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Spie Press Book

Fundamentals of Infrared Detector Materials
Author(s): Michael A. Kinch
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Book Description

The choice of available infrared (IR) detectors for insertion into modern IR systems is both large and confusing. The purpose of this volume is to provide a technical database from which rational IR detector selection criteria evolve, and thus clarify the options open to the modern IR system designer. Emphasis concentrates mainly on high-performance IR systems operating in a tactical environment, although there also is discussion of both strategic environments and low- to medium-performance system requirements.

Book Details

Date Published: 8 November 2007
Pages: 186
ISBN: 9780819467317
Volume: TT76

Table of Contents
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1 Introduction 1
2 IR Detector Performance Criteria 5
2.1 Photon Detectors 5
2.1.1 IR detector operating temperature 5
2.1.2 IR detector sensitivity 7
2.2 Thermal Detectors 9
3 IR Detector Materials: A Technology Comparison 13
3.1 Intrinsic Direct Bandgap Semiconductor 13
3.2 Extrinsic Semiconductor 16
3.3 Quantum Well IR Photodetectors (QWIPs) 18
3.4 Silicon Schottky Barrier Detectors 23
3.5 High-Temperature Superconductor 26
3.6 Conclusions 27
4 Intrinsic Direct Bandgap Semiconductors 31
4.1 Minority Carrier Lifetime 32
4.1.1 Radiative recombination 32
4.1.2 Auger recombination 33
4.1.3 Shockley-Read recombination 34
4.2 Diode Dark Current Models 34
4.3 Binary Compounds 35
4.3.1 Indium antimonide: InSb 35
4.4 Ternary Alloys 37
4.4.1 Mercury cadmium telluride: Hg1-xCdxTe 37
4.5 Pb1-x SnxTe 42
4.5.1 Minority carrier lifetime 43
4.5.2 Dark currents 44
4.6 Type III Superlattices 45
4.6.1 Superlattice bandstructure 45
4.6.2 Band offsets and strain 47
4.6.3 Interdiffusion in HgTe/CdTe superlattices 48
4.6.4 Misfit dislocations 48
4.6.5 Absorption coefficient 49
4.6.6 Effective mass 51
4.6.7 Minority carrier lifetime 52
4.7 Type II Superlattices 53
4.7.1 Minority carrier lifetime 54
4.8 Direct Bandgap Materials: Conclusions 57
4.8.1 HgCdTe 57
4.8.2 InSb 57
4.8.3 PbSnTe 58
4.8.4 Type III superlattices 59
4.8.5 Type II superlattices 59
4.8.6 Final thoughts 59
5 HgCdTe: Material of Choice for Tactical Systems 61
5.1 HgCdTe Material Properties 61
5.1.1 Material growth 61
5.1.2 HgCdTe annealing 65
5.1.3 HgCdTe properties 67
5.2 HgCdTe Device Architectures 75
5.2.1 DLHJ architecture 76
5.2.2 Bump-bonded ion implant architecture 77
5.2.3 Vertically integrated photodiode (VIP and HDVIP) architectures 77
5.3 ROIC Requirements 81
5.3.1 Detector performance: Modeling 82
5.3.2 Dark current in HgCdTe diodes 82
5.3.3 1/f noise 87
5.4 Detector Performance 89
5.5 HgCdTe: Conclusions 91
6 Uncooled Detection 93
6.1 Thermal Detection 93
6.2 Photon Detection 95
6.2.1 HOT detector theory 95
6.2.2 HOT detector data 101
6.2.3 HOT detector contacts 103
6.2.4 HOT detector options 103
6.3 Uncooled Photon vs. Thermal Detection Limits 105
6.4 Uncooled Detection: Conclusions 107
7 HgCdTe Electron Avalanche Photodiodes (EAPDs) 109
7.1 McIntyre's Avalanche Photodiode Model 110
7.2 Physics of HgCdTe EAPDs 112
7.2.1 High-energy scattering rates 113
7.2.2 Electron impact ionization rate in HgCdTe 115
7.3 Empirical Model for Electron Avalanche Gain in HgCdTe 121
7.4 Room-Temperature HgCdTe APD Performance 129
7.5 Monte Carlo Modeling 131
7.6 Conclusions 133
8 Future HgCdTe Developments 135
8.1 Dark Current Model 135
8.1.1 N-side 136
8.1.2 P-side 137
8.2 The Separate Absorption and Detection Diode Structure 139
8.3 Multicolor and Multispectral FPAs 141
8.4 High-Density FPAs 143
8.5 Low Background Operation 143
8.5.1 LWIR 14 m at 40K 143
8.5.2 Low background operation at a cutoff of 25 m 144
8.6 Higher Operating Temperatures 145
8.6.1 High-gain APDs 147
8.7 Conclusion 148
Epilogue 149
Appendix: Mathcad Program for HgCdTe Diode Dark
Current Modeling 151
References 165
Index 169


The choice of available infrared (IR) detectors for insertion into modern IR systems is both large and confusing. The purpose of this volume is to provide a technical database from which rational IR detector selection criteria can evolve, and thus clarify the options open to the modern IR system designer. Emphasis will be mainly on high-performance IR systems operating in a tactical environment, although there will be limited discussion of both strategic environments and low- to mediumperformance system requirements.

