Determination of the Sb/Si(111) interfacial structure by back-reflection x-ray standing waves and surface extended x-ray-absorption fine structure

Based on the surface extended x-ray-absorption fine-structure (SEXAFS) absolute surface coordination numbers, Woicik et al. [Phys. Rev. B 43, 4331 (1991)] argued that Sb trimers adsorb on the Si(111) surface in a modified ``bridge'' configuration rotated by 60\ifmmode^\circ\else\textdegree\fi{} relative to the atop ``milk stool'' geometry proposed by Abukawa et al. [Surf. Sci. Lett. 201, 513 (1988)]. Using the accurate near-neighbor bond lengths as determined from SEXAFS, these models predict perpendicular distances of 1.76 and 2.60 \AA{} between the top Si atoms and the Sb layer, respectively. New back-reflection x-ray standing wave data find the Sb atoms to reside 2.53\ifmmode\pm\else\textpm\fi{}0.1 \AA{} above the Si surface, in clear support of the milk stool geometry. Polarization-dependent SEXAFS relative coordination numbers support the milk stool model as well.