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 angstrom 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, angstrom.