7 - Layered structures and superlattices

Summary

A number of crystal growth techniques have been developed for the production of specimens which have the form of a succession of layers. In molecular-beam epitaxy (MBE) beams of atomic or molecular species passing through ultra-high vacuum impinge on a single-crystal substrate and in the right conditions crystal growth occurs epitaxially, that is, with the crystal structures in register. In metallo-organic chemical vapour-phase deposition (MOCVD) growth occurs by deposition from a flowing vapour. Both MBE and MOCVD are used to produce semiconductor specimens in which, for example, single-crystal layers of GaAs and A1_xGa_1-xAs alternate. Metallic specimens, including those in which one or both constituents may be magnetic, are made by sputtering. A review of MBE is given by Joyce (1985). Parker (1985) and Chang and Ploog (1985) include detailed chapters on MBE as well as on the physics of the resulting specimens, while the latter also contains a chapter on MOCVD.

The growth techniques can be used to prepare specimens consisting of alternating layers of thickness d₁ of constituent 1 and thickness d₂ of constituent 2. Specimens can be prepared so that d₁ and d₂ have any value from two or three atomic spacings up to the order of 100 nm typically.