  We use first-principles calculations based on density functional theory to
determine and understand the driving force of the observed stacking fault
expansion in SiC. We verify the suggestion based on recent experiments that the
free energy difference between the faulted and the perfect structures is
responsible for this expansion, and show that its origin lies in a large
entropy associated with soft vibrational modes of the faulted SiC structure.
These soft modes involve shearing of SiC on a long length-scale and are absent
in related group IV semiconductors, such as Si, Ge and C.
