过渡金属掺杂Mg_xZn_(1-x)O单晶薄膜的外延制备与性能研究
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摘要
随着科学技术的不断进步,作为信息技术基础的集成电路,按照Moore定律,其集成密度迅速增加,而相应的单个器件,其尺寸在逐渐减小。随之而来的便是,器件单位面积上能耗过高等等各种问题。而众所周知,电子同时具有电荷和自旋两种属性。在为传统微电子工业寻找出路的过程中,人们自然会想到,能否利用电子的自旋属性来实现信息的处理、传输以及存储,或者将电子的电荷和自旋两种属性结合起来加以充分利用?在这一背景下,自旋电子学(Spintronics)这门主要研究如何在固体材料中有效控制电子的自旋,以实现各种新型电子器件的新兴交叉学科,引起了人们广泛的关注。关于自旋电子学的研究,有着丰富的物理内涵,其兴起与发展不仅具有极其重要的学术意义,而且有着广泛的应用前景。
实现各种自旋电子学器件,一个关键就是要在半导体材料中实现自旋极化的电流。而要达成这个目的,目前看来有两种方法最具吸引力：一是将自旋极化的电流自铁磁性材料注入到非磁性的半导体材料中,二是将磁性引入到半导体中制成磁性半导体材料。因此,不管直接作为替代材料,还是作为自旋注入源,磁性半导体在自旋电子学中都注定起着至关重要的作用。一种优良的磁性半导体材料需要同时具备多种特性,比如说高的居里温度(室温以上)、高的饱和磁化强度、结构及性能稳定、制备工艺与传统微电子工业匹配等等。然而,要满足这诸多要求并不容易,从上世纪60年的Eu基硫族化合物,到80年代的过渡金属掺杂Ⅱ-Ⅵ族半导体材料,再到90年代Mn掺杂Ⅲ-Ⅴ族磁性半导体材料,人们在此领域展开了广泛而持久的研究。然而,所有这些材料体系都有一个很大的问题,即居里温度太低(远低于室温)。直到2000年左右,人们在氧化物磁性半导体中实现了室温铁磁性,这一难题才得以突破。
氧化物铁磁性半导体因其具有室温铁磁性,在实现自旋电子学器件的实用化方面有着巨大的潜力。然而,就目前众多研究工作的结果来看,在氧化物磁性半导体研究领域中依然有许多重要的问题急需解决。一方面,该类材料中的铁磁性是否是本征铁磁性,其输运电荷与铁磁性之间的关系究竟如何?人们目前就此尚未达成共识。这一问题的澄清具有显而易见的重要性,其对未来改善材料的性能、设计以及实现各种自旋电子学器件具有重要的指导意义。然而因为材料体系比较复杂、制备工艺有所差异等原因,不同课题组所报道的结果往往差别很大甚至相互矛盾。另一方面,就材料性能而言,虽然人们已经在多种氧化物磁性半导体中实现了高的居里温度,但是饱和磁化强度普遍偏低。从应用角度看,如何有效提高该类材料的饱和磁化强度等磁学性能,是摆在人们面前的又一个亟需解决的问题。
MgxZn1-xO作为一种极其重要的光电功能材料,有诸多优异的特性,例如：从3.4eV到7.8eV可调节的带隙；随Mg含量增加而显著降低的载流子浓度及迁移率；不引入Mg相关的带隙态；Mg与Zn之间是等化合价(Isovalent)取代等。因此,研究过渡金属Co、Mn等掺杂的MgxZn1-xO这一材料体系,为人们深入了解氧化物磁性半导体中丰富的物理内涵提供了一个很好的平台。同时,该材料体系本身,在未来开发多功能自旋电子学器件方面也有着光明的应用前景。具体而言：
(1)Co掺杂ZnO是具有代表性的氧化物磁性半导体材料,而掺Mg对ZnO的电输运性质有显著影响。因此,研究Co、Mg共掺杂ZnO,为人们研究氧化物磁性半导体中输运电荷与铁磁性之间的相互关系提供了一个很好的途径。
(2)MgxZn1-xO材料随Mg含量增加存在一个从六角纤锌矿结构到而心立方结构的转变,而其面心立方的结构及晶格参数与众多过渡金属一元氧化物(TMmonoxide)匹配很好。因此,在该体系中,人们有可能实现高含量过渡金属元素掺杂的单晶材料,从而实现高性能的氧化物磁性半导体。
(3) MgxZn1-xO/ZnO异质结构中的能带偏移(Band offset)是一个人们广泛关注的问题,其对开发和设计各种光电功能器件以及新型自旋电子学器件有重要的意义。通过研究过渡金属元素(Mn等)在MgxZm1-xO材料中所形成的带隙态的分布以及其与价带、导带的关系,我们可以对MgxZn1-xO/ZnO异质结构中的能带偏移加以研究。
(4)从材料制备角度看,MgxZn1-xO材料在热平衡条件下,Mg、Zn相互之间的取代难以达到较高的程度,因此探索高质量MgxZn1-xO基氧化物磁性半导体的制备工艺,对于其在未来自旋电子学领域中的广泛应用有着重要的意义。
在本论文中,我们利用分子束外延技术制备了过渡金属元素Co、Mn掺杂的MgxZn1-xO外延单晶薄膜材料,并且通过RHEED、XRD、Raman、AFM、XPS等手段对材料的结构、形貌及成份等进行了系统的表征测试；利用透射谱等手段研究了材料的光学性质；通过AGM、MOKE、SQUID等手段对材料的磁性以及电输运性质进行了深入的研究。主要的内容与结果如下：
