The color of the emission of zinc gallate (ZnGa2O4) oscillates between ultraviolet and blue by hydrogen ambient reduction and air ambient oxidation heat treatments. The photoluminescence spectra and electron paramagnetic resonance signals show that ultraviolet emission of reduced ZnGa2O4 always accompanies 680 nm emission originating from single oxygen vacancies (VO*). The increasing difference in binding energy between Ga3+ and O2− in reduced ZnGa2O4 indicates that the configuration of octahedral sites is distorted due to VO* generation and it becomes more ionic which shifts the emission band from 430 to 360 nm. The x-ray diffraction patterns and Raman scattering spectra show that β-Ga2O3 from ZnGa2O4 is formed in both reduction and oxidation processes which suggests the vaporization of Zn ions. We propose a model in which the origin of 360 nm emission is the Ga–O transition at distorted octahedral sites with VO* in ZnGa2O4, whereas 430 nm emission originates from the Ga–O transition of regular octahedral sites without VO* in ZnGa2O4.

Zn 1 − x Cd x O thin films were deposited on (0001) sapphire substrates by pulsed-laser deposition. Structural and optical properties of Zn1−xCdxO films were strongly correlated to the processing conditions. The composition of the films varied nonmonotonically as a function of deposition temperatures due to the difference of vapor pressure between Cd and Zn species. The optical energy bandgap of Zn1−xCdxO thin films, measured by photoluminescence and transmittance, changed from 3.249to3.291eV. The change of the optical properties was mainly attributed to the change of the stoichiometry of Zn1−xCdxO, as determined by Rutherford backscattering spectroscopy.

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