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

An emission line which depends on the simultaneous presence of excitons and similar to 10(10) cm(-2) photoexcited electrons in a 20nm quantum well is shown to be due to free negatively charged excitons (''trions''). We have measured the temperature dependence of the exciton and trion lifetimes and of the exciton/trion ratio, and demonstrate that the 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 exciton. Both lifetimes increase with temperature, as expected for a free particle. The intensity of the trion line relative to the exciton line is a good measure of the electron density.