First-principles Studies Of Doping Effects On The Magnetic And Electronic Properties Of Strontium Titanate And Gallium Oxide

Doping can manipulate and improve the performance of functional materials.As a typical perovskite oxide functional material,strontium titanate(SrTiO3)has attracted much attention because of its high static dielectric constant,large electron effective mass and large Seebeck coefficient,which has led to one of the most fascinating electronic ceramic materials.Due to its high efficiency,stability and strong reducing ability in the photocatalytic field,SrTiO3 can also be employed as a feasible photocatalyst.On the other hand,the wide band gap semiconductor material gallium oxide(Ga2O3)has also becoming one of the hot research topics due to its higher optical band gap(4.2-4.9eV),larger breakdown electric field,excellent physical and chemical properties,simple production process,and large-scale production.Therefore,β-Ga2O3 has potential applications in solar-blind ultraviolet photodetectors,photodiode,gas sensor and other applications.Both materials are intrinsic wide-bandgap n-type semiconductors through experimental synthesis,which severely limits their applications in the optoelectronic devices.Therefore,effective dopants and/or doping methods to achieve p-type or magnetic manipulation are particularly important for expanding their applications in electronic devices.In this thesis,we conduct research on strontium titanate and gallium oxide based on first-principles calculations to explore the effects of different dopants on the magnetic and electronic structure of these two materials,and draw the following conclusions:1.The magnetic,structural stability and electronic properties of strontium titanate with seven different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation(GGA)and the GGA+U approach.Our results show that the structural stability,electronic properties and magnetic properties of C-doped SrTiO3 strongly depend on the distance between carbon dopants.In both GGA and GGA+U calculations,the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors,which can be ascribed to the formation of a C-C dimer pair accompanied by stronger C-C and weaker C-Ti hybridizations as the C-C distance becomes smaller.As the C-C distance increases,C-doped SrTiO3 changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations,while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method.2.We also perform DFT with the GGA method to explore the magnetic and electronic coupling properties of boron-doped SrTiO3 within the framework of different dopant-dopant separations.We find that the ground magnetic state of B-doped SrTiO3 strongly depends on the dopant-dopant separation distance,i.e.,the ground magnetic state of different structures transforms from nonmagnetic to ferromagnetic/antiferromagnetic as the distance of the two doped boron atoms increases.When the two doped boron atoms have the smallest dopant-dopant separation distance,the structure is characterized by the lowest energy among all structures,and a dimer pair is visible.Strong ferromagnetic coupling is observed when two doped boron atoms align linearly along the B-Ti-B axis,which can be theoretically associated with the structure,e.g.,correlated with the bending of Ti-B-Ti and O-Ti-O units.Therefore,the symmetry of the local structure makes an important contribution to the generation of spin-polarized magnetic moment.Our study also demonstrates that the O-Ti-O unit is easier to deform than the Ti-B-Ti unit.When the dopant-dopant distance is less than 5 A,a semiconductor or insulator electronic structural character tends to be generated,while metal properties are present for the dopant-dopant distances beyond 5 A.The bands originating from B 2p states lead to widening instead of narrowing of the band gap when the two doped boron atoms are nearest neighbors,while the induced isolated energy levels can somehow contribute to enhancement of the photocatalytic activity under visible light.However,when the two doped boron atoms are the farthest away from each other,a significant band narrowing of approximately 0.67 eV is triggered,introducing more holes for the spin-down channel than the spin-up channel.Therefore,the calculated results indicate that we can theoretically manipulate the magnetism and band gap via different B-B dopant distances.3.Based on first-principles spin-polarized density functional theory calculations,the structural,magnetic,and electronic peculiarities and trends of 3d transition metal(TM)-doped β-Ga2O3 are investigated,in which one Ga atom is substituted by a TM atom(Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,or Zn,corresponding to the number of 3d electrons from 1 to 10)at either the octahedral or tetrahedral Ga coordinated site.Our results show that the valence difference between the dopant and the host cation and the symmetry of the local structure make important quantitative contributions to the magnetic moment.One obvious trend is that as the atomic number of the dopant increases,the percentage of the magnetic moment induced by the 3d TM atom drastically decreases,while the percentage provided by the O atom significantly increases,regardless of the substitution at octahedrally or tetrahedrally coordinated sites.We predict that except for Sc-and Co-doped Ga2O3,which show nonmagnetic ground states,the 3d TM-doped defective configurations energetically favor ferromagnetic ground states.Additionally,the impurities of Sc,Ti,V,Cr,Mn,Co,Ni and Cu prefer to reside at the octahedrally coordinated sites,whereas the impurities of Fe and Zn tend to occupy the tetrahedral sites.The 3d TM-doped β-Ga2O3 configurations change from n-type to p-type as the dopant atomic number increases,which can be ascribed to the lower atomic 3d-orbital energy levels,as well as the change from introducing electron/electrons to introducing hole/holes with increasing 3d TM atomic number.Moreover,as the 3d TM atomic number increases,near the Fermi level,the 3d orbitals of the TM are first occupied;then,the 3d states of the TM overlap obviously with those of O 2p,and subsequently,O 2p states exhibit a leading role.4.Study of 4d transition metal elements(Y,Zr,Nb,Mo,Tc,Ru,Rh,Pd,Ag and Cd)doped β-Ga2O3 based on first-principles calculations show that all 4d transition metal dopants prefer to occupy the octahedral Ga coordinated sites.The trend of the electronic structure of 4d doping is similar to that of 3d doping.For example,as the atomic number of the dopant increases,the percentage of the magnetic moment induced by the 4d TM atom drastically decreases,while the percentage provided by the O atom significantly increases,regardless of the substitution at octahedrally or tetrahedrally coordinated sites;the 4d TM-doped β-Ga2O3 structures transform from n-type to p-type as the dopant atomic number increases.The research on SrTiO3 in this thesis focuses on the searching for effective doping methods to achieve different magnetic and electronic structures,which can hopefully provide some guidelines for the application of spintronic devices and for the design of new materials;the study of 3d and 4d transition metal doping of Ga2O3 aims at the investigatation of effective dopants,revealing the trend of electronic structure and magnetic properties due to the introduction of different dopants,which can hopefully offer some guidelines for the doping engineering of Ga2O3 to realize effective p-type doping.