First Principles Study On Phonon And Optical Properties Of Binary Compounds And Ternary Mixed Crystal Semiconductors

Ⅲ-Ⅴ and Ⅱ-Ⅵ semiconductor materials are widely used in light-emitting diodes,semiconductor photodetectors,thin film solar cells,diode lasers,high electron mobility transistors（HEMTs）and other optoelectronic fields due to their advantages of good high temperature stability,wide band gap and high thermal conductivity.With the emergence of low-dimensional structures（such as superlattices,quantum well,etc.）,through effective doping,to change the composition to adjust the forbidden band width,the phonon frequency,the optical dielectric constant,and other important physical parameters,can further improve the performance of the semiconductor material,greatly optimize the application value of the semiconductor and broaden its application fields,have very important sense to develop new optoelectronic materials.However,due to the complexity of calculation and the disorder of crystal structure,only a few components of the optical,phonon and thermodynamic properties of ternary mixed crystals have been studied,so this thesis adopts the first principles method based on density functional theory to carry out for the elatedproperties of some ternary mixed crystal of semiconductor materials.This article carries out from three aspects:Firstly,the structural,mechanical properties and optical properties of InGaAs mixed crystal of sphalerite were calculated.The results show that the relationship between the calculated lattice constants and In composition is almost linear obeys Vigard’s law.The elastic constants of InGaAs satisfy the mechanical stability.The bulk modulus,shear modulus and Young’s modulus decrease monotonically and nonlinearly with the increase of In components.The static dielectric constant,the static reflflectivity and the static refractive index of the optics increase with the In composition in the range of the composition x we consider from 0 to 1.The results of absorption spectra show that the absorption edge of In_0.125Ga_0.875As is redshifted due to the incorporation of In components.With the increase of In components,the energy loss function of In_0.125Ga0.₈₇₅As has a maximum value at 15.4 e V,which is the so-called plasma frequency.And in low photon energy region,In_0.125Ga_0.875As larger than In_0.875Ga_0.125As of optical conductivity.Secondly,we use the Local-density approximation（LDA）and generalized gradient approximation（GGA）exchange correlation functions to calculate the structural,mechanical,phonon and thermodynamic properties of wurtzite Cd X（X=S,Se,Te）.And it turns out that cadmium chalcogenides in the wurtzite structure should be mechanically stable and dynamically stable.We also found that the values of bulk modulus B,shear modulus G and Young’s Modulus E decrease gradually from X=S to Se,Te,suggesting weaker ability to resist deformation,stiffness is reduced,the Cd S is more hard compound material than other materials.The Born effective charge are anisotropic.GGA calculations corresponding phonon frequency values below LDA,and strongest anisotropy of wurtzite Cd S.Optical phonon frequency and the gap of the acoustic and optical phonon branches clearance between sensitive to anionic/cationic quality ratio.The dependence of thermodynamic properties on temperature is further studied by using quasi-harmonic Debye model.Finally,the structural,mechanical properties,phonon and thermodynamic properties of wurtzite ZnCdS mixed crystal were calculated.The results show that the calculated lattice constants a,c show a linear relationship with the Zn composition,which satisfies Vigard’s law.The elastic constants of ZnCdS satisfy the stability criterion,indicating that they are mechanically stable.In the range of composition x from 0 to 1 that we consider 0.125 of the steps,the lack of virtual frequency of phonons indicates that the dynamics of the structure is stable,and the phonon frequency is dependent on the Zn composition.And optical frequency increases with the increase of zinc composition.In the thermodynamic function,the heat capacity is close to the experiment at low and high temperature,as the temperature increases,the free energy decreased and the entropy and enthalpy increased gradually.