We investigated the effect of H-2 ratio in carrier gas on oxygen concentration, crystallinity and threading dislocation density (TDD) in GaN layers synthesized from Ga2O vapor and NH3. SIMS analysis revealed that the oxygen concentration in GaN layers decreased with increasing H-2 ratios, and achieved 8.72 x 10(17) atoms/cc with a growth rate of 50 mu m/h when the H-2 ratio was 800%. The full width at half maximum (FWHM) and TDD decreased with increasing H-2 ratios. To explain these results, we explored the growth mode at the initial growth stage in H-2 ambient, and found that the nucleation rate of the GaN crysial decreased in H-2 ambient, resulting in the improvement of crystallinity. (c) 2014 Elsevier BM. All rights reserved