新型磁性半导体材料与金刚烃的软X射线光谱学研究
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摘要
电子同时具有电荷与自旋的属性,并且长期以来其电荷与自旋属性都在各自的领域内发挥着重要的作用。首先,传统的微电子学主要研究和利用电子作为电荷载体的输运性质,在此基础上研制的各种功能型器件已经构成了现代信息技术的基石。其次,传统的磁学则主要研究和利用电子自旋的属性,并且在微波通讯,信息存储等方面占据重要地位。而近些年出现的自旋电子学,则主要关注固体中电荷与自旋之间的相互作用,并希望同时利用电子的这两个属性,从而研制开发新型的自旋电子学器件。自旋电子学的产生有其深刻的历史背景。首先,众所周知的摩尔定律简单而准确地描述了在过去微电子技术的迅猛发展,如半导体芯片集成度的大幅度提高,以及相应的半导体器件尺寸的缩小。然而,按照此趋势发展,在不久的将来,半导体器件的尺寸将缩小到纳米量级,器件单位面积能耗的急剧上升导致的严重热损伤问题,以及纳米尺度下的量子限域效应,将逐渐成为按照传统原理运行的微电子器件发展道路上不可逾越的障碍。这也促使人们去研究运算速度更快、器件尺寸更小、能耗更低的器件。而且显而易见的是新型器件的运行原理与传统的微电子器件相比,将会产生革命性的变化。此时,具有热损耗小,断电信息不消失等特点的电子的自旋属性便吸引了研究人员的注意。随着各种基于电子自旋运行的原型器件的提出,自旋电子学也成为了近年来发展非常迅速的一个新兴的研究领域。而要想实现性能优异的自旋电子学器件,必须首先寻找这样一种材料或者结构,在其中电子的运动将与材料的自旋磁矩之间存在强的耦合作用。具有室温铁磁性和高自旋极化率载流子的磁性半导体便是符合该要求的材料之一。
典型的所谓稀释磁性半导体的制备方法是在现有半导体母体材料中掺杂少量的过渡金属元素,过渡金属元素以替位掺杂的形式进入半导体材料的晶格中,并通过局域的磁性离子与载流子自旋之间的耦合作用,在材料中产生宏观的铁磁性及自旋极化的载流子。低温分子束外延技术制备的Mn掺杂GaAs是目前研究比较成熟的稀释磁性半导体体系。GaAs基稀释磁性半导体的成功为演示及验证科学家提出的自旋电子学原型器件的工作原理提供了原材料。然而,经过全世界的科研工作者十多年的不懈努力,该系列稀释磁性半导体的居里温度最高只能达到200K,远低于实际器件在室温工作的要求。为了获得具有室温铁磁性的材料,研究人员开始转向其他半导体材料基的稀释磁性半导体的研究,研究的范围包括各种氧化物和如氮化物,如ZnO、TiO2、GaN等,以及Ⅳ族半导体Si和Ge等。然而,在大量的研究报道中,不同研究组得到的结果却各不相同甚至互相矛盾。室温稀释磁性半导体的问题在于科研人员都试图通过各种非平衡生长工艺来提高过渡族元素在传统半导体中的掺杂量,进而提高材料的居里温度和饱和磁化强度,然而过渡族元素在半导体晶格中低的溶解度,从根本上限制了替位掺杂量的提高。
在这样的背景下,我们课题组突破了半导体晶格的限制,利用低温交替沉积原子层厚度的半导体层与过渡金属层的创新性工艺,转向无序浓磁半导体的研究。课题组成员多年的研究工作表明,该非平衡生长工艺制备的无序浓磁半导体具有稳定的室温铁磁性,对于室温自旋电子学器件有潜在的应用价值。然而,随着我们对无序浓磁半导体认识的加深,一些重要的问题也凸显出来。例如,在之前的室温氧化物浓磁半导体中还没有测量到表征材料载流子自旋极化的反常霍尔效应。并且,虽然无序浓磁半导体的性质与铁磁金属半导体颗粒膜的性质差别很大,这也暗示了我们的浓磁半导体的磁性起源不可能来自铁磁金属颗粒,但是我们并没有给出材料铁磁性起源的直接实验证据。这些问题也正是我们在本篇论文中希望解决的问题。
在本论文的工作主要分三部分,见正文第三、四、五章。第一部分包括室温非晶浓磁半导体的制备、结构表征、磁性和软X射线光谱研究。主要创新工作如下：(1)我们改进了材料的制备工艺,通过在样品制备腔室中引入少量的氧气,来调控氧化物基浓缩磁性浓磁半导体的性质。我们并最终选择了氧化铟作为母体半导体材料,制备了一系列具有不同载流子浓度而相同钴铟组分比例的钴铟氧浓缩磁性浓磁半导体。(2)该系列的浓缩磁性浓磁半导体都具有室温铁磁性,并且其饱和磁化强度随样品制备时氧分压的增加而减小,与此同时样品的电阻也随氧分压的增加而增大。(3)基于同步辐射的掠入射X射线衍射结果显示,氧分压最小并且磁性最强的样品为非晶结构,而氧分压的增加导致了样品中结晶相的出现,与此相对应,样品的磁性减弱。高分辨电镜数据也再次证明了以上所讲氧分压对材料结构的影响,而且显示了在样品中,不论结晶相还是非晶相中,钴与铟元素的分布都是均匀的。对样品结构表征的结果为磁性起源与样品的非晶相之间建立了直接的关联。(4)软X射线吸收谱显示,氧分压的增加,也对应于钴元素L边吸收谱上特殊吸收峰的出现,并且软X射线磁圆二色谱证明了样品中产生该特殊吸收峰的结晶相对磁性没有贡献,因此样品的磁性起源于其中的非晶相。软X射线磁圆二色谱的结果还同时显示了样品的磁性起源不同于钴金属。
第二部分主要是浓磁半导体的正常霍尔效应和反常霍尔效应研究。主要创新工作如下：(1)该系列的无序浓磁半导体的电输运性质经历了金属绝缘体转变,并且远离转变点的绝缘样品在低温下显示变程跃迁导电性。这也为研究一些重要的物理问题,例如变程跃迁区的反常霍尔效应提供了条件。(2)我们首次报道了在实验上观测到的变程跃迁区反常霍尔效应随温度变号的现象,并且发现反常霍尔电阻与纵向电阻的标度关系与现有理论预测不符。依据实验结果,我们指出了温度不同,费米能级以下发生跃迁的局域态的能级不同,而不同局域态电子的自旋-轨道耦合相互作用不同,导致了新奇的标度关系；而在以往的理论处理中,这一项都被当作常数来对待。因此,我们的实验结果对于完善人们对变程跃迁区反常霍尔效应的认识具有重要的意义。
