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Structural characterization of nitrogen doped diamond-like carbon films deposited by arc ion plating
Applied Surface Science 241 (2005) 295–302 www.elsevier.com/locate/apsusc
Structural characterization of nitrogen doped diamond-like carbon films deposited by arc ion plating Y.S. Zoua,*, Q.M. Wanga, H. Dua, G.H. Songb, J.Q. Xiaoa, J. Gonga, C. Suna, L.S. Wena a
Institute of Metal Research, Division of Surface Engineering of Materials, Chinese Academy of Sciences, Shenyang 110016, China b Shengyang University of Technology, Shenyang 110023, China Received in revised form 8 July 2004; accepted 8 July 2004 Available online 17 September 2004
Abstract Nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating (AIP) technique under different N2 volume percentage in the gas mixture of Ar and N2. The deposited films were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectra indicate that the ID/IG ratio increases with increasing the N2 volume percentage. XPS analysis shows a strong influence of the N2 volume percentage on the N atom concentration and the chemical bonding states in the deposited films. Nitrogen content of the deposited films increased with the increasing of N2 volume percentage. The maximum N concentration and N/C atomic ratio are up to 12.7 at.% and 0.162 at the 90 vol.% N2, respectively. From decomposition of XPS C 1s peaks, it shows that the nitrogen doped diamond-like carbon films consist of amorphous carbon–carbon bonding (sp2C–C and sp3C–C), N atoms bonded to sp3-hybridized C atoms (sp3C–N) and N atoms bonded to sp2-hybridized C atoms (sp2C–N). The total content of sp3 bonding decreases with increasing N2 volume percentage. XPS N 1s spectra show that there exist the N–sp2C and N–sp3C bonding in the deposited nitrogen doped diamond-like carbon films. As the N2 volume percentage increases, the N–sp3C bonding content increases, but the N–sp2C bonding content decreases. # 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Uw; 81.15. z; 82.80.Pv; 68.55. a Keywords: Nitrogen doped diamond-like carbon; Arc ion plating; X-ray photoelectron spectroscopy; Microstructure; Bonding structure
1. Introduction
* Corresponding author. Tel.: +86 24 83978231; fax: +86 24 23843436. E-mail address: [email protected] (Y.S. Zou), [email protected] (C. Sun).
Diamond-like carbon (DLC) films have been widely utilized in recent years due to their special properties such as high hardness, high wear resistance, low friction coefficient and chemical inertness. In DLC film, the sp3 bonds are responsible for the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.043
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mechanical properties, whereas sp2 bonds, which lie in near the Fermi level, control the electronic and optical properties of the film. Recently, there are a few of attempts to improve the properties of DLC films by the addition of elements, such as silicon [1,2], nitrogen [3–5], fluorine [6] and some metals [7,8]. In general, the addition of those elements reduces the internal stress, electrical resistivity and friction coefficient of diamond-like carbon films. For example, the thermal stability of the carbon films increases [9] and the friction coefficient decreases as hydrogenated carbon films are doped with nitrogen. Arora et al. [10] demonstrated that nitrogen incorporation into DLC films could lower the compressive stress. It has been reported that nitrogenated carbon films have a better friction behavior than hydrogenated carbon films based on contact sliding tests with tripad [11]. So the nitrogenated diamond-like carbon films have been received increasing attention in many applications, especially in the magnetic recording industry. Many methods have been used to prepare nitrogenated diamond-like carbon films, including reactive sputtering [12,13], laser ablation of carbon in a nitrogen environment [14], ionic nitrogen beam irradiation [15], nitrogen ion implantation of carbon films [16,17], chemical vapor deposition combined with an electron cyclotron resonance plasma [18] and arc ion plating [19,20]. Because of the high ionization efficiency, high deposition rate and convenience of deposition parameters controlling, arc ion plating is a promising technique and the most perspective PVD method for industrial manufacture of DLC films. There have been considerable researches into the deposition of the carbon nitride films since the theoretical predictions that crystalline b-C3N4 might be harder than diamond [21]. However, up to now, there has been no successful result on depositing films with N content as predicted b-C3N4 phase, and N/C atomic ratio varied between 0.1 and 1 [22]. Marton et al. [23] and Sjo¨ stro¨ m et al. [24] investigated the microstructure and properties of the CNx films by distinguishing their characteristic binding energies of core level electrons of C and N atoms, and indicated the presence of N atoms bonded to C atoms in tetrahedral coordination (N–sp3C) and N atoms bonded to C atoms in trigonal coordination (N– sp2C). Although the properties of the nitrogenated diamond-like carbon films have been widely studied,
there is little knowledge about the chemical bonding states of the carbon and nitrogen atoms. In this paper, nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating using a pure graphite target and a mixed gas of Ar and N2. The main aim of this research is to study the existence state of N atoms and the influence on sp3/sp2 ratio in the deposited films. The C 1s and N 1s spectra were decomposed to study the N/C atomic ratio and their chemical bonding states of the deposited films. The influence of the N atom on the sp3/sp2 ratio and chemical bonding states of the nitrogen doped diamond-like carbon films is systematically investigated.