(1)我们利用分子束外延技术,探索了非热平衡条件下过渡金属Co、Mn掺杂MgxZn1-xO单晶薄膜的制备工艺。针对六角纤锌矿结构和面心立方结构MgxZn1-xO薄膜以及在其中掺杂过渡金属元素Co、Mn,我们仔细研究了衬底类型、衬底预处理方法、缓冲层、生长温度以及制备氧分压等,对晶体薄膜生长的影响。我们发现,衬底的预处理,以及预生长一层ZnO或者Mg0缓冲层,对于制备相应结构的高质量单晶薄膜材料非常关键。以SrTiO3(001)衬底上制备MgxZn1-xO为例,通过对衬底进行适当处理以及预生长一MgO缓冲层的方法,我们制备了高质量的Mg0.45Zn0.55O单晶薄膜。其生长过程遵循外延关系：Mg0.45Zn0.55O (001)[100]//MgO (001)[100]//SrTiO3(001)[100],而样品在5×5μm2范围内rms约为0.36nm。
(2)我们制备了(Zn1-xMgx)1-yMnyO (0≤x≤0.3,0≤y≤0.08)单晶薄膜材料,并且系统研究了其结构、带隙以及Mn2+相关带隙态。结果表明,样品具有六角纤锌矿结构并且其带隙随Mg含量增加而显著增大。我们还发现,在(Zn1-xMgx)1-yMnyO薄膜中,Mn2+相关带隙态覆盖了一个相对较宽的能量范围,而且其对电荷转移跃迁有贡献。我们基于Mn2+相关带隙态的参考能量位置,计算得到了ZnO/MgxZn1-xO异质结构中的能带偏移,这对未来开发新型光电功能器件以及自旋电子学器件来说具有重要的意义。
(3)我们制备了Zn0.85-xMgxCo0.150(0≤x≤0.3)单晶薄膜材料,并对其结构、磁性以及电输运性质进行了系统的研究。研究结果表明,随着样品中Mg含量的增加,电输运性质显著改变而饱和磁化强度基本不变。我们发现没有任何反常霍尔效应信号被探测到,表明样品中载流子是非自旋极化的。另外,我们只探测到几乎可以忽略的负磁电阻信号,表明样品中没有明显的自旋相关散射或隧穿过程,同时也证明样品中输运电荷与过渡金属Co的局域磁矩之间s,p-d交换作用很弱。这些结果都表明,在Zn0.85-xMgxCo0.15O薄膜中,输运电荷与产生局域磁矩的电荷之间没有交换相互作用。
(4)我们制备了高过渡金属Co含量的Coy(MgxZn1-x)1-yO(0.55≤x≤0.7,0≤y≤0.5)单晶薄膜材料,并对其结构、带隙以及磁性进行了系统的研究。我们发现,样品显示出具有较高饱和磁化强度的室温铁磁性。同时,该薄膜材料具有明显的磁光克尔效应,并且其带隙可以通过调节Mg、Zn的比例进行调节。另外,我们还发现,通过控制生长条件或者后续的退火处理等,可以对样品的铁磁性进行调控。我们推测,氧空位传递的局域Co磁矩之间的铁磁性耦合,是在该薄膜材料中产生室温铁磁性的一种比较可能的机制。
With the great progresses of science and technology in past few years, the integrated circuit, as the foundation of modern information technology, shows higher and higher integration density. Accordingly, the size of each device decreases gradually, and problems such as very high energy consumption per unit area comes out. It is well known that the electron simultaneously possesses the charge and spin degrees of freedom. On the process of finding the way for conventional microelectronics, people would be naturally considering that whether we can use the spin of electron to realize the processing, transmission, and storage of information, or comprehensive utilize the charge and spin degrees of freedom in one device. Under this background, a new field named spintronics, which focus on the effective manipulation of spin of electron to realize new electronic device, has attracted people's great attention. The research of spintronics involves abundant physical contents, and its'rising and developing will contribute not only to the development of science, but also to the wide application of numerous novel spin-electronic devices.
