The Phase of Pb/Ge(111): A Surface X-Ray Diffraction Study

This report describes the results of several surface X-ray diffraction
measurements of a chemisorbed overlayer of Pb on the Ge(lll) surface. Three phases of Pb/Ge(lll) exist in the monolayer regime: the a- and B-phases with a V3xV3R30° unit cell, and a high-temperature IX1 phase. In the 1X1 phase of Pb/Ge(lll), isotropic x-ray scattering is observed consistent with a two-dimensional liquid phase. Measurements of the integer-order Bragg reflections through the V3xV3R30° -» 1X1 transition confirm the liquid-like nature of the 1X1 phase, and show that the liquid layer is modulated by the periodic potential of the substrate. By measuring variations of the (2/3,2/3) surface Bragg reflection from the V3x V3R300 phase as a function of temperature and coverage, a simple phase diagram for Pb/Ge(lll) is deduced. Below 1/3 ML (where 1 ML is one Pb atom per Ge surface atom) the a-phase coexists with the 1X1 phase. Between 1/3ML and 4/3ML, a- and B-phases form a twophase system displaying phase separation. Analogies with simple theoretical phase diagrams are emphasized.