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Electron. Mater. Lett., Vol. 10, No. 1 (2014), pp. 43-49
Deposition of Cu2ZnSnS4 Thin Films by Magnetron Sputtering and Subsequent Sulphurization
Arun Khalkar,1 Kwang-Soo Lim,
1 Seong-Man Yu,
1 Shashikant P. Patole,
1,2 and Ji-Beom Yoo
1,2,*
1SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Korea
2School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea
(received date: 4 December 2012 / accepted date: 8 March 2013 / published date: 10 January 2014)
Top-down magnetron sputtering with subsequent, separate sulphurization was used to deposit Cu2ZnSnS4
(CZTS) absorber layers for solar cells. Cu, ZnS and SnS targets were used to deposit the absorber layersonto soda lime glass substrates. The sputtering system was first calibrated for individual Cu, ZnS and SnSdeposition. CZTS thin film was then deposited by co-sputtering followed by annealing at 530°C in sulphurousconditions at atmospheric pressure for 30 minutes. Scanning electron microscopy, x-ray diffraction, Ramanand UV-visible absorption spectroscopy were used to characterize the absorber film. It was found to haveproperties potentially suitable for use in high-efficiency solar cells. These include phonon peaks correspondingto quaternary CZTS, a high absorption coefficient of 1.1 × 105 cm−1, a direct optical band gap of 1.5 eV,a kesterite CZTS phase and stoichiometric ratios of Cu/[Zn+Sn] = 0.82 and Zn/Sn = 1.19.
Keywords: Cu2ZnSnS4 thin film, magnetron sputtering, optical absorption coefficient, direct optical band gap
1. INTRODUCTION
Cu2ZnSnS4 (CZTS) is a quaternary semiconductor with a
direct band gap of around 1.4 - 1.5 eV and a large optical
absorption coefficient of 104 cm−1.[1,2] Its constituent elements
are readily available at the industrial scale and are generally
not hazardous, making CZTS a promising absorber for use
in photovoltaic devices. Solar cells employing CZTS absorber
layers have shown efficiencies of up to 8.4%.[3] CZTS
absorber layers can be prepared by various methods including
sputtering, evaporation, electro deposition, sol gel techniques
and spray pyrolysis.[1-10] Industrial-scale production, low-
cost fabrication, high crystallinity, homogeneity (crack and
void-free film deposition), uniformity and high device
efficiency have yet to be achieved to allow the widespread
use of this material. Chemically produced films have tended
to be inhomogeneous and shown voids and cracks. Such
inhomogeneity deteriorates the performance of photovoltaic
devices by impeding the uniformity and the efficacy of the
extraction of photo-generated charge carriers.[11] Chemical
syntheses also require the use of hazardous chemicals, which
can pose health and environmental problems. Vacuum
deposition, such as by sputtering or evaporation, can achieve
high-quality thin films more cleanly and more safely than
chemical methods.[1,2] Elements, alloys and compounds with
various melting points can be easily sputtered and deposited.
Sputtering can achieve well-adhered films with compositions
close to that of the source material. Sputtering targets are
also generally stable, and thus represent long-lived deposition
sources. For the deposition of CZTS thin films, co-sputtering
allows the facile control and manipulation of the films'
compositions to produce precisely engineered band gaps for
efficient photon harvesting, suppressed electron-hole recom-
bination and effective extraction of electrons and holes. Itao
et al. first reported the deposition of CZTS by sputtering in
1988.[9] Katagiri et al. prepared CZTS thin films by RF
magnetron co-sputtering with subsequent vapour phase
sulphurization at 580°C; the devices showed 6.77% effi-
ciency.[1] The single-step preparation of CZTS films by
reactive sputtering to incorporate sulphur during deposition
has also been attempted.[10] The grain properties (size, texture
and boundaries) of CZTS films prepared by sputtering have
been studied by Li et al. and Oo et al..[11,12] The use of high-
temperature annealing for post-sulphurization can result in
the contamination of the source targets and the system
generally by the highly reactive H2S gas in the reactive
sputtering system. Nevertheless, sputtering represents a
promising low-cost, low-environmental impact, industrially
scalable technology for CZTS thin film deposition and solar
cell device fabrication and further study to develop its
applicability is desirable.
