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Spie Press Book

State-of-the-Art Infrared Detector Technology
Author(s): Michael A. Kinch
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Book Description

Scientists, engineers, managers, and policy makers who are currently involved in the funding of infrared R&D and subsequent system design and manufacture are confronted with a choice between two competing materials technologies, HgCdTe and III-V alloys. This book examines both the current and future performance of infrared focal plane arrays that use the various device architectures associated with these two materials technologies. All spectral bands from long wavelength (LWIR) through mid-wavelength (MWIR) to short wavelength (SWIR) are considered, ultimately with a view to achieving background and diffraction-limited system performance at room temperature for all wavelengths.


Book Details

Date Published: 6 October 2014
Pages: 280
ISBN: 9781628412895
Volume: PM248

Table of Contents
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Table of Contents

Introduction
List of Acronyms

1 Cooled Infrared Detector Architectures
1.1 Extrinsic Photoconductors
1.2 Intrinsic Photoconductors
1.3 Si Schottky Barrier Photodiodes
1.4 Metal-Insulator-Semiconductor (MIS) Photodiodes
1.5 Photodiodes
1.6 Barrier Layer Photoconductors
1.7 Bandgap-Engineered Devices
1.8 Electron Avalanche Photodiodes
1.9 PIN Photodiodes
1.10 Summary
References

2 Infrared Focal Plane Array Considerations
2.1 Modulation Transfer Function
2.2 Noise Equivalent Flux or Noise Equivalent Temperature Difference
2.3 Fast Adjustment of Tilt and Height
      2.3.1 Detector dark current
      2.3.2 Excess noise
2.3 Collection Efficiency
2.4 Summary
      2.4.1 Photodiodes
      2.4.2 Barrier layer photoconductors
References

3 Dark Current Considerations
3.1 Minority Carrier Lifetime
      3.1.1 Radiative recombination
      3.1.2 Auger recombination
      3.1.3 Shockley–Read recombination
      3.1.4 Minority carrier generation rate
3.2 Metallurgical Photodiode
3.3 Barrier Layer Photoconductor
3.4 Graded-Gap Heterojunction Photodiode
3.5 Summary
References

4 Noise Considerations
4.1 Experimenting with Collimation: Autocollimation
4.2 Systemic 1/f Noise
      4.2.1 McWhorter's systemic 1/f noise model for the photodiode
      4.2.2 McWhorter's systemic 1/f noise model applied to other architectures
4.3 Isolated Defect 1/f Noise: Dislocations
4.4 Summary
References

5 Infrared Materials for Focal Plane Arrays
5.1 II-VI Alloys
      5.1.1 HgCdTe material growth
      5.1.2 HgCdTe material annealing
      5.1.3 Band structure
      5.1.4 Optical properties
      5.1.5 Transport properties
      5.1.6 Minority carrier lifetime
      5.1.7 Dark current generation rates
      5.1.8 Tunnel currents in HgCdTe
      5.1.9 Electron avalanche multiplication in HgCdTe
      5.1.10 1/f noise
      5.1.11 Minority carrier diffusion lengths
      5.1.12 Surface passivation
      5.1.13 Electrical contacts
      5.1.14 Physical properties
      5.1.15 Type III superlattices
5.2 III-V Alloys
      5.2.1 Material growth
      5.2.2 Band structure
      5.2.3 Optical properties
      5.2.4 Transport properties
      5.2.5 Minority carrier lifetime
      5.2.6 Dark current generation rates
      5.2.7 Systemic 1/f noise
      5.2.8 Minority carrier diffusion lengths
      5.2.9 Surface passivation
      5.2.10 Electrical contact
      5.2.11 Physical properties
      5.2.12 III-V type II superlattices (T2SLs)
5.3 IV-VI Alloys
      5.3.1 Minority carrier lifetime
      5.3.2 Doping concentrations
      5.3.3 Dark currents
      5.3.4 Systemic 1/f noise
5.4 Summary
      5.4.1 II-VI alloy
      5.4.2 III-V alloy
      5.4.3 IV-VI alloys
References

6 HgCdTe FPA Technologies
6.1 Photodiodes
      6.1.1 Bump bond hybridization
      6.1.2 Verifying Metal via hybridization
      6.1.3 Two-color FPAs
6.2 Barrier Layer Photoconductors
      6.2.1 Monocolor nBn
      6.2.2 P+/N/N double-layer heterojunction photodiode
      6.2.3 Two-color nBn
6.3 Avalanche Gain HgCdTe FPAs
6.4 Type III Superlattice FPA Technologies
6.5 Summary
References

7 III-V Detectors
7.1 Photodiodes
      7.1.1 Monocolor alloy photodiodes
      7.1.2 Monocolor T2SL photodiodes
      7.1.3 Two-color T2SL photodiodes
7.2 III-V Barrier Layer Photoconductors
      7.2.1 Monocolor barrier layer photoconductor with alloy absorber
      7.2.2 Monocolor barrier layer photoconductor with T2SL absorber
      7.2.3 Two-color barrier layer photoconductor with T2SL absorber
7.3 III-V P+/N/N Double-Layer Heterojunction Photodiode
7.4 Summary
References

