【作者】 王晓；

【导师】 连洁；

【作者基本信息】 山东大学 ， 光学工程， 2014， 博士

【摘要】 近年来纳米技术作为产业革命主导技术已成为全世界的共识。进入21世纪,各国都把发展纳米技术作为未来政治、经济领域中最具有挑战性的关键技术之一近几年,随着电子器件的进一步小型化与亚微米甚至纳米技术的发展,一个跨越半导体和磁性材料的全新研究领域已成雏形,这个全新的领域称之为自旋电子学。以自旋电子学为基础的新型器件如高密度非易失性存储器、磁传感器、电隔离器等已经开始应用,正在研制的自旋晶体管和自旋量子计算机等也将有广阔的应用前景。未来自旋晶体管和自旋场效应管的材料基础是半导体-磁体混合系统,由于失配率低易生长,能够完成自旋极化电流从磁性薄膜到半导体的注入,砷化镓外延纳米铁膜成为半导体-磁体混合系统的代表,受到广泛的关注。当铁膜厚度减小,直至纳米级厚的纳米膜时,其结构和磁性将发生较大变化,表面层可强烈影响其磁性。迄今为止,对纳米铁膜磁各向异性的尺寸效应,尤其是非磁性表面层对其磁特性影响及其来源还远未搞清楚,国内外只有很少的研究聚焦在材料表面层对铁薄膜磁光及磁学性能的影响上。本来在铁薄膜上外延表面层只是为防止铁膜的氧化,保护铁磁薄膜的磁学特性。然而意大利的Madami研究小组在研究Cu覆盖的纳米铁膜磁学性质时,发现表面层Cu膜对纳米铁膜磁单轴各向异性较强烈的抑制作用,即使这种表面层只有0.2nm厚；另外,美国的Falco研究小组,多年从事纳米磁性薄膜的研究工作,他们也发现Al表面层抑制了纳米铁膜的磁各向异性,使铁膜的有效磁化强度变小；以上现象说明表面层已影响了磁性铁膜—半导体混合系统的磁学性质,也必将影响自旋极化电流的输运,是不能忽视的问题。这一问题的解决对于材料的生长和以后自旋晶体管的实现有重要意义。除了表面层之外,衬底材料也对铁薄膜的磁光性质有着很大的影响。在材料生长中,为了实现晶格匹配需要使用不同的衬底。当衬底材料为光学各向异性时,光在其表面的反射会变的十分复杂。当光束透过磁性层进入各向异性衬底时,由于材料光学各向异性的影响,会产生。光与e光两种透射光。这使得光在材料中传播的时候产生多束反射光,而且反射光的偏振态较为复杂。这导致了难以在实验中测量到偏振态的变化,而且理论上也没有对此种类型材料的分析方法。目前国内外对各向异性衬底材料对铁薄膜磁光及磁学性能的影响研究很少。而通过研究磁性薄膜-各向异性衬底界面的磁光性质,则可以揭示衬底各向异性对材料磁光特性的作用机理,丰富表面磁光技术的应用范围,并为自旋电子器件的制作与性能优化提供指导作用。随着电子器件的进一步小型化,IT行业有了很大的发展。但是目前无法解决电子芯片越变越小带来的热冷却问题,这一现状制约了IT行业的发展。而拓扑绝缘体材料在信息产业领域的应用则有望解决这一问题。拓扑绝缘体有很多独特的性质,它不同于传统意义上的绝缘体和金属,其体材料是有能隙的绝缘体,而其表面是无能隙的金属态。拓扑绝缘体材料(Bi2Te3, Bi2Se3, Sb2Te3)有体材料绝缘特性以及表面的金属特性,这些性质是由于材料内部奇数个的狄拉克锥决定的。当一束光入射到磁性材料表面时,会产生透射光与反射光。在外界磁场的作用下,反射光产生的偏振态变化被称为磁光克尔效应,透射光引起的偏振态变化称为法拉第效应。克尔效应与法拉第效应打破了材料内部的时间反演对称。当时间反演对称被打破后,在拓扑绝缘体的狄拉克点处会出现带隙,在材料表面会展现出强烈的磁电效应。而不同材料厚度、入射角、以及磁耦合强度等都会对拓扑绝缘体材料的宏观磁光特性产生影响。因此研究拓扑绝缘体材料的宏观磁光性质可以为工业应用中选择合适的材料生长厚度、磁耦合强度等参量提供参考作用。随着薄膜制备技术的发展,亚微米甚至纳米尺度薄膜的制备技术越来越成熟,纳米尺度的磁性薄膜材料应用也越来越广泛。在对纳米磁性材料研究时,除了对材料本身性质的表征外,对其表征技术与表征设备的研究也非常重要。磁光椭偏技术结合磁光克尔效应与椭偏技术,它实现了在一台仪器中对磁性材料磁光性质与光学性质的同时测量,可以同时测得磁光耦合系数与复折射率并给出材料的磁化方向。在使用磁光椭偏仪对纳米薄膜进行测试时,由于材料太薄导致了反射光偏振态的变化很小,其携带的磁致反射信号也就很小。这导致了信号信噪比较低,在实验中难以测得稳定的磁致信号。因此纳米薄膜的磁光椭偏技术面临着微弱信号检测以及提高信噪比两大问题。这些问题的解决可以提高磁光椭偏仪的测试能力,并进一步完善纳米薄膜的表征手段。如上文所述,磁性材料-半导体混合系统与拓扑绝缘体材料在未来电子器件领域有着重要的应用,而对磁性薄膜材料的表征技术的研究则会推动自旋电子器件等产品的发展。本文以嵌埋于半导体衬底和非磁性金属膜之间的纳米铁膜、拓扑绝缘体以及磁光椭偏技术为主要研究对象。研究了表面层、衬底材料以及入射角对磁性材料磁学性质的影响,通过对Al/Fe/GaAs纳米薄膜材料进行测试,讨论了表面层厚度以及材料对磁性薄膜磁学性质的影响。研究了拓扑绝缘体材料的宏观磁光性质,通过对拓扑绝缘体材料Bi2Te3与Bi2Se3进行数据模拟计算,分析了材料厚度、入射角以及磁耦合强度等对其磁光性质的影响。研制了一台磁光椭偏仪,实现了对纳米尺度薄膜材料的磁光性质测量。研究了入射角精度对磁光耦合系数测试精度的影响。通过对Ta/Fe/GaAs薄膜材料进行测试,研究了薄膜厚度对磁光耦合系数的影响。本文的主要内容有以下几个方面：第一、研究了三层膜结构纳米薄膜材料的磁光理论,并给出了该结构材料反射矩阵与纵向磁光偏转角的求解方法。利用该理论进行数据模拟计算,得到磁光偏转角与入射角以及覆盖层材料的关系,并通过数据拟合求得了该材料的磁光耦合系数。在完成理论模拟之后,使用磁光克尔效应仪对分子束外延法制备的覆盖层厚度不同的Al/Fe/GaAs纳米薄膜材料进行测试,测得该系列材料的磁光偏转角,即可得到磁光偏转角随表面层材料厚度变化的关系,结果显示表面层Al膜对纳米铁膜磁光效应较强烈的抑制作用,而且Al膜厚度越大抑制越强烈。该结论为在材料生长与实验测试中获得最大磁光偏转角提供了理论与实验指导。第二、研究了磁性材料/光学各向异性衬底结构材料的磁光理论,并使用该理论模拟了衬底介电张量的三个参数在决定磁光偏转角与入射角的关系时的重要作用。给出了磁光偏转角随介电张量三个参数εx εv εz的变化规律,这为材料生长制备时选择合适的衬底材料提供了参考依据。在此之后,将TiO2等光学各向异性晶体作为衬底,分别将其视为各向同性与各向异性材料进行数据模拟计算。数据模拟结果显示三个介电常数相差越大,其对磁光偏转角的计算影响就越大。以上结论表面衬底晶体的各向异性对薄膜磁光特性有的巨大影响。第三、研究了新型材料拓扑绝缘体的宏观磁光性质,通过对三维拓扑绝缘体材料Bi2Te3与Bi2Se3进行数据模拟计算,发现法拉第偏转角随频率变化曲线的峰值位置会受到磁场耦合强度的影响。对于克尔偏转角而言,它会受到材料厚度、磁耦合强度以及入射角的影响。除此之外,除去已被发现的低频区的-π/2的巨克尔偏转角之外,在非低频区域还有一个-π/2的巨克尔偏转角。对于Bi2Se3在非低频区域的克尔角显示出明显的红移现象。对比Bi2Te3与Bi2Se3两种材料的法拉第角与克尔角可以发现,二者的法拉第角基本相同,但Bi2Se3在非低频区域的巨克尔角的光谱范围则明显比Bi2Te3宽。第四、研制了一台磁光椭偏仪,解决了该仪器最关键的降低光源噪声与实现微弱信号检测的问题。通过将多层膜材料反射矩阵引入到磁光椭偏技术中,实现了对多层膜结构纳米磁性薄膜的磁光椭偏测试。