Significantly enhanced tunneling electroresistance in two-dimensional ferroelectric tunnel junctions realized by reversible Schottky-Ohmic contact switching

Achieving large tunneling electroresistance (TER) is the key to the application of two-dimensional (2D) ferroelectric tunnel junctions (FTJs) in high-density memory devices. In this work, we show that an aided layer of 2D metal materials with ultrahigh work function can lead to significantly enhanced TER effect in 2D FTJs. To implement the idea, we design a 2D van der Waals heterostructure consisting of a metallic ${\mathrm{NbS}}_2$ monolayer and a ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ ferroelectric, and propose a FTJ based on this heterostructure. By performing density functional theory and nonequilibrium Green's function method calculations, we find that the type of interfacial contact in the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure can be flexibly switched between Ohmic and Schottky contact by altering the ferroelectric polarization. Accordingly, the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJ exhibit giant TER ratio of up to ${10}^{13}\%$, and its TER ratio increases with the tunneling barrier length. These findings not only provide a feasible strategy for realizing large TER effect in 2D FTJs, but also offer promising candidates to designing nonvolatile memories.

Figure 1

(a) Top and side views of the ${\mathrm{NbS}}_2$ and ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayers. Green, yellow, cyan, brown, and red balls represent Nb, S, Sc, C, and O atoms, respectively. (b) Side views of the optimized ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure in the $P\uparrow$ and $P\downarrow$ polarization states, respectively. The layer-resolved band structures of (c) ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2\text{−}P\uparrow$ and (d) ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2\text{−}P\downarrow$ heterostructures.

Figure 2

(a) The minimal energy pathway of ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure from $P\uparrow$ to $P\downarrow$ states. (b) Schematic diagram of the work function and possible charge transfer between ${\mathrm{NbS}}_2$ monolayer and ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ in different polarization states. (c) The plane-averaged differential charge density ($▵\rho$) and the corresponding 3D differential charge density. The plane-averaged charge density along the $z$ direction $\rho$($z$) is defined as $\rho$($z$) = $\int \mathrm{\Delta }\rho (x,y,z)dxdy$.

Figure 3

The atomic structures of ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJs with (a) $P\uparrow$ and (b) $P\downarrow$ states. The solid black region indicates the source/drain electrode and the dashed green region represents the central scattering region.

Figure 4

The zero-bias transmission spectrums of ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJs at tunneling barrier lengths of (a) N = 3, (b) N = 5, (c) N = 7 and (d) N = 9, respectively.

Figure 5

(a) The TER ratio of ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJs with different tunneling barrier lengths. (b) The TER ratio of the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJs as a function of electron energy. (c) The scattering states at Fermi level for ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJs when N = 3.