First-principles Study On The Electronic Structure And Magneto-optical Properties Of ZnO Co-doped With Rare Earth Elements (La、Nd、Sm)

At room temperature,ZnO is a new type of third-generation II-VI multi-functional semiconductor material with a wide band gap and high exciton binding energy.Remarkable achievements have been made in the research of Zno-based thin magnetic semiconductors.However,the intrinsic magneto-optical properties of ZnO are simple and untunable,among which the representative ones are:Curie temperature is low,the explanation of magnetic source and magnetic coupling mechanism is not uniform.The results show that rare-earth doped ZnO thin magnetic semiconductor materials have excellent magneto-optical properties by means of intrinsic defects and doping adjustable room-temperature ferromagnetism and photocatalytic properties of ZnO,and are widely used in spintronic devices,photocatalysis,solar cells and other fields.Based on first principles of the density functional theory,GGA+U method is chosen to study the impact of rare-earth elements（La,Nd,Sm）doping of ZnO on its magnetic,optical and electrical properties under spin polarization conditions.The main research contents include:（1）The photoelectric properties of La,Nd,and Sm single doped ZnO systems.The results show that the crystal cell of the doped system is distorted,the formation energy is less than zero,and the doped system can be formed stably.All band gaps are narrowed,impurity energy levels appear in the band gap,and Fermi energy levels enter the conduction band,showing n-type semiconductor features.Especially near the Fermi level,La-5d,Nd-4f and Sm-4f state electrons introduced donor impurity levels overlap with Zn-4s states in the conduction band,resulting in the band-tail effect.With the increase of doping concentration,the number of occupied electrons contributed by La-5d,Nd-4f and Sm-4f states to the band gap increases,and orbital hybridization increases,leading to further reduction of the band gap.The charge density difference calculation results show that the polarity of La-O bond,Nd-O bond and Sm-O bond is strong,indicating strong covalency..The doping atoms have a greater impact on the surrounding O atoms,and most electrons are concentrated around the La-O bond,Nd-O bond and Sm-O bond.The calculation results of optical properties indicate that the Nd doped system has the largest average static dielectric constant,and the best photocatalytic activity.The optical band gap of all doped systems is widened,and the absorption edge is blue shifted.In the visible range,the addition of La,Nd and Sm significantly improves the photoconductivity of ZnO semiconductor,and the absorption spectrum is redshifted.（2）The magnetic properties of La,Nd,and Sm single doped ZnO systems.The La doped ZnO system shows no ferromagnetism,while the Nd and Sm doped ZnO system show ferromagnetism,and the magnetism mainly comes from the strong hybridization between Nd-4f,Sm-4f and O-2p orbital electrons.With the increase of Nd and Sm doping concentration,the magnetism increases,so they can be the preferred materials for the spin polarization devices based on DMS materials..（3）The magnetic,optical and electrical properties of the La,Nd and La,Sm co-doped ZnO systems.The crystal cells of the co-doped system are distorted,and the formation energy is less than zero.The formation energy of Zn₃₀La₁Nd₁O₃₂system is smaller,indicating that the doping is easier and the structure is more stable.The band gaps of these two co-doped systems become narrower,and their fermi levels enter the conduction band,indicating that these systems present n-type semiconductor properties.The charge density difference calculation results show that the electron cloud overlap between the doped atoms and the surrounding O atoms and the covalence of La-O bond,Nd-O bond and Sm-O bond are weakened,while the ionic property is enhanced.The calculation results of optical properties demonstrate that the average static dielectric constant of Zn₃₀La₁Nd₁O₃₂is large,the intensity of the photogenerated electric field is strong,and the charge binding capacity is the highest.The optical absorption edge of the co-doped system has a blue shift.In the range of visible light,the co-doped La,Nd,La and Sm significantly improve the optical conductivity of ZnO semiconductor,and the absorption spectrum has a red shift.（4）The magnetic properties of La,Nd and La,Sm co-doped ZnO systems are studied.The co-doped systems are ferromagnetic,and their magnetism mainly comes from the contribution of electrons in Nd-4f states,Sm-4f states and La-4d states,and the magnetic exchange with La-5s states,O-2p states and Zn-4s states leads to strong spd-f hybridization.The Nd-4f states and Sm-4f states show obvious spin splitting phenomenon,and contribute the most to the magnetic moment of doping system,which can be the preferred materials for spin polarization devices based on DMS materials.