Surface electronic structures of the Eu-induced Si(111)-(3×2)and -(2×1)reconstructions

Abstract

The electronic structures of the $\mathrm{Eu}∕\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces have been investigated by angle-resolved photoelectron spectroscopy. On the $(3\ifmmode\times\else\texttimes\fi{}2)$ surface, we identify six surface states in the gap and a pocket of the bulk band projection. Among the five surface states observed in the bulk band gap, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(111)-$(3\ifmmode\times\else\texttimes\fi{}1)$ surfaces. The dispersions of the two other surface states observed in the band gap agree well with those observed on the $\mathrm{Ca}∕\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface, which has basically the same structure as that of monovalent atom adsorbed Si(111)-$(3\ifmmode\times\else\texttimes\fi{}1)$ surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure. In the case of the $(2\ifmmode\times\else\texttimes\fi{}1)$ surface, two semiconducting states are observed in the bulk band gap. The difference in binding energy of these two states at the $\overline{\ensuremath{\Gamma}}$ point agrees well with that of the surface states obtained theoretically for a clean Si(111)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface with a Seiwatz structure, and the dispersion of the upper state shows good agreement with the corresponding theoretical surface state. These observations indicate that the two surface states in the band gap originate from Si atoms that form a Seiwatz chain. The present results support the structures of the $\mathrm{Eu}∕\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces proposed in the literature.