Near atomic images of twin boundaries and stacking faults in a 15R SiC sample: twin law and growth mechanism

In recent years, the study of the main polytypes of Silicon Carbide and their synthetic homologous have received renewed interest in the material science field due to the physical and electrical properties that enable SiC to be employed in high temperature, high power and high frequency electronic devices. The main focus of numerous previous studies has been the characterization of the grown-in structural defects that can damage device performance [1]. However, from a crystallographic point of view, several questions such as polytype stability, stacking fault and twinning formation, foreign polytype inclusions and interface between polytypes in syntactic coalescence, need still to be resolved taking into account the low stacking fault energy that characterizes the most important SiC polytypes. Hence, in this work an original contribute has been offered in analysing by means of HRTEM the local stacking microstructure of twinning, macrosteps on the twin boundaries and stacking faults found in a 15R twin –related lamellae [2]. HR images revealed that the perfect structure of 15R polytype (point group R3m), (23)3 was locally interrupted by numerous adjoining stacking faults parallel to (0001) with stacking of the coupled (22) and (33) bilayers superimposed on the twin boundaries. Instead, the twin boundaries showed a zig-zag pattern (32) which passed to (23) through an isolated (33), 6H like, sequence. The electron diffraction patterns taken exactly above both the twin interfaces indicate classifying of the twin found in this study as a “friedelian” reticular merohedric twinning. However, two indistinguishable twin operations matched the observed features: a reflection through rational plane (0001) that generates the ’m2’ twin point group, and 180°-rotation around [0001] with composite symmetry 6’mm’. Since individual Si and C atoms and even the SiC bilayer polarity could not be established from these HR images, the real twin law was deduced by taking into account that the coherent structural match at the interface. From a geometrical point of view, the stacking sequence will match that which was experimentally observed if a 180° rotation around [0001] is visualized in terms of only some bilayers rather than considering the rotation of the whole sequence of the 15R polytype. This type of operation produces the insertion of (33), a one cell thick 6H like sequence, while preserving the nearest-neighbour geometric relationship between adjacent SiC bilayers and their polarity. Similarly, the stacking faults can be explained by imaging a 180°-rotation around [0001] of a single bilayer along a slip plane. Hence, it follows that the formation of both, stacking faults and twins, should rely on similar processes. Actually, the growth mechanism was achieved by adding material to the self-perpetuating steps formed at the sites of a single or multiple cooperating screw dislocation and a lateral step flow [1]. The local increase of nitrogen dopant content in the growth surface modified the velocity of the steps, determining step bunching and the formation of macrosteps by the coalescence of multiple SiC growth steps. When the steps were too high, the danger of step bunching increased, macro-steps formed and local super-saturation changes occurred resulting in stacking fault and twinning formation.