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

Abstract

We present a short review of the photoluminescence and its excitation spectra 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 density is reduced below n e∼5×10 10 cm −2, the spectra evolve from a 2DEG – unbound heavy hole recombination PL, into exciton lines. Magneto-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 order to learn on the 2DEG spatial distribution in the QW plane. The low-density electrons were detected by their cyclotron resonance at a microwave frequency. Some one-side modulation-doped, single QW structures, with a very high 2DEG mobility, show a smooth transition (for n e≤5×10 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 excitonic phase is suggested to originate from different degrees of in-plane electron localization.