Gregory Brill

High-Performance Long-Wavelength Infrared HgCdTe Focal Plane Arrays Fabricated on CdSeTe Compliant Si Substrates

At the U.S. Army Research Laboratory, a new ternary semiconductor system CdSe_xTe_1-x,/Si(211) is being investigated as an alternative substrate to bulk-grown CdZnTe substrates for HgCdTe growth by molecular beam epitaxy. Long-wavelength (LW) photovoltaic devices fabricated on this compliant substrate material show diffusion limited performance at 78 K, indicating a high-quality material. The measured R₀A at 78 K on ¿_co = 10 ¿m material is on the order of 340 ¿ · cm². In addition to single devices, we have fabricated 256 × 256 2-D arrays with a 40-¿m pixel pitch on LW-HgCdTe grown on CdSe_xTe_1-x/Si(211) compliant substrates. The data show an excellent quantum efficiency operability of 99% at 78 K under a tactical background flux of 6.7 × 10¹⁵ ph/cm²s. The most probable dark current at peak distribution is 5.5 × 10⁹ e-/s and is very consistent with the measured R₀A values from single devices. This work demonstrates that CdSe_xTe_1-x/Si(211) substrates provide a potential roadmap for more affordable robust third-generation focal plane arrays.

Development of high performance radiation hardened antireflection coatings for LWIR and multicolor IR focal plane arrays

High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation. In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures. We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.

Pronounced Auger suppression in long wavelength HgCdTe devices grown by molecular beam epitaxy

Intrinsic carriers play a dominant role especially in the long wavelength (8-12 μm cut-off) HgCdTe material near ambient temperatures due to high thermal generation of carriers. This results in low minority carrier lifetimes caused by Auger recombination processes. Consequently, this low lifetime at high temperatures results in high dark currents and subsequently high noise. Cooling is one means of reducing this type of detector noise. However, the challenge is to design photon detectors to achieve background limited performance (BLIP) at the highest possible operating temperature; with the greatest desire being close to ambient temperature operation. We have demonstrated a unique planar device architecture using a novel approach in obtaining low arsenic doping concentrations in HgCdTe. Results indicate Auger suppression in P+/π/N+ devices at 300K and have obtained saturation current densities of the order of 3 milli Amps-cm2 on these devices.

Comparison of the Schaake and Benson Etches to Delineate Dislocations in HgCdTe Layers

The morphology and classification of etch pits in molecular beam epitaxy-grown (211) HgCdTe/CdTe/Si layers were investigated using the Schaake and Benson etch pit density (EPD) etches. The two EPD etches were compared and shown to have a 1:1 correlation in the etch pits that were produced. Close examination of the shape of the etch pits via scanning electron microscopy shows that several distinguishable classifications of etch pits are revealed using both etches. Samples subjected to thermal cycle annealing (TCA) treatment show a nonuniform reduction in etch pit populations according to the classification defined in this study. In particular, a class of etch pits called “fish shaped” are completely absent after TCA and can account for up to one-third of the total reduction in EPD.

Molecular beam epitaxy grown long wavelength infrared HgCdTe on compliant Si substrates

At the Army Research Laboratory (ARL), a new ternary semiconductor system CdSexTe1-x/Si(211) is being investigated as an alternative substrate to bulk-grown CdZnTe substrates for HgCdTe growth by molecular beam epitaxy. Under optimized conditions, best layers show surface defect density less than 400 cm-2 and full width at half maximum of X-ray double crystal rocking curve as low as 100 arc-sec with excellent uniformity over 3 inch area. LW-HgCdTe layers on these compliant substrates exhibit comparable electrical properties to those grown on bulk CZT substrates. Photovoltaic devices fabricated on these LWIR material shows diffusion limited performance at 78K indicating high quality material. Measured RoA at 78K on λco = 10 μm material is on the order of 340 Ω-cm2. In addition to single devices, we have fabricated 256x256 2-D arrays with 40 μm pixel pitch on LW-HgCdTe grown on Si compliant substrates. Data shows excellent QE operability of 99% at 78K under a tactical background flux of 6.7x1015 ph/cm2sec. Most probable dark current at the peak distribution is 5.5 x 109 e-/sec and is very much consistent with the measured RoA values from single devices. Initial results indicate NETD of 33 mK for a cut-off wavelength of 10 μm with 40 micron pixels size. This work demonstrates CdSexTe1-x/Si(211) substrates provides a potential road map to more affordable, robust 3rd generation FPAs.

