on a p+ Si substrate. An external efficiency of 1% at 1.3 btm and 17% at 0.85 pm at 4 V bias has been obtained by this group. The dark currents of these non-passivated mesa detectors with a lOOxlOOl~m area have been quite high (7 m A / c m 2 at 0 . 5 V bias). The latter structure showed its m a x i m u m photoresponse at 0.85 l~m, the peak wavelength could be shifted to 1.55 pm applying an external bias voltage. The first integrated waveguide/detector device on SIMOX substrate has been realized - as already mentioned - by A. Splett from the group of K. P e t e r m a n n at the Technical University of Berlin with external efficiency of rlext,-~11% at 1.3 p6. A very good review work of realized components on Si based integrated optics has been given by B. Schfippert from the same group, s Passive key components such as modulator, waveguide coupler and TE/TM splitter realized on Si substrates are reviewed, in addition a waveguide/detector combination

Infrared silicon~germanium detectors 411

with an MBE grown active SiGe OW layer has been fabricated by this group as described above. Another approach showing the feasibility of the latter idea has been the use of G e l _ y C y structures with carbon percentages as low as ()~ 3.2 Itm) in Figure 11.16(a). The absorption of the detector yields its upper limit of the photocurrent, i.e. the QE of the detector and provides a fundamental material property from which, together with the photocurrent (i.e. quantum efficiency), the recombination rate of excited carriers inside the material can be determined. Other authors have reported absorption data for SiGe/Si MOW samples shown in Figure 11.16(b). 17 The three different MOW samples in Figure 11.16(b) have 1() wells with 3()nm thickness with a Ge concentration of x-().26 and different doping levels from 4 x 10 ~ cm-~ up to 4• 1019 cm -3. As expected and clearly confirmed in Figure 11.16(b) the highest absorption is for the MOW sample with the highest doping level, i.e. for the sample with the highest free carrier concentration. Even more important is the evaluation of the dark current and the photocurrent of the detector at its designed operating temperature, namely 77 K. As can be seen from the absorption data, the high boron doping level of 5 • 102{~cm- 3 leads to a strong absorption of the incident light due to free carrier absorption. Figure 11.17(a) and (b) show the QE and the dark current of a series of HIP samples with different germanium fractions (4 3()8-4 336' from x-().2 5 to x=0.5" see Figure 11.12( a 1 ))" First of all we see that the cutoff wavelength and the position of the peak responsivity decreases with increasing Ge fraction, due to the higher threshold energy. The relatively large valence band offset between the heavily p-doped Sil_xGex layer and the Si substrate for x=().5 (43()8) results in a cutoff wavelength of approximately 3 It, whereas the low Ge fraction of sample 432 5 leads to a broad absorption of free carriers in the entire measured spectrum. QEs of up to 0.43% at 2.7Itm were measured for sample 4:335 containing a Sio.6sGeo.3s layer. As shown in Figure 11.17(b), the dark current densities of these samples vary accordingly in a wide range between 10 -9 A/cm 2 (4 308) and 10 -2 A/cm 2 (432 5) at 77 K according to the effective Richardson equation that describes the thermal emission current over the barrier.

jdark- A*T2exp ~RT

14)

424 Handbook of Infrared Detection Technoloqies

Figure 11.17 Quantum efficiency ( OE ) ( a ) and dark current (b) ~[mesa detectors from an HIP series having one SiGe ~vell with different Ge content in the well (4 ~2 ~. 4 ~ ~ ~. 4 ~ ~6. 4 ~08: Figure 11.12(a 1 ).

Here A* is the effective Richardson constant, k~ is the Boltzmann constant, ~t is the thermal barrier height and T is the temperature. The threshold energy of the HIP structures was determined from the temperature dependent measurement of the dark current where I n ( j D / T 2) is plotted versus 1 / ( k l ~ T ) (Figure 11.17(c) and from the slope of the curve in this Richardson plot, the thermal barrier energy is determined. The optical barrier ~'o is determined using the Fowler equation (see equation (3*)) written in a slightly modified form, that correlates the measured optical barrier ~ with the efficiency rl of the detector (Figure 11.17(d)):


425

Figure 11.17 (c) Richardson plot and (d) Fowler plot of these HIP samples. All QE spectra have been taken at 77 K. Note that the low temperature dark current level depends on the (;e content as discussed in text.

,

/7 -- (71

(by-

r

hi~

2

(3*)

Here hv is the photon energy and C1" is the modified Fowler emission coefficient. 14 The results are listed in table 11.2 below: The inelastic scattering of photo-excited holes prior to the emission over the barrier, results in a slightly higher optical barrier ~o compared to the thermal

426 Handbookof Infrared Detection Technologies Table 11.2

Thermal and optical barriers as d e t e r m i n e d from R i c h a r d s o n and Fowler plots

Sample

x

Thermal barrier qlt {Richardson )

Optical barrier ~,, (Fowler)

4325 4535 4536 4308

0.25 0.35 0.40 O.50

N/R 191 meV 286 meV 390 meV

157 meV 216 meV 299 meV 405 meV

N/R no results due to veryhigh dark current. barrier 1//t. To maximize the detectivity of such IR detectors, high q u a n t u m efficiency and a low dark current are desirable. From this result we conclude that the optimum g e r m a n i u m fraction in the Sil_xGex layer is between O. 3 and O. 35 where the detectors achieve high responsivities between 3 and 4 lam (0.4% at 7 7 K and 5 V reverse bias)combined with low dark currents (jd,-~l() -s A/cm-2). Because detector performance of a given SiGe structure is mainly influenced by crystal quality and growth conditions of the epitaxial layers, we have also studied the effect of various changes in this process: particularly the boron doping level and the growth temperature of the Sil_xGex-layers, which are discussed in the following section. The incorporation of electrically active boron atoms into the Sil_xGex-lattice is limited; at high doping levels, more and more boron atoms are incorporated on interstitial lattice sites which causes additional strain and lattice defects. To evaluate the optimum doping level and growth temperature of conventional HIP-structures, four samples with different growth parameters were analysed. All of them contain a heavily doped 1Onto thick Sio.~,~Geo.35-layer separated by the thin ( 2 . 5 n m ) undoped Si~.6sGeo.3s setback layer on an undoped Si-buffer-layer ( 3 0 0 n m ) and a 50 facm Si substrate (Figure 11.12(a5)): 4677 (p++=5xl0X~ -3) and 4 6 7 8 ( p + + = 2 x l 0 2 ~ -3) were grown at 550~ 4679 (p++=5xl0a~ -3) and 4 6 8 0 ( p + + = 2 x l 0 2 ~ -3) at 450~ Figure 11.18(a) and (b) show the experimental results of the photoresponse and the dark current from the HIP detectors. One can see that changing the doping level and growth temperature results in a different absolute magnitude of the photoresponse in the range between 2 lam and 4 lam. At the peak response position of 2.7 lam, the q u a n t u m efficiency varies from ().21% (4678) to 0.33% (4680), whereas the difference in the long wavelength section of the spectrum is only small and results from slightly different barrier heights caused by fluctuation in the Ge-flow during the MBE growth process. The small difference in the threshold energies also induces the shift in the dark current curves of Figure 11.18(b). Generally, samples grown at higher temperatures (46 77 and 46 78) show less photocurrent t h a n samples 4 6 7 9 and 4 6 8 0 grown at lower temperatures, because reduced surface migration during the growth process leads to a better electrical activation 5o of the doping atoms which causes higher absorption and enhanced photocurrent. On the other hand, a change of doping concentration from p + + = 5 x l 0 2 ~ cm -3 to p++=2x 1() 2~ cm -~ at lower growth temperatures,

Infrared silicon~germanium detectors 427

Figure 11.18 Quantum efficienc!!(a) and dark current (b) the ofdetector samples 4677-4680.

does not actually result in less photocurrent, because defect densities decrease and crystal quality improves with lower doping concentration, which leads to a reduced trapping and recombination of photo-generated carriers and thus to a higher efficiency (see also remarks in the theory Section 3.5). The same result can be obtained by absorption measurements from these samples: According to ref. 17 the absorbance of heavily doped HIP-structures is given by

428


Figure 1 1 . 1 8 (c) Absorption measurements at 77 K - plotted as absorbance divided b!t 22 - versus NAd (product of doping concentration and thickness of the active ~vell). (d) OE spectrum of a PtSi detector and a SiGe HIP detector in comparison.

Infraredsilicon/,qermaniumdetectors 429 ABS - 22 NAdS*

(5)

where NAd is the total quantity of dopant atoms. S* includes constant terms like effective mass, optical constants, velocity of light etc. Thus the slope of the plot ABS/,;, 2 over NAd should be constant. Results of the absorption m e a s u r e m e n t s of samples with different doping levels but the same growth temperature are presented in Figure 11.18(c)" ABS/,;. 2 increases linearly with increasing NAd in the low-doping section of this curve. At a m a x i m u m of about 3 x 1()2~ c m - ~ the absorbance cannot actually be further increased by higher doping concentrations or thicker Sil_xGex-layers as is evident from the connection line between the data points. This result has also been previously reported by other groups 17 who also obtained absorbance data for higher N~. From this result we can conclude that best detector performance is achieved with a doping concentration of about 2 - 3 • 1()2~ c m - ~ and growth temperatures of ,~4 5()~ where the activation of the incorporated boron atoms is optimized. A comparison of the QE between a platinum-silicide (PtSi) and a SiGe HIP detector with x=0.25 (p++=Sxl()2~cm -~. ds~;,,-15nm) is shown on Figure 11.18(d). Whereas the PtSi detector still has a higher QE for wavelength smaller than 3.5 lxm compared to the HIP, we can clearly see that the SiGe HIP detector has a broader photoresponse spectrum with a tunable cutoff wavelength. The ability of the Ge-composition to tailor the photoresponse curve has important advantages for the MWIR detector since PtSi has a fixed m a x i m u m responsivity at 2.7 ~m with a steep cutoff slope which means that for the range beyond 4 lain, PtSi detectors cannot be used any more Inote that the ordinate scale is logarithmic in Figure 11.18(d). As mentioned earlier, we have used an undoped Si~_xGex-doping setbacklayer between the heavily doped Si~_xGe~ layer and the undoped Si layer in contrast to conventional HIP-structures. ~4~ s This thin doping setback layer forms an additional barrier for excited carriers, resulting in higher threshold energies than determined for samples without these setback layers. This effect is caused by the reduction of the charge densities at the interface of the heavily doped Si~_xGex and the undoped Si-layer which confines the holes within the active layer. Figure l l . 1 9 ( a ) shows the comparison between the measured q u a n t u m efficiencies of these two different variants. The 15 nm thick Si~. sGeo. slayer (p++-5 • 102o c m - 3) of sample 3794 is grown directly on the undoped Sibuffer layer, whereas an additional 1.5 nm thick, undoped Sir162 setback layer (p,-~1 • 1()1 r c m - 3) is added at the Si/SiGe interface of sample 4165 (Figure 11.12(a2)). Though the g e r m a n i u m and boron doping in the active layers are equal for both samples, the threshold energy is increased from 161 meV to 254 meV due to the doping setback-layer (determined from the corresponding Fowler-plots). Therefore the photoresponse of sample 4 1 6 5 drops m u c h faster and the OE decreases from ().2% to ().()7% at 4 ~m. But, as expected, the dark current of these structures is also extremely influenced by the additional layer. At temperatures of 77 K and reverse bias of 5 V, the dark current density drops about four orders of magnitude from 2 x 1 ()- ~ A/cm 2 to 2 • 1 ()-7 A/cm 2

430


Figure 11.19 Comparison of quantum efficienc!l (a) and dark current (b) of two HIP samples (p++-~ 102~ c m - ~. dsi~;,,=l O nm ) with ( 4 1 6 ~ ) and without ( 3 7 9 4 ) an undoped doping setback la!ler. ( c ) Comparison of QE of two samples with (4 7 ~8 ) and without (4 7 ~ 7) top-doping setback la!ler.

at 2V due to the additional doping setback-layer (Figure l l.19(b)). As a result, the detectivity D* increases over two orders of magnitude from 1.5• 10 8 c m H z l / 2 / W to 3• 1() lr 1 2 / W at 4 13m which has been calculated assuming the shot noise approximation (see equation (6) below). We measured the dark current and the OE (Figure 11.2()(a+b)) in comparison to the HIP's m u c h lower doped SiGe MOW detectors and from that we evaluated

lnflared silicon~germanium detectors 431

Figure 11.19 (continued).

the detectivity from samples 1 9 4 7 - 1 9 4 9 at 77 K and 5 V bias. The result can be seen in Figure 11.20(c). Quite surprisingly, the OW sample 1949 with only one well exhibits the highest efficiency (QE~,().()7%) which is nevertheless about an order of magnitude lower than the value for the HIP samples. The lowest photocurrent is measured for the N=I() well of sample 1947. This sample, however, also shows the lowest dark current in this series. This results in the highest detectivity D* of 8x 1() ~ cmHzl 2/W at 4.2 ~tm which is confirmed in Figure 11.20(c) where D* has been evaluated using the shot-noise approximation according to

D*- rl ~ q hu

2jd

(6)

Here q is the elementary charge, hv is again the photon energy and Ja the dark current density. To get further increase in detector performance we improved the active layer composition and developed some new layer concepts. The main problem with conventional HIP-detectors is the reduced Debye length in the heavily doped Si/Sil_xGex layers. For doping densities of about 3 • 1 ()2~ cm- 3 LD is less than ] nm, so the applied electric field can only support the transport of excited carriers in a small region next to the Sil_xGex-layer interfaces. Most of the excited carriers have undirected momentum and reach the Si-interface by multiple elastic scattering events if their mean free path is sufficiently high. 17 By introducing an internal electric field by means of a germanium or boron gradient in the Sil_xGex-layer, the momentum would have a preference direction and the escape probability of excited carriers is therefore increased. Figure 11.21(a) shows the comparison between three different types of layer concepts: sample 4680 (as described above), 4684 containing a l ( ) n m

432


Figure 11.20 Dark current ( a ). quantum e~cienc!l (b) and detectivit!l ( c ) of the ,\lOW series 194 7 - 1 9 4 9.

In.l?ared silicon~germanium detecwrs

4 33

Figure 11.21 OE (a) and dark current (b) of HIP samples with a boron ( 4 6 8 5 ) and a (;e (4684)-gradient samples ( 4 6 8 4 . 4 6 8 5 ) . Comparison of phowcurrent with and ~vithout A1 top mirror (c) (see text for mesa fabrication).

Sio.6sGeo.3~-layer with a (from bottom to top, see );=0.3 to x=0.5 in the p++-2• All

boron-gradient from 3.9• 1() 2() c m - 3 to 1 • 1() TM cm -3 Figure 1 1 . 1 2 ( a 3 ) and 4 6 8 5 with a Ge-gradient from Sil_xGe• layer but with constant boron doping of samples were grown with the above-mentioned,


1,21 0,8 9 .

.

.

0,6 .

0,4

energy (eV)

.

0,35

0,35

0,30

~ ' " ~,,.,,~:...

o~" 0,25 i >, 0

c .o_

E C

0,30

with mirror and cavity-oxide

~...

0,25

0,20

0,20

0,15

0,15

0,10

0,10

0,05

0,05

r

without mirror

0,00

9 . ...... 9..........

9

. 1

~

2

.

,

0,00

.

.

~

3 4 wavelength (pm) (c)

.

