SCD has developed a range of advanced infrared detectors based on III-V semiconductor heterostructures grown on GaSb. The XB<i>n</i>/XB<i>p</i> family of barrier detectors enables diffusion limited dark currents, comparable with MCT Rule-07, and high quantum efficiencies. This work describes some of the technical challenges that were overcome, and the ultimate performance that was finally achieved, for SCD’s new 15 μm pitch “Pelican-D LW” type II superlattice (T2SL) XB<i>p</i> array detector. This detector is the first of SCD's line of high performance two dimensional arrays working in the LWIR spectral range, and was designed with a ~9.3 micron cut-off wavelength and a format of 640 x 512 pixels. It contains InAs/GaSb and InAs/AlSb T2SLs, engineered using k • p modeling of the energy bands and photo-response. The wafers are grown by molecular beam epitaxy and are fabricated into Focal Plane Array (FPA) detectors using standard FPA processes, including wet and dry etching, indium bump hybridization, under-fill, and back-side polishing. The FPA has a quantum efficiency of nearly 50%, and operates at 77 K and F/2.7 with background limited performance. The pixel operability of the FPA is above 99% and it exhibits a stable residual non uniformity (RNU) of better than 0.04% of the dynamic range. The FPA uses a new digital read-out integrated circuit (ROIC), and the complete detector closely follows the interfaces of SCD’s MWIR Pelican-D detector. The Pelican- D LW detector is now in the final stages of qualification and transfer to production, with first prototypes already integrated into new electro-optical systems.
When incorporated into the active layer of a "XB<i>p</i>" detector structure, Type II InAs/GaSb superlattices (T2SLs) offer a high quantum efficiency (QE) and a low diffusion limited dark current, close to MCT Rule 07. Using a simulation tool that was developed to predict the QE as a function of the T2SL period dimensions and active layer stack thickness, we have designed and fabricated a new focal plane array (FPA) T2SL XB<i>p</i> detector. The detector goes by the name of "Pelican-D LW", and has a format of 640 ×512 pixels with a pitch of 15 μm. The FPA has a QE of 50% (one pass), a cut-off of ~9.5 μm, and operates at 77K with a high operability, background limited performance and good stability. It uses a new digital read-out integrated circuit, and the integrated detector cooler assembly (IDCA) closely follows the configuration of SCD’s Pelican-D MWIR detector.
Over the past few years, a new type of High Operating Temperature (HOT) photon detector has been developed at SCD, which operates in the blue part of the MWIR atmospheric window (3.4 - 4.2 μm). This window is generally more transparent than the red part of the MWIR window (4.4 - 4.9 μm), and thus is especially useful for mid and long range applications. The detector has an InAsSb active layer and is based on the new "XBn" device concept, which eliminates Generation-Recombination dark current and enables operation at temperatures of 150K or higher, while maintaining excellent image quality. Such high operating temperatures reduce the cooling requirements of Focal Plane Array (FPA) detectors dramatically, and allow the use of a smaller closed-cycle Stirling cooler. As a result, the complete Integrated Detector Cooler Assembly (IDCA) has about 60% lower power consumption and a much longer lifetime compared with IDCAs based on standard InSb detectors and coolers operating at 77K. In this work we present a new large format IDCA designed for 150K operation. The 15 μm pitch 1280×1024 FPA is based on SCD's XBn technology and digital Hercules ROIC. The FPA is housed in a robust Dewar and is integrated with Ricor's K508N Stirling cryo-cooler. The IDCA has a weight of ~750 gram and its power consumption is ~ 5.5 W at a frame rate of 100Hz. The Mean Time to Failure (MTTF) of the IDCA is more than 20,000 hours, greatly facilitating 24/7 operation.
InAs/GaSb Type II superlattices (T2SLs) are a promising III-V alternative to HgCdTe (MCT) for infrared Focal Plane Array (FPA) detectors. Over the past few years SCD has developed the modeling, growth, processing and characterization of high performance InAs/GaSb T2SL detector structures suitable for FPA fabrication. Our LWIR structures are based on an XB<sub>p</sub>p design, analogous to the XB<sub>n</sub>n design that lead to the recent launch of SCD’s InAsSb HOT MWIR detector (T<sub>OP</sub>= 150 K). The T2SL XB<sub>p</sub>p structures have a cut-off wavelength between 9.0 and 10.0 μm and are diffusion limited with a dark current at 78K that is within one order of magnitude of the MCT Rule 07 value. We demonstrate 30 μm pitch 5 × 5 test arrays with 100% operability and with a dark current activation energy that closely matches the bandgap energy measured by photoluminescence at 10 K. From the dependence of the dark current and photocurrent on mesa size we are able to determine the lateral diffusion length and quantum efficiency (QE). The QE agrees very well with the value predicted by our recently developed k · p model [Livneh et al, Phys. Rev. B86, 235311 (2012)]. The model includes a number of innovations that provide a faithful match between measured and predicted InAs/GaSb T2SL bandgaps from MWIR to LWIR, and which also allow us to treat other potential candidate systems such as the gallium free InAs/InAsSb T2SL. We will present a critical comparison of InAs/InAsSb vs. InAs/GaSb T2SLs for LWIR FPA applications.
