Based on density functional theory analysis, a research team led by Prof. ZHENG Xiaohong proposed that the double barrier structure can greatly enhance the tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs), and demonstrated that the double barrier ferroelectric tunnel junction (DB-FTJ) can realize multi-state storage.

The research results were recently published in npj Computational Materials.

FTJs have gained significant attention as potential non-volatile memory devices. The structure of FTJs consists of metal electrodes on both sides and the intermediate ferroelectric tunnel barrier. Reversing the polarization direction of the ferroelectric material leads to a large change in conductance, creating high and low conductance states that can be utilized as ON and OFF states in binary memory units. One crucial research focus is to develop new methods for achieving higher TER ratio, which quantify the conductance change between the two polarization states.

In this research, the team designed the Pt/BaTiO₃/LaAlO₃/Pt/BaTiO₃/LaAlO₃/Pt DB-FTJ and performed density functional theory calculations to simulate its transport properties. It is found that the switching between the ferroelectric left and right polarization states produces a huge TER ratio of 2.210×10⁸% in the DB-FTJ proposed (which indicates that there is a huge difference in transmission coefficient between the two polarization states), which is at least three orders of magnitude larger than that of the Pt/BaTiO₃/LaAlO₃/Pt single barrier ferroelectric tunnel junction (SB-FTJ).

The basic idea is rooted in two facts. Firstly, the transmission coefficient of a double barrier structure, which consists of two individual barriers in series, is related to the product of the transmission coefficients of the two individual barriers. Secondly, the square of positive numbers larger than 1 will increase exponentially. These principles are perfectly revealed in the DB-FTJ.

Researchers also proposed that two extra polarization states with head-to-head and tail-to-tail ferroelectric polarizations can be achieved by separately controlling the polarization direction of each barrier, which leads to multiple resistance states.

This study demonstrated that, in the design of FTJs, the double barrier structure can greatly enhance the TER ratio of FTJs and make them promising for multi-state data storage.

Fig. 1 Atomic structure, electronic structure and transport properties of the DB-FTJ. (Image by XIAO Wei)

Fig. 2 Atomic structure and transport properties of SB-FTJs. (Image by XIAO Wei)