Giant tunnel electroresistance in two-dimensional ferroelectric tunnel junctions constructed with a ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ van der Waals ferroelectric heterostructure

Two-dimensional (2D) ferroelectric materials have attracted great attention in recent years due to their thin thickness, high stability, and switchable polarization states. In particular, ferroelectric tunnel junctions (FTJs) constructed from 2D ferroelectric materials have been shown to have very high tunnel electroresistance (TER) ratios. In this work, we design a ferroelectric tunnel junction composed of ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ vertical van der Waals (vdW) heterostructure based on two different 2D ferroelectric materials with out-of-plane polarization. Through density functional calculations combined with a nonequilibrium Green's function technique, it is found that the TER ratio as high as ${10}^7\%$ can be achieved. Analysis shows that it originates from the difference in the work functions of the contact surfaces of the two ferroelectric materials, which makes charge transfer occur or not occur between them and further leads to a metalinsulator switching of the ferroelectric vdW heterostructure upon the reversion of the applied electrical field. The results suggest the importance of ferroelectric vdW heterostructure in the design of FTJs and a feasible design scheme characterized by proper choice of the work functions and band gaps of the component materials.

Figure 1

(a) The electrode supercell with the lattice vectors in the $xz$ plane as $a=(6.90,0.00,0.00)$ Å and $c=(3.45,0.00,5.97)$ Å. (b) The channel region of the FTJs, with the transport direction along the $z$ direction. The structures of the FTJ with ferroelectric polarizations of both ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ and ${\mathrm{In}}_2{\mathrm{Se}}_3$; (c) pointing downward ($P_{\downarrow \downarrow }$) and (d) pointing upward ($P_{\uparrow \uparrow }$), with the blue dashed line region representing the electrode supercell and the red dashed line region representing the channel region. $n$ is the number of pristine ${\mathrm{Sc}}_2{\mathrm{CO}}_2$/${\mathrm{In}}_2{\mathrm{Se}}_3$ supercells included in the channel region.

Figure 2

The layer-resolved band structure for the state: (a) $P_{\downarrow \downarrow }$, (b) $P_{\downarrow \uparrow }$, (c) $P_{\uparrow \downarrow }$,; and (d) $P_{\uparrow \uparrow }$. The Fermi level is set to 0 eV.

Figure 3

The switching pathway between $P_{\downarrow \downarrow }$ and $P_{\uparrow \uparrow }$. $\lambda =0.0$ and $\lambda =1.0$ represent the $P_{\downarrow \downarrow }$and $P_{\uparrow \uparrow }$ states, respectively. The energy of $P_{\downarrow \downarrow }$ is set to 0 eV.

Figure 4

The band structure of (a) ${\mathrm{In}}_2{\mathrm{Se}}_3({\mathrm{As}}_{Se-i}$), (b) the $P_{\downarrow \downarrow }$ state of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$/${\mathrm{In}}_2{\mathrm{Se}}_3({\mathrm{As}}_{Se-i}$), and (c) the $P_{\uparrow \uparrow }$ state of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$/${\mathrm{In}}_2{\mathrm{Se}}_3({\mathrm{As}}_{Se-i}$).

Figure 5

The transmission functions of the $P_{\downarrow \downarrow }$ and $P_{\uparrow \uparrow }$ states when the transport channel length. (a) $n=1$, (b) $n=3$, (c) $n=5$, and (d) $n=7$. The Fermi level is set to 0 eV.

Figure 6

(a) The TER ratio at the Fermi level as a function of the channel length $n$. (b) The TER ratio as a function of electron energy at $n=7$.

Figure 7

The layer-resolved projected density of states (PDOS) of the central region of the FTJ for (a) the $P_{\downarrow \downarrow }$ state, (b) the $P_{\uparrow \uparrow }$ state.

Figure 8

(a) The schematic model for the formation of the contact potential between two different materials; (b) the band alignment of two different semiconductors in contact before a common Fermi level is established; (c) the effective potentials of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ (blue solid line), and ${\mathrm{In}}_2{\mathrm{Se}}_3$ (red solid line) along the vertical direction of the vacuum layer (in the figure, the out-of-plane polarization in each separate material points from right to left); and (d) the work functions of ${\mathrm{Sc}}_2{\mathrm{CO}}_{2,}$ and ${\mathrm{In}}_2{\mathrm{Se}}_3$ in the $P_{\downarrow \downarrow }$ state (left panel) and the $P_{\uparrow \uparrow }$ state (right panel).

Figure 9

The band structures with HSE functional for (a) the $P_{\downarrow \downarrow }$ state and (b) the $P_{\uparrow \uparrow }$ state. (c) The work functions of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ and ${\mathrm{In}}_2{\mathrm{Se}}_3$ in the $P_{\downarrow \downarrow }$ state (left panel) and the $P_{\uparrow \uparrow }$ state (right panel).