Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
H130
0013-4651/2007/155共2兲/H130/6/$23.00 © The Electrochemical Society
Synthesis and Characterization of Nanocrystalline SnS Films
Grown by Thermal Evaporation Technique
M. Devika,a,b N. Koteeswara Reddy,b D. Sreekantha Reddy,a Q. Ahsanulhaq,b
K. Ramesh,c E. S. R. Gopal,c K. R. Gunasekhar,d and Y. B. Hahnb,z
a
Department of Physics, Sri Venkateswara University, Tirupati–517 502, India
School of Semiconductor and Chemical Engineering, BK21 Center for Future Energy Materials and
Devices, Chonbuk National University, Jeonju–561 756, South Korea
c
Department of Physics and dDepartment of Instrumentation, Indian Institute of Science,
Bangalore–560 012, India
b
The SnS films were grown on glass substrates using the thermal evaporation technique at different substrate temperatures 共Ts兲
varied from 20 to 300°C, and their physical properties were studied with appropriate techniques. While increasing Ts, the sulfur
content in the films decreased and the Sn to S atomic percent ratio increased from 1.01 to 1.42. The structural studies showed that
most of the crystallites in the films were grown along 关111兴 direction and their grain size increased between ⬃18 and 41 nm with
the increase of Ts. The SnS films grown at Ts = 300°C exhibited considerably low electrical resistivity of ⬃43 ⍀ cm with an
average grain size of ⬃40 nm. These films also exhibited a direct optical bandgap of ⬃2.0 eV with a high absorption coefficient,
⬃106 cm−1. These results indicate that the physical properties of nanocrystalline SnS films are comparable to the properties of
bulk as well as microstructured SnS films and are suitable for photovoltaic or nanoquantum-well device applications.
© 2007 The Electrochemical Society. 关DOI: 10.1149/1.2819677兴 All rights reserved.
Manuscript submitted July 9, 2007; revised manuscript received October 29, 2007. Available electronically December 18, 2007.
Tin monosulfide 共SnS兲 is a narrow bandgap semiconductor material that belongs to IV-VI group and acts as an absorber layer in
heterojunction devices. SnS thin films grown with a thickness of
0.5 ␮m exhibit a direct optical bandgap of ⬃1.36 eV with an absorption coefficient of ⬃105 cm−1.1 The electrical and optical properties of SnS films can be tailored by doping with the suitable
dopants.2,3 SnS is a less toxic compound, and its constituent elements are abundant in nature. Besides these properties, its structure,
even at liquid nitrogen temperature, is stable and, also, the variation
of its optical properties with temperature is marginal.4 Because of
the above peculiar properties, SnS films have been widely used in
diverging fields, such as solar cells,5-7 photoconductors,8 semiconductor sensors,9 microbatteries,10 and solid-state lubricants.11 The
SnS films have been prepared using different techniques, such as
spray pyrolysis,12 pulse electrodeposition,13 plasma-enhanced
chemical vapor deposition,14 epitaxial,15 thermal evaporation,16
E-beam evaporation,17 etc.
Recently, nanostructured materials have attracted great attention
due to their exciting properties, which are different from their corresponding micro and bulk properties. Thus, the synthesis of nanostructured materials has been an active and challenging subject in
materials science and other fields.18 Moreover, the bandgaps of these
nanomaterials are particle-size dependent19 and, hence, the particle
size significantly influences the properties of the materials.20 In this
view, only a few studies have been reported on nanocrystalline SnS
material.21-26 Even though the investigations on SnS nanostructures
or nanoparticles are interesting, most of the above studies are limited to only the synthesis and analysis of a few properties. To date,
to the best of our knowledge, there has been no report on the electrical and optical properties of nanocrystalline SnS films. In our
earlier studies, it was observed that the films grown with a low
thickness 共t ⬍ 100 nm兲 exhibited some interesting properties as
compared to the films grown with higher thicknesses.27 The lower
thickness films exhibited slightly high absorption coefficient 共␣
⬃ 105 cm−1兲 as compared to the films grown with other thicknesses.
