| As a new generation of wide band gap semiconductor, the Ⅲ-nitrides semiconductor, such as GaN, has been widely studied and applied in the luminous field and high electron mobility transistors. In terms of the hetero-epitaxial thin films or heterostructure device design, the presence of heterojunction stress on materials properties and device performance has not been ignored. This paper used the first principles method of density functional theory (DFT) to simulate the impact of strain on the electronic structure and physical parameters of the ferroelectric material BaTiO3, GaN, AlN, InN and its alloys, in order to get the specific knowledge of strain in the changes of material properties to provide the necessary reference for further heterostructure device research and design.First, because of the dispersion of the experimental values of the elastic constants of GaN, AlN and InN, It is calculated to obtain the elastic constants of hexagonal and cubic structure. The biaxial strain model, which is strained in c plane and fully relaxed in the c axis direction, was established and determine the ratio c/a of GaN, AlN and InN were-0.49,-0.56and-0.78respectively. In large biaxial strain, the elastic constants no longer remain the same. And the changes of elastic constants in biaxial strain were known in hexagonal GaN.Second, the similarities and differences of the electronic structure between the cubic phase and hexagonal GaN, AlN and InN were investigated. It was found that the cubic phase AlN has non-direct band gap while others have direct band gap. There are strong s-p hybrid orbitals in GaN and InN, while the p-d hybrid orbitals played a dominant role in AlN. The strain model was used to analyze the strain effects in the electronic structure and electronic effective mass difference of the hexagonal GaN, AlN and InN, and compared the differences of strain effects in different materials. Due to the small band gap of InN, the electron effective mass no longer decreases and in turn increases rapidly when the tensile strain is larger than1%. Because of Non-degenerate conduction band minimum of Ⅲ nitrides, the strain only changed the curvature of the valley, not leading to the subband splitting, and the influence of strain on electron effective mass is less than the Si. In-plane uniaxial strain changed the symmetry of the hexagonal structure and increased the anisotropic of electronic structure. The lattice of cubic phase BaTiO3(111) surface is well lattice matched with the hexagonal GaN, and the heterostructure stress makes BaTiO3electron effective mass decreases rapidly and the emergence of the spontaneous polarization.Then, used the supercell method, the lattice constant and electron effective mass values of hexagonal AlGaN, AlInN and InGaN were obtained and consistent with other theoretical calculations. The properties of AlGaN follow Vegard’s law satisfactorily, while due to large difference between In cation and Ga cation, AlInN and InGaN has large deviation of Vegard’s law. Strained with the GaN substrate, the stress will vary with alloy components, so the stress and composition had combined effect in electron effective mass of the alloy. Electron effective mass of AlGaN decreases linearly with Al mole fraction, while with small In mole fraction of AlInN and InGaN alloys under strain, the electron effective mass of InGaN increases rapidly and decreases about AlInN.Finally, in the framework of density functional theory, the application of the local density approximation of density functional perturbation method examines the impact of biaxial strain on the phonon structure and the dielectric constant of the hexagonal GaN. Biaxial strain of GaN mainly shifted the phonon energy band structure and linearly decreased the LO, TO and LO-TO phonon energy at f point, while linearly increased the Born effective charges, high-frequency and static dielectric constant. The strain has largest influence in static dielectric constant of GaN. |
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