(001)GAAS SUBSTRATE PREPARATION FOR DIRECT ZNSE HETEROEPITAXY

We have investigated the influence of the (001) GaAs substrate prepara tion on the first stages of ZnSe heteroepitaxial growth by molecular b eam epitaxy. We show that three different GaAs reconstructions occur d epending on the ex situ substrate preparation, the Se residual pressur e in the growth chamber and the temperature of heating. After deoxidat ion, an epiready substrate leads to a (2 x 1)-reconstructed surface at high temperature (-600 degrees C) which turns into an unreconstructed surface when cooling down to the growth temperature (280 degrees C). An etched substrate, on the other hand, exhibits a (2 x 3) or a (4 x 3 ) reconstruction depending on the temperature reached during deoxidati on. Both reconstructions are stable upon cooling down to the growth te mperature. Direct nucleation of ZnSe on such deoxidized substrates lea ds to three-dimensional (3D), quasi two-dimensional (2D) and purely 2D growth modes on the unreconstructed, (2 x 3) and (4 x 3) reconstructe d surfaces, respectively. Very pronounced oscillations of the reflecti on high-energy electron diffraction intensity are observed during nucl eation on the (4 x 3) surface. Excellent agreement is obtained between simulated and experimental x-ray rocking curves for pseudomorphic lay ers grown on a (4 x 3) starting surface. In addition, their low-temper ature photoluminescence spectra are dominated by free exciton recombin ations without any defect-related line. Our results thus demonstrate t hat we have achieved a substantial improvement of ZnSe heteroepitaxy o n bare GaAs substrates. (C) 1997 American Institute of Physics.