摘要
微电子学主要研究和利用电子作为电荷载体的输运性质,是现代信息技术的基石。然而,随着集成化程度的提高,半导体器件的尺寸将进入纳米尺度,器件单位面积的能耗将急剧上升并导致严重的热损伤问题的出现。另外,在纳米尺度下,量子限域效应将起主导作用,电子的能带理论将不再完全适用。在这种情况下,人们开始关注电子的另一个内禀属性---自旋,希望能将电荷和自旋这两个属性同时加以利用,研制开发出一种新型的电子器件,该类器件与现有的半导体器件相比,具有运算速度更快、器件尺寸更小、能耗更低、断电信息不消失以及可用于量子计算等一系列的优点。这个新兴的研究领域即为自旋电子学,是近些年来发展非常迅速的一个研究方向。要实现性能优异的自旋电子器件,最首要的任务就是要开发出一种既具有半导体的带隙又具有磁性材料的自旋子带劈裂的新材料,为此,制备出具有室温铁磁性和高自旋极化度的铁磁性半导体材料是非常关键的。
典型的铁磁性半导体的制备方法是在现有的半导体材料中进行过渡金属元素的掺杂,使之以替位掺杂的形式进入半导体的晶格,通过磁性离子之间的铁磁耦合作用,使得半导体材料在原有带隙的基础上,进一步产生铁磁性。20世纪90年代,Ohno等人在GaAs中成功实现了Mn掺杂,但其居里温度最高只能达到170K,无法满足器件的应用需求。2000年,T.Dietl等人基于传统的Zener模型通过理论计算预言了Mn掺杂的宽禁带半导体GaN及ZnO会具有居里温度高于300K的铁磁性,这一结果一经报道,立即引发了人们对于氧化物基铁磁性半导体的研究热情,大量的实验和理论工作陆续开展起来。研究最多的是ZnO和TiO_2体系,但不同研究组所报道的结果却各不相同甚至互相矛盾,以致于直至现在,关于过渡金属掺杂的氧化物体系中是否具有本征铁磁性的问题仍然没有定论。同时,对于过渡金属掺杂的氧化铟体系而言,尽管目前对于该体系的研究尚处于起步阶段,但文献所报道的结果中大部分都发现了铁磁性的存在,部分还发现了反常霍尔效应等独特的性质,且铁元素在氧化铟基体中的溶解度高达20%。在这样的背景下,同时结合氧化铟基体材料所具有的优良的光电特性及气敏性质,我们最终选择了铁掺杂氧化铟作为本论文的研究体系,并尝试解决如下问题:铁掺杂氧化铟中能否实现高温本征铁磁性?铁磁性的来源是什么?
样品的制备我们采用脉冲激光沉积的方法,实验所需要的陶瓷靶用固相反应烧结的方法制备而成。我们首先在r向的蓝宝石衬底上生长了多晶的铁掺杂氧化铟薄膜样品,X射线衍射表明样品是沿(440)晶向择优生长的织构,未发现铁、铁的氧化物以及其他铟铁化合物的第二相,成分测试表明薄膜中铁的原子含量为7%,X射线光电子能谱表明铁元素以离子形式存在,这说明铁原子以替位掺杂的形式进入了氧化铟的晶格。透射电镜所给出的结果表明,晶格整齐有序排列,未发现第二相存在的迹象;薄膜的生长呈现柱状生长模式,柱状晶粒的大小在200nm左右;铁元素在薄膜的生长方向分布均匀,而在沿薄膜和衬底的界面方向分布不均匀,有贫铁区的间隔存在,且贫铁区所对应的刚好是柱状晶粒的边界。磁性测量表明,样品的居里温度高达927K,显示出强的垂直膜面磁各向异性。磁场垂直膜面时,样品的饱和磁化强度可达1.35μ_B/Fe。927K的居里温度说明掺杂的铁离子之间具有很强的间接交换作用。强磁各向异性可能是由样品内部晶场势与自旋轨道耦合共同作用的结果,可以作为本征铁磁性的非常重要的依据。我们认为该磁垂直各向异性与样品沿(440)方向择优的柱状生长模式相关。
随后,我们又在YSZ(100)衬底上首次尝试了铁掺杂氧化铟薄膜的外延生长。样品的整个生长过程都用RHEED监控,RHEED图像一直显示尖锐条纹,没有杂点出现,XRD的θ-20只观察到了In_2O_3(400)、(600)、(800)三个峰,说明我们成功实现了铁掺杂氧化铟薄膜的外延生长。AFM测试结果显示薄膜样品的生长为垂直于衬底表面的柱状生长模式,柱状晶粒大小约为200nm。磁性能测试表明,外延生长的铁掺杂氧化铟薄膜具有更强的磁垂直各向异性。这样,我们进一步验证了铁掺杂氧化铟薄膜中的磁垂直各向异性是与样品结构相关的内禀机制。
