Epitaxial growth mechanisms and structure of CaF[sub 2]/Si(111)

The early stages of interface formation between CaF[sub 2] and Si(111) have been studied, [ital in] [ital situ], by a combination of reflection high-energy electron diffraction, x-ray diffraction, and core-level photoemission. The results are combined with [ital ex] [ital situ] transmission-electron-microscopy measurements to show that the initial growth mode changes from Volmer-Weber to Stranski-Krastanow, depending on the substrate temperature. The crossover is correlated with a submonolayer transition from the Si(111)-(7[times]7) to a (3[times]1) reconstruction. This is accompanied by fluorine dissociation at the interface. Both initial growth modes can lead to a uniform CaF[sub 2] epilayer and subsequent growth on this surface is layer by layer. Using x-ray crystal truncation-rod analysis, we have examined the CaF[sub 2]/Si(111) surface and interface structure. For films grown at temperatures above the (7[times]7)[r arrow](3[times]1) transition, the Ca atom in the CaF layer at the interface is located in a single [ital T][sub 4] bonding site. Finally, we have observed a structural transition at the interface from the as-grown structure to a ([radical]3 [times] [radical]3 )[ital R]30[degree] reconstruction, which appears to be incommensurate. The dynamics of this transition and the possible mechanisms will be discussed.