Exciton-phonon interactions, exciton binding energy, and their importance in the realization of room-temperature semiconductor lasers based on GaN

Temperature dependence of the linewidths of free-exciton A and B transitions was investigated. Experimental linewidths were fitted to a theoretical model considering various interactions of excitons with phonons in addition to inhomogeneous broadening. It was shown that acoustic phonon scattering must also be considered to explain the emission linewidth broadening, in contrast to a recent report on luminescence linewidths in GaN. These exciton--acoustic-phonon interactions also explain the fast energy relaxation of free excitons to the bottom of the exciton band, which leads to generally observed short free-exciton lifetimes in GaN. The exciton--longitudinal-optical (LO) -phonon coupling constant was found to be extremely large. This was explained as being due to the Fr\"ohlich interaction and the polar nature of GaN. The binding energy of both A and B excitons was found to be 26 meV. The relevance of exciton-phonon interactions and the binding energy of free excitons in achieving room-temperature exciton-based semiconductor lasers was discussed. Though exciton--LO-phonon interaction was very strong in GaN, it was still possible to observe room-temperature excitons since the exciton binding energy is very large.