First-principles Study Of Electronic Transport Properties Of Two-dimensional Van Der Waals Tunnel Junction Devices

In recent years,with the rapid development of artificial intelligence and the Internet-of-Things,a large amount of information data needs to be stored and processed.Recently,traditional tunnel junctions based on bulk materials have become difficult to meet people’s needs for high-density and small-scale electronic devices.With the successful preparation of two-dimensional van der Waals（vdW）magnetic materials and ferroelectric materials,two-dimensional vdW tunnel junctions have provided new opportunities for the development of new high-performance microelectronic devices.On the one hand,tunneling magnetoresistance（TMR）and tunneling electroresistance（TER）are the key performance index of tunnel junctions.How to design two-dimensional vdW tunnel junction devices to achieve high TMR and TER is currently an urgent problem to be solved.On the other hand,with the development of microchip design,the devices that only achieve single performance can no longer meet the needs of logic circuits,so it is necessary to achieve multiple resistive states and functions in a single device to achieve high device integration.This thesis has been designed multiple two-dimensional vdW tunnel junction devices and investigated electronic transport characteristics based on the first principles calculations combined with the nonequilibrium Green’s function method.High TMR and TER values have been achieved in these tunnel junction devices,and various physical effects such as resistance states,negative differential conductance（NDC）,and diode characteristics have been achieved.The main research results of this thesis are as follows:1.In this work,using first-principles calculations,it is demonstrated that bilayer CoBr₂ is intrinsically a magnetic semiconductor with intralayer ferromagnetic（FM）and interlayer antiferromagnetic（AFM）couplings and the interlayer AFM coupling in bilayer CoBr₂ is independent on the stacking orders.Afterwards,graphite/bilayer CoBr₂/graphite spin-filter vdW magnetic tunnel junction（sf-vdW MTJ）is constructed,and a high TMR ratio of 2420%is achieved in this sf-vdW MTJ at zero bias due to the large difference between electronic structure of interlayer AFM and FM states of the CoBr₂ barrier.It is noticed that the bias can obviously increase TMR of sf-vdW MTJ.In particular,a giant TMR ratio of up to about 38000%can be achieved in this sf-vdW MTJ at 0.2 V bias.The calculation results of this work not only reveal the physical mechanism of the huge TMR in the sf-vdW MTJ composed of bilayer antiferromagnetic barrier,but also providing a theoretical guidance for the experimental preparation of high TMR devices.2.In this work,we propose a symmetrical vdW antiferroelectric multiferroic tunnel junction（AFMFTJ）device Fe₃GeTe₂/h-BN/bilayer In₂Se₃/h-BN/Fe₃GeTe₂（FGT/BN/IS/BN/FGT）,and investigate the spin-dependent transport of this vdW AFMFTJ by using nonequilibrium Green’s function combined with density-functional theory.Due to the existence of three polarization orientations of the bilayer In₂Se₃ and the two magnetic arrangements of the magnetic electrode Fe₃GeTe₂,multiple resistance states can exist in this tunnel junction.The calculated TMR can reach 695%and TER can reach 10⁴%.By calculating the bias related characteristics of current,it is found that in FGT/BN/IS/BN/FGT vdW AFMFTJ,as the bias gradually increases,the tunneling current first increases and then decreases,and then increases again,exhibiting a significant NDC phenomenon.When the polarization of In₂Se₃ and magnetic states of electrodes change,the NDC also changes,which means that the NDC effect in this tunnel junction can be regulated by external electric and magnetic fields.By calculating the conductive channel and density of states,this NDC effect comes from the change in the matching degree between the conductive channel of the electrode and the electronic structure of the ferroelectric barrier.The vdW AFMFTJ we designed has multiple resistance states and NDC effects,which has great guiding significance for achieving multifunctional microelectronic devices in experiments.3.In this work,the spin-dependent transport properties of the Fe₃GaTe₂/GeI₂/Fe₃GaTe₂/GeI₂/Fe₃GaTe₂/Fe₃GaTe₂ vdW double barrier magnetic tunnel junction（vdW DBMTJ）are studied by using the nonequilibrium Green’s function combined with density functional theory.It is found that due to a significant difference in resistance between different magnetic states,a maximum TMR of the vdW DBMTJ is as high as 3.61×10⁹%,and the resistance area（RA）is only 3.27×10^-1Ω·μm².Notably,through calculated current-voltage curve,it is found that for a ferromagnetic metal/insulator/ferromagnetic metal/insulator/ferromagnetic metal double barrier tunnel junction,if the magnetic moments of the two electrodes are in opposite directions,then when positive and negative biases are applied to the tunnel junction,the changes of current with the positive and negative biases will be asymmetrical,which can produce a diode characteristic that exhibits positive conduction and reverse cutoff.Our calculation results discover a polymorphic storage device with diode characteristics,which is helpful for the development of high-performance electronic devices.