For comparison, the ␣ vs photon energy plots of SnS films grown at
three different thicknesses are shown in Fig. 1. Here, the shaded
portion represents the variation of ␣ with photon energy at above the
fundamental absorption edge 共FAE兲. It can be seen that the ␣ value
of thinner films is higher than that of thicker films and, also, the
absorbability of thicker films is limited only for a short range of
z
E-mail: [email protected]
photon energy. This interesting and constructive behavior of thinner
SnS films attracted us to investigate nanocrystalline SnS films and
study their physical properties in view of optoelectronic device applications. A variety of techniques that have been used for thin-film
depositions, such as thermal evaporation, sputtering, ion implantation, chemical vapor deposition, and pulse laser deposition, can be
adopted for the preparation of nanocrystalline films.28 Therefore, in
this study we used the thermal evaporation technique for the deposition of SnS films. The synthesis of nanocrystalline SnS films at
different growth temperatures and their physical behavior are reported and discussed here.
Experimental
SnS films have been deposited on Corning 7059 glass substrates
with a thickness of ⬃50 ± 5 nm using the thermal evaporation technique. The depositions were carried out at different substrate temperatures 共Ts兲 varied from 20 to 300°C under a high vacuum of
Figure 1. 共Color online兲 Variation of ␣ with photon energy of SnS films
grown with three thicknesses.
Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
10−6 Torr. A SnS compound of 4N purity was evaporated from a
molybdenum 共100 A兲 boat by applying a constant voltage of 30 V
with a current of 65 A. For all depositions, the distance between the
source and substrate was fixed as 14 cm and the rate of deposition
was maintained as ⬃2 Å/s. The thickness of the film was monitored
with the help of a digital thickness monitor 共model: DTM-101兲. A
1 kW radiant heater was used to heat the substrates, and a temperature controller was employed to control the substrate temperature
with an accuracy of ±2°C.
The as-deposited SnS films were characterized with appropriate
techniques as follows. The elemental composition of the films was
examined using an energy dispersive analysis of X-rays system attached with field-emission-scanning electron microscopy 关共FESEM兲, model: Hitachi S-4700兴. The structural properties have been
studied using the X-ray diffraction 共XRD兲 spectra collected from the
X-ray diffractometer 共model: Philips X’Pert Pro兲. The surface topology and morphology of the films were examined using FE-SEM and
atomic force microscope 关共AFM兲, model: Digital Instrument
Nanoscope-E兴 at room temperature. In order to study the current
transport mechanism in the films, the current-voltage 共I-V兲 measurements were carried out at room temperature. They were measured
under dark, using the van der Pauw method in the dc voltage range
of 0.0–1.0 V. Here, the voltage was supplied and output current was
measured with a high-resistance electrometer 共model: Keithley
6517 A兲. The temperature-dependent conductivity studies were also
carried out in the temperature range of 20–200°C. Transmittance
共T%兲 and reflectance 共R%兲 spectra of the SnS films were recorded
using the Fourier transform infrared spectrometer 共model: Bruker
IFS 66 V/s兲 in the wavelength range of 350–2500 nm.
Results and Discussion
The SnS films grown at different substrate temperatures 共Ts兲 are
smooth and pinhole free. The adherence of the films to the surface of
the substrate increased with the increase of Ts. The deposited films
appeared light gold in color and are highly transparent. The thickness t of the films, evaluated using gravimetric technique 共t
= m/␳A, where m is the mass of deposited film, ␳ is the density of
deposited material, and A is the area of the coated film兲, varied
between 45 and 55 nm with Ts. The thickness of the films was also
confirmed from FE-SEM cross-sectional measurements.
Composition analysis.— The composition analysis of the asdeposited SnS films showed that the sulfur content in the films decreased from 49.7 to 40.1 atom % with the increase of Ts. This
indicates that the films grown at lower substrate temperatures are
sulfur rich as compared to the films grown at higher substrate temperatures. While increasing Ts, the evaluated Sn to S atomic percent
ratio of the films increased from 1.01 to 1.49. The variation of Sn to
S atomic percent ratio as a function of Ts is shown in Fig. 2. The rate
of increase of Sn, S ratio with Ts is found to be ⬃1.7 ⫻ 10−3 /°C.
This variation in elemental composition of the films with Ts is
mainly attributed to the adatom mobility and/or reevaporation of
evaporated sulfur atoms due to high vapor pressure of sulfur.29 At
lower substrate temperatures, the adatom mobility of the evaporated
atoms on the surface of the substrate is less and the rate of reevaporation from the surface is also negligible. Therefore, a maximum
number of deposited sulfur atoms might react with tin atoms and
results in a nearly stoichiometric SnS films. While increasing Ts, the
nonstoichiometric nature of the films increased due to increase of
the above factors; adatom mobility as well as reevaporation of sulfur
atoms from the hot substrate made the films more sulfur deficient.