第三部分主要是金刚烃Tetramantane中芯激子与辐射损伤效应的研究。主要创新工作如下：我们利用软X射线吸收谱研究了金刚烃同分异构体[121]与[123]tetramantane分子的辐射损伤效应以及尺寸关联的芯激子现象。软X射线吸收谱显示金刚烃中观测到的是表面激子,而且激子的束缚能比体金刚石中要大近一个数量级。我们发现该芯激子态对软X射线辐射非常敏感,并且[121]与[123]tetramantane分子表现出不同的辐射损伤效应。我们还建立了一个两步模型来解释观测到的跟辐射损伤相关的光化学反应。
The electron has two degrees of freedom of charge and spin, which have been playing important roles in separated fields for a long time. The traditional microelectronics, which mainly studies and utilizes the transport properties of electron as a charge carrier, has become the cornerstone of modern information technology, while the traditional magnetics, which focuses on the electron spin property, occupy a central place in microwave communication and information storage. The newly-developed spintronics, however, studies the interaction between charge and spin of electrons in solids, and novel spintronic devices, which take advantage of both charge and spin, are expected as new generation devices by many scientists. The development of spintronics has its profound historical background. It is well known that the chip integration is becoming more and more dense while the size of the semiconductor devices shrink fast, which can be described by the Moore's law. However, this high-speed development will soon meet the insuperable obstacle because the device size will enter nanoscale, in which the energy consumption per unit area will rise rapidly and result in serious heat damage problems and the quantum confinement effect rather than electron band theory will play a leading role. This challenging situation prompts the growing researches on revolutionized devices that are operated with totally different principle comparing with traditional microelectronic devices, with the anticipation of creating devices with faster arithmetic speed, smaller size and lower energy consumption. Along with the proposal of many prototype devices based on electron spin property, spintronics has become one of the hottest research area. To realize the goals of spintronics, the foremost mission is to develop a new material or structure, in which the charged electron motion is strongly coupled to the electron spin moment. The diluted magnetic semiconductor that possesses both room temperature ferromagnetism and spin-polarized carriers is the very material that meets all the requirements.
