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Minority-carrier lifetimes and diffusion lengths have been deduced from a comparison of band structure simulations and experimental measurements on mid-wave infrared InAsSb XBn and long-wave infrared InAs/GaSb type II superlattice (T2SL) XBp barrier detectors with low diffusion-limited dark current close to mercury cadmium telluride Rule 07 and high quantum efficiency. For the XBn devices, a lifetime of 1.9 μs was observed with a corresponding diffusion length of 14.5 μm. In contrast, the T2SL exhibited a much shorter lifetime of 7.5 ns, but the diffusion length of ∼ 7 μm was long enough to ensure that almost 90% of the photocarriers are collected. The lifetime appears to be Auger limited in the case of n-type InAsSb, but for the p-type T2SL, Shockley–Read–Hall (SRH) traps appear to dominate. In the second case, possible scenarios for the dominance of SRH recombination are discussed to identify pathways for further performance optimization.

The minority carrier lifetime in p-type InAs/GaSb type II superlattices (T2SLs) is quite short, typically in the region of tens of nanoseconds. In spite of this, T2SLs are becoming a viable alternative to Mercury Cadmium Telluride as the sensing material of choice for high end MWIR and LWIR infrared detectors. For example, SCD now manufactures a 640 × 512 format, 15 μm pitch LWIR focal plane array detector, with a quantum efficiency close to 50%, a pixel operability of >99.5%, and a dark current only about one order of magnitude larger than the state of the art Rule 07 value. A key to the very high performance of this detector is the use of an XBp barrier architecture that both suppresses the G-R current and allows stable passivation to all steps of the fabrication process. Since both the dark-current and photo-current in the XBp structure are diffusion limited, measurements of these quantities as a function of the device dimension provide an excellent vehicle for estimating the minority carrier lifetime and diffusion length, when performed in conjunction with k⋅p calculations of the T2SL density of states. Typical lifetime results are presented, which are consistent with values found by others using direct measurements. Diffusion lengths are reported in the range 3–7 μm, although these are not necessary limiting values.

The InAs/InSb/GaSb/AlSb family of III-V alloys and superlattice materials offer unique possibilities for band structure engineering, because they can be grown on GaSb or InSb substrates with high quality and satisfactory control of strain, doping and composition. The band profiles and oscillator strengths are also quite predictable, enabling full simulation of detector performance from a basic knowledge of layer and stack thicknesses. In conventional III-V p-n devices, Shockley-Read-Hall (SRH) traps generate a significant flow of thermal carriers in the device depletion region. At SCD, we have overcome this problem by developing XBn and XBp barrier device architectures that suppress these depletion currents, leading to higher operating temperatures or lower dark currents. Our first barrier detector product was launched in 2013 and operates at 150K. It uses a mid-wave infrared (MWIR) XBn device with an InAsSb absorber well matched to the most transparent of the atmospheric windows, at wavelengths between 3 and 4.2μm. However to span the full MWIR and to sense the long-wave infrared (LWIR) spectrum, we have investigated InAs/GaSb type II superlattices (T2SLs), because they offer full tunability. In this work we show that minority carriers in n-type T2SLs are localized and diffuse by variable range hopping, even when the period is short and the valence miniband has a width of 30-40 meV. Unfortunately, this leads to sub-micron diffusion lengths and a low quantum efficiency (QE) of ~20% in a full MWIR XBn device. On the other hand, p-type layers exhibit “metallic” minority carrier transport with much longer diffusion lengths, typically ~7 μm in our LWIR device layers. The successful development of p-type devices has led to our second barrier detector product, which uses an XBp LWIR T2SL and operates at 77K with a cut-off wavelength of 9.5 μm, a focal plane array (FPA) QE of ~50% and background limited performance up to ~90K at F/3. Moreover, the FPA operability is typically above 99.5%, based on stringent production-line criteria. Together with high spatial uniformity and good temporal stability, these barrier detectors are already a realistic alternative to MCT photodiode arrays, and further products operating at other wavelengths will be launched in due course.

