Theoretical status of diamond and its defects, excited states and atomic motion

Abstract

The key technical problems for diamond concern control: controlled growth (quality, composition, shape and texture), controlled doping (lasing species, electrically active species, redistribution and anneal of implanted species) and controlled interfaces (contacts, passivation, adhesion and friction). Theory has contributed to all these. Here I shall concentrate on electronic and motional aspects of defects. Several key phenomena involve features of the excited states, and I emphasize what can be obtained from the wealth of optical data. For diamond, optical data—involving a bound excited state—are especially fruitful; one feature to emerge is the strength and nature of the Jahn-Teller effect, which is both observed and measured through the satellite lines that it produces, so that the neutral vacancy is one of the best-understood Jahn-Teller centres. Three further features follow from examination of other properties. The first is the role of electron-electron correlation. The second is the question of whether all asymmetric defects are indeed examples of the Jahn-Teller effect. Isolated nitrogen is a key case, because of the families of nitrogen-related defects in diamond, and the important recent results of the Exeter group conclude, probably rightly, that wholly different mechanisms are involved. Thirdly, the nature and contributions to the energy in diamond and in ZnSe can be usefully contrasted, with again different reasons for the asymmetry of the singly charged cation vacancy.I shall also comment on some of the more general aspects of defect phenomena, including diffusion and reorientation behaviour. In particular, the motion of hydrogen and the muon studies need special comment, since it is likely that the diffusion mechanism is not simple classical diffusion (although this cannot yet be ruled out; further, other classical diffusion channels are available) and is perhaps closer to mechanisms now accepted for diffusion in of hydrogen in metals and indeed to reorientation of the V− centre in ZnSe.