Oxide films grown on GaAs(110) at 20 K: Stability during Cr overlayer formation

Abstract

GaAs oxide films were formed by x-ray illumination of condensed O2 on GaAs(110) at 20 K, and their stability was investigated during subsequent Cr overlayer formation with atoms at 20 and 300 K and with preformed clusters. High resolution synchrotron radiation photoemission results for overlayer formation at 300 K show that Cr atoms reduce the GaAs oxides to form Cr oxides, Cr—As, and Cr—Ga bonding configurations. These reactions first involve the As2O5-like species and then progress to the As2O3-like and Ga2O3-like oxides, reflecting the relative stability of the oxides and their spatial distributions. For overlayer formation at 20 K, the Cr atoms initially reduced the As-oxide surface layers but had little effect on the Ga oxides, and a nonuniform metallic overlayer was formed after 1.5 Å Cr deposition. These low-temperature results demonstrate that kinetic constraints imposed at 20-K limit Cr diffusion and, hence, oxide reduction. In contrast, the deposition of Cr clusters onto oxidized GaAs produced a metallic layer with minimal oxide disruption. This different final state can be understood by noting that the bonding of Cr atoms in metallic Cr clusters establishes an activation barrier for reaction that is not present for atom deposition.