Fabrication of ZnF2 thin films and their vacuum ultraviolet transparency

• ZnF2 thin films deposited on both sapphire and SiO2-glass substrates.
• (100)-oriented epitaxial film was obtained on (0001)-sapphire substrate.
• The optical band gap of the ZnF2 film was 6.7 eV.
• Potential of the film as transparent electrodes in the VUV region is discussed.

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 SiO2-glass, (112¯0)-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.