Effect of hydrofluoric acid concentration on the
evolution of photoluminescence characteristics
in porous silicon nanowires prepared by
Ag-assisted electroless etching method
Item Type Article
Authors Najar, Adel; Anjum, Dalaver H.; Hedhili, Mohamed N.; Ng, Tien
Khee; Ooi, Boon S.; Ben Slimane, Ahmed; Sougrat, Rachid
Citation Najar A, Slimane AB, Hedhili MN, Anjum D, Sougrat R, et al.
(2012) Effect of hydrofluoric acid concentration on the evolution
of photoluminescence characteristics in porous silicon nanowires
prepared by Ag-assisted electroless etching method. Journal of
Applied Physics 112: 033502. doi:10.1063/1.4740051.
Eprint version Publisher's Version/PDF
DOI 10.1063/1.4740051
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Journal Journal of Applied Physics
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Link to Item http://hdl.handle.net/10754/312973
Effect of hydrofluoric acid concentration on the evolution of photoluminescence
characteristics in porous silicon nanowires prepared by Ag-assisted electroless
etching method
A. Najar, A. B. Slimane, M. N. Hedhili, D. Anjum, R. Sougrat, T. K. Ng, and B. S. Ooi
Citation: Journal of Applied Physics 112, 033502 (2012); doi: 10.1063/1.4740051
View online: http://dx.doi.org/10.1063/1.4740051
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/112/3?ver=pdfcov
Published by the AIP Publishing
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Effect of hydrofluoric acid concentration on the evolution
of photoluminescence characteristics in porous silicon
nanowires prepared by Ag-assisted electroless etching method
A. Najar,
1
A. B. Slimane,
1
M. N. Hedhili,
2
D. Anjum,
2
R. Sougrat,
2
T. K. Ng,
1
and B. S. Ooi
1,a)
1
Photonics Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal,
Kingdom of Saudi Arabia
2
Advanced Nanofabrication, Imaging and Characterization Core Laboratory, King Abdullah University
of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia
(Received 3 February 2012; accepted 2 July 2012; published online 1 August 2012)
We report on the structural and optical properties of porous silicon nanowires (PSiNWs) fabricated
using silver (Ag) ions assisted electroless etching method. Silicon nanocrystallites with sizes <5nm
embedded in amorphous silica have been observed from PSiNW samples etched using the
optimum hydrofluoric acid (HF) concentration. Th e strongest photoluminescence (PL) signal has
been measured from samples etche d with 4.8 M of HF, beyond which a significant decreasing in
PL emission intensity has been observed. A qualitative model is proposed for the formation of
PSiNWs in the pre sence of Ag catalyst. T his mo del affirms our observations in PL enhancement
for samples etched using HF < 4.8 M and the eventual PL reduction for samples etched beyond
4.8 M of HF concentration. The enhancement in PL signals has been associated to the formation
of PSiNWs and the quantum confinement effect in the Si nanocrystallites. Compared to PSiNWs
without Si-O
x
, th e HF treated samples exhib ited significant blue PL p eak shift of 100 nm. T his
effect has been correlated to the f ormation of defect states in the surface oxide. PSiNWs
fabricated using the electroless etching method can find useful applications in optical sensors
and as anti-reflection layer in silicon-based solar c ells.
V
C
2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4740051]
I. INTRODUCTION
Nanoscale silicon has been intensively investigated and
explored for its applications in microelectronics, photonics,
and biomedical sensors.
1–3
Specific efforts have been con-
centrated in the development of new silicon nanostructures,
including quantum dots, nanowires, or porous silicon (PS).
Porous silicon has attracted much attention, especially in
enhancing photo-emission. Much research efforts have been
invested to realize an optical device with porous silicon,
4–6
but the inefficiency
7
and instability
8
of optical characteristic
in PS still remain. In addition, silicon nanowires (SiNWs)
are also ideal candidate for the study of physics of low-
dimensional systems. It has potential impact in realizing
nanoscale interconnects and functional device elements in
future nanoscale electronic and optoelectronic devices.
9,10
The field of SiNWs synthesis represents an exciting and
rapidly expanding research area. Considerable efforts have
been devoted to the development of versatile and controlla-
ble methods for the synthesis of SiNW. These methods can
be broadly classified as: (i) bottom-up, and (ii) top-down
approaches. The bottom-up approach involves the construc-
tion of desirable nanostructures from the basic components,
i.e., from the atomic level to the nano- or micro-scale wires.
