Bound Excitons in GaP

Three sharp lines $A$, $B$, and $C$ are seen in absorption and fluorescence near the band edge of GaP at 20°K and below. The lines are associated with excitons bound to defects. The Zeeman splitting of the lines allow the $g$ values of the holes and electrons to be determined. $A$ and $B$ both arise from an exciton bound to an ionized donor, the energy difference between $A$ and $B$ coming from electron-hole $\mathrm{jj}$ coupling. Both lines cooperate strongly with phonons, and this together with the splitting indicates a tightly bound state, yet it lies very close to the indirect exciton. Consequently $A$, $B$ involves another exciton, probably made from an electron and hole both at k=0. Line $C$ is produced by an indirect exciton bound to a neutral donor, probably sulfur. Despite the possibility of valley-orbit splitting of the conduction band, the spectrum of $C$ is quite simple.