It is shown that critical thicknesses of In0.65Ga0.35As epilayers, grown by molecular beam epitaxy on lattice matched InAlAs/InP heterostructures, are affected by surface steps and excess arsenic, i.e., nonstoichiometry, in the overgrowth. The critical thickness is reduced in the presence of steps, but is observed to increase when the excess arsenic atoms are present in the layer. It is argued that the generation of dislocation loops required for misfit dislocations is facilitated at the step edges. The resulting misfit dislocations augment the strain relaxation produced by the glide of threading dislocations in the epilayers. On the other hand, the internal strains associated with the nonstoichiometry strengthens the lattice, thus making dislocation glide difficult which delays the relaxation process. ©1995 American Institute of Physics.