This method is useful for the fabrication of low-dimensional
heterostructure based devices in large quantities.
11,12
Using
bottom-up, SiNWs were first obtained by vapor–liquid–solid
(VLS) method,
13
followed by an etching step to create nano-
wires. The VLS method has been implemented in a variety
of techniques, such as pulsed laser deposition (PLD),
14,15
gas-phase molecular beam epitaxy (GS-MBE),
16
chemical
vapor deposition (CVD),
17,18
laser ablation,
19,20
and oxide-
assisted growth techniques.
21
Top-down approach seeks to fabricate SiNWs from high
quality single crystal silicon wafer or thin film. Silicon nano-
wires have also been realized using lithographically defined
patterns, or spin-coating of nanospheres as etched mask,
22
followed by etching of the nanowires using plasma process-
ing technique. The fabrication of silicon nanowires using the
metal-assisted electroless etching method has also been
adopted.
23–25
The silver (Ag) ions in an ionic solution of
hydrofluoric acid (HF) and hydrogen peroxide (H
2
O
2
) have
been used to prepare the arrays of SiNWs from single crystal
wafers.
26,27
The effects of various process parameters such
as the etchant concentration of H
2
O
2
, etching time and post-
etch treatment on the morphology and optical properties of
the SiNWs have also been investigated.
28
The fabrication of
nanowires using this method does not require complex sam-
ple preparation steps. Furthermore, this technique is effec-
tive, having high throughput and low cost.
In this paper, we explore the effect of HF concentration
on the formation of porous silicon nanowires (PSiNWs) with
the aim of establishing a better understanding of the
a)
Author to whom correspondence should be addressed. Electronic mail:
boon.ooi@kaust.edu.sa.
0021-8979/2012/112(3)/033502/6/$30.00
V
C
2012 American Institute of Physics112, 033502-1
JOURNAL OF APPLIED PHYSICS 112, 033502 (2012)
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formation mechanisms of mesopores and nanocrystalline
structures in the PSiNWs. A qualitative model based on the
SEM observations with inferences from PL, high resolution
TEM, and XPS will be developed.
II. EXPERIMENTS
PSiNWs were fabricated by Ag assisted electroless etch-
ing method from a n-type Si wafer (100) with a resistivity of
0.01–0.02 X cm. The Si wafers were cleaned using acetone
followed by ethanol for 5 min in an ultrasonic bath. Next, the
wafers were immersed in a piranha solution H
2
SO
4
/
H
2
O
2
¼ (3:1) for 20 min to remove the organic deposits from
the surface. The cleaned wafers were transferred into HF/
AgNO
3
solution with a concentration of 4.8 M/0.005 M for
Ag-deposition, followed by rinsing with de-ionized water.
Then, the treated Si samples were etched in the HF/H
2
O
2
so-
lution for 45 min. Finally, samples were rinsed again for
10 min with HNO
3
solution to dissolve the excessive Ag
nanoparticles (Ag NPs), leaving behind traces of Ag for cata-
lyzing the etching reaction. Four samples were etched using
HF concentration of 1.8, 2.8, 4.8, and 5.8 M, respectively,
with a fixed H
2
O
2
concentration of 0.5 M.
The surface morphology of PSiNWs was investigated
using a FEI’s Magellan 400 FEG SEM operating at 5 keV
beam energy. The crystal structure, size of Si crystallites,
and the average pore size were measured using the FEI’s
TitanG
2
80–300 TEM. X-ray energy-dispersive spectroscopy
(EDS) analysis of PSiNWs was carried out with an x-ray de-
tector from EDAX (EDAX, Mahwah, NJ) attached to the
TEM. Energy filtered TEM (EFTEM) was also performed to
separate the crystalline phase of Si from its amorphous oxide
phase using a GIF Tridiem
TM
post-column energy filter from
Gatan. The photoluminescence (PL) measurements were per-
formed at room temperature using the Jobin Yvon LabRAM
ARAMIS system. A 8 mW diode-pumped solid-state (DPSS)
laser emitting at 473 nm was used as the PL excitation
source. The XPS studies were carried out in a Kratos Axis
Ultra DLD spectrometer equipped with a monochromatic Al
Ka x-ray source (h ¼ 1486:6eVh)in1 10
9
Torr vac-
uum. The spectrometer dispersion was adjusted to give a
binding energy of 932.63 eV for metallic Cu
2
p
3/2
. Samples
were mounted in floating mode in order to avoid differential
charging.