,

5

. 6

Figure 1 1.21 (continued).

undoped Sio.6sGeo.3 s-setback-layer and a 3()()nm undoped (weakly p-doped) Sibottom contact layer. Under thermal equilibrium conditions, the built-in gradients create an internal electric field with the desired direction. Though SIMS-measurements confirm the intended values for doping concentration and Ge content, the optical measurements indicate different barrier heights for each sample" the graded structures seem to have lower threshold energies than sample 4 6 8 0 with no gradient at all. The values evaluated by Fowler analysis for the given samples are 215 meV for 468(), 203 meV for 4684 and 200 meV for sample 4685. These results are in good agreement with the above made prediction concerning the effect of the built-in electric field. By adding a preference direction to the momentum of excited holes before and after scattering events in the active Si~_xGex-layer, the total amount of scattering events before reaching the Si-interface is reduced compared to the ungraded structure. Thus the effective barrier height of graded samples decreases because of less energy loss of the excited carriers. Nevertheless, the absolute magnitude of the measured QE cannot be increased by introducing a boron or germanium gradient in the heavily doped Sil_xGex-layer. All samples have a peak response at 2.6 pm with values from 0.24% to 0.28% for the graded samples. 4 6 8 0 with a QE of 0.33~ That result becomes more clear if we compare the mean free path length of excited holes within the doped Si~_xGex-layer and its thickness" Strong et al. 17 have determined the values L,,,~ 1.0 nm for elastic scattering and Lie between 140 and 250 nm for inelastic scattering. Thus. at a given Sil _xGex-layer thickness of about 10 nm, even with no internal electric field at all, most excited carriers with sufficiently high energy would be emitted over the barrier by an accidental redirection of their momentum. Furthermore, the dark current density of graded

Infrared silicon/gernmnium detectors 435

samples as shown in Figure 11.21(b) is much higher than for the samples without grading. That is caused by the barrier lowering and the additional momentum of injected holes as mentioned above. To improve the detector performance, we also used an Al-mirror on top of the detector mesa combined with a cavity oxide below (see Figure 11.15(b)). The thickness of the oxide is optimized for peak wavelengths of about 4 ~tm. Due to constructive interference the photoresponse of the IR-detectors can be increased by a factor of 2-3 (wavelength dependent)S~ compared to samples without AImirror and cavity oxide. The reflected wave at the top mirror (Figure 11.21(c)) shows the comparison in OE between two different processing variants of the same sample: 4822 is a conventional HIP detector with a heavily doped (p++-3 x lO 2~ cm -3) Sio.TGeo.3 layer. This sample has been processed with and without the A1 top mirror (see Figure 11.15(b)) and the appropriately chosen cavity oxide, the result of it can be seen in Figure 11.2 l(c). One can see the increase in QE for wavelengths between 2.5 and 5 l~m by the additional top mirror. The thickness of the cavity oxide beneath the Al-mirror was chosen to be 650 nm for maximum amplification at ,-~4 l~m. At this wavelength the response increases by a factor of about three (().17% with mirror, ().O6% without). The peak wavelength is shifted from 2.3 ~tm to 3.1 ~m with values up to ().34%. Consequently, the Fabry-Perot resonator also produces a minimum in the response spectrum, where the incoming wavelength satisfies the )v/2 condition, for sample 4822, roughly at 2 ~ m as expected. For optimum detector performance it is necessary to adjust the thickness of the cavity oxide to the response of the original spectrum to make sure that the reflector minimum does not coincide with the peak wavelength of the detector system. Finally, to get higher photoresponse in the long wavelength regime and reduce the dark current of the detectors, we fabricated multi-well HIP structures. These samples consist of three rather thin (,-~5 nm), heavily doped Sil_xGexlayers separated by thin, undoped (~1 • 101" cm-3) SiGe and Si layers (Figure 11.12(b l-b3)). However, these undoped layers are only a few nm thick (~6 nm) and prevent the increase of dark current due to the additional potential barriers. Due to the heavy well doping these samples are not comparable with common MOW structures, where thick Si-buffer layers (,-~5Onm)separate moderately doped (p~,,10 is cm -3) Sil_xGex layers. Some of the samples were grown with a Ge-gradient in the active layers and an overall reduced Ge content in the active layers compared to the above mentioned 'simple' HIP structures. Figure 11.12(b2) shows the layer design of such a multi-well HIP-structure: sample 4740 contains three Sil_xGex wells, each having an individual linear Ge gradient (Ge fractions are given on the right side) and different thickness, to adjust for the energy-level difference of the ground state to the top of the Si valence band edge with changing Ge content in each well. They are doped only in the centre region (p++-3• and are separated by 6 n m thin undoped Si layers. Sample 4823 has a different Ge concentration in each of the three wells (stepwise Ge grading) and only one undoped Sil_xGex spacer layer in addition to the undoped Si layer (Figure 11.12(bl )). Finally we have grown the '3- well' sample 4825 with constant Ge-content in each well (Figure 11.12(b3)).


Figure 11.22 Comparison of photocurrent spectra (a) and dark clirrent (b) of' J-well' HIP samples (4823, 4740and4825).

Experimental results of the measured quantum efficiencies and dark current densities are presented in Figure 11.22(a) and Figure 11.22(b). The threshold energies were determined to be 140 meV (48 2 3 ), 190 meV (482 5) and 20 5 meV (4740). Especially, sample 4823 shows an improved photocurrent in the long wavelength regime with a broad response within the whole spectrum. Remarkable is the high quantum efficiency (up to ().4% at 4~m) of 4823


437

Figure 11.22 (c) Comparison of C)Eand the dark current densit!l (d) of the 'l-well' and '~-well' samples 4 8 1 9 and 4 8 2 3 with the same integral number o.fcarriers processed with additional top mirror on the mesa.

combined with a low dark current (conventional HIP-detectors with such low threshold energies show dark current densities up to j,r.~().O 1 A/cm2). The peak response of sample 4 7 4 0 is shifted to smaller wavelengths (().43% at 3 l.tm) combined with less photocurrent in the long wavelength regime due to the overall higher Ge content and broader Sil_xGe-~ wells. The increased dark current of sample 4 8 2 5 compared to 4823 is in contrast to the higher optical

438


barrier which has been determined by the Fowler plot. Leakage current over the mesa surface may be responsible for this effect. Nevertheless, all multi-well samples show increased photocurrent in the long wavelength regime (4-5 Bm) combined with rather low dark current densities. So, detector performance of common 'l-well' HIP structures can be increased by the use of the multi-wells which are separated by thin, undoped Si layers. The effect of linear or stepwise Ge grading within the wells seems to be negligible. Further increase of photoresponse in the desired wavelength region could be achieved by optimizing thickness and depth of the Sil_xGex wells. Now it is interesting to compare the '3-well samples' with a common 'l-well' HIP-structure. To take into account the effects caused by the undoped SiGe spacer layers at the top and bottom of the active region, we have grown a ' 1-well' HIP sample (4819) with a l O n m heavily doped (p++=3• 1()2{~cm-3) Si{j.TGeo.3 layer surrounded by two 3.5 nm undoped Si{~.7Ge~.3 setback layers onto a Sibottom contact layer (1OOnm, p - = l • Figure 11.22(c) shows the comparison between the measured OE of 4819 with one well (constant Ge content, x=O.3) and 4823 with three small separated wells (and decreasing Ge content). The spectrum of the photocurrent between 1.5 and 3.5 ~m is nearly equal, reaching a peak at 3.2 ~m with a maximum OE of 0.45% for both samples. At higher wavelengths the 3-well sample 4823 achieves more OE due to the combination of thin SiGe wells combined with the decreasing Ge content throughout all wells. Note that the thickness of each well is different, to adjust for the energetic position of the ground state of each well in the entire structure (for the higher Ge-content wells the ground state is shifted up to the bottom of the Si valence band which can be compensated by increasing the well thickness). Because the total number of doping atoms is nearly the same for the two samples, the amount of excited and collected carriers of 482 3 indicates that all three wells must contribute to the measured photocurrent, thus scattering and recapture of photogenerated holes in a following well is of minor importance. On the other hand, we have seen no further improvement in detector performance, if we use samples grown with more than five wells. This means that, for the given level of doping and Ge content, the optimum number of separated, heavily doped wells lies between three and five. In addition, the lower dark current density of 4823 compared to the 'l-well' sample 4819 is also remarkable (Figure 11.22(d)), because the higher cutoff wavelength of sample 4823 (kco[4823]=6,71 l.tm, )V~o [4819]=5,631.tm, determined by Fowler analysis) should result in an increased dark current. 11.3.5 Calculation of optical properties of SiGe HIP detectors

Band structures of common Si/SiGe HIP structures are well known and reported in a number of papers. 15-17.52 A simple sketch of the band diagram showing the valence band profile of a heavily p-doped (2 x2() 2c~cm- 3) SiGeo.3s well has been sketched in Figure 11.11. Note that the Fermi level lies within the valence band edge of the well at 77 K, above the well there is the continuum of unoccupied states. To get additional information about the effect of the above mentioned Ge

Infrared silicon/germaniun7 detectors

439

Figure 11.23 Simulation of the band structure and electric Jieht for "gradient'samples 4 6 8 4 ( a ) and 468 ~ (b).

and B gradients within the active Sil_xGe• layer of our detector structures, it is necessary to determine carrier densities and potential profiles in thermal equilibrium. These simulations were made with a computer program, solving iteratively Schroedinger and Poisson equations for a given Sil_•215 structure,

440 Handbookof Infrared Detection Technolo#ies doping profile, and external bias voltage. For the gradient structures 4 6 8 4 and 4 6 8 5 - mentioned in Section 11.3.4 - the following results of the valence band profile and built-in electric field have been achieved, which is shown in Figure 11.23(a) and (b). Only the first 30 nm of the structure are shown in the plots the 10 nm heavily doped Si] _• layer, 2.5 nm undoped Si 1_xGe• and part of the intrinsic Si-bottom contact layer. The simulation does not include an external bias. One can clearly see the valence band discontinuity between the Si~_xGex layers and the Si-bottom contact layer at 12.5 nm for the two gradient structures of samples 4 6 8 4 and 4685. The calculated threshold energies of 232 meV (4684) and 226 meV (4685) agree quite well with the experimental results described in the previous chapter. The valence band profiles are quite similar for both samples except the band bending in the low doping section of sample 4684. Because the Sil_xGex-layer of 468 5 is heavily doped (p++=2 x 1()2~ cm- 3), the Ge gradient does not actually result in considerable band bending. The desired effect of the built-in gradients is shown in the upper part of the graphs in Figure 11.23(a) and (b) where the resulting electric field is shown. As expected, both samples have positive field strengths in the active Si~ _~Gex layer that should give photoexcited carriers an additional m o m e n t u m towards the Si interface. Simulations of conventional HIP structures without gradient do not show any field in this part at all. The undoped Sil_xGe~-doping-setback-layer and the Sibarrier cause strong negative peaks of the field at ~ 1 ()nm. In addition to these band structure simulations, we present calculations on the effect of undoped Sil_xGex-spacer-layers that separate the heavily doped Sil_xGex-layers from the Si layers, as mentioned above. We concentrate our attention on the density of states, which should give useful information for the discussion of dark current, and also present calculations of the optical lineshape produced by these systems comparing our calculations with the experimental results. Starting with pseudopotential calculations, including effects due to spin, non-parabolicity and mixing between valence minibands, we introduced a screening Coulomb potential, representing the high concentration of impurities. The alloy concentration in the well is taken to be Sio.7Ge{j. 3 throughout. For doping concentrations of 3 x 102o c m - 3 a self-consistent method was used to calculate the density of states within the well. We model highly-doped SiGe heterostructures where 'dilute models' do not apply due to the large number of impurity potentials per unit volume. We use as a starting point our full-scale pseudo-potential calculation of the valence band where spin and the non-parabolicity of the valence minibands are implicitly included. 53-56 We model the impurities using an added Coulomb potential to represent the impurities, self-consistently screened assuming the random phase approximation. The inclusion of dopants into semiconductor heterostructures can be tackled in three ways. The calculation to establish the miniband structure could be performed and the doping included only through the position of the Fermi level, chosen such that the number of holes introduced by the doping are accommodated in the top valence band levels, s7'~ Thus no account is taken of the extra potential


441

introduced by the impurities in establishing the miniband structure and hence the spread of acceptor levels into a wide band as the doping increases is not included. Secondly, we have our large cluster calculation presented here, where the additional potential due to the presence of the impurities is explicitly included when calculating the miniband dispersion. This results in a full description of the spread in energy of the acceptor states in the high doping limit, allows a more realistic Fermi level to be established, and provides acceptor states into which excitation can occur within the absorption process. Further improvement would require a full ab initio calculation, s€ i.e. a many-electron calculation, which greatly complicates the situation and is outside our scope for large clusters at present. The impurities are considered to be randomly distributed t h r o u g h o u t the well layers of our cluster, which contains some 47 000 atoms. We consider doping concentrations of up to 5 • 1 ()2~ c m - ~ which results in one in every 100 atoms within the alloy well layers being replaced with an impurity atom. We also considered a range of doping concentrations from 1 x 1() ~8 cm-~ to 5 x 102o cm -3. In Figure 11.24 we show the change in the density of states of a 12.5 nm wide Sio.TsGeo.2s alloy well including an undoped 2.5 nm spacer layer, equivalent to the structure of sample 4325 shown in Figure l l . 1 2 ( a l ) , as the doping concentration is increased. We can clearly see the formation of a wide band of acceptor states as the number of impurities increases. For 5 x 1019 c m - 3 this band is some 200 meV wide increasing to 800 meV width for 5 • 102o cm-3. The full description of this acceptor band is one of the key features of our model, and, to our knowledge, has not been previously achieved. The position and nature of these impurity states is crucial to the resulting optical lineshape, as, at the low temperatures considered there, it is these impurity states into which the carriers are excited in the optical absorption process.

Figure 11.24 The change in the densit!l of states with doping concentration,for a Sio.zsGeo.2s allo# well, 12.5 nm wide, incorporating a 2.5 nm undoped spacer la!ler, equivalent to the structure of sample 432 5. The zero of energy is taken to the top of the unperturbed valence band at the zone centre. The vertical lines indicate the top and bottom of the valence well.

442 Handbookof Infrared Detection Technologies We next consider four different situations based on the structure 4 8 2 5 , namely a 6 nm Sio.7Geo. 3 alloy well: 9 9 9

9

Structure 1. A doped region of 3.5 nm with a 2.5 nm undoped spacer, as in experimental structure 4 8 2 5 as shown in Figure 1 1 . 1 2 ( b l ). Structure 2. A 3.5 nm doped region with 1.25 nm undoped spacers on either side of the doped region. Structure 3. No doping setback layers (=spacers), a 6 nm well with doping distributed t h r o u g h o u t the well set to give the same number of impurities as present in structures 1 and 2. Structure 4. No spacers, a 6 nm well with doping distributed t h r o u g h o u t the well at a concentration of 3 • 1 ()2c~c m - 3.

Structures 3 and 4 both have doping distributed t h r o u g h o u t the well, i.e. no spacer layers. However, structure 3 has the same number of impurities as structures 1 and 2, thus a lower actual doping concentration, whereas structure 4 has the same concentration of impurities, therefore, as the doping width is greater, it has a larger n u m b e r of actual impurities present. Experimental results presented in Figure 11.19 for structures with and without spacer layers show little change in the q u a n t u m efficiency spectra on introducing spacers but a dramatic change, of more than three orders of magnitude, in the dark current density. In Figure 11.25(a) we show the change in the density of states produced by the different spacer configurations. We see that there are minor changes to the distribution of the states for the different spacer configurations. In particular there is a change to the distribution of states around the bottom of the well, (zero energy on the diagram). For the structures with no spacers, structures 3 and 4, there is a peak in the density of states just below the well, (zero energy on the diagram). For the structures with spacers the density of states in this region is flatter, with no peak. The states positioned at the bottom of the well are of particular importance to the dark current characteristics of the structure, as it is these states from which carriers can be thermally excited into the continuum, thus contributing to the dark current. However, it seems unlikely that this small redistribution of the states could result in the dramatic reduction in dark current that was observed experimentally. Our calculations of the normal incidence absorption response for structures with and without spacers, presented in Figure 11.25(b), similarly show only small changes in the optical lineshape with n u m b e r and width of spacers. As structure 4 has doping spread t h r o u g h o u t the well, i.e. a larger doping width, it therefore contains more impurities than structures 1, 2 and 3, hence the greater magnitude of the response. The change from one to two spacers results in a shift of the peak absorption of 0.5 l~m. This can be accounted for by a slight difference in the spread of energies t h r o u g h o u t the acceptor band for the two different structures. The main difference in the optical lineshape between the spacer and non-spacer configurations, is the lower magnitude of the absorption above 3 l~m. There is no obvious change in the density of states that would account for this difference. However, the charge densities of the acceptor states will be confined

hffrared silicon~germanium detectors

443

Figure 11.25 (a) The change in the densit!t of states with the spacer width and position. For details of the structure configurations, see text. The vertical lines indicate the top and bottom of the valence band well. The zero of energz3 is taken to be the top of the unperturbed valence band at the zone centre. ( b ) The variation of the normal incidence absorption response with structure, calculated at T - O K. The structures are labelled as described in the text.

within the doped region, therefore for structures with doping setback layers present there will be a smaller degree of overlap between these acceptor states and the states towards the bottom of the well. which are in general centrally located.

