The emission peak at 694 nm is caused by 2E→4A2 transitions of 3d electrons of Cr3+ which occupies the octahedrally coordinated
Ga3+ sites in the host lattice [25]. In addition, we can see that the intensity of the emission band is enhanced as the oxygen pressure is changed from 0 to 1 Pa. It may be caused by the good crystallinity, which can be confirmed by the XRD results from Fig. 1.
The PL and PLE spectra of ZnGa2O4:Cr3+ powder are shown in Fig. 6(b). From Fig. 6(a) and (b), we can see that the film and powder
samples exhibit similar PL and PLE spectra.