Nanowires (NWs) with their quasi-one-dimensionality often present different structural and opto-electronic properties than their thin-film counterparts. The thinner they are the larger these differences are, in particular in the carrier-phonon scattering and thermal conductivity. In this work, we present femtosecond transient absorbance measurements on GaAs0.8P0.2 NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain the hot-carriers at a higher temperature for longer times than thicker NWs. We explain the observation suggesting that in thinner NWs, the build-up of a hot-phonon bottleneck is easier than in thicker NWs because of the increased phonon scattering at the NW sidewalls which facilitates the build-up of a large phonon density. The large number of optical phonons emitted during the carrier relaxation processes generate a non-equilibrium population of acoustic phonons that propagates less efficiently in thin NWs. This makes the possible acoustic-to-optical phonon up-conversion process easier, which prolongs the LO phonon lifetime resulting in the slowdown of the carrier cooling. The important observation that the carrier temperature in thin NWs is higher than in thick NWs already at the beginning of the hot carrier regime suggests that the phonon-mediated scattering processes in the non-thermal regime play a major role at least for the carrier densities investigated here (8x1018-4x1019 cm-3). Our results also suggest that the boundary scattering of phonons at crystal defects is negligible compared to the surface scattering at the NW sidewalls.
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