STUDY OF THE GEOMETRIC STRUCTURE AND VIBRATIONAL AMPLITUDES AT THE SI(111)-(SQUARE-ROOT-3 X SQUARE-ROOT-3) R30-DEGREES-GA SURFACE

We have used medium-energy ion scattering to investigate the geometric structure and vibrational amplitudes at the Si(111)-(square-root 3 x square-root 3)R30-degrees-Ga surface. The geometric structure of the Si(111)-(square-root 3 x square-root 3)R30-degrees-Ga surface has previously been studied by other techniques. Our structural model is in substantial agreement with the findings of these investigations. The Si(111)-(square-root 3 x square-root 3)R30-degrees-Ga reconstruction is relatively mild, with Ga atoms sitting above second layer Si atoms and Si atom displacements that are small compared to those on the Si(111)-(7 x 7) surface. This reconstruction, therefore, provides an excellent opportunity to study atomic vibrational amplitudes at the surface. We find that the surface vibrational amplitudes are enhanced relative to the bulk amplitudes, and that these amplitudes decay rapidly away from the surface. Furthermore, the amplitudes normal to the surface are larger than the intraplanar amplitudes, consistent with the notion that the interplanar restoring forces are smaller than the intraplanar restoring forces at the surface.