First-principle Study Of Electronic Properties Of Doped MgAl₂O₄

Transparent conducting oxides are a class of materials combining properties of electronic conductivity and optical transparent in visible light range,which are needed in numerous applications including flat panel displays,solar cells and transparent electrodes.The spinel structure containsnot only edge-sharing networks which are viewed as conductive channels and less affected by the scattering of ionized impurity atoms but a large number of vacant site for doping.Therefore,Oxides with spinel structure are expected for high charge carrier mobility.Doping of transparent conducting oxides is a effective method of controling and improving their optical and electronic performance.As an important research tool and widely used in material field,the first-principle method is applied in studies of electronic properties of materias with native defects or impurity dopants.In this study,a first-principle calculation was proposed to investigate electronic properties of native defects in MgAl₂O₄ and elements doping of MgAl₂O₄.The main works as well as obtained results of this thesis are summarized as follow:1.The calculated valence band of MgAl₂O₄ is composed mainly of O 2pstates while the conduction band is compounded of Mg-3s and Al-3s,3p states.The formation energies of neural acceptor defects of O_i44 and Mg_Al and neural donor defects of V_O and Al_Mg are relatively low under O-rich condition and O-poor condition,respectively.It was found that O_i4,Mg_All and V_O,introducing deep levels in the band gap,can hardly produce charge carrier but can be a source of compensation.Although the antisite defect Mg_Al acts as a shallow acceptor,the formation energy become higher as the Fermi level approaches the VBM,which limits the solubility of Mg _Al.Therefore,native defects can not enhance the conductivity of MgAl₂O₄.2.The electronic property of s-block elements doping of MgAl₂O₄ has been studied.The results of formation energy indicate that O-poor conditions are favorable for s-block elements doping of MgAl₂O₄ and formation energies of H_i44 and H_i8 are lower than other impurities.O atoms of local structures of HO₄ and HO₆ are pushed outward when H occupies the interstitial site.The calculated s-block element dopants are deep donors because the CBM of MgAl₂O₄ are high,originating from the cationorbitals and O-2p orbitals form covalent chemical bonds and the CBM levels are raised by their resultant anti-bonding nature.Therefore,it is difficult to introduce shallow donors in MgAl₂O₄.3.The electronic property of nonmetal elements doping of MgAl₂O₄ has been studied.The results of formation energy indicate that O-rich conditions are favorable for nonmetal elements doping of MgAl₂O₄ and formation energies of N_i44 and F_i8 are lower than other impurities.N atom move toward neighbouring local structures of MgO₄ and AlO₆ when N occupies the tetrahedral interstitial site.The neighbouring O atoms were pushed outward because of the repulsion between O atoms and F atom when F occupies the octahedral interstitial site.N_i4 acts as a deep acceptor while F_i88 acts as a shallow acceptor with transition level?0/1-=E_VBM+0.53eV.Furthermore,native donor defects and impurities of V_O,Al_Mgand F_Otogether with complex defects ofF_i8+V_O,F_i8+Al _Mg and F_i8+F_O unlikely form in lattice because of their high formation energies under O-rich condition.On the other hand,the formation energies of F_i8 and 2F_i8 are negative so that they can form spontaneously under O-rich condition,indicating possible p-type conductivity in MgAl₂O₄ spinel by F doping.