NOVEL EXCITONIC TRANSITIONS IN N-TYPE GAAS ALGAAS QUANTUM-WELLS/

We report on a novel peak, the F-line, observed in photoluminescence s pectra of GaAs/AlGaAs quantum wells (QWs) with various donor layer pos itions and concentrations. The F-line is well-defined and red shifted by approximately 1.3 meV (dependent on the experimental conditions) re latively the free exciton (FE) in a 200 Angstrom wide QW. The F-line e xhibits a strong magnetic field dependence. The enhanced intensity wit h increasing field is due to an increasing wave function overlap cause d by ; the enhanced localization of the involved charge carriers. In a ccordance, the derived thermal activation energy for the F-line is mag netic field dependent. The F-line exhibits a diamagnetic shift as expe cted 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, be tween the heavy hole- and light hole-FE in PL excitation spectra. The E-line also exhibits a striking magnetic field and temperature depende nce. The observed properties of the F-line with a striking dependence on the excitation intensity, magnetic field and temperature are consis tent with the observation of an exciton bound at the negatively charge d D- donor state or a negatively charged X(-) exciton. (C) 1995 Academ ic Press Limited