THEORY OF LATTICE RESOLUTION IN HIGH-ANGLE ANNULAR DARK-FIELD IMAGES

The theoretical interpretation of lattice resolution in high-angle ann ular dark-field images produced in a scanning transmission electron mi croscope (STEM) has been a subject of controversy. A first-order pertu rbation theoretical analysis is presented here, which shows that the c ontrast in the image arises from large-angle multiphonon, incoherent s cattering, which is atomic number dependent. The lattice resolution is a consequence of coherently filling the objective aperture, and dynam ical elastic diffraction preceding the large-angle multiphonon scatter ing is not a necessary requirement. Elastic scattering to the higher o rder Laue zone (HOLZ) is also shown to be negligible, compared with th e incoherent scattering. Calculations from application of the theory a re also presented. They show that lattice images formed using the high -angle annular dark-field detector are sensitive to atomic number and are relatively insensitive to defocus. Although high-angle annular dar k-field lattice imaging appears to be simple, scattering into the high -angle detector can only be approximately described by an incoherent i maging model.