Microstructure and electrical properties of zinc oxide thin film varistors prepared by RF sputtering

The thin film varistors of ZnO-Bi2O3 multilayer junctions were fabricated by RF sputtering. The nonlinear I-V characteristics and nonlinear coefficient, alpha, under reverse bias were found to be effected by the composition and structure of the varistor multilayers. The threshold voltage is predominantly determined by the microstructure and thickness of the Bi2O3 layer in thin film varistors, while that can be tuned by altering the donor density in ZnO, which was achieved by varying Al doping concentration or sputtering conditions. The higher leakage current and lower nonlinear coefficient associated with the ZnO layer doped with Al (ZnO:Al) can be improved by inserting another ZnO layer doped with selective transition metal impurities between ZnO:A1 and Bi2O3 layers. The microstructure and defects of the multilayers were investigated in detail and related to the performance of the electrical properties.