First Principles Study Of MoSi₂N₄/two-dimensional Metal Heterojunctio

Since the successful exfoliation of graphene in 2004,two-dimensional（2D）materials have attracted much attention worldwide for their excellent electronic,optical,and mechanical properties.Van der Waals heterostructures built from two 2D materials can not only retain the inherent electronic properties of each 2D material layer,but also bring new physical properties,which have been widely used in the fields of nanoelectronics and optoelectronic device fabrication.Given the successful synthesis of the novel 2D semiconductor material MoSi₂N₄（MSN）,we use seven 2D metals（graphene,XA₂（X=Nb,Ta,V;A=S,Se））as electrodes in contact with MSN to form semiconductor-metal heterostructures and systematically investigate the electronic and interfacial properties of the seven MSN/2D metal heterostructures using the first-principles calculations based on the density functional theory.The electronic and interfacial properties of the seven MSN/2D metal heterostructures and the effects of vacancy defect on the electronic properties of the intrinsic MSN and MSN/graphene heterostructures are studied.Finally,the Schottky barrier of MSN/graphene heterostructure is modulated by biaxial strain and external electric field to explore the critical points of p-type to n-type Schottky contact transitions and even Ohmic contact transitions,and to provide theoretical guidance for the experimental preparation of Schottky devices based on the MSN/graphene heterostructure.The main conclusions of this work are as follows:（1）The electronic and interfacial properties of heterostructures formed by seven MSN/2D metal contacts are systematically investigated.We propose a new van der Waals stacking strategy,which achieves efficient Ohmic contacts just by simple van der Waals stacking at the contact interfaces of four 2D metals:Nb S₂,Ta S₂,Ta Se₂,and VS₂with MSN.While p-type Schottky contacts are formed at the contact interfaces of graphene,Nb Se₂,and VSe₂ with MSN.In both MSN/XA₂ contact models,we find similar phenomena of charge transfer:charges are accumulated in the XA₂ layer and are depleted in the MSN layer.While in the MSN/graphene contact model,charges are accumulated in the MSN layer and are depleted in the graphene layer,exhibiting a charge transfer phenomenon opposite to that of the XA₂ metal.By linear fitting,the values of n-type and p-type Fermi level pinning（FLP）factor S of MSN are calculated to be 0.42 and-0.63,respectively.These values are higher than some 2D materials,showing a weak Fermi level pinning effect.By calculating the tunneling probability,we find that the graphene contact model has a higher tunneling probability than other2D metal contact models,implying a higher charge injection efficiency.（2）Four vacancy defects models,including N-in vacancy,N-out vacancy,Si vacancy and Mo vacancy,are constructed to investigate the effects of the vacancy defects on the electronic properties and magnetic properties of the intrinsic MSN and perfect MSN/graphene heterostructure.We also investigate the origin of the magnetic properties.First,we calculate the vacancy defect formation energy,and we find that the values of formation energy of all vacancy defects are negative for both defective MSN and defective MSN/graphene,implying that all vacancy defects are stable.We also find that the introduction of N-out and Si vacancy defects transforms the intrinsic MSN from semiconductor to metal.The introduction of N-in vacancy defect transforms the intrinsic MSN from semiconductor to semimetal.And the system remains semiconductor when Mo vacancy defect is introduced.The introduction of N-out and Mo vacancy defects does not change the magnetic properties of the original system for both intrinsic MSN and MSN/graphene heterostructure.However,the N-in and Si vacancy defects make the original nonmagnetic system magnetic.（3）The modulation effects of biaxial strain and external electric field of MSN/graphene heterostructure are investigated.Compressive strain and negative applied electric field can realize the transition from p-type Schottky contact to n-type Schottky contact in MSN/graphene heterostructure.When the compressive strain increases to-10%,the transition from Schottky contact to Ohmic contact is realized.When the tensile strain and positive applied electric field are applied,the MSN/graphene heterostructure interface remains p-type Schottky contact.