Chemistry Journal Vol. 1, No. 2, 2015, pp. 10-14 http://www.publicscienceframework.org/journal/cj
Silver Nanoparticles Effect on Silicon Nanowires Properties R. Benabderrahmane Zaghouani*, S. Aouida, N. Bachtouli, B. Bessais Photovoltaic Laboratory, Research and Technology Centre of Energy, BorjCedria Science and Technology Park, Hammam-Lif, Tunisia
Abstract Thiswork focuses on optical characterization of silicon nanowires (SiNWs) used for photovoltaic applications. We report on the elaboration of silicon nanowires by Metal Assisted Chemical Etching (MACE) technique using silver (Ag) as metal catalyst. The obtained resultsshow that the SiNWsoptical properties are sensitive to their elaboration conditions specially the cleaning protocol. The reflectivity was found to be dependent on wavelength and increased from ultra-violet to red wavelength for all tested samples. The comparison between samples with different cleaning protocol shows that, in the UV spectral region, SiNWsmore contaminated with Ag nanoparticles present a pronounced decrease of the reflectivity. This optical behavior was attributed to metallic nanoparticles persisting in the silicon nanowires structures presenting surface plasmon resonance energy in a vicinity of UV spectral region. Keywords SiNWs, MACE, Reflectivity, Silver Nanoparticles, Surface Plasmon Resonance Received: March 5, 2015 / Accepted: March 20, 2015 / Published online: March 24, 2015 @ 2015 The Authors. Published by American Institute of Science. This Open Access article is under the CC BY-NC license. http://creativecommons.org/licenses/by-nc/4.0/
1. Introduction Silicon-based solar cells remain the main candidate in photovoltaic solar energy conversion [1]. However, a large part of the solar cell conversion-efficiency limits are attributed to the optical losses in silicon material. That’s why the scientific community isinterested in developing concepts and technologies that enable reducing optical losses and thus enhance the solar cell efficiency [2-4]. Conventionally, the surface reflectivity of silicon-based solar cells can be reduced by texturing the front side of the cell [5-7]and/or using appropriate antireflection (AR) coatings[8,9]. During the last decade, important efforts have been dedicated to the use of silicon nanowires structures (SiNWs) in photovoltaic applications. SiNWs deposited on the surface of siliconbased solar cells could act as an efficient antireflection layer. Such structures present highly light absorption behavior which could attain 97% in UV-visible spectral regionsattributed to light confinement of incident light in the * Corresponding author E-mail address:
[email protected] (R. B. Zaghouani)
nanowires structures, as for porous silicon. Nowadays, silicon nanowires have been integrated in many otherdevices and applications such as silicon nanowire field-effect transistor (SiNW-FET) in microelectronics applications and biological sensorsthanks to their high internal surface [10,11].Many techniques are proposed in order to elaborate homogenous silicon nanowires: bottom up and top down approaches. One of the mostly used methods in the bottom up approaches the vapor Liquid Solid (VLS) method which needs the use of hard equipments like the CVD or PECVD. In this work, we have elaborated silicon nanowires with a top down method: the Metal Assisted Chemical Etching (MACE). This method is simple and can lead to homogenous silicon nanowires. Gold and silver are the most used metals as catalysts. Gold is known to diffuse and create deep-level defects in the silicon band gap [12]. These defects act as recombination centersdeteriorating the electrical properties of
11
R. Benabderrahmane Zaghouani et al.: SilverNanoparticlesEffect on SiliconNanowiresProperties
silicon. In this study, wechoose to work with silver (Ag). We report on the effect of Ag nanoparticles (Ag-Nps) on SiNWs optical behavior. Therefore, we study the sensitivity of SiNWs optical response with experimental conditions specially the cleaning protocol.