SCD introduced its first cooled two-dimensional (2D) Mid-Wave Infrared (MWIR) detector more than twenty years ago. The InSb Focal Plane Array (FPA) had a 30 μm pitch and approximately 80,000 pixels. We have since introduced 2D MWIR detectors with pitches of 25, 20, 15 and 10 μm and an increasing number of pixels, up to 3 M-pixel. By decreasing the pixel size, it is possible to increase their overall number in a given die size, leading to a higher resolution and opening the way for new applications such as persistent surveillance. On the other hand, maintaining the same number of pixels leads to a much smaller die size, which enables detectors to be manufactured with a lower Size, Weight, Power and Cost (SWaP-C). Reducing the pixel dimensions can also reduce the size of the optics to support lower SWaP-C at the system level. The novel XBn-InAsSb technology enables High Operating Temperature (HOT) MWIR FPA, normally operated at 150K, for lower SWaP and longer lifetime cooler, detector and system. To date, HOT MWIR detectors of several formats, with 15 and 10 μm pitch, are integrated in numerous electro-optical systems for many defense and commercial applications. Achieving even smaller pixel size while maintaining the same electro-optical performance, namely high quantum efficiency, low dark current, low cross talk, and high array uniformity, is a serious technological challenge. In this work, we present SCD's new HOT MWIR detector (CRANE) with 2560×2048 format and 5μm pitch.

SCD has developed advanced technologies for small pitch, large format and high operating temperature detector arrays, based on III–V materials and novel barrier architectures. These detectors operate with background limited performance in the Long-, Mid- and Short-wave infrared, with high uniformity and good stability. In all cases the pixel operability is above 99.5%.

HgTe/CdTe and InAs/GaSb/AlSb superlattices both exhibit a topological insulator transition. In each case, there is an inversion of the s- and p-band ordering for layer thicknesses above a critical value. The resulting topological phase is a 2D bulk insulator at zero temperature, with edges that conduct massless carriers whose direction of motion is locked to their direction of spin. These 1D edge states exhibit essentially dissipationless transport over coherence lengths greater than one micron, with a quantized conductance of e2/h per edge. When a current passes, opposite spins are separated to the two sample edges, giving rise to the so-called quantum spin Hall effect. Effects such as these may be exploited in future low temperature spintronic devices. The edge states in HgTe/CdTe differ from those in InAs/GaSb/AlSb in several ways, due to the type II band alignment and weaker electron–hole hybridization of the III-V superlattice. The former exhibit a simple exponential decay over thousands of Angstroms, while the latter are more strongly confined to the edge, with an oscillating wave function whose period increases with the edge state momentum. In any calculation, the edge state dispersion and the nature of the wave-function depend strongly on the boundary conditions used. A k · p model is presented using standard boundary conditions for the wave function and its derivative, which yields spin polarized edge states with a finite amplitude at the sample edge. The interaction between states at opposite sample edges is also considered.

Semi-Conductor Devices’ long-wave infrared 640 × 512/15-μm pitch type II superlattice detector is based on an XBp design with an InAs/GaSb absorbing layer and an InAs/AlSb barrier layer. The barrier architecture ensures a low, diffusion-limited, dark current and allows stable passivation to all fabrication steps. It is shown that the dark current is about 10 times the Rule 07 value and corresponds to a minority carrier lifetime of about 10 ns, while the quantum efficiency can approach within 10% of the HgCdTe value for realistic detector parameters. Detectors are now being manufactured with a reasonable yield for an operability above 99.5%, and a stable and reproducible noise equivalent temperature difference of

XBn or XBp barrier detectors exhibit diffusion-limited dark currents comparable with mercury cadmium telluride Rule-07 and high quantum efficiencies. In 2011, SemiConductor Devices (SCD) introduced “HOT Pelican D”, a 640 × 512/15-μm pitch InAsSb/AlSbAs XBn mid-wave infrared (MWIR) detector with a 4.2-μm cut-off and an operating temperature of ∼150 K. Its low power (∼3 W), high pixel operability (>99.5%) and long mean time to failure make HOT Pelican D a highly reliable integrated detector-cooler product with a low size, weight and power. More recently, “HOT Hercules” was launched with a 1280 × 1024/15-μm format and similar advantages. A 3-megapixel, 10-μm pitch version (“HOT Blackbird”) is currently completing development. For long-wave infrared applications, SCD’s 640 × 512/15-μm pitch “Pelican-D LW” XBp type II superlattice (T2SL) detector has a ∼9.3-μm cut-off wavelength. The detector contains InAs/GaSb and InAs/AlSb T2SLs, and is fabricated into focal plane array (FPA) detectors using standard production processes including hybridization to a digital silicon read-out integrated circuit (ROIC), glue underfill and substrate thinning. The ROIC has been designed so that the complete detector closely follows the interfaces of SCD’s MWIR Pelican-D detector family. The Pelican-D LW FPA has a quantum efficiency of ∼50%, and operates at 77 K with a pixel operability of >99% and noise equivalent temperature difference of 13 mK at 30 Hz and F/2.7.

