Effect Of Mn Doping And Point Defects On Magnetic And Optical Properties Of ZnO By First Principles Study

ZnO based diluted magnetic semiconductors have been widely used in the research field of solar cells,spintronic devices and photocatalysis due to their excellent magneto-optical properties.And the magneto-optical properties of ZnO can be effectively improved by adjusting Mn doping and point defects.However,Mn doping and point defects are sensitive to sample preparation conditions,which leads to the poor reproducibility of experimental data.As a result,it is difficult to control Mn doping and point defects in experiments.In addition,there is no unified conclusion about the magnetic source mechanism,and Curie temperature of ZnO based diluted magnetic semiconductors.First principles have certain advantages in studying the above problems systematically.Therefore,the effects of Mn doping and point defects on the magneto-optical properties of ZnO were systematically studied by first principles.The research contents and results are as follows:Firstly,the effect of point defects on the magneto-optical properties of pure ZnO was studied.The results show that in zinc-rich conditions,the formation energy of ZnO with interstitial zinc(Zn_i)and oxygen vacancies(V_O)is lower.Inder oxygen-rich conditions,the formation energy of ZnO with Zinc vacancies(V_Zn)and interstitial oxygen(O_i)is lower.The study found that ZnO with Zn_i and V_O has no magnetism,while the magnetic order state of ZnO with V_Zn and O_i is antiferromagnetic.The donor impurity energy level is formed in the band gap of ZnO with Zn_i and V_O,and the absorption spectrum is red shifted.Moreover,the impurity level coincides with the Fermi level,resulting in a trap effect,which reduces the recombination rate of electrons and holes.This is beneficial to the design and preparation of novel ZnO based photocatalysts.The band gaps of ZnO with V_Znand O_i are widened,and the absorption spectra is blue-shifted.Secondly,the effect of the Mn doping in different site(Mn substituting Zn expressed Mn _Zn,and Mn in interstitial site expressed Mn_i)and V_Zn/V_O on the magneto-optical properties of ZnO was studied.Mn in Mn doped ZnO without V_Zn/V_O has the characteristics of aggregation distribution,while Mn in Mn doped ZnO with V_Zn/V_O has the characteristics of uniform distribution.The Mn _Zn doped ZnO with V_Zn and Mn_i doped ZnO with V_O have long-range ordered ferromagnetism,and the Curie temperature is higher than room temperature.The former also has obvious semi-metallic characteristics,which is of great significance to the design and preparation of new spin-electron injection source dilute magnetic semiconductors.The magnetic properties of the doping systems are derived from the double exchange interaction between Mn3d,O2p and Zn4s states mediated by carriers.This is consistent with the average field approximation theory and the dual exchange mechanism theory.The absorption spectrum of Mn _Zn doped ZnO is red-shifted,while the absorption spectrum of Mn_i doped ZnO is blue-shifted.The absorption spectra of Mn _Zn doped ZnO with V_Zn/V_O and Mn_i doped ZnO with V_O are red-shifted,and the effect of V_O on the absorption spectra is more obvious.Thirdly,the effects of Mn doping and V_Zn/V_O at different ratio on the magneto-optical properties of ZnO are studied.Mn doped ZnO with Mn:V_Zn=2:1 exhibits stronger half-metallic ferromagnetism.The magnetic properties of Mn doped ZnO with V_O have nothing to do with V_O,but only depend on the doping amount of Mn.The average field calculation results show that Mn doped ZnO with V_Zn/V_O have room temperature ferromagnetism.The V_O and V_Zn both have effect on the visible absorption spectrum of ZnO,but V_Zn has little effect.Impurity energy levels are formed in the band gap of the ZnO:Mn with V_O,which leads to trap effect and promotes the photocatalytic properties of the system.Moreover,the higher the V_O concentration,the narrower the optical band gap,indicating the adjustment of the ratio of Mn:V_O plays a key role in the adjustment of photocatalytic properties of Mn doped ZnO.Finally,the effects of Mn doping and V_Zn/V_O with different valence states on the magneto-optical properties of ZnO are studied.Mn²⁺doped ZnO with neutral V_Zn(expressed V⁰_Zn)system possesses the lowest formation energy and the largest magnetic moment(10?_B).In addition,this system also has room temperature ferromagnetism and characteristics of half-metallization,and its magnetic properties are mainly derived from the double exchange interaction between the hole-mediated O2p and Mn3d and the spin polarization of itinerant electrons.This is very valuable for the design and preparation of a new type of dilute magnetic semiconductor with high magnetic moment and high Curie temperature.Neutral V_O(expressed V_O⁰)and V⁰_Zn have great effect on the visible light absorption spectrum,and the effect of V_O⁰ is greater in the wavelength range of 400-509nm,while the influence of V⁰_Zn is greater in the wavelength range of 509-760 nm.