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XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 1420–1423 www.elsevier.com/locate/solmat
XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering E. Ech-chamikh, A. Essafti, Y. Ijdiyaou, M. Azizan Laboratoire de Physique des Solides et des Couches Minces, Faculte´ des Sciences Semlalia, Universite´ Cadi Ayyad, B.P. 2390, 40001 Marrakech, Morocco Available online 1 December 2005
Abstract Amorphous carbon nitride (a-C:N) thin films were deposited by reactive radiofrequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto silicon substrates, from a graphite target of very high purity, in an atmosphere of pure nitrogen. The chemical properties of these films were studied by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the a-C:N films reveal that nitrogen is well incorporated in the amorphous carbon network. The atomic percentage of nitrogen in the a-C:N films, calculated from the XPS spectrum, is about 32%. In addition to C–C and CQC bonds, the analysis of the chemical shifts of C 1 s and N 1 s core level peaks show that nitrogen is bonded to carbon in CQN double bonding and CRN triple bonding configurations. The content of the CRN triple bonds is found to be more important than the CQN double bonds. r 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon nitride; XPS; RF sputtering
1. Introduction Amorphous carbon nitride (a-C:N) thin films have been subjected to intense research developments since Cohen and Liu predicted that the b–C3N4 phase would be harder than diamond [1]. This crystalline phase necessitate severe conditions to be grown by the usual deposition techniques. However, the a-C:N films are easy to be grown by different techniques and they have become potential candidates for eventual use in mechanical, Corresponding author. Tel.: +212 64252331; fax: +212 44 437410.
E-mail address: [email protected] (E. Ech-chamikh). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.10.007
ARTICLE IN PRESS E. Ech-chamikh et al. / Solar Energy Materials & Solar Cells 90 (2006) 1420–1423
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optical, electrical and electronics applications [2–9]. The properties of this material depend strongly on the chemical bonding of carbon and nitrogen atoms because of the different possible hybridizations of carbon atoms (sp, sp2 and sp3). Therefore, a study of the chemical bonding types in this material could give information on some properties. In the present work, we present the main results obtained on RF sputtered a-C:N films, by X-ray photoelectron spectroscopy (XPS).
2. Experimental details a-C:N thin films used in this study were prepared by RF sputtering in an ALCATEL SCM 451 deposition system equipped with an ALCATEL ARF 601 RF generator operating at 13.56 MHz. More details on the deposition chamber have been reported in previous work [10]. Prior to each deposition the vacuum chamber was evacuated down to a low pressure of 5 10 5 Pa. The a-C:N films were deposited, at room temperature onto polished silicon and on corning C7059 glass substrates, from a graphite target in an atmosphere of pure nitrogen. The pressure of nitrogen was adjusted to be maintained at 1 Pa and the RF power was maintained at 250 W. Before the deposition, the graphite target was pre-sputtered for 15 min in order to eliminate superficial contamination resulting from exposure to air. The XPS measurements were performed ex-situ in an ultra high vacuum system with a base pressure of 2 10 8 Pa, using a double-pass cylindrical mirror spectrometer. XPS spectra were obtained using 1253.6 eV photons from an Mg anode Xray source at a pass energy of 50 eV. The binding energy values of the photoelectron peaks were measured with an accuracy of 0.05 eV.
Intensity (au)
N 1s
O 1s C 1s
600
500
400 300 Binding Energy (eV)
Fig. 1. Typical XPS spectrum of as deposited a-C:N thin films.
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3. Results and discussion Fig. 1 shows the XPS spectrum of an as-deposited a-C:N film. In addition to carbon and nitrogen, this spectrum shows the presence of a small peak of oxygen. The presence of a small amount of oxygen (about 6%) is due to the surface contamination during the exposure of samples in ambient air before the analysis. The atomic concentrations of carbon and nitrogen as calculated from the integrated intensities of the C 1 s and N 1 s peaks are 62% and 32%, respectively. In order to study the chemical bonding of C and N atoms in the a-C:N films we have recorded the C 1 s and N 1 s core level spectra which are reported in Fig. 2. The C 1 s spectrum consists in a quite large peak with a marked shoulder on the high-binding energy side, indicating the presence of carbon atoms in different bonding states. A best deconvolution of the experimental C 1 s core level spectrum necessitates at least four C≡N
N 1s
Intensity (au)
C=N
404
400 396 Binding energy (eV)
392
C-C
C 1s C=N
Intensity (au)
C≡N
C=O
292
288
284
280
Binding energy (eV) Fig. 2. C 1 s and N 1 s core level XPS spectra: (??) experimental spectra, (—–) deconvolution results.
