Scanning tunneling microscopy and spectroscopy of gold on the GaAs(110) surface

The geometric and electronic structure of Au on the GaAs(110) surface is studied with the scanning tunneling microscope. At low metal coverage, the binding site for the Au adsorbates is found to be beside a surface Ga atom. The adsorbates usually occupy every second unit cell on the surface. In tunneling spectroscopy, a characteristic band gap state is observed for the adsorbed Au atoms, located ∼1.0 eV above the valence-band maximum. A resonant state in the valence band is also observed. The valence-band resonance and band gap state are interpreted as the first and second electron states of the Au–Ga bond, respectively. Both states form bands, and the surface Fermi level is pinned between these bands. At higher metal coverage, the Au atoms form clusters, which have a distinct epitaxial relationship with the substrate. Spectroscopy at high metal coverage reveals a continuum of states throughout the band gap, associated with the metallic nature of the clusters.