Adsorption sites of Br on Si(211) investigated with X-ray standing wave fields

The Br/Si(211) interface was investigated using the X-ray standing wave technique. The samples were prepared in situ under ultra high vacuum conditions, and three different Bragg reflections were employed ((111), (220) and (422)), which allowed to measure the adsorption distance from three different bulk lattice planes. In all studied cases the Br adsorption reverted the 2 × 1 substrate reconstruction into 1 × 1. The results support that Br atoms are adsorbed at least at two different sites. The first one corresponds to the conventional on top site of the [111]-like terraces of the surface. The second one corresponds to adsorption at the [100]-like steps of the Si(211) surface. We have determined the relative occupation of each site and a model for them is proposed on the basis of the geometrical structure of the surface