Theoretical Investigation Of Electronic And Optical Properties Of The Group-Ⅲ Nitride Gan And AlN

Group-Ⅲ nitride semiconductors have been recognized as the most promising materials for optical devices in the short-wavelength region, for instance, AlN, GaN and InN. Due to their superior intrinsic properties such as the wide band gap, high electron saturation velocity, high thermal conductivity and physical and chemical stability, group III nitrides is suitable for application in high-speed, high-temperature and high-power electronic devices. The world-wide interest in group III nitrides has been increasing in the last decade.The external pressure effect on the elastic, energy band structures, density of states and optical properties of GaN and AlN under high pressure have been calculated and analyzed using the first-principle ultra-soft pseudo-potential approach of the plane wave based on density functional theory. The calculated conclusions offer theoretical data for the design and application of GaN and AlN. The main results and contribution of the dissertation are as follow:(1) The external pressure effect on the elastic, electronic and optical properties of zinc-blende (ZB) and wurtzite (WZ) structure GaN have been investigated using the first-principle ultra-soft pseudo-potential approach of the plane wave. The external pressure levels are extended from0to100GPa. We fully study the mechanical stability of ZB and WZ GaN, the present results satisfying the mechanical stability criteria for the cubic and hexagonal crystal, respectively. The calculated results indicate that the dependence of the elastic constants, bulk modulus, and energy gaps on the applied pressure is presented, and the results are in good agreement with the comparable experimental and theoretical values. The energy band structure and density of states under high pressure are analyzed and it is found that the band gaps for AlN are broadened with the increasing external pressure. Furthermore, the dielectric function, optical reflectivity, refractive index, absorption coefficient and energy-loss function of GaN are discussed for radiation up to50eV. In order to obtain more accurate calculations of the optical properties, the band gap is adjusted with scissor operator values of1.72eV. The optical properties for WZ GaN at (100) and (001) direction polarized have calculated. The present results suggest that GaN has high ultra-violet radiation shielding performance.(2) The elastic, electronic and optical properties of zinc-blende (ZB), wurtzite (WZ) and rock-salt (RS) structure AIN have been investigated using the first-principle ultra-soft pseudo-potential approach of the plane wave. For the ZB structure, the dependence of the elastic constants, bulk modulus and shear modulus on the applied pressure are presented, the external pressure levels are extended from0to70GPa and the results are in good agreement with the comparable experimental and theoretical values. The energy band structure and density of states under high pressure are analyzed and it is found that the band gaps are broadened with the increasing external pressure. Furthermore, the optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are discussed for radiation up to50and40eV by analysis of the energy band structures and density of states calculated. The optical properties for WZ AIN at (100) and (001) direction polarized have calculated, too. The common trends and the differences between the three structures of AIN are presented. The optical properties indicate that AIN is a good shielding materials of ultra-violet radiation.