Atomic Structure of the Sb-Stabilized GaAs(100)-(2×4) Surface

The microscopic structure of the Sb stabilized GaAs(100)-( $2\times 4$) surfaces is investigated combining reflectance anisotropy spectroscopy with first-principles total energy minimization and tight-binding calculations of optical properties. We show that the model accepted so far, containing three Sb dimers in the outermost layer, is not a stable surface geometry. Our results reveal a coexistence of Sb and Ga dimers on the Sb-stabilized $(2\times 4)$ surface.