Arsenic overlayer on Si(111): Removal of surface reconstruction

The sputtered and annealed (111) surface of silicon exhibits a strong 7 x 7 reconstruction. Evidence from a number of techniques shows that this large unit cell has a number of distinctly different surface atomic sites. The reconstruction of the surface is driven by a lowering of the energy of the dangling-bond orbitals, which are half-filled on the ideal surface. If the dangling bonds were to become saturated, the driving force for the reconstruction would be reduced, and a nearly ''ideal'' surface should result. This can be approximated by an As terminated surface, with As atoms substituting for the top atomic layer of Si. The As is then 3-fold coordinated, which it prefers, with 2 electrons in a ''lone-pair'' orbital. This model, with each Si second layer atom bonded to 3 As atoms above and one Si below, predicts the observed removal of the 7 x 7 reconstruction upon As exposure. It also predicts that the core-level spectra of the Si atoms should exhibit only one non-bulk site, well shifted from the bulk peak, as opposed to the many different sites for the 7 x 7 surface. This configuration has also been predicted and observed for the Ge(111) surface