Effect Of La - N And Gd - N Doping On The Electronic Structure And Absorption Spectra Of ZnO

Zn O is widely used in industry, because of its characteristics of being non-toxic, inexpensive and stable. Zinc Oxide(Zn O), as a direct-bandgap semiconductor material, its band gap being 3.37 e V at room temperature, its exciton binding energy 60 me V, has very good chemical stability. Zn O has attracted wide attention from domestic and foreign academic communities in terms of optoelectronic materials.Rare earth elements has rich electronic states. More and more people use rare earth to change the performance of semiconductor in recent years. Rare earth doped Zn O or rare earth and nonmetal codoped Zn O will change the electronic structure and optical property of Zn O, also the photocatalytic ability.Base on First principle, we use ultrasoft pseudopotentials of density functional theory(DFT) and generalized gradient approximation(GGA)of CASTEP(MS 6.0) to set up two different concentrations N/La 、Gd-N doped Zn O models, and N-La codoped Zn O models. After geometry optimization, we calculate the formation energy, the energy band distribution, the TDOS, the PDOS, the electron density difference and the absorption spectrum.The calculation results show that when La, Gd, or N doped Zn O respectively, there will be an impurity band. The impurity valence bands of doped La and Gd are near to the forbidden zone of middle area, while the impurity band of doped N is at the top of the valence band and degenerate with the valence band. The density of state says that the impurity band are from La-4f state, Ga-4f state and N-2p state. With the increased La and Gd doping density, donor level moves toward deep energy while Zn O bandgap slightly narrowing. With the increase of N-doping density, acceptor level did not change significantly, but its bandgap narrows. When La-N, Gd-N codoped Zn O, impurity levels contributed by the La-4f state, Gd-4f state of and N-2p state respectively. Comparing with the single doped situation, the donor level is shallower but not obvious. The more codoped quantities will make the donor level deeper and band gap narrower. Compared with a single doping case, acceptor level and valence band of degenerate is more obvious, donor site is slightly shifted to shallow levels, but is still in the band the middle area, light moves not distinctly. Doping concentration of total donor level to deep level mobile, bandgap narrowing further.The results show that Zn O doped with La, Gd and codoped with La-N, Gd-N will make the absorption spectrum redshift, but when La-N and Gd-N codoped Zn O, the absorption spectrum has most redshift. From the formation energy we know that codoped system need more energy to form, but the codoped system is more stable. From electronic structure, it is concluded that the attraction between La-N and Gd-N makes the repellant between La –Zn、Gd- Zn and N-O less, which let both impurity bands shallower. The lives of carriers become longer. And the two impurity bands come to be the capture traps of hole and electron, which make the hole and electron are harder to compound. So the carriers’ lives are further longer. Synergistic effect of La-N, Gd-N codoped Zn O makes the photocatalytic ability of Zn O better. Although more codoped quantities make better redshift, the impurity level deeper. According to the semiconductor theory, deep level is effective in the complex center, which is not effective in tassing carrier to the surface, and the carriers’ lives are reduced. Above all, Zn O Photocatalyst preparated by La-N, Gd-N codoped Zn O are better than Photocatalyst preparated by La, Gd, N-doped Zn O. However, we should both consider the redshift and carriers’ lives, when preparing photocatalysis material with La-N, Gd-N codoped Zn O, and mixed the proper amount of density. Due to the special electronic structure of rare earth La, Gd, it makes 4f state electrons shielded by outer electrons, weakened interactions hybridization between the 4f state electrons and other electrons. Therefore, shallow-shift trend is not obvious. when La-N and Gd-N codoped with donor energy level due to synergetic effect.