SINGLE JUNCTIONS IN ZNO VARISTORS STUDIED BY CURRENT-VOLTAGE CHARACTERISTICS AND DEEP-LEVEL TRANSIENT SPECTROSCOPY

The electronic properties of individual grain boundaries in ZnO varistors were characterized by current-voltage (I-V) measurements and high-temperature zero-bias deep level transient spectroscopy (DLTS). A single-junction electrode pattern was designed using photolithography in order to study these properties. It was found that interface trap energy levels and capture cross sections vary with the polarities of trap filling pulses. The behavior indicates that the grain boundary potential barriers are not symmetric. Asymmetry was also observed in I-V measurements. Intergranular differences in chemical composition, distribution of chemisorbed oxygen, and grain boundary microstructure were suggested to be responsible for the asymmetry in electronic properties.