Influence of excess electrons and magnetic fields on Mott-Wannier excitons in GaAs quantum wells

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Abstract
We describe an experimental study of the interband optical properties of GaAs quantum wells as a function of their excess electron density and an applied magnetic field. With a negligible density of excess electrons in the well, the spectra show sharp resonances due to neutral Mott-Wannier-type excitons X. Upon increasing the density slightly the excitons capture an excess electron to form negatively charged excitons X-. The transition energy separation of X and X- agrees with the expected binding energy of the second electron. At a density roughly consistent with that determined for a homogeneous well, we observe a sharp transfer of oscillator strength from X to X-. With further density increase, X is completely quenched from the spectra, while X- evolves adiabatically into the Fermi-edge singularity observed for dense electron gases. Application of a magnetic field perpendicular to the well causes a large enhancement of the second electron binding energy of X-, due to the enhanced Coulomb interaction which arises because the outer electron orbital is forced closer to the core of the exciton. Furthermore, excited states of X- for which the spin wavefunction is symmetric upon interchange of the two electrons (spin triplet), rather than antisymmetric (spin singlet) as in the ground state, are observed to bind at finite magnetic fields. Under magnetic field the single X- transition becomes increasingly circularly polarized in excitation spectra, owing to the spin polarization of its electron initial state at low temperatures.