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.
KeywordsSilicon nanowire Optical confinement Light scattering Solar cells 73.25.+i 77.55.df 78.67.Uh
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.
Results and discussion
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.
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