CHEMISORPTION OF ANTIMONY ON GAAS(110)

We report results obtained by a systematic study of Sb chemisorption on the relaxed GaAs(110) surface, using density-functional theory within the local-density approximation and norm-conserving, fully separable, ab initio pseudopotentials. The GaAs(110) surface is simulated by a slab geometry wherein the atomic structure of the Sb atoms at the preferred adsorption positions and the top three substrate layers is optimized by minimizing the total energy. Sb coverages of THETA = 1/2 and THETA = 1 are considered, corresponding to one or two Sb atoms per surface unit cell, on the average. We study nine different bonding configurations in detail. The results are interpreted in terms of the strong adsorbate-substrate bonds and the Sb-Sb interaction. For the energetically favored epitaxial continued layer structure in the THETA = 1 case, the atomic positions are found in good agreement with results of low-energy electron diffraction and x-ray standing wave analyses. However, the epitaxial on top structure, which seems to fit somewhat better to the scanning tunneling microscopy (STM) data, is some tenths of an eV higher in energy. In the THETA = 1/2 case we give a detailed analysis of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting adsorption energies and equilibrium geometries indicate a tendency to form two-dimensional Sb clusters for submonolayer coverage. The accompanying electronic properties (surface band structure, photothreshold, etc.) are discussed within the context of experimental data available from STM, photoemission spectroscopy, etc.