Investigations On The Transport Properties Of Van Der Waals Junction Based On Transition Metal Dichalcogenides

The two-dimensional transition metal chalcogenides(TMDCs)are a set of atomic thick semiconductors with dangling bond free surfaces.TMDCs are ideal materials for constructing van der Waals junctions,Therefore,are considered to be competitive candidates for the next generation of semiconductor materials.A typical van der Waals junction is mechanically stacked two layers of monolayers that partly overlapped without chemical bonding.Such unique structure makes the charge transport different from that in traditional semiconductor devices.As a result,research on interlayer charge transport of van der Waals structure and how it could be tuned are important for developing TMDCs van der Waals junctions.In this paper,first-principles calculation based on density functional theory and the non-equilibrium Green’s function are implemented to systematically study the electronic structure and charge transport behavior of the TMDCs van der Waals junction.The property of valleys and interlayer coupling are examined.Due to the breaking space inverse symmetry of monolayer TMDCs,the degenerate states are not equal in K(K’)and Q(Q’)valleys.Therefore,the bands split in these degenerate valleys because of the spin-orbital coupling.When two identical monolayer TMDCs stack together in both AA and AB sequence,the interlayer coupling give rise to various splitting in different valleys.According to the interlayer coupling caused splitting,the interlayer coupling strength in these valleys can be ordered in a sequence of Π>Q(Q’)>valence band K(K’)>conduct band K(K’).Such results of interlayer coupling could be fundation of deeper understanding and tuning strategy of electronic structure and interlayer charge transport behavior.Then we suggest that by applying in-plane strain or adding point defect,the charge carriers could be transfer from K(K’)valley to Q(Q’)valley resulting in enhancement of interlayer transport.First principle calculation is performed.It is found that in-plane compress strain could effectively lower the energy level of Q(Q’)valley while increase the energy level of K(K’)valley,resulting carrier transfer from interlayer transport forbidden conduct band K(K’)valleys to permitted Q(Q’)valleys.Thus,the interlayer electron transport is improved.Point defects including V-S,iV-S,Mo-S and iMo-S can also deliver free electrons from K(K’)valleys to Q(Q’)valleys via K-Q inter-valley scattering,promoting electrons,which contribute to interlayer current,pass through the interface van der Waals gap.The edge band bending introduced by unbounded edges is studied in MoS2-WS2 van der Waals heterojunction.We calculate the influence of several types of edges on band structure and find out how edge band bending evolves with gate voltages.The results show that S terminated edge leads to upward band bending while Mo/W edge leads to downward band bending.Good interlayer transport of both electron and hole could be achieved by use S(Mo/W)terminated edge in p(n)type layer.Positive gate voltage would rise up edge band while negative gate voltage would lower edge band.The tuning effect of out-plane electric field and layer number is investigated in van der Waals heterojunctions.By calculating the transmission spectrum of MoS2-WS2 van der Waals heterojunction under variable out-plane electric field,it is found that the interlayer transport channels of Γ,valence band K(K’),conduct band K(K’)and conduct band Q(Q’)valleys opens respectively as the electric field amplified.Then the interlayer transport property of WSe2-ReS2 tunneling junction is calculated,it is found that there exist two modes of K-Γ and Γ-Γ band to band tunneling.Base on such result,we suggest that optimization of interlayer current could be achieved by adding the number of WSe2 source from one to two.As the layer number of WSe2 increased,the band gap of tunneling junction is narrowed along with indirect-direct band shifting(from K-f indirect band gap to f-f direct band gap).As a result,the open gate voltage is remarkably reduced,meanwhile,136-990μA/μm band to band tunneling current is achieved.