Electron Density Dependence of the Excitonic Absorption Thresholds of GaAs Quantum Wells

Abstract

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 1s 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 cm–2. On the other hand the 2s/continuum threshold disappears at the lowest excess electron densities.