Metal to semiconductor transition on Ag/Ge(111): Surface electronic structure of the √3 × √3, √39 × √39, and 6×6 surfaces

Abstract

The √3 × √3, √39 × √39, and 6×6 phases of Ag/Ge(111) have been studied by angle-resolved photoemission and low-energy electron diffraction. The √3 × √3 surface, formed at a one-monolayer (ML) Ag coverage, shows a metallic behavior with two partially occupied surface bands resulting from a tiny amount of extra Ag atoms on the surface. The √3 × √3 surface transforms into a √39 × √39 periodicity, below ∼250 K, when a small amount of Ag is added to the surface. The presence of the additional Ag atoms leads to an increased filling of two partially occupied surface bands. By depositing ∼0.2 ML of Ag on the√3 × √3 surface, it transforms into a 6 ×6 periodicity. We observe an interesting transition from the metallic #x221A;3 × √3, √39 × √39 phases to a semiconducting phase for the 6×6 surface, with a gap of around 0.2 eV with respect to the Fermi level. On the 6×6 surface, the lower band of the partially occupied surface bands is pulled down entirely below the Fermi level while the upper band is missing in the photoemission spectra. These changes result in the observed band gap. © 2001 The American Physical Society.