EFFECT OF A COMPOSITION DISCONTINUITY ON THE EVOLUTION OF A BIMETAL INTERFACE STUDIED BY PHOTOEMISSION MICROSCOPY - AU PATCH DEPOSITED ON AAG SI(111) SURFACE/

The evolution of a 2 ML Au rectangular patch deposited on a Ag/Si(111) -(root 3 x root 3)R30 degrees surface containing three-dimensional Ag islands was studied by scanning photoemission microscopy. The surface composition and electronic structure of the Au patch and of the transi tion region between the patch and the Au-free Ag/Si area were characte rized on a submicron scale at room temperature and after annealing at various temperatures up to 993 K. The spatial expansion of the Au patc h due to Au surface diffusion and the chemical state of Ag and Au in t he different interface zones were examined by chemical mapping and by analysis of the Ag 3d and Au 4f core level and valence band spectra, t aken at selected spots of the surface. From the analysis of the Au 4f core level line shape, we identified a chemical state for Au at the fo refront of the Au migration on the root 3-Ag/Si surface and assigned i t to an adsorption site on top of Si trimers. This was the dominating Au species for low (< 0.2 ML) Au coverages. Following the evolution of the Au patch upon annealing we discovered substantial differences in its composition and structural changes compared to a continuous 2 ML A u film deposited on a root 3-Ag/Si surface. The important role of the Ag and Au surface diffusion in the evolution of an interface containin g compositional discontinuities was discussed.