2. Experimental procedures The nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating. Before being located into the reactor chamber, the Si (1 0 0) substrates were ultrasonically cleaned in alcohol and acetone solution, and then rinsed in deionized water. Target was high purity graphite (99.999 at.%). The distance between the target and substrate was kept at approximately 240 mm. Prior to deposition, the chamber was evacuated to a pressure of 7 10 3 Pa, and then Ar was introduced for sputtering cleaning to remove the oxide and contaminant layer on the substrate at pulse bias of 800 V and duty cycle of 30% for 5 min. Mixed gas of N2 and Ar was fed into the chamber during deposition, and the flow ratio of the N2 to mixed gas (N2/(Ar+N2)) varied from 30 to 90% by independently adjusting the mass flow of Ar and N2. The substrate was biased at 200 V and duty cycle at 20% when the films were prepared. The surface morphologies of the nitrogen doped diamond-like carbon films were observed by atomic force microscopy (Quesant Incorporation, Q-250 AFM) in tapping mode. The root mean square (RMS) of the roughness was automatically calculated by the digital image-processing package of the AFM system. The nitrogen doped diamond-like carbon films were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Raman spectra were obtained with a micro-Raman spectrometer (Jobin Yvon HR800) using 514.5 nm Ar line as an
Y.S. Zou et al. / Applied Surface Science 241 (2005) 295–302
excitation source. The spectra resolution was 1 cm 1. To minimize the damage during measurement, the laser-input power is fixed at 10 mW. All the XPS measurements were carried out with ESCALAB 200 IXL spectrometer with a monochromatic X-ray source of Mg Ka (1253.6 eV). The chamber was pumped to a pressure of 1 10 8 Pa and the measurement was carried out at room temperature. A step with 0.05 eV energy intervals was used to acquire the XPS spectra of the C 1s and N 1s. The energy scale was calibrated using the reference binding energies of Cu 2p3/2 at 932.5 eV and Cu 3p3/2 at 75.0 eV. To minimize uncertainties in the binding energy measurements, all samples were mounted on a copper block using conductive silver colloid paint. All obtained spectra were calibrated using the signal from the implanted Ar 2p3/2 peak at 241.3 eV. The samples were analyzed without Ar+ bombardment for sputter cleaning before XPS analysis. The film compositions were determined by analyzing XPS spectra and using published sensitivity factors.
