Dynamical equilibrium between excitons and trions in CdTe quantum wells in high magnetic fields

The formation of two-dimensional negatively charged excitons (negative trions) out of excitons and free electrons is found to be determined by a dynamical equilibrium. This dynamical equilibrium consists of a chemical equilibrium, relating the trion, exciton, and electron populations, modified by finite formation and recombination times of (charged) excitons, as is evidenced by a magnetic-field-dependent photoluminescence (PL) and far-infrared study of doped CdTe/CdMgTe quantum wells. The data show that the trion formation is entirely driven by the occupation of the spin-split trion, exciton and electron levels. Incorporation of the proposed trion formation scheme into a rate equation model gives a proper description of the experimental data, leading to values of the formation, recombination, and spin-flip times of trions and excitons that are in good agreement with results of time-resolved experiments in the literature. The model elucidates the effect of the heavy-hole splitting on the polarization degree of the trion PL in a magnetic field and the influence of the dark excitons on the trion formation.