Metallization and incomplete melting of a semiconductor surface at high temperature

We present a first microscopic study of the state of a semiconductor surface, clean Ge(111), close to the bulk melting temperature. Both electronic states and ionic motion are fully treated via first-principles molecular dynamics. Results indicate a clear dynamical disordering, confined, however, to the first atomic bilayer. This region acquires a liquidlike diffusion, and is metallic. Lack of melting of the second and deeper bilayers, found to be in good quantitative agreement with recent x-ray data, indicates an incomplete wetting of the semiconducting solid by its own metallic melt. Previously conflicting data on Ge(111) are also reconciled within this picture.