Thick high quality gallium nitride (GaN) layers presenting a dislocation density reduced to 6×106 cm−2
were grown by hydride vapour phase epitaxy (HVPE). Scanning electron microscopy (SEM) characterizations, X-ray double diffraction (XRD) measurements, photoluminescence and reflectivity experiments, both at 4.5 K have been carried out to reveal the optical and the structural properties of the GaN epilayers. The strain relaxation was studied as a function of increasing the thickness. Special emphasis has been placed to the control of the parasitic nucleation on the reactor wall upstream the substrate, which consistently appears for long-time growth experiments. The amount of the parasitic deposit was quantitatively assessed by theoretical modelling. Attention was paid to accurate characterizations of 450 μm thick GaN layers obtained via thickening through a regrowth step after growth interruption and strain relaxation on cooling the templates.