Electronic And Spin Transport Properties Of The Transition Metal Oxides Modified By Low-energy Ion Irradiation

Physical properties of materials are closely related to their atom and electron structures and properties at the surfaces,and surface modification by introducing defects has a significant effect on material properties.Transition metal oxides have enough flexibility for defects due to their diversity of structure and oxidation,which makes this kind of materials useful in defect engineering.As a surface modification tool,low-energy ion irradiation has many advantages,such as low penetration depth,high controllability,easy operation and so on,which is an important way to explore new effects in materials.In recent years,it has been found that transition metal oxides,modified by low-energy ion beam bombardment,exhibit some new physical properties,such as the metal-insulator phase transition,resistivity memory effect,two-dimensional electron gas?2DEG?and so on.These changes indicate that such materials have great application potentials in sensor,memorizer and field-effect devices.Thus,the work,aiming at the change of electronic and spin transport properties for transition metal oxides modified by low-energy ion irradiation,is theoretically and practically valuable to develop new electronic devices.Based on the above background,the surfaces of monocrystalline and amorphous transition metal oxides were modified with different degrees by low-energy Ar⁺ion beam bombardment in this dissertation.First,the variations of surface morphology,electronic and spin transport properties of SrTiO₃?STO?and TiO₂ were investigated before and after the bombardment.Oxygen vacancies were introduced into the oxide surface layer by ion irradiation,and their contribution to the formation of quasi-two-dimensional surface layer was discussed.Secondly,thermally stimulated discharge was utilized to study the trap structures of amorphous oxide TiO₂ and Ta₂O₅films surface modified by the ion irradiation.The main results are divided into four parts as follows.1.low-energy Ar⁺ion irradiation could introduce oxygen vacancy defects into the oxide surface due to the preferential sputtering of oxygen.The extra electrons provided by oxygen vacancies may be partly trapped within the Ti 3d band accompanying with the variation of oxidation state for Ti from+4 to+3.Therefore,the binding energy peak of Ti³⁺and PL peaks attributed to oxygen vacancy defects were observed at the irradiated surface of STO?001?and TiO₂?001?.Meanwhile,the bond between metal and oxygen was broken after oxygen removed partly,which led to the amorphization of the oxide surface,and a disordered damage surface layer with thick of 3⁵ nm was depicted in transmission electron microscopy images.These results indicate that low-energy ion beam bombardment is an effective method for surface modification of transition metal oxides,the process of which can generate disordered nano-scale defect layers on single crystal oxide surface and cause the variation of electronic properties due to the introduction of oxygen vacancies.2.The electrical properties of the surface with different bombardment time were detected in STO?001?and TiO₂?001?,and both of their resistance decreased with increasing irradiation time.However,there were obvious differences in their electronic transport properties.For the irradiated STO,the in-situ measurement the conductance during low-energy Ar⁺ion irradiation revealed extra electrons provided by oxygen vacancies or Sr-O divacancies were the main reasons for the formation of conducting surface.The conduction was governed by electron hopping from one localized state to another at the disordered surface of the STO irradiated less than 5 min.For the STO irradiated with 10 min,a metal-insulator transition was observed during lowering temperature.When the bombardment time was more than 10 min,the temperature dependence of resistance of the irradiated STO displayed proportionality to T^???>2?,which implied the generation of electron-doped quasi-two-dimensional surface layer.For the TiO₂ modified by ion beam with a short time,its transport behavior at low temperature could be interpreted by the multiple trap and release model,which demonstrated a quasi-two-dimensional conductive layer was generated in TiO₂ by low-energy ion irradiation.When the bombardment time was longer,the conduction of TiO₂ was respectively governed by the Mott's variable-range hopping at low temperature and the weak localization effect at the relatively high temperature.The theoretical analysis indicated that the difference of the surface structure for two single crystal oxides,such as Ti-O-Ti bond angle,was the main reason for their different electronic transport properties.Thus,it could be seen that the electrical transport properties of the modified transition metal oxides were closely related to the microstructure.3.The inverse Rashba-Edelstein effect?IREE?was detected the in the ion-irradiated oxides,which originates from the Rashba spin-orbit coupling in the quasi-two-dimensional conductive layer.The pure voltage of the IREE was obtained by the two-step measurement with sample flipping.Spin transport parameters,such as spin mixing conductance and the IREE length,were extracted by the voltage curves detected at different microwave frequencies,and the values of these parameters were in the same order of magnitude for that obtained in these widely investigated 2DEG systems.These results indicated that the spin transport properties of the transition metal oxides modified by ion beams were less related to the microstructure,but were determined by the spin-orbit coupling dominated by the surface electric field.In contrast,the spin transport properties of Nb,Nd-doped STO are proportional to the spin-orbit coupling from impurity atoms.4.Due to the Anderson localization,the surface of amorphous Ta₂O₅ and TiO₂films did not transit into metallic state after ion bombardment.However,discharge was observed on the surface of the modified oxide films during heating by the thermally stimulated method,which indicated that vacancy defects were generated by ion bombardment and a number of surface charges were injected into the film surface simultaneously.Based on the relationship between the current density and the temperature,the activation energy of trap was estimated about 0.121 eV for Ta₂O₅ and0.228 eV for TiO₂,and their relaxation time was 585 s and 287 s,respectively.The small values of these parameters implied the surface charges injected by ion irradiation are mainly trapped in the shallow traps and easy to be removed from the film surface.The existence of surface charges implies that this kind of materials modified by ion bombardment have potential application in charge storage and transport action.In short,low-energy ion beam bombardment is an important way for surface modification.Moreover,transition metal oxides provides an important platform for investigation of relationship between structure and physical properties.The present work will ahead light on the development of new multi-functional oxide devices.