Giant tunneling electroresistance in two-dimensional ferroelectric tunnel junctions with out-of-plane ferroelectric polarization

Ferroelectric tunnel junctions (FTJs) have been intensively studied in recent years due to the great potential in nonvolatile memory devices and two-dimensional (2D) FTJs have started to catch attention lately because of their atomic thickness and their significance in miniaturizing FTJ device sizes. In this work, we propose a mechanism for building 2D FTJs based on the large difference between the two work functions of a 2D ferroelectric polar material with out-of-plane polarization. When it forms a van der Waals (vdW) vertical heterostructure with another 2D material there will be two kinds of interfaces, according to which surface of the 2D polar material is contacted. Depending on the relative work functions of the contacted surfaces of the two materials, charge transfer may or may not occur between them, thus the 2D polar material may become conducting or be still insulating, resulting in two distinct conducting states (“ON” and “OFF”). We demonstrate the feasibility of this proposal by the example of a vdW heterostructure FTJ constructed with graphene and 2D ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$ with out-of-plane polarization. Specifically, based on density functional calculations, we show that excellent tunnel electroresistance (TER) effect with TER ratio $\sim 1\times {10}^8\%$ is achieved, suggesting a promising route for applying 2D ferroelectric materials with out-of-plane polarization in ferroelectric memory devices.

Figure 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/${\mathrm{In}}_2{\mathrm{Se}}_3$ vdW vertical heterostructure. The channel is 2D ${\mathrm{In}}_2{\mathrm{Se}}_3$. Panels (c) and (d) are the top views of the lead for upward and downward polarizations, respectively.

Figure 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 $I\text{−}V$ curves for both polarization directions, with the inset showing the TER at low bias.

Figure 3

The averaged LDOS distribution over the $xy$ plane of the central scattering regions as a function of electron energy $E$ and position $z$ for (a) $P_{\uparrow }$ and (b) $P_{\downarrow }$ and the corresponding LDOS contribution of the ${\mathrm{In}}_2{\mathrm{Se}}_3$ layer extracted from the total LDOS: (c) $P_{\uparrow }$ and (d) $P_{\downarrow }$. The black dotted box represents the transport channel region. The white dashed lines indicate the Fermi level and the black solid curves indicate the CBM and the VBM of the ${\mathrm{In}}_2{\mathrm{Se}}_3$ layer.

Figure 4

The layer-resolved band structure of graphene/${\mathrm{In}}_2{\mathrm{Se}}_3$ vdW heterostructure for (a) $P_{\uparrow }$ and (b) $P_{\downarrow }$. The insets show the locally magnified region indicated by the dashed line box around the $G$ point.

Figure 5

The effective potential along the $y$ direction (the direction of vacuum layer) of ${\mathrm{In}}_2{\mathrm{Se}}_3$ (red solid line) and graphene (green solid line). The “up” side and “dn” side marked on the figure are the two sides of ${\mathrm{In}}_2{\mathrm{Se}}_3$. The black arrow is the polarization direction of ${\mathrm{In}}_2{\mathrm{Se}}_3$. The inset indicates the work functions of graphene, “up” side and “dn” side of ${\mathrm{In}}_2{\mathrm{Se}}_3$.

Figure 6

The schematic diagram of transport process for the case: (a) $P_{\uparrow }$ and (b) $P_{\downarrow }$. The blue arrows show the possible transmission channels and the red cross means the blocking of the corresponding transmission channel.