The dynamics of electrostatic potential barriers at grain boundaries in Nb-doped bicrystals were investigated by using a combination of bulk and in situ transmission electron microscopic electrical measurements across isolated grain boundaries, coupled with electron holography under an in situ applied bias. The Nb bulk-doped bicrystals exhibited a positive grain boundary potential which was suppressed reversibly under an applied bias that was greater than the non-linearity threshold in the current-voltage curve. This suppression was interpreted as reflecting a breakdown of the potential barrier to current transport. Results on Nb bulk-doped bicrystals were compared to those in which Mn had been added as a grain boundary-specific dopant. Such acceptor doping of the grain boundary caused an appreciable increase in the grain-boundary resistance, and an extension of the non-linear regime. Static electron holography revealed a relatively flat potential profile across the grain boundary; thus probably indicating the compensation of donor states - at the grain boundary core - with Mn acceptors. In this case, the phase profile under an applied bias exhibited a reversible so-called dip at the grain boundary. This was interpreted as being due to the activation of grain boundary trap states because the Mn-acceptor dopants trapped extra electrons (majority charge carriers) at the grain-boundary core. This introduced a negative grain boundary potential, and decreased current transport until the threshold bias was exceeded.

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3. K.D.Johnson, V.P.Dravid: Interface Science, 2000, 8[2-3], 189-98