29,30
Charge neutralization was required for all sam-
ples. Binding energies were referenced to the C 1s binding
energy of adventitious carbon contamination which was
taken to be 284.80 eV. The measured data were analyzed by
fitting the individual peaks with a Gaussian (70%)-Lorent-
zian (30%) function.
III. RESULTS AN D DISCUSSIONS
In the presence of Ag catalyst, an increase in HF or
H
2
O
2
concentration in electroless etching method is analo-
gous to an increase in the current density in electrochemical-
based methods.
31,32
In both cases, increasing the H
2
O
2
or HF
concentration increases the oxidation rate and dissolution
rate, respectively, resulting in nanostructures with varying
optical properties.
33
Figures 1(a) to 1(h) show the cross-
section SEM micrographs and TEM images acquired from
the NW samples grown with HF concentration of 1.8 M,
2.8 M, 4.8 M, and 5.8 M in a fixed H
2
O
2
concentration of
0.5 M. From Fig. 1(i), a typical 3D-tomography observation
was conducted using a TEM and 3D reconstructions were
FIG. 1. (a)–(h) SEM and TEM micrographs of PSiNWs fabricated with
increasing HF concentration from 1.8, 2.8, 4.8, up to 5.8 M. (i) 3D tomogra-
phy TEM image of nanowires in (f).
033502-2 Najar et al. J. Appl. Phys. 112, 033502 (2012)
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achieved using a simultaneous iterative reconstruction algo-
rithm of consecutive 2D slices in Fig. 1(f). The pore sizes
present a distribution from 10 to 50 nm, with an estimated
measurement error of 10%, and these pores go inside the
nanowire showing similar structure to porous silicon. An av-
erage distance between two neighboring pores of lower than
5 nm has been observed. These mesoporous structures are
expected to show strong quantum confinement effects. The
variation of NWs length, and NWs diameter with increasing
HF concentrations were measured and analyzed using SEM
and TEM, and the results are tabulated in Table I. The NWs
length shows significant changes, while the NWs diameter
varies from 80 nm to 210 nm for samples indicating the dom-
inant effect of vertical etching enhanced by Ag catalyst with
preferential etching along the low atomic density plane in
[001]. An increase in aspect ratio from about 9 to 12 and 27
has been calculated for NWs samples etched with 1.8 M,
2.8 M, and 4.8 M HF concentration (Figs. 1(a), 1(c), and
1(e)). However, the aspect ratio reduces to about 9 for NWs
etched with (Fig. 1(g) ) signifying a change in etching mecha-
nism with faster lateral chemical etch.
To identify the causes of the transition point between
the vertical -dominant to lateral-dominant etching in the
chemical process, the pore-density of the nanowires have
been examined thoroughly. The TEM images show clearly
that samples prepared with 4.8 M of HF concentration
present relatively high pore density compared to samples
prepared with 5.8 M of HF. The monotonic increase in the
pore-density evidently showed that lateral oxidation process
at a fixed H
2
O
2
concentration dominates at low HF concen-
tration of 4.8 M due to a limited HF dissolution rate. As
HF concentration increases, the HF dissolution rate increases
to the extent that the PSiNWs apexes were effectively dis-
solved at the highest HF concen tration of 5.8 M, forming
conical PSiNWs. Hence, the reaction (dissolution) rate domi-
nates the lateral PSiNWs chemical etching process at high
HF concentration, while the diffusion of HF etching species
and byproducts dominates at low HF concentration. In the
following paragraphs, we further explain the role of Ag
nanopartiules and ionic Ag
þ
in the formation of conical
PSiNWs, the transport of etchant and other by products in
the nanowires.
TABLE I. Effect of HF concentration on PSiNWs length, NWs diameter,
and aspect ratio analyzed from SEM micrographs.
HF concentration
PSiNWs physical parameters 1.8 M 2.8 M 4.8 M 5.8 M
NWs length (lm) 1.3–1.8 2–2.2 4.8–5.3 1.6–1.8
NWs diameter (nm) 160–210 100–180 80–200 150–200
Aspect ratio
(longest length/largest diameter)
9 12 27 9
FIG. 2. (a-i)–(d-i) The schematic illustration of the proposed qualitative etching model for the formation of PSiNWs using Ag-assisted HF/H
2
O
2
electroless
etching technique. (a-ii)–(d-ii) the corresponding SEM micrographs, prepared using various HF concentrations.
033502-3 Najar et al. J. Appl. Phys. 112, 033502 (2012)
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