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 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.