This work reports the deposition of CZTS thin films by
magnetron sputtering and sulphurization. The system was
calibrated for Cu, SnS and ZnS targets (of 99.9% purity) to
achieve uniform films at high deposition rates. Co-sputtering
followed by annealing under H2S at 530°C temperature
resulted in CZTS thin films with potential applicability in
DOI: 10.1007/s13391-013-2238-8
*Corresponding author: jbyoo@skku.edu ©KIM and Springer
44 A. Khalkar et al.
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
high-efficiency solar cells.
2. EXPERIMENTAL PROCEDURE
A top-down magnetron sputtering system with sputter
targets placed above the substrate (Fig. 1, SNTEK Semi-
conductor & Nano Technology Pvt. Ltd. Model No. RSP5004)
was designed and installed for the deposition of CZTS thin
films. Three sputter targets of Cu (99.9%), SnS (99.9%) and
ZnS (99.9%) each sized two inches (RND Pvt. Ltd., Korea)
were respectively connected to DC (maximum 1 kW), RF1
(maximum 600 W, frequency 13.56 MHz) and RF2 (maximum
600 W, frequency 12.56 MHz) power sources. The targets
were placed ~120 mm above the substrate. Samples of up to
50 × 50 mm could be loaded into the chamber without
breaking the vacuum using a ‘load lock system’, which
allowed the vacuum inside the chamber to be maintained
below 10−6 Torr. Inside the chamber, the substrate holder
rested on a rotating stage (capable of rotating at up to
20 rpm) and was connected to a resistive heater (capable of
achieving up to 600°C). Samples could simultaneously
rotate and be heated during deposition.
Uniformity testing was performed on samples deposited
on 50 × 50 mm soda lime glasses (SLG). These were first
cleaned ultrasonically in acetone for 20 minutes and then in
de-ionized water. They were then dried with nitrogen gas.
Ketone tape strips were masked diagonally onto the SLG
samples before the separate depositions of Cu, SnS and ZnS.
Samples were loaded into the sputter chamber using the
‘sample transfer arm’ and argon gas was introduced at 50
sccm with a working pressure of 5 mTorr. The individual Cu,
SnS and ZnS targets each had 100 W power applied. Each
material was deposited on to separate SLGs at room
temperature for 20 minutes. During deposition, the substrate
holder was rotated at 12 rpm. After deposition, samples were
removed from the sputter chamber and the ketone tape was
detached. Films' uniformity and thickness were monitored at
different places using a α-step surface profilometer (KLA-
Tencor Ltd.).
Deposition rates were assessed by separately depositing
Cu, SnS and ZnS on to cleaned SLG substrates at 5 mTorr
for 20 min under various applied powers at room temperature.
After deposition, the Cu, ZnS and SnS films were annealed
at 500°C, 350°C and 100°C respectively for 10 minutes
under N2 at atmospheric pressure to ensure good crystallinity.
The samples were removed from the annealing system and
then characterized. The films' thickness and surface morph-
ology were measured by scanning electron microscopy
(SEM; JSM7401F, JEOL). Their crystallinity was analysed
by x-ray diffractometer (XRD; D8 Discover, Bruker).
After confirmation of uniformity test, deposition rate and
good crystallinity of individual Cu, SnS and ZnS films, the
CZTS precursor films were deposited on to SLG substrates
by co-sputtering Cu, SnS and ZnS targets at a working
pressure of 5 mTorr with an argon flow rate of 40 sccm. The
DC power to the Cu target was 15 W, the RF powers to the
SnS and ZnS targets were 40 W and 80 W, respectively.
