The coalescence of adjacent a-plane GaN stripes grown by low angle incidence microchannel epitaxy is studied to produce wide flat GaN layers with a low dislocation density. Coalescence was successfully achieved ,resulting in an extremely flat and smooth GaN layer being obtained using NH3-based metal-organic molecular beam epitaxy. Transmission electron microscopy observations revealed that lateral grown areas above SiO2 masks had a very low dislocation density, although several dislocations were observed where two adjacent stripes intersect. In contrast, there is an extremely high dislocation density in areas grown above openings, which were directly transferred from the template. Voids are occasionally formed where adjacent stripes intersect. The coalescence mechanism (including void formation) is studied. Inter-surface diffusion of Ga adatoms plays an important role in coalescence and flattening the surface. (c) 2012 Elsevier B.V. All rights reserved.