Structural analysis.— The smoothened XRD spectra of SnS
films deposited at three temperatures are shown in Fig. 3. The films
grown at room temperature appeared to have very poor crystallinity
or were nearly amorphous in nature. The crystallinity of the films
increased with the increase of Ts. As usual, a bump in the XRD
spectrum of the films between 15 and 40° is observed. This bump is
suppressed with the increase of Ts owing to the improvement in
H131
Figure 2. Sn/S atomic percent ratio vs substrate temperature of nanocrystalline SnS films.
crystallinity of the films. Here, the bump is due to amorphous glass
substrate. The SnS films grown at different substrate temperatures
exhibited one strong peak at around 2␪ = 31.62°. The evaluated
d-spacing value of this peak is comparable to the SnS JCPDS data
共card no. 39-0354兲. Hence, the crystallites in the films are exclusively oriented along the 关111兴 direction. These results point out that
the SnS films are crystallized orthorhombically with lattice parameters of a = 0.433, b = 1.119, and c = 0.398 nm. In the present
study, we could not find any other tin sulfide phases, such as SnS2
and Sn2S3, which were observed at lower substrate temperatures in
our earlier studies.1 This might be attributed to the rate of evaporation of the material. The average grain size of the films, evaluated
using Scherrer’s formula, increased from 18 to 41 nm with the increase of Ts, as shown in Fig. 4. Therefore, the films can be treated
as nanocrystalline or nanostructured films. Noticeably, the films
grown at higher substrate temperatures contained large-sized grains
and their sizes are comparable to the thickness of the film. To understand the genuine crystallinity of SnS films, the XRD data were
Figure 3. 共Color online兲 XRD spectra of nanocrystalline SnS films grown at
three different substrate temperatures.
Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
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Figure 4. Variation of average grain size and DPO of nanocrystalline SnS
films with substrate temperature.
Figure 5. XRD spectrum of annealed nanocrystalline SnS films at
T = 100°C for 1 h.
analyzed using the X’Pert HighScore software supplied by Philips
X’Pert Pro Co., Ltd., and the degree of preferred orientation 共DPO兲
of the films was calculated. In this analysis, we observed a minor
peak with 7–8% of intensity at 2␪ = 66.64° that belongs to 共080兲
plane along with 100% 共111兲 peak. Therefore, the DPO of the films
was calculated using the equation, DPO = I共111兲/I共080兲. While increasing Ts, the DPO of the films increased up to Ts = 150°C and,
above this temperature, it decreased. The variation of DPO with Ts
is also shown in Fig. 4. Similar behavior was observed for the SnS
films grown with 0.5 ␮m thickness with respect to substrate
temperature.1
Satyanarayana et al.30 reported that the interatomic distance in
the semiconductor materials increases when their particles size decreased to nanoscale. However, we could not find any change in
d-spacing values of SnS films even though the grain size of the films
is at nanolevel. The observed d-spacing values of SnS films are
given in Table. I along with standard data of SnS material. Su et
al.,21 An et al.,22,23 Li et al.,24 Shen et al.,25 and Chen et al.26 also
observed similar results in SnS nanostructures. Therefore, it can be
concluded that the effect of particle size on d-spacing values of SnS
films may be negligible. The difference between the observed results
of nanocrystalline SnS films grown at Ts = 300°C and the data reported elsewhere27 is either due to rate of deposition or annealing of
the SnS films because the later films were postannealed at 100°C for
1 h under a vacuum of 10−6 Torr. For clarification of above doubt,
the present films also annealed at 100°C for 1 h under a vacuum of
⬃10−5 Torr. The XRD spectrum of these annealed films is shown in
Fig. 5. It can be seen that the peak height of the annealed SnS films
Table I. The observed and standard d-spacing value of (111) peak
of nanocrystalline SnS films grown at different substrate
temperatures.
d-spacing values 共nm兲 correspond to 共111兲 peak
Ts
20
50
100
150
200
250
300
Observed
Standard
0.284
0.283
0.283
0.282
0.283
0.282
0.283
0.284
increased and the average grain size is found to be ⬃132 nm. These
results, therefore, clarify that the process of annealing of SnS films
may not be a cause for the above difference.
