Improved LWIR sensors are needed for defense applications. We report an advance in sensor technology based on diodes in type-II strained layer superlattice structures built in the InAs/GaSb/AlSb materials system. A key feature of the devices is a pair of complementary barriers, namely, an electron barrier and a hole barrier formed at different depths in the growth sequence. The structure is known as CBIRD. This work is a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, with design and growth being performed at JPL, and processing and testing at RVS. We have analyzed the current-voltage characteristics as functions of temperature and junction area, and have measured the spectral response and quantum efficiency as functions of bias voltage. From the temperature dependence of the dark current in a typical case, we infer that the effective barrier height is 0.175 eV. This indicates that dark current is limited by the barriers rather than diffusion or GR mechanisms occurring within the absorber region where the bandgap is 0.13 eV. The barriers prove to be very effective in suppressing the dark current. In the case of a detector having a cutoff wavelength of 9.24 mu m, we find R(0)A > 10(5) ohm cm(2) at 78 K, as compared with about 100 ohm cm(2) for an InAs/GaSb homojunction of the same cutoff. For good photo response, the device must be biased to typically -200 or -250 mV. In this condition we find the internal quantum efficiency to be greater than 50%, while the RA remains above 10(4) ohm cm(2). Thus, the device shows both high RA and good quantum efficiency at the same operating bias. We have also measured the capacitance of the CBIRD device as functions of bias and frequency to help characterize the behavior of the barriers. A 256x256 focal plane array was fabricated with this structure which showed at 78K a responsivity operability of more than 99%.

We have demonstrated the use of bulk antimonide based materials and type-II antimonide based superlattices in the development of large area mid wavelength infrared (MWIR) focal plane arrays (FPAs) as well as smaller format long wavelength infrared (LWIR) arrays. Barrier infrared photodetectors (BIRDs) and superlattice-based infrared photodetectors are expected to outperform traditional III-V MWIR and LWIR imaging technologies and are expected to offer significant advantages over II-VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow InAs/GaSb superlattice pin photodiode and bulk InAsSb structures on GaSb substrates. The coupled quantum well superlattice device offers additional control in wavelength tuning via quantum well sizes and ternary composition. Furthermore, we have fabricated mid-wavelength 1024x1024 pixels superlattice imaging FPAs, 640x512 MWIR arrays based on the BIRD concept, and 256x256 LWIR arrays based on pin superlattice structures. These initial FPA have produced excellent infrared imagery.

Long wavelength infrared (LWIR) focal plane arrays (FPAs) built on Type-II strained layer InAs/GaSb superlattice materials are emerging as an alternative to LWIR HgCdTe. We have made progress in the development of this technology in a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, resulting in successful devices with LWIR cutoff wavelengths. We report here two investigations related to wafer processing and superlattice material characteristics. The critical interface between the superlattice and the silicon dioxide passivation was examined at the atomic scale by high resolution transmission electron microscopy (HRTEM), showing a conformal coating on an InAs/GaSb mesa sidewall, which undulates with the superlattice periodicity due to differential etching. Electron energy loss spectroscopy (EELS) showed that oxides of the superlattice elements were present but minimal, and some occasional arsenic precipitates were observed at the passivation interface. Our previous analysis of the current–voltage curves was extended further to reveal the minority carrier lifetimes responsible for producing the generation–recombination (GR) and the diffusion dark currents. Lifetimes at 78 K were found to be 6 and 20 ns in the GR and diffusion processes, respectively. Lifetimes from both mechanisms track together with temperature. A HgCdTe diode was analyzed in the same manner for comparison.

We have analyzed by electron microscopy techniques the effect of the deposition of a SiO2 passivation layer on an InAs/GaSb type-II superlattice (SL) mesa with applications as a photodetector. Our images reveal good conformal coverage by the SiO2 upon an undulating edge of the SL mesa. However, we have observed scarce As clusters at the interface between the SL mesa and the passivation layer and some degree of oxidation of the mesa sidewall. The strong reduction in surface leakage currents demonstrates that the observed imperfections do not have a substantial detrimental effect on the passivation capabilities of the SiO2 layer.

Advanced LADAR receivers enable high accuracy identification of targets at ranges beyond standard EOIR sensors. Increased sensitivity of these receivers will enable reductions in laser power, hence more affordable, smaller sensors as well as much longer range of detection. Raytheon has made a recent breakthrough in LADAR architecture by combining very low noise similar to 30 electron front end amplifiers with moderate gain >60 Avalanche Photodiodes. The combination of these enables detection of laser pulse returns containing as few as one photon up to 1000s of photons. Because a lower APD gain is utilized the sensor operation differs dramatically from traditional "Geiger mode APD" LADARs. Linear mode photon counting LADAR offers advantages including: determination of intensity as well as time of arrival, nanosecond recovery times and discrimination between radiation events and signals. In our talk we will present an update of this development work: the basic amplifier and APD component performance, the front end architecture, the demonstration of single photon detection using a simple 4 x 4 SCA and the design and evaluation of critical components of a fully integrated 8x8 linear mode photon counting camera under development in support of the Ultra-Sensitive Detector (USD) program sponsored by AFRL-Kirtland.

