Direct evidence for grain boundary potential barrier breakdown via in situ electron holography

Static and dynamic grain boundary potential barrier effects are directly observed at high spatial resolution for a varistor of model structure and chemistry. Grain boundary mechanisms for nonlinear electrical behavior are investigated for Nb-doped SrTiO₃ bicrystals by in situ high-resolution electron holography under applied current coupled with electrical measurements. For the static case, holography reveals a positive grain boundary barrier of about 0.45 V, which is indicative of positive grain boundary charge adjoined by negative space charge regions. Under high applied current, in situ holography records the breakdown of this grain boundary barrier in accord with the macroscopic varistor effect, which is reflected in bulk I-V experiments.