2. Experimental Details To elaborate silicon nanowires, the substrates used are borondoped single crystalline silicon (100) with a resistivity of ~13 ohm.cm and a thickness of 300 µm. Before SiNWs elaboration, silicon substrates were cleaned with acetone for 1 min followed by immersion in ethanol for 1 min and rinsed in deionized water in order to eliminate organic greases. Finally, the substrates are etched in 5% HF for 1min to eliminate native oxide.Silicon nanowires were prepared by Metal Assisted Chemical Etching technique (MACE). The method used is one-step process consisting of silver nanoparticles deposition and silicon etching (Fig.1 (a)). The silicon substrates are immersed in a mixture of 10 ml HF (48%), 10 ml AgNO3 solution and 1 ml H2O2 (30%) at room temperature for 40 min.At the end of the experience, the substrates are covered by a thick silver layer, the dendritic layer, which has an important role in the etching mechanism (Fig.1 (b)) [13,14]. Finally, the substrates are rinsed with water to stop the etching and then rinsed with nitric acid (HNO3) to eliminate silver (Fig.1(c)). The surface reflectivityof elaborated SiNWs was analyzed using a PerkinElmer Lambda 950 UV/VIS spectrometer.The morphological features of SiNWs were investigated using a JEOL JSM-5400 Scanning Electron Microscopy (SEM).
3. Results and Discussion 3.1. Silicon Nanowires Elaboration in HF/AgNO3/H2O2 Solution The elaboration of silicon nanowires by MACE consists on metal nanoparticles deposition and silicon etching. Metal nanoparticles are deposited by Electroless Metal Deposition (EMD) whichis a commonly used technique for metal-plating from a solution containing chemical reducing agents. Many research works have explained the formation of silicon nanowires by different mechanisms and have attributed different roles to metal nanoparticles depending on the process used and the experimental results. Qui et al. have proposed a chemical etching mechanism by using Ag-Nps [15]. They have suggested thatAg-Npsare protecting silicon during etching. Their conclusions are based on SEM characterization and X-ray analysis showing the presence of Ag-Npson the top of SiNWs.However, Peng et al. were confused about the role of silver. In 2003, theyexplained that silver nanoparticles protect the silicon during etching [16]but later in 2005, they suggested that Ag-Nps are catalyzing the nanowires formation [17]. Bai et al. have also proposed that silicon nanowires formation in HF/AgNO3/H2O2 solution is catalyzedby silver nanoparticles [13]. Our experimental results confirm the catalytic role of silver nanoparticles which cover the sidewalls of the formed SiNWs. In fact, silicon nanowires are elaborated in HF/AgNO3/H2O2 solution. The first reaction consists on the capture of electrons by Ag+ions from silicon surface leading to Ag atoms and then to Ag nanoparticles deposition (Eq. 1). The second reaction is dealing with silicon etching by the formation and dissolution of silicon oxide. The total reaction is depicted by (Eq. 2).
Ag + + 1é → Ag ( s ) 2−
Si + 2 Ag+ + 6F − + 2H + → SiF6 + 2 Ag + H2
Fig. 1. (a) Schematic illustration of the formation of SiNWs in HF/AgNO3/H2O2; (b) image of SiNWs sample covered by a dendritic layer; (c) image of SiNWs sample after elimination of the dentritic layer. Fig. 2. SEM image of a silver dendritic layer.
(Eq. 1) (Eq. 2)
Chemistry Journal Vol. 1, No. 2, 2015, pp. 10-14
As the etching time increases, a lateral growth of silver nanoparticlesis occurring leading to a dendritic layer.Fig.2 presents a SEM image illustrating this layer at the silicon surface showing the agglomeration of Ag-Nps. The presence of H2O2 in the etching solution may oxidize the dendritic layer generating more Ag+ ions in the solution. Despite the fact that dendritic layer is known to be transparent to Ag+ ions, its oxidation by H2O2 even with small amounts permits the formation of homogeneous SiNWs. Ag+ ions, going through the dendritic layer, oxidize silicon successivelyetched by HF.These two reactions continue until the formation of quasi-regularSiNWs covered by silver dendrites (Fig.1 (a)).This layer is eliminated by the reactivity ofnitric acid with silver structurefollowing (Eq. 3). 3 Ag + 4 HNO3 → 3 AgNO3 + NO + 2H 2 O (Eq. 3)
Fig. 3. SEM cross-section view of SiNWs formed in HF/AgNO3/H2O2 solution during 40 min.
