Localized and delocalized two-dimensional excitons in GaAs-AlGaAs multiple-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 200 Å 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.