Semiconductors and Semimetals, Volume 18 Mercury Cadmium Telluride

SEMICONDUCTORS AND SEMIMETALS VOLUME 18 Mercury Cadmium Telluride

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SEMICONDUCTORS AND SEMIMETALS Edited b y R . K . WILLARDSON ELECTRONIC MATERIALS DIVISION COMINCO AMERICAN INCORPORATED SPOKANE, WASHINGTON

ALBERT C . BEER BAlTELLE COLUMBUS LABORATORIES COLUMBUS, OHIO

VOLUME 18 Mercury Cadmium Telluride

1981

ACADEMIC PRESS A Suhsidia ry o j Harco iirt Bruce Jovunovic h , Pu hl is hers

N e w York London Puris Sun Diego Sun Fruncisco Siio Puulo Sydney Tokyo Toronto

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L'OPYRICHI' 198 1 , ItY ACADEMIC PRESS, I N r . AI.1. RIGHTS RESERVED. NO PART 01: THIS I'UBI.ICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCI.UI)INC; PHOTOCOPY, RECORDING, OR ANY INI~ORMATIONSTORAGE AND RETRIEVAL SYSI'EM, WITHOUT PERMISSlON i N WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC.

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Uniied Kirigiloni Ediiiori published h y ACADEMIC PRESS, INC. (LONDON) LTD.

24/28 O v a l Road, London N W l

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Library o f Congress Cataloging i n Publication Data Main entry under t i t l e : Saniconductors and semimetals. Includes hiblioqraphical references and index. Contents: v. 1-2 Physics o f 111-V cmpounds-v . 3. Optical properties o f 111-V cmpounds--[etc.l --v. 18. Mercury cadmim t e l l u r i d e . 1. Semiconductors--Collected works. 2. S m i m e t a l s --Collected works. I . Willardson, Robert K . 11. Beer, Albert C . , j o i n t ed. 111. Title. G610.9.547 537.6'22 65-26048 ISBN 0-12-752118-6 ( V . 18) AACR2

PRINTED IN THE UNITED S I A I t S OF AMERICA 81 82 83 84

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Contents LISTOF CONTRIBUTORS . PREFACE. . . . . NOMENCLATURE.

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Chapter 1 The Emergence of (Hg.-.Cd. )Te as a Modern Infrared Sensitive Material Paul W. Kruse I . Historical Overview . . . . . . . . I1. Review of the Electrical. Optical. and Structural Properties References . . . . . . . . . .

Chapter 2 Preparation of High-Purity Cadmium. Mercury. and Tellurium H . E . Hirsch. S . C . Liang. and A . G . White I . Introduction . . . . . . I1 . Purification Processes . . . . . I11. Purification of Cadmium . . . . IV . Purification of Mercury . . . . V . Purification of Tellurium . . . . VI . Special Products for (CdHg)Te Preparation References . . . . . . .

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Chapter 3 The Crystal Growth of Cadmium Mercury Telluride W . F . H . Micklethwaite I . Introduction . . . . . . . . . . I1. I11. IV . V.

Crystal Growth by the QuencWRecrystallization Method Liquid/Solid Growth . . . . . . . Epitaxial Growth . . . . . . . Other Considerations . . . . . . . References . . . . . . . . . V

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CONTENTS

Chapter 4 Auger Recombination in Mercury Cadmium Telluride

Paul E . Peterscri I . Introduction . . I1 . 111. IV . V.

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Auger Lifetime in Nondegenerate Material . Auger Lifetime in Degenerate Material . Experimental Results . . . . . Summary . . . . . . . Appendix . . . . . . . References . . . . . . .

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Chapter 5 (HgCd)Te Photoconductive Detectors R . M . Broudy cind V . J . Mazurczyck I . Introduction . . . . . . . . I1 . Performance Parameters . . . . . . 111. Simple Photoconductivity . . . . . . IV . Photoconductive Device Analyses . . . . V . Photoconductive Device Design . . . . . VI . Technology of (HgCd)Te Detectors . . . . References . . . . . . . . .

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Chapter 6 Photovoltaic Infrared Detectors M . B . Reine. A . K . Sood. und T . J . Tridwell I. I1. 111. IV .

Introduction . . . . . . Theory of p-n Junction Photodiodes . . Hg,-,Cd, Te Junction Photodiode Technology Summary and Conclusions . . . . References . . . . . . .

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Chapter 7 Metal-Insulator-Semiconductor Infrared Detectors M . A . Kinch I . Introduction

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I11 . (HgCd)Te MIS Experimental Data . . . IV . (HgCd)Te MIS Photodiode Technology . . V . (HgCd)Te Charge Transfer Device Technology . VI . Summary . . . . . . . . References . . . . . . . .

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

R. M. BROUDY, Honeywell Electro-Optics Operations, Lexington, Massachusetts 02173 (157) H. E. HIRSCH, Technical Research, Cominco Ltd., Trail, British Columbia V l R 4L8, Canada (21) M. A. KINCH,Texas Instruments Znc., Dallas, Texas 75265 (313) PAULW. KRUSE,Honeywell Corporate Technology Center, Bloomington, Minnesota 55420 (1) S. C. LIANG,Electronic Materials Division, Cominco American Zncorporated, Spokane, Washington 99216 (21) V . J . MAZURCZYCK,* Honeywell Electro-Optics Operations, Lexington, Massachusetts 02173 (157) W. F. H. MICKLETHWAITE, Electronic Materials, Cominco Ltd., Trail, British Columbia V l R 4L8, Canada (47) PAUL E. PETERSEN, Honeywell Corporate Technology Center, Bloomington, Minnesota 55420 (12 1) M. B. REINE,Honeywell Electro-Optics Operations, Lexington, Massachusetts 02173 (201) A. K. SOOD,Honeywell Electro-Optics Operations, Lexington, Massachusetts 02173 (201) T. J. TREDWELL,? Honeywell Electro-Optics Operations, Lexington, Massachusetts 02173 (201) A. G. WHITE,Technical Research, Cominco Ltd., Trail, British Columbia V l R 4 L 8 , Canada (21)

* Present address: Bell Telephone Laboratories, Holmdel, New Jersey 07733. t Present address: Research Laboratories, Eastman Kodak Company, Rochester, New York 14650.

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Preface A fascinating chapter in compound semiconductor alloys was opened in 1958 when W . D. Lawson and co-workers at the Royal Radar Establishment at Malvern, England discovered that Hg,-,Cd,Te alloys were semiconductors with bandgaps that could be varied from 0.0 to 1.605 eV as x increased from 0.17 to 1 .O. The demonstration of long-wavelength photoconductivity in these alloys led the way for subsequent development of infrared detectors. Studies of the basic properties of this alloy were carried out at various laboratories such as the Laboratoire de Magnetisme et le Physique des Solides in France and the Honeywell Corporate Research Center and Lincoln Laboratory in the United States. Now, work on this alloy system and devices made from it has expanded to over 60 laboratories and production facilities ranging from the Ioffe Institute in the USSR and the Academy of Sciences in Poland to Technion in Israel and Fujitsu in Japan. During the past two decades (HgCd)Te alloys have found widespread applications as infrared detectors, which are vital in a variety of military, space, and industrial systems. Although these alloys are not as well known to the commercial market as silicon, GaAs, Gap, InP, (GaAl)As, and Ga(AsP), the dollar volume now is actually comparable to the total for the latter five compounds. “Semiconductors and Semimetals” first summarized the accumulated knowledge of (HgCd)Te and closely related alloys in Volume 5. More recently (HgCd)Te and the closely related alloy (HgCd)Se were treated in a majority of the chapters in Volume 16. For example, Chapter 2 was devoted to the crystal growth and electrical and physical properties of (HgCd)Se, while Chapter 3 concentrated on magnetooptical properties of (HgCd)Te, including a review of the most accurate determinations of band parameters and transport properties. The subject of Chapter 4 was nonlinear optical effects-mostly those arising from large values of the third-order electric susceptibility in (HgCd)Te. A great variety of useful devices results from phenomena such as resonant four-photon mixing, optical phase conjugation, and the spin-flip Raman laser, with its useful tuning capabilities. The present volume, devoted entirely to (HgCd)Te, deals with the practical production and use of these alloys in photoconductive and photovolix

X

PREFACE

taic infrared detectors and arrays. The first chapter provides a historical overview as well as a review ofthe electrical, optical, and structural properties. For the convenience of the reader, a number of figures from Volume 5 of this treatise are reproduced. An especially useful feature is the information on carrier mobilities and on the longitudinal and transverse phonon frequencies. The second chapter is concerned with techniques for purifying cadmium, mercury, and tellurium. While the technology for preparing these elements with individual impurities less than one part per billion is known, doing it reproducibly for all impurities has not been possible. An associated problem is quantitative analysis at the one-part-per-billion level. Spark source mass spectrometry can provide a semiquantitative analysis for many impurities, but numerous interferences preclude some. Flameless atomic absorption is excellent for some individual impurities, but cannot help with the important group V and VII elements, which are acceptors and donors, respectively, in (HgCd)Te. In Chapter 3, the phase diagram and its implications regarding crystal growth are discussed. Each of the various methods of crystal growth and the results of many investigators are reviewed. Most of the photoconductive devices currently in production utilize crystals grown by a recrystallization method, because it is cost effective in providing the required uniformity of composition. On the other hand, materials with the best purity have been obtained from melt-grown crystals due to the additional purification obtained by impurity segregation during crystal growth. In both photoconductive and photovoltaic detectors, the device performance depends critically on the lifetime of the photoexited carriers. Chapter 4 contains a detailed discussion of carrier lifetime, recombination mechanisms, and the effects of the light-hole and nonparabolic bands, as well as degeneracy. Excellent agreement is shown between experiment and theory using an electron-electron Auger process in which an electron recombines with a heavy hole as being dominant in rz-type (Hg,.,Cd,.,)Te. It is important to note that in characterizing a sample, some investigators refer to experimental data obtained at about 110 K or the peak lifetime, while others are using measurements at 78 K where the lifetime is approaching a minimum. In p-type (HgCd)Te it is expected that the lighthole band will be a major factor in Auger recombination, but more and better experimental data are required to clarify its role. The present generation of (HgCd)Te detectors are photoconductive with linear arrays up to 180 elements. They are the eyes of practical and widely used infrared systems involved in thermal imaging, surveillance and other military, space, and commercial applications. The objective of Chapter 5 is to present an up-to-date description of the theory and basic

PREFACE

xi

principles as applied to these detectors in a form suitable to aid designers and engineers. The next generation of (HgCd)Te detectors will be photovoltaic with two-dimensional focal plane arrays containing thousands of detector elements. Apropos of this, Chapter 6 is a review of the present status ofp-n junctions and Schottky barrier photodiodes, including the theory of operation as well as the fundamental and practical performance limits. Fabrication of arrays utilizing both diffusion and ion implantation' is treated, with emphasis on the latter. Especially valuable is a summary of the electrical and optical properties, including those of minority carriers derived from measurements on p - n junctions. Future generations of focal plane arrays may utilize the technologically challenging intrinsic monolithic approach, where the focal plane contains not only tens of thousands of detectors, but also signal processing functions such as time delay and integration, multiplexing, array staring mode operation, antiblooming, and background subtraction. Detection and signal processing in (HgCd)Te are the subjects of Chapter 7, where the general theory of metal-insulator-semiconductor devices is reviewed and compared with experimental data. The extension of single-level (HgCd)Te technology into the multilevel capability required for charge transfer device operation is discussed. Specific consideration is given (HgCd)Te infrared sensitive charge-coupled shift register performance. The metal-insulator-semiconductor photodiode is relatively easy to fabricate since it does not involve processing extremes. Device quality is determined almost completely by the properties of the substrate material. Charge-coupled-device and charge-injection-device technologies in (HgCd)Te are practical now, and the development of monolithic infrared sensitive integrated circuits to perform advanced signal processing is proceeding. The editors are indebted to the many contributors and their employers who make this treatise possible. They wish to express their appreciation to Corninco American Incorporated and Battelle Memorial Institute for providing the facilities and environment necessary for such an endeavor. Special thanks are also due the editors' wives for their patience and understanding.

R. K . WILLARDSON ALBERT c. BEER

Nomenclature It should be noted that Chapters 2 and 3 as well as current Canadian and European literature follow the older European nomenclature [Report of the German Commission for Nomenclature, Meyer, H e f v . Chim. Acta 20, 159-175 (1937)], where the names of the elements of the inorganic compound with two cations are arranged with the more electropositive cation first, or in the case of two anions they are given in alphabetical order, e.g., (CdHg)Te, (GaAl)As, In(AsP). The other chapters refer to (HgCd)Te following the newer American nomenclature, where according to the American Version of the International Union of Pure and Applied Chemistry Inorganic Rules as published in J . Am. Chem. Soc. 82, 5525 (1960) the following nomenclature for inorganic chemistry is applicable: 1. In formulas the electropositive constituent (cation) should always be placed first, e.g., CdTe. 2. Cations shall be arranged in order of increasing valence. 3. The cations of each valence group shall be arranged in order of decreasing atomic number, e.g., (HgCd)Te, (PbSn)Te. 4. Anions containing the smallest number of atoms shall be cited first. 5 . In the case of two ions containing the same number of atoms they shall be cited in order of decreasing atomic number, e.g., Ga(AsP).

xii

SEMICONDUCTORS A N D SEMIMETALS, VOL. 18

CHAPTER 1

The Emergence of Hg,-,Cd,Te Infrared Sensitive Material

as a Modern

Paul W . Kruse I. HISTORICAL OVERVIEW . . . . . . . . . . . 11. REVIEWOF THE ELECTRICAL, OPTICAL, A N D STRUCTURAL

PROPERTIES.

. . . . . . . . . . . . . . . Effective Muss Ratio . .

1. Energy-Band Siructure 2. Forbidden Energy Gap

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3 . Electron 4. Fermi Energy and Intrinsic Concentruiion 5 . Electron Mobility . . . . . . . 6 . Hole Mobiliiy . . . . . . . . 7. Optical Absorption Edge . . . . . 8. Phonon Frequencies . . . . . . 9. Lattice Constunt and Density . . . REFERENCES. . . . . . . . .

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I. Historical Overview? The usual course of development of a new semiconductor material begins with university research and ends with industrial exploitation. Such has not been the case with Hg,-,Cd,Te. From the outset development has occurred largely within industry and at national laboratories. It is only recently that university research has taken place. The basis for this anomaly lies in the unique role which Hg,-,Cd,Te plays in infrared detection for military applications. Although radar technology came of age during World War 11, infrared technology was in its infancy, consisting principally of active infrared image converters and single element PbS cells (Cashman, 1946). Neither was capable of passive thermal imaging, an emerging military need. In the decade following the war, the lead salt family, including PbS, PbSe, and PbTe, was exploited (Cashman, 1959). In addition to their use in missile guidance, PbSe and PbTe, with absorption edges at 77 K near 5 p m , were potentially useful for 3-5 pm thermal imaging. During the latter part of the period it was determined that InSb, a member of the newly discovered 111-V compound 1- As viewed from the limited perspective of the author.

1 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-7.52118-6

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PAUL W. KRUSE

semiconductor family, held promise as a material useful for thermal imaging (Reike et ul., 1959). Dopants for Ge were also discovered which had excitation energies useful for thermal imaging; those of greatest interest for military applications were Ge: Au and Ge: Hg (Levinstein, 1959). As the 1950s ended, those materials under active investigation for military thermal imaging systems were Ge: Hg and InSb. The former had the virtue of responding throughout the 8-12 pm atmospheric window, but required cooling to around 30 K. The latter operated at 77 K but operated only in the 3-5 Frn interval. Thus, the need existed for a new material which would combine 8-12 pm response and 77 K operation. A search therefore began for such a material. One candidate was gray Sn, which was thought to be a narrow-gap (i.e., 0.1 eV) semiconductor. It turned out that gray tin was semimetallic and unstable at room temperature. HgSe, another candidate in the late 1950s, was believed to be a semiconductor, but studies showed that it too was semimetallic (Blue and Kruse, 1962b). HgTe, a third candidate, was thought to be a narrow-gap semiconductor, but it also turned out to be semimetallic (Harman et al., 1958). What was really needed was an “InSb-like” material whose properties were similar to InSb but whose energy gap was about half as large. It was realized that an intrinsic photoconductor was better than an extrinsic one in terms of reduced cooling requirements, and that a direct-gap material was superior to an indirect one in terms of free-carrier lifetime, and thus speed of response. The sought-for material should be radiative lifetime limited in order to minimize the cooling needed to attain the photon noise, or BLIP, limit (Kruse er a / . , 1962b). All of these characteristics were sought in the as-yet undiscovered material. In 1959, Lawson, Nielsen, Putley, and Young published a paper reporting that the alloy system Hg,-,Cd,Te exhibited semiconducting properties over much of the composition range (Lawson et a/., 1959). The forbidden energy gap was found to be dependent on the composition variable x, ranging from a wide-gap semiconductor for x = 1 to a semimetal at x = 0. This was widely recognized as an InSb-like material which appeared promising, and it was therefore selected for investigation. Studies began at laboratories in the U.S. (Kruse et af., 1962b; Harman et al., 19611, France (Bailly et al., 1963; Rodot and Henoc, 1963), Poland (Galazka, 1963), and the Soviet Union (Kolomiets and Mal’kova, 1963). Because of the potential military application, secrecy surrounded some of these efforts. Early investigations, from 1961- 1965, were concerned with determining a method for preparing crystals of Hg,-,Cd,Te having the proper x value to have an absorption edge at 12 pm at the temperature of opera-

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

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tion. This was the optimum; if the edge were too short or too long, then the BLIP-limited value of D*(300 K) would be too 1ow.t It was realized that the optimum spectral response was obtained by convoluting the atmospheric transmission spectrum with the thermal emission spectrum of the earth (Kruse et al., 1962b). The initial problem to be faced was one of determining accurately the energy gap as a function of composition and temperature. This was not at all easy to do. There was the difficulty of determining the composition itself. Determining the composition by measuring the density was sufficiently accurate for a large sample, but it assumed that the composition was uniform throughout the measured volume. The x value of very small volumes could be measured with an electron beam microprobe, but the accuracy was poor. Measurement calibration data were also insufficient. Crystal growth was a major problem, especially because of explosions. Because of the high vapor pressure of free Hg, open-tube methods were not employed. The initial approach was the Bridgman technique, in which the elements were sealed within a quartz ampoule that was heated above the liquidus temperature appropriate to the composition, then lowered through a temperature gradient (Woolley and Ray, 1960; Blair and Newnham, 1961). The quality of the thick-walled quartz from which ampoules were constructed was inconsistent, and explosions were frequent. A sidearm at a lower temperature to establish the Hg vapor pressure was sometimes employed (Harman, 1967). Because of the health hazard, it was necessary to seal the tubes and furnaces in steel liners, with proper venting to remove Hg vapor in case of an explosion. It was soon realized that constitutional supercooling gave rise to a dendritic growth pattern within the crystal, in which there was a microscopic web of high-x material within a low-x surrounding. The initial attempts to avoid this employed a rocking furnace to thoroughly mix the melt. The ampoule was then lowered through a very large temperature gradient at a very slow rate. Such attempts were only partially successful. It was then discovered that a high-temperature anneal (just below the solidus temperature) would remove the dendritic structure. Electrical defects that resulted from deviations from stoichiometry were another problem. It was discovered that they could be controlled by a low-temperature anneal. Optical and galvanomagnetic studies were underway from the very beginning. The optical studies were directed toward determining the energy gap by measuring the energy of the absorption edge. The early edges t D*(300 K) is the signal-to-noise ratio measured in a I-Hz bandwidth in response to 1 W of radiant power from a 300-K blackbody incident on a detector normalized to 1 cm2 sensitive area.

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P A U L W . KRUSE

were soft, and often of unusual shape, because the relatively large samples employed in the spectrometers incorporated composition gradients (Kruse and Blue, 1963; Blue, 1964). Even so, such measurements revealed that at 77 K, the desired temperature of detector operation, the required x value for a 12-pm absorption edge was about 0.20 (Scott, 1969; Schmit and Stelzer, 1969). The galvanomagnetic studies were mostly measurements of the Hall coefficient and resistivity as functions of temperature for a given composition (Blue and Kruse, 1962b; Galazka, 1963); thus energy gap, carrier concentration, and mobility could be determined. The early studies concentrated on n-type samples. It was found that Hg in excess of that required for stoichiometry needed to be loaded into the ampoule. Too much excess resulted in droplets and voids in the crystal. Too little resulted in p-type samples. The electron density of n-type ~ . samples of x = 0.20 material was usually about 1-2 x loi5 ~ m - Very unusual shapes were found in some of the Hall curves, e.g., double crossovers, and Hall coefficients that showed a dip at low temperatures. Some of these anomalies were later determined to arise from an n-type surface on lightly doped p-type material (Scott and Hager, 1971). The preparation and evaluation of infrared detectors made from the crystals was carried out in parallel with these studies. Attention was directed toward photoconductivity in n-type samples. Here too, spurious effects showed up early. Composition and purity gradients in the individual detector elements gave rise to the bulk photovoltaic effect (Kruse, 1965). These spurious effects confused the interpretation of the spectral response and responsivity measurements of the early detectors. Despite all of these obstacles, progress was rapid. By 1965 Hg,-,Cd,Te photoconductive infrared detector technology had advanced sufficiently so that prototype detectors could be made for thermal imaging systems. Photoconductivity continued to be of major interest; lifetime studies were undertaken (Ayache and Marfaing, 1967). Airborne thermal mappers based upon single elements or small linear arrays of InSb and Ge: Hg had been developed earlier and placed into limited production in the U.S. New mappers were forthcoming based upon small arrays of Hgo.,,,Cdo.20,Te detectors. Yield of high performance detectors was a problem; even today there are yield problems. During the second half of the 1960s much interest was devoted to the preparation of epitaxial layers of Hg,-,Cd,Te. A close-spaced method utilizing evaporation of HgTe upon a CdTe substrate was reported (Cohen-Solal et d.,1965). Another employed a temperature gradient between source and substrate (Tufte and Stelzer, 1969). Thin films of Hg,-,Cd,Te were also prepared by sputtering (Kraus ef al., 1967). The 1970s were a period in which (Hg,Cd)Te technology made rapid ad-

1 . Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

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vances. Photoconductive detector technology matured, especially that for Hg,,,,Cd,,,,,Te used in 8-12 p m airborne mappers and in 8-12 p m FLIRs (forward looking infrared systems) (Kinch and Borrello, 1975). Other compositions suitable for use in the 1-3, 3-5, and 15-30 p m range were also investigated (Reine and Broudy, 1977). Photovoltaic detector development also advanced rapidly, including those types suitable for optical heterodyne receivers for CO, laser communications systems operating at 10.6 pm. During the early 1970s, (Pb,Sn)Te detector technology was also rapidly advancing (Melngailis and Harman, 1970). However, interest in (Pb,Sn)Te waned for various reasons, including its high dielectric constant and relatively large thermal expansion coefficient, and it is no longer a viable competitor for military infrared systems. Today (Hg,Cd)Te ranks as one of the most thoroughly studied semiconductors (Long and Schmit, 1970; Harman and Melngailis, 1974; Dornhaus and Nimtz, 1976). From an investment point of view, it is the third most important semiconductor, outranked only by Si and GaAs. Mercury cadmium telluride detector linear arrays of 60- 180 elements are in mass production in the U.S. for the common modular FLIR. It is the preferred material for second generation FLIRs, in the form of a (Hg,Cd)Te photovoltaic detector array bonded to a Si CCD chip. Hg,-,Cd,Te PV detector/% CCD hybrid matrix arrays are also under investigation for terminal homing missile seekers. In other military applications, arrays having spectral responses as short as 2-3 p m for detecting missile plumes, and as long as 15-30 p m for detecting spaceborne objects are under development. Civilian applications of Hg,-,Cd,Te thermal imaging systems are also being explored, including thermography for early detection of breast cancer. During the 198Os, Hg,-,Cd,Te technology, including research development and production, will continue to expand. Infrared detector development and production will increase rapidly. New phenomena and applications will emerge; recent examples are CCD shift registers (Chapman et ul., 1978), injection lasers (Harman, 1979), and nonlinear optics (Kruse et al., 1979). This trend will clearly continue; by the end of the 1980s, Hg,-,Cd,Te will be established as one of the most useful of all semiconductors. 11. Review of the Electrical, Optical, and Structural Properties

There exists a wealth of published data concerning the properties of Hg,-,Cd,Te (Dornhaus and Nimtz, 1976). Because of compositional nonuniformities and measurement inaccuracies, some of the early data

6

PAUL W . K R U S E

have been superseded by better values. The following brief overview includes data which are believed accurately to represent the compositional dependence of selected parameters. 1. ENERGY-BAND STRUCTURE The band structure near the r point for three different values of forbidden energy gap is illustrated in Fig. 1 (Overhof, 1971). The left-hand part illustrates the semimetallic behavior found in HgTe and Hg,-,Cd,Te for which x is less than -0.16 at 0 K. The re state, which is the conduction-band minimum in CdTe and other zinc-blende semiconductors, lies at a lower energy than the restate, which is the valence-band maximum in CdTe. Thus the energy gap Eo is negative in HgTe. The usual light-hole valence band becomes the conduction band and the conduction band becopes the light-hole valence band. Because of the k p interaction (Kane, 1966) the conduction band and light-hole valence bands are nonparabolic. The central part of Fig. 1 illustrates the region near the r point when the forbidden energy gap is slightly positive. Here the light-hole valence band is the normal Ts state and the conduction band, the normal re.The conduction and light-hole valence bands are nonparabolic and symmetric, with the free-electron and light-hole masses at the re and Tspoints very small and equal. The right-hand part of Fig. 1 illustrates the region near the r point when

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

7

the energy gap is relatively wide. The band structure is very similar to that of InSb, with a small amount of conduction-band nonparabolicity, which decreases and ultimately vanishes as the energy gap widens with increasing x value.

2. FORBIDDEN ENERGY GAP The dependence of the forbidden energy gap, i.e., the Ta-Ts transition, upon composition at 0 and 300 K is illustrated in Fig. 2 (Long, 1968). The temperature coefficient of the energy gap for CdTe is negative, which is

I. 6

1.4

1.2

1.0 c

-2,

a 0.8 Q

W

0.6 lr

w

z W

0.4

0.2 0 -0.2 -0.4

I HgTe

I

I

1

X

I

CdTe

FIG.2. Energy gap as a function of composition in Hg,-,Cd,Te. 0, interband magnetoreflection at 77 K; x , interband magnetoreflection at 4 K; f , optical absorption at 300 K; A,A, photovoltaic studies at 77 and 300 K; V, photoluminescence at 12 K. [From Long (1968).]

8

PAUL W . KRUSE

0.50 0.4 5

0.40 0.35

0.30 0.25 m

w

0.20

0.15 0.10

0.05 0 T (K)

FIG. 3. Dependences of the energy gap E , and the long wavelength limit A,, of Hg,,Cd,Te as functions of temperature and composition. [From J . L. Schmit and E. L. Stelzer ( I 969).]

the usual case for most semiconductors, but is positive for compositions rich in HgTe. Figure 3 illustrates the compositional and temperature dependences in more detail for x values equal to or less than 0.40, the region of most interest (Schmit and Stelzer, 1969). The left-hand ordinate is the forbidden energy gap expressed in electron volts, whereas the right-hand one is the corresponding absorption edge wavelength or photodetector long-wavelength limit. The data illustrate that the composition Hgo.,esCdo.sosTe is the proper choice for an infrared detector operating at 77 K having an energy gap of 0.10 eV (long-wavelength limit of 12.4 pm). The analytic expression for the data illustrated is

E, (eV)

=

1 . 5 9 ~- 0.25

+ 5.233(10-4)T(1-2.08x) + 0 . 3 2 7 ~ ~ (1)

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

9

where Eg is the energy gap in electron volts, x is the composition variable, and T is the absolute temperature.

3. ELECTRON EFFECTIVE MASSRATIO The dependence of the electron effective mass ratio upon composition is illustrated in Fig. 4 (Long, 1968). The values illustrated are at the conduction band edge. The ratio goes to zero at the semiconductorsemimetal transition. In the region of most interest, the right-hand half of the figure, the effective mass is small and directly proportional to the gap, as predicted by the Kane model (Kane, 1966). More detailed data concerning the effective mass is illustrated in Fig. 5 (Schmit, 1970). The values shown have been calculated based upon the I

I

0.025

0.020

mz -

0.015

m0

0.010

0.005

0

0 HgTe

0.I

X

0.2

0.3

FIG.4. Dependence of conduction band edge effective mass ratio upon composition at 0 K in Hg,-,Cd,Te. 0, interband magnetoreflection at 77 K; X, interband magnetoreflection at 4 K; cyclotron resonance at 4 and 77 K; A,oscillatory magnetoresistance at 4 K . [From D. Long (1968).]

+,

10

PAUL W . KRUSE X

0.07

0.70 w

2

0.65

0.06

0.60

0.05

0.55 0.50

0.04

0.45 0.40

0.03

0.35

0.30 0.28

0.26 0.24

0.02

0.22

0.20 0.IB

0.16

0.0I

0

0

50

100

150 200 TEMPERATURE ( K )

250

300

350

FIG.5 . Temperature dependence of the electron effective mass ratio of Hg,-,Cd,Te. [From J. L. Schmit (1970).]

Kane model and the measured dependence of energy gap upon composition illustrated in Fig. 3. The electron effective mass values illustrated in Fig. 5 are referred to by Schmit as the parabolic equivalent effective mass, i.e., the electron effective mass which would have to be employed in the standard expression for the intrinsic concentration in order to give the intrinsic concentration value predicted by the Kane model. 4. FERMI ENERGYA N D INTRINSIC CONCENTRATION

The temperature dependences of the reduced Fermi energy and intrinsic concentration with composition as an independent parameter are illustrated in Figs. 6 and 7 (Schmit, 1970). The calculations upon which the figures are based employ the measured dependence of the energy gap upon composition and temperature illustrated in Fig. 3 and expressed in Eq. (1). A nonparabolic conduction band was used. The valence band was approximated by a single parabolic band with hole effective mass equal to 0.55 m, (free-electron mass).

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

11

+3

F

0

-15 TEMPERATURE ( K )

FIG.6. Temperature dependence of the intrinsic reduced Fermi energy (measured from the conduction-band edge) of Hg,-,Cd,Te. [From J. L. Schmit (1970).]

5 . ELECTRON MOBILITY

Figure 8 illustrates the dependence of the free-electron mobility upon composition at 4 K (Long and Schmit, 1970). The parameter ,uIis the Hall mobility, i.e., the Hall coefficient divided by the resistivity. The curves are theoretical, with account taken of the dependence of free-electron mass upon concentration (Fig. 4); the scattering is assumed to be by singly ionized impurity or defect centers of density equal to the extrinsic electric concentration. Measured data points are also shown in the figure. Extremely high values of mobility for high-purity samples are observed near the semiconductor-semimetal transition. The dependence of the free-electron mobility at 4.2 K upon composition and free-electron concentration is illustrated in Fig. 9 (Scott, 1971). At this temperature, the mobility is determined by scattering from ionized impurities or defect centers, as has been seen in the calculations of Fig. 8. The values illustrated were calculated from the known dependence of electron effective mass upon composition (Fig. 4). The dependence of the electron Hall mobility upon composition and temperature, determined experimentally for n-type samples in which the free-electron concentration was less than 2 x 1015 cmP3, is illustrated in

12

P A U L W . KRUSE X

10'

lo1'

*-

10'6

E v

z

50

1015

w

I-

z u

0

z a 0

1014

0 m

2 a I-

z

10':

1012

10"

50

100

150

200

250

300

350

TEMPERATURE ( K )

FIG.7. Temperature dependence of the intrinsic carrier concentration in Hg,-,Cd,Te. [From J . L. Schmit, Honeywell Corporate Technology Center, personal communication, based upon revised data from J . L. Schmit. (1970).]

Fig. 10 (Scott, 1972). Below about 30 K, the value for Hp0.80Cd0.20Te is about 3 x 105 cm2/V sec, which is extremely high for semiconductors. The room-temperature mobility of Hgo.aoCdo.20Te is about 1 X lo4 cm2/V sec. As the x value increases, the mobility decreases monotonically. For x 2 0.25, the mobility increases as the temperature increases between 4 and 20 K. Over this composition and temperature range, singly ionized donor impurity scattering dominates.

1. Hg,-,Cd,Te

5 3

AS A MODERN INFRARED SENSITIVE MATERIAL

13

r

2

I o6

7

5

Y

:

3

c

0

-

0 u) 0

2

>

5

" 10

5

-

7

4

I

FIG.8. Hall mobility of electrons as a function of composition in Hg,-,Cd,Te at approximately 4 K.[From D. Long and J. L. Schmit (1970).]

Figure 11 illustrates the dependence of free electron Hall mobility upon composition at 300 K for n-type Hg,-,Cd,Te samples in which the freeelectron concentration was less than 2 x lOI5 (Scott, 1972). The highest value, about 3.5 x lo4 cm2/V sec, is obtained near x = 0.08, i.e., near the semiconductor-semimetal transition at room temperature, where the electron effective mass has its minimum value. 6. HOLE MOBILITY Most of the mobility data are for n-type samples. Figure 12 illustrates some data on the hole mobility determined from Hall effect and resistivity measurements on p-type samples (Schmit and Scott, 1971; Scott et af., 1976). The hole mobility at room temperature ranges from

I

14

PAUL W . KRUSE

I

I

I l l

I

15

I0

I

I

l

i

l

I

16

I0

l

i I'

7

CONDUCTION-ELECTRON CONCENTRATION ( c m V 3 )

FIG.9. Electron mobility at 4.2 K as afunction of composition in Hg,-,Cd,Te. [From M. w. Scott (1971).]

40-80 cm2/V sec. The temperature dependence is relatively small. As indicated, the p-type samples are relatively impure compared to the n-type ones of Fies. 10 and 11.

'"F

J W

CdTe

HgTe

MOLE

FRACTION CdTe

FIG.1 1 . Electron mobility at 300 K as afunction of composition. [From M. W. Scott (1972).]

T (K) FIG. 10. Temperature dependence of the electron Hall mobility in Hg,-,Cd,Te as a function of composition. [From M. W. Scott (1972).]

lo3

c

I00 TEMPERATURE ( K )

10

FIG. 12. Temperature dependence of the hole Hall mobility in Hg,,Cd,Te as a function of composition. [From J . L. Schmit and M. W. Scott (1971); M. W. Scott, E. L. Stelzer, and R. J . Hager (1976). Numbers (1)-(6) identify the samples.]

X

:

0.21 0.23 0.25 0 0

0

0 0

9

o 0 0 0

L

w

o o

0 0

o o

0

oo 0

ooo

a

0

1

0

0

0

0

0

0

:

0

0

0

I

I

01

0 2

0

0

0

l

0 3

o

n

0 0

0 0

0 0

0

0 0

0

O

0

0

0

0

,"

o

O 0

0

0

0 0

0

x

d x x x x

0

0

0 0 0

x x x

0

0 0

0 0

:,"

z

00

0

x o x x o x o x o x

O

o

0

0

0 0

0

0

I

0 4 ENERGY ( e V )

0

0 0

1-

0 5

0 6

0 7

FIG. 13. Optical absorption coefficient as a function of composition in Hg,-,Cd,Te.at room temperature. [From M. W. Scott (1969).]

1 . Hg,-,Cd,Te

I10

AS A MODERN INFRARED SENSITIVE MATERIAL

0

0.2

HgTe

04

06 X

0.0

17

I CdTe

FIG. 14. Longitudinal and transverse phonon frequencies in Hg,-,Cd,Te at 71 (+,O) and 300 K (@,a). [From R . Dornhaus and G. Nimtz (1976).]

7. OPTICALABSORPTION EDGE Figure 13 illustrates the dependence of the optical absorption coefficient upon photon energy for various compositions (Scott, 1969). The edges are steep, as expected for a direct-gap semiconductor. Early results (Blue, 1964) showing edges with a more shallow dependence upon energy probably were obtained from samples of nonuniform composition.

.

8. PHONONFREQUENCIES

Figure 14 depicts longitudinal and transverse phonon frequencies as functions of composition at 77 and 300 K (Dornhaus and Nimtz, 1976). Most of the data illustrated were originally published by Baars and Sorger (Baars and Sorger, 1972). The LO and TO frequencies were deduced by Kramers -Kronig analysis of reflectivity measurements.

PAUL W . KRUSE

18 I

\ 5

t

i

I

1

I

- 6.0

I

-

,DENSITY

7.5

* ,i

-1.0

6.475

0

l I-n

z

>

- 6.5 t9

0 V 6.470

n t J

-

0

6.465

ATTIC€ 6.460

0

6.0

CONSTANT

I

1

I

I

1

I

I

I

I

0I

02

03

0.4

0.5

06

0.7 0.7

0.8

09

5.5

1.0

X

FIG.15. Lattice constant and density of Hg,_,Cd,Te as a function of composition. [From D. Long and J. L . Schmit (1970).]

9. LATTICE CONSTANT AND DENSITY Figure 15 illustrates the dependence of lattice constant and density upon composition in Hg,-,Cd,Te (Long and Schmit, 1970; Woolley and Ray, 1960; Blair and Newnham, 1961). There is a small deviation from Vegard's law, i.e., the lattice constant is not quite linear with composition. ACKNOWLEDGEMENT Many colleagues at the Honeywell Corporate Technology Center and the Honeywell Electro-Optics Operation have worked for almost two decades in Hg,-,Cd,Te technology. Among them are Dr. Donald Long, Mr. Joseph L. Schmit, Dr. M. Walter Scott, Dr. Obert N . Tufte, Mr. Ernest L. Stelzer, Mr. Robert J. Hager, Dr. Marion Reine, Dr. Robert Broudy, and Mr. Robert Lancaster. Thanks to Darlene Rue for typing the manuscript.

REFERENCES Ayache. J. C., and Marfaing, Y. (1967). C . R. Acad. Sci. Paris B265,568. Baars, J., and Sorger, R. (1972). Solid Sfare Cornmutt. 10, 875. Bailly, F., Cohen-Salal, G., and Marfaing, Y.(1963). C. R . A m d . Sci. Paris 257, 103. Blair, J., and Newnham, R. (1961). "Metallurgy of Elemental and Compound Semiconductors," Vol. 12, p. 393. Wiley (Interscience), New York.

1 . Hg,-,Cd,Te

AS A MODERN I N F R A R E D SENSITIVE MATERIAL

19

Blue, M. D. (1964). Phys. Rev. 134, A226; in Phys. Semicond. 1, 233. Blue, M. D., and Kruse, P. W. (1962a). Bull. A m . Phys. Soc. Ser. I1 7, 202. Blue, M. D., and Kruse, P. W. (1962b). J . Phys. Chem. Solids 23, 577. Cashman, R. J . (1946). J . O p t . So(..A m . 36, 356. Cashman, R. J . (1959). Proc. Inst. Radio Eng. 41, 1471. Chapman, R . A. et a / . (1978). Appl. Phys. L e t f . 32, 434. Cohen-Solal, G., Marfaing, Y., Bailly, F . , and Rodot, M. (1965). C. R. Acad. Sci. Paris 261, 931. Dornhaus, R., and Nimtz, G. (1976). The properties and applications of the Hg,-,Cd,Te alloy system. In “Springer Tracts in Modem Physics, Solid State Physics” (G. Hohler, ed.), Vol. 78. Springer-Verlag, Berlin. Galazka, R. R. (1963). A d a Phys. Pulon. 24, 791. Harman, T. C. (1967). In “Physics and Chemistry of 11-VI Compounds” (M. Aven and J. S. Prener, eds.), p. 784. Wiley, New York. Harman, T. C. (1979). J . Elecrron. Muter. 8, 191. Harman, T . C., and Melngailis, I. (1974). Narrow gap semiconductors, Appl. Solid Sfrite Sci. 4. Harman, T. C., Logan, M. J., and Goering, H. L. (1958). J . Phys. Chrm. Solids 7, 228. Harman, T . C., Strauss, A. J., Dickey, D. H., Dresselhaus, M. S . , Wright, G. B., and Mavroides, J. G. (1961). Phys. Rev. Lett. 7 , 403. Kane, E. 0. (1966). The k p method, Semicund. Srmimer. 1. Khan, M. A., Kruse, P. W., and Ready, J . F. (1980). Optics L e f t . 5, 261. Kinch, M. A., and Borello, S. R. (1975). Infrared Phys. 15, 1 1 1 . Kolomiets, B. T., and Mal’kova, A. A. (1963). Fiz. Tverd. Tele 5 , 1219 [English Transl.: Sov. Phys. Solid State 5, 8891. Kraus, H. , Parker, S . G., and Smith, J. P. (1967). J . Electrochem. Soc. 114, 616. Kruse, P. W. (1965). Appl. O p t . 4, 687. Kruse, P. W., and Blue, M. D. (1963). Bull. A m . Phys. SOC. Ser. I1 8, 246. Kruse, P. W., Blue, M. D., Garfunkel, J. H., and Saur, W. D. (1962a). Injrured Phys. 2,53. Kruse, P. W., McGlauchlin, L. D., and McQuistan, R. 9 . (1962b). “Elements of Infrared Technology,” Chapter 9. Wiley, New York. Kmse, P. W . , Ready, J . F . , and Khan, M. A. (1979). Injrured Phys. 19, 497. Lawson, W. D., Nielsen, S. , Putley, E. H., and Young, A. S. (1959). .I. Phys. Chem. Solids 9, 325. Levinstein, H. (1959). Proc. Inst. Radio Eng. 47, 1478. Long, D. (1968). “Energy Bands in Semiconductors.” Wiley, New York. Long, D., and Schmit, J. L. (1970). Mercury cadmium telluride and closely related alloys, Semicond. Semimet. 5 . Melngailis, I., and Harman, T. C. (1970). Single crystal lead-tin chalcogenides, Semicond. Semimet. 5 . Overhof, H. (1971). Phys. Sturus Solidi B45, 315. Reine, M. B., and Broudy, R. M. (1977). A review of (Hg,Cd)Te infrared detector technology. In Proc. SPIE Tech. Symp., 2/st, Sun Diego, Culifortliu, August. SPIE, Bellingham, Washington. Rieke, F. F., DeVaux, L. H. and Tuzzolino, A . J. (1959). Proc. Inst. Rudiu Eng. 47, 1475. Rodot, H., and Henoc, J. (1963). C. R . A c a d . Sci. Paris 256, 1954. Schmit, 3. L., (1970). J . Appl. Phys. 41, 2876. Schmit, J. L., and Scott, M. W. (1971). Honeywell Corporate Technology Center, unpublished data. Schmit, J. L., and Stelzer, E. L. (1969). J . Appl. Phys. 40,4865.

