Correct determination of low-temperature free-exciton diffusion profiles in GaAs

We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a $1.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the $(FX)\ensuremath{-}2\ensuremath{\hbar}{\mathrm{\ensuremath{\Omega}}}_{\text{LO}}$ replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.