The unintentional carbon doping concentration of GaN films grown by low pressure metal organic chemical vapor deposition （LP-MOCVD） depends strongly on the growth rate. The concentration of carbon is varied from 2.9 × 1017 to 5.7 × 10^18 cm-3 when the growth rate increases from 2.0 to 7.2 μm/h, as detected by secondary ion mass spectroscopy. It is shown that the presence of N vacancies give rises to high carbon concentration. We show that a reduction of the carbon concentration by one order of magnitude compared to the regular sample with nearly same growth rate can be achieved by operating at an extremely high NH3 partial pressure during growth. The intensity ratios of yellow and blue luminescence to band edge luminescence in the samples are found to depend significantly on carbon concentration. The present results demonstrate direct and quantitative evidence that the carbon related defects are the origin of yellow and blue luminescence.