InAs/GaSb and InAs/InAsSb type II superlattices have been proposed as promising alternatives to HgCdTe for the photon-absorbing layer of an infrared detector. When combined with a barrier layer based on an InAs/AlSb superlattice or an AlSbAs alloy, respectively, they can be used to make diffusion-limited “barrier” detectors with very low dark currents. In this work we compare theoretical simulations with experimental bandgap and photoabsorption data for such superlattices, spanning from the mid to the long-wave infra-red (2.3–12 μm). The spectral response of detectors based on these materials is also simulated. The simulations are based on a version of the k · p model developed by one of the authors, which takes interface contributions and bandgap bowing into account. Our results provide a way of assessing the relative merits of InAs/GaSb and InAs/InAsSb superlattices as potential detector materials.

For the highest end mid-wave-infrared applications, SCD, France offers a family of cryogenically cooled detectors with background limited performance (BLIP). The matured InSb planar technology is implemented in a variety of focal plane arrays, from a 320 x 256 format with a 30 µm pitch to a 1280 x 1024 format with a 15 µm pitch, all of which are operated at 77K. A major challenge is to reduce the cooling requirements. Then substantial reductions in size, weight, and power (SWaP) can be achieved by using a smaller cooler and Dewar assembly. SCD’s new epi-InSb detectors, grown by molecular beam epitaxy, have a BLIP temperature of ~100 K at F/3. This enhanced operating temperature reduces the required cooling power by ~20 % compared with the conventional 77 K operation. For a very high operating temperature, we have developed the new XBn-InAsSb detector with a 4.2 µm cut-off wavelength. This detector exhibits a BLIP temperature of ~160K at F/3 and a reduction in cooling power of ~60 %. These HOT detectors enable an improved range of solutions, including faster cool-down time and mission readiness, longer mission times, and higher cooler reliability. We can also exploit their reduced dark current to obtain an enhanced signal to noise ratio at lower operating temperatures. The well-established 25 µm pitch family of uncooled µ-Bolometer detectors has two basic formats, 384 x 288 and 640 x 480, and several sensitivity grades. The very high sensitivity 25 µm pitch detector has been demonstrated at F/2.4 for mid-range systems. The wide-band detector is optimized for both the long-wave-infrared and mid-waveinfrared spectral bands. Recently we developed the new 17 µm pitch family of detectors. The 640 x 480 format is a leading candidate for applications such as thermal weapon sights, driver vision enhancers and other mid-range IR systems. The 17 µm family is currently being expanded with the high sensitivity grade and with the addition of two new formats: the compact 384 x 288 for low SWaP applications, and the large 1024 x 768 format for applications requiring high resolution and a wide field of view.

InSb is a III–V binary semiconductor material with a bandgap wavelength of 5.4 μm at 77 K, well matched to the 3–5 μm MWIR atmospheric transmission window. When configured as a Focal Plane Array (FPA) detector, InSb photodiodes offer a large quantum efficiency, combined with excellent uniformity and high pixel operability. As such, InSb arrays exhibit good scalability and are an excellent choice for large format FPAs at a reasonable cost. The dark current is caused by Generation–Recombination (G–R) centres in the diode depletion region, and this leads to a typical operating temperature of ∼80 K in detectors with a planar implanted p–n junction. Over the last 15 years SCD has developed and manufactured a number of different 2-dimensional planar FPA formats, with pitches in the range of 15–30 μm. In recent years a new epi-InSb technology has been developed at SCD, in which the G–R contribution to the dark current is reduced. This enables InSb detector operation at 95–100 K, with equivalent performance to standard InSb at 80 K. In addition, using a new patented XBnn device architecture in which the G–R current is totally suppressed, epitaxial InAsSb detectors have been developed with a bandgap wavelength of 4.2 μm, which can operate in the 150–170 K range. In this short review of the past two decades, a number of key achievements in SCD’s InSb based detector development program are described. These include High Operating Temperature (HOT) epi-InSb FPAs, large format megapixel FPAs with high functionality using a digital Read Out Integrated Circuit (ROIC), and ultra low Size, Weight and Power (SWaP) FPAs based on the HOT XBnn architecture.

Tunnelling between a three-dimensional emitter contact and a two-dimensional electron system (2DES) is studied in magnetic fields aligned perpendicular to the barriers of a double-barrier heterostructure. The differential conductance around the Fermi energy exhibits a magnetic-field-dependent pseudogap. This pseudogap is shown to be thermally activated and to depend on the two-dimensional electron density. We attribute this pseudogap to an extra energy that an electron tunnelling from the emitter into the 2DES has to overcome as a result of the correlated state of the 2DES.

