Surface states on clean Ge(111) surfaces

Surface states on Ge(111)c(2X8) are detected using ellipsometry, field effect and surface conductivity measurements. Optical transitions at 1.7, 1.9, 2.3, and 3.0 eV are observed. Surface states in the gap are detected with DC field effect measurements. We found a relationship between the surface conductivity and the ellipsometric parameter ψ at the optical transitions during oxygen adsorption. A maximum in the surface conductivity is found for a coverage of 0.15 monolayer oxygen, at the same point Surnev detected (with work function measurements) a change of the elementary dipole moment.