Contrasted electronic properties of Sn-adatom-based (√3X√3)R30° reconstructions on Si(111) -: art. no. 115407

We have investigated the electronic structure of the two-dimensional solid solution SixSn1-x/Si(111)-(root3 x root3)R30 degrees 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 (root3 x root3)R30 degrees symmetry shows drastic evolution with increasing Sn-adatom concentration, including a semiconductor to metal transition. The Si0.5Sn0.5/Si(111)-root3 or mosaic phase has a single empty surface state localized at 0.56 eV above E-F at Gamma. With an overall bandwidth of approximate to0.15 eV, this 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 Angstrom with Sn adatoms higher than Si adatoms. In the Sn-rich limit SixSn1-x/Si(111)-root3 with x less than or equal to0.05, we follow an empty state U-1' throughout most of the surface Brillouin zone except near the (K) over bar point where it clearly crosses the Fermi level. A second, empty surface state U-2' is detected 1.67 eV above E-F. Once correlation effects suggested by the small bandwidth of U-1' are adequately taken into account, we explain our KRIPES results in the framework of a dynamical fluctuations model as originating from an underlying (3 x 3) structure. Finally, results pertaining to intermediate Sn-adatom concentrations are interpreted in view of the two limiting cases.