利用该仪器对磁控溅射方法制备的Ta/Fe/GaAs三层膜材料进行了测试,研究发现磁光耦合系数具有厚度独立性,不会随着磁性材料的厚度变化而变化。第五、研究了入射角精度对磁光耦合系数精度的影响,通过将有不同误差值的入射角数据带入公式计算,发现入射角度微弱的变化会对磁光耦合系数造成较大的影响。磁光椭偏仪对入射角的精度要求非常高。当入射角度的误差达到1度时,磁光耦合系数的变化将达到9%。因此将改造的分光光度计引入该仪器,用以提供高精度的入射角度。本文对纳米尺度薄膜的磁光特性进行了研究,其创新性如下：1.研制了一台磁光椭偏仪,该仪器包括光源系统、光路系统、磁场系统、探测系统、控制与数据处理系统。通过锁相放大器处理信号可以有效的降低外界噪声对信号的影响并实现对微弱信号的检测。通过电脑编程编写控制软件,可以实现对积分时间与采集次数等参数的控制。通过软件可以控制数据采集与数据处理,降低数据噪声。使用该仪器对三层膜纳米磁性材料进行测试,研究了磁光耦合系数的厚度依赖性。结果显示磁光耦合系数与材料的厚度无关,为一个不会厚度变化的常数。2.研究了各向异性衬底对磁性薄膜的磁光性质的影响,给出了其磁光偏转角的理论求解方法,并使用此方法研究了衬底介电张量的三个参数在决定磁光偏转角与入射角的关系时的重要作用。结果显示衬底材料的ε,与εz在克尔角与入射角的关系中发挥着重要作用。研究了磁光偏转角随介电张量三个参数εx εy εz：的变化规律。结果显示当入射角确定后,三个介电参数εxεy在εz克尔角的确定中发挥着巨大的作用。εy决定了p态克尔偏转角随入射角变化曲线的峰值位置,通过改变εx εz则可以对其进行更准确的微调。3.研究了三层膜纳米材料的磁光效应,给出了不同覆盖层材料、覆盖层厚度以及入射角等对铁磁性薄膜的磁光特性的影响。研究结果表明表面层对磁铁—半导体混合系统的磁学性质有很大的影响,这为材料生长与性质分析提供了理论参考作用。更多还原

【Abstract】 Recently, nanotechnology has been perceived as one of the leading techniques in industry revolution by the whole world. In21th century, many countries have treated nanotechnology as one of the key technologies in the domain of the economy and politics of the future world. Over the years, with the development of electron device miniaturization and nano-technique, a new area striding across semiconductor and magnetic material has appear. This new area is called spintronics. New devices based on spintronic such as high density nonvolatile memory, magnetic sensor and electrical isolator have been used in many areas. Spin transistor and spin quantum computer under development have promising prospects in the future too.In the future, the basic material of spin transistor and spin field-effect tube is magnetic film and semiconductor hybrid system. Fe/GaAs film has some advantages such as low miss match rate, easily fabricated and the injection of spin polarized current from magnetic film to semiconductor. So Fe/GaAs film becomes the representation of magnetic film and semiconductor system. When the thickness of Fe film reduces to nanometer size, the structure and magnetic will change. The overlayer will change the magnetic strength of Fe film. Till now, research on the scale effect of the nanoscale magnetic film magnetic anisotropy, especially the influence of nonmagnetic surface layer on the magneto-optical property is still not clear. The overlayer is intended to protect the magnetic material and avoid it from oxidation. But the Madami group in Italy found that the overlayer Cu has strong constraint on the magnetic anisotropy of nanoscale magnetic film, even the surface layer is only0.2nm. The Falco group in America also found that the Al overlayer can reduce the magnetic anisotropy of Fe nano-film. Researches above show that the overlayer has affected the magnetic property of magnetic film and semiconductor system. Inevitably, the spin polarized current transport will also be affected. Solving this problem has strong significance on the film growth and the realization of spin transistor. Except the surface layer, the substrate also has strong influence on the magneto-optical property of Fe ultrathin film. In film growth process, different substrates are chosen to satisfy the lattice matching condition. When the substrate is optical anisotropy, the reflection