The strain relaxation behavior for a series of GaNyAs1, films "Its studied by transmission electron microscopy, high-resolution X-ray diffraction and atomic force microscopy, Samples consisting of 200 nm thick GaN1As1-y epitaxial layers with 0.025 <= y <= 0.065 (i.e. various misfit strain) Were grown on (100) GaAs substrates by molecular beam epitaxy at 460 C. The GaN0.025As0.975 film shows no misfit dislocations and remains pseudoniorphic well beyond the Matthews and Blakeslee's critical thickness. which can be explained bv the high activation energy for a homogeneous dislocation nucleation at a smooth tilm surface. In samples with large N content (gamma > 0.04) relaxation of the built-in strain proceeds through morphological changes involving formation of surtace cusps, followed by stacking faults and microtwins. The surface nucleation or go partial dislocations is shtmn to he feasible at the low growth temperature in the presence of cusps due to the stress concentration. The surface rougghness is anisotropic between the two < 011 > directions in the low strained films, and this anisotropy Ofthe Surface rnorpholog decreases with increase in N concentration. (c) 2005 Elsevier B.V. All rights reserved.