We investigated the properties of a GaN epilayer grown by metalorganic vapour phase epitaxy on a c-plane bulk GaN substrate obtained by ammonothermal growth. X-ray diffraction measurements showed that the epilayer and substrate were fully relaxed, had a miscut angle of 0.3 +/- 0.05 degrees towards m and had omega rocking curve width values of 20-30 arcsec, limited by the instrumental broadening. Scanning capacitance microscopy data of the sample in cross-section indicated that the substrate had n-type conductivity with a carrier concentration of at least 10(19) cm(-3). Combined optical Nomarski microscopy, atomic-force microscopy and scanning electron microscope-cathodoluminescence studies showed the presence of large hexagonal pyramids on the surface, each associated with one or two dislocations with a screw-component threading from the substrate. This observation leads us to calculate a lower limit of the threading dislocation density of 3 x 10(2) cm(-2). We predict that the formation of such hexagonal hillocks during epitaxy can be avoided with a slightly larger miscut angle of 0.4 degrees or 0.5 degrees. Another type of defect observed were ridge-like surface structures with narrow arrays of edge-type threading defects with a local density of 10(9) cm(-2). However, the absence of threading defects below the regrowth interface at a ridge suggested that this type of structure is linked to (polishing) damage to the substrate surface and is therefore rated as an avoidable problem. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved