MAGNETOOPTICAL STUDIES OF SCREENED EXCITONS IN GAAS/ALXGA1-XAS MODULATION-DOPED QUANTUM-WELLS

Asymmetric GaAs/AlxGa1-xAs quantum wells were studied by photoconductivity and photoluminescence in magnetic fields up to 15 T. The sample chosen had only one occupied electric subband, with a high density of 8.2 X 10(12) cm-2, which allowed a study of the empty E2-H1 transitions by photoconductivity. This showed that there was a small but finite binding energy of 1.5+/-0.5 meV for the 1s state, which increased very rapidly at high fields in agreement with theory, due to the changing effects of the screening on the exciton. No evidence was seen for excitonic contributions to any higher-Landau-level transitions. Photoluminescence was used to study the E1-H1 transition, which was shown to be due to free-carrier recombination.