MICROPHOTOLUMINESCENCE STUDIES OF SINGLE QUANTUM DOTS .1. TIME-RESOLVED EXPERIMENTS

Photoluminescence measurements with combined spatial, temporal, and sp ectral resolution are performed on single GaAs/GaxAl1-xAs quantum dots . The complete spatial quantization leads to a spectrum of discrete em ission lines. A series of structures with various confinement strength is investigated, as a function of excitation wavelength, excitation p ower, and temperature. In all cases, a fast rise of the luminescence i s observed. Several independent results show that Coulomb scattering p lays a major role within the early stage of energy relaxation. At liqu id-helium temperature, a strikingly different recombination dynamics i s observed for dots with various lateral potential. For weak lateral c onfinement, energy relaxation is directly observed in the time depende nce of the luminescence spectrum. In contrast, in the sample with stro ngest confinement, independent recombination of the discrete lines occ urs. Increasing the excitation-bower, higher-energy lines appear and t he spectral weight shifts systematically from the lowest to the higher -energy lines. For this variation, which corresponds to an increase in the estimated number of electron-hole pairs in the single dot from ab out 1-2 to 200, the peak energies hardly change. We have also performe d detailed calculations of the energy spectrum and the relaxation and recombination times of excitons in quantum dots. The experimental resu lts are surprisingly well interpreted assuming the formation of an exc iton gas obeying the Pauli exclusion principle.