Preparation And Physical Properties Of Iron-Doped Indium Oxide Ferromagnetic Semiconductors

Microelectronics,which mainly studies and utilizes the transport properties of electron as a charge carrier,is the cornerstone of modern information technology. However,as the degree of integration increases,the device size will enter nanoscale,in which the energy consumption per unit area will rise rapidly and result in serious heat injury problems.Furthermore,the quantum confinement effect,instead of electronic energy band theory,will play a leading role.Against such a background,researchers begin to pay attention to spin,another intrinsic property of electron,and expect the combined utilization of both the charge and the spin to develop a new generation of electronic device.In contrast with the current semiconductors,the new kind of device has many advantages such as faster arithmetic speed,smaller size,lower energy consumption,information preservation under power-off and potential applications in quantum computation.This new research area namely spintronics,is an important direction of future information technology development.To realize the goals of spintronics,the foremost mission is to develop a new material possessing both the band-gap of semiconductor and spin sub-band splitting of magnetic material simultaneously,and ferromagnetic semiconductor is just a kind of this new material.The preparation method of typical ferromagnetic semiconductor is to dope transition element into the current semiconductor system.It is expected that the transition element may enter the crystal lattice by substituting some cations' positions, and through ferromagnetic coupling between transition metal ions,ferromagnetism may appear in semiconductor on the basis of the original band-gap.In 1990s,Ohno et. al.successfully doped Mn element into GaAs.The Curie temperature,however,is 170K,which does not meet the requirements of application.In the year of 2000,based on the traditional Zener model,T.Dietl et.al.calculated and predicted a Curie temperature of higher than 300K in Mn doped wide band-gap semiconductors such as GaN and ZnO.After this report,the research enthusiasm toward oxide-based ferromagnetic semiconductor has been immediately aroused and a lot of experimental and theoretical works have been evolved.The most studied systems are ZnO and TiO₂. However,results reported by different groups are different or even contradictory.To date,the question whether intrinsic ferromagnetism exists in transitional element doped oxide systems is still inconclusive.Meanwhile,although the research on In₂O₃-based ferromagnetic semiconductor is still in its early stage,many groups have reported the existence of ferromagnetism,and some has even revealed unique properties such as anomalous Hall effect and a solubility as high as 20%of iron in In₂O₃.Based on such a background,also with a view to the excellent opto-electric and gas sensitive properties of In₂O₃,we finally choose Fe-doped In₂O₃ as the study system of this thesis,and try to solve the questions below:whether high temperature intrinsic ferromagnetism can be realized in iron doped indium oxide and what is the origin of ferromagnetism.Pulsed laser deposition is used for sample preparation and the ceramic targets are sintered using solid state reaction method.We first grew polycrystalline Fe-doped In₂O₃ film on r-cut sapphire.XRD indicates the sample has a preferential growth texture along(440) direction,without any secondary phases such as iron,iron oxides and other indium-iron compounds.Composition analysis indicates iron content in the film is 7%and XPS demonstrates the valence state of iron is +2 and +3,which means Fe atoms have entered the crystal lattice of In₂O₃ in a substitutional way.The result by TEM indicates:the crystal lattice has a well-oriented cubic structure without any secondary phases;the film presents a columnar growth mode with the grain size of approximately 200nm;iron is homogeneously distributed in the growth direction of the film,while inhomogeneously along the interface between the film and the substrate, with casual existence of poor-iron areas which are the boundaries of the crystal columns.Magnetic measurements indicate that the samples have a high Curie temperature of 927K and a strong magnetic anisotropy with the easy axis perpendicular to the film surface.The high Curie temperature demonstrates the strong indirect interactions between the doped Fe ions.The strong anisotropy is probably the result of co-action of crystal-field potential and spin-orbit coupling,and also an important feature of intrinsic ferromagnetism.We believe this anisotropy is related to the preferential columnar growth mode along(440) direction.We also tried epitaxial growth of Fe-doped In₂O₃ on YSZ(100).The whole growth process was monitored by RHEED.Sharp stripes without any unexpected dots can be observed during the whole process.Only(400),(600),(800) peaks of In₂O₃ are observed in the XRDθ-2θscan,which indicates that epitaxial growth of iron-doped indium oxide is successfully realized.AFM measurement shows a columnar growth mode perpendicular to the substrate,with the grain size of approximate 200nm.The magnetic measurements indicate the epitaxially grown Fe-doped In₂O₃ has a stronger perpendicular magnetic anisotropy than that of the polycrystalline samples.Thus,we demonstrate further that the perpendicular magnetic anisotropy is a manifestation of intrinsic ferromagnetism which is related to the sample structure.For a further study of the origin of ferromagnetism,we prepared samples with different Sn contents,which is aimed at changing the density of carriers in the samples. Also,we changed the oxygen atmosphere during growth and carried out post annealing, which are aimed at changing the density of oxygen vacancies to observe how the ferromagnetism changes.The results indicate that after Sn doping,although the carrier density in the samples are greatly increased,ferromagnetism remains nearly uninfluenced.Different oxygen atmosphere,however,leads to an obvious change in the ferromagnetism.A nonmonotonic change of ferromagnetism with the oxygen atmosphere during growth is found.Under an optimal pressure,samples will have a strongest ferromagnetism.The later alternate annealing in vacuum and air also successfully regulated the ferromagnetism,which made it successfully transformed between the high magnetization state and the low magnatization state.The perpendicular magnetic anisotropy remains unaffected after annealing is conducted. Thus,we can see clearly that the ferromagnetism in our samples is closed related to the oxygen vacancies,rather than the carrier density caused by Sn dopingUltimately,we modified the bound magnetic polaron model proposed by Coey, and well explained the origin of ferromagnetism in Fe-doped In₂O₃.In brief,studies in this thesis have achieved such results:First,Fe-doped In₂O₃ films with texture structure along(440) direction were successfully prepared by PLD.High temperature ferromagnetism and strong perpendicular magnetic anisotropy are realized.This ferromagnetism is not related to impurities,but is an intrinsic property caused by iron doping into the crystal lattice of indium oxide.Second,eptaxial growth of Fe-doped In₂O₃ was successfully realized.The epitaxial film has stronger perpendicular magnetic anisotropy,which further proves the intrinsic nature of the ferromagnetism.Third,high saturation magnetization was achieved.Under an optimal condition, the saturation magnetization of the sample could reach nearly 3.0μ_B/Fe.Fourth,the relation between ferromagnetism in Fe-doped In₂O₃ and carriers as well as oxygen vacancies was systematically studied by experiments.A nonmonotonic change of ferromagnetism with the oxygen atmosphere during growth is discovered. Meanwhile,controllable ferromagnetism through annealing is also realized.Fifth,based on the related experimental results,the bound magnetic polaron model was modified and well explained the origin of ferromagnetism in Fe-doped In₂O₃.