Inna Lukomsky

XBn barrier photodetectors based on InAsSb with high operating temperatures

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

Type II superlattice technology for LWIR detectors

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 SCDs 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.

InAs/GaSb Type II superlattice barrier devices with a low dark current and a high-quantum efficiency

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.

High operating temperature epi-InSb and XBn-InAsSb photodetectors

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 XBnn bariode technology, in which the bulk G-R current is totally suppressed. This technology includes nBnn and pBnn devices, as well as more complex structures. In all cases, the basic unit is an n-type AlSb1-yAsy / InAs1-xSbx 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 turn.

Reducing the cooling requirements of mid-wave IR detector arrays

The mid-wave IR (MWIR) band in the atmosphere is important for thermal sensing because it spans the optical wavelengths (3–5 m) at which all room-temperature objects emit significant quantities of electromagnetic radiation. Detectors that operate in this range are useful for security applications such as night vision cameras. High-performance photodiode focal plane array (FPA) MWIR detectors are usually made from the semiconductors mercury cadmium telluride (HgCdTe) or indium antimonide (InSb). However, a common limiting factor of detector performance is dark current, which arises from the thermal excitation of charge carriers across the semiconductor bandgap. Reducing dark current is key to advancing MWIR detector performance. A detector is described as operating in the diffusion limit if the dark current results from minority carriers—such as holes in an n-type semiconductor—that are excited in the photon-absorbing active layer and diffuse with Brownian-like motion to the collecting contact. A diffusion-limited current can be achieved in the best HgCdTe FPAs, which are typically grown on expensive cadmium zinc telluride substrates.1 In contrast, even the best InSb FPAs are generation-recombination (G-R) limited.2 In this limit, Shockley-Read-Hall traps3, 4—created by imperfections in the semiconductor crystal lattice—provide energy states that lie in the semiconductor bandgap. That is, they act as ’stepping stones’ for thermally excited electrons and holes to pass through. In the depletion region—a thin layer at the diode p-n junction—a built-in electric field exists, which separates the Figure 1. Indium antimonide integrated detector/cooler assembly. The gold shield (top) is the upper part of the Dewar assembly, which contains the detector. The large round engine (right) is part of the Stirling cooler.