In a previous study on epitaxial h-BN growth on c-plane sapphire with AlN buffer, the polytype transition from h-BN to r-BN was suggested to be caused by stress relaxation from the lattice mismatch with the underlying AlN layer. It has been reported that the critical thickness required for the onset of dislocations in h-BN is around 4 nm. However, our experimental findings indicate that h-BN growth in region II can exceed 4 nm in thickness, while the transition from h-BN to r-BN in region I is localized. This inconsistency in growth suggests that stress relaxation from the AlN buffer is unlikely to be the only factor contributing to the nucleation of h-BN and r-BN or the transition from h-BN to r-BN polytypes. From the literature, we find several examples of how surface carbon influences the nucleation of BN polytypes. Epitaxial growth of BN films by magnetron sputtering on graphene-covered Ru(0001) substrates resulted in r-BN growth, while growth on Ru(0001) resulted in a few-layer h-BN growth. In a CVD study using ZrB2(0001)/4H-SiC (0001) as substrates for BN growth with TEB, r-BN nucleated directly on a carbon-containing interlayer formed on the ZrB2. The growth turned from r-BN to t-BN as the film thickness increased. In another CVD study using TEB and 4H-SiC substrates, TEM showed that r-BN nucleates directly, without any trace of h-BN, on the Si-rich (0001) plane , in contrast, t-BN nucleates on the carbon-rich (0001 � ) side of the 4H-SiC. From our results and these examples from the literature, we suggest that r-BN, rather than h-BN, nucleate on surfaces that contain carbon. Furthermore, our results indicate that the co-deposition of carbon species during the growth of h-BN induces a polytype transition to r-BN. A study compared the thermodynamic stability of r-BN to h-BN films at CVD conditions using van der Waals corrected first principles to indicate that the h-BN lattice is less liable to expansion compared to the r-BN lattice. In the presence of carbon, this suggests that r-BN has a higher tolerance to carbon impurities than h-BN and could explain why the h-BN growth transitions to r-BN growth when the carbon content increases.