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

The growth and degradation of ultrathin alkali layers on the Si(111)7×7 surface are studied via second-harmonic generation (SHG) in pp-geometry and resistivity measurements. During the growth of a Na layer on Si(111) at room temperature a pronounced SH maximum is observed at 0.5 ML due to Na-induced, isolated dipoles. A larger signal increase occurs in the multilayer regime. 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 surface, respectively.