Boron nitride films were grown on 2-inch sapphire substrates using the MOCVD technique. The growth was performed using a pulsed source injection mode with triethylborane (TEB) and ammonia (NH3) with a high V/III ratio, which corresponds to the self-terminated growth mode. Two different carrier gases, H-2 and Ar, were used to investigate their influence on the growth mechanism, while all the other growth parameters were kept the same. The BN films thus obtained were examined using XRR, SIMS, Raman spectroscopy, ATR spectroscopy, SEM and AFM. A difference was found in the surface roughness. The BN films grown in an H-2 atmosphere had a surface roughness three times smoother than those grown in Ar. However, the main difference was brought to light by SIMS measurements. The SIMS measurements revealed that growth under an H-2 flow leads to a reduction in carbon concentration by more than four orders of magnitude. It was also shown that the boron distribution in the BN film grown under an H-2 flow is very uniform. It is discussed that this mechanism is related to the reactions between TEB and NH3 in the presence of H-2. The elimination of carbon by using hydrogen reveals its role in BN growth, which is indeed essential.