Thermodynamics and diffusion of free trions in mixed type GaAs/AlAs quantum wells

An emission line which depends on the simultaneous presence of excitons and <2×10¹⁰ cm^-2 free photoexcited electrons in a 20 nm quantum well is shown to be due to the recombination of free negatively charged excitons ('trions'). The binding energy is 1.3 meV and the lifetime is 1/4 that of the exciton. The increase in the radiative decay times of excitons and trions with increasing bath temperature is consistent with that of free particles. More directly, from time- and spatially-resolved measurements, we find that trions are localized at 5 K, and with increasing bath temperature both excitons and trions become mobile. The diffusivity of trions and excitons increases linearly with bath temperature, and at 10 K, the trion diffusivity is found to be factor of two lower than the exciton diffusivity. Our results are the first direct experimental evidence for free trions in GaAs quantum wells.