Microcontact impedance measurements of individual highly resistive grain boundaries:: General aspects and application to acceptor-doped SrTiO3

The electrical properties of grain boundaries in polycrystalline materials can vary from boundary to boundary and conventional impedance measurements yield only averaged information. Local measurements at individual grain boundaries using microelectrodes can partly overcome this restriction. Finite element calculations on the impedance of such microelectrode measurements are performed to reveal the interrelations between the measured grain boundary impedance and the grain boundary properties (grain boundary resistivity, permittivity and geometry). We will discuss how far adjacent grain boundaries influence the results. In particular, the calculations revealed that even in the case of identical grain boundaries a considerable part of the current between two microelectrodes may flow across neighboring grain boundaries. It is shown how these additional current paths can be taken into account with respect to a reliable data analysis. Experiments on SrTiO3 polycrystals demonstrate that spatially resolved impedance investigations on single grain boundaries are possible in this way. The results are interpreted in terms of a double Schottky barrier at the grain boundaries. (C) 2000 American Institute of Physics. [S0021-8979(00)05205-1].