RESOLUTION AND CONTRAST OF CRYSTALLINE OBJECTS IN HIGH-RESOLUTION SCANNING TRANSMISSION ELECTRON MICROSCOPY.

The image contrast and specimen resolution of point scatterers embedded in thick crystalline material are calculated for the high-resolution scanning transmission electron microscope. A static periodic potential is assumed for the crystal, thereby neglecting inelastic scattering. Owing to the reciprocity theorem, the results are also valid for the fixed-beam electron microscope. To account for thick crystalline layers the propagation of a convergent incident electron wave within the crystal is calculated, employing the dynamic theory of electron diffraction. The analytical formulas are evaluated numerically for different thicknesses of crystalline aluminum layers. The results indicate that the crystalline foils can be considered as fiber-optic plates for electrons. When the atom rows are parallel to the axis of the incident beam, the electrons channel along the atom rows, so beam broadening is largely avoided for thin crystals. At the defocus of the objective less is chosen optimally, the beam broadens only in proportion to the square root of the crystal thickness.