GaN substrates grown from solution under high N-2 pressure without an intentional seeding are almost dislocation free single crystals in the form of thin (about 100 mu m) hexagonal platelets with a lateral size up to 1 cm. They were used as substrates for HVPE which allows very fast growth of GaN in the c-direction. Both n-type GaN crystals (lattice mismatch to pure GaN of 10(-4) due to high electron concentration) and high resistivity GaN:Mg crystals (lattice matched to pure GaN at RT) have been used. Most of the layers were grown on the Ga-polar (0 0 0 1) surface of the substrates. It is shown that at the applied growth conditions, near dislocation free GaN can be deposited only up to a critical thickness dependent on the type of the substrate used. The possible mechanisms of the relaxation processes is discussed on the basis of defect selective etching and micro Raman measurements for the cleaved cross-sections of the HVPE-GaN/pressure-GaN samples. It is also shown that stable (in terms of flat crystallization front and material continuity) growth of GaN on GaN substrates is possible at a rate as high as 500 mu m/h. In this way, the single crystals of GaN with thickness exceeding 2 mm were grown. These crystals were sliced along non-polar crystallographic planes. Thus, both {1 0 (1) over bar 0} and {1 1 (2) over bar 0}-oriented platelets have been obtained to be used for epitaxial growth of quantum structures by MBE. (c) 2005 Elsevier B.V. All rights reserved.