Channeling effects from impurity atoms in the high-angle annular detector of the stem

We have examined the contrast in the high-angle annular detector of a scanning transmission electron microscope from high-atomic-number impurity atoms in a low-atomic-number substrate. We find significant variations when the incident electron beam is near a Bragg condition. Analysis shows that the contrast, which depends strongly on the deviation parameter. actually arises from strain fields surrounding individual impurity atoms. In principle, with very coherent illumination, direct determination of impurity lattice sites is possible. The contribution of very small strain fields in the high-angle annular detector can dominate the signal from point defects and may aid in their detection, but in any case cannot be neglected in Rutherford scattering for determining atomic concentrations.