Valley-orbit splittings for the donor states in GaP

We report on a calculation of the role of the "camel's-back" structure in determining valley-orbit splittings for the low-lying states in GaP:S. We find a splitting of about 49 meV between the lowest-lying state of ${\Gamma }_1\left(A_1\right)$ symmetry and that of ${\Gamma }_3\mathrm{}\left(E\right)\mathrm{}$ symmetry. The relative oscillator strength ratios for the two-electron transition luminescence lines are estimated. Both the line positions and oscillator strength ratios for these states are in reasonably good agreement with the experimental data.