Strongly enhanced light trapping in a two-dimensional silicon nanowire random fractal array

Abstract

We report on the unconventional optical properties exhibited by a two-dimensional array of thin Si nanowires arranged in a random fractal geometry and fabricated using an inexpensive, fast and maskless process compatible with Si technology. The structure allows for a high light-trapping efficiency across the entire visible range, attaining total reflectance values as low as 0.1% when the wavelength in the medium matches the length scale of maximum heterogeneity in the system. We show that the random fractal structure of our nanowire array is responsible for a strong in-plane multiple scattering, which is related to the material refractive index fluctuations and leads to a greatly enhanced Raman scattering and a bright photoluminescence. These strong emissions are correlated on all length scales according to the refractive index fluctuations. The relevance and the perspectives of the reported results are discussed as promising for Si-based photovoltaic and photonic applications.

Light trapping: a ‘black forest’ of silicon nanowires

An easy-to-fabricate ‘forest’ of silicon nanowires with a greatly enhanced ability to trap light has been developed by researchers in Italy. Materials that strongly trap light are highly desired for boosting the efficiency of solar cells. Barbara Fazio of the Institute for Chemical and Physical Processes in Messina and co-workers have made a two-dimensional array of silicon nanowires that efficiently traps light over the entire visible spectrum as well as into the near-infrared. They show that the random fractal arrangement of the nanowires causes light to bounce around many times within the array, thereby increasing the absorbance of light. The nanowire arrays can be readily fabricated by cheap, silicon-compatible processes. They also exhibited enhanced Raman scattering and photoluminescence, making them potentially useful for a new class of light-emitting devices.