Phonon-wind-driven transport of photoexcited carriers in a semiconductor quantum well

The in-plane transport of photoexcited carriers in GaAsAl0.3Ga0.7As multiple quantum wells is measured optically with 10 ps and 4 μm resolution. The local carrier density is measured by the bleaching of the excitonic absorption line. The expansion of the photoexcited carriers following a 6-ps pump pulse is found to be a strong function of excitation power and ambient temperature. At low powers, with initial carrier densities n0≤2×1011 cm-2, the spatial distribution is well described by a single expanding Gaussian profile corresponding to an effective diffusion constant Deff(n0,T). The power and temperature dependence suggests a phonon-wind driving mechanism. At higher densities (n0>2×1011 cm-2) a narrow spatial profile appears which we attribute to ‘‘thermal confinement’’ of carriers within a phonon hot spot. After some delay, the ‘‘confined’’-carrier distribution is observed to undergo a rapid expansion. This unusual effect is attributed to the ‘‘explosion’’ of the phonon hot spot, as predicted by Kazakovtsev and Levinson.