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Reflectance properties of silicon moth-eyes in response to variations in angle of incidence, polarisation and azimuth orientation

Asa Asadollahbaik, Stuart A. Boden, Martin D. B. Charlton, David N. R. Payne, Simon Cox, and Darren M. Bagnall

Open Access Open Access

Abstract

We report a study of the optical properties of silicon moth-eye structures using a custom-made fully automated broadband spectroscopic reflectometry system (goniometer). This measurement system is able to measure specular reflectance as a function of wavelength, polar incidence angle and azimuth orientation angle, from normal to near-parallel polar incidence angle. The system uses a linear polarized broadband super-continuum laser light source. It is shown that a moth-eye structure composed of a regular array of protruding silicon rods, with finite sidewall angle reduces reflectance and sensitivity to incident wavelength in comparison to truly cylindrical rods with perpendicular sidewalls. It is also shown that moth-eye structures have omnidirectional reflectance properties in response to azimuth orientation of the sample. The importance of applying the reflectometer setup to study the optical properties of solar cell antireflective structures is highlighted.

© 2014 Optical Society of America

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Figures (11)
Fig. 1
Fig. 1 He-Ion microscope image of a wing of Cephonodes Hylas from 45° tilt (Scale bar 1μm). Inset shows a high magnification image of the same section of the wing (scale bar 200nm).

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Fig. 2
Fig. 2 Reflectometer setup, photo and schematic diagram

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Fig. 3
Fig. 3 Angle resolved specular reflectance of silicon measured by reflectometer (a and b) and calculated by Fresnel equations (c and d) at s polarisation and p polarisation

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Fig. 4
Fig. 4 Comparison of the theoretical value of angular reflectance of silicon with experimental value at wavelengths of 500nm, 650nm and 800nm extracted from three runs of the measurement.

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Fig. 5
Fig. 5 Top view (top) and side view (bottom) SEM images of moth-eye structures fabricated by nano-imprinting lithography. An outline of pillars are provided. The diameter of the top and bottom of the pillars is noted on the image. (a) Wafer 1 (cylindrical rods), Isotropic etch for 90s; (b) Wafer 2 (tapered rods), Isotropic etch for 90s, oxidation 5mins, oxide strip. (scale bar:100nm)

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Fig. 6
Fig. 6 Comparison of the reflectance of silicon moth-eyes at normal incidence using probe measurement vs reflectometer measurement, (a) wafer 1 (cylindrical rods) and (b) wafer 2 (tapered rods). (c) mean average of normal incidence reflectance of silicon moth-eye structures measured by reflectometer.

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Fig. 7
Fig. 7 Angle resolved specular reflectance of silicon moth-eye structures: (a) and (b) wafer 1 (cylindrical rods), (c) and (d) wafer 2 (tapered rods).

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Fig. 8
Fig. 8 Total angular reflectance, (a) Wafer 1 (cylindrical rods), (b) Wafer 2 (tapered rods) in comparison with silicon.

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Fig. 9
Fig. 9 Photograph of silicon moth-eye samples, wafer 1 (cylindrical rods) and wafer 2 (tapered rods), showing the reflection reduction in silicon caused by the moth-eye structures.

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Fig. 10
Fig. 10 WSR of Silicon, Silicon moth-eyes, wafer 1 (cylindrical rods) and wafer 2 (tapered rods), and SLAR (Si3N4) and DLAR (SiO2/TiO2).

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Fig. 11
Fig. 11 Comparison of the angular reflectance of silicon moth-eye wafer 2 (tapered rods) and PERL+DLAR structure taken from [5] at the wavelength of 632nm and the AOI of 2 – 83°. The azimuth angle of the Si moth-eye is varied between 0°, 30° and 60° and for the PERL+DLAR is varied between 0°, 45° and 90°.

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Tables (1)

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Table 1 The laser power and integration time set for reflectance measurements performed on silicon moth-eye samples using the reflectometer

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

R moth – eye = I moth – eye − I dark I source − I dark
R w ( θ ) = ∑ λ R ( λ , θ ) . PFD ( λ , θ ) ∑ λ PFD ( λ , θ )
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