Combined (1×2)?(1×1) transition and atomic roughening of Ge(001) studied with surface x-ray diffraction

Surface x-ray-diffraction measurements are presented that show a reversible (1×2)→(1×1) phase transition of the Ge(001) surface. The variation of the (1×2) superlattice reflection intensity with temperature gives a transition temperature of Tc=955±7 K. The data are interpreted as being due to the creation of adatoms and vacancies on the surface with consequent break up of surface dimers. X-ray reflectivity indicates a corresponding loss of height-height correlation across the surface. A simple three-level model is used to describe the reflectivity, and the results are compared with a simple Monte Carlo simulation of the transition