Low-energy electron-loss spectroscopy and Auger-electron-spectroscopy studies of noble-metal—silicon interfaces: Si-Au system

We report complete low-energy electron-loss spectroscopy (LEELS) and Auger-electron-spectroscopy measurements on the Si(111)-(2×1)—Au system at different thicknesses of Au deposited at room temperature and for different annealing cycles. Two overlayer thickness ranges correspond to two different stages of interface formation. In the submonolayer range gold atoms stick on the silicon surface without intermixing and give rise to strong interface LEELS features. A thickness of two monolayers is the lower limit over which intermixing occurs and an Au-Si alloy is formed. The alloy composition changes with overlayer thickness up to 20 ML. Above this value pure gold begins to grow with some silicon outdiffused to the free surface. LEELS data show that in the alloyed phase the shallowest $d$ electrons are those mostly involved in bond formation with silicon $\mathrm{sp}$ electrons. The hybrid bonds correspond to new electronic states at 3.7 eV under $E_F$ for the silicon-rich Au-Si mixed phase.