HAADF-STEM imaging: from qualitative to quantitative interpretation of atomic resolution HAADF-STEM images
published: Jan. 18, 2008, recorded: October 2007, views: 14173
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In our presentation an overview on qualitative and quantitative HAADF-STEM technique will be given and illustrated by examples of characterization of various inorganic ceramic materials, such as CaTiO3-La(Mg,Ti)O3 solid solution, GaN blue laser diode, bulk CaTiO3 and AO-doped SrTiO3 (A=Sr,Ca,Ba). The specimens for the HAADF-STEM observations were prepared by high-energy and/or low-energy ion milling and were observed in a FEG JEOL-2010F (Cs=0.48 mm). The probe semi-angle was 10 mrad. The inner and outer annular angles of the HAADF detector were 100 and 220 mrad, respectively. The HAADF-STEM image simulations were carried out using a calculation scheme developed by Watanabe. Our results showed that differences as small as 2% in the average atomic number Z can readily be detected by HAADF-STEM imaging. In qualitative interpretation of atomic-resolution HAADF-STEM images we compared intensity ratios between different atom columns or used intensity profiles to show the difference in the chemical composition between individual atom columns. In this way we could qualitatively interpret the ordering and/or partial ordering of solute atoms in bulk materials, evaluate the occupancy of atom columns in special structures and study the segregation of impurities along grain boundaries. Quantitative interpretation required image simulations and matching of the processed experimental images with the calculated ones. However, in order to calculate HAADF-STEM images the exact structure of the observed structural phenomena should be known, i.e., the positions of the atoms, in order to create proper supercells for calculations. The realistic values of the Debye-Waller factor should also be used in calculations. After image calculations appropriate matching algorithms with the experimental images were applied in order to determine the best fit between calculated and experimental image.
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