ARTICLE IN PRESS E. Ech-chamikh et al. / Solar Energy Materials & Solar Cells 90 (2006) 1420–1423
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Gaussian components. These components, located at 284.6, 286.1, 287.3 and 288.8 eV in binding energy can be attributed, respectively, to C–C and/or CQC, CQN, CRN and CQO bonding types [11,12]. The integrated intensity of the CRN component is more important than the CQN one. This result suggests that carbon atoms are preferentially bonded to nitrogen atoms as CRN triple bonds. The presence of the peak at 288.8 eV associated to CQO bonds are in accordance with the oxygen contamination of the surface observed in the large survey spectrum (Fig. 1). On the other hand, the N 1 s core level spectrum consists in a relatively large and asymmetric peak centered at 398.8 eV (Fig. 2). Therefore, we have deconvoluted this peak considering two Gaussian components one of them being located at 398.6 eV associated to NRC bonding type [13]. The second component located at 400.1 eV can be attributed to the N atoms in NQC double bonding configuration as has been reported by Chowdhury et al. [14]. The integrated intensity of the NQC component is small compared to the CRN component. This result confirms that the C atoms are preferentially bonded to N atoms in the CRN triple bonding configuration as deduced above from the deconvolution of the C 1 s spectrum. 4. Conclusion Amorphous carbon nitride (a-C:N) thin films were obtained by reactive RF sputtering from a graphite target in a plasma of pure nitrogen. The chemical properties of these films have been studied by XPS. The obtained a-C:N films contain 32% of nitrogen and the N atoms are mainly bonded to C atoms in CQN double bonding and CRN triple bonding configurations with a dominance of CRN bonds. Acknowledgment This research was partially supported by the Moroccan-Spanish Program (Project no. P18/00). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. C.J. Torng, J.M. Sivertsen, J.H. Judy, C. Chang, J. Mater. Res. 5 (1990) 2490. D. Li, Y.-W. Chung, M.-S. Wong, W.D. Sproul, J. Appl. Phys. 74 (1993) 219. J. Vlcek, K. Rusnak, V. Hajek, L. Martinu, Diamond Relat. Mater. 9 (2000) 582. He.-X. Han, B.J. Feldman, Solid State Commun. 65 (1988) 921. J.J. Cuomo, P.A. Leary, D. Yu, W. Reuter, M. Frisch, J. Vac. Sci. Technol. 16 (1979) 299. Z.J. Zhang, J.L. Huang, S.S. Fan, C.M. Lieber, Mater. Sci. Eng. A 209 (1996) 5. L. Valentini, J.M. Kenny, G. Carlotti, M. Guerrieri, G. Signorelli, L. Lozzi, S. Santucci, Thin Solid Films 389 (2001) 315. X. Yan, T. Xu, G. Chen, S. Yang, H. Liu, Q. Xue, J. Phys. D: Appl. Phys. 57 (2004) 1. M. Khaidar, A. Essafti, A. Bennouna, E.L. Ameziane, M. Brunel, J. Appl. Phys. 65 (1989) 3248. E. D’Anna, M.L. De Giorgi, A. Luches, M. Martino, A. Perrone, A. Zocco, Thin Solid Films 347 (1999) 72. E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Thechnol. 125 (2000) 124. W. Kulish, C. Popov, L. Zambov, New Diamond Front. Carbon Technol. 11 (2001) 53. A.K.M.S. Chowdhury, D.C. Cameron, M.S.J. Hashmi, Surf. Coat. Technol. 112 (1999) 133.
