Dr. rer. nat. Christian Wiktor
The phenomenon of solid state dewetting describes the gradual transition of homogeneous thin films into nanosized particles well below the melting point of the material . Through in situ Transmission Electron Microscopy (TEM) studies promise to improve our understanding of the process and to help create more stable and smaller electronic or magnetic devices , but also to generate defined nanoparticle arrays .
At the CENEM in situ TEM is used to gain insight into the kinetics of solid state dewetting and the underlying microscopic processes, including edge retraction, grain coarsening and texture evolution [4,5]. The exceptionally high quality of the obtained TEM data is achieved by using chip based heating systems. In comparison with conventional TEM heating holders, sample drift is greatly reduced and temperature can be controlled more precisely.
Applied TEM techniques include electron diffraction, high angle annular dark field scanning TEM, hollow cone dark field TEM and high-resolution TEM. The plethora of information contained in the obtained in situ data is accessed by detailed statistical analysis.
Another thin film phenomenon studied by in situ TEM is the crystallization of Si thin films through metal induced layer exchange. As the process occurs way below the melting point of Si, the process requires less energy than others. In a typical sample, a substrate is first coated with a metal film. The second layer is an oxide generated either by the superficial oxidation of the first layer or by depositing another metal oxide. The last layer is amorphous Si. When heated Si atoms diffuse through the oxide layer into the metal. Here Si crystallizes upon reaching a critical concentration. The metal on the other hand is pushed up and forms isolated islands on the crystalline Si . The grain size of the crystalline Si is controlled by the oxide layer between the metal and the initially amorphous Si .
If the reaction can be observed by a given in situ TEM techniques depends on the choice of metal. HAADF-STEM can be easily used if the atomic number of the metal is sufficiently different to the one of Si. In the most typical combination of Al and Si however this is not possible . Here spectroscopic imaging techniques are applied like the local acquisition of energy dispersive X-ray spectra in STEM mode.
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