We present a model to describe the mechanisms involved in tunneling between the quasiconfined X sub-bands of the AlAs layers in GaAs/AlAs double-barrier structures at pressures up to the type-II transition. The model involves self-consistent Schrödinger-Poisson calculation of the potential profiles within the device for a given relative alignment between the two X-like quantum wells, and thus allows prediction of the bias positions at which certain resonant tunneling processes will occur. By systematic variation of the parameters involved, in particular the charge distribution between the two AlAs layers, these predictions have been fitted to the measured vertical transport characteristics of a series of samples of different AlAs layer width. In the cases of those samples with X ground states that are transverse in nature, very good agreement has been obtained. The level of insight afforded by the model opens up alternative methods for the determination of important band-structure parameters, such as the light effective mass of the X minima in AlAs, and the ΓGaAs-XAlAs conduction-band offset. It also proves to be an extremely sensitive probe of the degree of symmetry between the AlAs layer widths of near-symmetric devices, and we are thus able to measure an asymmetry of around one monolayer in each of our devices. This explains entirely the asymmetry exhibited by the low bias Xt(1) → Xt(1) resonances of some nominally symmetric double-barrier structures, which has been a source of some debate in recent years.

We report some of the highest 77 K peak to valley ratios (PVRs) for single heterojunction InAs( n )/GaSb( p ) resonant conduction diodes. The devices were grown with low background doping ( n ≈ p ≈ 10 16 cm −3 ) on (100) oriented substrates by atmospheric pressure MOVPE, and were prepared by switching the precursors in a predetermined order, to have “InSb” or “GaAs” like interfaces. We observe a stronger resonance with a weaker temperature dependence when the interface is “GaAs” like. In all samples, the voltage of the resonance and the peak current both decrease with hydrostatic pressure due to the pressure induced decrease of band overlap. Our results are consistent with a shift of −10 meV/kbar, and overlaps of 120 ± 20 meV and 250 ± 50 meV respectively for “InSb” like and “GaAs” like interfaces, and are in agreement with high pressure parallel transport results in superlattices with “InSb” like interfaces.

The growth, by MOVPE, of a range of antimonide-based material systems suitable for providing devices responsive to 2-4 mu m wavelength radiation is reported. Photodetectors with external quantum efficiencies of 60% at 2.2 mu m have been fabricated from an InGaSb homojunction. In order to examine the possibility of tuning the wavelength of emission or detection by using a strained single quantum well (SSQW) of InGaSb/GaSb this has been grown in the depletion region of a GaSb homojunction. These novel heterostructures have been grown to produce devices without the need for conventional doping. Using the crossed-gap alignment of InAs/GaSb the authors can form a diode-like structure. The most promising devices have a turn-on voltage VTO of 0.7 V (1mA), and a typical reverse voltage VR=-12 V (0.1 mA) and a best VR of -12 V (10 mu A) for a 100 mu m diameter device with some evidence of avalanche breakdown in the structure. Abrupt doping junctions have been formed between GaSb and GaAs:Si substrates. The mismatch between the layers is ameliorated by using a low-temperature buffer layer to improve the interface. Avalanche breakdown starts at -4.5 V in these structures and reverse bias currents of

We examine in detail the first high pressure resonance related to tunneling between the lowest transverse X-state in each AlAs layer, of GaAs/AlAs ‘double barrier’ structures with equal AlAs thicknesses grown by Molecular Beam Epitaxy (MBE) calibrated using Reflection High Energy Electron Diffraction (RHEED), and we compare the results with previously reported measurements on Metalorganic Vapour Phase Epitaxy (MOVPE) grown structures. The resonance, which does not conserve in-plane momentum, is much sharper and more asymmetric between forward and reverse biases in MBE structures, and the sense of the asymmetry in these structures, which can be as large as 6:1, always appears to be the same relative to the polarity of the substrate. At the same time momentum conserving resonances, such as the ambient pressure Γ-resonance, remain highly symmetric in all samples, confirming that the two AlAs layers have closely equal thicknesses. The lack of in-plane momentum conservation results in great sensitivity to differences in roughness at the two types of interface (AlAs grown on GaAs or GaAs grown on AlAs), and to differences in roughness related to the method of growth (MBE and MOVPE).

