Analysis of defect formation in Nb-doped SrTiO 3 by impedance spectroscopy
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The thermal activation energies for conduction of Nb-doped SrTiO3 grains and grain boundaries have been investigated by impedance spectroscopy. First, to observe the effect of electrode/SrTiO3 bulk interface, the varied impedances of SrTiO3 single crystal were measured with temperatures. The activation energy of an electrode/bulk interface was determined to be 1.3 eV, whereas that of bulk was 0.8 eV. When the impedances of Nb-doped SrTiO3 ceramics were measured, it was suggested that the more precise impedance values of a single grain and a single grain to grain junction be obtained using a microelectrode method. The activation energies for a grain, a grain boundary, and an electrode/bulk interface were determined to be about 0.8, 1.3, and 1.5 eV, respectively. From these measured results, it was suggested that the activation energy, 0.8 eV, measured in grain was originated from oxygen vacancies and the activation energy, 1.3 eV, in grain boundary was from strontium vacancies.
I. INTRODUCTION
SrTiO3 is one of the most important electroceramic materials. Since semiconductive or conductive characteristics in SrTiO3 can be obtained by doping donors or acceptors, SrTiO3 is widely used as a basic material in a broad range of electronic applications.1–3 Among these, recently, a new type of SrTiO3 has been reported, which has not only a high dielectric constant but also nonohmic electrical characteristics.4 Since the dielectric constants of SrTiO3 is several hundred times larger than that of ZnO, which is widely used in a base material of varistor, SrTiO3 varistors acquire a large surge capability and a sharp pulse response.5 More recently, SrTiO3 has been used as a suitable substrate for depositing thin films of various oxides such as high-Tc superconducting materials as YBa2Cu3O7−␦.6,7 And grain boundary Josephson junctions in high-temperature superconducting thin films grown on SrTiO3 bicrystal substrates have been studied extensively for fundamental science and for many applications.8,9 Since these components are generally known to be used for the electrical characteristics of a grain boundary, many scientists have investigated the origin of electrical characteristics for the SrTiO3 grain boundary. While it is accepted that there is a high acceptor state at the grain boundary, which creates back-to-back Schottky barriers, as yet there is no generally accepted explanation for its origin. In this study, we investigated that a type of defect formed in grains and grain boundaries by thermal activation energies calculated from the variances of imped192
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J. Mater. Res., Vol. 16, No. 1, Jan 2001 Downloaded: 14 Mar 2015
ance values. In order to acquire more precise values, a microelectrode method was applied, when an impedance spectroscopy was performed. It was suggested that more precise impedance values can be measured using the microelectrode method in our previous work.10
II. EXPERIMENTAL PROCEDURE
The specimens were prepared from commerically available SrCO3, TiO2, and Nb2O5 (艌99.9%, a
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