Carrier scattering and excitonic effects on electron-hole-pair diffusion in nondoped and p-type-modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$ As quantum-well structures

We have performed lateral-diffusion measurements of photoexcited electron-hole pairs at various excitation densities for nondoped and p-type-modulation-doped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures at 77 K, using an all-optical time-of-flight technique with a single-mode optical-fiber probe. Systematic measurements at various electron and hole densities revealed remarkable deviation from the conventional ambipolar diffusion, indicating the important contribution of carrier-carrier scattering and excitonic effects to the diffusion.