High quality Alx>0.5Ga1-xN layers were grown on (0001) sapphire substrates by metal organic chemical vapor deposition utilizing an AIN nucleation layer. The growth conditions of the nucleation layer were observed to have a large impact on both the surface morphology and the crystalline quality of the AlxGa1-xN layers. Low temperature nucleation layers with an optimum thickness of similar to 15 nm gave the best quality AlxGa1-xN layers. Additionally the annealing of the nucleation layer was seen to have a dramatic effect on the crystalline quality of the overgrown Al(x)Gal(1-x)N layers. The quality of the films as measured by atomic force microscopy and x-ray diffraction was largely independent of the V/III ratios of the AlxGa1-xN layers, although Al incorporation and growth rate were affected. Two distinct regimes were identified for the Al incorporation in the overgrown AlxGa1-xN as a function of ammonia flow rate; the Al incorporation first decreases with increase in the ammonia flow rate and then increases with a continued increase in the ammonia flow rate. It is shown that the residence time of the species in the growth system controls the amount of the precursor prereaction independently of the actual flow rates and V/III ratio affecting the Al incorporation in the solid phase. (C) 2007 American Vacuum Society.