Optical spectra associated with transitions that create of annihilate charged excitons X~- of X+ can be observed in quantum well heterostruc- tures 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 sta- bilise trion states in concentrated 2D electron or hole systems is discussed.
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