CHEMISORPTION OF ANTIMONY ON GAAS(110)

We report results obtained by a systematic study of Sb chemisorption o n the relaxed GaAs(110) surface, using density-functional theory withi n 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 pref erred adsorption positions and the top three substrate layers is optim ized by minimizing the total energy. Sb coverages of THETA = 1/2 and T HETA = 1 are considered, corresponding to one or two Sb atoms per surf ace unit cell, on the average. We study nine different bonding configu rations in detail. The results are interpreted in terms of the strong adsorbate-substrate bonds and the Sb-Sb interaction. For the energetic ally 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 t o the scanning tunneling microscopy (STM) data, is some tenths of an e V higher in energy. In the THETA = 1/2 case we give a detailed analysi s of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting adsorption energ ies and equilibrium geometries indicate a tendency to form two-dimensi onal Sb clusters for submonolayer coverage. The accompanying electroni c properties (surface band structure, photothreshold, etc.) are discus sed within the context of experimental data available from STM, photoe mission spectroscopy, etc.