Over the past few years, a new type of High Operating Temperature (HOT) photon detector has been developed at SCD,
which operates in the blue part of the MWIR window of the atmosphere (3.4-4.2 μm). This window is generally more
transparent than the red part of the MWIR window (4.4-4.9 μm), especially for mid and long range applications. The
detector has an InAsSb active layer, and is based on the new "XBn" device concept. We have analyzed various electrooptical
systems at different atmospheric temperatures, based on XBn-InAsSb operating at 150K and epi-InSb at 95K,
respectively, and find that the typical recognition ranges of both detector technologies are similar. Therefore, for very
many applications there is no disadvantage to using XBn-InAsSb instead of InSb. On the other hand XBn technology
confers many advantages, particularly in low Size, Weight and Power (SWaP) and in the high reliability of the cooler
and Integrated Detector Cooler Assembly (IDCA). In this work we present a new IDCA, designed for 150K operation.
The 15 μm pitch 640×512 digital FPA is housed in a robust, light-weight, miniaturised Dewar, attached to Ricor's
K562S Stirling cycle cooler. The complete IDCA has a diameter of 28 mm, length of 80 mm and weight of < 300 gm.
The total IDCA power consumption is ~ 3W at a 60Hz frame rate, including an external miniature proximity card
attached to the outside of the Dewar. We describe some of the key performance parameters of the new detector,
including its NETD, RNU and operability, pixel cross-talk, and early stage yield results from our production line.
In MWIR photodiodes made from InSb, InAs or their alloy InAs1-xSbx, the dark current is generally limited by
Generation-Recombination (G-R) processes. In order to reach a background limited operating temperature higher than
~80 K, steps must be taken to suppress this G-R current. At SCD we have adopted two main strategies. The first is to
reduce the concentration of G-R centres, by changing from an implanted InSb diode junction to a higher quality one
grown by Molecular Beam Epitaxy (MBE). Our epi-InSb diodes have a background limited performance (BLIP)
temperature of ~105 K at F/4, in 15 to 30 μm pitch Focal Plane Arrays (FPAs). This operation temperature increase
delivers a typical saving in cooling power of ~20%. In order to achieve even higher operating temperatures, we have
developed a new XB<sub>n</sub>n bariode technology, in which the bulk G-R current is totally suppressed. This technology
includes nB<sub>n</sub>n and pB<sub>n</sub>n devices, as well as more complex structures. In all cases, the basic unit is an n-type AlSb<sub>1-y</sub>As<sub>y</sub> /
InAs<sub>1-x</sub>Sb<sub>x</sub> barrier layer / photon-absorbing layer structure. These FPAs, with 15 to 30 μm pitch and a cut-off
wavelength of ~ 4.1 μm, exhibit a BLIP temperature of ~ 175K at F/3. The cooling power requirement is reduced by
~60% compared with conventional 77K operation. The operation of both our diode and bariode detectors at high
temperatures results in an improved range of solutions for various applications, especially where Size, Weight, and
Power (SWaP) are critical. Advantages include faster cool-down time and mission readiness, longer mission times, and
higher cooler reliability, as well as very low dark current and an enhanced Signal to Noise Ratio (SNR) at lower
operating temperatures. This paper discusses the system level performance for cut-off wavelengths appropriate to the
sensing materials in each detector type. Details of the radiometric parameters of each detector type are then presented in
A bariode is a new type of "diode-like" semiconductor photonic device, in which the transport of majority carriers is
blocked by a barrier in the depletion layer, while minority carriers, created thermally or by the absorption of light, are
allowed to pass freely across the device. In an n-type bariode, also known as an XB<sub>n</sub>n structure, both the active photon
absorbing layer and the barrier layer are doped with electron donors, while in a p-type bariode, or XB<sub>p</sub>p structure, they
are both doped with electron acceptors. An important advantage of bariode devices is that their dark current is
essentially diffusion limited, so that high detector operating temperatures can be achieved. In this paper we report on
MWIR n-type bariode detectors with an InAsSb active layer and an AlSbAs barrier layer, grown on either GaSb or
GaAs substrates. For both substrate types, the bariodes exhibit a bandgap wavelength of ~ 4.1 μm and operate with
Background Limited Performance (BLIP) up to at least 160K at F/3. Different members of the XBnn device family are
investigated, in which the contact layer material, "X", is changed between n-InAsSb and p-GaSb. In all cases, the
electro-optical properties of the devices are similar, showing clearly the generic nature of the bariode device