20

P A U L W . KRUSE

Scott, M. W. (1969). J . Appl. Phys. 40, 4077. Scott, M. W. (1971). Honeywell Corporate Technology Center, unpublished data. Scott, M. W. (1972). J . Appl. Phys. 43, 1055. Scott, M. W., and Hager, R. J . (1971). J . App'pl. Phys. 42, 803. Scott, M. W., Stelzer, E. L . , and Hager, R. J. (1976). J . Appl. Phys. 47, 1408. Tufte, 0. N . , and Stelzer, E. L. (1969).J . Appl. Phys. 40, 4559. Woolley, J . C., and Ray, B. (1960).J . Phys. C h m . Solid.\- 13, 151.

SEMICONDUCTORS A N D SEMIMETALS, VOL. 18

CHAPTER 2

Preparation of High-Purity Cadmium, Mercury, and Tellurium H . E . Hirsch, S. C . Liang, and A . G . White I. INTRODUCTION. . . . . . . . . . . . . . . . . . .

. . .

. . . . . . . . . . . . . . . . . . . . . . . . 1 . Electrolysis . . . . . . . . . . . . . . . . . . . . 2. Distillation . . . . . . . . . . . . . . . . . . . . 3. Zone Refining. . . . . . . . . . . . . . . . . . . Iv. P U R I F I C A T I O N OF M E R C U R Y . . . . . . . . . . . . . . 4. Chemical Methods . . . . . . . . . . . . . . . . V. PURIFICATION OF TELLURIUM . . . . . . . . . . . . . 5. Introduction . . . . . . . . . . . . . . . . . . . 6. Chemical Refining: Crystullization und Precipitation . I . Hydride Process . . . . . . . . . . . . . . . . . 8. Chloride Refining . . . . . . . . . . . . . . . . . 9. Solvent Extruction. . . . . . . . . . . . . . . . . 10. Electrolytic Purification . . . . . . . . . . . . . . 11. Distillation . . . . . . . . . . . . . . . . . . . . 12. Zone Refining. . . . . . . . . . . . . . . . . . . VI. SPECIAL PRODUCTS FOR (CdHg)Te PREPARATION . . . . 13. Mercury . . . . . . . . . . . . . . . . . . . . . 14. Cadmium and Tellurium . . . . . . . . . . . . . . 15. General . . . . . . . . . . . . . . . . . . . . . REFERENCES. . . . . . . . . . . . . . . . . . . . . 11. PURIFICATION PROCESSES . . . 111. PURIFICATION OF CADMIUM..

. .

. . . .

. .

. . . . . .

. .

. . . . . .

. .

. .

. .

. .

. . . .

. .

. .

. .

. .

21 23 24 25 25 29 32 33 34 34 36 36 36 36 37 37 31 38 39 41 43 45

I. Introduction

The term “high purity” can mean different things. In this chapter, it is defined to mean a purity of 69 (99.9999%)o r better, and in any case, not less than 59 (99.999%), as is commonly understood in the high-purity metal trade. Purity cannot be defined without defining the method of analysis. Indeed, even analysis is meaningless without regard for the end use, and realistically, end use is a method of analysis. For instance, where high-purity metals, e.g., aluminum, copper, etc., are used in low-temperature (4 K) magnet applications, the residual resistivity reflecting the transition-element impurities becomes important, whereas the total impurity content is not. In the case of indium used for 21

Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-752118-6

22

H . E . HIRSCH, S. C . LIANG, A N D A . G . WHITE

doping silicon to make high-performance infrared detectors, the shallow-level impurities affect detector performance even though undetectable by other analytical means (lO-'Orange). In preparing compound semiconductors, often the isoelectronic impurities can be tolerated well above 1 ppm, whereas the carrier-generating impurities must be reduced to a level that would give less than l O I 3 carriers/cm3 in the final semiconductor. The identification of such impurities is difficult, if possible at all. The only analytical method is the determination of the extrinsic carrier concentration; where such measurements indicate no harmful effect or the concentration is at such a low level as not to affect the end use, the material is judged good. Thus, the only useful method of analysis, when dealing with truly high-purity materials, is the measurement of the factors or parameters which affect the end use. The material suppliers do not claim to be able to determine the precise amount of all the low-level impurities. The designations 59 or 69 have been chosen because it is believed that the metals processed and supplied to customers are essentially that pure. For routine quality control, the emission spectrometric method is the principal analytical tool. To assure consistency, at Cominco when establishing the production process all methods appropriate to the subject are used periodically on a continuing basis. These include emission and spark mass spectrographic analyses, chemical analysis, polarographic analysis, atomic absorption, residual resistivity, and other electrical measurements. The 69 purity allows a total impurity concentration of 1 ppm, as measured by emission spectrograph. While this requirement is demanding, for semiconductor applications it is not a very high purity. Specifically, the n-type cadmium mercury telluride (CdHg)Te single crystal used for making high-performance photoconductive infrared detectors peaking around 12 km must routinely exhibit fewer than 1015carriers/cm3 and preferably approaching 1014carriers/cm3. Such a carrier concentration corresponds to one part in 300 million, or 3 x lo+'. With regard to the net carrier concentration one might therefore designate the purity of the (CdHg)Te as 89. For such a product, it is important that the starting metals Cd, Hg, and Te have a purity better than 69. To deliver metals of adequate purity to allow production of (CdHg)Te routinely having a total concentration of active elements of less than lo-* is no simple matter, particularly in view of the added complication that oxygen is an active element in (CdHg)Te. Users must also possess and deploy adequately sophisticated technology. Since metal purity for such applications cannot be assured by any of the conventional analytical techniques, a direct but lengthy and costly approach is used. A portion of each production lot is taken and processed to (CdHg)Te single crystals. The

2.

PREPARATION OF HIGH-PURITY

Cd, Hg,

AND

Te

23

crystals must exhibit fewer than 1015carriers/cm3. While this method as closely as possible guarantees product quality, it does not always assure the desired quality because at such high purity the metals are inordinately active as collectors and scavengers. Packaging processes must also be observed to the most minute detail. A user who is unwilling to take an equal amount of care after receiving the metals will obviously not receive the benefit of the manufacturer’s efforts. When designing a purification process, one principal concept is followed. Because all purification processes are based on the difference in response of the host metal and the impurities to a given treatment and no element in the periodic table is so unique that no other elements will respond in the same way to the treatment, free energy consideration dictates that no single treatment can remove all impurities to less than level and still leave the host metal mostly unaffected, i.e., to give a reasonable yield. On the other hand, it is not difficult to find a treatment method based on one physicochemical principle to remove one group of elements, and another method based on a different principle to remove a different group. The two groups may overlap and include all impurities of concern, yet both groups will exclude the host metal. Thus, as a rule at least two processes based on significantly different prinCiples are used-for instance, electrolysis and zone refining-to complement each other. The combination is also chosen to take advantage of the primary metallurgy. A knowledge of the origin and processing history of the metal is a great advantage, which can accrue to the producer who has control of primary metals sources. Short of this, a reliable source of primary metal is absolutely necessary in order to obtain the specified purity on a routine basis. A detailed discussion of primary metallurgy is outside the scope of this paper. In the following descriptions, therefore, we shall present only the different methods used for purifying Cd, Hg, and Te. Combinations of these methods are used in various laboratories to obtain high-purity materials. 11. Purification Processes

Strange as it may seem, the preparation of high-purity metals is relatively simple. The only knowledge required is that of the basic chemical and physical properties of the subject metal. Processes mentioned here include distillation (including sublimation), electrolysis (including chemical displacement), wet chemical treatment, and zone refining. The processes are basically simple and the methods of application are straightforward. The specified purity is readily obtained if all necessary care against con-

24

H . E . H I R S H , S . C . L I A N G , A N D A . G. WHITE

tamination is taken. For instance, the human hand can be a source of contamination. Finger marks on high-purity metal surfaces are virtually indelible. The choice of gloves must be made with care. While baby powder will greatly reduce the discomfort of the wearer, if used it also introduces many elements foreign to the metal. Less noticeably, if the gloved hands, while processing the metal, should pick up a work tool in between motions, such actions may transfer contaminants from the tool to the metal. Undoubtedly, such constraints will slow down the work. In fact, they are some of the reasons why high-purity metals are costly. However, ignoring such requirements can be disastrous. No less important is the technique of comminuting a high-purity metal into a form which can be used in a practical manner without sacrificing purity. The most common technique is melting and casting. Materials for the melting and casting vessels must be chosen to cause no recontamination. High-density, high-purity graphite (such as UC grade ECL or equivalent), pyrolytic graphite, and quartz (sometimes Vycor) vessels, if properly cleaned, have all been successfully used. Melting and casting of cadmium and tellurium for (CdHg)Te semiconductor applications must be done in protective atmospheres such as argon, hydrogen, nitrogen, or vacuum. Mechanical cutting tools may also be used if the knowledge of avoiding contamination is properly applied. An instructive review of the requirements for maintaining purity during handling and exposure has been given by Zief ( 1978). 111. Purification of Cadmium

Cadmium occurs in nature as a minor constituent of zinc and zincbearing polymetallic ores. Production in the western world from 1967 to 1977 has been in the range of 10,000-14,000 metric ton per year. Virtually all cadmium is recovered as a coproduct in zinc plants, which may also treat zinc- and cadmium-bearing fumes and dusts from lead and copper smelters. The efficiency of cadmium recovery in electrolytic zinc plants has already reached a realistically high level ( 270%), and the future growth in the supply of cadmium will be tied to the demand for zinc. Recycling of cadmium is relatively insignificant now and will likely remain so in the foreseeable future. The purification practice for commercial grade cadmium has been surveyed by Lund and Sheppard (1964). The purity of current commercial cadmium metal is generally 99.96% or better. A typical analysis of Cominco commercial grade cadmium is shown in Table I. The relatively high purity of the commercial'grade metal reflects the significant differences in certain physical and chemical properties between

2.

PREPARATION OF HIGH-PURITY

Cd, Hg, AND Te

25

TABLE I TYPICAL ANALYSIS OF COMINCO COMMERCIAL GRADECADMIUM

Pb cu Ni Zn Al

TI Fe

co Mg Cd by difference

0.0025% 0.0025% 0.0010% 0.0002% 0.0003% 0.000 1% 0.0002% \

'.

c

-\

Q

u

\

I-" I00

\

50 ' : ; z l

\.

10'

'

'

'

'

I

'

'

'

'

FIG.12. Cd,Hg,-,Te solidus in the Hg comer of the Gibb’s triangle. 395°C; AT 5°C; - A T 0.5”C. [From Wong and Eck (1980).]

-

-

t

- 5-8

pm; T =

3.

THE CRYSTAL GROWTH OF CADMIUM MERCURY TELLURIDE

59

FIG. 13. Liquidus isotherms for Hg comer of the Cd-Hg-Te ternary at ( 1 ) 404°C. (2) 435"C, and (3) 454°C. [From Vanyukov et al. (1977).]

Atomic '/c,Cd

4.

CONSEQUENCES OF THE PHASE

DIAGRAMS

Consider first the case of a perfect pseudobinary melt. From the wide separation of the liquidus and solidus curves in Fig. 2 along any given isotherm, it is apparent that directional freezing of the liquid alloy will result in the formation of solid alloy solutions varying continuously in composition along the growth axis. Tiller (1963) has given Eq. (4) as the condition for the avoidance of constitutional supercooling in the course of freezing such an alloy:

C, = initial melt composition, k = segregation coefficient, m = slope of the liquidus at C,, D = diffusion constant in the liquid, V = solidification velocity, and G = temperature gradient ahead of the interface.

From Fig. 2 the liquidus slope can be estimated to be about 600 K/moie fraction. Schmit and Speerschneider (1968) have reported the segregation

60

W . F. H. MICKLETHWAITE

coefficient k to be 2-4 for initial cadmium telluride mole fractions of 0.02-0.10. Bartlett et ul. (1969) consider the case where the pseudobinary stoichiometry is not preserved. Because of the very high mercury partial pressure in this system, it is very difficult to control the stoichiometry exactly, and frequently the melts are metal deficient immediately before freezing. These authors suggest that for the case of tellurium segregation, the liquidus slope is approximately - 1000 K/mole fraction and considering a typical case of 1 mole % tellurium excess, k is approximately 0.1. Comparing these numbers with those earlier numbers for cadmium telluride segregation in the pseudobinary case, it seems reasonable that tellurium might cause constitutional supercooling before cadmium telluride would. These authors show several results which support this. The criticality of the metal/tellurium balance cannot, therefore, be overemphasized. For acceptable crystal growth, it is absolutely mandatory that this be controlled during all phases of the growth situation. 5.

PURl7-Y

Because of the very strong divergence of the liquidus/solidus lines in the phase diagram, little or no purification is possible by a normal freeze. The classical purification technique of zone refining is made doubly difficult by pronounced segregation into the two binary constituents and the simultaneous, very high mercury overpressure. Some workers have resorted to the expedient of zone refining the two binary subcompounds mercury telluride and cadmium telluride in their own separate furnaces and then combining them in the cadmium-mercury-telluride growth ampoule. Because of the technical difficulties of charging the precise stoichiometric amounts of the two binaries while simultaneously maintaining purity, most laboratories have resorted to directly combining the three elements taken from the purest sources available made usually by repeated distillation or zone refining. Kruse and Schmit (1973) give typical analyses of materials charged to a growth ampoule. Mercury typically contains about 8 ppb of impurities, mainly copper, silver, and gold. The cadmium used contains a total of about 200 ppb, mainly magnesium and silver, while the tellurium used contains perhaps 7 ppm of impurities, mainly excess oxygen and selenium. Since the subject of purification of these elements is discussed in Chapter 2 of this book, it will not be treated further here. Lin (1975) has given analyses typical of the resulting ternary compound for cadmium telluride mole fractions between 0.2 and 0.4. His data are reproduced in Table I. Steininger (1977) lists similar data for material grown by three methods: solid-state recrystallization method, solution-slush

3.

T H E CRYSTAL GROWTH OF CADMIUM MERCURY TELLURIDE

61

TABLE I DETERMINATION OF TRACE IMPURITIES IN TYPE AND n-TYPE (HgCd)Te BY EMISSION SPECTROMETRY A N D ATOMIC ABSORPTION Concentration (ppm) Impurity

P-tYPe

n-type

Weight of impurity barely detectable (ppm)

cu

0.13-0.61 0.39-0.77 1.2 -1.0 0.8 -1.5 1.3 -1.0

0.1-> 1, and in the remainder of the discussion the approximation I + b = b will be used. The device is usually placed in a simple series circuit with a load resistor whose conductance is much smaller than that of the device. A change in the latter’s conductance, G, produced by a change in the arrival rate of photons, will result in a signal voltage across the load resistor given by AVL = Vb AGIG,

(14)

where V , is the dc bias voltage on the detector. Combining Eqs. (2), (9), (12), and (14), and recognizing that PA= Q,Ahv, gives the responsivity in steady state: %A

=

[71(A)/Lwd)(A/hc)v,T/n,,

115)

where no is the average thermal equilibrium carrier density and d is the thickness of the photoconductor. Under the assumptions of no drift or diffusion and equal recombination, the time dependence of the average excess carriers is described by

aP/at = Q J ~= - A P / t .

(16)

Solving for M ( t )gives AP(r) = Q&qTe-‘’’, or in the frequency domain

+ (27”7)’]-’’*.

A P ( f ) = QsAr)T[l

(18)

The frequency dependent responsivity can therefore be written

+ (2flfT)2]-”2,

%!~(f) = %!~(o)[l

(19)

where WA(0)is the steady-state value given by Eq. (15).

4. Noise

(I. Johnson Noisc The conductance of the detector defined by Eq. (9) is of course an average value. The total current in the detector is made up of many individual vector components whose contributions are constantly changing

5. (HgCd)Te

PHOTOCONDUCTIVE DETECTORS

165

even if the number of charge carriers remain constant. The rms noise per unit bandwidth Af, due to thermal fluctuations, is independent of the material and device characteristics, and is given by the well-known expression for Johnson noise =

( 4 k T / @ Af.

(20)

b . Generation -Recombination Noise

The average number of carriers in the detector is determined by the balance between the generation and recombination processes. Fluctuations in these quantities results in another type of noise appropriately called generation-recombination noise. For a semiconductor where direct recombination dominates, the noise per unit bandwidth Af is (Van Vliet, 1958, 1967; Long, 1970)

where ( A N 2 )is the variance in the number of majority carriers and no is the average density. For a two-level system the variance has been evaluated in general and is given by (Long, 1970)

( A N 2 ) = g7,

(22)

where g is the generation rate and 7 is the relaxation time constant defined by Eq. (11). Equation (21) is correct both for steady-state as well as thermal equilibrium conditions. A key and simplifying assumption is often made that thermal and optical generation-recombination processes are independent. The variances are therefore uncorrelated and combine linearly so that (AN2) = (g&lerrn+

(g7)0,t.

(23)

The first variance is given by (Van Vliet, 1958, 1967; Long, 1970)

and the second by (g7)opt =

Ph =

PhLwd,

(25)

where P b represents the optically generated background hole density. Using the above relations, the g-r noise is found to be

166

R. M . B R O U D Y A N D V . J . M A Z U R C Z Y K

The full equation is practically required only over a small range of temperature, where the semiconductor changes from intrinsic to extrinsic. Outside of that range either optical or thermal generation dominates, and Eq. (26) reduces to the g-r noise for one or the other single generation process. Holes generated by the background flux are relatively independent of temperature when the detector is an extrinsic semiconductor and po may often be made negligible by reducing the temperature so that P b dominates. Under these conditions the noise is photon limited since it is determined by fluctuations in the photon arrival rate; then, as discussed in Part 11, computation of D* using Eqs. (l), (2), and (26) yields the BLIP limit given by Eq. (8). c. l / f Noise

The phenomenon of l/f noise will always determine the low frequency limit of device performance, although it is not related to the same fundamental principles that control the other noise mechanisms. At present, there is no rigorous theory of llfnoise. Nevertheless, it is well recognized that it is related to practical device structures, and may be reduced by improved device design and fabrication techniques. For convenience, the existence of l/f noise may be modeled in terms of the “corner frequency” f o , which is defined as the frequency at which the I/f noise power equals the g-r noise power. llfnoise at any frequency can then be modeled from the relation:

G,f= (fo/f)%-r(O),

(27)

where V&O) is the generation-recombination noise at the plateau region at frequencies below the high rolloff and well above the llfcorner frequency. Both V,,, and Vg-r depend on bias, temperature, and background flux in possibly different ways, so that fo may be a function of these parameters. llfnoise is treated in more detail in Part IV.

d . Ampl$er Noise The noise contribution of the amplifier can be modeled by the presence of a voltage noise generator of rms magnitude e, and a current noise generator i, . The voltage generator appears in series with the amplifier input while the current noise generator is in parallel with the input. These are usually considered to be white noise sources. When a detector of resistance r d is placed across the input of the amplifier, the noise (aside from detector Johnson noise) generated will be

Vf; = ef; + if;r:.

(28)

5. (HgCd)Te

PHOTOCONDUCTIVE DETECTORS

167

With typical (HgCd)Te photoconductive detectors the value of rd is usually less than 100 a. This low resistance, combined with an upper usable frequency range of perhaps 10 MHz allows one to ignore capacitance at the input. However, individual cases should be examined carefully to insure the validity of this assumption. e . Total Noise

The individual noise contributions from the detector and amplifier (V,) are uncorrelated and therefore add in quadrature. The total noise of the detector is given by

v: = vq + v;-*+ (Vl,f)Z + v:.

(29)

A characteristic noise spectrum is shown in Fig 2 to illustrate the contribution of the various noise components.

5. DETECTIVITY Combining equations of the previous sections gives the result,

The expression for D* has been put into this form to emphasize the fact that the maximum performance is given by DgUpand to isolate those factors which cause the performance to deviate from that value.

lif NOISE

g-r CORNER FREQUENCY g-r

NOISE

I/f CORNER F R E O U E N C Y

AMPLl F IE R [NOISE

FREQUENCY

FIG. 2. Characteristic noise spectrum of a photoconductive detector.

168

R. M. BROUDY A N D V. J. MAZURCZYK

At higher temperature where the detector is an intrinsic semiconductor (i.e., no = po) the g-r noise is sufficiently large that the condition V&,(f>

>>

v; + v;

(3 1)

can be achieved with moderate bias levels (except at high frequencies). Performance in the midrange frequencies, therefore, is determined entirely by the factor

which is strongly temperature dependent. Since the performance of the detector in this region is relatively poor, it is used under these conditions only when cooling capacity is limited. With decreasing temperature no becomes constant and dominates p o . The detector then becomes an extrinsic semiconductor, and D* is given by

D” = D&,p(l + ~ o / p b ) ~ ” ’ [ l

+ ( f o / f ) + (Vj +

V~)/VPr(f)]-”’.

(33)

It is obvious that maximum performance is achieved when po/pb> 1 , and Vo, takes the simple well-known form for an illuminated floating junction (Beck and Sanborn, 1979)

+ sqQsA/Zo>.

Voc = fkT/q) Using Eqs. (79), (82), (83), and (84),

(84)

takes the convenient form

Alternately, the photoconductive gain G,, may be used in addition to, or in place of %N from Eq. (85) by noting that %N =

(A/hc)sqGw.

(86)

It should be noted that Eq. (85) has essentially the same form as Eq. (77). Beck and Sanborn (1979) have calculated the temperature and background dependence of & and G, from Eq. (85) for the diffusion limited junction case in the low voltage bias limit for x = 0.39 (HgCd)Te detectors with the following set of typical device parameters and operating conditions: QB = lo", 10l2,and 1013 photons/cm2 sec, N A = 6 x 10l6 in transverse junction, N D = 6 x 1014 cm-3 in body of photoconductor, q = 1.0, d =7 x cm, device thickness, E, = 17.2, dielectric constant, 7, = 1 x sec, bulk lifetime.

196

R . M . BROUDY A N D V. J . MAZURCZYK

,-OB =! x 1 0 t 2 photons/sec ,in2

og

photons/sec cmZ

t

t.0

0

"

'

2

1

4

"

"

6

1

"

8

"

~

1 0 4 2 t 4

tOOO/T ( K - 0 FIG. 16. Calculated gain of the transverse junction device versus temperature and background for the diffusion limited case at low bias for an x = 0.39 (HgCd)Te photoconductor.

The results are shown in Fig. 16. It should be pointed out that the above derivations represent only a first-order evaluation, since the unavoidable effects have not been considered of contacts and longitudinal potential variation along the JFET. Inclusion of these effects (Beck and Sanborn, 1979) will modify the background dependences as well as bias dependences. Similar approaches (Beck and Sanborn, 1979) for the externally initiated transverse field devices may be based on MOSFET theory. VI. Technology of (HgCd)Te Detectors

Modern (HgCd)Te photoconductive detectors are generally fabricated in arrays by methods of which many are quite similar to processes used for silicon integrated circuits, including photolithography, etching, vacuum and sputtering metallization, insulator deposition, and wire bonding. In addition, special techniques applicable to (HgCd)Te may be used, such as slab bonding and subsequent lapping and etching. Array performance close to the theoretical limit has been achieved in many cases.

5. (HgCd)Te

PHOTOCONDUCTIVE DETECTORS

197

A typical set of key steps for array fabrication are the following: (1) Start with good material: Develop methods for material evaluation and selection. (2) Obtain the material in slices sufficiently thick for ease of handling: Prepare the backside. (3) Bond the slab to a substrate: Epoxy is generally used. (4) Lap, polish, and etch the material almost to final thickness (usually close to cm) chosen to be thick enough to absorb almost all of the optical radiation and thin enough to minimize bias current. (5) Delineate the array using a photolithographic process with etching. (6) Accumulate the surface: This process will often also be performed between (2) and (3) above. (7) Metallize after further photolithography for contact and possibly active area delineation. (8) Wire bond for external electrical contact.

For similar reasons to those well known in integrated circuit pro-

FIG. 17. Section of a linear (HgCd)Te photoconductive array. Active area dimensions are 0.00125 x 0.002 in. (L x W) for each element.

R. M. BROUDY A N D V. J. MAZURCZYK

198

-

4

-

3 -

-

0

0

X

i.“

2 -

I

1

.. . FIG.18. D*performance under reduced background levels for a 60-element array with the configuration shown in Fig. 17.

cessing, scrupulous attention must be given to process technique, procedures, and cleanliness for all of these steps. Fig. 17 shows a photograph of section of a modern multielement PC array that has been prepared according to this procedure. The D* of this array is presented in Fig. 18, which shows values approaching the theoretical limit for the reduced background of this measurement.

REFERENCES Beck, J. D., and Sanborn, G. S . (1979). Air Force Materials Laboratory Rep. AFML-TR-79. Borello, S., Kinch, M., and Lamont, D. (1977). fnfrared Phys. 17, 21. Broudy, R. M. (1974). NASA Rep. CR-132512. Broudy, R. M. (1976). Frequency characteristics of high performance (HgCd)Te detectors, Proc. Infrared Informar. Symp. Detector Specidly Group. Broudy, R. M., and Beck, J. D. (1976). Pror. Infrured Informar. Symp. Detector Speciulty Group. Broudy, R. M., Mazurczyk, V. J., Aldrich N. C., and Lorenze, R. V. (1975). Advanced (HgCd)Te array technology, Proc. Infrured Informat. Symp. Defector Sperialty Group. Burgess, R. E. (1954). Physica 20, 1007. Burgess, R. E. (1955). Proc. Phys. Soc. London B68, 661. Burgess, R. E. (1956). Proc. Phys. Soc. London B69, 1020. Domhaus, R., and Nimtz, G. (1976). The properties and applications of the Hg,-,Cd,Te alloy system, in “Springer Tracts in Modem Physics,” Vol. 78, pp. 1-119. SpringerVerlag, Berlin and New York. Eisenman, W. L., Meniam, J. D., and Potter, R. F., (1977). Operational characteristics of

5. (HgCd)Te

PHOTOCONDUCTIVE DETECTORS

199

infrared photodetectors, in “Semiconductors and Semimetals” (R. K. Willardson and A. C. Beer, eds.), Vol. 12, Chapter 2. Academic Press, New York. Finkman, E., and Nemirovsky, Y. (1979). J. Appl. Phys. 50, 4356. Hooge, F. N. (1969). Phys. Lett. 29A, 129. Kinch, M. A., and Borello, S. R. (1975). Infrared Phys. 15, 11 1. Kinch, M. A., Brau, M. J., and Simmons, A. (1973). J. Appl. Phys. 44, 1649. Kinch, M. A., Borello, S. R., Breazale, B. H., and Simmons, A. (1977). Infrared Phys. 17, 137. Kinch, M. A., Borrello, S. R., and Simmons, A. (1977). Infrared Phys. 17, 127. Kingston, R. H . (1978). “Detection of Optical and Infrared Radiation.” Springer-Verlag, Berlin and New York. Kittel, C. (1961). “Elementary Statistical Physics,” p. 145. Wiley, New York. Kolodny, A., and Kidron, I. (1981). Infrured Phys. (to be published). Kruse, P. W. (1979). The photon detection process, in “Infrared and Optical Detectors,” Chapter 1. Springer-Verlag, Berlin and New York. Kruse, P. W., McGlauchlin, L. D., and McQuistan, R. B. (1962). “Elements of Infrared Technology: Generation, Transmission and Detection.” Wiley, New York. Levinstein, H. (1970). Characterization of infrared detectors, in “Semiconductors and Semimetals” (R. K. Willardson and A. C. Beer, eds.), Vol. 5 , Chapter I . Academic Press, New York. Long, D. L. (1970). Infrared Phys. 7 , 169. Long, D. (1977). Private communication, who refers to the work of J. R. Hauser and P. M. Dunbar, Solid State Electron. 18, 716 (1975). Long, D. (1977). Photovoltaic and photoconductive infrared detectors, in “Topics in Applied Physics” (R. J. Keyes, ed.), Vol. 19, Optical and Infrared Detectors. SpringerVerlag, Berlin and New York. Long, D., and Schmit, J. L. (1970). Mercury-cadmium telluride and closely related alloys, in “Semiconductors and Semimetals” (R. K. Willardson and A. C. Beer, eds.), Vol. 5 , Chapter 5 . Academic Press, New York. Mazurczyk, V. J., Graney, R. N., and McCullough, J. B. (1974). High performance, wide bandwidth (Hg,Cd)Te detectors, Opt. Eng. 13, 307. Milton, A. F. (1973). Proc. Infrared Informat. Symp. Detector Specialty Group. Rittner, E. S . (1956). In Photoconduct. Conf. (R. Breckenridge, B. Russell, and E. Hautz, eds.), p. 215ff. Wiley, New York. Shacham-Diamand, Y. J., and Kidron, I. (1981). Infrared Phys. 21, 105. Schmit, J. L. (1970). J . Appl. Phyc. 41, 2867. Schmit, J. L., and Stelzer, E. L . (1969). J . Appl. Phys. 40, 4865. Smith, D. (1981). Submitted for publication. Van der Ziel, A. (1959). “Fluctuation Phenomena in Semiconductors.” Butterworth, London. Van Vliet, K. M. (1958). Proc. IRE 46, 1004. Van Vliet, K. M. (1967). Appl. Opt. 6 , 1145. Van Vliet, K. M., and Fassett, J. R. (1965). Fluctuations due to electronic transistions and transport in solids, in “Fluctuation Phenomena in Solids” (R. E. Burgess, ed). Academic Press, New York. Williams, R. L. (1968). Infrared Phys. 8, 337.

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SEMICONDUCTORS AND SEMIMETALS, VOL. 18

CHAPTER 6

Photovoltaic Infrared Detectors M. B. Reine, A . K . Sood, und T . J . Tredwell I. INTRODUCTION. . . . . . . . . . . . . . . . . . . . I . Applications jbr Hg,-,Cd,Te Photodiodes . . . . . . 2. Scope of This Chapter . . . . . . . . . . . . . . . 3. Summary (if Hg,-,Cd,Te Properties , . . . . . . . . 11. THEORY OF p-n JUNCTION PHOTODIODES . . . . . . . . 4. Current- Voltuge Characteristics . . . . . . . . . . 5. Photocurrent in p -n Junctions . . . . . . . . . . . 6. Noise Mechanisms . . . . . . . . . . , . . . . . 1. Responsivity, Noise Equivalent Power, and Detectivity 8 . Lateral Collection in Small-Area Junctions. . . . . . 9. Response Time . . . . . . . . . . . . . . . . . , 10. Auger Lifetime in p-Type H g , - , C d , T e . .. . . . . . 111. Hg,-,Cd,Te JUNCTIONPHOTODIODE TECHNOLOGY. .. . 1 1 . Ion Implantation . . . . . . . . . . . . . . . . . 12. Diffused Photodiodes . . . . . . . . . . . . . . . 13. Type Conversion in Hg,-,Cd,Te by Other Techniques. 14. Minority-Carrier Properties of p-Type Hg,-,Cd,Te . . 15. I/f Noise in Hg,-,Cd,Te Photodiodes. . . . . . . . 16. Schoitky Barrier Photodiodes . . . . . . . . . . . . 1V. SUMMARY A N D CONCLUSIONS . . . . . . . . . . . . . REFERENCES. . . . . . . . . . . . . . . . . . . , .

. .. .. .

. . . .

. . . . . . . .

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

. . . . . . . . . .

20 1 202 204 205 201 201 221 232 235 238 240 243 246 247 272 283 286 291 301 303 305

1. Introduction

The semiconductor alloy Hg, -,Cd,Te is a remarkably versatile infrared detector material. Photoconductors, junction photodiodes, and several types of infrared-sensitive metal-insulator-semiconductor (MIS) devices, including charge coupled devices (CCDs), charge injection devices (CIDs), and recently metal-oxide-semiconductor field effect transistors (MOSFETs), have been realized in this material. Each of these quite different types of devices has certain advantages for infrared detection, depending on the particular application. In this chapter we review the status of Hg,,Cd,Te photovoltaic infrared detector technology. We concentrate almost exclusively on p - n junc20 1

Copyright 0 1981 by Academic Press. tnc. All rights of reproduction in any form reserved. ISBN 0-12-752118-6

202

M . 0. REINE, A . K . SOOD, AND T. J . T R E D W E L L

tion photodiodes, but will review also recent work on Hg,-,Cd,Te Schottky barrier photodiodes. Although there have been several interesting papers on photovoltaic effects in graded-bandgap Hg,-,Cd,Te structures (Marfaing and Chevallier, 1971a,b; Cohen-Solal and Marfaing, 1968), these effects are not included in this review. Photovoltaic effects were mentioned briefly in the first paper on Hg,-,Cd,Te by Lawson rt ul. (1959), and were attributed to an unintentional p-n junction at or near a contact to a photoconductive sample. In a later study of Hg, ,Cd,Te detectors, Kruse (1965) reported photovoltaic signals in n-type photoconductive samples at 77 K and attributed these to the bulk photovoltaic effect as described by Tauc (1962). The first intentional p-n junctions in Hg,-,Cd,Te appear to be those of Vdrid and Granger (1965) who observed injection luminescence at about 3.7 pm from these junctions at 77 K (see also Rodot rt ul., 1966). Since this first report of p - n junctions in Hg,-,Cd,Te by Verit and Granger (1965), over 100 papers, reports, and other publications dealing with Hg,-,Cd,Te photodiodes have appeared in the open literature. Cutoff wavelengths have ranged from 1.0 to beyond 48 pm. Junctions have been formed by ion implantation, impurity diffusion, and both outdiffusion and in-diffusion of mercury. Avalanche gains of about 20 have been reported in Hg,,Cd,Te photodiodes with 2-pm cutoff wavelengths. Hg, -,Cd,Te photodiodes have been reviewed previously by Long and Schmit (1970), Melngailis (1973), by Harman and Melngailis (1974), and by Reine and Broudy (1977). I . APPIKATIONSFOR Hg,-,Cd,Te

JUNCTION PHOTODIODES

Initially the Hg,-,Cd,Te junction photodiode attracted interest for use as a high-speed detector for COz laser radiation around 10.6 pm. Operating at 77 K in the heterodyne mode, this type of photodiode has recently achieved a noise equivalent power which is only a factor of 1.4 above the theoretical quantum limit at a frequency of 2 GHz. Operation at these high frequencies is possible because of the low junction capacitance achievable in Hg,-,Cd,Te, which in turn is a result of its relatively low static dielectric constant. As pointed out by Melngailis (1973), Hg,-,Cd,Te is superior to Pb,-,Sn,Te for wide-bandwidth applications because of the much larger static dielectric constant of Pb,-,Sn,Te. Hg, -,Cd,Te junction photodiodks continue to be developed for advanced 10.6-pm direct detection and heterodyne applications. At present, efforts are being directed toward a 10.6-pm heterodyne detector that operates at a temperature of around 200 K so that it can be conveniently cooled by a thermoelectric cooler. More recently, interest has focused on Hg, junction photo- -,Cd,Te .

6.

PHOTOVOLTAIC INFRARED DETECTORS

203

diodes for use with silicon CCDs in hybrid mosaic focal plane arrays for direct detection, principally in the 3-5 and 8- 12 pm spectral regions. Hybrid mosaic focal planes utilizing Hg,-,Cd,Te photodiodes have been reviewed by Broudy and Reine (1977) and by Broudy el al. (1978, 1980). A hybrid mosaic focal plane array consists of a two-dimensional array of large numbers of Hg,-,Cd,Te junction photodiodes in contact with a silicon CCD chip. Each photodiode is connected electrically to one CCD stage. Charge that is photogenerated in and separated by each photodiode is injected into each CCD stage. Once it is within the silicon CCD chip, this charge is transferred laterally to the edge of the chip and is available for further processing. In this application, the photovoltaic (PV) mode is preferred over the photoconductive (PC) mode. These mosaic focal plane arrays will contain tens of thousands of detector elements or more and will operate at cryogenic temperatures, so that minimizing power dissipation is important. Unlike the photoconductor, the photodiode requires no dc bias power. Because of its relatively high impedance, the photodiode matches directly into the input stage of a silicon CCD, whereas the photoconductor would require a buffer preamplifier, which itself would dissipate power and occupy valuable real estate on the silicon CCD chip. Hybrid mosaic focal plane arrays utilizing Hg,,Cd,Te junction photodiodes and silicon CCD chips are being developed in two different configurations: the so-called planar processed configuration shown in Fig. la and the backside-illuminated configuration shown in Fig. lb. In the planar hC Insulator

Layer

Interconnect Metallization

Silicon CCD Chip

Input Node

CCD

Silicon CCD Chip

._

\"put Nodes

FIG. 1. Schematic illustrations of the (a) planar processed and (b) backside-illuminated hybrid mosaic focal plane schemes utilizing a silicon CCD processing chip together with Hg, -,Cd,Te junction photodiodes.

204

M.

€3.

REINE,

A. K. SOOD, A N D T. J . TREDWELL

processed configuration, the silicon CCD chip is actually used as a substrate. A Hg,-,Cd,Te wafer is epoxied to the silicon chip, is lapped and etched to an appropriate thickness, usually on the order of 10-15 p m , junctions are formed and metal films are evaporated to connect each junction to the CCD stage immediately below it. In the backside-illuminated configuration, a mosaic of Hg, -,Cd,Te junctions is processed separately on an optically transparent substrate. Contact bumps of indium are plated on each junction and on each CCD input stage. Finally, the Hg,-,Cd,Te mosaic is flipped over and connected to the silicon chip by means of the contact bumps. In the backside-illuminated configuration, the Hg,-,Cd,Te active layer could be grown by liquid phase epitaxy onto a CdTe substrate. In both the planar-processed and backside-illuminated configurations, the Hg,,Cd,Te active layer must be fairly thin, on the order of 10- 15 pm, which in turn is on the order of or less than a minority-carrier diffusion length. This requirement of thinness has important consequences on photodiode performance. The choice of the PV mode over the PC mode for hybrid mosaic focal planes is not because of any inherent superiority of the PV mode in terms of sensitivity or operating temperature. Long (1977) in fact has pointed out that the theoretical limits to signal-to-noise ratio and operating temperature are the same for all intrinsic quantum detectors, no matter whether they are photovoltaic or photoconductive. His arguments can be shown to hold true also for the MIS detector described by Kinch in this volume. 2. SCOPEOF THISCHAPTER This chapter is organized into four parts. Part 11 reviews the theory of operation and the fundamental and practical limits to the performance of p-n junction infrared photodiodes. In the thirty years since Hg,,Cd,Te Shockley’s classic paper (Shockley, 1949) there have been many excellent textbooks on p-n junctions, among the most notable being those by Grove (1967) and Sze (1969), to which we make frequent reference. The purpose of Part I1 is not to repeat what is already well written, but rather to summarize the information needed to understand the operation and predict the performance of Hg,-,Cd,Te infrared photodiodes. Part I1 borrows liberally from the excellent discussions of Hg,-,Cd,Te photodiodes by Long (1977) and of photodiodes in general by Kingston (1978). Part I11 summarizes the state of the art in Hg,-,Cd,Te p - n junction technology. There have been impressive advances recently with the use of ion implantation; the review of these is followed by a review of photodiodes formed by diffusion of both impurities and mercury. Recent results

6.

