Ab initio cluster calculations of hydrogenated GaAs(001) surfaces

Hydrogen adsorption on the (2x4) and (4x2) reconstructions of gallium arsen ide (001) has been studied by internal reflectance infrared spectroscopy an d ab initio cluster calculations with density-functional theory. The calcul ations are made on Ga5As4H11,13, Ga4As5H11,13, and Ga7As8H19 clusters, whic h model the arsenic- and gallium-dimer termination of the semiconductor sur face. Excellent agreement has been achieved between the vibrational frequen cies predicted by the theory and those observed in experiments. On the (2x4 ), hydrogen adsorbs on arsenic dimers to form isolated and coupled arsenic- monohydrogen bonds, and arsenic-dihydrogen bonds. Conversely, on the (4x2), hydrogen adsorbs on gallium dimers to form terminal and bridged gallium hy drides. The latter species occur in isolated or coupled structures involvin g two or three Ga atoms.