Observation of an electrical breakdown at ZnO Schottky contacts in varistors

We report an electrical breakdown (EBD) at metal/ZnO junctions in varistors with Ag-Pd and Pd electrodes. It was found that these junctions show Schottky behaviour with a mean Schottky barrier height (SBH) of 0.70 +/- 0.07 eV for Bi-based varistors with Ag-Pd (80-20 %) electrodes and a mean SBH of 0.47 +/- 0.03 eV for Pr-based varistors with Pd electrodes. All investigated electrode-to-grain junctions (EGJ) exhibited an EBD of the Schottky barrier in the reverse current direction, whereas the mean EBD voltage for the Bi-based varistors is 3.5 V and 2.5 V for the Pr-based varistors. Observation of electroluminescent light at the EGJ interface clearly indicates that the EBD is related to electron-hole generation, similar to the EBD at the grain boundaries in the ZnO microstructure. This shows that a similar good but asymmetric varistor effect, as it is observed at grain boundaries, can be observed at EGJ.