First Principals Study Of Electronic Transport Properties For MoS₂ Thin Films Nano Device

With the size of integrated circuit decreasing continuously,further reducing the silicon-based integrated circuit will face many technical difficulties,so it is necessary to study new technology for integrated circuit.Two basic approaches can be applied to study new technology for integrated circuit.The first approach is abandoning the existing physical mechanism and looking for a new physical mechanism,but the progress is slow.The second one is maintaining the existing physical mechanism and discovering new materials to replace silicon.Graphene may replace silicon,but the problem that the graphene need opening the band gap,which has not been well solved,limits the development of graphene.Monolayer MoS₂found in recent years has not only similar structure and properties to graphene,but also natural band gap.Can monolayer MoS₂replace silicon?There are still many problems need to study.Using the first principles computing,this dissertation studies the electronic transport properties of MoS₂thin films nano device:1.multi-phase properties and phase transition principle of monolayer MoS₂;2.electronic properties of MoS₂heterostructure;3.tuning the electronic property of monolayer MoS₂adsorbed on metal Au substrate;4.electronic transport properties of MoS₂thin films nano device.The major contributions and innovations in this paper can be summarized as follows.1)study multi-phase properties and phase transition principle of monolayer MoS₂.Using the first principles based on density functional theory,and introducing van der Waals force,this dissertation studies the electronic properties of the three phases:2H-MoS₂,1T-MoS₂and ZT-MoS₂,and 2H-1T phase transition principle.Firstly,the geometric structure of the three phases is determined by the structure relaxation.Total energy shows that the stability of the three phases is 2H-MoS₂>ZT-MoS₂>1T-MoS₂.Band structure and density of states calculations confirm that 1T-MoS₂ has metal property and ZT-MoS₂has semiconductor property with 0.01 eV band gap.Then using the deformation potential theory to calculate the mobility of 2H-MoS₂and 1T-MoS₂.The electron and hole mobility of 2H-MoS₂are between 100-400cm²V^-1s^-1.Compared to 2H-MoS₂,the electron and hole effective mass of ZT-MoS₂reduce significantly,leading to the mobility of ZT-MoS₂increases significantly,up to 10⁴cm²V^-1s^-1.The high mobility of ZT-MoS₂makes up the defect of 2H-MoS₂,and expands the application of the monolayer MoS₂.Finally this dissertation computes 2H-1T phase transition energy curve and finds that density concentration is key factor in phase transition of monolayer MoS₂.2)study electronic properties of MoS₂heterostructure.Using the first principles based on density functional theory,and introducing van der Waals force,this dissertation studies the electronic properties of MoS₂/WS₂and MoS₂/Graphene heterostructure.Firstly,this dissertation systematically summarizes two heterostructure interface models:interface match model and interface unmatch model,and builds the transformation matrix table of interface unmatch model.Then using two functionals:PBE and optPBE-vdW,this dissertation calculates the adsorption energy's change along distance between two layers and determine the distance through comparison.Band structure analysis finds that MoS₂/WS₂heterostructure has indirect band gap and interlayer coupling leads to a larger energy splitting at G point.But band structure of MoS₂/Graphene heterostructure is just a sum of MoS₂and Graphene band structure.Density of states analysis finds that MoS₂/WS₂heterostructure belongs to type II heterostructure,and MoS₂/Graphene heterostructure belongs to type I heterostructure.Difference charge analysis finds that the charge transfer in the heterostructure is small.3)study tuning the electronic property of monolayer MoS₂adsorbed on metal Au substrate.Using the first principles based on density functional theory,this dissertation studies tuning the electronic property of monolayer MoS₂adsorbed on metal Au substrate.The research focuses on four points:adsorption energy,band structure,density of states and difference charge density.Adsorption energy determines the arrangement of sulfur atoms layer when monolayer MoS₂adsorbed on metal Au substrate.Adsorption energy points out that the adsorption structure is not stable and this is accordance with experiment.Band structure analysis points out that Au substrate and monolayer MoS₂form Schottky contact and pinning effect arises simultaneously.Density of states analysis points out that conductivity of monolayer MoS₂increases as a result of Au substrate's effect.Difference charge density not only further confirms the above conclusion,but also points out that conductive channel of monolayer MoS₂will appear at the interface.4)study electronic transport properties of MoS₂thin films nano device.Using the the non equilibrium green function theory,this dissertation studies electronic transport properties of monolayer MoS₂nano device and MoS₂/WS₂heterostructure nano device.Firstly for monolayer MoS₂and MoS₂/WS₂heterostructure,device models on the Zigzag and Armchair direction are established respectively.Then the electrodes and the center scattering areas of devices are calculated in turn.Finally physical quantities related to the electron transport are calculated.Electronic transmission coefficient shows that conductive ability of nano device in the Zigzag and Armchair direction is different.Conductive ability in the Zigzag is stronger than the Armchair direction.Conductive ability of monolayer nano-device is stronger than the heterostructure nano device.