8 A Technology Comparison
8.1 HOT MWIR FPAs
      8.1.1 MTF
      8.1.2 NETD
      8.1.3 Operability
      8.1.4 HOT MWIR summary
8.2 HOT LWIR FPAs
      8.2.1 MTF
      8.2.2 NETD
      8.2.3 Operability
      8.2.4 HOT LWIR summary
8.3 SWIR FPAs
      8.3.1 MTF
      8.3.2 Noise equivalent irradiance (NEI)
      8.3.3 Operability
8.4 A Technology Comparison: Conclusions
References

9 The Future of Infrared FPA Technology
9.1 MTF
9.2 NETD
      9.2.1 MWIR FPAs
      9.2.2 LWIR FPA
      9.2.3 SWIR FPAs
9.3 Operability
      9.3.1 Systemic 1/f noise: surface passivation
      9.3.2 Isolated defect 1/f noise: dislocations
9.4 Summary
      9.4.1 Conclusions
      9.4.2 Issues
9.5 Room Temperature Detection: Photon versus Thermal
9.6 A Final Thought
References

Appendix A: Reverse-Biased Heterojunctions
Appendix B: Shockley–Read Bandgap States
Index

Introduction

The future of infrared focal plane array (IRFPA) technology is seemingly in a state of flux. The current third generation of IR systems utilizes large staring focal plane arrays of photodiodes fabricated with the ternary II-VI alloy HgCdTe, and the binary III-V alloy InSb as their workhorses, in formats of increasing size, complexity, and functionality. InSb has been limited to MWIR applications operating at temperatures not much in excess of 80 K. The various x compositions of the ternary alloy Hg1–xCdxTe cover the entire IR spectrum with a capability of operating at temperatures greatly in excess of 80 K. These cooled technologies are currently expensive and for many future potential applications are unattractive due to their prohibitive size, weight, and power signature. For this reason, significant paradigm shifts are required in IR technology in order to achieve the desired end goals of cost and performance, beginning with the IR focal plane array itself.

First and foremost, the drive is on to minimize the pixel pitch of the array. A reduction in the pixel pitch within the limits of diffraction for the cutoff wavelength in question will result in a direct saving in the size of the FPA, the optics, and the cooler. This enables a corresponding saving in the size, weight, and power of the IR system. An approximate rule of thumb suggests that the system size will vary approximately as the cube of the pixel pitch. Pixel pitches of 12 μm have already been reported for various hybridization technologies such as metalized vias and indium bumps. Efforts are now underway to reduce the pixel pitch down to 4 μm. To achieve the full benefit in image resolution of such a reduction in pitch will be extremely demanding on the chosen device architecture, both from a fabrication point of view and with regard to limiting potential cross-talk or modulation transfer function (MTF) issues. If the appropriate device architecture and hybridization technology can be developed to accommodate such a pitch, this development would enable a 2000 × 2000-pixel IRFPA to be fabricated on a 400 mil × 400-mil chip of IR material.

Secondly, IRFPAs are being driven to operate at significantly higher temperatures for all of the relevant spectral bands, from the LWIR band (8- to 14-μm cutoff) through the MWIR midwavelength band (3- to 5-μm cutoff) to the SWIR band (1.5- to 2.5-μm cutoff). The ultimate temperature goal is determined by the cold shield efficiency associated with the system optics and cutoff wavelength, assuming that the necessary well capacity is available in the detector unit cell to accommodate the integrated charge. The basic individual detector requirement is then simply twofold: first, that the detector dark current be less than the system background flux current and second, that the detector 1/f noise be insignificant relative to the shot noise on the background flux.

There are currently two significant schools of thought with regard to achieving these future system requirements. The schools can be divided into two broad IR materials technology categories, namely, III-V and II-VI IR semiconductors. With this in mind, it is of interest to consider the history of the evolution of these two technologies. III-V binary alloys were first compounded in the early 1950s, and InSb was among the earliest materials to be utilized for IR detection, even though it was limited specifically to the MWIR band. The perceived requirement for detection in the other main IR atmospheric window, the LWIR band, led to the development of alternative narrow-bandgap ternary alloy systems composed of II-VI and IV-VI semiconducting materials. This ternary alloy market eventually came to be dominated by the II-VI compound Hg1–xCdxTe , with a variable bandgap across the composition range from the semi-metal HgTe (x = 0) with a –0.3-eV bandgap to the semiconductor CdTe (x = 1) with a 1.5-eV bandgap. In this manner, the composition of the ternary alloy was varied to cover the complete range of the IR spectrum, from a 0.9-μm cutoff wavelength to values in excess of 20 μm.

