This paper reports a study of the effect of NH3 flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH3 flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (10 (1) over bar0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH3 flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (10 (1) over bar0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy. (c) 2009 Elsevier B.V. All rights reserved.