Study Of Transport Properties Of Two-dimensional Ferroelectric Van Der Waals Heterostructures

Ferroelectric materials have ordered spontaneous polarization and the spontaneous polarization vector changes in response to an applied electric field.A ferroelectric tunnel junction(FTJ)is a device based on the quantum mechanical tunneling effect.The tunneling effect occurs when the thickness of the intermediate ferroelectric layer decreases to the nanometer scale.The reversal of the polarization direction of the ferroelectric layer causes a change in the height and width of the tunneling barrier,resulting in different chances of electron tunneling,and the emergence of high and low resistance states can be used as logical states "0" and "1".Therefore,it has great potential for applications in non-volatile memory.One of the central issues in the study of FTJs is how to obtain a high tunnel electroresistance(TER)ratio.Currently,the most studied are FTJs constructed by three-dimensional(3D)ferroelectric materials,however,tunneling cannot occur when the ferroelectric layer of 3D ferroelectric materials is very thick,while ferroelectricity will disappear when the ferroelectric layer is very thin,and the surface suspension bond will lead to structural instability,so the ferroelectric layer can neither be too thick nor too thin.The need for device miniaturization in turn requires ferroelectric materials to be as thin as possible to increase the storage density.The rise of two-dimensional(2D)ferroelectric materials in recent years can be a good solution to the above-mentioned challenges faced by 3D ferroelectric materials.It has the advantages of thin thickness,no hanging bonds,and high stability,and occupies an important position in FTJs.However,the existing studies on 2D FTJs have used a ferroelectric material to construct FTJs.With the rise of 2D van der Waals(vdW)heterostructures,what are the properties of 2D vdW ferroelectric heterostructures?In particular,can we use two different 2D ferroelectric materials to construct FTJs?Do the constructed FTJs have excellent tunnel electroresistance properties?To address these questions,we have carried out the following studies.1.We constructed ferroelectric heterostructures using different 2D out-of-plane polarized ferroelectric materials,Sc2CO2 and In2Se3,and investigated the variation of their electronic structures with the inversion of the direction of ferroelectric polarization in each layer.Then a FTJ with As-doped In2Se3 and Sc2CO2 forming a Sc2CO2/In2Se3(AsSe-i)vdWs vertical heterostructure as the left and right electrodes and an undoped Sc2CO2/In2Se3 vdWs vertical heterostructure as the transport channel was constructed.By increasing the length of the transport channel,it is found that the TER becomes progressively larger with the growth of the central region,eventually showing a huge TER ratio of 107%at the channel length n=7.The analysis reveals that this stems from the difference in the contact surface work function of the two ferroelectric materials,which makes the charge transfer occur or not between the two ferroelectric materials.The results show the importance of the ferroelectric van der Waals heterostructure in the design of ferroelectric tunnel junctions,and a feasible design scheme featuring a reasonable selection of the component material work function and band gap is proposed.2.We further investigated the photogalvanic effect in the Sc2CO2/In2Se3 vdWs ferroelectric heterostructure and found that the photocurrent with both upward polarization directions(P↑↑)is much smaller than that with both downward polarization(P↓↓)when the photon energy is relatively small,and the analysis shows that in the P↑↑ insulated state,the photon energy needs to overcome a band gap to achieve the excitation of the electron.In contrast,in the P↓↓ metallic state,the photon energy can be excited directly from the valence band to the conduction band,so the photocurrent is much larger.The results illustrate that the dramatic changes in electrical properties caused by 2D ferroelectric heterostructures can be detected not only by means of applied voltage but also by means of photocurrents generated by light illumination.