Fig. 4 shows the optical transmittance spectra of ZnGa2O4:Cr3+ films grown on quartz substrates at oxygen pressures of 0 and 1 Pa, respectively. The transmittance spectra show that the two films exhibit high transmittance in the 200–900 nm range. Transmission, however, falls very sharply in the 200–300 nm range due to the onset of fundamental absorption. The absorption edge of the 1-Pa film sample is shifted to a shorter wavelength with respect to that
of the 0-Pa film sample, which indicates that the optical energy gap
is enhanced.