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The composite structure of SiO2@AgNPs@p-SiNWs based on silicon nanowires (SiNWs) produced by metal-assisted chemical etching (MaCE) method has been designed to realize the significant reflection suppression over a broad wavelength range (300 - 2500 nm). Especially, the reflectivity of the structure even below 0.3% at a wide range of 620 - 1950 nm can be achieved. It also has been demonstrated that SiO2 capers play a dominant role in the significant reflection suppression of the composite structure.
©2013 Optical Society of America
1. Introduction
Due to the high refractive index of silicon, great incident light loss (up to 40%) occurs by reflecting back from the silicon/air interface [1], limiting its applications in photovoltaic cells and other optoelectronic devices [2–6]. In order to suppress the surface reflection over a broad wavelength range, antireflection layers with a graded-refractive-index are employed and have been investigated intensely [7–15]. Multilayered thin films fabricated by oblique-angle deposition can match the refractive indices of air and the substrate by controlling the porosity of each layer, showing excellent antireflection characteristics [7–9]. However, the various physical and chemical properties of the coating materials will affect the adhesion, the thermal mismatch, and the stability of the thin-film stacks [16,17]. Surface nano-texturing is another effective way to suppress reflection over a wide range for silicon. Lots of methods have been developed to fabricate nano-textured silicon, such as cryogenic deep reactive-ion etching [10,11], electron cyclotron resonance plasma etching [12,13], laser-assisted chemical etching [14], and fast atom beam etching [15]. Most of these methods, however, are often either too complicated or difficult for making sub-wavelength structures with optimum anti-reflection property over a wide range of wavelengths over large areas, therefore, they are not suitable for high-throughput and inexpensive industrial production.
Recently, large-scale silicon nanowire (SiNW) arrays fabricated by metal assisted chemical etching (MaCE) have been used as the antireflection layer for photovoltaic cells or other optoelectronic devices [18–22]. The MaCE method is very cheap and simple, which is suitable for high-throughput and inexpensive industrial production. However, the sudden changes of refractive indices at air/SiNW and SiNW/silicon substrate interfaces lead to higher optical reflection for silicon substrate textured with SiNWs than that with silicon nanocones (SiNCs) with a graded-refractive-index [23,24]. To improve the optical antireflection property of the silicon substrate textured with SiNWs, scattering particles, such as Al2O3, are deposited on the surface of SiNWs to lengthen the light propagation path, thus improving the light absorption [2].
Herein, we design a composite structure of SiO2@Ag nanoparticles (AgNPs)@porous silicon nanowires (p-SiNWs) with excellent optical antireflection property over a wide range of wavelengths (300 - 2500 nm). The SiO2@AgNPs@p-SiNWs structure is prepared by annealing the SiNWs fabricated by MaCE method at 550 °C in ambient air. This inexpensive and high-throughput method allows the utilization of nano-structured silicon in antireflection layer to be greatly enhanced.
2. Experiment
Vertically aligned SiNWs were fabricated from n-type (100) silicon wafers with a resistivity of 0.010-0.018 Ω/cm by MaCE method using silver particles as the catalysts [25,26]. After being cleaned with acetone, ethanol, and H2SO4: H2O2 solution in a 3:1 volume ratio, silicon wafers were dipped in diluted HF solution to remove the native oxide. The cleaned silicon wafers were immersed in 0.02 M AgNO3/4.8 M HF solution for 30 s at 25 °C to obtain silver particles as the catalysts. The silicon wafers were then etched in 0.30 M H2O2/4.8 M HF solution for 60 min at 25 °C to fabricate SiNWs. Finally, the as-prepared SiNWs (original SiNWs) were rinsed with de-ionized water and blown dry in N2. In order to study the effect of annealing time on the optical property of the SiNWs, the original SiNWs were annealed at 550 °C in ambient air for different time. The diffusion reflectance spectra of the samples were measured by spectrophotometer (SHIMADZU UV-3150). Morphology and microstructure investigations of the samples were performed by scanning electron microscopy (SEM) (HITACHI S-4800) and transmission electron microscopy (TEM) (JEOL JEM-1230).
3. Results and discussion
Figure 1(a) is a typical cross-sectional SEM image of original SiNWs, which are highly uniform and vertical to the silicon substrate with a length of about 32.1 μm. As the catalysts, large silver particles, several hundred nanometers in size, are located at the interface of SiNWs and the silicon substrate [Fig. 1(b)]. After the 60min-annealing, significant morphological evolution of silver occurs: tiny AgNPs uniformly appear on the surface ofSiNWs; meanwhile, large silver particles disappear (consumed), as shown in Fig. 1(c). Figure 1(d) is the corresponding TEM image of an individual 60min-annealed SiNW with magnified partial views shown in Figs. 1(e) and 1(f). The average size of AgNPs is about 5.7 nm, covered by a thin layer of SiO2 caps with an average thickness of about 4.8 nm. Interestingly, we found that most of the AgNPs located at the edge of SiNWs have a sector-like cross-section with the vertex pointing to the center of SiNWs, as shown in Fig. 1(f), in which contours of AgNPs and SiO2 caps are profiled by dark-blue and light-blue dashed curves, respectively. At the same time, SiNWs become porous (p-SiNWs), as shown in Fig. 3(g). So far, we have finished the fabrication of the composite structure of SiO2@AgNPs@p-SiNWs. It is worthwhile to note that only one step is necessary to fabricate this structure.
