By density functional calculations combined with a nonequilibrium Green¡¯s function technique, the scientists obtain a giant TER ratio of around 1 ¡Á 108% in two-dimensional ferroelectric tunnel junctions with out-of-plane ferroelectric polarization, which is higher than that of most current 3D FTJs.
Ferroelectric tunnel junctions (FTJs), which use ferroelectric materials as the central tunnel barrier and metal or semiconductor as leads, have been widely studied in both experiments and theory due to their potential applications in nonvolatile memory devices.
The tunnel electroresistance(TER) ratio is a very important parameter characterizing the performance of data storage in the study of FTJs. Thus far, there have been numerous studies about how to design high-performance FTJs with a very large TER ratio.
However, most of the attention has been paid to FTJs adopting thin films of three-dimensional (3D) ferroelectric materials as the tunnel barrier. It is well known that there is a limit in critical thickness of 3D ferroelectric materials for the observation of spontaneous polarization because of the charge accumulation on the surfaces, which is in contradiction with requirements in the device miniaturization.
Thus, how to decrease the critical thickness of the ferroelectric thin films is a key problem in the further development of high performance FTJs. An emerging direction in the study of ferroelectric materials these years is the search of two-dimensional (2D) ferroelectric materials, which provides a natural good candidate for construction of devices with atomic thickness. If 2D ferroelectric materials can be adopted to construct high-performance FTJs with a large TER ratio, it will greatly reduce the thickness of the FTJ device. As a matter of fact, the application of 2D ferroelectric materials in FTJs has already started to catch research attention lately.
Recently, ZHENG Xiaohong¡¯s research group at Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institutes of Physical Science design a 2D out-of-plane ferroelectric tunnel junction adopting the graphene/In2Se3 van der Waals(vdW) vertical heterostructure as left/right lead and In2Se3 as the central ferroelectric tunneling barrier. By density functional calculations combined with a nonequilibrium Green¡¯s function technique, the scientists obtain a giant TER ratio of around 1 ¡Á 108%, which is higher than that of most current 3D FTJs. Further analysis indicates that the big difference (∼2.396 eV) in work functions of the two surfaces of the In2Se3 slab leads to different charge transfer from graphene to In2Se3 and different electron filling of the bands of In2Se3 around the Fermi level when it contacts with graphene, which leads to the change between the metallic and insulating nature of In2Se3. Thus, an extremely high TER ratio can be easily achieved. The findings demonstrated the great potential of novel application of 2D ferroelectric material with out-of-plane ferroelectric polarization.
Fig. 1 The structure of the FTJ with (a) upward polarization and (b) downward polarization. The structure is divided into three parts: left (L) and right (R) leads, and the central scattering region (C). The left/right leads are graphene/In2Se3 vdW vertical heterostructure. The channel is 2D In2Se3. Panels (c) and (d) are the top views of the lead for upward and downward polarizations, respectively. (Image by KANG Lili)
Fig. 2 The transmission function for both polarization directions, with the Fermi level set to 0 eV; (b) the TER ratio as a function of electron energy; (c) the I-V curves for both polarization directions, with the inset showing the TER at low bias. £¨Image by KANG Lili£©
Link to the Paper: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.014105