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Effect of Sulfurization Time on the Formation of CuInS2 Thin Films
Rare Metal Materials and Engineering Volume 44, Issue 4, April 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(4): 0805-0807.
ARTICLE
Effect of Sulfurization Time on the Formation of CuInS2 Thin Films Wang Ligang,
Wang Yanlai,
Yao Wei,
Zhu Jun,
Xu Jingang
Key Laboratory of Semiconductor Photovoltaic Technology of Inner Mongolia Autonomous Region, Inner Mongolia University, Hohhot 010021, China
Abstract: CuInS2 thin films were prepared by the sulfurization of Cu-In precursor films through magnetron sputtering. The obtained films were characterized using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), UV–vis spectrophotometer and Hall Effect measurement. The results show that while the sulfurization time increases from 10 min to 30 min, the compositions of the thin films are almost stable, the crystallization becomes better and better, the resistivity increases, and the band gap of the film is in the range of 1.42~1.55 eV. Key words: CuInS2 thin films; sulfurization time; photoelectric property
With the development of economy, energy has become the principle problem which many countries are faced with in the world. Research and development of the clean and renewable energy is a critical concern in international academic circles. Being inexhaustible and free from pollution, solar energy has drawn much attention since 1950s. Many efforts have been devoted to the research and the application of solar cell recently. Chalcopyrite compound copper indium disulfide (CulnS2) existing at low temperature is one of the most promising candidates as absorber materials for photovoltaic applications due to its direct band gap of 1.55 eV and high absorption coefficient of 10-5 cm-1 [1] which ensures that only a few microns of CulnS2 (CIS) are enough to absorb most of the sunlight. As for the preparation, many methods have been developed to deposit CIS thin film so far including three source molecular beam epitaxy[2], RF sputtering[3], sulfurization [4,5], coevaporation from elemental source [6], single source evaporation [7], electrodeposition [8], spray pyrolysis [9-11], chemical bath deposition (CBD) [12], etc. But there is still a contradiction between the pollution-free process and the development of quality and properties. Therefore, the main emphasis or the purpose of the present work is to deposit good quality CIS thin films by sulfurization
of Cu-In precursors which were created by DC magnetron sputtering onto glass-substrates. During the sulfurization process, the solid sulfur powder was used as sulfur source instead of H2S or other toxic gases. Effects of sulfurization time on the microstructure and optical properties of the CuInS2 samples were studied.
1
Experiment
In the first stage, the simple chamber was evacuated to a base pressure of 4 ×10-4 Pa before the high-purity (99.999%) argon gas was introduced to provide the plasma at a pressure of 1.2 Pa. Via DC magnetron sputtering of Cu-In alloy (with Cu/In atomic ratio 1:1) on a clean glass-substrate with the size of 15 mm × 20 mm, the target Cu-In precursor films were deposited. The sputtering power was set to 15 W. This process was conducted at ambient temperature. In the second stage, sulfurization of the prepared precursor films was carried out under N2 atmosphere at 400 °C for 10, 20, and 30 min in a tube type resistance furnace. The final thin films were investigated using a variety of characterization techniques including XRD (D8 advance) with Cu-Kα (λ=0.15406 nm) radiation, SEM (QUANTA400), EDS (Kevex SuperDry), UV-vis spectrophotometer (Lambda750S) and Hall effect measurement (HMS-3000).
