Position- ensitive STEM etectors for igh- ensitivity hase etection

The most commonly-used mode of imaging in the scanning transmission electron microscope (STEM) has made use of the annular dark-field (ADF) detector. Recently, however, there has been increased interest in bright-field (BF) modes. By reciprocity, a small BF detector with collection angles that are about an order of magnitude smaller than the convergence angle of the probe (see Fig. 1) provides an imaging mode that is equivalent to phase contrast imaging in the conventional high-resolution transmission electron microscope (HRTEM). A small detector is necessary to maintain sufficient coherence for efficient phase contrast, but leads to low efficiency of use of incident electrons in the STEM configuration because most of the incident electrons are not collected.