Structural Defects and Polytypism in Moissanite and Synthetic SiC Crystals

In recent years, defect characterisation of SiC polytypes has been developed significantly, due to the increasing importance of such polytypes for high temperature, high-power and highfrequency electronic devices [1]. On the other side, moissanite- SiC is a rare mineral that was discovered in 1905 within a meteorite and only recently as a rock-forming mineral (8.4 vol%) in one unique specimen of a terrestrial volcanic rock [2]. Besides it has been found in kimberlitic pipes and can provide some light on the redox conditions of the Earth Mantle. Finally SiC is a particularly interesting species of presolar grain because the polytype distribution can be related to different parameters of the expanding stellar atmospheres of asymptotic giant branch-AGB-stars. In the frame of a research program in progress on SiC, growth defects of bulky gem-crystals of 6H-SiC were studied by X-ray Diffraction Topography-XRDT and it was found that the main types of defects were dislocations parallel and perpendicular to the growth directions, and micro-channels [3]. This piece of work is devoted to the study of structural defects and degree of order of natural and synthetic moissanite. Natural and synthetic samples have been investigated by XRD Topography and TEM, focusing on the relationships between defects and growth conditions. XRDT analyses of synthetic 6H-SiC allowed the characterization of dislocations and channels to be made and other polytypes for further investigation by TEM to be localized. All studied sample are characterized by the presence of linear defects, dislocations and micro-channels, uniformly distributed in the crystal. Moreover samples grown by means of Physical Vapour Transport-PVT method, show the same linear defects with different character, strictly related to growth conditions. TEM images and electron diffractions (EDs) strongly differentiate natural from synthetic samples. ED patterns with [100] incidence of natural crystals are consistent with the 6H polytype and do not show streaks along the [001] stacking direction. This result is confirmed by structure refinement from area detector single crystal X-ray data [4]. Synthetic samples are comparatively much more disordered. Conventional images show high density of (001) faults, not observed in natural samples. Consistently, ED patterns of the [100] zone are streaked along c*. Atomic resolution imaging shows that synthetic samples mainly consist of (001) stacking sequence described as (32)3 [5]. Locally mixed stacking sequence described by notation 23(3233)5, probably referred to a long period polytype, are present. [1] Neudeck, P. G., Journal of Electronic Materials, 1995, 24, 283. [2] Di Pierro S., Gnos E., Grobety B. H., Armbruster T., Bernasconi S. M. and Ulmer P., Am. Mineral., 2003, 88, 1817. [3] Agrosì G., Fregola R.A., Monno A., Scandale E., Tempesta G., Materials Science Forum, 2005, 483-485, 311. [4] Capitani G.C.*, Tempesta G., Di Pierro S., Scandale E., 2006, This congress. [5] Zhdanov G.S., Minervina Z.V., Compt. Rend. Acad. Sci. URSS 1945, 48. Industrial mineralogy 23rd European Crystallographic Meeting, ECM23, Leuven, 2006 Acta Cryst. (2006). A62, s64 Pag