Vibrational modes of sulfur defects in GaP

First principles calculations carried out on GaP containing sulfur donors ³²S_P (95%) and ³⁴S_P (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.