Mechanically tuned conductivity at individual grain boundaries in polycrystalline ZnO varistor ceramics

Abstract

Mechanically induced changes in electrical properties have gained increasing interest over the last decade. The research field piezotronics, which describes the change in potential barrier height (e.g., at Schottky contacts) by piezoelectric charges, could lead to promising applications such as sensors or strain trigged transistors. In this contribution, the changes of potential barriers by mechanical stress at several grain boundaries of polycrystalline ZnO have been quantified. Rationalization is provided by concurrent measurement of crystal orientations of both neighboring grains. The highest changes in barrier height could be observed at ZnO–ZnO grain boundaries exhibiting varistor properties. In this case, the barrier height can be almost completely reduced by positive piezoelectric charges. Furthermore, an increase in barrier height is observed with negative piezoelectric charges. The developed physical model suggests an equivalent impact of externally applied voltage and induced positive piezoelectric charge on the barrier properties. In turn, not only the barrier height is modified by piezoelectric charges but also the breakdown voltage. This has, nevertheless, been only indirectly verified in experiments. These allow rationalizing the proposed model. Furthermore, a statistical study reveals a change in the distribution of breakdown voltages with changing stress conditions in the varistor ceramic.