Research On The Transport And Anisotropic Properties Of Novel SiC Based On First Principles

It is well known that SiC is a typical third-generation semiconductor,which has much characteristics,such as wide band gap,high hardness,high thermal conductivity and electron saturation drift speed.Consequently,SiC has large application in high temperature,high frequency,high voltage,and high power electronic devices.In this work,we will utilize the first-principle calculations to explore and research novel SiC semiconductor suitable for device fields,and combined with simulation results to research and analyze the stability,mechanical properties,electrical properties and transport properties of materials.The research content of this thesis is mainly divided into the following aspects:(1)Four new three-dimensional SiC crystal structures are put forward,and among them,that the Pm-SiC is monoclinic symmetry,while the Pnnm-SiC,Pbca-SiC,and Pbcn-SiC all belong to orthorhombic symmetry.The geometry optimization of SiC structures in different pahses is executed by PBE method,and the optimized structures are computed to analyze properties of SiC materials.The formation enthalpy,phonon spectra,and elastic constants results show that SiC in different phases have thermodynamic and mechanical stability.Moreover,the elastic modulus and Vickers hardness results show that Pnnm-SiC has the largest hardness,while Pbcn-SiC has the smallest hardness,and SiC in different phases are all brittle semiconductors.Analysis of the two-and three-dimensional Young's modulus,shear modulus,and Poisson's ratio shows that SiC in different phases all have anisotropy,and Pm-SiC has the obvious anisotropy,while Pbca-SiC has the weakest anisotropy.It is known from the energy band structures that Pm-SiC is a quasi-direct band gap semiconductor with a band gap value of 2.640 e V;Pnnm-SiC and Pbca-SiC are both indirect band gap semiconductor,and the band gap values are 3.120 e V and 3.724 e V,respectively;Pbcn-SiC is a direct band gap semiconductor with a band gap value of 3.639 e V.In addition,the effective mass results show that SiC in different phases all exhibit anisotropy,and the minimum value of electrons effective mass is Pbcn-SiC with a value of 0.262 m₀,and the minimum value of holes effective mass is obtained in Pm-SiC with a value of-0.187 m₀.The carrier mobility results show that SiC in different phases all exhibit anisotropy,among them,the maximum value of electron mobility is Pbcn-SiC with a value of 8.78×10⁴ cm²V^-1s^-1,while the maximum value of hole mobility is Pbca-SiC with a value of 3.37×10³ cm²V^-1s^-1.So Pbcn-SiC and Pbca-SiC semiconductors have good ability to transport electrons and holes,while the ability of Pm-SiC and Pnnm-SiC to transport electrons and holes is relatively poor.(2)The structural,mechanical,anisotropic,and electrical properties of SiC in different phases are researched by applying pressure.In the pressure range of 0-50 GPa,the optimized results show that the lattice constant and the unit cell volume continue to decrease with the increasing pressure,and the unit cell density increases with the increasing pressure.Compared with Pbcn-SiC semiconductor,Pnnm-SiC,Pm-SiC and Pbca-SiC have stronger incompress.In addition,the four SiC crystal structures still have dynamic stability and mechanical stability under certain pressure conditions.Analyzing the bulk modulus of SiC materials under given pressure conditions,it is known that the bulk modulus of SiC in different phases increases with the increasing pressure.When the pressure is 50 GPa,the largest bulk modulus is Pnnm-SiC with a value of 375 GPa.The shear modulus values of Pnnm-SiC,Pbca-SiC and Pbcn-SiC show a trend of first increasing and then decreasing with the increasing pressure,and the maximum value of the shear modulus is Pnnm-SiC obtained at a pressure of 50 GPa and the value is 188 GPa.In addition,the shear modulus value of Pm-SiC has a trend of first increasing,then decreasing,then increasing and final decreasing with the increasing pressure.Moreover,the maximum value of shear modulus of Pm-SiC is186 GPa when the pressure is 40 GPa.The results of Young's modulus show that the value of Pnnm-SiC at a pressure of 50 GPa is the largest and the value is 480 GPa,while the value of Pbcn-SiC at zero pressure is the smallest and the value is 352 GPa.Therefore,the hardness of Pnnm-SiC semiconductor is the largest while the Pbcn-SiC semiconductor has the smallest hardness.The results of Poisson's ratio show that four different SiC semiconductors all can undergo a transition from brittleness to ductileness under certain pressure conditions.Analyzing the two-and three-dimensional Young's modulus,it is found that SiC in different phases have mechanical anisotropy,and the degree of anisotropy increases with the increasing pressure.In addition,by analyzing the maximum and minimum ratio of Young's modulus under different pressure conditions,it is known that the mechanical anisotropy of Pm-SiC is the most significant,while the mechanical anisotropy of Pbca-SiC is the weakest.In the pressure range of 0-50 GPa,the band gap value of the four kinds of SiC in different phases decreases with the increasing pressure,and the band gap values of Pnnm-SiC and Pbca-SiC is less affected by pressure.So Pnnm-SiC and Pbca-SiC have strong electrical stability,not easily affected by the working environment.In addition,the band gap value of Pbcn-SiC semiconductor by applying pressure is transformed from ultraviolet light region to the visible light region,and Pbcn-SiC has a greater potential application in the field of light-emitting devices.Through researching the new SiC crystal structures under certain pressure conditions,it is known that Pm-SiC,Pnnm-SiC,Pbca-SiC and Pbcn-SiC have the advantages of high hardness,high anisotropy,strong incompressibility,tunable direct band gap and high carrier mobility.Therefore,it has broad application prospects in the fields of high voltage resistance,high frequency,high power and light-emitting devices.