The ambipolar carrier diffusion in bulk GaAs is studied by using an ultrafastpump-probe technique with a high spatial resolution. Carriers with a point-likespatial profile are excited by a tightly focused pump laser pulse. Thespatiotemporal dynamics of the carriers are monitored by a time-delayed andspatially scanned probe pulse. Ambipolar diffusion coefficients are deducedfrom linear fits to the expansion of the area of the profiles, and are found todecrease from about 170~$\mathrm{cm}^2 \mathrm{s}^{-1}$ at 10 K to about20~$\mathrm{cm}^2 \mathrm{s}^{-1}$ at room temperature. Our results areconsistent with those deduced from the previously measured mobilities.
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