On the influence of crystal orientation on the high resolution image contrast of polytypes

High resolution electron micrographs of polytypes reveal under suitable experimental conditions the stacking sequence of the crystal structure. However, in these experimental images an extra intensity modulation is frequently observed as is the case for hexagonal polytypes and 18R-type structures; this is intuitively not expected on the basis of the structure for perfectly oriented crystals. Moreover, the kinematically forbidden reflections are much stronger than predicted by computer simulations assuming perfect orientation of the crystal. It will be shown that these anomalies can be associated with a small misorientation of the crystal foil.