Surface-structure analysis of sulfur-passivated GaAs(111)A and GaAs(111)B by x-ray standing-wave triangulation

The positions of sulfur atoms adsorbed on GaAs(111)A and GaAs(111)B surfaces are determined by x-ray standing-wave (XSW) triangulation studies. On the GaAs(111)A surface, which is Ga terminated, sulfur atoms are on top of the first-layer Ga atoms, at the so-called T sites. On the GaAs(111)B surface, which is As terminated, sulfur atoms exchange with the first-layer As atoms and form bonds with three Ga atoms. These models are consistent with the results of photoemission studies. The S-Ga bond length of 2.41 Å on GaAs(111)B, determined from XSW results, is in very good agreement with that derived from the first-principles calculation, suggesting that the substrate surface relaxation is less than the experimental error of 0.04 Å.