444 Handbookof Infrared Detection Technologies Structure 1 is equivalent to the structure of sample 4825. The experimental q u a n t u m efficiency for the sample is presented in Figure 11.22(a). We can see that some of the gross features of our calculated curve are in good agreement with the experimental results, namely the turn-on of the absorption occurs at 2 l~m and peaks at around 4 I~m. The experimental curve sharply drops off at longer wavelength, whereas the calculated curve flattens off. The absorption m e c h a n i s m is excitation from occupied valence states into unoccupied acceptor states. As our calculation produces a large n u m b e r of closely spaced levels around the Fermi energy for the distribution of impurities considered, a large response at low energy (long wavelength) will always occur. A well defined peak would only arise if there was a non-uniform distribution of acceptor states, i.e. the acceptor states form discrete bands. This may result if the added impurities form clusters rather than being distributed randomly t h r o u g h o u t the well - an interesting topic for further investigation. The reasons for the dramatic reduction in the dark current with the introduction of the spacer layers is still not fully understood on a microscopic level. The presence of the spacer layers could cause further confinement of the wavefunctions of states lying within the well but within a kRT spacing from the continuum. The leakage of the wavefunctions out of the well for states near the c o n t i n u u m results in a 'ladder' of states which can be easily populated by thermally generated carriers resulting in a high dark current. If the confinement of the wavefunctions is enhanced by the presence of spacer layers, then this effect, and hence the dark current, may be reduced. However, the interaction of the excited carriers with the interface, the non-ideal nature of the real interface, i.e. roughness, islanding 6~ etc, may also be important factors. In a paper by Shaw 5s on the fundamental physics of SiGe structures, it was demonstrated that the microscopic interaction between the defects and the Si-Ge interfaces results in changes to the electronic wavefunctions that can drastically affect the optical properties of these systems, and concludes that localized interface features will play an important role in determining both the optical and transport characteristics of SiGe structures. At the extremely high doping concentrations considered here, where up to 1% of the original atoms are replaced by the dopant boron atoms, the interaction of these dopants with the interfaces may well be highly significant. Further investigation is needed to determine the ultimate effect of the interaction. This is a first attempt at modelling these highly doped structures, and although some success was achieved, there needs to be more investigation of how the impurities are included for these very high doping concentrations, the sensitivity of the optical response to the type and position of these impurities, as well as a more detailed investigation of the effect of the interaction of the defects with the interface. 77.3.6 R~sum~ and outlook for SiGe MWIR detectors

We have discussed several types of Si/SiGe HIP and MQW detectors grown by MBE and fabricated as single mesa as well as focal plane array detectors. Various structural designs such as multi-well, Ge and B gradient and the use of doping

hffrared silicon/germanium detectors 445

setback layers on one and on both sides of the highly doped SiGe well have been applied and the detector characteristics have been compared. Broad photoresponse curves with a m a x i m u m OE of ~O.5% at a peak wavelength between 3 l.tm and 4 l.tm, at 77 K have been obtained. By using doping setback layers in addition to an appropriate passivation to suppress surface leakage currents detectivities of up to 9• 1()11 cmv/Hz/W can be achieved, a value which, to our knowledge, has not been obtained by any other III-V detectors for this wavelength range. Reports of higher QEs by other authors must be carefully analyzed. The reported m a x i m u m OE of 8% from ref. 14 has been measured at a m u c h lower temperature of 30 K. In addition, these structures had unacceptably high dark currents for device fabrication purposes. Other authors report very similar values for the OE and the dark current as we have for comparable structures, a6'17 SiGe/Si MQW samples have a lower doping level compared to SiGe HIPs therefore, in general, lower dark currents and higher detectivities are possible in these structures. A detectivity of D*> ~l()l(~cmv/Hz/W has been achieved for a ] 0 well OW sample. However. with an order of magnitude lower photoresponse compared to a standard HIP structure. In comparison to PtSi SiGe, HIPs do not show a higher OE between 2.5 and 3 ~ m (Figure 11.18(d)). However, by changing the well geometry and/or composition, one is able to change the cutoff wavelength and tune the m a x i m u m of the photoresponse curve over a rather broad wavelength range (2 ~tm,-~4 ~tm). Since the barrier height strongly determines the dark current at a fixed temperature (see Richardson equation (2)) SiGe HIP's with rather short cutoff wavelength have also lower dark currents and higher detectivities. Furthermore, there is strong evidence that in multi-well HIP samples with three wells, all wells contribute to the photocurrent. This means that by proper choice of the well geometry and separation, the carrier trapping during transport of the excited carriers to the contacts can be overcome, which gives some potential for further improvements of the SiGe HIP detectors. The SiGe material system is also unique, not only because of the already mentioned ability to realize near infrared as well as mid-infrared and longinfrared detectors in the same material system but also because of the favourable selection rules of the quantized states for normal incident radiation, which do not require expensive and complicated coupler gratings as in the case of A1GaAs/ GaAs OW infrared photoconductors (QWIP). This makes the SiGe intersubband detectors for the LWIR spectral range attractive. In addition the compatibility of SiGe detectors to the Si FPA fabrication process could - provided there are further improvements in the O E - make SiGe MWlR detectors an attractive alternative to the commercially available silicide on one side and the III-V OWIP detectors on the other side.

Acknowledgements I would like to acknowledge the work of m a n y of my colleagues, for XRD m e a s u r e m e n t s we are grateful to Mr. H.-J. Herzog, fo RBS m e a s u r e m e n t s to Mr.

446


K. Schmidt (from KFA Jiilich) and for the sample growth to Mr. Horst Kibbel. For reading the manuscript and numerous measurements we are especially indebted to Mr. Johannes Konle.

References 1. E. Kasper, H.-J. Herzog, H. Jorke and G. Abstreiter, Mat. Res. Syrup. Proc. 1 0 2 , 3 9 3 (1988). 2. H. Presting, T. Zinke, A. Splett, H. Kibbel and M. Jaros, Appl. Phys Lett. 69, 2376(1996). 3. T. Lin andJ. Maserijan, Appl. Phys. Lett. 57, 1422 (1990). 4. J. Uschmann, H. Presting, T. H. Kibbel, K. Thinke and R. Sauer, Thin Solid Films 2 9 4 , 3 4 0 - 3 4 2 (1997). 5. T. Fromherz, E. Koppensteiner, M. Helm. G. Bauer, J. F. Ntitzel and G. Abstreiter, Extended Abstracts of the International Cotfference on Solid State Devices and Materials, Makuhari (Japan), p. 41 ()-412 ( 1993 ). 6. A. Splett, T. Zinke, K. Petermann, E. Kasper, H. Kibbel, H.-J. Herzog and H. Presting, Photonics Technology Letters PTL-6, 42 5 ( ] 994). 7. J. Engvall, J. Olajos, H. Grimmeiss, H. Kibbel and E. Kasper, Appl. Phys. Lett. 63,491(1993). 8. B.Schtippert, J. Schmidtchen, A. Splett and K. Petermann, Integrated Optics in Silicon, Conference on Microsystem Technologies ed. by H. Reidel, p. 27B (1993)" B. Sch(ippert, J. Schmidtchen, A. Splett, U. Fischer, Th. Zinke, R. Moosburger and K. Petermann, Journal ofLightwave Technolog!t 14, 2 311 (1996)" see also R. A. Soref, Si based optoelectronics, Rev Article, Proc. IEEE 81, 1687 ( ] 993). 9. B. Jalali, S. Y e g n a n a r a y a n a n , T. Yoon. T. Yoshimoto, I. Rendina and F. Coppinger, Advances in Silicon-on-Insulator Optoelectronics, IEEE Jollrnal of Selected Topics in Quantum Electronics, Special Issue on Silicon-Based Optoelectronics, 4(6), 9 3 8 - 9 4 7 (December 1998 ). ] O. R. Breiter, W. Cabanski, R. Koch, W. Rode, J. Ziegler, K. Eberhardt and R. Oelmaier, SPIE Proceedings 3 3 7 9 , 423, (1998) and R. Breiter, W. Cabanski, K. H. Mauck, W. Rode, J. Ziegler, K. Eberhardt and R. Oelmaier, SPIE Proceedings 3 3 7 9 , 344 (1998). 11. S. D. Gunapala, T. N. Krabach, S. V. Bandara, J. K. Liu and M. Sundaram, SPIEProceedings 3 0 6 1 , 2 9 2 ( 1 9 9 7 ) a n d S. D. Gunapala, S. V.Bandara, J. K.Liu, W. Hong, M. Sundaram, R. Carralejo, C. A. Shott, P. D. Maker and R. E. Miller, SPIE Proceedings 3 0 6 l, 722 ( 1997). 12. H. Schneider and Eric Lerkins, Semicond. Sci. and Technol. 10 1329-1338 (1995). 13. H. Schneider, C. Mermelstein, R. Rehm, C. SchOnbein, A. Sa'ar and M. Walther, Phys. Rev. B57, 15()96 ( 1998 ). 14. T. L. Lin, T. George, E. W. Jones, A. Ksendzov and M. L. Huberman, Appl. Phys. Lett. 6 0 , 3 8 0 (1992).

hlfrared silicon/germanimndetectors 447 15. T. L. Lin, J. S. Park, S. Gunapala, E. W. Jones and H. M. Del Castillo, Opt. Eng. 3 3 , 7 1 6 (1994). 16. B.-Y. Tsaur, C. K. Chen and S. Marino, Opt. Eng. 33, 72 (1994). 17. R. Strong, R. Misra and D. W. Greve, ]. App1. Phys. 82, 5191 ( 199 7): and R. Strong andD. W. Greve, ibid 5199 (1997). 18. F. D. Shepherd, V. E. Vickers and A. C. Yang, Schottky barrier photodiode with a degenerate semiconductor active region, U.S. patent No. 3 603 84 7 filed Sept. 7,1971 19. S. Lury, A. Kastalsky and J. C. Bean, IEEE Transactions on Electron Devices 31,1135(1984). 20. E. Kasper, H. Kibbel and H. Presting, Thin Solid Fil,ls 183, 87 (1989). 21. H. Presting, H. Kibbel, M. Jaros, R. M. Turton, U. Menczigar, G. Abstreiter and H. G. Grimmeiss, Semicond Sci. atld Techllol. 7, 112 7 ( 1992 ). 22. E. A. Fitzgerald, Y.-H. Xie, M. L. Green, D. Brasin, A. R. Kortan, J. Michel, Y.-J. Mii and B. E. Weir, Appl. Phys. Lett. 59, 811 ( 1991 ). 23. G. Masini, L. Colace, G. Assanto, H.-C. Luan and L. Kimerling, IEEE Trans. on Electron Devices 48, 1092 (20() 1 ). 24. L. Colace, G. Masini, F. Galluzi, G. Assanto, G. Capellini, L. DiGaspare, E. Palange and F. Evangelisti, App1. Phys. Lett. 72, 3175 (1998). 25. G. Masini, V. Cencelli, L. Colace, F. De Notaristefani and G. Assanto, to be published App1. Phys. Lett. (2002). 26. M. Jaros, Semiconductors and Sel~lilnetals 32, 1 75 (1990). 2 7. M. Gail, G. Abstreiter, J. Olajos, J. Engvall, H. G. Grimmeiss, H. Kibbel and H. Presting, Appl. Phys. Lett. 66, 29 78 ( 1995 ). 28. U. Gnutzmann and K. Clausecker, Appl. Phys. 3, 9 ( 19 74). 29. V. d. Merwe, Surf. Sci. 31, 198 ( 19 72 ). 30. R. People and J. C. Bean, App1. Phys. Lett. 47, 322 (1985): Erratum: APL 4 9 , 2 2 9 (1986). 31. W. A. Brantley, J. Appl. Phys. 44, 534 ( 19 73 ). 32. K. B. Wong and M. Jaros, Semicond. Sci. and Technol. O, 790 ( 1991 ). 33. G. Abstreiter, K. Eberl, K. Friess, W. Wegscheider and R. Zachai, J. Cryst. Growth 9 5 , 4 3 2 (1989 ). 34. E. Kasper and K. W6rner, ]. Electrochem. Society 1 3 2 , 2 4 8 1 (1985). 35. H. Kibbel andE. Kasper, Vacll,m 4 1 , 9 2 9 (199()). 36. M. Copel, M. C. Reuter, M. Horn v. Hoegen and R. M. Tromp, Phys. Rev. B42, 1 ] 682 (1990): E. T. Croke, T. C. Mc.Gill. R. J. Hauenstein and R. H. Miles, Appl. Phys. Lett. 56, 367 ( 1989 ). 3 7. H. Presting and H. Kibbel, Thin Solid Fihns 222, 2 1 5 - 2 2 0 ( 1992 ). 38. H. Temkin, J. C. Bean, T. P. Pearsall, N. A. Olsson and D. V. Lang, AppI. Phys. Lett. 49, 155 (1986). 39. H. Temkin, T. P. Pearsall, J. C. Bean, R. A. Logan and S. Lury, App1. Phys. Lett. 48, 963 (1986). 40. H. Temkin, A. Antreasyan, N. A. ()lsson, T. P. Pearsall and J. C. Bean, Appl. Phys. Lett. 4 9 , 8 0 9 (1986 ). 41. F. Y. Huang, X. Zhu, M. O. Tanner and K. L. Wang, Appl. Phys. Lett. 67, 566(1995).

448 Handbookof Infrared Detection Technologies 42. F. Y. Huang and K. L. Wang, AppI. Phys. Lett. 69, 2 3 3 0 (1996). 43. T. Tashiro, T. Tatsumi, M. Sugiyama, T. Hashimoto and T. Morikawa, IEEE Transactions on Electron Devices 44. "545 ( 1997). 44. S. B. Samavedam, S. M. T. Currie, T. A. Langdo and E. A. Fitzgerald, Appl. Phys. Lett. 73, 2125 (1998). 45. X. Shao, S. L. Rommel, B. A. Orner, H. Feng, M. W. Dashiell, R. T. Troeger, J. Kolodzey, P. R. Berger and T. Laursen, Appl. Phys. Lett. Y2, 1860 (1998). 46. L. Colace, G. Masini and G. Assanto, IEEE I. Quantum Electronics 3 5, 1843 (1999). 4 7. F. D. Shepherd, Infrared Internal Emission Detectors. State of the Art, ed by W . H . Makky. Proc. SPIE 1 7 3 5 , 25()-261 (1992). 48. H. Presting and M. Jaros, SiGe IR detector with ballistic carrier injection, patent filed at the German patent office, Oct. 1998. 49. H. Presting, J. Uschmann and H. Kibbel, Thin Solid Films 3 2 1 , 1 8 6 - 1 9 5 (1998). 50. C. M. Parry, S. M. Newsstead, R. D. Barelow, P. Augustus, R. A. Kubiak, M. G. Dowsett, T. E. Whall and E. H. C. Parker, APL 58, 481 (1990). 51. R. T. Carline, D. J. Robbins, M. B. Stanaway, W. Y. Leong, App1. Phys. Lett. 68,544(1996). 52. R. Strong, D. W. Greve, R. Misra, M. Weeks and P. Pellegrini, Thin Solid Films 2 9 4 , 3 4 3 (1997). 53. E. Corbin, C. Williams, J. P. Hagon. M. Jaros and H. Presting, Thin Solid F i l m s 2 9 4 , 186 (1997). 54. E. Corbin, K. B. Wong and M. Jaros, Phys. Rex,. B 50, 2339 (1994). 55. M. J. Shaw and M. Jaros, Semicondllctors and Semi-metals 56, ] 69 (1999). 56. K. B. Wong, M. Jaros, I. Morrison and J. P. Hagon, Phys. Rex,. Lett. 60, 2221 (1988). 57. E. Corbin and M. Jaros, Semicond. Sci. and Technol. 1 2 , 1 6 4 1 ( 1997). 58. D. C. Herbert, Semicond. Sci. and Technol. 13, 1090 (1998). 59. M. J. Shaw, P. R. Briddon and M. Jaros, Phys. Rex,. B, 54, 16781 (1996). 60. M. Jaros and A. W. Beavis, App1. Ph!ls. Lett. 6 3 , 6 6 9 ( 1993).

