Theoretical studies of halogen–semiconductor-surface interactions: The Cl/GaAs(110) system

We have employed a semiempirical molecular-orbital method to simulate the adsorption of chlorine (Cl) atoms on GaAs(110) for three cases: monatomic Cl, molecular Cl, and 0.16-monolayer (ML) coverage of Cl. For the single Cl atom, the equilibrium bonding site has the Cl atom at the on-top site of the surface As atom. The ${\mathrm{Cl}}_2$ molecule on GaAs(110) displays dissociative adsorption with both Cl-Ga and Cl-As bonding configurations. For the third case, Cl adsorption on GaAs(110) leads to enhanced relaxation beyond that of the already relaxed surface. In this geometry, the surface As atoms gain charge at the expense of the surface Ga atoms and are pushed further away from the surface plane while the Ga atoms relax inward. The microscopic structure of 0.16-ML coverage of Cl displays both Ga-Cl and As-Cl bonding configurations. For the Ga-Cl bonding configuration, the Cl atom is adsorbed at the surface Ga dangling-bond site while for As-Cl, the Cl atom is sited atop the surface As atom, offset slightly toward the dangling-bond direction. The Cl adsorbed at the Ga site is lower in height than Cl atop the surface As atom relative to the substrate and the Cl-Cl nearest-neighbor distance calculated is 3.60 Å, quite close to the value of 3.46 Å observed experimentally with the scanning tunneling microscope (STM). These results compare favorably with the findings of Stepniak, Rioux, and Weaver from recent photoelectron spectroscopy experiments, as well as the structural information obtained from STM.