THERMAL IONIZATION OF EXCITONS IN GAAS/ALGAAS QUANTUM-WELL STRUCTURES

Photoluminescence and photoluminescence excitation spectra have been performed on GaAs/AlGaAs quantum well structures in the temperature range 4-300 K. Sharp exciton resonances are present up to room temperature and can be ascribed to localized excitons for T≤50-70 K and to free excitons at higher values of T. Nevertheless, a line-shape analysis of the PL spectra clearly shows the presence of band-to-band recombination. A fit based on a simple statistical model reproduces with high accuracy the photoluminescence spectrum line shape and allows to evaluate the relative densities of excitons and free carriers generated by the exciton dissociation. We find that the ratios of the relative densities can be interpreted on the basis of the law of mass action for describing the thermal equilibrium between excitons, electrons, and holes.