Surface topology and morphology analysis.— The surface
analysis of the as-deposited films demonstrates how the top surface
of the films changes with Ts. The FE-SEM images 共recorded at a tilt
angle of 45°兲 of SnS films grown at Ts = 20, 150, and 300°C are
shown in Fig. 6. Figure 6a shows that the crystallites on the surface
of the films grown randomly and formed a meshlike network with
deep holes. While increasing Ts, these deep holes disappeared and
formed a continuous and uniform surface at Ts = 150°C that can be
seen in Fig. 6b. With further increase of Ts, the size of the crystallites on the surface of the films increased and formed a continuous
surface with a high surface roughness 共Fig. 6c兲. Therefore, the crystallites on the surface of the films are initially forming meshlike
network and, while increasing Ts, the meshlike surface is slowly
converting into an irregular granular surface via a smooth and uniform surface. These changes were clearly observed in AFM studies.
AFM images 共recorded in the area of 500 ⫻ 500 nm兲 of SnS films
grown at Ts = 20, 150, and 300°C are shown in Fig. 7. The rootmean-square 共rms兲 roughness of the films, evaluated from AFM
analysis, decreased from 3.25 nm with the increase of Ts and
reached a minimum value of 2.87 nm at Ts = 150°C. With further
increase of Ts, the rms roughness increased and reached a maximum
value of 7.9 nm at Ts = 300°C. The average grain size of the films
obtained from AFM analysis is consistent with the data obtained in
XRD studies.
Electrical properties.— The variation of electrical resistivity of
the SnS films with Ts is shown in Fig. 8. The films resistivity drastically decreased from 3.4 ⫻ 104 to 43.4 ⍀ cm with the increase of
Ts. At higher substrate temperatures 共⬎150°C兲, the change in resistivity of the films with Ts appears marginal. However, the resistivity
in this region also decreased linearly with Ts is shown in Fig. 8 in
the inset. High resistivity of SnS films grown at Ts = 20°C is due to
poor crystallinity and smaller grain size in the films. While increasing Ts, the size of the grains increased and led to low resistivity. It is
interesting to note here that the resistivity of the nanocrystalline SnS
films grown at Ts = 300°C is low and also comparable to the values
observed in microstructured SnS films.1 This difference might be
attributed to improved crystallinity of the present films as compared
to the microstructured films. The activation energy Ea of nanocrys-
Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
Figure 6. FE-SEM pictures nanocrystalline SnS films grown at three substrate temperatures.
talline films was evaluated from the slope of the ln关n共␴兲兴 vs T−1
plots. The activation energy increased sharply from 0.16 eV with the
increase of Ts up to Ts = 150°C and above this temperature, its
variation is marginal. This can be seen from Fig. 9, which represents
the variation of activation energy with Ts. The films grown at Ts
= 300°C showed an activation energy of ⬃0.27 eV. The sharp increase in Ea of the films is attributed either on grain size and/or Sn
content in the films.
H133
Figure 7. 共Color online兲 AFM pictures of nanocrystalline SnS films grown at
three substrate temperatures.
Optical properties.— The optical behavior of SnS films was
studied using transmittance 共T%兲 and reflectance 共R%兲 data measured in the wavelength range of 350–2500 nm. Here, the observed
reflectance of SnS films is very small 共⬍7%兲 and, hence, it is neglected. The transmittance of the films is high, ⬃89%, and constant
at higher wavelengths 共⬎1000 nm兲. Therefore, only the T% vs
wavelength spectra of SnS films recorded in the wavelength range of
350–1000 nm is shown in Fig. 10 for the films grown at three sub-
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Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
Figure 8. Variation of electrical resistivity of nanocrystalline SnS films with
substrate temperature.
strate temperatures. The overall transmittance of the films in the
lower wavelength range 共⬍1000 nm兲 decreased with the increase of
Ts, which indicates the absorbability of the nanocrystalline films
increases with Ts. It can also be observed that the transmittance of
the films drastically decreased at below the wavelength of 1000 nm
and, hence, its corresponding incident photon energy 共h␯兲 is treated
as a FAE. The absorption coefficient ␣ of the films at above FAE
was evaluated using the equation, ␣ = ln共1/T兲/t. At above FAE, the
films exhibited ␣ on the order of ⬃106 cm−1. This is a noticeable
point and is quite high as compared to the data observed in
microfilms.1
The bandgap of SnS in few reports is classified as direct31,32 and
in others as indirect.33,34 In order to determine the optical bandgap
of nanocrystalline SnS films, the ␣ dependence h␯ equation:35
␣h␯ = A共h␯ − Eg − Ep兲x, where Ep is the phonon energy and Eg is
the optical bandgap, was used. For direct transition 共Ep = 0兲, x
= 1/2 for allowed transition and 3/2 for forbidden transition, and for
Figure 9. Activation energy of nanocrystalline SnS films as a function of
substrate temperature.