为了进一步深入的探讨该铁磁性的来源,我们分别通过在样品中进行不同浓度的Sn掺杂改变样品内的载流子浓度、及改变生长过程中的氧气氛和后续退火改变样品内氧空位的浓度来观察样品的铁磁性会有怎样的变化。结果表明,在样品中进行Sn掺杂后,虽然样品内部的载流子浓度大幅度提高了,但是铁磁性却基本没有受到影响。而在改变生长过程中的氧气氛后,样品的铁磁性发生了很明显的改变。样品的铁磁性随氧气压呈非单调的变化,在最优化的生长氧分压下,样品的铁磁性最强。后续的在真空和空气中的交替退火同样也成功的调控了铁磁性,使得铁磁性能够在高磁矩状态和低磁矩状态间相互转化,并且样品的磁垂直各向异性不随着反复退火而消失。这说明,氧空位的浓度对铁掺杂氧化铟的铁磁性有严重的影响和调控制、可后控作用。最后,根据所得到的实验规律和相关的测量数据,我们对Coey所提出的束缚磁极化子模型进行了必要的修正,比较好的解释了铁掺杂氧化铟薄膜中铁磁性的作用机理。
总之,本论文的研究工作主要取得了如下成果:
第一,用脉冲激光沉积的方法成功制备了具有高温铁磁性(Tc=927K)以及强垂直磁各向异性的沿(440)方向的织构铁掺杂氧化铟薄膜。该铁磁性与杂质相无关,是铁掺杂进入氧化铟晶格之后引起的一种本征的铁磁性。
第二,成功实现了铁掺杂氧化铟薄膜的外延生长,该外延薄膜具有更强的磁垂直各向异性,进一步验证了该铁磁性是与结构相关的本征铁磁性。
第三,获得了较高的饱和磁化强度,在最佳情况下,样品的平均原子磁矩接近3μ_B/Fe。
第四,系统的研究了铁掺杂氧化铟薄膜中的铁磁性与普通载流子及氧空位之间的关系。发现了铁磁性随制备过程中氧分压的非单调变化,同时也成功的通过退火调控了样品的铁磁性在高磁矩状态和低磁矩状态之间的转化。
第五,根据实验所得规律和相关实验数据,对Coey所提出的束缚磁极化子模型进行了修正,较成功的解释了铁掺杂氧化铟薄膜样品中铁磁性的作用机制。
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 TiO_2. 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 In_2O_3-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 In_2O_3.Based on such a background,also with a view to the excellent opto-electric and gas sensitive properties of In_2O_3,we finally choose Fe-doped In_2O_3 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 In_2O_3 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 In_2O_3 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 In_2O_3 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 In_2O_3 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 In_2O_3 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 doping
Ultimately,we modified the bound magnetic polaron model proposed by Coey, and well explained the origin of ferromagnetism in Fe-doped In_2O_3.
In brief,studies in this thesis have achieved such results:
First,Fe-doped In_2O_3 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 In_2O_3 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 In_2O_3 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 In_2O_3.
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