Daniel Aronov

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

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.