Fabrication of nanocomposites based on silicon nanowires and study of their optical properties

Abstract

This article presents the results on a fabrication of silicon nanowires by a two-step metal-assisted chemical etching. Morphological and optical properties of the obtained silicon nanostructures depending on the type and resistivity of the initial silicon wafer have been studied. In addition, we have used the silicon nanowires as templates for zinc oxide and silver deposition. The resulting nanocomposites have been shown to demonstrate intensive photoluminescence and activity in the surface enhanced Raman scattering respectively. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Phys. Status Solidi C 13, No. 4, 146–150 (2016) / DOI 10.1002/pssc.201510219
Fabrication of nanocomposites based
on silicon nanowires and study
of their optical properties
Stanislau Niauzorau*, Kseniya Girel, Alexander Sherstnyov, Eugene Chubenko, Hanna Bandarenka,
and Vitaly Bondarenko
Belarusian State University of Informatics and Radioelectronics, P. Brovka str. 6, 220013 Minsk, Belarus
Received 18 October 2015, revised 21 December 2015, accepted 15 February 2016
Published online 26 February 2016
Keywords silicon nanowires, metal-assisted chemical etching, silver, SERS, zinc oxide, photoluminescence
* Corresponding author: e-mail openzstas@gmail.com
This article presents the results on a fabrication of silicon
nanowires by a two-step metal-assisted chemical etching.
Morphological and optical properties of the obtained sili-
con nanostructures depending on the type and resistivity
of the initial silicon wafer have been studied. In addition,
we have used the silicon nanowires as templates for zinc
oxide and silver deposition. The resulting nanocompo-
sites have been shown to demonstrate intensive photolu-
minescence and activity in the surface enhanced Raman
scattering respectively.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Metal-assisted chemical etching
(MACE) can be successfully used to fabricate different sil-
icon (Si) nanostructures. Comparing to other nanotechnol-
ogy approaches of the Si nanostructures formation, MACE
is a lithography free, low cost and simple method that al-
lows to grow ordered arrays of Si nanowires (SiNWs) [1]
demonstrating very prospective properties for optoelec-
tronics [2], solar cells [3], biomedical and chemical sensors
[4], energy storage devices [5, 6], etc.
By present time, many works have been performed to
reveal dependence of a SiNWs morphology on the MACE
regimes (etching time, temperature, and illumination) as
well as the metal used and the type of an initial Si substrate
and the etching solution composition [1-7]. Nevertheless,
complex research in this area has not been realized at full
rate yet.
Moreover, SiNWs can present very suitable templates
for a deposition of various materials resulting in a fabrica-
tion of nanocomposites with well-controllable structural
parameters [8, 9], which are favorable for a number of
practical applications (especially for optical devices).
Today two ways of the MACE method are used: (i)
one- and (ii) two-step. In the first case, deposition of metal
nanoparticles (NPs) and SiNWs formation simultaneously
occur in a solution containing metal ions and HF. Silver
(Ag) is known as the most popular metal for the MACE
process. However, Ag tends to crystallize into dendrites
upon a prolonged wet chemical deposition. That is why the
one-step technique often results in a fabrication of non-
uniform SiNWs. During the two-step MACE, the chemical
etching of Si follows the metal NPs deposition. Therefore,
the second way provides better control of the structural pa-
rameters of SiNWs.
In this paper, we presented results on the SiNWs for-
mation by the two-step MACE of p- and n-type Si wafers.
Total reflectance and photoluminescence (PL) spectra of
the obtained SiNWs were also studied. Finally, we used
different SiNWs as templates for a deposition of zinc oxide
(ZnO) and Ag to fabricate nanocomposites with peculiar
optical properties. ZnO in known as a wide-gap semicon-
ductor with outstanding optical properties which can be
useful for development of the light emitting diodes [10]. It
is very attractive to fabricate effective photoluminescent
ZnO/SiNWs nanocomposites as such structures may be in-
tegrated with Si wafers. On the other hand, Ag nanostruc-
tures demonstrate very intensive surface plasmon reso-
nance in the visible range. This phenomenon is widely
studied for many optical applications including surface en-
hanced Raman scattering (SERS) spectroscopy [11, 12].
