Electrical properties in semiconductive SrTiO_3 bicrystals

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SrTiO_3 is one of most useful electroceramic materials. It is well known that SrTiO_3 polycrystals exhibit unique electrical properties, and then, it is often used for varistic devices, condensers and so on. Their electrical properties are considered to be due to electrical potential barriers formed at grain boundaries. Such electrical properties were revealed to depend on grain boundary characters. But precise mechanism has not been clarified yet. In this study, the mechanism of electrical transport phenomena across grain boundaries were investigated from a viewpoint of charging-up of point defects.In order to control grain boundary structure, bicrystals were used, which were prepared by hot-joining technique. In low angle boundaries, grain boundary structure consisted of grain boundary dislocations from HRTEM studies. Their density is a function of tilt angles : it increased with tilt angles. Non-linearity in current-voltage behaviors was found to increase with tilt angles. Namely, non-linearity, i.e., a height of potential barriers, is a function of the number of grain boundary dislocations. By investigating the relation between non-linearity coefficient and the number of grain boundary dislocations, the number of trapped electrons related to one grain boundary dislocation could be estimated to be about 10^5/cm.On the other hand, it was found that non-linearity changes with cooling rate from annealing temperature. The magnitude of non-linearity increases with a decrease of cooling rate. The important point is that low angle boundaries have grain boundary dislocations as their grain boundary structure. Namely, the structure does not change unless the tilt angle changes. The change in the non-linearity caused by a variation of cooling rate is not resulted from a change in grain boundary structure. With these facts, it could be concluded that the origin of extra states is due to charging-up of cation type defects.