PHOTOVOLTAIC INFRARED DETECTORS

205

on Schottky barrier photodiodes and Ilfnoise in reverse-biased p - n junction photodiodes are reviewed. Part I11 also contains a summary of what information various measurements on p - n junctions have yielded about the minority-carrier properties of p-type Hg,-,Cd,Te. Nearly all research and development on Hg,-,Cd,Te has been funded by various Department of Defense and National Aeronautics and Space Administration contracts. We have tried to reference all unclassified contract reports relevant to Hg,-,Cd,Te junction photodiode technology, and have added to each such reference either the "AD" number for ease of obtaining copies from the Defense Documentation Center or the appropriate NASA reference number.

3. SUMMARY OF Hg,-,Cd,Te

PROPERTIES

A summary of the most important electrical and optical properties of Hg,,Cd,Te with alloy compositions between 0.19 and 0.55 is contained in Table I. The values shown should be regarded as highly approximate. We include it to give the reader a general idea of how these properties vary with alloy composition and temperature over those ranges which will be encountered in this chapter. The energy gap Eg(x,T)was taken from the empirical expression of Schmit and Stelzer (1969). The cutoff wavelength A,, is that wavelength at which the photodiode response has dropped to 50% of its peak value. The values in Table I were approximated by the simple relationship

A,,

= hC/&

>

(1)

where h is Planck's constant and c is the speed of light. If A,, is in pm and Eg in eV, then A,, (pm) = 1.24/Eg (eV).

(2)

The values for the peak wavelength A, were assumed to be about 10% less than those for Ac,. Values for the intrinsic carrier concentration ni(x,T) were calculated from the semiempirical expression of Schmit (1970). The values for the electron mobility pe were taken from the data of Scott (1972) for n-type samples with carrier concentrations less than 2 X lOI5 cmP3. Similarly, the values for the hole mobility p h were taken from data for p-type samples. The values for x = 0.2 are from data (see Fig. 44) for gold-doped samples with net acceptor concentrations of 0.4-2 X 10"j cmP3. The values for x = 0.3 were taken from the data of Riley et al. (1978b) with net acceptor concentrations of 0.8-2 x IOl5 ~ m - The ~ . data for x = 0.40 and 0.55 were taken from the data of Scott et al. (1976) for samples with net acceptor concentrations around 1 X 10'' cmP3. Values for the band-edge effective mass ratios m, ,mlh, and m h h for the conduction

TABLE I

SUMMARY OF Hg,-,Cd,Te ELECTRICAL A N D OFTKAL PROPERTIES (ALL VALUESSHOWN ARE HIGHLY APPROXIMATE) Hg,-,Cd,Te alloy composition Temperature

x

-

T

(K)

Energy gapo Cutoff wavelength Peak wavelength

E.

(eV)

Electron mobility in n-type Hole mobility in p-type

I4

I

77

~~

170

0.079

0.107 11.6 10.5

14.4

A,

c(h

-

0.19

0.094 13.2 12.0

0.080 15.4 14.0

(cmz/V sec) 2 x 10s (cmz/V sec) 1,4W

2 x 105 8006

-

0.006

Conduction and light-hole massg m, ,mlh Heavy-hole mas9 hh Kane's matrix elements EL'

(eV)

19

19

Static dielectric constanta High-frequency dielectric constant!

G

-

17

17

c.,

-

12.5

12.5

From Schmit and Stelzer (1969). * From Schmit (1970). From Scott (1972). See Fig. 44b. From Riley ef nl. (1978b). From Scott cr al. (1976). a From Weiler (1981). From Domhaus and Nimtz (1976).

'

0.008

0.006

0.5

0.251 4.9 4.4

0.260 4.8 4.3

0.275 4.5 4.1

0.425 2.9 2.6

0.433 2.9 2.6

I x 108 3 x 1W2 3 x lo1* 6 x I@*

6

5x104 500"

3 x IW 1w

2x104

3w

1x104 200"

4x109

1w

X

10"

0.656 1.9 1.7

I

0.007

11

IOl3

2 x Iff 2w

0.5

12

X

110

6.

PHOTOVOLTAIC INFRARED DETECTORS

207

band and the light- and heavy-hole valence bands and the value for Kane's matrix element Ep were obtained from the review of Hg,,Cd,Te magnetooptical data by Weiler (1981). The dielectric constants were taken properties by Dornhaus and Nimtz from the review of Hg,-,Cd,Te (1976). 11. Theory of p - n Junction Photodiodes 4. CURRENT-VOLTAGE CHARACTERISTICS

The current-voltage (or I-V) characteristics of a p - n junction photodiode determine its dynamic impedance and its thermally generated noise. In this section we consider those mechanisms which produce dark current in a nonilluminated photodiode, in which the only departure from thermal equilibrium is due to a voltage externally applied to the device. Photocurrent will be discussed in Section 5. In many direct detection applications the photodiode is operated at zero-bias voltage. The photodiode dynamic resistance at zero-bias voltage is denoted by R, and is related to the current-voltage characteristic W )by

A frequently encountered figure of merit for a photodiode is its RJ product, which is simply R, given by Eq. (3) multiplied by the junction area A . If J = Z/A is the current density, then the R,A product is given by (&A)-' =

1

v=o

.

(4)

Equation (4) shows that the R,A product is independent of junction area, which is one of the reasons for the widespread use of R,,A as a figure of merit. This will not be the case when the dimensions of the junction area are comparable to a minority-carrier diffusion length, and considerable care will be needed in using the R,A product as a figure of merit because of the effects of lateral diffusion. These effects will be discussed in Section 8. a.

Diffusion Current

Diffusion current is the fundamental current mechanism in a p - n junction photodiode. It arises from the random thermal generation and recombination of electron-hole pairs within a minority-carrier diffusion length on either side of the space-charge region. Calculations of diffusion current are relatively straightforward since, unlike space-charge region g-r cur-

208

M . B . R E I N E , A . K . SOOD, A N D T. J . TREDWELI.

rent or interband tunneling current, it is largely insensitive to the actual details of the space-charge region. Diffusion current is the dominant junction current in Hg,-,Cd,Te photodiodes at higher temperatures. For example, diffusion current is usually dominant in 12-pm Hg,,,Cd,,,Te photodiodes at temperatures of roughly 77 K and above,. and for 4-5 pm H&.,Cdo.,Te photodiodes at temperatures of roughly 190 K and above. In this section we treat the p-n junction as simply as possible in what is generally referred to as the depletion approximation. Figure 2 shows a cross section of a simple n-on-p junction photodiode. We assume it can be divided into three regions: an electrically neutral n-type region of thickness a, a space-charge region of width W, and an electrically neutral p-type region of thickness d. We assume that the transition layers between adjacent regions are negligibly thin, that the n and p regions are uniformly doped, and that all of the applied voltage V is dropped across the space-charge region. We also restrict ourselves to the so-called low injection case in which departures of the carrier concentrations from their thermal equilibrium values in the n and p regions are small compared to the majority-carrier concentrations. Finally, we assume that the carrier distributions are nondegenerate so that the thermal equilibrium carrier concentrations n,(z) and p o ( zj in each region obey n,(z)po(zj = nf -

1 I

=

z=o

(5)

z=d

-a-W

FIG.2. Cross section of the simple n-on-pjunctions photodiode, showing the quasineutral n region ( - a - W < z < - W ) , the space-charge region (- W < z < 0). and the quasineutral p region (0 < x < d ) .

6.

PHOTOVOLTAIC INFRARED DETECTORS

209

With these assumptions, the minority-carrier concentrations in the regions satisfy the following familiar boundary conditions at the edges of the space-charge region (Hauser, 1971):

where pno and npo are the thermal equilibrium values of the minoritycarrier concentrations in the n and p regions, and where e is the electron charge, k is Boltzmann’s constant, and T i s the diode temperature. Within the space-charge region, these assumptions lead to the following relation for the nonequilibrium carrier concentrations (Moll, 1958):

which of course reduces to Eq. ( 5 ) for thermal equilibrium (V = 0). Consider first the p region. When thermal equilibrium is disturbed, the carrier concentrations in the p region are given by

We have assumed space-charge neutrality in the p region, so we have An(z,t)

=

Ap(z,t).

(10)

The minority-carrier concentration n(z,t) obeys the following continuity equation in the p region:

where the electron current density J , is an

J , = eD,--, az where D, is the electron diffusion coefficient. In Eq. (1 l), g(n,p) and r ( n , p ) are the electron generation and recombination rates per unit volume due to internal mechanisms such as radiative, Auger, o r Shockley-Read generation and recombination processes, whereas G,, represents external mechanisms such as photogeneration of electron-hole pairs due to incident nonequilibrium radiation.

210

M. B . REINE, A . K . SOOD, A N D T. J . TREDWELL

Consider steady-state solutions to Eq. (1 1). In general g ( n , p )and r ( n , p ) are functions of both n(z) and p ( z ) . For An(z) small compared to both ppo and nnc, we can expand (r - g ) in a Taylor series about npo and ppo; the first two terms in such an expansion are r - g = [ r h o ,ppo)- g(npor ~ p o ) l

The first term is zero since r = g in thermal equilibrium. If we define the minority-carrier lifetime T~ as

--

7

(14)

then the steady-state excess minority-carrier concentration is a solution to An 0 = D c -d 2 A n - -. dz2

7,

In order to solve Eq. (15) for An(z), we need another boundary condition in addition to the one at z = 0 given in Eq. (6b). For the moment let us assume that the boundary of the p region at z = d is far away. Then the other boundary condition is An (z

+. 00) +.

0,

(16)

and the solution for An(z) is

where the minority-carrier diffusion length L, is L,

=

G.

(18)

The boundary condition in Eq. (16) is equivalent to assuming that the p-region thickness d is large compared to L ,, in which case the nature of the interface at z = d does not affect the p-side diffusion current. The p-side diffusion current density J , is obtained from Eq. (12) evaluated at z = 0:

which is the familiar relationship of Shockley (1949). Here the subscript ~0 denotes that d >> L,.

6.

PHOTOVOLTAIC I N F R A R E D DETECTORS

211

The R,,A product due to p-side diffusion current is

(RJ)P m

kT 1 T ez n p o Le

=-->,

Using Eq. ( 5 ) with ppo = NA where NA is the net acceptor concentration on the p side, and using the Einstein relation

4= ( k T / e ) p e ,

(21)

one can rewrite Eq. (20) as

The dominant temperature dependence of (RoA),, is due to nf, and a versus 1/T would be nearly a straight line with semilog plot of (&A), a slope given approximately by E,/kT. Now consider diffusion current from the n side. Using the boundary condition in Eq. (6a) and assuming that the n-region thickness a is much larger than a minority-carrier diffusion length Lh given by L

h

=

G

,

(23)

where D h and T h are the minority-carrier diffusion coefficient and lifetime in the n region, one obtains an expression analogous to Eq. (20) for the RJ product due to n-side diffusion current:

This can also be written as nm

where we have set n,, = N,,, with N Dbeing the net donor concentration on the n side, and where we have used the Einstein relation Dh

=

(kT/e)ph.

(26)

The interfaces at z = - a - Wand z = d in Fig. 2 will have an effect on the diffusion current from the n side and the p side when they are within a diffusion length from the edges of the space-charge region. Consider the p region. The excess steady-state minority-carrier concentration An(z) is a solution to the continuity equation in Eq. (15), and the boundary condition at z = 0 is given by Eq. (6b). The boundary condition at z = d can be

212

M . B . REINE, A . K . SOOD, A N D T. J . TREDWELL

expressed conveniently in terms of the surface recombination velocity

s,: J,

(Z =

d ) = eD,-a

I

dz

z=d

= -eS, An(d).

The solution for Ahn(z) is

, (28) where p is defined as

P

=

S,Le/D,

($)I

(29)

= Sp/(Le/Te)r

and is just the ratio of the surface recombination velocity to the diffusion velocity LJT,. The resulting R,,A product is

[

1 + p tanh

(R0A)P = (ROA),,

p

e

+ tanh ($) ’

(30)

where &A), is given in Eq. (20). Equation (30) is plotted in Fig. 3 for various values of p. What it shows is that having an interface within a diffusion length of the depletion region can either enhance or degrade the R,,A product, depending on magnitude of S, relative to the diffusion velocity D e / L e .The effect of the n-side surface at z = - u - w in Fig. 2 can be treated in exactly the same way, with (&A), given by an expression like Eq. (30) with S, (the surface recombination velocity at z = - a - W), Lh and the n-region thickness u substituted appropriately. Minority-carrier diffusion lengths as long as 45 pm in p-type Hg,.,Cd,.,Te and 100 p n in p-type H&.,Cdo,,Te have been measured for moderately low carrier concentrations. These lengths are well in excess of the p-region thicknesses required for most mosaic focal plane array applications. Figure 3 shows the importance of having the thin (i.e., d > d, the R A product for p-side diffusion current in Eq. (30) becomes

d. With

(&A),

kT

N A

re

= --- ,

e2 n: d

which shows that reducing the volume in which diffusion current is generated increases the corresponding R,A product by a factor ( L , / d ) . There are three ways to approach the ideal boundary condition of Eq. (31). As suggested by Long (1977), one can implant or diffuse acceptors into p-type Hg,-,Cd,Te to form a p + region of higher carrier concentration, resulting in a p+-p hi-lo junction which is relatively impermeable to minority-carrier flow but which is ohmic to majority-carrier flow and can be used as a backside contact to the p region. This n+-p-p+junction configuration has been discussed by Tredwell and Long (1977), Long (1977), Longer al. (1978), and Sood er al. (1979a,b,c). Second, through an appropriate surface treatment, one can adjust the surface potential to reduce

214

M . B . REINE, A. K. SOOD, A N D

T. J . TREDWELL

the surface recombination velocity S, . Third, the p-region Hg, -,Cd,Te layer can be grown via LPE onto a substrate, such as CdTe, with a wider bandgap. In the graded transition region the electric fields will be such as to repel both minority and majority carriers away from the interface and back into the p region. This type of boundary has been discussed by Lanir P t a / . (1979a,b) and by Lanir and Shin (1980). Progress in reducing the p-region diffusion volume will be reviewed in Section 11,e of this chapter. Diffusion current contributions from the n and p regions add to give the total diffusion current, and the resulting R,,A product due to diffusion current from both sides is 1

1

R,,A - (R,,AIn

1 +-(RoA),'

(33)

It is important to know the relative magnitudes of the n- and p-side contributions. For the ideal case of perfectly electrically reflecting boundaries at z = - ( a + W)and at z = d and L, >> d and Lh >> a , we have

If this ratio is much larger than unity, then p-side diffusion current will essentially determine the RoA product. The relatively small conduction-band effective mass in Hg,-,Cd,Te causes the electron distributions to be appreciably degenerate for the doping levels encountered in photodiodes, particularly in the ion implanted n-on-p configuration which has been almost universally adopted for direct detection applications. Equations (24), (25), and (34) are not entirely valid for n-side diffusion current when the n side is statistically degenerate. The effect of strong degeneracy in the n region is to make the nope product considerably less than n:. The boundary condition in Eq. (6a) at z = - W needs to be modified for degeneracy in the manner discussed by Marshak and Shrivastava (1979). Furthermore, the rapidly varying implanted donor profile should also be taken into account; this would have the effect of producing an electric field in the n region which might appreciably affect carrier transport through this region. This effect and its possible consequences were recently discussed by Redfield (1979) for the case of a strongly asymmetric silicon n+-on-pjunction. This effect should be much more pronounced in asymmetric n+-on-p Hg,-,Cd,Te junctions due to the much smaller conduction-band density of states. An additional complication in analyzing heavily doped n regions in Hg,-,Cd,Te is the uncertain behavior of the minority-carrier lifetime due to the Auger electron-electron collision mechanism in strongly degenerate n-type Hg,,Cd,Te.

6.

215

PHOTOVOLTAIC INFRARED DETECTORS

Because of these uncertainties, there have been no reported theoretical calculations of the n-side diffusion current for implanted n+-on-p Hg,,Cd,Te photodiodes. The implanted n regions are about 0.3-0.4 pm thick, whereas the p region is usually greater than 10 pm thick, so that geometry favors p-side diffusion current. Furthermore, in their study of 8- 14 pm photodiodes for high-temperature operation, Tredwell and Long (1977) argue qualitatively that n-side diffusion current should be small compared to p-side diffusion current in n+-on-p Hg, -,Cd,Te photodiodes. They point out that the Auger electron-electron collision generation rate, which determines the minority-carrier lifetime in n-type Hg,-,Cd,Te for x = 0.2, is greatly reduced when states at the bottom of the conduction band become filled. Long (1977) has calculated the upper theoretical limit for the R,A product due to diffusion current for a Hg,-,Cd,Te photodiode, subject to two assumptions: first, that n-side diffusion current can be neglected compared to p-side diffusion current; second, that the dominant recombination mechanism in technologically perfected p-type Hg,-,Cd,Te crystals (i.e., those entirely free of Shockley-Read recombination centers) is radiative recombination. This second assumption is subject to question in light of the recent theoretical work of Casselman and Petersen (1979a,b), who have shown that there is a hole-hole Auger collision mechanism which may be an important recombination mechanism in p-type Hg,,Cd,Te. The strength of this mechanism is still uncertain and there has been no clear experimental observation of it yet. Section 10 of this chapter discusses this mechanism in more detail, as does Petersen in Chapter 4. With these two assumptions, the upper theoretical limit to the RJ product due to diffusion current in a Hg,-,Cd,Te photodiode is (R,A), given by Eq. (32), which in turn assumes that L, >> d and that there is a perfectly electrically reflecting p-side boundary at z = d in Fig. 2. For re we take the radiative lifetime ?-,ad which, for small departures from thermal equilibrium, is given by (Blakemore, 1962)

where the approximation is for ppo = N A >> npo. The coefficient B has been evaluated by Hall (1960) for radiative recombination in simple parabolic bands: 1 3/2 1 1 300 3/2 B = 5.8 x dC m,> (1 + F,, (36) mc + -)(y) m, +

where Eg is in eV, B is in cm3/sec, T is in K, and where

cm is

the high-

216

M . B. REINE, A. K . SOOD, A N D T. J . T R E D W E L I

10) N

1-77 K

c: a 4

1 T 3OOK

150 K I0 ;

r

l

o

:

'

I ,LLLLL11.18

6

4

7

10

I2

14

16

c u i o n WAVELENGTH iMmi

FIG.4. The theoretical upper limit to the R,A product for a Hg,-,Cd,Te photodiode due to p-side diffusion current with radiative recombination, as calculated from Eq. (32) for a p region thickness d of 10 p m and for a perfectly electrically reflecting boundary.

frequency dielectric constant and m, and m , are the conduction- and valence-band effective mass ratios. For m, we take the heavy-hole mass ratio value of 0.5. We calculate m, from the following expression (Weiler, 1981)

(37)

m,

with F = 1.6, Ep = 19 eV, and A Sorger, 1972) E&)

=

9.5

=

1 eV. For e,(x) we use (Baars and

+ 3.5 [(0.6

-

~)/0.43].

(38)

We take &(x,T) from Schmit and Stelzer (1969) and nl(x,T) from an improved version of the expression of Schmit (1970). Note that the upper theoretical limit to (&A), is independent of acceptor concentration N,, . The upper theoretical limit to the RJ product so calculated is shown in Fig. 4 plotted versus cutoff wavelength [obtained from E,(x,T) via Eq. (I)] for various temperatures, and by the solid lines in Fig. 5 plotted versus reciprocal temperature. A p-region thickness d of 10 p n was chosen. b. Generlition -Recombinuiion Current from the Space-Charge Region

Impurities or defects located within the space-charge region can act as generation and recombination (g-r) centers of the Shockley-Read type

6.

PHOTOVOLTAIC INFRARED DETECTORS

217

r-

u 3 L3

0

a

LL

a PI

100DIT l K ~ ’ l

FIG.5. R A products due top-side diffusion current (solid lines), calculated from Eq. (32) for the same conditions as in Fig. 4, and due to space-charge region g-r current (dashed = 0.1 psec and eVbl = E,. lines), as calculated from Eq. (48) for W = 0.1 p m ,

(Shockley and Read, 1952) and thereby produce junction current. The importance of this current mechanism was first pointed out by Sah et a / . (1957), who showed that space-charge region g-r current could be more important than diffusion current at low temperatures, even though the width of the space-charge region is much less than a minority-carrier diffusion length. Space-charge region g-r current varies with temperature roughly as ni, whereas diffusion current varies as 4 . Diffusion current dominates at relatively high temperatures; diffusion current decreases as the temperature is lowered but space-charge region g-r current decreases less rapidly, so that a temperature is finally reached at which the two are

218

M . B . REINE, A. K . SOOD, A N D T. J . TREDWELL

comparable and below which space-charge region g-r current dominates. There are other junction current mechanisms which also decrease with decreasing temperature less rapidly than n:, such as surface generation-recombination and interband tunneling, which are discussed in the following sections, and so some care is needed in unambiguously determining which current mechanisms are important in a given photodiode at lower temperatures. For g-r centers located at an energy Et above the top of the valence band, the steady-state net recombination rate U(z)through these centers is

U(Z)= R ( z )- G(z) np

?po(n

+ nl) + ?no(p + P I )

nf

-

+ nl) + no(^ + PI)’

~po(n

(39)

where n = n ( z ) and p = p ( z ) are the nonequilibrium electron and hole concentrations within the space-charge region, and where

p1 = N , e x p ( s ) ,

where N , and N, are the conduction- and valence-band effective densities

of states, C , and C, are the capture coefficients for electrons and holes, and Nt is the number of g-r centers per unit volume. Under the simplifying assumptions discussed at the beginning of Section 4,a the product of the carrier densities n(z) and p ( z ) is roughly independent of position within the space-charge region and follows the approximate Shockley relation given in Eq. (7). Hence, the g-r centers provide net recombination for forward bias [i.e., U(z) > 01 and net generation for reverse bias [U(z) < 01* The junction current density Jg-rdue to these centers is obtained by integrating Eq. (39) over the space-charge region (see Fig. 2):

I, 0

J , - ~= e

~ ( z dz. )

(44)

In order to do this integral, n(z) and p ( z ) must be known within the space-

6.

PHOTOVOLTAIC INFRARED DETECTORS

219

charge region. Sah et al. (1957) assume that the potential varies linearly with distance over the space-charge region and obtain the following result:

where v b i is the built-in voltage of the p - n junction such that eVbi is the difference between the Fermi levels on the n side and the p side for zero-bias voltage applied to the junction. This equation is valid for reverse-bias voltage and for forward-bias voltage values that are less than vbi by several ( k T / e ) . The function f ( b ) is given approximately by

where

b

=

exp

(g)[y + (?)I, cosh

In

(47)

where Eiis the position of the intrinsic above the top of the valence band. When Et = Ei and T~,,= T~,,, V(z)in Eq. (39) has its maximum value for a given voltage V and the recombination center has its maximum effect. The RJ product due to g-r current originating in the depletion layer, as obtained from Eq. ( 4 9 , is

b is one for For the most effective g-r center (i.e., Et = Ei and 7n0= T~,,), V = 0 and f(0) = 1, so (RoA)g-r varies with temperature as n i l , as opposed to the R& product due to diffusion current which varies with temperature as n;* . Plots of (RJ)g-r calculated from Eq. (48) with f ( b ) = 1 for several Hg,,Cd,Te alloy compositions are shown by the dashed lines in Fig. 5 . For these curves we set T ~ , = , T~ = 0.1 psec and eV,, = E g . For W we use a value of 0.1 pm, which corresponds to an effective space~ . that both 7, and charge region doping Ns of around 1 x 10l6~ m - Note T ~ ,depend , on the concentration Nt of Shockley-Read centers, which will vary from crystal to crystal and may also depend on the junction formation method. The dashed curves in Fig. 5 are meant to illustrate the dependence on temperature of (RJ),-,as compared to (RJ), rather than predict the values of (RJ)g.-r likely to be encountered in Hg,-,Cd,Te photodiodes.

220

M . B. REINE, A . K . SOOD, A N D T. J . TREDWELL

Less effective g-r centers will have El # Ei. If IEl - EiI > kT, then the approximation for h > 2 of

f(b) = (In b ) / b

(49)

can be used to give

This results in (R,J)g-r increasing more rapidly than n;l with decreasing temperature. The ratio of the R,J products for depletion layer g-r current [Eq. (48)] and p-side diffusion current [Eq. (20)] is

where we have set f(b) = 1. Now ( L J W ) is on the order of 100 and (eVhi/k7')is also large, so a temperature for which tzi < N A x lop3or less will have to be reached for depletion layer g-r current to be dominant over diffusion current. Choo (1968) extended the theory of Sah et al. (1957) to the case of a strongly asymmetric p-n junction in which the doping levels on the n and p sides are quite different and also considered the case in which rn0and T~ may be quite different. Choo was able to show that significant differences from the simple Sah et al. treatment could occur for forward-bias voltage. However, near zero-bias voltage these differences are small and so Eq. (48) ought to give a reasonable estimate for even for strongly asymmetric junctions. Finally, it should be pointed out that generation and recombination processes which do not involve impurity or defect levels can also occur in the depletion region. Long (1977) considered both radiative and Auger electron-electron collision processes occurring in a depletion region free of Shockley-Read levels. In both cases, resulting R,A products had the same nc2 temperature dependence as conventional diffusion current (in the nondegenerate case) and were larger than the diffusion R A product by the ratio (L,/ W ) . c . Surfice Lecikrrge Current

Dark current in an ideal p-n junction is due to carrier generation and recombination in the quasineutral regions (diffusion current, which was discussed in Section 4,a) and in the space-charge region (g-r current, which was discussed in Section 4,b). Actual devices often have additional

6.

221

PHOTOVOLTAIC INFRARED DETECTORS

dark current, particularly at low temperatures, which is related to the surface. The semiconductor surface and its oxide and overlying insulator affect the junction current both through fast interface states, which act as g-r centers, and through fixed charge in the oxide and insulator, which modifies the surface potential on both sides of the junction. The combination of fast interface states and fixed charge result in a variety of surface-related dark current mechanisms. These surface-related currents frequently dominated diode characteristics in the initial development of germanium and silicon technology and have been widely studied (Sah, 1962; Grove and Fitzgerald, 1965, 1966) in these semiconductors. Figure 6 illustrates some of these surface-related dark current mechanisms. In the absence of oxide charge (Fig. 6a.) an annular area A, of the surface defined by the junction perimeter and the junction depletion layer width Wbwill lie within the depletion layer. Fast interface states in this depleted region of the surface will generate carriers. The magnitude of the surface generation current Z, is given by

where the surface recombination velocity so for a depleted surface is re-

f

kLC-!

OX IDE/INSULATOR

f

METALLURGICAL JUNCTION

F L c

i Id I

\ I 1 1 FIG. 6. Effect of fixed oxide/insulator charge on the effective junction space-charge region: (a) flat-band condition; (b) positive fixed charge causes inversion of the p side and formation of a n-type surface channel; (c) negative fixed charge causes accumulation of thep side and a narrow field induced junction at the surface; (d) larger amount of negative fixed charge causes inversion of the n side and formation of a p-type surface channel.

222

M . B. REINE, A . K . SOOD, A N D T. J . TREDWELL

lated to the number of fast interface states ZVss(E)per unit surface area per unit energy interval by

where C, and C , are defined in Eqs. (42) and (43). When sufficient positive fixed charge is present, the p side can become inverted and an n-type surface channel can form as shown in Fig. 6b. The space-charge region will become larger, and additional current can be collected by this field induced junction. The channel length L, will depend on the bias voltage applied to the junction, on the surface conductivity, and on the amount of current flowing across the field induced junction. The generation current due to Shockley-Read centers in the surface channel is given by an expression analogous to Eq. (45): where W, is the channel width and A, is the channel area. When negative fixed charge is present, the p side becomes accumulated and, as shown in Fig. 6c,a narrow field induced junction can occur at the surface, offering a region in which interband tunneling can occur. A larger amount of negative fixed charge can invert the n side (Fig. 6d) and form a p-type surface channel. In addition to thermal generation and recombination processes occurring at the surface and within surface channels, there is another class of surface-related current mechanisms which frequently limit junction performance. These currents are usually termed surface breakdown or surface leakage. They are characterized by ohmic or breakdownlike current -voltage characteristics and are nearly temperature independent. Surface breakdown occurs when a region of high electric field is created by a narrowing of the depletion layer where it intersects the surface. Figure 6c illustrates cases in which the depletion layer is narrowed by surface charge. The fixed charge strongly accumulates the lightly doped p side of the junction at the surface, pinching off the depletion layer. Since the electric field across the junction is given by ( V + VbJ/ W, where V,, is the built-in voltage and W the depletion layer width, surface breakdown can occur at low or even at zero applied voltage. In Fig. 6d the fixed charge inverts the heavily doped n side of the junction. Because of the high carrier concentration, the depletion layer between the inversion charge and the heavily doped region is very narrow and the electric field across the depletion layer correspondingly large. Again, breakdown would occur at comparatively low junction voltages. This latter form of surface channel

6.

PHOTOVOLTAIC INFRARED DETECTORS

223

breakdown was observed by Grove and Fitzgerald (1965) in silicon p+-n junctions in which the oxide was heavily contaminated by sodium. The breakdown may proceed by avalanche, tunneling, or microplasma, with the latter two being more likely in narrow-gap semiconductors. Tunneling and microplasma are both nearly temperature independent. This is in contrast to the thermal generation currents, which decrease rapidly with temperature as the first or the second power of the intrinsic carrier concentration ni . Both thermal generation in surface channels and surface breakdown are observed in Hg,-,Cd,Te photodiodes, frequently in the same device. In such devices, thermal generation dominates at moderate temperatures with the dark current proportional to n i ; at lower temperatures the thermal generation in the channel decreases to a value less than the temperature-independent surface breakdown and the latter dominates device characteristics. In order to investigate surface-related current mechanisms, an insulated gate electrode is often fabricated around the junction perimeter in order to externally control the surface potential. Gate-controlled junctions are discussed in the textbook by Grove (1967). Experiments with Hg, -,Cd,Te gate-controlled photodiode structures are described in Section 11,d of this chapter. Figure 7 shows the qualitative behavior of the various surface-related current mechanisms as functions of applied gate voltage V , for a gate-controlled n-on-p junction. VG>0

0

P

ACCUMULATION

I

I

P

I

lNVERSION Tunneling Across ieid Induced

J

c

5

5

Tunneling Across Field Induced Junction

1

I

0 3 VI VI

"

Y

X

Y

---FIG. 7. Qualitative behavior of the various excess (leakage) current mechanisms in a narrow-bandgap gate-controlled photodiode, shown as functions of gate voltage V,. The actual current versus gate voltage curve observed would be the sum of the currents from the various mechanisms.

224

M . B. REINE, A . K. SOOD, A N D 7’. J . TREDWELL.

d . Interband Tunneling Current

Interband tunneling is an important junction current mechanism to consider in the design of high impedance Hg, -,Cd,Te photodiodes, particularly for cutoff wavelengths in the 8- 12 pm range and beyond, and particularly for low temperatures where thermally generated junction currents are small. In this section we consider the effect of interband tunneling current on the junction resistance R, at zero-bias voltage. It is well known that tunneling can influence the reverse bias current-voltage characteristics of a junction even if another current mechanism is responsible for R, near zero-bias voltage. This is usually referred to as Zener breakdown or internal field emission. Interband tunneling can occur uniformly over the metallurgical junction area as well as across field induced junctions which are due to surface inversion or accumulation, as was discussed in the preceding section. There are two general types of tunneling transitions to be concerned with in Hg,-,Cd,Te. These are illustrated in Fig. 8: direct tunneling of an electron in an energy conserving transition from one side of the spacecharge region to the other (transition a), and trap-assisted tunneling in which impurities or defects within the space-charge region act as intermediate states (transitions b and c). Direct interband tunneling calculations for Hg,,Cd,Te MIS structures pulsed into deep depletion have been performed by Anderson (1977); these and their implications are discussed by Kinch in Chapter 7 of this volume. The importance of trap-assisted tunneling in Hg,_,Cd,Te MIS structures was suggested by Chapman ut (11. (1978), and is also discussed by Kinch in this volume. Calculations of the effect of trap-assisted tunneling on long-wavelength Hg, -,Cd,Te p - n junctions at low temperatures were recently reported by Wong (1980). n

P

FIG.8. Energy diagram for an n+-on-pjunction with a small reverse-bias applied, showing a direct energy-conserving tunneling transition (a) and two trap-assisted tunneling transitions (b) and (c). The trap level energy is Et. The n-side conduction band is degenerate, the p-side valence band is nondegenerate.

6.

PHOTOVOLTAIC INFRARED DETECTORS

225

Direct interband tunneling across p - n junctions has been extensively treated in the literature (Duke, 1969) for the case of tunnel diodes which generally consist of strongly degenerate n and p regions and which show a pronounced region of negative differential resistance at forward bias. This case is almost never encountered in Hg,,Cd,Te photodiodes. While the n region is usually degenerate even at relatively modest donor concentrations, the p region is almost always nondegenerate. However, even though the p region is nondegenerate, direct interband tunneling can still be an important mechanism, such as in the so-called backward diode (Sze, 1969, pp. 193-197), provided that the electric field in the spacecharge region is large enough. The junction current density Jt(V) due to direct interband transitions between the conduction band on the n side and the light-hole valence band on the p side, as calculated by Kane (1961), is of the form Jc = Be-AD(V),

(55)

where

@I2

e.

= eFh/21~

(57)

Here F is the electric field in the space-charge region, which Kane assumed was uniform, m* is the effective mass characterizing the edges of the conduction and light-hole bands, h is Planck's constant, and El is a kinetic energy associated with particle motion in the plane perpendicular to the tunneling direction:

El = e

m .

(58)

The factor D ( V ) in Eq. ( 5 5 ) represents the availability of initial and final states at the same energy on opposite sides of the space-charge region to participate in tunneling transitions. At zero-bias voltage, D(0) is zero and there is no net junction current. For the tunnel diode case in which both sides of the junction are strongly degenerate, the low-temperature approximation (Kane, 1961) for D ( V ) near zero-bias voltage is usually used: D(V) = eV,

(59)

and the R,A product due to tunneling current is, from Eqs. (4) and (55): --1

(&A),

- eB, exp(-A,)

226

M . B . REINE, A . K . SOOD, A N D T. J . TREDWELL

where the subscripts on A and B mean they should be evaluated at V = 0. The dominant dependence on temperature here is that of the energy gap Eg(T)through A,. Since Eg in Hg,-,Cd,Te decreases with decreasing temperature, this would say that ( R A ) ,should also decrease with decreasing temperature. For the more relevant case in which the n side is degenerate but the p side is nondegenerate, the simple approximation in Eq. (59) cannot be used. Small but finite acceptor activation energies are generally observed in p-type Hg,-,Cd,Te (see Section 14,a of this chapter), and so the freezeout of free holes in the p-side valence band at lower temperatures needs to be taken into account. If this is done, the approximate expression for D ( V ) near V = 0 is

where p , is the free hole concentration on the p side and valence-band effective density of states. (&A), then becomes

and one can see how the exponential dependence of p o on temperature, at low temperatures when freezeout is important, could cause (&A), to increase with decreasing temperature. This can be seen in Fig. 9, in which (R,A), is plotted as a function of reciprocal temperature for various acceptor activation energies Ea for direct interband tunneling transitions in a n+-on-p Hg0.,Cdo.,Te junction. Arbitrary units were chosen because of uncertainty in the space-charge region electric field F, which enters into the exponential via the term A in Eq. (56) and affects drastically the magnitude of (&A), . As temperature increases, ( R d ) ,increases sharply as the bands become "uncrossed" and direct energy-conserving tunneling transitions no longer occur. As temperature decreases, (&A), again increases for values of acceptor activation energy E A above about 0.003 eV due to hole freezeout in the p-side valence band. An acceptor activation energy of about 10 meV has been reported for p-type Hg.&d0.,Te (see Fig. 44). Behavior of R,,A qualitatively similar to that in Fig. 9 was obtained by Wong (1980) in his calculations of trap-assisted tunneling transitions in long-wavelength Hg,-,Cd,Te n-on-p junctions. It is not expected that interband tunneling, whether it be via energy conserving or trap-assisted transitions, will present a fundamental limitation to Hg,-,Cd,Te photodiode performance at low temperatures. The magnitude of the tunneling current depends strongly on the space-charge

6.

PHOTOVOLTAIC INFRARED DETECTORS

227

IOWiT I K - ' I

FIG.9. ( R J ) , versus reciprocal temperature for direct energy-conserving tunneling in an n+-on-p Hg,,.,C$,,Te junction, for several values of acceptor activation energy E A , and for N D = 1 x 10'' ~ r n - ~N;A = 4 x lot5 ~ r n - ~ .

region electric field, and suitable choices of doping concentrations and junction profile can lower this electric field throughout the bulk of the space-charge region to the point where tunneling currents will probably be small. More important is the surface where, as discussed in Section 3, field induced junctions can occur with much stronger electric fields. Careful control of surface potential will be necessary to minimize the occurrence of such field induced junctions.

5.

PHOTOCURRENT IN

p - n JUNCTIONS

Infrared radiation of wavelength A shorter than the cutoff wavelength A, is absorbed by the photodiode and produces electron-hole pairs. If the absorption occurs within the space-charge region, the electron-hole pairs are immediately separated by the strong electric field and contribute to photocurrent in the external circuit. If the absorption occurs within a

228

M . B . REINE, A . K . SOOD, A N D T . J . TREDWELL

diffusion length of the space-charge region in the n region or the p region, the photogenerated electron-hole pairs first must diffuse to the space charge region where they are then separated by the electric field and contribute to photocurrent in the external circuit. If no current can pass externally between the n and p terminals of the photodiode, then an open-circuit photovoltage (the photovoltaic effect) appears across the junction when it is illuminated. If the n and p terminals are connected to a very low impedance, the photodiode is short circuited and a short circuit photocurrent will flow when the photodiode is illuminated. If Q is the nonequilibrium steady-state photon flux in photons/cm2 sec incident on the photodiode, then the steady-state photocurrent density JdQ) is

where q is called the quantum efficiency of the photodiode. It is the number of electrons contributing to photocurrent per incident photon and has a maximum value of unity unless there is some. avalanche gain present. The quantum efficiency q is a function of the wavelength of the incident radiation and depends on the photodiode geometry and on the diffusion lengths for minority carriers within the quasineutral regions. Unless the photon flux Q is large enough to make the photogenerated excess minority carrier concentrations comparable to the majority-carrier concentrations (i.e., the so-called large injection case), the quantum efficiency q in Eq. (63) will be independent of photon flux and the photocurrent will be a linear function of Q . The current-voltage relation J(V,Q)for an illuminated photodiode with an applied bias voltage V is usually written as

where Jd(V)is the current-voltage relation for the unilluminated photodiode which was discussed in Section 4. Equation (64) says that the current through an illuminated photodiode is just the dark current (which depends only on V) minus the photocurrent (which depends only on Q ) . The applicability of this simple linear superposition of two independent currents has been recently examined in connection with Si and GaAs photovoltaic solar cells. Lindholm ef ul. (1979) considered ajunction in which the dark current was due to diffusion current and space-charge region g-r current and showed that Eq. (64)was valid so long as the low injection case obtained and the space-charge region did not contribute appreciably to both dark current and photocurrent. Subsequently, Tarr and Pulfrey (1979) showed that Eq. (64) could apply under certain circumstances even

6.

229

PHOTOVOLTAIC INFRARED DETECTORS

when space-charge region dark current and photocurrent were significant. So long as dark current and photocurrent are linearly independent, the quantum efficiency 7 can be calculated in a straightforward’manner as a function of wavelength for a specified photodiode geometry, optical absorption coefficient a@),and minority-carrier properties. This has been done by a number of authors including Melngailis and Harman (1970a) in and Pb,-,Sn,Se infrared photodiodes, their discussion of Pb,-,Sn,Te Van de Wiele (1976) in a more detailed treatment of photodiode quantum efficiency, and Hovel (1975) in his review of solar cells. We want to examine the effect of photodiode geometry and material properties on cutoff wavelength for the special case of ion implanted n-on-p Hg,-,Cd,Te photodiodes in which the n region and the spacecharge region are fairly thin. The geometry is shown in Fig. 10, where we assume that the p region is semiinfinitely thick. As shown by Van de Wiele (1976), the steady state photogenerated excess minority-carrier concentration An(z) in the p region is

(aLe = l), (65b)

0.2

r

E

1

2

l

I

I

3

4

5

ZIL,

FIG. 10. Normalized photogenerated excess minority-carrier concentration in the p region, plotted from Eq. (65) versus normalized distance for several values of cuL,.

230

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL

and the quantum efficiency, neglecting front surface reflections and any absorption in the n region and the space-charge region, is 7) =

a L e / ( a L e+ 1).