3. Results and discussion From SEM observation of the cross-section of the deposited films, the thickness of the N-doped DLC film is measured to be approximately 300 nm. Fig. 1 shows the AFM surface morphology of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2. The surface was very smooth without any cracks. According to surface morphologies of the other N2 volume percentage, the surface morphology was almost not affected by the N2 volume percentage in the present deposition condition. The average roughness of the films is in the range of 2.8–4.9 nm under the N2 volume percentage varied form 30 to 90. Fig. 2 shows the visible Raman spectra of the films deposited under different N2 volume percentage. All the spectra of the nitrogen doped diamond-like carbon films are similar to that of diamond-like carbon film. After the subtraction of the photoluminescence background, the Raman spectra can be deconvolved into two Gaussian peaks, a G peak at around 1560 cm 1 and a D peak at around 1360 cm 1, and the intensity ratio of D to G peak (ID/IG) was determined. The G peak centered at 1560 cm 1 is derived from the zone centre E2g bond stretching mode
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of graphite, and is present in all sp2-bonded carbons. The D peak centered at 1360 cm 1 is a disorder activated K zone boundary mode, due to the A1g symmetry breathing motion of six-fold aromatic rings, which requires the presence of such rings [25]. The disorder in the films due to the clustering of sp2bonded atom is reflected by the D peak intensity. So the sp3/sp2 ratio can be indirectly evaluated according to the intensity ratio of D to G peak. Fig. 3 shows the ID/IG ratio of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage. As shown in Fig. 3, the nitrogen doped diamond-like carbon films have a higher ID/IG ratio than that of the diamond-like carbon film deposited in pure Ar, and the ID/IG ratio increases with increasing the N2 volume percentage in the atmosphere. The increase in the ID/ IG ratio is due to the effect of N incorporation into the films, which indicates the increases of vibrating modes in the D band of the nitrogen doped diamond-like carbon films and the evolution of sp2 clusters with increasing the N2 volume percentage. Hence, the Raman spectra results here suggest that the structure of the nitrogen doped diamond-like carbon films become increasingly graphitic with N2 volume percentage increasing. The global N/C atomic ratio and N atom content were evaluated from the XPS measurement and published sensitivity factors. Fig. 4 shows the N/C atomic ratio and N atom content in the deposited films as the function of the N2 volume percentage in the gas mixture. From Fig. 4, it can be seen that the N/C atomic ratio and N atom content increase rapidly in the range of 0–50 vol.% N2, then increase slightly with the N2 volume percentage further increasing. The results shown in Fig. 4 indicate that the nitrogen gas flow ratio affects the film composition. In the range of N2 volume percentage studied, the highest N/C atomic ratio and N atom content was 0.162 and 12.7 at.% for the film deposited under 90 vol.% N2, respectively. The result is consistent with previous observation [26]. In order to identify the bonding state of the nitrogen doped diamond-like carbon films, the XPS spectra on the C 1s and N 1s spectra were analyzed. Fig. 5 shows the typical XPS C 1s spectra of the films deposited under various N2 volume percentages. Compared with the spectrum of the diamond-like carbon film deposited in pure Ar, the C 1s spectra of the nitrogen doped diamond-like carbon films are broad and
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Fig. 1. AFM surface morphology of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2.
asymmetric at the higher binding energy. It suggests that there are several bonding configurations related to carbon atoms in the deposited films. Using Gaussian function fitting after the inelastic background subtraction, the C 1s spectra were deconvolved into five
sub-peaks corresponding to the different bonding state of C atoms. Fig. 6(a) shows the typical deconvolved XPS C 1s spectra of the nitrogen doped diamond-like carbon films deposited under 30 vol.% N2, and those deconvolutions are in good agreement with previous
Fig. 2. The Raman spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
Fig. 3. The ID/IG ratio of the nitrogen doped diamond-like carbon films as a function of N2 volume percentage.
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Fig. 4. The N/C atomic ratio and N atom content in the nitrogen doped diamond-like carbon films as a function of N2 volume percentage.
reported results [22,12]. In C 1s spectra, the peaks at binding energies of 284.3 and 285.3 eV are contributed to the sp2C–C and sp3C–C bond atoms, respectively. The two peaks at 286.3 and 287.5 eV correspond to sp2-hybridization bonding structures of C atoms bonded to N atoms (sp2C–N) and sp3hybridization bonding structures of C atoms bonded to N atoms (sp3C–N), respectively. The peak with the highest binding energy value in C 1s spectra was assigned to the bonding states of C with O. It is possible produced by surface oxidation during the exposure to the atmosphere. The N 1s spectra of the films deposited under various N2 volume percentages are shown in Fig. 7.
