THE GROWTH OF AG ON SI(100)-2X1

The growth of Ag on Si(100)-2 x 1 has been studied by Auger electron spectroscopy and surface reflectance spectroscopy. The adsorption of Ag atoms leads initially to the formation of an intermediate Ag film characterized by an optical absorption around 2.3 eV which increases with the Ag coverage up to a saturation value. We propose an adsorption configuration where each Ag atom is bound to one Si surface atom; which corresponds to a theoretical saturation coverage equal to 1 monolayer. The 2.3 eV absorption is interpreted as due to an optical transition involving electronic interface states corresponding to the covalent bonding between Ag and Si atoms, which gives a semiconductor character to the interface. The metallic character appears for coverages larger than the saturation value; as demonstrated by the large increase of the reflectivity in the near infrared and by the excitation of surface plasmons located in three-dimensional flat Ag islands growing upon the intermediate film.