Absorption spectra have been measured for a 10 nm CdTe(0.3%Mn) quantum well with electron concentration n_e variable up to 1.5×10^{11}cm^{-2}. Following recent theory appropriate to low n_e, here ≈0.1/π a_B^2, the spectra are interpreted in a "strong exciton" model, where the initial oscillator strength of the excitonic resonance (X) is conserved, with screening and phase-space filling effects negligible. As n_e increases in zero-field and as the filling factorν increases in magnetic field, the intensity of X is transferred to: (i) trion processes, namely exciton-one electron scattering and the trion resonance (T), and (ii) quatron processes, namely exciton-two electron scattering and trion-one electron scattering. In magnetic field, the three- and four-body scattering processes become discrete, combined "exciton and cyclotron" and combined "trion and cyclotron" excitations that take all the intensity of X and T for ν≾1 and ν>2, respectively.
Optical spectra associated with transitions that create or annihilate charged excitons X^{-} or X^{+} can be observed in quantum well heterostructures containing an electron gas or a hole gas, respectively. A review is given of properties of trion states in CdTe quantum wells in zero field and of the magnetic field-dependence of the circular polarization and oscillator strength of the trion optical resonance. The possibility that disorder is needed to stabilise trion states in concentrated 2D electron or hole systems is discussed.
The phenomenon in which giant enhancement of exciton magnetic moments occurs due to translational motion was found for light hole excitons in ZnTe/ZnMgTe quantum well structures. Decreasing diamagnetic shifts as the number of the exciton quantized state increases were found for the first time.
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