Theoretical Study On Physical Properties Of V, Nb-doped ZnO By First-principles Calculations

ZnO based semiconductor materials, as the one of the representative third-generation semiconductors, have attracted extensive attention due to the excellent physical performance, environmental friendship and so on. Recent years, along with the increasing requirements of miniaturized and integrated device, researches on doped ZnO material is particularly important. Whether V or Nb, the adjacent elements of the VB group in the periodic table, is the appropriate doping elements for the requirements remains to be seen. In this thesis, the geometrical structure, electronic band structure, electrical property, surface magnetic properties and optical properties of V or Nb doped ZnO have systematically investigated by the first principle calculation with appropriate correcting algorithm.Firstly, the calculations of the geometrical structure and electronic band structure on doped ZnO show that both V and Nb doping could lead to lattice expansion. Although the either V ion or Nb ion has a smaller ionic radius than Zn ion. The HSE algorithm does get a far more accurate band structure compared with the PBE algorithm. Both dopings form impurity bands under the conduction band minimum which indicates a typical n-type doping. The differences between the two systems are that V doping induces quite strong and localized impurity peaks. However, the V impurity peaks decrease with the increasing doping concentration while the Nb impurity peaks increase.Secondly, the electrical property of doping systems was investigated based on the effective mass. Both V and Nb doping have a significant influence on the electron effective mass which improves the electrical conductivity. A appropriate doping concentration, which is lower in Nb doping than V doping, may result in the lowest resistivity. The existence of O vacancy could enhance the electrical conductivity and the Zn vacancy has the opposite effects. Thirdly, the magnetic properties of V or Nb doped ZnO surface with and without intrinsic defect are studied systematically. It shows that surface effect indeed enhance, but not introduce, the net magnetic moment and increase the Curie temperature. V doping produces a lager net magnetic moment than Nb doping which mostly contributed by V ion. However, Nb doping could cause spin polarization on more O ions than V doping. The calculations of magnetic coupling reveal that the ground state of V doped ZnO is ferromagnetic, but for Nb doped ZnO it’s anti-ferromagnetic which can’t lead to macro magnetism. The net magnetic moment will increase with the O vacancy and decrease with the Zn vacancy.Finally, the optical properties of V or Nb doped ZnO system are systematically investigated through the HSE hybrid functional. The calculated optical properties indicate that low concentration V doping could enhance the absorption of visible light and high concentration will result in good transparency, whereas the Nb doping shows a continuous increase along with the doping concentration. This absorption comes from the intraband transition from impurity band to conduction band. Moreover, V doped ZnO prefers an absorption on (001) direction, but Nb doped ZnO prefer the (100) one.According to the above four aspects of research, it could be concluded that V doped ZnO can achieve a good comprehensive performance for diluted magnetic semiconductor or photocatalysis materials. While Nb doped ZnO can easily obtain highly electric conductivity and transmissivity which is suitable for transparent conducting materials.