To realize the numerous spin-electronic devices, the realization of spin-polarized currents in semiconductors is fundamentally crucial. On this issue, now there are two ways seem to be attractive:one is to inject spin-polarized currents from a ferromagnetic material into nonmagnetic semiconductors, the other is to make magnetic semiconductors. Thus, the magnetic semiconductors are destined to play key role in spintronics, either be used as source of spin-injection, or be used as the replacement of conventional semiconductors. A fine magnetic semiconductor should concurrently possess several properties such as high Curie temperature, high saturation magnetization, stable structure and properties, and compatible preparation technology with conventional microelectronics. However, it is not easy to full fill all these requirements, and people have made wide and timeless studies on this area. The studied main material systems have changed a lot in the past few decades, from Eu chalcogenide in1960s to transition metal (TM) doped Ⅱ-Ⅵ semiconductors in1980s, and then to Mn-doped III-V semiconductors in1990s. All these material systems come against a big problem of low Curie temperature, and this challenge haven't be broken until2000s when people obtained room temperature ferromagnetism in oxide magnetic semiconductors.
As oxide magnetic semiconductors show Curie temperature above room temperature, it should have great potential in the future applications of spin-electronic devices. However, on the results of numerous reports nowadays, there are several important issues need to solve in the oxide magnetic semiconductors. On the one side, whether the ferromagnetism in this system is intrinsic ferromagnetism and what the correlation between conductive carriers and ferromagnetism is in this system? No consensus has been reached about these questions. The clarification of these questions is of apparently important to design various spin-electronic devices in the near future. Unfortunately, the conclusions of different groups are quite different and even contradictory due to the complicacy of the system and the differences of preparation technology. On the other side, although people have realized room temperature ferromagnetism in oxide magnetic semiconductors, the saturation magnetization is usually rather low, greatly hindering their applications. Thus, how to effectively improve the magnetic properties of oxide magnetic semiconductors is another challenge before people.
As a very promising material for many optoelectronic devices, ternary MgxZn1-xO possesses several unique properties, e.g., tunable band gap from3.4eV to7.8eV (band gap engineering), dramatic reduction of both carrier concentration and carrier mobility with increasing Mg concentration, no mid-gap states introduced by Mg incorporation, isovalent substitution of Mg to Zn, and so on. Thus, studying the system of TM-doped MgxZn1-xO provides us routes to access the abundant physical connotations of oxide magnetic semiconductors. Moreover, this system itself possesses promising applications in designing future multifunctional spin-electronic devices. To be specific, our works were based on the following concerns:
1) It is known that Co-doped ZnO is a representative oxide magnetic semiconductor material, and doping ZnO with Mg would greatly affects the electrical transport properties of ZnO. Thus, codoping Co and Mg into ZnO provides us a very good platform to access the intrinsic correlation between the conductive carriers and the observed ferromagnetism.
2) MgxZn1-xO material shows structural evolution from hexagonal structure to face-centered-cubic (fcc) structure with increasing Mg concentration, and its fcc-structure matches very well with many TM monoxides. Thus, the solid solubility of TM elements in this system may reach a very high level, and then we might acquire an oxide magnetic semiconductor with high-performance.
