Chromium doped ZnGa2O4 thin films: An X-ray absorption near edge structure (XANES) and X-ray excited optical luminesce (XEOL) study

We report the thermally controlled synthesis of Cr doped ZnGa2O4 thin films and the evolution of their morphology, crystallinity, and optical luminescence tracked with X-ray Absorption Near Edge Structure (XANES) and X-ray Excited Optical Luminescence (XEOL). It is found that the as-prepared ZnGa2O4 thin films doped with/without Cr are amorphous and exhibits no luminescence while annealing induces crystallization which markedly improves crystallinity at higher temperatures. SEM, XRD and O K-edge, Zn and Ga L-edge, and Cr K-edge XANES show disorder to order phase transition upon annealing which is accompanied by the appearance of the N2 line (696 nm) characteristic of the luminescence from Cr-doped ZnGa2O4 where Cr3+ occupies the octahedral Ga3+ site. The N2 line correlates with the crystallinity of the sample in that the higher the annealing temperature, the better the crystallinity and the brighter the N2 line. It is also interesting to note that the optical luminescence from the host ZnGa2O4, which appears in the blue (420 nm), is completely quenched and the energy is transferred to the N2 luminescence in the red (696 nm). These results and their implications are discussed.