Chapter 12

PolySiGe uncooled microbolometers for thermal IR detection Chris Van Hoof and Pier De Moor

12.1 Introduction This section introduces polySiGe bolometers and its relative place in the field of resistive bolometers. Even though other chapters in this book are devoted to microbolometers, the subject of microbolometers is very briefly discussed at the end of this section for reasons of portability and to ensure uniform terminology. 12.1.1 Uncooled resistive microbolometers

Uncooled thermal imagers using (mainly resistive) microbolometers have caused a boom in thermal imaging because of reduced sensor system cost. 1 The yearby-year improvement in noise-equivalent-temperature-difference (NETD) from approximately lOOmK in 19932 to 18mK in 2001 for imagers with 50 gm• 50 ~tm pixels and the simultaneous improvement in higher-resolution imagers (also using 25 l~m• ~m pixels) has increased their competitiveness in high-end application areas such as military and medical. The first decade of microbolometer development focused on vanadium oxidebased resistive microbolometers, following the Honeywell reports and patents. 2 The potential of this material system and the ensuing concentrated research was largely reponsible for the observed improvements in the technology. In spite of the superlative uncooled performance of VOx microbolometer arrays, uniform sputter deposition of low-noise VOx material on foundry CMOS wafers proved a challenge and the optimum deposition process remains difficult (e.g. due to the tight oxygen content control needed). Especially when low-cost thermal imaging for, e.g., automotive night vision became a possible

450 Handbookof Infrared Detection Technologies mass-market application, a far lower-cost solution was needed than could be achieved using Vox microbolometers. This lead to a cost versus performance trade-off which had to be made for all parts of the system and mainly concerned the sensor array, the optics and the packaging. In the latter field it has lead to the development of zero-level v a c u u m packaging R&D and wafer-scale packaging, 3 and in the former to low-cost molded optics development. To reduce the sensor die cost (in essence a yield factor), it was necessary to re-evaluate the alternative bolometer schemes as lower-cost candidates. Many materials have served as resistive elements in microbolometers, metals (e.g. Ti4), crystalline material (pn diodes in SIS), polycrystalline material, 6 amorphous material (a-Silicon 7). However, low-cost thermal sensing dies imply, not only adequate sensor material, but also surface-micromachining compatibility and either CMOS back-end compatibility or direct front-end integration. The latter category covers bulk-micromachined crystalline Si, Si pn diode, polySiGe, and the former category concerns a-Si and Ti microbolometers. At present, the quest for low-cost uncooled imaging has not been concluded and several options which all share CMOS process compatibility are being developed in parallel: on one hand Ti-metal bolometers and on the other hand amorphous or polycrystalline Si or SiGe alloys. The choice for either system is not an easy one since both have advantages and drawbacks. Whereas metal microbolometers feature low TCR (0.1-0.2% 4) this is compensated by the low noise in these metals and the very high wafers-scale uniformity. In addition, thermal isolation of metal microbolometers is generally worse t h a n that of their polycrystalline or amorphous counterparts and the lowest NETDs reported for metal bolometers are 2 ()0 mK. The higher TCR (2-5 %) in polycrystalline or amorphous bolometers is accompanied by increased noise, but the thermal isolation levels achieved in these microbolometers are superior (e.g. below 8E-8 W/K thermal conductance) leading to reported NETDs below 60 mK for a-Si. The fact that both systems are easier to fabricate than Vanadium oxide and are more reliable materials, compensates for their higher NETD compared to Vanadium oxide. PolySiGe was indicated as a promising material system because it features low thermal conductance, high TCR and moderate noise levels combined with low stress, suitable for surface micromachining. Furthermore, it is a CMOS frontend material used in BiCMOS processes and its process modules are available in the IMEC pilot line. Therefore, compared to nearly all microbolometer processes, deep-submicron lithography is used which (among other outcomes) leads to superlative uniformity in direct material parameters (TCR, noise) and indirect process-related properties (thermal isolation)over entire 8" wafers. This chapter is devoted to a discussion of the merits and shortfalls as well as the present state-of-the-art of polycrystalline SiGe microbolometers and is organized as follows: In Section 12.2, the different deposition techniques and mechanical properties of polySiGe are presented. Section 12.3 presents polySiGe bolometer pixels and their electro-optical properties (such as TCR, noise, thermal isolation,

PolySiGe uncooled microbolometers for thermal IR detection

451

uniformity .... ) Section 12.4 concerns array development, readout requirements and system development. Section 12.5 discusses zero-level v a c u u m packaging development of polySiGe microbolometer arrays. Section 12.6 concludes and provides an outlook for the future. 12.1.2 M i c r o b o l o m e t e r t e r m i n o l o g y

A bolometer is a thermal infrared detector. In order to obtain significant heating it must be thermally insulated from its heat sink (typically the substrate) by a support having a low thermal conductance G. The active element in a bolometer is typically a resistor with a large temperature coefficient of the resistance (TCR). As a consequence, a significant change in resistance occurs when the detector is heated by incident infrared radiation. A basic thermal block diagram of such bolometer is shown in Figure 12.1. The effect of varying incident thermal radiation creates a temporary thermal imbalance since the pixel absorbs the radiation and warms up faster than the heat sink. The heat capacity C of the pixel determines the response rate and the thermal balance is described by dT

ot P - C-d-i+ G ( T - To)

(1)

where P is the incident infrared power, :x the absorption coefficient and To is the heat-sink temperature. As a consequence, in steady-state, the temperature increase of the bolometer is proportional to absorbed power and inversely proportional to the thermal conductivity: high thermal isolation leads to more heating. By solving the above differential equation, the thermal time constant for reaching the new thermal equilibrium is given by r = C / G . It is clear that the signal is m a x i m u m when pixel heating is m a x i m u m and this is achieved by optimizing the absorption efficiency and decreasing the thermal conductance G.

Figure 12.1 Thermal block diagram of a microbolometer listing the key parts and the physical quantities that govern the microbolometer behavior: thermal capacitance, thermal conductance, incident power, temperature of pixel and heat sink.

452


Given a constant temperature rise, the higher the TCR of the pixel material, the higher the signal will be. For a practical implementation, it is only the thermal isolation requirement (G) which sets microbolometers apart from other temperature sensors which need to measure the surrounding temperature rather than incident radiation. Thermal isolation can be obtained by removing most of the underlying material as is done in bulk silicon micromachining. A superior technique used in nearly all microbolometer implementations involves surface micromachining: a sacrificial layer is removed in the last process step. thereby creating a suspended structure. A typical microbolometer is shown in Figure 12.2. 12.1.3 M i c r o b o l o m e t e r

process options

Two distinct types of microbolometers exist (Figure 12.3): either self-suspended microbolometers or bolometers supported by an insulating membrane. In the self-suspended microbolometer system, the resistive sensor provides at the same time structural support and electrical contact. Although this puts a requirement of low-stress on the material, it generally allows to maximize the thermal isolation of the pixels. In the second approach, the microbolometer material still has to fulfill certain mechanical requirements, but the support is created by a membrane (e.g. silicon nitride) which is typically below the sensor, but can also be above the bolometer pixel material. Because electrical contacts to the pixel are

Figure 12.2 (a) Translation of the basic bolometer diagram of Figure 12.1 into a practical scheme indicating the different parts. (b) SEM photograph of a :,012 • :~0 12m pol!lSiGe microbolometer suspended b!! two approximately 50 12m long legs.

Figure 12.3 Two types of microbolometers: microbolometers ~vith separate membrane for suspension and self-suspended microbolometers. The bolometers supported b!l a membrane typicall!t have separate sensor and interconnect materials (left). ~vhile the electrical and mechanical fi~nctions are combined in one material in the self-suspended bolometer (right).

Pol!lSiGe uncooled microbolonwters for thermal IR detection

4 53

required, the thermal isolation of such microbolometers is generally worse, but it is the only way to make, for example, YBaCuO-based microbolometers, since this material is not suited as structural material for micromachining. Ti-metal, a-Si and polySiGe bolometers are examples of self-suspended microbolometers, whereas all other schemes use a separate electrical interconnect and m e m b r a n e suspension.

12.2 Structural, thermal and electrical properties of polySiGe This section discusses the mechanical, thermal, and electrical properties of polycrystalline silicon-germanium thin films in relation to the deposition conditions. 8-1~ SiGe is well established as a front-end material as high-speed HBTs use SiGe epitaxial base layers and polySiGe base electrodes within a modified BiCMOS front-end process. The high deposition temperature is perfectly compatible with the front-end CMOS but presents a formidable challenge for the MEMS sensor processing as this type of processing ideally takes place after the full CMOS process and therefore requires lower deposition temperatures. Low-temperature deposition is discussed further in this section. For application of polySiGe in microbolometers, the Ge content is increased with respect to that of SiGe HBT (i.e. from 10% to 30%) which decreases the thermal c o n d u c t i v i t y of the material.

12.2.1 Deposition of polySiGe Polycrystalline silicon-germanium alloys are typically grown in a chemical vapor deposition reactor. The reactor used at IMEC is an ASM Epsilon I consisting of a horizontal, lamp-heated quartz chamber with a SiC coated graphite susceptor. Layers were initially grown on 6" silicon wafers and since 2()()() on 8" silicon wafers, covered where needed by 2 ~m of sacrificial TEOS. Samples can be grown at atmospheric pressure (APt or at a reduced pressure (RP) of 40 Torr. The deposition temperature is around 650 ~ For the deposition a mixture of germane and dichlorosilane is used and the proportion of the gases is adjusted to give a thin film containing 30% of germanium. The growth rate in such conditions amounts to O. 5 nm/s. As practical bolometers are between 1 O0 and 250 nm thick, the deposition time is limited to an acceptable 3-9 minutes. Thicker bolometers have also been made (up to 1 ~m). The nucleation of polySiGe on top of the sacrificial oxide layer and using the above gases is very slow. To improve the nucleation, a thin polySi nucleation layer is used which is approximately 10 nm thick and grown at 6()() ~ Our initial work used boron implantation to achieve the desired electrical resistivity of the polySiGe layers and of the supporting legs. After ion implantation (3x 1() 1~ B/cm 2 for the pixels and 1() l~' B/cm e for the legs), a hightemperature anneal was done (at approximately 85()~ The low-dose implantation was later replaced by in situ doping which provides more constant doping concentration in the pixel and does not require a high-temperature

454


anneal step. The low leg resistance is still achieved by high-dose implant followed by anneal at 650 ~ 12.2.2

Structural properties

As data ofpolySiGe are limited, it is instructive to compare Si and Ge. The melting point of Si is 1415 ~ and that ofGe 9 3 7 ~ The density of Si is 2.32 g/cc and that of Ge 5.33 g/cc. The modulus of Young of polySi is 173 GPa and that of polyGe is 132 GPa (each about 10% lower than their monocrystalline counterparts). The fracture strength of polySi is 2.6 GPa and that of polyGe is 2.2 GPa. Combining these data gives a rough indication that polySiGe requires lower temperature processing than poly Si and may have adequate mechanical properties. For a material to be used as structural MEMS material, the stress in the layers is of prime importance. The stress of the layers can be measured prior to processing by analyzing wafer curvature. The resulting stress in layers grown by RP or AP CVD is shown in Figure 12.4 as a function of subsequent annealing temperature. The stress in AP CVD polySiGe layers is typically compressive but becomes nearly zero for annealing temperatures around 90() ~ The stress in RP CVD polySiGe is tensile and almost independent of annealing. By varying the pressure, the stress in the polySiGe can be further tuned. Although low stress is important, its importance for microbolometer application should not be exaggerated, as the polySiGe is just one of the materials in the pixel stack (microbolometers also feature isolator and absorber material). As a consequence, the residual stress or stress gradient in the multilayer stack is critical. It is, however, possible to compensate stress and stress gradients in multilayer stacks and this is demonstrated in Section 12.3.

Figure 12.4 Stress of different materials measured as a function of annealing temperature. Low stress is obtained particularly in RPCVD pol!tSiGe. This method was used for all actual device work and deposition took place at 62 5-6 50 ~

Pol!lSi(;e uncooled microbohmleters for thermal IR detection

455

12.2.3 Thermal properties Compared to metals and monocrystalline silicon, polycrystalline material shows much lower thermal conductivity #. Whereas aluminum and monocrystalline silicon have a thermal conductivity of approximately 200 W/mK and 1 O0 W/mK, that of polycrystalline silicon is 18 W/mK (due to phonon scattering at the grain boundaries). By adding germanium, phonon scattering at point defects further reduces the thermal conductivity, and polySiGe with 30% Ge achieves a thermal conductivity of 3 W/mK (i.e. 2 5 times less than monocrystalline silicon and more than 50 times lower than aluminum). It will be shown in the next section that polySiGe shows similar or slightly superior electrical performance over polySi, so the key advantage of higher thermal isolation remains. It is instructive to calculate the thermal conductance for a typical poly SiGe pixel. Assuming a beam section of 1 square micron, and a beam length of 50 micron, the thermal leak through (typically) two beams equals 1.2 x 1 ()-7 W/K. When reducing the poly SiGe thickness, beam sections below 1 square micron are possible, resulting in a thermal conductance close to the radiation limit (the thermal leak through radiation). 12.2.4 Electrical properties

As polySiGe is to be used as a resistive element, the interrelation between resistivity, temperature coefficient of the resistance (TCR) and noise is essential. Data have been collected for ion-implanted AP CVD and for P,PCVD polySiGe. The electrical properties of implanted or in s i t u doped polySiGe are found to be identical. The 500 nm thick samples were implanted at a fixed energy of 7:3 KeV by a boron doping dose varying from 1()11 boron/cm 2 to 101 ~ boron/cm 2 and annealed at 850 ~ for one hour. Sheet resistance and temperature coefficient of the resistance have been measured by monitoring the current as a function of temperature at fixed bias. Care has been taken to avoid contact resistance effects on resistivity determination. Figure 12.5 shows the resistivity of APCVD polySiGe layers and the temperature coefficient of the resistance (TCR) versus implantation dose. It is possible to achieve 4% TCR in highly resistive layers, but high resistivity leads to high ]/f noise. The figure shows that a practical 1% TCR is achieved for approximately 1 f~cm resistivity. A similar result is obtained in RPCVD polySiGe. This general trend of resistivity and TCR versus implantation dose is similar to the one observed in poly Si: one can distinguish a low doping region where the sheet resistance is doping independent and a region where sheet resistance sharply decreases with the increase of doping. We note that the sheet resistance starts to decrease steeply when the doping exceeds 1 ()1~ boron/cm 2. which corresponds to an average doping density of about 2 x 1 ()~ 7 boron/cm 3. Since this is similar to observations in polySi, this indicates that grain size and grain defect density are similar in the two materials. The comparative values of TCR and resistivity of polySiGe and polySi are summarized in Table 12.1 where polySi was annealed for one hour at 850 ~ and

456


Figure 12.5 Dependence of APCVD polySiGe resistivit!l ( II ) and of TCR ( 9 ) on boron implantation dose. For low-dose implant, the TCR is as high as 4% but practical (lower noise) la!ters fi'ature approximatel!l 1% TCR and 1 f2cm resistivity.

T a b l e 12.1 D e p e n d e n c e of sheet resistance and TCR in AP poly SiGe and AP poly Si o n doping doses. The electrical properties are s i m i l a r . Doping Dose

1.5 x 1019 3 x 10 ~~ 9 x 1013

Sheet resistance

TCR (%)

AP poly SiGe

AP poly Si

AP poly SiGe

AP poly Si

3 5 0 KF2/[-] 173 K~2//-/ 19.5 KF2/[-1

4.3 Mr2/[-] 58 5 K f 2 / ~ 2 4 . 7 Kf2/[--1

-2.54% -2.1% -().96%

- 3.29% -2.2% - 1.1%

polySiGe at 650~ The table also demonstrates that for the same doping dose, poly SiGe has lower noise since 1/f noise scales with resistivity. Further evidence is obtained from noise measurements on these layers. Figure 12.6(a) compares noise spectra ofpolySi and polySiGe for 1 ~tm thick samples. At low frequencies, the noise spectrum is dominated by 1/f noise as can be expected in polycrystalline (or amorphous) materials. However. the 1/f noise in polySiGe is a factor of two lower than that of polySi. Figure 12.6(b) compares layers of polySiGe with different resistivities and it is indeed observed that the 1/f noise scales with the square-root of the resistivity. As both the bolometer signal (given by TCR which depends on resistivity) and bolometer noise (dominated by 1/f noise which depends on resistivity) are resistivity-dependent, an optimum signal-to-noise ratio has be found which is a compromise between noise and TCR. However, the problem is more complex than that as noise of the readout circuitry has to be included. This figure depends on the readout architecture and this in turn is affected by the absolute resistance of the bolometers which has to be read out by the circuit. In practice, it will be necessary for polySiGe bolometers to achieve read out noise below or equal to the

Pol!lSiGe uncooled microbolometers for thermal IR detection

4 57

$ 9

0%% ee e 9

0,5

0,1

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~

1

~

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. . . . .

1

S

20 Frequency (Hz) (a)

200

0.01 1 Frequency(KHz)

100

(b)

Figure 12.6 (a) Noise spectra for 50 l~m• ~0 l~m. 1 I~m thick pol!tSiGe samples: O A P poly Si. 9 AP poly SiGe. The 1/f noise of the pol!lSiGe la!ters is approximatel!l half that of the pol!!Si. (b) Noise spectrum of polySiGe layers with varying resistivit!l. B!I changing the resistivit!l a fi~ctor of 10. the 1If noise scales with the square-root of the resistivitz3.

bolometer 1/f noise. This contrasts with metal bolometers where the readout noise is dominant.

