Scanning transmission electron microscopy of thin specimens.

The intensity distributions in bright field and dark field images of thin specimens obtained in scanning transmission electron microscopy are evaluated in terms of the phase of object approximation for coherent and partially coherent illumination. Previous treatments, based on a weak phase object approximation are shown to contain unwarranted assumptions in some cases, resulting in predictions of limited validity. The probable errors due to the neglect of higher order terms in power series expansion and in the assumption that the dark-field signal from an annular detector is proportional to the total elastic scattering are evaluated. Questions of optimizing the signal intensity and contrast are examined by considering the detector configuration in relationship to the form of the convergent beam diffraction pattern formed in the detector plane and the implications for the use of diffraction pattern information to enhance the imaging process are considered.