Far infrared response of silicon nanowire arrays

The reflection, transmission and absorbance spectra of silicon nanowire arrays ( NWAs), as a function of the length of the nanowires, are investigated in a wavelength range of 15 mu m < lambda < 200 mu m, using Fourier transform infrared spectroscopy in vacuum. The NWAs are fabricated using metal-assisted electroless chemical etching. The wire length is varied between 20 mu m and 140 mu m, which is of the same order of magnitude as the wavelength, and their spectra are compared to bulk Si. At high frequencies the absorbance spectra of the NWAs show molecular resonances due to adsorption of molecules involved in the fabrication process but also due to the oxide quality that wraps the nanowires and changes as a function of nanowire length. Transmission characteristics show an increasing shift in absorption band edge towards the far infrared for longer wires and a transition from specular to diffuse reflection at a nanowire length of approximately 60 mu m.