【作者】 林亮；

【导师】 韩圣浩；

【作者基本信息】 山东大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 自旋电子学是现代凝聚态物理学极具研究潜力的领域之一。与传统的电子学不同,自旋电子学将电子的自旋特性和电荷特性相结合,其核心内容是研究自旋极化电子的注入、输运、探测及调控,其目的是将器件的电特性、光特性和磁特性等组合在一起,实现新型的自旋电子器件。自旋电子学是电子学的重大发展,不仅导致了高密度存储器的出现,而且还导致了一些基础性的物理革命,如自旋流、自旋压、自旋霍尔效应等新物理概念或现象的出现。自旋电子学器件包括铁磁金属或磁性半导体与绝缘体、超导体、导体、半导体等构型的复合。目前研究发现,注入电流的自旋极化与注入层和传输层的电阻之比密切相关,而电阻不匹配正是传统材料难以实现高自旋注入效率的主要原因所在。为了与当今的微电子技术相结合,人们对半导体中的自旋现象进行了广泛的研究,但是传统无机半导体大多数是非磁性物质不含磁性粒子,并且晶格结构与磁性材料不同。与传统无机半导体相比,有机半导体具有自旋一轨道相互作用和超精细相互作用较弱、自旋扩散长度较长的优点,并且由于其自身“软物质”的特点晶格匹配问题较小,所以有机半导体有潜力成为自旋阀中自旋注入和输运的良好材料。就自旋注入有机半导体来讲,采用有机铁磁半导体作为自旋注入的电极被认为是解决晶格匹配和电导匹配问题的最佳方法之一。因此,有机磁性半导体材料的研究成为了新的研究热点,这必将促进全有机的自旋阀器件发展,从而开辟自旋电子学新的篇章。有机小分子半导体材料种类繁多、重量轻,具有丰富的结构和物理特性,其制备工艺简单、可塑性强,可以通过官能团修饰、杂化、掺杂等多种方法调控性能。有机小分子半导体材料在有机自旋电子器件中多被用来作为中间传输层,也被用作为制备有机磁性半导体材料而进行过渡金属掺杂的基体材料。2002年,Dediu研究组首次报道了有机材料中的自旋注入和输运,他们采用半金属La_0.67Sr_0.33MnO₃（LSMO）作为极化电子的注入层,中间传输层为有机半导体材料sexithenyl（T6）,发现了负磁电阻效应,表明有机层内存在自旋注入,两电极间存在自旋极化的电流。2004年,Xiong等人首次成功制备LSMO/Alq₃/Co有机自旋阀器件,低温下测得约40%的负磁电阻效应；Majumdar等人采用LSMO作自旋极化电极,研究了LSMO/polymer/Co结构中的自旋极化注入现象；Yoo等人采用有机磁体V（TCNE）x作为自旋极化电极,研究了V（TCNE）x/rubrene/LAO/LSMO结构的自旋阀,观察到了磁电阻现象,向全有机自旋器件的实现迈出了重要一步。最近,在过渡金属掺杂的Alq₃中发现了室温铁磁性。Co掺杂的Alq₃是通过对纯的Co金属和Alq₃粉末进行热共蒸发来合成的。在5%的Co（原子比Co/Al=0.5）掺杂的Alq₃中发现了明显的铁磁现象,其磁矩大约为0.33μB/Co。在过渡金属掺杂的8-羟基喹啉铝（Alq₃）中发现的室温铁磁性开启了有机自旋小分子半导体材料研究的大门。虽然经过多年研究,研究人员对有机自旋小分子半导体材料和器件的理解逐渐加深。但还有很多问题亟待解决,例如,失效层问题,室温磁电阻效应,过渡金属掺杂有机小分子的结构等。因此,本论文针对有机自旋小分子半导体材料与器件制备及特性研究,主要研究内容和结论如下：1. LSMO/Alq₃/Co有机自旋阀中的正磁电阻研究首先我们采用热蒸发法制备了LSMO/Alq₃/Co结构自旋阀,在低温100 K时观察到正12%的磁电阻效应。出现的正磁阻效应是指低阻态出现在两个铁磁电极磁化方向平行时,高阻态出现在两个铁磁电极磁化方向反平行时。在LSMO/Alq₃/Co结构的有机自旋阀中观察到的正负磁电阻效应来源于对应Co的不同电子自旋态分布,这个自旋态分布与有机层厚度、有机/Co界面、Co晶格结构和杂质等因素相关。2.有机自旋阀的失效层研究采用原子力显微镜和卢瑟福背散射方法研究了有机自旋阀的起因。原子力显微镜的研究结果表明LSMO薄膜的突起和Alq₃薄膜中自生长的针孔是导致器件短路的主要原因。我们还发现LSMO薄膜的表面起伏对随后生长的薄膜、LSMO/Alq₃界面和Alq₃/Co的界面有很大影响。此外,还发现厚度为1-4 nm的Alq₃薄膜是可以作为有机隧道自旋阀的势垒层,前提是小的工作区域和绝缘层的加入。卢瑟福背散射的研究结果表明顶铁磁电极Co只注入距离有机层表面约20 nm的深度。3. LSMO/Alq₃-Co/Co有机自旋电子器件中自旋输运研究在LSMO/Alq₃-Co/Co有机自旋电子器件中,我们发现磁阻随着温度变化发生信号反转。大小约为10%的负磁电阻在10K低温下被观察到。随着温度升高,磁电阻经历符号改变。在室温,一个正9.7%磁阻被观察到,这个正磁阻随着外磁场单调变化。而在没有铁磁电极的Alq₃-Co的复合物中只观察到约0.1%的室温磁阻。室温磁阻的增强是因为有自旋极化的载流子注入到Alq₃-Co的纳米复合层中。4.Co掺杂Alq₃薄膜的结构研究采用同步辐射表面掠入射X射线超精细结构（GIXAFS）和傅里叶红外吸收谱（FTIR）方法研究了过渡金属Co掺杂Alq₃薄膜的结构特性。掠入射X射线超精细结构研究表明存在多价态Co-Alq₃复合物,并且掺杂的Co原子倾向于定位在N和O原子中心位置,并与N和O原子成键。傅里叶红外吸收谱研究表明掺杂的Co原子和neridianal型Alq₃相互反应,而不是形成无机化合物。5.8-羟基喹啉铁（Feq₃）的磁性和光学性质研究采用实验和理论方法研究了Feq₃的结构、磁学特性和光致发光特性。与没有磁性并发黄绿光的Alq₃薄膜相比,Feq₃薄膜在5 K的低温下显示顺磁行为,室温发射波长为392 nm的紫光。第一性原理计算表明Feq₃分子的自旋态密度在费米能级处多数自旋和少数自旋发生劈裂。Feq₃的总磁距约为1μB,主要来源于Fe的3d局域态,另有一小部分来源于Fe原子周围的非金属原子（C、N和O）。铁磁态和反铁磁态的能量差非常小约为1-2 meV,这与实验上观察到的顺磁行为相对应。更多还原

【Abstract】 As a very promising research area in condensed matter physics, spintronics has attracted a lot of interests during the past few decades. Different from the classical electronics, spintronics involves both the charge and spin characters of an electron. Fundamental studies of spintronics include investigations of spin injection, transport, detection, and manipulation. Its goal is to understand the interactions among the electrical, optical, and magnetic characteristics and to achieve novel spin electronic devices. Spintronics is a significant development of electronics, which has not only led to the emergence of high-density memory, but also resulted in some fundamental physical revolution, such as the spin current, spin valves, spintronic Hall Effect and so on. Spintronics devices include the combinations of magnetic metals or magnetic semiconductors with insulator, semiconductor, conductor, or superconductor. It was found recently that the spin polarization with injected current is closely related to the ratio of the resistances in injecting layer and transport layer. The resistance mismatch in the traditional materials hinders them from achieving high spin injection