InGaN films with up to 19% indium composition are demonstrated for the first time by NH3-based MOMBE. Significant variations in alloy composition, growth rate, and III-alkyl desorption with substrate temperature, V/III, and Ga/In are reported. Desorbing III-alkyl species in the presence of TMIn are identified as predominantly III-methyl species resulting from mixed alkyl interactions at the growth interface. The results indicate that while reduced temperature growth is possible with NH3, growth is N-limited and control of active N, In/In-methyl surface coverage, and III-alkyl desorption set an upper limit to indium incorporation. Moreover, these three interdependent variables provide sensitive, exploitable mechanisms for intentional and controllable lateral composition inhomogeneity enabling novel three-dimensional device structures. (C) 2008 Elsevier B.V. All rights reserved.