功能薄膜界面的电子显微学研究

IMR OpenIR

	功能薄膜界面的电子显微学研究
	汪雪
学位类型	博士
导师	马秀良
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	外延生长功能薄膜透射电子显微术界面结构显微结构 Epitaxial Growth Functional Thin Films Transmission Electron Microscopy Interface Structure Microstructures
摘要	"功能薄膜在现代电子和通讯器件中具有广泛的应用。由于界面的电学和磁学性能在很大程度上控制着器件的使用效能，而这些界面的性质又受到界面的原子结构、成分和化学键的强烈影响，对于薄膜界面的研究非常重要。随着现代高分辨电子显微术和高空间分辨分析电子显微术的发展，我们有可能在原子或纳米尺度研究薄膜中界面的显微结构和成分组成。本文利用（扫描）透射电子显微术、高角环形暗场成像术、X射线能量色散谱、电子能量损失谱分析和能量过滤成像技术对于几种功能薄膜的显微结构和成分组成进行了研究。研究的重点在于界面的结构。主要包括以下几个方面：采用两步法通过激光分子束外延技术在Si (001)衬底上生长的高K栅介质Er2O3薄膜，在30 nm和100 nm两种厚度下均由具有(111)织构的多晶晶粒组成，织构关系为Er2O3 {111} // Si (001)。在薄膜和衬底之间的界面上发现了厚度为2-3 nm的含有有序小岛的非晶界面层。界面层的成分主要由Er和O构成，仅含有非常少量的Si。这种类型的界面将对提高薄膜的电学性能有重要的意义。采用激光分子束外延技术在SrTiO3 (001)衬底上、在两种非优化氧压条件（1 Pa和0.3 Pa）下外延生长的单相多铁性BiFeO3薄膜，均含有镶嵌在BiFeO3薄膜基体中的反铁磁的α-Fe2O3第二相。α-Fe2O3与BiFeO3的取向关系被确定为(001)α-Fe2O3//(111)p-BFO，[010] α-Fe2O3//[-110]p-BFO。随着氧压的降低，α-Fe2O3相的密度和尺寸增加，形状变得不规则。在α-Fe2O3粒子与BiFeO3薄膜基体的界面上分布有周期性排列的位错，说明α-Fe2O3粒子以半共格的形式镶嵌在BiFeO3薄膜中。在1 Pa下生长的薄膜与SrRuO3/SrTiO3基体的界面上没有发现失配位错，说明薄膜中的失配应变通过形成α-Fe2O3相的形式已经完全得到释放。相比之下，在0.3 Pa下生长的薄膜中除了α-Fe2O3相外，还在薄膜与衬底的界面处发现了亚铁磁性的Fe3O4析出物。Fe3O4与BiFeO3的取向关系被确定为(001)Fe3O4//(001)p-BFO， [010]Fe3O4//[010]p-BFO。薄膜与SrRuO3层的界面上发现了高密度的失配位错，说明薄膜中的失配应变是通过形成失配位错和α-Fe2O3相两种形式释放的。薄膜中Fe化合价为+3价。讨论了第二相的形成机理和它们对电学和磁性性能的影响。指出可通过调控生长氧压来制备具有潜在应用价值的纳米复合薄膜。通过对脉冲激光沉积和磁控溅射技术的精确控制，成功实现在SrTiO3 (001)衬底上Fe/BaTiO3复合多铁性薄膜的外延生长。Fe与BaTiO3之间的外延关系为(001)Fe//(001)BTO， [-110]Fe//[010]BTO。Fe/BaTiO3界面是半共格的，具有周期性排列的失配位错网络。通过[010]和[110]两个方向的高分辨成像，确定了界面位错为不全位错，柏氏矢量为a/2<100>，位错线的方向为<010>。通过建立界面的几何模型提出位错网络的形成机理。在分析了各层薄膜的残余应变的基础上，讨论了BaTiO3薄膜层的厚度对Fe/BaTiO3界面位错形成的影响。在实验高分辨成像的基础上，结合像模拟计算，研究了Fe/BaTiO3复合多铁性双层外延薄膜中的Fe/BaTiO3界面的原子结构。发现存在两种类型的界面配置。确定界面的终结面为BaO面，一种界面是Fe原子位于由Ba-Ba和O-O原子组成的中心空洞的上方，另一种是Fe原子同时直接位于Ba和O原子的上方。这两种界面之间具有一定的平移关系，两种类型界面是由aFe/2<110>或者aFe/2<001>界面位错引入的。这两种界面原子结构的确定，可以为理论研究Fe/BaTiO3界面磁电耦合效应提供结构信息。"
其他摘要	"Functional thin films have many potential applications in modern electronic and communication devices. Since the electronic and magnetic properties of interface control the performance of the devices to a large extent, and since these interfacial properties are strongly influenced by the atomic structure, composition and bonding at the interface, the investigations on the interface of thin films are of vital importance. With the development of modern high resolution transmission electron microscopy and high spatial resolution analytical microscopy, it is highly possible to study the structures and compositions of the interface of thin films at an atomic or/and nano scale. In this dissertation, microstructures and microanalysis of several functional thin films were investigated by means of high resolution (scanning) transmission electron microscopy, high-angle annular dark-field imaging, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy and energy filtered imaging. The interface structures are the key point in our research. The main results are as follows: Erbium oxide (Er2O3) films, as a promising high-k gate dielectric, were deposited on Si (001) substrates by laser molecular beam epitaxy by two step method. Both Er2O3 films with thicknesses of 30 nm and 100 nm, respectively, are composed of polycrystalline with dominant (111) textures of Er2O3 {111} // Si (001). Amorphous layers dotted with small ordered islands with the thickness of 2-3 nm were observed at the interfaces between the films and the Si substrates. It was found that the layers contain much of Er and O, and a very small