Evaluation of Laminated Thin-Film ZnO Varistor by PLD

Laminar-type thin-film ZnO varistors were fabricated on sintered alumina substrates using visible light (532nm) pulsed laser deposition (PLD). The structure of the laminar-type thin-film varistor is Ni / Co-added ZnO / impurity layer / Co-added ZnO / Ni. Many droplets were observed on the deposited Bi2O3+MnO2 compared with the deposited Co-added ZnO thin film. Moreover, for droplets on the Bi2O3+MnO2 layer, the content of Mn was higher than that of Bi. The V-I characteristics of the deposited ZnO+CoCl2 or Bi2O3+MnO2 thin film were ohmic. However, V-I characteristics of laminar-type thin film including the Bi2O3+MnO2 impurity layer deposited for 30min showed nonlinearity. The non linearity index α was approximately 2 and the varistor voltage was approximately 1V. Thermal annealing in N2 gas atmosphere at 700°C for 10 min was carried out to improve the crystallinity of the thin film. After annealing, both the varistor voltage and the current at which nonlinearity appeared decreased. Moreover, the value of non linearity index α was approximately 2.8.