(66)

The quantum efficiency will depend on wavelength because the optical absorption coefficient a(A)does. Recent data for the optical absorption coefficient versus wavelength in Hg,-,Cd,Te for 0.205 < x < 0.220 and for temperatures between 80 and 300 K have been published by Finkman and Nemirovsky (1979). The cutoff wavelength A,, is defined as that wavelength at which the quantum efficiency has dropped to half its short-wavelength value. For values of a such that aLe >> 1, Eq. (66) says that 7) is essentially unity. From Eq. (66), the cutoff wavelength occurs when

(67)

a(A,,) = l/LI?-

In the semiinfinitely thick photodiode of Fig. 10, the cutoff wavelength depends not only on the alloy composition and the temperature, but also depends somewhat on the p-side minority-carrier diffusion length. But the dependence of A,, on L, is not large because the absorption coefficient varies so rapidly with wavelength near the absorption edge. For example, for an alloy composition of x = 0.210 at a temperature of 80 K, the data of Finkman and Nemirovsky (1979) would give a cutoff wavelength of 12.7 p m (a = 400 cm-') for a diffusion length of 25 p m according to Eq. (67), and would give a cutoff wavelength of 13.1 prn (a = 200 cm-l) for a diffusion length of 50 pm. This is roughly a difference of 0.4 pm for a factor of 2 change in diffusion length. The general behavior of An(z) as given by Eq. (65) is shown in Fig. 10. One can see from Eq. (65a) that the maximum value of An(z) occurs at zmax =

Le ln(aLe)/(aLe

= In(aLe)/a

-

1)

for aLe >> 1,

(68) (69)

and the maximum value of An is An(zmax) = [aQ~e/(aLe+ 111 exP(-azmaJ

(70)

So for a photon flux of 1 x loi7 ph/cm2 sec, a lifetime of 0.5 Fsec, an absorption coefficient of 5 x 103 cm-' and a diffusion length of 25 pm, Anmax is 1.6 X 10l2~ m - Here ~ . one can see that extremely large photon fluxes are needed to cause departures from the low injection region.

6.

PHOTOVOLTAIC INFRARED DETECTORS

231

For the case considered here, the p region will determine the cutoff wavelength. At wavelengths shorter than the cutoff wavelength, absorption within the n region and the space-charge region will become more important, even though those regions are quite thin compared to L, ,because the radiation will penetrate less as the absorption coefficient increases. If the n region is heavily doped, the Burnstein-Moss effect will tend to shift the absorption edge in this region to shorter wavelengths. Two other photodiode geometries are important to consider, the front-side-illuminated n-on-p photodiode with a p region of finite thickness (Fig. la) and the backside-illuminated n-on-p photodiode (Fig. Ib). In both cases, we usually want the p-region thickness d to be much less than a diffusion length in order to reduce p-side diffusion current as much as possible. In any event, the p region in. the backside-illuminated case needs to be at most a diffusion length thick so that nearly all photogenerated carriers reach the space-charge region before recombining. If d > 2kT/vezQ&.

(82)

At reverse bias such that JeVl >> kT, the noise current from Eq. (79) and (80) is

and the photodiode shot noise is a factor of fi lower than the zero-bias case. Equation (79) is a special case of a somewhat more general expression given by Guggenbuehl and Strutt (1957) (see also van der Ziel and Chenette, 1978): I; ( V ) = [4kT Re( Y) - 2eZ(V)] Af,

(84)

234

M . B. REINE, A. K . SOOD, A N D T . J . TREDWELL

where Y is the photodiode admittance and Z(V) is the total dc current at the photodiode terminals, which is given by Eq. (77) for the ideal case considered here. It is sometimes important to take into account the dependence of the photodiode admittance Yon frequency. For an ideal photodiode in which only diffusion current from the p side is important, the ac admittance Y(V,w) is (see, for example, Sze, 1969, pp. 107- 109)

where a(w) and b(w) are dimensionless functions of COT,, where w is the angular frequency and T~ is the minority-carrier lifetime on the p side:

b(w) =

For low frequencies, a + 1 and b + 0 and Eq. (85) reduces to Eq. (80) for V = 0. For high frequencies, a ( w ) increases as ( W T , ) ~ / and ~, Re( Y) and the noise also increase. If the minority-carrier lifetime on the p side is 0.5 psec, these effects will begin to occur around 300 kHz. Hence, these effects are not important for most direct detection applications, but may be significant for high-frequency heterodyne applications. h. Noise with Space-Charge Region G - R Current

The noise in a p - n junction in which the dominant current mechanism is space-charge region g-r current has been treated by Lauritzen (1968), by van Vliet (1976) and by van Vliet and van der Ziel(1977), and has been summarized by van der Ziel and Chenette (1978). At zero-bias voltage, of course, the expression for (R&)g-r given in Eq. (48) can be substituted into Eq. (76) to give the noise current. As in the case of diffusion current, space-charge region g-r current is the sum of two currents:

zg-r = I , - I , ,

(88)

J -W

(90)

where

Zg

=

eA

G(z)d z ,

6.

PHOTOVOLTAIC INFRARED DETECTORS

235

where R(z) and G(z) are given in Eq. (39). For reverse biases such that eV >> kT, the recombination current component I,. is negligible. For a single trap level with Et = E, and 7,, = rpo= T , , the space-charge g-r current is

Van der Ziel and Chenette (1978) show that the mean squared noise current at low frequencies is

z

-

=

2eZg Af,

(92)

i.e., just the shot noise on the generation current Zg. At frequencies higher than the emission rates of the traps, the mean squared current is slightly lower:

I", = 3-(2eZg)Af.

(93)

c . l / f Noise

l/f noise has been most extensively investigated in silicon p-n junctions and MOS transistors. McWhorter (1957) measured l/f noise in semiconductor filaments and suggested that it originated in trapping of carriers by surface states. Later, Nobel and Thomas (1961) found experimentally that exposure of germanium filaments to various atmospheres resulted in variations in low-frequency noise and suggested that atmospheric exposure altered surface trapping by affecting the surface potential. Watkins (1954) found that surface treatment affected llfnoise in germanium junctions and Atalla et al. (1959) observed similar effects in silicon junctions. The first quantitative investigation of the correlation between surface state density and l/f noise was made by Sah and Hielscher (1966). A number of investigators (Terman, 1962; Nicollian and Goetzberger, 1967; Nicollian and Melchior, 1967; Prier, 1967; Christensson et al. 1968; Christenssen and Lundstrom, 1968) have since related l/f noise in MOS transistors to charge trapping in surface states. The first direct demonstration of the relationship between surface states and l/f noise in p-n junctions was reported by Hsu et al. (1968) and by Hsu (1970a,b). In those experiments a correlation between surface state density and l/f noise was observed in silicon gate-controlled diodes. Recent experiments on 1/ f noise in Hg,-,Cd,Te photodiodes are summarized in Section 15 of this chapter. 7. RESPONSIVITY, NOISEEQUIVALENT POWER, A N D DETECTIVITY The figures of merit usually used to characterize the sensitivity of infrared photodiodes in the direct detection mode are detectivity and noise

236

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL

equivalent power NEPA. These and other useful infrared detector figures of merit and their methods of measurement are reviewed by Eisenman c t ul. (1977). Suppose the photodiode is uniformly illuminated by a rms signal photon flux Qsof monochromatic radiation of wavelength A. From Eq. (63) the rms signal photocurrent I, is (94)

1s = ~ Q s - 4 ,

where A, is the photosensitive area. The rms signal radiation power P Areceived by the detector is

PA= (hc/A) Q A . The current responsivity A / W:

(95)

is just the ratio of I , to P Aand has units of =

(96)

(A/hc)q e .

The noise equivalent power NEPA is just that rms power of incident monochromatic radiation of wavelength A necessary to produce a signal-to-noise ratio of unity when the noise is normalized to unit bandwidth. The signal-to-noise ratio S I N for the detector described above is

where is the mean squared noise current in the bandwidth Af. The noise equivalent power obtained by setting ( S I N ) = 1 and normalizing the noise current to unit bandwidth is

The units of NEPA are W& The detectivity D f is related to the NEPA through a normalization to unit detector photosensitive area:

The units of D f are cm Hz’’*/W. At zero-bias voltage, the photodiode noise current is given by Eq. (81) and the detectivity from Eqs. (96) and (99) is =

A

1 rle [(4kT/R,A,)

+ 2qe2QB]1’2 ‘

6.

PHOTOVO LTA I c INFRARED DETECTORS

237

When the photodiode thermal noise is dominant, Eq. (100) reduces to

where the subscript th denotes the thermally limited case. In this equation one can see the direct relation between the R A c product and the thermally limited detectivity. Note that the area A, which appears is the optical collection area of photosensitive area of the photodiode. When the background photon flux noise is dominant, Eq. (100) reduces to

which is the well-known detectivity for a background limited infrared photodetector (BLIP). In the heterodyne mode of operation, different figures of merit are used to characterize the sensitivity of the photodiode. Consider the ideal case of a p - n junction photodiode in which only diffusion current is important. The diode is uniformly illuminated by two monochromatic coplanar radiation beams of constant amplitude: a local oscillator (LO) beam of photon flux Qm and a signal beam of photon flux Q,. The resulting photocurrent I p h ( t ) in the photodiode from these two ideally mixed beams is (Kingston, 1978, Chapter 3; Spears and Kingston, 1979) Zph(f)

= ZLO

+ 1s + 27)(wif) a

s

where Zm and Z, are the dc photocurrents due to

c

cos wifl,

QLO

and

Qs

(103) individually, ( 104a)

( 104b)

and where the ac term is due to the coherent mixing of the two beams. The modulation frequency wif is wif =

lwL0

-

4,

(105)

where hum and hw, are the photon energies of the LO and signal beams, and wif is assumed to lie below the upper-frequency response cutoff of the ) the dc phophotodiode. Note that the dc quantum efficiency ~ ( 0governs tocurrents ZLo and I s , but the ac quantum efficiency ?(wit) governs the obmodulated photocurrent. The rms heterodyne signal current Is,h(~if) tained from Eq. (103) is

238

M. B . REINE, A. K . SOOD, A N D T. J . TREDWELL

The noise current in the photodiode can be obtained from Eq. (79). Assume that Q,N >> Q s . Then the mean squared noise current is

where the first term represents photodiode thermal noise and the second term is the shot noise on the dc local oscillator photocurrent. The figure of merit which characterizes the photodiode hetrodyne sensitivity is the heterodyne noise equivalent power NEPh,A which is defined as that signal power necessary to produce a signal-to-noise power ratio of unity: From Eqs. ( 9 9 , (106), and (107) this gives

The units of NEPh,A are W/Hz. The theoretical quantum limit for the heterodyne noise equivalent power results from Eq. (109) by setting both q(0) and q(qf)equal to unity and assuming ZLo >> Isat:

NEPh,A = hc/A.

(1 10)

For A = 10.6 pm, this has the value of 2 x W/Hz.Equation (109) shows that one wants to design a heterodyne photodiode such that the dc local oscillator photocurrent ZLo can be made to dominate the dark current and, as pointed out by Spears and Kingston (1979), such that q ( O ) / q 2 ( w ) is as close to unity as possible.

8. LATERALCOLLECTION

IN

SMALL-AREA JUNCTIONS

When the dimensions of the junction are so small that they are comparable to a minority-carrier diffusion length, then the lateral diffusion of both photogenerated and thermally generated current to the junction edges needs to be explicitly taken into account. This situation is frequently encountered in infrared system applications, particularly with two-dimensional mosaic arrays. Often lateral collection effects can be used advantageously to increase the photosensitive area of a junction and allow the actual junction area to be reduced. This in turn reduces junction capacitance, space-charge region g-r current and leakage currents originating at the junction perimeter. There are three different geometrical areas to consider when lateral collection is important. These are shown in Fig. 1 I for a planar n-on-p junction photodiode with a shallow n region, which is generally the case for

6.

PHOTOVOLTAIC INFRARED DETECTORS

la1

239

bl

FIG.11. Cross sections and top views of an n-on-p photodiode showing diffusion collection volumes for the cases of (a) no optical mask and (b) with an optical mask. Surface generation and recombination at the front p surface are assumed to be negligible.

implanted photodiodes. Surface generation and recombination at the front p surface are ignored. Thejunction area A , is just the area of the implanted n region. The volume of the space-charge region is just AjW, where W is the space-charge region width. The collection area A, is larger than A, by approximately the area of an annular region of width L, around the junction perimeter. This would be the photosensitive area determined in a high resolution spot scan of the photodiode in Fig. 1la. It is also the area to be used for diffusion current calculations. The diffusion volume is given approximately by A,L, . Sometimes there is an optical mask used to define the optically active area of the detector. This is shown in Fig. 1lb, in which the optical area A,, defined by the mask is smaller than the collection area A, ; in this case a high resolution spot scan would measure A,, as the photosensitive area. However, the area to be used for diffusion current calculations is still the collection area A,. One can see that the situation with A, > A,, in Fig. l l b is not optimum for signal-to-noise ratio. The effective volume contributing to diffusion current is larger than necessary. The background limited detectivity will be the same, but one will have to cool the photodiode to a lower than necessary temperature to achieve it. Because the junction area Aj is larger than necessary, there will be more space charge region g-r current and junction space-charge capacitance will be larger. Because the junction perimeter is larger, there will be more leakage current from mechanisms located at the junction edge. These effects have been considered analytically by Holloway (1978, 1979) and by Gurnee et al. (1979). If the surface recombination velocity at the front p-type surface is large

240

M . 3. REINE, A. K . SOOD, A N D T. J . TREDWELL

enough, lateral collection of photogenerated electron-hole pairs will not be important. The effects of lateral collection on the photocurrent of small area, uriiformly illuminated photodiodes, including a finite surface recombination velocity s at the front p surface, were treated by Shappir and Kolodny (1977). For aL, > 1, which is usually the case for A < A,,, they show that maximum lateral collection occurs when s is small enough so that s < vD, where vl, is the diffusion velocity L e / 7 , . The amount of lateral collection decreases for s > vD; there is essentially no lateral collection for values of s about equal to or larger than the quantity D,a. If the p-surface recombination velocity is large, then the area of the p-side surface immediately adjacent to the junction edge could be a source of dark current, since it is within a diffusion length of the junction. An optical spot scan of the photodiode would indicate no appreciable lateral collection because photogenerated pairs would recombine at the surface. But when the photodiode is biased, the p surface would be a source of minority-carrier current. 9. RESPONSETIME In this section we consider those mechanisms which limit the ability of the photocurrent in Hg,,Cd,Te photodiodes to remain in phase with modulated infrared signal radiation when the modulation frequency becomes large. High-frequency limitations of photodiodes are discussed in general by Sze (1969, Chapter 12), and have been considered specifically for Hg,-,Cd,Te photodiodes by Cohen (1972), by Koehler and McNally (1974) and by Kingston (1978). The considerable interest in the behavior of the signal response of Hg,-,Cd,Te photodiodes at high frequencies has been mostly due to applications at 10.6 pm involving CO, laser heterodyne detection for laser radar or optical communications. There are basically three effects which determine the upper-frequency response of a photodiode to modulated signal radiation: (a) the time required for electron-hole pairs photogenerated in the quasineutral n or p regions to diffuse to the space-charge region; (b) the transit time required for photogenerated carriers to drift across the space-charge region; (c) the RC time constant associated with the junction capacitance and resistance combined with the impedance of the external circuit. In practice, it is generally found that either the first or the third of these determine the upper frequency response of Hg, -,Cd,Te photodiodes. u . Diffusion Effects in the Quasineutral Regions

Consider the case of the n-on-p photodiode shown in Fig. 10 and again assume that essentially all the incident signal radiation is absorbed in the quasineutral p region. The instantaneous generation rate per unit volume

6.

PHOTOVOLTAIC INFRARED DETECTORS

24 1

due to signal radiation modulated at an angular frequency o is G(z,t) = cuQ exp(-az

+ iot).

(111)

The photogenerated excess minority-carrier concentration An(z,t) is a solution to Eq. (1 l ) and is subject to the boundary condition

An(0,t) = 0,

(112)

and that An(z,t) vanish for large z. The solution for An(z,t) leads to a photocurrent density .Ip&) given by Jph(f)

= e q ( o > QexpCibt -

where the phase angle C#I is given by

413,

( 1 13)

.

and where the ac quantum efficiency is given by

Here a ( w ) and b(w) are the dimensionless functions of frequency given in Eqs. (86) and (87). In the limit of low frequency, a -+ 1 and b + 0, and q ( w ) reduces to the dc quantum efficiency given in Eq. (66). For high frequencies such that 07, >> I , q ( w ) reduces to q(w) + C Y L e / G .

(1 16)

An estimate of the frequencyf, above which the quantum efficiency decreases substantially below its dc value is obtained by equating Eqs. (66) and (116):

f, = ((~L,)~/27r7, = cu2D,/27r.

(1 17)

Let us estimate f, for p-type Hgo,,Cdo.,Te at 77 K. If the minority-carrier mobility can be taken as the electron mobility in low carrier concentration n-type Hg,,,Cd,.,Te (Table I), which is about 3 x lo5 cm2/V sec, then Eq. (21) gives a value for D, of 2 x lo4 cm2/sec. The frequency& will depend strongly on wavelength through a(X). For wavelengths not too much shorter than the cutoff wavelength, we can let a be about 500 cm-’, andf, turns out to be about 800 MHz. By analogy, one can see that the upper frequency response limit due to diffusion effects in n-type (HgCd)Te will be significantly less than that in p-type because of the much smaller hole mobility. A two order of magnitude smaller hole mobility will lead to a correspondingly smaller cutoff frequency & . The problem of the frequency response of a p region of finite thickness,

242

M . B . REINE, A. K . SOOD, A N D T. J . TREDWELL

including effects of surface recombination, has been treated by Sawyer and Rediker (1958). 6. Drift Ejjects in the Space-Charge Region

The effect of the finite transit time of photogenerated carriers on photodiode frequency response was considered by Gartner (1959) and is discussed in Sze (1969, Chapter 12). The transit time t, for carriers of drift velocity v d across a space-charge region of width W is t, =

w/v,.

(1 18)

If W is 1 pm and V d is limited by lattice scattering to a value of about 1 X lo7cm/sec, then the transit time is about 1 x lo-" sec. One would expect to see transit time effects in the frequency response at a frequency around (27rQ-l or at 16 GHz.

c . Junction Capacitance Effects The effects of junction capacitance, dynamic resistance, and series resistance, together with external circuit impedance, on the high-frequency response of 10.6-pm Hg,,,Cd,.,Te photodiodes have been analyzed by Peyton el al. (1972), by Koehler and McNally (1974), by Shanley et al. (1977) and by Shanley and Perry (1978a,b). Here we give a simplified discussion in which we assume that the only significant capacitance is the junction space-charge region capacitance Cj and the only significant resistance is the external load resistance RL. In this case, the upper-frequency limit f, is =

I/'LrRLCj.

(119)

For Cj we assume CJ = ~SEdZJIW,

where 8, is the static dielectric constant, E, is the permittivity of free space, Aj is the junction area, and W is the space-charge region width. For W we assume the usual abrupt junction expression W

=

d2&&,(VW- V ) / e N B ,

(121)

where NR[= NAND/(NA + N,)] is the effective doping concentration in the space charge region, and where Vbl is the built-in junction voltage given by

For a Hg0.206Cdo.,g4Te photodiode, the cutoff wavelength will be about

6.

PHOTOVOLTAIC INFRARED DETECTORS

243

12 km at 77 K, ni is about 5 x 1013cm-3 and E, is about 17 (Dornhaus and and N A = 1 X lo'' ~ m - Eq. ~ , (122) Nimtz, 1976). For ND = 2 x 1014 gives eVbi as 0.059 eV. Figure 12 contains plots of the cutoff frequency& versus applied reverse-bias voltage for a Hg,,to6Cdo.,aaTephotodiode. The cutoff frequencyf, was calculated from Eqs. (1 19)-(122) for a load resistance of 50 SZ, for ajunction area of Aj of 1 X lop4cm2,for N A = 1 X 10'' and for various values of ND.Also shown are the space-charge region width W and the capacitance per unit area Cj/Aj. In Fig. 12, the curves are not extended beyond what are reasonable values of reversebias voltage for 12-pm Hg,-,Cd,Te photodiodes. Still, one can see that cutoff frequencies of 2 GHz can be realized with the lowest n-side doping concentrations at reasonable reverse-bias voltages. It should be noted that any junction series resistance and stray capacitance will tend to lower & from these values (Shanley and Perry, 1978a,b). lo. AUGERLIFETIMEI N p-TYPE Hg,,Cd,Te It has been known for some time now (Peterson, 1970; Kinch et af., 1973) that an Auger electron-electron collision process involving two conduction band electrons has been an important recombination mechanism in n-type Hg,-,Cd,Te, particularly for x around 0.2 and at higher

0.Wl

0.01

0.1

1

10

REVERSE BIAS VOLTAGE (Vl

FIG.12. Calculated cutoff frequencyf, [from Eq. (1 19)] for a Hgo,,,C&,,,,Te photodiode at 77 K (&,, = 12 pm) with an aread, of 2 x cm2. The right-hand scales correspond to the depletion width W from Eq. (121) and to the junction capacitance per unit area C,/A, from Eq. (120).

244

M . B. REINE, A . K . SOOD, A N D T . J . TREDWELL

temperatures. Tredwell and Long (1977) showed that this electronelectron Auger process, which we denote as “Auger process 1,” was generally not expected to be significant in p-type Hg,,Cd,Te. Recently, Casselman and Petersen (1979, 1980) surveyed those Auger processes which could occur in p-type Hg,-,Cd,Te and found that there was one Auger hole-hole collision process involving a transition from the light-hole valence band to the heavy-hole band which might, on theoretical grounds, be expected to be important in p-type Hg,-,Cd,Te. Following Casselman and Petersen (1979, 1980), we call this process “Auger process 7.” In this section we derive the dependence of the Auger process 7 lifetime on excess carrier concentration. Using detailed balance arguments and assuming nondegenerate electron and hole distributions, Blakemore (1962, Chapter 6) shows that the recombination and generation rates rl and g , per unit volume for the electron-electron Auger process 1 in Fig. 13 are TI

= (1/27,)(n2p/n:),

gi = n/271,

(123)

where T~ is the Auger process 1 lifetime for an intrinsic semiconductor at thermal equilibrium. Let the electron and hole concentrations n ( t ) and p ( t ) undergo small departures from their steady-state values 7i and is: n(t) = F +

An(&

p(t)

= p + A&).

( 1 24)

Further assume that An(t) = A&). Then the incremental lifetime qnc which characterizes the exponential decay of An@) to zero is obtained from ( r - g ) as follows:

E

L

FIG. 13. Recombination transitions for the electron-electron collision (Auger process 1) and hole-hole collision (Auger process 7) mechanisms. The arrows indicate electron transitions, and i and f denote initial and final states for recombination. [From Casselman and Petersen (1979, 1980).]

Heavy-

Auger 1

Auger 7

6.

PHOTOVOLTAIC INFRARED DETECTORS

245

The incremental lifetime for Auger process 1 is then

Similar arguments can be applied to the hole-hole Auger process 7 (Casselman and Petersen, (1979, 1980) to give

l @ r, = 2~~ nT '

g7

=

e277

7

and the incremental lifetime for Auger process 7 is

The total incremental lifetime for both Auger processes is obtained by adding Eqs. (126) and (128):

where

T~ is

defined as

_1 -- _1 +-.1 7i 71

77

Consider now small departures from thermal equilibrium;

n = no and p

=

po can be substituted into Eq. (129), with the result

For an intrinsic semiconductor, one can see that Eq. (131) reduces to Eq. (130). The symmetry between Auger 1 and 7 is clear from Eq. (131). For heavily n-type material, Auger process 1 becomes more important, and Auger process 7 becomes more important for heavily p-type material. This behavior can be seen in Fig. 14, which is a qualitative representation of Eq. (13 1). The longest lifetime occurs when and Fig. 14 was drawn with the assumption that T~ < r7 so that the longest Auger lifetime would occur for p-type material. The problem of determining how important Auger process 7 is in p-type . though the necessary Hg,,Cd,Te lies in obtaining a value for T ~ Even regions of the Hg,-,Cd,Te band structure are probably known well enough, there are overlap integrals involved in any calculation of 77 (just

246

M . B. REINE, A . K . SOOD, AND T. J . TREDWELL

FIG.14. Qualitative behavior of the incremental Auger lifetime for processes 1 and 7 for small departures from thermal equilibrium, from Eq. (131).

as in T J which are difficult to evaluate with sufficient confidence to enable an a priori assessment of the magnitude of 77 (Casselman, 1981). The symmetry of the two mechanisms, as evident from Eqs. (129)-( 1301) will make it particularly difficult for experiments to determine independent values for T , and 77. For example, consider the photoconductive lifetime data of Kinch et al. (1973) on n-type Hgo.,Cd,.,Te samples. Their conclusion that Auger process 1 was operative in their samples was based in large part on the temperature dependence of the photoconductive lifetime between 125 and 200 K, where the samples were intrinsic. Both T~ and r7 have nearly the same temperature dependence. The intrinsic Auger lifetime is given in Eq. (130), so that the data of Kinch et al. (1973) in the intrinsic range can only provide an experimental value for [ 7 1 7 7 / ( 7 1 + T T ) ] rather than for either T~ or 71 separately. 111.

Hg,-,Cd,Te Junction Photodiode Technology

A wide variety of techniques has been used to form p-n junctions in Hg,,Cd,Te. Both n-on-p and p-on-n junctions have been formed, and type conversion has involved introduction of foreign atom impurities, controlled deviations from stoichiometry, and occasionally combinations of both. Junction formation techniques have included ion implantation of donors and acceptors, high energy proton bombardment, diffusion of donors and acceptors, in-diffusion and out-diffusion of mercury, creation of p-type surface layers on n-type material by intense pulsed laser radiation, and in situ cosputtering of donors and acceptors in sputtered Hg, ,Cd,Te layers.

6.

PHOTOVOLTAIC INFRARED DETECTORS

247

The most technologically significant of these techniques for high performance infrared detectors appear to be ion implantation and Hg indiffusion. For direct detection applications, the highest R A products for Hg,-,Cd,Te photodiodes to date have been reported for boron-implanted n-on-p photodiodes. For high-frequency 10.6-pm heterodyne applications, n-on-p planar photodiodes, formed by Hg in-diffusion into p-type Hgo,,Cdo.,Te containing a low concentration of donors, have given the widest bandwidths so far. 11. ION IMPLANTATION

Ion implantation has been used to form a variety of junction configurations in Hg,,Cd,Te during the past 10 years. Most of the implant work has been focused on forming n-on-p junction photodiodes for direct detection applications. This work and the work on forming p+-p and n+-n hi-lo junctions are reviewed in this section. Ion implantation has also been used to form n+-p-p--p+ avalanche photodiodes in Hgo.,Cdo.,Te (Koehler and Chiang, 1975) and, recently, n-channel MOS transistors in Hgo.71Cdo.zsTe (Kolodny et al., 1980) and n-channel insulated-gate field effect transistors in Hgo.,ssCdo.zlsTe(Nemirovsky et al., 1980). a. Summary of Ion-Implantation Work in Hg,-,Cd,Te Table I1 summarizes implantation work reported in Hg,-,Cd,Te to date. The proton bombardment results of Foyt et al. (1971) are included for comparison. Type conversion via high-energy electron irradiation is considered later in Section 13,a. The electrical behavior of the various implanted species shown in Table I1 is based on the conclusions of Johnson and Schmit (1977), who studied the doping properties of selected impurities in Hg,,Cd,Te. In their work, impurities were introduced either by diffusion or by melt doping during crystal growth. Their conclusions can be summarized as follows: (1) Ag, Cu, Au, and Li are fast diffusing acceptors; (2) Ga and In are fast diffusing donors; (3) A1 and Si are donors requiring high temperatures for diffusion; (4) Sn is inactive when diffused at or below 300°C; ( 5 ) P and As are slowly diffusing acceptors; and (6) Br is a slowly diffusing donor. has been devoted to the forMost of the implant work in Hg,,Cd,Te mation of n-on-p junction photodiodes. The first use of high-energy particle bombardment for type conversion in Hg, -,Cd,Te was reported by Foyt et al. (1971) who used proton bombardment to form n+-on-pjunction photodiodes. Type conversion in this work was most probably due to cre-

TABLE I1

SUMMA RY OF IOK IMPLANTATION WORKI N Hg,-,Cd,Te Implant species

Doping behavior in Hg,-,Cd,Te"

Protons

-

Hg

Unimportant as a donof

Implant energy (keV) Implant dose 200 200 100 30

5 x 10'4

Damage anneal

)

None

2 x 1014 1.6 x 1014 1012-

1013

Hg,-,Cd,Te alloy composition

0.25

Device configuration n+-on-p

0.31

0.5

NoG

Reference Foyt er al. (1971) [see also Elliott er al. (1972)l

0.2-0.5

n-on-p

Fiorito et a / ., 1973, 1975, 1978 [see also Fiorito el a!. (1976, 1977)]

0.20, 0.23

n-on-p

Marine and Motte (1973)

2wc

0.29

n-on-p

McNally (1974)

250"C, 5-10 min

0.2

p-on-n

Koehler (1977) Shanley et al. (1977)

0.2-1

n-on-p

Igras (1977)

~

Al

Donor

250

5 x 10'5

In

Donor

500

1013- 1014

Au

Acceptor

200

1014

In, A1 Hg

Donor Unimportant as a donof ? Acceptor AcceptoP

300"C, 1 h

~

Zn P, Au N

30- 140

10'"lO'j

None

No type conversion in n-type

(Continued.)

B

Dono?

110

3 x 1013-3 x 10l5

Al

Donor

3

Ne

Neutral

300 -

Si

Donor

-

CI B B

Donor' Dono? Donor'

60- 100 40 and l W

1014

B

Donold

110

3.7 x loLo-3.7 x

B

DonoP

100

P

Acceptor

P As

Acceptor Acceptor

B

DonoP

P

Acceptor

B

DonoP

20-250

B

Donor'

100-150

X

1013-3 -

X

lOI5

Some samples at 200°C

-

-

0.3

n+-on-p

Bratt (1978)

n +-on-p n+-on-p Sood and Tredwell (1978a)

0.2

N o type conversion in p-type n+-on-p n+-on-p n + / n-on-p

Yes; no conditions given

0.28

n-on-p

Riley et al. (1978b) (see Figs. 15 and 16)

I x 1013

None

0.33

n+-on-p

Lanir et al. (1978, 1979)

100

1 x 1013

300"C, 22 h

0.3

p-on-n

Chapman et a / . (1978)

150 200

1 x 10'5

150-200"C, 2 h

0.2

P+-P PI-P

Sood ef a / . (1979a) [see also Sood er a / . (1979b,c)]

I x

150"C, 2 h

0.39

n+/n-on-p

Tobin (1979); Schmit ef al. (1979)

250"C, 3 days in Hg vapor

0.2

p-on-n

See footnote e

n-on-p

Riley et a / . (1979) (see Fig. 17)

n-on-p

Wang et a l . , 1979

40 and l W 280

- 1015 I x 1015

1015

1.1014-3 x

lOI5

I x 1015

- 10'3

I

0.2

125-175"C, 1-3 h 150-2WC, 0.5-3 h

25O-45O0C, 4 h

-

0.2

TABLE XI (continued.)

B Be

Dono?

B

DonoP

-100

-5 x

1012

None

0.2

n-on-p

Chu er al. (1979); Bubulac er al. (1979)

20-150°C. 24 h in

0.21

n+-on-n

Margalit et al. (1979)

0.29

n-on-p

Kolodny and Kidron

?

150

1 x 10L1-lx lOI5

vacuum

Ar

Inert

In

Donor

P Au

Acceptor Acceptor

B A1 Ar P

Donold Donor Inert Acceptor

Hg

Unimportant as a donof

a

100 -

1 x IOL3-3 X

300 300

1 x 10'4 I x 1014

300

1 x lOI3-2 x lOI4

IOI5

65"C, 12 h

-

(1980)

25OoC, 30 min

Based on the conclusions of Johnson and Schmit (1977). Extrapolated from the conclusions of Johnson and Schmit (1977). Schmit and Stelzer (1978); Vydyanatb ef al., (1979). Double Implant. Private communication from P. R. Bratt, K. I. Riley, and A. H.Lockwood.

0.2

-

Bahir et al. (1980)

6.

PHOTOVOLTAIC INFRARED DETECTORS

25 1

ation of damage which was intentionally not annealed out after bombardment. Ion implantation of mercury to form n-on-p Hg,-,Cd,Te junction photodiodes was reported by Fiorito et af. (1973) and has been discussed by them in a series of subsequent papers (Fiorito et a/., 1975, 1976, 1977, 1978). No work damage anneals were performed, and damage is probably responsible for type conversion here also, particularly in view of the lack of evidence for interstitial mercury being a significant native donor in Hg,-,Cd,Te (Schmit and Stelzer, 1978; Vydyanath and Nelson, 1981). Implantation of the donors aluminum (Marine and Mott, 1973) and indium (McNally, 1974) with post-implant work damage anneals formed n-on-p Hg, ,Cd,Te junction photodiodes, but the roles of damage versus activated impurity donors were not delineated. Igras et a / . (1977) reported type conversion in p-type Hg,-,Cd,Te with implanted In, Al, Hg, and with Zn, but did not obtain type conversion of n-type Hg, -,Cd,Te via implantation of phosphorus, nitrogen, and gold. No post-implant damage anneals were performed, which probably accounts for the failure to achieve type conversion in n-type Hg,-,Cd,Te. Similar results were obtained by Kolodny and Kidron (1980), who formed n-on-p junctions in p-type H&.,,Cdo.2eTe by implantation of boron and aluminum as well as by implantation of phosphorus and argon, with damage anneals at 65°C for 12 h. They concluded that their implanted layers are n-type due to damage rather than to the implanted species. Margalit et al. (1979) studied the electrical and annealing properties of implanted n+ layers on low carrier concentration (about 5 x 1014 ~ m - ~ ) n-type Hgo.,,Cdo.,,Te wafers. They observed nf layer formation with implantation of boron, argon, indium, phosphorous, and gold. The boron implantations were performed through 2000-A insulating layers of either CdTe or ZnS. The implant energy was 150 keV and doses ranged from 1 x 10" - 1 x 1015cmP2.They found that the sheet electron concentration in the implanted (i.e., unannealed) n+layers, as determined from conductivity and Hall effect measurements on van der Pauw samples, was not very sensitive to either the implant species or the implant dose, for doses ranging between 5 x 10"-1 x 1015 cm-2. For example, sheet electron concentrations of around 1 X 1013cm-2 were obtained at an implant dose of 1 x 10l2cm-2, and sheet electron concentrations around 5 x lOI3ern+ were obtained at a dose of 1 x lOI5 cm-2. From these results, Margalit et af. conclude that the implanted n+ layers are due to implant damage rather than to substitutional doping. Margalit et a/. studied the effects of annealing samples after implant at temperatures ranging from 20-120°C for 24 h in vacuum. Samples implanted with gold at 5 X l O I 4 cmP2and with boron at 1 x 1015 cm-2 showed only a slight reduction in sheet electron concentrations at an annealing temperature of 120°C. Margalit et af. point

252

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL

out that implanted n+ layers, in addition to providing n+-on-pjunctions, could also prove useful for accumulating the surface of n-type Hg,,Cd,Te photoconductors to reduce surface recombination and to aid in forming ohmic contacts to n-type Hg,-,Cd,Te. There have been some reports of p-on-n junction photodiodes formed via acceptor ion implantation into n-type Hg,-,Cd,Te, although with work damage anneals at significantly higher temperatures than those of Margalit et ul. (1979) and Kolodny and Kidron (1980). Koehler (1977) and Shanley ef u / . (1977) reported the use of gold implantation to form p-on-n Hgo.,Cdo.2Tephotodiodes, with damage anneals at 250-300°C for 5 - 10 min being required for junction formation. Both Rileyt and Sood el a/. (1979a,b,c) have reported formation of p-on-n Hg,.,Cdo.2Te junctions via phosphorus implantation. In addition, formation of p+-p hi-lo junctions in p-type Hgo,,Cdo.2Tevia both phosphorous and arsenic implantation has been reported by Sood et c d . (1979a,b,c). Chapman et al. (1978) have formed p-on-n Hg,,Cdo,,Te junctions by phosphorous implantation. In all of this work, the implanted layers were n-type prior to annealing, and converted to p-type only after a work damage anneal. Implantation of boron into p-type Hg,-,Cd,Te has received considerable attention in the past several years and has become the most commonly used implant species for the formation of n+-on-p Hg,-,Cd,Te junction photodiodes. It is not known that boron is indeed a donor in Hg,-,Cd,Te, but boron is in the same column of the periodic table as aluminum, gallium, and indium which are known to be donors (Johnson and Schmit, 1977). Its small mass makes it a desirable choice from implant damage considerations. Riley et a / . (1978b) have reported SIMS analysis data (Fig. 15) for boron implanted at an energy of 110 keV into Hg0.,Cdo.,Te and Hgo.,Cdo.,Te. They show that their data for the concentration profile N ( x ) of implanted boron atoms follow generally the Gaussian behavior given by

W )= N,,, e W - i x

-

R,Y12(AR,)2),

(133)

and where cp is the boron ion dose, and where the mean projected range R, and the standard deviation AR, about the mean can be calculated from LSS theory (Mayer and Marsh, 1969; Pickar, 1975). For boron implanted into Hg,_,Cd,Te at 110 keV, R, is about 0.24 pm and AR, is about t Private communication from P. R. Bratt, K . J. Riley, and A . H. Lockwood.

6.

PHOTOVOLTAIC INFRARED DETECTORS

A I. 0

3 . 7 ~ 10l5 cmT2

110 k a v

0 0.4

3. 7 x 1014

110 keV

0 0.2

I . OX

.-

0.4

loL4 cm-2

253

110 keV

Calculated from U S Theory

Depth (rm)

FIG.15. SIMS analysis profiles of boron implants in Hg,,Cd,Te. of Eq. (135). [From Riley et al. (1978b).]

The solid lines are plots

0.15 pm. The solid curves in Fig. 15 were calculated from Eq. (135). Good agreement between Eq. (135) and SIMS data for boron implanted into H&,,Cdo.,Te at an energy of 100 keV and a dose of 4 x lOI4 cm-, has also been reported by Wang et a/. (1979). Riley et a / . (1978b) also reported data for the dependence of carrier concentration on implant dose in boron implanted n-type layers on p-type Hg,,7zCdo.zeTesamples. Their data, shown in Fig. 16, were obtained from both van der Pauw Hall samples and capacitance-voltage measurements on MIS samples. Evidence for additional donors due to implant damage is shown by the data for unannealed samples being about an order of magnitude above the straight line calculated from Eq. (136). The data for annealed samples are in good agreement with Eq. (136) for implant doses below about 1 x 1013 ~ m - ~ . as a Riley et a / . (1979) studied boron implantation into Hg,-,Cd,Te function of implant energy over the 20-250 keV range. Their data for the mean projected range Rp as determined by SIMS analyses of the implant profiles are shown in Fig. 17. After implantation, half of each sample was

254

M. B. REINE, A. K . SOOD, AND T . J . TREDWELL

DOSE

II+lCm2)

FIG.16. Donor concentration in n-type boron-implanted layers in H&.72C&.2aTeplotted versus implant dose. The straight line labeled nmaxwas calculated from Eq. (136). [From Riley et a / . (1978b).]

annealed for 4 h at the temperatures shown in Fig. 17. Except at the lowest implant energies, the data for both annealed and unannealed samples agree well with the values expected from LSS theory. Moreover, the data show that the boron profile is quite stable up to anneal temperatures of 450°C. Studies of implant damage in Hg-implanted H&&d,,Te via Rutherford back scattering (RBS) and proton-induced x-ray (PIXE) channeling experiments have been reported by Bahir et al. (1980). They used 300-keV Hg ions at implant doses ranging from 1 X 1013 cm-, to greater than 2 X 1014cm-*. From their RBS data, they observed a buildup of the damage until it reached a saturation level, beyond which no further changes in the RBS spectra were observed. The saturation dose was 2 X

6.

Irn

10

PHOTOVOLTAIC INFRARED DETECTORS

20

50

I00 ENERGY lkeVI

200

255

503

FIG.17. The mean projected range R, for boron implants into Hg,-,Cd,Te for various implant energies and for a dose of 1 x loL5cm-*. Data were determined via SIMS analysis. Data for both unannealed and annealed (for 4 h and at the temperatures indicated) samples are shown. 0, annealed; A, unannealed. [From Riley et a / . (1979).]