Fig. 1 Cross-sectional SEM images of original SiNWs before (a, b) and after (c) annealing at 550 °C in ambient air for 60 min. (d) The corresponding TEM image of (c). (e) The magnified view of the white square in (d). (f) The magnified view of the white square in (e). Dark-blue dashed curves indicate the AgNPs, and light-blue dashed curves show the contour of SiO2 caps.
More importantly, the composite structure of SiO2@AgNPs@p-SiNWs shows excellent optical antireflection characteristics. Figure 2(g) shows photographs of original SiNWs before (left) and after (right) annealing at 550 °C in ambient air for 60 min. The sample with annealed SiNWs looks extremely black, exhibiting excellent reflection suppression due to enhanced absorption of the front surface. In order to quantitatively characterize the reflection of these samples, their diffusion reflectance spectra were measured. The original SiNWs have a high reflectivity of over 10% at wavelength range of 550 - 1200 nm, as shown in Fig. 2(a). The reflection is much suppressed compared with that of silicon wafer due to the increased surface area of nano-textured silicon [17,27]; however the reflection suppression is not efficient enough. Further reflection suppression is realized by a simple method of annealing at550 °C in ambient air. After 60min-annealing, the reflection of SiO2@AgNPs@p-SiNWs is significantly suppressed to less than 1% at a wide range of 600 - 1200 nm [Fig. 2(f)], as well as at the range of 1200 - 2500 nm (the inset of Fig. 2). The remarkable reflection suppression comes from lengthening of the optical path by multiple scattering among p-SiNWs decorated with tiny nanoparticles [28]. The detailed reflectance evolution of the composite structure during annealing also has been studied. With increasing the annealing time, the color of the samples gradually turns dark, which is an indication of improvement in reflection suppression. It is well known that when a spherical nanoparticle is much smaller than the wavelength of the incident light (λ), the scattering cross section can be written as
where α is the polarizability of the particle [28]. For spherical nanoparticles covered by a semiconductor with permittivity of εs, the polarizability is given bywhere r and εm are the radius and permittivity of the particle, respectively. Based on these two equations, we can infer larger spherical nanoparticles have larger scattering cross section, thus stronger scattering effect. As the nanoparticles grow larger and denser with the increasing in annealing time (the detailed morphological evolution of AgNPs with annealing time can be found in our previous work [29]), the scattering effect of nanoparticles is becoming more efficient, leading to a gradual decrease in the reflectivity of the annealed SiNWs [Figs. 2(b) –2(f)]. Moreover further prolonging the annealing for another 60 min has no visible influence on the morphology of the composite structure, as well as on its reflection (not shown), indicating the 60min-annealing results in a stable morphology of the uniform tiny nanoparticles.Fig. 2 (a)-(f) Reflectance spectra for SiNWs annealed at 550 °C under varying annealing times: 0 (original SiNWs), 4, 6, 10, 30, and 60 min, respectively, at wavelength range of 300-1200 nm. The inset shows reflectance spectrum of 60min-annealed SiNWs at wavelength range of 1200-2500 nm. Curves around 870 nm are not shown in all spectra due to strong spectrum oscillations caused by detector changes. (g) Photographs of original SiNWs before (left) and after (right) annealing at 550 °C for 60 min.
It has been confirmed in Fig. 1 that the composite structure of SiO2@AgNPs@p-SiNWs consists of SiO2 caps, AgNPs and p-SiNWs. These three materials have all been proved to be efficient light trapping materials that have been intensively investigated in the field of photovoltaic cells [30–35]. In order to figure out the mechanism of the optical antireflection of the annealed samples, the original SiNWs were treated with different processes. Figures 3(a)–3(f) show schematic diagrams of surface morphologies of SiNWs after these treatments. All annealing processes were conducted under the same condition, i.e. at 550 °C in ambient air for 60 min. After annealing, the original SiNWs [Fig. 3(a)] are transferred to a composite structure of SiO2@AgNPs@p-SiNWs [Fig. 3(b)], which shows significant reflection suppression with reflectivity less than 1% at a wide range of 600 - 1200 nm [Fig. 2(f)]. If the SiO2 caps of the composite structure were removed by an immersion in HF as shown in Fig. 3(c), the reflectivity of AgNPs@p-SiNWs increases to more than 3% at the range of 600 - 1200 nm, especially hugely at wavelengths longer than 1100 nm, to which single-crystalline silicon is transparent because it has a band gap of 1.07 eV [Fig. 3(h)]. The scattering cross section of SiO2 caps is found to be several times larger than that of AgNPs after removal of SiO2 caps at long wavelengths by applying Eqs. (1) and (2), responsible for the lower reflectivity of SiO2@AgNPs@SiNWs than that of AgNPs@SiNWs. It is worthwhile to note that the radii of curvature of the SiO2 caps and AgNPs can be determined by TEM images easily, and