Received date: April 22, 2014 Foundation item: National Natural Science Foundation of China (51364025) Corresponding author: Wang Yanlai, Ph. D., Associate Professor, School of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, P. R. China, Tel: 0086-471-4993141, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
805
Wang Ligang et al. / Rare Metal Materials and Engineering, 2015, 44(4): 0805-0807
2
Results and Discussion
The chemical compositions of the precursor films sulfurized for 10, 20 and 30 min obtained using EDS analysis are shown in Table 1. It can be seen that thin films prepared for different sulfurization time are slightly In-riched，which actually can restrain the formation of CuxS effectively. There is report that CuxS is observed obviously in copper-rich composition films[13]. The ratio of Cu/In is reduced with sulfurization time increasing, but it is almost 1:1. Fig.1 shows XRD patterns of thin films obtained for different sulfurization time. CuInS2 phase is observed in the films sulfurized for 10 min mixed with Cu16In9 and CuIn5S8 phases which indicates that the sulfurization reaction is not conducted sufficiently. When the sulfurization time increases to 20 min, CuIn5S8 and Cu16In9 phases disappear and the peaks of CuInS2 in the XRD pattern for the sample are intense and narrow, which implies a good crystallinity. However, it is also noted that an additional reflection showing up corresponds to In2S3 phase. Depending on the atomic concentration of metallic precursors, the secondary phase could be In 2S3 for In-rich precursors. With the sulfurization time being prolonged to 30 min, the crystallinity becomes even better. Table 1
The plan-view SEM images of sulfurized films at 400 °C for 10, 20, and 30 min are shown in Fig.2a~2c. Because of the sufficient supply of S for CuInS2 formation, the thin film exhibits a smooth surface with fine grains. But some small particles can also be observed on the surfaces of all the samples, which might be In2S3 particles deduced from the discussion about the XRD patterns mentioned above. As shown in Fig.3, the absorption coefficients of all thin films are above 104 cm-1. And the Eg values for different sulfurization time sequences of the thin films annealed at 400 °C in Ar atmosphere can be estimated to be 1.42~1.55 eV. These values are consistent with the value of 1.53 eV for CIS single crystal at room temperature [14]. The electrical properties of all samples are listed in Table 2 showing that all obtained samples are P-type semiconductors. Resistivity increases within the sulfurized time varying from 10 min to 30 min, while both mobility and conductivity decrease with the sulfurization time increasing to 30 min. The change trends may be resulted from the decrease of binary metal compound Cu16In9 and the disappearance of In2S3 in a
Chemical composition (at%) of CuInS2 thin films sulfurized for different time
T/°C Time/min
Cu/In/S ratio, at%
Eg/eV
400
10
24.11 26.52 49.37
Cu
In
S
0.49:0.54:1
1.42
400
20
23.81 26.84 49.35
0.48:0.54:1
1.48
400
30
23.42 28.43 48.15
0.49:0.59:1
1.55
b
c
Cu16In9(250)
70
Fig.2 SEM images of thin films sulfurized for 10 min (a), 20 min (b),
50 2θ/(º)
60
70
CuInS2(312)(116)
c
60
70
Fig.3
10 20 min
8 6
30 min
10 min
4
(αhv)2/×108eV2·cm-2
50
XRD patterns of thin films sulfurized for 10 min (a), 20 min (b), and 30 min (c)
and 30 min (c)
CuInS2(312)(116)
CuInS2(220)(204)
b
CuInS2(220)(204)
40
60
Absorption Coefficient, α/×104 cm-1
30
CuIn5S8(440)
CuIn5S8(511)
Cu16In9(921)
CuInS2(112)
20
50
40
CuInS2(112) CuInS2(220)(004)
30
In2S3(107)
Intensity/a.u.
20
Fig.1
40
CuInS2(112) CuInS2(220)(004)
30
In2S3(107)
Intensity/a.u.
20
Cu16In9(002)
Cu16In9(020)
Intensity/a.u.
a
10 min 20 min
120
30 min 80 40 0 0.8 1.2 1.6
2.0
hv/eV
2 400
600
800 1000 1200 Wavelength, λ/nm
1400
Plots of α against λ for thin films. Inset is a plot of (αhν)2 against hν for the absorption spectra
806
Wang Ligang et al. / Rare Metal Materials and Engineering, 2015, 44(4): 0805-0807
Table 2 Time/ min 10 20 30
Electrical properties of CuInS2 thin films sulfurized for
1 Pathan H M, Lokhande C D. Appl Surf Sci[J], 2004, 239(1): 11
different time Bulk concen- Mobility/ tration/cm-3 cm2·(V·s)-1 6.257E+18 3.389E-1
2 Eberhardt J, Cieslak J, Metzner H et al. Thin Solid Films[J],
5.400E+16 3.697E+16
2.629E-1 1.333E-1
Resistivity/ Ω·cm 2.994E+0
Conductivity/ S·cm 3.396E-1
3 Ghribi F, El Mir L, Dahman H et al. Sens Lett[J], 2011, 9(6):
4.397E+2 1.266E+3
2.272E-3 7.897E-4
4 Siemer K, Klaer J, Luck I et al. Sol Energy Mater Sol Cells[J],
the components of thin films. It has been reported that the value of electrical resistivity of CIS thin film varies from 104 Ω·cm to 10-2 Ω·cm for the sample with indium excess to copper excess films [15]. Sulfur is more well-distributed in the thin films and the sulfurization reaction is completed with the sulfurization time increasing. Meanwhile, the binary impurity phase (Cu16In9 with lower electrical resistivity) disappears and the crystallinity of thin film is improved. Therefore, the electrical resistivity is increasing and an optical band gap of CuInS2 thin film is widening to be close to the ideal optical band gap with the sulfurization time increasing.
