Coherence of Electron Source vs HRTEM
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.



In ideal EMs (electron microscopies), e.g. in point-like monochromatic TEM (transmission electron microscopy), we can assume that the electron beam is generated from a point-like source and the energy variation ΔE is zero. This electron source is fully coherent. Even though the TEM specimen can be a crystal and thus, the incident electron beam k0 is split into different Bragg-diffracted components kg, resulting in a characteristic phase shift, the phase relation between the diffracted beams is maintained (the elastic electron scattering is coherent) [1]. This coherent relationship induces an interference pattern forming a HRTEM image.

In images formed by elastic scattering (e.g. HRTEM images), contrast delocalization is referred to as the loss of image contrast due to lens aberrations or called blurring effect. In this case, the image information is blurred and displaced from their true locations in the TEM specimen. In other words, the contrast delocalization can also be expressed as the lateral displacement of spatial frequencies in the image. This displacement increases with spatial frequency significantly. It is rarely a problem on the microscopes with LaB6 guns due to their limited coherence in illumination. However, it is a significant imaging artifact in HRTEM images taken from microscopes with field emission guns (FEGs) due to their high coherence that causes a strong contribution of the high spatial frequencies. Note that the coherence of the electron source is important for high resolution TEM imaging but is not important for all other applications such as Z-contrast STEM imaging, and EDS and EELS measurements. For the same reason, field-emission guns works on HRTEM imaging better than other electron sources (see page4196).

Theoretically, due to the large d (the diameter of the specimen over which the illumination is coherent, see page1708), parallel-beam TEM with α ≈ 0 mrad can be used to record lattice fringes with diffraction contrast (so-called HRTEM).








[1] L. H. Schwartz and J. B. Cohen, (1987) Diffraction from Materials, 2nd ed., Berlin: Springer-Verlag.



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