1014cm-2. From the position in energy of a “knee” in the RBS energy spectra obtained from implanted samples, they infer that the damage extends to a depth of about 1500 A into the sample. This is much deeper than the calculated range of about 500 A for 300-keV Hg ions in Hg,,Cd,Te. They found that the crystal structure could be nearly completely restored by annealing the implanted samples at 250°C for 30 min. The use of a double boron implant to tailor the boron profile within the implanted region of n+-on-p Hg,-,Cd,Te photodiodes was first reported by Sood and Tredwell(1978a). They specifically wanted to move the peak of the implanted profile towards the sample surface so that surface inversion would be less likely. They used sequential boron implants at 100 keV -~ a 1000-A thick ZnS layer, and 40 keV at doses of 1 x 1015 ~ r n through followed by annealing at temperatures between 150-200°C for times ranging between 0.5-3 h. SIMS analysis data reported by Sood et al. (1979~)for the n+/n boron atom profile introduced by such a double implant are shown in Fig. 18. These data are for a p-type Hg,,,,Cd,,.,Te sample with a native defect concentration of about 2-5 x lOI7

256

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL TIME (CYCLE NUMBER)

0

20

10

30

40

50

60

70

1

\

0 0 0

BACK -

GROUND \

P

, I000 8

\

+

- L I

10

0

\ \ \

I

I

I

I

0.75

0.5

0.75

1.0

DEPTH IN

Hyo

Te (urn)

FIG.18. SIMS analysis data for the boron atom profile introduced by adouble implant in a p-type Hgo,,Cdo,,Tesample with a 1000-A thick ZnS layer. 150°C anneal temperature ( 1 h). N A = 2.0-5.0 x I O l 7 cm-J. [From Sood P I trl. (1979c).]

which was annealed after implant at 150°C for 1 h. One can see that the boron atom distribution has roughly the double-Gaussian shape expected 1979~) were obfor the two sequential implants. Similar data (Sood rt d., tained for similarly implanted p-type Hg,.,Cd,.,Te samples with lower native defect acceptor concentrations (1 - 2 x 10l6 cm-3) which were post-implant annealed at 150°C for times ranging from 1-3 h ; no changes were observed in the boron atom profiles for the different anneal times. Double boron implants for n+/n-on-p photodiode configurations were also reported by Tobin (1979), Schmit ct 01. (1979) and Riley rf cil. (1979). b. Ion-Implanted Hg,-,Cd,Te

Photodiode Performance

There has been significant progress during the past three years in achieving high performance ion implanted Hg,-,Cd,Te photodiodes for direct detection applications. Space-charge region g-r and surface leak-

6.

PHOTOVOLTAIC INFRARED DETECTORS

257

age currents have been reduced to the point where diffusion current generally limits the R A products of 8-12 pm photodiodes at temperatures down to 77 K and of 3-5 pm photodiodes at temperatures around 170-200 K. In this section we summarize the state-of-the-art performance of implanted Hg,-,Cd,Te photodiodes with emphasis on these wavelengths and temperatures. The device configuration which has been almost universally adopted so far for 8-12 pm photodiodes for temperatures down to about 77 K and for 3-5 pm photodiodes for temperatures above 170 K is the n+-on-p or n-on-p configuration. There are both practical and theoretical reasons for this choice. As discussed in the preceding section, implanting n-type layers in p-type Hg,-,Cd,Te is more convenient than implanting p-type layers in n-type because damage anneals are less critical. Implant damage appears to consist of donorlike defects, and some workers (cf. Table 11) have entirely omitted the post-implant anneal, particularly at lower doses.

I o4

103/T

(K-I)

FIG.19. R A product versus 1000/Tfor a boron-implanted n+-on-p H&.,,5Cdo,3,5Tephotocmz. [From Tobin (1979).] diode. A, = 6.44 x

258

M. B. REINE, A. K . SOOD, A N D T. J . TREDWELL

In addition, one needs to be less concerned about the effects of the post-implant anneal on the stoichiometry of p-type substrates than of n-type substrates. Theoretical reasons for the choice of the n-on-p or the n+-on-p configuration include the significantly longer minority-carrier diffusion length in p-type Hg,-,Cd,Te and the possibility of achieving longer minority-carrier lifetimes in p-type Hg,-,Cd,Te than in n-type Hg,-,Cd,Te of comparable carrier concentration. Data for n+-on-p Hg,,6,Cdo.39Te photodiodes fabricated by a double boron implant in p-type material for a range of acceptor concentrations have been reported by Tobin (1979) (see also Schmit et af., 1979). The R,,A product for one of these elements is shown in Fig. 19. For temperatures above about 200 K the data vary with temperatures as $ , which is characteristic of diffusion current. The solid line in Fig. 19 was calculated from Eq. (22) for diffusion current from the p side. The fit between the data and this expression is excellent over three orders of magnitude. The dashed line is the R,,A product expected for space-charge region g-r current and is a plot of Eq. (48) forf(b) = l, and varies with temperature as n l l . Data for several arrays of these implanted d - 0 n - p Hgo.6,Cdo.,,Te photodiodes are shown in Fig. 20 plotted versus net acceptor concentra-

7

FIG.20. R,,A product data at 193 K for arrays of boron-implanted n+-on-p Hg,,.,,Cdo,asTe photodiodes plotted versus net acceptor concentration. [From Tobin, (1979).]

6.

259

PHOTOVOLTAIC INFRARED DETECTORS

tion (NA- ND).The data roughly follow the dashed line which has a slope of+. This would be expected from Eq. (22) if the lifetime T, varied with net acceptor concentration as ( N A - N J 1 . Minority-carrier lifetime data obtained on these photodiodes indeed confirmed this dependence of lifetime on ( N A - ND)at this temperature, as will be discussed in Section 14 of this chapter. Studies of boron-implanted n+-on-p Hgo.,Cdo.,Te photodiodes as a function of p-side acceptor concentration have been reported by Riley ef al. (1978b). Their results for R,A at 193 K versus the effective space-charge region doping NB[= N , N , / ( N , + ND)]are shown in Fig. 21. In their photodiodes, NB was determined by fits to the straight-line parts of junction capacitance versus voltage data plotted as C? versus V. The N Bvalues so determined were generally about eight times lower than the p-type base concentrations determined from Hall data. The relative insensitivity of R A at 193 K to N Bis attributed to the minority-carrier lifetime T, varying

-1

E

L

l

I

i

I

I

J

ctci

0.11014

Abrn - 5.7 urn

1015

1016

101'

NAND ,c,,-3,

N F D

FIG.21. &A product data at 195 K for boron-implanted H&,,Cdo,3Tephotodiodes plotted versus the space-charge region effective doping concentration N B . [From Riley er a / . 11978b).]

260

M . B . REINE, A . K . SOOD, A N D T. J . TREDWELL

FIG.22. Zero-bias capacitance per unit junction area C , / A , at 78 K for boron-implanted Hg,,,Ccb3Te photodiodes plotted versus space-charge region effective doping concentration Nn. A,x = -0.30. C , / A , a [ ( A J A N J / ( N k+ N,)JUaff. [From Riley et al. (1978b).]

as N i 2 , which behavior was observed in photoconductive decay measurements and is discussed in Section 14. Data for the zero-bias junction capacitance per unit area C,/Aj at 78 K versus N B are shown in Fig. 22 and vary as Wd2, as expected from Eqs. (120) and (121). Data for the reverse-bias breakdown voltage at 78 K versus NB are shown in Fig. 23. At this temperature, the reverse-bias characteristics are fairly soft and a breakdown voltage is not well defined. In Fig. 23 the breakdown voltage BV is defined as that reverse-bias voltage for which the reverse current is twice the sum of the short circuit photocurrent and the saturation current. What the data of Fig. 23 show is that the strength of the reverse current Reverse-bias leakage current mechamechanism varies roughly as are still not well understood, but the data of Fig. 23 nisms in Hg,,Cd,Te indicate that lower space-charge region doping concentrations NBare desirable, and this trend is consistent with tunneling through or avalanche multiplication within the space-charge region. Data for the R,A products at 193 K for near-infrared boron-implanted Hg, ,Cd,Te photodiodes are shown plotted versus cutoff wavelength in

6.

26 1

PHOTOVOLTAIC INFRARED DETECTORS

la 0

-

-> s >

0.1 1

17

NAND (cm-31 NA+ND

FIG.23. Reverse-bias breakdown voltage at 78 K for boron-implanted Hg,-,Cd,Te photodiodes plotted versus space-charge region effective doping concentration N s . BV = voltage where I = 2& + Zsat). BV = [ ( NAND) / ( NA + N,)]-0.56.[From Riley et al. (1978b).]

Fig. 24. These data are all for fairly small junction areas, on the order of 0.002 x 0.002 in2. to 0.003 X 0.005 in2. All are probably limited by diffusion current at this temperature. The solid curve is a plot of Eq. (32) for a p-region thickness of 10 pm and for the radiative lifetime given by Eqs. (35) and (36). The dashed curve is a plot of (R,A)pmgiven by Eq. (20) with N A = 1 x 10l6 cmP3,T~ = 2 psec and L, = 50 pm. Of course, other values could have been chosen, since it is the product (NA7,/L,) which enters into (R,A),, . Data for R,A versus reciprocal temperature for one of these elements are shown in Fig. 25. The cutoff wavelength is 4.08 pm at 193 K. The solid line is a plot of (R,A),, in Eq. (20) showing that diffusion current is dominant down to 170 K. Quite large R,,A products at 77 K for near-infrared implanted Hg,,Cd,Te photodiodes have been reported by Riley et af. (1978b) and by Gurnee et al. (1979). The I-V characteristics at 77 K and at 0" FOV for a boron-implanted Hg, ,Cd,Te photodiode with a cutoff wavelength of 4.2 pm at 77 K are shown in Fig. 26 (Gurnee et al., 1979). This element had an optical collection area A , of (25 pm)2 beneath a sputtered CdTe mask. At 77 K the zero-bias resistance R, was about 3.2 x 1013 Q, giving

-

1-

-

l

L

2pec

l

l

I

i

\

m

Le'W I

I

I

I

1

I

I

1

\ I

FIG.24. R,A products at 193 K for boron-implanted Hg,-,Cd,Te photodiodes versus cutoff wavelength. [From:0 , T . J . Tredwell (unpublished data): A, Riley er al. (1979);and 0 , Lanir ef a/. (1979b).] TEMPFRATURE

IOLWT

(K)

(K-')

FIG. 25. R d versus reciprocal temperature for a boron-implanted n+-p Hg,-,Cd,Te photodiode, x = 0.326; h,,(193 K) = 4.08 p m ; A, = 3 x 5 mil2. [From T. J . Tredwell, unpublished data.]

VOLTAGF f V I

FIG. 26. I-V characteristics at 77 K for a boron-implanted n+-on-p Hg,-,Cd,Te diode with a cutoff wavelength of 4.2 pm. [From Gurnee ei d.(1979).]

photo-

Theoretical Limit for Radiative Lifetime and d - 1 0 ~ m

10-41

1

1

a. o

I

I

I

I 10.0

1

I

I

1 I 12.0

I

I

CUTOFF WAVELENGTH (Hmi

FIG.27. R J , products at 145 K for 10 x 10 mil2 boron-implanted n+-on-p Hg,-,Cd,Te photodiodes versus cutoff wavelength. [Data from Sood et a / . (1979a).] The solid line is calculated from Eq. (32) for T~ = radiative lifetime and d = 10 pm. The dashed line is calculated from Eq. (20) for N , = 1 X 10l6 c m P , T~ = 0.3 psec, and L, = 50 p m .

264

M . B . REINE, A. K . SOOD, A N D T . J . TREDWELL

an R& product of about 2 x lo8 Q cmZ.The doping concentration of the p-type Hg,-,Cd,Te used was in the 1nid-10'~cmP3 range. Riley et d. (1978b) report a R,A product at 77 K of 1 x lo8 fk cm2 for a boronimplanted Hg,,Cd,Te photodiode with a cutoff wavelength at 77 K of 4.0 pm; they report the value of N B as being 8 x 1014c ~ n - ~ . Data for the R,A, products of long-wavelength, boron-implanted Hg,-,Cd,Te photodiodes with A j = 10 x 10 mil2 are shown in Fig. 27 for a temperature of 145 K (Sood rt ul., 1979a) and in Fig. 28 for a temperature of 77 K (Sood and Tredwell, 1978a; Longshore, 1979). As in Fig. 24, the solid lines are plots of the theoretical upper limit for p-side diffusion current with radiative recombination and a p-region thickness of 10 p m , and the dashed lines are plots of ( R d ) , , from Eq. (20) for the values of (NA7,/L,) shown. The data at 77 K show more spread and less agreement with Eq. (2O), probably due to the onset of leakage current mechanisms. I-V characteristics are shown in Figs. 29 and 30.

Fic;. 28. R,A, products at 77 K for photodiodes versus cutoff wavelength. line is calculated from Eq. (32) for 7 ,= calculated from Eq. (20) for N , = 1 x

10 x 10 mil' boron-implanted n+-on-p Hg,-,Cd,Te [Data from Sood and Tredwell (1978a,b).] The solid radiative lifetime and d = 10 p m . The dashed line is loL6C I - I - ~ , 7, = 0.15 p e c , and L, = SO p m ,

6.

PHOTOVOLTAIC INFRARED DETECTORS

265

FIG.29. I-V characteristics at 77 K (left) and 145 K (right) of a boron-implanted n'-on-p Hg,-,Cd,Te photodiode. This element had a junction area of 10 x 10 milz, a quantum efficiency of 60%, and a cutoff wavelength of 9.75 pm at 77 K and 8.7 pm at 145 K . The &Aj product at 77 K was 61 R cm-z and was 0.1 R cm2 at 145 K . [From Sood and Tredwell (1978a).]

Capacitance versus reverse-bias voltage data (Sood er af., 1978a) at 77 K for a boron-implanted n+-on-p Hg,,Cd,Te photodiode with a junccm2 are shown in Fig. 31. The str ;ght line is a tion area Aj of 6.25 x plot ofjunction capacitance as given by Eqs. (120) and (121) with the intercept giving Vbi = 130 mV and the slope giving NB= 3 X 1015~ m - The ~. intercept at C = 0 gives a value for C, of about 35 p F and a value for C , / A , of 0.056 pF/cm2. Hall data on thep-type Hg,,,Cdo.,Te samples gave ~, due to a hole concentration at 77 K of about 2 x 1 O I 6 ~ m - presumably native defect acceptors, which is appreciably larger than the value for NB obtained from the fit to the capacitance data in Fig. 31. Similar discrepancies between base acceptor concentration from Hall data and spacecharge effective doping NBfrom C2- V analysis have been reported for p-type Hgo.,Cdo,,Te by Bratt (1978) and by Riley el al. (1978b). Bratt

FIG. 30. I-V characteristics of a boron-implanted n+-on-p Hg,-,Cd,Te photodiode at 77 K. This element had ajunction area A , of 10 x 10 mil', a quantum efficiency of 62%. a cutoff wavelength of 9.0 p m and a R d , product of 410 cm2. [From Sood and Tredwell (1978b).]

266

M . B . REINE, A. K . SOOD, A N D T. J . T R E D W E L L

REVERSE B I A S VOLTAGE ( V I

FIG.31. Reciprocal of the capacitance squared versus reverse-bias voltage at 77 K for a 10 X 10 mil2 boron-implanted n+-on-p Hg,,,C$.,Te photodiode with a cutoff wavelength of 9.0 pm, a quantum efficiency of 66%, and a &A product of 910 R cm2. The intercept of the straight line through the data points gave V,, = 130 mV, and the slope gave N A N D / ( N A + ND) = 3 x loi5 cmF'. [From Sood and Tredwell (1978a).]

(1978) suggested that compensation of the base acceptors by the implanted donor atoms may occur within a narrow range near the spacecharge region. c. Large-ArPa Implanted Photodiodes Laser tracking and guidance applications frequently require relatively large area detectors, on the order of 2 x 2-5 x 5 mm2, usually in a quadrant array form. Boron-implanted n-on-p large area Hg, -,Cd,Te photodiodes have been reported for use at 2.06 F m (Bratt, 1978), at 3.85 pm (Bratt and Vanderwyck, 1977; Bratt, 1978) and at 10.6 p m (Riley et al., 1978a,b). A summary of the performance data for these detectors is contained in Table 111. AH data are for T = 77 K unless otherwise noted. Pulse response times were measured with a GaAs laser, with effective load resistances of 20-50 a. As expected, response times were limited by the RC products and usually decreased with reverse bias. Spot scan uniformities of 2% to -C 6% over the active areas were obtained.

*

d. Gate-Controlled Hg,-,Cd,Te

Photodiodes

The use of insulated field plates to externally vary the surface potential in the vicinity of a p-n junction is a standard diagnostic technique for localizing and identifying surface leakage current mechanisms. The effectiveness of this technique in silicon technology has been well documented

TABLE I11 LARGE-AREA BORON-IMPLANTED n-on-p Hg,-,Cd,Te

cutoff wavelength ( pm)

Peak wavelength ( Pm)

Junction area A j (cm')

Zero-bias capacitance per unit area (KFlcm')

2.2-2.5

2.1 -2.4

0.0645

0.04-0.08

3.8-4.3 11.0-11.5

3.6-4.1 10.2-10.6

0.284 0.0115

0.015-0.02 0.01'

~

200-400 (V = 0) 140 =

-1.6

R dJ

Series resistance

(0cm-Z)

'Q)

Detectivity cm Hzl@/W

Reference

~

~

~

~

(V

All data are for T = 77 K unless otherwise noted. Bratt (1978). Bratt and Vanderwyck (1977). Riley et nl. (1978a). Estimated from smaller-area devices. With load resistances of 20-50 Q. Data for T = 192 K.

Zero-bias resistancearea product

Pulse response time' sec)

