Polarization-dependent formation of biexcitons in (Zn,Cd)Se/ZnSe quantum wells

The dynamics of biexciton formation and the spin-relaxation process of excitons in (Zn,Cd)Se/ZnSe quantum wells are investigated systematically using time-resolved photoluminescence spectroscopy. Excitons with well-defined spin orientation were created using linearly or circularly polarized light, respectively, while the energy of the exciting laser pulse was tuned to the transition of the ground state of the heavy-hole exciton. For linearly polarized excitation, the rise time of the biexciton luminescence signal directly reflects the biexciton formation coefficient. For circularly polarized laser pulses, a strong increase of the rise time of the biexciton photoluminescence signal is observed, which is attributed to the necessity of an exciton spin-flip process in the case of circular polarization in order to form biexcitons. The experimental data were evaluated numerically by using a system of rate equations. Generating resonantly localized excitons, we obtain a biexciton-formation coefficient of about 5.2×${10}^{\mathrm{-}11}$ ${\mathrm{ncm}}^2$/ps and an exciton spin-flip time of 70 ps. Increasing the excitation energy (i.e., creating more mobile excitons), we obtain a drastically reduced spin-flip time, indicating that exchange interaction is the leading exciton spin-relaxation mechanism.