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

Deposition of 1 monolayer (ML) of Ag on the clean Ge(111) surface, followed by annealing at 300°C for 2 min, results in a sharp √3×√3 low-energy electron diffraction (LEED) pattern. This surface transforms into a √39×√39 surface, as observed by LEED, due to a tiny amount of additional Ag atoms when the temperature is below approximately −20°C. The presence of the additional Ag leads to an increased filling of two partially occupied surface bands. By depositing approximately 0.2 ML of Ag on the √3×√3 surface, it transforms into a 6×6 periodicity. The addition of Ag leads to an interesting transition from the metallic surfaces (√3×√3 and √39×√39) to the semiconducting 6×6 surface with a gap of around 0.2 eV with respect to the Fermi-level. On the 6×6 phase, the lower one of the partially occupied surface bands of the √3×√3 and √39×√39 surfaces seems to be entirely pulled down below the Fermi-level, while the upper one is missing. The electronic structures of the different Ag/Ge(111) surfaces are also discussed in comparison with the Ag/Si(111) surfaces.