THERMODYNAMICS OF FREE TRIONS IN MIXED-TYPE GAAS ALAS QUANTUM-WELLS/

An emission line which depends on the simultaneous presence of exciton s and similar to 10(10) cm(-2) photoexcited electrons in a 20nm quantu m well is shown to be due to free negatively charged excitons (''trion s''). We have measured the temperature dependence of the exciton and t rion lifetimes and of the exciton/trion ratio, and demonstrate that th e trion is a free particle analogous to the negative positronium ion. The binding energy is 1.3 meV and the lifetime is 1/4 of that of the e xciton. Both lifetimes increase with temperature, as expected for a fr ee particle. The intensity of the trion line relative to the exciton l ine is a good measure of the electron density.