Shrinking the pixel size in advanced infrared Focal Plane Array (FPA) detectors allows either a reduction in the system size for the same number of pixels, or an increase in the pixel count for the same focal plane area. Smaller pitch and increased pixel count enables new applications such as long range surveillance, advanced Search and Track, missile warning, persistent surveillance, and infrared spectroscopy. In the last two decades SCD has followed this path of reducing the pixel size in InSb detectors for Mid-Wave Infrared (MWIR) applications, developing and manufacturing FPAs from 30μm down to 10μm pitch. The Blackbird InSb detector with 1920×1536/10μm format was introduced in 2013. Modern electro-optical systems are also designed towards a more compact, low power, and lower cost solution compared with traditional systems. In order to meet these requirements, detectors are being developed to work at Higher Operating Temperatures (HOT). In the last few years SCD has introduced 15μm pitch MWIR detectors based on the novel XBn-InAsSb technology, which enables outstanding electro-optical performance at temperatures as high as 150K. Two XBn FPA formats were developed and are now in production: 640×512/15μm and 1280×1024/15μm. Following the above trends, SCD is currently developing a 10μm XBn pixel, designed to operate at 150K with performance similar to the mature 15μm pixel. In this paper we present results from XBn FPA test devices, where the XBn array is flip-chip bonded to a Readout Integrated Circuit (ROIC) with a 10μm pitch. Test measurements in a laboratory Dewar at 150K demonstrate dark currents of 250fA, quantum efficiency greater than 70%, pixel operability of higher than 99.5%, and excellent array uniformity.

SCD has developed a range of advanced infrared detectors based on III-V semiconductor heterostructures grown on GaSb. The XBn/XBp 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) XBp 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.

XBnn mid-wave infrared (MWIR) detector arrays aimed at high operating temperature (HOT) applications, also known as barrier detectors or “bariodes”, are based on device elements with an InAsSb/AlSbAs heterostructure. There is no depletion layer in the narrow bandgap active layer of such devices, suppressing the usual Generation-Recombination (G-R) and Trap Assisted Tunneling (TAT) mechanisms for dark current that exist in standard narrow bandgap diodes. This yields lower dark currents in bariodes than in diodes with the same bandgap wavelength. InAsSb-bariode detectors, grown on lattice matched GaSb substrates have been shown previously to exhibit low dark current densities of ∼10-7 A/cm2 at 150 K. In this communication we show crystallographic and electro-optical characteristics of bariode structures grown on GaAs. Although the 7.8% mismatch causes a high density of dislocations, the devices still demonstrate electr-optical performance comparable with equivalent structures grown on GaSb, both for test devices and for focal plane array detectors (FPAs) with a 640×512 pixel format and a 15 μm pitch. This is in contrast to the behavior reported for InAsSb pin photodiodes grown on lattice mismatched substrates. The large leakage currents seen in the latter and attributed to a TAT mechanism, do not occur in the InAsSb-based bariodes grown on GaAs.

For 2D topological insulators with strong electron-hole hybridization, such as HgTe/CdTe quantum wells, the widely used 4 x 4 k.p Hamiltonian based on the first electron and heavy hole sub-bands yields an equal number of physical and spurious solutions, for both the bulk states and the edge states. For symmetric bands and zero wave vector parallel to the sample edge, the mid-gap bulk solutions are identical to the edge solutions. In all cases, the physical edge solution is exponentially localized to the boundary and has been shown previously to satisfy standard boundary conditions for the wave function and its derivative, even in the limit of an infinite wall potential. The same treatment is now extended to the case of narrow sample widths, where for each spin direction, a gap appears in the edge state dispersions. For widths greater than 200 nm, this gap is less than half of the value reported for open boundary conditions, which are called into question because they include a spurious wave function component. The gap in the edge state dispersions is also calculated for weakly hybridized quantum wells such as InAs/GaSb/AlSb. In contrast to the strongly hybridized case, the edge states at the zone center only have pure exponential character when the bands are symmetric and when the sample has certain characteristic width values., Comment: 35 pages, 15 figures

Oxide removal processes for InSb(100) and GaSb(100) substrates utilizing exposure to molecular hydrogen at moderate temperatures were studied in an experimental UHV chamber and were compared to the common method of thermal oxide desorption (TOD). Molecular hydrogen cleaning (MHC) deoxidizes the substrate surfaces effectively at temperatures of ∼100 and ∼200°C lower than that in TOD of GaSb and InSb, respectively. Closely related hydrogen based procedures for the cleaning of these materials before molecular beam epitaxial growth were developed. In these procedures a very low flux of atomic hydrogen was used in order to achieve short, yet mild, deoxidation of the substrates both with and without an Sb flux. These mild atomic hydrogen cleaning (MAHC) methods give smoother, cleaner, and more perfect InSb and GaSb surfaces after a shorter cleaning cycle at temperatures of 100–200°C lower than that in TOD. An Sb flux is not necessary in MAHC, but if used it gives a smoother substrate surface, and a well ordered...

When incorporated into the active layer of a "XBp" 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 XBp 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.