12.2.5 High-temperature versus low-temperature polySiGe The deposition conditions described in Section 12.2.1 are in line with a CMOS front-end but clearly conflict with CMOS post-processing. Several groups have demonstrated low-temperature polySiGe deposition for MEMS applications. 11 Thin films of polySiGe were deposited in an Oxford Plasma Technology reactor at 2 Torr which allows to achieve sufficiently high growth/deposition rates in spite of the reduced substrate temperature. As modern CMOS back-end processes can withstand temperatures of 450 ~ ~ for an extended period of time, 12 the deposition temperature was between 4 5 0 ~ and :360 ~ and the deposition time between 20 minutes and one hour. Practical layers were those deposited at 520 ~ which featured 0.8 f2cm resistivity and a stress below 50 MPa. These films have higher Ge content (50%) than their high-temperature counterparts. 13

12.3 PolySiGe bolometer pixels 12.3.1

Process development

The polySiGe bolometer processing is schematically shown in Figure 12.7 and is run in an 8" CMOS pilot line. Although a typical bolometer process encompasses between 50 and 100 lot turns, the process can be summarized as follows: First, a sacrificial silicon oxide (i.e. TEOS) layer is deposited and patterned. Then, polySiGe is deposited on the wafer followed by silicon nitride isolator deposition. In a third stage, the polySiGe/silicon nitride stack are each patterned

458


Figure 12.7 Schematic process flow of pol#SiGe microbolometers. 1) TEOS deposition,patterning and etching, 2) deposition, patterning and etching of pol!tSiGe SiN. 3 ) Metal contact and absorber definition.

and the bolometer legs undergo high-dose implant. Then, metal contacts are deposited and patterned and the IR absorber is deposited on top of the pixel. Finally, the sacrificial TEOS layer is removed (either in a HF solution or in vapor HF, see further). The SEM photograph of a single-pixel microbolometer can be seen on Figure 12.2 (Section 12.1.2). While the initial process and material development was carried out on 6" wafers, 90% of the current microbolometer process is done in our (sub-) 100 nm CMOS pilot line on 8" Si wafers. Therefore, we benefit from sub-micron stepper lithography and advanced deposition/etching technology resulting in excellent uniformity of the material. 14 In a typical design, each 8" wafer contains 1300 arrays with more than 80 000 pixels in total. Roughly half of the pixels are being probed using an automatic probe station. First, the resistance variation was studied. On full 8" wafers, no single pixel having a resistance out of a +2% range around average was found. A +1% resistance uniformity requirement results still in a yield of 99.9%. The typical resistance uniformity over a 128 element linear array is shown in Figure 12.8. We measured a 0.4% peak-to-peak and +().1% standard deviation nonuniformity. However, these values include the m e a s u r e m e n t uncertainty. This excellent resistance uniformity is the result of both a very good poly SiGe deposition control and an accurate lithography and etching. This is only possible thanks to the advanced equipment of the 8" IMEC CMOS pilot line. The high level of uniformity allows longer integration time and significantly simplifies the offset and gain correction problems in a current integration based read-out scheme.

Pol!tSiGe uncooled microbolometers for thermal IR detection

459

Figure 12.8 Typical resistance measurements of a linear array of 128 detectors: (a) the discrete steps in the measured values are due to digitizing of the measured voltage. ( b ) histogram of the measured resistance.

Using m e a s u r e m e n t s of resistance at 2() and 40 ~ the TCR of the device can be calculated. Its absolute value being around 1% (depending on the doping), the uniformity over an 8 " wafer is measured to be only a few percent. This includes, however, a possible error due to the temperature stability of about O. 5 %. Last but not least, the noise uniformity over a 128 element linear array was measured. Figure 12.9 displays the 1/f noise at 1 Hz for a typical device. The noise uniformity over the array is about 6% for the lffnoise. The Johnson noise contribution is about 2 orders of magnitude lower at 1 Hz, and moreover its variation is proportional to the excellent resistance uniformity. 12.3.2 Absorber comparison and trade-offs

It is possible to rely on polySiGe absorption without the need of an additional absorber but this compromises absorption and hence signal, in the case of thin polySiGe films. As a consequence, nearly all bolometer developments include suitable absorber layers. A practical approach consists of a thin metal absorber with sufficiently high sheet resistance. ~s Although almost any highly resistive metal could be used (and NiCr resulted in the observed absorption spectrum), the CMOS front-end compatibility limits the options to materials such as TiN and TaN. Even though the thin absorber does not absorb all radiation, part of the remainder is reflected by the silicon substrate and hence an unoptimized quarterwavelength stack is created consisting of absorber, the a i r / v a c u u m gap under the pixel, and the silicon substrate. For this reason, the thickness of the sacrificial layer should indeed be a quarter wavelength of the radiation to be detected ( 8 - 1 2 l.tm): as a consequence, the TEOS absorber is between 2 ~m and 2.5 ~m thick. This results in approximately 60% absorption over the 8-121.tm wavelength range (Figure 12.10(a)). If the application permits, a superior absorber can be constructed by means of a quarter-wave stack on top of the bolometer pixel. Such quarter-wave


Figure 12.9 Typical uniformity of the 1ff noise at 1 Hz for 128 detectors. The non-uniformit!l is about 6%.

Figure 12.10 (a) Schematic implementation and absorbance spectrum (b) for a thin metal fihn based absorber, and the implementation (c) of a quarter wavelength absorber and its associated absorption spectrum (d). Thanks to the vacuum gap which creates an unoptimized quarter-wavelength stack, the thin-metal absorber achieves high absorption while maintaining low thermal capacitance.

absorbers have resulted in nearly 100% peak absorption although they come with a penalty of increased thermal mass and therefore increased time constant (Figure 12.10(c.d)). 12.3.3 Pixel optimization

Optimizing the pixel response starts simply with the basic requirement that the pixel has to be operated in vacuum. This is the only way to minimize conductive heat loss through the air surrounding the device. If conductive heat loss is further mimimized by using long narrow pixel legs, the pixel temperature and hence the pixel resistance will change as much as possible during IR irradiation.

Pol#SiGe uncooh'd microbolometers for thermal IR detection

461

Figure 12.11 shows the thermal conductance G of a polySiGe pixel (60 ~ m • ~m side, 1 ~m thick, two 30 l~m long legs with 0.8 l~m2 cross-section) as a function of ambient pressure surrounding the pixel. The thermal isolation increases over a factor of 200 between atmospheric pressure and 1 0 - 3 mbar. 1~ The polySiGe bolometer of Figure 12.11 shows a thermal conductance G of 2 x 10 -7 W/K. During the course of the polySiGe development (see further), this value was further improved to below 5x1() -~ W/K. Figure 12.11 also demonstrates the use of bolometer as vacuum gauge (micropirani) as over four orders of magnitude of pressure difference can be resolved. The complexities of v a c u u m packaging of MEMS devices in general, and microbolometers in particular, are discussed in detail in Section 12.5. Optimization of microbolometers for imaging purposes results in conflicting requirements: in order to increase the sensitivity, the thermal conductance G should be minimized. However, for a given device volume, this results in a larger time constant. As a consequence, the pixel response becomes too slow for the imaging frame rate (i.e. typically 30 Hz or higher). Therefore the only practical solution to this problem is to reduce the device thickness. Changing the pixel size is not an option since this is governed by the required temperature resolution, spatial resolution and focal-plane (i.e. optics) size. Figure 12.12 shows the microbolometer pixel heating as a function of time during pulsed excitation. As the TCR is negative, the voltage signal decreases over time with an exponential time constant. For a 1 lam thick polySiGe pixel having a G of 1 • 10 -7 W/K the thermal time constant is approximately 40 ms whereas for a 0.2512m thin polySiGe pixel with similar thermal isolation the time constant is 10 ms.

Figure 12.11 Thermal conductance G of a microbolometer as a h~nction of the ambient pressure.


Figure 12.12 Pixel response during pulsed biasing of a 1 I~m thick bolometer showing a time constant of 40 ms, and a 0.25 ~m thick bolometer showing a time constant of 10 ms. Because the TCR of polySiGe is negative, the resistance decreases and hence the voltage signal decreases as a flmction of time.

Reducing the thickness of the bolometers results in a large decrease in mechanical stiffness of the pixel. The pixel deflection is inversely proportional to its m o m e n t of inertia which in turn is proportional to the cube of the pixel thickness. The stiffness, however, is critical for the release process (i.e. the etching of the sacrificial layer) and for the presence of strain and strain gradients in the pixel material. As a consequence, it is virtually impossible to realize extremely thin devices. A pragmatic solution to the release process is presented in Section 12.3.4 and a fundamental solution to overcome the loss of stiffness is discussed in Section 12.3.5.

12.3.4 Vapor-HF processing Many surface micromachined devices use a silicon oxide as sacrificial layer and this is also the case in the polySiGe bolometer process. In the last process step, this sacrificial layer is removed by etching. This is undoubtedly the most critical step in a surface-micromachining process, as shown in progress in Figure 12.13. A conventional technique consists of etching in an HF solution followed by rinsing or, better, freeze-drying to avoid condensation. Condensation has proven detrimental as it leads to sticktion: the surface tension forces in liquid droplets pull down thin microbolometers and once bonded to the substrate, the long beams do not have sufficient stiffness to restore the pixel. In addition to the condensation and sticktion, wet HF attacks a l u m i n u m interconnects and bond pads. The problem of sticktion during wet release has been solved by the development of a completely dry process consisting of vapor-HF performed at elevated temperatures. 17 The process tool is a modified Gemetec PAD-Fume vapor HF wafer cleaning system with heated wafer chuck. It has proven to be

PolySiGe uncooled microbolometers for thermal IR detection

463

Figure 12.13 Subsequent steps in the release etch of the sacr(ficial oxide (black) underneath the pol!lSiGe pixel. The gradual etch of the sacrificial oxide is clearl!i seen as well as absence qf residue due to the clean vapor HF process.

very efficient in releasing extremely thin microbolometers with nearly unity yield. The chemical reaction shows that water is produced in any case: Si02 + 4 HF ---+ SiF4 + 2 H20

(2)

However, by heating the wafer to 30 ~ ~ the condensation can be avoided. Pixels with a thickness of only 160 nm could be successfully released as is shown in Figure 12.14. The curvature of the device is due to the combination of an extremely low m o m e n t of inertia and the presence of a strain gradient in the pixel. Apart from the pixel yield, the vapor HF process is very clean as can be seen from the SEM photographs. An added advantage of vapor-HF processing is that other device materials such as a l u m i n u m and TaN are hardly attacked during the release etch. 12.3.5 Stiffness e n h a n c e m e n t

techniques

Theory

A fundamental problem of extremely thin device layers, both for microbolometers and for surface-micromachined MEMS in general, is their behavior as a result of strain and strain gradients. Particularly since a device may consist of a stack of materials. The very low m o m e n t of inertia of the microbolometers suspended on long legs makes them prone to curvature. The ensuing touching of the bolometers to the substrate reduces the thermal isolation and renders the pixel useless. This is demonstrated in Figure 12.15 below where a linear array of fully-processed 0.2 5 lam polySiGe microbolometers is shown. The compressive stress gradient between absorber/isolator and pixel causes the observed buckling and touching of pixel to substrate. There are two solutions to such a problem. The first is pragmatic and consists of minimizing stress in the materials or compensating stresses in the material stack. The second, and by far superior, solution is to enhance the m o m e n t of

464 Handbook of Infrared Detection Technolo#ies

Figure 12.14 160 nm thin poI!tSiGe microbolomeler reh'ased using vapor-HF at elevated temperatures.

Figure 12.1 5 Detail of a linear arra!t of full!! processed 0.2 ~, 12m thin microbolonwters. The stress gradient in the material stack causes the devices to touch the substra~e. This stress #radient is due to the three materials used (polySiGe, SiN, TAN).

inertia of a pixel by design and therefore make the pixel less sensitive to stress gradients. Typically, a combination of both techniques will lead to the most robust MEMS devices. Let us briefly consider the pragmatic solution of minimizing stress and stress gradients. The situation encountered in Figure 12.15 stems from a stress of roughly 60 MPa in the polySiGe and of the order of 1 GPa in both the TaN absorber and the SiN isolator, although with opposite sign. By modifying the TaN deposition parameters, the stress in the TaN could almost completely compensate for that in the SiN layer and the pixels remain suspended. The second and more generally applicable solution is to overcome the bending of the pixels altogether by increasing structural stiffness by increasing the moment of inertia of legs (and also of the pixel}, is Similarly to civil engineering, this can be accomplished by using the well-known I-profile (see Table 12.2) which has the highest moment of inertia for a given beam cross-section. The

PolySiGe uncooledmicrobolometersfor thermal IR detection 465 Table 12.2 files

S c h e m a t i c b e a m s e c t i o n s a n d t h e i r m o m e n t o f i n e r t i a for d i f f e r e n t b e a m pro-

reader should briefly dwell on the complexities of realizing such an I-profile in a surface micromachining based MEMS process. As this is indeed most impractical, another type of profile should be sought which combines easy manufacturability with excellent stiffness and low thermal conductivity. This is found to be the Uprofile (see Table 12.2). The m o m e n t of inertia of a U-profile is identical to that of the optimal I-profile (for the same section). More importantly, it can be processed in a very simple way which is by incomplete removal of the sacrificial layer in a second patterning step and timed etch. This is shown in Figure 12.16. This extra etch step defines the lower part of the U-profile. Thanks to its excellent step coverage, subsequent polySiGe deposition creates a U-shape in this narrow trench. A polySiGe etch will, in a following step, define the width of the U-profiled legs. The quantitative stiffness improvement can be calculated. Comparing the Uprofile to a rectangular profile with thickness H and width B, the stiffness reduction R 1 of a rectangular profile is given by:

Ij

BH 3 - b h 3

b(h) =

B

3 (3/

The reduction of the beam section R 2 is R2 .

Si B H - bh . . . . Sr BH

1

b 1l BH

(4)

Since the goal is to have m a x i m u m thermal insulation, hence a minimal section, in combination with a m a x i m u m stiffness, the ratio M of m o m e n t of inertia I over section S should be maximized. W h e n comparing regular rectangular legs to U profiles, the figure of merit m is given by

m -

(S/


Figure 12.16 Schematic process flow for U-profile fabrication: patterning and partial etch of the sacrificial layer (top), conformal polySiGe deposition (middle). patterning and etching of the pol~3SiGe(bottom).

This figure of merit is plotted in Figure 12.17. It compares the I/S for regular beam with that for a U-profile beam as a function of layer thickness t. The different curves reflect different beam width B and beam height H. For all curves, the increase in figure of merit becomes more pronounced for thinner layers, which is relevant since m a x i m u m thermal isolation is obtained for thinnest layers. For rather thick devices (0.5 lum), the figure of merit is roughly 10 whereas for l O O n m (50 nm) thin layers, the figure of merit is 200 (1000). This implies a 200-fold (respectively ]()()()-fold) reduced deflection of the beams for a given force on the beam.

Implementation

We have experimentally verified the stiffness e n h a n c e m e n t on l O O n m thin cantilever beams (Figure 12.18) where standard cantilevers and U-profile cantilevers with increasing length alternated. Whereas the regular cantilevers progressively curved up due to a stress gradient, the U-profile cantilevers remain straight. Let us now consider a practical example such as a 501amx5Olam microbolometer. Such a bolometer featuring classical rectangular beams of width 1 lam and device layer thickness 0.2 lam has a very low beam section of 0.2 l.tm2, resulting in high thermal insulation. If we construct a 50 nm thin Uprofile of the same width and a 2 nd sacrificial layer etch depth of 1 lam, we obtain an identical beam section of 0.2 l.tm2. However, due to the increased m o m e n t of inertia, the deflection for a given force is reduced by a factor of 50. Yet another advantage becomes clear. The time constant of the bolometer is reduced by a factor of 4 (from 33 ms to 8 ms) since the pixel thickness is reduced by that factor. The reduced time constant is very important for 2D imaging applications. In conclusion, the use of U-profiles not only enhances the mechanical stiffness of the devices, but also considerably reduces the thermal time constant. SEM photographs of several polySiGe pixels and details of the profiles are shown in Figures 12.19 and 12.20.

Pol!lSi(;e uncooled microbolometers fi~r thermal IR detection

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10000

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Figure 12.17 Gain in st(fl)wss over section, plotted a.s.ligure of merit m vs. the la!ler thickness.

Figure 12.18 1 O0 nm thin cantilevers of increasing length, alternativel!l with and without U-profiles.

Figure 12.19 Detailed SEM photograph of a U-proJile beam with the following dimensions: length 7 ~ I~m. top width: 1.2 #m. depth" 1 I~m.

468

Handbook of Infrared Detection Technolo~jies

Figure 12.20 SEM photograph of a linear arra!t leaturiplg pixels with d(flerent U-lwofih' pattern. From upper left to lower right: no U-profih, s. onl!l U-prqfih's in the beams. U-profih's in both beams and pixel.

Table 12.3 C o m p a r i s o n of different p a r a m e t e r s for a r e c t a n g u l a r a n d a U-profile b e a m w i t h identical section

I

Beam shape"

~'---~"~*

Section"

G(W/K) Deflection (a.u.): Timeconst.=-

C G

~ t

II.u5.mn

ii ,~m

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(). 2 l.lm 2 3• s

1

().()2= 1 / 5()

3) ms

8 ms=l/4

Figure 12.20 shows four neighboring 13() nm thin polySiGe bolometer pixels. The one on the left features no U-profiles and is heavily curved. The second one features U-profile legs which effectively straighten the legs but the unsupported parts of the pixel still curve. The other two feature U-profiles in both the legs and in the pixel and it is obvious that the added stiffness keeps the entire bolometer perfectly suspended. One could argue that pixels that curve upwards are not dead since they do not touch the heatsink. However. the bending changes the optical properties (projected cross-section, quarter-wave effect with silicon surface) and should therefore be avoided.

