Step flow and polytype transformation in growth of 4H-SiC crystals

4H-SiC crystals containing polytype defects are investigated by optical microscopy, atomic force microscopy, and Raman scattering, aiming at understanding the mechanism of polytype transformation during growth processes. It is observed that the crystal surfaces around the facet are uneven and contain many macroscopic triangular domains, consisting of wide triangular terraces and giant macrosteps. Nucleation and growth on the wide terraces are demonstrated to be responsible for the polytype transformation. A possible polytype transformation mechanism is put forward, which can explain the stabilizing effect of nitrogen on 4H-SiC growth.