Vol.2, No.3, 197-200 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.23030
Effect of anneal temperature on electrical and optical
properties of SnS:Ag thin films
Hong-Jie Jia1, Shu-Ying Cheng1,3, Xin-Kun Wu1, Yong-Li Yang2
1College of Physics and Information Engineering, and Institute of Micro-Nano Devices and Solar Cells, Fuzhou University, Fuzhou,
China
2Ministry of Education Key Laboratory of Analysis and Detection Technology for Food Safety, and Department of Chemistry,
Fuzhou University, Fuzhou, China
3Corresponding author at: College of Physics and Information Engineering, Fuzhou University, Fuzhou, China;
Received 14 September 2009; revised 13 January 2010; accepted 25 January 2010.
ABSTRACT
SnS and Ag films were deposited on glass sub-
strates by vacuum thermal evaporation tech-
nique successively, and then the films were
annealed at different temperatures (0-300℃) in
N2 atmosphere for 2h in order to obtain sil-
ver-doped SnS ( SnS:Ag ) films. The phases of
SnS:Ag films were analyzed by X-ray diffraction
(XRD) system, which indicated that the films
were polycrystalline SnS with orthogonal struc-
ture, and the crystallites in the films were ex-
clusively oriented along the（111）direction. With
the increase of the annealing temperature, the
carrier concentration and mobility of the films
first rose and then dropped, whereas their re-
sistivity and direct band gap Eg showed the
contrary trend. At the annealing temperature of
260℃, the SnS:Ag films had the best properties:
the direct bandgap was 1.3 eV, the carrier con-
centration was up to 1.132 × 1017 cm-3, and the
resistivity was about 3.1 Ωcm.
Keywords: Sns:Ag Films; Thermal Evaporation;
Annealing; Electrical And Optical Properties
1. INTRODUCTION
Emphasis on cost-competitive photovoltaic cells has
been involved in the study of low-cost, non-toxic mate-
rials. SnS could be of interest for photovoltaic cells,
since its optical energy gap of 1.3 eV [1,2] is close to
the optimum energy gap 1.5 eV of solar cells, and it has
a high absorption coefficient ( > 104 cm-1) and a high
conversion efficiency of about 25% [3,4]. In addition,
the constituent elements Sn and S are non-toxic and
abundant in nature.
In spite of the above advantages, the electrical proper-
ties of SnS thin films still need to be improved in order to
make good SnS thin film solar cells. W. Albers et al. [5]
investigated Sb- and Ag-doped SnS single crystals and
observed n-type conductivity with carrier concentration of
~1019 cm−3 in Sb-doped SnS crystals and p-type conduc-
tivity with ~1018 cm−3 concentration in Ag-doped SnS
crystals. Devika et al. [6] investigated Ag-doped SnS
films grown by thermal evaporation technique and ob-
served that the resistivity of the SnS layers reached a
minimum value of 6.98 Ω cm at 15 atom％ of Ag. How-
ever, some properties of SnS:Ag thin films are unclear and
waiting for further study. In particular, how the annealing
temperatures affect the microstructure and physical prop-
erties of SnS:Ag thin films prepared by vacuum thermal
evaporation technique has not been investigated. There-
fore, in this paper we investigate the influence of anneal-
ing temperatures on the films in order to improve optical
and electrical properties of SnS:Ag thin films.
2. EXPERIMENTAL
SnS and Ag films were deposited on glass substrates by
vacuum thermal evaporation technique successively. The
thermal evaporation system is DMDE-450 deposition
equipment (made in China). The SnS powder with 99.5%
purity and the Ag grains with 99.9% purity were used as
source materials and were loaded onto a ceramic crucible
and a molybdenum boat, respectively. The chamber was
evacuated down to 5.6 × 10-3 Pa. The source-to-substrate
distance was about 10 cm. The as-prepared films were
annealed at different temperatures of 210, 260 and 300 ℃
in N2 atmosphere for 2h respectively so that Ag-doped
SnS films can be obtained.
The structure of the films was characterized by a
Philips X’Pert-MPD X-ray diffraction (XRD) system
with a Cu K radiation source. The surface roughness was
analyzed by a CSPM5000 Scanning Probe Microscopy
(AFM), and the thickness of the films was measured by

