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 spectroscopy. 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 show that a 2D mass action law, describing the coexistence of free carriers and excitons, explains the time evolution of the exciton photoluminescence linewidth. This demonstrates the existence of a thermodynamic equilibrium between excitons and free carriers at each time delay following the excitation. The consequence is a very short exciton formation time (less than or similar to 10 ps) in the density range investigated (10(9) - 10(10) cm(-2)), implying a bimolecular formation coefficient gamma higher than 14 cm(2) s(-1).