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

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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.