STRUCTURE OF THE SULFUR-ADSORBED CU3AU(110) SURFACE

The sulfur-adsorbed Cu3Au(110) surface (precise bulk composition is 71.7% Cu and 28.3% Au) has been studied with RHEED and Auger spectroscopy. The sulfur is segregated from inside of the bulk crystal. It has been found that the composition and the structure of the surface depend on the surface sulfur concentration. The ratio of Au and Cu concentration on the surface drops drastically in a uniform way with the increase of the sulfur coverage. At different sulfur coverages (2 x 2), (2 x 3), (2 x 4) and (3 x 2) structures and some incommensurate (3 x 2)* structures have been observed. It is found that under low sulfur coverage the original (2 x 1) lattice at the first layer of the clean surface remains mostly unchanged, although some Au atoms are lost from the surface due to the sulfur adsorption. Under very high sulfur coverage the surface appears to be similar to the sulfur-adsorbed Cu(110) surface, in which the sulfur atoms are adsorbed at the hollow site of the substrate lattice. The linear relation of surface composition changes implies that the hollow site model is valid for all the surfaces with different sulfur concentration. The different reconstructions are formed by different periodical distributions of occupied and unoccupied sites, which depend on the sulfur coverage.