Dislocation reduction in LWIR HgCdTe epitaxial layers grown on alternate substrates
Mercury cadmium telluride (HgCdTe) is used in ultrahigh performance infrared (IR) detectors and focal plane arrays (FPAs) used by both the DoD and NASA. This material has been shown to be unique in many ways in comparison to other IR materials. For example, it is possible to scan over the entire infrared spectrum (i.e. LWIR/MWIR/SWIR) by controlling the growth composition of HgCdTe, which is accomplished via an atomically controlled method called Molecular Beam Epitaxy (MBE). MBE of HgCdTe on silicon substrates has been used to routinely grow complex structures, including multi-junction heterostructures and superlattices. However, while the growth of HgCdTe/Si is more-or-less under control for SWIR/MWIR, LWIR devices are another story. This inconsistency, in general, comes from the fact that unlike SWIR/MWIR HgCdTe/Si devices which can operate with a relatively large amount of dislocations, the LWIR devices are strongly affected by dislocations. These dislocations are attributed to the large lattice mismatch (~19%) and difference in thermal expansion coefficients between the silicon substrate and the HgCdTe layer which creates strain and ultimately large defect densities. Here, we develop an innovative process to reduce the number of defects on HgCdTe/Si - leading to better HgCdTe device performance, especially in the LWIR region.
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