Binding and migration paths of Au adatoms on the GaAs(001) surface

The binding and the migration paths of an isolated Au adatom on the GaAs (001)-beta2(2x4) reconstructed surface have been investigated by first-principle total-energy calculation, in the Car-Parrinello scheme. The potential energy surface calculated tor the An adatom shows that the most interesting Au binding sites are located at short-bridge sites next the As-As dimers of the surface. Similar binding sites were found for Ga adatoms on the same surface. However. the Au chemical binding is different from that of Ga. A Ga adatom forms strong covalent Ga-As bonds with a marked ionic character when interacting with the As dimers, while the Au-dimer interaction is characterized by the formation of weaker pure covalent Au-As bonds. Accordingly, Au adatoms do not break the As-As dimers at variance with the case of Ga adatoms. The characteristics of the Au binding also account for an anisotropic an migration that results to be faster along the dimer rows than perpendicular to them.