Optical detection of vertical transport in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices: Stationary and dynamical approaches

We propose an experimental method for the optical detection of transport along the growth axis of GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices (SL) that combines stationary photoluminescence excitation experiments (PLE) and time-resolved picosecond spectroscopy. We show that both the absorption coefficient of the structure and the effective diffusion length of the photogenerated excitations can be deduced from PLE experiments performed on structures with two enlarged test wells (EW) grown on both sides of the SL. The diffusion coefficients of the excitations involved in the low-temperature diffusion processes are then deduced from time-resolved experiments. A one-dimensional diffusion model is proposed taking into account two kinds of diffusive excitations: free-exciton states and localized-exciton states. We show that the capture process of the SL excitations by the enlarged wells must be taken into account in order to fit the dynamical experiments with capture times in the range 1–5 ps. The dependence of the free-exciton states’s diffusion coefficient on the miniband dispersion of the structure is clearly demonstrated. Finally, temperature effects are discussed such as the influence of the free-exciton cooling on the diffusion process; the nonthermalization of the EW excitations with the lattice is also revealed.