Atomic resolution imagery composition measure applied to superarrays

Published : 27 February 2019

For crystalline materials sensitive to electron beam radiation damage, it is necessary to quantify the chemical composition at the atomic scale while minimizing the electron dose. The usual analytical techniques in the transmission electron microscope (TEM) can not be used because of the high probe current and the relatively long acquisition time. On the other hand, atomic imaging, more precisely using the high-angle annular dark field (STEM-HAADF), is performed at a reduced dose and exhibits contrasts proportional to the atomic number of the elements. In addition, the TEMs Titan on the PFNC are equipped with an aberration corrector to acquire state-of-the-art HAADF images in terms of atomic resolution. However, for the contrast in these images to be quantitatively related to the chemical composition of the material, controlled TEM acquisition conditions and electronic scattering simulations must be developed. In parallel, another imaging technique in the TEM is attracting growing interest: ptychography, or “4D data STEM”. This technique, consisting in acquiring a diffraction pattern for each position of the incident electron beam, provide the projected potential in the sample. The development of the quantitative aspect of these imaging techniques has many applications: the one targeted in this thesis is the understanding of the atomic order of GeTe / Sb2Te3 superlattices, materials considered as the most promising for phase change memories (PCRAM).

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