Subwavelength Si nanowire arrays for self-cleaning

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www.rsc.org/materials | Journal of Materials Chemistry

Subwavelength Si nanowire arrays for self-cleaning antireflection coatings†‡ Yu-An Dai,a Hung-Chih Chang,a Kun-Yu Lai,a Chin-An Lin,a Ren-Jei Chung,b Gong-Ru Lina and Jr-Hau He*a

Downloaded by National Taiwan University on 29 March 2011 Published on 18 October 2010 on http://pubs.rsc.org | doi:10.1039/C0JM00524J

Received 26th February 2010, Accepted 31st August 2010 DOI: 10.1039/c0jm00524j Galvanic wet etching was adopted to fabricate Si nanowire arrays (NWAs) as a near-perfect subwavelength structure (SWS), which is an optically effective gradient-index antireflection (AR) surface and also exhibits super-hydrophobicity with an extremely high water contact angle (159 ). Fresnel reflection and diffuse reflection over the broad spectrum can be eliminated by a Si NWA AR coating. Moreover, Si NWA SWSs show polarization-independent and omnidirectional AR properties. The wavelength-averaged specular and diffuse reflectance of Si NWA SWSs are as low as 0.06% and 2.51%, respectively. The effects of the surface profile of this biomimetic SWS on the AR and superhydrophobic properties were investigated systematically.

Introduction Recently researchers are increasingly turning toward nature for design inspiration to mimic a variety of the functions of living species. It is critical to clarify the effects of the surface profile of the biomimetic structures to optimize the desired functionality.1–3 For instance, subwavelength structures (SWSs) for antireflection (AR)4–7 are inspired by the corneas of moth-eyes, which are comprised of an array of protuberant structures.8 Such AR behavior arises from the graded refractive index profile between the air and the surface (of the eye or substrate). As compared to the conventional multilayer coating, the polarization-insensitive, broadband, and omnidirectional aspects of the graded-index SWS make it an ideal candidate for AR coatings (ARCs) for solar utilization.9 Recently, the availability of nanofabrication has enabled the engineering of SWSs with desired AR characteristics. However, current developments of SWS ARCs are greatly impeded by either expensive and complicated top-down fabrications,10–13 or thermally unstable bottom-up methods.7,14 For practical applications with low cost, thermal stability, and durability, the interest in nano-scaled AR textures have been extended to disordered structures using a simple, yet scalable, wet etching method.15 Wet etching is an appropriate method to fabricate a large amount of uniform SWSs with defined type, doping level, and crystal orientation, which is in demand for the design and analysis of ARCs for various optical and optoelectronic devices, such as solar cells.16,17 A solar cell experiences a range of wavelengths and angles of incidence (AOI) over an entire day. To ensure high solar collection efficiency, an ideal ARC has to maintain low reflectance for broadband and wide AOI. However, the Fresnel

a Institute of Photonics and Optoelectronics, Department of Electrical Engineering, National Taiwan University, Taipei, 106, Taiwan (ROC). E-mail: [email protected]; Fax: +886-2-2367-7467; Tel: +886-23366-9646 b Graduate Institute of Biotechnology, National Taipei University of Technology, Taipei, 106, Taiwan (ROC) † The research was supported by the National Science Council Grant No. NSC 96-2112-M-002-038-MY3, NSC 96-2622-M-002-002-CC3, and Aim for Top University Project from the Ministry of Education ‡ Electronic supplementary information (ESI) available: SEM images of Si NWAs. See DOI: 10.1039/c0jm00524j

