Solar Energy Materials & Solar Cells 89 (2005) 85
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Solar Energy Materials & Solar Cells 89 (2005) 85“94
Optical and photoelectrical properties of b-In
2
S
3
thin films prepared by two-stage process
Rahana Yoosuf, M.K. Jayaraj
Ã
Department of Physics, Optoelectronics Device Laboratory, Cochin University of Science and Technology,
Kochi 682022, India
Received 11 December 2004; accepted 10 January 2005
Available online 8 March 2005
Abstract
b-In
2
S
3
films were grown on glass as well as on quartz substrates by rapid heating of metallic
indium films in H
2
S atmosphere. The effect of sulfurization temperature and time on the
growth, structural, electrical and photoelectrical properties of b-In
2
S
3
films has been
investigated. Highly oriented single-phase b-In
2
S
3
films were grown by the sulfurization
technique. The morphology and composition of films have been characterized. The optical
band gap of b-In
2
S
3
is found to vary from 1.9 to 2.5 eV when the sulfurization temperature is
varied from 300 to 6001C or by increasing the sulfurization time. The electrical properties of
the thin films have also been studied; they have n-type electrical conductivity. The
photoelectrical properties of the b-In
2
S
3
films are also found to depend on the sulfurizing
temperature. A high photoresponse is obtained for films prepared at a sulfurizing temperature
of 600 1C. b-In
2
S
3
can be used as an alternative to toxic CdS as a window layer in photovoltaic
technology.
r 2005 Elsevier B.V. All rights reserved.
Keywords: Solar cells; b-In
2
S
3
; Buffer layer; Sulfurization; Chalcogenization; Thermal evaporation
ARTICLE IN PRESS
elsevierlocate/solmat
0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2005.01.004
Ã
Corresponding author. Tel.: +91 48 4257 7404; fax: +91 48 4257 7595.
E-mail address: mkj@cusat.ac.in (M.K. Jayaraj).Page 2

1. Introduction
I“III“VI
2
ternary chalcopyrite semiconductors are widely being used for solar cell
applications and its photovoltaic efficiency is predicted to yield 27“32% [1].
Photovoltaic conversion efficiencies higher than 18% have been achieved by
fabricating Mo/Cu(In, Ga)Se
2
/CdS/TCO structured device [2]. Recently, indium
sulfide is widely being studied for optoelectronic and photovoltaic applications [3].
The Cu (In, Ga)Se
2
-based solar cells with In
2
S
3
buffer layer have shown photovoltaic
conversion efficiency (15%) near those obtained by a device fabricated with CdS
buffer layer. The wide band gap (42.5 eV) of In
2
S
3
thin films suggests that it can act
as a better buffer layer than CdS having a band gap of 2.4 eV. The use of cadmium-
free buffer layer is desirable for environmental safety reasons.
Indium sulfide (In
2
S
3
) exists in different modifications depending upon
temperature and pressure with phase transitions at 420 and 750 1C [4]. The three
known polymorphic forms of In
2
S
3
are a, b and g. The a form is a defect face-
centered cubic, b is a defect spinel and g is a layered structure [5]. Of the three
modifications, b-In
2
S
3
single crystal is the stable form with tetragonal structure.
b-In
2
S
3
is an n-type semiconductor with a direct band gap of 1.98 eV [6].
b-In
2
S
3
thin films of band gap ranging from 2 to 2.7 eV have been grown by
various techniques like atomic layer chemical vapor deposition (ALCVD) [7], low-
pressure metal-organic chemical vapor deposition (MOCVD) [8], atomic layer
epitaxy (ALE) [9,10], spray pyrolysis [11“13], chemical bath deposition (CBD) [14],
annealing of elemental layers [15], by evaporating metal onto molybdenite [16],
physical vapor deposition (PVD) [17,18] and sulfurization of metallic electroplated
indium [19].
To the authorsâ„¢ knowledge, there is only one report on the preparation of In
2
S
3
films by the chalcogenization process [19]. The precursor for this chalcogenization
process was the electrodeposited metallic indium. In the present investigation, we
report on the study of the chalcogenization of thermally evaporated indium film. The
sulfurization was carried out at various annealing temperatures ranging from 250 to
600 1C in the presence of H
2
S. The In
2
S
3
films were characterized by studying the
structural, optical and electrical properties. The composition and the morphology of
the films and the dependence of processing temperature were also investigated.
2. Experimental details
Thin films of indium were coated on the glass as well as on quartz substrates by
vacuum thermal evaporation. High-purity indium (99.999%) was evaporated from
molybdenum boat under high vacuum (3 Â 10
Ã5
mbar). The thickness of the indium
layer was monitored using a quartz digital thickness monitor.
In
2
S
3
films prepared by the sulfurization of indium films deposited at room
temperature were found to be less adhesive. Better adhesion for the In
2
S
3
films was
obtained when the precursor indium films were deposited at a substrate temperature
of 75 1C. The In
2
S
3
films were prepared by heating the indium films under H
2
S
ARTICLE IN PRESS
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
86Page 3

