Epitaxial Calcium and Strontium Fluoride Films on Highly Mismatched Oxide and Metal Substrates by MOCVD: Texture and Morphology

The growth of epitaxial CaF 2 and SrF 2 thin films on single crystalline r -cut sapphire, MgO (001) and biaxially textured Ni−W polycrystalline tape by low-temperature MOCVD is reported. A novel and efficient combination of alkaline-earth and fluorine precursors was used for deposition. A comprehensive study regarding the out- and in-plane orientation of the films and their surface morphology is presented using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), energy-dispersive X-ray analysis (EDX), field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and electron backscatter diffraction (EBSD). The grown films are shown to have different crystallographic orientation depending on the film−substrate lattice mismatch and growth rate. Both types of films were obtained with pure c -axis orientation on all used substrates by either choosing the appropriate deposition conditions or postdeposition treatment. Epitaxial relations for all grown films are determined. A film−substrate interaction is described for the case of Ni−W substrate; a way to avoid it is proposed and successfully implemented. Certain growth conditions are shown to result in a unique three-dimensional ordered nanogrid structure of the films, making them perfect nanotemplated substrate for the epitaxial growth of other functional layers.