amount of Si. This kind of interface layer may play a very important role in the electrical properties of the films. Multiferroic BiFeO3 thin films were epitaxially grown on SrRuO3 buffered SrTiO3 (001) substrates by laser molecular beam epitaxy under two non-optimized oxygen pressures (1 Pa and 0.3 Pa). Both films contain secondary phase of antiferromagnetic α-Fe2O3 embedded in the BiFeO3 film matrix. The orientation relationships between α-Fe2O3 and BiFeO3 were determined to be (001)α-Fe2O3//(111)p-BFO, [010] α-Fe2O3//[-110]p-BFO. By lowering oxygen pressure, the density and the size of α-Fe2O3 phase increases whereas the regularity decreases. Approximately periodic misfit dislocations exist at the interface between α-Fe2O3 particles and the BiFeO3 film matrix, indicating that the α-Fe2O3 particles are semi-coherently embedded in the films. In the films grown under the oxygen pressure of 1Pa, less misfit dislocations were detected at the interfaces between the BiFeO3 films and the SrRuO3/SrTiO3 substrates, implying that the misfit strains in the films are fully relaxed by the formation of α-Fe2O3 phase. In contrast, besides α-Fe2O3 phase, ferromagnetic Fe3O4 precipitates were also found in BiFeO3 films grown under 0.3 Pa and accumulate in the areas near the film/substrate interfaces. The orientation relationships between Fe3O4 and BiFeO3 were determined to be (001)Fe3O4//(001)p-BFO, [010]Fe3O4//[010]p-BFO. High density misfit dislocations were observed near the interfaces between the BiFeO3 and SrRuO3 layers, implying that the misfit strains in this heteroepitaxy system is relieved by the formation of misfit dislocations as well as by the formation of α-Fe2O3 phase. Fe exists in the +3 oxidation state in these films. The possible formation mechanisms of these secondary phases and their effect on electric and magnetic properties were discussed. The control of the growth pressure may be used to prepare the nano-composite thin films. Epitaxial Fe layers were successfully grown on top of epitaxial layers of BaTiO3 on SrTiO3 (001) substrates by a careful control of the pulsed laser deposition and magnetron sputtering procedures. The epitaxial relationship between Fe and BaTiO3 films were determined to be (001)Fe//(001)BTO, [-110]Fe//[010]BTO.The interface is semi-coherent with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] directions observations, the interfacial dislocations were found to be partial with Burgers vectors a/2<100> and line directions of <010>. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both the BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed. The atomic structure of Fe/BaTiO3 interface in Fe/BaTiO3 bilayer films was investigated by high-resolution TEM imaging and its simulations. The interface is composed of two types of interface structure. Both types of interface is terminated by BaO plane, and one is Fe atoms on top of the hollow of Ba-Ba and O-O atoms, whereas the other is Fe atoms on top of Ba and O atoms simultaneously. They have certain of translation relationship with each other, and two types of interface are introduced by interfacial dislocations of aFe/2<110> and aFe/2<001>. The determination of the two types of interface structure can provide guidance for theoretical study of magnetoelectric effect of Fe/BaTiO3 interface.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64467
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	汪雪. 功能薄膜界面的电子显微学研究[D]. 北京. 中国科学院金属研究所,2012.