The XBn/XBp family of barrier detectors enables diffusion limited dark currents comparable with HgxCd1-xTe Rule-07 and high quantum efficiencies. SCD’s XBp type II superlattice (T2SL) detector contains InAs/GaSb and InAs/AlSb T2SLs, and was designed for the long wave infrared (LWIR) atmospheric window using k · p based modeling of the energy bands and photo-response. 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 hybridisation, under-fill, and back-side polishing. The 640 × 512 pixel, 15 μm pitch, detector goes by the name of ‘Pelican-D LW’ and exhibits a quantum efficiency of ~ 50 per cent with background limited performance at an operating temperature of 77 K. It has a cut-off wave length of ~ 9.5 μm, with a pixel operability of above 99 per cent. The detector gives a very stable image with a residual non uniformity of below 0.04 per cent over its useful dynamic range. A new digital read-out integrated circuit has been designed so that the complete detector closely follows the configuration of SCD’s MWIR Pelican-D detector.

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 XBpp design, analogous to the XBnn design that lead to the recent launch of SCD’s InAsSb HOT MWIR detector (TOP= 150 K). The T2SL XBpp 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 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.

The in-plane dispersion of the XX,Y states in a GaAs/AlAs “double-barrier structure” is measured by varying the angle of an in-plane magnetic field. When XX,Y (1) states in the emitter AlAs layer tunnel into collector X X,Y (m>1) states, a characteristic dumbbell shape is observed for the bias shift of the resonant tunneling peak versus magnetic field angle, with the major axis along [1 1 0] or [ 1 1 0] . This corresponds to an elliptical constant energy surface in the collector AlAs layer which is rotated by 45° with respect to the bulk Fermi surface. We explain the new symmetry by XX–XY interface band mixing which is closely analogous to the widely studied Γ–XZ mixing. Our results provide new insight into the microscopic origin of both types of mixing.

The real and imaginary components of the GaAs refractive index at temperatures between 20-700 degrees C have been obtained. Measurements were made by comparing the variable angle reflectivity of p-polarized and s-polarized 633 nm wavelength light from a deoxidized GaAs surface. By using these temperature-dependent optical constants for GaAs, modeling has allowed the behavior of surface photoabsorption (SPA) signals with temperature and oxide layers present to be predicted for different angles of incidence. The experimentally observed SPA signals during deoxidization of GaAs show strong qualitative agreement with these calculations at each of the angles of incidence considered. The measurement of data and application to modeling provides a platform for the measurement of temperature-dependent optical data for other III-V materials and for the investigation of deoxidation mechanisms.

We determine the exciton exchange splitting in a wide GaAs quantum well. Our method is based on applying a magnetic field parallel to the layers and measuring the oscillator strength ratio of the Zeeman split lines in two linear polarizations. We develop a theoretical model to describe the effect of the magnetic field on the exciton spectrum, and use it to determine the exchange splitting in a 22-nm quantum well to be $22\ifmmode\pm\else\textpm\fi{}3 \ensuremath{\mu}\mathrm{eV}.$ These measurements also allow us to make an accurate determination of the value of $q=0.03,$ the Luttinger parameter which appears in the cubic term of the valence band Zeeman Hamiltonian.

We have investigated the 2D 2D resonant tunneling in coupled X-band double quantum well structures, which can be achieved in GaAs/AlAs heterostructures by using high hydrostatic pressure. Until recently, there has been clear observation of 2D 2D resonant tunneling between confined transverse XX,Y states but not between longitudinal XZ states. In this paper, we demonstrate the existence of such resonant tunneling in samples with very thin well and barrier layers. The existence of detectable 2D 2D resonant tunneling between XZ states, even in a structure with a barrier thickness of 40 A, is striking. However, by modelling the transport in terms of quantum beats between symmetric and anti-symmetric double well states, we show that Γ–XZ mixing can produce enhancements of up to ≈102.

Surface photoabsorption (SPA) is an in situ technique for monitoring III–V semiconductors in an MOVPE reactor. When used in conjunction with UV absorption, it gives information about the strain associated with the growth of ideal (one monolayer) and nonideal (more than one monolayer) GaAs interfaces in InAs/GaSb superlattices. Raman scattering has been used as an ex situ technique for quantifying the strain in these superlattices. It is found that the LO frequency blue shifts by 3 cm −1 when nonideal interfaces are grown.