atmosphere at different temperatures varying from 250 to 600 1C. The sulfurization
set up requires about 30“45 min to reach the set temperature. The slow heating of
indium films to set temperature resulted in the evaporation of indium and hence the
films obtained were very thin. To minimize indium loss, the metallic precursors were
introduced rapidly into the furnace in the presence of H
2
S, which has already
attained the set temperature. The duration of sulfurization was varied from 10 min to
3 h keeping the sulfurization temperature constant at 300 1C. The sulfurization of
indium films for 45 min and above resulted in single-phase b-In
2
S
3
. A band gap of
2.38 eV has obtained for the films sulfurized at 300 1C for a duration of 2 h. The
thickness of the In
2
S
3
films decreases with increase in sulfurization time. So further
chalcogenization experiments were carried out keeping the sulfurization time
constant for 45 min but for various sulfurization temperatures ranging from 250
to 600 1C. Single-phase b-In
2
S
3
films were obtained for the sulfurization temperature
at 300 1C and above. The thickness of the In
2
S
3
films was measured by using
gravimetric method by means of a sensitive microbalance and found to be in the
range 500“600 nm. Precursor indium films had a thickness of 150 nm, which was
measured using a quartz thickness monitor.
The In
2
S
3
thin films are further characterized by studying the structural, optical
and electrical properties. Crystallinity of the In
2
S
3
films was measured using a
Rigaku X-ray diffractometer with Cu-K
a
radiation. Compositions of the films were
analyzed using energy-dispersive X-ray analysis (EDX). The surface morphology of
the films was evaluated using the scanning electron microscopy (SEM) technique.
Optical transmittance measurements were performed using a Hitachi U-3410 model
uv“vis“nir spectrophotometer. The electrical characterization was done by
current“voltage measurements using Keithley Source Measure Unit (SMU 236)
with two silver electrodes in planar geometry. The type of carriers responsible for
conduction in In
2
S
3
films has been determined by the hot probe method and found to
be n-type.
3. Results and discussions
The crystallinity of the films was studied by recording the X-ray diffraction
pattern (XRD). The sulfurization of the indium films at 300 1C in H
2
S atmosphere
for 10 min yielded only a single peak of b-In
2
S
3
(0 0 1 2) and showed poor
crystallinity (Fig. 1). The films obtained by 30-min sulfurization at 300 1C showed
two major diffraction peaks (1 0 9) and (1 0 3) of b-In
2
S
3
along with the (1 1 1) peak
of InS phase. This may be because the duration of sulfurization was insufficient for
the complete conversion of InS to In
2
S
3
. Another diffraction peak (0 0 3)
corresponding to In
6
S
7
was also present, but it disappeared on annealing the
samples in air for 30 min. Single-phase In
2
S
3
was obtained when the sulfurization
time was 45 min at 300 1C, hence for further studies, the sulfurization time was fixed
as 45 min.
The XRD pattern of In
2
S
3
films prepared at different sulfurization temperatures
for 45 min is shown in Figs. 2(a) and 2(b). X-ray diffraction studies show that
ARTICLE IN PRESS
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
87Page 4

