Effect Of Defects And Ion Doping On Large Abnormal Electricalresistance And Magnetic Properties Of Zero Band Gap Semiconductor PbPdO₂

Zero band gap semiconductor has become a potential material for fabrication of spintronic devices and has a very broad application prospect due to its unique band structure.In this thesis,PbPdO₂ nanoparticles and thin films were prepared by sol-gel method and pulsed laser deposition（PLD）method,and the structure,electrical transport and magnetic properties of PbPdO₂-based semiconductor with zero band gap were systematically studied.Because PbPdO₂ material has zero band gap structure,its ferromagnetic characteristics and positive or negative CER effect regulated by the doped ion species make it a very broad application prospect in magnetic memory,spin detector,spin generator,spin diode and other electronic devices.The research work consists of the following four parts:（1）CER effect of PbPdO₂ thin films with different Pb vacancy（002）preferred orientations.PbPdO₂ thin films with（002）preferred orientation were prepared by pulsed laser deposition（PLD）,and the temperature dependences of resistance and resistivity,R（T）andρ_I（T）,were measured under different applied DC currents.A positive CER effect induced by the current was firstly observed in the PbPdO₂ films.In particular,it is novelty found that the positive CER value of PbPdO₂with I=10μA and T=10 K reached about 300%.Meanwhile,A critical temperature T_c=260 K for all applied currents was found.As T>T_c,the band gap of the film is enhanced by the combined effect of the temperature and current.At the same time,Pb and O vacancies,and the evolution of oxygen valance states in the PbPdO₂ film were observed by energy dispersive spectrometer（EDS）,electron paramagnetic resonance（EPR）and in-situ x-ray photoelectron spectroscopy（XPS）.Especially,the charge transport between O^1-and O^2-was confirmed by in-situ XPS.Finally,based on first-principles calculation,an internal electric field model and its induced potential barrier were established,which well explains the positive CER effect and the critical temperature T_c.At the same time,the electron transport behaviors have been studied of PbPdO₂@5mol%,PbPdO₂@10mol%and PbPdO₂@15mol%films with different Pb vacancy percentage.It is found that they all have CER effect,but the critical temperature T_c has obvious difference of different Pb vacancy percentage.（2）Effect of ion-doped on electrical and magnetic properties of PbPdO₂ thin films.Ion-doped PbPdO₂ thin films with（002）preferred orientations were prepared by PLD.The selected ions are divided into metal cations（V,Gd）and nonmetal ions（F,N）.The experimental results show that cationic-doped Pb Pd_0.9V_0.1O₂ and Pb Pd_0.9Gd_0.1O₂ films have positive CER effect,room temperature ferromagnetism,metal-insulator transition temperature higher than room temperature and the same critical temperature（T_c=260 K）;V ion doping film have a greater CER effect.Moreover,V and Gd ions doping further enhanced the magnetic properties of PbPdO₂ samples.PbPdO₂@F and PbPdO₂@N films were prepared by F ion and N ion doping on the surface of PbPdO₂ films with（002）preferred orientation,respectively.The experimental results show that both samples show negative CER effect.The main reason is that the doping of F and N causes a large number of defects in the films structure,and the current induction reduces the defect content and improves the conductivity of the samples.In addition,a large number of defects result in resistivity that is largely temperature independent over a wide range of temperatures;Both PbPdO₂@F and PbPdO₂@N films exhibit room temperature ferromagnetism.The introduction of F and N can change the Pd-O coupling in the films and produce Pd^（2-δ）+,which enhances the ferromagnetic properties of the films.（3）Influence of Co and Co-F co-doping on defect-induced intrinsic ferromagnetic properties of PbPdO₂ nanoparticles.Co-doped and Co-F co-doped PbPdO₂nanoparticles were synthesized via sol-gel method.The experimental results reveal that,Pb and O vacancies exist in all samples;the valence state of magnetic atom Co is identified as mixed Co²⁺and Co³⁺;all samples exhibit both ferromagnetism and paramagnetism;the incorporation of fluorine results in an evident increase in the magnetic moment of Pb Pd_0.875Co_0.125O₂@F;a jump of magnetization at about 265 K can be found in both FC and ZFC curves of Pb Pd_1-xCo_xO₂（x=0,0.05,0.10and 0.115）.Moreover,the origin of the magnetism can be considered as follows:the ferromagnetism for Pb Pd_1-xCo_xO₂（0<x≤0.15）results from the combined effect of O^1-and magnetic Co ions;fluorine substitution in an oxygen lattice site and the fluorine doping in the interstitial sites can induce both distinct roles on the valence of oxygen,Co and Pd ions,which changes evidently the magnetic properties of Pb Pd_1-xCo_xO₂.Fortunately,the magnetic properties of PbPdO₂,Co-doped PbPdO₂,Co-F co-doped PbPdO₂ can be verified by first-principles calculations.（4）Low temperature magnetoresistance characteristics of PbPdO₂ thin films.The low temperature quantum transport properties of PbPdO₂@5mol%,PbPdO₂@10mol%and PbPdO₂@15mol%films are investigated.With the increase of temperature,the MRs of all samples exhibit a weak antilocalization（WAL）to weak localization（WL）transition.From the analysis of the changes of the fitting factorαand the coherent scattering length Lφwith the temperature and the amount of Pb vacancy,it is found that the Pb vacancy has an important effect on the quantum transport,which is closely related to the change of SOC caused by the Pd-O hybridization induced by the Pb vacancy.The results of quantum transport experiments with obvious WAL and WL effects suggest that PbPdO₂ should be a topological insulator,which needs further theoretical and experimental study.