Measurement of the chemical composition of nanostructures by quantification of Z-contrast in scanning-transmission electron microscopy

在项目中，我们开发了一种使用扫描-透射电子显微镜(STEM)精确分析样品中化学成分的方法。这种基于实验高角环形暗场(HAADF)强度与模拟的直接比较的方法已经应用于许多不同的材料，到目前为止，强度是通过积分由相机长度和探测器几何形状定义的一定角度范围内的散射强度来测量的。在这个更新方案中，我们的目标是测量作为散射角的函数的STEM强度。散射强度的角度依赖关系不仅受化学浓度的影响，还受散射过程的类型(如热扩散散射或原子静态位移散射)和晶格扭曲的影响。因此，用选定的探测角度测量的散射强度包含允许同时测量两个或更多实验参数(例如，厚度和化学浓度)的附加信息。为此，我们建议在显微镜的HAADF探测器的正上方安装电动隔膜，这种新方法将能够同时测量单个元素(例如In In InGaN)的局部化学浓度和局部样品厚度或两个元素(例如InAlGaN中的In和Al)的局部化学浓度。我们使用原型隔膜进行的初步研究表明，我们的方法允许在没有应变诱导伪影的情况下使应变场可见或执行定量化学分析。研究进一步表明，在小于40mrad的角度下，实验和模拟的端面光强分布存在差异。该方案的一个主要主题是小角散射强度的量化，包括等离子体激元散射的研究及其在我们的模拟软件中采用不同的参数实现。该方法将通过其应用于研究量子点、四元层、偏析效应、非均匀组成的纳米粒子、纳米泡沫金和SiGe MOSFET而为各种合作和项目做出贡献。因此，该项目将极大地促进对半导体纳米结构和催化剂的理解。

In project we developed a method to precisely analyse the chemical composition in a sample using scanning-transmission electron microscopy (STEM). The method wich is based on a direct comparsion of the experimental high-angle annular dark field (HAADF) intensity with simulations was already applied to many different materials.So far, the intensity was measured by integrating the scattered intensity over a certain angular range which is defined by the camera length and the geometry of the detector. In this renewal proposal we aim at a measurement of STEM intensity as a function of the scattering angle. The angle-dependence of the scattered intensity is not only influenced by the chemical concentration but also by the type of the scattering process (e.g. thermal-diffuse scattering or scattering at static atomic displacements) and by distortions of the crystal lattice. Therefore, the scattering intensity measured with selected detection angles contains additional information which allows for measuring two or more experimental parameters (e.g. thickness and chemical concentration) at the same time. For this purpose we suggest to install a motor-driven diaphragm directly above the HAADF detector in the microscope.The new method will be able to measure the local chemical concentration of a single element (e.g. In in InGaN) and the local specimen thickness or the local chemical concentrations of two elements (e.g. In and Al in InAlGaN) at the same time. Our priliminary studies using a prototype diaphragm show that our method allows for either making strain fields visible or performing quantitative chemical analyses without strain-induced artifacts. The studies further show a difference in the angle-dependend intensity distribution between experiment and simulation at angles smaller than 40 mrad. A main topic of this proposal is the quantification of the small-angle scattering intensity, which includes the investigation of plasmon scattering and its implementation into our simulation software using different apporaches.The method will contribute to various cooperations and projects by its application to investigate quantum dots, quaternary layers, segregation effects, nano particles of non-homogeneous composition, nano-pourous gold and SiGe MOSFETs. Thus, the project will significantly contribute to the understanding of semiconductor nanostructures and catalysists by the suggested method.