Helium atom scattering from the Si(111) surface at high temperatures

Abstract

Helium atom scattering has been used to investigate the structure of the Si(111) surface in the temperature range from 900 to 1600 K. Even below the well-known (7\ifmmode\times\else\texttimes\fi{}7) to ``(1\ifmmode\times\else\texttimes\fi{}1)'' transition the adatoms become mobile, and, when the transition is reached near 1140 K, the specular- and integral-order diffraction peaks have sudden intensity changes, some up and others down, while the seventh-order peaks disappear. Above the transition the adatoms remain, moving rapidly on, and supported by, the ordered but relaxed, outer bilayer of the surface. A second transition, first reported by Ishizaka and co-workers occurs near 1470 K. The loss of all diffraction peaks and the attenuation of the specular peak indicate a completely disordered surface as the temperature approaches the melting point.