An ultrathin two-dimensional vertical ferroelectric tunneling junction based on CuInP₂S₆ monolayer

Ferroelectric (FE) materials, especially ABO₃ FE perovskite oxides, have been extensively studied for their important applications in memory devices, electronics and sensors. However, the integration of FE perovskite oxides into miniaturized memory and electronic devices has been impeded by the critical thickness limitation, as out-of-plane ferroelectricity in most FE perovskite oxides will disappear when the oxide thin film thickness is below a critical value. On the other side, CuInP₂S₆ (CIPS) nano-flake, a prototypical two-dimensional (2D) FE material, has recently been demonstrated to display stable out-of-plane ferroelectricity at the atomic layer thickness by experiment, which offers a new candidate for developing FE devices in the 2D nanoscale regime. Herein, after investigation of the structural and ferroelectric properties of 2D CIPS layers, especially the interactions between out-of-plane polarization and the corresponding depolarization field using first-principles calculations, we reveal that out-of-plane ferroelectricity can even persist in the CIPS monolayer, which is only 3.4 A in thickness. Moreover, in order to explore the potential application of 2D FE CIPS layers as minimized FE devices, we design an ultrathin ferroelectric tunneling junction (FTJ) composed of a graphene/CIPS monolayer/graphene vertical van der Waals (vdW) heterostructure. Our transport simulations based on the non-equilibrium Green's function formalism predict that such an ultrathin FTJ device can still exhibit the typical tunneling electroresistance (TER) effect, where tunneling current strongly depends on the direction of FE polarization. Our work not only elucidates the origin of stable out-of-plane ferroelectricity appearing in 2D CIPS layers, but also demonstrates the practical application of a CIPS based 2D FTJ as a miniaturized, multi-functional and low-power consumption memory device for modern electronics.