Alkali adsorption and its effect on hydrogen saturated Si(111) surface

The coverage-dependent effects of alkali additives on hydrogenated Si(lll)-7x7 surface are monitored using high-resolution electron energy-loss spectroscopy. A —0.2-monolayer (ML) coverage of alkali causes a large broadening in the quasielastically scattered 4.8-eV electrons from the surface. No appreciable broadening is observed either at high alkali coverages (— 1 ML) or when the deposition of —0.2 ML of alkali is followed by hydrogen saturation. Quasielastic broadening is explained in terms of single-particle excitations of low-density conduction electrons. Within the first ML of alkali coverage the H-Si stretch loss energy decreases almost linearly with coverage by — 20 meV. The increase in the width of the stretch loss peak is correlated with the inhomogeneous distribution of H-Si dipoles with respect to the surface layer, and the increase in the intensity of the stretch loss excitation is correlated with the formation of the image dipoles.