Negatively and positively charged excitons in $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

We report the observation of the positively charged exciton and of the triplet state of the negatively charged exciton in modulation doped GaAs quantum wells. Applying a gate voltage at high magnetic fields we find that the photoluminescence line of the two-dimensional electron gas smoothly transforms into a negatively charged exciton and not into a neutral exciton. The Zeeman splitting of this line exhibits an abrupt change at the metal-insulator transition.