MAGNETOPHOTOLUMINESCENCE MEASUREMENT OF THE FORMATION TIME OF AN EXCITON IN ALXGA1-XAS GAAS QUANTUM-WELL STRUCTURES/

A magnetophotoluminescence technique is adopted to study the exciton f ormation processes in AlxGa1-xAs/GaAs multi-quantum-well structures, w ith the cyclotron periods of carriers in the quantum wells as measures of time. The exciton-related photoluminescence intensities decrease w ith increasing external magnetic field along the growth direction. It is found that the exciton formation time is dominated by the time duri ng which the electron and hole move toward each other under the Coulom b force between them. Through the fitting of experimental and theoreti cal results, exciton formation time is directly obtained. In addition, it is found that the formation time of an exciton is a function of ca rrier density, and that excitons are formed after the carrier system i s cooled down.