Light-induced reflectivity transients in black-Si nanoneedles

The change in reflectivity of black-Si (b-Si) upon optical excitation was measured by the pump-probe technique using picosecond laser pulses at 532 (pump) and 1064nm (probe) wavelengths. The specular reflection from the random pattern of plasma-etched b-Si nano-needles was dominated by the photo-excited free-carrier contribution to the reflectivity. The kinetics of the reflectivity were found to be consistent with surface structural and chemical analysis, performed by scanning and transmission electron microscopy, and spectroscopic ellipsometry. The surface recombination velocity on the b-Si needles was estimated to be ~102cm/s. Metalization of b-Si led to much faster recombination and alteration of reflectivity. The reflectivity spectra of random b-Si surfaces with different needle lengths was modeled by a multi-step refractive index profile in the Drude formalism. The dip in the reflectivity spectra and the sign reversal in the differential reflectivity signal at certain b-Si needle sizes is explained by the model. •Black-Si reflectivity changes upon optical excitation were measured.•Reflection from b-Si was dominated by photo-excited carrier contribution.•The reflectivity kinetics were consistent with surface structural analysis.•The surface recombination velocity on the needles was estimated at ~100cm/s.•Reflectivity spectra were modeled versus probe wavelength and needle length.