Initial stages of CaF2 growth and wetting layer formation on Si[001]

It has been recently found that different types of CaF2 nanostructures can be formed on Si[001] substrates depending on the growth conditions. Below 500°C, nearly square-shaped dots nucleate on bare Si[001] surface; above 650°C, at submonolayer coverage, a so-called "wetting layer" forms. At higher coverage, formation of CaF2 stripes of a few nanometers in height and width takes place on top of the wetting layer. Interestingly, CaF2 [001]∥Si[001] in the dots and CaF2 [110]∥Si[001] in the stripes. In this work, initial stages of CaF2 epitaxial growth on Si[001] were studied by atomic force microscopy and photoemission spectroscopy with emphasis on formation and chemical composition of the wetting layer.