Photoemission studies of the silicon-gold interface

Abstract

The early stages of the formation of the Au-Si interface have been studied with photoelectron spectroscopy of the valence-band and core levels. In this study, the Si sample was prepared by cleavage in ultrahigh vacuum and Au was deposited in a controlled manner at room temperature. By slowly increasing the Au coverage on the surface, the silicon surface states were depleted rapidly (by a factor of 2 at an Au coverage of 0.2 monolayers) without any observable change (g0.1 eV) in the Fermi-level pinning position. Furthermore, at low coverages, the binding energies of the Au core levels and the $5d$ peaks in the valence band were similar to those of atomic Au; however, the width of the structure indicated that the Au is strongly interacting with Si. Measurement of the strength of the Au core levels gives evidence for penetration of a fraction of Au into Si. Thus, at low coverages, the Au is probably dispersed in and on the Si, causing the removal of the surface states and production of new states in the band gap. As the Au coverage is increased, the Au $4f$ and Si $2p$ core levels shift in such a way as to suggest the formation of an alloy with variable composition at the Au-Si interface. At the highest Au coverages (above 15 monolayers), a small amount (less than 1 monolayer) of Si was observed on the surface of the deposited Au overlayer. Thermal annealing of a thin Au (50 monolayers of Au) on Si at 500\ifmmode^\circ\else\textdegree\fi{}C resulted in an increased Si concentration at the surface with the photoelectron spectra resembling those at low coverages (about 1.5 monolayers), thus indicating a high diffusion coefficient of Si through the Au layer. When less than 1 monolayer of ${\mathrm{O}}_{2}$ was adsorbed onto the cleaved Si surface prior to the deposition of Au, the intermixing of Au and Si was significantly reduced.