Surface dipole and Fermi-level position on clean, oxygen-, and water-covered cylindrical Si crystals: A photoelectron spectroscopy study

Photoelectron spectroscopy, including emission from the Si 2p core level, was used to investigate the orientation dependence of the photoionization threshold {ξ}({α}) and the surface position of the valence-band edge below the Fermi level, EVFS({α}), on a cylindrically shaped Si crystal with [11{}0] as its axis. The average {ξ} values for (001), (113), (111), and (110) are 5.33, 5.32, 5.26, and 5.26 eV, respectively, and thus differ only very little as expected for a covalent, nonpolar crystal. Increasing corrugation on stepped, faceted, defect-rich, and high-index surfaces increases {ξ} values, in contrast to what is observed on metal surfaces. The orientation dependence of {ξ} is interpreted in terms of a relaxation- or reconstruction-related surface polarity which is increased at edges. Water adsorption generally reduces {ξ}. In the range (001)--(113) the shape of {ξ}({α}) is conserved, whereas it is changed in the range (112)--(111)--(110), consistent with fundamentally different adsorption mechanisms as observed earlier. After oxygen adsorption, {ξ}({α}) is strongly changed, indicating a strong distortion of the surface by bond breaking. On the clean surface, the Fermi-level--pinning position EVFS({α}), which is given by the surface-state structure, varies strongly. After oxygen exposure it is pinned at a nearly constant position {~}0.1 eV above the midgap, probably by defect-related surface states. For water adsorption, again two ranges can be distinguished with a similar pinning as for oxygen in the range (112)--(111)--(110) and a reduced but still strong variation between (001) and (113).