On the peculiarities of bright/dark contrast in HRTEM images of SiC polytypes

Tilt induced changes in HRTEM images obtained near [1 1 0] zone of cubic centrosymmetric Si and noncentrosymmetric SiC were studied by the use of multislice image simulations. The contrast of Si images was shown to change symmetrically with the tilt with low brightness variations while SiC demonstrates strong anisotropy along [0 0 1] direction and strong variations of contrast. The comparison of the brightness of crystals situated at twin orientation reveals that the above anisotropy leads to an anomalously high bright/dark (B/D) contrast of SiC twins appearing at extremely low misorientations from the exact [1 1 0] zone. It was also shown that the contrast of 6H–SiC can be treated to some extent as a sequence of independent twinned cubic modules. Limits of this representation were studied and the role of forbidden reflections in the image formation was clarified. A method was demonstrated to determine the polarity of the SiC crystal utilising the anisotropic tilt dependence of contrast.