Annealing-atmosphere control of paramagnetic defects in ZnO microparticles
In this work, we used temperature-dependent electron paramagnetic resonance (EPR) spectroscopy to reveal the effect of high-temperature annealing in N2 and air on paramagnetic and electronic properties of ZnO microparticles. EPR spectral simulations have shown that N2 annealing produces Zni, VO+, and shallow-donor centers, whereas Zni, VO+, and Hi-related centers are observed after annealing the ZnO microparticles in an air atmosphere. The resolved paramagnetic centers exhibit distinctly different behavior over 5-297 K. The shallow-donor and Hi signals are rapidly quenched below approximately 50 K, while the Zni contribution shows a nonmonotonic temperature dependence. In contrast, the VO+ signal persists up to room temperature and increases with temperature, consistent with charge redistribution among donor-related defects involving deep oxygen-vacancy states. The cavity Q-factor generally decreases with increasing temperature for both annealed samples, indicating atmosphere-dependent changes in the microwave losses. These results show that the annealing atmosphere controls the paramagnetic defect population, the temperature dependence of donor-related centers, and the associated electronic response of ZnO microparticles.