Computing Studies On Magnetic Properties And Electronic Structure Of Rare Earths (Gd, La, Y) Doped Materials Based On Density Function Theory

The research in this dissertation is based on the Density Function Theory (DFT) in First-principles. The calculation software is CASTEP model in Materials studio. This dissertation focus on the structure, magnetic properties, electronic structure, and optical properties of 2×2×2 ZnO supercell which doped rare earth (Re=Y, La, Gd), Vo and rare earth (Re=Y, La, Gd) co-doped, or doped rare earth (Re=Y, La, Gd) and C/N. This dissertation expects to give some instructions for experiment in producing p-type ZnO dilute magnetic semiconductors (DMS), and to explain physical mechanism behind the phenomenon by comparing and analysising the computed data. The results shown as follow:Magnetic moment of Y-doped ZnO supercell is 0.02μB. La-doped ZnO supercell causes spin polarization of localized atomic, but the supercell does not have magnetic. Magnetic moment and band gap of Gd-doped ZnO supercell are 7μB and 0.133eV respectively. The main reason is the effect of half-full 4f electron shell. Overall, the effect of Vo for improving magnetic properties of rare earth doped ZnO supercell is limited. The improving of magnetic moment is mainly from the doped rare earth itself. Rare earth doped in ZnO supercell usually performance as the donor form which appear as+3 valence and Vo also exists as donor form. This causes the present of ZnO semiconductor is n-type and makes the Fermi level of rare earth doped ZnO supercell come into the top level of valence band at different degree. Thus, either rare earth elements doped or Vo and Re (Re=Y, La, Gd) co-existed supercell can not achieve p-type ZnO.The band gap of rare earth Re (Re=Y, La, Gd) and N co-doped in the ZnO supercell increased effectively, but magnetic moment of the supercell improved little. Magnetic moment of the supercell is from the rare earth mostly. By calculating, Gd-N co-doped ZnO makes the supercell with high magnetic moment 6.96μB and the wide band gap 0.608eV. This is due to N exists as the acceptor impurity that absorbed the free electrons of rare earth elements effectively. Thus, the Gd-N co-doped ZnO supercell is benefit to realize p-type and high magnetic moment.Rare earth Re (Re=Y, La, Gd) and C elements co-doped ZnO supercell can highly effect magnetic moments of the supercell. Especially the magnetic moments of Y-C and La-C co-doped supercell are highly improved than Y/La doped supercell. And the magnetic moment of Gd-C co-doped ZnO supercell is up to 8.0μB. However, the band gap of ZnO supercell which co-doped with C and rare earth are disappeared. This is mainly due to the larger radius of rare earth Re (Re=Y, La, Gd) that causes rare earth elements loss of valence electrons easily. Secondly, C atomic radius is greater than N, and atomic number smaller than N. Thus although the C-2p orbital can accommodate four electrons, its binding force on the electrons is smaller than N. Again, even though C doped supercell can absorb and reduce the free electrons generated by the rare earth elements, but the electron hole also increases the number of electronic carriers which lead to the electrical conductivity of the supercell improved. All those cause the disappearion of ZnO's band gap.In addition, the optical properties in low-energy region of ZnO which N/C replaced O doped changed greatly. Those changes are mainly due to N/C doped ZnO supercell that brought impurity level at Fermi level. The optical properties of ZnO supercell that doped Rare earth mainly changed the location of energy level, intensity of the peak and the energy in high region.