Diffusion-free ultrafast carrier dynamics in silicon nano-pillars

We have investigated ultrafast carriers dynamics in crystalline silicon nano-pillars structure using a pump-probe reflectivity method with 800 nm, 150 fs laser pulses and fluence in the range of {17 - 170} mJ/cm^2. Dimensions of the structure allow us to eliminate contribution from the diffusion process to the relaxation dynamics of the excited carriers. Strong intensity-dependent time-resolved reflectivity change, \Delta R(\tau_d), was monitored in the submelting regime. At strong pumping \Delta R(\tau_d) can reach up to 8%, a several times higher than for a bulk silicon. From the measurements we deduced recombination time of up to 10 ns at carriers density of ~1 x 10^20 cm^-3, while electron-phonon interaction occurs during 350-400 fs and it is independent of the concentration.