Interaction of cobalt atoms with an oxidized Si(111)7 $\times$ 7 surface

The interaction of cobalt atoms with a Si(111)7 $\times$ 7 surface subjected to in situ oxidation in an oxygen atmosphere at a pressure of 10$^{-5}$ Pa, an exposure of 20 L, and a temperature of 500$^\circ$C is studied by high-decomposition (100 meV) photoelectron spectroscopy using synchrotron radiation. This surface treatment is shown to form an oxide film, which has a complex composition, occupies about 80% of the substrate surface, and has a thickness of $\sim$6 $\mathring{\mathrm{A}}$. At room temperature and a coverage of up to six monolayers, cobalt atoms are found to migrate to free (unoxidized) silicon surface spots rather than being adsorbed on the formed oxide layer. In these spots, a thin layer of cobalt disilicide first forms and a Co–Si solid solution then grows on it. Some cobalt atoms penetrate under the oxide layer and form a three-component Co–Si–O interfacial phase and a metastable cobalt disilicide with a CsCl-type structure at the SiO$_x$–Si interface.