Field-Induced Point Defect Redistribution in Metal Oxides: Mesoscopic Length Scale Phenomena

MOBALLEGH, ALI. Field-Induced Point Defect Redistribution in Metal Oxides: Mesoscopic Length Scale Phenomena. (Under the direction of Prof. Elizabeth C. Dickey). The spatial redistribution of charged point defects under direct-current (DC) biasing can have significant implications for electroceramic device performance and lifetime. The transport behavior of point defects is regulated by the boundary conditions of the electrodes, which can block electronic charge and/or ion transfer across the interface to varying degrees. When the electrodes are impermeable to mass transport, there will be an accumulation of point defects in the near-electrode region that can lead to significant modifications in the local electronic carrier concentrations. Such defect redistribution is responsible for the longterm increases in leakage current in many capacitor devices via modification of the interface Schottky barrier at the reverse-biased cathode. While this leakage current enhancement is detrimental in capacitor devices, the phenomenon of lattice defect migration can be utilized to form novel functional behaviors, such as resistive switching in metal-oxides via modulation of the Schottky barrier or formation of nonstoichiometric filaments oriented along the applied field direction. The present work aims to understand the phenomenon of defect redistribution as a function of the initial defect chemistry state and electrode boundary conditions under the degradation process, using single-crystal rutile TiO2 as a model material. Experiments are performed as a function of degradation voltage and crystallographic orientation since the self-diffusion coefficients of oxygen vacancies and titanium interstitials are known to be highly anisotropic in rutile. Rutile single crystals are equilibrated at specific oxygen partial pressures and temperatures to define the initial defect chemistry state. Platinum electrodes, which form Schottky contacts and are largely impermeable to oxygen transfer, are deposited on opposite faces of the crystal. The samples are then subjected to up to 200 V/cm electric field at 200 ̊C while the leakage current is continuously monitored. To understand spatial variations in chemistry and possible changes in microstructure, we utilize a combination of cathodoluminescence spectroscopy (CL), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). After electrical degradation, correlating electrical characterization measurements with electron microscopy analyses provides insight into the redistribution of point defects as a function of electric field and time. Diode-like rectification behavior was observed in crystals subjected to an applied voltage in the low electric field regime (< 75V/cm). One-dimensional and homogenous defect redistribution along both <010> and <001> results in accumulation of point defects and the formation of highly reduced substoichiometric regions near the cathode, which leads to the Schottky barrier degradation. The CL spectroscopy shows that titanium interstitials dominate the point defect redistribution process in this region. The reversibility of the rectification behavior, examined for both crystallographic directions, shows that the process can be influenced by the anisotropy of rutile. At degradation fields on the order of 56 V/cm at 200°C, although the degradation of Schottky barrier is mostly reversible along <010>, formation of extended structural defects is not recovered during the application of a reverse bias and results in an irreversible rectification behavior along <001> direction. We also identify electric field regimes (> 175 V/cm) in which the concentrations of point defects become large enough to induce higher-dimensional defects such as dislocations and the formation of Magnéli phases. We find that the condensation of point defects into Magnéli phases at the electrodes depletes point defect concentration in the bulk, thus increasing the bulk resistivity. The Magnéli phases formed near the cathode are found to be stable, and not reversible, at 200 ̊C for the times and fields studied. The defect condensation processes have significant impacts on the overall I-V behavior of the material and the ability to switch the total resistance. The electroformation of conductive filaments, another dominant degradation mechanism, is observed on samples with slightly higher initial bulk resistivity (about 80 Ω.cm) at field levels as low as 150 V/cm. The redistribution of ionic carriers leads to heterogeneity in the chemistry of TiO2 in the form of nonstoichiometric filaments oriented along the applied field direction. The CL spectra taken from conductive filaments and nonfilaments regions indicate a noticeable increase in the concentration of the titanium interstitials of the filaments. We demonstrate that the filaments can be disconnected from the electrodes with subsequent reverse-polarity biasing, demonstrating switching on a macroscopic crystal. This work was supported by the National Science Foundation under grant number