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 overla yer exhibits an incommensurately modulated structure parallel to the h eterointerface, consisting of a triangular network of discommensuratio ns, which separate domains with two different Ca site occupations H3 a nd T4. The 2D spatial modulation of the overlayer is induced by the co mpetition between the strong covalent-like Ca-Si bonding at the interf ace and the ionic Ca-F bonding in the film. Remarkably, the in-plane p eriodic lattice distortion generated at the interface propagates many atomic layers through the CaF2 film. Specular reflectivity and standin g wave measurements suggest that the CaF2-Si interface is unstable tow ards 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 inter face.