- Nano Express
- Open Access
Optical assessment of silicon nanowire arrays fabricated by metal-assisted chemical etching
© Kato et al.; licensee Springer. 2013
- Received: 18 February 2013
- Accepted: 15 April 2013
- Published: 7 May 2013
Silicon nanowire (SiNW) arrays were prepared on silicon substrates by metal-assisted chemical etching and peeled from the substrates, and their optical properties were measured. The absorption coefficient of the SiNW arrays was higher than that for the bulk silicon over the entire region. The absorption coefficient of a SiNW array composed of 10-μm-long nanowires was much higher than the theoretical absorptance of a 10-μm-thick flat Si wafer, suggesting that SiNW arrays exhibit strong optical confinement. To reveal the reason for this strong optical confinement demonstrated by SiNW arrays, angular distribution functions of their transmittance were experimentally determined. The results suggest that Mie-related scattering plays a significant role in the strong optical confinement of SiNW arrays.
- Silicon nanowire
- Optical confinement
- Light scattering
- Solar cells
Silicon nanowire (SiNW) arrays demonstrate considerable promise as an absorber layer for solar cells because of their advantages such as quantum size effect  and strong optical confinement [2–6]. Many researchers have investigated the optical properties of SiNW arrays fabricated by several methods such as metal-assisted chemical etching (MAE) [7–9], vapor–liquid-solid method , laser ablation , thermal evaporation , and reactive ion etching . Some researchers have reported the control of diameter and density of SiNW arrays using self-assembled close-packed 2-D arrays of nano/microparticle arrays or nanopatterns, and so on. Recently, SiNW solar cells have been extensively investigated for the utilization of their optical confinement [14–16] properties. Vertically aligned SiNW arrays exhibit low reflection and strong absorption  and can be used in antireflection coatings or as the active layer in solar cells [17, 18]. The optical properties of such arrays investigated thus far have included the influence of silicon substrates. The optical properties of vertically aligned SiNW arrays have been theoretically evaluated by several researchers [3, 4, 19]. On the other hand, Bao et al. reported that SiNW arrays with random diameter show significant absorption enhancement . According to this paper, we focused on SiNW arrays fabricated by the MAE method to enhance absorption in SiNW arrays with random diameter. To apply these arrays to large-area solar cells, many researchers have adopted SiNW arrays by MAE method, and SiNW arrays prepared by the MAE method tend to have nanowires with a broad range of diameters and may contain bundles of nanowires that adhere to each other due to the wet etching process . Although the optical properties of SiNW arrays have been reported, their light-scattering properties have been scarcely investigated. It is essential to investigate the light-scattering properties of SiNW arrays in order to understand their high optical confinement. In this study, we have investigated the optical properties of SiNW arrays prepared by MAE. Since the SiNW arrays prepared by this method are deposited on silicon substrates, it is difficult to measure the optical properties of SiNW arrays in isolation from the substrate. To remove the effect of the substrate, the SiNW arrays were peeled from the substrate. We present experimentally determined angular distribution functions (ADFs)  of the transmittance of SiNW arrays composed of SiNWs of different lengths. The effects of light scattering were also investigated.
The silver nanoparticles were fabricated by electroless silver plating. Si wafers (p-type, (100), 2 to 10 Ω·cm) were immersed in a silver coating solution composed of 0.015 M AgNO3 and 4.8 M HF for 1 min to cover the surface with silver nanoparticles. The size of the silver nanoparticles appears in the range of 20 to 60 nm. The silver nanoparticle-coated Si wafers were placed in an etching solution composed of 4.8 M HF and 0.15 M H2O2 at room temperature. The length of the resulting SiNW arrays was controlled by the etching time. In this time, the etching time was varied from 5 to 10 min. After etching, the wafers were dipped in a HNO3 aqueous solution for 10 min to remove all remaining silver nanoparticles. The wafers were then immersed in a 5% HF solution to remove the oxide layer. After preparation of the SiNW arrays, polydimethylsiloxane (PDMS) solution  was spin-coated on the arrays at 200 rpm and baked at 150°C. The transmittance of the 2-mm-thick PDMS coating was more than 90% in the range from 400 to 1,100 nm and exhibited a refractive index of about 1.4. The SiNW arrays thus embedded in the PDMS coating were mechanically peeled from the substrate with a razor blade.
We succeeded in measuring the key optical properties of SiNW arrays that were prepared with metal-assisted chemical etching and separated from the substrates by peeling. The absorptance of a SiNW array composed of 10-μm-long nanowires is much higher than the theoretical absorptance of a 10-μm-thick flat Si wafer. Therefore, SiNW arrays demonstrate a strong optical confinement effect. To investigate the reason why SiNW arrays demonstrate such a strong optical confinement, their scattering properties were observed. For an array with 10-μm-long SiNWs, the range of high transmittance was expanded to high scattering angles for wavelengths above 1,000 nm. Since high-angle scattering leads to the enhancement of photocurrent, the 10-μm-long SiNW array demonstrates strong light confinement for wavelengths above 1,000 nm. This enhancement of light scattering may be due to Mie-related light scattering because the ADF of this array is similar with the scattering patterns calculated by Mie-related theories.
This work was supported in part by JST, PRESTO, and the Nissan Foundation for Promotion of Science.
