DECHANNELING CONTRAST IN ANNULAR DARK-FIELD STEM

The image contrast in high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) for crystalline specimens oriented so that the incident beam axis is parallel to a principal axis or plane of the crystal is considered to be produced by a process of channelling of the electrons along rows or planes of atoms plus a process of high-angle scattering by the atoms. Crystal defects or the termination of a crystal at a face can change the channelling conditions and hence give image contrast. It is shown that at MgO crystal faces and at dislocations in Al the de-channelling of the electron beam produces an expansion of the diffraction pattern and hence an increase of ADF image intensity. It is suggested that the HAADF STEM technique may be particularly valuable for the study of defects in thick crystals and of surface structures.