Physica E 28 (2005) 264–272
Role of an electrolyte and substrate on the stability
of porous silicon
Shailesh N. Sharma
Ã
, R.K. Sharma, S.T. Lakshmikumar
Materials Division, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110012, India
Received 14 March 2005; accepted 21 March 2005
Available online 6 June 2005
Abstract
Porous silicon (PS) layers were prepared by anodization on polished and textured substrates of (1 0 0) Si for a fixed
anodization time at different current densities in different HF-based electrolytes. Highly stable, mechanically strong,
hydrogen-passivated surface and thick porous silicon films have been obtained using HF:ethanol-based electrolyte on
textured silicon substrates. Porous silicon formed using HF:ethanol as an electrolyte exhibits superior properties
compared to porous silicon formed using HF:H
2
O
2
-based electrolyte at the same current density, time of anodization
and type of substrate. Porous silicon films formed on textured substrates exhibits higher porosity and photoluminescence
efficiency, negligible PL decay, better mechanical strength, adherence to the substrate, non-fractured surface
morphology and lower stress compared to porous silicon formed on polished silicon substrates at the same current
density for both ethanol and H
2
O
2
-based electrolytes, respectively. Use of textured silicon substrate and ethanol-based
electrolyte is a key parameter for the formation of tailored-made porous silicon films for device applications.
r 2005 Elsevier B.V. All rights reserved.
PACS: 61.43.Gt; 81.05.Rm; 82.45.Gj
Keywords: Porous silicon layers; HF-electrolytes: Si substrates

formation of a high-quality oxide surface layer is
now accepted as a good solution to the formation
of a stable surface and improved luminescent
properties [8]. Embedding the nanocrystalline
silicon particles in an optically transparent med-
ium is another way of isolating the surface from
the ambient and providing a stable luminescence
[9]. Recently, the use of alkyl-terminated mono-
layers as a mean of stabilizing the PS surface has
received attention where Si–H bonds at the surface
during PS formation are replaced by a hydrophilic
alkyl termination [10].
The electrolyte composition is one of the most
important fabrication parameter for well-defined
porous layers. The pore dimensions and porosity
change with different ratios of electrolytes. Var-
ious electrolytes have been used for the fabrication
of porous silicon viz, HF, ethanol, H
2
O
2
and
HNO
3
[1,11,12]. HF is mainly used for the
dissolution of silicon, ethanol is basically used to
reduce the surface tension of the electrolytic
mixture since surface wetting is important for
good pore uniformity. Recently thrust has been
given on H

H
5
OH and HF–H
2
O
2
) and
current density formed on textured and polished
Si substrates, respectively. The emphasis is mainly
on the development of PS with high and stable PL,
control of pore size distribution and therefore a
better control on the formation process.
2. Experimental
Boron-doped p-type Si wafers of (1 0 0) orienta-
tion, 8–10 ohmcm resistivity and 400 mm thickness
were used for preparing PS. The wafers were
polished in 40% NaOH for 2 min. These wafers
were textured using 2% NaOH at 85 1C for 30 min.
For forming the back contact, Ag–Al paste was
screen printed on the wafer and dried at 250 1C.
The wafer was then heated to 750 1C for 2 min in
an IR furnace. PS was formed by the standard
anodization process using Si as the anode and Pt as
the counter electrode in an acid resistant container.
The anodization was carried out at 20–50 mAcm
À2
for 30 min, in two different electrolytes. The first is
a mixture of HF and C
2
H

the laser radiation and PL measurements were
carried out at regular intervals.
3. Results and discussion
Good porous silicon films exhibiting high
photoluminescence intensity could be formed on
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S.N. Sharma et al. / Physica E 28 (2005) 264–272 265
both textured and polished substrates at various
current densities corresponding to both electro-
lytes A and B, respectively. The porosity (45–80%)
and thickness (12–96 mm) of PS films were
estimated from gravimetric measurements [14].
Fig. 1 shows porosity values as a function of I
d
for PS films formed on textured and polished
substrates corresponding to both electrolytes A
and B, respectively. As shown in Fig. 1, porosity of
PS films increases with increase in current density.
As evident from Fig. 1, PS films corresponding to
electrolyte B exhibits higher porosity as compared
to the corresponding films of electrolyte A for both
textured and polished substrates.
Fig. 2(A) shows the weight loss of PS films
prepared using electrolyte A at different I
d
,asa
function of time of ultrasonic treatment. There is a
substantial weight loss of PS samples on polished
substrates when subjected to an ultrasonic treat-
ment for an hour by which time the entire porous

