Mechanically-Induced Resistive Switching in Ferroelectric Tunnel Junctions (original) (raw)
Recent advances in atomic-precision processing of oxide ferroelectricsmaterials with a stable polarization that can be switched by an external electric fieldhave generated considerable interest due to rich physics associated with their fundamental properties and high potential for application in devices with enhanced functionality. One of the particularly promising phenomena is the tunneling electroresistance (TER) effectpolarization-dependent bistable resistance behavior of ferroelectric tunnel junctions (FTJ). Conventionally, the application of an electric field above the coercive field of the ferroelectric barrier is required to observe this phenomenon. Here, we report a mechanically induced TER effect in ultrathin ferroelectric films of BaTiO 3 facilitated by a large strain gradient induced by a tip of a scanning probe microscope (SPM). The obtained results represent a new paradigm for voltage-free control of electronic properties of nanoscale ferroelectrics and, more generally, complex oxide materials.
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Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale
Nano Letters, 2009
Stable and switchable polarization of ferroelectric materials opens a possibility to electrically control their functional behavior. A particularly promising approach is to employ ferroelectric tunnel junctions where the polarization reversal in a ferroelectric barrier changes the tunneling current across the junction. Here, we demonstrate the reproducible tunneling electroresistance effect using a combination of Piezoresponse Force Microscopy (PFM) and Conducting Atomic Force Microscopy (C-AFM) techniques on nanometer-thick epitaxial BaTiO 3 single crystal thin films on SrRuO 3 bottom electrodes. Correlation between ferroelectric and electronic transport properties is established by the direct nanoscale visualization and control of polarization and tunneling current in BaTiO 3 films. The obtained results show a change in resistance by about two orders of magnitude upon polarization reversal on a lateral scale of 20 nm at room temperature.
Ultrathin BaTiO3-Based Ferroelectric Tunnel Junctions through Interface Engineering
Nano Letters, 2015
The ability to change states using voltage in ferroelectric tunnel junctions (FTJs) offers a route for lowering the switching energy of memories. Enhanced tunneling electroresistance in FTJ can be achieved by asymmetric electrodes or introducing metal− insulator transition interlayers. However, a fundamental understanding of the role of each interface in a FTJ is lacking and compatibility with integrated circuits has not been explored adequately. Here, we report an incisive study of FTJ performance with varying asymmetry of the electrode/ferroelectric interfaces. Surprisingly high TER (∼400%) can be achieved at BaTiO 3 layer thicknesses down to two unit cells (∼0.8 nm). Further our results prove that band offsets at each interface in the FTJs control the TER ratio. It is found that the off state resistance (R Off) increases much more rapidly with the number of interfaces compared to the on state resistance (R On). These results are promising for future low energy memories.
Nature Materials, 2013
The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts 1-3 . One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions 4-6 (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect 7-10 . Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface 11-17 . Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO 3 tunnelling barriers and La 0.7 Sr 0.3 MnO 3 electrodes exhibit a TER enhanced by up to ∼10,000% by a nanometre-thick La 0.5 Ca 0.5 MnO 3 interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La 0.5 Ca 0.5 MnO 3 , driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.
Scaling of electroresistance effect in fully integrated ferroelectric tunnel junctions
Applied Physics Letters, 2016
Systematic investigation of the scalability for tunneling electroresistance (TER) of integrated Co/ BaTiO 3 /SrRuO 3 ferroelectric tunnel junctions (FTJs) has been performed from micron to deep submicron dimensions. Pulsed measurements of the transient currents confirm the ferroelectric switching behavior of the FTJs, while the hysteresis loops measured by means of piezoresponse force microscopy verify the scalability of these structures. Fully integrated functional FTJ devices with the size of 300 Â 300 nm 2 exhibiting a tunneling electroresistance (TER) effect of the order of 2.7 Â 10 4 % have been fabricated and tested. Measured current density of 75 A/cm 2 for the ON state and a long polarization retention time of ON state (>10 h) show a lot of promise for implementation of high-density BaTiO 3-based FTJ memory devices in future.
Theoretical Approach to Electroresistance in Ferroelectric Tunnel Junctions
Physical Review Applied
In this paper, a theoretical approach, comprising the non-equilibrium Green's function method for electronic transport and Landau-Khalatnikov equation for electric polarization dynamics, is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, measured current-voltage characteristic curves in both inorganic (Co/BaTiO3/La0.67Sr0.33MnO3) and organic (Au/PVDF/W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co/BaTiO3/La0.67Sr0.33MnO3 systems is addressed by considering the interface termination effects using the effective contact ratio, defined through the effective screening length and dielectric response at the metal/ferroelectric interfaces. Finally, our approach is extended to investigate the role of a CoOx buffer layer at the Co/BaTiO3 interface in a ferroelectric tunnel memristor. It is shown that, in order to have a significant memristor behavior, not only the interface oxygen vacancies but also the CoOx layer thickness may vary with the applied bias.
Giant electrode effect on tunnelling electroresistance in ferroelectric tunnel junctions
Nature Communications, 2014
Among recently discovered ferroelectricity-related phenomena, the tunnelling electroresistance (TER) effect in ferroelectric tunnel junctions (FTJs) has been attracting rapidly increasing attention owing to the emerging possibilities of non-volatile memory, logic and neuromorphic computing applications of these quantum nanostructures. Despite recent advances in experimental and theoretical studies of FTJs, many questions concerning their electrical behaviour still remain open. In particular, the role of ferroelectric/electrode interfaces and the separation of the ferroelectric-driven TER effect from electrochemical ('redox'-based) resistance-switching effects have to be clarified. Here we report the results of a comprehensive study of epitaxial junctions comprising BaTiO 3 barrier, La 0.7 Sr 0.3 MnO 3 bottom electrode and Au or Cu top electrodes. Our results demonstrate a giant electrode effect on the TER of these asymmetric FTJs. The revealed phenomena are attributed to the microscopic interfacial effect of ferroelectric origin, which is supported by the observation of redox-based resistance switching at much higher voltages.
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