Identification of the basic structure of the Ag/Si(111)-(6×1) surface : Observation of a low-temperature c(12×2) phase

The surface structure of the so-called Ag/Si(111)-(6×1) surface is studied by low-energy electron diffraction (LEED), high-resolution core-level photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. A c(12×2) phase is observed in LEED after cooling the room-temperature (6×1) phase to 100 K. In the Si 2p core-level spectra, no significant difference is observed between the two surfaces. In the valence-band spectra, five surface states are observed on both the (6×1) and c(12×2) surfaces. None of these surface states crosses the Fermi level. The binding energies and dispersions of the surface states observed on the (6×1) surface are quite similar to those of the c(12×2) surface. These results indicate that the basic structure of this Ag/Si(111) surface has a c(12×2) periodicity, and that the (6×1) structure results from thermal vibrations of the surface atoms. Moreover, we assign two of the surface states to bonding states between Ag and Si atoms, and one of them to a π-bond state