Electronic excitations on the alkali-metal (Na,Cs)-adsorbed Si(111)7x7 surfaces

Electronic excitation spectra of the alkali-metal (AM) (Na,Cs)-adsorbed Si( 111)7 x 7 surfaces have been investigated by using high-resolution electron energy loss spectroscopy. We find that AM induces metal-semiconductor tran sitions at the early stage of adsorption by completely filling half-filled Si adatom dangling-bend states. A loss peak of energy less than 0.31 eV app ears as a precursor of the semi conducting phases, which disappears with in creasing coverage, resulting in the formation of energy gaps of 0.43 eV and 1.08 eV for Cs and Na, respectively. Both Cs- and Na-adsorbed surfaces bec ome metallic at saturation, in sharp contrast with the semiconducting K-sat urated surface. We discuss the physical origin of the AM-induced loss peaks and its implications for the electrical phase transitions.