3
Conclusions
With the sulfurization time increasing from 10 min to 30 min, the following change trends of the thin films can be seen: compositions of thin films remain nearly stable; the crystallization is becoming better and better; the resistivity is increasing; and the band gap is in the range of 1.42~1.55 eV.
References
2009, 517(7): 2248 2186 2001, 67(1): 159 5 Antony A, Asha A, Yoosuf R et al. Sol Energy Mater Sol Cells [J], 2004, 81(4): 407 6 Tsuboi N, Tamogami T, Kobayashi S. Jpn J Appl Phys[J], 2011, 50(5): 05FB03 7 Zhao Y, Peng X, Ding Y et al. Mater Sci Semicond Process[J], 2013, 16(6): 1472 8 Chen H, Yeh Y M, Liao C H et al. Ceram Int[J], 2014, 40(1): 67 9 Rafi M, Arba Y, Hartiti B et al. Adv M[J], 2013, 15(11-12): 1328 10 Acik I O, Otto K, Krunks M et al. Anal Calorim[J], 2013, 113(13): 1455 11 Meshram R, Thombre R, Suryavanshi B. Adv Appl Sci Res [J], 2012, 3(3): 1271 12 Garskaite E, Pan G T, Yang T C K et al. Sol Energy[J], 2012, 86(9): 2584 13 Chen Y, He X, Zhao X et al. Mater Sci Eng B[J], 2007, 139(1): 88 14 Yan Y H, Liu Y C, Fang L et al. Trans Nonferrous Met Soc China[J], 2008, 18(5): 1083 15 Ortega-Lopez M, Morales-Acevedo A. Thin Solid Films[J], 1998, 330(2): 96
807
ARTICLE
Effect of Sulfurization Time on the Formation of CuInS2 Thin Films Wang Ligang,
Wang Yanlai,
Yao Wei,
Zhu Jun,
Xu Jingang
Key Laboratory of Semiconductor Photovoltaic Technology of Inner Mongolia Autonomous Region, Inner Mongolia University, Hohhot 010021, China
Abstract: CuInS2 thin films were prepared by the sulfurization of Cu-In precursor films through magnetron sputtering. The obtained films were characterized using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), UV–vis spectrophotometer and Hall Effect measurement. The results show that while the sulfurization time increases from 10 min to 30 min, the compositions of the thin films are almost stable, the crystallization becomes better and better, the resistivity increases, and the band gap of the film is in the range of 1.42~1.55 eV. Key words: CuInS2 thin films; sulfurization time; photoelectric property
With the development of economy, energy has become the principle problem which many countries are faced with in the world. Research and development of the clean and renewable energy is a critical concern in international academic circles. Being inexhaustible and free from pollution, solar energy has drawn much attention since 1950s. Many efforts have been devoted to the research and the application of solar cell recently. Chalcopyrite compound copper indium disulfide (CulnS2) existing at low temperature is one of the most promising candidates as absorber materials for photovoltaic applications due to its direct band gap of 1.55 eV and high absorption coefficient of 10-5 cm-1 [1] which ensures that only a few microns of CulnS2 (CIS) are enough to absorb most of the sunlight. As for the preparation, many methods have been developed to deposit CIS thin film so far including three source molecular beam epitaxy[2], RF sputtering[3], sulfurization [4,5], coevaporation from elemental source [6], single source evaporation [7], electrodeposition [8], spray pyrolysis [9-11], chemical bath deposition (CBD) [12], etc. But there is still a contradiction between the pollution-free process and the development of quality and properties. Therefore, the main emphasis or the purpose of the present work is to deposit good quality CIS thin films by sulfurization
of Cu-In precursors which were created by DC magnetron sputtering onto glass-substrates. During the sulfurization process, the solid sulfur powder was used as sulfur source instead of H2S or other toxic gases. Effects of sulfurization time on the microstructure and optical properties of the CuInS2 samples were studied.
