AlxGa1?xN/GaN hetero-structures were grown on SiN-treated (00.1) sapphire substrate by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE). Characterization of the grown structures was performed in-situ by laser reflectometry and ex-situ by secondary ion mass spectrometry (SIMS) measurements. Al SIMS profile showed some tailing to GaN layers which is associated to Al diffusion. The trimethylaluminium (TMA) effects on the growth rate, Al composition and Al diffusion coefficient were discussed. Al diffusion coefficients (DAl) into GaN were calculated. The results suggest that Al diffuses faster near the AlN/GaN interface.