(001) GaAs substrate preparation for direct ZnSe heteroepitaxy

Abstract

We have investigated the influence of the (001) GaAs substrate preparation on the first stages of ZnSe heteroepitaxial growth by molecular beam epitaxy. We show that three different GaAs reconstructions occur depending on the ex situ substrate preparation, the Se residual pressure in the growth chamber and the temperature of heating. After deoxidation, an epiready substrate leads to a (2×1)-reconstructed surface at high temperature (∼600 °C) which turns into an unreconstructed surface when cooling down to the growth temperature (280 °C). An etched substrate, on the other hand, exhibits a (2×3) or a (4×3) reconstruction depending on the temperature reached during deoxidation. Both reconstructions are stable upon cooling down to the growth temperature. Direct nucleation of ZnSe on such deoxidized substrates leads to three-dimensional (3D), quasi two-dimensional (2D) and purely 2D growth modes on the unreconstructed, (2×3) and (4×3) reconstructed surfaces, respectively. Very pronounced oscillations of the reflection high-energy electron diffraction intensity are observed during nucleation on the (4×3) surface. Excellent agreement is obtained between simulated and experimental x-ray rocking curves for pseudomorphic layers grown on a (4×3) starting surface. In addition, their low-temperature photoluminescence spectra are dominated by free exciton recombinations without any defect-related line. Our results thus demonstrate that we have achieved a substantial improvement of ZnSe heteroepitaxy on bare GaAs substrates.