CaF sub 2 Si Heteroepitaxy: Importance of Stoichiometry, Interface Bonding and Lattice Mismatch.

Using x-ray standing waves in UHV and synchrotron radiation the early stages of growth of CaF sub 2 on Si(111) were investigated. A new and subtle arrangement of the Si(111)/CaF sub 2 interface emerges from this study. For deposition at high substrate temperatures, the first monolayer of CaF sub 2 dissociates, and CaF is formed at the surface with Ca in the T sub 4 and H3 site. At lower temperature, a larger fraction of the CaF sub 2 remains undissociated, and a third Ca site 7 fold F coordinated in addition to H sub 3T sub 4 sites is consistent with the standing wave experiments. These results have a direct bearing on the interfacial electronic behavior and its dependence on deposition conditions.