Vibrational modes of sulfur defects in GaP

Abstract

First principles calculations carried out on GaP containing sulfur donors ${}^{32}{\mathrm{S}}_{\mathrm{P}}$ (95%) ${\mathrm{and}}^{34}{\mathrm{S}}_{\mathrm{P}}$ (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 ${}^{32}\mathrm{S}$ ${\mathrm{and}}^{34}\mathrm{S}$ at frequencies close to those found experimentally. Modes within the gap are also predicted for neutral sulfur at frequencies within a few ${\mathrm{cm}}^{\ensuremath{-}1}$ of their charged-state counterparts. However, the ${\mathrm{S}}_{\mathrm{P}}^{0}$ donor has a very low apparent charge (i.e. oscillator strength), its calculated integrated absorption cross section being only $\ensuremath{\sim}3%$ of that for the ${\mathrm{S}}_{\mathrm{P}}^{+}$ defect. These results support an earlier explanation of the failure to detect gap modes from ${\mathrm{S}}_{\mathrm{P}}^{0}$ in infrared measurements. Calculated and observed apparent charges for the ${\mathrm{S}}_{\mathrm{P}}^{+}$ 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.