He diffraction study of the structural phase transition on the Ge(111) surface at 550 K

A structural phase transition on the Ge(111) surface is investigated using thermal energy He scattering by observing the temperature dependence of several diffraction peaks. The presence of strong quarter-order peaks confirms that the Ge(111) surface has three orientational domains of a c(2 × 8) reconstruction at room temperature. As the temperature rises, all the superlattice peaks decrease in intensity without broadening and disappear above 550 K. In addition there is one striking feature which has not been observed in earlier low energy electron diffraction and reflection high energy electron diffraction experiments. The half-order peak shows continuous broadening and a shift of its maximum toward the specular peak as the temperature increases above 550 K. Consideration of several possible explanations for such a peak shift suggests that the transition, which may be driven by a charge density wave, is to an incommensurate phase.