Defect dynamics in ZnO varistors under electrical stress: A potential method to improve electrical properties

ZnO-Bi₂O₃ based varistors are prepared by solid-state sintering and defect dynamics in the varistors under AC and DC stress are investigated using dielectric spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. By combining experimental evidence, we find that the dynamics of the occupation of the interface states by electrons is influential in ZnO varistors under AC stress and at the early stage of DC stress, which has not been noticed before. The process is beneficial to the enhancement of electrical properties of varistors by modifying double Schottky barriers at grain boundaries. After 302 h of AC stress, the breakdown field (E_B) increases from 236.2 V/mm to 248.7 V/mm and the leakage current density (J_L) decreases from 0.062 μA/cm² to 0.025 μA/cm². After 184 h of DC stress, in the same direction as the stress E_B increases from 235.2 V/mm to 243.7 V/mm and J_L decreases from 0.065 μA/cm² to 0.059 μA/cm².