Stability of superlattices under pressure

Under pressure, covalent semiconductors transform to more closely packed metallic (or small band gap) structures, with large reductions in volume ~15-20%. Recent experiments on AlAs/GaAs epitaxial multilayers have revealed that the analogous phase changes in superlattices exhibit several remarkable properties. Depending on layer width, transitions may occur: a) discretely within individual AlAs or GaAs layers, b) collectively within many similarly composed layers, or c) within the entire superlattice as a whole. Furthermore, the zincblende phase of AlAs can be superpressed far above its bulk transition threshold; this superpressing is greater in thinner layers and is limited by the stability of GaAs. These properties reflect the importance of the epitaxial interface for the total energy of a superlattice. A model is presented that considers the energy competition between destroying the interface by producing misfit defects, or preserving it by forming strained layers (in the high pressure phase.) Recent total energy calculations by Martin suggest that, for sufficiently thin layers, one of several novel strained-layer metal/semiconductor structures could be stable at high pressure. Conditions for creating such structures in various materials systems, and for retrieving them metastably at 1 atm., are discussed.