Theoretical Study Of Metal-multilayered Two-dimentional MoS₂ Heterojunctions

The potential of two-dimensional(2D)semiconductors in ultrathin electronics and optoelectronics has stimulated intensive research of their fundamental properties in the last more than ten years.For 2D semiconductor connecting to external circuit,a bottleneck problem to realize high-performance 2D devices is the unexpectedly high resistance at the metal-2D semiconductor interface.Recently,layer-number depend performance and Schottky-Mott limit are reported in metal–semiconductor junctions(MSJ)with multilayered 2D semiconductors(Mm SJ),which brings light to the realization of Ohmic contacts.But till present,people have little knowledge about the layer-number dependence.Another important research direction of 2D semiconductor is semiconductor heterojunctions with type-II band alignment,which play an important role in carrier-photon conversion.However,the momentum-mismatch is an intractable problem which reduces the efficiency of photon excited electron-hole pairs with a radiative transition.Based on state-of-the-art density functional theory calculations,firstly the contact resistances between monolayer Mo S2 and metal surfaces can be reduced by in-plane strain.The Schottky barrier(SB)and tunneling barrier(TB),both of which contribute to the interface resistance,are lowered significantly by tensile strain.Especially,the SB can reduce to zero with increasing tensile strain.The mechanism of SB height reduction under tensile strain is attributed to the increase of the electron affinity energy of Mo S2 since the monolayer Mo S2 conduction band minimum(CBM)is in anti-bonding.From this mechanism,the SB height in other semiconducting transitional metal dichalcogenides(TMDCs)with anti-bonding CBM could also be reduced to zero by tensile strain.Secondly,taking Mm SJ with Mo S2 as a typical example,the Fermi-level pinning(FLP)factor depends sensitively on the layer-number of Mo S2,especially for few-layer(less than 5 layers)Mo S2.Strong pinning arises right at the metal-1st-layer-Mo S2 interface while depinning occurs between Mo S2 layers.The depinning effect between Mo S2 layers makes FLP decrease as a function of layer-number,and makes p-type SB contact favored in Mm SJ than that in metal-2D monolayer Mo S2 junctions,especially for junctions with large work-function metals.Thirdly,to solve the intractable problem of momentum-mismatch in 2D semiconductor heterojunctions,2D multilayered van der Waals(vd W)semiconductor homojunctions(m SHs)supported on 2D metals could offer a universal approach to obtain type II band alignment with the advantage of wide range momentum-space-match in the Brillouin zone by band-nesting effect.There are two advantages in m SHs than 2D semiconductor heterojunctions: 1)momentum-matched band alignments are easy to achieve due to the inherent lattice-orientation-match between vd W layers of the same material in homojunction,and moreover,a wide-range momentum-space-match can be obtained by metal-induced Fermi-level rigid shift to achieve ‘parallel' band dispersions(due to the 1st-layer of m SH is degenerately doped);and 2)largely tunable band offsets make band alignment change from Type II to Type III to inversed Type II due to charge redistribution at the interface.Moreover,the m SH is better to be supported on 2D metal rather than three-dimensional(3D)bulk metal,with the advantage of free of metal-induced-gap-states and easily obtained n-and p-type Schottky-barrier-free contacts.Fourthly,a detailed mechanism study about the nature of bonding and electron redistribution at the metal-semiconductor(M-S)and semiconductor-semiconductor(S-S)interfaces of Mm SJ is revealed.Multiple mechanisms simultaneously contribute to the electron redistribution at M-S and S-S interfaces,and,the dominant mechanism depends on both the metal electrode's dimension(2D vs 3D)and their work function.At 3D M-S interfaces,the pushback effect and metal-induced gap states play a dominant role.At 2D M-S interfaces,the covalent-like quasi-bonding feature appears for 2D metal with medium work function,while charge transfer plays the main role for 2D metals with extremely large or small work functions.The S-S interface inherits the electron-redistribution behavior at the M-S interface in Mm SJ with 2D metal,while a depinning effect appears across the S-S interface in Mm SJ with 3D metal.Our discoveries about strain shed a new and general light toward minimizing the contact resistance of semiconducting TMDCs-metal based contacts which can also prove applicable to other 2D semiconductors with an anti-bonding CBM(or banding VBM for p-type contacts).The most important innovation of this thesis is that the importance of the layer-number of 2D semiconductors in MSJ is highlighted,this is unnoticed before.Our work of layer-number engineering in Mm SJ promotes an emerging field that layer-number can be used as a new degree of freedom for manipulating the pinning factor and SB in Mm SJ and sheds light on the recent controversial experimental observations.Besides,vd W m SHs supported on 2D metals,with unexpected excellent properties than heterojunctions,stimulates experimental and theoretical studies for their various applications.Therefore,the detailed mechanism study about interfaces in Mm SJ provides general insights and new concepts to better understand and use of Mm SJ.