50
60
70
80
(d)
(c)
(b)
(a)
Porosity (%)
Current Density I
d
(mA cm
-2
)
Fig. 1. Porosity of PS as a function of current density (I
d
); (a)
textured substrate, electrolyte B; (b) polished substrate,
electrolyte B; (c) textured substrate, electrolyte A; (d) polished
substrate, electrolyte A.
0204060
0.3450
0.3455
0.3460
0.3465
0.3470
0.3475
0.3480
0.3485
(c)

(A)
(
B
)
Fig. 2. Weight loss of porous silicon samples prepared at
different current densities (I
d
) for (A) electrolyte A and (B)
electrolyte B; (a) textured substrate, I
d
¼ 20 mA cm
À2
; (b)
polished substrate, I
d
¼ 20 mA cm
À2
; (c) textured substrate,
I
d
¼ 35 mA cm
À2
; (d) polished substrate, I
d
¼ 35 mA cm
À2
; (e)
textured substrate, I
d
¼ 50 mA cm

A and B, respectively (Figs. 3(A) and (B)). These
results are in accordance with quantum confine-
ment effects [1]. It is known that the peak position
of the PL intensity is blue shifted when HF-H
2
O
2
is used as the electrolyte [15]. A marginal shift in
PL peak position towards low l side is also
observed upon texturization (Figs. 3(A) and (B)).
Visual observation shows that the porous silicon
films corresponding to electrolyte A formed on
textured surfaces appear more uniform and strong
as compared to the corresponding films prepared
using electrolyte B. The PS films at higher current
densities (I
d
X35 mA cm
À2
) on polished substrates
shows a break off in PL curves as these films
are powdery in nature and hence unstable corre-
sponding to both electrolytes A and B. PS
films prepared using B are more powdery in nature
and shows peeling-off tendency particularly for
films prepared on polished substrates. This is
even more obvious for films formed at higher I
d
(X50 mA cm
À2

O
2
-
based electrolyte is used (Figs. 4(A) and (B)). To
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0
1
2
3
4
5
6
7
(a)
(f)
(e)
(d)
(c)
(b)
PL Intensity (a.u.)
500 550 600 650 700 750 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0

; (d) Polished substrate, I
d
¼ 35 mA cm
À2
(e)
textured substrate, I
d
¼ 50 mA cm
À2
and (f) polished substrate,
I
d
¼ 50 mA cm
À2
.
S.N. Sharma et al. / Physica E 28 (2005) 264–272 267
ensure the reproducibility of this PL decay,
measurements were done repeatedly and for
several hours and the PL decay trend was found
to be the same. This is a direct evidence for the
formation of stable surface and correlates with the
superior mechanical stability of porous silicon
formed on textured substrates.
SEM was used to identify the surface morphol-
ogy of the porous silicon formed on textured and
polished Si-substrates at different current den-
sities for electrolytes A and B, respectively. Silicon
nanowires are not visible at these magnifica-
tions. Figs. 5 (A) and (B) show the surface of
porous silicon formed on polished silicon at

results in increased porosity and the inability of
the silicon nanowires to withstand the stress leads
to cracking.
The surface morphology of PS films formed on
textured substrates is significantly different as
compared to polished substrates. Figs. 6(A) and
(B) shows the surface morphology of porous
silicon formed on textured substrates at I
d
¼
35 mA cm
À2
corresponding to electrolytes A and
B, respectively. Here, the smooth surface morphol-
ogy consists of randomly sized and spaced
pyramids homogeneously distributed on the sur-
face. The pyramids appear to be more sharply
separated but no macroscopic cracking is observed
even for electrolyte B-based sample unlike in the
case of PS film formed polished silicon substrate
for the same current density (Figs. 6(A) and (B)).
This surface morphology does not essentially
differ from the textured silicon substrate (not
shown) and is not affected by current density. On
polished silicon substrates, PS layers showed a
tendency to have a mechanically weak structure at
higher current densities (I
d
$50 mA cm
À2

(d)
(c)
(a)
PL Intensity (a.u.)
Time
(
mins.
)
(A)
(B)
Fig. 4. PL decay of porous silicon samples prepared at different
current densities (I
d
) as a function of time of laser exposure for
(A) electrolyte A and (B) electrolyte B; (a) textured substrate,
I
d
¼ 20 mA cm
À2
; (b) polished substrate, I
d
¼ 20 mA cm
À2
; (c)
textured substrate, I
d
¼ 35 mA cm
À2
; (d) polished substrate,
I

