Anisotropic polarization-induced conductance at a ferroelectric–insulator interface

Yi Zhang 1, Haidong Lu2, Lin Xie3, Xingxu Yan1, Tula R. Paudel2, Jeongwoo Kim4, Xiaoxing Cheng5, Hui Wang 4, Colin Heikes 6, Linze Li1, Mingjie Xu1, Darrell G. Schlom6,7, Long-Qing Chen5, Ruqian Wu4, Evgeny Y. Tsymbal2, Alexei Gruverman 2* and Xiaoqing Pan1,4*

1Department of Materials Science and Engineering, University of California, Irvine, CA, USA.
2Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE, USA.
3National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu, China.
4Department of Physics and Astronomy, University of California, Irvine, CA, USA.
5Department of Materials Science and Engineering, Penn State University, University Park, PA, USA.
6Department of Materials Science and Engineering, Cornell University, Ithaca, NY, USA.
7Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA.
*e-mail: agruverman2@unl.edu; xiaoqing.pan@uci.edu

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emer-gent phenomena in complex oxide heterostrutures. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO3/SrTiO3 (LAO/STO) interface. This is caused by the polar discontinuity and coun-teracts the electrostatic potential build-up across the LAO film3. The ferroelectric polarization at a ferroelectric/insu-lator interface can also give rise to a polar discontinuity. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG). While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces, experimental evi-dence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polariza-tion orientation. By probing the local conductance and ferro-electric polarization over a cross-section of a BiFeO3–TbScO3(BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theo-retical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.

https://www.nature.com/articles/s41565-018-0259-z

 https://news.uci.edu/2019/01/25/uci-scientists-develop-method-for-observing-nanocrystal-formation-at-atomic-resolution/