Fig. 5. The XPS C 1s spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
Fig. 6. (a) Deconvolved with Gaussian function XPS C 1s spectra of nitrogen doped diamond-like carbon film deposited under 30 vol.% N2 and (b) deconvolved with Gaussian function XPS N 1s spectra of nitrogen doped diamond-like carbon film deposited under 30 vol.% N2.
Fig. 7. The XPS N 1s spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
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N 1s spectra have a broad peak corresponding to some bonding configurations related to N atoms. These spectra were deconvolved into three sub-peaks with Gaussian function that correspond to different chemical bonding states. Fig. 6(b) shows the typical deconvolved XPS N 1s spectra of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2. According to the published data [24,27], the peak at the binding energy of 398.3 eV is attributed to N atoms bonded to sp3-coordinated C atoms (similar to b-C3N4), and the second peak at the binding energy of 400 eV is attributed to substitutional N atoms in an sp2 graphite-like configuration bonded to sp2-coordinated C atoms (N–sp2C bond). Whereas the small peak located at higher binding energy side can be assigned to N atoms bonded to the O atoms, which related to incorporation of oxygen into the sample due to exposure to the atmosphere. Marton et al. [23] as well as Lu and Komvopoulos [12] have found that the total sp3 carbon bonding included sp3 coordinated C atoms bonded to N atoms in a configuration resembling that of b-C3N4 (sp3C–N) and C atoms with sp3-hybridized C atoms (sp3C–C), whereas the total sp2 carbon bonding included N atoms bonded to sp2-coordinated C atoms (sp2C–N) and C atoms with sp2-hybridized C atoms (sp2C–C). Based on the assignment and deconvolution of different carbon bonding states, the contents of sp2 and sp3 bonds in the deposited film were calculated. Fig. 8 shows the total sp2 and sp3 bonds contents as a function of N2 volume percentage. As the N2 volume
2
3
Fig. 8. The total sp and sp bonds content as a function of N2 volume percentage.
percentage increases from 30 to 90, the total sp3 content decreases from 29.9 to 21.0%, while the total sp2 bonds content increases from 61.0 to 76.6%. Those results indicate that the incorporation of nitrogen atom into the film promotes the development of sp2 bond. Compared with the result of the diamond-like carbon film deposited in pure Ar, the N incorporation into the diamond-like carbon films obviously affects the total sp2 and sp3 bonds contents in the film. Fig. 9(a) and (b) shows the variation of every bonding content as a function of N2 volume percentage. From Fig. 9(a), it can be seen that the total sp3 bonding content decreases with increasing the N2 volume percentage mainly due to the sp3C–C bonding content decreasing, although the content of sp3C–N bonding slightly increases with increasing of N2 volume percentage. In Fig. 9(b), it can be seen that
Fig. 9. (a) The content of total sp3, sp3C–C, sp3C–N as a function of N2 volume percentage and (b) the content of total sp2, sp2C–C, sp2C–N as a function of N2 volume percentage.
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4. Conclusion
Fig. 10. The relationship between the N–sp3C and N–sp2C content as a function of N2 volume percentage.
the total sp2 bonding content in the deposited film increases with increasing the N2 volume percentage. Compared with the sp2C–C content, the content of sp2C–N bonding slightly affects the total sp2 bonding content in the range of N2 volume percentage from 30 to 90. Consequently, the structure is affected by the N atomic content, and this is major reason for the variation of total sp3 and sp2 bonding contents. Fig. 10 shows the relationship between the N–sp3C and N–sp2C contents as a function of N2 volume percentage according to the peak intensity of the N 1s spectra. The result is in agreement with those from C 1s spectra. With increasing the N2 volume percentage, the N–sp2C bonding content decreases, while the N–sp3C bonding content increases. The decrease of N–sp2C atom content and the increase of N–sp3C atom content are related to the total amount of N atoms in nitrogen doped diamond-like carbon films. It is obvious that the N–sp2C bonds dominate in the deposited films in the low N2 volume percentage. With N2 volume percentage increasing, the N–sp3C bonds begin to dominate in the films. The content ratio of the N–sp3C to N–sp2C increases monotonously with increasing N2 volume percentage. A similar tendency was also observed for the films deposited by the reactive dc magnetron sputtering method [28]. From the variation tendency, it can be suggested that the nitrogen atoms tend to be easily bound to sp2hybridized carbon atoms at the lower nitrogen atom content, and then nitrogen atoms will be easily bound to sp3-hybridized carbon atoms when more nitrogen atoms are incorporated.