architecture. Focal Plane Array detectors have been made with a pitch of 15 or 30μm. We present radiometric
performance data and images from our Blue Fairy (320×256) and Pelican (640×512) detectors, operating at
temperatures up to 180K. We demonstrate for both GaSb and GaAs substrates that detector performance can be
achieved which is close to "Rule 07", the benchmark for high quality, diffusion limited, Mercury Cadmium Telluride
We demonstrate the suppression of the bulk generation-recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark currents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn+). This results in a significant shortening of the device cutoff wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10−7 A/cm2 at 150 K. Such a device with a 4-μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have made nBnn focal plane array detectors (FPAs) with a 320 × 256 format and a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns.
The XB<sub>n</sub><i>n</i> high operating temperature (HOT) detector project at SCD is aimed at developing a HOT (~150K) mid-wave
infrared (MWIR) detector array, based on InAsSb/AlSbAs barrier detector or "bariode" device elements. The essential
principle of the XB<sub>n</sub><i>n</i> bariode architecture is to suppress the Generation-Recombination contribution to the dark current
by ensuring that the depletion region of the device is contained inside a large bandgap <i>n</i>-type barrier layer (BL) and
excluded from the narrow bandgap <i>n</i>-type active layer (AL). The band profile of the XB<sub>n</sub><i>n</i> device leads to effective
blocking of electron transport across the BL while maintaining a free path for the holes, thus assuring a high internal
quantum efficiency (QE). Our devices exhibit a very large minority carrier lifetime (~700 ns), leading to a very low
dark current of <10<sup>-6</sup> A cm<sup>-2</sup> at 150K, which is essentially diffusion limited. We compare bariode devices with both a <i>p</i>-type
GaSb contact layer (CL) and an n-type InAsSb CL (termed C<sub>p</sub>B<sub>n</sub><i>n</i> and <i>n</i>B<sub>n</sub><i>n</i>, respectively). Apart from a ~0.3V
shift in the operating bias, the optical and electrical properties of both architectures are virtually identical,
demonstrating the generic nature of the XB<sub>n</sub><i>n</i> barrier detector family. We have fabricated FPAs from <i>n</i>B<sub>n</sub><i>n</i> bariode
arrays bonded both to a 320×256, 30 μm pitch Read-Out Integrated Circuit (ROIC) and a 640×512, 15 μm pitch ROIC.
For lattice matched FPAs the cut-off wavelength at >50% of maximum response is ~ 4.1 μm. We show an image
registered at 150K with a 640×512/15 μm Pelican FPA, using f/3.2 optics. The operability at 150K is >99.5% and the
measured NETD, limited only by shot and Read-Out noise, is 20 mK for a 22 ms integration time. At this f/number, the
detector has a background limited performance (BLIP) up to ~165K.
An XBn photovoltaic device has a band profile similar to that of a standard homojunction p-n diode, except that the
depletion region is made from a wide bandgap barrier material with a negligible valence band offset but a large
conduction band offset. In this notation, "X" stands for the n- or p-type contact layer, "B", for the n-type, wide bandgap,
barrier layer, and "n", for the n-type, narrow bandgap, active layer. In this work, we report on the fabrication of XBn
devices, which were grown by Molecular Beam Epitaxy (MBE) on GaSb substrates. Each structure has an InAsSb
active layer of thickness ~1.5μm and a 0.2-0.5μm thick AlSbAs barrier layer. Good growth uniformity was achieved
with lattice matching of better than 500ppm. Selected layers have been processed into devices which operate with a
high internal quantum efficiency at a bias of ~0.1-0.2V, and which exhibit a very low dark current due to the strong
suppression of the current component due to bulk Generation-Recombination processes. From dark current
measurements, a minority carrier lifetime of >670nS has been estimated in devices with an active layer doping of
~4×10<sup>15</sup>cm<sup>-3</sup>. In optimized, lattice matched, devices with this doping and an active layer thickness of 4μm, a cut-off
wavelength of ~ 4.0 - 4.1μm is expected at 160K, with a dark current density of ~10<sup>-6</sup> A cm<sup>-2</sup> and a quantum efficiency
of >70% (λ<4μm). These figures correspond to BLIP operation at 160K with a photocurrent to dark current ratio of ~4
Recently, a new "XB<i>n</i>" device architecture, based on heterostructures, has been proposed as an alternative to a
homojunction photodiode. The main difference is that no depletion layer exists in any narrow bandgap region of the
device. Instead, the depletion layer is confined to a wide bandgap barrier material. The Generation-Recombination (G-R)
contribution to the dark current is then almost totally suppressed and the dark current becomes diffusion limited.
