Electrical properties of ZnO/Bi/sub 2/O/sub 3/ two-layer composites

A ZnO/Bi/sub 2/O/sub 3/ 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/Bi/sub 2/O/sub 3/ two-layer specimens increases with the thickness of the Bi/sub 2/O/sub 3/ layer and thus can be controlled by changing this layer. This is explained in terms of enhancement of the electric field at the ZnO/Bi/sub 2/O/sub 3/ interface by space-charge accumulation in the Bi/sub 2/O/sub 3/ layer and at the interface. TSC for reverse-bias poling voltage flows in the direction of polarization at temperatures >120 degrees C. This is explained by the space-charge accumulation in the Bi/sub 2/O/sub 3/ bulk and at the interface.<>