Band-gap variation and a self-redox effect induced by compositional deviation in ZnxGa2O3+x:Cr3+ persistent phosphors
We made a systematic investigation on the optical properties of non-doped and Cr-doped ZnxGa2O3+x (0.98 ≤ x ≤ 1.02) spinel crystals. Absorption, photoluminescence excitation, and persistent luminescence excitation spectra indicated that the bottom of the conduction band (CB) was affected (i) by compositional deviation from stoichiometry or (ii) by atmosphere control in a similar way. By constructing an energy level diagram, a broad distribution of trap depth in a Cr3+-doped ZnxGa2O3+x sample with a composition of excess Zn or in one prepared in an O2 atmosphere was attributed to lowering of the CB bottom, which resulted in faster decay of persistent luminescence in these two samples. Defect chemical reactions revealed that the anti-site species of Zn′Ga may be the reason for lowering of the CB bottom. A self-redox effect of Cr ions induced by the compositional deviation was found and explained by the defect reactions. The investigation indicated that the composition with slight Zn deficiency is important to obtain a narrow distribution of trap depths and longer persistent luminescence, however the self-reduction effect of Cr ions in the Zn deficient composition also needs to be considered.