Metastable defects in silicon: hints for DX and EL2?

A review is given of defects that display metastability in silicon, with emphasis on those that have been identified and the various mechanisms that they reveal for the phenomenon. Pair defects described include interstitial-iron-substitutional-group-III-acceptors and ones formed by interstitial carbon with substitutional group V donors or substitutional carbon. Interstitial hydrogen, boron and silicon and substitutional nitrogen and oxygen are taken as examples of isolated single-atom defects that display on-centre to off-centre instabilities. It is argued that this single-atom instability can be understood in terms of a predictable Jahn-Teller effect and that this concept may provide useful insight into the DX and EL2 phenomena in the III-V materials and their alloys.