INFLUENCE OF EXCESS ELECTRONS AND MAGNETIC-FIELDS ON MOTT-WANNIER EXCITONS IN GAAS QUANTUM-WELLS

We describe an experimental study of the interband optical properties of GaAs quantum wells as a function of their excess electron density a nd an applied magnetic field. With a negligible density of excess elec trons in the well, the spectra show sharp resonances due to neutral Mo tt-Wannier-type excitons X. Upon increasing the density slightly the e xcitons 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 c onsistent with that determined for a homogeneous well, we observe a sh arp transfer of oscillator strength from X to X(-). With further densi ty increase, X is completely quenched from the spectra, while X(-) evo lves adiabatically into the Fermi-edge singularity observed for dense electron gases. Application of a magnetic field perpendicular to the w ell causes a large enhancement of the second electron binding energy o f X(-), due to the enhanced Coulomb interaction which arises because t he outer electron orbital is forced closer to the core of the exciton. Furthermore, excited states of X(-) for which the spin wavefunction i s symmetric upon interchange of the two electrons (spin triplet), rath er than antisymmetric (spin singlet) as in the ground state, are obser ved to bind at finite magnetic fields. Under magnetic field the single X(-) transition becomes increasingly circularly polarized in excitati on spectra, owing to the spin polarization of its electron initial sta te at low temperatures.