Dominance of deep over shallow donors in ZnO-based varistors

Abstract

Broadband admittance spectroscopy measurements of ZnO-based varistors are analyzed in terms of charge transport theory through double Schottky barriers, hence obtaining empirical evidence about the nonmajority of the shallow donor in n-type ZnO. The dominant defect species is found to be a deep donor, which agrees with recent first-principles calculations. This result consistently explains the observed frequency-domain non-Debye and time-domain nonexponential electrical response, and invalidates a key assumption underlying various electrical characterization studies of this material. We find two deep levels with fine structures. These fine structures are attributable to fluctuating chemical environments around the defects.