SURFACE DIPOLE AND FERMI-LEVEL POSITION ON CLEAN, OXYGEN-COVERED, 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 \ensuremath{\xi}(\ensuremath{\alpha}) and the surface position of the valence-band edge below the Fermi level, ${E}_{\mathrm{VFS}}$(\ensuremath{\alpha}), on a cylindrically shaped Si crystal with [11\ifmmode\bar\else\textasciimacron\fi{}0] as its axis. The average \ensuremath{\xi} 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 \ensuremath{\xi} values, in contrast to what is observed on metal surfaces. The orientation dependence of \ensuremath{\xi} is interpreted in terms of a relaxation- or reconstruction-related surface polarity which is increased at edges. Water adsorption generally reduces \ensuremath{\xi}. In the range (001)\char21{}(113) the shape of \ensuremath{\xi}(\ensuremath{\alpha}) is conserved, whereas it is changed in the range (112)\char21{}(111)\char21{}(110), consistent with fundamentally different adsorption mechanisms as observed earlier. After oxygen adsorption, \ensuremath{\xi}(\ensuremath{\alpha}) is strongly changed, indicating a strong distortion of the surface by bond breaking. On the clean surface, the Fermi-level\char21{}pinning position ${E}_{\mathrm{VFS}}$(\ensuremath{\alpha}), which is given by the surface-state structure, varies strongly. After oxygen exposure it is pinned at a nearly constant position \ensuremath{\sim}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)\char21{}(111)\char21{}(110) and a reduced but still strong variation between (001) and (113).