Using SiNWs as templates for Ag deposition opens an op-
portunity to define a size, a shape and a spatial location of
the Ag nanostructures and in this way to manage their

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plasmonic properties. Here we found that the ZnO/SiNWs
nanocomposites show a strong PL intensity while the
Ag/SiNWs nanocomposites demonstrate the SERS-activity.
2 Experimental details
2.1 SiNWs growth SiNWs were fabricated on p- and
n-type (100)-oriented Si wafers with a resistivity of
12 and 0.01 Ωcm respectively. A chemical cleaning of the
Si wafers was performed for 10 min with a hot (75 °C) so-
lution of NH4OH, H2O2 and H2O mixed in a volume ratio
of 1:1:4. Ag was chosen as a catalyst metal for the further
Si etching. The deposition of Ag on the Si surface was per-
formed by the immersion technique according to the fol-
lowing procedure. The Si wafers were dipped into an
aqueous solution of 10 mM AgNO3 and 2.5 M HF for 3-30
min. As a result, Ag NPs were formed on the Si surface.
The obtained samples were etched in aqueous solutions of
5 M HF and 0.3 M H2O2 for 1-60 min. All procedures were
performed at the room temperature (21 °C).
2.2 Nanocomposites fabrication Just before the
nanocomposites formation the experimental samples were
dipped into a diluted nitric acid (HNO3) for 30 s to remove
Ag NPs from the SiNWs bottom. The nanocomposites
were formed on the SiNWs obtained during 60 min of the
MACE chemical etching.
ZnO was deposited on SiNWs based on the n-type Si
wafers by the wet electrochemical method. The electro-
chemical bath was composed of dimethylsulfoxide
(DMSO), ZnCl2, KCl, polyoxyethilene (POE) and H2O2.
The current density of 0.3 mA/cm2 was applied for 60 min.
The temperature of the solution was about 100 °C.
Ag/SiNWs nanocomposites were formed by the
SiNWs dipping into an aqueous solution of 3 mM AgNO3
and 0.5 M HF for 30 min at the room temperature.
2.3 Measurements The morphology and structure of
the samples were studied with the scanning electron micro-
scope (SEM) Hitachi S-4800 that provided a 1 nm resolu-
tion. The elemental composition of samples was deter-
mined using SEM Cambridge Instruments Stereoscan-360
equipped with the Link Analytical AN 10000 energy dis-
persive X-ray (EDX) analyzer. The diameter of the focused
electron beam was no more than 1 µm, the atomic mass
accuracy did not extended 0.1%, and the depth of the anal-
ysis was 1.3-1.5 µm under 20 keV. An equipment used to
conduct electrochemical processes was the potentio-
stat/galvanostat AUTOLAB PGSTAT302n. PL-spectra of
SiNWs and ZnO/SiNWs nanocomposites were recorded
using complex included Xe Newport 6271 lamp (excitation
at 350 nm), focusing optical system, monoсhromators SO-
LAR DM 160 and MS 7504i. Prior to study of the
Ag/SiNWs SERS-activity the drop of 10-6-10-12 M rhoda-
mine 6G (R6G, organic dye) solution was poured on the
sample surface and air-dried. SERS-spectra were recorded
with the 3D scanning confocal microscope Confotec
NR500 using 473 nm laser.
3 Results and discussion
3.1 Metal-assisted chemical etching Prior to the
chemical etching it was necessary to study the structure of
Ag NPs deposited on the Si surface. SEM images (are not
presented) revealed that Ag NPs of 50-500 nm diameter
were formed on the Si surface. We also observed the pits
under Ag NPs as a result of the Si oxide formation and its
following etching with HF during the immersion deposi-
tion. The diameter of the pits varied in the range of 50 –
100 nm.
