摘要
电子同时具有电荷和自旋两种自由度。传统的微电子学主要研究电子作为电荷载体在固体材料中的运动规律及其应用。微电子技术是发展最快的技术之一,是现代信息技术的基础。各种新型高性能器件的不断涌现,使电子信息产业乃至整个世界发生了深刻变革。然而,随着集成化程度的提高和器件尺寸的不断缩小,工艺极限和量子限制效应的影响逐渐显著。于是,人们开始关注电子的另一内禀属性---自旋,自旋电子学应运而生,并引发了信息技术领域的革命,磁盘存储技术和磁敏传感器技术取得了飞速发展。与传统的半导体器件相比,自旋电子学器件具有速度快、能耗低、非易失等优点。
随着自旋电子学的进一步发展和新型材料的不断涌现,对电子自旋自由度调控的方式逐渐多样化,主要可分为：磁场调控,电场调控,光调控等。多场调控的需求,使人们对具有多功能特性的材料体系表现出极大的兴趣,比如磁性和半导体特性结合的磁性半导体材料,铁磁和铁电(或铁弹)结合的多铁性材料等。在本论文中,我们选择了磁性半导体材料和多铁性材料这两类不同的多功能材料体系,开展了自旋电子学相关的研究。
电子自旋可以通过磁场直接调控,其中巨磁电阻效应已经在硬盘磁读头、磁性传感器等方面得到应用。基于隧穿磁电阻效应的器件将在磁随机存储器、自旋晶体管等方面有重要的应用前景。磁性半导体材料用于隧穿磁电阻器件,使器件不但具有非易失性、非破坏性、寿命长等特点,还易与传统半导体器件集成。Mn掺杂GaAs是研究最多且相对成熟的磁性半导体体系,但由于其居里温度(191K)远远低于室温而无法实行工业应用。ZnO是第三代宽禁带半导体,Co掺杂ZnO的居里温度高于室温,是最有潜力的磁性半导体材料。本论文通过优化工艺条件,利用分子束外延技术制备了具有室温铁磁性的高品质(Zn,Co)O薄膜,结合半导体光刻技术,成功制备了完全外延的(Zn,Co)O/(Zn,Mg)O/(Zn,Co)O磁性隧道结,并且在5K1.8T磁场下观察到了85.6%的正磁电阻,这是目前在ZnO基磁性隧道结中报道的最大的隧穿磁电阻。其巨大的隧穿磁电阻是(Zn,Co)O/(Zn,Mg)O界面处自旋反转散射的抑制和sp-d交换相互作用导致的导带电子自旋劈裂共同作用的结果。隧道结的结电阻与T-4/3成线性关系,表明其隧穿机制为通过势垒层中2个局域态的直接非弹性跃迁。
电场调控磁矩和电子自旋的途径之一是通过磁和铁电序参量相互耦合的多铁性材料体系。多铁性材料不但同时具有铁磁性(或反铁磁)和铁电性,还具有铁磁—铁电序参量相互耦合的磁电耦合效应,使其同时体现铁磁性和铁电性材料的优点,有望实现铁磁—铁电高密度多态存储器。利用铁电的快速写入特性和铁磁的快速读出特性,有望在电场调控的快速读和写的磁性存储器中得到应用。虽然多铁性材料受到了人们的极大关注,但是对于多铁性材料的样品制备、物理性能和磁电耦合机理的研究仍然处于初级阶段。自然界中的单相多铁性材料比较稀少,BiFeO3(BFO)是目前唯一在室温下具有铁电性和反铁磁性的多铁性材料。从实际应用出发,还可以利用铁磁—铁电(压电、铁弹)构建复合多铁性材料。本论文研究了BFO单晶薄膜外延的工艺条件,以及应力与外延BFO薄膜丰富的晶体结构、铁电性和磁性的关联相互作用。研究了基于无铅压电薄膜的xCoFe2O4(CFO)/(1-x) K0.5Na0.5NbO3(KNN)多铁性异质结构的磁介电效应。
由于BFO薄膜的合成窗口较窄,常伴有Bi203,Bi2Fe409,Fe203等杂相,严重影响了对其本征性质的研究。本论文通过对生长工艺的的探索和优化,利用氧等离子体辅助的分子束外延技术在STO(001)、STO(111)以及斜切的STO单晶衬底成功制备了高质量的BFO薄膜,系统研究了其结构、漏电流、磁性、铁电性和压电性等。优化的主要生长参数为：氧气压～10-6mbar,Bi和Fe的束流比7：1,衬底温度：430—450℃。
对于TiO2终止面的STO(001)衬底,当厚度为50nm以下时,BFO是完全应变的四方相,其与STO基片之间的外延关系为：(001)[100]BFO//(001)[100]STO。当BFO薄膜的厚度大于80nm时,除了c/a较小的R-like相(c/a-1.04)外,还发现了少量c/a较大的T-like相(c/a-1.2)。BFO具有不规则的电畴结构,在10V电压下可以实现电畴的翻转。BFO薄膜具有较小的漏电流,当电场为-100kV/cm时,BFO/STO(001)的漏电流密度为3.4×10-5A/cm2,并在Pt/BFO/Nb-STO三明治结构中发现了双极性的电致电阻效应。
BFO在STO(111)衬底上为三维岛状生长。研究表明BFO/STO(111)为菱方畸变的钙钛矿结构,其结构随薄膜厚度变化不明显。BFO/STO(111)为均匀的单畴结构。与BFO/STO(001)相比,BFO/STO(111)具有更小的漏电流,当电场为-100kV/cm时,BFO/STO(111)的漏电流密度为4.5×10-6A/cm2。且介电常数随频率的变化更为缓慢,介电损耗也更小。这归因于BFO/STO(111)中更少的缺陷(如氧空位、铋空位等)。
在斜切STO(001)衬底上,外延的BFO薄膜均为单斜MA结构,BFO的晶格常数随厚度稍有变化,但变化微乎其微。斜切面上的BFO具有比BFO/STO(001)更小的漏电流密度,电流密为2.81×10-7A/cm2@100kV/cm,且BFO只有两种极化取向。在c方向的超结构,可以归结于氧八面体的旋转。同时氧八面体的旋转也导致Fe的磁矩的偏转,从而使得斜切衬底上的BFO薄膜具有微弱的室温铁磁性。
利用射频磁控溅射技术制备了无铅压电KNN薄膜,研究了制备工艺和退火对KNN结构和物性的影响。并制备了体积恒定的复合多铁性双层膜xCFO/(1-x)KNN,研究了其磁性、介电特性和磁介电效应。结果表明：当生长温度为800℃时,可制备出钙钛矿结构的KNN无铅压电薄膜,退火处理可大幅提高KNN的介电性能。在xCFO/(1-x)KNN薄膜中,CFO保持了其优良的磁性能,发现了巨大的以界面应力为媒质的磁介电效应。对于0.6CFO/0.4KNN薄膜,9kOe下的磁介电系数为7%。
An electron has two degrees of freedom:charge and spin. In conventional information technology we only took advantage of the charge property of electrons, while the spin degree of freedom was ignored. Adding the spin degree of freedom to conventional semiconductor charge-based electronics or using the spin degree of freedom alone will add substantially more capability and performance to electronic products. The advantages of these new devices would be nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.
