Surface states of the clean and oxidized Ge(001) surface studied with normal-incidence ellipsometry

Abstract

With an optical probe, the anisotropy in the electronic structure of the clean, single-domain Ge(001)2×1 surface was studied by recording the change in optical response upon either adsorption of molecular oxygen or upon ${\mathrm{Ar}}^{+}$-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 ${\mathit{D}}_{\mathrm{down}}$ 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 ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$O saturation exposure, it has been possible to deduce an unoccupied state at 0.7 eV above ${\mathit{E}}_{\mathit{F}}$ on the monolayer oxygen-covered surface after ${\mathrm{N}}_2$O 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.