Au on Ag/Si(111)-(root 3x root 3)R30 degrees: A spectromicroscopy study of a bimetal-silicon interface

We present a scanning-photoemission-microscopy study of the Au on Ag/Si(111)-(root 3x root 3)R30 degrees interface containing two different phases: a two-dimensional (2D) ordered surface with additional three-dimensional Ag islands. Our submicrometer lateral resolution makes it possible to characterize these two phases in function of Au coverage and annealing temperature. On the 2D (root 3x root 3)R30 degrees-Ag/Si regions no major disruption of the ordered structure occurs for Au coverages up to 2/3 ML, where a (2 root 3x2 root 3)R30 degrees low-energy electron-diffraction (LEED) pattern can be observed after annealing at 470 K. If this Au coverage is exceeded, exchange processes between Ag and Au atoms take place on the 2D phase, resulting in an amorphous Au/Si interface with small Ag clusters on top; annealing this interface restores the (root 3x root 3)R30 degrees LEED pattern which, on the basis of our core-level and valence-band spectra, we assign to a (root 3x root 3)R30 degrees reconstruction similar to the initial one. This suggests the transport of Au toward the 3D phase, where Ag-Au alloying is the dominating process. Strong Au-Ag interactions take place on the 3D islands already at low Au coverages and even at room temperature. The Si skin, which is originally present on the 3D islands, is penetrated by Au and is also maintained after annealing. Finally, the different electronic properties of the two phases, in particular the evolution of the initially semiconducting 2D phase, could be shown by detecting their valence-band electronic structure.