In the past50-plus years, it is mainly external magnetic field that controls the magnetization orientation. Recently, new enabling phenomena blossom that permit spintronic functionality to prevailing in the absence of external magnetic fields. The new ways to control the magnetization orientation include control via electric fields and photonic fields. Multi-field control makes multifunctional materials more and more important. In this thesis, we mainly focus on multifunctional materials such as magnetic semiconductor and multiferroics.
Magnetic field can be used to switch the magnetization of a memory element. This is an example of a tunneling magnetoresistance (TMR) effect. Magnetic tunnel junction (MTJ) devices are nonvolatile and non-destructive. Magnetic semiconductor based MTJ devices are easy to integrate with conventional semiconductor devices. One of the most popular diluted magnetic semiconductor (DMS) materials,(Ga,Mn)As, has already been investigated in the context of spintronics applications. Large TMR of a few hundred percent at4K, which can be amplified to1500-fold at1.7K, has been observed in a (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction. However, the curie temperature Tc of (Ga,Mn)As (～191K) is far below room temperature (RT). This inhibits its application in RT spintronics devices. For practical application, DMS materials with RT ferromagnetism are very demanding. As one of the most promising candidates to obtain RT ferromagnetism,(Zn,Co)O is intensely studied. In this thesis, high quality (Zn,Co)O films were grown on Al2O3(0001) substrates by oxygen plasma-assisted molecular beam epitaxy (OPAMBE). The effect of oxygen pressure and concentration of Co dopant on the crystal quality, morphology, magnetism and MR were studied. It shows that (Zn,Co)O films grown at substrate temperature of450°and oxygen partial pressure of～-8.0×10-7mbar exhibit flat surfaces and robust magnetization. Fully epitaxial ZnO-based (Zn,Co)O/(Zn,Mg)O/(Zn,Co)O MTJs were grown under optimal growth condition. The MR behavior and spin injection through (Zn,Mg)O barrier were investigated. An enhanced positive tunnel magnetoresistance (TMR) ratio of85.6%is observed at1.8T at5K, which can be attributed to the high quality of two epitaxial (Zn,Co)O/(Zn,Mg)O interfaces in our MTJs. The MR can be explained by the resultant contributions of both positive MR and the reduction of spin-flip scattering at interfaces. The junction resistance at zero magnetic field is linear with respect to temperature power law T-4/3between5K and70K, indicating that carriers tunnel through (Zn,Mg)O barrier via two localized states.
