Dynamical analysis of low-energy electron diffraction intensities from GaAs(110)-p(1×1)-Sb(1 ML)

The deposition of Sb and GaAs(110) at room temperature produces a stable, ordered, saturated adsorbate structure at a coverage of approximately one monolayer (1 ML, i.e., one Sb for each Ga and As surface species). An analysis of the atomic geometry of this GaAs(110)-p(1×1)-Sb(1 ML) overlayer system was performed by the comparison of dynamical calculations of elastic low-energy electron diffraction (LEED) intensities with those measured at room temperature. A nonrelativistic model embodying energy-independent Slater exchange was utilized in the analysis to achieve compatibility with previous studies of the GaAs(110) substrate. Five qualitatively distinct classes of geometrical models were examined, each corresponding to a different hypothesis concerning the nature of the Sb-GaAs chemical bond. Only one of these, a model in which the Sb adsorbates occupy sites corresponding to both the Ga and As species on the surface of unrelaxed GaAs(110), provided a reasonable description of the measured LEED intensities. Refinement of this class of structures led to a best-fit atomic geometry which provides a description of the LEED intensity data comparable to that achieved for the clean (110) surfaces of compound semiconductors, i.e., GaAs(110), InSb(110), CdTe(110), InP(110), GaP(110), ZnS(110), and ZnTe(110). In this geometry, chains of Sb adsorbates characterized by an Sb Sb bond length of 2.8 0.1 reside upon a nearly unrelaxed GaAs(110) substrate. The Sb species bonded to the Ga substrate atoms lie 0.1 0.075 above the Sb bonded to the substrate As. The Ga Sb bond length is 2.6 0.17 and the Sb As bond length is 2.7 0.17. The uppermost As in the substrate is relaxed toward the bulk GaAs by 0.1 relative to the uppermost Ga, which retains its bulk (i.e., unrelaxed) position relative to the substrate. This structure corresponds to bond angles of approximately 104°between the Sb adsorbates and the substrate species to which they are bonded and to bond angles of 91°between the Sb species in the zigzag overlayer chain. It constitutes a saturated monolayer because the valence of each adsorbed Sb is completely satisfied with two electrons participating in the nearest-neighbor bonds, one in the bond to the substrate, and the other two occupying the lone-pair charge density corresponding to the missing bond directed out of the surface.