Recently, in a paper published on Physical Review Applied, a research team in the Hefei Institutes of Physical Science (HFIPS), Chinese Academy of Sciences (CAS) studied the interfacial control of transport properties of perovskite oxide ferroelectric tunnel junctions (FTJs) and proposed a new scheme to achieve a giant tunneling electroresistance (TER) in FTJs.

According to ZHENG Xiaohong, leader of the team, TER ratio up to 10⁵ % was obtained by introducing a negative polar atomic layer at one of the interfaces of the symmetric Pt/BaTiO₃/Pt FTJ.

FTJ is a tunnel junction in which a thin ferroelectric film is sandwiched between two metal electrodes. The resistance is highly dependent on the polarization direction of the ferroelectric barrier. Two greatly different states with high and low resistances respectively can be obtained by reversing the polarization direction with an external electrical field.

The FTJs have important applications in non-volatile random access memories. With advantages of high data storage density, fast read/write speed and low power consumption, it has attracted extensive research interest as memory elements. The difference between the high and low resistance states is usually characterized by TER ratio. Therefore, how to obtain a high TER ratio is always one of the key issues in the study of FTJs.

In this research, scientists proposed a new scheme to realize giant TER ratio by introducing a negative polar atomic layer at one interface of the FTJ.

In the symmetric Pt/BaTiO₃/Pt FTJ, a negative NaO₂ or LiO₂ interface is formed by replacing Ti with Na or Li atoms at the right interface of Pt/BaTiO₃/Pt tunnel junction. Then a 10⁵ %TER ratio was achieved due to this additional NaO₂ or LiO₂ layer.

The mechanism is rooted in the great difference in the potential change in the ferroelectric barrier arising from the negative polar interface in the two polarized states.

When the ferroelectric barrier is left polarized, the bands of the barrier at each atomic layer increase from left to right. Meanwhile, due to Coulomb repulsion, the negatively charged NaO₂ or LiO₂ interface further pushes up the bands of the barrier, and near the right interface region, the valence band maximum (VBM) rises above the Fermi energy, leading to partial metallization.

On the contrary, in the right polarization state, although the Coulomb repulsion at the NaO₂ or LiO₂ interface still exists, the band of the ferroelectric barrier itself decreases from left to right. Due to the cancellation between them, the valence band distribution in the whole barrier is relatively flat and the VBM is always below the Fermi energy, without the occurrence of partial metallization. The occurrence and disappearance of partial metallization in the two polarization states change the effective barrier width significantly and lead to the low and high resistance states, with a giant TER ratio achieved subsequently.

The study indicates that a negatively charged polar interface based on interfacial substitution is a feasible scheme to achieve large TER ratio in FTJs and provide important reference for the design of high-performance FTJs.

The above work was supported by the National Natural Science Foundation of China, and all calculations were performed at the Center for Computational Science of CASHIPS, the ScGrid of Supercomputing Center and the Computer Network Information Center of Chinese Academy of Sciences.

 Figure 1. The schematic diagrams of the atomic structures in the left and right polarization states of Na_Ti-FTJ. (Image by XIAO Wei)