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

Abstract

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 (Zni and VO) 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 Zni 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 Zni and ex-Zni 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 VO 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.