Localized and delocalized two-dimensional excitons in GaAs- AlGaAs multi-quantum well structures.

We have measured the homogeneous linewidth gamma(h) and the diffusion constant D of resonantly excited excitons confined to GaAs layers 50 to 200A wide. We find that gamma(h) and D increase sharply as the exciton energy increases through the center of the inhomogeneously broadened absorption line. Below the center gamma(h) is thermally activated. We conclude that the excitons are effectively localized below the line center and delocalized above it as predicted by classical percolation theory.