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Ferroelectric Tunnel Junctions: Effects of In-Plane Domain Walls and Polar Interfaces
Ferroelectric tunnel junctions (FTJs), promising for applications in nanoscale electronics exhibit a tunneling electroresistance (TER) effect. This dissertation explores new physical mechanisms controlling the TER effect via first-principles density functional theory calculations. The formation of in-plane ferroelectric domain walls (DWs) in FTJs and their effect on TER are studied. It is demonstrated that a head-to-head charged DW can be formed in La0.5Sr0.5MnO3/BaTiO3/La0.5Sr0.5MnO3 FTJs due to symmetric polar interfaces resulting in a two-dimensional electron gas at the DW. The predicted in-plane DW is metastable but can be stabilized by proper engineering of the La1−xSrxO/TiO2 interface stoichiometry. This state becomes a global minimum for x ≤ 0.4, resulting in fully reversible triple polarization states as derived from the proposed phenomenological model. Transport calculations reveal a highly conductive DW state, due to resonant tunneling, and two less conductive uniform polarization states. This phenomenon manifests a new DW TER effect. A tail-to-tail DW in thin BiFeO3 films between SrRuO3 electrodes has been experimentally reported. A theoretical model involving oxygen vacancies and symmetric interfaces is introduced to explain this. While oxygen vacancies provide the necessary positive charge to stabilize the DW, the symmetric interfaces form the required electrostatic potential to keep the oxygen vacancies intact. This model is confirmed by the experimental measurements. Effects of polar interfaces on the mechanism of tunneling and TER are explored. Polar interfaces determine position of the Fermi energy and thus control if tunneling occurs via electrons or holes. Transition between electron/hole tunneling reverses the TER effect. It is theoretically predicted and experimentally confirmed that depending on the interface termination, TiO2/La0.7Sr0.3O or BaO/MnO2, Pt/BTO/La0.7Sr0.3MnO3 FTJs support either electron or hole tunneling, leading to opposite TER. Two-dimensional antiferroelectric tunnel junctions are proposed based on bilayer In2X3 (X = S, Se, Te) barriers, supporting switching between ferroelectric/antiferroelectric states. In-plane/out-of-plane tunneling across In2S3 bilayers is explored. The tunneling barrier height is shown to be changed upon ferroelectric-antiferroelectric order transition, leading to a giant TER and multiple non-volatile resistance states.
Condensed matter physics|Materials science|Computational physics
Li, Ming, "Ferroelectric Tunnel Junctions: Effects of In-Plane Domain Walls and Polar Interfaces" (2021). ETD collection for University of Nebraska - Lincoln. AAI28419733.