Meanwhile, the decrease of Eg with the increase of thickness is likely to be attributed to the increase of lattice constant [7,21–23]. From the XRD results, we suggest that the two absorption edges observed in trace (a) are due to poor crystallinity of ZnGa2O4:Cr3+ film.
Fig. 6 shows the PL and PLE spectra of ZnGa2O4:Cr3+ powder and thin film phosphors grown on quartz substrates at oxygen pressure of 0 and 1 Pa, respectively. Fig. 7 shows an energy-level diagram of ZnGa2O4:Cr3+ based on the measurement results.