GIA, ~

~~~~~~~~~~

PHOTODIODE PERFORMANCE SUMMARY"

0.17-5.7

V)

100 35-110 (V = 0) 8 (V = -1.5 V)

X

104

(0.2-1.9 X 103)' 200-600 1.2-2.8

50-70

7 4.3

2.4-8.5

X

10"

(0.4-3.1 X lo''), 3-4 x 10" 3.5-4.6 X 10'O

h

b, c

d

268

M . B . REINE, A. K. SOOD, A N D T . J . TREDWE1.L

by Grove (1967) and others. It was discussed briefly in Section 4,c of this chapter. The insulated field plate has also been used to provide external control of the surface potential in high-impedance InSb and InAsSb infrared junction photodiode arrays. Leonberger et al. (1971) and Hurwitz et al. (1972) found that insulated field plates on proton-bombarded planar n-on-p InSb photodiodes could reduce surface leakage current considerably at 77 K. More recently, insulated field plates were used in a backside-illuminated impurity-diffused mesa p-on-n InSb hybrid 32 X 32 mosaic array (Hoendervoogt ut nl., 1978) and in backside-illuminated InAs,.,Sb,., 32 x 32 mosaic arrays (Tennant et a / . , 1979). The first experiments with gate-controlled Hg,,Cd,Te photodiodes were reported by Sood and Tredwell (1978) (see also Sood and Tobin, 1980). Their results for the R,A product versus temperature of a boronimplanted gate-controlled n+-on-p planar Hg, ,Cd,Te photodiode are shown in Fig. 32; a cross section of the device is also shown. A thin layer of ZnS insulated the evaporated field plate from the p region. For zero gate voltage, R& saturates with decreasing temperature to a value of about 2 0 cm2 at 77 K. Applying a positive gate voltage between the gate and the common p-side contact increased R,A at 77 K by two orders of magnitude; more positive gate voltage again caused RJ to decrease. The best explanation for these data seems to be that the bands in the p region just below the field plate are at or close to flat-band conditions at + 4.5 V , are inverted for less positive gate voltages, and are accumulated for more positive gate voltages. When the p side is inverted, there is more junction current due to g-r centers within the field-induced junction and due to interband tunneling transitions across the field-induced junction. When the p side is accumulated, there is more current due to interband tunneling across the field-induced junction between the n* and p' regions at the surface. Effects similar to those were also seen in experiments with boron-implanted n+-on-p gate-controlled Hgo.66Cdo,3,Tephotodiodes at 77 K by Gurnee et crl. (1979). The effect of gate voltage on the R d product can be seen more clearly in Fig. 33, in which the R,Aj product'at 40 K of a gate-controlled boron-implanted H&.,Cdo.2Tephotodiode is plotted versus p-side surface potential. This photodiode had a cutoff wavelength of 14.2 pm at 40 K and the same configuration as shown in Fig. 32. Conversion from gate voltage V , to surface potential cps was made by assuming a flat-band voltage of + 5.4 V, a p-side acceptor concentration of 2 x 10l6 and an oxide/insulator thickness of 0.6 p m . A strong decrease in R,A, occurs for both accumulation and strong inversion of the p-side surface, with an optimum gate voltage which just depletes this surface. Experiments on gate-controlled implanted n+-on-p mesa Hgo.,2Cdo.2sTe

6.

269

PHOTOVOLTAIC INFRARED DETECTORS

-

V=I.OV

pn/=

0-0-0

v =o

oYo 9 0 - 0 -

/O-

,

0

Y

0

o/o

In-2

6

7

8

9

10

II

I2

13

IM)O/T (K)-I

Fic. 32. R A product versus reciprocal temperature for a 10 x 10 mil2 gate-controlled boron-implanted n+-on-p Hg,,,Cd,,,Te photodiode with a cutoff wavelength at 80 K of 9.5 Frn, a p-side acceptor concentration of about 2 x 1OI8 ~ m - and ~ , a n-side donor concen~ m - Data ~ . are shown for various values of positive voltage V, aptration of about 1 x plied to the gate electrode. [From Sood and Tredwell (1978a); see also Sood and Tobin (1980).]

photodiodes have been reported recently by Kolodny and Kidron (1980). They employed an evaporated indium field plate which overlapped the junction at the surface. The insulator was ZnS. The flat-band voltage, as determined by C-V measurements between the field plate and the p-side common contact, was +4.5 V, which corresponded to a negative fixed charge density of about 3 x 10" electrons/cm2. They observed excess junction current for gate voltages on either side of the flat-band voltage, which they also interpreted in terms of tunneling current across fieldinduced junctions. e . Phntodiodrs with Reduced Diffusion Volumes

As discussed in Section 4,a of this chapter, reducing the volume of the p-side region in which diffusion current is generated should reduce the

270

M . B. REINE, A . K . SOOD, A N D T. J . TREDWELL

SURFACF POTENTIAL

'p,

(eV)

FIG.33. R d , product at 40 K versus gate voltage for a gate-controlled boron-implanted n-on-p Hgo.,Cdo,,Te photodiode. Conversion from gate voltage to surface potential was made assuming a flat-band voltage of +5.4 V, a p-side acceptor concentration of 2 x 1OI8 cm-3, and an insulator thickness of 0.6 prn. [From S . P. Tobin, J. W. Marciniec, and A . K . Sood (private communication).]

amount of diffusion current and should increase the photodiode R,,A product due to diffusion current. This reduction of diffusion current will occur only if the p region terminates in an electrically reflecting boundary. There have been two experiments to achieve this boundary in so far, one using an acceptor-implanted p + - p hi-lo junction Hg,,Cd,Te (Sood er al., 1979a,b,c) and one using the graded bandgap transition LPE layer and its CdTe substrate region between a p-type Hg,,Cd,Te (Lanir et a)., 1979b; Lanir and Shin, 1980). Data for the zero bias resistances at 145 K for linear arrays of n+-p-p+ Hg,,Cd,Te photodiodes with a tapered variation in the p-region thickness along the length of the array are shown in Fig. 34. The n+ regions were 3 x 5 mil2 in area and were formed by boron implantation.

6.

PHOTOVOLTAIC INFRARED DETECTORS

271

P-REGION THICKNESS ( u r n )

FIG.34. Zero-bias resistance at 145 K versus p-region thickness for five linear arrays of n+-p-p+ Hg,-,Cd,Te photodiodes formed by boron and phosphorous implantation. [From Sood et a / . (1979c).]

The common p+-p hi-lo junction, which also acted as a common p-side contact, was formed by phosphorous implantation. The solid lines in Fig. 34 are plots of Eq. (30) for /3 = 0 (i.e., perfectly electrically reflecting boundary):

with L, set equal to 25 pm and with chosen for each array to give the best fit to the data. The data do show a trend consistent with Eq. (135). However, two of the arrays show additional features which probably reflect variations in base material properties along the length of the arrays. The (&A),, values chosen to fit Eq. (135) are all lower than the best val-

272 HgCdTeICdTe INTERFACE

M . B . REINE, A . K . SOOD, A N D T. J . TREDWELL

p-n JUNCTION

FIG.35. Electron-beam-induced current (EBIC) scan of a boron-implanted n+-on-p H&.,Cd,,Te photodiode formed on a LPE layer grown onto a CdTe substrate. [From Lanir and Shin (1980).]

U 10 urn

ues reported for n+-on-p Hg,,Cd,Te photodiodes with thick p regions (cf. Fig. 27). Boron-implanted n+-on-pjunction photodiodes formed on relatively thin (10-30 pm) Hgo.,Cdo.,Te LPE layers on CdTe substrates have been reported by Lanir et (11. (1979b) and Lanir and Shin (1980). Electronbeam-induced current (EBIC) data for one of these devices at 200 K are shown in Fig. 35. These data were obtained by scanning a 20-keV electron beam across a cleaved surface perpendicular to the junction plane and the Hgo,,Cdo.,Te/CdTe interface plane. The p-type Hgo,,Cd,.,Te layer was grown from a Te-rich solution at a temperature around 500°C. The graded-bandgap transition region at the film-substrate interface was estimated to be less than 3 pm thick. The EBIC data of Fig. 35 indicate a minority-carrier diffusion length of around 60 pm in the p region. Electron-hole pairs created in the CdTe substrate do not contribute to the EBIC scan due to their short diffusion length of about 3 pm. By fitting the normalized EBIC profiles away from the interface region to an expression similar to Eq. (30). Lanir and Shin (1980) deduce a value for p in Eq. (29) of about 0.05 at 80 and 210 K for the Hg,,,Cdo,,Te/CdTe interface.

12. DIFFUSED PHOTODIODES Junction photodiodes have been formed in Hg, ,Cd,Te by diffusion of acceptors into n-type material, by diffusion of donors into p-type matenal, and by converting thin layers of p-type material to n-type (or from n-type to p-type) by appropriate annealing in a Hg atmosphere. Such anneals are believed to proceed by diffusion of Hg either into the sample to fill and thereby reduce the concentration of metal vacancy acceptor sites, or out of the sample to create metal vacancy acceptor sites. A layer of the sample at the surface will convert from p-type to n-type during Hg in-

6.

PHOTOVOLTAIC INFRARED DETECTORS

273

diffusion if the impurity donor concentration in the sample exceeds the impurity acceptor concentration and if the in-diffusion temperature and time are chosen so as to reduce the metal vacancy acceptor concentration below this level. Analogous arguments apply for Hg out-diffusion. Contrary to much earlier speculation, it does not appear that native donor defects play an important role at the concentration levels normally encountered in infrared detectors (Schmit and Stelzer, 1978; Vydyanath et af., 1979; Vydyanath, 1980). were The first p-n junction photodiodes reported in Hg,,Cd,Te n-on-p junctions formed in p-type Hg0.35Cd0.65Te with a native defect acceptor concentration of 2 x 1017 cmP3 by Hg in-diffusion (VCriC and Granger, 1965). Subsequently the formation of n-on-p junctions via Hg in-diffusion was reported in Hg,-,Cd,Te with alloy compositions of 0.15 < x < 0.28 (VtriC and Ayas, 1967), of x = 0.45 (Figurovskii et al., 1979), and of x = 0.2 (Cohen-Solal and Riant, 1971). The longestwavelength Hg, -,Cd,Te photodiodes reported to date were Hg indiffused n-on-p devices which had cutoff wavelengths at 4.2 K of 35 pm (x = 0.179) and beyond 48 pm (x = 0.171) (Melngailis and Harman, 1970b). There is very little information in the literature on diffusion of Hg in Hg,,Cd,Te. Spears (1977) reports forming an n-type region about 5 pm deep in p-type Hgo.,Cdo.,Te wafers (with a hole concentration at 77 K of about 2 X 1017cm-3) by a Hg anneal at 240°C for 30 min. For similar diffusion conditions, Shanley and Perry (1978a) estimate a junction depth of about 2 pm for p-type Hgo.,Cdo.2Teof comparable acceptor concentration. a.

Diffusion of Indium and Gold in Hgl-,Cd,Te

The diffusion properties of several impurities in Hg,,Cd,Te have been discussed by Johnson and Schmit (1977) (see also Schmit and Johnson, 1977). In this section we summarize some recent data on indium diffusion in Hg,-,Cd,Te and review some data on gold diffusion which have not been widely reported. Diffusion of indium into n-type Hg,.,Cdo.,Te was reported recently by Beck and Sanborn (1979). Films of indium were evaporated onto the 111B surfaces of n-type samples with donor concentrations of 2-3 x lo1, ~ m - ~ . This was followed by anneals at 71°C and 95°C for 1 h in a dry nitrogen atmosphere. The increase in donor concentration was measured via C-V measurements on MIS samples on a beveled surface. Their data for the net indium donor concentration following a l-h anneal at 95°C are shown in Fig. 36. The data fit reasonably well the conventional complementary error function distribution. Beck and Sanborn fit these data and data at

214

M . B. REINE, A . K . SOOD, A N D T . J . TREDWELL

Dl PIH 111 LOW SlJRFACC IPm)

FIG.36. Data for indium diffusion into n-type H&,,Cd,..,Te at 95°C for 1 h. The data point of 1.5 X is an extrapolated value. [From Beck and Sanborn (1979).]

71°C to the usual expression for the diffusion coefficient D: (136) Do exp(- E,/kT), and obtained a value of 1.6 eV for the activation energy E, and an estimate for Do of about 10l2cm2/sec. Margalit and Nemirovsky (1980) report diffusion data for indium into ern+ at 77 K) Hg,-,Cd,Te with x = 0.215 and p-type (about I x x = 0.29 for the temperature range of 70- 160°C. Their results for Do and Ea for In are given in Table IV along with the results of Beck and Sanborn. Some data on gold diffusion have been reported by Soderman (1970b) for Hgo.,Cd,,Te and by Timberlake and Soderman (1972) for HgJdo.pTe. Radioactive gold (atomic weight 199 with a half-life of 3.15 days) was deposited onto the surfaces of n-type samples from a gold chloride (AuCI,) electroless plating solution. The gold was diffused at a temperature of 275°C. Radioactive tracer analysis measurements were performed as material was successively lapped away to determine the gold concentration profile. The resulting profile for the x = 0.4 sample is shown in Fig. 37. A similar profile was obtained for the x = 0.2 sample. D

=

6.

27s

PHOTOVOLTAIC INFRARED DETECTORS

TABLE IV DATAON THE DIFFUSION OF I N D I U M Hg,-,Cd,Te alloy composition

Sample type and doping concentration

0.4

n-type; 2-3 x 1014cm-3 p-type; 1 x lOI5 c m P

0.215-0.29

(I

INTO

D O

(cm*/sec) 1

X

5.25

lox2 X

E, (ev)

Hg,-,Cd,Te Diffusion temperature range ("C)

1.6

lOI4

0.37

?

0.01

Reference

71 -95

a

70-160

6

Beck and Sanborn (1979).

* Margalit and Nemirovsky (1980).

I

o

:

1015

2

O

l

3 DISTANCE FROM SURFACF (mils)

FIG. 37. Data for gold diffusion into n-type H&,.,C&,,Te at 275°C as determined from radioactive isotope tracer analysis. [From Soderman (1970b).]

276

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL

The data were interpreted in terms of two diffusion mechanisms, one much faster than the other. The fast component was described by an exponential profile and was attributed to diffusion along dislocations. The slow component was interpreted in terms of a complementary error function profile as expected from Fick's law. From the slow diffusion data, diffusion coefficients of about cm2/sec for x = 0.4 and about 5 x cmz/sec for x = 0.2 were deduced. The slow diffusion component was attributed to out diffusion of mercury atoms and the subsequent filling of mercury vacancies with gold atoms. Scott and Kloek (1973) reported a diffusion coefficient of about loa9 cm2/sec for gold diffused into Hg,.,Cd,.,Te at 300°C with no mercury overpressure. They also observed that the rate of gold diffusion decreased strongly with applied mercury overpressure, indicating that the gold diffusion mechanism involves mercury vacancies. Additional data on gold diffusion in Hg,,,Cd,.,Te as well as on copper diffusion in Hgo.,Cdo,Te were reported by Andrievskii et al. (1974). Their experiments on n-type samples with carrier concentrations at 77 K of 2-7 x 10l6 cm" gave a diffusion coefficient D for gold at 300°C of 5.2 x lop9 cm2/sec. From data on anneals between 200°C and 450"C, they deduced an activation energy E, of 0.4 e V and a diffusion coefficient Do of 5.6 x cm2/sec. Their diffusions were done in a Hg atmosphere and diffusion depths were estimated from the p - n junction depth as measured by thermoelectric probing.

b. Wide-Bandwidth 10.6-pm Photodiodes The most important application of Hg in-diffused n-on-p Hg, ,Cd,Te photodiodes so far has been as wide-bandwidth 10.6-pm COz laser detectors. Verit and Sirieix (1972) first pointed out that the Hg in-diffused n--on-p Hg,,,Cdo.2Te photodiode was useful for COz laser heterodyne detection out to frequencies beyond about 1 GHz. This type of detector has been the subject of considerable developmental effort since then, both in single-element form with useful bandwidths out to 2 GHz (Spears et ul., 1973, 1974; Spears and Freed, 1973; Koehler, 1976a,b; Shanley et al., 1977; Shanley and Perry, 1978a,b) as well as in quadrant array form (Spears, 1977; Shanley and Hanagan, 1980) and in 12-element array form (Spears and Hoyt, 1978). A cross section of this photodiode is shown in Fig. 38. Initially, a mesa configuration was used for this type of device. The planar version with its greater versatility for arrays is now generally used. An insulator, usually ZnS, is evaporated or sputtered over the surface of a p-type Hgo.,Cdo.,Te wafer. Openings in the ZnS film are opened photolithographically to define the active areas. The wafer is then annealed in a Hg atmosphere with

6.

PHOTOVOLTAIC INFRARED DETECTORS

277

\Charge Region

Common p-Side Contact

FIG. 38. Cross section of the Hg in-diffused planar n--on-p photodiode configuration usually used for wide-bandwidth 10.6-pm C02 laser detectors.

the ZnS film acting as a diffusion mask. n-type regions approximately 2 - 4 p m deep are formed where the ZnS film has openings. p-type Hg,.,Cd,,,Te of fairly high carrier concentration, on the order of 1 X 10'' cmP, is generally used. The n-type regions formed usually have low carrier concentrations, on the order of 1 x 1015 cmP3or less, so that nearly all of the space-charge region occurs on the n-side. Hence, the donor concentration in this region determines the space-charge region width and the junction capacitance. For wide bandwidth, one wants low capacitance and hence low donor concentration. Donor concentrations down to 2.5 X 1014 cmP3have been reported in photodiodes of this type (Shanley et al., 1977). However, low donor concentrations in the n region also cause an appreciable series resistance. Since for wide bandwidth one also wants low series resistance as well as low capacitance, the surface of the n region is usually accumulated or given a shallow n+doping. Spears (1977) reported the use of a thin layer of indium placed over the p-type Hg,.,Cdo.,Te surface prior to Hg in-diffusion, which produced a n+ surface layer on the n- region. An additional advantage of the n+ surface layer is to provide a donor concentration gradient which in turn sets up an electric field in the n region. This electric field can assist transport of photogenerated holes to the space-charge region. I-V curves at 77 K for a wide-bandwidth n--on-p Hg,&do.2Te photodiode are shown in Fig. 39. The R& product of about 0.1 n cm2 is quite low compared with those obtained in implanted n-on-p photodiodes of the same cutoff wavelength (Fig. 28) and is believed to be due to surface leakage current. Capacitance versus voltage data at 77 K for this photodiode are shown in Fig. 40 and indicate an n-side doping concentration of about 4 x 1014cmP3.In the heterodyne mode, this device had a noise equivalent W/Hz, at a frequency of 1.75 GHz, which is only a power of 6.2 x factor of 3 above the quantum limit of 2 x lo-*, W/Hz given in Eq. (1 10).

278

M . B . REINE, A . K. SOOD, A N D T. J . TREDWELL

FIG.39. I-V characteristics for a Hg in-diffused planar n--on-p Hg,,,Cdo.,Te photodiode at 77 K. The cutoff wavelength is 11.5 p m , the quantum efficiency at 10.6 p m is 34%. the the forward resistance is 16 R, the zero-bias resistance is junction area is 1.8 x lo-' or, 775 R, and the heterodyne bandwidth is 2.0 GHz. [From Shanley and Perry (1978a).]

c . Hgl-,Cd,Te 10.6-pm Photodindes at High Temperatures

Hg, -,Cd,Te photodiodes designed for 10.6-pm operation with a thermoelectric cooler at temperatures of 170 K and higher have been reported by Koehler and Chiang (1975) and by Koehler (1976a,b). These n-on-p photodiodes were formed by diffusion of indium into p-type Hg, -,Cd,Te For these temperawith acceptor concentrations of 0.6-2 x lo'' 08

1.8

0.6

--

1.41

g y1

1.58

0.4

%

2

2 4

-

Y

n

N

1.82

5 0.2

9

?, 23

3.01

0

REVERSE-BIAS VOLTAGE (Vl

FIG.40. Reciprocal of the capacitance squared versus reverse-bias voltage at 77 K for a wide-bandwidth Hg in-diffused n-on-p Hg,.,Cdo,,Te photodiode with A, = 1.8 X lo-' cm2. The straight-line fit to the data gave values for Vb, of 70 mV and for NBof 4 x lof4c ~ n - ~ . [From Shanley and Perry (1978a).]

6.

PHOTOVOLTAIC INFRARED DETECTORS

279

tures and bandgap, diffusion current is the dominant junction current mechanism. These photodiodes had an active area diameter of 150 pm (Aj = 1.8 x lop4cm,). The best device had a reverse saturation current of 8.8 mA (50 A/cm2) at 174 K and a quantum efficiency of 21% at 10.6 pm. Measurements on this device in the heterodyne mode at 174 K gave a noise equivalent power at 10.6 pm and 10 MHz of 8 X W/Hz for an applied local oscillator power of 0.5-1 mW, as compared to the quantum limit of 2 x W/Hz at this wavelength. The bandwidth was 23 MHz, less than that anticipated from junction capacitance considerations, and was attributed to minority-carrier diffusion in the p region. The junction saturation current density Jsatof 50 A/cm2 at 174 K reported by Koehler (1976a,b) corresponds to, according to Eq. (80), an RJ product of 3 x R cm2, whereas the ultimate theoretical limit (Tredwell and Long, 1977) is around I x R cm2 at this temperature. There is still interest in a thermoelectrically cooled 10.6-pm Hg, ,Cd,Te heterodyne photodiode. Further development could reduce the junction saturation current so that the photodiode noise can be dominated by local oscillator noise for lower local oscillator power levels, thus reducing the heat load on the thermoelectric cooler. The same approaches which are being developed to increase the photodiode RJ product for direct detection applications are useful here also. These include the use of p-type Hg,-,Cd,Te with long minority-carrier lifetime and photodiode configurations with reduced diffusion volume. Bandwidths limited by p-side diffusion times with values on the order of 100 MHz ought to be achievable. From Eq. (1 17) the p-region cutoff frequency fo is about 270 MHz at 170 K for CY = 2000 cm-' and for an electron mobility equal to the value of 3 x lo4 cm2/V sec measured by Scott (1972) for low carrier concentration n-type Hgo.8Cdo.zTeat 170 K. Shanley et al. (1980) have reported CO, laser heterodyne measurements on boron-implanted n+-on-p Hg,,,Cdo.,Te photodiodes at temperatures up to 145 K. Their data show that this type of device is capable of achieving bandwidths of approximately 475-725 MHz at 145 K. Noise equivalent powers of 3.2 X W/Hz at 77 K and 1.0 x W/Hz at 145 K were reported. d. Wide-Bandwidth 1-2 p m Hg,-,Cd,Te Room Temperature

Photodiodes at

Among the first applications considered for Hg,-,Cd,Te photodiodes was room temperature detection of pulsed laser radiation in the 1-2 pm spectral region. The feasibility of this type of detector was shown by Kohn and Schlickman (1969) who reported photovoltaic contact effects in Hg,-,Cd,Te samples with an alloy composition of x = 0.58 and a peak

280

M . B. REINE, A . K . SOOD, A N D T. J . TREDWELL

. 2 h

3

10.0

r

:: z E: m

75% QUANTUM E F F I C I E N C Y

8 1 .o

40% QUANTUM I

1.O

I

EFFICIENCY I

2. 0

I

.1

3.0

WAVELENGTH OF PEAK SPECTRAL RESPONSE (urn)

FIG.41. Detectivity (A) and current sensitivity (Le., current responsivity) ( x ) for n+-on-p Hg,-,Cd,Te photodiodes at 300 K, plotted versus the wavelength of peak response. The curve through the detectivity data points is a smooth fit to the data. The curves through the current sensitivity data points are plots of Eq. (96) for values of quantum efficiency of 0.40 and 0.75. [From Sodermdn and Pinkston (1972).]

response wavelength of 1.45 p m at room temperature. Scott (1970) reported response times around 40 nsec for reverse-biased diffused p-on-n Hg,-,Cd,Te photodiodes with alloy compositions ranging between x = 0.32-0.55. Soderman (1970a,b) fabricated gold-diffused p+-on-n cm2 and Hg,-,Cd,Te mesa photodiodes with junction areas of 3-5 x with peak response wavelengths of 1.8-2.3 p m at room temperature. and These devices had noise equivalent powers of 1-5 x lo-'* W/& response times of 30-60 nsec at room temperature. Soderman and Pinkston (1972) fabricated 1-3 pm n-on-p and p-on-n Hg, ,Cd,Te photodiodes by several different methods (impurity diffusion, alloying and proton bombardment); their data are shown in Figs. 41 and 42. The junction areas were about 5 X cm2. The response times

6.

PHOTOVOLTAIC INFRARED DETECTORS

281

t

'$

E

\Rd

I

E-

101

t-

I

I

1

I

2

Ap (,urn)

FIG.42. Junction resistance Rd (A) and junction capacitance C, ( X ) at zero-bias voltage for n+-on-p Hg,,Cd,Te photodiodes at 300 K , plotted versus the wavelength A, of peak response. The dashed curves are smooth fits to the data. [From Soderman and Pinkston ( 1972) .]

obtained for some of these devices at room temperature were approximately 20 nsec and were limited by the RC product and preamplifier bandwidth. With a very broadband preamplifier, the pulse response was limited to 0.5 nsec and a secondary time constant of about 5 nsec was attributed to carrier transit time. Probably the largest difficulty in developing Hg,-,Cd,Te photodiodes with peak spectral response wavelengths less than 2 pm has been the high mercury vapor pressures involved in conventional bulk growth of crystals from pseudobinary melts. [Crystal-growth techniques for Hg,-,Cd,Te have been reviewed recently by Nelson et al. (1980)l. For example, a cutoff wavelength of 1.5 pm corresponds to an alloy composition of x = 0.64. The liquidus temperature increases with alloy composition, being around 760°C for x = 0.2 and around 950°C for x = 0.6. Mercury vapor

282

M. B. REINE, A. K. SOOD, A N D T. J. TREDWELL

pressure increases exponentially with temperature to the point where conventional bulk crystal growth becomes quite difficult for material of wavelength less than about 2 p m . The use of liquid phase epitaxial (LPE) growth of Hg,-,Cd,Te from ternary melts (either Hg- or Te-rich solutions) overcomes these difficulties by allowing growth films at significantly lower temperatures (Mroczkowski and Vydyanath, 1980; Bowers et al., 1979, 1980; Schmit and Bowers, 1979). One would expect that LPE grown layers would be utilized in further development of 1-2 pm Hg,-,Cd,Te photodiodes. Piotrowski (1977) has reported data for Hgand acceptor-diffused Hg,-,Cd,Te photodiodes, fabricated on epitaxial graded-gap Hg,,Cd,Te layers grown on CdTe substrates, with peak response wavelengths down to 1.0 pm at room temperature. Fabrication TEMPERATURE IKI

300

lo8

I

250

170

200

I

I

I

150

125

I

110

-

I

/

5

N-

/

lo5k

/

I

in

I 2

10 I I 3

I 4

I

I

I

I

5 6 1WOiT IK-’I

I

I I 1

I

I 8

FIG. 43. RJ products versus reciprocal temperature for a 23-element planar Hg,,,C&.rrTe array. Element area was 4.5 X 4.5 mil2 (1.3 X lo-’ cm2), cutoff wavelength was 2.15 p m , and the zero-bias capacitance was 6 pF. [From Tredwell (1977).] The solid, nearly horizontal lines are calculated from Eq. (101) for A = 2 p m and 7) = 0.5.

6.

PHOTOVOLTAIC INFRARED DETECTORS

283

of p-n junctions in epitaxially grown Hg,-,Cd,Te has also been reported by Becla and Pawlikowski (1976) and by Pawlikowski and Becla (1975). Magnetic quantum oscillations in the photovoltaic response spectra of such n-on-p epitaxially grown Hg, -,Cd,Te junctions have been reported by Dudziak et al. (1978). Data for Hg-diffused n-on-p planar arrays of Hg,-,Cd,Te photodiodes with cutoff wavelengths between 2.0-2.3 pm have been reported by Tredwell (1977) for lower frequency applications. Data for a 23-element array with a cutoff wavelength of 2.15 pm are shown in Fig. 43. Junction zero-bias capacitances were about 6 pF, corresponding to a value for C,,/Aj of about 0.05 pF/cm2. In many of the devices, the reverse-bias breakdown mechanism involved an avalanche process; avalanche multiplications as large as 30 were observed in the photocurrent at large reverse bias. 13. TYPECONVERSION I N Hg,-,Cd,Te

BY

OTHERTECHNIQUES

a. Electron Irradiation of Hg,-,Cd,Te

High-energy electron irradiation is not a useful technique for p-n junction formation because the range of electrons is quite large. For example, the range of 2.5-MeV electrons in Hg,-,Cd,Te is about 1.2 mm. Electron irradiation studies are important, however, in that they give information devices and on the hardness (i.e., radiation resistance) of Hg,,Cd,Te they can be helpful in understanding the behavior of lattice defects in Hg, -,Cd,Te. Radiation hardness experiments on Hg, ,Cd,Te have recently been reported by Kalma and Cesena (1980). The effects of high-energy electron irradiation on the electrical and optical properties of Hg, -,Cd,Te have been studied by Melngailis er al. (1973) for initially p-type samples at about 8 K, by Mallon et al. (1973, 1975) for initially n-type samples at 10 and 80 K, and recently by Voitsehovski et al., (1979) for both initially n-type and initiallyp-type samples at 300 K. These experiments are summarized in Table V. In all cases, electron irradiation produced donor defects which were easily annealed out. Melngailis et al. (1973) found that the electrical properties of the irradiated samples could be restored to their initial values by annealing, and that most of the changes anneal out at temperatures around 55-75 K. They found a donor introduction rate at about 8 K of 28 cm-' for an initially p-type H&.,,Cdo.22Te sample. They found that electron irradiation broadened and shifted to longer wavelengths the photoluminescence spectrum at 8 K of an initially p-type Hg,,,,Cdo.31Te sample, and that most of these effects seem to anneal out at temperatures between 106-231 K. Similar behavior was reported by Mallon et al. (1973, 1975) in their

TABLE V

ELECTRON IRRADIATIONEXPERIMENTS IN Hg,-,Cd,Te Sample Maximum temperature Reirradiation Donor Electron electron during Preirradiation carrier Preirradiation introduction energy dose irradiation electrical Alloy concentration" mobility" rateb (cm-l) (MeV) (cm-2) type ( ~ r n - ~ ) (cm-*/v sec-I) composition (K) 2.5 5 5

5 2

4.7 x 1015

s

x

1014

4x

1014

8.5 x 1014 2 x 1018

25

P

2.5 x 10l6

780

P

4 x 10'6 2.6 x 1014 1.3 x 1015

300

80

n

10, 80

n

80

n

300

P P

n n n a

5.8 x 1014 1.5 x 1015

6 X IOl3 2.7 x 1OI6 9.6 x lOI6 1.7 x 10'5

117 2.5 x 104

1.2 x 1015 2.6 x 1015

2 x 104

Based on 80-K Hal! data. Measured at the temperature of sample irradiation.

322

1 x 105

0.22

0.3 I

0.188 0.20 0.20 0.26 0.26 0.24

0.26 0.28

28 -

5.7 16 (10K) 6 (80 K)

Reference Melngaiiis ef a / . (1973) Mallon et al. (1973) Mallon ef al. (1975) Leadon and Mallon (1975) Brudnyi er al. (1977)and Voitsehovski et al. (1979)

6.

PHOTOVOLTAIC INFRARED DETECTORS

285

experiments on low carrier concentration n-type Hg,,,Cd,.,Te samples at 10 K and 80 K. They reported donor introduction rates of about 16 cm-' at 10 K and 6 cm-l at 80 K. Anneals up to 340 K caused a nearly complete return of the irradiated sample electrical properties to their preirradiation values. They also found (Mallon et a f . , 1975) that gamma irradiation and high-energy neutron irradiation created donor defects which behaved the same as those produced by electron irradiation. The electron irradiation experiments of Voitsehovski et a f . (1979) were performed with the Hg,,Cd,Te samples at a temperature of 300 K during irradiation. For electron doses up to about 2 x lo1* cm-2, the initially n-type samples showed no significant change in carrier concentration. The initially p-type samples, however, converted to n-type at a dose of 5-8 x 10'' ~ m - These ~ . donor defects annealed out within the 360-430 K temperature range.

6. Sputtered H g , ,Cd,Te

Photodiodes

The growth of polycrystaline Hg,-,Cd,Te films onto either CdTe or Hg,-,Cd,Te substrates by means of triode sputtering in a mercury plasma has been reported by Cohen-Solal et al. (1974, 1976) (see also Zozime et target was cooled during sputtering. The a f . , 1975). The Hg,,Cd,Te CdTe or Hg,-,Cd,Te substrates were heated during sputtering in the 50-250°C range. The Hg,,Cd,Te films were doped either p or n during growth by cosputtering from either a gold or an aluminum target. Both p-on-n and n-on-p junction photodiodes were formed by sputtering the doped Hg, ,Cd,Te films onto substrates of the opposite conductivity type. Cohen-Solal et a f . (1974, 1976) report a value of 1.8 fl cm2 for the R d j product at 77 K of a sputtered p-on-n photodiode with a cutoff wavelength of 12.0 pm, which compares favorably with those obtained in implanted junctions (cf. Fig. 28). c. Junction Formation by Pulsed Laser Irradiation

The use of pulsed laser radiation to form p-on-n junctions in n-type Hg,,Cd,Te was reported by Lutsiv et al. (1978). Their starting material was n-type Hg,-,Cd,Te with x = 0.20-0.25 and with carrier concentrations at 77 K of 1-3 x IOl5 cmP3and mobilities of 1-2 x lo5 cm2/V sec. Samples were mounted on a heat sink in an evacuated ampoule during irradiation from either a ruby (0.694 pm) or neodymium (1.06 pm) laser. p-type surface layers about 10-30 pm deep were formed with laser intensities low enough to cause no visible surface damage. They postulate that the p-type surface layers result from metal vacancy acceptors which are formed when the laser pulse heats the sample surface to temperatures of about 500-600°C and which are subsequently quenched in during the

286

M. 9. REINE, A. K. SOOD, A N D T. J . TREDWELL

rapid sample cooling. Not enough information is given to assess the quality of the junctions formed, but the spectral response data show evidence of significant surface recombination effects. 14. MINORITY-CARRIER PROPERTIES OF

TYPE Hg,-,Cd,Te

The considerable interest in ion-implanted and diffused n-on-p Hg,-,Cd,Te photodiodes has focused both experimental and theoretical attention on the properties of p-type Hg,-,Cd,Te, particularly with respect to minority-carrier lifetime mechanisms. As discussed in Section 4,a of this chapter, the minority-carrier properties of the p region determine the diffusion current and the R,A product of 3-5 pm n-on-p photodiodes at temperatures above about 170 K and of 8-12 p m n-on-p photodiodes at temperatures above about 77 K. It is standard practice to characterize infrared photodiodes by measuring their I-V curves and R, versus temperature. But it is difficult to compare R,(T) to, say, Eq. (20) and then deduce values for the minority-carrier lifetime and mobility. One can at best get only the T e / p , ratio. But even this has considerable uncertainty, since the deduced value of 7,/pe depends on on It: and on N i , and usually ni and N , are not known accurately enough in a given photodiode to give appreciable confidence in the values of ~ ~ so /ob- p tained. In order to determine the minority-carrier properties of p-type Hg, -,Cd,Te, several techniques which have proven useful in widebandgap semiconductor technology have been applied. Measurements of the minority-carrier lifetime in p-type Hg, -,Cd,Te have been performed on implanted it+-on-p photodiodes by means of the reverse-bias step recovery technique. The electron-beam-induced current (EBIC) technique has been used with n+-on-p photodiodes to measure the minoritycarrier diffusion length in p-type Hg,-,Cd,Te. Recently the technique of deep-level transient spectroscopy (DLTS) has been used with implanted It+-on-p photodiodes to provide some information on trap energies, concentrations and emission rates in p-type Hg, -,Cd,Te. Nearly all minority-carrier lifetime data taken so far has been interpreted in terms of Shockley-Read recombination involving one or more deep levels within the forbidden energy bandgap. However, recent theoretical work by Casselman and Petersen (1979, 1980), which is discussed by Petersen in this volume and in Section 10 of this chapter, has shown that an Auger hole-hole collision process involving both the light- and heavy-hole valence bands may be an important recombination mechanism in p-type Hg, ,Cd,Te. There has been no reported experimental confirmation of this mechanism so far.

~

6.

PHOTOVOLTAIC I N F R A R E D DETECTORS

287

a . Electrical Properties The electrical properties of p-type Hg,-,Cd,Te with alloy compositions in the semiconductor range 0.21-0.60 have been studied by Elliott et al. (1972) and by Scott et al. (1976). Unlike n-type Hg,-,Cd,Te, for which no evidence of donor freezeout has been reported, p-type Hg,-,Cd,Te generally shows appreciable acceptor freezeout at low temperatures. From Hall data and photoluminescence data on p-type Hgo.,Cdo.3Tesamples with net acceptor concentrations around 3 X 10l6 cmP3, Elliott et a/. (1972) concluded that there existed a singly ionized acceptor level with an activation energy on the order of 20 mV. From Hall data together with far infrared photoconductivity and transmission data, Scott et al. (1976) report an acceptor activation energy for p-type Hgo.,Cdo.,Te samples of about 14 meV for net acceptor concentrations around 3 x 1015~ m - the ~; activation energy decreased with increasing net acceptor concentration and was about 3-4 meV for N A - N D = 1 x 10'' cmP3. Photoluminescence and cathodoluminescence data for both n-type and p-type HgO,,Cdo.,Tecrystals have been reported by Ivanov-Omskii et a/. (1978). Samples of both types showed an impurity luminescence band about 15 meV lower than the intrinsic (i.e., band-to-band) luminescence band. The impurity luminescence band was attributed to transitions from the conduction band to Hg vacancy acceptor levels located about 15 meV above the top of the valence band. It is interesting that evidence has been found for an acceptor level with similar properties in n-type H&.,Cdo,3Te. In experiments with n-type H&.,Cdo.,Te samp!es at 16 K with carrier concentrations of 2-5 x 1014 ~ m - Andrukhiv ~, et a/. (1979) report a photoluminescence peak which is shifted by about 18 meV below the band-to-band recombination peak. They interpret the peak as being due to transitions from the conduction band to an acceptor level lying about 18 meV above the top of the valence band. Data on the acceptor activation energies in p-type Hgo.,Cdo.,Te have only recently become available. In the past, difficulties were frequently encountered with n-type surface inversion layers on p-type Hg0.,Cdo,,Te samples of low to moderate acceptor concentrations which dominated the low-temperature Hall data (Scott and Hager, 1971; Wong, 1974). The recent data? in Fig. 44 on gold doped p-type H&.,Cdo.,Te samples show no evidence of such inversion layers, even for doping concentrations as low as 3.5 x lOI5~ m - From ~ . these data Bratt et al. deduced an acceptor actit Private communications from P. R. Bratt, K . J. Riley, and A . H. Lockwood.

288

M . B. REINE, A. K . SOOD, A N D T. J . TREDWELL

2w

102

I 104€-r t

50

40

30

20

I

I

I

I

1IT ( K - ' )

FIG.44. Hall coefficient (a) and mobility (b) versus reciprocal temperature for three gold-doped p-type Hgo.,Cdo.zTesamples of different net acceptor concentrations N A - N D : 0, 2 x 10l8 cmV3;A,6 x loLscmP; 0 , 3.5 x loL5~ m - The ~ . magnetic field was 1000 G. [From P. R . Bratt, K . J. Riley and A. H. Lockwood (private communication).]

6.

PHOTOVOLTAIC INFRARED DETECTORS

289

vation energy of about 10 meV. Tobin et a1.t obtained a value of 13 meV for the acceptor activation energy in p-type Hgo.,Cdo.,Te with N A - N D = 5 x 1015cm-3 from an analysis of the series resistance of boron-implanted photodiodes at temperatures below 30 K. b. Minority-Carrier Lifetime

Minority-carrier lifetime in implanted n+-on-p Hg, ,Cd,Te photodiodes has been measured for alloy compositions in the range 0.2-0.4 by the reverse-bias step recovery technique. The technique consists of abruptly switching a forward-biased n+-on-p junction into reverse bias and observing the transient current response. The technique is based on the analysis of p - n junction switching transients which was originally performed by Kingston (1954) and by Lax and Neustadter (1954) and later reformulated by Kuno (1964). The applicability of the technique for determination of minority-carrier lifetime has been recently discussed by Tobin (1979).$ In reverse-bias pulse recovery experiments on implanted n+-on-p photodiodes fabricated from p-type Hgo.,Cdo.,Te samples with net acceptor concentrations in the range 0.1-3 x 10l6 cmP3, Tobin (1979) found that the lifetime data could be explained by a single Shockley-Read recombination level at 140 meV above the top of the valence band. In p-type material, the net Shockley-Read recombination rate per unit volume from Eq. (39) is approximately

U

= h / T , =

An/Tno[l

+ pl/pO].

(137)

Setting

(138)

P o = N v exp(-EF/kT)

gives the following approximate expression for

T~ :

Data for 7, versus temperature are shown in Fig. 45 for a n+-on-p Hg0.61Cd0.39Te photodiode with a net acceptor concentration of 1.4 x lOI5 in the p region. The data agree well with Eq. (141) with E, = 140 meV and T,, = 90 nsec. Data for T , at 295 K determined via reverse-bias step recovery for n+-on-p Hg,.61Cdo.39Tephotodiodes are shown in Fig. 46 plotted versus t Private communication from S. P. Tobin, J. W. Marciniec, and A. K. Sood. t Tobin's work is also contained in Schmit et al. (1979) and is summarized in Tobin and Schmit (1979).

290

M . B . REINE, A. K . SOOD, A N D T . J . TREDWELL

FIG.45. Reverse-bias step recovery lifetime data versus reciprocal temperature for a boron-implanted n’-on-p Hgo,,,Cdo.3sTephotodiode with a net p-side acceptor concentration of 1.4 x lOl8 cm+. The maximum minority-carrier density An injected into the p region was about 2 x C I I - ~ . Lifetime was deduced from the measured data via the method of Kingston (1954). The upper curve is the calculated radiative lifetime and the curve through the data represents a Shockley-Read model. [From Tobin (1979).]

net acceptor concentration or, equivalently, hole concentration p o since po = N A - N Dat room temperature. Also, EF> Et at room temperature so that p1 >> p o . Hence the p ;I dependence shown by the data of Fig. 46 is to be expected from Eq. (139). At low temperatures, T~ = T,, which, according to Eq. (42) should vary as N ; ' . In fact the data for T, at low temperature did show roughly a N;l dependence on acceptor concentration, which led to the conclusion that the concentration N t of Shdckley Read centers was proportional to the acceptor concentration NA. As shown in Fig. 46 most of the Hgo.,,Cdo.39Te samples used in this study were p type due to native defect (metal vacancy) acceptors. Riley ~t a / . (1978a,b) have reported lifetime data for acceptor-doped p-type Hgo,,zCdo.zeTesamples measured via photoconductivity decay resulting from a GaAs laser pulse. They claim that the lifetime mechanism in their samples is due to Shockley-Read centers. They find that the lifetime T~ over the temperature range for which the material is extrinsic

6.

PHOTOVOLTAIC I N F R A R E D DETECTORS

291

An = o

FIG.46. Summary of reverse-bias step recovery lifetime data at room temperature versus net acceptor concentration for boron implanted n+-on-p Hg,,,Cd,,,,,Te photodiodes. The solid curves are the calculated radiative lifetimes for two values of injected minority-carrier density An. 0, quench/anneal material doped by stoichiornetry; A,quench/anneal material doped by copper: 0, zone-leveled material doped by stoichiometry. [From Tobin (1979).]

varies with acceptor concentration as re = 2 x 1024/Ni,

( 140)

for samples with N A between 6.5 x loi4 ~ r n and - ~ 3.1 x 1015~ m - Note ~. that this corresponds to measured lifetimes of about 2 psec at 77 K for N A = 6.5 x 1014 ~ r n -and ~ about 0.3 psec at 77 K for N A = 3.1 x 1015

~rn-~. Minority-carrier lifetime data obtained via the reverse-bias step recovery technique for boron-implanted Hg,,,Cd,,,Te photodiodes have been reported by Polla (1979) and by Sood et al. (1978a, 1979a,c; 1980). Data for an alloy composition x = 0.234 are shown in Fig. 47. The solid curve is a plot of Eq. (141) with Et = 56 meV. The reverse-bias step recovery technique has been used with both n-on-p Hg,-,Cd,Te photodiodes and with Schottky barrier photodiodes on p-type Hg,-,Cd,Te with alloy compositions in the range x = 0.2-0.4.

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M. B. REINE,

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Shockley-Read Et = 56 meV

-

1

6

8

10 lWOlT ( K - l l

12

FIG.47. Reverse-bias step recovery lifetime data versus reciprocal temperature for a boron-implanted n+-on-p Hg,.77Cd,,.rsTephotodiode with a cutoff wavelength of 8.4 p m at 80 K and with an estimated p-side acceptor concentration of 1-4 X 1 O I g The upper curve is the calculated radiative lifetime and the lower curve through the data represents a Shockley-Read mechanism. [From Sood et a / . (1979a).]

In nearly all cases the data for lifetime versus temperature can be reasonably well fit by Eq. (141). The values of the Shockley-Read energy levels Et so obtained are summarized in Fig. 48. Also plotted is the energy gap E J x ) for different temperatures (Schmit and Stelzer, 1969). From studies of photoconductivity in n-type Hgo.8Cdo.zTe,Kinch et a f . (1973) reported a Shockley-Read recombination level at an energy of about 30 meV above the top of the valence band in certain samples. This value agrees quite well with the data shown in Fig. 48. The samples in Kinch's work generally had carrier concentrations less than 4 x 1014 and Hall mobilities at 77 K less than 1 x lo5 cm2/V sec which, for an alloy composition of x = 0.2, could mean that these samples were actually p type with inverted surfaces (Scott and Hager, 1971; Wong, 1974).

c . Minority-Carrier Dufusion Length Use of the electron-beam-induced current (EBIC) technique for directly measuring the minority-carrier diffusion length in implanted n+-on-p Hg,-,Cd,Te photodiodes has been reported by Lanir er al.

6.

PHOTOVOLTAIC INFRARED DETECTORS

293

FIG. 48. Summary of the Shockley-Read trap energies Et as determined for p-type Hg,-,Cd,Te by measurements of reverse-bias step recovery lifetime versus temperature for n+-on-p junctions 0, 0 , [from Tobin (1979); Sood and Tredwell (1978a); Sood et al. (1978a)l and Schottky barrier photodiodes A [from Polla and Sood (19SO)l. Also shown is the energy gap Eg for 77 and 300 K as calculated from Schmit and Stelzer (1969).

(1978a,b, 1979a,b), by Polla (1979), by Tobin (1979), and by Iverson (1979). In this technique, a scanning electron microscope is used to generate electron-hole pairs near the junction and the electron-beam-induced current in the junction is measured as a function of the distance between the electron-beam spot and the junction edge. Under appropriate conditions, the electron-beam-induced current varies exponentially with the separation between the beam spot and the junction edge, with the characteristic length being the minority-carrier diffusion length L, . An EBIC image of the photodiode can be obtained by operating the scanning electron microscope in the scanning mode and letting the electron-beam-induced junction current modulate the intensity of the CRT display. This is sometimes useful for junction diagnostics. An EBIC photodiode at about image of a boron-implanted d - o n - p Hg, ,Cd,Te 80 K is shown in Fig. 49. The implanted junction area is 0.025 x 0.025

294

M. B . REINE,

A.

K. SOOD, A N D T. J. TREDWELL

FIG.49. Electron-beam-induced current (EBIC) image of a boron-implanted n+-on-p H&.8Cdo.zTephotodiode at 80 K. The implanted junction area was 10 x 10 mil2. (Courtesy of R . P. Murosako and D. L. Polla.)

cm2.The lead bonded to the center of the n+ region is evident. The bright annular region around the n+ region is about a diffusion length L, wide and shows the largest EBIC response. The n+ region is less bright, possibly due to higher recombination rates. Minority carrier diffusion length data obtained by Tobin (1979) via the EBIC technique for a boron-implanted Hg,,,,Cdo.,,Te photodiode are shown in Fig. 50. The p-region net acceptor concentration was 1.4 x loi5

0 1 1 1 70 80

1

I

I

I

I

100

140

180

2T'

260

d

31111

TEMPERATURE IKt

FIG.50. EBIC p-side diffusion length data versus temperature for the boron-implanted [From Tobin d - 0 n - p Hgo,,Cd,,,,Te photodiode of Fig. 45. N , - N D = 1.4 x l O I 5 (1979).]

6. lo5

295

PHOTOVOLTAIC I N F R A R E D DETECTORS

-

-

-

-

-P

SCOTT'S DATA:

-

>

\

MAJORIW-CARRIER MOBILITY in n-type

-5 t_

=

0"z

6E

k! Y

CALCULATED AMBIPOLAR

lo4

-

-

MINORITY-CARRIER MOBlLlW, colculoted

from

m e o w e d L, and Te

I

lo3. 10

I

1

I

1

I l l 1 100

I

, lo00

TEMPERATME (K)

FIG.51. Electron mobility in p-type H&.,,Cd,,,,Te as deduced from measured values for L, (Fig. 50) and T, (Fig. 45) (light solid line). Data for the electron mobility in n-type Hg,-,Cd,Te are indicated by the heavy solid lines (Scott, 1972). The dashed line is the effective mobility calculated from Eq. (144). [From Tobin (1979).]

~ m - The ~ . minority-carrier lifetime data versus temperature for this photodiode were given in Fig. 45. From the data for L, and 7, Tobin (1979) was able to compute the minority-carrier mobility pefrom the relationship Le = d ( k T / e ) PeTe

7

(141)

and compare p,,(T) determined in this way with the data of Scott (1972) for the majority-carrier mobility in n-type Hg,-,Cd,Te. This comparison is shown in Fig. 51. The solid curves are taken from Scott's data for n-type Hgo.5,Cdo.41Teand Hgo.64Cdo.3aTe samples with electron carrier ~ . dashed line is the calculated concentrations less than 2 x 1015~ m - The effective mobility pa in p-type Hgo.61Cdo.39Te calculated from (Smith, 1978, p. 194)

( n + p ) / [ ( n / ~ J+ ( p / ~ e ) I , (142) where p,(T) was extrapolated from Scott's data, p h was taken as 200 cmZ/V sec, N A was taken as 1.4 x 1015 and where n(T) = n f / p ( T ) . Agreement between pa(T) as calculated from Eq. (144) and pe(T)as deduced from data for L,(T) and T J T )via Eq. (143) is quite good for temperp a

=

296

M. B. REINE,

A . K . SOOD, A N D T . J .

TREDWELL

atures above about 170 K. Below 170 K there is significant disagreement with p, at 77 K being about a factor of 4 too low. This could be due to surface recombination affecting the values of L, measured via EBIC at the lower temperatures. However, excellent agreement between Scott’s mobility data and the mobility deduced via Eq. (143) from measured values for L, and 7, was obtained over the 77-300 K range for a sample with a somewhat higher acceptor concentration of 4.3 x 1015 cm-3 (Schmit et d,, 1979). EBIC data for L, taken on boron-implanted n+-on-p Hg,.,,Cd,.,,Te photodiodes of various net p-side acceptor concentrations are shown in Fig. 52. Data for L, at 195 K follow generally a (NA-ND)pl’z dependence, which which would be expected from Eq. (143) if 7, varies as (ZVn-ZV,J1, it generally did at 195 K in these photodiodes. Diffusion length data obtained via EBIC for a boron-implanted n-on-p Hg,,.mCdo.,2Tephotodiode were reported by Lanir er al. (1978a,b). They scanned a cleaved surface that was perpendicular to the junction plane. For a base acceptor concentration of 1 x lo1‘~ m - they ~ , report values for L, of 65 pm at 210 K and of 106 p m at 90 K. EBIC measurement of L, in a boron-implanted n+-on-p Hg,-,Cd,Te photodiode with a cutoff wavelength of 9.5 pm at 77 K have been reported by Polla (1979). For a sample at 81 K, he reports a value for L, of 45 pm. The base acceptor concentration was estimated to be around 1 x 1OI6 cmp3,

