MICROSTRUCTURAL EVALUATION OF STRAINED MULTILAYER INASSB/INSB INFRARED DETECTORS BY TRANSMISSION ELECTRON-MICROSCOPY

InSb/InAsSb strained layer superlattices (SLS) were grown on (001) InSb substrates by molecular beam epitaxy at 425-degrees-C. The active device consisted of an InAs0.15Sb0.85/InSb superlattice region embedded within a p-i-n junction. The large lattice mismatch between the active device and the substrate required the growth of a buffer. InAs0.15Sb0.85/InSb SLS, where the average As content was gradually increased, was used as a buffer. The buffer structure was varied to probe its microstructural effect on the capping device. Three distinct approaches (A, B, and C) were used to grow the buffer. Approach A was a four-step buffer where the average content of As in the superlattice was increased in four equal composition steps. This approach led to a crystal with an extensive network of threading dislocations and microcracks. Approach B was to change the average composition in five equal composition steps, thereby decreasing the misfit at the interfaces between composition steps. This led to a decrease in the threading dislocation density but microscopic cracks were still evident. The last approach (C) was to employ migration enhanced epitaxy (MEE) for the growth of the five-step buffer. Samples grown by employing MEE revealed no microcracks but they contained a high density of unusual ''wiggly'' dislocations at the buffer/device interface. Detailed microstructural analysis by transmission electron microscopy is presented.