GaN films were grown on c-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE) with a pulsed flow of HCl over Ga metal. NH3 gas supply was controlled to flow in a constant rate or in a modulated way. The surface morphology dependence of these films on the various flow modulation schemes was investigated. Depending on the duty cycle of NH3 flow, the surface morphology of GaN films was sensitively modified. This sensitive response of surface morphology of GaN films to the flow modulation was attributed to diffusion efficiency variation of Ga species under different gas environment. Under proper modulation conditions, flattened top-surface morphology of nucleated domains was found to be obtained. (C) 2009 Elsevier B.V. All rights reserved.