INTRINSIC LINEWIDTHS AND RADIATIVE LIFETIMES OF FREE-EXCITONS IN GAAS QUANTUM-WELLS

The giant oscillator strength for radiative transitions of excitons in quantum wells is largely due to the macroscopic polarization of a two-dimensional system. At low temperatures the large oscillator strength leads to a short radiative lifetime of free excitons. We investigate the photoluminescence linewidths and lifetimes of free excitons in a series of extremely high-quality GaAs quantum wells as a function of lattice temperature, excitation intensity, and quantum-well width. With only negligible defect states in our quantum-well sample, we are able to correlate the time-resolved data with temperature-dependent linewidth measurements on the series of quantum wells to estimate the homogeneous linewidth and acoustic-phonon scattering rate of free excitons. Our studies show that thermalization of the excitonic states, ionization into free carriers, and a reduction in the coherence volume of the exciton polarization due to defect scattering, lead to a decrease in the net radiative recombination rate.