自上个世纪90年代在具有钙钛矿结构的锰氧化物La1-xAxMnO3（A=Ca，Sr，Ba）尤其是薄膜中发现了庞磁电阻效应（CMR）以来，由于这种特性可以广泛地应用于读写磁头、磁传感器及磁记录等器件中，因而引起了人们极大的兴趣和广泛的关注。有关钙钛矿结构氧化物的显微结构和缺陷方面的研究对于理解其结构以及电磁学性能之间的相互关系有着十分重要的意义。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，使得我们可以在原子尺度研究材料的显微结构和成分分布，也使透射电子显微镜成为材料结构表征方面最为直接有力的工具。
本论文结合了透射电子显微术、高角环形暗场成像术以及X射线能量色散谱的线扫描技术对几种激光分子束外延方法生长的具有钙钛矿结构的氧化物功能薄膜的显微结构和化学成分分布进行了研究，主要结果包括以下几个方面：
研究了两种生长在Si（001）基体上的厚度分别为300nm和10nm的La0.9Sr0.1MnO3薄膜的结构特征和化学成分分布。研究结果表明，这两种样品中La0.9Sr0.1MnO3薄膜均由单相多晶体构成，这些多晶体颗粒以柱状结构生长，柱状晶垂直于界面并一直贯穿整个LSMO薄膜。La0.9Sr0.1MnO3薄膜具有正交点阵结构，在LSMO/Si的界面处发现具有纳米尺度的两个非晶层。成分分析表明在靠近Si基体一侧具有2-3nm厚度的非晶层是由Si和O组成，而且O元素在这里富集。另一个非晶层中含有La、Sr、Mn、O以及Si等五种元素，阐明了两种非晶层的形成原因是由界面处的化学反应引起的，经过快速反应阶段氧化硅非晶层生长较迅速，然后由于第二层非晶层的出现阻碍了扩散的顺利进行，从而抑制了氧化硅非晶层的生长，达到饱和。这个过程与薄膜的沉积厚度关系不大。
研究了外延生长在SrTiO3（001）基体上的厚度为200nm的BaSrNb0.3Ti0.7O3薄膜的显微结构特征。横截面样品中，薄膜存在以亮衬度带或暗衬度带为特征的柱状结构。薄膜与基体之间的界面明锐且平直，界面上和薄膜内部均没有第二相形成。薄膜内部化学成分分布很均匀。BaSrNb0.3Ti0.7O3薄膜和SrTiO3基体之间的界面上有近似周期性排布的失配位错。界面上所观察到的一些不全位错应该就是Burgers矢量为a[010]或a[001]的全位错沿着某一个方向上的投影。而所观察到的Burgers矢量为b = a [011]的全位错分解为具有Burgers矢量b = 1/2a [011]的两个相等的不全位错。薄膜的平面样品中，发现BaSrNb0.3Ti0.7O3薄膜内有着许多随机分布的反向畴，反向畴界呈现蛇形排布。阐明了BaSrNb0.3Ti0.7O3薄膜具有高导电性的原因主要在于Nb元素的掺杂，薄膜内部穿过位错的密度非常低以及还原性气氛—氮气的影响。
其他摘要
The recent renaissance of the perovskite-based La1-xAxMnO3 (A=Ca, Sr, Ba and Pb) compounds has triggered a lot of scientific and industrial attention because of the discovery of colossal magneto-resistance (CMR) in these oxides. Large magnitude of magneto-resistance makes these materials very interesting from the point of view of their potential application such as magnetic sensors, magnetic access random memories and hard-disk read heads. Studies of the structures and defects in perovskite-based oxides are of great significance for understanding the relationship between structure and properties of electromagnetism in the kind of materials. With the development of modern high resolution transmission electron microscopy (HREM) and high spatial resolution analytical microscopy, the studies on the microstructure and chemical components distribution of materials are at an atomic scale. And the transmission electron microscope (TEM) has been become one of most powerful tools for microstructure characterization of materials.
In this dissertation, microstructures and chemical components analyses of several kinds of perovskite-based oxide functional thin films deposited by laser molecular beam epitaxy have been investigated by means of HREM, high-angle annular dark-field imaging, and line-scan X-ray energy dispersive spectroscopy. The main results are as follows:
Microstructures and chemical component distribution in the thin films of La0.9Sr0.1MnO3 with the thickness of 300nm and 10nm, respectively, grown on Si (001) substrate, have been investigated. The La0.9Sr0.1MnO3 film has an orthorhombic structure and it is polycrystallined with grain size of 10-20nm. The grains of La0.9Sr0.1MnO3 display a columnar growth normal to the interface. Between the La0.9Sr0.1MnO3 film and Si substrate, two amorphous layers with nanometer dimensions are identified. The formation mechanism of these amorphous layers is proposed on the basis of EDS composition scanning. The amorphous layer with the thickness of 2-3nm close with Si substrate is made up of element Si and element O. And this amorphous layer is rich in oxygen. In the second amorphous layer, there are five elements (La, Mn, Sr, O and Si). The forming of these two amorphous layers is caused by chemical reactions on the interface between La0.9Sr0.1MnO3 thin films and Si substrate. The formation of the amorphous layers is independent on the thickness of films.
In the BaSrNb0.3Ti0.7O3 thin film grown on SrTiO3 substrate, it is found that the film is single-crystallized and epitaxially grown on the SrTiO3 substrate forming a flat and distinct interface. Columnar bands with dark contrast can be found in the film. No secondary phase exists in the film. And the film shows nearly homogeneous composition distribution. It can be seen that an array of dislocations is nearly periodically distributed along the interface. The partial dislocation found on the interface should also be a projected component of a perfect dislocation with Burgers vector of either a[010] or a[001]. The perfect dislocation with the Burgers vector b of a [011] is dissociated into two identical partials with the Burgers vector b of 1/2a [011]. In the plan-view specimen, it can be observed that high density of anti phase domains exist in the film. The domain boundaries reveal a wave-like pattern. The doping of Nb, low density of threading dislocations, and the effect of reducing atmosphere of N2 are believed to be responsible for the high conductivity of the BaSrNb0.3Ti0.7O3 film.
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