GaNAs layers were grown on 3 mu m-thick GaN epilayers with tertiarybutylarsine (TBA) for As incorporation. It was found that surface of even 2 nm-thick GaNAs was already granular on step-flow pattern of GaN template, which means that GaNAs growth mode was three-dimensional from the beginning. As incorporation in GaN was mainly determined by NH3 decomposition and As desorption. NH3 decomposition was dominant process to suppress incorporation of As adatoms on N-sites of GaNAs in case of low growth temperature and high As concentration over similar to 5 x 10(19) cm(-3). On the other hand, As desorption became more critical to limit As incorporation in case of high growth temperature and low As concentration below similar to 5 x 10(19) cm(-3). In this case, As adatoms were incorporated on Ga-sites of GaNAs and As double donor-related emission were observed from 900 degrees C GaNAs. (C) 2010 Elsevier B.V. All rights reserved.