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

Abstract

The surface structure of the so-called Ag/Si͑111͒-(6ϫ1) surface is studied by low-energy electron diffraction ͑LEED͒, high-resolution core-level photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. A c(12ϫ2) phase is observed in LEED after cooling the room-temperature (6ϫ1) phase to 100 K. In the Si 2p core-level spectra, no significant difference is observed between the two surfaces. In the valenceband spectra, five surface states are observed on both the (6ϫ1) and c(12ϫ2) surfaces. None of these surface states crosses the Fermi level. The binding energies and dispersions of the surface states observed on the (6 ϫ1) surface are quite similar to those of the c(12ϫ2) surface. These results indicate that the basic structure of this Ag/Si͑111͒ surface has a c(12ϫ2) periodicity, and that the (6ϫ1) 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 -bond state.

STM observation of the Si(111)-c(12*2)-Ag surface

“…As described above, the c(12×2)-Ag phase undergoes transitions to the 6×1-Ag above ∼100 K and subsequently to the 3×1-Ag above 500 K [10]. Contrary to only one paper on the c(12×2)-Ag [14], there has been a considerable number of studies devoted to structure determination of the 6×1-Ag and 3×1-Ag phases. In the following discussion, we argue the c(12×2)-Ag atomic structure based on those of the 6×1-Ag and 3×1-Ag phases because the STM images can be understood by some modulations of those of the 3×1/6×1 structures.…”

“…1 (a). For comparison, a sketch of a LEED pattern from the triple-domain c(12×2) surface [14] is given in Fig. 1 (c).…”

“…In order to find the relation between the present STM image and the previously reported low-energy electron diffraction (LEED) pattern [14], we have performed Fourier transform (FT) of the STM image. The FT pattern of (a) is presented in Fig.…”

“…The 3×1-to-6×1 transition is proposed to be associated with configuration change of the Si-Ag-Si bonds [13,15] and it is also induced by STM tips at room temperature [22,23]. The recent photoemission spectroscopy study has reported little change in the Si2p core levels during the 6×1-to-c(12×2) transformation, and has proposed that the transition is the orderdisorder type [14]. Contrary to the √ 3 × √ 3-Ag case, the ground-state structure of this 1/3 ML-superstructure, the c(12×2) phase, has remained unresolved.…”

“…The ground state of the Si(111)-√ 3 × √ 3-Ag surface has now been widely accepted to possess a so-called "InEquivalent Triangle (IET)" structure [11,12], while the structure of the Si(111)-3×1-Ag is well described by the "Honeycomb Chain Channel (HCC)" model [13]. Furthermore, these surfaces have been found to undergo various phase transitions with temperature [11][12][13][14][15] and the systems have now become important playgrounds to study exotic low-dimensional phase transitions on solid surfaces [16][17][18][19]. For example, the √ 3 × √ 3-Ag surface exhibits an empty-state STM image of a hexagonal-lattice pattern at low temperature but a honeycomb-lattice one at room temperature [11,12].…”