12.4 Readout and system development 12.4.1 Introduction

Due to the high-temperature processing of polySiGe, the IMEC polySiGe bolometers are not monolithic but hybrid arrays since postprocessing on backend CMOS cannot withstand the deposition temperature of polySiGe nor that of the sacrificial layer.

PolJISi(;e mlcooled microholometers Jbr thermal IR detection

469

As a c o n s e q u e n c e , their m a i n relevance is for linear arrays or small 2D arrays w h e r e the bolometer matrix is wired to b o n d p a d s w h i c h are read out by a dedicated ASIC (see Figure 12.21). This allows re-usability of an N - c h a n n e l r e a d o u t for a variety of 1D or small 2D arrays, w h i c h t h e n need a limited n u m b e r of CMOS lot t u r n s to produce. F u r t h e r m o r e . the r e a d o u t can be designed in a less expensive CMOS front-end process (e.g. (). 713m or (). 5 lJm). 12.4.2 Readout of polySiGe microbolometer arrays

The effect of the absorbed IR radiation is a resistance c h a n g e of a microbolometer. This c h a n g e must be detected from the r e a d o u t circuitry and in the case of a m i c r o b o l o m e t e r a r r a y the produced signal m u s t be multiplexed to a c o m m o n output. Various r e a d o u t schemes h a v e appeared in the literature as the i n t e g r a t i o n of the c u r r e n t flowing t h r o u g h the microbolometer, 1~ the m o d u l a t i o n of the phase of an RC-oscillator due to the resistance c h a n g e 2~ and a copper stabilization t e c h n i q u e for noise reduction. 21 Implementation o[ a direct current readout In this section a new readout scheme is presented that provides a pulsed current bias, measures and amplifies the voltage drop across each microbolometer and multiplexes this signal to a c o m m o n o u t p u t . 22'2~ With this scheme and in contrast to integrating readout, a continuous time measurement of the voltage drop across the microbolometer is performed. The advantage of this circuit is the high dynamic range that allows the accommodation of the large signal swings due to the self-heating effects of the microbolometers without adding extra circuitry. 24 Based on that scheme, a CMOS chip capable of reading linear arrays or small 2D arrays of microbolometers with pixel counts up to 144 has been fabricated and tested. The technology used is CMOS with ().7 Bm m i n i m u m feature size and 0-5 V power supply. The readout chip is connected with the sensor by means of a ceramic substrate using a Multi-Chip-Module (MCM) technology (see Figure 12.21 ). The same R()IC design can be used for monolithic sensors when integrated on the same substrate with the microbolometers. Each microbolometer has its own dedicated readout circuit referred to as pixel amplifier. The chip contains ] 4 4 pixel amplifiers, an 8 bit address decoder, an output driver and the bias stages. The genera] architecture is shown in Figure ]2.22, together with the connectivity with the sensor chip on the MCM. The pixel amplifiers are designed on a pitch of 5() l~m in order to match the pitch of the bolometers to be tested. This simplifies the connectivity and most importantly, minimizes the design effort for a future integration with the microbolometer array. Employing an address decoder instead of a shift register for the multiplexing a|]ows for dynamic windowing and, most importantly, eases the connection when a (small) 2D microbolometer array is used. The schematic of a pixel amplifier is shown in Figure 12.22. Pixe]s are selected according to the digital addresses that are applied in the address decoder. The m i c r o b o l o m e t e r is biased using a c u r r e n t mirror circuit. A pulsed c u r r e n t biasing

470

Handbook of Infrared Detection Technologie~

Figure 12.21 H!lbrid interconnection of a linear boh,neter arra!l (bottom chip) with its dedicated R()IC (top chip) using wire bonding and an MCAI substrate.

Figure 12.22 Schematic design and electrical scheme of the readout chip.

is performed that minimizes the self-heating of the microbolometer due to the power dissipated from the bias current. The output of the amplifier is: Vo :

v,.,,s -

IH(O)I (v,.,:r

-

n,o,)

(6)

where H(O) is the low frequency gain of the amplifier and Vl,,,1 is the voltage drop across the bolometer. The second opamp that is placed in a unity-gain configuration drives the output bus. The opamps are realized using differential pairs. The above equation implies that the signal difference between two pixels is the difference in the voltage drops across the two microbolometers multiplied by the low frequency gain of the amplifier. In other words, the pixel amplifier enhances the differences in the voltage drop of the microbolometers. In the current implementation, the pixel amplifiers can operate with a gain of 1 () or 1.

Pol!!Ni(h' ltm'ooh'd microl~olometer,,; for therlnal IH detection

471

Selection of gain 1 results in the direct output of the voltage over the microbolometers. The readout chip has been connected and tested with a linear array of 64 microbolometers based on poly Si-Ge. The microbolometers exhibit a typical resistance of 50 kOhm and they are biased with a pulsed current of 1 () up to 8() microAmps amplitude. Figure 12.2 3 shows the output during the selection of all consecutive pixels. The exponential drop is attributed to the self-heating of the microbolometers. In an imaging system, the actual signal corresponding to one pixel must be a sample of this waveform while this pixel is selected. In the current implementation, this sampling is performed off-chip because the whole output waveform is desired in order to study the characteristics of the microbolometers. An advantage of this readout scheme is the high dynamic range. Depending on the actual microbolometer resistance, the bias current can be adjusted in order to set the operating point within the input range of the pixel amplifiers ((). 5-4 V). The total input referred noise has been measured as 22()laV which in theory would allow to measure 3()()mK noise-equivalent temperature difference NETD (with f/1 optics) provided the bolometers are biased to 4V and show 6()% absorption, 1% TCR and 1• 7 W/K thermal isolation. This noise level exhibited by the sensor can be translated in a m i n i m u m resolvable resistance change of almost O. 9 Ohms.

Figltre 12.2 ~ Direct cttrrent readoltt oZltl~Ztt of a 64 eh'n~ent lirlear bolomewr arra!l, tnea~tlred at anlbient

pressltre atld in vacuttm. The itzset ,,;/tow,,,"the t!lpical e.rl~onential self-heating bo/ometer reSl~Onse aml the of]._ chip sampling.

472

R~,o V

Handbook of Infrared Detection Technolo#ies

1~

Rr~

/

V1/

,i II

] I

v.

/

Lv tv iv Lv

//

y

Figure 12.24 Scheme of a CTIA based microbolometer readout (left). A serial and paralh'l readout architecture for a linear arra# of microbolometers is shown ri~lht.

A very important issue in multi-element sensors is the response uniformity. For a uniformly bright scene the response of various pixels is not the same, mainly due to mismatches in bolometer resistance, amplifier gain and bias current. Therefore, a complete sensor system must utilize signal calibration. This can be performed on-chip or off-chip by software. The second approach has been taken in the present phase of this work. The non-uniformity due to the bias current has been minimized by the proper design of the current sources. According to data provided by the silicon foundry the relative error in the current is 0.3%. The gain uniformity is also high because the gain depends on capacitance ratios that are well controlled in CMOS technologies. The total ROIC non-uniformity has been measured as (). 3'Y~,.

Integration readout principle

The most commonly used readout scheme uses CTIA integration of the net IRinduced current. 19 This is accomplished by means of a reference bolometer or reference current source so only the change in IR radiation provides a signal. Even for the same readout noise, the dynamic range of this approach is superior to the direct readout scheme. For a linear array (or small 2D array), two different readout architectures are possible: serial or parallel. In the first the voltage over subsequent detectors is switched in a serial way. while the current is integrated in one CTIA. The parallel architecture provides a CTIA for each detector (see Figure ] 2.2 4).

12.5 Zero-level vacuum packaging 72.5.7 Introduction

Using the above poly SiGe based process on 8" enables low cost production of large quantities of IR imaging systems for. e.g., the automotive industry. An often overlooked cost is the packaging at low pressure (typically 1()-~ mbar).

Pol!lSi(;e uncooled microbolometers for thermal IR detection

473

Commercial hermetic packages are available, but their cost is prohibitively high for (by definition) low-cost automotive and other non-military applications. Even structural stiffness enhancement techniques cannot overcome the forces encountered during the final dicing and pick-and-placing of the dies into packages. Bolometer packaging, therefore, is a critical item. The requirement for vacuum packaging of microbolometers further complicates this issue as hermeticity of many electrical feedthroughs has to be guaranteed over an extended period of time. As a consequence, much effort is currently devoted to so-called zero-level hermetic packaging techniques where the microbolometer device is protected at an early stage of the process to improve yield and reduce cost. The additional vacuum requirement makes this approach even more challenging. When waferlevel or on-chip encapsulation takes place, this is commonly termed zero-level (or O-level) packaging. It means that a package is realized at the die level. This can be accomplished by wafer bonding, by flip-chip bonding or by surface micromachining. The electrical feedthroughs are routed on-chip. After realizing such a zero-level package, further standard (hence low cost) packaging techniques can be applied. IMEC has developed a patented zero-level packaging technique called IndentReflow Sealing (IRS) which is described in Section 12.5.2. An alternative approach using BCB instead of metals as soldering material currently under investigation is presented in Section 12.5.3. 12.5.2 Indent-Reflow Sealing using metal solder We proposed the so-called indent reflow sealing (IRS) process as a new O-level packaging approach for, e.g., MEMS.2s In this process, two chips are hermetically sealed using an electroplated ring (Figure 12.25). Because most MEMS devices including microbolometers have to stay clean, a fluxless solder joining is required. This is realized by using the eutectic bonding between tin lead (SnPb) and gold (Au). This process is such that no vacuum flip-chip bonder is needed. The IRS processing details are as follows. First, the under bump metallization (UBM) ring is formed on the bottom device wafer, i.e. the Ni/Au layer is deposited on a SiO2 isolation layer using electroless plating. The electrical connections are processed on the bottom chip under (and electrically isolated from) the plated

Figure 12.25 Scheme of the on-chip hermetic packaging assembl!l b!l indent-reflo~v sealing ( IRS).

474 Handbook of Infrared Detection Technoloc3ies rim. Figure 12.26 shows a linear array of microbolometers each with an electrical 'fan-out' structure u n d e r n e a t h the Ni/Au v a c u u m rim. On the cover substrate, the same geometry is created by electroplating PbSn. We have used Si, Ge (which is I1R t r a n s p a r a n t ) or glass (for application in the visible w a v e l e n g t h range or for d e m o n s t r a t i o n purposes) as cover material. 26 A groove is made in the ring to allow evacuation before the reflow. In the second step, the two substrates are prebonded in a flip-chip aligner. Then the cavity is evacuated in a reflow oven t h r o u g h the indent in the ring structure. As the indent is relatively large with respect to the overall volume of the cavity, both the evacuation and the filling is fast. Finally, the assembly is reflown at a typical t e m p e r a t u r e of only 2 4 0 ~ resulting in a eutectic solder bond rim sealing and hence a hermetic micropackage. This is s h o w n in Figure 12.2 7 for the case of a glass top cover. The fact that the actual sealing takes place after the flip-chip assembly, results in a marked cost reduction, since no v a c u u m flip-chip aligner is needed and,

Figure 12.26 Detail of the Ni/Au ring processed around a linear arra!! of microbolometers (a). and Pb/Sn electroplating pattern on the top (glass) substrate ( b ).

Figure 12.27 Nanoliter package formed b!! the IR S process rising glass as a top cover.

Pol!lNi(h' uru'ooh'd microbolometers lbr thermal IR detection

475

moreover, m a n y hermetic micropackages can be reflown in one operation. We realized cavities with typical surface of a few square millimeters and an inside height of around 10 micron, resulting in a hermetic volume of only several nanoliters. The total height of the system is marginally more than the thickness of the combined substrates.

12.5.3 Zero-level packaging using BCB Although metal-based reflow soldering seems the method of choice for v a c u u m packaging. BenzoCycloButene (BCB)is also being investigated as a sealing material. 27 Photosensitive BCB can be easily patterned, displays minimal outgasing and has a low reflow temperature ( < 2 5() :C). Zero-level packaging of microbolometers using BCB is a relatively simple process compared to the metal solder IRS technique. No further processing on the microbolometer wafer is required since the BCB rim can be processed on the top substrate. Moreover. since BCB is an insulator, there is no need for an insulating layer on top of the interconnects. Figure 12.28 shows a BCB-based zero-level package containing a linear array of microbolometers.

12.5.4 Hermeticity testing using microbolometers The hermeticity of BCB capped zero-level packages containing released microbolometers has been tested according to the military standard MIL-STD883D. Gross leaks are tested using tluorocarbon liquids with different boiling points. After immersion in a first liquid with low boiling point for several hours. the package is transferred to the second liquid (having a higher boiling point). When heating above the boiling temperature of the first liquid, the appearance of bubbles (due to the presence of the first liquid in the package) indicates a leak. With this method, leaks larger than 1 () 4 mbar.l/sec can be detected. The fine leak test procedure is as follows. First the package is exposed for several hours to

Figl~re 12.28 Example of a :ero-level package of a lim'ar arra!l of microl~olometers tisinfl BCB (a). Detail of the BCB rim viewed throztgh the glass top mtbstrate {17).

476


a He environment at high pressure (typically 3 bars). Next, the samples are transferred to a He leak detector. The latter has a He leak rate sensitivity down to approximately 10 -9 mbar.1/sec. No leaks were found when applying both the gross and fine leak tests to the BCB packaged microbolometers. In situ characterization of the microbolometers, however, proved the existence of a leak. As discussed already in Section 12.3.3, microbolometer characteristics can be used to measure (local) pressure. When pumping (and venting) a test chamber containing the capped microbolometers, the device response upon a pulsed bias was monitored (Figure 12.29). The typical exponential selfheating characteristic at low pressure, and a square signal at high pressure clearly indicated that the pressure inside the package was exactly following the pressure of the test chambers. Hence the package was leaking. This result conflicts with the hermiticity tests according to the MIL-STD-88 3D standard. However, from a careful theoretical analysis of the military standard leak testing procedure we concluded that it is not applicable to the small volumes typical for wafer-level packages. Depending on the volume, there is indeed an undefined leak regime which is not covered by the gross leak, nor by the line leak procedure. 27

12.6 Conclusions and outlook This chapter gave an overview of polySiGe microbolometer development. In the introduction, polySiGe was compared to other microbolometer process schemes. The structural, thermal and electrical properties of polySiGe were presented, from which it followed that polySiGe is a very suitable structural material for MEMS applications and a good microbolometer material as well. The latter stems from a very low thermal conductivity combined with a reasonably high TCR and

Figure 12.29 Schematic test setup (a) for testing the hermiticit!l of zero-h'vel packages using the pressure dependence response oll a pulsed bias of microl~oh, m, ters ( b ).

l)ol!lSi(;e uncooh'd microl~olometers for thermal IR detection

477

the former from low stress and stress gradients in the material. Since the detector processing takes place in an 8" CMOS pilot line, one can take full advantage of the submicron lithographic capabilities as well as the superlative uniformity: on an 8" silicon wafer, the pixel-to-pixel resistance non-uniformity is better than 2%. The different techniques to optimize pixel response were discussed. Whereas these, to a large extent, involve polySiGe bolometers, two generic approaches were highlighted: ultra-clean oxide sacrificial layer removal using vapor HF processing and the use of structural stiffness e n h a n c e m e n t to make stiffer MEMS components. Since current production techniques use a high-temperature deposition/ anneal process incompatible with CMOS postprocessing, polySiGe microbolometers are, in essence, hybrid sensors which can be integrated to a readout circuit by multi-chip-module technology. A direct current readout for polySiGe microbolometers was discussed in detail and conventional integrating readout was presented. Results on 64 element linear arrays were shown. MEMS packaging in general and microbolometer packaging in particular is a challenge due to the requirement of long-term v a c u u m packaging to ensure proper operation. Only wafer-scale or zero-level packaging can result in sufficiently low-cost packaging, provided that the technical challenges are solved. The zero-level packaging approach at IMEC using the patented indentreflow-sealing method (IRS) was presented. In view of the current high-temperature process technology used for polySiGe, these sensors are hybrids and therefore presently limited to linear arrays or small 2D arrays. As there is a definite market for small imagers, this hybrid approach may be very competitive, particularly given the increased flexibility. An Nchannel readout circuit can be connected to a variety of 1D and small 2D detector geometries.

Acknowledgements This work has only been possible thanks to m a n y co-workers in the Microsystems, Components and Packaging division of IMEC. as reflected by the list of references. It is particularly necessary to acknowledge A. Verbist, S. Sedky. P. Fiorini, A. Witvrouw, B. Du Bois, S. Kavadias, G. Ruttens, Y. Creten, P. Merken, M. Gastal, A. Jourdain, H. Tilmans, K. Baert, E. Beyne, R. Van Hoof for their contributions. Furthermore, the collaboration with XenICs (C. Goessens, V. Leonov, J. Vermeire) is acknowledged.