3) The band offsets in ZnO/MgxZn1-xO hetero-structures are of fundamental importance for designing numerous optoelectronic devices and novel spin-electronic devices. Through studying the distribution of TM-related mid-gap states and its interrelationship with the conduction and valence band in MgxZn1-xO, we could approach the band offsets in ZnO/MgxZn1-xO hetero-structures.
4) From the perspective of material preparation techniques, the substitution of Mg/Zn in ZnO/MgO is difficult to reach a high level under chemical equilibrium conditions. Thus, to determine the preparation techniques for high quality MgxZn1-xO-based oxide magnetic semiconductors is meaningful to realize its future applications in spin-electronic devices.
In this thesis, we have prepared epitaxial TM-doped MgxZn1-xO single crystal thin films by radio frequency oxygen plasma assisted molecular beam epitaxy (RF-MBE), and studied the structure, morphology and composition systematically by using reflection high energy electron diffraction, X-ray diffraction, Raman spectra, atomic force microscopy, X-ray photoemission spectroscopy and so on. We have also investigated the optical properties of the films by measuring optical transmission spectra, and studied the magnetism and electrical transport properties of films by using alternating gradient magnetometer, magneto-optic Kerr effect and superconducting quantum interference device. Our main results are as follows:
1) We have studied the preparation techniques of TM-doped MgxZn1-xO single crystal thin films by using RF-MBE. For hexagonal-and fcc-MgxZn1-xO fims and films doped with Co and Mn, we carefully investigated the influences of substate type, pretreatment methods of substrate, buffer layer, growth temperature and oxygen partial pressure, on the growth of the films. We found that the pretreatments of the substrates and pre-growth of a ZnO or MgO buffer layer are crucial to achieve the high quality single crystal films with corresponding structure. For instance, by pre-growth an MgO buffer layer on SrTiO3(001) substrate, we have acquired meta-stable Mgo.45Zno.55O single crystal films with high crystal quality, and we find that the growth follows epitaxial relationship of Mgo.45Zno.55O (001)[100]//MgO (001)[100]//SrTiO3(001)[100], and the rms roughness is0.36nm in an area of5×5
2) We have prepared (Zn1-xMgx)1-yMnyO (0 3) We have prepared Zno.85-xMgxCoo.150(0 4) We have prepared Coy(MgxZn1-x)1-yO (0.55≤x≤0.7,0≤y≤0.5) single crystal thin films with high Co concentration, and systematically investigated the structure, band gap and magnetism of the films. We found that the films showed room temperature ferromagnetism with high magnetization. Moreover, the films were found to show obvious magneto-optic Kerr effect and feature of band gap engineering. In addition, the magnetism of the films can be modulated on a large scale through adjusting the conditions when preparation or post annealing process. We speculate that oxygen vacancies mediated exchange coupling of Co spins is the possible mechanism for the ferromagnetism in the films.
