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Researches on electrical degradation of double-Schottky barrier in ZnO varistors are reviewed, aimed at the constitution of a full picture of universal degradation mechanism within the perspective of defect. Recent advances in study of ZnO materials by atomic-scale first-principles calculations are partly included and discussed, which brings to our attention distinct cognition on the native point defects and their profound impact on degradation.

The migration and neutralization of charged defect ions during the degradation of a double-Schottky barrier are observed by performing nondestructive pulsed electroacoustic measurements on ZnO bicrystals. This offers the possibility to experimentally access the predicted defect migration behavior and provides a solid foundation to validate the theoretical aging model for electroceramics. Theoretical modeling of the acoustic attenuation effects during measurements is also discussed to understand and validate the experimental results, gaining insight into the widely used acoustic technique.

ZnO bicrystals with specific coincidence site lattice twist boundaries are manufactured for investigation. Asymmetrical I-V behaviors occur with maximum nonlinear coefficient of forward direction tenfold of reverse one. Energy dispersion spectra measurement reveals the nonuniform distributions of elements such as bismuth and oxygen in the grain boundary region. This reflects a higher Schottky barrier formed at Zn-polar face than that at O-polar face, which is confirmed by capacitance-voltage method. Such natural asymmetry may result from the effect of spontaneous polarization of polar surfaces of ZnO. Finally, simulation results prove the validity of the asymmetrical model proposed.

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