Broadband antireflective silicon nanostructures produced by spin-coated Ag nanoparticles
© Kim et al.; licensee Springer. 2014
Received: 11 December 2013
Accepted: 24 January 2014
Published: 1 February 2014
We report the fabrication of broadband antireflective silicon (Si) nanostructures fabricated using spin-coated silver (Ag) nanoparticles as an etch mask followed by inductively coupled plasma (ICP) etching process. This fabrication technique is a simple, fast, cost-effective, and high-throughput method, making it highly suitable for mass production. Prior to the fabrication of Si nanostructures, theoretical investigations were carried out using a rigorous coupled-wave analysis method in order to determine the effects of variations in the geometrical features of Si nanostructures to obtain antireflection over a broad wavelength range. The Ag ink ratio and ICP etching conditions, which can affect the distribution, distance between the adjacent nanostructures, and height of the resulting Si nanostructures, were carefully adjusted to determine the optimal experimental conditions for obtaining desirable Si nanostructures for practical applications. The Si nanostructures fabricated using the optimal experimental conditions showed a very low average reflectance of 8.3%, which is much lower than that of bulk Si (36.8%), as well as a very low reflectance for a wide range of incident angles and different polarizations over a broad wavelength range of 300 to 1,100 nm. These results indicate that the fabrication technique is highly beneficial to produce antireflective structures for Si-based device applications requiring low light reflection.
Silicon (Si) is an important material used for optoelectronic device applications, such as sensors, photodetectors, and solar cells, due to its abundance in the earth's crust, low-cost, and mature fabrication technique[1–4]. For these devices, minimizing the light reflection on the surface thereby increasing the light transmission into the device is the key to increase the device performance. However, more than 30% of the incident light is lost through Fresnel reflection owing to the large refractive index difference between air (nair = 1) and Si (nSi approximately 3.8), and therefore, antireflective structures are indispensible to improve the device performance. Conventional multilayered thin-film antireflection coatings have been widely used to suppress the unwanted surface reflection losses. However, these coatings have serious drawbacks that are related to material selection, mechanical instability, and thermal mismatch. Furthermore, these antireflective coatings can suppress the reflections only over a narrow wavelength and incident angle range[5, 6]. Recently, bioinspired antireflective nanostructures with tapered features have attracted great interest for improving the performance of optical and optoelectronic devices due to their broadband and omnidirectional antireflection properties as well as long-term stability[1, 5–13]. A commonly used technique to produce such antireflective nanostructures on various materials is dry etching of nano-scale etch masks formed by electron-beam or interference lithography process[5, 6, 9, 10]. However, lithography-based nanopatterning method is not suitable for mass production because it is a time-consuming process requiring delicate and expensive equipment, reducing the cost effectiveness. Numerous research efforts have therefore been carried out to form nano-scale etch masks using a simple, fast, and cost-effective nanopatterning method in order to enhance productivity and thereby reduce the fabrication cost of antireflective nanostructures.
In this paper, we report a simplified fabrication technique for producing antireflective nanostructures having tapered profile on Si substrates without using any lithography steps. To achieve this goal, nano-scale silver (Ag) etch masks were formed using spin-coating Ag ink and subsequent sintering process. The significant advantage of the reported technique is that it requires only a low temperature and a short process duration to form the Ag etch masks[7, 11, 12]. Furthermore, the technique avoids the usage of any lithographic process, making it highly cost-effective for mass production. Prior to fabrication, the period- (i.e., distance between the adjacent nanostructures) and height-dependent reflection characteristics of the Si nanostructures were theoretically investigated using a rigorous coupled-wave analysis (RCWA) method in order to provide a guideline for producing a desirable Si nanostructure with broadband antireflection properties because the antireflection properties of these nanostructures are closely correlated with their geometry[6–12]. The Ag ink ratio and dry etching conditions, which affect the distribution, distance between adjacent nanostructures, and height of resulting Si nanostructures, were carefully adjusted, and optimal experimental conditions were found that can produce desirable antireflective Si nanostructures for practical applications. We found that the fabricated Si antireflective nanostructures have excellent antireflective properties over a wide wavelength range and polarization-independent antireflection properties.
Optical modeling of Si nanostructures
Fabrication of Si nanostructures
Results and discussion
The hemispherical reflectance spectra of the fabricated Si nanostructures for various Ag ink ratios in the wavelength range of 300 to 1100 nm are shown in Figure 3b. The hemispherical reflectance spectra were measured using a UV/VIS-NIR spectrophotometer (Cary 500, Varian, Inc., Palo Alto, CA, USA) with an integrating sphere kept at a near-normal incident angle of 8°. The reflection spectrum of bulk Si with an average reflectance of 36.8% is also included for comparison. It is evident that the Si nanostructures drastically reduced the reflection compared to that of the bulk Si over the entire wavelength range considered. The reflection minima shifts from the short-wavelength region to the long-wavelength region with an increasing Ag ink ratio (i.e., increasing the distance between adjacent Si nanostructures) as can be seen in Figure 1a[6, 8]. The Si nanostructures fabricated using an Ag ink ratio of 25%, 35%, and 50% showed an average reflectance of 6.4%, 8.5%, and 9.6%, respectively. This result indicates that controlling the Ag ink ratio is crucial to fabricate antireflective Si nanostructures having desirable antireflection properties. Although the Si nanostructures fabricated using Ag ink ratio of 25% exhibited the lowest average reflectance among the ones fabricated with three different Ag ink ratios, a 25% ink ratio resulted in the formation of too thin nanoparticles which were unable to withstand harsh etching conditions and long etching duration, as a result producing collapsed Si nanostructures. Therefore, Ag ink ratio of 35% was chosen to form Ag nanoparticles for the reminder of experiments.