References 1. P. W. Kruse and D. D. Skatrud, Uncooled Infrared Imaging Arrays and Systems, Semiconductors alld SelT~i171etals 47, Academic Press ( 199 7 ). 2. R. A. Wood, High-performance infrared thermal imaging with monolithic Silicon Focal Planes operating at room temperature, Intenmtiona] Electro11

478

Hamtbookof Infrared Detection Technologie,s

Devices Meetin# 1993. Technical Digest (Cat. No.93CH3361-3). IEEE, p.175 (1993). 3. H. Baltes, O. Brand and M. Waelti. Packaging of CMOS MEMS, Microelectronics Reliabilit!! 40, 1 2 5 5 - 1 2 6 2 (2()()()). 4. V. K. Jain and C. R. Jalwania, [Jncooled IR-sensor array based on MEMS technology, Proc. SPIE 3 9 0 3 , 2()6 (1999 ). 5. T. Ishikawa et al., Low-cost 32()x24() uncooled IRFPA using conventional silicon IC process, Proc. SPIE 3 6 9 8 , 556 ( 1999 ). 6. M. H. Unewisse, K. C. Liddiard el al., The growth and properties of semiconductor bolometers for infrared detection. Proc. SPIE 2 5 5 4 ( 4 3 ) , 4 3 - 5 4 (1995). 7. J.-L. Tissot et al., LETI/LIR's uncooled microbolometer development, Proc. SPIE3436, 605-610(1998). 8. S. Sedky, P. Fiorini el al.. Structural and mechanical properties of polycrystalline silicon g e r m a n i u m for micromachining applications, Jozlrttal of Microelectromechanical S!lstelns 7(4), 3 6 5 - 3 7 2 ( 1998 ). 9. S. Sedky, P. Fiorini et al., IR bolometers made of polycrystalline silicon germanium, Sensors and Actuators A A 6 6 . 1 9 3 - 1 9 9 ( 1998 ). 10. S. Sedky, P. Fiorini et al., Characterization and Optimization of Infrared Poly SeGe Bolometers. IEEE Trails. olt Eh'ctrolt Devices 4 6 ( 4 ), 67 "3-682 ( 1999 ). 11. A. Franke et al., Optimisation of poly-silicon-germanium as a microstructural material, Transdt~cers ' 9 9 , 4 9 2 - 4 9 5 ( 1999 ). 12. S. Sedky, A. Witvrouw et al.. Experimental determination of the m a x i m u m post-process annealing temperature for standard CMOS wafers. IEI~E Traits. Electron Devices 48(2), 3 7 7 - 3 8 5 (2()() 1 ). 13. S. Sedky, A. Witvrouw et al.. Effect of in silu boron doping on properties of silicon g e r m a n i u m films deposited by chemical vapor deposition at 4()() degrees C, Journal of Materials Research 16(9 ). 26() 7 - 2 6 1 2 (2()() 1 ). 14. V. Leonov et al., Optimization of design and technology for uncooled polySiGe microbolometer arrays in Illhared Detectors atut Focal Platte Arra!ls VII. Proceedings of SPIE 4 7 2 1 (2002). 15. A. D. Parsons and D. J. Pedder. Thin-film infrared absorber structures for advanced thermal detectors, J. Vac. Sci. Techttol. A6( 3 i. 1686 ( 1988 ). 16. P. De Moor, C. Van Hoof et al.. Process development of fast and sensitive polySiGe microbolometer arrays in Photodetectors: A laterials atut Devices VI. SPIE, pp. 9 4 - 1 0 0 (2001 ). 17. A. Witvrouw, B. Du Bois et al.. A comparison between wet HF etching and vapor HF etching for sacrificial oxide removal in MicrotTtachittitt# atut Microfabrication Process Tech17olo#!! ~I. SPIE 41 74. pp. 1 3()-141 (2()()()). 18. P. De Moor, C. Van Hoof et al., I~inear arrays of fast uncooled poly SiGe microbolometers for IR detection in hq)'ared Detectors atut Focal Platte At'rails VI. SPIE 4 0 2 8 , pp. 2 7 - 3 4 (2()()()). 19. A. Tanaka et al., Infrared focal plane array incorporating silicon IC process compatible bolometer, IEEE Trails. Electrott I)evices 43, 1 8 4 4 - 1 8 5 ( ) (1996). 20. U. Ringh, K. Liddiard et al., CMOS RC-()scillator Technique for Digital Readout from an IR Bolometer Array. the 8 th International Conference on

PohlSi(;e lltlcooh'd nlicrol~olonlelersjbr thernlal IR detectiotl 479 Solid-State Sensors and Actuators and eurosensors IX. Stockholm. Sweden. pp. 1 3 8 - 1 4 1 (1995). 21. C. Menolfi and Q. Huang. A I, ow-Noise CMOS I n s t r u m e n t a t i o n Amplifier for Thermoelectric Infrared detectors. IEEE ]oz~rt~al of Solid State Circllits ] 2 ( 7 ) ,

968-976(1997). 22. S. Kavadias et al., CMOS circuit for readout of microbolometer arrays, Electronics Letters ] 7(8), 4 8 1 - 4 8 2 (2()() 1 ). 23. S. Kavadias, M. Gastal et al., Hybrid l o n g - w a v e l e n g t h IR sensor based on a linear array of poly Si-Ge uncooled microbolometers with a CMOS readout in Sensors, Systelns, arid Next-Getleratiotl Satellites IV. SPIE 4 1 6 9 , pp. 3 4 8 - 3 5 5

(2001 ). 24. C. Jansson, U. Ringh and K. Liddiard. Theoretical analysis of pulse bias heating of resistance bolometer infrared detectors and effectiveness of bias compensation, Proc. SPIE 2 5 5 2 . 644-6 g2 ( 199g ). 2 5. H.A.C. Tilmans et al., The indent reflow sealing (IRS) technique- A method for the fabrication of sealed cavities for MEMS devices, ]ozirnal of Microeh'ctromechanical S!lste171s 9( 2 ). 2()6-21 7 (2()()()). 2 6. De Moor, P. et al., Hermetically sealed on-chip packaging of MEMS devices in Proceedings Europeatl Space CotllpotletltS Cotlh'retlce- ESCCON 2 0 0 0 , pp. 6 7 - 6 9 (ESA SP: 439)(2()0()).

2 7. A. Jourdain et al., Investigation of the hermeticity of BCB-sealed cavities for

housing (RF-)MEMS devices in ~\II.;,~,IS 2 0 0 2 . tlle 1 5 th hit. ConiC'fence Oll Micro Electro Mechanical Sllstenls. pp. 6 7 7 - 6 8 ( ) (2()()21.


Chapter 13

Fundamentals of spin filtering in ferromagnetic metals with application to spin sensors Henri-Jean Drouhin

13.1 Introduction A new field of solid-state technology, spintronics, is presently emerging. Electron devices based on electron spin manipulation may exhaust our capacities in key areas. Spin-dependent transport experiments have attracted broad interest because of the new physics involved and their possibility of application. Studies on magnetoresistance effects and related phenomena are nowadays an active field of research, stimulated by industrial challenges concerning high-density recording and magnetic sensors. ~ In 1995. M. Johnson proposed the concept of the ferromagnetic bipolar spin transistor made of three metal layers: a ferromagnetic emitter: a nonmagnetic metal base: and a ferromagnetic collector. 2 Today, the progress in fabrication technology enables us to associate the properties of ferromagnetic thin films with those of metals, semiconductors or insulators. The development of MRAM magnetic memories, based on tunnel junctions, is very promising. ~ The most popular transistor concept - so far never a c h i e v e d - is the Datta and Das transistor, analogous to the electro-optical modulator, based on the Rashba spin splitting of the conduction band in some two-dimensional semiconductor structures. 4 Various other devices have also been considered, for instance, unipolar devices, which are closely related to semiconductor electronics, s Very attractive new ideas, based on magnetization reversal triggered by a spin current are emerging, (' and the idea of possible stimulated emission of spin waves (SWASER) was proposed. 7 It appears that dealing with spin-polarized electron transport in ferromagnetic multilayers presents many similarities with the techniques used in the field of semiconductors. For instance, the well known notion of ambipolar diffusion of charge carriers, electrons and holes in semiconductors, s proves to be

482


perfectly adapted to describe the diffusion of carriers of different spins in metals. 9 Even the concept of doping (spin doping) could be extended to ferromagnetic metals. 1~ In this context, devices associating optical properties and magnetism have to be seriously considered. Atomic physics experiments have shown that optically-pumped spin filters can be achieved, ll It was also demonstrated that ferromagnetism order may be induced in magnetic semiconductor structures by near infrared excitation.l-' Photogenerated spin-polarized carriers in semiconductor q u a n t u m wells, under intense far-infrared excitation, can be directly converted into electrical currents, l~ An important step toward spin injection in semiconductors was made by Fiederling et al. 14 who demonstrated extremely efficient spin injection from a dilute magnetic semiconductor into a LED structure. Nevertheless, the understanding of spin injection from a ferromagnetic metal into a semiconductor remains under discussion and controversial. 1 S In this paper, I will describe ultra-thin ferromagnetic layers as spin-filters, analogous to optical polarizers. However, dealing with electrons offers more possibilities because the electron polarization possibly has both longitudinal and transverse components. The inelastic electron mean free path (IMFP) in metals, which characterizes the escape depth, plays a crucial role in these experiments, as in all electron spectroscopies. Its analysis in the low-energy regime (below a few tens ofeV) remained a puzzling question for decades. If it was thought that its energy dependence was following a 'universal curve', 1~' more recent experiments involving metals with d electrons, evidenced a clear material dependence. 17'1s Note that direct IMFP m e a s u r e m e n t s are extremely difficult because they imply the extraction of vanishingly-small ballistic-electron contribution from a large background of secondary and stray electrons. However, the situation is quite different in magnetic materials, because the spindependent part of the IMFP can be determined with accuracy. Indeed, reversing the sample magnetization, or eventually the primary beam polarization, eliminates parasitic effects. Today, electron dynamics in metals, and particularly magnetic metals, is actively studied using time-resolved two-photon photoemission experiments which give direct access to the electron lifetime for electrons up to a few eV above the Fermi level. TM All these results stimulated several analyses, ~s-29 most of them based on numerical calculations. In refs. 2 7 - 2 9 , I showed that an accurate analytical description of the IMFP is obtained in a model based on density-of-state effects, in the range 5-5()eV above the Fermi level. The result does not depend on the detail of the d bands, but only on the numbers of s and d electrons, and the various electron relaxation channels are disentangled. In the first part of this paper. I will analytically derive the relation between the IMFP and the electron numbers. In the second part, I will focus on the ferromagnetic bilayer structure, which associates a 'polarizer' and an 'analyzer'. This spin-valve structure plays a central role in the study of new p h e n o m e n a and presents some u n u s u a l properties, which are closely related to Stern and Gerlach experiments. In particular I will precisely define the parameters characterizing

Fundamentals of spin filtering in ferromr

metals ~vith ~pl~lic~ltion to spin sensors

483

the spin selectivity, k n o w n as the Sherman functions in spin polarimetry. 3(J Finally, I will discuss solid-state spin-detector structures, based on the exchange interaction, whereas conventional electron polarimeters make use of the spinorbit coupling. ~~ At the end, I will make a link between experiments involving hot electrons and transport experiments at the Fermi level.

13.2 Theoretical IMFP variation 13.2.7 The simplest m o d e l - mathematical bases of the calculation

In transition metals, the IMFP variation is mostly determined by density of state effects, whereas the transition matrix elements introduce weaker corrections. The crudest model was given by Sch6nhense and Siegmann, 1~ who e m p i r i c a l l y related the 'scattering cross-section' ~+(~-) for majority-spin (minority-spin) electrons to the numbers of holes per atom in the d bands. This m o d e l - which was an important contribution to the understanding of spin-dependent transport but turned out to be unphysical and led to strong deviations from the experimental d a t a - is based on the picture of a primary electron 'falling' into empty states in the d bands, with spin conservation. Thus, the majority(minority-) spin electron scattering cross-section should appear to be only related to the number of d holes in the corresponding spin subband. On the contrary, we must keep in mind that majority-spin electrons propagating in the metal can always lose a small a m o u n t of energy and excite minority-spin electrons inside the minority-spin band so that any correct expression for the majority-spin cross section should contain the number of holes in the minorityspin band. First, let us remember that. generally speaking, an electron scattering crosssection (3e is connected to the electron mean free path ;.~, by the relation ~.cc~e N = ] , where N is the density of scattering centers. In ref. 18, to make more evident that the d holes act as scattering centers, the scattering cross-section is somewhat improperly defined as the inverse of the IMFP. Here, we will keep this terminology to define notations for ease of comparison between different papers in the literature. The simplest realistic model, besides numerical calculations, is the density-ofstates model proposed in ref. 28. which only takes into account electronelectron scattering in the framework of the random-k approximation. 31.32 The band structure is described by a constant density of states n,.t, for the sp band, which is taken as unpolarized, on which is superimposed a positive continuous compact-support function nd~(u) describing the considered d band 9 n~(u)=na~(u)+nsp. The energy origin is set at the metal Fermi level, so that - EF specifies the energy location of the bottom of the sp-band. The d bands are bounded by energies E1 ~ (lower bound) and E_,~(upper bound), their common width is denoted as Wd (see Figure 1 3.1 ). The numbers Ne ~ and Nh ~ of electrons and holes in the d-bands are

484


Ep E

8'

......

E~

.~

-/~ -.-

o

_

;.:-".5;'..-: ":-:;'.:.:.:; i

"':2;":)-?;':;??" Wd

g'- E

~I ET

mu

Density of states Figure 13.1 Principle of the calculation. For each spin, the band structure is described bt3 a constant densitt3 of states in the sp band on which is superimposed a positive continuous compact-support function describing the d band. The d bands are bounded by energies E1 • and E2 +-. their common width isdenoted as Wa. The dotted area represents the occupied states, located below the Fermi level ( energ!t origin). In the figure, a primar!t electron at energy Epwith a minority spin. loses an amount of energ!t e, which is used to excite a secondar!t electron from a negative energy E'-e to a positive energy e'. with spin conservation. This occurs either in the minorit!t-spin band (direct processes) or in the majority-spin band (exchange-like processes ).

Ne -

j()-~c

n d~(u)du; N h~-

n d~(u)du

l+0 :x:

(1)

with NeC%Nh~ The d-band centers Ed~ and the centers of the emerged part of the d bands, Eh~, are defined as

j

+3c

5E~ -

un~(u)du;

N~Eg -

j

+~c

()

un~(u)du

(2)

This implies that the densities of states are normalized to individual atoms. The total d-hole number is .A/'h:Nh-+Nh +. whereas the hole asymmetry is AN'h=Nh---Nh +. Other relevant quantities are defined in the following way: Ed=-- (Ed-+Ed + )/2, .AfhEh = (Nh-Eh- +Nh+Eh + ), A2V'hEh'= ( Nh-Eh--- Nh+Eh +), and 7 N'h= [(Nh -)2 +(Nh +)2].

