Laser-Induced Fano Resonance Scattering in Silicon Nanowires

We have studied the impact of excitation laser power density on the Raman spectrum of small-diameter (5−15 nm) silicon nanowires. At low power densities, a Lorentzian line is observed at 520 cm-1, the same value as that of the zone center LO (TO) phonon in bulk silicon. With increasing laser illumination, the Raman band downshifts and asymmetrically broadens on the low-frequency side. Our results contradict the traditionally accepted notion that a downshifted and asymmetrically broadened line in Si nanowires is due to quantum confinement effects. Rather, we suggest that the downshifting can be due to a laser heating effect of the nanowire and that the asymmetric line shape is due to a Fano interference between scattering from the k = 0 optic phonon and electronic continuum scattering from laser-induced electrons in the conduction band.