This lowering of the dark current allows the device operating temperature to be raised relative to that of a standard
photodiode made from the same photon absorbing material, with essentially no loss of performance. At SCD we have
been developing XB<i>n</i> devices grown on GaSb substrates with an InAsSb photon absorbing layer and an AlSbAs barrier
layer. The results of optical and electrical measurements are presented on devices with a bandgap wavelength of about
4.1μm. Strong suppression of the G-R current is demonstrated over a range of almost two orders of magnitude in the
doping of the photon absorbing active layer (AL), while at the same time very high internal quantum efficiencies are
achieved. A model of the spectral response is developed which can reproduce the observed behaviour very well at 88K
and 150K over the whole AL doping range. In properly optimized devices, the BLIP temperature is shown to be in the
region of 160K at <i>f</i>/3.
The study of HgCdTe technology in Israel began in the mid 1970's under the leadership of the late Prof. Kidron and his
group at the Technion, Israel Institute of Technology. The R&D efforts were continued by other groups at the Technion
and other universities and research institutes in Israel, as well as by SCD. Many aspects of the technology of this material
were studied, including both bulk crystal and epitaxial growths and microelectronic fabrication methods, with an
emphasis on surface treatment and passivation. Various characterization methods were developed to study both the basic
and applied material and device properties. The efforts, reviewed in this article, matured at SCD as it commercialized the
HgCdTe technology, launching large-volume production lines of state-of-the-art linear and multi-linear TDI LWIR
detector arrays of various sizes from 10×1 to 480×6 elements. Over the years, SCD has supplied its customers with
thousands of both photoconductive (PC) and photovoltaic (PV) detectors, which are briefly presented in the paper.
Detectors composed of novel Antimonide Based Compound Semiconductor (ABCS) materials offer some unique
advantages. InAs/GaSb type II superlattices (T2SL) offer low dark currents and allow full bandgap tunability from the
MWIR to the VLWIR. InAs<sub>1-x</sub>Sb<sub>x</sub> alloys (x~0.1) also offer low dark currents and can be used to make MWIR devices
with a cut-off wavelength close to 4.2μm. Both can be grown on commercially available GaSb substrates and both can
be combined with lattice matched GaAlSbAs barrier layers to make a new type of High Operating Temperature (HOT)
detector, known as an XBn detector. In an XBn detector the Generation-Recombination (G-R) contribution to the dark
current can be suppressed, giving a lower net dark current, or allowing the same dark current to be reached at a higher
temperature than in a conventional photodiode. The ABCS program at SCD began several years ago with the
development of an epi-InSb detector whose dark current is about 15 times lower than in standard implanted devices.
This detector is now entering production. More recently we have begun developing infrared detectors based both on
T2SL and InAsSb alloy materials. Our conventional photodiodes made from T2SL materials with a cut-off wavelength
in the region of 4.6μm exhibit dark currents consistent with a BLIP temperature of ~ 120-130K at f/3. Characterization
results of the T2SL materials and diodes are presented. We have also initiated a program to validate the XBn concept
and to develop high operating temperature InAsSb XBn detectors. The crystallographic, electrical and optical properties
of the XBn materials and devices are discussed. We demonstrate a BLIP temperature of ~ 150K at f/3.
