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.
Highlights
The development of new materials for light trapping, emission and amplification of light is an ever-growing research field
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
The gold deposition morphology is imposed on the silicon substrate as a negative mask during the wet etching procedure[26]; as a consequence, the Si NW distribution is organized with this specific structure
Summary
The development of new materials for light trapping, emission and amplification of light is an ever-growing research field. A new strategy of designing two-dimensional (2D) random patterns of submicron size holes in thin films has been demonstrated[6,7] These new structures allow for strong and broad optical resonances, leading to in-plane multiple scattering phenomena, efficient light trapping and absorption enhancement beyond the theoretical limit dictated by ray optics[8,9,10]. In this scenario, the production of a fractal pattern presents the possibility of achieving a complex disorder with strong structural heterogeneities correlated on all length scales[11,12,13]. Plasmonic fractal-like structures have been proposed to improve photovoltaic device performances; through an efficient coupling of the incident light at different frequency bands into both the cavity modes and the surface plasmon modes[14], a broadband absorption enhancement can be reached[15]
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