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.