Deposition of Cu ZnSnS Thin Films by Magnetron …web.skku.edu/~nmdl/publication/2014/185....

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: [email protected] ©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


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.



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.



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.



[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.



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.

REFERENCES

1. H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W. S.

Maw, T. Fukano, T. Ito, and T. Motohiro, Appl. Phys. Express

1, 041201 (2008).

2. J. Li, Q. Du, W. Liu, G. Jiang, X. Feng, W. Zhang, J. Zhu,

and C. Zhu, Electron. Mater. Lett. 8, 365 (2012).

3. B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey,

and S. Guha, Prog. Photovolt: Res. Appl., DOI: 10.1002/

pip.1174 (2011).

4. A. Weber, H. Krauth, S. Perlt, B. Schubert, I. Kötschau, S.

Schorr, and H. W. Schock, Thin Solid Films 517, 2524

(2009).

5. A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I.

Kötschau, H.-W. Schock, R. Schurr, A. Hölzing, S. Jost, R.

Hock, T. Vob, J. Schulze, and A. Kirbs, Thin Solid Films

517, 2511 (2009).

6. K. Tanaka, M. Oonuki, N. Moritake, and H. Uchiki, Sol.

Energy Mater. Sol. Cells 93, 583 (2009).

7. N. Kamoun, H. Bouzouita, and B. Rezig, Thin Solid Films

515, 5949 (2007).

8. D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, and S.

Guha, Sol. Energy Mater. Sol. Cells 95, 1421 (2011).

9. K. Ito and T. Nakazawa, Jpn. J. Appl. Phys. 27, 2094

(1988).

10. V. Chawla and B. Clemens, IEEE 978, 1902 (2010).

11. J. B. Li, V. Chawla, and B. M. Clemens, Adv. Mater. 24,

720 (2012).

12. W. M. Hlaing oo, J. L. Johnson, A. Bhatia, E. A. Lund, M.

M. Nowell, and M. A. Scarpulla, J. Electron. Mater. 40,

2214 (2011).

13. K. Y. Chan and B. S. Teo, J. Mater. Sci. 40, 5971 (2005).

14. K. Y. Chan and B. S. Teo, Microelectronics J. 37, 1064

(2006).

15. K. Hartman, J. L. Johnson, M. I. Bertoni, D. Recht, M. J.

Aziz, M. A. Scarpulla, and T. Buonassisi, Thin Solid Films

519, 7421 (2011).

16. D. H. Hwang, J. H. Ahn, K. N. Hui, K. S. Hui, and Y. G.

Son, Nanoscale Res. Lett. 7, 26 (2012).

17. F. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y. Lain, Z.

Zhang, J. Li, and Y. Liu, Sol. Energy Mater. Sol. Cells 94,

2431 (2010).

18. B. Pracejus, The ore minerals under the microscope: an

optical guide, Elsevier, Atlases in Geoscience 3, 214 (2008).

19. K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, N.

A. Bojarczuk, D. Mitzi, and S. Guha, Appl. Phys. Lett. 97,

143508 (2010).

20. M. Altosaar, J. Raudoja, K. Timmo, M. Danilson, M.

Grossberg, J. Krustok, and E. Mellikov, Phys. Stat. Sol. (a)

205, 167 (2008).

21. J. I. Pankove, Optical Processes in Semiconductors, p. 35,

Dover, NY (1971).

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