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

With an optical probe, the anisotropy in the electronic structure of t he clean, single-domain Ge(001)2X1 surface was studied by recording th e change in optical response upon either adsorption of molecular oxyge n or upon Ar+-ion bombardment. Both methods were shown to result in an optically isotropic surface. It was possible to associate the measure d surface dielectric function with the energy positions and wave-funct ion parities of the occupied and unoccupied surface states known on th e clean surface. The unoccupied D(down) state has been observed experi mentally 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 uno ccupied state at 0.7 eV above E(F) on the monolayer oxygen-covered sur face after N2O exposure. This state is assigned to a bridge bond betwe en germanium and oxygen atoms in the first layer. Furthermore, we pres ent evidence that in the initial stage of molecular-oxygen adsorption one of the two oxygen atoms is immediately incorporated in a subsurfac e position.