Photoluminescence study of magnetic-field-induced excitonic transitions in Alx Ga1-x As GaAs asymmetric double quantum wells

We have observed magnetic-field-induced charged excitonic transitions from one-side-modulation-doped Alx Ga1-x As GaAs asymmetric double quantum wells (ADQWs) where two low-energy conduction subbands e1 and e2 are coupled strongly, whereas the heavy-hole ground state hh1 is localized mostly in a quantum well. In the presence of a magnetic field applied parallel to the growth axis, the photoluminescence spectrum of the e1-hh1 transition develops into a Landau fan for filling factors of ν>2. However, at ν<2, the lowest Landau-level (LL) transition reveals the charged-exciton behavior with an abrupt changeover in the field dependence of the transition energy, whereas the intensities of other LL transitions in the same Landau fan diminish. When the density of free electrons in the ADQWs is about 1.4× 1011 cm-2, a new charged excitonic e2-hh1 transition also appears in the σ+ polarization at ν<2.