Density-functional cluster study of K adsorption on GaAs(110) surface

Local density approximation of the density-functional theory has been used to investigate K adsorption on a GaAs(110) surface. The surface is modeled by finite. hydrogen-bonded clusters and two different formalisms (Slater-Vosko-Wilk-Nusair and Becke-Lee-Yang-Paar) for the exchange-correlation energies are used. All the clusters representing the GaAs(110) surface are found to be stable at the correlated levels of theory and the binding energy, in general, increases with the number of atoms in the cluster. The most stable cluster is found to be the three-layer cluster, Ga4As5H11, followed by Ga5As4H12 which were considered for the study of chemisorption. Of the three sites considered for K adsorption on an ideal surface, sites I and II, are found to be stable, whereas site III is unstable. Site II is the most favorable site for K adsorption with a chemisorption energy of 2.09 eV, which is comparable to the second-order many-body perturbation theory (MP2) value of 2.02 Angstrom. The equilibrium distance of the adatom is found to be 2.35 A from the surface. For the optimized surfaces, site II is again found to be the most stable site. For both sites I and II the height of the adatom and chemisorption energy are higher compared to ideal surfaces, whereas the lattice constants decrease. Our previous MP2 results agree with the current results fairly well. As far as a charge transfer is concerned, there is a significant charge transfer to GaAs surface upon potassium adsorption. In general, gallium atoms lose charge and arsenic. atoms gain charges. No significant change in a charge transfer is found in case of an optimized surface, in comparison to an ideal surface. The reductions in the highest occupied molecular orbital-lowest occupied molecular orbital gap upon potassium adsorption indicate the distinct possibilities of metallization. (C) 1999 American Vacuum Society. [S0734-2101 (99)06605-1].