Reconstruction and oxidation of the GaAs(110) surface

Abstract

Using ab initio methods (generalized valence bond with small clusters to represent the surface), we have studied reconstruction of the GaAs (110) surface and the initial steps of oxidation. The relaxation of the surface atoms (26° twist angle) is found to be in good agreement with the results of recent LEED experiments (27°±2° twist angle). The results indicate that the reconstruction is dominated by hybridization effects of orbitals localized at the surface. The calculations of chemisorbed O2 and O support the following picture: (i) Molecular O2 can bond only weakly (physisorb) to the perfect GaAs(110) surface. Chemisorption of the O2 requires a defect site which may then catalyze decomposition of the O2. (ii) There is an intermediate oxidized state at low coverages (before O is inserted in the GaAs bonds) in which O atom is chemisorbed to a surface As (donor–acceptor bond). For this intermediate oxide we calculate chemical shifts of 2.6 eV (deeper) in the 3d levels of the surface As and 0.8 eV (deeper) in the 3d levels of the surface Ga, which are in agreement with recent experiments. We propose several experiments to test various aspects of the initial stages in the oxidation process.