Effects Of Point Defects On The Magnetic And Optical Properties Of Polar ZnO:La/Y Surface And GaN/ZnO Interface

ZnO is a stable,inexpensive,safe and reliable semiconductor material with ideal band gap and exciton binding energy.Therefore,ZnO has great potential for development in the fields of spintronics and photocatalysis.In recent decades,many scholars have conducted extensive and in-depth research on ZnO,and the physical and chemical properties of ZnO have been greatly improved by modification methods such as doping and processing.Recent experimental studies have confirmed that La/Y doped ZnO can achieve room temperature ferromagnetism.However,the intrinsic magnetic source of ZnO has not been clearly defined,and there is considerable controversy between relevant research conclusions.Most of the previous theoretical studies have been carried out on bulk ZnO,but the research on the surface structure of ZnO has rarely been reported.In addition,ZnO-based interfacial systems have received widespread attention from scholars,and GaN has attracted much attention due to its similar lattice structure and electronic structure with ZnO.However,previous studies on GaN/ZnO heterojunctions have focused on their optical properties,ignoring their potential as magnetic materials,and neglecting the effects of magnetic ground states on optical properties.In order to solve these problems and promote the research and development of ZnO-based surface/interface materials,based on density functional theory of first principles,the La/Y doped ZnO?0001?polar surface with point defects and GaN/Zn heterojunction polar interface with point defects were established and calculated.On the basis of structural stability analysis,the physical and chemical properties of each system,such as electronic structure,electron spin property,net magnetic moment,differential charge density and absorption spectrum were further calculated.The effects of La/Y doping and point defects on the magnetic and optical properties of ZnO-based surfaces/interfaces were systematically studied.The specific conclusions obtained are as follows:For La doped ZnO?0001?polar surface systems,the 5d state introduced by La³⁺was located in the conduction band away from the Fermi level,so the simple La doping fails to cause spin-polarized electrons in the ZnO surface system,and the system is nonmagnetic.O vacancy could induce two weakly bound Zn-4s electrons,but the two electrons have opposite spin directions,therefore,O vacancy does not make the system ferromagnetic.The calculation results of formation energy indicate that La doping easily induces Zn vacancy in the system,while the two weakly bound O-2p electrons caused by Zn vacancy have the same spin direction.Therefore,Zn vacancies can make ZnO surface system ferromagnetic.The more weakly bound O-2p electrons,the greater the net magnetic moment of the system.In addition,the anomalous radiation magnetic moment has a slight influence on the total magnetic moment.When the La and Zn vacancies are 2:1,the system exhibits magnetic quenching.La doping and Zn vacancies induced by La can effectively reduce the band gap of ZnO surface,thereby increasing the absorption coefficient of visible light in the system.For Y doped ZnO?0001?polar surface systems,similar to the case of La doping,simple Y doping and O vacancies do not make ZnO ferromagnetic.However,in the experimental study,there may be a H atom that enters the ZnO lattice to become a interstitial impurity.These interstitial H atoms can be coupled with O atoms adjacent to the O vacancies to form bound magnetopolarons which make the system ferromagnetic.In addition,in the Y doped ZnO system,the Zn vacancies can still induce spin-polarized weakly bound O-2p state electrons,which becomes the source of the net magnetic moment.Y acts as a donor to compensate for the charge imbalance caused by the acceptor of Zn vacancies.Therefore,similar to the case of La doping,when the ratio of Y and Zn vacancies is 2:1,the system still exhibits magnetic quenching.Since the uppermost Zn polar plane is destroyed by impurities and vacancies,spin electrons induced by Zn vacancies are apt to appear in the O polar layer.In addition,although Y doping is not conducive to improving the visible light absorption of ZnO,the O-2p state electrons induced by Zn vacancies can effectively reduce the band gap of the system and make the absorption spectrum red-shifted.For the GaN/ZnO heterojunction polar interface with point defects,the magnetic properties of the system are determined by two factors:first,there are unpaired p-state electrons induced by cation vacancies in the system;second,there is the electric polarization intensity caused by the electric dipole moment in the interface.The spin polarization of unpaired p-state electrons is regulated by the electric polarization intensity.Unpaired p-state electrons are difficult to spin polarized when the electric polarization intensity is too weak.As the electric polarization intensity increases,the spin-polarization of unpaired p-state electrons is enhanced.However,excessive electric polarization intensity will cause partial p-state electron spin reversal,causing ferromagnetic-ferrimagnetic-antiferromagnetic transition in the interface.This indicates that the GaN/ZnO heterojunction polar interface is a potential tunable magnetic material.On the other hand,the defect level induced by cation vacancies can be degenerated with the intrinsic conduction band by the tailing effect,which reduces the band gap of the system and increases the absorption coefficient of visible light.However,the O-2p defect level induced by Zn vacancies may become a recombination center of hole-electrons,thereby reducing carrier lifetime.The conduction band is spin-splitting under the influence of the net magnetic moment contributed by the spin polarization of unpaired p-state electron,which widens the conduction band,which is beneficial to reduce the effective mass of electrons and improve the separation efficiency of holes and electrons.Therefore,the magnetic ground state may increase the activity of the photocatalyst to some extent.In addition,the electric polarization intensity in the interface also facilitates the separation of hole-electrons in the polar direction.