Quantum dots in strained layers-preventing relaxation through the precipitate hardening effect

The internal strain in epitaxial layers due to lattice misfit has long been recognized as a limiting factor in the design of semiconductor structures. In strained layer structures above a critical thickness h(c), this strain is relaxed by the introduction of misfit dislocations. Here, we show that the interaction between the strain fields of a self-assembled quantum dot and a dislocation can lead to a threading dislocation being trapped, or pinned, by the quantum dot. The strength of this interaction is always larger than the force exerted on the dislocation by a surrounding layer with lower misfit strain. This gives a significant increase in the critical thickness for relaxation h(c)((QD)). In layers between h(c) and h(c)((QD)), threading dislocations can at best move only small distances, effectively preventing relaxation. Furthermore it is not possible to destabilize such a layer by the deposition of strained layers above it. The classical critical thickness condition thus does not apply to these structures, and they can be produced with essentially no limits to thickness before relaxation occurs. This may be expected to have significant technological consequences and allow a much wider range of structures to be produced than is possible using current strained layer design rules.