Defects control and origins of blue and green emissions in sol-gel ZnO thin films

ZnO has broad application prospects in the field of LED lighting and display. However, the unclear luminescence mechanism limits the improvement of luminous efficiency and the realization of controllable luminescence. Herein, intrinsic defects (Zn_i and V_O) in sol-gel ZnO thin films were controlled by post-annealed in different atmospheres (air, N₂ and vacuum). The effect of annealing atmospheres on the intrinsic defects were quantitatively studied by XRD, Raman, XPS and EPR spectra, and the relationship between intrinsic defects and visible emissions were systematically investigated by excitation-dependent PL spectra under above bandgap and below bandgap excitations. Results show that N₂-annealed ZnO film with more Zn_i defects and fewer Vo defects exhibits much stronger blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). Zinc interstitials states (including Zn_i and ex-Zn_i states) are the initial states of the electronic transitions for the blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). In addition, transitions from the conduction band to the V_O level also emit a green peak at 525 nm, which can only be observed under above bandgap excitation. This work contributes to a better understanding of defect-induced blue and green emission in ZnO, which will help to achieve controllable luminescence and facilitate its commercial application as a luminescent material.