  Time-resolved and time-integrated circularly polarized photoluminescence of
excitons and trions in external magnetic fields up to 10 T has been studied in
undoped and n-type doped quantum well structures based on ZnSe. In an undoped
structure, a circular polarization of photoluminescence induced by magnetic
fields corresponded to the Boltzmann distribution of excitons on Zeeman
sublevels. The inverse spin orientation of excitons is observed in doped
samples with a carrier density of $3 \times 10^{10}$ cm$^{-2}$ and higher.
Model calculations show that the reason for the inverse spin orientation is the
effective depletion of the lowest-exciton Zeeman sublevel as a result of the
spin-dependent formation of trions. The trion formation time as a result of
exciton-electron binding was determined as 2 ps. This is noticeably shorter
than the characteristic time of the exciton-photon interaction. The observed
effect can be considered as a way of spin manipulation by electric fields.