sulfurization of indium films at 300 1C and above result in single-phase b-In
2
S
3
.
When the films were sulfurized at 600 1C films were oriented with the (1 0 3) plane
parallel to the substrate surface. These films show only (h 03 h) reflections while the
other films, which were prepared at lower sulfurization temperature, showed a
preferred orientation along the (1 0 9) plane.
The grain size of the films was calculated using Scherrer™s formula t ¼
0:9l=b cos y; where l is the wavelength of the X-rays used, b is the full-width at
half-maximum (FWHM) in radians for a particular peak and y is the Bragg angle.
The grain sizes of the films were in the range 22“30 nm.
The absorption coefficient a was deduced from the transmission spectra using
the relation I ¼ I
0
e
Ãat
; where t is the thickness of the film. The absorption edge of
the b-In
2
S
3
was examined using the relation given by Bardeen et al. [20]. The
coefficient a is related to the incident photon energy hn as ahn ¼ bðhn à E
g
Þ
n
where
E
g
is the energy band gap and n ¼
1
2
for direct transition. Fig. 3 shows the plot of
ðahnÞ
2
as a function of the energy for the films sulfurized at different temperatures
for 45 min.
The optical band gap measurements show that the sulfurization time affects the
band gap values of the films. Asikainen et al. [9] have reported the dependence of
optical properties of b-In
2
S
3
on the sulfurizing parameter. The films sulfurized at
350 1C exhibit the higher band gap of 2.58 eV. For the films sulfurized at 400 1C and
above, the band gap is around 2.39 eV, which agrees very well with values reported
for b-In
2
S
3
films prepared by the SILAR method [21].
ARTICLE IN PRESS
Fig. 1. X-ray diffraction patterns of b-In
2
S
3
prepared by the sulfurization of indium films for different
durations at 300 1C and the standard JCPDS data of b-In
2
S
3
.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
88Page 5

ARTICLE IN PRESS
Fig. 2. (a) X-ray diffraction patterns of b-In
2
S
3
prepared by the sulfurization of indium films for 45 min at
different temperatures and the JCPDS File (250390) of b-In
2
S
3
; (b) X-ray diffraction pattern of b-In
2
S
3
prepared by the sulfurization of indium films for 45 min at 600 1C.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
89Page 6

The S/In ratio of the films prepared has been calculated by the EDX measurement.
From the EDX measurements, it is observed that the S/In ratio increases with
increase of sulfurization temperature up to 400 1C and then it decreases. The film
sulfurized at 350 1C has an S/In ratio 1.58 that is nearly stoichiometric.
In the present study the band gaps of In
2
S
3
films are found to be in the
range of 2.37“2.58 eV, which depends on the sulfurization temperature (Fig. 4). The
optical band gap value of the films of In
2
S
3
reported in the literature varied from 2 to
2.4 eV [22]. A higher value of band gap (42.5 eV) has also been reported. This
broadening of band gap in In
2
S
3
films has been explained with the help of
different phenomena. Kim et al. [23] interpreted the band gap broadening of their
thin films by the presence of excess sulfur in the bulk. Another possibility
for band gap broadening can be attributed to the partial substitution of oxygen for
sulfur to form In
2
S
3Ãx
O
x
[17]. In the present study, the possibility of the presence of
traces of oxygen cannot be ruled out; however, no oxides were detected by XRD
or in the EDX spectra. The higher resistivity of the films also suggests the absence of
oxygen impurities in the films. Some authors have explained the broadening of the
band gap of the b-In
2
S
3
thin films on the basis of the quantum size
effect [24]. Although the average grain size of the films in the present study obtained
from the XRD data is $22 nm for a sulfurization temperature below 400 1C and
$30 nm at 400 1C and above, the variation of the band gap of the films with
sulfurization temperature may be attributed to the quantum size effect. The
sulfurization at temperature 400 1C and above causes an increase in the grain size to
30 nm and the band gap is lower compared to films sulfurized below 400 1C. A
similar observation of variation of band gap on annealing has been reported by
Yousfi et al. [10].
ARTICLE IN PRESS
Fig. 3. The plot of ðahnÞ
2
vs. the energy for b-In
2
S
3
films sulfurized for 45 min at different temperatures.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
90Page 7

