Growth and thermal destruction of sodium films on Si(111)7x7 surfaces as monitored via second-harmonic generation

The growth and degradation of ultrathin alkali layers on the Si(111)7 x 7 s urface are studied via second-harmonic generation (SHG) in pp-geometry and resistivity measurements. During the growth of a Na layer on Si(lll) at roo m temperature a pronounced SH maximum is observed at 0.5 ML due to Na-induc ed, isolated dipoles. A larger signal increase occurs in the multilayer reg ime. When the substrate temperature is gradually increased, SHG reveals the desorption of the multilayer at 390(5) K and of the monolayer at 550(5) K. This corresponds to adsorption sites on the Na film and on the silicon sur face, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.