We fabricated antireflective Si nanostructures by a simple nanofabrication technique using spin-coated Ag nanoparticles and a subsequent ICP etching process. Theoretical investigations based on RCWA method were carried out prior to fabrication to determine the effect of variations in height and period on the antireflection properties of Si nanostructures. Using the results from RCWA as a guideline, various Si nanostructures with different distribution, period, and height were fabricated by adjusting the Ag ink ratio and ICP etching conditions. It was found that the fabricated Si nanostructures significantly reduced the surface reflection losses compared to bulk Si over a broad wavelength range. Si nanostructures fabricated using a 35% Ag ink ratio and optimum ICP etching conditions showed excellent antireflection properties over a broad wavelength range as well as polarization- and angle-independent reflection properties. The antireflective Si nanostructures fabricated using this simple, fast, and cost-effective nanofabrication technique exhibits great potential for practical Si-based device applications where light reflection has to be minimized.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011–0017606).
- Liu Y, Sun SH, Xu Zhao L, Sun HC, Li J, Mu WW, Xu L, Chen KJ: Broadband antireflection and absorption enhancement by forming nano-pattered Si structures for solar cells. Opt Express 2011, 19: A1051-A1056. 10.1364/OE.19.0A1051View ArticleGoogle Scholar
- Pillai S, Catchpole KR, Trupke T, Green MA: Surface plasmon enhanced silicon solar cells. J Appl Phys 2007, 101: 093105. 10.1063/1.2734885View ArticleGoogle Scholar
- Rosan K: Hydrogenated amorphous-silicon image sensors. IEEE Trans Electron Devices 1989, 36: 2923–2927. 10.1109/16.40956View ArticleGoogle Scholar
- Song YM, Xie Y, Malyarchuk V, Xiao J, Jung I, Choi KJ, Liu Z, Park H, Lu C, Kim RH, Li R, Crozier KB, Huang Y, Rogers JA: Digital cameras with designs inspired by the arthropod eye. Nature 2013, 497: 95–99. 10.1038/nature12083View ArticleGoogle Scholar
- Yu P, Chiu MY, Chang CH, Hong CY, Tsai YL, Han HV, Wu YR: Towards high-efficiency multi-junction solar cells with biologically inspired nanosurfaces. Prog Photovoltaics in press in pressGoogle Scholar
- Boden SA, Bagnall DM: Tunable reflection minima of nanostructured antireflective surfaces. Appl Phys Lett 2008, 93: 133108. 10.1063/1.2993231View ArticleGoogle Scholar
- Lee Y, Koh K, Na H, Kim K, Kang JJ, Kim J: Lithography-free fabrication of large area subwavelength antireflection structures using thermally dewetted Pt/Pd alloy etch mask. Nanoscale Res Lett 2009, 4: 364–370. 10.1007/s11671-009-9255-4View ArticleGoogle Scholar
- Yeo CI, Kwon JH, Jang SJ, Lee YT: Antireflective disordered subwavelength structure on GaAs using spin-coated Ag ink mask. Opt Express 2012, 20: 19554–19562. 10.1364/OE.20.019554View ArticleGoogle Scholar
- Song YM, Jang SJ, Yu JS, Lee YT: Bioinspired parabola subwavelength structures for improved broadband antireflection. Small 2010, 6: 984–987. 10.1002/smll.201000079View ArticleGoogle Scholar
- Tommila J, Polojärvi V, Aho A, Tukianinen A, Viheriälä J, Salmi J, Schramm A, Kontio JM, Turtiainen A, Niemi T, Guina M: Nanostructured broadband antireflection coatings on AlInP fabricated by nanoimprint lithography. Sol Energy Mater Sol Cells 2010, 94: 1845–1848. 10.1016/j.solmat.2010.05.053View ArticleGoogle Scholar
- Zhang RY, Shao B, Dong JR, Huang K, Zhao YM, Yu SZ, Yang H: Broadband quasi-omnidirectional antireflection AlGaInP window for III-V multi-junction solar cells through thermally dewetted Au nanotemplate. Opt Mater Express 2012, 2: 173–182.View ArticleGoogle Scholar
- Leem JW, Chung KS, Yu JS: Antireflective properties of disordered Si SWSs with hydrophobic surface by thermally dewetted Pt nanomask patterns for Si-based solar cells. Curr Appl Phys 2012, 12: 291–298. 10.1016/j.cap.2011.06.022View ArticleGoogle Scholar
- Huang YF, Chattopadhyay S, Jen YJ, Peng CY, Liu TA, Hsu YK, Pan CL, Lo HC, Hsu CH, Chang YH, Lee CS, Chen KH, Chen LC: Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures. Nat Nanotechnol 2007, 2: 770–774. 10.1038/nnano.2007.389View ArticleGoogle Scholar
- Moharam MG, Gaylord TK: Rigorous coupled-wave analysis of planar-grating diffraction. J Opt Soc Am 1981, 71: 811–818. 10.1364/JOSA.71.000811View ArticleGoogle Scholar
- Lee JM, Kim BI: Thermal dewetting of Pt thin film: Etch-masks for the fabrication of semiconductor nanostructures. Mater Sci Eng A 2007, 449–451: 769–773.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.