Arsenic atom location on passivated silicon (111) surfaces.

The position of As atoms on a clean Si (111) surface has been determined with x-ray standing waves in UHV. The As atoms occupy exclusively the top half of the silicon (111) double plane and lie at 0.17angstroms above the unrelaxed bulk terminated silicon (111) plane. This value is in good agreement with recent total energy minimization calculations. A significant but limited stability of As passivated surfaces is observed upon exposure to various ambients. We also present general arguments showing how surface specificity can be achieved with x-ray standing wave measurements such as the above, where bulk symmetry site occupancy rules are broken.