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

Abstract

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