efficiency. Therefore, more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.In order to adapt spintronics to conventional microelectronics, plenty of researches have been done on spin in semiconductors. The conventional semiconductors used for devices and integrated circuits do not contain magnetic ions and are nonmagnetic. Moreover, the crystal structures of magnetic materials are usually different from that of the semiconductors used in electronics, which makes them incompatible with each other. Compared with conventional inorganic semiconductors, organic semiconductors have weak spin-orbit interaction, week hyperfine interaction, long spin diffusion length and ’soft’matter property, thus they become potential candidates for spin injection and transport applications in spin valves. In terms of spin injection in organic semiconductor, the use of organic magnetic materials as spin inject electrode is considered as the best way to solve the conductance and lattice mismatch problems in organic spin valves. Therefore, the study of organic magnetic semiconductor materials has become a new hotspot, which will promote the full development of the organic spin valve devices.Organic small molecule semiconductors have a variety of species and rich structural and physical properties. Moreover, their performance can be easily modified by functional group modification, hybridization, and doping. Organic semiconductors may open up a way to cheap, low weight, mechanically flexible and chemically interactive electronics. In the field of organic spintronics, organic small molecule semiconductors are used as the space layer of organic spin valves and the matrix materials for transition metals doping during the preparation of the organic magnetic materials.In 2002, Dediu’s group firstly reported the spin injection and transport in organic material. They used half-metal material LaxSr1-xMnO3 （LSMO） as the source of electrons and sexithenyl （T6） as the organic layer. The spin injection has been detected, and the current is found to be spin polarized. In recent years, a lot of experiments have confirmed the spin injection and transport in organic materials. For example, Xiong et al. have observed spin injection and transport in a LSMO/Alq₃/Co organic spin valve in 2004. The measured magnetoresistance can be as high as 40% at low temperature. Majumdar et al. have observed a magnetoresistance of 80% at 5 K and 1.5% at room temperature in a LSMO/polymer/Co organic spin valve. They also found that there is a thin spin-selective tunneling interface between LSMO and the polymer, which improves the spin injection. Yoo et al. have observed magnetoresistance in V（TCNE）x/rubrene/LAO/LSMO organic spin valves where the organic magnets V（TCNE）x were used as the spin injecting electrode.Recently, ferromagnetism was found in Co-doped Alq₃ synthesized by thermal coevaporation of pure Co metal and Alq₃ powders. Clear ferromagnetic behavior with a magnetic moment of～0.33μB/Co was observed in 5%（Co/Al atom ratio=0.5） Co-doped Alq₃. It may open up a way to the organic small molecule magnetic semiconductors.After years of works, researchers have grasped the main characteristics of the organic small molecule spintronic semiconductor materials and devices. However, there are still many issues to be solved, such as ill-defined layer, room temperature magnetoresistance, and the structure of transition metals doped organic small molecule materials. The detailed contents and the main results are given below: 1. Positive magnetoresistance in organic spin valvesPositive spin valve effect-low resistance for parallel electrodes configuration-has been observed in organic spin valves fabricated by vacuum thermal evaporation using half-metal perovskite manganites Lao.67Sro.33Mn03 