During deposition, the substrate holder was rotated at 12 rpm.
Depositions were carried out for 60 min at room temperature.
The post sulfurization of deposited CZTS films was carried
Fig. 1. Schematic of the magnetron sputtering system used for CZTS thin film deposition.
A. Khalkar et al. 45
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
out into the sulfurization chamber. Diluted H2S gas balanced
with 97 vol. % N2 was then introduced into the chamber at
100 sccm to achieve atmospheric pressure. Samples were
heated to 300°C, 400°C, 500°C and 530°C at 10°C/min and
then annealed for 30 min. After annealing, they cooled
naturally to room temperature under the same atmosphere.
The annealed films' surface morphology and thickness were
measured by SEM. The crystallinity of the annealed films
was analyzed using XRD. Raman spectra with excitation
wavelength of 514 nm were measured using a Renishaw
system (Renishaw, Gloucestershire, UK). Optical properties
were measure using a UV-VIS-IR spectrometer (UV-3600
Shimadzu, Japan).
3. RESULTS AND DISCUSSION
Before the deposition of CZTS films by co-sputtering, the
targets' deposition characteristics were assessed after their
individual sputtering. Uniformity testing examined the ability
of the sputter system to achieve uniform deposition over the
samples' areas. The individually deposited Cu, ZnS and SnS
films (Fig. 2(a-c)) used in the uniformity testing showed
colours characteristic of the metals: the Cu film appeared
red-brown, the ZnS film appeared blue and transparent and
the SnS film appeared dark blue and glossy. The diagonal
transparent regions were due to the ketone tape masks. Each
sample's thickness (Table 1) was assessed by taking mea-
surements at five different locations (the numbers in Fig.
2(a-c)). The Cu, SnS and ZnS films showed uniformities of
3.16%, 4.89% and 4.71%, respectively. Uniformity varied
due to the materials' different deposition rates, surface
morphologies, film characteristics and sputter type (DC or
RF).
To assess deposition rate, depositions were carried out at
various powers for each individual target (Fig. 3). The ZnS
target resulted in the lowest observed deposition rate; the
SnS target resulted in the highest. The ZnS target led to
deposition at 4.2 nm/min at 50 W, which linearly increased
to 8.33 nm/min at 100 W. Target power and deposition rate
were related by a slope of 0.083 and a linear correlation
coefficient (R) of 0.999. The Cu target led to deposition at
6 nm/min at 50 W, which linearly increased to 20 nm/min at
100 W, with a slope of 0.269 and R of 0.982. The SnS target,
with the highest deposition rates, achieved 18 nm/min at
50 W, which linearly increased to 22.8 nm/min at 100 W,
with a slope of 0.088 and R of 0.968. The observed linear
dependence of deposition rate on target power is advantageous
to allow the precise engineering of CZTS films. The observed
deposition rates could subsequently be used in the design of
CZTS films of certain compositions.
The surface morphology and crystallinity of each of the
Fig. 2. The deposited (a) Cu, (b) ZnS and (c) SnS samples used for uniformity testing.
Table 1. Ellipsometry data for Cu, SnS and ZnS thin films at the posi-tions marked in Fig. 2(a-c).
Cu SnS ZnS
Position Thickness (Å)
①
②
③
④
⑤
21802200215121302268
52375300550054855377
51285382563854695400
Uniformity (%) 3.16 4.89 4.71
Fig. 3. Effect of target power on the deposition rate.
46 A. Khalkar et al.
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
individually deposited films were assessed by SEM (Fig. 4).
In each case, the target power was 100 W and deposition
was for 20 min at 5 mTorr. The cross-section view of the Cu
film (Fig. 4(a)) shows its compactness. The approximately
350 nm thick film appears uniformly deposited and free
from voids. The top view of image shows its surface
morphology; small grains of around 40 nm are visible with
cracks/grain boundaries at distances of around 200 to 300 nm.