The Si wafer coated with Ag NPs was immersed in the
solution for the chemical etching. Ag has positive redox
potential in contrast to Si thus Ag NPs oxidize the Si atoms
takin their electrons. Then Si oxide is dissolved due to HF
presence in the solution. It leads to the Ag NPs penetrating
into the Si wafer. In our case as the Si orientation was
(100) Ag NPs moved in the vertical direction [7].
SEM cross section view of SiNWs formed on p-type
Si wafer is shown in Fig. 1. It is well-seen that SiNWs
are vertical and highly ordered. The length of SiNWs is
about 10 µm while their diameter varies in the range of
30-150 nm. The structure of the SiNWs based on the n-
type Si wafer was the same (not presented). Their length
and diameter were 8 µm and 50-200 nm, respectively.
Figure 1 SEM cross section view of SiNWs formed on the
p-type Si wafer by the Ag immersion deposition for 3 min fol-
lowed by chemical etching for 60 min.
The SiNWs were found to be partially covered with a
microporous Si layer because of adding H2O2 to the etch-
ing solution.
The SiNWs growth kinetics had a linear character both
for the p- and n-type Si wafers (Fig. 2). A slower etching
rate of SiNWs formed on the n-type Si wafer in contrast to
the p-type Si wafer can be explained by the less resistivity
(0.01 Ωcm for the n-type Si wafer vs. 12 Ωcm for that of
the p-type) [13].

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Figure 2 Dependence of the p- and n-type SiNWs length on the
etching time.
3.2 SiNWs optical properties The total reflectance
spectra of SiNWs formed on p-type Si wafer are presented
in Fig. 3A. SiNWs formed by MACE on the p-type Si wa-
fer demonstrate low total reflectance in the range from 400
to 1100 nm that can be explained by strong light absorp-
tion and scattering [14].
It can be seen that there is a length of SiNWs at which
the minimum reflectance is achieved. Further increase in
the SiNWs length does not make any contribution to the
reflective properties.
The PL-spectra of SiNWs are presented in the Fig. 3B.
There are observed broad bands on the PL-spectra of
SiNWs formed on p- and n-type Si wafers with maxima at
890 and 750 nm, respectively. SiNWs formed on the p-
type Si wafer demonstrate higher PL-intensity comparing
to SiNWs based on that of n-type. The PL-peak centered at
750 nm can be found due to the SiNWs coating with a thin
oxide layer. The Si oxidation is attributed to an air expo-
sure after the chemical etching. The PL-peak centered at
890 nm can be explained by an excitation from the na-
noporous surface of SiNWs and by SiNWs or nanocrystals
with dimensions less than 8 nm [15].
The Raman spectra of SiNWs formed by MACE on the
p-type Si wafer are presented in Fig. 4. The Raman inten-
sity increased strongly for SiNWs formed during 1 min of
the chemical etching (L = 1.1 µm) comparing to the initial
p-type Si wafer. It can be explained by plasmon effects in
the Ag NPs that were not removed after the chemical etch-
ing (there was no HNO3 treatment). The Raman intensity
of SiNWs went into decline as the etching time increased.
Raman peaks are broadening and gradually shifting to the
short-wavelength region according to the etching prolonga-
tion.
Figure 3 (A) The total reflectance spectra of SiNWs formed dur-
ing various etching time on p-type Si wafer; (B) the PL-spectra of
SiNWs formed on the p- and n-type Si wafers.
These facts can be explained by the presence of the Si
structures with nanoscaled dimensions. It leads to the de-
crease in the frequency of optical phonons in the vicinity
of the Brillouin zone center [16].
The peak at 520 cm-1 is attributed to the scattering
from crystalline silicon. It can be seen that upon the in-
creasing etching time a second peak at 480 cm-1 is emerged.
This peak is attributed to the scattering from an amorphous
Si (a-Si) [17]. The a-Si formation can be explained by
crystalline modifications at the etching time increase.

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Figure 4 Raman spectra of p-type Si and SiNWs/p-type Si
formed for various etching time.