Another new mean of manipulating magnetic moments and electron spins is electric field. One of the ways by which electric fields have recently been demonstrated to control magnetic properties is in multiferroic systems. Multiferroic systems couple magnetic and ferroelectric order parameters and are useful because electric fields can potentially be used to switch the magnetization. Multiferroics have aroused ever increasing interest worldwide. However, the preparation, physical properties and magneto-electric coupling of multiferroics are still under investigation. Multiferroic materials are classified into two main groups:single phase multiferroics and composite multiferroics. Until now, BiFeO3(BFO) is perhaps the only material that is both magnetic and strong ferroelectric at room temperature. For practical application, alternative artificial multiferroic composites are being extensively studied, which can be obtained by combining a ferroelectric phase and a ferromagnetic phase relying on strain, charge or magnetic interaction through the interface. These materials exhibit stronger ME coupling than single phase materials. Moreover, the ME coefficient is tunable by modulating the composite structures and component ratio. In this thesis, we prepared BFO thin films of different thickness on exact and miscut SrTiO3(STO)(001) and (111) substrates and systemically studied the structural and physical properties of BFO films. In addition, lead-free piezoelectric Na0.5K0.5NbO3(KNN) and CoFe2O4(CFO)/KNN films were deposited by radio-frequency magnetron sputtering. Their structural and physical properties have been studied.
High quality BFO films can only be obtained in a narrow window, out of which Bi2O3, Bi2Fe4O9, Fe2O3may form. These impurity phases hinder the characterization of the intrinsic properties of BFO. In this thesis, High quality epitaxial BFO films were fabricated by OPAMBE on SrTiO3(STO)(001),(111) and vicinal STO (001) substrates at growth temperature of450°and oxygen partial pressure of～8.0×10-7mbar with a fixed Bi:Fe flux ratio of8:1. The structural, magnetic, ferroelectric and piezoelectric properties were systematically studied.
For BFO films grown on TiO2-terminated STO (001) substrates, when the thickness is less than50nm, BFO is fully strained tetragonal phase with orientation relationship (001)[100]BFO||(001)[100]STO. In thicker BFO films (thickness>80nm), there is a little amount of tetragonal-like BFO phase with large c/a ratio (c/a-1.2) besides the rhombohedral-like BFO phase (c/a～1.04). BFO films have complex domain structure with four polarization variants and can be electrically switched by a voltage of10V. The leakage current of these BFO films is remarkably reduced. At an electric field of-100kV/cm, the leakage current density is～3.4×10-5A/cm2. Besides, bipolar resistance switching effect without forming process was observed in Pt/BFO/Nb-STO capacitors.
The BFO growth on STO (111) substrate was three-dimensional as can be expected given the high bond density of the (111)p pseudocubic faces. XRD results indicate that BFO films grown on STO (111) are rhombohedral. Comparing to BFO/STO(001), BFO/STO(111) has smaller leakage current density, which is4.5×10-6A/cm2At an electric field of-100kV/cm, weaker frequency dependent dielectric constant and lower dielectric loss due to less space charge which induced by defects in BFO film, such as oxygen vacancies and/or Bi vacancies. Moreover, BFO/STO(111) exhibits a single domain behavior.
Epitaxial BFO thin films are monoclinic MA when grown on TiO2-terminated vicinal STO (001) substrates. Leakage current measurements indicate that the leakage current density is largely reduced. At an electric field of-100kV/cm, the leakage current density is～2.81×10-7A/cm2The domain structure of BFO films is simplified by employing vicinal STO substrates, which only show two polarization variants. Weak room temperature ferromagnetism and periodic ordering doubled along the [001] direction are observed, which result from rotation of oxygen octahedral.
Lead-free piezoelectric KNN films were deposited by radio-frequency magnetron sputtering at different termperatures. It indicates that KNN hardly crystallize below800℃. Perovskite phase can be obtained at growth temperature of800℃. The annealing treatment in air remarkably enhances the dielectric constant due to denser grains and recombination of oxygen vacancies. CFO/KNN multiferroic bilayers with different volume ratio were deposited on conductive Nb-STO (001) single-crystal substrates. Their structure, magnetic, dielectric and magnetodelectric propreties were studied. Results indicate that the CFO phase maintains its good magnetic properties in CFO/KNN bilayers, while the dielectric constant of CFO/KNN decreases with increasing content of CFO ascribed to lower dielectric constant and higher conductivity nature of CFO comparing with KNN. Noticeable magnetodielectric effect was observed, indicating the presence of magnetoelectric coupling in CFO/KNN films. The magnetodielectric ratio increases with increasing volume fraction of CFO layer, showing a maximum of about7%for0.6CFO/0.4KNN at10kOe at2kHz. The magnetodielectric phenomenon can be attributed to strain-mediated effect between ferromagnetic CFO layer and dielectric KNN layer.
引文
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