Strain relaxation in GaNyAs1−y films on (1 0 0) GaAs

The strain relaxation behavior for a series of GaNyAs1−y films was studied by transmission electron microscopy, high-resolution X-ray diffraction and atomic force microscopy. Samples consisting of 200 nm thick GaNyAs1−y epitaxial layers with 0.025⩽y⩽0.065 (i.e. various misfit strain) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy at 460 °C. The GaN0.025As0.975 film shows no misfit dislocations and remains pseudomorphic well beyond the Matthews and Blakeslee's critical thickness, which can be explained by the high activation energy for a homogeneous dislocation nucleation at a smooth film surface. In samples with large N content (y>0.04) relaxation of the built-in strain proceeds through morphological changes involving formation of surface cusps, followed by stacking faults and microtwins. The surface nucleation of 90° partial dislocations is shown to be feasible at the low growth temperature in the presence of cusps due to the stress concentration. The surface roughness is anisotropic between the two 〈0 1 1〉 directions in the low strained films, and this anisotropy of the surface morphology decreases with increase in N concentration.