Structural and morphological evolution in magnetron co-sputtered (Zn, Cr)O films

In this study, x-ray diffraction, scanning electron microscopy, micro-Raman spectroscopy, x-ray absorption near-edge structure and particle-induced x-ray emission are used to characterize the microstructure of (Zn, Cr)O films prepared using a co-sputtering method. We found that the Cr ions did not substitute for the Zn sites but instead formed Cr nano-particles and secondary oxide phases (SOPs) of Cr₂O₃ and/or ZnCr₂O₄ in co-sputtered Zn_1-xCr_xO films with Cr content x ≥ 0.1. Evidence is presented for the evolution of SOPs formed in (Zn, Cr)O films with increasing Cr sputtering power. Based on the inspection of the Cr and Zn contents in (Zn, Cr)O films, we conclude that the formation of the Cr ₂O₃ phase is driven by a substantial increase in the atomic ratio of Cr/Zn, followed by the formation of a ZnCr₂O ₄ phase promoted by a higher content of Cr than of Zn in film with increasing Cr sputtering power. It seems that a strong preference of Cr for octahedral rather than tetrahedral coordination with oxygen would trigger the formation of SOPs rather than the substitution of Cr into Zn sites and could be an obstacle for achieving a real Cr-substituted ZnO dilute magnetic oxide.