3.3 ZnO/SiNWs nanocomposite Nanocomposite
materials based on ZnO and Si nanostructures are promis-
ing for their sensing [18] and optoelectronic [19] applica-
tions. SiNWs array is a convenient material for ZnO depo-
sition because of their developed surface area and good ad-
sorption properties. SiNWs formed on the n-type Si wafer
is more preferable for the ZnO electrochemical deposition
due to a great number of the free electrons (provided by the
dopant).
Figure 5 shows SEM cross-section view of SiNWs
based on the n-type Si wafer (chemical etching for 60 min)
after ZnO deposition. It is seen that a nucleation of ZnO
NPs occurs in the entire length of SiNWs. The average
size of ZnO NPs is about 20 nm. According to the EDX-
analysis atomic percentages of oxygen and zinc are
19.62 and 6.77 %, respectively. Such nonstoichiometric ra-
tio of zinc and oxygen can be associated with the Si oxida-
tion during the electrochemical deposition of ZnO.
Figure 5 SEM cross section view of the ZnO/SiNWs nanocom-
posite.
Figure 6 The PL spectrum of ZnO/SiNWs nanocomposite.
In Fig. 6 the PL-spectrum of the ZnO/SiNWs nano-
composite is presented. It shows strong emission peaks at
580 and 650 nm that can be explained by the oxygen va-
cancies and oxygen interstitial atoms in the ZnO nanocrys-
tals, respectively [20].
3.4 Ag/SiNWs nanocomposite Figure 7 shows the
cross sectional view of the Ag/SiNWs nanocomposite.
SiNWs were formed on the p-type Si wafer. Quantitative
analysis of SEM pictures revealed that Ag deposited as
NPs of 10-150 nm diameter prevalently on the top of
SiNWs. Figure 8 presents SERS-spectrum of molecules
adsorbed from 10-6 M R6G solution on the Ag/SiNWs sur-
face and on the surface of the pure SiNWs. The spectrum
has all bands typical for SERS-spectrum of this organic
dye [21]. No enhancement was observed for SiNWs with-
out Ag NPs (see the dashed line). We studied several con-
centration of the R6G down to 10-12 M. It was possible to
find the typical R6G bands even at 10-11 M. Therefore, the
detection limit provided by the Ag/SiNWs nanocomposite
corresponds to the best results on the SERS-active sub-
strates reported elsewhere [21, 22].
Figure 7 SEM cross section view of the Ag/SiNWs nanocompo-
site.

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Figure 8 SERS-spectrum of R6G molecules adsorbed on the sur-
face of the Ag/SiNWs nanocomposite. The dashed line is the
background from the sample of the pure SiNWs covered with the
R6G molecules.
4 Conclusion SiNWs with the length of 10 µm on
the p-type and 8 µm on the n-type Si wafers were formed
by the two-step MACE. The kinetics of SiNWs growth
was found to have the linear character. SiNWs demonstrate
the low total reflectance in the range from 400 to 1100 nm
due to their strong light absorption and scattering. SiNWs
formed on p- and n-type Si wafers demonstrate PL in the
visible and IR region, respectively. Raman spectroscopy of
SiNWs based on the p-type Si wafer showed that upon the
increasing etching time the Si Raman peak is broadened
and shifted to short-wavelength region. With the increasing
etching time the top of SiNWs is exposed the crystalline
modification according to the peak at 480 cm-1 in the Ra-
man spectra. The PL of ZnO/SiNWs nanocomposites was
also studied resulting in PL-spectrum with broad bands in
the visible range (peaks at 580 and 650 nm) that is ex-
plained by the oxygen vacancies and oxygen interstitial at-
oms in the ZnO nanocrystals, respectively. Finally, we
demonstrated significant SERS-activity of the Ag/SiNWs
nanocomposite provided the 10-11 M detection limit of the
R6G molecules.
Acknowledgements This work has been supported by the
Grant of the Ministry of Education of Republic of Belarus for
students, Belarusian State Research Programs “Nanotechnology
and Nanomaterials” (task 2.4.16) and “Electronics and photonics”
(task 2.3.06).
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