The effect of additional HCl flow on the growth of a-plane GaN layers on r-plane sapphire by hydride vapor phase epitaxy was investigated. Upon increasing the additional HCl flow rate, the surface roughness of the a-plane GaN layers, as measured by atomic force microscopy, reduced. The crystal qualify of a-plane GaN, however, deteriorated, as confirmed by high stacking fault density observed by transmission electron microscopy, relating to the increased nuclei density and mosaicity and a high full width at half maximum in the omega-scan X-ray rocking curve. These observations were attributed to the large difference of growth rare and etch rare along c-direction, and m-direction of a-plane GaN, which were originated from the surface energetics of the crystallographic planes of GaN. (C) 2014 Elsevier B.V. All rights reserved.