We have measured the effect of in-plane magnetic field on tunneling resonances between transverse X states in GaAs/AlAs double-barrier structures under high hydrostatic pressure. Current-voltage and conductance-voltage measurements performed at pressures just beyond the type-II transition, and at fields up to 15 T, reveal clear field dependences of resonances originating from the Xt(1)→Xt(1) and Xt(1)→Xt(2) tunneling processes. Their behavior is consistent with a Lorenz force analysis, and therefore probes the in-plane electron dispersion around the X minima. Differences between measurements with the magnetic field oriented parallel to the [100] and [110] crystal axes reflect the anisotropy of the X minima, a first analysis indicating that the field dependence is dominated by the two Xt minima with large wave vectors perpendicular to the magnetic-field direction. In support of this, Schrödinger-Poisson modeling of the shift in bias position of the Xt(1) →Xt(2) resonance provides a value for the effective mass parallel to the Lorenz in-plane momentum vector which is consistent with the heavy principal effective mass of the X minima.

Low-temperature photoluminescence (PL) studies of InAs self-assembled quantum dots (SAQD's) embedded in a GaAs matrix have been performed under hydrostatic pressure $P$ up to 70 kbar. A strong blueshift of the PL line from the SAQD's with $P$ up to 53 kbar changes to a relatively small redshift at higher $P.$ This is the fingerprint of a $\ensuremath{\Gamma}\ensuremath{-}X$ crossover. Above the crossover pressure, we find experimental evidence for type-II band alignment in the InAs SAQD/GaAs heterostructure system. This gives a reference point that allows us to determine independently the energies of the electron and hole levels in the QD.

We present the first identification of the direct participation of the GaAs interface (IF) mode in the phonon-assisted recombinations from type-II GaAs/AlAs short-period superlattices (SPSL). This is achieved by utilizing a novel first-order resonant Raman process associated with the type-II Xz–Γ band gap to enable a direct comparison, in the same sample, of zone boundary phonons with the photoluminescence (PL) phonon satellite energies. We present PL measurements on two complementary SPSLs to identify the reversal in the parity of the GaAs IF branch, where coupling is only allowed to the IF(+) branch. We also identify weak satellites, not previously reported, between the IF(+) and LA(X) satellites.

InAs/GaSb strained layer superlattices have been grown by atmospheric pressure MOVPE and the growth conditions optimised by observing, in real time, the in-situ UV absorption of the alkyls in the growth chamber. The Raman scattering of folded longitudinal acoustic phonons in the superlattices has been used as a probe of the periodicity of the superlattice. Atomic force microscopy has also been used to give information about the final surface morphology and RMS roughness of the superlattices. By combining all three techniques, optimum conditions have been found for the growth of short period InAs/GaSb superlattices. These have been used to sandwich a long period superlattice designed for transport measurements. The use of the short period superlattices eliminated additional conducting layers at each end of the semimetallic superlattice and produced structures where the hole and electron densities are equal. Such structures exhibit a dramatic new quantum transport effect where the Hall resistance goes to zero at high pressures and low temperatures.

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.

We have performed a pressuretuned resonance Raman scattering study in short-period GaSb/InAs SLs in the region of E1 and E1 + Δ1 in GaSb. Resonance profiles for polarized Raman scattering by both LO and elastic interface modes reveal two electronic transitions with two different types of Frohlich-interaction-induced scattering. The LO resonance at the E1 gap displays strong interference and we attribute it to intrinsic intrasubband Frohlich scattering, which is dipole allowed due to the difference in the degree of localization of the electron and hole wave functions. A new resonance enhancement peak, not present in the bulk, has been observed in between the E1 and E1 + Δ1 gaps of GaSb. This resonance appears to be dominated by extrinsic Frohlich scattering.

We present an alternative method for determining the shear deformation potential b of the valence band in III-V semiconductors. Instead of bulk semiconductors we use quantum well structures and apply uniaxial stress perpendicular to the growth direction. Using analytical solutions we show that the first confined hole state has a pronounced and characteristic nonlinear energy shift with respect to stress. This allows the hydrostatic and shear deformation potentials a and b to be determined independently from the stress dependence of the bandgap only, which can be measured by a wider range of experimental techniques than those necessary for obtaining deformation potentials from bulk material. Measurements on a (GaAs)10/(AIAs)10 superlattice yielded a value for b which is in good agreement with recent results from bulk measurements and hence support the validity of our proposed method.

