STRUCTURE OF SB MONOLAYERS ON GE(111)2X1 - A COMBINED STUDY USING CORE-LEVEL PHOTOEMISSION, X-RAY STANDING WAVES, AND SURFACE EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE

The atomic stucture of an annealed monolayer of Sb on the Ge (111) - (2 X 1) surface has been studied with core-level photoemission, x-ray standing waves (XSW), and surface extended x-ray absorption fine structure (SEXAFS). Phase analysis of the SEXAFS data taken over the Sb L3 edge using bulk Sb and GaSb as standards determines the bond lengths within the first Sb shell to be 2.87 +/- 0.03 angstrom and 2.67 +/- 0.03 angstrom for the Sb-Sb and Sb-Ge bonds, respectively. The XSW data for (111) planes, taken in the backreflection diffraction geometry, place the Sb layer 2.60 +/- 0.05 angstrom above the Ge(111) surface. A relatively high coherent fraction (0.85) indicates small disorder or buckling in the Sb overlayer. The Sb overlayer is found to quench the clean surface shifted components of the Ge 3d core-level spectrum, which indicates an ideal termination of the Ge (111) surface with all dangling bonds saturated by the Sb adatoms. Lack of chemically shifted components in both the Ge 3d and the Sb 4d core level spectra indicates a single bonding site for the Sb atoms. Although it is not possible to differentiate between Sb trimers [as found for the Sb/Si(111) interface] or zigzag chains (common for Sb/III-V interfaces) on the Ge surface, substitutional geometries, in which Sb replaces the topmost layer of the Ge surface bilayer [as found for As/Ge (111)], can be ruled out.