Crystal quality of GaN films grown by hydride vapor-phase epitaxy on porous and non-porous SiC substrates is studied. The effect of reduction of threading dislocation density in the films grown on porous substrates is discussed. We suggest that at the early stages of the growth, the porous substrate is capable to re-distribute the mismatch strain in the heterostructure. This results in specific mechanism of strain relaxation in the epitaxial film, which occurs through self-organization of a 'superlattice' of planar defects. At the later stages of the growth, these defects hinder the propagation of threading dislocations. (C) 2007 Elsevier B.V. All rights reserved.