Over the past few years SCD has developed a new InAlSb diode technology based on Antimonide Based Compound Semiconductors (ABCS). In addition SCD has lead in the development of a new standard of silicon readout circuits based on digital processing. These are known as the "Sebastian" family of focal plane processors and are available in 384 × 480 and 512 × 640 formats. The combination of ABCS diode technology with digital readout capability highlights an important cornerstone of SCDs 3<sup>rd</sup> generation detector program. ABCS diode technology offers lower dark currents or higher operating temperatures in the 100K region while digital readouts provide very low noise and high immunity to external interference, combined with very high functionality. In this paper we present the current status of our ABCS-digital product development, in which the detectors are designed to provide improved performance characteristics for applications such as hand-held thermal imagers, missile seekers, airborne missile warning systems, long-range target identification and reconnaissance, etc. The most important Detector-Dewar-Cooler Assembly (DDCA) parameters are reviewed, according to each specific application. Benefits of these products include lower power consumption, lighter weight, higher signal-to-noise ratio, improved cooler reliability, faster mission readiness, longer mission times and more compact solutions for volume-critical applications. All these advantages are being offered without sacrificing the standard qualities of SCDs InSb Focal Plane Arrays (FPAs), such as excellent radiometric performance, image uniformity, high operability and soft-defect cosmetics.
Antimonide Based Compound Semiconductors (ABCS) and a new family of advanced analogue and digital silicon read-out integrated circuits form the basis of the SCD 3rd generation detector program, which builds on the firm platform of SCDs existing InSb-FPA technology. We have devised a staged roadmap at SCD which begins with epitaxial InSb mesa diodes and gradually increases in technological sophistication. In the initial stages we have focused in particular on In<sub>1-z</sub>Al<sub>z</sub>Sb alloys grown on InSb by Molecular Beam Epitaxy (MBE). Some of our achievements with these materials are presented in this paper. For epitaxial InSb (z = 0), we demonstrate the performance of Focal Plane Arrays (FPAs) with a format of 320x256 pixels, at focal plane temperatures between 77K and 110K. An operability has been achieved which is in excess of 99.5%, with a Residual Non-Uniformity (RNU) at 95K of less than 0.03% (standard deviation/dynamic range) between 15 and 80% well fill. Moreover, after a two point Non-Uniformity Correction (NUC) has been applied at 95K, the RNU remains below ~0.1% at all focal plane temperatures down to 85K and up to 100K without the need to apply any further correction. This is a major improvement in both the temperature of operation and the temperature stability compared with implanted diodes made from bulk material. We also demonstrate rapid progress in the development of epitaxial InAlSb FPAs with comparable operability and RNU to the InSb FPAs but which exhibit lower dark current and offer a range of cut-off wavelengths shorter than in InSb. These FPAs are intended for temperatures of operation in excess of 100K.
Antimonide Based Compound Semiconductors (ABCS) and a new family of advanced analogue and digital silicon read-out integrated circuits form the basis of the SCD 3<sup>rd</sup> generation detector program, which builds on the firm platform of SCDs existing InSb-FPA technology. In order to cover the MWIR atmospheric window, we recently proposed the epitaxial alloys: InAs<sub>1-y</sub> Sb<sub>y</sub> on GaSb with 0.07 < <i>y</i> < 0.11 and In<sub>1-z</sub>Al<sub>z</sub> Sb on InSb with 0 < <i>z</i> < 0.03. In this paper we focus on the results of some of our recent work on epitaxial In<sub>1-z</sub>Al<sub>z</sub> Sb grown on InSb by Molecular Beam Epitxay (MBE). In epitaxial InSb (<sub>z</sub> = 0), we demonstrate the performance of Focal Plane Arrays (FPAs) with a format of 320x256 pixels, at focal plane temperatures between 77K and 100K. An operability has been achieved which is in excess of 99.5%, with a Residual Non-Uniformity (RNU) <i>at 95K</i> of less than 0.03% (standard deviation/dynamic range). Moreover, after a two point Non-Uniformity Correction (NUC) has been applied at 95K, the RNU remains below ~0.1% at all focal plane temperatures down to 85K and up to 100K without the need to apply any further correction. This is a major improvement in both the temperature of operation and the temperature stability compared with implanted diodes made from bulk material. We also demonstrate rapid progress in the development of low current epitaxial InAlSb photodiodes with high uniformity and low dark current that offer a range of cut-off wavelengths shorter than in InSb. Preliminary results are presented on FPAs with a cut-off wavelength in the range λ<sub>C</sub>~5μ.
