No abstract

Helium atom scattering has been used to investigate the structure of the Si͑111͒ surface in the temperature range from 900 to 1600 K. Even below the well-known ͑7ϫ7͒ to ''͑1ϫ1͒'' 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.

The scattering of helium atoms from a Ge(100) surface has been studied at temperatures to within 110 K of the Ge melting point (1210 K). Although the elastic scattering is greatly attenuated as the temperature increases, the (1/2 0) and (00) helium diffraction peaks remain detectable up to 1000 and 1100 K respectively. No increased attenuation of these diffraction peaks that would be expected to accompany the appearance of numerous adatoms and vacancies is observed. This makes doubtful the interpretation of the results of an earlier x-ray diffraction study. A slight change in the Debye–Waller attenuation of the (00) diffracted helium peak near 825 K is observed, possibly indicating a weak, dimer-preserving phase transition.