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The Deep Level Transient Spectroscopy (DLTS) technique was applied to characterize the interface states at the grain boundaries of ZnO varistors. Two types of commercially available varistors, Bi-type and Pr-type, were tested. The ordinary DLTS method for both varistors gave similar spectra showing two peaks. These two peaks were assigned as bulk traps located at approximately 0.2eV and 0.3eV below the conduction band. In addition to these experiments, a new technique of DLTS method, called zero-biased DLTS method, was tried for observing the interface states at the grain boundaries. By this method a peak for the interface states of ZnO varistors was detected at higher temperatures for the first time. The Arrhenius plots of the emission rate gave the depths of the interface states of 1.03eV and 0.94eV for Pr-type and Bi-type varistors, respectively. Numerical simulation of the DLTS peak revealed that the interface states are distributed monoenergetically. Other samples which contained one of Ce, Pr, Nd, Sm, Gd, and Tb showed similar peaks for the interface states. Although these peaks gave similar values around 1eV, the peak height depended on the element of the rare earth group. Furthermore, since the nonlinear exponent (α) depended strongly on the peak height, the concentration of the interface states should give a large effect on the nonlinearity of the ZnO varistors. The origin of the interface states was suggested to be identified as chemisorbed oxygen ion at the ZnO grain boundaries.

Characterization of the Interface States in ZnO Varistors by DLTS Method