Nitrogen doped diamond-like carbon films have been prepared on Si (1 0 0) substrates by arc ion plating under different N2 volume percentage. The N atomic content, N/C atomic ratio and bonding structure of the deposited films determined by XPS depend on the N2 volume percentage. Raman and XPS results indicate that the total sp3 bond content decreases with the increasing of N atom content in the nitrogen doped diamond-like films. The nitrogen atom content and N/C atomic ratio in the deposited films increase with increasing the volume percentage of N2 in gas mixture, and exhibit the maximum value of 12.7 at.% and 0.162 under the 90 vol.% N2, respectively. The XPS C 1s spectra are asymmetric and have tails at the higher energy side, which suggests there are several types of bonds of carbon atoms. The peaks at binding energies of 284.3 and 285.3 eV are attributed to sp2C–C and sp3C–C bonding state C atoms, respectively. The two carbon peaks at 286.2 and 287.5 eV in C 1s spectrum correspond to sp2C–N and sp3C–N bonding state C atoms. The incorporation of N results in two different N–C bonding state, including N–sp3C bond and N–sp2C bond. As the N2 volume percentage increases, the N–sp3C bonds content increases, whereas the N–sp2C bonds content decreases.
References [1] A.A. Ogwu, R.W. Lamberton, S. Morley, P. Maguire, J. McLaughlin, Physica B 269 (1999) 335. [2] K.R. Lee, M.G. Kim, S.J. Cho, K.Y. Eun, T.Y. Seong, Thin Solid Films 308–309 (1997) 263. [3] S. Bhattacharyya, C. Cardinaud, G. Turban, J. Appl. Phys. 83 (1998) 4491. [4] J. Schwan, V. Batori, S. Ulrich, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 84 (1998) 2071. [5] F.L. Freire Jr., D.F. Franceschini, Thin Solid Films 293 (1997) 236. [6] T.W. Mountsier, J.A. Samuels, Thin Solid Films 332 (1998) 362. [7] M. Wang, K. Schmidt, K. Reichelt, H. Dimigen, H. Hu¨ bsch, J. Mater. Res. 7 (1992) 667. [8] W.J. Meng, B.A. Gillispie, J. Appl. Phys. 84 (1998) 4314. [9] R. Hauert, A. Glisenti, S. Metin, J. Goitia, J.H. Kaufman, et al. Thin Solid Films 268 (1995) 22. [10] M.K. Arora, A.H. Lettington, D.R. Waterman, Diamond Relat. Mater. 8 (1999) 623.
302
Y.S. Zou et al. / Applied Surface Science 241 (2005) 295–302
[11] B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 (1998) 2491. [12] W. Lu, K. Komvopoulos, J. Appl. Phys. 85 (1999) 2642. [13] C. De Martino, F. Demichelis, G. Fusco, A. Tagliaferro, Diamond Relat. Mater. 5 (1996) 461. [14] X.-A. Zhao, C.W. Ong, Y.C. Tsang, Y.W. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 (1995) 2652. [15] J. Narayan, J. Reddy, N. Biunno, S.M. Kanetkar, P. Tiwari, N. Parikh, Mater. Sci. Eng. B 26 (1994) 49. [16] D.-H. Lee, H. Lee, B. Park, J. Mater. Res. 12 (1997) 2057. [17] N. Laidani, A. Miotello, A. Glisenti, C. Bettani, J. Perriere, J. Phys.: Condens. Mat. 9 (1997) 1743. [18] M. Zhang, Y. Nakayama, J. Appl. Phys. 82 (1997) 4912. [19] Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. [20] Z.M. Zhou, L.F. Xia, Surf. Coat. Technol. 172 (2003) 102.