I
d
¼ 10 mA cm
À2
, electrolyte A; (B) I
d
¼ 10 mA cm
À2
, electrolyte B; (C) I
d
¼ 35 mA cm
À2
, electrolyte A; (D) I
d
¼ 35 mA cm
À2
,
electrolyte B.
Fig. 6. Scanning electron micrographs of porous silicon prepared on textured substrates at I
d
¼ 35 mA cm
À2
; (A) electrolyte A; (B)
electrolyte B.
S.N. Sharma et al. / Physica E 28 (2005) 264–272 269
prepared samples, it is clear that there are a
number of distinct peaks with different intensities.
Figs. 7(a) and (b) shows FTIR absorption spectra
for PS samples prepared using electrolyte A at
I

À1
which is
attributed to Si–H stretching modes when the
silicon is backbonded to oxygen atoms [21] and at
$2117 cm
À1
due to Si–H stretching mode, broad
peak at $1192 cm
À1
and a satellite peak at
$1010 cm
À1
due to Si–O–Si stretching mode and
a weak contribution at $879 cm
À1
due to non-
stretching Si–H modes [20] and no signal of Si–H
wagging modes between 600 and 700 cm
À1
was
observed. It is worthwhile to note that there is no
signature of any O atoms backbonded to Si–H
related mode at $2250 cm
À1
for PS films prepared
on textured substrates (Fig. 7(a)). Another inter-
esting difference noted in the FTIR spectra of PS
films using electrolyte A prepared on textured and
polished substrates is the shift of Si–O related
mode from 1110 to 1192 cm

to Si–H wagging modes, respectively. However,
for the corresponding PS sample formed on
polished substrate, the FTIR spectrum (Fig. 7(d))
exhibits mainly Si–O-related modes at 2250 cm
À1
(O backbonded to SiH mode), a broad peak
comprising of peaks at $1161 and 1018 cm
À1
(Si–O–Si stretching mode) with weak contribu-
tions at $880 and 805 cm
À1
(Si–H-related bending
and wagging modes). Here in Fig. 7(d), the notable
feature is the absence of Si–H stretching at
$2100 cm
À1
and Si–H wagging at $630 cm
À1
.
Thus, silicon–hydrogen-related modes are stronger
for PS samples prepared on textured substrates
while silicon–oxygen-related modes are stronger
for the corresponding films prepared on polished
substrates for the same current density and
electrolyte. The effect of oxidation is felt more
for H
2
O
2
-based PS films particularly formed on

(d)
(c)
(b)
(a)
Absorbance (a.u.)
Wavenumber (cm
-1
)
Fig. 7. FTIR absorption spectra of porous silicon prepared at
current density I
d
¼ 20 mA cm
À2
; (a) textured substrate,
electrolyte A; (b) polished substrate, electrolyte A; (c) textured
substrate, electrolyte B; (d) polished substrate, electrolyte B.
S.N. Sharma et al. / Physica E 28 (2005) 264–272270
nanocrystalline Si causes shrinkage of the Si-core
due to the breaking of Si–Si bonds resulting in a
blue-shift in PL spectra [11]. However, apart from
interpretation in terms of quantum confinement in
silicon clusters that decrease in size upon oxida-
tion, the PL blue shift can also be related to Si–O
species or due to defects and the silica networks on
which OH groups are absorbed as suggested by
others [23]. These results are in accordance with
our PL and SEM studies where a significant PL
decay and cracked surface morphology was
observed for PS films formed on polished sub-
strates which underlines the importance of tex-

exposed at the boundaries. This may lead to
partial merging of nanopores and the formation of
a high porosity region which can deform and
release the stress at dimensions small enough to
prevent macroscopic crack formation and fragility.
Thus high porosity of PS films formed on textured
substrates can be explained. However, in case of
PS films formed on polished substrates, the etching
is not preferential but random thus resulting in
lower porosity of PS layers. However, the proper
choice of both the substrate (textured) and the
electrolyte (ethanol-based) in conjunction can have
a profound effect in improving the luminescent
properties and stability of porous silicon films.
4. Conclusions
The visual observation of mechanically strong,
stable surface bond configuration, smooth surface
morphology and hydrogen-passivated PS surfaces
essentially conforms the viability of textured
substrates and ethanol-based electrolyte as a
requisite condition for the formation of highly
luminescent, thick and stable porous silicon films.
Porous silicon using ethanol-based electrolyte is
superior to porous silicon formed using H
2
O
2
-
based electrolyte at the same current density on
both textured and polished substrates, respec-

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