Abstract
It is currently possible to grow nearly perfect strained-layer semiconductor heterostructures. Experimental evidence shows that misfit dislocation sources become active almost immediately as the layer thickness exceeds the equilibrium critical thickness. These dislocation sources do not seem related to obvious causes, such as threading dislocations or homogeneous nucleation at surface structures. It is argued here that the dislocation sources are inhomogeneous in nature and result from local stress concentrations near the surface.