COMBINED (1X2)-](1X1) 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 X 2) --> (1 X 1) phase transition of the Ge(001) surface. The variation of the (1 X 2) superlattice reflection intensity with temperature gives a transition temperature of T(c) = 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.