Interface and overlayer structure of epitaxial CaF2 thin films on Si(111): An X-ray scattering study

Using X-ray scattering techniques we investigated a 50 Angstrom thick CaF2 film, epitaxially grown on a Si(1 1 1) substrate. The CaF2 overlayer exhibits an incommensurately modulated structure parallel to the heterointerface, consisting of a triangular network of discommensurations, which separate domains with two different Ca site occupations H3 and T4. The 2D spatial modulation of the overlayer is induced by the competition between the strong covalent-like Ca-Si bonding at the interface and the ionic Ca-F bonding in the film. Remarkably, the in-plane periodic lattice distortion generated at the interface propagates many atomic layers through the CaF2 film. Specular reflectivity and standing wave measurements suggest that the CaF2-Si interface is unstable towards the formation of an intermediate CaSi2 layer. In situ temperature-dependent strain measurements revealed that the strong interfacial Ca-Si bending prevents a complete in-plane lattice relaxation up to 700 degrees C while the overlayer expands considerably normal to the interface.