The structural characteristics and growth regimes of AlGaN grown by ammonia-based metal-organic molecular beam epitaxy (NH3-MOMBE) on GaN templates were investigated. The NH3 utilization efficiency for the growth of AlGaN was estimated to be 2-2.5 times greater than the growth of GaN. Increasing the Al gas phase composition lead to an increase in the utilization efficiency as a result of increased NH3 catalyzation. Despite the increased pyrolysis of ammonia, AlGaN films grown at 860 degrees C had significant active species desorption, leading to slower growth rates as well as lower calculated utilization efficiencies. AlGaN films grown with constant At gas phase compositions showed an increase in the solid At composition when grown more metal-rich, because of the preferential At incorporation over Ga in N-limited growth environments. AlGaN surface morphologies became smoother with higher V/III ratios since surface pitting, which is attributed to decoration of threading dislocations, was reduced with increasing NH3 flux. (C) 2009 Elsevier B.V. All rights reserved.