on the surface is quite complex. When the light passes through magnetic film and reaches the substrate, there will be two refracted lights, ordinary light and extraordinary light. These two lights have different polarizations and light paths. This make it difficult to detect the polarization change of the reflected light. By studying the magneto-optical property of magnetic film-anisotropic substrate system, the action mechanism of substrate constraint can be known. Meanwhile, this research can expand the application of magneto-optical effect and provide a reference to spin-electron device application.Recently, with the development of electron device miniaturization, the IT industry has rapidly development. But now it is still quite difficult to reduce the excessive heat caused by the device miniaturization. This status restricts the development of IT industry. But the application of topological insulator（TI） is expected to solve this problem. Topological insulator is different from the traditional semiconductor and conductor. It has metallic surface and insulator bulk. Topological insulators （Bi2Te3, Bi2Se3, and Sb2Te3） have insulating bulks and metallic surfaces with an odd number of Dirac cones that are responsible for most unique TIs properties. When a light incidents to the magnetic film surface, there will be two lights, reflected light and transmitted light. The polarization change of reflected light is called Kerr effect and that of the refractive light is called Faraday effect. The Kerr and Faraday magneto-optical （MO） effects allow light to directly realize time-reversal symmetry （TRS） breaking in a solid. When TRS is weakly broken, a band gap is induced at the Dirac points of a topological insulator and the surface states exhibit strong magnetoelectric effects. So the research in magneto-optical property of topological insulator is quite important for its application in spin electron device. The film thickness, incident angle and magnetic coupling strength are all quite important in determining the magneto-optical property of TI. This research can provide the proper film thickness and magnetic coupling strength in industry application.With the development of film preparation technique, nanoscale film preparation can be realized, In nanoscale magnetic film research, besides the film property characterization, the characterization technique investigation is also quite important. Magneto-optical ellipsometry combines magneto-optical Kerr effect and ellipsometry technique. It realizes the detection of optical property and magneto-optical property in one single experiment. When the magneto-optical ellipsometry is applied in ultrathin film measurement, the polarization change of the reflected light is quite tiny. So it is difficult to detect the magnetic induced signal. So the application of magneto optical ellipsometry faces two key factor, tiny signal detection and signal to noise ratio enhancement. Solving these two problems can raise the measurement ability and complete the nanoscale magnetic film characterization technique.As is shown above, magnetic film and semiconductor system and topological insulator will play important