Early IR imaging systems utilized extrinsically doped Ge as the detecting material and operated at 28 K. However, the development in the 1960s of the semiconductor alloys HgCdTe [1] and PbSnTe [2], with their tunable bandgaps that covered the complete IR spectrum from 1 to 20µm, led to the birth of the first generation of modern high-performance IR systems in the early 1970s, with the advent of the so-called Common Module, first developed by Texas Instruments. The heart of this system was a simple 180-element linear parallel-scan HgCdTe photoconductive array mounted on a cold finger operating at approximately 77 K. The IR image formed at the focal plane of the system was scanned across the simple linear array of detectors by a rotating mirror, thus generating one line of the IR scene at a time, with an available integration time (or noise bandwidth) determined by the system line time, and the image was formatted by the subsequent off-focal plane electronics. This first-generation imaging system was thus mechanically complex, but geometrically simple from a detector, and hence array producibility, point of view. The array bias and amplifier electronics were mounted off the cold finger at an elevated temperature. HgCdTe had proved to be the material of choice at this time because of issues with PbSnTe associated with its high dielectric constant and inferior mechanical properties. The use of the direct bandgap alloy HgCdTe resulted in high absorption coefficients for the IR, and for the first time enabled the use of thin material, and hence the deployment of standard semiconductor photolithographic techniques, in IR focal plane array fabrication. The Common Module has survived in one form or another for some 30 years, and is still in production today.

The 1990s fostered the evolution of the second generation of IR systems, in which the detector bias and signal electronics were incorporated onto the cold finger itself. This was typically achieved by hybridizing the IR-sensitive material with the silicon processor. Scanning arrays, with their simple linear geometry and relative mechanical complexity, still dominate the marketplace. The degree of detector complexity is increasing, as ever-higher performance is demanded of the scanning IR system. This is typically achieved by increasing the number of elements in the cross-scan direction of the focal plane array (FPA), and by the incorporation in the scan direction of time, delay, and integrate (TDI) techniques, which essentially result in a linear array with an increased integration time equal to integral multiples of the available line time. HgCdTe is still the unambiguous material of choice, and indeed has no rival in the scanning FPA marketplace. However, the ever-increasing complexity of second-generation scanning FPAs has in turn led to the development of staring FPAs of increasing size and density, particularly for applications in which mechanical complexity cannot be tolerated. The mechanical complexity of the scanning array is replaced in the staring array by a complexity of design in both the IR device and the associated hybridized silicon electronics. It is this aspect of second-generation FPAs that has led to the most concern regarding the possible deployment of alternative detector material technologies, other than HgCdTe, for various, specific staring-system applications. This concern stems from the perception, which is in many cases mistaken, that materials and processing issues prohibit the use of HgCdTe in the fabrication of production quantities of large-area FPAs in which the IR material is hybridized to the silicon readout integrated circuit (ROIC).

Third-generation high-performance IR FPAs are already a gleam in the eye of the IR system designer, and will almost certainly be staring architectures. They will demand capabilities such as multicolor (at least two, and maybe three or four different spectral bands) with possibly simultaneous detection in both space and time, ever larger array sizes, of say 2000×2000, and operation at higher temperatures, even to room temperature, for all cutoff wavelengths, to name but the obvious.

A successful IR materials technology must be capable of addressing any and all of these second- and third-generation IR focal plane issues.With this in mind, the current arsenal of IR materials technologies is examined from a strictly fundamental standpoint, in which the materials and device physics of each situation are allowed to determine the limitations of possible system implementation. This methodology allows for each of the IR technologies to be considered strictly on its fundamental merits, with no diversions provided by issues associated with a suitable reduction of the technology to practice. Problems, such as nonideal contacts or surfaces, obviously will exist and will exacerbate the situation, but will only serve to confuse a meaningful fundamental comparison of these technologies.

This book is a byproduct of the many years of experience in IR technology acquired by the author, primarily while employed at the original Texas Instruments Central Research Laboratories in Dallas, Texas. The author would like to acknowledge the expertise of all the scientists that worked there during his tenure, and the significant role that they played in the development of what has become a critical technology in the defense of the United States. Giants walked those corridors in the form of George Heilmeyer, Bob Stratton, Dennis Buss, Dick Chapman, Dick Reynolds, Al Tasch, Roland Haitz, Bob Bate, Jerry Hynecek, Seb Borrello, Bill Breazeale, Grady Roberts, Kent Carson, Herb Schaake, John Tregilgas, Francois Padovani, Larry Hornbeck, Don Shaw, Dean Collins, Bill Wisseman, and Turner Hasty, to name but a few. In many ways, this book is a tribute to the stimulating environment provided by their talent and innovation, in this and many other fields. The author feels privileged to have belonged to that community. Unfortunately, the Texas Instruments Central Research Laboratory, and many like it, no longer exist on the technology playing fields of the United States. The country is much the poorer for their demise.

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