First-principles calculations, carried out on material which contained S donors, 32SP (95%) and 34SP (5%), showed that both the neutral and ionized donors were located on substitutional sites and had weak S-Ga bonds. For the S impurity in its positive charge

state, the calculations gave gap-modes, for 32S and 34S, that were at frequencies which were close to those found experimentally. Modes within the gap were also predicted for neutral S at frequencies within a few /cm of their charged equivalents. On the other hand, the SP0 donor had a very low apparent charge and its calculated integrated absorption cross-section was only some 3% of that for the SP+ defect. These results supported an earlier explanation for the failure to detect gap modes, arising from SP0, in infra-red measurements. The importance of taking due account of the differing geometries that applied to theoretical calculations and to infra-red experiments was stressed.

Vibrational Modes of Sulfur Defects in GaP. R.S.Leigh, M.J.L.Sangster, R.C.Newman, J.P.Goss, R.Jones, V.J.B.Torres, S.Öberg, P.R.Briddon: Physical Review B, 2003, 68[3], 033304 (4pp)