Noble-metal adsorption on Si(111): Medium-energy ion-scattering results for the Ag (3×3)R30 reconstruction

The geometric structure of the Si(111)-(3 × 3) R30 Ag surface has previously been studied by numerous surface-science techniques. Despite the abundance of data, no model for the surface has emerged as a clear favorite. Commonly proposed models feature honeycomb structures of either Ag or Si, with two or three Ag atoms per 3 × 3 unit cell. Results from medium-energy ion scattering conclusively show that each unit cell contains three Ag atoms. In addition, the surface is highly reconstructed, with atomic reordering of the Si atoms of similar magnitude to the clean (7×7) surface. Conventional models of the surface do not incorporate sufficient displacements of silicon atoms. Two new models are compatible with our results. One is a model constructed from silicon adatoms and vacancies. The other is a model based on x-ray scattering results, consisting of a subsurface silver honeycomb-chained trimer. © 1989 The American Physical Society.