The morphology of the indium sulfide thin films was examined by the SEM
(Fig. 5). The grains are seen as spherically structured when the sulfurization
temperature is 350 1C. As the sulfurization temperature increases to 400 1C, the
surfaces are found to be more orderly and closely packed. Films obtained are found
to be formed of clusters when the sulfurization temperatures are 500 and 600 1C.
The resistivity ðrÞ of In
2
S
3
films is found to increase with sulfurization temperature
(Fig. 6). The low resistivity values of In
2
S
3
films prepared below 400 1C may be due
to the presence of impurity phases or incompletion of sulfurization. At higher
sulfurization temperature, the resistivities of b-In
2
S
3
films are in the range of
10
6
O cm. Similar values have been obtained for ALE-grown In
2
S
3
films [9].
Fig. 7 shows the variation of photosensitivity (I
L
ÃI
D
/I
D
) with sulfurization
temperature. I
L
is the current through the sample under illumination; I
D
is the dark
current. It is observed that photosensitivity increases with sulfurization temperature.
Maximum photoresponse was obtained for the highly oriented In
2
S
3
films prepared
by sulfurization at 600 1C. The increase in the photosensitivity can be attributed to
the improvement in crystallinity with increase in sulfurization temperature as
indicated by XRD (Fig. 2(b)).
4. Conclusions
b-In
2
S
3
thin films were prepared by the reactive sulfurization of evaporated
indium thin films at various sulfurization temperatures ranging from 250 to 600 1C.
Single-phase b-In
2
S
3
thin films can be obtained by sulfurizing the indium films above
300 1C for 45 min. The thickness of the In
2
S
3
films decreases with increase in
sulfurization time. High sulfurization temperature resulted in a clustered surface.
ARTICLE IN PRESS
Fig. 4. Variation of band gap and S/In of In
2
S
3
films with sulfurization temperature.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
91Page 8

ARTICLE IN PRESS
Fig. 5. SEM pictures of b-In
2
S
3
thin films sulfurized at (a) 350 1C, (b) 400 1C and © 600 1C.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
92Page 9

The resistivity of b-In
2
S
3
was found to increase from 10
2
to 10
6
O cm with the
increase in the sulfurization temperature. The maximum band gap of 2.58 eV was
obtained for the b-In
2
S
3
sample sulfurized at 350 1C, which is nearly stoichiometric.
ARTICLE IN PRESS
Fig. 6. Plot of resistivity ðrÞ vs. sulfurization temperature.
Fig. 7. Variation of photosensitivity of In
2
S
3
films with sulfurization temperature.
R. Yoosuf, M.K. Jayaraj / Solar Energy Materials & Solar Cells 89 (2005) 85“94
93Page 10

This wider band gap, n-type b-In
2
S
3
can be used as an alternative to toxic CdS as a
window layer in photovoltaics.
Acknowledgements
This work was supported by the Kerala State Council for Science and Technology
Environment Department (KSCSTE), Government of Kerala. The authors thank
KSCSTE for the financial assistance under SARD program. One of the authors
(RY) thanks the Ministry of Non-Conventional Energy Sources (MNES) for the
NRE research fellowship.
References
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[17] N. Barreau, S. Marsillac, D. Albertini, J.C. Berne˜de, Thin Solid Films 403“404 (2002) 331.
[18] N. Barreau, S. Marsillac, J.C. BerneËœde, T.Ben. Nasrallah, S. Belgacem, Phys. Stat. Solidi. 184 (2001)
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