Our results show that the carbon, originating from the employed organoboranes, influences the epitaxial growth conditions for h-BN and r-BN films. There is no literature available that compares TMB and TEB for BN epitaxy. However, a study was conducted to compare h-BN films grown from TMB and B2H6, which showed that TMB-grown BN films contained 60 times more carbon. This study supports our results that dissociating organoboranes bring carbon to the growing BN surface, which deteriorates the epitaxial growth conditions for h-BN and r-BN. Regarding the organoboranes used in our study, TMB is reported to react with H2 to release CH4 and BH3 in up to three consecutive gas phase reactions, all with negative Gibbs free energy. The more carbonrich organoborane, TEB is reported to β-eliminate one ethyl ligand as C2H4 and approach the surface as B(C2H5)2H. From the results above, TEB is expected to provide more carbon to the BN surface, while TMB is expected to produce more carbon in the gas phase. A higher BN growth rate has been reported when using TMB vs TEB. Additionally, we grew thicker crystalline BN films using TMB vs TEB, even after considering the difference in growth rate. This suggests that surface poisoning from carbon accumulation occurs faster for the TEB process, compared to the TMB process. As TEB brings a more carbon-rich species to the surface, faster accumulation would deteriorate the epitaxial growth conditions at lower BN thickness. For the epitaxial BN layers, the well-ordered h-BN and the triangular pyramidal shaped r-BN structures, a low carbon content is observed. Further evidenced by the thicker h-BN growth observed for region II for the films grown using TMB vs TEB as seen in S2-S4.