Frist Principle Calculations On Electronic Energy Band Structure And Optical Properties Of Ternary Mixed Crystal Al_xGa_1-xAs

With the rapid development of semiconductor technology, III-V group compound semiconductors, such as GaAs, have draw profound attention, because of its applications in fabricating hetero-structures and tunable devices, optoelectronic devices etc. In recent years, to optimize and enlarge the application of semiconductor devices, ternary mixed crystals (TMCs) of semiconductors, for instance AlxGa1-xAs, whose properties may vary over a wide range by changing the composition, has been widely applied in nano-electronic devices such as superlattices, nanowires, quantum wells, dots and wires. However, only a few theoretical calculations of the TMC AlxGa1-xAs energy band structures and optical properties have been carried out for a couple of composition values because of the computational complexities. In order to understand the composition dependence of the electronic properties completely, the theoretical studies for the whole composition range are invoked. In this thesis we study the electronic energy band structures and optical properties of ternary mixed crystal AlxGa1-xAs, by the first-principle calculations within the framework of the density functional theory (DFT) for the whole composition range.The thesis includes three parts, as follows:Firstly, the band structure, density of states and dielectric function are obtained for the pure zinc blende structure GaAs, and the phonon spectrum, Born effective charge and thermodynamic properties are calculated by the LDA and GGA approximations. Secondly, to obtain a full understanding for the structural, electronic and optical properties of the TMC AlGa1-xAs, we have discussed the composition dependences of the lattice constants, electronic band structures, dielectric function and the fitting formula for the TMC AlGa1-xAs (0≤x≤1). Finally, the band gap is corrected using the method of HSE.The results show that GaAs belongs to direct band gap semiconductor. It is found that the phonon frequency values calculated with LDA are higher than those with GGA in phonon dispersion curves. Our results for phonon frequencies along the high symmetry points are in agreement with available experiment data and other theoretical results. The lattice constants and dielectric function vary with the composition almost linearly following the Vegard’s law. The electron band gap at Γ point exhibits non-linear increase behavior versus the composition and gap bowing parameter b is0.56. The band gap calculated by HSE method increases and closes to the experimental value. From density of state, we found that the Al-3s and3p states shift to high energy region in the conduction band with increasing the Al concentration. It leads to an increase of the band gap and the blue shift phenomenon to appear.