DYNAMICAL EQUILIBRIUM BETWEEN EXCITONS AND FREE-CARRIERS IN QUANTUM-WELLS

We have investigated the exciton and free carrier populations dynamics in quantum wells by low temperature time-resolved photoluminescence s pectroscopy. When the excitation energy is set above the free carrier quantum well gap, excitons are formed from random binding of electrons and holes. On the basis of this bimolecular formation process, we sho w that a 2D mass action law, describing the coexistence of free carrie rs and excitons, explains the time evolution of the exciton photolumin escence linewidth. This demonstrates the existence of a thermodynamic equilibrium between excitons and free carriers at each time delay foll owing the excitation. The consequence is a very short exciton formatio n time (less than or similar to 10 ps) in the density range investigat ed (10(9) - 10(10) cm(-2)), implying a bimolecular formation coefficie nt gamma higher than 14 cm(2) s(-1).