The potential for abrupt interfaces in Cd xHg 1− xTe using thermal and photo-MOVPE

Abstract

This paper reviews the current progress in the growth of abrupt structures in the infrared detector alloy Cd x Hg 1− x Te, with special reference to metalorganic vapour phase epitaxy (MOVPE). Recent results on the growth of heterostructures using the interdiffused multilayer process (IMP) are described for epitaxy onto GaAs as well as CdTe substrates. It is envisaged that useful heterostructure devices can be grown where the interface widths are of the order of 0.3 μm. For more abrupt structures, lower growth temperatures are needed and this has been demonstrated using the new photolytic MOVPE process. Epitaxial growth at temperatures as low as 200°C has enabled measured interface widths of approximately 100 Å to be realised for a HgTe/CdTe structure. Lower growth temperatures also reduce the rates of diffusion of dopants such as Ga from a GaAs substrate. Ga concentrations of just 0.05 ppma have been measured 500 Å from a CdTe/GaAs interface. Detailed Hall measurements on photo-MOVPE HgTe and HgTe/CdTe structures have shown that high quality epitaxial layers can be grown. A study of the limitations on the electrical quality has shown some influence of impurity diffusion in very thin layers but for thicker layers ( > 1 μm) this has little effect. The possibility of using photo-MOVPE for the growth of CdTe/HgTe superlattices has been explored by predicting the extent of interdiffusion at 200°C and 150°C. Even at 150°C, the predicted interdiffusion in just 10 min is significant. However, interdiffusion may depend critically on the dislocation structure and strain at the interface. Structural studies on thin epitaxial layers shows the critical dependence of structure, and strain, on the substrate orientation and layer thickness.