It was recalled that this material remained an insulator even when large concentrations of P were incorporated into it. It was suggested here that this was so because of the formation of P-vacancy complexes which were deep acceptors and compensated any donor. The complex was analyzed by using a first-principles cluster method. It was concluded that, in the ionized state, the defect was diamagnetic and could not give rise to any internal optical transitions; although broad-band donor-acceptor transitions were expected, and observed, in material which was co-doped with N.

R.Jones, J.E.Lowther, J.Goss: Applied Physics Letters, 1996, 69[17], 2489-91