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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.

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.

This article describes new imaging capabilities and technologies developed for infrared focal plane arrays (FPAs) at SCD. One of the new technologies is the patterning of the back surface of the FPA, whose front surface is bonded to a silicon readout integrated circuit (ROIC). Another is the hybridization of a spectral filter to the same back surface. Increased image resolution has been achieved by using an opaque mask on the backside of the FPA with small central apertures. The reduced fill factor of the sensor leads to lower crosstalk between neighboring pixels and a higher Nyquist frequency. A highly detailed multi-mega pixel image is obtained when the sensor is micro-scanned relative to the imaging optics. Spectral filtering was achieved by hybridization of a designated filter to the backside of the FPA. The filter was glued to the FPA with high accuracy achieving single pixel resolution. System implementation of these SWIR sensor cameras has been demonstrated at imec and is reported in this paper. First results are reported for a continuously varying monolithic filter deposited onto the FPA, which has a high spectral dispersion. We report electro-optical measurements on several different sensors and describe some of their key parameters.

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.

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.

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.

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.

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.

Short wavelength Infra Red (SWIR) imaging has gained considerable interest in recent years. The main applications among others are: active imaging and LADAR, enhanced vision systems, low light level imaging and security surveillance systems. In this paper we will describe SCD's considerable efforts in this spectral region, addressing several platforms: 1. Extension of the mature InSb MWIR product line operating at 80K (cut-off wavelength of 5.4mm). 2. Extension of our new XBnn InAsSb "bariode" technology operating at 150K (cut-off of 4.1mm). 3. Development of InGaAs detectors for room temperature operation (cut-off of 1.7mm) 4. Development of a SNIR ROIC with a low noise imaging mode and unique laser-pulse detection modes. In the first section we will present our latest achievements for the cooled detectors where the SWIR region is combined with MWIR response. Preliminary results for the NIR-VIS region are presented where advanced substrate removal techniques are implemented on flip-chip hybridized focal plane arrays. In the second part we will demonstrate our VGA, 15mm pitch, InGaAs arrays with dark current density below 1.5nA/cm2 at 280K. The InGaAs array is hybridized to the SNIR ROIC, thus offering the capability of low SWaP systems with laser-pulse detection modes.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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 XBnn structure, both the active photon absorbing layer and the barrier layer are doped with electron donors, while in a p-type bariode, or XBpp 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 (MCT) devices.

Pelican-D is a new digital 640x512 / 15μm InSb detector developed by SCD to serve a number of applications. The Readout Integrated Circuit (ROIC) has a digital output which can be calibrated to a signal resolution in the 13-15 bit range. Besides the digital output, the detector has some additional advantages over other MWIR detectors of the same format. The high frequency of data output, which supports a full image frame rate of over 300Hz, is very useful in systems that track fast evolving events such as Missile Warning Systems (MWS), Missile Seekers and some Thermographic applications. Another important characteristic of the detector is related to an operation mode with relatively low readout noise. This mode of operation is especially beneficial in applications where the background radiation is low such as in long range surveillance systems. For imaging systems where very high sensitivity is required, the ROIC can be coupled to an epi-InSb detector array and have a dark current at 77K that is lower by a factor of 15 with respect to standard InSb. Alternatively, Pelican-D with epi-InSb can be operated at 95K with a standard dark current and sensitivity. Such an elevated operating temperature enables the use of cryogenic coolers of relatively low size, weight and power for applications such as Hand-held cameras, miniature gimbaled systems, and light UAVs. In this work we present in detail the characteristic performance of the new detector and its applications.

The XBnn 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 XBnn 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 n-type barrier layer (BL) and excluded from the narrow bandgap n-type active layer (AL). The band profile of the XBnn 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 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 ~4×1015cm-3. 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 ~10-6 A cm-2 and a quantum efficiency of >70% (λ

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. InAs1-xSbx 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.

Lifetimes of TPPS4 (meso-tetraphenylporphine tetrasulfonate) triplet states were measured for liquid solutions of different acidity and viscosity and as a function of acceptor concentration for different acceptors (bovine serum albumin, tryptophan and furfuryl alcohol). Triplet lifetimes were estimated by monitoring the decay of TPPS4 delayed fluorescence of E-type. The lifetime of delayed fluorencence depends on the concentration of O2, since the latter is an effective quencher of the triplet state. The lifetime is shown to be influenced mainly by degree of aggregation state of TPPS4 and, therefore, by the pH of the solution, decreasing with pH and for each pH remaining constant over a wide range of acceptor concentrations. The monomeric species is found to have the longest triplet lifetime in aqueous phosphate-buffered saline solution at neutral pH, especially when bound to albumin, despite of the low viscosity and protonated nature of the medium.

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 cur- rents 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 cut- off 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/cm 2 at 150 K. Such a device with a4 -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 madenBnnfocalplane arraydetectors(FPAs)with a 320 ×256 formatand 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. C 2011 Society of Photo-Optical Instrumentation Engineers