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 s tanding waves (XSW), and surface extended x-ray absorption fine struct ure (SEXAFS). Phase analysis of the SEXAFS data taken over the Sb L3 e dge using bulk Sb and GaSb as standards determines the bond lengths wi thin 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 fo r (111) planes, taken in the backreflection diffraction geometry, plac e the Sb layer 2.60 +/- 0.05 angstrom above the Ge(111) surface. A rel atively high coherent fraction (0.85) indicates small disorder or buck ling in the Sb overlayer. The Sb overlayer is found to quench the clea n surface shifted components of the Ge 3d core-level spectrum, which i ndicates an ideal termination of the Ge (111) surface with all danglin g bonds saturated by the Sb adatoms. Lack of chemically shifted compon ents in both the Ge 3d and the Sb 4d core level spectra indicates a si ngle bonding site for the Sb atoms. Although it is not possible to dif ferentiate between Sb trimers [as found for the Sb/Si(111) interface] or zigzag chains (common for Sb/III-V interfaces) on the Ge surface, s ubstitutional geometries, in which Sb replaces the topmost layer of th e Ge surface bilayer [as found for As/Ge (111)], can be ruled out.