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reflection predicts a large reflectance at large AOI, except for those near the Brewster angle. This presents a fundamental constraint against the requirements of a low reflectance at all AOI for solar application. Moreover, little has been reported about the diffuse reflection, which depends on the surface morphology of the nanostructured ARCs. The diffuse reflection could even be severe compared to its specular counterpart, which poses serious limitations on AR performances. Hence the studies of diffuse reflection are needed to provide valuable information to optimize the performance of ARCs. Another biomimetic example is the self-cleaning effect by the formation of the super-hydrophobic surface, which is observed on lotus leaves as well as many other kinds of leaves.18 The surface features combining AR and super-hydrophobic effects allow the solar cells to keep contamination away so that efficiency is maintained. In this study, we demonstrate that the randomly grown Si nanowire arrays (NWAs) fabricated by galvanic wet etching at room temperature can accomplish a near-perfect ARC to eliminate not only specular, but diffuse reflection significantly with super-hydrophobic surfaces with a contact angle as high as 159 . Combining small discontinuities of refractive index at air/Si NWA layers interfaces with a small magnitude of refractive index gradient of Si NWA layers leads to excellent AR performance to eliminate not only Fresnel reflection, but also diffuse reflection over a broad range of wavelengths. Moreover, the AR performance of Si NWAs is almost independent of the polarization of the incident light over a wide range of AOIs, showing excellent omnidirectional AR performance. This fabrication is compatible with standard industrial manufacturing and paves the way for developing self-cleaning ARCs for a large variety of the applications ranging from solar cells and photodetectors to optical components and flat panel displays.

Experimental Single crystalline p-type (001) Si substrates with r ¼ 8–12 U cm were used here. The Si substrates were first cleaned in acetone, followed by a HF dip to remove the native oxide from the surfaces. The cleaned Si substrates were then immersed into an aqueous HF solution containing Ag nitrate and treated for the This journal is ª The Royal Society of Chemistry 2010


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desired times at room temperature. The concentrations of HF and AgNO3 were 4.6 M and 0.02 M, respectively. The substrate-bound NWAs were mechanically scraped, sonicated in ethanol, and deposited on carbon-coated copper grids for TEM characterization. Morphological studies of grown Si NWAs have been performed with a JEOL 2100F TEM operating at 200 kV and a JEOL JSM-6500 field emission SEM. Optical reflection measurements were measured for nearly normal light incidence (8 offset) covering the spectral regions from 250 nm to 850 nm with a standard UV-vis spectrometer (JASCO ARN-733) and an integrating sphere. In this measurement, the noise level is 0.002%. For the specular reflectance spectra, the coherent reflectance of a collimated incident light beam was determined by collecting the specularly reflected cone of light within an acceptance angle of 8 . The diffuse reflectance was determined by subtracting the specular reflectance from the total reflectance. The measurement of the contact angle (CA) was accomplished by observing 10 ml water droplets on the surface, and the presented CA values are the average of 10 repeats. A CCD camera was used to capture static images to determine the CAs. The advancing (and receding) CAs were obtained by slowly and gradually dispensing (withdrawing) the droplets on the surface with a needle, and measuring the largest (smallest) CAs before the contact line moved. The measurement of advancing and receding CAs were performed by the movie-capturing feature in the CA apparatus.

Results and discussion Galvanic wet etching was used to fabricate single crystalline Si NWAs at room temperature. The formation of Si NWAs is initiated by Ag-induced etching on the Si surface exposed to the AgNO3/HF solution via the electrochemical redox reaction process in which both cathodic and anodic reactions occur simultaneously at the surface of Si while the charge is exchanged through the substrate.19–21 Firstly, Ag ions in the vicinity of the Si surface capture the electrons from the valence band of Si, and consequently Ag nanoparticles (NPs) are deposited and act as local cathodes oxidizing the Si beneath. Subsequently, the HF etchants dissolve the Si oxide underneath Ag NPs into SiF62. This localized etching at the Ag/Si interface extends further along the preferred direction leading to the eventual formation of Si NWAs. Longer Si NWAs were obtained with longer etching times. Fig. 1(a)–(i) shows the cross-sectional scanning electron microscopy (SEM) images of Si NWAs etched for 10 min to 24 h. The length of Si NWAs vs. etching time shown in Fig. 1(j) indicates the linear relationship, allowing an empirical determination of the formation rate of the Si NWAs. The growth rate is estimated to be 50 nm min1. However, the growth rate of Si NWAs is not always constant with etching time. For example, the length of Si NWAs etched for 24 h is 40 mm, suggesting that the growth rate was lowered during long-time etching. We suggest that Si surfaces in the trough areas between NWAs are hard to expose to the HF solution with increasing etching time, leading to a slow dissolution rate of SiO2 underneath the Ag NPs. In addition, it was found that etching times play a role in not only the length, but also the density of the Si NWAs. The fill factor of Si NWAs over the entire wafer surface was characterized by plane-view SEM images, as presented in Fig. S1 in the ESI.‡ It is obvious that the fill factor of the Si NWAs at the air/ This journal is ª The Royal Society of Chemistry 2010

