Lattice imaging studies on structure and disorder in SiC polytypes

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Abstract
Crystal structure of a large period polytype as well as some complex polytypes like 40H, 150R, and their intergrowth structures have been determined on the basis of one dimensional lattice images and confirmed finally by establishing a match between observed and calculated X-ray intensities. Tilted beam two dimensional lattice images using selected reflections to produce chevron shaped fringes (by deriving first the imaging conditions in case of a simple structure 6H) simulating layer stacking sequence have been used to determine the structure of a new large period polytype 411R and its intergrowth structures. Critical evaluation of these techniques for structure determination has been discussed. A notable feature emerging from the lattice imaging studies is the existence of intergrowth structures causing the formation of superperiod structures (> 100 nm). Finally in this study the origin of one dimensional disorder was found as a result of either due to large periodicities or due to non-random disorder and in very few cases due to random disorder.

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