Enhanced first-order Raman scattering from arrays of vertical silicon nanowires

Vertical ordered silicon nanowire arrays with diameters ranging from 30 to 60 nm are fabricated and display enhanced Raman scattering. The first-order 520 cm(-1) phonon mode shows no significant shift or peak broadening with increasing laser power, suggesting that the excellent defect-free diamond crystalline structure and thermal properties of bulk silicon are maintained. The Raman enhancement per unit volume of the first-order phonon peak increases with increasing nanowire diameter, and has maximum enhancement factors of 7.1 and 70 when compared to the original silicon on insulator (SOI) and bulk silicon wafers, respectively. For the array with 60 nm diameter nanowires, the total Raman intensity is larger than that of the SOI wafer. The results are understood using a model based on the confinement of light and are supported by finite difference time domain (FDTD) simulations.