Excitons in GaAs/AlGaAs quantum wells containing a low density two-dimensional electron gas

We present a short review of the photoluminescence and its excitation spect ra of GaAs/AlGaAs quantum wells (QW) that contain a low-density (10(10) < n (e) < 10(11) cm(-2)) two-dimensional electron gas (2DEG), and are observed at low temperatures and under a perpendicular magnetic field. As the densit y is reduced below n(e) similar to 5 x 10(10) cm(-2), the spectra evolve fr om a 2DEG - unbound heavy hole recombination FL, into exciton lines. Magnet o-optical studies are presented of two types of QW structures, in which n(e ) is varied by photoexcitation: modulation-doped GaAs/AlGaAs QWs and mixed types I-II GaAs/AlAs QWs. They exhibit an excitonic phase. in the presence of free electrons, consisting of both neutral (X) and negatively charged (X -) excitons. The intensity ratio between the X and X- lines is used in orde r to learn on the 2DEG spatial distribution in the QW plane. The low-densit y electrons were detected by their cyclotron resonance at a microwave frequ ency. Some one-side modulation-doped, single QW structures, with a very hig h 2DEG mobility, show a smooth transition (for n(e) less than or equal to 5 x 10(10) cm(-2)) from a 2DEG - unbound heavy hole recombination band into an X line only. The difference between the two observed cases of the excito nic phase is suggested to originate from different degrees of in-plane elec tron localization. (C) 2000 Elsevier Science B.V. All rights reserved.