The preparation method of a typical diluted ferromagnetic semiconductor was to dope transition element into the current used semiconductor system. It is expected that the transition elements may enter the crystal lattice by substituting some cation's positions, and through ferromagnetic coupling between localized transition metal ions and itinerant carriers, ferromagnetism and spin-polarized carrier may appear in semiconductors. Mn doped GaAs prepared by low temperature molecular beam epitaxy method is one of the most matured diluted ferromagnetic semiconductor systems. This GaAs based diluted ferromagnetic semiconductor has provided the playground to demonstrate the feasibility of spintronic prototype devices proposed by scientists from all around the world. However, the Curie temperature, can only reach as high as200K, which does not meet the requirements of room temperature application. In order to obtain material that has room temperature ferromagnetism, researchers turn to study other semiconductor based diluted ferromagnetic semiconductor, which includes oxides, nitrides and IV group semiconductors, such as ZnO, TiO2, GaN, Si, Ge and so on. However, results reported by different groups are different or even contradictory. This implies a gloomy future for those diluted magnetic semiconductors to be used in spintronic devices. Over the last several decades, researchers have been attempted to increase the doping level of transition elements in traditional semiconductors by different unequilibrium growth method in order to increase the Curie temperature and saturated magnetization of the material, however, the low solubility of transition metal elements in semiconductor lattice, has limited the further increases of the institutional doping level.
Given this research background, our group attempted to break the limitation of semiconductor lattice and turned to the exploration of disordered condensed magnetic semiconductors by using the creative thin film growth method that alternatively deposit an atomic-thin layer of semiconductor and an atomic-thin layer of transition metal elements. Our previous research results suggested that, this condensed magnetic semiconductors prepared by the nonequilibrium growth method has room temperature ferromagnetism and has promising future in room temperature spintronic devices. However, as our knowledge of disorder condensed magnetic semiconductors increases, some important questions emerges. For example, we did not observe anomalous Hall effects in previous room temperature magnetic oxides. Secondly, although the disordered condensed magnetic semiconductor has huge difference with ferromagnetic metal and semiconductor granular films, which implied the ferromagnetic origination is unlikely to come from the ferromagnetic metal clusters, however, we have not yet give the direct experimental prove to the ferromagnetic origination of the material. Those are also the questions that we attempted to solve in this work.
(1) In this work, we improved the material growth method and introduce oxygen into the sample preparation chamber to control the properties of the condensed magnetic oxides. We finally chose In2O3as the material and prepared a series of In1-xCoxO magnetic semiconductor with same In and Co composition but different carrier densities.(2) All the samples have room temperature ferromagnetism while their saturated ferromagnetization decreased as the oxygen pressure increased, and at the same time, their resistivity increases.(3) The graze incident X-ray diffraction results indicated that, the sample with strongest ferromagnetism and most carrier concentration is amorphous, while oxygen induced a crystal phase in the sample that has no ferromagnetization. The high resolution transmission electron microscopy also confirmed the influence of the oxygen to the sample structure as mentioned above and indicated the cobalt and indium elements distribute homogeneously in the sample. The X-ray diffraction data has built direct connection between the ferromagnetism and the amorphous phase in the sample.(4) Furthermore, soft X-ray absorption spectra show that the introduction of oxygen also give rise to a new cobalt L-edge absorption feature, which shows no ferromagnetism as confirmed by soft X-ray magnetic dichroism spectra. Moreover, the soft X-ray magnetic dichroism results also show the magnetic properties of the sample is different from cobalt metal.