[21] A. Liu, M.L. Cohen, Science 245 (1989) 841. [22] H.-S. Jung, H.-H. Park, Thin Solid Films 377–378 (2000) 320. [23] D. Marton, K.J. Boyd, A.H. Albayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (1994) 118. [24] H. Sjo¨ stro¨ m, L. Hultman, J.-E. Sundgren, S.V. Hainsworth, T.F. Page, G.S.A.M. Theunissen, J. Vac. Sci. Technol. A 14 (1996) 56. [25] A.C. Ferrari, B. Kleinsorge, G. Adamopoulos, J. Robertson, W.I. Milne, V. Stolojan, L.M. Brown, A. LiBassi, B.K. Tanner, J. Non-Cryst. Solids 266–269 (2000) 765. [26] Y. Aoi, K. Ono, E. Kamijo, J. Appl. Phys. 86 (1999) 2318. [27] C.W. Ong, X.A. Zhao, Y.C. Tsang, C.L. Choy, P.W. Chan, J. Mater. Sci. 32 (1997) 2347. [28] N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, J.-E. Sundgren, Phys. Rev. B 59 (1999) 5162.
Structural characterization of nitrogen doped diamond-like carbon films deposited by arc ion plating Y.S. Zoua,*, Q.M. Wanga, H. Dua, G.H. Songb, J.Q. Xiaoa, J. Gonga, C. Suna, L.S. Wena a
Institute of Metal Research, Division of Surface Engineering of Materials, Chinese Academy of Sciences, Shenyang 110016, China b Shengyang University of Technology, Shenyang 110023, China Received in revised form 8 July 2004; accepted 8 July 2004 Available online 17 September 2004
Abstract Nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating (AIP) technique under different N2 volume percentage in the gas mixture of Ar and N2. The deposited films were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectra indicate that the ID/IG ratio increases with increasing the N2 volume percentage. XPS analysis shows a strong influence of the N2 volume percentage on the N atom concentration and the chemical bonding states in the deposited films. Nitrogen content of the deposited films increased with the increasing of N2 volume percentage. The maximum N concentration and N/C atomic ratio are up to 12.7 at.% and 0.162 at the 90 vol.% N2, respectively. From decomposition of XPS C 1s peaks, it shows that the nitrogen doped diamond-like carbon films consist of amorphous carbon–carbon bonding (sp2C–C and sp3C–C), N atoms bonded to sp3-hybridized C atoms (sp3C–N) and N atoms bonded to sp2-hybridized C atoms (sp2C–N). The total content of sp3 bonding decreases with increasing N2 volume percentage. XPS N 1s spectra show that there exist the N–sp2C and N–sp3C bonding in the deposited nitrogen doped diamond-like carbon films. As the N2 volume percentage increases, the N–sp3C bonding content increases, but the N–sp2C bonding content decreases. # 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Uw; 81.15. z; 82.80.Pv; 68.55. a Keywords: Nitrogen doped diamond-like carbon; Arc ion plating; X-ray photoelectron spectroscopy; Microstructure; Bonding structure
1. Introduction
* Corresponding author. Tel.: +86 24 83978231; fax: +86 24 23843436. E-mail address: [email protected] (Y.S. Zou), [email protected] (C. Sun).
