Point Defects in Metal Oxides and Their Interactions with Electric Fields

Lattice point defects and their coupling with electronic defects dominate the overall electrical properties of electroceramic materials, and thus great effort is expended on controlling the defect equilibria via oxygen activity during processing and/or doping.  In device applications, because the lattice defects are typically charged, the applied voltage provides a strong driving force for defect migration. When the electrodes are impermeable to mass transport, the spatio-temporal redistribution of defects can cause time-dependent increases in conductivity in many electroceramic devices. Our research into this phenomenon combines electrical transport measurements with electron microscopy analyses to understand the mesoscopic redistribution of point defects as a function of temperature, electric field and time. In systems for which electronic transport is limited by injection at the electrode, we find that the accumulation of the charged defects can modulate the electrode Schottky barrier, allowing for greater carrier injection. At high electrical potentials, the non-stoichiometry in the near-electrode regions can become very large, inducing defect clustering and ordering. The implications of this defect redistribution process and its reversibility are discussed within the context the overall electrical transport characteristics.
 
J.N. Baker, P.C. Bowes, D.M. Long, A. Moballegh, J.S. Harris, E.C. Dickey, D.L. Irving, “Defect Mechanisms of Coloration in Fe-doped SrTiO₃ from First Principles,” Applied Physics Letters, 110 (12) 122903 (2017). DOI: 10.1063/1.4978861.
J.-J. Wang, H.-B. Huang, T.JM Bayer, A. Moballegh, Y. Cao, A. Klein, E.C. Dickey, D.L. Irving, C.A. Randall, L.-Q. Chen, “Defect chemistry and resistance degradation in Fe-doped SrTiO₃ single crystal,” Acta Materialia, 108, 229-240 (2016). DOI: 10.1016/j.actamat.2016.02.022
A. Moballegh and E.C. Dickey “Electric-Field-Induced Point Defect Redistribution in Single-Crystal TiO₂ and Effects on Electrical Transport,” Acta Materialia, 86 (2015) 352–360  (2015). DOI: 10.1016/j.actamat.2014.11.032

Bio: Elizabeth Dickey a Professor of Materials Science and Engineering and the Director of the Center for Dielectrics and Piezoelectrics at North Carolina State University. Her research aims to develop processing-structure-property relationships for materials in which the macroscopic physical properties are governed by point defects, grain boundaries or internal interfaces. Particular emphasis is placed on understanding the role of these material defects on electrical and chemical transport in dielectric materials.  Her research involves using an array of analytical techniques, in particular electron microscopy and spectroscopy, to understand the structure and chemistry of materials at the nanometer length scale. She has over 150 peer-reviewed journal publications in these areas, which have over eleven-thousand citations. In 1999 she received the Presidential Early Career Award for Scientists and Engineers (PECASE) for her work on metal-ceramic interfaces.  In 2010 she became a fellow of the American Ceramic Society, has served on the Board of Directors and was awarded the Fulrath Award by the Society in 2012 in recognition of her technical contributions related to the characterization of functional ceramics and composites. She has served as an editor for Microscopy and Microanalysis and is currently an editor for the Journal of the American Ceramic Society.