Experimental studies of AlN and AlGaN deposition in a 3 x 2 '' Thomas Swan close-coupled showerhead reactor have revealed the effect of growth rate reduction at enhanced ammonia and group-III flow rates. Detailed modeling was made of gas-phase chemistry, taking into account parasitic reactions as well as formation and growth of nanoparticles. The kinetics of interactions between trimethylaluminium (TMAl) and ammonia explains the presence of nanoparticles at high ammonia/TMAl flow rates, and their absence at low ammonia/high TMAl or high ammonia/low TMAl flow rates. It is shown that the growth efficiency can be increased by reduction of the carrier gas flow rate at relatively low V/III ratios, or by total reactor pressure decrease. (c) 2005 Elsevier B.V. All rights reserved.