Mercury cadmium selenide for infrared detection

Samples of HgCdSe alloys were grown via molecular beam epitaxy on thick ZnTe buffer layers on Si substrates. Two Se sources were used: an effusion cell loaded with 5N source material that produced a predominantly Se6 beam and a cracker loaded with 6N material that could produce a predominantly Se2 beam. The background electron concentration in as-grown samples was significantly reduced by switching to the Se cracker source, going from 1017–1018 cm−3 to 3–5 × 1016 cm−3 at 12 K. The concentration remained low even when the cracking zone temperature was lowered to produce a predominantly Se6 beam, which strongly suggests that a major source of donor defects is impurities from the Se source material rather than Se species. Secondary ion mass spectroscopy was performed. Likely donors such as F, Br, and Cl were detected at the ZnTe interface while C, O, and Si were found at the interface and in the top 1.5 μm from the surface in all samples measured. The electron concentration for all samples increased when ann...

Microstructural Characterization of HgCdSe Grown by Molecular Beam Epitaxy on ZnTe/Si(112) and GaSb(112) Substrates

Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure of HgCdSe thin films grown by molecular beam epitaxy on ZnTe/Si(112) and GaSb(112) substrates. The quality of the HgCdSe material was dependent on the growth temperature and materials flux, independent of the substrate. Samples grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and increased dislocation densities. Improved preparation of the GaSb buffer layer should be developed for future HgCdSe growth on GaSb(112) substrates.

Design and development of high-performance radiation-hardened antireflection coatings for LWIR HgCdTe focal plane arrays

High Performance LWIR Focal Plane Arrays are critical for many space applications. Reliable LWIR focal plane arrays are needed for these applications that can operate in space environment without any degradation. In this paper, we present various LWIR detector array architectures currently being evaluated for LWIR applications. These include backside-illuminated configurations for HgCdTe fabricated on CdZnTe and Silicon substrates. To optimize the LWIR device performance, minimize the anti-reflection losses, and significant reduction in the effects of solarization in space, innovative Anti-reflection coatings are needed, that will enhance the performance of the LWIR detector / focal plane arrays. We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 8-14 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.

Comparison of cathodoluminescence and photoluminescence of CdSeTe films grown on Si by molecular beam epitaxy

For the first time, cathodoluminescence of CdSexTe1-x (with x = 0-1) films grown by molecular beam epitaxy on (211) Si substrates were systematically studied and compared with photoluminescence. The Se mole fraction was consistently determined by x-ray rocking-curve diffraction, wavelength-dispersive spectroscopy, and Rutherford backscattering. The band gap energy, as determined by both cathodoluminescence and photoluminescence, was found consistent with literature. The band gap energy varied parabolically with composition as predicted by theory. The results suggest cathodoluminescence can be used to conveniently map composition fluctuations such as Se segregation in CdSexTe1-x films, with higher spatial resolution than photoluminescence.

MBE growth of ZnTe and HgCdSe on Si: a new IR material

Growth of ZnTe and HgCdSe on Si has been pursued using molecular beam epitaxy (MBE) as a new class of IR materials. Besides, ZnTe/Si can also be used as a lattice-matching, large area and low cost alternate substrate for other III-V and II-VI compound semiconductors, such as GaSb based type-II superlattice materials around 6.1A. We report in this paper our systematic studies on MBE growth conditions for ZnTe(211) on Si and highlights of MBE growth of HgCdSe on ZnTe/Si. A close to optimal growth window has been established for MBE growth of ZnTe(211)/Si(211) to achieve high crystalline quality, low defect and dislocation densities as well as excellent surface morphology. Using this baseline MBE growth process, we are able to obtain ZnTe(211)/Si wafers with X-ray full-width at half-maximum (FWHM) as low as 70 arcsec, low dislocation density (~105 cm-2) and defect density (1000 cm-2).