Electron holography study of active interfaces in zinc oxide varistor materials

Abstract

Electron holography measurements on individual interfaces in ZnO varistor material have been performed in the transmission electron microscope and correlated with the interfacial microstructure. The interfacial microstructure, including the local chemistry, has been determined using scanning transmission electron microscopy in combination with energy-dispersive x-ray spectrometry. The grain boundary charge was directly determined from electron holography to be negative, with a positively charged depletion region across it. The phase information has been analyzed to reveal several aspects of the double Schottky barrier, including the barrier height and the depletion region width. Variation in sample thickness was compensated for using a standard electron energy-loss log-ratio method combined with a total-inelastic mean free path calculated from scattering theory. From a potential line trace across the interface the barrier height was measured to be 0.71±0.20 V and the depletion region width estimated to approximately 150 nm. Evaluation of the mean inner potential of a ZnO grain from the phase map yielded a value of |V0|=21±4.2 V. Calculations of the mean inner potential using a nonbinding approximation and the Doyle–Turner scattering amplitudes, yielded, however, a lower value, |V0|=16.1 V. An asymmetric spatial extent of the depletion region was observed. These findings are compared to previously reported results from active interfaces in ZnO varistor materials.