Fundamentals of spin filtering in ferromagnetic metals with application to spin sensors

485

We consider a primary electron at energy Ep, with a spin ~=+. This electron loses an a m o u n t of energy e, which is used to excite a secondary electron from a negative energy e'-e to a positive energy d, with spin conservation. Under these assumptions, the transition rate from Ep to (Ep-e) for the primary electron, r~'(g), is expressed as r~(g) = n~(Ep - g) [co(~cm(e) + cord~-c~-c~(g)]

(3)

E

~'~" (~) -

I

n ~'(e')n ~'' (e' - g)de'

(4)

0

In the expression of r~(e), m and mr are taken as constant transition matrix elements characterizing direct and exchange processes. From physical arguments, it will finally be reasonable to consider them as equal. The n~(u) functions are the relevant densities of states. Although this is not necessary at this step, we more generally define the ~)~'(e) functions for cs'-_+c~. We also define d~d~'(e), which is ~ ' ( e ) calculated for the only d-bands, i.e. with nsp-O. The overall spin-dependent scattering rate at energy Ep, R~(Ep) is R~ (Ep) -

i

Ep

r~ (~)d~

(5)

O

It can be s h o w n that the qb~"(e)-functions have the following properties: (i) For small e, qb~'(g),-m~(()) n~'(())e. This results from a straightforward mathematical limit and holds in the vicinity of e=0. Physically, significant deviations can be expected at growing ~, in particular if the d-band density of states is zero at the Fermi level, because the occupied d bands lie at a lower energy, which is the case for gold, copper or silver, for which the top of the d bands lie respectively at 1.8, 1.7, and 3 eV below the Fermi level. 33 A sharp increase of the electron scattering cross-section is expected w h e n the energy transfer is large e n o u g h to allow excitation of the d electrons. In such a case, E2 ~ and E2 ~; are negative and, if ~ only slightly exceeds (-E2~S'), whe have ~ ' ( ~ ) ~ n s p 2 ~+H(~+E2 ~') nsp nd~'(E2 ~') (~+E2~'), where H is the Heaviside function. (ii) For large values of ~, and more precisely when ~ ~>W ~ ' = S u p (]EI~[, ]E2C~], IE,~"I, IE2~"], ]E2~'-E~" l, IE2~"-E,~'})]. we find qb~ ' (1r - nsplr 4- nsp A consequence is that the value of qb~'~'(g) does not depend on the band considered: also note that, ifn~p=0, ~'~'(e)=0d~'~"(g)=()when g>~Sup ({Ee~'-EI~"I, IE2~S'-EI~I). This results from a straightforward integration, after observing that, w h e n g is large enough, either g' or g ' - g is out of the support of the d-band density of states. (iii) W h e n u >~Sup(IEe~'-E, ~'l, IE_,~"-E, ~'l),

486

Handbook of Infrared Detection Technoh}gies

I

ll ~ 0 0 '

0

"t-'d

(e)de - NhN o ~o' \

(7)

]

To derive this relation, let us define the G{u} function as follows

G(u)-

i

ll {}

(a)de

'+'d

(8)

When u~>Sup{lE2~'-El'~' l, IE2'~'-E~I}, we have the relation dG{u)/ du={J)d~ Thus, G{u) keeps a constant value that we evaluate for u=+cx:. G(+ec) -

j+3C dg i,g dv n d{3"(v)n d~ (v ()

{}

=

de ()

e) (9)

dv H ( e - v ) n ~ ( v ) n f ( v - e) 0

Using Fubini's theorem and with the change w = v - a , we obtain

j0

C(+oc) - .+ ::x;dvn~(v) i+~ daH(~- v)nf ( v - 8) -

{}

dvnj(v)

()

(lo)

( - d w ) H ( - W ) n d (w)

Finally, we find

G(+cxD) --

(i

()

dv n dc~(v)

)(3"

--x.

dw n d

)

__ NhNe

(11)

For very small primary energies. R~'{Ep) is easily evaluated (using (i)} and we obtain

I(R-(Ep)2 + R+(EP))

~Ep-(n-({}) + n-((})) co n-((})-+n*(())+({or - co)n-({})n+ ({})]

(12)

If co=mr, or if n + ( 0 ) - 0 (perfect ferromagnet), or if n+{()}-n-(()){non-magnetic metal), the spin asymmetry of the IMFP is the asymmetry in the density of states at the Fermi level. In the case of a metal with all-occupied and identical d-bands (E2+=E2-=E2; nd+=nd-=nd), the overall scattering rate is R ( E p ) - n~p [conspE~ + H(Ep + E2)co nd(E2)(Ep + E2) 2]

(13)

It is possible to identify the origin of the scattering process by substituting co with the relevant matrix element of the scattering channel, in this case (2rc//i) [M~s[2


487

when only sp states are involved or (2re~h)IMsdssl 2. when a d state is involved. This approach will be systematically developed in the next section. R(Ep)

27r , [ , , 2 ,] ----fi-- nsp nsp M~I-Ep + H(e + E2)nd(E2)lMsdlss (Ep + E,)-

(14)

The onset of the d-state contribution was analyzed by Echenique et al., who treated the d bands as 'boxes' (square bands). 21 Nevertheless, note that equation (14) differs by a factor of( 1/4) from the expression originally derived by Berglund and Spicer, where the spin is not taken into account, ~1 and also given in ref. 21. This factor arises because, in our model, the scattering events occur with spin conservation, so that only half of the total sp density of states ( 2 nsp) is available as a final electron state. For a large primary energy [Ep>~Sup (~,7++,~1- )+Sup((), E2-, E2+)], R~ is calculated after cutting the integration domain at energy W=Sup (W++.W - - ) W'(Ep) -

r~

-

()

r~(8)d~ + ()

r~

(15)

W

When ()~~W, in the domain Ep>/Ev+2 Sup (0, E2+, E2-). In a square band model, this condition implies that Ep ~>Ev+2Nh-Wd/5. In the calculation of R ~', we perform the integration on the amount of energy loss c after cutting the domain at energies W, EF, EF+SUp (0, E2e~) for direct (involving the +~ channel) and exchange-like (involving the - ~ channel) processes, EM, and Ep [with Ep-EM ~>Sup (0, E2 +, E2-)]. We obtain the important relations

492


R

~

10 nsp (2~fli)

1 (Ep + Eh _ EF)IM~dj2] =.A/'h [2 ( 1 - - ~ ) l M : d ] 2 ---f~nsp + n~p(gp - Ed)IM~I-+~n;EF

+gh

Hh)

Up --?EF

,1

O'd--.,~ e

1

(34)

,

1 - 10 I~d(Ep)]-+-n~pEF]M~d(Ep)]*"~ds 5 ss

o.o_5N.:p[(Ep-E~)]M~dl2 1 .~p(Ep 1)M~:] EF

Ia:Ssl-

IM~d] 3 f- ~ "A/e ~ . . . .

M,~ds

(35)

A.,/~h ~ Msd 2 sd 2 sd ~ -Jr-.Ale Mds(gP)[ .,~eJV.hj M~tds I~/[sd2*''ds] (36)

+ A/.~p 2 EF + (1 -- O.17)Wd -- Ep MSdss2

EF Ao" -- I.,~/" ]Mds sd(E_P) 2 e M ds,

M~dl2

M~d2

VFO~IA.~ h v

(3 7)

In eq. (34), the energy domain where IMsd~]2 has to be evaluated is only indicated in the term proportional to Nh c~. This leads to a simpler expression, whereas the full matrix-element energy dependence is kept in eq. (37). The comparison of the regimes Ep ~<EF and Ep>~Ej: shows that, in materials with a large hole number, the spin-averaged cross-section is strongly reduced when increasing the energy, because the excitation of electrons from the sp band into the empty d states is no longer possible (IM~dds(Ep)]2 in eq. (2 5) is multiplied by (./V'e/10) in eq. (34). This unusual effect was in fact observed in Gd (see the discussion in ref. 28). 13.2.3 An intuitive derivation

In this section, we do not perform any detailed calculation but we simply suppose that the scattering rate is proportional to the product of the numbers of states involved in the process, an intuitive idea. We introduce a hierarchy of the scattering events as a function of the number of d states they involve (they have the larger density of states) and classify them as a function of the amount of energy transfer. We consider an energy domain where Ep exceeds the d-band width, so that the relevant events are of the following types (see Figure 13.2): (i) Events involving 2 d states and large energy transfers. The primary electron, initially in an sp state, falls into the empty d states of the same spin, undergoing a large energy loss (close to Ep), and excites an electron with either


Ep

Ep

~I

I

49 3

lk

E I

J

~xO ~0

-'l

i....'..

;..;.-;

iii;.ili'".-... j'..'..

_EF

_

iili

Density of states Figure 13.2 Simple picture. Left: (ii) process. The primar!! electron, with either spin. injected at energy Ep into the sp band loses a small amount of ener#!! e which is used to excite a secondar!l electron at energ!t e' from the occupied d states into the empt!l d states, with spin conservation. Right: ( i ) process. The primar!l electron (here with a minorit!l spin)falls into the erupt!! minoritJI-spin d states. Secondar!l eh'ctrons can be excited from the occupied d states into the empt!l sp states or. (f tlp Ee, electron excitation from the occupied sp states into the empty d states becomes impossible, due to energy conservation, so that this contribution reduces to IMSdds(Ep}r2Nh+(Ne++Ne-)=lMSdds(Ep)lZNh+./V'e 9Most of the spin asymmetry of the scattering rate arises from this term. (ii) Events involving 2 d states and small energy transfers. The primary electron loses a small energy amount and falls into an sp state. It excites an electron with either spin from the occupied d states into the empty d states. The corresponding contribution to the scattering rate is proportional to IMSdsdl2 [Nh+Ne++Nh-Ne-]. This term does not contribute to the spin asymmetry of the scattering cross-section as the scattering process is equally probable for majority- and minority-spin electrons.

494 Handbook of Infrared Detection Technoloqies (iii) Events involving one d state and large energy transfers. The primary electron falls into the empty d states of the same spin, undergoing a large energy loss (close to Ep), and excites an electron with either spin from the occupied sp states into the empty sp states. These events introduce additional spin dependence. When Ep/> EF, they contribute to the scattering rate proportionally to IMSdss(Ep)]2 Nh+A/'esp, where the number of sp electrons is ./V'eSP=2 nsp Ev. When Ep~Ev, as the number of accessible occupied sp states is (Ep/EF)A/'~sp. their contribution reduces to IMsd,~,~(Ep)]2 Nh+(Ep/Ev)A/'r ~p. (iv) Other scattering events, involving one d state, where the primary electron falls into the empty sp states and excites an electron with either spin from. or into, the d states. When Ep~<Ev. their contribution is proportional to (Nh++Ne++Nh-+Ne-)nspEp ]Msd~]2=l() nsp Ep ]MSdssl2. because all d states are involved. When Ep >/EF, the events with an energy transfer smaller than Ev yield the total contribution 10 nsp El-. IMsdssl2. Concerning the events involving an energy transfer larger than Ev. the contribution of the empty d states is forbidden so that they yield the additional term (Ne++Ne-)nsp (Ep-Ev) [MSdss]2 =(10-A/'h)nsp(Ep-Ev) tM~dssl2, with A/'h=(Nh++Nh - ). Thus, the overall contribution is proportional to 1() n~p El: ]M~d,~,~12+(l()--A/'h) nsp (Ep-Ev) ]M~dss]2=10nspEplM~d~s]2-A/'hnsp {Ep-Ev)IM~'~.~I2.These events do not contribute to the spin asymmetry. (v) Events involving only sp states. Obviously~ these do not contribute to the spin asymmetry. When Ep ~<El-., they yield the familiar quadratic contribution: (1/2) (nspEp) (2nspEp)tMSSss]2: the (1/2) factor arises from integration on the amount of energy transfer. When Ep >/El-, the contribution of the events with an energy transfer smaller than El-, is then (n~pEv) (2n~pEv) IM~~,~,~I2. The additional contribution of the events involving an energy transfer larger than Ev is expressed as: [n~p(Ep-Ev)] A/'~~p ]M~~,~]2 because the number of occupied sp states involved in the transitions is .A/'esp . Finally, we obtain the overall contribution (nspEl:) (nspEv)]MSSss]2+[nsp (Ep-Ev)] J~'esp ]MSSssl2=nsp [Ep-( 1/2 )Ev].A/'esp IMSSssl2. Consequently, when Ep ~<Et:, the scattering rate for majority- and minorityspin electrons is proportional to

1 Af.sp2(Ep)4-'e ~

)

ss 2+Sj~ ,p~gp Mss sd]2-]- (1 "~'h'/~'e AJ~.h) ]msd ]Mssl sd2] -

-

__b_N~ [101 Mds(Ep) sd I2-qL.A/'~P~ Up ]Mss sd(Ep)I 2]

(38)

When Ep >/EF, the scattering rate is proportional to ~'-e

~--

+X1 (.Af.hA/ee _

Mss I- + 5"/V'~,PLF Mss - ~-/v hJ~ c

Ev

(39)

~/lsd(Ep) 12+./~p ]M:~t A./V'~)Im:~l2+Nil~_[j~eel,,,ds ,, (Ep) ]2]

These relations coincide with eqs. (2 5) and (34). except for some small terms proportional to Ed, Eh+, and Eh. which are missing here.

Fundatnet~tals of spitl filterin# itl ferrotna~,ltu'tic tm'tals with applicatiotl to spit1 setlsors

49 5

13.2.4 Comparison with the Schdnhense and Siegmann model In ref. 18, S c h 6 n h e n s e a n d S i e g m a n n empirically propose the relation (40)

(5• -- cy()+ OdN~:

w h e r e c~() a n d (3d respectively refer to sp and d c o n t r i b u t i o n s a n d are a s s u m e d to be m a t e r i a l i n d e p e n d e n t . C o n s e q u e n t l y , the s p i n - a v e r a g e d cross-section is

(~}

-

~- + ~+ 2

ZCh

(41)

= cs(i + ~ d - ~

and A ~ = ~aAN'h

(42)

The IMFP a s y m m e t r y A is expressed as A -

c~- - ~+ c~- + ~s+

=

)v+ - )vX+ + Z,-

=

An (cs()/~d) + 5 -- n

1 = -

~

A.A/'h ./~'h

(43)

-(~,,/~d) +7-

These expressions h a v e to be c o m p a r e d to eqs. { 2 1 - 2 3 ) . For a m a t e r i a l with a small hole n u m b e r a n d at m o d e r a t e p r i m a r y e n e r g y , we observe t h a t the S c h 6 n h e n s e a n d S i e g m a n n expressions o v e r e s t i m a t e Acy a n d A by a factor of two. For a s t r o n g f e r r o m a g n e t , eq. (43) predicts a m a x i m u m spin a s y m m e t r y A=IO()(~) (Jk/'h=/XJV'h=Nh-. O'()=()). w h e r e a s eq. (23) yields A ~ 5 ( ) % . This c a n be simply u n d e r s t o o d by the simple picture given in the p r e c e d i n g section. Indeed, m o s t of the spin a s y m m e t r y o r i g i n a t e s from the (i) events. We refer to their c o n t r i b u t i o n as OdINh+. A large c o n t r i b u t i o n to the s p i n - a v e r a g e d s c a t t e r i n g cross-section o r i g i n a t e s from the (ii) events. We refer to this c o n t r i b u t i o n as r A/'h/2. Thus, the overall c o n t r i b u t i o n of the (i) a n d (ii) e v e n t s to the s c a t t e r i n g AA/'h/2. ~d' a n d ~"d h a v e n o w a rate is GdfNh-++O"'dA/'h/X=(Gd'+G"d) Nh-+• simple physical m e a n i n g as they express the c o n t r i b u t i o n to the s c a t t e r i n g crosssection of the t r a n s i t i o n s with a large a m o u n t a n d a small a m o u n t of e n e r g y transfer. Obviously, we h a v e AC~=CSd'AA'rh a n d Gd=r 9If all the t r a n s i t i o n m a t r i x e l e m e n t s are equal, Gd/Gd '= 2( 1 --7/1 ()), w h i c h explains the m a i n t e r m s in eq. (23). On the c o n t r a r y , the S c h 6 n h e n s e a n d S i e g m a n n f o r m u l a overlooks the (ii) e v e n t s (i.e. c o r r e s p o n d s to c~"d=(), or e q u i v a l e n t l y to M'~dsd=()) SO t h a t it c a n n o t fit b o t h Ac~ a n d (cs} data, even a p p r o x i m a t e l y , w i t h a single set of p a r a m e t e r s , 3~, a n d has to be rejected.

13.3 Experimental study of

A c~

Cobalt a n d iron t h i n films h a v e been extensively studied a n d h e r e a f t e r we focus on these materials. In the following, e x p e r i m e n t a l data from several g r o u p s are

496 Handbookof Infrared Detection Technoloqies analyzed in the framework of the above described model. The spin-dependent IMFP component is of particular interest as it mostly originates from electronelectron interaction and can be measured with accuracy. The dark squares and crosses are Co data from ref. 37 (ll, 2.5 nm-thick layer, X, 4 nm-thick layer). The spin-dependent IMFP in Co was measured at several energies in refs. 38 and 39 and these data correspond to the circles ( 9 and to the cross (+) respectively. Similar measurements were performed in Fe, and the Acy(Fe) values have been scaled by multiplication by A.A/'h(Co)/A.Afh(Fe)=0.77, a good approximation in the relevant energy domain (from eq. (37), assuming that IMSdds(Ep)] has strongly decreased; see below). The corresponding points are indicated by the triangles (A) (ref. 17), (V) and ( A ) ( r e f . 39). Low-energy measurements performed on Co and giving high Acs values (Acy~,().8 nm -1 at 1.7 eV), which have been obtained through a very reliable technique, are indicated by 7-1, after ref. 40. Recent results obtained by the BEEM technique in the range 1-2 eV, taken from ref. 41, are indicated by the A symbol. The very-low-energy determination of Filipe et al. 42 in Fe is marked by the symbol ~ (without any scaling, see the discussion below). To determine the Cyd~ value in Co, we use eq. (3 ] ) and the data from ref. 37. For a first estimate, we consider the measurements of Acy at E p : 6 e V (almost the lowest energy where the model strictly applies; 6.7:3 eV). Because the different samples give somewhat different results, and as stray effects will reduce the spin asymmetry, we use the largest values and take Acy(Co)=0.50 nm -1. Neglecting the term proportional to ,Ale sp in eq. ( 31 ) and taking [MSdd~(6 eV)12/lMSddsl2 : 1 , we -1, i.e. CydC~().()43nm -1. At Ep=lSeV, Acy is obtain (vv/v) cyd~ strongly reduced, being of the order of (). 15 n m - l : assuming that the exchange matrix element at this energy is small, we deduce from eq. (37), [M~d~s(1:5eV)12/ IMSddsl2 ~

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