实现各种自旋电子学器件,一个关键就是要在半导体材料中实现自旋极化的电流。而要达成这个目的,目前看来有两种方法最具吸引力：一是将自旋极化的电流自铁磁性材料注入到非磁性的半导体材料中,二是将磁性引入到半导体中制成磁性半导体材料。因此,不管直接作为替代材料,还是作为自旋注入源,磁性半导体在自旋电子学中都注定起着至关重要的作用。一种优良的磁性半导体材料需要同时具备多种特性,比如说高的居里温度(室温以上)、高的饱和磁化强度、结构及性能稳定、制备工艺与传统微电子工业匹配等等。然而,要满足这诸多要求并不容易,从上世纪60年的Eu基硫族化合物,到80年代的过渡金属掺杂Ⅱ-Ⅵ族半导体材料,再到90年代Mn掺杂Ⅲ-Ⅴ族磁性半导体材料,人们在此领域展开了广泛而持久的研究。然而,所有这些材料体系都有一个很大的问题,即居里温度太低(远低于室温)。直到2000年左右,人们在氧化物磁性半导体中实现了室温铁磁性,这一难题才得以突破。
氧化物铁磁性半导体因其具有室温铁磁性,在实现自旋电子学器件的实用化方面有着巨大的潜力。然而,就目前众多研究工作的结果来看,在氧化物磁性半导体研究领域中依然有许多重要的问题急需解决。一方面,该类材料中的铁磁性是否是本征铁磁性,其输运电荷与铁磁性之间的关系究竟如何?人们目前就此尚未达成共识。这一问题的澄清具有显而易见的重要性,其对未来改善材料的性能、设计以及实现各种自旋电子学器件具有重要的指导意义。然而因为材料体系比较复杂、制备工艺有所差异等原因,不同课题组所报道的结果往往差别很大甚至相互矛盾。另一方面,就材料性能而言,虽然人们已经在多种氧化物磁性半导体中实现了高的居里温度,但是饱和磁化强度普遍偏低。从应用角度看,如何有效提高该类材料的饱和磁化强度等磁学性能,是摆在人们面前的又一个亟需解决的问题。
MgxZn1-xO作为一种极其重要的光电功能材料,有诸多优异的特性,例如：从3.4eV到7.8eV可调节的带隙；随Mg含量增加而显著降低的载流子浓度及迁移率；不引入Mg相关的带隙态；Mg与Zn之间是等化合价(Isovalent)取代等。因此,研究过渡金属Co、Mn等掺杂的MgxZn1-xO这一材料体系,为人们深入了解氧化物磁性半导体中丰富的物理内涵提供了一个很好的平台。同时,该材料体系本身,在未来开发多功能自旋电子学器件方面也有着光明的应用前景。具体而言：
(1)Co掺杂ZnO是具有代表性的氧化物磁性半导体材料,而掺Mg对ZnO的电输运性质有显著影响。因此,研究Co、Mg共掺杂ZnO,为人们研究氧化物磁性半导体中输运电荷与铁磁性之间的相互关系提供了一个很好的途径。
(2)MgxZn1-xO材料随Mg含量增加存在一个从六角纤锌矿结构到而心立方结构的转变,而其面心立方的结构及晶格参数与众多过渡金属一元氧化物(TMmonoxide)匹配很好。因此,在该体系中,人们有可能实现高含量过渡金属元素掺杂的单晶材料,从而实现高性能的氧化物磁性半导体。
(3) MgxZn1-xO/ZnO异质结构中的能带偏移(Band offset)是一个人们广泛关注的问题,其对开发和设计各种光电功能器件以及新型自旋电子学器件有重要的意义。通过研究过渡金属元素(Mn等)在MgxZm1-xO材料中所形成的带隙态的分布以及其与价带、导带的关系,我们可以对MgxZn1-xO/ZnO异质结构中的能带偏移加以研究。
(4)从材料制备角度看,MgxZn1-xO材料在热平衡条件下,Mg、Zn相互之间的取代难以达到较高的程度,因此探索高质量MgxZn1-xO基氧化物磁性半导体的制备工艺,对于其在未来自旋电子学领域中的广泛应用有着重要的意义。
在本论文中,我们利用分子束外延技术制备了过渡金属元素Co、Mn掺杂的MgxZn1-xO外延单晶薄膜材料,并且通过RHEED、XRD、Raman、AFM、XPS等手段对材料的结构、形貌及成份等进行了系统的表征测试；利用透射谱等手段研究了材料的光学性质；通过AGM、MOKE、SQUID等手段对材料的磁性以及电输运性质进行了深入的研究。主要的内容与结果如下：
(1)我们利用分子束外延技术,探索了非热平衡条件下过渡金属Co、Mn掺杂MgxZn1-xO单晶薄膜的制备工艺。针对六角纤锌矿结构和面心立方结构MgxZn1-xO薄膜以及在其中掺杂过渡金属元素Co、Mn,我们仔细研究了衬底类型、衬底预处理方法、缓冲层、生长温度以及制备氧分压等,对晶体薄膜生长的影响。我们发现,衬底的预处理,以及预生长一层ZnO或者Mg0缓冲层,对于制备相应结构的高质量单晶薄膜材料非常关键。以SrTiO3(001)衬底上制备MgxZn1-xO为例,通过对衬底进行适当处理以及预生长一MgO缓冲层的方法,我们制备了高质量的Mg0.45Zn0.55O单晶薄膜。其生长过程遵循外延关系：Mg0.45Zn0.55O (001)[100]//MgO (001)[100]//SrTiO3(001)[100],而样品在5×5μm2范围内rms约为0.36nm。
(2)我们制备了(Zn1-xMgx)1-yMnyO (0≤x≤0.3,0≤y≤0.08)单晶薄膜材料,并且系统研究了其结构、带隙以及Mn2+相关带隙态。结果表明,样品具有六角纤锌矿结构并且其带隙随Mg含量增加而显著增大。我们还发现,在(Zn1-xMgx)1-yMnyO薄膜中,Mn2+相关带隙态覆盖了一个相对较宽的能量范围,而且其对电荷转移跃迁有贡献。我们基于Mn2+相关带隙态的参考能量位置,计算得到了ZnO/MgxZn1-xO异质结构中的能带偏移,这对未来开发新型光电功能器件以及自旋电子学器件来说具有重要的意义。
(3)我们制备了Zn0.85-xMgxCo0.150(0≤x≤0.3)单晶薄膜材料,并对其结构、磁性以及电输运性质进行了系统的研究。研究结果表明,随着样品中Mg含量的增加,电输运性质显著改变而饱和磁化强度基本不变。我们发现没有任何反常霍尔效应信号被探测到,表明样品中载流子是非自旋极化的。另外,我们只探测到几乎可以忽略的负磁电阻信号,表明样品中没有明显的自旋相关散射或隧穿过程,同时也证明样品中输运电荷与过渡金属Co的局域磁矩之间s,p-d交换作用很弱。这些结果都表明,在Zn0.85-xMgxCo0.15O薄膜中,输运电荷与产生局域磁矩的电荷之间没有交换相互作用。