as the bottom electrode, cobalt as the top electrode, and tris （8-hydroxyquinoline） aluminum （Alq₃） as the organic semiconductor spacer. A positive giant magnetoresistance （GMR） of～12% has been observed at 100 K. We considered that the origin of the negative and positive GMR in LSMO/Alq₃/Co organic spin valve comes from the fact that the spin-resolved electronic structures of Co is related to thickness, interface, crystalline structure, impurity, and other possible factors.2. Origin of the ill-defined layer in organic spin valvesThe origination of ill-defined layer in organic spin valves was investigated by atomic force microscopy （AFM） and Rutherford backscattering （RBS） analysis. It was found that conductive bulges of LSMO film and self-grown pinholes in Alq₃ film other than Co inclusions could lead to the formation of ill-defined layer. The morphology of LSMO substrate had a strong influence on that of Alq₃ film, LSMO/Alq₃ and Alq₃/Co interfaces. Moreover, Alq₃ film with the thickness of 1～4 nm could be barriers, which was explained by small active area and added insulated layer in organic magnetic tunnel junctions. The results of RBS measurement showed that the top FM electrode Co only diffuse into the Alq₃ film less than 20 nm below the suface of the organic layer.3. Alq₃-Co nanocomposites based organic hybrid devicesLarge room-temperature （RT） magnetoresistance （MR） and temperature-dependent MR inversion have been observed in Alq₃-cobalt nanocomposites-based organic-inorganic hybrid devices. Negative MR-high resistance for parallel electrodes configuration-due to magnetization reversal of ferromagnetic （FM） electrodes has been observed at low temperature. As the temperature increases, the MR undergoes a sign change. At room temperature, a positive MR of～9.7% with the resistivity dropping monotonously with increasing magnetic fields has been observed. The RT MR is about two orders of magnitude of that in organic-FM nanocomposites measured with nonmagnetic electrodes. The enhancement of RT MR is attributed to the injection of spin polarized carriers into Alq₃-Co nanocomposites. 4. The structure of Co-doped Alq₃ filmThe structural properties of Co-doped Alq₃ have been studied by grazing incidence X-ray absorption fine structure （GIXAFS） and Fourier transform infrared spectroscopy （FTIR）. GIXAFS analysis suggests that there are multi-valences Co-Alq₃ complexes and the doped Co atoms tend to locate at the attraction center with respect to N and O atoms and bond with them. The FTIR spectra indicate that the Co atoms interact with meridional （mer） isomer of Alq₃ rather than form inorganic compounds.5. The magnetic and optical properties of tris-8-hydroxyquinoline iron （Feq₃）The structure, magnetic properties, and photoluminescence （PL） of Feq₃ films are experimentally investigated. The corresponding properties and origins are examined and studied by means of first-principles density functional theory. Our experiments show that in contrast to the nonmagnetic behavior and the green emission of Alq₃ film, the magnetic property and photoluminescence of the Feq₃ film display paramagnetic behavior at 5 K and violet emission （392 nm） at room temperature, respectively. Our calculated electronic structure of Feq₃ molecule indicates clear exchange splitting between the majority and minority spin channels. The total magnetic moment of Feq₃ is about 1μB, which mainly derives from the localized Fe 3d orbital with a little contribution of the neighboring nonmetal （C, N, and O） atoms. The observed paramagnetic behavior is due to the small energy difference （1-2 meV） between ferromagnetic and antiferromagnetic coupling in Feq₃. And the electronic structures indicate the violet emission comes from the transition between LUMO and HOMO.更多还原