Lasing in ZnSe/ZnS0.18Se0.82 superlattices

We investigated lasing in highly excited ZnSe/ZnS0.18Se0.82 superlattices. Despite the considerable exciton stability and the strong exciton-phonon coupling, we found that lasing originates from electron-hole plasma recombination. Exciton recombination is found to dominate the spontaneous emission up to carrier densities very close to the stimulated emission threshold. The modest enhancement of the exciton binding energy caused by the negligible conduction band offset prevents excitonic gain in ZnSe/ZnS(x1-x)heterostructures. The impact of these findings for the interpretation of various spectroscopic experiments is discussed in the framework of a phase diagram model in which exciton and free-carrier phases coexist at high density in the quantum well.