Based On Density Functional Theory Of Electronic Structure And Properties Of Strontium Titanate

Strontium titanate (SrTiO₃), a typical perovskite material, has been attracted widely for its fundamental research and perspective applications in fields of ferroelectricity, grain-boundary barrier layer capacitor, oxygen-gas sensor, epitaxial growth substrate for high temperature superconductor thin films, optical switch and so on. At present, althought tremendous theoretical and experimental investigations have been done on electronic structure, doping, conductivity model, defect states, suefacs and inteface of SrTiO₃, there are still a lot of divarications, such as the effect of doping and defect on the semiconducting mechanics, optical properties of SrTiO₃. So it is necessary to use the first principles method to explore. Electronic structures, intrinsic oxygen vacancies and doping were investigated by using a first-principles ultra-soft pseudo-potential approach of the plane wave based upon the Density Functional Theory (DFT) using the CASTEP package of Materials Studio in this thesis. The main contents are as the following:1,The detailed investigation of the electronic structure, band structure, density of states (DOS) and electronic density difference of paraelectric SrTiO₃ in the cubic phase has been performed. The energy levels of high symmetry points in the Brillouin zone were listed. The population analysis, DOS and electron density difference all indicates that there is a high degree of covalency in the Ti-O bond and an ionicity in the Sr-O bond.2,The electronic structure calculations of SrTiO₃ and SrTiO_0.875 were investigated. Due to the O vacancy electron doping, the Fermi level moves into CBs for SrTiO_2.875 and the system shows metallic behavior. The bottom of the CBs is distorted and the rigid band model is not proper to describe the O vacancy doping effect on SrTiO_2.875. At the same time, a defect energy level appears at 0.3eV below the Fermi level at theΓpoint near the bottom of the CBs. Since the electrons resulted from the O vacancy are localized by the Ti atoms in SrTiO_3-δ withδ=0.125, the linear relationship between electron doping concentration by O oxygen vacancy and the density of free carriers will be not valid anymore.3,Both n-type and p-type doping for SrTiO₃ were carried out. The properties of n-type calculation results indicate that there are lots of free electrons provided by the impurity atoms in the bottom of conduction band and the conductivity of SrTiO₃ is well improved.With regarde to p-type doping, the calculated results reveale that the In doping introduces the p-type carriers into the system, the Fermi level moves into the valence bands(VBs). The transmittance improves greatly and the cut-off wavelength moves to blue-light region with increasing doping concentration. At the same time, the In doped SrTiO₃ is highly transparent with the transmittance 90% in the wavelength range from 300nm to 600nm.