Effect Of Biaxial Strain On Half-metallicity Of Ternary Alloys Based On Zinc-blende Zno And Gaas: A First-principles Study

As it is known to all, electron not only has the charge, but also has a spin. Fullapplication of the spin and charge characters of electrons will make the human society enterinto a new era of information technology. At present, the real application of the electronspintronic devices, is only achieved in magnetic recording head and magnetic storage deviceusing giant magnetoresistance effect. A natural question: whether the conventionalsemiconductor materials, for example, Si, GaAs, ZnO, can be coupled with spin flow, so as todevelop spintronic devicesand with using the spin and charge attributes of electron? If thiscan come true, there will have a hope to produce spin field effect transistors, spin leds, spinresonance tunnel device, THz frequency optical switches, modulators, encoders, decoders andother new devices could be developed for quantum computing and quantum communication,leading to a new revolution in information technology. In order to achieve this goal,researchers are developing spintronics devices of new materials by the following twoapproaches mainly: one is the traditional semiconductor doped with magnetic ions, making itbecome a magnetic semiconductor. Another is preparation of half metal ferromagnets instructure matching with traditional semiconductors, which can be obtained through injectingspin flow to the semiconductors wiht high efficiency of spin flow.In the lab, films are often prepared by the state-of-art molecular bean epitaxial （MBE）technology, where the in-plane lattice constants of a film will decrease or increase along withthe changes of the substrate lattice. The film will be affected by bixial strains, but notisotropic strains. With appropriate choices of substrate lattice, it is expected that the stabilityof heavy doped Zinc-Blende（ZB） structures of interesting semiconductors, such as GaAs:TMand ZnO:TM could be robust. Therefore, it is important to understand how the biaxial strainaffects the electronic structure and half-metallicity. The electronic structure, magnetic andhalf metallic properties of transitional metal （TM）-doped zinc-blende ZnO and GaAs（TM=Cr, Mn, Fe, Co, Ni） thin films with biaxial strains on （001） plane are studied by densityfunctional theory （DFT） and beyond. Here, we focus on two simple layer-by-layer deltadoping structures with the TM substituting along （100） planes （type-I） and （001） planes（type-II）. We find that the Fe, Co, and Ni-doped GaAs, Mn and Fe-doped ZnO, and Co-doped ZnO（II） show antiferromagnetic （AFM） states, while Ni-doped ZnO（I） and Cr-doped GaAsshow ferromagnetic （FM） coupling independent of the biaxial strain within25%along （001）plane. The systems of Cr-doped ZnO, Co-doped ZnO（I）, Ni-doped ZnO（II） and Mn-dopedGaAs undergo a phase transition from AFM to FM when the substrate lattice constantchanges. The Co-doped ZnO（I）, Ni-doped ZnO and Cr-doped GaAs systems are demonstratedof half metallicity with a GGA description. The Cr-doped ZnO and Mn-doped GaAs systemsalso show robust half-metallicity with a large spin flip gap by a GGA+U description, althoughtheir half metallicity disappears with the standard GGA description.Subsequently, considering the effects of bixial strain on intrinsic defects insemiconductors, we have studied the intrinsic defects in ZnO and GaAs of ZB structure withbiaxial strain using a64-atom supercell. Under oxygen-rich condition, the formation energyof zinc vacancies with biaxial compress strain will be lowered, indicating that with biaxialcompress strain zinc vacancies are relatively easy to appear under oxygen-rich condition andpossibly the p-type carriers. Under Zn-rich condition, when the Fermi energy is near to thevalance band maximum （VBM）, oxygen vacancies have lower formation energy than otherdefects, indicating that oxygen vacancies are easy to appear under Zn-rich condition. Whenthe Fermi energy is near to the conduction band minimum （CBM）, zinc vacancies are easy toappear in ZnO. We found that the formation of defects are remarkly affected by biaxial strains.Under Ga-rich condition, gallium interstitial defects （Ga_i） are easily to appear when theFermi energy is near to VBM. When the biaxial strain changes from-5%to5%, the formationenergy ofGa_idecreases linearly. When the Fermi energy is near to CBM, gallium vacancies(V_Ga) is easily to appear, When the biaxial strain changes from-5%to5%, the formationenergy ofV_Gaincreases linearly, which indicates thatV_Gais much easier to form in GaAsunder the compress strain. Under As-rich condition, the formation energy ofV_Gais the lowestone of all defects with compress strain. As the compress strain increases, the formation energyofV_Gadecreases linearly. Under As-rich condition, when the Fermi energy is near to VBM,the formation energy ofGa_iis the lowest one of all the defects under tensile strain and Ga_iis easy to appear in GaAs. When the Fermi energy is near to CBM, the formationenergy ofV_Gais lower and linearly increases with the increasing of tensile strain.Finally, we apply the method of formation energy and chemical potential to someresearches on the effect of the defects in lithium ion batteries, which are commonly applied inthe information industry. The structures of LiTiPO5and LiTi2（PO4）3, as well as the possibilityof oxygen vacancies formation in the systems are studied by first-principles calculations. It isfound that oxygen vacancies can be formed in LiTiPO5and LiTi2（PO4）3under oxygen poorcondition. The formation of oxygen vacancies introduce a defect band within their band gaps,which is expected to improve the electronic conductivity of LiTiPO5and LiTi2（PO4）3significantly. Meanwhile, a great concentration of oxygen vacancies may increase thedischarge voltage of LiTiPO5and LiTi2（PO4）3.