Structure of Ge(100) Surfaces for High-Efficiency Photovoltaic Applications

While much is known about the Ge(100) surface in a UHV/MBE environment, little has been published about this surface in an MOCVD environment. The main objective of this study is to determine the structure of the surface of Ge substrates in the typical MOCVD reactor immediately prior to and following the heteronucleation of GaAs and other lattice-matched III-V alloys, and to determine theconditions necessary for the growth of device-quality epilayers. In this paper we present the first STM images of the MOCVD-prepared Ge surfaces. Although many of the observed features are very similar to UHV- or MBE-prepared surfaces, there are distinct and important differences. For example, while the As-terminated surfaces for MBE-Ge and MOCVD-Ge are virtually identical, the AsH3-treatedsurfaces in an MOCVD reactor are quite different. The terrace reconstruction is rotated by p/2, and significant step bunching or faceting is also observed. Time-dependent RD kinetic studies also reveal, for the first time, several interesting features: the transition rate from an As-terminated (1x2) terrace reconstruction to a stable AsH3-annealed surface is a function of the substratetemperature, substrate miscut from (100) and AsH3 partial pressure, and, for typical prenucleation conditions, is relatively slow. These results explain many of the empirically derived nucleation conditions that have been devised by numerous groups.