,,

I

FIG.52. EBIC data for L, taken on boron-implanted n+-on-p Hgo.,,Cdo,,,Te photodiodes of various p-side acceptor concentrations. 0,T = 195 K; A, T = 77 K . [From Tobin (1979).]

6.

PHOTOVO LTAIC INFRARED DETECTORS

297

d. Deep-Level Transient Spectroscopy Studies The deep-level transient spectroscopy (DLTS) technique (Lang, 1974a,b) has been used extensively in recent years for studying the nature and behavior of various impurity centers in elemental and compound semiconductors. Its application to the study of impurity centers in GaAs and GaP has been discussed by Lang and Logan (1975). In a recent article, Miller et al. (1977) summarized recent advances in the technique itself and discussed briefly the results obtained on impurity centers in 111-V compound semiconductors. In 11-VI compounds, there has also been some recent work on the study of impurity centers in CdTe by Legros et al. (1978) using the thermally stimulated capacitance measurement technique. The first use of the DLTS technique in narrow-bandgap semiconductors has been on Hg,,Cd,Te and was reported recently by Polla and Jones (1980). They investigated impurity centers in p-type Hg,-,Cd,Te with ion implanted n+-p junctions and showed the presence of a deep recombination center in p-type H&.,,Cd,,,,Te. Their data suggest the presence of an electron trap approximately 0.049 eV above the valence band, which is in excellent agreement with the Shockley-Read level energies determined by reverse-bias pulse recovery lifetime measurements (cf. Fig. 48). The DLTS technique appears quite promising for measuring the position, nature and concentration of various hole and electron traps in the Hg, ,Cd,Te semiconductor alloy system. Understanding and identification of these impurity states will help in improvement of the electronic properties of Hg,,Cd,Te (such as lifetime and diffusion length) and hence will lead to improved device performance. 15.

NOISE

IN

Hg,,Cd,Te

PHOTODIODES

A study of llfnoise in implanted n+-on-p Hg,,Cd,Te photodiodes has recently been reported by Tobin et al. (1980) (see also Gurnee et al., 1979). By measuring l/f noise while varying background photon flux, temperature, reverse-bias voltage, and the potential applied to an insulated gate electrode, they showed that the llfnoise was independent of both photocurrent and diffusion current, but was linearly proportional to surface leakage current. Moreover, they showed that the rms llfnoise current I,,,, as a function of measurement frequency f,reverse-bias voltage V, applied gate voltage V, and temperature T was given by the following relationship: (143) ~ zn,ex(f,v,vg ,T) = [ ~ e x ~ s ( v ~ v g ~ ) / d ~ ~ W where Z, (V,V,,T) is the photodiode surface leakage current, which does

298

M. B. REINE,

A.

K. SOOD, A N D T. J . TREDWELL

depend strongly on reverse-bias voltage, gate voltage, and temperature, and where the dimensionless coefficient aeXhad a value of about 1 x lop3 for all photodiodes measured in their study. In Eq. (145) Af is the noise bandwidth which is assumed small compared to$ Their results on the effects of photocurrent and reverse-bias voltage can be understood with reference to Fig. 53 which shows schematically the I-V curves of a photodiode in the dark and with illumination. In thermal equilibrium (point A) no llfnoise was observed down to frequencies of 0.1 Hz. When the photodiodes were illuminated but kept at zero-bias voltage (point B), the total photodiode noise current increased by an amount 2eZphdue to the added shot noise in the photocurrent Zphbut no llfnoise was observed. When the photodiodes were reverse biased without illumination (point C) so that a reverse current comparable to Zph flowed, l/f noise was usually observed. However, when the reverse-biased photodiodes were illuminated (point D), no additional l/f noise was observed. These results showed that llfnoise was not simply related to the total current in the photodiode, but depended on the mechanism by which the current was generated. Thus they were able to rule out the metalsemiconductor contacts and current flow through the quasineutral regions and the space-charge region of the photodiode as mechanisms for l/f noise.

k-”o

Current

Illuminated

B

FIG. 53. I-V curves for a photodiode in the dark and with a net photocurrent -Iph flowing due to illumination. No llfnoise was observed when the photodiode was at points A (thermal equilibrium) and B. llfnoise was observed at points C and D, and its magnitude was independent of Iph.[After Tobin cr al. (198Q.l

6.

PHOTOVOLTAIC INFRARED DETECTORS

10-'0

IO-~

10-8

299

lo-'

DARK CLJRHFNT AT 50-mV REVERSE RlAS ( A )

FIG. 54. I/f noise current plotted against dark current for a reverse-biased n+-on-p H&.,C&,,Te array at temperatures between 83-160 K. In this temperature range, the dark current was due to a surface leakage mechanism. The straight line has a slope of unity. 0 , 83 K; 0, 100 K; A, 120 K ; 0,140 K ; 0,160 K. [From Tobin et a / . (1980).]

Figure 54 shows the Ilfnoise current plotted versus dark current, both measured at a reverse-bias voltage of - 50 mV, for an implanted n+-on-p Hg,.,Cdo,,Te photodiode array over the 83- 160 K temperature range. The junction areas ranged from 1.3 x 10-5-4.8 x lop4cmz. Over this temperature range, the dependence of dark current on junction area indicated that the dark current was due to a surface g-r mechanism. The solid line is a plot of Eq. (145) with aeXset equal to 1 x lop3.At temperatures above 180 K, the dark current in these photodiodes became dominated by diffusion current and the correlation between llfnoise and dark current was no longer observed. Figure 55 shows dark current and Ilfnoise current, both measured at a reverse bias of -50 mV, as functions of temperature for one of the Hg,.,Cdo,Te photodiodes. At temperatures between 110 and 180 K the dark current is dominated by surface leakage current and varies with temperature roughly as the intrinsic carrier concentration ni(T). Over this temperature range the l/f noise current displays the same temperature

300

M . B . REINE, A. K . SOOD, A N D T . J . TREDWELL

Io

-~

10-10

10-11

Io-I?

I000IT (K-I)

FIG. 55. Dark current (circles) and l/f noise current (squares) versus lOOO/T for two reverse-biased n+-on-p Hh,,Cd,,,Te photodiodes. 0,dark current; 0,l/f noise. [From Tobin er al. (1980).]

dependence as the dark current. Below 110 K the diode in Fig. 55 is limited by a temperature-independent surface current leakage; the l/f noise continues to follow the temperature dependence of the dark current. At temperatures above 180 K, the dark current of the diodes in Fig. 55 is limited by diffusion current. The I/fnoise current, however, continues to display the same temperature dependence as the surface generation current. These data indicate that it is only the surface generation component of the dark current which is related to llfnoise. The relationship in Eq. (145) obtained by Tobin et al. (1980) for implanted n+-on-p Hg,,Cd,Te photodiodes is similar to that observed by Hsu et (11. (1968; Hsu, 1970a,b) for gate-controlled silicon junctions. Hsu proposed a model in which the current fluctuations were a result of a modulation of the surface generation current by fluctuations in surface potential. The data on Hg,,Cd,Te photodiodes could be explained by such a model. However, a better understanding of the nature of the surface

6.

PHOTOVOLTAIC INFRARED DETECTORS

301

leakage current in Hg,,Cd,Te photodiodes is required to gain a deeper understanding of l/f noise in these devices. 16. SCHOTTKY BARRIERPHOTODIODES The previous sections of this chapter have been concerned exclusively with p-n junction photodiodes in Hg,-,Cd,Te. In this section we review some recent work in Schottky barrier diodes in Hg,,Cd,Te. Schottky barrier diodes have been studied quite extensively in several semiconductors (Henisch, 1957; Rhoderick, 1978) and can be quite suitable for use as visible and infrared detectors. The rectification properties of a Schottky barrier diode are determined by the work function of the metal and the height of the metal-semiconductor barrier. The electrical properties of the Schottky barrier diodes in most of the semiconductors studied so far are strongly influenced by surface preparation and by thin interface layers on the semiconductor prior to the deposition of the metal. Several papers have discussed these effects in silicon (Turner and Rhoderick, 1968) and in gallium arsenide (Childs et al., 1978). Barrier formation can be quite complex for several reasons. Chemical reactions at the metal-semiconductor interface can play a role in establishing a barrier. This could become especially important if there is a thin interfacial layer initially present on the semiconductor surface. Schottky barrier diodes have been formed on such ternary semiconductors as GaAs,P,, (Neamen and Grannemann, 1971) and In,Ga,,As (Kajiyama er al., 1973). Schottky barrier infrared photodiodes have been formed on the narrow-bandgap semiconductor Pb,,Sn,Te (Melngailis, 1973; Harman and Melngailis, 1974). The only reported work on Schottky barriers in Hg,,Cd,Te has been by Polla (1979) and by Polla and Sood (1978, 1980). They utilized Schottky barrier structures primarily to determine the minority-carrier properties of p-type Hg,-,Cd,Te for 0.2 Ix s 0.4. Polla (1979) fabricated Schottky barrier diodes on bulk-grown crystals of p-type Hg,-,Cd,Te with net acceptor concentrations of 0.5-2 x 10ls ~ m - ~Aluminum, . manganese and chromium were selected as barrier metals on the basis of the conclusions of Johnson and Schmit (1977), which are summarized in Section 1 1 ,a of this chapter. Polla (1979) showed that all three of these metals formed Schottky barriers on p-type Hg,-,Cd,Te. Aluminum is a known donor in Hg,,Cd,Te; however, Johnson and Schmit (1977) showed that it diffuses quite slowly. They found no signs of diffusion of aluminum from a sputtered layer about 1 p m thick on p-type Hg,.,Cdo.,Te samples after a heat treatment at 290°C for 2 h. Current-voltage characteristics at 77 K for an aluminum Schottky bar-

302

M . B. REINE, A . K. SOOD, A N D T . J . TREDWELL

FIG. 56. Current-voltage characteristics at 77 K for an aluminum Schottky barrier on p-type Hg,.,,Cdo,,eTe. [From Polla and Sood (1980).]

ner on p-type Hq,7,Cd,,,2Te are shown in Fig. 56. The electrical properties at 77 K of this diode and of a chromium Schottky barrier on p-type Hg,,78Cd0.32Teare summarized in Table VI (Polla and Sood, 1980). The barrier metals were thermally evaporated in a circular geometry of area 1.14 x lop3 cm2. The barrier metal layers were not thin enough to be transparent, but cutoff wavelengths were able to be determined from the measured photoresponse due to carriers photogenerated near the edge of the barrier metal. The reverse bias characteristics in these devices TABLE VI ELECTRICAL PROPERTIES AT 77 K OF Two Hg,-,Cd,Te Hg,-,Cd,Te alloy composition x Energy gap E , Cutoff wavelength A,, Acceptor concentration N,, Barrier metal Zero-bias resistance R , R J , (A, = 1.14 x cmz) Barrier height (a) from I-V data (b) from C-V data

SCHOTTKY BARRIERDIODES

0.22 0.136 eV 9.1 p m 2 x 10'6 cm-ti

0.32 0.288 eV 4.3 p m 8 x cmP

Aluminum

Chromium 4.4 x 104 n 50.6 fi cmz

0.101-0.104 eV

Q.272-0.275 eV 0.220 eV

1.3 x 104 n 1.49 cmz

0.071 eV

6.

PHOTOVOLTAIC INFRARED DETECTORS

303

showed excess leakage currents which were perhaps due to edge breakdown. Attempts were made to determine values for the barrier height both from detailed current-voltage and from capacitance-voltage measurements. The barrier height values deduced from current -voltage measurements were consistently higher than those deduced from capacitancevoltage measurements. This discrepancy in barrier height has also been observed for Schottky barriers in silicon (Tongson el al., 1979) and in GaAs (Childs et al., 1978). The explanation for this behavior is that surface states block majority carriers traveling from the semiconductor to the metal under forward bias. The shielding effect is not present for capacitance - voltage measurements since the majority-carrier current is small and the barrier height is determined by modulating the depletion layer width. Hence, barrier height values deduced from capacitance-voltage data should be more reliable than those from current -voltage data. IV. Summary and Conclusions

As this chapter has indicated, there is considerable interest in Hg,-,Cd,Te junction photodiodes for a wide range of infrared detector applications. The performance levels achieved so far in these devices indicate that this interest will continue. For 3-5 pm applications at operating temperatures around 190 K, and for 8-12 pm applications at around 77 K , ion implanted n-on-p Hg, ,Cd,Te junction photodiodes have already demonstrated sufficiently high R,,A products.? For these combinations of cutoff wavelength and operating temperature, p-side diffusion current generally dominates the I-V characteristics near V = 0. Efforts are being directed toward further reduction of p-side diffusion current by reducing the p-side diffusion volume and by using p-type Hg,-,Cd,Te with long minority-carrier lifetime. Efforts are also being directed toward the further reduction of various other junction current mechanisms in order to reduce I/’noise at reverse bias and to increase the overall yield of diffusion-limited junctions photodiode arrays. Issues such as long-term device reliability are beginning to be addressed. Improved surface passivation techniques are being explored in order to achieve better control of surface potential and lower surface state densities. For applications at lower temperatures where conventional diffusion current is negligible, various leakage current mechanisms have generally t The applicability of photovoltaic Hg,-,Cd,Te detector mosaic arrays for advanced thermal imaging systems has been reviewed recently by Grant and Hutcheson (1979).

304

M . B . REINE, A. K . SOOD, A N D T . J . TREDWELL

kept junction impedances below their desired values so far. This is particularly so for junctions with longer cutoff wavelengths, and is presumably due to the ease with which interband tunneling can occur across the narrow bandgaps involved. Here efforts are being directed toward identifying the origins of leakage currents, with particular emphasis on control of surface potential and surface state density and on device designs which are less conducive to interband tunneling. Recently? there has been encouraging progress in this area with zero-bias impedances of 1 x lo9 0 being measured at 4.2 K in boron-implanted n-on-p Hg,-,Cd,Te junction photodiodes with cutoff wavelengths around 17 pm.The junction area for these laboratory device was I x cm2, which gives R,Aj products around 1 x 1W R cm2. Whereas photoconductive Hg,-,Cd,Te detector technology has exclusively been based on n-type material of low carrier concentration (usually ~ m - ~photovoltaic ), Hg,,Cd,Te detector technology less than 5 x has to date relied essentially on p-type material with carrier concentrations generally in the 5 x lOl5-I x 10'' cm-3 range. Improved understanding and control of p-type Hg,-,Cd,Te are critical to the continued development of Hg,-,Cd,Te junction photodiode technology. For example, the roles of various impurities, acceptor dopants, and native defects in determining the rninority-carrier lifetime in p-type Hg,-,Cd,Te need to be better understood. The DLTS and reverse bias step recovery techniques should prove useful in this regard. There needs to be an experimental determination of the strength of the theoretically proposed Auger 7 recombination process in p-type Hg,-,Cd,Te. In addition to being the basis for n-on-p Hg, ,Cd,Te junction photodiode technology for hybrid mosaic focal planes, p-type material will also be quite important to monolithic Hg,-,Cd,Te IR/CCDs (Kinch rt al., 1980) because of its large minority-carrier mobility. Finally, it is important that there be increased basic and applied research in the interrelated areas of Hg,-,Cd,Te crystal growth, defect chemistry, surface science, and junction formation, passivation, and contact technology. There remains much to be done in each of these areas besemiconductor alloy system can fore the full potential of the Hg,,Cd,Te be realized.

ACKNOWLEDGMENTS The preparation of this chapter was aided by many pleasurable, informative, and stimulating discussions which the authors had with their colleagues at the Honeywell Electro-Optics

t Data on Honeywell laboratory devices taken by W. L. Eisenman, D. Arrington and C. Sayre of the Naval Ocean Systems Center.

6.

PHOTOVOLTAIC INFRARED DETECTORS

305

Operations and the Honeywell Corporate Material Sciences Center. In particular, we thank Messrs. D. L . Polla and S. P. Tobin, and Drs. T. N. Casselman, P. LoVecchio, and J. F. Shanley for providing papers and other information prior to their publication for use in this chapter; and we acknowledge valuable discussions on space-charge region g-r current with Drs. P. H. Zimmermann and R. J. Briggs. We also thank the following individuals for providing preprints, unpublished information, and in some cases original figures which could be used in preparing this chapter: Professors R. Kalish and I. Kidron of Technion-Israel Institute of Technology, Dr. M. Lanir of Rockwell International Science Center, Dr. K. J. Riley of Santa Barbara Research Center, and Professor M. H. Weiler of the Massachusetts Institute of Technology.

REFERENCES Anderson, W. W. (1977). Infrared Phys. 17, 147. Andrievskii, A. I., Teodorovich, A. S., and Shneider, A. D. (1973). Fiz. Tekh. Provodn. 7, 1659 [English transl.; Sov. Phys. -Semicond. 7, 11 12 (1974)l. Andrukhiv, M. G., Mal’tseva, V. A., Ivanov-Omskii, V. I., Ogordnikov, V. K., and Totieva, T . Ts. (1979). Fiz. Tekh. Provodn. 13,362 [English transl.: Sov. Phys. -Semicond. 13, 210 (1979)l. Atalla, M. M., Tannenbaum, E., and Schiebner, E. J. (1959). Bell System Tech. J . 38,749. Baars, J., and Sorger, F. (1972). Soiid State Commun. 10, 875. Bahir, G., Bernstein, T., and Kalish, R. (1980). Rad. Effects 48, 247. Beck, J. D., and Sanborn, G. (1979). (Hg,Cd)Te: Trapping and Surface Processing. Final Rep. for Air Force Materials Laboratory Contract F33615-76-C-5070, AFML-TR-794046, Volume 1. Becla, P., and Pawlikowski, J. M. (1976). Infrared Phys. 16, 457. Blakemore, J. S. (1962). “Semiconductor Statistics.” Pergamon, Oxford. Bowers, J. E., Schmit, J. L., and Mroczkowski, J. (1979). Proc. Meeting IRIS Specialty Group Infrared Detectors ( U ) 1, 55. Bowers, J . E., Schmit, J. L., Speerschneider, C. J., and Maciolek, R. B. (1980). IEEE Trans. Electron Devices ED-21, 24. Bratt, P. R. (1978). Development of Large Area PV HgCdTe Detectors for 2.06 p m and 3.85 p m . Interim Rep. for U.S. Army Night Vision and Electro-Optics Laboratory Contract DAAB07-76-(2-0803 (DDC AD A067306). Bratt, P. R., and Vandenvyck, A. H. B. (1977). Proc. Meeting IRIS Specialty Group Infrared Detectors ( U ) 1, 221 (DDC AD B025619). Broudy, R. M., and Reine, M. B. (1977). Proc. SOC. Photo-Opt. Instrum. Eng. 124, 62. Broudy, R . , Gurnee, M., Tredwell, T., and White, J. (1978). Proc. SOC.Photo-Opt. Instrum. Eng. 132, 10. Broudy, R. et a / . (1980). Proc. Soc. Photo-Opt. Instrum. Eng. 217, 69. Brudnyi, V. N., Voitsekhovskii, A. V., Grechukh, Z. G., Krivov, M. A., Lilenko, Yu. V., Limarenko, L . N., Pashkovskii, M. V., Petrov, A. S.,and Potapov, A. I. (1977). Fiz. Tekh. Provodn. 11, 1540 [English transl.: Sov. Phys.-Semicond. 11, 9051. Bubulac, L. O., Shin, S., Tennant, W. E. and Wang, C. C. (1979). Jpn. J. Appl. Phys. Suppl. 19-1, 495. Casselman, T. N. (1980). J . Appl. Phys. 52, 848. Casselman, T. N., and Petersen, P. E. (1979). Proc. Meeting IRIS Specialty Group Infrared Detectors ( U ) 1, 37. Casselman, T. N., and Petersen, P. E. (1980). Solid Stare Commun. 33, 615.

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SEMICONDUCTORS A N D SEMIMETALS, VOL. I8

CHAPTER 7

Metal-Insulator-Semiconductor Infrared Detectors M . A . Kinch

. . .

. . . . . . I . General MIS Theory . . 2. (HgCd)Te MIS Theory .

. . . . . . .. . . . . (HgCd)Te MIS EXPERIMENTAL DATA . . 3. Thermal Equilibrium Mode . . . . . . 4 . Dynamic M o d e . . . . . . . . . . . .

. . . . . . . .. . . . . . . .. . . . . (HgCd)Te MIS PHOTODIODE TECHNOLOGY . . . 5.Theory . . . . . . . . . . . . . . . . . . 6. Performance Datu . . . . . . . . . , . . . 7. Surface-Controlled Photoconductor . . . . .

.. . . . . ... . . . . . . . . . .. . .. , . . . . . . . . . . . . . ... . . . . . . . . . . . . . ( H g C d ) T e CHARGE TRANSFER DEVICETECHNOLOGY .. . . 8. (HgCd)Te CTD Theory and Design. . . . . . . . . . . . 9. (HgCd)Te CCD Performance D a t a . . . . . . . . . . . . SUMMARY . . . . . . . . . . . . . . . . . . .. . . . . REFERENCES. . . . . . . . . . . . . . . . . . . . . . .

I. INTRODUCTION. . 11. MIS T H E O R Y . .

111.

IV.

V.

VI.

. . . .

. . . .

. . . . . . . .

.

. . . . . . . . . . .

,

313 315 315 322 339 339 346 349 349 354 360 364 364 371 376 377

I. Introduction The realm of discrete intrinsic infrared detectors has long been dominated by photoconductive devices, and to a lesser extent metallurgically formed photodiodes. This dominance has continued even into current generation focal planes, in which arrays of photoconductive elements are fabricated in (HgCd)Te utilizing conventional photolithographic techniques, with the number of elements, each with its own preamplifier, typically being less than 200. The characteristics of these devices have now advanced to the stage in which background limited operation (BLIP) is expected even for systems operating at greatly reduced bandwidths and background flux levels. However, the performance parameters that will be demanded of the next generation of infrared systems are such as to require a significant increase in the number of detectors employed in the 313 Copyright @ 1981 by Academic Press. Inc. All rights of reproduction in any form reserved. ISBN 0-12-752118-6

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A . KINCH

focal plane to values in excess of lo4, and photoconductive detectors will not be able to meet this need. Signal processing on the focal plane itself will be mandatory, with such signal processing functions envisioned as time delay and integration (TDI) for scanning systems, multiplexing, area array staring mode operation, antiblooming, and background subtraction, to name but a few. To this end both charge coupled (CCD) and charge injection (CID) device technology will be employed. Three different approaches have been proposed to meet these onfocal-plane signal processing requirements: ( I ) The hybrid (Long0 et ul., 1978), which represents a marriage of present-day technologies, utilizing conventional intrinsic infrared photodiode arrays and silicon signal processing electronics electrically and mechanically mated together. (2) Extrinsic silicon (Nummedal er nf., 1979, which utilizes the material in which signal processing is most advanced but whose infrared capabilities are severely limited. (3) The intrinsic monolithic (Chapman et al., 1978) approach, which utilizes the materials that are at the forefront of present-day infrared detector technology but whose signal processing capabilities are largely unproven. The intrinsic material approach, although perhaps technologically the most challenging, offers the distinct advantages of the highest temperature of operation consistent with the system spectral window, a high quantum efficiency and hence a low cross-talk capability, a minimal power dissipation on the focal plane, and a minimal number of focal plane interconnects. Although signal processing in (HgCd)Te is as yet in its infancy high-quality materials (Kinch er af.,1973) and single-level metalinsulator-semiconductor technologies (Tasch et uf., 1970; Kinch 1974) have been developed over the years in support of the present-day infrared systems effort utilizing photoconductive (HgCd)Te. The surface technology which is required to reduce surface recombination velocity and I/f noise effects in photoconductive detectors has also been utilized to fabricate MIS photodiode detector arrays in (HgCd)Te, and it is this technology which forms the foundation on which this chapter is based. The general theory of metal-insulator-semiconductor (MIS) devices as applied to (HgCd)Te is reviewed briefly in Part I1 and compared with experimental data in Part 111. The utilization of this predominantly single-level MIS technology to detect infrared radiation in the conventional discrete manner, in the form of an MIS photodiode, is discussed in Part IV together with a brief excursion into the realm of the surfacecontrolled photoconductor. The extension of this single-level (HgCd)Te MIS technology into the multilevel capability required for charge transfer

7.

METAL-INSULATOR-SEMICONDUCTOR

I N F R A R E D DETECTORS

315

device (CTD) operation is described in Part V from both a theoretical and experimental standpoint with specific regard to (HgCd)Te infrared sensitive CCD shift register performance. 11. MIS Theory

1 . GENERAL MIS THEORY

The simple metal-insulator-semiconductor device is shown in Fig. 1 and consists of a metal gate separated from a semiconductor surface by an insulator of thickness to, and dielectric constant E,, . The surface potential of the semiconductor is controlled by the bias voltage applied to the metal I

,-METAL

‘OX

INSULATOR

f

SEMICONDUCTOR

’r

FIG.1. Metal-insulator-semiconductor (MIS) structure.

gate with respect to the substrate, and the energy-band diagram for an n-type semiconductor biased to the threshold of strong inversion is shown in Fig. 2. A quantitative expression for the potential r#~as a function of distance from the surface x is given by a solution of the one-dimensional Poisson’s equation which for nondegenerate statistics and thermal equilibrium is given by

The electric field at any point is

where

nno and ppo represent the majority- and minority-carrier densities in the

316

M. A . KINCH

INSULATOR-’

FIG.2. Energy-band structure for n-type semiconductor MIS structure biased to threshold of strong inversion.

bulk of the semiconductor, and E is the semiconductor dielectric constant. The space charge per unit area at the semiconductor surface is given by Gauss’s law, namely

Qs= E E ~ =E 2 ~ (~EE,,~T)”~F(C#J).

(4)

A typical variation of the space-charge density as a function of surface potential c$~ for n-type 0.25-eV (HgCd)Te (cutoff wavelength, A, = 5 pm) with no = 2 x l O I 5 cm-3 is shown in Fig. 3 for a temperature of 77 K. For the surface is accumulated and the function F ( 4 ) depositive values of C& fined by Eq. (3) is dominated by the first term, and Qs exp(q4,/2kT). For values of bSbelow flat band the second term in Eq. (3) dominates and Qs l ~ $ ~ and l ~ / the ~ , semiconductor space charge is determined by the ionized impurities in the depletion region formed at the surface. At larger negative values of surface potential the minority-carrier density at the surface is much greater than the bulk majority-carrier concentration, and the space charge is dominated by the fourth term in Eq. (3), such that Qs exp(ql+,l/2kT). Strong inversion is seen to occur in Fig. 3 for values of surface potential I+,] > 1244, where 4Frepresents the bulk Fermi potential, as defined in Fig. 2. This condition for strong inversion is equivalent

-

-

-

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

-0 3

-0.I

-0.2

0

317

+0.05

0,( e v i

FIG.3. Variation of space-charge density Qsas a function of surface potential 0.25-eV n-type (HgCd)Te with no = 2 X 1015 c m P , T = 77 K.

QS,

for

to

or nno

= Pno exp(-q4PV/kT) = ~ s r

(5)

where ps represents the surface concentration of minority carriers. The differential capacitance associated with the semiconductor space-charge region is given by

M.

318

A . KINCH

Again for ds > 0, the surface is accumulated and Eq. (6) approximates to C,,, 0: exp(q+,/2kT), so that for values of +s in excess of a few ( k T / q )the space-charge capacitance is high. In the other extreme of strong inversion Eq. (6) becomes Cinva exp(ql&l/2k7‘), and again for +s > few ( k T / q ) the capacitance associated with the inversion layer is very large. For values of surface potential in the depletion-weak inversion range where the space charge is dominated by depletion, Eq. (6) reduces to c d = ( e ~ ~ q N / 2 + ~ ) ~ ’ * , the familiar depletion region capacitance (= EE,/ W, as shown in Fig. 2). In the idealized MIS device biased to strong inversion the inversion layer is assumed to be in the form of a sheet of charge Qinvimmediately at the surface and the associated depletion region is assumed devoid of charge carriers. The applied gate voltage VG is dropped partly across the insulator and partly across the space-charge region, such that by Gauss’s law =

EOXEO

to,

Qinv+ q N W ,

(7)

where N is the net impurity concentration in the depletion layer W. The surface potential +s associated with the applied gate voltage V , is given by (Macdonald, 1964) =

v;:+ v, - (2VAV0 + v“,)”’,

(8)

with Vo = q N E E o / C , x Vh = (VG - VFB) + Qinv/Cox C o x = &oxEo/fox where VFBrepresents the device flat-band voltage. The total capacitance of the idealized MIS device is the series combination of the insulator capacitance Coxand the space-charge capacitance Cd, namely 9

c = coxcd/(cox + c d ) *

9

9

(9)

The theoretical variation of C versus V , for such an idealized MIS device (with VFB = 0) is shown in Fig. 4, for a particular set of device parameters on n-type 0.25-eV (HgCd)Te, with no = 1015 cmP3. The general features follow from a consideration of Eqs. (6) and (9). At positive gate voltages the surface is accumulated and Cd (= C,,,) is large; thus the total capacitance is essentially that of the insulator Cox.As the gate voltage is reduced to values such that +s < 0, a depletion layer is formed with capacitance , ) ”the ~ , total capacitance decreases. Upon given by Cd = ( E E , ~ N / ~ +and further decreasing V G the surface is biased into strong inversion and the space-charge capacitance is now Cinv,which is very large, and the total capacitance increases until it again approximates Cox.Thus, the measured

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

319

m L

standard

A

I

-

high frequency

-*. I -0.8

pulsed

1

I

-0.6

-0.4

1

0 -0.2 Gate Vollaqe (VI

1 t0.2

I t0.4

I 10.6

FIG.4. Capacitance versus gate voltage for Eg 0.25-eV n-type (HgCd)Te at 77 K . no = lot5~ m - r~# ;~=~ 0.092 eV; Cox= 2.1 x lo-’ F/cm2. Vectors are indicated by overarrows in figures and by bold letters in text. [Chapman (1978).]

device capacitance exhibits a minimum and this curve is designated low frequency in nature. This terminology is used because throughout the above theory it has been tacitly assumed that the carriers can follow the applied voltages. This is certainly true for the accumulation and depletion regimes but is not necessarily correct for the minority carriers in the inversion layer. These carriers have a finite response time to an applied ac signal that is dependent on the availability of minority carriers to the surface region, i.e., the inversion layer response time is inversely related to the minority-carrier dark current in the device. At sufficiently high ac signal frequencies the inversion layer cannot follow the applied ac voltage even though it is in equilibrium with the dc gate bias and the capacitance-voltage curve exhibits a high-frequency response, as shown in Fig. 4, with a limiting capacitance given by CoxCd/(CoX + Cd). This variable frequency response is typically represented by a finite resistance connecting the majority- and minority-carrier bands in a device equivalent circuit. Also shown in Fig. 4 is a curve labeled “pulsed” response. This curve represents the measured capacitance of the MIS device when biased into deep depletion by a voltage pulse which is sufficiently fast that minority carriers cannot appear at the surface of the semiconductor in the time available. The measured capacitance is then given by Coxin series with Cd, where Cd is determined by the depletion layer associated with the surface potential +s obtained by solving Eq. (8) with Qinv = 0. Figure 4 also includes the effects of degeneracy and nonparabolicity on the total capacitance in the accumulation region ( V , large and positive), and these

320

M . A. KINCH FIXED OXIDE CHARGE

+

1

]+

I I I

t

I I

SLOW (INTERFACE)

-

I -

-

-

I

HgCdTe

I I le- -100 OXIDE

A

FIG.5 . Surface states of (HgCd)Te.

result in a reduction in the value of C,,, as compared to the theory of Eq. (6). designated by the standard curve in Fig. 4. Surface states have been ignored thus far in the above theory. The oxide-semiconductor interface is shown in Fig. 5 , and the three main types of state of importance in (HgCd)Te are indicated, namely fixed oxide charge, and slow and fast surface states. The magnitude of the fixed oxide charge is important in determining the device flat-band voltage. The slow surface states, typically within a tunneling distance of the interface, manifest themselves as hysteresis effects in capacitance-voltage curves due to minority-carrier trapping. The fast interface states cause deviations of measured C-V characteristics from those values predicted above, depending upon the applied frequency, temperature, and surface potential, and can also cause excessive dark currents. The kinetics of generationrecombination through fast surface states is identical to that through bulk Shockley-Read centers (Shockley and Read, 1952), and is depicted in Fig. 6 for an n-type substrate biased into depletion. The thermal generation rate out of Nfscan be represented by associated resistances R,,# and Rp2 to the respective band edges, and these impedances vary exponentially with energy from the appropriate band. Also included in Fig. 6 are the minority-carrier dark current contributions due to diffusion from the neutral bulk region and generation-recombination in the depletion region. When biased to depletion-inversion the MIS device can be accurately represented (Lehovec and Slobodskoy , 1964) by the equivalent circuit shown in Fig. 7a, which includes the aforementioned effects due to finite inversion layer response time and surface states. The equivalent circuit is greatly simplified for the MIS device biased into strong inversion. In this

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

321

n - TYPE

Jdif

FIG.6 . Surface of an n-type semiconductor biased into depletion, including fast surface states.

case R,,g + m, RP,*+ 0 , and ( w C , ~ ~ 0.3 T~~ must become important because of the relative dependence of these two mechanisms on bandgap E g . Indeed, the lifetime data for x = 0.275 shown in Fig. 10 cannot be fitted by rAalone and indicates a possible contribution due to rR which is a factor of 5 weaker than the theoretical values. The measured value of rAis a factor of 1.5 larger than the indicated theoretical values in Figs. 9 and 10. Evidence of Shockley -Read recombination is observed in some n-type Hg,-,Cd,Te devices, particularly in the more compensated material with

-

M. A. KINCH

324

FIG. 9. Experimental and theoretical dependence of minority-carrier lifetime 7 versus 1B/T for n-type Hg,,Cd,Te with x = 0.195.

to-’,

;

I

I

I

I

I

5

6

7

8

9

10

103/T (KdJ

FIG. 10. Experimental and theoretical dependence of minority-camer lifetime 7 versus = 0.275.

1V/Tfor n-type Hg,-,Cd,Te with x

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

325

N D - N A < 3 x l O I 4 ~ m - The ~ . temperature variation of 7 indicates in some cases levels in the range of 20-30 mV from the valence band. In others the lifetime is temperature invariant which can indicate a Shockley-Read center at the intrinsic level or a very high density of recombination centers at an arbitrary level in the energy gap. Auger recombination will not be important in p-type material and the limiting band-to-band recombination mechanism will be radiative. No direct measurements of T~ have been reported for bulk p-type material, although various authors have attempted to fit measured photodiode dark current (Polla and Sood, 1978; Lanir et al., 1978; Sood and Tredwell, 1978) data with lifetimes predicted by radiative limitations, with values approaching T~ to within a factor of 2 for po concentrations in the range 2-5 x 10l6 ~ m - The ~ . calculated radiative lifetime should be identical to that for n-type material. b . ( H gCd)Te MIS Cha ract eris t ics The above band structure parameters for (HgCd)Te give a density of states in the conduction band, N , = 6.5 x 1OI6 (Eg)3’2~ m - where ~ , Eg is in eV, and a density of states in the valence band N , = 2 X 10l8 cm-3 at a temperature of 77 K. The gross dissimilarity in conduction-band and heavy-hole valence-band effective masses results in an intrinsic level considerably above mid-gap for all reasonable temperatures which must be taken into account when calculating #IF. Degeneracy and nonparabolic effects have already been mentioned in describing the theoretical capacitance-voltage curve of an idealized MIS device on 0.25-eV n-type (HgCd)Te at 77 K shown in Fig. 4.The capacitance in accumulation is seen to rise less steeply toward the asymptotic value of C,, than predicted from classical theory for the conduction band. In experimental (HgCd)Te MIS devices there can occur three basic deviations from the theoretical C-V curve shown by the curve designated “thermal” in Fig. 11. Firstly, a finite background flux aBcan increase the minority-carrier density in the vicinity of the semiconductor surface above its thermal equilibrium value and the pnoappearing in the expression for Q s , namely Eq. (4), must be replaced by pa. The effect of the background flux is to essentially forward bias the depletion region induced under the field plate, thus increasing its associated capacitance, and is illustrated by the low-frequency curve labeled QB. The highfrequency capacitance is also increased over and above the thermally limited case, because the maximum potential dropped across the depletion region is now (2#IF- V,), where V , is the forward bias induced by the finite background flux. The second phenomenon that can affect the measured C-V characteristic is the presence of fast surface states, and this is

326

M. I

A. KINCH

I

I

I

!

I

___-- ---oulsed I

i

-0.4

I -0.3

I

-0. 2

I

-0.I

I

I

0

0. I

2

V G IVI

Fic. 11. Capacitance versus gate voltage for 0.25-eV n-type (HgCd)Te MIS device including effects due to background flux QB and fast surface states N f 3 .T = 77 K ; C,, = F/cm*; no = 7.5 x lo" ~ r n - ~[Chapman . (1978).] 2.1 x

also shown in Fig. 11 for a uniform density of surface states across the bandgap of Nf,= 1, and 2 x 10" ern+ V-l. The effect of these fast interface states is twofold; the charge trapped in the states as the dc gate voltage is increased modifies the surface potential corresponding to that gate bias and results in a horizontal shift of the C-V curve in the direction of increasing fixed oxide charge, and also if the states are capable of following the applied signal they will result in an increase in the space-charge capacitance, i.e., a vertical shift in the C-V curve. The states included in Fig. 11 are assumed to follow the applied signal. The frequency response of these surface states is described phenomenologically by the RC time constant of the state to the majority-carrier band, as indicated by the device equivalent circuit of Fig. 7a. For an n-type substrate, assuming that Rd >> Rnc, we thus have (Nicollian and Goetzberger, 1967) T , , , ~ = RnaC8,and at the Fermi level Rn,*C8= ( y , p ~ ~ )where - ~ , yn is the capture coefficient of the surface state for electrons, and n, is the surface concentration of electrons. The surface state capacitance is q N f , . It is of interest to estimate the frequency response of surface states located at the intrinsic level at 77 K for particular Hg,-,Cd,Te compositions. For A, = 5 pm, measurements of Shockley-Read lifetime in bulk material indicate

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

327

values of recombination center capture coefficients in the range , R,,C, = (.ynni)-l = 1-10 10-1°-10-9 cm3/sec, and ni lo9 ~ r n - ~hence sec. Thus, fast surface states located at the intrinsic level will not, follow applied ac signals. For A, = 12.5 pm, however, ni 3 X 1013 at 77 K, and R,,C, = 33-330 psec. Intrinsic level surface states can thus follow applied signals in the 5-500 KHz frequency range, and some dispersion in capacitance -voltage characteristics will become apparent for any significant density of surface states in this material. The third factor that can affect experimental capacitance-voltage characteristics is that of a finite distribution in fixed oxide charge (McNutt and Sah, 1974; Chang and Johnson, 1978). This effect is due to the fact that the density of fixed charge at the surface is not strictly uniform and is typically represented by a Gaussian with a finite width. The effect of this distribution is similar to that of fast surface states, and a width of a few kT can result in significant smearing out of the capacitance-voltage characteristics. The impact of surface states (Nicollian and Goetzberger, 1967) and distribution of fixed oxide charge (Brews, 1972) on conductance-voltage characteristics in the depletion-weak inversion regime is well documented; however, for typical (HgCd)Te MIS devices at 77 K little evidence is found of these effects. For the strong inversion regime their importance is also minimal. The equivalent circuit for the MIS device in strong inversion is shown in Fig. 7b and our discussions of conductance will be limited mainly to this mode of operation. The conductance of the MIS device in strong inversion, following the simple theory of Eq. (lo), gives a direct indication of the minority-carrier dark current associated with the depletion region under the gate, for both the low- and highfrequency signal regimes. The minority-carrier dark current perhaps represents the single most vital parameter with regard to the successful operation of MIS devices as detectors of infrared radiation. This applies to operation in the thermal equilibrium mode as discussed at length above, in which the dark current determines the MIS diode impedance, and to operation in the dynamic, or integrating mode, in which the minority-carrier dark current determines the storage time of the device. In the dynamic mode of operation the MIS gate is pulsed into deep depletion and the surface potential approximates the gate voltage for reasonable values of insulator capacitance. The capacitance of the device is given by the “pulsed” C-V characteristic shown in Fig. 11, and the band bending at the surface of an n-type substrate is shown in Fig. 12a, with the depletion region reaching far into the semiconductor. Thereafter, the potential well begins to fill with minority-carrier holes, generated either thermally or photoelectrically via background or signal photons incident on the device. As the potential well fills the sur-

-

-

328

M.

A . KlNCH

Metal

Insulator

I

n-type semiconductor

la1

-w+ n-type semiconductor (bl

FIG. 12. MIS device (a) immediately after voltage pulse V, and (b) after reaching thermal equilibrium.

face potential, given by Eq. (€9, collapses until eventually it reaches thermal equilibrium with a greatly reduced depletion region width, the greater part of the applied gate bias now being dropped across the insulator. The suitability of such a device for reading optical information obviously depends upon the time taken to relax back to thermal equilibrium; this is defined as the storage time of the MIS device given by Tst

= COXV/Jd

7

(1 1)

where CoxVessentially represents the well capacity of the device per unit area, and Jd is the total minority-carrier dark current density. c'.

Minority-Carrier Dark Current

The dark current density in an n-type (HgCd)Te MIS device biased into deep depletion is given by

where the first term represents the current due to minority carriers generated in the neutral bulk and diffusing to the depletion region; the second is due to minority carriers generated in the depletion region of width W; the third is due to generation out of fast surface states represented by a surface recombination velocity s; the fourth is due to the incident background photon flux; the fifth is due to tunneling of carriers from the valence band to the conductance band across the bandgap. These same currents are important for MIS equilibrium mode device operation but the expressions appropriate for zero bias are necessary, as opposed to those above which are equivalent to reverse-bias operation. For this reason the

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

329

performance of MIS equilibrium diodes is often defined by reference to device &A product, where R, is the diode impedance at zero bias, and A the device area. A theoretical consideration of these various dark current contributions appropriate for (HgCd)Te is as follows. (i) Diffusion Current. Minority-carrier diffusion current for an n-type substrate is given by Jdif

=

qnfLp/noT~ 3

(13)

where Lp is the minority-carrier diffusion length and T~ the bulk minority-carrier lifetime. For high-quality material the lifetime is limited by an Auger band-to-band recombination mechanism, and is given by 7 A

= 2nf7A~/no(no + pol,

(14)

where T A is ~ the Auger lifetime for intrinsic material. Thus from Eqs. (13) and (14), for no >> p o t Jdif

=~ n l [ ~ ~ k ~ / ~ q ~ A i I ~ ” ~

(15)

and p p is the hole mobility. The Auger-limited diffusion current thus has no adjustable parameters; all of the quantities in Eq. (15) are well defined and Jdlfis calculable for all temperatures and compositions of n-type (HgCd)Te. Such a calculation is shown in Fig. 13 for a range of compositions 0.20 < x < 0.35, whose

T(K)

FIG. 13. Auger-limited diffusion current versus temperature for various compositions of n-type Hg,-,Cd,Te.

M.

330

A. K I N C H

bandgaps cover the cutoff wavelength range 13.5 < A, < 4 pm. This calculated value of dark current for n-type material represents a lower limit at any specific temperature unless geometrical artifacts such as the use of substrates with thicknesses > n c , hence

ue =

uh

=

- NRypPl~Nc/(~OPl + ON,),

(32)

and again the rate will optimize when the denominator is a minimum, occurring for yppl = ON,, giving

ue = u h = - ~ N R Y ~ P I 9

(33)

which can be significantly larger than the thermally limited case, as p 1 >> n l , depending on where the Shockley-Read level is relative to the valence band. Similar arguments apply to surface states, and NR is replaced by Nfs(kT/q).The magnitude of the tunneling rate ON, is obviously critical in determining the importance of this effect. The tunnel diode theory of (Sah, 1961) indicates that the tunneling rate out of the NR centers is given by

where all quantities have been previously defined except for M, which represents the matrix element of the trap potential energy between the unnormalized trap-state wave function and the band-edge Bloch wave function, normalized in unit volume. The experimentally determined value of V cm3. Assuming a the quantity W ( m * / m , )for silicon is found to be similar value for (HgCd)Te, Eq. (34) gives

) in volts, and E in V/cm. For h, = 12.5 km, for where E, and (E, - E ~ are instance, Eg = 0.1 eV and the critical field for significant direct tunneling is approximately 7 x lo3 V/cm. The generation rate of Eq. (32) will maximize at the value of ct given by the solution of yppl = O N , , where y p for (HgCd)Te lies in the range 10-lo-lO-g cm3/sec. Utilizing Eq. (35) and a value for yp = 5 x 10-lo cm3/sec the optimum level is given by .zt = 42.5 mV, for E = 7 X 101 V/cm and T = 77 K. The value of ON, = yppl = 2 x lo6 sec-l, and p1 = 4 x 1015 ~ m - The ~ . generation rate given and (ypNR)-' again represents the bulk by Eq. ( 3 3 ) is Ue = - (NRypp1)/2

M.

338

A . KINCH

Shockley-Read lifetime associated with minority-carrier holes in (HgCd)Te. High-quality material will have values of (ypNR)-l in excess of 5 psec, in which case the tunnel current associated with the generation rate of Eq. (33) is given by Jtunnel

= tqNRypp1W = 6.4 X 10 W A/cmZ.

(36)

The value of W will be given by that part of the depletion region over which (i) the combined thermal-tunnel transition is allowed, and (ii) the electric field is a maximum. For MIS devices pulsed into deep depletion W may be a significant fraction of the total depletion region, but for devices in thermal equilibrium the relevant W will be a much smaller fraction of the space-charge region. However, if the reasonable value for W = 10-5-10-4 cm is assumed, then the current due to the above mechanism will be Jtunnel= 6.4 X 10-*-6.4 x lop3 A/cm2. To achieve tunnel currents smaller than this will require the use of material with ShockleyRead lifetimes in excess of 5 psec, or operation at temperatures below 77 K, assuming that the above calculation from N R centers is the right order of magnitude. The validity of the calculation rests mainly on the assumed value for M ,the matrix element of the trap potential between the trap-state wave function and the band-edge Bloch function. Similar reasoning will apply to fast surface states and band-tail states. For instance, the current generated from tunneling via a uniform density of fast surface states across the bandgap will be given by Jtunnei

= -!iqypP~[N&T/q] = 1.75

X

l0-''Nfs A/cm2.

(37)

A/cmZ for Ni,= 10l2 cm-2/V, so surface Thus, .Itunnel = 1.75 x states must also be kept to minimum. d. Storage Time

The dark currents discussed at length above can be utilized to predict upper limits for R,,A products of thermal equilibrium diodes or for storage times of dynamic mode devices. Infrared detector operation is typically required at the highest possible temperature compatible with system performance, and this temperature will not be too far removed from the intrinsic range for any cutoff wavelength. Under these circumstances the dominant dark current mechanism, particularly for n-type substrates, is via diffusion from the bulk of the semiconductor, with some possible contribution from the various tunneling mechanisms at cutoff wavelengths beyond 10 pm, depending on the desired well capacity. Utilizing the earlier diffusion current theory Fig. 19 shows the calculated dependence of storage time on temperature for n-type (HgCd)Te of two specific cutoff wavelengths, namely A, = 5 pm and A, = 10 pm. These calculations are for a constant cutoff wavelength as opposed to a constant composition