DC sputtered Cu films' morphology and grain boundaries
are mainly affected by the target power, working pressure
and deposition time.[13,14] The cross-section view of the SnS
film (Fig. 4(b)) shows it to be compact, free from voids and
600 nm thick. The top view images shows distinct elongated
grain-like crystals. Magnification revealed these grains to
comprise attached flake-like structures. The different directional
growth rates in the flakes of such as SnS crystals have been
attributed to the effects of argon pressure.[15] The cross-
section image of the ZnS films (Fig. 4(c)) shows a compact,
250 nm thick film. The top view image shows the film to
comprise the small grains of below 10 nm, characteristic of
ZnS films deposited by magnetron sputtering.[16]
The crystallinity of the annealed films was studies using
XRD (Fig. 5). Each showed crystallographic properties
corresponding to their source materials without contamination.
The Cu film (Fig. 5(a)) showed XRD peaks at 2θ = 43.3°,
50.4° and 74.1°, corresponding respectively to the (111),
(200) and (220) planes of face-centred cubic structured Cu
(JCPDS card number 03-1005). The SnS film (Fig. 5(b))
showed XRD peaks at 2θ = 26.6°, 31.3°, 38.8° and 50.6°,
corresponding respectively to the (021), (040), (131) and
(151) planes of orthorhombic end-centred SnS (JCPDS card
number 32-1361). Sn or sulphur related phases were not
observed, indicative of the SnS film's high quality. The ZnS
film (Fig. 5(c)) showed a single XRD peak at 2θ = 28.9°,
Fig. 4. SEM images of deposited films of (a) Cu, (b) SnS and (c) ZnS. Left, cross-section views; right, top views.
A. Khalkar et al. 47
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
corresponding to the (111) plane of cubic face-centred ZnS
(JCPDS card number 80-0020). The single observed plane
indicated that the film showed preferential growth in the
(111) plane orientation, i.e. parallel to the glass substrate.[16]
Distinct, separate Zn or sulphur related phases were not
observed, indicative of the high quality of the single crystalline
ZnS film. These results indicate that the sputtering system
could deposit high-quality Cu, SnS and ZnS thin films.
CZTS precursor films were deposited by co-sputtering.
They were then annealed at different temperatures in the
sulphurization chamber as described in the experimental
section. XRD patterns of samples annealed at 300°C, 400°C,
500°C and 530°C are shown in Fig. 6. For the annealing
temperature 300°C, 400°C shows the secondary phases of
CuZn5, Sn2S3 and ZnSnO3 along with CZTS phase. To
eliminate these phases annealing temperature increased up to
500°C which shows no secondary phases except ZnSnO3.
The ZnSnO3 phase may be present due to the oxidation of Zn
and Sn during sample exposure to the open air conditions.
Finally, the sample annealed at 530°C shows CZTS peaks at
2θ = 28.5°, 32.9°, 47.3°, 56.1°, 58.9° and 76.5° respectively
corresponding to the (112), (200), (220), (312), (224) and
(332) planes of tetragonal, body-centred, kesterite CZTS
(JCPDS card number 26-0575). The JCPDS reference data
show relative peak intensities of the (112) (200) and (220)
planes of 100%, 90% and 25% respectively. However
respective peak intensities of 100%, 33% and 20% were
observed here indicating that the CZTS film favoured
growth in the (112) direction.[17] EDX showed the samples
annealed at 530°C to have compositional ratios of Cu/
Fig. 5. XRD patterns of annealed films of (a) Cu, (b) SnS and (c) ZnS.
Fig. 6. XRD patterns of CZTS thin film annealed at 300, 400, 500 and530°C.