Diamond-like carbon (DLC) films have been widely utilized in recent years due to their special properties such as high hardness, high wear resistance, low friction coefficient and chemical inertness. In DLC film, the sp3 bonds are responsible for the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.043
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mechanical properties, whereas sp2 bonds, which lie in near the Fermi level, control the electronic and optical properties of the film. Recently, there are a few of attempts to improve the properties of DLC films by the addition of elements, such as silicon [1,2], nitrogen [3–5], fluorine [6] and some metals [7,8]. In general, the addition of those elements reduces the internal stress, electrical resistivity and friction coefficient of diamond-like carbon films. For example, the thermal stability of the carbon films increases [9] and the friction coefficient decreases as hydrogenated carbon films are doped with nitrogen. Arora et al. [10] demonstrated that nitrogen incorporation into DLC films could lower the compressive stress. It has been reported that nitrogenated carbon films have a better friction behavior than hydrogenated carbon films based on contact sliding tests with tripad [11]. So the nitrogenated diamond-like carbon films have been received increasing attention in many applications, especially in the magnetic recording industry. Many methods have been used to prepare nitrogenated diamond-like carbon films, including reactive sputtering [12,13], laser ablation of carbon in a nitrogen environment [14], ionic nitrogen beam irradiation [15], nitrogen ion implantation of carbon films [16,17], chemical vapor deposition combined with an electron cyclotron resonance plasma [18] and arc ion plating [19,20]. Because of the high ionization efficiency, high deposition rate and convenience of deposition parameters controlling, arc ion plating is a promising technique and the most perspective PVD method for industrial manufacture of DLC films. There have been considerable researches into the deposition of the carbon nitride films since the theoretical predictions that crystalline b-C3N4 might be harder than diamond [21]. However, up to now, there has been no successful result on depositing films with N content as predicted b-C3N4 phase, and N/C atomic ratio varied between 0.1 and 1 [22]. Marton et al. [23] and Sjo¨ stro¨ m et al. [24] investigated the microstructure and properties of the CNx films by distinguishing their characteristic binding energies of core level electrons of C and N atoms, and indicated the presence of N atoms bonded to C atoms in tetrahedral coordination (N–sp3C) and N atoms bonded to C atoms in trigonal coordination (N– sp2C). Although the properties of the nitrogenated diamond-like carbon films have been widely studied,
there is little knowledge about the chemical bonding states of the carbon and nitrogen atoms. In this paper, nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating using a pure graphite target and a mixed gas of Ar and N2. The main aim of this research is to study the existence state of N atoms and the influence on sp3/sp2 ratio in the deposited films. The C 1s and N 1s spectra were decomposed to study the N/C atomic ratio and their chemical bonding states of the deposited films. The influence of the N atom on the sp3/sp2 ratio and chemical bonding states of the nitrogen doped diamond-like carbon films is systematically investigated.
2. Experimental procedures The nitrogen doped diamond-like carbon films were deposited on Si (1 0 0) substrates by arc ion plating. Before being located into the reactor chamber, the Si (1 0 0) substrates were ultrasonically cleaned in alcohol and acetone solution, and then rinsed in deionized water. Target was high purity graphite (99.999 at.%). The distance between the target and substrate was kept at approximately 240 mm. Prior to deposition, the chamber was evacuated to a pressure of 7 10 3 Pa, and then Ar was introduced for sputtering cleaning to remove the oxide and contaminant layer on the substrate at pulse bias of 800 V and duty cycle of 30% for 5 min. Mixed gas of N2 and Ar was fed into the chamber during deposition, and the flow ratio of the N2 to mixed gas (N2/(Ar+N2)) varied from 30 to 90% by independently adjusting the mass flow of Ar and N2. The substrate was biased at 200 V and duty cycle at 20% when the films were prepared. The surface morphologies of the nitrogen doped diamond-like carbon films were observed by atomic force microscopy (Quesant Incorporation, Q-250 AFM) in tapping mode. The root mean square (RMS) of the roughness was automatically calculated by the digital image-processing package of the AFM system. The nitrogen doped diamond-like carbon films were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Raman spectra were obtained with a micro-Raman spectrometer (Jobin Yvon HR800) using 514.5 nm Ar line as an
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excitation source. The spectra resolution was 1 cm 1. To minimize the damage during measurement, the laser-input power is fixed at 10 mW. All the XPS measurements were carried out with ESCALAB 200 IXL spectrometer with a monochromatic X-ray source of Mg Ka (1253.6 eV). The chamber was pumped to a pressure of 1 10 8 Pa and the measurement was carried out at room temperature. A step with 0.05 eV energy intervals was used to acquire the XPS spectra of the C 1s and N 1s. The energy scale was calibrated using the reference binding energies of Cu 2p3/2 at 932.5 eV and Cu 3p3/2 at 75.0 eV. To minimize uncertainties in the binding energy measurements, all samples were mounted on a copper block using conductive silver colloid paint. All obtained spectra were calibrated using the signal from the implanted Ar 2p3/2 peak at 241.3 eV. The samples were analyzed without Ar+ bombardment for sputter cleaning before XPS analysis. The film compositions were determined by analyzing XPS spectra and using published sensitivity factors.
