Fabrication of ZnF₂ thin films and their vacuum ultraviolet transparency

ZnF₂ thin films were deposited on SiO₂-glass and Al₂O₃-single crystal substrates using electron-beam evaporation, and the morphology of the films was studied in relation to the kind of substrate and deposition conditions. While the films deposited on SiO ₂-glass, (112̄0)-Al₂O₃ substrates exhibited an almost non-oriented polycrystalline nature, the films deposited on (0001)-Al₂O₃ substrates were epitaxially grown (100)-oriented films. The full-width at half maximum of the rocking curve of (200)-diffraction for the (100)-oriented film deposited at 400 C was 150″. This value was comparatively small compared with previously reported (100)-oriented SnO₂ and TiO₂ films and showed the high orientation and crystallinity of the ZnF₂ film. From the transmission spectra from near-infrared (NIR) to vacuum ultraviolet (VUV), the optical band gap of the ZnF₂ was evaluated to be 6.7 eV, and the films exhibited high transparency in the wide range from NIR to VUV. We discuss the potential of the film as a transparent conducting material in the VUV region.