| In order to study the individual grain boundary double Schottky barrier structure of the ZnO varistor, photolithographically prepared microelectrodes were used to probe single grain boundaries on the surface of a commercial ZnO varistor material. Characterizations of current vs. voltage (I-V) over a range of temperature, impedance spectroscopy, and deep level capacitance transient spectroscopy (DLTS) were carried out on a large number of single grain boundary junctions. Results showed that there are large variations among the individual grain boundaries in terms of the I-V characteristics, the barrier height and its pinning against external bias voltage, and the parameters of the interface states giving rise to the grain boundary electrical barrier. Asymmetry in I-V characteristics and capacitance transients was observed and explained in terms of the involvement of bulk parameters of the two adjacent ZnO. The electronic structure of the boundary between a ZnO grain and the triple junction region was also discussed, based on its asymmetry in both the I-V and the barrier height pinning characteristics between the two sides of the boundary. Capacitance transient analyses, DLTS measurements using incremental bias magnitudes, and impedance spectroscopic analyses all appear to indicate that the energy of the interface states at a single grain boundary spreads over a range. The degree of this energy spread was found to be on the order of magnitude of several tenths of an electron volt and to decrease with increasing temperature. Possible reasons for such energy broadening were examined, including grain boundary dislocations and/or impurities associated with dislocations. |