1
Experiment
In the first stage, the simple chamber was evacuated to a base pressure of 4 ×10-4 Pa before the high-purity (99.999%) argon gas was introduced to provide the plasma at a pressure of 1.2 Pa. Via DC magnetron sputtering of Cu-In alloy (with Cu/In atomic ratio 1:1) on a clean glass-substrate with the size of 15 mm × 20 mm, the target Cu-In precursor films were deposited. The sputtering power was set to 15 W. This process was conducted at ambient temperature. In the second stage, sulfurization of the prepared precursor films was carried out under N2 atmosphere at 400 °C for 10, 20, and 30 min in a tube type resistance furnace. The final thin films were investigated using a variety of characterization techniques including XRD (D8 advance) with Cu-Kα (λ=0.15406 nm) radiation, SEM (QUANTA400), EDS (Kevex SuperDry), UV-vis spectrophotometer (Lambda750S) and Hall effect measurement (HMS-3000).
Received date: April 22, 2014 Foundation item: National Natural Science Foundation of China (51364025) Corresponding author: Wang Yanlai, Ph. D., Associate Professor, School of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, P. R. China, Tel: 0086-471-4993141, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
805
Wang Ligang et al. / Rare Metal Materials and Engineering, 2015, 44(4): 0805-0807
2
Results and Discussion
The chemical compositions of the precursor films sulfurized for 10, 20 and 30 min obtained using EDS analysis are shown in Table 1. It can be seen that thin films prepared for different sulfurization time are slightly In-riched，which actually can restrain the formation of CuxS effectively. There is report that CuxS is observed obviously in copper-rich composition films[13]. The ratio of Cu/In is reduced with sulfurization time increasing, but it is almost 1:1. Fig.1 shows XRD patterns of thin films obtained for different sulfurization time. CuInS2 phase is observed in the films sulfurized for 10 min mixed with Cu16In9 and CuIn5S8 phases which indicates that the sulfurization reaction is not conducted sufficiently. When the sulfurization time increases to 20 min, CuIn5S8 and Cu16In9 phases disappear and the peaks of CuInS2 in the XRD pattern for the sample are intense and narrow, which implies a good crystallinity. However, it is also noted that an additional reflection showing up corresponds to In2S3 phase. Depending on the atomic concentration of metallic precursors, the secondary phase could be In 2S3 for In-rich precursors. With the sulfurization time being prolonged to 30 min, the crystallinity becomes even better. Table 1
The plan-view SEM images of sulfurized films at 400 °C for 10, 20, and 30 min are shown in Fig.2a~2c. Because of the sufficient supply of S for CuInS2 formation, the thin film exhibits a smooth surface with fine grains. But some small particles can also be observed on the surfaces of all the samples, which might be In2S3 particles deduced from the discussion about the XRD patterns mentioned above. As shown in Fig.3, the absorption coefficients of all thin films are above 104 cm-1. And the Eg values for different sulfurization time sequences of the thin films annealed at 400 °C in Ar atmosphere can be estimated to be 1.42~1.55 eV. These values are consistent with the value of 1.53 eV for CIS single crystal at room temperature [14]. The electrical properties of all samples are listed in Table 2 showing that all obtained samples are P-type semiconductors. Resistivity increases within the sulfurized time varying from 10 min to 30 min, while both mobility and conductivity decrease with the sulfurization time increasing to 30 min. The change trends may be resulted from the decrease of binary metal compound Cu16In9 and the disappearance of In2S3 in a
Chemical composition (at%) of CuInS2 thin films sulfurized for different time
T/°C Time/min
Cu/In/S ratio, at%
Eg/eV
400
10
24.11 26.52 49.37
Cu
In
S
0.49:0.54:1
1.42
400
20
23.81 26.84 49.35
0.48:0.54:1
1.48
400
30
23.42 28.43 48.15
0.49:0.59:1
1.55
b
c
Cu16In9(250)
70
Fig.2 SEM images of thin films sulfurized for 10 min (a), 20 min (b),
50 2θ/(º)
60
70
CuInS2(312)(116)
c
60
70
Fig.3
10 20 min
8 6
30 min
10 min
4
(αhv)2/×108eV2·cm-2
50
XRD patterns of thin films sulfurized for 10 min (a), 20 min (b), and 30 min (c)
and 30 min (c)
CuInS2(312)(116)
CuInS2(220)(204)
b
CuInS2(220)(204)
40
60
Absorption Coefficient, α/×104 cm-1
30
CuIn5S8(440)
CuIn5S8(511)
Cu16In9(921)
CuInS2(112)
20
50
40
CuInS2(112) CuInS2(220)(004)
30
In2S3(107)
Intensity/a.u.
