ELECTRON-MICROSCOPY OF INTERFACES BETWEEN TRANSFORMING POLYTYPES IN SILICON-CARBIDE

High resolution transmission electron microscopy, utilizing tilted illumination lattice imaging techniques, has been used to examine the structure of interfaces between transforming polytypes in cubic silicon carbide samples subsequent to annealing heat treatments. Though structure projection images have not been possible, detailed observation of the twinned cubic, 3C/6H and 3C/4H interfaces have been made including stacking sequence information observed with ∼ 0·25 nm lattice fringes. In contrast to the apparently strain‐free coherent conditions found at twinned cubic and 3C/6H interfaces, 3C/4H interfaces were observed to be coherent but with a short‐range strain field periodic over every twelve layers of the cubic structure. The 3C/6H transformation has been clearly observed to initiate at twin interfaces and to proceed by the propagation of steps of unit‐cell height along the coherent interface. The implications of these observations for phase transformations in silicon carbide, together with the necessary associated structural rearrangements, are discussed. Finally, some preliminary observations (utilizing impurity‐sensitive SEM contrast, together with the optical examination of thin foils in polarized light) concerning possible impurity redistributions and anomalous grain growth effects during polytypic phase transformations are presented. 1979 Blackwell Science Ltd