Strength of strained quantum wells and other small scale structures

The strength of a material increases either when the structure is small or when only a small volume is under strain. The term ‘size effect’ covers generically all the ways in which this may happen. One manifestation of the size effect is in epitaxial growth of strained layers, for which critical thickness theory provides a satisfactory explanation. We have extended critical thickness theory to the bending and torsion of foils and wires of soft metals, and have built instruments for measuring the stress-strain curves of soft metal foils with unprecedented accuracy to test this. Experimentally, semiconductor epitaxial growth provides structures with tailored internal strain distributions, ideal for helping to understand these problems. We have found that internal strains can reduce the strength of a superlattice by a factor of two at room temperature, but on the other hand can increase the strength by a factor of a hundred at high temperature. Nanoindentation on the semiconductor structures also reveals the size effect very clearly. All of these effects are clearly related to the finite volume required for the initiation of plasticity. New data is crucial to reconciling the various theoretical approaches to these problems.