Composition and morphology studies of ultrathin CaF2 epitaxial films on silicon

Thin CaF2 films with thicknesses between 1 and 100 nm were grown on Si(1 1 1) at UHV conditions by evaporation from a CaF2 source. The correlation of film composition and growth morphology with the deposition parameters was studied by various techniques such as heavy-ion elastic recoil detection analysis (ERDA), RBS/channeling, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and auger electron spectroscopy (AES). The composition and morphology of the CaF2 films strongly depend on the substrate temperature during deposition. Epitaxial growth is observed at deposition temperatures higher than 450°C. The F/Ca ratio of the films in the monolayer range is found to deviate appreciably from the stoichiometric composition (F/Ca=2) suggesting that the interface composition does not obey the rules of bulk equilibrium chemistry. AES results show the formation of different bonding configurations (in particular of silicide-like bonds) at the CaF2/Si interface.