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

Abstract

The coverage-dependent effects of alkali additives on hydrogenated Si(111)-7×7 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.