Dedicated to Professor Dr. K. W. BOER on the occasion of his 70th birthday In 11-VI material based laser diode structures, the role of excitonic processes for stimulated emission is still rather controversially discussed. In this paper, the manifestation of biexciton recombination in high-density luminescence of ZnCdSe/ZnSe quantum wells is investigated by intensity-and polarization-dependent spectroscopy. In spite of their evidence in such spectra, their contribution to stimulated emission in laser diodcs with moderately confining wells is not confirmed, instead, indication of strong Coulomb correlation is given when using the new technique of pump-and-probe excitation spectroscopy. A strong electrooptic effect yielding saturation is found in laser diodes exactly at their lasing wavelength thus being promising candidates for integrated laser-modulation devices.

Although lasing is meanwhile achieved in various ZnSe-based heterostructures, the underlying mechanisms are still seriously under debate. The gain spectra of various MBE-grown samples are measured using the variable-stripe-length method. A description of the gain spectra for low particle densities is possible in terms of the model of Ding et al. The responsible gain mechanism is assumed to be an induced recombination of strongly exchange-interacting localized excitons, which, however, may be a questionable precondition. By means of time-dependent measurements the localizing character of the active material is demonstrated. However, several indications are given that under certain circumstances more than one process contribute to lasing, in particular biexciton recombination seems to be involved in the gain mechanism.

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