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This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.

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Arrays of x-ray microcalorimeters will enable broadband, high-resolution x-ray spectroscopy to study and substantiate black holes, dark matter, and other celestial phenomenon. At EPIR we continue to achieve growth of high-quality, low-doped, single-crystal HgCdTe, and HgTe epilayers on Si and CdZnTe to be employed by NASA in these instruments. Excellent low-temperature heat capacities (with no significant electronic term) have been demonstrated in integrated devices, with both HgTe and HgCdTe showing improvement over the HgTe used previously. Goal resolutions £4 eV have been achieved with good yield for both HgTe and HgCdTe.

We report the development of high performance low cost SWIR infrared detectors from MBEgrown HgCdTe on 3-inch CdTe-buffered silicon substrates. The experimental findings demonstrate that despite the large lattice mismatch between HgCdTe and Si substrate, the materials and detector performances are sufficiently better than those reported for III-V mixed crystals. High minority carrier lifetime of the order 3 µs at room temperature was measured on the as grown material. Photodetectors fabricated from this material produced low dark current densities on the order of 10 -6 A/cm 2 and 10 -3 A/cm 2 at 200K and 300K. Quantum efficiency exceeding 70% at 2.0 µm, without antireflective coating, was measured on single element detectors. Further, 320 X 256, 30 µm pitch FPA's have been fabricated with this HgCdTe on Si material and dark current operability of ~ 99.5% (mean dark current of 30 pA/Pixel) at 200K has been demonstrated.HgCdTe is an attractive material that historically has been of interest primarily as the absorber material in photovoltaic detector applications at mid and long wavelengths. Recently there is growing interest in using HgCdTe for shorter wavelengths, because it provides the natural ability to tune the bandgap by adjusting the alloy composition for a broad range of wavelengths. So far, most research and development on NIR and SWIR imaging has focused on ternary and quaternary III-V mixed crystals such as InGaAsP, InGaAs/InP, and AlGaAsSb. However, III-V materials exhibit degraded performance for cutoff wavelengths longer than 1.7 µm due to an increasing lattice mismatch with the growth substrate 1,2,3 . HgCdTe on the other hand maintains a nearly constant lattice parameter over the entire range of alloy composition. This study investigates the application of HgCdTe materials grown on Si substrates for extended SWIR band operation beyond 1.7μm. The bandgap of HgCdTe material can be tuned throughout the SWIR 4 , MWIR 5 and LWIR 6 spectral regions with negligible change in the lattice parameter. The challenge in designing a HgCdTe photodetector is optimizing cutoff wavelength so that it is long enough to utilize the atmospheric transmission windows of interest, while keeping it short enough to achieve low dark currents at the desired operating temperature.

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CdZnTe-based heterojunction p-i-n or M-p-n detectors using HgTe/HgCdTe superlattice contacts are modeled and designed to reduce leakage currents under high electric fields and thereby improve x-ray and c-ray detector performance. The employment of an n-type HgTe/HgCdTe superlattice as a contact layer can theoretically result in significantly less leakage current compared with a contact layer using either bulk semiconductor or metal contacts. The benefits arise from the ability to design HgTe/HgCdTe superlattices to have large carrier effective masses in the electric field direction, which results in low carrier velocities. Nevertheless the density of states is lower than that of a comparable bulk semiconductor, which results in low carrier concentrations.