permittivities can be found in references [36,37]. However at the range of 380 - 460 nm, the reflectivity of AgNPs@p-SiNWs decreases a lot with a valley at 410 nm, at which the reflectivity decreases by ~40% from 1.83% to 1.14%. This characteristic absorption band around 410 nm is caused by surface plasmon resonance of the AgNPs [38,39]. Meanwhile, no obvious plasmon band around 410 nm is observed for SiO2 coated SiNWs [Fig. 2(f)], indicating plasmon scattering of the AgNPs is weakened by the outer layer of SiO2 capers probably due to the highly efficient scattering effect of the uniform SiO2 capers [40]. If the large silver particles of the original SiNWs were removed by an immersion in HNO3 solution as shown in Fig. 3(e), the reflectivity of bare SiNWs [Fig. 3(j)] decreases by ~50% compared with that of the original SiNWs [Fig. 2(a)], because the large size silver is an efficient reflection material [2,41]. As we can see, the reflection suppression of SiO2 coated sample is actually very weak around 410 nm with respect to that of bare SiNWs, as the reflectivity decreases only by 6.6% from 1.96% to 1.83% at wavelength of 410 nm. For further investigation, SiO2 caps and AgNPs, both of them were removed by an immersion in HF and HNO3 solution respectively, and then nanopores on the SiNWs were exposed to the air as shown in Fig. 3(d). Figure 3(g) is a typical TEM image of an individual p-SiNW withnumerous nanopores uniformly distributing on the surface of SiNW. The corresponding reflection spectrum is shown in Fig. 3(i), which even shows a little better antireflection property than that of AgNPs@p-SiNWs at the range of 600 - 1000 nm, indicating the scattering effect of porous surface is stronger than that of AgNP decorated porous surface at these wavelengths, and are not fully utilized in the existence of AgNPs (otherwise, the reflectivity of AgNP@p-SiNWs should be lower than that of p-SiNWs at these wavelengths). The p-SiNWs are good antireflection materials with reflectivity less than 3.3%, about half of that of bare SiNWs at the range of 600-1000 nm. In addition, plasmon band around 410 nm disappears because of the removal of AgNPs. These results prove that SiO2 capers play a dominant role in the significant reflection suppression of the composite structure of SiO2@AgNPs@p-SiNWs. For comparison, bare SiNWs were also annealed to form SiO2@bare SiNWs as shown in Fig. 3(f), with corresponding reflectance spectrum and TEM image shown in Figs. 3(k) and 3(l). Compared with that of bare SiNWs, the reflectivity of SiO2@bare SiNWs shows no obvious decrease, indicating a thin flat SiO2 (~7.5 nm) doesn’t affect the optical property of SiNWs.
Fig. 3 (a)-(f) Schematic diagrams of surface morphologies of SiNWs after various treatments. (g) The corresponding TEM image of (d). (h)-(k) Reflectance spectra for SiNWs after various treatments, corresponding to (c), (d), (e), and (f), respectively. (l) The corresponding TEM image of (f).
Great antireflection also can be realized for annealed SiNWs even for the very short SiNWs, which is a great advantage for cost-effective photovoltaic cells and other optoelectronic devices. Figure 4 shows reflectance spectra for 8.5μm–SiNWs (a, b) (etching time, 15 min) and 2.1μm-SiNWs (c, d) (etching time, 4 min) before (a, c) and after (b, d) annealing at 550 °C in ambient air for 60 min. It shows that the reflectivity of annealed SiNWs decreases drastically compared with that of original SiNWs. For 60min-annealed 8.5μm–SiNWs, the reflectivity is below 1% at the whole detected range of 300 - 2500 nm [Fig. 4(b)]. In particular, at the range of 620 - 1950 nm, the reflectivity is below 0.3%. The lowest reflectivity is 0.189% at wavelength of 806 nm. The reflectivity is still strongly suppressed (<1%) for SiNWs with a length of only 2.1 μm at short wavelengths of 300 - 1300 nm [Fig. 4(d)]. However, as the wavelength increases, the reflectivity of 60min-annealed 2.1μm–SiNWs increases quickly to nearly 10%, due to reflection from SiNW/silicon substrate interface [27]. For short SiNWs, few large particles are remained after the annealing as shown in Fig. 4(e). But it has been found these few residual large silver particles have no obvious influence on the reflectivity of the composite structure (not shown).
Fig. 4 Reflectance spectra for 8.5μm-SiNWs and 2.1μm-SiNWs before (a, c) and after (b, d) annealing at 550 °C in ambient air for 60 min; (e) The corresponding SEM image of (d).
4. Conclusion
In conclusion, significant reflection suppression over a broad wavelength range (300 - 2500 nm) is realized by designing the composite structure of SiO2@AgNPs@p-SiNWs, which is a great advantage for cost-effective photovoltaic cells and other optoelectronic devices. It has also been demonstrated that SiO2 capers play a dominant role in the significant reflection suppression of the composite structure.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Nos. 60976012 and 51272232), Program for New Century Excellent Talents in University, the Fundamental Research Funds for the Central Universities and the Science and Technology Innovative Research Team of Zhejiang Province (2009R50010).
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