Elastic Scattering of Electrons at Large Angles
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Small fraction of incident electrons are scattered through large angles and the extreme case is 180° backscattering as shown in Figure 4755. The scattering angle has been discussed in Section Elastic Scattering Angle of Electrons. In electron microscopes, a collection angle higher than 50 mrad allows acquisition of mainly incoherent electrons. The incoherent images are directly interpretable.

Elastic scattering of electrons from an atomic nucleus for a large-angle collision and a 180 ° collision

Figure 4755. Elastic scattering of electrons from an atomic nucleus for a large-angle collision and a 180 ° collision.

Dark-field images in TEMs/STEMs are constructed using electrons scattered at relatively large (≥ 30 mrad for STEM) angles and are dominated by elastic and thermal diffuse scattering. Scattering in the range of high angles is dominated by Rutherford (elastic) scattering and thermal diffuse (quasi-elastically) scattering (TDS). Such scattering is very sensitive to the atomic number (Z) of the scattering atoms and therefore provides information of the local chemical composition [1]. The scattering cross section is proportional to Z2. In electron microscopic imaging, high-angle annular dark-field (HAADF) method is used to remove the complexity of conventional bright-field scattering in HRTEM and the associated diffraction complications.

 

 

 

 

[1] Characterization of III–V semiconductor interfaces by Z -contrast imaging, EELS and CBED, Hubert Lakner, Bernd Bollig, Stefan Ungerechts and Erich Kubalek, J. Phys. D: Appl. Phys. 29 1767–1778 (1996).


 

 

 

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