Interface states in ZnO varistor with Mn, Co, and Cu impurities
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The interface states in ZnO with impurities of transition-metals, Mn, Co, and Cu, were investigated by the DLTS (deep-level transient spectroscopy) measurements in ZnO/PrCoO^/ZnO junctions as model systems of ZnO ceramic varistors and by the SCF-Xa-SW molecular orbital calculations using simplified cluster models. The DLTS signals, correlated to the doping of Mn and Co, are obtained with ZnO/PrCo^/ZnO junctions. The signals correspond to the interface states due to the transition-metal doping. X a calculations indicate that the interface states attributed to the doping of transition-metals, Mn, Co, and Cu, in ZnO are created between the valence band and the conduction band, which consist of transition-metals 3d character. The impurities of transition-metals affect interface states as well as the adsorbed excess oxygen.
I. INTRODUCTION The ZnO-based ceramics with small amounts of metal oxides such as Bi, Pr, and Co exhibit nonlinear voltage-current (V-I) characteristics.1'2 These devices are widely used for protection against transient voltage surges in electronic circuits. An extensive research effort has been aimed at characterizing ZnO grain boundary regions in order to explain the mechanism of the breakdown V-I characteristics of these materials." A model of a double Schottky barrier with a defect (DSB-defect model) 8 proposed by Pike9 provides a consistent explanation for all the observed prebreakdown and breakdown phenomena. Realistic calculations of the steady state and time dependent properties were worked out by Blatter and Greuter.10'11 In the DSB-defect model, interface states are most important for the "varistor effect". Interface states in varistors can be detected by monitoring the recovery of the nonequilibrium grain-boundary barrier capacitance,12'13 by the V-I deconvolution technique,14 and by the deep-level transient spectroscopy (DLTS) method15 or the isothermal capacitance transient spectroscopy (ICTS) method.16 From these measurements, it was reported that a large density of interface states are observed 0.9-1.0 eV15 and 0.6-0.7 e V 13 ' 16 ' 17 below the conduction-band edge. Furthermore, they indicated experimentally that the larger density of interface states leads the higher nonlinearity of V-I characteristics,14'15 and that the degradation in varistor is caused by the change of interface states.18 However, the physical origins of interface states are still unclear. The candidates for the origin of interface states are adsorbed excess oxygen,15'19 a 'Address
correspondence to this author. address: Department of Applied Physics, Osaka University, Suita, Osaka 565, Japan.
b) Present
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http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 1, Jan 1994
Downloaded: 13 Mar 2015
impurities of Mn or Co, 8 dangling bonds in ZnO lattice,20 and strain effects caused by large ions such as Bi and Pr in ZnO. 13 In experiments on ceramic varistors, it seems to be difficult to clarify the interface phenomena. This is, in part, due to the many different factors such as grain size, density, cryst
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