  In this study we report on a novel two-step epitaxial growth technique that
enables a significant improvement of the crystal quality of nitrogen-polar GaN.
The starting material is grown on 4{\deg} vicinal sapphire substrates by metal
organic vapour phase epitaxy, with an initial high-temperature sapphire
nitridation to control polarity. The material is then converted into a regular
array of hexagonal pyramids by wet-etch in a KOH solution, and subsequently
regrown to coalesce the pyramids back into a smooth layer of improved crystal
quality. The key points that enable this technique are the control of the array
geometry, obtained by exploiting the anisotropic behaviour of the wet-etch
step, and the use of regrowth conditions that preserve the orientation of the
pyramids' sidewalls. In contrast, growth conditions that cause an excessive
expansion of the residual (000-1) facets on the pyramids' tops cause the onset
of a very rough surface morphology upon full coalescence. An X-ray diffraction
study confirms the reduction of the threading dislocation density as the
regrowth step develops. The analysis of the relative position of the 000-2 GaN
peak with respect to the 0006 sapphire peak reveals a macroscopic tilt of the
pyramids, probably induced by the large off-axis substrate orientation. This
tilt correlates very well with an anomalous broadening of the 000-2 diffraction
peaks at the beginning of the regrowth step.