20
Fig.1
40
CuInS2(112) CuInS2(220)(004)
30
In2S3(107)
Intensity/a.u.
20
Cu16In9(002)
Cu16In9(020)
Intensity/a.u.
a
10 min 20 min
120
30 min 80 40 0 0.8 1.2 1.6
2.0
hv/eV
2 400
600
800 1000 1200 Wavelength, λ/nm
1400
Plots of α against λ for thin films. Inset is a plot of (αhν)2 against hν for the absorption spectra
806
Wang Ligang et al. / Rare Metal Materials and Engineering, 2015, 44(4): 0805-0807
Table 2 Time/ min 10 20 30
Electrical properties of CuInS2 thin films sulfurized for
1 Pathan H M, Lokhande C D. Appl Surf Sci[J], 2004, 239(1): 11
different time Bulk concen- Mobility/ tration/cm-3 cm2·(V·s)-1 6.257E+18 3.389E-1
2 Eberhardt J, Cieslak J, Metzner H et al. Thin Solid Films[J],
5.400E+16 3.697E+16
2.629E-1 1.333E-1
Resistivity/ Ω·cm 2.994E+0
Conductivity/ S·cm 3.396E-1
3 Ghribi F, El Mir L, Dahman H et al. Sens Lett[J], 2011, 9(6):
4.397E+2 1.266E+3
2.272E-3 7.897E-4
4 Siemer K, Klaer J, Luck I et al. Sol Energy Mater Sol Cells[J],
the components of thin films. It has been reported that the value of electrical resistivity of CIS thin film varies from 104 Ω·cm to 10-2 Ω·cm for the sample with indium excess to copper excess films [15]. Sulfur is more well-distributed in the thin films and the sulfurization reaction is completed with the sulfurization time increasing. Meanwhile, the binary impurity phase (Cu16In9 with lower electrical resistivity) disappears and the crystallinity of thin film is improved. Therefore, the electrical resistivity is increasing and an optical band gap of CuInS2 thin film is widening to be close to the ideal optical band gap with the sulfurization time increasing.
3
Conclusions
With the sulfurization time increasing from 10 min to 30 min, the following change trends of the thin films can be seen: compositions of thin films remain nearly stable; the crystallization is becoming better and better; the resistivity is increasing; and the band gap is in the range of 1.42~1.55 eV.
References
2009, 517(7): 2248 2186 2001, 67(1): 159 5 Antony A, Asha A, Yoosuf R et al. Sol Energy Mater Sol Cells [J], 2004, 81(4): 407 6 Tsuboi N, Tamogami T, Kobayashi S. Jpn J Appl Phys[J], 2011, 50(5): 05FB03 7 Zhao Y, Peng X, Ding Y et al. Mater Sci Semicond Process[J], 2013, 16(6): 1472 8 Chen H, Yeh Y M, Liao C H et al. Ceram Int[J], 2014, 40(1): 67 9 Rafi M, Arba Y, Hartiti B et al. Adv M[J], 2013, 15(11-12): 1328 10 Acik I O, Otto K, Krunks M et al. Anal Calorim[J], 2013, 113(13): 1455 11 Meshram R, Thombre R, Suryavanshi B. Adv Appl Sci Res [J], 2012, 3(3): 1271 12 Garskaite E, Pan G T, Yang T C K et al. Sol Energy[J], 2012, 86(9): 2584 13 Chen Y, He X, Zhao X et al. Mater Sci Eng B[J], 2007, 139(1): 88 14 Yan Y H, Liu Y C, Fang L et al. Trans Nonferrous Met Soc China[J], 2008, 18(5): 1083 15 Ortega-Lopez M, Morales-Acevedo A. Thin Solid Films[J], 1998, 330(2): 96
807