Pulsed laser deposition growth of ultra-wide bandgap GeO₂ film and its optical properties

In this paper, we report the direct growth of ultra-wide bandgap GeO₂ film on the m-plane sapphire substrate by pulsed laser deposition. Raman scattering and x-ray diffraction measurements confirm that the obtained GeO₂ film has a (001)-oriented rutile structure mixed with the amorphous phase, and the film has an out-of-plane strain of -0.28% along the c direction. Transmittance spectra and x-ray photoelectron spectroscopy measurements determine that the transparent GeO₂ film has an ultra-wide bandgap of about 5.1 eV. Room temperature photoluminescence spectrum exhibits a broad blue-green emission band dominated by two peaks at about 2.4 and 2.8 eV. With the temperature decreasing to 21 K, the peak intensities increase exponentially accompanied by a slight blue-shift in the peak position. We believe that these findings will pave the way for applications of the wide-bandgap GeO₂ film in power and optoelectronic devices.