The current-voltage (I-V) characteristics at 77 K are presented up to a pressure of 16 kbar for an AlAs/GaAs single-barrier structure and an AlAs/GaAs double-barrier structure behaving like a single-barrier structure. The current is first seen to rise with pressure and then fall at higher pressure close to and beyond the type I to type II transition-a signature for the involvement of the X profile. We present one model to account for this behaviour in both the low-pressure regime and the new high-pressure regime. For the latter we stress the importance of the pinning of the Fermi level in the contacts by the lowest confined X state in the AlAs region, which leads to band bending and the formation of depletion barriers in the contacts. Additionally, we contrast the behaviour of these indirect-gap barrier structures with that of a direct-gap Al0.35Ga0.64As/GaAs structure over the same pressure range, and discuss the possible origin of the falling background current in AlAs/GaAs double-barrier structures displaying X-X interwell and mixed X- Gamma resonances in the high-pressure regime.

We present results from a photoluminescence study of a GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattice between 0 and 37 kbar. At 29 kbar, a type-I to type-II transition is observed, and this transition pressure is in good agreement with the predictions of the Kronig-Penney model. In the type-II regime, we detect direct X-\ensuremath{\Gamma} recombination and have observed three distinct phonon satellites from structures having alloy layers. By studying the laser-power dependence of the photoluminescence spectra, we demonstrate that the ${\mathit{X}}_{\mathit{z}}$ and ${\mathit{X}}_{\mathit{x}\mathit{y}}$ level separation is just 8 meV, again in close agreement with theoretical predictions. Consideration of the relative intensity of the satellites shows that the strongest satellite at high laser powers is associated with the ${\mathit{X}}_{\mathit{x}\mathit{y}}$ states and overlaps three further satellites coupled to ${\mathit{X}}_{\mathit{z}}$. These spectra provide experimental evidence that the ${\mathit{X}}_{\mathit{x}\mathit{y}}$ states can have a significant effect on the optical emission from GaAs/(AlGa)As superlattices in the type-II regime, even when the conduction-band edge has ${\mathit{X}}_{\mathit{z}}$ character.

A single heterojunction of n -InAs/p-GaSb behaves as a two dimensional electron gas (2DEG) back to back with a two dimensional hole gas (2DHG), due to overlap between the GaSb valence band and the InAs conduction band. An unusual feature is the possibility of growing a ‘ light ’ GaAs or a ‘ heavy ’ InSb interface. The two interface types are predicted to give rise to different electronic and vibrational properties, while the existence of monolayer steps or interdiffusion could give rise to a mixed interface character. In this paper we demonstrate that vertical transport measurements as a function of pressure are a sensitive way of probing the electronic structure of the interface. On biasing InAs negative with respect to GaSb, electron transfer takes place from the 2DEG to the 2DHG, and the current initially increases with bias. At a critical bias, quantum confinement raises the energy of the 2DEG above that of the 2DHG and a region of Negative Differential Resistance (NDR) then occurs. The size of the NDR feature and the critical bias are both reduced by hydrostatic pressure, showing that the band overlap decreases. For a number of samples grown with a heavy interface by atmospheric pressure metalorganic vapour phase epitaxy the NDR vanishes at a threshold, P c ~ 12 ± 2 kbar. This threshold is consistent with a band overlap of 120 ± 20 meV and a shift of −10 meV/kbar. The NDR properties are found to be very sensitive to the detailed sequence of switching of the precursors during interface growth, as well as to the interface type.

When a typical double barrier GaAs AlAs structure is pressurised above the type I-type II transition at about 13 kbar, electrons are transferred from the doped GaAs emitter and collector layers into the undoped AlAs layers. This can be verified by capacitance measurements as a function of pressure and bias. In the type II regime, new X-related tunneling resonances were first reported in 1990, for 40 A barriers at 77 K, but were wrongly attributed to tunneling between longitudinal X-states. In this paper, tunnel current measurements are reported as a function of temperature between 4.2 and 150 K, and AlAs thickness between 30 and 50 A. From these measurements it can be deduced that the originally reported resonance was between the ground transverse X-states in each AlAs layer, because this resonance is suppressed at 4.2 K for barriers thinner than about 50 A. Resonant tunneling processes of two types are discussed: X 1 → X t ( n ), and X 1 (1) → X t ( n ) + P with n ≥ 1, where P represents a single or multiple phonon emission process for conservation of parallel momentum. The second type of process is distinguished from the first because it is not suppressed at 4.2 K, even for barriers thinner than 50 A, consistent with an initial longitudinal state. The suppression of the first type of process for low temperatures and thin barriers indicates that the ordering of the longitudinal and transverse states changes at AlAs thicknesses of about 50 A, due to competition between quantum confinement and biaxial strain. This process also shows an unusually strong asymmetry between forward and reverse bias for n = 1, even in structures which exhibit highly symmetric resonant tunneling characteristics at ambient pressure. It is suggested that the behaviour is related to differences in the roughness of the normal and inverted interfaces.