12
silver during silicon etching [13]. 3.2. Optical Behaviorof Silicon Nanowires In order to investigate optical behavior of silicon nanowires, we have studied their reflectivity. In Fig. 5(a), we compare the reflectivity of silicon substrate with a SiNWs sample. We notice that the SiNWs decrease the total reflectivity as regards to the bare silicon thanks to light trapping by multiple reflections on SiNWs lateral surfaces. SiNWs reflectivity is wavelength dependant reaching its minimum in the UV spectral region. This behavior observed may be attributed to Ag-Nps. Silver nanoparticles are known to exhibit a plasmonic effect and present surface plasmon resonance energy in the UV spectral region [18] when illuminated by light with suitable wavelengths. This effect is a consequence of the oscillation of the conduction electrons of the metal under excitation enhancing the electric field leading to an absorption increase and then to a reflectivity decrease [19].As mentioned above, Ag-Npsare found to be on the bottom and on the sidewalls of SiNWs even after silver dendrites elimination(Fig. 4).To confirm theseobservations; we have investigated the sensitivity of SiNWs optical response with the cleaning protocol. In Fig.5(b), we compare the total reflectivity of SiNWs cleaned in a concentrated HNO3 solution during 1min (sample A) andSiNWs cleaned in dilutedsolution (=) during 1 min (sample B).Sample B, suspected to be more contaminated with AgNps, exhibits a pronounced decrease of the reflectivity in the UV spectral regioncomparedto sample A. Fig.6 shows crosssection SEM images of sample A (Fig.6(a)) and sample B (Fig.6(b)). We observe that sample B presents effectively a higher density of Ag-Nps than sample A.These results support the hypothesis of metallic nanoparticles contribution on SiNWs optical response.
Fig. 4. SEM cross-section view of SiNWs showing the presence of silver nanoparticles.
Fig.3 showsa cross-section SEM view of the obtained SiNWswith a length about 15 µm after the dendritic layer dissolution. Fig. 4 presents a magnification of the SiNWsillustrating the presence of Ag nanoparticles on their sidewalls. This observation confirms the catalytic effect of
a
13
R. Benabderrahmane Zaghouani et al.: SilverNanoparticlesEffect on SiliconNanowiresProperties
To confirm thisbehavior, we studied the reflectivity of the dendritic structures. Fig.7 shows the surface reflectivity of three dendritic layers presenting different densities (D1>D2>D3).We notice that the curves shape is similar to those obtained for SiNWs structures. The reflectivity of different layers is a wavelength-dependant decreasing from visible to ultraviolet wavelengths. It exhibitsa sharp drop around 350 nm reaching a minimum in the UV region. The drop observed is related to the optical response of Ag-Nps in the UV spectral region attributed to the plasmonic effect. This optical response presents an evidence of silver nanoparticles contribution in SiNWs reflectivity. b Fig. 5. (a) Reflectivity spectra of SiNWsand bare silicon substrate (b): Reflectivity spectra of SiNWs: sample A and sample B rinsed in concentrated and diluted HNO3, respectively.
4. Conclusions In this work, we have presented the elaboration of silicon nanowires structuresthat could be integrated in silicon-based solar cells as antireflection layer. SiNWs are formed by chemical etching technique assisted by silver. We reported a detailed study on the effect of silver nanoparticles on the optical response of silicon nanowires. These nanoparticles are persisting in the structure even after cleaning with HNO3and are influencing the SiNWs reflectivity. They present a plasmonic effect when excited by light leading to a reflectivity decrease. This property of Ag-Nps seems to be interesting in order to verify the efficiency of the cleaning protocol.
a
Fig. 7. Total reflectivity of three dendritic layers presenting different densities D1>D2>D3.
