ELECTRONIC-STRUCTURE OF CLEAN AND HYDROGEN-CHEMISORBED GE(001) SURFACES STUDIED BY PHOTOELECTRON-SPECTROSCOPY

The electronic structure of the clean and the hydrogen-chemisorbed Ge(001) surface has been studied with photoelectron spectroscopy. Angle-resolved valence-band and high-resolution core-level spectra were recorded for both the room-temperature, 2 X 1, and the low-temperature, c(4 X 2), reconstructions of the clean surface. The electronic structure for the two phases was found to be very similar. The Ge 3d core-level spectra are decomposed with two surface-shifted components and the different origins of the components are discussed. The valence-band electronic structure of the clean, room-temperature, 2 X 1 and the hydrogen-induced 2 X 1:H reconstructions has been studied in detail by polarization-dependent angle-resolved photoemission. The surface-state dispersions E(k(parallel-to)BAR) in the [010] and [110] directions are mapped out. For the clean surface these dispersions are compared with surface-state bands obtained from a self-consistent calculation, using the local-density approximation and scattering theory, applied to a semi-infinite crystal with a 2 X 1 reconstructed surface consisting of asymmetric dimers. In spectra recorded from freshly annealed samples a structure is observed just above the Fermi level. This structure can be explained by thermal excitation to an empty dangling-bond band. The valence-band maximum was determined to be less than 0.1-0.2 eV below the Fermi-level for the clean 2 X 1 surface. On the hydrogen-chemisorbed 2 x 1:H surface two strong hydrogen-induced surface states are observed in the energy region 4.4-55 eV below E(F). The symmetry properties of these states, investigated by their polarization dependence were found to be similar to the two corresponding states of the monohydride Si(001)2 X 1:H surface.