Vibrational modes of sulfur defects in GaP
First principles calculations carried out on GaP containing sulfur donors 32SP (95%) and 34SP (5%) show that both the neutral and ionized donors are located on substitutional sites and have weak S-Ga bonds. For the sulfur impurity in its positive charge state the calculations give gap modes for 32S and 34S at frequencies close to those found experimentally. Modes within the gap are also predicted for neutral sulfur at frequencies within a few cm−1 of their charged-state counterparts. However, the S0
p donor has a very low apparent charge (i.e. oscillator strength), its calculated integrated absorption cross section being only similar to 3% of that for the S+
p defect. These results support an earlier explanation of the failure to detect gap modes from S0
p in infrared measurements. Calculated and observed apparent charges for the S+
p donor are compared, and the importance of taking due account of the different geometries that apply to the theoretical calculations and infrared experiments is emphasized.