Circular polarization of excitonic luminescence in CdTe quantum wells with excess electrons of different densities

The circular polarization of excitonic luminescence is studied in $\mathrm{CdTe}/{\mathrm{Cd}}_{1-x}{\mathrm{Mg}}_x\mathrm{Te}$ quantum wells with excess electrons of low density in an external magnetic field. It is observed that the circular polarization of X and $X^{-}$ emissions has opposite signs and is influenced by the excess electron density. If the electron density is relatively high so that the emission intensity of the negatively charged excitons $X^{-}$ is much stronger than that of the neutral excitons X, a stronger circular polarization degree of both X and $X^{-}$ emissions is observed. We find that the circular polarization of both $X^{-}$ and X emissions is caused by the spin polarization of the excess electrons due to the electron-spin-dependent nature of the formation of $X^{-}.$ If the electron density is relatively low and the emission intensity of $X^{-}$ is comparable to that of X, the circular polarization degree of X and $X^{-}$ emissions is considerably smaller. This fact is interpreted as due to a depolarization of the excess electron spins, which is induced by the spin relaxation of $X^{-}.$