First-Principles Study On Electronic Structures And Devices Of The Two Dimension Group Ⅳ-Ⅵ Monochalcogenides

Two-dimensional（2D）semiconductors,having an atomic-scale uniform thickness,dangling-bond-free interface,and easily being vertically stacked,are considered potential alternatives to silicon.2D semiconductor as the channel will contact metal electrodes so as to inject suitable types of carriers for lack of a suitable doping method.A Schottky contact（barrier）is always formed at the interface of 2D semiconductors and electrodes due to the metal-induced gap states（MIGS）and gap states,decreasing the carriers’transport,and thus degrading the performance of 2D semiconductor devices.Therefore,it is important to select a suitable electrode with an ohmic contact or quasi-ohmic contact（with a small Schottky barrier height（SBH））at the interface.Herein,we report a family of 2D semiconductor materials,having a proper bandgap,binary Ⅳ-Ⅵ chalcogenides MXs（M=Sn,Ge;X=S,Se,Te;SnS,SnSe,SnTe,GeS,GeSe,and GeTe）.The fundamental characteristics of the MXs,the change of MXs under the biaxial strains,the interfacial properties of MXs FET with metals electrodes,and GeSe MOSFET are investigated by using quantum transport calculations and density functional theory.（1）The intrinsic band structure and the effect of a biaxial strain of MXs are simulated.When MXs or MXs devices are prepared in the laboratory,the substrate and electrode materials will change the lattice constants,which is equivalent to imposing a kind of strain on MXs.Therefore,a simulation of-5%～5%biaxial strain on the MXs is made.The indirect-to-direct-bandgap-transformation of GeSe,Ge Te,SnSe,and SnTe can be obtained while the strain is enforced.Moreover,the bandgap and variation Fermi level of monolayer MXs induced by compressive strain are larger than the corresponding tensile strain.（2）We systematically study the contact properties of MXs FET（two-interface model）.Firstly,GeSe-FET with electrodes（Cu,Ag,Ti,Au,Pd,and Pt）are simulated.With an ohmic and quasi-ohmic contact in the vertical interface,strong Fermi level pinning and anisotropic Schottky contact are in the lateral interface with a pinning factor of 0.14 in the armchair direction.Then,use the Graphene（Gr）electrode and insert Gr between the GeSe and a Cu electrode to improve the performance of GeSe FET.The Gr electrode has a dramatic lateral p-type（quasi p-type）ohmic contact in the armchair（zigzag）direction of GeSe FET,and Gr-Cu hybrid electrodes have a quasi p-type ohmic contact in the zigzag direction.Secondly,we report the contact characteristics of the MXs FET with Gr/Ag/Au electrodes.At the vertical interface,the MXs FETs form van der Waals（vd W）contact type after contacting with the Gr electrode,and an ohmic contact is formed after contacting with Ag and Au electrodes.At the lateral interface,the SnTe（armchair and zigzag）,Ge S（zigzag）,and GeSe（zigzag）FETs with Gr electrode get a desired p-type ohmic contact or quasi p-type ohmic contact,suggesting high device performance in such a MXs device.Our simulation provides a theoretical foundation for the choice of suitable electrodes in future2D MXs devices.（3）The transfer characteristics of underlap sub 5 nm GeSe MOSFETs with different gate lengths along the armchair and the zigzag directions are researched.A p-type（zigzag）device is superior to other types（n-and p-type（armchair）and n-type（zigzag））.The on-state current of p-type devices（zigzag）,even at a 1 nm gate length,can fulfill the requirements of high-performance applications for the next decade in the International Technology Roadmap for Semiconductors（ITRS）.The optimal values of I_on for p-type devices（zigzag）at 1-4 nm gate-length are higher than those of black phosphorene devices.To the best of our knowledge,these GeSe MOSFET have the smallest gate length that can fulfill the ITRS HP on-state current requirements among reported 2D material FET.