We propose a new origin of unintentional gallium (Ga) incorporation during the epitaxial growth of AMIN layers. We observed substantial amount of Ga in AlInN layers on GaN-free underlying layers and wafer carrier grown by metalorganic chemical vapor deposition, even though the Ga precursor was not introduced during the growth. The Al(Ga)InN layers were characterized by high-resolution X-ray diffraction and Rutherford backscattering spectrometry. The mole fraction of Ga in the Al(Ga)InN layers showed strong dependence on conditions of the growth chamber, making it difficult to maintain controlled alloy compositions only by growth process parameters. Also, Ga incorporation was not observed under the same growth parameters and conditions, when the indium (In) precursor was absent. The formation of In-Ga eutectic system between metallic Ga from the deposition on the chamber surrounding surfaces and adsorbed In is suggested as a possible pathway for the Ga incorporation. Finally, we performed an intentional chamber coating process prior to growth of AlInN to achieve repeatable Al(Ga)InN growth with stable alloy compositions. (C) 2013 Elsevier B.V. All rights reserved