Characterization of mercury cadmium telluride/cadmium zinc telluride(211)B heterostructures and mercury telluride/mercury cadmium telluride superlattices.

摘要

In this dissertation, microstructural investigations on different types of defects in Hg_1−xCd_xTe grown by molecular beam epitaxy (MBE) are described. MBE growth of HgCdTe is not straightforward. Hg_1−xCd_xTe epilayers usually suffer from various different types of defects depending on the growth conditions. Transmission electron microscopy (TEM) is ideal to study these defects. However, it has not been fully utilized, presumably due to the difficulties of preparing high-quality TEM samples. In this research, TEM sample preparation procedures were first established, and then extensive TEM and high-resolution electron microscopy (HREM) as well as small-probe microanalysis have been carried out to investigate the microstructure and origin of major defects in MBE-grown HgCdTe alloys. First, surface defects, such as surface crater defects, surface ridges, and surface volcanos, were studied. High-resolution observations revealed the origin and microstructure of surface crater defects. The microstructure of surface ridges and volcanos was studied by cross-sectional TEM. The surface ridges appeared to be caused by faceting of HgCdTe, leading to faceting of the CdTe capping layer and twin formation within the layer. The surface volcanos had threading defects underneath, but these were confirmed to nucleate within the HgCdTe epilayer. Second, growth defects such as low-temperature (Hg-rich) structural defects, epilayer/substrate interface dislocations, and precipitates were studied. It was found that MBE growth lower than about 180°C increased dislocation and loop densities. The microstructure of HgCdTe/CdZnTe(211)B and the effect of the CdTe buffer layer were studied and it was shown that growth conditions had more impact on the interface microstructure than lattice mismatch due to the CdTe buffer layer. The microstructure of precipitates and their surrounding strain field were observed on the atomic scale. Small-probe analysis showed that the precipitates appeared to be slightly Hg-deficient, although the precise nature of the precipitates is still unclear. The microstructure of HgTe/HgCdTe superlattices (SLs) and the effects of annealing were studied and correlated with optical and electric properties.