In the period up to 1980, the IR world in the U.S. was dominated by these two materials technologies, InSb and Hg1–xCdxTe , which were developed almost exclusively within the confines of the defense industry. The first-generation Common Module, developed in 1972, was the brainchild solely of Texas Instruments, utilizing LWIR Hg0.8Cd0.2Te photoconductors configured in a 180-element linear scanning array format, on a 50-μm pitch. This technology came to dominate the IR systems business for the next 20 years and is still in production today.

In 1981, DARPA concern at the secretive and inefficient nature of the IR R&D effort within the U.S. resulted in the introduction of the U.S. Workshop on the Physics and Chemistry of II-VI Materials series, the first of which was held in Minneapolis. The next nine years saw the science of II-VI IR materials proceed at a breakneck pace because of strongly coordinated efforts between the laboratories of academia and industry. The II-VI vapor phase growth technologies of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) were developed as a direct result of this effort. Significant advances were made in the understanding of the surface science of HgCdTe. The role of native defects in II-VI semiconductors was characterized and modeled. Models were also developed to describe the electronic band structure of the ternary alloy HgCdTe. The HgTe/CdTe type III superlattice was also conceived as an alternative IR materials technology to the ternary alloy. As a consequence of all of this, in the late 1980s, the second generation of IR systems was born, based on HgCdTe with scanning and staring array formats, and InSb with a staring array format.

After a brief unsuccessful foray into surface-mode charge-coupled devices (CCDs) and charge-injection devices (CIDs), the architecture of choice for second-generation systems became the bipolar photodiode. At this point, DARPA funding of the science of HgCdTe essentially ceased, and the remainder of the U.S. Defense agencies seemingly reached the conclusion that it had invested enough in this technology. HgCdTe had proven to be a somewhat difficult material, whose successful growth and development was confined primarily to research laboratories that were vertically integrated into the confines of the larger defense contractors. This did not apparently lend itself easily to the government's desired end-goal, namely, the fabrication of inexpensive large-area staring IRFPAs with foundry-like processing. R&D funding was still available for HgCdTe FPAs, but essentially for development work, and was again provided mainly through the major prime U.S. defense contractors. IR system development reverted back to its somewhat secretive and inefficient pre-1980 mode and still operates in this manner, even to this day.

In 1990, the U.S. Government switched its research emphasis to III-V materials, as an alternative technology option to HgCdTe, to attain its stated goal of inexpensive large-area IRFPAs amenable to fabrication by the horizontal integration of material foundries and processing centers of excellence. The various III-V materials technologies that have since received government funding have all involved bandgap engineering of one form or another. This concept typically involves the fabrication of superlattices to tailor the bandgap of the material to detect the desired IR radiation. There are essentially two types of III-V superlattice, namely, type I and type II. Type I superlattices quantize electron motion in one dimension and utilize the bandgaps generated in the majority carrier band to absorb IR radiation, albeit somewhat inefficiently. Such type I superlattices are majority carrier photoconductors, with all of the trappings that entails. Quantum well infrared photoconductors (QWIPs) and quantum dots (QDOTs) are two particular manifestations of this technology. Type II superlattices (T2SLs), on the other hand, quantize carrier motion in both the conduction and valence bands of adjacent semiconductor layers, and the necessary IR bandgap is generated between the interacting levels of the conduction band of one layer and the valence band of the adjacent layer. As such, type II superlattices are minority carrier devices. Yet another topical application of bandgap engineering is the barrier layer photoconductor concept, in which a barrier is introduced to prevent current flow in the majority carrier band of an IR absorber but allows unimpeded flow in the minority carrier band. This concept has recently been applied to resurrect the performance of III-V FPAs, allowing them to operate at considerably higher temperatures than their photodiode counterparts simply by the elimination of depletion regions in the absorber volume. The absorber layers of these devices can be either a MWIR ternary alloy such as InAsSb or a LWIR T2SL.

The science of bandgap engineering has proceeded at a rapid rate. Type I superlattices, designed in-house, are fabricated in the III-V vapor phase foundries set up to support flourishing electro-optics businesses, based primarily on arsenic-based semiconductors. Type II superlattices (T2SLs), on the other hand, are based primarily on antimony-based materials, and such foundries are not as readily available, initially leading to the development of growth chambers within the confines, mainly, of government and academic laboratories. The sophisticated physics associated with the bandgap engineering concept has resulted in a great deal of interest in these technologies within academic and national laboratories. The mandatory open nature of this research in academia has allowed the science associated with bandgap engineering to flourish in much the same manner as the DARPA-led era of HgCdTe in the 1980s.

The trade-offs between these two competing III-V and II-VI IR materials technologies, with regard to the roles that they might play in the development of the ultimate in photon detection, namely, operation at room temperature, form the basis of this book. It should be pointed out that, although the majority of the physics employed herein is considered accepted practice in today's IR world, a number of the proposed models that are utilized are somewhat original and possibly controversial. It is the author's hope that these models will be held up to earnest examination in subsequent investigations and, if found wanting, will perhaps inspire the development of alternative, more exact concepts to replace them. After all, this is how real progress is made.

Michael A. Kinch
September 2014


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