Exciton states and spin relaxation in CdSe/ZnSe self-organized quantum dots

Abstract

The nature and the relaxation properties of the localized electronic states responsible for the radiative recombination in the ensemble of self-organized CdSe/ZnSe quantum dots (QDs) are studied and discussed. It has been found that due to the complicated topological relief of the localizing potential in the lateral plane of the QDs, both ground and metastable states of dots contribute to the emission at helium temperatures. The metastable states are subjected to further energy relaxation, with the rate being dependent on the temperature. The temperature dependence of the relaxation rate is responsible for the anomalous temperature shift of the luminescence band maximum. By the study of polarization properties of the emission at resonant excitation by linearly or circularly polarized light we have shown that, in the samples studied, the photoluminescence (PL) of quantum dots arises due to the radiative recombination of both excitons and trions formed in the charged dots. The exciton emission comes mostly from the metastable states responsible for the high-energy part of the PL band, whereas the trion emission corresponds to the ground states of the quantum dots and forms the low-energy wing of the PL band. The main attention is given to the theoretical and experimental study of the spin-relaxation processes of the electronic states in QDs. We have demonstrated that the broadening of localized electronic levels caused by multi-phonon processes strongly influences the spin relaxation. Such broadening removes the limitations on the energy of phonons assisting the one-phonon transitions between spin sublevels. Taking into account the multi-phonon broadening of spin sublevels we were able to describe theoretically the temperature dependence of the polarization degree of PL spectra of CdSe/ZnSe quantum dots and to restore from experimental data the characteristic function, describing the interaction of localized electronic states with phonons.