Towards a unified view of polytypism in silicon carbide

Abstract

In this study, a detailed survey has been made of 200 silicon carbide crystals grown by the modified Lely technique. All 200 crystals were examined using, at minimum, the combination of X-ray oscillation photography and one synchrotron topograph. This has now produced a reliable and unique database on polytypism, one-dimensional disorder, defect content, and in particular their spatial relationships. Over half (114) the crystals were multi-polytypic, some 37 displaying complex polytype combinations of which 16 included a long-period phase. For some 32 crystals, where shape and size permitted, synchrotron edge-reflection topographs could be obtained leading to depth profile models of the polytype content; 9 of these have shown up a very interesting class of configuration, the idealised form being termed a ‘sandwich’ in which a longer-period polytype layer inside the crystal is surrounded each side by shorter-period polytypes: the reverse configuration, or long-period polytype layers existing on their own, were not found. The various findings from this survey are discussed in terms of their implications for polytype formation/transformation, one-dimensional disorder, and crystal-growth conditions. The new data are then reconciled with previous studies (mainly bulk observations) and current theories in terms of the effect of temperature, supersaturation, cooling rate, impurity content and the presence of nucleation sites.