The Study On The Structure-property Relationship And The Mechanical Control Mechanism In Two Dimensional Semiconductors

Two-dimensional?2D?semiconductors have two outstanding characteristics,which are atomic thickness and intrinsic band gap.These features make great potential applications for 2D semiconductors in the next generation nano-electronic devices.The electronic structure and carrier transport of 2D materials are closely related to their structures,and the mechanical control in carrier mobility is also one of the hot topics in the research of semiconductors.Our work focuses on 2D semiconductors including phosphorene,group ?-? semiconductors and monolayer GeAsSe to investigate the relationships between structures and performance.For monolayer and bilayer semiconductors,the strains engineering and stack engineering have been studied to control the carrier mobilities in 2D system,respectively.The conclusions reached in this article are as follows,1.The 2D semiconductor?-CP has two layers of graphene-like honeycomb structures.Based on the first-princple calculation results,we fitted the mechanical parameters,including Young modulus,shear modulus and Poisson's ratio,of monolayer?-CP with the energy method.The results indicated Young modulus of monolayer?-CP is high up to 445.72GPa along armchair direction,which is a half of graphene.The shear modulus is 178.53GPa and the Poisson's ratio is only 0.064.2D?-CP exhibts much higher anti-deformation abilities,when compared to the soft materials such as phosphorene.Along the in-plane directions,the distribution of Young modulus approximates the ellipse.Young modulus of monlayer?-CP has the maximum and the minimum values at the armchair and zigzag directions,respectively.In general,monolayer?-CP shows small anisotropy in mechanical properties.2.As a typical representative in 2D semiconductors,phosphorene has many excellent physical and chemical properties.First-priciple calculations were performed to investigate the relationships between geometry structures and electronic properties.Due to the dihedral angles in geometric structure,the ultimate strains of phosphorene is 30%and 27%along armchair and zigzag directions,respectively.In addition,phosphorene shows different mechanical properties in different directions.The mechanical anisotropy is caused by the anisotropy in geometry structure.We have found the uniaxial strains will dramatically change the electrionic structure,local electrons distribution and band structures in phophorene.Under uniaxial strains,phosphoene is proved to make direct-indirect band gap transition and semiconductor-metal transition.What's more,we found the effective mass of carriers in phosphorene will change suddenly under uniaxial strains.The main reasions are the strain-induced band gap decreasing and the transformation in conduction band minimum?CBM?or valence band maximum?VBM?.The effective masses of electrons in phosphorene whithout strains are 1.244_em and 0.146_em along zigzag and armchair directions,respectively.When applied 0.06 zigzag strains in phosphorene,the effective mass along zigzag and armchair directions change to 0.16_em and 2.78_em,respectively.In the zigzag strains of 0.06⁰.12,the effective mass along armchair direction will decrease,while the zigzag direction keeps unchanged.The effective mass of electrons will decrease from 1.27_em to 0.32_em when 0.06 armchair strain was applied,and the armchair effective mass keeps unchanged.At 300K,the carrier density in phosphene will first decrease gradually ate_x?28?0⁰.04ande_y?28?0⁰.06,and then increase with the strains.3.In this section,we have proposed a new group of 2D materials which are composed of IV and V elements with the ratio of 1:1 in space group of P???m2.The group ?-? materials?except?-CN?show narrow band gaps ranging from 1.9eV to2.7eV.The main features in group ?-? materials are the high and anisotropic mobility.The intrudction of armchair strains will lead to the increase of ?-? bonds.Results indicated the electronic structures of monolayer group ?-? materials are very sensitive to the armchair strains.Under a certain strain along armchair direction,the CBM of group ?-? materials will be gradually replaced by a new band state.The density of states suggests the CBM and VBM in group ?-? materials are mainly formed by p states in IV and V elements.The introduction of stain will directly affect chemical bonds and further modify the orbital hybridization.The replacement in CBM will further lead to changes in effective mass of electrons,and the deformation potential?DP?constants will also be changed accordingly.Based on DP theroy,the CBM replacement will finally lead to sudden change in mobiliy along armchair direction in group ?-? materials.For example,the armchair mobility of?-SiAs will increase from 2.3cm²s^-1v^-1 to 197.5cm²s^-1v^-1 under strains.For monolayer?-CAs,the carrier mobility can decrease from 11817.9cm²s^-1v^-1 to 0.4cm²s^-1v^-1 after strain engineering.The strain engineering induced controllable mobility in group ?-? materials will bring great advantages in the applications of advanced nano-electronic devices.4.Bulk GeAsSe has a very low cleavage energy and a large Van der Waals?VdW?gap which indicate the preparation of monolayer will be very easy.Monolayer GeAsSe shows great anisotropy in band structure,effective mass of carriers,stiffness and mobility.At 300K,the mobility of monolayer along?100?direction is only1.6cm²v^-1s^-1.For bilayer GeAsSe,we have built four kinds of GeAsSe models based on stack engineering and results indicated they exhibt different electronic properties.For example,type I,II and III bilayers show indirect band gaps,while type IV exhibits direct band gap.Type I and II show high anisotropy in the distribution of electrons effective mass,while type III and IV exhibit isotropy.We find the stack engineering between layers can be used to control the mobility of system.For example,the mobility of bilayer II along Y direction is 192cm²v^-1s^-1,while the mobiliy along X direction is only 0.96cm²v^-1s^-1.It indicates the aniostropic mobility of GeAsSe can be further enhanced through stack engineering.The stack engineering between layers in 2D materials provides a new way to improve electronic properties.