silver nanoparticles effect on silicon nanowires...
TRANSCRIPT
Chemistry Journal
Vol. 1, No. 2, 2015, pp. 10-14
http://www.publicscienceframework.org/journal/cj
* Corresponding author
E-mail address:[email protected] (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.
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