Surface and bulk electronic structure of Ge(111)c(2×8) and Ge(111):As 1×1

The surface band dispersion of the annealed (111) surface of germanium has been determined experimentally using angle-resolved photoemission. The c(2×8) symmetry seen in low-energy electron diffraction is not seen in the surface-state dispersion, which exhibits a 2×2 periodicity. This result is interpreted as indicating that the Ge(111) c(2×8) surface unit cell contains 2×2 subunits. The surface-state dispersion and bandwidth are consistent with a 2×2 adatom model for the surface. An examination is made of the effect of the surface on the near-surface electronic structure. Results for the heavily reconstructed Ge(111) c(2×8) surface are compared with those for the Ge(111):As 1×1 surface. This latter surface was chosen for comparison because the arsenic termination removes the reconstruction and leads to an extremely stable surface with a geometry close to that of an ideal (111) surface. Spectral features corresponding to bulk initial states were compared with calculated values and found to agree throughout most of the surface Brillouin zone. Discrepancies were found near K¯ and attributed to the effect of back-bond formation. At finite values of $k_{?}$, a free-electron parabola with its zero at 9.70 eV below the top of the bulk valence band is the final state which best explains the bulk data. At normal emission, on the other hand, a Bloch conduction-band state was found to be more appropriate. The effect of secondary cone emission on the photoemission is discussed.