HELIUM ATOM SCATTERING FROM THE SI(111) SURFACE AT HIGH-TEMPERATURES

Helium atom scattering has been used to investigate the structure of t he Si(111) surface in the temperature range from 900 to 1600 K. Even b elow the well-known (7x7) to ''(1x1)'' transition the adatoms become m obile, and, when the transition is reached near 1140 K, the specular-a nd integral-order diffraction peaks have sudden intensity changes, som e up and others down, while the seventh-order peaks disappear. Above t he transition the adatoms remain, moving rapidly on, and supported by, the ordered but relaxed, outer bilayer of the surface. A second trans ition. first reported by Ishizaka and co-workers occurs near 1470 K. T he loss of all diffraction peaks and the attenuation of the specular p eak indicate a completely disordered surface as the temperature approa ches the melting point.