(4)我们制备了高过渡金属Co含量的Coy(MgxZn1-x)1-yO(0.55≤x≤0.7,0≤y≤0.5)单晶薄膜材料,并对其结构、带隙以及磁性进行了系统的研究。我们发现,样品显示出具有较高饱和磁化强度的室温铁磁性。同时,该薄膜材料具有明显的磁光克尔效应,并且其带隙可以通过调节Mg、Zn的比例进行调节。另外,我们还发现,通过控制生长条件或者后续的退火处理等,可以对样品的铁磁性进行调控。我们推测,氧空位传递的局域Co磁矩之间的铁磁性耦合,是在该薄膜材料中产生室温铁磁性的一种比较可能的机制。
With the great progresses of science and technology in past few years, the integrated circuit, as the foundation of modern information technology, shows higher and higher integration density. Accordingly, the size of each device decreases gradually, and problems such as very high energy consumption per unit area comes out. It is well known that the electron simultaneously possesses the charge and spin degrees of freedom. On the process of finding the way for conventional microelectronics, people would be naturally considering that whether we can use the spin of electron to realize the processing, transmission, and storage of information, or comprehensive utilize the charge and spin degrees of freedom in one device. Under this background, a new field named spintronics, which focus on the effective manipulation of spin of electron to realize new electronic device, has attracted people's great attention. The research of spintronics involves abundant physical contents, and its'rising and developing will contribute not only to the development of science, but also to the wide application of numerous novel spin-electronic devices.
To realize the numerous spin-electronic devices, the realization of spin-polarized currents in semiconductors is fundamentally crucial. On this issue, now there are two ways seem to be attractive:one is to inject spin-polarized currents from a ferromagnetic material into nonmagnetic semiconductors, the other is to make magnetic semiconductors. Thus, the magnetic semiconductors are destined to play key role in spintronics, either be used as source of spin-injection, or be used as the replacement of conventional semiconductors. A fine magnetic semiconductor should concurrently possess several properties such as high Curie temperature, high saturation magnetization, stable structure and properties, and compatible preparation technology with conventional microelectronics. However, it is not easy to full fill all these requirements, and people have made wide and timeless studies on this area. The studied main material systems have changed a lot in the past few decades, from Eu chalcogenide in1960s to transition metal (TM) doped Ⅱ-Ⅵ semiconductors in1980s, and then to Mn-doped III-V semiconductors in1990s. All these material systems come against a big problem of low Curie temperature, and this challenge haven't be broken until2000s when people obtained room temperature ferromagnetism in oxide magnetic semiconductors.
