Grain-boundary interface electron traps in commercial zinc oxide varistors

A systematic study of the electronic characteristics of grain-boundary interface states was performed on a commercial zinc oxide varistor material using zero-bias deep level transient spectroscopy, current-voltage, high-frequency capacitance, and infrared reflectance techniques. Interface states, which act as deep acceptors, were found to exist at 0.97±0.12 eV below the conduction-band edge. The capture cross section is approximately 4×10−15 cm2. Characteristics of the interface states were determined as a function of the voltage of the trap filling pulse, which was varied from 0.9 to 2.0 V per grain boundary. It is shown that the trap energy is independent of the voltage of the pulse, while the apparent trap density increases with increasing voltage from 1.9 to 7.4×1011 cm−2. The results presented here indicate that this interface level is monoenergetic.