We report on the status of focal plane arrays (FPAs) based on GaSb/InAs type-II superlattice diodes grown by molecular beam epitaxy (MBE) and designed for infrared absorption in the 2-5μm and 8-10μm bands. Recent LWIR devices have produced differential resistance-area product greater than 100 Ohmcm2 at 80K with a long wavelength cutoff of approximately 10μm. The measured quantum efficiency of these front-side illuminated devices is close to 25% in the 8-9 μm range. MWIR devices have produced detectivities as high as 8x1013 Jones with a differential resistance-area product greater than 3x107 Ohmcm2 at 80K with a long wavelength cutoff of approximately 3.7μm. The measured quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2-3μm range at low temperature and increases to over 60% near room temperature. Initial results on SiO2 and epitaxial-regrowth based passivation techniques are also presented, as well as images from the first lot of 1kx1k MWIR arrays and our latest 256x256 LWIR arrays.

We report progress in the development of long wavelength infrared (LWIR) focal plane arrays (FPAs) built on type-II strained layer InAs/GaSb superlattice materials. Work at Raytheon Vision Systems and Jet Propulsion Laboratory has led to successful devices with cutoff wavelengths in the 10 to 12 μm range. Pixels have been formed by wet etching and surface passivation by plasma-deposited silicon dioxide. We present test results on arrays hybridized with indium bump bonding to silicon readout integrated circuits, as well as analyses of current-voltage characteristics of individual diodes. In particular, we find that, at temperatures below about 70 K the leakage current is dominated by generation-recombination effects near zero bias and by trap-assisted tunneling in reverse bias. Although other authors have demonstrated imaging for SWIR and MWIR type-II superlattice devices, to our knowledge no one has done so prior to 2006 in the LWIR range. We have obtained both still and video imaging with 256×256 arrays with 30-μm pixels operating at 78 K, having high operability and a cutoff wavelength of 10.5 μm.

Raytheon is developing HgCdTe APD arrays and sensor chip assemblies (SCAs) for scanning and staring LADAR systems. The nonlinear characteristics of APDs operating in moderate gain mode place severe requirements on layer thickness and doping uniformity as well as defect density. MBE based HgCdTe APD arrays, engineered for high performance, meet the stringent requirements of low defects, excellent uniformity and reproducibility. In situ controls for alloy composition and substrate temperature have been implemented at HRL, LLC and Raytheon Vision Systems and enable consistent run to run results. The novel epitaxial designed using separate absorption-multiplication (SAM) architectures enables the realization of the unique advantages of HgCdTe including: tunable wavelength, low-noise, high-fill factor, low-crosstalk, and ambient operation. Focal planes built by integrating MBE detectors arrays processed in a 2 x 128 format have been integrated with 2 x 128 scanning ROIC designed. The ROIC reports both range and intensity and can detect multiple laser returns with each pixel autonomously reporting the return. FPAs show exceptionally good bias uniformity <1% at an average gain of 10. Recent breakthrough in device design has resulted in APDs operating at 300K with essentially no excess noise to gains in excess of 100, low NEP <1nW and GHz bandwidth. 3D LADAR sensors utilizing these FPAs have been integrated and demonstrated both at Raytheon Missile Systems and Naval Air Warfare Center Weapons Division at China Lake. Excellent spatial and range resolution has been achieved with 3D imagery demonstrated both at short range and long range. Ongoing development under an Air Force Sponsored MANTECH program of high performance HgCdTe MBE APDs grown on large silicon wafers promise significant FPA cost reduction both by increasing the number of arrays on a given wafer and enabling automated processing.

We report on the status of GaSb/InAs type-II superlattice diodes grown by molecular beam epitaxy (MBE) and designed for infrared absorption in the 2-5 mu m and 8-12 mu m bands. Recent LWIR devices have produced detectivities as high as 8x10(10) Jones with a differential resistance-area product greater than 6 Ohmcm(2) at 80K with a long wavelength cutoff of approximately 12 mu m. The measured quantum efficiency of these front-side illuminated devices is close to 30% in the 10-11 mu m range. MWIR devices have produced detectivities as high as 8x10(13) Jones with a differential resistance-area product greater than 3x10(7) Ohmcm(2) at 80K with a long wavelength cutoff of approximately 3.7 mu m. The measured quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2-3 mu m range at low temperature and increases to over 60% near room temperature. Initial results on SiO2 and epitaxial-regrowth based passivation techniques are also presented, as well as images from the first lot of LWIR arrays.

We report on progress in the development of a device fabrication process for type-II strained layer superlattice IR detectors, composed of InAs/GaSb or InAs/GaInSb. Steps of the process include etching the mesas, cleaning up the surface, and applying a surface passivation treatment. Certain etchants have been evaluated and calibrated. The surface has been studied with single wavelength ellipsometry and results have been compared with modeled ellipsometry values, revealing effects of surface residues and surface roughness. An initial investigation of ammonium sulfide treatment for surface passivation has been made. Initial measurements of the IR transmission of the GaSb substrate have also been made to determine how much thinning is needed for back side illuminated operation of the IR detectors.

We present current versus voltage characteristics for two metal-insulator-semiconductor systems: lead-insulator-mercury cadmium telluride and silver-insulator-silicon. A previously developed new technique allows us to suspend the metal <20 Å above the semiconductor so that the insulator is simply the space between them. The Si studies at room temperature show features suggestive of electron tunneling between the metal and semiconductor, while the HgCdTe studies in liquid helium conclusively show the Pb superconducting energy gap, demonstrating that contactless tunneling studies of semiconductor surfaces are possible.

An adjustable metal-air-semiconductor capacitor was fabricated using a Pb disk suspended 1700–3600 Å above an n-type Si 〈111〉 surface. Experimental differential capacitance versus voltage and differential conductance versus voltage curves are similar to those previously obtained for metal-oxide-semiconductor capacitors.