Chemistry Journal

Vol. 1, No. 2, 2015, pp. 10-14

http://www.publicscienceframework.org/journal/cj

* Corresponding author

E-mail address:rabia.benabderrahmane@gmail.com (R. B. Zaghouani)

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 silicon-

based 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

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 boron-

doped single crystalline silicon (100) with a resistivity of ~1-

3 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).

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.

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).

)(1 sAgéAg →++ (Eq. 1)

2

2

6 2262 HAgSiFHFAgSi ++→+++ −+−+ (Eq. 2)

Fig. 2. SEM image of a silver dendritic layer.

Chemistry Journal Vol. 1, No. 2, 2015, pp. 10-14 12

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 3 23 4 3 2Ag HNO AgNO NO H O+ → + + (Eq. 3)

Fig. 3. SEM cross-section view of SiNWs formed in HF/AgNO3/H2O2

solution during 40 min.

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

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 (<HNO3:H2O>=<1:1>) during 1 min (sample

B).Sample B, suspected to be more contaminated with Ag-

Nps, exhibits a pronounced decrease of the reflectivity in the

UV spectral regioncomparedto sample A. Fig.6 shows cross-

section 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.

a

13 R. Benabderrahmane Zaghouani et al.: SilverNanoparticlesEffect on SiliconNanowiresProperties

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.

a

b

Fig. 6. (a) SEM cross-section view of sample A (b): SEM cross-section view

of sample B.

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.

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.

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

Chemistry Journal Vol. 1, No. 2, 2015, pp. 10-14 14

[4] 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

[5] 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

[6] 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

[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 Single-Crystal 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.