Reactions at the Gd-Si(111)7×7 interface: Promotion of Si oxidation

The interactions of various Gd-Si(111) interfaces and surface phases of Gd silicides with oxygen have been investigated by uv and soft-x-ray photoelectron spectroscopy using synchrotron radiation and Auger-electron spectroscopy. At the ‘‘reacted’’ Gd-Si interfaces the room-temperature oxidation of Si is substantially enhanced. In contrast, the epitaxial Gd disilicide–type phases, which are formed after annealing the room-temperature interfaces at elevated temperature, are much less reactive towards ${\mathrm{O}}_2$; however, reaction can be initiated after an initial induction period. A pronounced dependence of the oxidation rate on Gd coverage is found, which emphasizes the importance of the concentration of Si atoms having reacted with Gd. The Gd component of the interface is also oxidized, and the oxidation product, which is characterized by a Si 2p core-level shift of ≃3–3.3 eV, is insulating with a band gap of several electron volts.