GaN epitaxial layers were grown on Si (111) substrates by metal organic chemical vapor deposition.Carbon concentrations in the films grown in different ambients were measured by secondary ion mass spectrometry.The results show that the carbon incorporation is strongly dependent on the H2 flow rate when the NH3 flow rate is small,but insensitive to the H2 flow rate when the NH3 flow rate is sufficient large.We conclude that H2 can inhibit the dissociation of NH3 and result in a less active N source;an insufficiently active N source causes more N vacancies,which enhances carbon incorporation.