Novel excitonic transitions in n-type GaAs/AlGaAs quantum wells

Abstract

We report on a novel peak, the F-line, observed in photoluminescence spectra of GaAs/AlGaAs quantum wells (QWs) with various donor layer positions and concentrations. The F-line is well-defined and red shifted by approximately 1.3 meV (dependent on the experimental conditions) relatively the free exciton (FE) in a 200 Å wide QW. The F-line exhibits a strong magnetic field dependence. The enhanced intensity with increasing field is due to an increasing wave function overlap caused by the enhanced localization of the involved charge carriers. In accordance, the derived thermal activation energy for the F-line is magnetic field dependent. The F-line exhibits a diamagnetic shift as expected for an excitonic transition and splits into four components with increasing magnetic field. Another associated higher energy peak, the E-line, is observed preferably in the presence of a magnetic field, between the heavy hole- and light hole-FE in PL excitation spectra. The E-line also exhibits a striking magnetic field and temperature dependence. The observed properties of the F-line with a striking dependence on the excitation intensity, magnetic field and temperature are consistent with the observation of an exciton bound at the negatively charged D- donor state or a negatively charged X- exciton.