Type II superlattices (T2SLs) based on alternating layers of InAs and GaSb exhibit rather unique properties, including a zero band gap at a critical value of the layer thicknesses. In this respect, T2SLs bear a close relationship to the alloy, Hg1-xCdxTe (“MCT”), where the band gap vanishes at a critical value of the composition parameter, x. A 15 μm pitch T2SL Long Wave Infrared array detector has recently been developed by SCD, based on a new XBp barrier architecture and a new and robust passivation process. This detector is made entirely from III-V materials and exhibits performance comparable to high quality MCT detectors.
The SCD T2SL XBp detector contains both an InAs/GaSb active layer (AL) and an InAs/AlSb barrier layer. A k.p simulation method is described which can predict both the quantum efficiency and dark current with reasonable precision, from a basic definition of the superlattice period and the AL stack thickness. Results are compared with simulations for type I HgTe/CdTe superlattices. The method introduces a number of novel features including the use of an interface matrix, and a way of calculating the Luttinger parameters from standard reference values.
For layer thicknesses greater than the critical values, both InAs/GaSb/AlSb and HgTe/CdTe superlattices undergo a transition to a Topological Insulator (TI) phase. The TI phase exhibits unusual spin polarized transport and optical properties which may be useful in future spintronic and THz devices.
Keywords: Infrared detector, Focal plane array, Type II superlattice, XBn, XBp, Mercury cadmium telluride, Topological insulator.
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