roles in future electronics area, and research on magnetic film characterization will promote the development of spin electronics devices. In this dissertation, the magnetic film and semiconductor system, topological insulator and magneto-optical ellipsometry technique are investigated. The influence of surface layer, substrate material and incident angle on magneto-optical property of magnetic layer is investigated. After measurement on nanoscale Al/Fe/GaAs film, it can be found that magneto-optical property can be constrained by the surface layer. After that, the magneto-optical property of topological insulator is studied. Results show that the film thickness, incident angle and external magnetic coupling strength play important role in determining the magneto optical property of topological insulator Bi2Te₃与and Bi2Se3. At last, a magneto-optical ellipsometry is developed which can realize the measurement of multilayer film. Simulations show that the incident accuracy has strong influence on magneto optical coupling constant accuracy. Using this device, the Ta/Fe/GaAs is investigated. Results show that the magneto optical coupling constant is independent, it will not change with the film thickness. The work of this dissertation will be described in the following aspects:1. The magneto-optical theory of three layers nanoscale film is investigated. Based on this model, the reflection matrix and the longitudinal magneto-optical Kerr rotation formula can be obtained. Using this formula, simulations were done. Results showe that the magneto-optical Kerr rotation can be affected by incident angle and overlayer material. Then the magneto-optical coupling constants can be obtained by numerical inversion. After that, using magneto-optical Kerr rotation instrument, the Al/Fe/GaAs films prepared by MBE with different overlayer thickness were measured. Then the relation between Kerr rotation and film thickness can be given. Results show that the surface layer Al has strong constraint on the magneto-optical property of the magnetic layer, and this constraint increases with the surface layer thickness. This conclusion gives a reference to get a huge Kerr rotation in film growth and experiment.2. The magneto-optical theory of two-layer system with optical anisotropic substrate is investigated. Using this theory, we found that the three dielectric parameters ε_x ε_v ε_z play important roles in determining the relation between Kerr rotation and incident angle. After that, simulations were done on Fe film with TiO2anisotropic crystal as substrate. Then the Kerr rotation was calculated with TiO2crystal treated as anisotropic and isotropic substrate, respectively. Results show that the influence on Kerr rotation depend on the difference between three dielectric parameter. The substrate anisotropic has strong impact on the magneto-optical property of Fe-substrate system.3. The macroscopic magneto-optical effects of new material topological insulator were investigated. Simulations were done on3D topological insulators Bi2Te3and Bi2Se3. Results show that the Faraday rotation peak position will change with magnetic coupling strength. For Kerr rotation, the