Contrasted electronic properties of Sn-adatom-based(3×3)R30°reconstructions on Si(111)

Abstract

We have investigated the electronic structure of the two-dimensional solid solution ${\mathrm{Si}}_{x}{\mathrm{Sn}}_{1\ensuremath{-}x}/\mathrm{Si}(111)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ at room temperature, with a particular emphasis on the empty states, using both global $[{k}_{//}$-resolved inverse photoemission spectroscopy (KRIPES)] and local probes (scanning tunneling microscopy and spectroscopy, STM and STS), as well as DFT-LDA calculations. This adatom overlayer with a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry shows drastic evolution with increasing Sn-adatom concentration, including a semiconductor to metal transition. The ${\mathrm{Si}}_{0.5}{\mathrm{Sn}}_{0.5}/\mathrm{Si}(111)\ensuremath{-}\sqrt{3}$ or mosaic phase has a single empty surface state localized at 0.56 eV above ${E}_{F}$ at $\overline{\ensuremath{\Gamma}}.$ With an overall bandwidth of $\ensuremath{\approx}0.15 \mathrm{eV},$ this ${\mathrm{sp}}_{z}$-type state localized on Si adatoms does not cross ${E}_{F}:$ the mosaic phase is semiconducting, with a bandgap between 0.3 and 0.5 eV. This phase is characterized by a large corrugation of 0.75 \AA{} with Sn adatoms higher than Si adatoms. In the Sn-rich limit ${\mathrm{Si}}_{x}{\mathrm{Sn}}_{1\ensuremath{-}x}/\mathrm{Si}(111)\ensuremath{-}\sqrt{3}$ with $x<~0.05,$ we follow an empty state ${U}_{1}^{\ensuremath{'}}$ throughout most of the surface Brillouin zone except near the $\overline{K}$ point where it clearly crosses the Fermi level. A second, empty surface state ${\mathrm{U}}_{2}^{\ensuremath{'}}$ is detected 1.67 eV above ${E}_{F}.$ Once correlation effects suggested by the small bandwidth of ${U}_{1}^{\ensuremath{'}}$ are adequately taken into account, we explain our KRIPES results in the framework of a dynamical fluctuations model as originating from an underlying $(3\ifmmode\times\else\texttimes\fi{}3)$ structure. Finally, results pertaining to intermediate Sn-adatom concentrations are interpreted in view of the two limiting cases.