Solar Energy Materials & Solar Cells 90 (2006) 1420–1423 www.elsevier.com/locate/solmat
XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering E. Ech-chamikh, A. Essafti, Y. Ijdiyaou, M. Azizan Laboratoire de Physique des Solides et des Couches Minces, Faculte´ des Sciences Semlalia, Universite´ Cadi Ayyad, B.P. 2390, 40001 Marrakech, Morocco Available online 1 December 2005
Abstract Amorphous carbon nitride (a-C:N) thin films were deposited by reactive radiofrequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto silicon substrates, from a graphite target of very high purity, in an atmosphere of pure nitrogen. The chemical properties of these films were studied by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the a-C:N films reveal that nitrogen is well incorporated in the amorphous carbon network. The atomic percentage of nitrogen in the a-C:N films, calculated from the XPS spectrum, is about 32%. In addition to C–C and CQC bonds, the analysis of the chemical shifts of C 1 s and N 1 s core level peaks show that nitrogen is bonded to carbon in CQN double bonding and CRN triple bonding configurations. The content of the CRN triple bonds is found to be more important than the CQN double bonds. r 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon nitride; XPS; RF sputtering
1. Introduction Amorphous carbon nitride (a-C:N) thin films have been subjected to intense research developments since Cohen and Liu predicted that the b–C3N4 phase would be harder than diamond [1]. This crystalline phase necessitate severe conditions to be grown by the usual deposition techniques. However, the a-C:N films are easy to be grown by different techniques and they have become potential candidates for eventual use in mechanical, Corresponding author. Tel.: +212 64252331; fax: +212 44 437410.
E-mail address: [email protected] (E. Ech-chamikh). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.10.007
ARTICLE IN PRESS E. Ech-chamikh et al. / Solar Energy Materials & Solar Cells 90 (2006) 1420–1423
1421
optical, electrical and electronics applications [2–9]. The properties of this material depend strongly on the chemical bonding of carbon and nitrogen atoms because of the different possible hybridizations of carbon atoms (sp, sp2 and sp3). Therefore, a study of the chemical bonding types in this material could give information on some properties. In the present work, we present the main results obtained on RF sputtered a-C:N films, by X-ray photoelectron spectroscopy (XPS).
2. Experimental details a-C:N thin films used in this study were prepared by RF sputtering in an ALCATEL SCM 451 deposition system equipped with an ALCATEL ARF 601 RF generator operating at 13.56 MHz. More details on the deposition chamber have been reported in previous work [10]. Prior to each deposition the vacuum chamber was evacuated down to a low pressure of 5 10 5 Pa. The a-C:N films were deposited, at room temperature onto polished silicon and on corning C7059 glass substrates, from a graphite target in an atmosphere of pure nitrogen. The pressure of nitrogen was adjusted to be maintained at 1 Pa and the RF power was maintained at 250 W. Before the deposition, the graphite target was pre-sputtered for 15 min in order to eliminate superficial contamination resulting from exposure to air. The XPS measurements were performed ex-situ in an ultra high vacuum system with a base pressure of 2 10 8 Pa, using a double-pass cylindrical mirror spectrometer. XPS spectra were obtained using 1253.6 eV photons from an Mg anode Xray source at a pass energy of 50 eV. The binding energy values of the photoelectron peaks were measured with an accuracy of 0.05 eV.
Intensity (au)
N 1s
O 1s C 1s
600
500
400 300 Binding Energy (eV)
Fig. 1. Typical XPS spectrum of as deposited a-C:N thin films.
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3. Results and discussion Fig. 1 shows the XPS spectrum of an as-deposited a-C:N film. In addition to carbon and nitrogen, this spectrum shows the presence of a small peak of oxygen. The presence of a small amount of oxygen (about 6%) is due to the surface contamination during the exposure of samples in ambient air before the analysis. The atomic concentrations of carbon and nitrogen as calculated from the integrated intensities of the C 1 s and N 1 s peaks are 62% and 32%, respectively. In order to study the chemical bonding of C and N atoms in the a-C:N films we have recorded the C 1 s and N 1 s core level spectra which are reported in Fig. 2. The C 1 s spectrum consists in a quite large peak with a marked shoulder on the high-binding energy side, indicating the presence of carbon atoms in different bonding states. A best deconvolution of the experimental C 1 s core level spectrum necessitates at least four C≡N
N 1s
Intensity (au)
C=N
404
400 396 Binding energy (eV)
392
C-C
C 1s C=N
Intensity (au)
C≡N
C=O
292
288
284
280
Binding energy (eV) Fig. 2. C 1 s and N 1 s core level XPS spectra: (??) experimental spectra, (—–) deconvolution results.
