CHEMISORPTION OF ALUMINUM ON GAAS(110)

We have performed total-energy pseudopotential calculations for a vari ety of structures for Al adsorbed on the GaAs(110) surface. For the co verage of one monolayer we find only very small differences of the ads orption energy for a number of competing structures. In contrast to re cent LCAO calculations, we find all these structures to be either meta llic or to have only a very small band gap. Total energy calculations are also made for a Ga-Al exchange reaction and for different structur al models for half a monolayer coverage including a 2 x 1 reconstructi on. In all cases we find the adsorption energy to be less than the coh esive energy of bulk Al in FCC structure or the energy gain due to an exchange reaction, indicating the tendency for cluster formation, as o bserved experimentally, or penetration into the substrate. By means of density-of-states calculations it is shown that the Schottky barrier is already formed for the ordered half-monolayer case and is completed for one monolayer of Al on GaAs. The band gap is found to open for an asymmetric dimer in a 2 x 1 reconstruction which may explain recent e xperimental observations.