Excitons and free carrier lasing in II-VI quantum wells

The exciton stability in ZnCdSe/ZnSe quantum wells has been investigateci by means of absorption, pump and probe transmission, and magnetophotoluminescence experiments in undoped and modulation doped samples. The well width and the composition dependence of the exciton binding energy has been measured and calculated variationally in order to estimate the typical carrier densities for the occurrence of excitonic bleaching. The absorption saturation as a function of the carrier density has been directly measured in modulation doping samples with different sheet carrier densities or by absorption saturation in undoped samples. The obtained information on the exciton ionization have been correlated to the occurrence of stimulated emission in the investigated samples. Free carrier recombination seems to dominate the stimulated emission of ZnCdSe QWs with little exciton binding energy, whereas a dominant excitonic character is found in the lasing of deep and narrow ZnCdSe QWs. Stimulated emission experiments in high magnetic fields performed on various samples confirm this attribution.