SURFACE-STATES OF THE CLEAN AND OXIDIZED GE(001) SURFACE STUDIED WITH NORMAL-INCIDENCE ELLIPSOMETRY

With an optical probe, the anisotropy in the electronic structure of the clean, single-domain Ge(001)2X1 surface was studied by recording the change in optical response upon either adsorption of molecular oxygen or upon Ar+-ion bombardment. Both methods were shown to result in an optically isotropic surface. It was possible to associate the measured surface dielectric function with the energy positions and wave-function parities of the occupied and unoccupied surface states known on the clean surface. The unoccupied D(down) state has been observed experimentally and it is positioned 0.4 eV above the Fermi level. Comparing the changes in the anisotropy of the surface electronic structure upon O2 and N2O saturation exposure, it has been possible to deduce an unoccupied state at 0.7 eV above E(F) on the monolayer oxygen-covered surface after N2O exposure. This state is assigned to a bridge bond between germanium and oxygen atoms in the first layer. Furthermore, we present evidence that in the initial stage of molecular-oxygen adsorption one of the two oxygen atoms is immediately incorporated in a subsurface position.