Microstructure and Electrical Properties of Zinc Oxide Thin Film Varistors Prepared by RF Sputtering
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E6.2.1
Microstructure and Electrical Properties of Zinc Oxide Thin Film Varistors Prepared by RF Sputtering Keng-Ming Chang1, Chuan-Pu Liu1, and Chon-Ming Tsai2 1
Department of Materials Science and Engineering, National Cheng-Kung University, Tainan,
Taiwan 2
Besdon Technology Corp. Taipei, Taiwan
ABSTRACT The thin film varistors of ZnO-Bi2O3 multilayer junctions were fabricated by RF sputtering. The nonlinear I-V characteristics and nonlinear coefficient, α, 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:Al and Bi2O3 layers. The microstructure and defects of the multilayers were investigated in detail and related to the performance of the electrical properties.
INTRODUCTION Nonlinear I-V (current-voltage) characteristic is the most important feature of varistors. It was usually described by I=KVα, where K is a constant and α is the nonlinear coefficient. It was believed that the nonlinear I-V characteristic of varistors was governed by the vicinity of grain boundary [1-3]. The mechanism of the nonlinear behavior has been ascribed to the formation of double Schottky barrier (DSB) proposed by Pike [3, 4]. The potential barrier is formed in order to equilibrate the Fermi level by charge transfer from bulk-electron-trap state to acceptor interface states. The theoretical calculations provide a consistent explanation for all of the experimentally observed nonlinear behavior. Applications of varistors are determined by the threshold voltage and nonlinear coefficient. Zinc oxide based ceramic varistors, which were fabricated by sintering with several additives of heavy metal oxides such as Bi2O3, Pr6O11, and CoO possess highly nonlinear I-V characteristics [5, 6]. Their threshold voltage can be adjusted by the number of grain boundary between two electrodes and were used widely for protection against transient voltage surges in electrical circuit. Actually, the connections between adjacent grain boundaries of the ZnO ceramic varistors are in complex manner both serially and parallel [1]. Since it is difficult to clarify the influence of individual features on the nonlinear characteristics, the precise control over the varistors properties by the traditional process is also difficult. Moreover, there is a general trend forward the shrinkage of the components in electronic devices, and it will become more difficult to fabricate smaller varistors by tape-casting due to the lack of precision in electrode printing and green type cutting. Thin film varistors of ZnO-Bi2O3 multilaye
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