(1) The series of disordered condensed magnetic semiconductors experience metal-insulator transition and the samples that is far way from the transition point show variable range hopping conduction at low temperature. This offers a good playground for many important physical research, such as anomalous Hall effect in variable range hoping.(2) We reported firstly the experimental observation of sign change of anomalous Hall resistivity in variable range hopping. We also observed that the scaling relationship between the anomalous Hall resistiviy and the longitudinal resistiviy disagrees with the existed theoretical prediction. According to our experimental results, we pointed out that the spin-orbital coupling of electrons in the variable range hopping region changes as temperature varies, which however has been considered as a constant in previous theory. As a result, our experiment is of significant importance for a better understanding of anomalous Hall effects in the variable range hopping.
At last, we also studied the radiation damage effects and size-dependent core exition of two diamondoid isomers,[121] and [123] tetramantane molecules. The soft X-ray absorption results show the binding energy of the surface exciton that observed in tetramantane diamondoid is almost one order larger than that in the bulk diamond. We also found this core exciton state is very sensitive to the soft X-ray irradiation and [121] and [123] tetramantane showed different radiation damage effects. A two step model was introduced to explain those radiation related photochemical reactions.
典型的所谓稀释磁性半导体的制备方法是在现有半导体母体材料中掺杂少量的过渡金属元素,过渡金属元素以替位掺杂的形式进入半导体材料的晶格中,并通过局域的磁性离子与载流子自旋之间的耦合作用,在材料中产生宏观的铁磁性及自旋极化的载流子。低温分子束外延技术制备的Mn掺杂GaAs是目前研究比较成熟的稀释磁性半导体体系。GaAs基稀释磁性半导体的成功为演示及验证科学家提出的自旋电子学原型器件的工作原理提供了原材料。然而,经过全世界的科研工作者十多年的不懈努力,该系列稀释磁性半导体的居里温度最高只能达到200K,远低于实际器件在室温工作的要求。为了获得具有室温铁磁性的材料,研究人员开始转向其他半导体材料基的稀释磁性半导体的研究,研究的范围包括各种氧化物和如氮化物,如ZnO、TiO2、GaN等,以及Ⅳ族半导体Si和Ge等。然而,在大量的研究报道中,不同研究组得到的结果却各不相同甚至互相矛盾。室温稀释磁性半导体的问题在于科研人员都试图通过各种非平衡生长工艺来提高过渡族元素在传统半导体中的掺杂量,进而提高材料的居里温度和饱和磁化强度,然而过渡族元素在半导体晶格中低的溶解度,从根本上限制了替位掺杂量的提高。
在这样的背景下,我们课题组突破了半导体晶格的限制,利用低温交替沉积原子层厚度的半导体层与过渡金属层的创新性工艺,转向无序浓磁半导体的研究。课题组成员多年的研究工作表明,该非平衡生长工艺制备的无序浓磁半导体具有稳定的室温铁磁性,对于室温自旋电子学器件有潜在的应用价值。然而,随着我们对无序浓磁半导体认识的加深,一些重要的问题也凸显出来。例如,在之前的室温氧化物浓磁半导体中还没有测量到表征材料载流子自旋极化的反常霍尔效应。并且,虽然无序浓磁半导体的性质与铁磁金属半导体颗粒膜的性质差别很大,这也暗示了我们的浓磁半导体的磁性起源不可能来自铁磁金属颗粒,但是我们并没有给出材料铁磁性起源的直接实验证据。这些问题也正是我们在本篇论文中希望解决的问题。
在本论文的工作主要分三部分,见正文第三、四、五章。第一部分包括室温非晶浓磁半导体的制备、结构表征、磁性和软X射线光谱研究。主要创新工作如下：(1)我们改进了材料的制备工艺,通过在样品制备腔室中引入少量的氧气,来调控氧化物基浓缩磁性浓磁半导体的性质。我们并最终选择了氧化铟作为母体半导体材料,制备了一系列具有不同载流子浓度而相同钴铟组分比例的钴铟氧浓缩磁性浓磁半导体。(2)该系列的浓缩磁性浓磁半导体都具有室温铁磁性,并且其饱和磁化强度随样品制备时氧分压的增加而减小,与此同时样品的电阻也随氧分压的增加而增大。(3)基于同步辐射的掠入射X射线衍射结果显示,氧分压最小并且磁性最强的样品为非晶结构,而氧分压的增加导致了样品中结晶相的出现,与此相对应,样品的磁性减弱。高分辨电镜数据也再次证明了以上所讲氧分压对材料结构的影响,而且显示了在样品中,不论结晶相还是非晶相中,钴与铟元素的分布都是均匀的。对样品结构表征的结果为磁性起源与样品的非晶相之间建立了直接的关联。(4)软X射线吸收谱显示,氧分压的增加,也对应于钴元素L边吸收谱上特殊吸收峰的出现,并且软X射线磁圆二色谱证明了样品中产生该特殊吸收峰的结晶相对磁性没有贡献,因此样品的磁性起源于其中的非晶相。软X射线磁圆二色谱的结果还同时显示了样品的磁性起源不同于钴金属。