References [1]
Green MA. Crystalline and thin-film silicon solar cells: state of the art and future potential. Sol. Energy 2003; 74:181-192
[2]
Green MA. Photovoltaics: technology overview. Energy Policy 2000; 28: 989-998
[3]
Green MA. Thin-film solar cells: review of materials, technologies and commercial status. J. Mater. Sci: Mater Electron 2007; 18: S15-S19
b Fig. 6. (a) SEM cross-section view of sample A (b): SEM cross-section view of sample B.
Chemistry Journal Vol. 1, No. 2, 2015, pp. 10-14
[4]
[5]
[6]
Kang MH,RyuK, Upadhyaya A, Rohatgi A. Optimization of SiN AR coating for Si solar cells and modules through quantitative assessment of optical and efficiency loss mechanism. Prog. Photovolt: Res. Appl 2011; 19: 983-990 Bachtouli N, Aouida S, HadjLaajimi R, Boujmil MF, Bessais B. Implications of alkaline solutions-induced etching on optical and minority carrier lifetime features of monocrystalline silicon. Appl. Surf. Sci 2012; 258: 8889-8894 Macdonald DH, Cuevas A, Kerr MJ, Samundsett C, Ruby D, Winderbaum S, Leo A. Texturing industrial multicrystalline silicon solar cells. Sol. Energy 2004; 76: 277-283
[7]
Sopori BL, Pryor RA.Design of antireflection coatings for textured silicon solar cells. Solar Cells 1983; 8: 249-261
[8]
Chiao SC, Zhou JL, Macleod HA.Optimized design of an antireflection coating for textured silicon solar cells. Appl. Opt 1993; 32: 5557-5560
[9]
Richards BS. Comparison of TiO2 and Other Dielectric Coatings for Buried-contact Solar Cells: a Review. Prog.Photovolt: Res. Appl 2004; 12: 253-281
[10] Chen KI, Li BR, Chen YT.Silicon nanowire field-effet transistor-based biosensors for biomedical diagnosis and cellular recording investigation. Nano Today 2011; 6: 131-154 [11] Lu N, Gao A, Dai P, Wang Y, Gao X, Song S, Fan C, Wang Y. Ultra-sensitive nuclei acids detection with electrical nanosensors based on CMOS-compatible silicon nanowire field-effect transistors. Methods 2013; 63: 212-218
14
[12] Bullis WM. Properties of gold in silicon. Solid-State Electronics 1996; 9: 143-168 [13] Bai F, Li M, Song D, Yu H, Jiang B, Li Y. One-step synthesis of lightly doped porous silicon nanowires in HF/AgNO3/H2O2 solution at room temperature. Journal of Solid State Chemistry 2012; 196: 596-600 [14] Smith ZR, Smith RL, Collins SD. Mechanism of nanowires formation in metal assisted chemical etching. Electrochimica Acta 2013; 92:139-147 [15] Qiu T, Wu XL, Yang X, Huang GS, Zhang ZY.Self-assembled growth and optical emission of silver-capped silicon nanowires.App. Phys. Lett 2004; 84: 3867 [16] Peng KQ, Yan YJ, Gao SP, ZhuJ. Dendrite-assited growth of silicon nanowires in electroless metal deposition. Adv. Function. Mater 2003;13: 16 [17] Peng KQ, Wu Y, Fang H, Zhong X, Xu Y, Zhu J.Uniform, Axial- Orientation Alignement of One Dimensional SingleCrystal Silicon Nanostructure Arrays. Angew. Chem. Int. Ed 2005; 44: 2737 [18] RazaaS, Stengera N, Kadkhodazadeh S, Fischer SV, Kostesha N, Jauho AP, Burrows A, Wubs M, Mortensen NA. Blueshift of the surface plasmon resonance in silver nanoparticles: substrate effects. Nanophotonics 2013; 2: 131–138 [19] Atwater HA, PolmanA. Plasmonics for improved photovoltaic devices. Nature Mater 2010; 9: 205-213.