spectral range reduces with increasing film thickness, magnetic coupling strength and incident angle. There are two giant Kerr non-low-frequency area. For the Bi2Se3film, the giant Kerr rotation in the non-low-frequency area shows an obvious red shift. In addition, it is observed that the Faraday rotation peak position of Bi2Se3is the same as that of Bi2Te3, so the Faraday rotation can be only determined by magnetic coupling strength. The giant Kerr rotation spectral range in the non-low frequency area is larger than that of Bi2Te3.4. A magneto-optical ellipsometry instrument was established and the key problems in this system which are reducing the noise of the light source and realizing the detection of tiny signal were solved. By introducing in the multilayer reflective matrix, magneto-optical ellipsometry can be extended to measurement of multilayer film. Then using this instrument, the Ta/Fe/GaAs multilayer films prepared by magnetron sputtering were measured. Results show that the magneto-optical coupling constant Q is independent on the film thickness.5. The influence of incident angle on magneto-optical coupling constant was investigated. Taking incident angle with different errors into simulation, results show that the little error of incident angle will cause a great change of magneto-optical coupling parameter. When the incident angle error reaches1°, the change of Q reaches9%. So the incident angle accuracy is quite important to this instrument. To improve the system accuracy, a modified spectrometer is introduced into this system to give a precise incident angle.In this dissertation, the magneto-optical properties of nanoscale films are investigated. The innovations are shown as below:1. A magneto-optical ellipsometry instrument was established which contains the light source system, optical system, electromagnet system, detection system and controlling and data processing system. Using the lock-in amplifier, the noise of the system can be successfully reduced and the tiny useful signal can be detected. Through the software, the integral time and number of excitations can be controlled by the computer. Through this, the noise can be reduced. After that, the instrument was used to measure the Ta/Fe/GaAs multilayer films to investigate the relation between magneto-optical coupling constant and film thickness.2. The magneto-optical theory of two-layer system with anisotropic crystal as substrate was investigated. Using this theory, we found that the three dielectric parameter ε_v ε_z, play important roles in determining the relation between Kerr rotation and incident angle. After that, the relation between kerr rotation and dielectric parameter is discussed. Results show that ε_y determines the peak position of kerr rotation and ε_x ε_z. can slightly change it.3. The magneto-optical theory of three layers nanoscale film was investigated. Results show that the magneto-optical Kerr rotation can be affected by incident angle, overlayer material and overlayer thickness. This means that the surface layer has strong impact on the magnetic-semiconductor system, this can not be neglected in film preparation and property characterization.更多还原