GEOMETRIC STRUCTURE OF THE SI(111)-(SQUARE-ROOT-3 X SQUARE-ROOT-3)R30-DEGREES-AU SURFACE

We have used medium-energy ion scattering to investigate the geometric structure of the Si(111)-(square-root 3 x square-root 3)R30-degrees-Au surface. The ion scattering data show unambiguously that the Au atoms are not located in Si bulk lattice sites and the Si lattice in the surface region is severely distorted. By comparing computer simulations of the experiment for various models of the Si(111)-(square-root 3 x square-root 3)R30-degrees-Au surface, it is found that none of the previously proposed models are compatible with our ion scattering data. A new model involving twisted Au trimers on top of a missing top layer Si lattice is proposed to explain our experimental results.