Epitaxial growth mechanisms and structure ofCaF2/Si(111)

The early stages of interface formation between ${\mathrm{CaF}}_2$ and Si(111) have been studied, in situ, by a combination of reflection high-energy electron diffraction, x-ray diffraction, and core-level photoemission. The results are combined with ex 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)-(7×7) to a (3×1) reconstruction. This is accompanied by fluorine dissociation at the interface. Both initial growth modes can lead to a uniform ${\mathrm{CaF}}_2$ epilayer and subsequent growth on this surface is layer by layer. Using x-ray crystal truncation-rod analysis, we have examined the ${\mathrm{CaF}}_2$/Si(111) surface and interface structure. For films grown at temperatures above the (7×7)→(3×1) transition, the Ca atom in the CaF layer at the interface is located in a single ${\mathit{T}}_4$ bonding site. Finally, we have observed a structural transition at the interface from the as-grown structure to a (√3 × √3 )R30° reconstruction, which appears to be incommensurate. The dynamics of this transition and the possible mechanisms will be discussed.