Fig. 1 The cross-sectional SEM images of Si NWAs etched for (a) 10 min, (b) 20 min, (c) 30 min, (d) 60 min, (e) 90 min, (f) 120 min, (g) 200 min, (h) 300 min, and (i) 24 h. The scale bar in (a)–(i) is 1 mm. (j) The length of Si NWAs vs. etching time.

Si NWA interface decreases with etching time. The prolonged reaction time enhances the chance of formation of Ag NPs on the Si, which results in more dissolution of Si. As a result, longer etching time leads to more sparse Si NWAs, as shown in the schematic of Si NWA formation in Fig. 2(a)–(e). A typical transmission electron microscopy (TEM) image of a Si nanowire (NW) (Fig. 2(f)) shows the diameter increases from the tip to the bottom of a Si NW, confirming this speculated mechanism. Moreover, Fig. S2(a) and S2(b) in the ESI‡ are the high-magnification crosssectional SEM images showing that the fill factor of Si NWAs etched for 5 h is increased from top to bottom. The gradient of the fill factor can be confirmed by the following two observations: (i) in Fig. S2(a), two or more NWs are merged into a thicker one at the bottom, resulting in the large fill factor at the bottom with respect to the one at the top; (ii) in Fig. S2(b), there are J. Mater. Chem., 2010, 20, 10924–10930 | 10925


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Fig. 2 (a)–(e) A schematic representation of the formation of Si NWAs. The prolonged reaction time enhances the chance of formation of Ag NPs on the Si and results in more dissolution of Si, which leads to the fact that the fill factor of Si decreases with the depth of Si NWAs. (f) A typical TEM image of a Si NW showing that the diameter increases from the tip to the bottom of the Si NW.

Fig. 3 (a) A low-magnification TEM image of a Si NW and the corresponding selected area electron diffraction pattern (inset). (b) An HRTEM image taken near the edge of the individual NW.

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short NWs in the regions close to the bottom.‡ Since these short NWs are not uniform in length, the densities of the NWs and therefore fill factor were found to gradually increase from top to bottom. Consequently, the fill factor of Si varies with the depth, i.e. the fill factor at the bottom regions of Si NWAs is larger than that at the tip regions of Si NWAs. To further analyze the structure of Si NWAs, TEM characterization was performed. As shown in Fig. 3 (a), the Si NWs are single crystalline, demonstrated by a low-magnification TEM image and the corresponding selected area electron diffraction pattern, which can be indexed as the [110] zone axis of Si. The axial orientation of a NW can be characterized to be parallel to the crystallographic [001] direction of Si. As shown in Fig. 3 (b), a high resolution TEM (HRTEM) image taken near the edge of the individual NW shows Si NWAs fabricated by galvanic wet etching are rough as compared with the smooth surfaces of Si NWs using a vapor–liquid–solid method. 22 The roughness of the Si NWs is 2 nm. We ascribe this roughness to the randomness of the lateral oxidation, slow HF etching and faceting of the correlattice.23 The measured interplanar distance of 5.4 A sponds to the Si (001) plane, indicating that the Si NWAs grew along the [001] direction again. This anisotropic etching can be interpreted in terms of the lattice configuration of Si surfaces.19–21 The (001) plane presents two covalent bonds symmetrically directed into the reactive solution, leading to a geometry that sterically prefers etching Si atoms along the [001] direction.

Fig. 4 (a) Specular reflectance spectra and (b) diffuse reflectance spectra of the Si NWAs.

