Tunable Metallic Conductance in Ferroelectric Nanodomains
Peter Maksymovych,1 Anna N. Morozovska,2,3 Pu Yu,4 Eugene A. Eliseev,3 Ying-Hao Chu,4,5 Ramamoorthy Ramesh,4 Arthur P. Baddorf,1 and Sergei V. Kalinin1
1 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, TN, 37831
2 Institute of Semiconductor Physics, National Academy of Science of Ukraine,41, pr. Nauki, 03028 Kiev, Ukraine
3 Institute for Problems of Materials Science, National Academy of Science of Ukraine,3, Krjijanovskogo, 03142 Kiev, Ukraine
4 Department of Materials Science and Engineering and Department of Physics, University of California, Berkeley, CA 94720
5 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010
We have utilized nanoscale ferroelectric switching to activate metallic conduction through an otherwise insulating lead-zirconate titanate, PbZr0.2Ti0.8O3 (PZT) film. This is the first time metallic conduction channels due to ferroelectric properties have been demonstrated in an oxide material, even though theoretical proposals hinting at such a possibility date back to the 1970’s [1–3]. Our experimental measurements and theoretical analysis have also revealed that the magnitude of metallic conductivity is tunable by changing the size of the nanodomain and, correspondingly, the tilt angle of its domain walls. This new manifestation of the well-known ferroelectric field effect  results in colossal and tunable resistive switching synchronized with ferroelectric switching. Finally, our work demonstrates that the topology of the ferroelectric polarization can create dramatically different electronic conduction mechanisms in the exact same region of a ferroelectric material, and that flexoelectric coupling can be important for the conductivity of ferroelectric domain walls.
Ferroic interfaces, such as ferroelectric domain walls and vortices, are unique nanoscale entities because they are defined by lattice symmetry rather than its chemical composition. As a result, they can be created, patterned and erased with applied fields repeatedly within the same volume of the material. There is however a dearth of experimental examples of distinct electronic properties of ferroic interfaces, in contrast to rich electronic behaviors (2DEG, superconductivity) of chemically-defined interfaces between complex oxides. Recent experiments in BiFeO3  and PZT  revealed interfacial conductivity of ferroelectric domain walls, but so far their electronic behavior has been that of a leaky dielectric and distinct conduction channels associated with the domain walls have remained elusive. In contrast to previous works, we measured the electronic properties of ferroelectric topologies created at the instance of ferroelectric switching. It is here that we found the long-sought metallic conductivity. We have observed large current densities (up to 30,000 A/cm2), colossal resistive switching ratios (up to 10,000:1) and temperature-independent conductance from 100‒400K in a relatively thick film of an insulating oxide. These properties pave the way to new applications for ferroic interfaces in oxide nanoelectronics, such as information storage and neuromorphic computing. At the same time, extending our studies onto mutliferroics, mixed-phase and anti-ferroelectrics may reveal a whole new family of previously unknown electronic properties.
Published in Nano Letters, DOI: 10.1021/nl203349b. Research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. SVK was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Material synthesis at Berkeley was partially supported by the SRC-NRI-WINS program as well as by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the U. S. Department of Energy under contract No. DE-AC02-05CH1123.
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