第二部分主要是浓磁半导体的正常霍尔效应和反常霍尔效应研究。主要创新工作如下：(1)该系列的无序浓磁半导体的电输运性质经历了金属绝缘体转变,并且远离转变点的绝缘样品在低温下显示变程跃迁导电性。这也为研究一些重要的物理问题,例如变程跃迁区的反常霍尔效应提供了条件。(2)我们首次报道了在实验上观测到的变程跃迁区反常霍尔效应随温度变号的现象,并且发现反常霍尔电阻与纵向电阻的标度关系与现有理论预测不符。依据实验结果,我们指出了温度不同,费米能级以下发生跃迁的局域态的能级不同,而不同局域态电子的自旋-轨道耦合相互作用不同,导致了新奇的标度关系；而在以往的理论处理中,这一项都被当作常数来对待。因此,我们的实验结果对于完善人们对变程跃迁区反常霍尔效应的认识具有重要的意义。
第三部分主要是金刚烃Tetramantane中芯激子与辐射损伤效应的研究。主要创新工作如下：我们利用软X射线吸收谱研究了金刚烃同分异构体[121]与[123]tetramantane分子的辐射损伤效应以及尺寸关联的芯激子现象。软X射线吸收谱显示金刚烃中观测到的是表面激子,而且激子的束缚能比体金刚石中要大近一个数量级。我们发现该芯激子态对软X射线辐射非常敏感,并且[121]与[123]tetramantane分子表现出不同的辐射损伤效应。我们还建立了一个两步模型来解释观测到的跟辐射损伤相关的光化学反应。
The electron has two degrees of freedom of charge and spin, which have been playing important roles in separated fields for a long time. The traditional microelectronics, which mainly studies and utilizes the transport properties of electron as a charge carrier, has become the cornerstone of modern information technology, while the traditional magnetics, which focuses on the electron spin property, occupy a central place in microwave communication and information storage. The newly-developed spintronics, however, studies the interaction between charge and spin of electrons in solids, and novel spintronic devices, which take advantage of both charge and spin, are expected as new generation devices by many scientists. The development of spintronics has its profound historical background. It is well known that the chip integration is becoming more and more dense while the size of the semiconductor devices shrink fast, which can be described by the Moore's law. However, this high-speed development will soon meet the insuperable obstacle because the device size will enter nanoscale, in which the energy consumption per unit area will rise rapidly and result in serious heat damage problems and the quantum confinement effect rather than electron band theory will play a leading role. This challenging situation prompts the growing researches on revolutionized devices that are operated with totally different principle comparing with traditional microelectronic devices, with the anticipation of creating devices with faster arithmetic speed, smaller size and lower energy consumption. Along with the proposal of many prototype devices based on electron spin property, spintronics has become one of the hottest research area. To realize the goals of spintronics, the foremost mission is to develop a new material or structure, in which the charged electron motion is strongly coupled to the electron spin moment. The diluted magnetic semiconductor that possesses both room temperature ferromagnetism and spin-polarized carriers is the very material that meets all the requirements.
