Electrical properties of ZnO/Bi2O3 two-layer composites

A ZnO/Bi₂O₃ two-layer composite formed by RF sputtering is studied as a model of a single-grain boundary in a ceramic ZnO varistor to clarify the mechanisms of degradation and nonlinear conduction and to realize a thin-film varistor. The current-voltage characteristics and the thermally stimulated currents (TSC) are measured. The composite under a reverse bias shows highly nonlinear conduction comparable to that in an actual ceramic varistor. The breakdown voltage of ZnO/Bi₂O₃ two-layer specimens increases with the thickness of the Bi₂O₃ layer and thus can be controlled by changing this layer. This is explained in terms of enhancement of the electric field at the ZnO/Bi₂O ₃ interface by space-charge accumulation in the Bi₂O₃ layer and at the interface. TSC for reverse-bias poling voltage flows in the direction of polarization at temperatures >120°C. This is explained by the space-charge accumulation in the Bi₂O₃ bulk and at the interface