Over the past 27 years, SCD has developed and manufactured more than 30 types of Infrared Detector, both with support from the Israeli MOD and in cooperation with institutions and companies such as the Technion, Soreq NRC, RICOR and RAFAEL. SCD's current production line includes Hg<sub>1-x</sub>Cd<sub>x</sub>Te (MCT) devices with up to 480x6 elements operating in Time Delay and Integration (TDI) mode and InSb Focal Plane Arrays (FPAs) with up to 640x512 elements, all available in various configurations including fully integrated Detector-Dewar-Cooler (DDC) packages. Such DDCs have been designed to range from the very small to the very large. At one end the Piccolo DDC is a small, low weight and power detector, ideal for compact low cost imagers such as handheld IR cameras. At the other end, we manufacture a very long (2048x16) bi-directional TDI InSb detector designed for "whiskbroom scanning" systems. This device consists of four modules precisely butted on a single substrate, with each 512x16 module connected to a single signal processor. In 2003, SCD announced its new breakthrough Digital Read Out Integrated Circuit (ROIC) technology: Digital DDC or D<sup>3</sup>C. This readout system, with excellent performance and increased flexibility is the first in a series of new imaging solutions that SCD is developing to meet future demands of noise and power reduction, combined with greater wavelength selectivity. To continue along this path we have also been developing our new ABCS (Antimonide Based Compound Semiconductor) technology, which we first reported in 2002. The ABCS program, combining SCD's existing strengths in InSb FPA systems with new concepts in bandgap engineering and smart structure design, is aimed at multispectral IR detectors operating at higher temperatures. This review discusses some of the key trends at SCD as described above. After surveying the performance of SCD's current InSb technology, SCD's evolution towards the next generations will be described, including the achievements and potential of the D<sup>3</sup>C and ABCS systems.
We propose that the antimonide family of semiconductors should be considered in some cases as a serious alternative to Mercury Cadmium Telluride (MCT) for the active region of next generation IR detectors, based on epitaxial materials. Among the alloys, epitaxial InAs<sub>1-y</sub>Sb<sub>y</sub> on GaSb with 0.07 < y < 0.11 and In<sub>1-z</sub>Al<sub>z</sub>Sb on InSb with 0 < z < 0.03 together span important regions of the MWIR atmospheric window, yet exhibit strains of less than 0.15%. Both InSb and GaSb are binary substrates available in high quality. The sensitivity of bandgap to composition in In<sub>1-z</sub>Al<sub>z</sub>Sb is similar to that in MCT. However, in InAs<sub>1-y</sub>Sb<sub>y</sub> this sensitivity is more than halved. In growth from the gas phase, the constraints on temperature stability are about 3 - 5 times lower than in MCT. Together, these characteristics make it easier to achieve high uniformity, particularly in InAs<sub>1-y</sub>Sb<sub>y</sub>. Finally, high quality superlattices based on InAs/Ga<sub>1-x</sub>In<sub>x</sub>Sb can be grown by lattice matching to GaSb. This epitaxial material is emerging as an attractive alternative to MCT with a high degree of spatial uniformity and with an ability to span cut-off wavelengths from 3-20m in a single material system.
The transition to second generation backside-illuminated dense LWIR FPAs requires consideration of issues not previously relevant in first generation modules: unlike in front illuminated arrays, the MTF (or effective area) of a pixel is no longer close to the ideal sinc function. The cutoff wavelength, quantum efficiency and crosstalk depend on the thickness and composition grading of the epitaxial layer. The tradeoff between resolution and sensitivity demands extensive engineering and optimization of the array configuration. The transition was accomplished by comparisons of simulations with experimental results. Expectations of performance indicators, such as MTF, quantum efficiency and crosstalk were obtained by detailed Monte-Carlo simulations. The results were used to configure the focal plane array. This paper discuses the basic assumptions and simulation results and compares them with the performance of actual detectors and various test structures.
The traveling heater method (THM) is usually characterized by crystal defects such as grain boundaries and dislocations. The need for low cost HgCdTe FPA systems requires high photodiode yield. This demands understanding the crystal defect-diode relationships and necessitates a sorting method that is able to sort the as grown THM wafers before process according to the probability of achieving large photodiode arrays. This paper discusses the influence of crystals defects on photodiode performance and presents a sorting method which is under development. Oriented  THM HgCdTe crystals were grown and long wave N<SUP>+</SUP>P photodiode arrays were fabricated on the A (metal) face. It is found that individual or clusters of high current diodes which deviate drastically from their neighbors -- in current magnitude and in their slope on a Weibull distribution -- could be explained by a correspondence of excessive leakage and low angle sub-grain boundaries which cross the diode location. The distribution of single and multiple defects is compared to models based on isolated point defects and line defects. Yield implications of these results as a function of array design are described.