Angle-resolved STEM using an iris aperture: Scattering contributions and sources of error for the quantitative analysis in Si

• Published in: Ultramicroscopy Vol. 221; p. 113175
• Main Authors: Grieb, Tim, Krause, Florian F., Müller-Caspary, Knut, Firoozabadi, Saleh, Mahr, Christoph, Schowalter, Marco, Beyer, Andreas, Oppermann, Oliver, Volz, Kerstin, Rosenauer, Andreas
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01-02-2021
• Subjects: Angle-resolved STEM; Inelastic scattering; Low-angle scattering; Phonon correlation; Plasmon excitation; Quantitative STEM; Quantitative STEM; Phonon correlation; Plasmon excitation; Inelastic scattering; Angle-resolved STEM; Low-angle scattering
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2020.113175

•Experimental angle-resolved STEM series of silicon were recorded using an iris aperture above the ADF detector•The thicknesses of four lamella steps were determined by comparing the scattering intensity in different angular intervals with simulations.•For angles <50 mrad, the thickness was strongly overestimated when frozen-lattice reference simulations in the Einstein approximation were used.•Simulations, which take phonon correlation, inelastic scattering and amorphous layers into account, are able to describe the experiment.•The influence of parameters such as mistilt, defocus or decentering in diffraction space on the quantitative determination was investigated.
The angle-resolved electron scattering is investigated in scanning-transmission electron microscopy (STEM) using a motorised iris aperture placed above a conventional annular detector. The electron intensity scattered into various angle ranges is compared with simulations that were carried out in the frozen-lattice approximation. As figure of merit for the agreement of experiment and simulation we evaluate the specimen thickness which is compared with the thickness obtained from position-averaged convergent beam electron diffraction (PACBED). We find deviations whose strengths depend on the angular range of the detected electrons. As possible sources of error we investigate, for example, the influences of amorphous surface layers, inelastic scattering (plasmon excitation), phonon-correlation within the frozen-lattice approach, and distortions in the diffraction plane of the microscope. The evaluation is performed for four experimental thicknesses and two angle-resolved STEM series under different camera lengths. The results clearly show that especially for scattering angles below 50 mrad, it is mandatory that the simulations take scattering effects into account which are usually neglected for simulating high-angle scattering. Most influences predominantly affect the low-angle range, but also high scattering angles can be affected (e.g. by amorphous surface covering).