Exotic structures of tetrahedral semiconductors

Recent experimental and theoretical studies of exotic forms of tetrahedrally coordinated semiconductors are reviewed. These unusual phases are synthesized as long-lived metastable forms of the elemental semiconductors silicon and germanium by the application and subsequent removal of high pressure. Rather than being simply crystallographic oddities, the bonding arrangements in these phases shaw many similarities to those found in amorphous semiconductors. As a result, these dense structures have been used as so-called 'complex crystal' models for the amorphous state. Advances in experimental and computational techniques have recently allowed for detailed study of the structural, electronic and vibrational properties of these phases to be made under variable temperature and pressure conditions. In view of the considerable difficulties associated with performing theoretical studies of non-crystalline solids, the BC8 and ST12 structures are useful in that an understanding of their properties provides insight into the essential physics of amorphous tetrahedral semiconductors.