Optical orientation and the Hanle effect of neutral and negatively charged excitons in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

We report on the optical orientation of negatively charged excitons (trions) in [001] ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells (QW’s). The spin orientation of equilibrium electrons reveals itself in the polarized luminescence of both excitons and trions. The Hanle effect of the trions differs substantially from that of the neutral excitons. It is found experimentally that the electron-spin-relaxation time in a 14-nm QW exceeds 10 ns at $T=4.2\mathrm{}\mathrm{K}.$