Low-energy-electron-loss spectroscopy of Ge surfaces

Energy-level schemes of intrinsic surface states of annealed (111) 8 × 8 and (100) 2 × 2 Ge surfaces are deduced from the $d$-core-electron and valence-electron excitation spectra. The results compare favorably with tight-binding calculations of the relaxed surface. Empty dangling bond states are observed at the valence-band edge. Their density is estimated to be 2 × ${10}^{14}$/${\mathrm{cm}}^2$ and 4 × ${10}^{14}$/${\mathrm{cm}}^2$ for the (100) and (111) surfaces, respectively. Oxygen-adsorption experiments indicate two different adsorption sites of approximately equal number, but drastically differing activity. The more active sites are associated with the empty dangling bonds, which become saturated near a third and half monolayers, respectively, for the (100) and (111) surface. The oxygen chemisorbs to form quasimolecular GeO complexes independent of surface orientation. The results qualitatively favor the rumpled surface or Haneman model for the (111) 8 × 8 surface reconstruction and suggest a vacancy model for the (100) 2 × 2 surface.