Y.W. Hong, M.J. Ha, J.H. Paik, J.H. Cho, Y.H. Jeong, J.S. Yun, W.I. Park
Korea Institute of Ceramic Engineering & Technology,
Keywords: ZnO, Bi2WO6, varistor, defect, grain boundary, dielectric function
Summary:Polycrystalline ZnO has been applied to various electrical and optical applications such as gas sensors, piezoelectric transducers, phosphors, transparent conducting films, and varistors. To control the figure of merits of these devices it is essential to understand the behavior of defects and interface states in doped ZnO, especially in ZnO varistor. All of these applications are either affected by, or depend on, defects and grain boundary properties according to various dopants (Bi, V, Pr, Co, Mn…oxides). It is known that cobalt oxide added to ZnO improves the nonlinear current-voltage characteristics, which is presumably due to the formation of the Schottky-type double barriers at the grain boundaries. Also it is found that the presence of a large Bi, V or Pr ions induces the concentration of native defects of acceptor type in the grain boundary of doped ZnO. However, it is not known about the relationship between the defects and grain boundary properties in the simple three component system ZnO-Bi2WO6-Mn3O4 revealed good nonlinear current-voltage characteristics. In this study, we investigated the effects of Mn3O4 doping and Bi2WO6 additive (liquid phase sintering aid) in ZnO (it’s a new varistor system developed by us) on the defects and grain boundary properties using dielectric functions (Z*, M*, ε*, Y*, and tanδ). It will provide information about the relationship between the formations of donor or acceptor defects at the grain or grain boundary for the double Schottky barrier. Experimentally Mn-doped ZnO-Bi2WO6 sintered at several temperatures in air have two kinds of defects (Zn interstitial and O vacancy) according to its doping levels, a single grain boundary (apparent activation energy Ea=0.94~1.05 eV), and a good varistor properties (nonlinear coefficient α>69). The origin of varistor behavior in this system has been discussed in relation to grain boundary traps and defect chemistry based on the experimental results.