The preparation method of a typical diluted ferromagnetic semiconductor was to dope transition element into the current used semiconductor system. It is expected that the transition elements may enter the crystal lattice by substituting some cation's positions, and through ferromagnetic coupling between localized transition metal ions and itinerant carriers, ferromagnetism and spin-polarized carrier may appear in semiconductors. Mn doped GaAs prepared by low temperature molecular beam epitaxy method is one of the most matured diluted ferromagnetic semiconductor systems. This GaAs based diluted ferromagnetic semiconductor has provided the playground to demonstrate the feasibility of spintronic prototype devices proposed by scientists from all around the world. However, the Curie temperature, can only reach as high as200K, which does not meet the requirements of room temperature application. In order to obtain material that has room temperature ferromagnetism, researchers turn to study other semiconductor based diluted ferromagnetic semiconductor, which includes oxides, nitrides and IV group semiconductors, such as ZnO, TiO2, GaN, Si, Ge and so on. However, results reported by different groups are different or even contradictory. This implies a gloomy future for those diluted magnetic semiconductors to be used in spintronic devices. Over the last several decades, researchers have been attempted to increase the doping level of transition elements in traditional semiconductors by different unequilibrium growth method in order to increase the Curie temperature and saturated magnetization of the material, however, the low solubility of transition metal elements in semiconductor lattice, has limited the further increases of the institutional doping level.
Given this research background, our group attempted to break the limitation of semiconductor lattice and turned to the exploration of disordered condensed magnetic semiconductors by using the creative thin film growth method that alternatively deposit an atomic-thin layer of semiconductor and an atomic-thin layer of transition metal elements. Our previous research results suggested that, this condensed magnetic semiconductors prepared by the nonequilibrium growth method has room temperature ferromagnetism and has promising future in room temperature spintronic devices. However, as our knowledge of disorder condensed magnetic semiconductors increases, some important questions emerges. For example, we did not observe anomalous Hall effects in previous room temperature magnetic oxides. Secondly, although the disordered condensed magnetic semiconductor has huge difference with ferromagnetic metal and semiconductor granular films, which implied the ferromagnetic origination is unlikely to come from the ferromagnetic metal clusters, however, we have not yet give the direct experimental prove to the ferromagnetic origination of the material. Those are also the questions that we attempted to solve in this work.
(1) In this work, we improved the material growth method and introduce oxygen into the sample preparation chamber to control the properties of the condensed magnetic oxides. We finally chose In2O3as the material and prepared a series of In1-xCoxO magnetic semiconductor with same In and Co composition but different carrier densities.(2) All the samples have room temperature ferromagnetism while their saturated ferromagnetization decreased as the oxygen pressure increased, and at the same time, their resistivity increases.(3) The graze incident X-ray diffraction results indicated that, the sample with strongest ferromagnetism and most carrier concentration is amorphous, while oxygen induced a crystal phase in the sample that has no ferromagnetization. The high resolution transmission electron microscopy also confirmed the influence of the oxygen to the sample structure as mentioned above and indicated the cobalt and indium elements distribute homogeneously in the sample. The X-ray diffraction data has built direct connection between the ferromagnetism and the amorphous phase in the sample.(4) Furthermore, soft X-ray absorption spectra show that the introduction of oxygen also give rise to a new cobalt L-edge absorption feature, which shows no ferromagnetism as confirmed by soft X-ray magnetic dichroism spectra. Moreover, the soft X-ray magnetic dichroism results also show the magnetic properties of the sample is different from cobalt metal.
(1) The series of disordered condensed magnetic semiconductors experience metal-insulator transition and the samples that is far way from the transition point show variable range hopping conduction at low temperature. This offers a good playground for many important physical research, such as anomalous Hall effect in variable range hoping.(2) We reported firstly the experimental observation of sign change of anomalous Hall resistivity in variable range hopping. We also observed that the scaling relationship between the anomalous Hall resistiviy and the longitudinal resistiviy disagrees with the existed theoretical prediction. According to our experimental results, we pointed out that the spin-orbital coupling of electrons in the variable range hopping region changes as temperature varies, which however has been considered as a constant in previous theory. As a result, our experiment is of significant importance for a better understanding of anomalous Hall effects in the variable range hopping.
At last, we also studied the radiation damage effects and size-dependent core exition of two diamondoid isomers,[121] and [123] tetramantane molecules. The soft X-ray absorption results show the binding energy of the surface exciton that observed in tetramantane diamondoid is almost one order larger than that in the bulk diamond. We also found this core exciton state is very sensitive to the soft X-ray irradiation and [121] and [123] tetramantane showed different radiation damage effects. A two step model was introduced to explain those radiation related photochemical reactions.
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