Influence of layer thickness on the quality of mercury cadmium telluride epilayers grown by the interdiffused multilayer process

Abstract

The interdiffused multilayer process (IMP), in which alternate submicron layers of CdTe and HgTe are grown by metal–organic vapor-phase epitaxy (MOVPE) and are allowed to interdiffuse during the deposition of subsequent layers, has been used to grow mercury cadmium telluride epitaxially on CdTe substrates. A thorough investigation by electrolyte electroreflectance (EER) shows that epilayers of 1×1 cm grown by this method have a reasonable lateral homogeneity with lateral fluctuations in the alloy composition being less than ±0.008. The homogeneity in depth of the composition of the layer is of the order of ±0.005, and the interface region is quite abrupt (less than 3000 Å in thickness). These characteristics are quite attractive and compare well with state of the art liquid-phase epitaxy (LPE) materials. In order to understand better the potential of this relatively novel growth method we have investigated the dependence of the quality of the layers on the combined thickness of the CdTe and HgTe layers, each pair of which constitutes a cycle. Materials grown from a small number of thick cycles were found to be highly strained or downright inhomogeneous. Those grown from a high number of very thin cycles also were found to be highly strained and to have relatively large fluctuations in composition with the depth of the layer. However, for intermediate cycle thicknesses of the order of 2500 Å the epilayers were found to be strain-free, and the density of polarizable defects was found to be negligible; i.e., those materials were found not to be impurity doped and to be relatively free of charged extended defects. For this reason, the best IMP materials are at this time superior to LPE materials. On the other hand, they were found to exhibit a significant amount of alloy clustering, which is not surprising given the growth method, and thus still are inferior to bulk or molecular beam epitaxy (MBE) n-type materials.