The deep level transient spectroscopy studies of a ZnO varistor as a function of annealing

Abstract

Deep level transient spectroscopy measurements were performed to study the annealing-induced changes in trap centers in ZnO varistors and to shed more light on varistor stability mechanism. Two electron traps, Ec-0.26 eV and Ec-(0.2–0.3) eV, were observed in the unannealed samples in large quantities (7–9×1014 cm−3). The density of the Ec-0.26-eV trap gradually decreases to 4.7×1013 cm−3 at the annealing temperature of 600 °C. Beyond the 600 °C anneal, the Ec-0.26-eV trap begins to grow along with the appearance of another electron trap at Ec-0.17 eV. The minima in the Ec-0.26-eV trap density, coupled with our observation that unannealed devices are unstable whereas 600 °C annealed devices are most stable, suggest that the ZnO varistor instability is related to the Ec-0.26-eV trap. Our data support the ion migration model for device instability where the Ec-0.26-eV defect may be the Zn interstitial or the migrating ion.