Electrical properties in semiconductive SrTiO_3 bicrystals

SrTiO_3是一种重要的电子陶瓷材料。SrTiO_3多晶具有独特的电学性质，常用于各种器件、电容器等，其电学性质主要是由于晶界形成的势垒引起的。这种电性能被揭示依赖于晶界字符。但确切的机制尚未阐明。本研究从点缺陷充电的角度研究了晶界电输运现象的机理，为了控制晶界结构，采用了热连接技术制备的双晶材料。在低角度晶界，晶界结构由晶界位错HRTEM研究。它们的密度是倾斜角度的函数：它随着倾斜角度而增加。发现电流-电压行为的非线性随着倾斜角度的增加而增加。即，非线性，即，势垒的高度是晶界位错数量的函数。通过研究非线性系数与晶界位错数之间的关系，可以估计与一个晶界位错相关的俘获电子数约为10^5个/cm。另一方面，发现非线性系数随着从退火温度开始的冷却速率而变化。非线性的大小随冷却速率的减小而增大。重要的一点是，低角度晶界具有晶界位错作为其晶界结构。即，除非倾斜角改变，否则结构不改变。冷却速率的变化引起的非线性的变化不是由晶界结构的变化引起的。根据这些事实，可以得出结论，额外态的起源是由于阳离子型缺陷的充电。

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.