As oxide magnetic semiconductors show Curie temperature above room temperature, it should have great potential in the future applications of spin-electronic devices. However, on the results of numerous reports nowadays, there are several important issues need to solve in the oxide magnetic semiconductors. On the one side, whether the ferromagnetism in this system is intrinsic ferromagnetism and what the correlation between conductive carriers and ferromagnetism is in this system? No consensus has been reached about these questions. The clarification of these questions is of apparently important to design various spin-electronic devices in the near future. Unfortunately, the conclusions of different groups are quite different and even contradictory due to the complicacy of the system and the differences of preparation technology. On the other side, although people have realized room temperature ferromagnetism in oxide magnetic semiconductors, the saturation magnetization is usually rather low, greatly hindering their applications. Thus, how to effectively improve the magnetic properties of oxide magnetic semiconductors is another challenge before people.
As a very promising material for many optoelectronic devices, ternary MgxZn1-xO possesses several unique properties, e.g., tunable band gap from3.4eV to7.8eV (band gap engineering), dramatic reduction of both carrier concentration and carrier mobility with increasing Mg concentration, no mid-gap states introduced by Mg incorporation, isovalent substitution of Mg to Zn, and so on. Thus, studying the system of TM-doped MgxZn1-xO provides us routes to access the abundant physical connotations of oxide magnetic semiconductors. Moreover, this system itself possesses promising applications in designing future multifunctional spin-electronic devices. To be specific, our works were based on the following concerns:
1) It is known that Co-doped ZnO is a representative oxide magnetic semiconductor material, and doping ZnO with Mg would greatly affects the electrical transport properties of ZnO. Thus, codoping Co and Mg into ZnO provides us a very good platform to access the intrinsic correlation between the conductive carriers and the observed ferromagnetism.
2) MgxZn1-xO material shows structural evolution from hexagonal structure to face-centered-cubic (fcc) structure with increasing Mg concentration, and its fcc-structure matches very well with many TM monoxides. Thus, the solid solubility of TM elements in this system may reach a very high level, and then we might acquire an oxide magnetic semiconductor with high-performance.
3) The band offsets in ZnO/MgxZn1-xO hetero-structures are of fundamental importance for designing numerous optoelectronic devices and novel spin-electronic devices. Through studying the distribution of TM-related mid-gap states and its interrelationship with the conduction and valence band in MgxZn1-xO, we could approach the band offsets in ZnO/MgxZn1-xO hetero-structures.
4) From the perspective of material preparation techniques, the substitution of Mg/Zn in ZnO/MgO is difficult to reach a high level under chemical equilibrium conditions. Thus, to determine the preparation techniques for high quality MgxZn1-xO-based oxide magnetic semiconductors is meaningful to realize its future applications in spin-electronic devices.
In this thesis, we have prepared epitaxial TM-doped MgxZn1-xO single crystal thin films by radio frequency oxygen plasma assisted molecular beam epitaxy (RF-MBE), and studied the structure, morphology and composition systematically by using reflection high energy electron diffraction, X-ray diffraction, Raman spectra, atomic force microscopy, X-ray photoemission spectroscopy and so on. We have also investigated the optical properties of the films by measuring optical transmission spectra, and studied the magnetism and electrical transport properties of films by using alternating gradient magnetometer, magneto-optic Kerr effect and superconducting quantum interference device. Our main results are as follows:
1) We have studied the preparation techniques of TM-doped MgxZn1-xO single crystal thin films by using RF-MBE. For hexagonal-and fcc-MgxZn1-xO fims and films doped with Co and Mn, we carefully investigated the influences of substate type, pretreatment methods of substrate, buffer layer, growth temperature and oxygen partial pressure, on the growth of the films. We found that the pretreatments of the substrates and pre-growth of a ZnO or MgO buffer layer are crucial to achieve the high quality single crystal films with corresponding structure. For instance, by pre-growth an MgO buffer layer on SrTiO3(001) substrate, we have acquired meta-stable Mgo.45Zno.55O single crystal films with high crystal quality, and we find that the growth follows epitaxial relationship of Mgo.45Zno.55O (001)[100]//MgO (001)[100]//SrTiO3(001)[100], and the rms roughness is0.36nm in an area of5×5
2) We have prepared (Zn1-xMgx)1-yMnyO (0
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