The Story So Far ….
A significant gap in our knowledge of the understanding of polytypism in silicon carbide exists, caused by the lack of experimental data on the spatial distribution of polytype coalescence and in particular the regions between adjoining polytypes. Few observations detailing the relative location of polytypes and the exact position of one-dimensional disorder and the existence of long period polytypes with respect to them have been reported, see for example Takei & Francombe 1967 [1].
It has been shown by Fisher & Barnes 1990 [2] that it is possible to
accurately locate the position of polytypes along the crystal c-axis using the
method of synchrotron edge topography. Using this methodology a unique
database has been compiled from the results of a survey, to date, of 135 such edge profiles distinguishing polytypes from regions of disorder.
With improved resolution and the observation of new features at polytype boundaries Barnes et.al. 1991 [3], as well as a broad classification scheme Kelly et al. 1995 [4] the authors have been able to describe polytype coalescence in terms of a non-degenerate sandwich model.
This model incorporates one-dimensionally disordered layers, long period polytypes and defect bands as essential ingredients. These features are illustrated here with some examples.