H. J. Jia et al. / Natural Science 2 (2010) 197-200
198
a Veeco Dektak 6M stylus profiler. The transmission and
reflectance spectra were carried out with a Varian Cary
500 UV−VIS−NIR spectrophotometer in the range 400 –
1600 nm. Based on Van der Pauw method, the electrical
properties were determined by a HMS-3000 Hall meas-
urement system.
3. RESULTS AND DISCUSSION
3.1. Structural Analysis
Figure 1 (a)-(d) show the XRD patterns of the samples
as-prepared and annealed at different temperatures
(samples S1-S4 corresponding to unannealing, 210, 260
and 300℃), respectively. It can be seen from Figure 1
(a)-(c) that all the diffraction peaks are attributed to SnS
(JCPDS39-354) phase with orthorhombic structure. It
indicates that the films are polycrystalline SnS with a
strong {111} preferred orientation. With the increase of
the annealing temperature, the main diffraction peaks of
the samples become stronger. Sample S3 exhibits the
best crystallization. But the crystallization of sample S4
becomes weaker and there is an obvious SnO2 peak at 2θ
= 33.17°. Maybe a partial of SnS was oxidized into tin
dioxide (SnO2) at the higher annealing temperature be-
cause of a low vacuum annealing condition. Therefore,
when the annealing temperature is higher, there is
probably a SnO2 phase in the SnS:Ag films.
3.2. Optical Properties
Figure 2 shows absorption coefficient vs photon energy
(h  ) curves of the SnS:Ag thin films at different anneal-
ing temperatures. As a whole, with the increasing of
photon energy, the absorption coefficient increases rap-
idly and then almost stabilizes at h
 ﹥2.5 eV. The
maximum absorption coefficients of all the films are
greater than 1.3 × 105 cm-1. With the increase of the an-
nealing temperature, the absorption coefficient increases
correspondingly. Because the improved crystallization
and uniformity, and the reduced defect density in the
films will make the scattered light loss decrease, thus
leading to the increase of the absorption coefficient.
However, when the annealing temperature is equal to or
greater than 300℃, the absorption coefficient decreases
probably due to the presence of SnO2 in the films.
Figure 3 shows a curve of (  h  )2 vs. h  for sample S3
and the estimated Eg value of 1.30 eV (here only show
sample S3 for simplicity). The Eg values of all the four
samples are shown in Table 1. With the increase of the
annealing temperature, the Eg first drops and then rises.
Because, with the increasing of the annealing temperature,
Ag atoms are easier to be diffused and doped in the poly-
crystalline SnS films, and the doped-Ag can drop the
band gap [6]. But the Eg value of sample S4 is larger
than that of sample S3, this is perhaps due to the presence
of SnO2 (3.4 ~ 4.6eV) [7] in sample S4.
20 40 60
0
30
60
90
120
150
180
210
240
(141)
(211)
(112)
(042)
(131)
(021)
(120)
(101)
Intensity(counts)
2( degr ee)
(111)
a. unaneal i ng
20 40 60
0
30
60
90
120
150
180
210
240
(042)
(061)
(122)
(112)
(211)
(002)
(131)
(021)
(120)
(101)
Intensity(counts)
2( degree)
(111)
b.T=2100C
20 40 60
0
30
60
90
120
150
180
210
240
(042)
(122)
(112)
(211)
(002)
(131)
(120)
(021)
(111)
Intensity(counts)
2(degree)
(101)
c.T=2600C
20 40 60
0
30
60
90
120
150
180
210
240
(042)
(122)
(112)
(211) (002)
(141)
(131)
(111)
(101)
(021)
Intensity(counts)
2(degree)
(120)
d.T=3000C
SnO2(101)
Figure 1. XRD patterns of SnS:Ag thin films at different annealing temperatures.