Figure 10. 共Color online兲 T% vs wavelength spectra of nanocrystalline SnS
films grown at three substrate temperatures.
indirect transition, x = 2 for allowed transition and 3 for forbidden
transition. Therefore, the allowed direct and indirect optical bandgaps of SnS films were evaluated from the plots of 共␣h␯兲2 vs h␯ and
共␣h␯兲1/2 vs h␯. Figure 11a shows 共␣h␯兲2 vs photon energy plots of
SnS films grown at three substrate temperatures. The plot of 共␣h␯兲2
vs h␯ of SnS films is a straight line, and the intercept of energy axis
at 共␣h␯兲2 = 0 gives the direct energy bandgap. The variation of
direct bandgap with substrate temperature is shown in Fig. 11b.
Similarly, the indirect bandgap of the films was evaluated from
共␣h␯兲1/2 vs h␯ plots, as shown in Fig. 12a, and its variation with Ts
is shown in Fig. 12b. Figures 11b and 12b reveal that the bandgaps
of SnS films initially decreased with the increase of Ts up to 150°C
and with further increase of Ts, both the parameters slightly in-
Figure 11. 共Color online兲 共a兲 共␣h␯兲2 vs photon energy and 共b兲 direct bandgap vs substrate temperature plots of nanocrystalline SnS films.
Journal of The Electrochemical Society, 155 共2兲 H130-H135 共2008兲
H135
showed an optical bandgap of ⬎2.0 eV with a high absorption coefficient of ⬃106 cm−1. These results indicate that the physical
properties of nanocrystalline SnS films are comparable to the properties of bulk as well as microstructured SnS films and suitable for
photovoltaic device applications.
Acknowledgments
Dr. N. Koteeswara Reddy thanks the Korean Government 共MOHRD兲 for its support to carry out the postdoctoral studies under
project no. BK21 during 2007–2008.
Chonbuk National University assisted in meeting the publication costs of
this article.
References
Figure 12. 共Color online兲 共a兲 共␣h␯兲1/2 vs photon energy and 共b兲 indirect
bandgap vs substrate temperature plots of nanocrystalline SnS films.
creased. The direct 共indirect兲 bandgap of the SnS films varied in
between 2.43 and 2.02 eV 共1.28 and 0.6 eV兲 with the increase of Ts
and showed a minimum value of 2.02 eV 共0.6 eV兲 at Ts = 150°C.
The bandgap of nanocrystalline SnS films is high as compared to the
microstructured SnS films, which might be attributed to the thickness, composition, and/or crystallinity of the films. In general, a low
crystalline or amorphous and thinner films exhibit higher bandgaps
than the crystalline and/or thicker films.36 Therefore, the high Eg
value of SnS films grown at Ts = 20°C might be due to lower crystallinity of the films and the decrease of Eg with Ts is due to improvement in the crystallinity and/or the decrease of sulfur content.
However, a slight variation in bandgap of the films at higher substrate temperatures 共⬎150°C兲 may be attributed to DPO of the
films. The DPO of the films decreased with Ts at above 150°C and
leads the bandgap to higher values.
Conclusion
The SnS films were grown using the thermal evaporation technique at different substrate temperatures 共20–300°C兲, and other
deposition parameters were kept as constant. The physical properties
of as-deposited SnS films were studied with appropriate techniques,
and the observed results are summarized as follows:
The as-deposited films are highly transparent and appeared light
gold in color. The adherence of the films to the surface of substrate
increased with the increase of Ts. The composition analysis of the
films showed that the films grown at Ts = 20°C are nearly stoichiometric. While increasing Ts, the nonstoichiometric nature in the
films increased due to the loss of sulfur content in the films. The
structural studies revealed that the as-deposited SnS films were
nanocrystalline and crystallized orthorhombically along 关111兴 direction. The grain size of these films increased from 18 to 41 nm with
the increase of Ts. The films grown at Ts = 150°C showed lower
rms surface roughness than the films deposited at other substrate
temperatures. The SnS films grown at Ts = 300°C showed a lower
electrical resistivity of ⬃43.4 ⍀ cm than the films grown at other
temperatures due to high grain size. The nanocrystalline SnS films
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