  Ab initio self-consistent total energy calculations using second order
Moller-Plesset perturbation theory and Hay-Wadt effective core potentials with
associated basis sets (HWECP's) for gallium and arsenic have been used to
investigate the chemisorption properties of atomic aluminum on the Ga-rich
GaAs(100)-(2x1) and beta(4x2) surfaces. Finite sized hydrogen saturated
clusters with the experimental zinc-blende lattice constant of 5.654 ang. and
the energy optimized Ga dimer bond length of 2.758 ang. have been used to model
the semiconductor surface. To investigate the effects of the core electrons of
aluminum in the adsorption process, we have represented the Al adatom with both
HWECP's and an all electron 6-311++G** basis set. Detailed energetics of
chemisorption on the (100) surface layer including adsorption beneath the
surface layer at an interstitial site are investigated. Mulliken population
analysis and highest occupied molecular orbital-lowest unoccupied molecular
orbital (HOMO-LUMO) gaps are reported for all considered sites of
chemisorption.
