Formation and identification of secondary oxide phases in co-sputtered ZnO:Cr films

The secondary oxide phases in ZnO:Cr films have been observed to evolve from an initial Cr2O3 phase near the film surface to a final ZnCr2O4 phase throughout the film as the Cr content is increased. Two absorption bands, corresponding to the 4A2→4 T1 and 4A2→4 T2 transitions of Cr3+, can be observed in the Cr2O3-rich sample, whereas the ZnCr2O4-rich sample is opaque at wavelengths ≤ 500 nm. Our results suggest that the formation of secondary oxide phases in ZnO:Cr films is inevitable, leading to a much lower solubility of Cr.