Electron density dependence of the excitonic absorption thresholds of GaAs quantum wells

We study the evolution of the interband optical excitation spectra of remotely doped GaAs/Al0.33Ga0.67As quantum wells (QWs) with excess electron density using photoluminescence excitation (PLE) and electroreflectance (ER) spectroscopies. At the lowest electron densities the PLE spectra resemble those of undoped QWs, with a discrete peak due to the Is neutral exciton (X) and a higher energy band due to the 2s/continuum. At very dilute excess electron densities the excitons bind a single excess electron to form a trion or negatively charged exciton (X-), producing a second peak in the PLE a few meV below that due to X. With increasing density the X- peak shifts to higher energy, tracking the position of the k-vector conserving Fermi edge transition observed on the high energy side of the photoluminescence band, to form the single threshold seen at high carrier density. The peak originating from X at low density shifts more rapidly to higher energy with increasing density and weakens. although it can be resolved to densities of up to 10(11) cm(-2). On the other hand the 2s/continuum threshold disappears at the lowest excess electron densities.