XRDT AND TEM STUDY OF DEFECTS AND POLYTYPISM IN NATURAL MOISSANITE AND SYNTHETIC SIC CRYSTALS

Structural defects and degree of order of natural and synthetic moissanite have been investigated by XRD Topography and TEM. XRDT analyses of synthetic 6H-SiC wafers allowed to study extended defects and to identify and localize coalescence of polytypes. The observed linear defects are micro-channels and dislocations. Axial screw dislocations, either parallel or slightly inclined to the c-axis, suggest that the growth mechanism for the bulk crystals was mainly by spiral growth. Moreover, the study of line orientations of the dislocations allow to reconstruct the growth sector evolution of the sample. Therefore, the convex shape of the growing surface is attained by the development of growth sectors (10l) neighbouring growth sector (001). The coalescence of a thin lamella of a different polytype has been also localized and investigated. The contrast analysis and the diffraction pattern of the lamella are consistent with a 15R-SiC crystal. Such coalescence indicates local variations of growth conditions. TEM images and selected area electron diffractions (SAEDs) strongly differentiate natural from synthetic samples. SAED patterns with [010] incidence of natural crystals are consistent with the 6H polytype and do not show streaks along the [001] stacking direction. Synthetic samples are comparatively much more disordered. Conventional images show high density of (001) faults, not observed in natural samples. Consistently, SAED patterns of the [100] zone are streaked along c*. High resolution (HR) imaging shows that synthetic samples mainly consist of (001) stacking sequence described as (32)3. Locally mixed stacking sequence described by notation 23(3233)5, probably referred to a long period polytype, are present.