Evolution of GaAs quantum well excitons with excess electron density and magnetic field

We present an experimental study of excitonic optical transitions in GaAs quantum wells (QWs) as a function of their excess electron density and an applied magnetic field. There is a dramatic weakening of the neutral excitonic transitions (X) upon adding 10¹⁰ cm^?2 excess electrons to the QW, accompanied by strengthening of the transition due to the negatively-charged exciton (X^?) to lower energy. Increasing the density further causes X to be completely quenched from the spectra, while X^? evolves smoothly into the Fermi-edge singularity. A qualitatively different evolution with an applied magnetic field is observed for X and X^?, the former showing similar behaviour to that in undoped QWs. Assignment of the X^? transition is confirmed by its partial circular light polarization in excitation spectra taken at field, caused by the higher population of excess electrons in the lower energy spin state at low temperature. There is a large enhancement of the second electron binding energy with magnetic field, which stabilizes excited (spin-triplet) X^? states.