Epitaxy of CdS on SrF2

The epitaxy of wurtzite CdS on (111)-cleavage planes of Sr${\mathrm{F}}_2$ is examined theoretically and experimentally. The theoretical treatment is based on the assumption that the predominent interaction across the film-substrate interface is of an ionic type. The energy of a unit of the CdS films interacting with the substrate is calculated by lattice summation as a function of position and orientation. Based on a minimum-energy criterion, the calculation shows that it is more favorable for a sulfur layer rather than a cadmium layer to be immediately adjacent to the substrate. Further, the calculation yields the positions on an atomic scale of the atoms of the deposit relative to those of the substrate. Epitaxial films of CdS on Sr${\mathrm{F}}_2$ were grown using a chemical transport reaction and were shown to have the wurtzite structure with the [0001] direction of the deposit parallel to the [111] direction of the substrate. The [1¯100] direction of the film is perpendicular to the [10¯1] direction of the substrate. An experimental investigation of the film surfaces using an ion-scattering method verified the prediction that a sulfur layer is immediately adjacent to the substrate.