ARTICLE IN PRESS E. Ech-chamikh et al. / Solar Energy Materials & Solar Cells 90 (2006) 1420–1423
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Gaussian components. These components, located at 284.6, 286.1, 287.3 and 288.8 eV in binding energy can be attributed, respectively, to C–C and/or CQC, CQN, CRN and CQO bonding types [11,12]. The integrated intensity of the CRN component is more important than the CQN one. This result suggests that carbon atoms are preferentially bonded to nitrogen atoms as CRN triple bonds. The presence of the peak at 288.8 eV associated to CQO bonds are in accordance with the oxygen contamination of the surface observed in the large survey spectrum (Fig. 1). On the other hand, the N 1 s core level spectrum consists in a relatively large and asymmetric peak centered at 398.8 eV (Fig. 2). Therefore, we have deconvoluted this peak considering two Gaussian components one of them being located at 398.6 eV associated to NRC bonding type [13]. The second component located at 400.1 eV can be attributed to the N atoms in NQC double bonding configuration as has been reported by Chowdhury et al. [14]. The integrated intensity of the NQC component is small compared to the CRN component. This result confirms that the C atoms are preferentially bonded to N atoms in the CRN triple bonding configuration as deduced above from the deconvolution of the C 1 s spectrum. 4. Conclusion Amorphous carbon nitride (a-C:N) thin films were obtained by reactive RF sputtering from a graphite target in a plasma of pure nitrogen. The chemical properties of these films have been studied by XPS. The obtained a-C:N films contain 32% of nitrogen and the N atoms are mainly bonded to C atoms in CQN double bonding and CRN triple bonding configurations with a dominance of CRN bonds. Acknowledgment This research was partially supported by the Moroccan-Spanish Program (Project no. P18/00). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. C.J. Torng, J.M. Sivertsen, J.H. Judy, C. Chang, J. Mater. Res. 5 (1990) 2490. D. Li, Y.-W. Chung, M.-S. Wong, W.D. Sproul, J. Appl. Phys. 74 (1993) 219. J. Vlcek, K. Rusnak, V. Hajek, L. Martinu, Diamond Relat. Mater. 9 (2000) 582. He.-X. Han, B.J. Feldman, Solid State Commun. 65 (1988) 921. J.J. Cuomo, P.A. Leary, D. Yu, W. Reuter, M. Frisch, J. Vac. Sci. Technol. 16 (1979) 299. Z.J. Zhang, J.L. Huang, S.S. Fan, C.M. Lieber, Mater. Sci. Eng. A 209 (1996) 5. L. Valentini, J.M. Kenny, G. Carlotti, M. Guerrieri, G. Signorelli, L. Lozzi, S. Santucci, Thin Solid Films 389 (2001) 315. X. Yan, T. Xu, G. Chen, S. Yang, H. Liu, Q. Xue, J. Phys. D: Appl. Phys. 57 (2004) 1. M. Khaidar, A. Essafti, A. Bennouna, E.L. Ameziane, M. Brunel, J. Appl. Phys. 65 (1989) 3248. E. D’Anna, M.L. De Giorgi, A. Luches, M. Martino, A. Perrone, A. Zocco, Thin Solid Films 347 (1999) 72. E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Thechnol. 125 (2000) 124. W. Kulish, C. Popov, L. Zambov, New Diamond Front. Carbon Technol. 11 (2001) 53. A.K.M.S. Chowdhury, D.C. Cameron, M.S.J. Hashmi, Surf. Coat. Technol. 112 (1999) 133.