Hernandez-Martin D, Gallego F, Tornos J, Rouco V, Beltran J I, Munuera C, Sanchez-Manzano D, Cabero M, Cuellar F, Arias D, Sanchez-Santolino G, Mompean F J, Garcia-Hernandez M, Rivera-Calzada A, Pennycook S J, Varela M, Muñoz M C, Sefrioui Z, Leon C, Santamaria J
GFMC, Universidad Complutense de Madrid, 28040 Madrid, Spain.
2D-Foundry Group, Instituto de Ciencia de Materiales de Madrid ICMM-CSIC, 28049 Madrid, Spain.
Phys Rev Lett. 2020 Dec 31;125(26):266802. doi: 10.1103/PhysRevLett.125.266802.
The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometer-thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a La_{0.7}Sr_{0.3}MnO_{3} bottom electrode and BaTiO_{3} ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than 10^{6}% at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphic computing.
铁电体在超薄层中的持久性关键取决于极化电荷的屏蔽或补偿，否则会破坏铁电态的稳定性。在表面，带电缺陷在触发新型混合电化学 - 铁电态的屏蔽机制中起着关键作用。然而，在界面处，铁电态与电化学态之间的耦合尚未得到探索。在这里，我们利用铁电隧道结纳米厚势垒中氧空位分布的动态形成，来证明氧化物界面处电化学自由度与铁电自由度之间的相互作用。我们制备了具有La₀.₇Sr₀.₃MnO₃底部电极和BaTiO₃铁电势垒的铁电隧道结。我们使用极化策略来促进金属顶部电极处氧空位的产生和传输。产生的氧空位控制铁电极化的稳定性并改变其矫顽场。反过来，铁电极化控制氧空位的电离，其程度远高于热力学平衡极限，从而在界面处引发肖特基势垒的形成，该势垒可通过铁电开关打开和关闭。电子自由度与电化学自由度之间的这种相互作用产生了非常大的电阻值（低温下超过10⁶%），并能够在同一结中实现顺时针和逆时针开关模式之间的可控切换（因此，电阻符号发生变化）。在电化学自由度与电子自由度之间发现的强耦合，为铁电隧道结中电阻开关与铁电开关之间日益激烈的争论提供了线索，此外，还可能成为存储器件和神经形态计算中新颖概念的来源。