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

339

TlKl

FIG.19. Auger-limited storage time versus temperature for constant cutoff wavelengths A, = 5 pm, 10 pm. n-type (HgCd)Te: C A V = 2 x lo-' C/cm2.

and utilize the Auger-limited diffusion current data contained in Fig. 13 together with the mobility data of Fig. 8. The assumed value for well capacity is 2 x coulomb/cm2. Included in Fig. 19 on the right-hand axis is the equivalent background flux for any specific storage time, assuming a quantum efficiency q = 0.5. In other words, the current generated by the indicated background flux levels, namely qqaB,will limit storage time values to T , ~= 2 x 10-7/(qq@B).As a specific example consider a 5-pm device operating in a background flux environment of 5 X 1014photons/cm2 sec (anf/2.5 system); the storage time limit will be 5 X lop3sec. For the MIS device to be background limited the thermally generated dark currents must be less than qq@* which according to the A, = 5 pm curve will necessitate operation at temperatures below 155 K. Similar arguments apply to other cutoff wavelengths. III. (HgCd)Te MIS Experimental Data

3. THERMAL EQUILIBRIUM MODE A simple single-level MIS device on (HgCd)Te typically incorporates a layer of thermally evaporated zinc sulfide, with thicknesses between

M.

340

A. KlNCH

1000 A and 10,000 A, on top of a layer of native oxide (Catagnus and Baker, 1976) to form the overall insulator, although the native oxide alone can be employed with a layer of 700- 1300 A. Zinc sulfide is used because of its excellent infrared properties and dielectric strength, and the native oxide for its high-quality interface properties. A variety of metals have been employed as gate material, and for infrared response measurements transparent electrodes consisting of thin layers of metal (-100 A) are utilized. Photoconductive-quality (HgCd)Te is grown by the solid-state recrystallization technique which is based on quenching a high-purity melt of the proper composition followed by recrystallizing the resulting polycrystalline ingot at a temperature just below the solidus temperature. The result is a homogeneous, high-purity (determined by the purity of the starting constituents) boule consisting of a few large crystallites of the desired alloy. The majority of ingots prepared in this manner contain to a greater or lesser degree some substructure, or low angle grain boundaries, although some are entirely free of this defect and are single crystal. No direct evidence has yet been found that the degree of substructure, unless extremely bad, has any deleterious effect on MIS performance. Bearing this in mind, no attempt will be made throughout these experimental sections to correlate device performance with known crystal orientation. The measured capacitance-gate voltage characteristic at 77 K and f = 1 KHz for a typical 5-pm n-type (HgCd)Te device fabricated on 700 8, of native oxide is shown in Fig. 20. The device is 12 x 12 mil in area and the oxide capacitance is -2.3 x lo-' F/cmZwhich indicates an oxide dielectric constant of 18.3. The flat-band voltage occurs at negative gate volt-

/+I

200

-

150

-

G w E

s Y

w

U

-

100-

50

-

I

I

FIG. 20. Capacitance (C) and dC/dV versus gate voltage for 0.25-eV n-type (HgCd)Te at 77 K , f = 103 Hz.

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

341

ages which, if work function differences are ignored, implies a fixed positive charge density in the oxide -10l2 cm+. This value of fixed positive charge is typical of the (HgCd)Te-oxide interface and has been verified for a variety of oxide-ZnS thickness combinations. Hysteresis effects are observed at 77 K and indicate a slow surface state density for hole trapping -1 to 5 x 1Olo cm-2. No evidence of mobile charge hysteresis is observed. The measured dC/dV curve shown in Fig. 20 can be utilized together with Amelio's theory to provide a value for the fast surface state density capable of responding to the applied signal frequency of 3 x 10" cm-2/V, assuming that the distribution in fixed oxide charge is negligible. Typical values obtained for Nfs by this technique on both n-type and p-type material lie in the range 5 X lolo-5 x 10" cm-2/V, and for the most part are frequency independent exhibiting no strong features across the accessible part of the bandgap. The measured conductance for the device in Fig. 20 is essentially zero, indicating that under these conditions (77 K, thermal background) the dark current associated with the depletion region under the gate is zero. This is substantiated by the fact that the highfrequency behavior of Fig. 20 is followed even at a frequency of 10 Hz. No frequency dispersion of capacitance is normally observed over the range 10- lo6 Hz, indicating that fast surface states are not contributing to the device capacitance even in the depletion range of surface potential. This is somewhat surprising in view of the predicted magnitude of the equivalent circuit R,,C, discussed earlier, and suggests that the value of 3 x 10" cm+/V for Nfsobtained by the Amelio technique is not correct, as a fast surface state density of this magnitude should be readily apparent in the C-V characteristics in the depletion range. Conductance measurements at higher device temperatures also typically show an absence of the conductance dispersion effects normally associated with surface states densities Nfs> 1Olo cm+/V. This is not always the case and a measured C-V characteristic for an n-type 0.25-eV (HgCd)Te device at 146 K is shown in Fig. 21 which indicates the presence of surface state dispersion in the frequency range 103-104 Hz, even though no effects due to these states were obvious at 77 K. The use of the higher temperature for 0.25-eV material enables one to probe levels deeper into the bandgap at the applied signal frequency simply because of the increased thermal transition rates from Nfsto the majority-carrier band. A somewhat different approach can be utilized to investigate transitions from Nfs to the minority-carrier band which involves the use of background photon flux. The incident photon flux effectively lowers the resistance Rd connecting the minority- and majority-carrier bands in the equivalent circuit Fig. 7a to a negligible value. Thus, in the gate bias region corresponding to weak inversion Rpa 2&, as the depletion width is thereafter constant. Surface recombination currents of this magnitude are also unlikely, requiring surface recombination velocities > 103 cm/sec, a value totally out of context with measured recombination velocities on n-type (HgCd)Te photoconductive detectors fabricated using similar surface processing techniques. It is thought that the dominant current contribution is due to tunneling, although it cannot be of the direct interband variety, because at the indicated gate bias voltages the conduction and valence bands do not overlap. Tunneling via

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

345

bandgap states is possible, however, in the manner discussed earlier in Section 2. Following that theory, even for high-quality material with T~ = 5 psec, tunnel currents A/cm2 will be generated for average electric fields of 7 x 103 V/cm effective over a distance cm. A thermal transition is involved in this process which indicates that this tunnel component should be strongly temperature dependent, however, attempts to measure this effect are masked somewhat by the fairly strong decrease of the (HgCd)Te bandgap (and hence increased tunneling probability) with temperature. The peak conductance, however, does show a very marked increase as the temperature is reduced even to as little as 55 K, indicating that the above mechanism is indeed the dominant one. Again in this material, as in the 0.25-eV (HgCd)Te, little evidence is found of dispersion in either capacitance or conductance measurements that could be attributed to fast interface states. This would again indicate that the observed slope dC/dV is mainly due to a distribution in fixed oxide charge, and that the surface state density of the typical (HgCd)Te-native oxide interface is in the low 1O'O cm-2/V range. At intermediate compositions of (HgCd)Te between the 0.1- and 0.25-eV bandgap values the general trends reported above hold. A specific example of x = 0.22 is shown in Fig. 25, which has a measured cutoff wavelength of A, = 9.8 pm at 77 K. Tunneling effects are again in evidence but are not as severe as the A, = 12.5 pm case. The indicated RJ product at the conductance peak is -1.2 X 102 SZ cm2.

FIG. 25. Capacitance (0 and conductance (G) versus gate voltage for x = 0.22 n-type (HgCd)Te at 77 K . no = 7 x 10"

~rn-~.

ih, I c l

U 0

-7

-6

-5

-

-3 VG IVI

-2

-I

346

M. A. KINCH

4. DYNAMIC MODE

Operation of MIS devices in a pulsed or dynamic mode provides a direct measurement of the storage time of the well formed under the gate, and hence the magnitude of the device dark current under these operating conditions. The discussion of dark current and storage time in Section 2 indicated that (HgCd)Te MIS devices should be limited by diffusion currents over a wide temperature range. For n-type substrates the upper limit on storage time, T , ~ ,is given by an Auger band-to-band recombination mechanism and the theoretical predictions were summarized in Fig. 19. The absence of adjustable parameters in the theory of Auger-limited diffusion currents at any specific temperature suggests that a high confidence factor can be placed in these calculations. This has been verified by measurements of storage time versus temperature on a variety of compositions, one of which is shown in Fig. 26. The composition is x = 0.30, which corresponds to a cutoff wavelength at 77 K of A, = 4.9 prn, and at 200 K of A, = 4.5 pm. The measured storage time closely tracks the theoretical prediction over the range 130-200 K. Below 130 K a weaker dependence on temperature is observed which could be attributed to generation currents associated with the depletion region. It is interesting to note that significant storage times are available in the thermoelectric cooler range of temperatures, namely 170-200 K, with measured values in the 100-psec range for this composition. The magnitude of the various dark current components at 77 K can be

ID

FIG.26. Experimental and theoretical storage time versus temperature for x n-type Hg,-,Cd,Te. A, = 4.9 km at 77 K; C A V = 6 x lo-* C/crnWa.

- 0.295

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

347

estimated from the theory summarized in Section 2, and as a specific example let us consider the composition x = 0.285 (A, = 5.4 p m at 77 K, which is identical to InSb). For reasonably low gate voltages, tunnel currents can be ignored and at zero background the dark current is given by Ea.- (12): . . J~ = qni

[%+ 270" + I

For (HgCd)Te, ni = 7 x lo9 ~ m - T~~ ~ ; = 105 sec; p p = 450 cm2/V sec. Assuming a voltage swing of 0.5 V, and material parameters no = 3 X 1015 ~ m - T~, ,= 2 x sec, then the depletion region width is W = cm. The thermal terms become [2~~~V/q= n ~6.1 ] ~x/ ~

gnW , = 2.0 x lo-* A/cm2, 270

7

= 5.6 X 10-lo s A/cmZ.

Thus, the dominant dark current, for reasonable surface recombinatim velocities (s < 102 cm/sec), is due to generation in the depletion region, and for a native oxide device with Cox 2 X lo-' F/cm2, and a gate voltage pulse of 0.5 V the predicted storage time will be rSt= C,,AV/Jd = 5 sec. The accuracy of the above calculation has been verified by storage time measurements on an MIS device fabricated from just such material. The measured storage time is illustrated in Fig. 27 and is - 5 sec.

-

HORIZONTAL ( I 5 ~ c / c i i i ]

FIG.27. Measured storage time at 77 K for x = 0.285 (Ac with n, = 3 x lOl5 ~ m - ~ .

- 5.4 Fm) n-type Hg,-,Cd,Te

M.

348

A. KINCH

Storage time measurements on narrow-gap (HgCd)Te indicate, as expected, that tunneling plays a significant role in this material. The theoretical prediction for a cutoff wavelength A, = 10 prn given in Fig. 19 for Auger-limited diffusion currents indicates that Tst values in excess of 100 psec should be possible at temperatures in the liquid nitrogen range. The measured dependence of T,$ versus temperature for a composition x = 0.23 (A, = 9.3 pm at 50 K) is shown in Fig. 28, for a fixed gate voltage pulse -0.75 V. For temperatures above 100 K the values of rStare in approximate agreement with diffusion current theory for a ShockleyRead limited minority-carrier lifetime. This Shockley -Read limited diffusion current can be estimated at temperatures above 110 K from thermal equilibrium MIS conductance measurements. The calculated storage time based on this estimate is indicated at 112 K in Fig. 28, and agrees very well with the measured value of TSt. At temperatures below 100 K the measured 7,t exhibits a strong departure from the diffusion-limited curve. This is attributed to tunnel current contributions to the device dark current. In devices pulsed into depletion both direct interband tunneling and tunneling via bandgap states is possible, however, the observed plateau in TSt versus temperature indicates that the dominant mechanism is via bandgap states. If direct tunneling dominates then due to the decrease of the (HgCd)Te bandgap with temperature one would expect the measured

50

60

70

an

90

Tprriperarure

ion

iio

120

(K)

FIG.28. Storage time versus temperature for x = 0.23 (A, = 9.3 p m at 50 K) n-type F/crn. Hg,-,Cd,Te. V,,,, = 0.75 V; no = 5 x 10" ern-? Cox= 4 X

7.

METAL-INSULATOR-SEMICONDUCTOR I N F R A R E D DETECTORS

349

storage time at constant gate bias (or well capacity) to show a marked decrease at lower temperatures. However, the bandgap state tunneling involves a thermal transition and the two effects might be expected to cancel each other out depending upon the position in the energy gap of the relevant Shockley-Read center. At lower values of gate bias voltage than 0.75 V the measured storage time increases as the temperature is decreased, and values -100 psec are observed at 55 K for this material. This lends further credence to the importance of the tunneling mechanism involving bandgap states. IV. (HgCd)Te MIS Photodiode Technology 5 . THEORY

The MIS photodiode (Phelan and Dimmock, 1967; Kinch, 1974) is merely a photon detector that utilizes the equilibrium depletion region induced under the transparent gate of an MIS structure biased into strong inversion to collect minority carriers generated by incident photons from less than a diffusion length away. It will become apparent that for all practical purposes the MIS photodiode behaves as an open-circuit photodiode that is capacitively coupled to the outside world. The potential diagram for an n-type MIS device biased to strong inversion is shown in Fig. 29 for the two cases of (a) thermal equilibrium and

FIG.29. Potential diagram for n-type MIS device biased to inversion (a) at thermal equilibrium, and (b) with incident back-

I

ground photon flux.

I

(a1

M.

350

A . KINCH

(b) an incident photon flux OBsufficient to give a hole quasi-Fermi level From the general MIS theory discussed in Section 1 we have from Eq. ( 5 ) that strong inversion begins when

@. significantly different from the electron Fermi level

a.

p S = P , exp( - q+PVlkT) 2 no ,

i.e., when the surface concentration of holes becomes greater than the bulk majority-carrier concentration. For the case of an incident photon flux, surface inversion occurs for lower values of +s than the thermal equilibrium case because the valence band crosses the hole quasi-Fermi level Ef for smaller values of surface potential, i.e., +f(inv) < &"(invj. The associated depletion width W will also be narrower for the incident photon flux case. The relationship betweenp, and +,(inv) is given by Eq. (5) to be A+,(inv) = ( k T / q ) Apolpo;

(38)

thus a change of Ap, due to a change in photon flux results in a corresponding change in A&(inv), given by Eq. (38), and a change AW in maximum depletion layer width. This effect can be utilized to detect infrared radiation in the following manner. Consider the expression for space-charge density at the surface given by Eqs. (3) and (4), namely,

+

(e-q@8/kT

+& kT -

y2. (39)

The dependence of Qs on +s is shown in Fig. 30 with p , as a variable parameter (due to incident photon flux, say). Let us assume that the MIS gate is biased to a voltage V corresponding to a surface potential appropriate for strong inversion and then the gate is floated. Any change in incident photon flux after this point in time will result in a change in surface potential given by Eq. (39). In strong inversion Qs = ( 2 ~ ~ , k T p , e - ~ @ J ~ ~ ) ~ ~ ~ ,

(40)

and for the floating gate we have, differentiating Eq. (40) and assuming Qs is constant, A+s =

(kT/q)A~olpo,

(41)

which is equivalent to Eq. (38). The charge on the gate is constant, hence the voltage drop across the insulator must be constant and therefore A+s = AV, the change in voltage on the floating gate. Thus a change in photon flux has been translated into a change in voltage on the MIS gate

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

351

$s (ev)

FIG.30. Variation of space-change density in 0.25-eV n-type (HgCd)Te as a function of surface potential for no = 2 x loxscm-3, T = 77 K, for various minority-camer concentrations.

given by Eq. (41). This expression can be recast into more familiar terms. Consider the case of the MIS device limited by diffusion currents, in which case the impedance of the diode region is given by RdAd = kT/qJdif,

which from Eq. (13) = kTTp/q2poLp,

(42)

where L, is the minority-carrier diffusion length, T, the minority-carrier lifetime, and Ad the device area. Assuming that Apo is due to an incident signal photon flux A> RdCd and is passed through to the preamplifier. This voltage will decay with a characteristic time RLCoxuntil the insulator capacitor charges up to the point where no further current flows. A large value of RLC,, essentially constitutes the floating gate of our earlier discussion. In ac terminology, if the incident photon flux is modulated at a frequency o m ,then the ac signal appearing at XX is given by

For WmRdCd -K 1, and omRLco,>> 1 , Eq. (44) reduces to L\v = qq A@B R,,Ad, which again is simply the signal voltage across the open circuit diode. The theoretical frequency dependence of the MIS photodiode given by Eq. (44) is shown in Fig. 32. With an incident background flux OBon the MIS detector, the impedance at sufficiently low temperatures will be determined by aB.For a diffusion-dominated depletion region the current flowing is given by I

=

Z,(exp(qV/kT) - 1) -

7)@&d

=0

for the open-circuit case. Hence, for V > k T / q , we have

(45)

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

353

log f

FIG.32. Dependence of MIS photodiode signal on frequency.

Thus, from Eqs. (43) and (46),

AV = (kT/q) A@B/@B

(47)

and the signal voltage is independent of quantum efficiency. Therefore, although the device responsivity is less than for a photodiode operating at zero-bias voltage (in that mode of operation the diode impedance is determined by the thermal dark currents flowing in the depletion region), it is independent of quantum efficiency. Equations (46) and (47) can be combined to give the familiar expression for the BLIP specific detectivity, namely,

D* = (1/2h~)[7/@B]~”.

(48)

Arguments similar to those above can be made for nonideal depletion regions (e.g., dominated by generation-recombination currents in the depletion region) with essentially similar results. Theoretical D* values will only be achieved provided the detector noise dominates all other noise sources, and this will be true provided the value used for RL is sufficiently large. In fact the noise voltage output across the XX terminals is approximately given by VN

%

4kTdRd Af

+ l/joCox12Af + 4kTRLIRd IRL + Rd + ~/jWCoxl~

4kTdRd Af

4kT Af + RLO~C~,, ’

(49)

if wCOxRL> 1 ; T in Eq. (49) refers to the temperature of the load resistor

354

M.

A.

KINCH

7

"TR2

FIG.33. MIS photodiode coupled to Si CCD via gate of input MOSFET.

R, ,and Td to that of the detector. Thus the noise from Rd will be observed across XX provided RL

'( T / T d ) ( l / R d ~ 2 ~ x ) *

(50)

The equivalent circuit of Fig. 31b includes the input impedance RA and input capacitance C A associated with the following preamplifier. For optimum device performance it will also be required that RA > RL and CA < cox.

Thus far we have limited our discussion to preamplification of the detector voltages prior to any form of signal processing. Various schemes have also been proposed (e.g., Bate et ai., 1973) to utilize the compatibility of the MIS photodiode with silicon CCD signal processors for signal multiplexing prior to preamplification. Just one such scheme is illustrated in Fig. 33 in which the output from the MIS photodiode is coupled to the gate of the MOSFET input to a silicon CCD shift register, and thus modulates the charge input to the CCD wells. The bias voltage of the MIS device is periodically reset via the MOSFET switch V,, to the value required for surface inversion. The limitation on the usefulness of this signal multiplexing scheme will be determined by a consideration of the thermal noise of the CCD input compared to the noise of the MIS photodiode. 6. PERFORMANCE DATA A typical M I S photodiode is illustrated in Fig. 34. The insulator in this instance is 600 A thick. The thick insulator underneath the bonding pad is shown as photoresist, but obviously other insulating materials may be used. Ohmic contact to the n-type substrate is made via a large-area indium bonding pad. It is meaningful to calculate the magnitude of the MIS photodiode

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

355

Photons

htilk contact Au h a l l h o n d I

n d i uiii bond

t nad

\

6 0 0 - i oxide

depletion reqion n - t y p e (HqCd)Te

FIG.34. MIS photodiode structure

parameters for thermal background operation in the temperature range in which diffusion dominates the device dark current, say for T > 130 K for a 0.25-eV (A, = 5 pm) n-type (HgCd)Te substrate. The impedance associated with the diode region is given by Eq. (42) Rd

=

kT7p/q2PoLpAd

(42)

3

and in the best available material T~ is limited by an Auger band-to-band recombination process, and is given by Eq. (14) TP

where

T A represents ~

(14)

= 2TAin?/no(no -k P o ) ,

the Auger lifetime for intrinsic material. Thus,

where all the parameters are well defined at any specific temperature. As an example consider operation at 190 K, when ni = 2 x lOI4 ~ m - p~p,= 200 cm2/V sec-l, and rAi= 4 X sec; thus R d d = 8 fl cm2, and for a 1.5 X cm2 device Rd = 5.3 X 104 a. The dependence of R d d on temperature as given by Eq. (51) is shown in Fig. 35, assuming a constant cutoff wavelength A, = 5 pm. The capacitance per unit area associated with the depletion region is given by Cd/& = EEo/ W, where the depletion width W is given by

W 2= 4 ~ ~ , + d n , q , +F

being the bulk Fermi potential. For n-type 0.25-eV material

(52)

E

- 19.5,

M.

356

A. KINCH

-

-

-

-

- in' - -

--

3 \

r. , -

r .

-

I

" *o

10'2

-

- 10

a.

r D

I DO

i 70

1'10

I60

I A0

200

220

(K)

r)

FIG.35. L)' and Rg, versus temperature for Auger-limited 0.2s-eV n-type (HgCd)Te. = 1 .O; A, = 5.0 Nm,

-

and for no = lOI5 cm", & 25 mV and W = 3.4 x lo+ cm, giving CdAd = 5.3 X lop8F/cm2. Thus the capacitance of a 1.5 x cm2 device is C, = 8 pF, and the response time under these conditions is RdCd = 4.25 X lO-'sec. The value of the insulator capacitance is determined by the thickness of the insulator, which is typically 700 8, of native 18.3. Hence, &/Ad = oxide, with a static dielectric constant E,, 2.3 x 10'' F/cm2. The value of the device components in the equivalent circuit of Fig. 31b for this 0.25-eV n-type (HgCd)Te MIS device of area 1.5 X 10" cm2 operating at 190 K will thus be Rd = 5.3 X lW 0;Cd = 8.0 pF; and Cox= 34.5 pF. The signal voltage appearing across RL is that given by Eq. (44),and provided RL is large enough, the noise voltage V, across RL will be determined by Rd. The condition on RL is given by Eq. (50) to be RL >> 6.1 x lOI4f2 a, wherefis in Hz,assuming that T = 300 K Thus V , will be determined by Rd at f = 1V Hz if RL >> 6.1 X 108 R. The magnitude of V, under these circumstances will be given by Eq. (49), namely (4kTd& Af)1'2 = 2.4 X V/Hz112.The detectivity is thus given by Eqs. (44) and (49):

-

D*

=

AV(Ad

Af)"2/v,. h @hVAd ~

= (174/2hV)[RdAd/kTl1'*,

(53)

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

357

hence D* = 10” cm I - I P / W for the above set of parameters, assuming the quantum efficiency q = 1.0. The dependence of D* on temperature given by Eqs. (51) and (53) is included in Fig. 35. The only reported data on (HgCd)Te MIS photodiodes (Kinch, 1974) dealt with n-type substrates of photoconductive quality material. The dependence of MIS photosignal magnitude on applied gate bias for a typical 0.25-eV device at 77 K is shown in Fig. 36, for a background photon flux level of 4 x I O l 4 photons/cm2 sec. Also included is the measured dependence of capacitance and conductance at 5 x lo4 Hz, and the inversion layer exhibits a high-frequency response at this frequency. The photosignal, however, is measured at 700 Hz where umRdCd< 1, and it is readily apparent that the photosignal appears as the surface is driven into inversion and thereafter remains at a constant magnitude, as does the measured device conductance at both high and low frequencies. Figure 37 shows the measured dependence of photosignal on inverse temperature for the same device. The indicated temperature dependence follows closely the theory of Eq. (51) for the composition x = 0.285 although the magnitude is low by a factor -2.5 for the measured quantum efficiency 7 = 0.7. The photosignal reaches a background limited value for T < 130 K with an indicated peak responsivity Rh = 1.2 x lo6 V/W, which is

VG

(v)

FIG.36. Capacitance ( C ) ,conductance ( G ) ,and photosignal (V.1 versus gate voltage for 0.25-eV n-type MIS photodiode at 77 K.

358

M . A. KINCH

T/(T&02G,). For a typical value of Rd = 2 x lo4 R, at 77 K with Cox= 30 pF, the noise associated with Rd will only dominate at lo3 Hz for RL > 5 x log R, with an equally stringent limitation on R A . The best value reported for a 0.1-eV device is D* = 3 x 10’O cm H Z ” ~ / Wmeasured at

360

M . A. KINCH

b

C u Y

'F Y 0

-I

5

-1 0

-0 5

0

0 5

1.0

I.5

vc ( \ I ) FIG.39. Capacitance versus gate voltage for a 14-element 0.1-eV n-type (HgCd)Te MIS photodiode array at 77 K , f = lo8 Hz.

lo4 Hz with a 22" FOV (+B = 4 x lois photons/cm2 sec) and a quantum efficiency 7 = 0.2. The associated peak responsivity was R A = 3 x 104 V/W with Ad = 1.5 X cm2. 7. SURFACE-CONTROLLED PHOTOCONDUCTOR It has long been realized that the effective photoconductive lifetime of a material is strongly dependent on surface conditions, or more precisely on fast surface state density N,, and surface potential &. Consider the energy level diagram in Fig. 40. If the density of fast surface states is suffi-

E:

E"

Ftc. 40. Simplified band model for sur. face of (HgCd)Te photoconductor.

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

361

ciently high then electron-hole recombination will proceed through these states at a more rapid rate than through the dominant recombination process in the bulk (which is determined by an Auger, radiative, or Shockley-Read mechanism). This recombination rate is defined in terms of the surface recombination velocity s given by

where yn and yp represent the familiar recombination coefficients of the surface states for electrons and holes, respectively. Ersis the surface state Equation (54) defines the surface recombinenergy, and uo = ln(-yp/yn)1’2. ation velocity for a monoenergetic surface state at Efs. If the surface states form a continuum, as indicated in Fig. 40, then it is obvious from Eq. (54) that the most effective states for surface recombination will be those at the intrinsic level E i , as this minimizes the denominator, provided that yp = yn. In the equivalent circuit of Fig. 7a representing the surface depletion region this condition is equivalent to R,,8 = Rp,8.The surface recombination velocity will be symmetrical with surface potential about this position. However, it must be pointed out that the surface potential can have a drastic influence on bulk photoconductive lifetime even if no surface states are present. This becomes obvious if one considers the dark currents flowing in the surface region of Fig. 40, which is depicted at strong inversion. Electron-hole pairs photogenerated within the depletion region or within a diffusion length of it will be physically separated as shown, and they will recombine at a rate determined by the RdCdof the depletion region w. At thermal equilibrium this time constant will be controlled by the dominant current source as discussed at length in Section 2, namely due to (a) diffusion, (b) depletion region, (c) surface, or (d) tunnel currents. (c) is the contribution described by the surface recombination velocity expression Eq. (54).The effect of these current contributions can be qualitatively estimated in the following manner. The incident photon flux density A@B changes the voltage drop across the depletion region, and hence its width, by essentially forward biasing the region in the manner described in Eqs. (38) and (43). This voltage change is given by

A v = 7)q A@B R&j.

(43)

The change in depletion width A W corresponding to this change in barrier height is given by

362

M.

A . KINCH

This change AW in the width of the depletion layer releases noAW electrons per unit area in the n-type bulk region of the photoconductor. This is equivalent to an effective change in majority-carrier concentration in the photoconductor of thickness t of

Ano =

no AW t

'

which by Eqs. (43) and ( 5 5 ) is Ano = -R~ 7 A@B = RdCd W t TP

(57)

where Apo represents the concentration of minority carriers generated by the signal flux A@* if the bands at the surface are flat, and rP is the bulk minority-carrier lifetime. Therefore, from Eq. (57), if the depletion layer response time T = RdCd> then the sensitivity of the photoconductive element is increased over and above the bulk value. If RdCd C T~ then a decrease in photoconductive sensitivity will result. To this end the quantity RdCd/rpmay be considered a trapping gain (Kinch and Boyd, 1973). The variation of photoconductive signal with inverse temperature for the surface-controlled photoconductor will depend on the nature of the dominant current contribution. For a diffusion current dominated surface region RdCd0~ nF2, and the photosignal will exhibit an activation energy of E,; for depletion currents on the other hand RdCda nrl, and the thermal activation energy will be E,/2. The contributions due to surface and tunnel currents should be relatively temperature independent. An example of surface-controlled photoconductivity is shown in Fig. 41 I

I

Capacitance

-45

-40

-j5

-?O

-20

-75

vc,

-15

-10

-5

0

(V)

FIG.41. Photoconductive signal and capacitance versus gate voltage for 0. I-eV n-type (HgCd)Te photoconductor at 77 K .

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

363

for 0.1-eV n-type (HgCd)Te at 77 K. The photosignal is measured on a photoconductive element 20 pm thick fabricated with the upper surface completely controlled by a transparent field plate on a 1-pm layer of ZnS + native oxide. The flat-band voltage for this MIS structure is - 27 V, and the measured bulk lifetime in accumulation ( V , more positive than - 27 V) is 3 psec. The measured photosignal is relatively flat in the region of surface accumulation. However, as VG is increased to larger negative values the photosignal decreases dramatically and is reduced by more than an order of magnitude for a strongly inverted surface condition. This is because a typical RdCd time constant for 0.1-eV material is 5 2 x lo-' sec at 77 K. There is also an indication in Fig. 41 of the classical surface recombination velocity effect of Eq. (54), given by the slight minimum in photosignal observed in the region of field plate bias at which the surface is approximately intrinsic (VG - 29 V). The wider-gap composition of (HgCd)Te at 77 K, on the other hand, will exhibit a photosignal enhancement upon surface inversion, relative to the accumulated condition, because the values of RdCd associated with the depletion region are considerably in excess of 7p(which is typically in < 7p < 4 x the range sec). An example of this photosignal enhancement is shown in Fig. 42 for a composition x 0.30 at 77 K with an indicated minority-carrier trapping gain -70. The measured dependance of the inverted surface photosignal on inverse temperature for this x 0.30 composition is shown in Fig. 43, and the measured thermal acti-

-

-

-

-

-

-

IO?/T IK-')

FIG.42.

FIG.43.

FIG.42. Photoconductive signal versus gate voltage for x toconductor at 77 K . FIG.43. Photoconductive signal versus lff/Tfor x verted, and (b) accumulated surfaces.

- 0.30 n-type Hg,-,Cd,Te

- 0.30 n-type Hg,-,Cd,Te

pho-

for (a) in-

364

M. A. K l N C H

vation energy is approximately E g / 2 indicating that the dominant current associated with the depletion region is due to generation-recombination currents. The saturation of photosignal at lower temperatures is due to incident background flux. For comparison purposes the measured dependance of photosignal with inverse temperature for an accumulated surface condition is shown in Fig. 43 (Borrello et a / . , 1973).

V. (HgCd)Te Charge Transfer Device Technology 8. (HgCd)Te CTD THEORY A N D DESIGN

The (HgCd)Te MIS photodiode can be utilized in the dynamic, or integrating, mode to detect infrared radiation and it is perhaps this mode of operation which holds the greatest promise for this device. The performance improvements demanded of the next generation of infrared systems require a significant increase in the typical number of detectors employed in the focal plane to values in excess of lo4, thus ruling out conventional arrays of photoconductors of photodiodes with individual preamplifiers as used in current infrared systems. This detector requirement will necessitate that a considerable amount of signal processing be performed on the focal plane; possible signal processing functions envisioned are time delay and integration (TDI) for scanning systems, multiplexing outputs, area array staring mode operation, antiblooming, and background subtraction. To this end both charge coupled (CCD) (Boyle and Smith, 1970) and charge injection (CID) (Burke and Michon, 1976) device technology on the infrared sensing material itself can be employed, and both have been developed at Texas Instruments Incorporated. The charge-coupled device is somewhat the more challenging of the two in that a considerable number of charge transfers are involved in signal readout which necessitates a high quality of both material and semiconductor-oxide interface technology, and it is this structure that will be described at length here to demonstrate the feasibility of (HgCd)Te charge transfer devices. A simplistic example of focal plane architecture appropriate for a scanned infrared system is shown in Fig. 44. The current generation linear array of photodetectors is essentially replaced by a linear array of “superdetectors,” or columns containing N individual detectors. These columns are in fact CCD shift registers fabricated on infrared sensitive material and are aligned with the direction of image scan such that the potential wells collecting charge from the various pixel elements of the scene can be clocked down the shift register in synchronization with the mechanical rate of scan. In this manner each pixel is integrated over a considerably longer period of time than the conventional linear array, in fact, for

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

365

I iiiaqr

Scan D i rec t i o n

FIG.44. Second generation scanning focal plane.

where T~~~~is a horizontal line time. The columns are then fed in parallel into a multiplexer again fabricated on (HgCd)Te, and the signal is fed out serial fashion. In the ideal case all of the columns of the focal plane are multiplexed with a single CCD shift register and fed out through a single preamplifier. However, in reality the number of columns that can be handled by a single parallel- serial multiplexer will be determined by such considerations as charge transfer efficiency, CCD noise, and data rates, and compromises will have to be made for large focal planes. Thus, the concept of the (HgCd)Te CCD shift register combines the functions of photon detection, time delay and integration (in the case of the scanned system), and signal multiplexing into the same advice. The integration of signal and noise in the charge domain not only results in a significant increase in system sensitivity but also occurs prior to preamplification with a considerable relaxation of preamplifier area and power requirements. The single-level (HgCd)Te MIS technology discussed throughout this chapter has recently (Chapman et al., 1978; Kinch et al., 1980) been extended into the multilevel capability required for the fabrication of CCD shift registers. The reported designs have incorporated both 8- and 16-bit four-phase stepped insulator geometries with overlapping gates and an electrostatically controlled channel stop. Figure 45 shows the longitudinal and transverse sections of the CCD design, together with the thicknesses of the various layers of insulator (ZnS) employed. The high quality of semiconductor-insulator interface required for successful CCD operation was achieved with a native oxide -700 A thick. The gate lengths employed were 10 p m with various channel widths between 50-125 pm. The above design is dominated by two basic features of CCD operation in intrinsic infrared sensitive materials, namely, the high field breakdown associated with the narrow bandgaps in question, and the well capacity

366

M.

A . KINCH

Potential Profile

(bl

FIG.45. (HgCd)Te CCD (a) longitudinal section and (b) cross section. [Kinch ef al. (1980).]

required for the photon fluxes involved. An upper limit is placed on the clock voltages that can be employed, depending on the doping level of the substrate, consistent with the predicted breakdown field for the composition in question. The tunneling calculations earlier indicate that this field is 3 V/pm for A, = 5 pm, and 1 V/pm for A, = 10 pm. These values of surface electric field are sufficient to generate a tunneling current equal to approximately 10% of the background flux generated current for a typical system with f/2.5. This limit on clock voltages is determined by both normal and tangential field considerations, and as such makes the choice of a stepped insulator virtually mandatory for maximum well capacity CCD operation at these wavelengths. Channel stops can be formed in one of three ways, namely by the use of a heavily doped region (typically an ion implant), or a stepped insulator, or an electrostatic shield. The heavily doped region is ruled out by high field breakdown considerations at the channel edge, and the stepped insulator design requires the use of thick gate metals for edge coverage with inherent yield problems. The electrostatic shield (field plate) channel stop does not suffer from these disadvantages and provides the added benefit of a field stop for infrared radiation when opaque. A variety of metals have been employed for gate fabrication, with aluminum the most common for opaque, and nickel for transparent metallizations. Contact to the various levels is made by via etching and evaporated indium bonding pads. A photograph of the 16-bit, 2-mil wide

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

367

FIG.46. Micrograph of lbbit, 2-mil wide shift register. [Kinch et al. (1980).]

channel shift register is shown in Fig. 46, and a cross-section schematic in Fig. 47 together with the silicon output buffer circuit. Input to the register is made by biasing the input gate to high field breakdown, and the output signal is sensed by the floating gate circuit shown in Fig. 47. Operation of the floating gate output is as follows. The output transfer gate is operated “on” so that when 44 is turned off the signal charge transfers over this output gate onto the floating gate. Imme-

Reset

Staye 16

I

I

-

&

FIG. 47. Schematic of 16-bit shift register with floating gate output circuit. Substrate bias = V,. [Kinch et al. (1980).]

368

M . A . KINCH

diately prior to this the floating gate is preset to reference voltage Vrervia a voltage pulse applied to the gate of the reset MOSFET, and then floated. The input of charge under the floating gate from #4 changes the voltage there, which is then sensed by the source follower circuit. After sensing, the floating gate is discharged prior to the arrival of the next signal pulse. This is accomplished in Fig. 47 by the application of a positive injection pulse (for ap-channel device) to the floating gate, which dumps the charge into the substrate where it recombines within a minority-carrier lifetime (0.1 - 10 psec). The floating gate is then preset again to Vretand the cycle repeats. The signal on the floating gate is detected after an ac-coupled correlated double sampling circuit (White er d.,1974) which clamps immediately before injection and samples the voltage on the floating gate immediately after the injection pulse. A typical timing sequence for this mode of operation is shown in Fig. 48. The performance parameters associated with (HgCd)Te CCD operation can be predicted utilizing any of the standard texts (e.g., Sequin and Tompsett, 1975) and will not be treated here, with the exception of the expected value of the charge transfer efficiency. The primary source of charge transfer loss at low frequencies in CCDs operating with a fat-zero charge is trapping in fast interface states at the edge of the CCD well. Simple theory (Kosonocky and Carnes, 1973) for the loss at the edges pre-

clamp preset

inject

t

T I 1-1 k

FIG.48. Typical clock sequence for (HgCd)Te CCD operation. [Kinch et a / . (1980).]

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

369

dicts a fractional loss per transfer, for p-channel (HgCd)Te, given by

edge linear dimension; density of fast surface states; signal charge per unit area; emission coefficient for holes from surface states; clock frequency; gate voltage in escess of threshold; surface potential; semiconductor dielectric constant.

For 0.25-eV (A, = 5 pm) (HgCd)Te at 77 K, assuming that Cox= 2.3 x F/cm2, then we have Et

=

1.58 x 10-4

w

10'0

(59)

( J - ) ( ~ ) ( ~ ) l i 2 ,

where Wc is the channel width in mils, and Nfs is in units of cm-2/V. The signal charge is assumed 0.8 of full well capacity, and k4 = upvthNvr where upis the surface state capture cross section for holes, Vth 5 X lo6 cm/sec, N, 2x ~ m - and ~ , we have assumed up lo-'' cm2, giving k4 lo* sec-'. Equation (59) assumes four-phase operation at a frequency f = 5 X 104 Hz, with a clock voltage of V = 5 V riding on a substrate bias 0.5 V in excess of threshold. At higher frequencies with optimum design the transit time across a CCD gate is determined by fringing field drift. Drift theory (Carnes et af., 1971) applied to (HgCd)Te with a combination oxide-ZnS insulator for a four-phase structure gives for the transit time of a carrier across a gate:

-

-

-

-

where for 0.25-eV n-type (HgCd)Te at 77 K the appropriate parameters are

L. = gate length = 0.4 mil; p = surface mobility for holes = 250 cm2/V sec; fox = insulator thickness = 3400 A; V = clock voltage on electrodes = 5 V; W = depletion layer thickness = [ 2 ~ ~ ~ + ~ / l t ~ q ] ' ~ ~ . +s

is the surface potential under the transferring electrode, which is as-

370

M. A . KINCH

4

2

1

VG - VFB (VI

FIG.49. Empty well surface potential versus gate voltage for typical (HgCd)Te CCD level ~. r f a / . (1980).] geometries. no = 1OIs ~ r n - [Kinch

sumed to carry a gate voltage equal to half the clock voltage (relative to flat band). The expression for empty well surface potential is given by Eq. (8):

4s =

vo

+ (VG

-

VFd

- [c + 2v01,(vG-

vFB)]”*,

(8)

where v o

= q~notf/E’x%.

The variation of #Bs with (V, - VFB)is shown in Fig. 49 for typical firstand second-level insulator thicknesses employed. For a substrate bias 0.5 V in excess of flat band and 5-V clocks, the value of #Bs under the transfer electrode will be approximately 2.0 V, providing W = 2.15 X cm. For no = lOI5 cmP3, Eq. (61) gives rtr= 6.7 X sec. The charge transfer loss is related to the fringing field drift time by (Carnes et al., 1971) .sc =

7.4 exp(-t,,/0.33~~~) = 7.4 exp[-l/(l.33fc~,,)].

(61)

The loss per transfer based on Eqs. (59), (60), and (61) is shown in Fig. 50 for the particular set of CCD parameters listed. The sharp increase in transfer loss at higher frequencies can be greatly reduced by (1) reducing the gate length, (2) decreasing the doping level, and hence increasing the maximum employable gate voltage V and depletion depth W, or (3) using

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

lo-4

371

-

lo-[ I

o4

I o4

I

I

I l l

I

I

l i t Io6

I0 5

I

I

I I , 10'

Frequency ( H I )

FIG.50. Theoretical charge transfer inefficiency versus frequency for 0.25-eV p-channel (HgCd)Te CCD at 77 K, with typical operating parameters. Nf, = 5 x 10'O cm-* V-I; Cox= 2.3 x IO-'F cmP; no = lOI5 cmP; V = 5 V; I* = 250 cm2/V sec. [Kinch er al. (1980).

n-channel devices, for which the surface mobility is expected to be in excess of 2 X lo4 cm2/V sec for 0.25 eV material. The effect of reducing the gate length is shown in Fig. 50 for a realistic value of L = 0.25 mil, together with a calculation for different channel widths. It is apparent from Fig. 50 that good transfer efficiencies in n-type substrates can be achieved up to frequencies of los Hz provided that the fast surface state density can be held at reasonably low values (NfsI5 x 1010 crn-z/V). 9. (HgCd)Te CCD PERFORMANCE DATA

The CCD shift register performance data reported thus far has been concerned entirely with n-type (HgCd)Te substrates of composition x 0.30 (A, 5 pm at 77 K), with typical doping levels of no = IOl5 ~ m - No ~. diodes have been incorporated into the shift registers. Input is by high field breakdown of the signal gate, and output sensing is by the floating gate technique described earlier, followed by substrate injection of the signal charge.

-

-

M.

372

A.

KINCH

I

L , 1 s t Level

7nd LPvrl

15

10

~5

0

-8

-6

-4

-2

0

FiPld Plate Bias I V I

FK. 51. Capacitance-voltage characteristics for the gates of a 16-bit. 0.25-eV (HgCd)Te CCD measured at 77 K , f = 103 Hz.[Kinch er a / . (1980).]

n-type

A typical set of capacitance-voltage curves at 77 K for a 16-bit shift register with a 2-mil wide channel is shown in Fig. 51, measured at a frequency of lo3 Hz and a thermal background environment. The characteristics exhibit all of the properties described earlier for 0.25-eV n-type material and the device uniformity is seen to be very good. The absence of dark current results in a considerable degree of charge storage even for the slowest ramp speeds employed, as evidenced by the vertical displacement of the forward and reverse C-V curves on all gates. The capacitance curve in the direction of increasing negative bias voltage is reminiscent of a deep depletion characteristic. The output observed from the correlated double sampler (CDS) of a 16-bit, 5-mil wide shift register with 0.4-mil opaque gates operated in a four-phase mode is shown in Fig. 52, together with the high field breakdown pulses applied to the signal gate. Charge transfer is clearly demonstrated by the delay of the CDS output relative to the input signal by the required number of bits. The fat-zero necessary for optimum transfer efficiency is introduced into this device by biasing the clock voltages to val-

7.

METAL-INSULATOR-SEMICONDUCTOR INFRARED DETECTORS

373

FIG.52. ( 1 ) Signal input and (2) signal output from the correlated double sampling (CDS) unit of a 16-bit, four-phase, 5-mil wide (HgCd)Te CCD (0.25-eV n-type) at 77 K , clock frequency = 50 kHz.[Kinch ef a / . (19801.1

ues sufficiently close to high field breakdown to generate the desired charge. This fat-zero can also be introduced by pulse biasing the input signal gate. The CCD clock frequency employed in Fig. 52 is 5 X lo4 Hz, and an expanded photograph of this pulse train is shown in Fig. 53. This signal represents approximately 0.8 of full well as indicated by signal saturation, and the charge transfer efficiency as measured from the leading and trailing edge losses is 0.9994 for this particular four-phase device. There is no evidence of fixed charge loss at the leading edge; both the leading and trailing edges are essentially symmetrical. The highest transfer efficiency report thus far (Kinch et al., 1980) is 0.9995 for a 16-bit, 2-mil wide channel, with 0.4-mil gates, operating in a four-phase mode, with device parameters approximating those used in Eq. (60). If the measured transfer loss is due to fast surface states then N,,= 6.3 x 1O'O cm-'/V. This value is somewhat below the norm obtained for this material by dC/dV techniques, as discussed in Section 3, and tends to confirm the hypothesis that a finite distribution of fixed oxide charge is responsible for the apparent density of surface states calculated by this method. As discussed earlier, this observation is in line with the absence of capacitance and conductance dispersion effects in typical MIS devices fabricated on (HgCd)Te. The magnitude of the signal voltage is given by an analysis of the equiv-

M. A . KINCH

374

FIG.53. CDS output from 16-bit, four-phase (HgCd)Te CCD with 0.4-mil gates, showing CTE = 0.9994. [Kinch et UI. (1980).]

alent circuit of the floating gate output shown in Fig. 54. The floating gate signal voltage AVs is related to the input signal charge AQs (from 44gate) injected onto the node between the oxide capacitance Coxand the depletion layer capacitance Cd by

AVs =

+

[AQsCox/(Cox

cp)l/[cd

+ CoxCp/(Cox

-I- Cp>I3

(62)

where C , represents the parasitic capacitance of the circuit including the input capacitance of the source follower, the overlap capacitances between the floating gate and its adjacent electrodes, the capacitance associated with the injection circuit, and the capacitance of the interconnect between the (HgCd)Te IRCCD and the silicon IC. For the present multilevel technology status Cox Cd