48 A. Khalkar et al.
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
[Zn+Sn] = 0.82 and Zn/Sn = 1.19, close to those required for
high-efficiency CZTS solar cells.[1,8]
SEM images were recorded for the film annealed at 530°C
(Fig. 7). The cross-sectional view (Fig. 7(a)) shows a compact,
thin film of around 1.0 μm thickness that is free from voids
or irregularities. It appears homogeneous and free from
grains or grain boundaries. The top view image (Fig. 7(b))
shows the film to consist of densely packed grains without
cracks. The SEM images indicate that these sputtered films
are of better quality than films prepared chemically, which
consistently contain voids and irregular grain structures.[3]
The homogeneous distribution of the source elements at the
atomic scale during deposition and annealing likely led to
the films' compactness.[10] In white light, the film appeared
olive-grey, consistent with its kesterite characteristics.[18]
The optical properties of the CZTS film annealed at 530°C
were examined by Raman and UV-visible absorption spec-
troscopy (Fig. 8). The Raman spectrum (Fig. 8(a)) shows
peaks at 252, 287, 334, 351 and 368 cm−1, corresponding
very closely to published Raman data for quaternary
CZTS.[18-20] The optical absorption coefficient (α) was
calculated using the equation: where, t is the
film's thickness, R is reflectance and T is transmittance. As
the incident photon energy increased from 1.29 to 2 eV, the
optical absorption coefficient increased from 0.7 × 104 to
1.1 × 105 cm−1 (Fig. 8(b)). The plateau followed by the sharp
edge in the absorption indicates that the film behaved as a
semiconductor; it absorbed exceptionally high numbers of
photons in the visible range and appears suitable to be used
as the film in photovoltaic devices. Plotting (αhν)2 with
respect to incident photon energy allows the calculation of
semiconductors' direct optical energy band gaps (Eg) via the
equation (αhν)2 = (hν − Eg).[21] The optical energy band gap
was calculated to be 1.5 eV at (αhν)2 = 0 (Fig. 8(c)), in
agreement with previous reports.[7,8] The film deposited by
magnetron sputtering showed an exceptionally high absorption
coefficient, a desirable band gap, the correct phase and
stoichiometry, demonstrating its potential to be used as
absorber layers in photovoltaic devices.
α1
t---ln
1 R–( )T
---------------=
Fig. 7. SEM images of CZTS thin film annealed at 530°C: (a) cross-section and (b) surface morphology.
Fig. 8. Optical properties of CZTS thin film annealed at 530°C: (a)Raman spectrum, (b) optical absorption coefficient, and (c) opticalband gap.
A. Khalkar et al. 49
Electron. Mater. Lett. Vol. 10, No. 1 (2014)
4. CONCLUSIONS
Top-down magnetron sputtering of Cu, ZnS and SnS
targets and subsequent, separate sulphurization were used to
deposit high-quality CZTS absorber layers for potential use
in high-efficiency solar cells. The sputtering system was first
calibrated via the individual deposition of Cu, ZnS and SnS.
The deposited thin films showed the uniformity, morphology
and phase of high-quality film. The CZTS thin film deposited
by co-sputtering and annealed at 530°C in H2S at atmospheric
pressure for 30 minutes demonstrated the uniform deposition
of homogeneous materials free from grains or grain boundaries
and voids. XRD confirmed the film's kesterite CZTS phase.
Raman spectra showed phonon peaks corresponding to
quaternary CZTS. Optical absorption coefficients up to
1.1 × 105 cm−1 were observed an order of magnitude higher
than previously reported. A direct optical energy band gap
(Eg) of 1.5 eV was observed. Room-temperature deposition
and post-treatment at 530°C likely resulted in the high-
quality CZTS that showed potential to be used in future
photovoltaic devices.
ACKNOWLEDGEMENTS
This work was supported by the ‘Global Leading Technology
Program’ of the Office of Strategic R&D Planning (OSP)
funded by the Ministry of Knowledge Economy, Republic of
Korea (No.S-2012-1226-000). One of the authors, SPP is
grateful to the Korean government for awarding him the
BK-21 fellowship.
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