  Ab initio self-consistent total energy calculations using second order
Moller-Plesset perturbation theory and Hay-Wadt effective core potentials for
gallium and arsenic have been used to investigate the chemisorption of atomic
oxygen on the Ga-rich GaAs (100) (2 x 1) and beta(4 x 2) surfaces. Finite sized
hydrogen saturated clusters with the experimental zinc-blende lattice constant
of 5.654 angstroms and the energy optimized surface Ga dimer bond length of
2.758 angstroms have been used to model the semiconductor surface. We present
the energetics of chemisorption on the (100) surface layer including adsorption
beneath the surface layer at two interstitial sites. Chemisorption energies,
nearest surface neighbor Ga-O bond lengths, and homo-lumo gaps are reported for
all considered sites of chemisorption and compared with published results in
the literature on O adsorption on the GaAs surface. Results are also compared
with our previous results on hydrogen chemisorption on the same GaAs surface.
Possibilities of transition of the surface from a semi-conducting state to a
semi-insulating state are also discussed.
