Enhancement of electron-hole pair mobilities in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

We have investigated the lateral transport of excitons and free carriers in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells over a wide temperature range, between 10 and 230 K. For all temperatures we observe an increase of the ambipolar diffusivities with increasing well widths. According to different transport phenomena, the entire temperature range can be divided into three parts. For 40<T<180 K, we obtain an isothermal diffusion for the lateral motion of excitons. The experimental data can quantitatively be explained by the contributions of different scattering mechanisms: interface-roughness, barrier-disorder-alloy, acoustic-deformation-potential, and polar-optical-phonon scattering. Raising the temperature in the range 180<T<230 K, we find an unexpected increase of the diffusivities for small well widths (below 5 nm). This can be described by the thermal dissociation of excitons into free carriers revealing higher diffusivities. For decreasing temperatures in the range 10<T<40 K, we observed considerably enhanced diffusivities in thin quantum wells. We attribute this to locally elevated phonon populations or temperature gradients in the optically excited sample volume.