First-principles Investigations On Transport Characteristics Of Two-dimensional Ferroelectric Tunnel Junctions

Ferroelectric tunnel junctions(FTJs),which are constructed by sandwiching a thin ferroelectric material between two leads,are also arousing great interest due to the tunneling electroresistance(TER)effect and their great potential as non-volatile memories.Here,the TER effect arises from the switchable spontaneous polarization by an electrical field in the ferroelectric materials.By far,the most common ferroelectric materials studied in FTJs have a three dimensional(3D)structure and especially those with a perovskite structure(ABO3)receive the most attention.It is well known that the spontaneous polarization in the ferroelectric materials will lead to the charge accumulation on their surfaces.When the thickness of the ferroelectric thin film is less than a critical value,the positive and negative charges on the two surfaces will neutralize with each other.This means that,for 3D FTJs,the ferroelectricity will fade away when the thickness of the ferroelectric materials is decreased below a critical thickness.Such a limit in the critical thickness of the ferroelectric layer is in contradiction with the technological demand of on-going device miniaturization.An emerging direction in the study of ferroelectric materials these years is the search of two-dimensional(2D)ferroelectric materials and many of them with either in-plane or out-of-plane spontaneous polarization have been predicted by theory and even prepared in experiments.2D materials provide 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.Therefore,we use the density functional theory of first principles combined with the non-equilibrium Green's function method to systematically study the transport characteristics of several different two-dimensional ferroelectric tunneling junctions and the mechanisms by which they can produce TER effect.Its main contents include:1.Realizing giant tunneling electroresistance in two-dimensional graphene/BiP ferroelectric tunnel junction:In this work,using first-principles calculations,we demonstrate that a giant TER effect of around 623%,which is comparable to 3D FTJs,can be realized through the ferroelectric tunnel junction constructed with the 2D ferroelectric materials BiP and B/N-doped graphene.The analysis of the effective potential and electronic structure indicates that the large TER ratio arises from the unsymmetrical screening effects of the B/N-doped vertical van der Waals graphene/BiP leads.Our findings demonstrate the great potential of novel application of 2D ferroelectric BiP in FTJs.2.Giant tunneling electroresistance in two-dimensional ferroelectric tunnel junctions with out-of-plane ferroelectric polarization: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 In₂Se₃ with out-of-plane polarization.Specifically,based on density functional calculations,we show that excellent tunnel electroresistance(TER)effect with TER ratio?1×108%is achieved,suggesting a promising route for applying 2D ferroelectric materials with out-of-plane polarization in ferroelectric memory devices.3.Domain-wall Induced Giant Tunneling Electroresistance Effect in Two-dimensional Graphene/In₂Se₃ Ferroelectric Tunnel Junctions:By far,all present schemes for realizing giant TER ratio with 2D FTJs are based on the polarization reversal of the whole ferroelectric layer upon an electrical field.In this work,we explore the quantum transport properties of the 2D FTJs with the partial reversal of polarization,namely,the formation of domain walls(DWs)by constructing two kinds of FTJs.One is in a uniform-polarization state and the other one is a state with domain walls.Structural relaxation confirms the stability of the domain-wall state.By quantum transport calculation,we obtain a TER ratio as high as 2.75×104%.Further analysis of the electronic structure shows that there is charge accumulation or charge depletion at the two DWs.Such asymmetric interface polarization charges result in a built-in electrical field and thus affect the distribution of the effective potential along the transport direction.This leads to partial metal-insulator transition around the DWs and finally the giant TER ratio.Our results indicate that DWs may greatly affect the quantum transport and provide a new mechanism for realizing giant TER effect in 2D FTJs.4.Giant tunnel electroresistance in ferroelectric tunnel junctions with metal contacts to two-dimensional ferroelectric materials:To better integrate with the present semiconductor technology,it is necessary to consider metal contacts in the construction of FTJs with 2D FEMs.However,due to the unknown interaction between traditional metals and 2D FEMs,it is not clear whether ferroelectricity still persists when the 2D FEMS are in contact with metals and whether the corresponding FTJs exhibit high TER effect as demanded for memory devices.To probe this,we construct FTJs with top contact between Au(010)and In₂Se₃,a 2D FEM with out-of-plane ferroelectric polarization.By density functional calculations combined with a non-equilibrium Green function technique,we find that not only the ferroelectricity still persists in the metal/FEM contact,but also a giant TER ratio as high as 104%is achieved.The giant TER arises from the change of the metal/FEM contact from a Schottky type to an Ohmic type accompanying with the ferroelectric polarization reversal.In the meantime,the tunnel barrier height between Au(010)and In₂Se₃ is zero,which means good ability of electron injection from metal to semiconductor and low contact resistance.Our study suggests that,by properly selecting the metal materials,giant TER ratio and high performance can be achieved in FTJs constructed with 2D FEMs and metal contacts.