Given that in our results, the epitaxial r-BN on h-BN film grew to a thickness of only 70 nm in 60 minutes for the TMB process, rendering a growth rate of 1.16 nm/min and supplemented by the study analyzing the time evolution for the surface morphology of BN films , we conclude that the carbon accumulation eventually poisons the surface, terminating further BN growth. This suggests that either our BN film stopped growing much before the end of the 60 min CVD process or nucleated at a gradually decreasing rate as carbon accumulated on the surface. Support for such surface poisoning by carbon is also found in a study analyzing the influence of growth temperature on the crystallinity of BN in CVD with TEB on c-plane sapphire with an AlN buffer. The crystallinity then improved up to a temperature of 1500 °C but decreased significantly at 1600 °C. The suggested explanation was that the crystallinity was affected by the change in the crystallinity of the AlN buffer layer. With support from carbon poisoning in this study, we argue that at higher temperatures, TEB should undergo a higher degree of thermal decomposition, releasing more carbon species, which are more active as CVD precursors at more elevated temperatures, increasing the potential of TEB to bring carbon species to the BN surface.