- Kurokawa Y, Kato S, Watanabe Y, Yamada A, Konagai M, Ohta Y, Niwa Y, Hirota M: Numerical approach to the investigation of performance of silicon nanowire solar cells embedded in a SiO2 matrix. Jpn J Appl Phys 2012, 51: 11PE12. 11PE12–4 11PE12-4View ArticleGoogle Scholar
- Hu L, Chen G: Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications. Nano Lett 2007, 7: 3249–3252. 10.1021/nl071018bView ArticleGoogle Scholar
- Zhu J, Yu ZF, Burkhard GF, Hsu CM, Connor ST, Xu Y, Wang Q, McGehee M, Fan S, Cui Y: Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays. Nano Lett 2009, 9: 279–282. 10.1021/nl802886yView ArticleGoogle Scholar
- Lin CX, Povinelli ML: Optical absorption enhancement in silicon nanowire arrays with a large lattice constant for photovoltaic applications. Opt Express 2009, 17: 19371–19381. 10.1364/OE.17.019371View ArticleGoogle Scholar
- Tsakalakos L, Balch J, Fronheiser J, Shih MY, LeBoeuf SF, Pietrzykowski M, Cordella P, Korevaar B, Sulima O, Rand J, Davuluru A, Rapol U: Strong broadband optical absorption in silicon nanowire films. J Nanophotonics 2007, 1: 013552. 10.1117/1.2768999View ArticleGoogle Scholar
- Kosten ED, Warren EL, Atwater HA: Ray optical light trapping in silicon microwires: exceeding the 2n(2) intensity limit. Opt Express 2011, 19: 3316–3331. 10.1364/OE.19.003316View ArticleGoogle Scholar
- Zhang ML, Peng KQ, Fan X, Jie JS, Zhang RQ, Lee ST, Wong NB: Preparation of large-area uniform silicon nanowires arrays through metal-assisted chemical etching. J Phys Chem C 2008, 112: 4444–4450.View ArticleGoogle Scholar
- Li XL: Metal assisted chemical etching for high aspect ratio nanostructures: a review of characteristics and applications in photovoltaics. Current Opinion in Solid State & Mater Sci 2012, 16: 71–81. 10.1016/j.cossms.2011.11.002View ArticleGoogle Scholar
- Shin JC, Zhang C, Li XL: Sub-100 nm Si nanowire and nano-sheet array formation by MacEtch using a non-lithographic InAs nanowire mask. Nanotechnology 2012, 23: 305305. 10.1088/0957-4484/23/30/305305View ArticleGoogle Scholar
- Hochbaum AI, Fan R, He RR, Yang PD: Controlled growth of Si nanowire arrays for device integration. Nano Lett 2005, 5: 457–460. 10.1021/nl047990xView ArticleGoogle Scholar
- Zhang YF, Tang YH, Wang N, Yu DP, Lee CS, Bello I, Lee ST: Silicon nanowires prepared by laser ablation at high temperature. Appl Phys Lett 1998, 72: 1835–1837. 10.1063/1.121199View ArticleGoogle Scholar
- Pan H, Lim S, Poh C, Sun H, Wu X, Feng Y, Lin J: Growth of Si nanowires by thermal evaporation. Nanotechnology 2005, 16: 417–421. 10.1088/0957-4484/16/4/014View ArticleGoogle Scholar
- Liu HI, Maluf NI, Pease RFW, Biegelsen DK, Johnson NM, Ponce FA: Oxidation of sub-50 Nm Si columns for light-emission study. J Vac Sci Technol B 1992, 10: 2846–2850.View ArticleGoogle Scholar
- Chen C, Jia R, Yue HH, Li HF, Liu XY, Wu DQ, Ding WC, Ye T, Kasai S, Tamotsu H, Chu J, Wang S: Silicon nanowire-array-textured solar cells for photovoltaic application. J Appl Phys 2010, 108: 094318. 10.1063/1.3493733View ArticleGoogle Scholar
- Shiu SC, Chao JJ, Hung SC, Yeh CL, Lin CF: Morphology dependence of silicon nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction solar cells. Chem Mater 2010, 22: 3108–3113. 10.1021/cm100086xView ArticleGoogle Scholar
- Kayes BM, Atwater HA, Lewis NS: Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J Appl Phys 2005, 97: 114302. 10.1063/1.1901835View ArticleGoogle Scholar
- Stelzner T, Pietsch M, Andra G, Falk F, Ose E, Christiansen S: Silicon nanowire-based solar cells. Nanotechnology 2008, 19: 295203. 10.1088/0957-4484/19/29/295203View ArticleGoogle Scholar
- Sivakov V, Andra G, Gawlik A, Berger A, Plentz J, Falk F, Christiansen SH: Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters. Nano Lett 2009, 9: 1549–1554. 10.1021/nl803641fView ArticleGoogle Scholar
- Bao H, Ruan XL: Optical absorption enhancement in disordered vertical silicon nanowire arrays for photovoltaic applications. Opt Lett 2010, 35: 3378–3380. 10.1364/OL.35.003378View ArticleGoogle Scholar
- Krc J, Zeman M, Kluth O, Smole E, Topic M: Effect of surface roughness of ZnO:Al films on light scattering in hydrogenated amorphous silicon solar cells. Thin Solid Films 2003, 426: 296–304. 10.1016/S0040-6090(03)00006-3View ArticleGoogle Scholar
- Plass KE, Filler MA, Spurgeon JM, Kayes BM, Maldonado S, Brunschwig BS, Atwater HA, Lewis NS: Flexible polymer-embedded Si wire arrays. Adv Mater 2009, 21: 325–328. 10.1002/adma.200802006View ArticleGoogle Scholar
- Bohren CF, Huffman DR: Absorption and Scattering of Light by Small Particles. New York: Wiley; 1983.Google Scholar
- Mie G: Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Ann Phys 1908, 330: 377–445. 10.1002/andp.19083300302View ArticleGoogle Scholar
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