H. J. Jia et al. / Natural Science 2 (2010) 197-200
199
Table 1. The bandgap and absorption edge of SnS:Ag thin films at different annealing temperatures.
Samples Annealing Temperature/℃ Band gap Eg /eV Absorption Edge /nm
S1 unannealed 1.34 925.4
S2 210 1.32 939.4
S3 260 1.30 953.8
S4 300 1.52 815.8
Table 2. Hall measurement results for SnS:Ag thin films at different annealing temperatures.
Samples temperature/℃ Bulk concentration/cm-3 Mobility/
（cm2.v-1.s-1）
Resistivity/
（Ωcm）
Average Hall Coeffi-
cient/(m2c-1)
S1 unannealed 7.242 × 1014 14.3 601.1 8.631 × 103
S2 210 3.212 × 1016 16.3 11.9 1.945 × 102
S3 260 1.132 × 1017 17.8 3.1 5.522 × 101
S4 300 2.601 × 1016 15.7 15.3 2.402 × 102
1.0 1.5 2.0 2.5 3.0 3.5
0.0
5.0x104
1.0x105
1.5x105
Absorption coefficient()cm-1
h


/
e
V
00C
2100C
2600C
3000C
Figure 2.  vs. h  curves of SnS:Ag thin films at
different annealing temperatures.
1.01.52.02.53.0
0.0
5.0x1010
1.0x1011
1.5x1011
2.0x1011
2.5x1011
(hv)2(eV2.cm-2)
h


(
eV
)
S3
T=2600C
Eg=1.30
Figure 3. (  h  )2 vs. h  curve of the SnS:Ag thin film
annealed at 260℃.
3.3. Electrical Properties
At room temperature, semiconducting properties of the
films were measured by a HMS-3000 Hall measurement
system. The results are listed in Table 2. Compared with
the unannealed sample, the semiconducting properties of
the annealed samples have been improved. With the in-
crease of the annealing temperature, the carrier concen-
tration and mobility increase whereas the resistivity de-
creases except that of films annealed at 300.℃ When the
annealing temperature equals to 260 , the carrier co℃n-
centration and mobility reach the maximum values of
1.132 × 1017 cm-3 and 17.8 cm2.v-1.s-1, respectively. But,
when the annealing temperature is up to 300, the ca℃r-
rier concentration decreases to 2.601 × 1016 cm-3 with a
mobility of 15.7 cm2.v-1.s-1. The phenomenon can be
explained by the fact that, with the increase of the an-
nealing temperature, better crystallization and greater
grain size in the films lead to the decrease of defects
density and crystal-boundary, therefore the resistivity
decreases. However, higher temperature urges crystal
lattice vibration stronger and results in some crystal lat-
tice defects. These defects become dispersion centers,
causing the increase of resistivity of the films. In addi-
tion, the presence of SnO2 can also cause the resistivity
of the films increasing. Notably, the average Hall coeffi-
cients of all the samples are positive, which proves that
the SnS:Ag films are of p-type conduction.
4. CONCLUSIONS
The SnS:Ag thin films were deposited on glass sub-
strates using thermal evaporation technique and post-
annealing, and effect of annealing on the films was in-
vestigated. The above results indicate that appropriate
annealing temperature can increase the grain size of the
films, improve the uniformity and crystallization of the
films, decrease the resistivities of the films, and increase
the absorption coefficients of the films. However, if the
annealing temperature is higher than 300,℃ crystalliza-
tion of the films become weaker and the films can be
oxidized due to low vacuum, thereby electrical and op-
tical properties of the films become poor. At an anneal-
ing temperature of 260, the ℃SnS:Ag films have the
best properties: the direct bandgap is 1.3eV, the carrier
concentration is up to 1.132 × 1017 cm-3, and the resistiv-
ity is about 3.1 Ωcm.
5. ACKNOWLEDGMENT
The project-sponsored by SRF for ROCS, SEM (LXKQ0801) is grate-
fully acknowledged. The authors also wish to express their gratitude to

H. J. Jia et al. / Natural Science 2 (2010) 197-200
200
funding from Fujian Provincial Department of Science & Technology
and Department of Education, China (2008I0019, 2009J01285,
JB09008, JB09010).
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