Dynamics of Oxygen Vacancies and Defect Complexes in the Perovskite Oxide Structure
Open Access
- Author:
- Maier, Russell Alan
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 18, 2014
- Committee Members:
- Clive A Randall, Dissertation Advisor/Co-Advisor
Susan�Trolier�McKinstry, Committee Chair/Co-Chair
Patrick M Lenahan, Committee Member
Shujun Zhang, Committee Member
Long Qing Chen, Committee Member - Keywords:
- oxygen vacancy
EPR
impedance
defect complex
ionic conductivity - Abstract:
- A comprehensive description of the methodology necessary to completely characterize and analyze the concentration and kinetic parameters related to oxygen vacancies in oxide dielectrics is presented. An emphasis is made on the characterization of defect complexes formed by the nearest neighbor pairing of acceptor dopant or impurity ions with oxygen vacancies. By using a range of experimental techniques backed by a thorough re-analysis of studies available in the literature, a new model for the local interaction of oxygen vacancies with acceptor ions is presented. An important conclusion of the presented material is that the method used to measure transport processes related to conductivity has a critical influence on the kinetic parameters that are obtained. General expressions for vacancy mobility are typically applied over wide ranges of experimental and material parameters. As depicted in Figure 1, however, the measurement technique that is used will typically probe orders of magnitude in length scale. Figure 1 Local interaction of oxygen vacancies with acceptor defect centers at various length scales. The interaction of oxygen vacancies with their local surroundings is influenced by changes in temperature, defect concentration, and applied electric bias. By defining an oxygen vacancy potential energy landscape, a unified model for oxygen vacancy mobility can be derived that explains the dynamics of oxygen vacancies at all measurable length scales. Large Length Scales (Oxygen Vacancy Mass Transport) A new scaling law that can be used to predict mean time to failure of multilayer ceramic capacitors is introduced. The effectiveness of the common practice of using an Eyring equation to predict the rate of voltage accelerated resistance degradation in ceramic capacitors has recently been shown to decline when it is applied to the testing of samples with thin dielectric layers. Using an electrochemically driven mass transport description of oxygen vacancy migration, the diffusion equation for oxygen vacancies can be derived. This equation, when combined with the full expression for voltage-driven vacancy hopping, produces a better highly accelerated lifetime scaling law that better explains failure mechanism in capacitors with thin layers. Low temperature conductivity data for acceptor doped SrTiO3 and BaTiO3 are presented. Ionic mobility and diffusivity measurements are typically made at high temperatures in order to speed up the kinetics of sample equilibration with the atmosphere as well as the mass transport of oxygen vacancies. The interpretation of high temperature data can be difficult because additional charge compensation mechanisms must be accounted for, such as intrinsic Schottky disorder and impurity redox reactions. To fit high temperature conductivity data, many unknown mass action and mobility parameters must be used to explain a single conductivity curve. Additionally, the measurement of ionic mobility is difficult because high temperature conductivity is typically controlled by electronic carriers, leaving a small transference number for ionic conductivity. High concentrations of oxygen vacancies can be quenched into a sample at low temperatures, ensuring that low temperature conductivity is controlled by ionic conduction. This technique was used to measure the mobility of oxygen vacancies in acceptor doped BaTiO3 and SrTiO3. The results of these experiments provided values for the enthalpy of migration of oxygen vacancies in the range of 0.7-0.8eV. These measured values are substantially less than the values typically measured using high temperature conductivity methods. However, these lower activation energy values agree well with measurements made using tracer diffusion experiments. Intermediate Length Scales (Distributed Dipole Relaxation) In the past, defect dipole-related relaxations have been measured using thermally stimulated depolarization measurements in the SrTiO3 system as well as other materials in the paraelectric phase. Here, data are presented that show, under dc bias, nearest neighbor defect dipoles do not align in the cubic phase of a material. This result carriers strong implications, not only for the way some dielectric relaxations are characterized, but also for the analysis of ferroelectric aging mechanisms. EPR analysis reveals that defect dipoles do not align under an applied field and that their association energies are on the order of 0.3eV. A proposed model is introduced for the potential energy landscape of the oxygen vacancy sublattice in order to describe the temperature and concentration dependence of oxygen vacancy defect association. This model is used to explain the thermally stimulated depolarization current measured in the iron doped SrTiO3 system. It is determined that, even though nearest neighbor defect dipole alignment is not likely in this system, a net polarization of a distributed defect dipoles can explain the depolarization current and EPR behavior. Small Length Scales (Nearest Neighbor Defect Interaction) Finally, data on the defect association of oxygen vacancies with manganese and iron acceptors in BaTiO3 single crystals is presented. A new resonance feature is found in manganese doped BaTiO3 samples that is argued to be a signal related to defect complexes. It is determined, using Newman superposition analysis, that defect association is minimal for these particular systems doped with acceptor concentrations of 0.5mol%. These nearest neighbor defect complexes only exist in small concentrations; they are not the majority Mn2+ related defect center. These defect complexes are experimentally shown to orientate along the direction of spontaneous polarization in the ferroelectric phase. The time-dependent orientation of these defect centers can be observed using in situ EPR techniques. The relative signal intensity of these defect complexes, however, when compared to the signal intensity of fully coordinated manganese centers suggest that nearest neighbor oxygen vacancy defect association is minimal in this system. As a result, higher order defect complexes (defect complexes with oxygen vacancies in next nearest, next next nearest, etc.. neighbor positions) must be considered when modeling oxygen vacancy transport in the vicinity of an acceptor point defect center. In conclusion, a first approximation potential energy landscape model, based on the assumption that Coulombic interactions provide a larger driving force for defect association compared to strain interactions, can be used to describe the effect of temperature, dopant concentration, and applied field on the distributed site occupancy of oxygen vacancies in the vicinity of a counter charged ion. In the past, experimental techniques like EPR and bulk conductivity experiments have not been able to describe the local distribution of oxygen vacancies in the perovskite lattice. Predicting the probability of site occupancy using the model introduced here allows a single, unified, physical description of the local lattice potential which describes the low temperature conductivity and depolarization phenomena. The potential landscape model can also be used to explain the differences between defect association behavior in SrTiO3 and BaTiO3. Proposed future EPR work will lead to an improved association model that can account for the combination of strain as well as electrostatic driving forces.