The morphology of compressive InxGa1-xAs/In0.52Al0.48As layers grown on (100)‐InP substrates by molecular beam epitaxy was observed by transmission electron microscopy. A preliminary analysis of the network of misfit dislocations at the interface in layers with a thickness of 0.5 μm and xIn between 54% and 63% led to a further study of the onset of stress relaxation for layers with composition xIn=60% and thickness ranging from 5 to 25 nm. A critical thickness was found for plastic relaxation at 20 nm≪tc≪25 nm. Following a model of excess stress, a mechanism for the nucleation of dislocations according to the sequence 90°partial→60°perfect→30°partial is proposed. © 1995 American Institute of Physics.