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To understand the importance of the geometrical factors of the Si NWA SWSs to AR properties, the specular and diffuse reflectance spectra of the Si NWA layers were measured over a large spectral region from 250 to 850 nm. The spectra are shown in Fig. 4(a), indicating that the polished Si exhibits specular reflectance >35% while specular reflectance is effectively decreased by forming a Si NWA layer over the broadband regions. Broadband AR can be related to the gradient of the fill factor from the top to the bottom of the Si NWA layers, resulting in the gradient refractive index effect.6,7,24 A perfect Si cylinder NWA with uniformity should be equivalent to a homogeneous coating with a constant index of refraction. For a perfect cylinder structure, AR properties could not employ over a broad range of wavelengths, and a minimum in the reflectance would be expected at a wavelength for which the optical thickness is l/4. However, such behavior has been not observed in all of our samples. The wavelength-averaged reflectance can be decreased to 0.06% from 250 nm to 850 nm after the Si is nanostructured. Due to the feature size of Si NWAs (such as the diameter of Si NWAs and the distance between the NWAs) is too small with respect to the wavelength of incident light, light propagation is governed by the effective refractive index of the NWAs. The continuous profile of the refractive index of Si NWA layers was proposed by the randomized deposition of Ag NPs with prolonged reaction times, as shown in Fig. 2. A long etching time leads to sparse Si NWAs with long length, leading to the low effective refractive index at the air/ Si NWA interfaces with a smaller magnitude of index gradient from the top to the bottom based on the effective medium theory.25 An effective refractive index at the interface of the air and the Si NWAs estimated by averaging the refractive indices of air (n ¼ 1) and Si (n ¼ 3.7) weighted by volume is decreased with etching time as shown in Fig. S1.‡ For the Si samples etched for 24 h, the lowest refractive index at the interface of the air and the Si NWAs, and the smallest magnitude of effective refractive index gradient due to the longest length of Si NWAs result in the lowest broadband specular reflectance. As a good antireflector, not only specular reflectance but also diffuse reflectance should be eliminated. Fig. 4(b) shows the diffuse reflectance of Si NWAs. For the short Si NWAs, due to roughness, the scattering results in large diffuse reflectance; the diffuse reflectance is virtually over 10% for Si NWAs etched for 90 min, which is lower than that of polished Si, can be observed. For example, the wavelength-averaged diffuse reflectance (2.51%) for the Si NWAs etched for 24 h is much lower than that of polished Si (4.66%). Moreover, the two reflectance This journal is ª The Royal Society of Chemistry 2010

Fig. 5 The specular reflectance as a function of the AOI for (a) TE and (b) TM lights. (c) The ratio of the reflectance of the TE-polarization to that of the TM-polarization, denoted as ITE/ITM.

maxima at 275 and 365 nm for polished Si are caused by the interband transitions.29 This principal spectral feature is also seen for the nanostructured surfaces and not shifted substantially with the etching times, indicating that the Si NWAs are not extensively damaged by the wet etching process.30 It is known that at normal incidence the reflection is independent of the polarization. However, the reflection strongly depends on the polarization when the AOI is non-zero. Fig. 5(a) and (b) show that the reflectance of Si NWA layers depends as a function of the AOI for different lengths of Si NWAs for TE and TM polarized lights, where TE and TM denote planes of incidence perpendicular and parallel to the electric field, respectively. The angular reflectance of all Si NWA structures is much lower than that of polished Si substrates, confirming the excellent J. Mater. Chem., 2010, 20, 10924–10930 | 10927


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Fig. 6 Static CAs on Si NWAs as a function of NWA length. The insets are the water droplets on the NWAs prepared with different etching durations.

AR characteristics. The reflectance for TE waves is decreased with NWA length. A similar reduction occurs for TM waves. The reflectance of the Si NWAs etched for 24 h is lower than 0.06% and 0.07% up to an AOI of 60 for TE and TM polarizations, demonstrating that the Si NWAs can effectively suppress the reflectance over the wide AOI ranges. The Brewster angle (qB) at which minimal reflectance occurs for TM polarization is decreased with NWA length and indiscernible in NWA structures with long length. For example, the qB for Si NWA etched for 24 h with h ¼ 40 mm is indistinguishable. As shown in Fig. 5(c), the polished Si exhibits a maximum of the ratio of the reflectance of the TE polarization to that of the TM polarization (ITE/ITM) > 100 at 75 , while a maximum of ITE/ITM drops significantly to 150 ) and the small hysteresis (