3. Results and discussion From SEM observation of the cross-section of the deposited films, the thickness of the N-doped DLC film is measured to be approximately 300 nm. Fig. 1 shows the AFM surface morphology of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2. The surface was very smooth without any cracks. According to surface morphologies of the other N2 volume percentage, the surface morphology was almost not affected by the N2 volume percentage in the present deposition condition. The average roughness of the films is in the range of 2.8–4.9 nm under the N2 volume percentage varied form 30 to 90. Fig. 2 shows the visible Raman spectra of the films deposited under different N2 volume percentage. All the spectra of the nitrogen doped diamond-like carbon films are similar to that of diamond-like carbon film. After the subtraction of the photoluminescence background, the Raman spectra can be deconvolved into two Gaussian peaks, a G peak at around 1560 cm 1 and a D peak at around 1360 cm 1, and the intensity ratio of D to G peak (ID/IG) was determined. The G peak centered at 1560 cm 1 is derived from the zone centre E2g bond stretching mode
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of graphite, and is present in all sp2-bonded carbons. The D peak centered at 1360 cm 1 is a disorder activated K zone boundary mode, due to the A1g symmetry breathing motion of six-fold aromatic rings, which requires the presence of such rings [25]. The disorder in the films due to the clustering of sp2bonded atom is reflected by the D peak intensity. So the sp3/sp2 ratio can be indirectly evaluated according to the intensity ratio of D to G peak. Fig. 3 shows the ID/IG ratio of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage. As shown in Fig. 3, the nitrogen doped diamond-like carbon films have a higher ID/IG ratio than that of the diamond-like carbon film deposited in pure Ar, and the ID/IG ratio increases with increasing the N2 volume percentage in the atmosphere. The increase in the ID/ IG ratio is due to the effect of N incorporation into the films, which indicates the increases of vibrating modes in the D band of the nitrogen doped diamond-like carbon films and the evolution of sp2 clusters with increasing the N2 volume percentage. Hence, the Raman spectra results here suggest that the structure of the nitrogen doped diamond-like carbon films become increasingly graphitic with N2 volume percentage increasing. The global N/C atomic ratio and N atom content were evaluated from the XPS measurement and published sensitivity factors. Fig. 4 shows the N/C atomic ratio and N atom content in the deposited films as the function of the N2 volume percentage in the gas mixture. From Fig. 4, it can be seen that the N/C atomic ratio and N atom content increase rapidly in the range of 0–50 vol.% N2, then increase slightly with the N2 volume percentage further increasing. The results shown in Fig. 4 indicate that the nitrogen gas flow ratio affects the film composition. In the range of N2 volume percentage studied, the highest N/C atomic ratio and N atom content was 0.162 and 12.7 at.% for the film deposited under 90 vol.% N2, respectively. The result is consistent with previous observation [26]. In order to identify the bonding state of the nitrogen doped diamond-like carbon films, the XPS spectra on the C 1s and N 1s spectra were analyzed. Fig. 5 shows the typical XPS C 1s spectra of the films deposited under various N2 volume percentages. Compared with the spectrum of the diamond-like carbon film deposited in pure Ar, the C 1s spectra of the nitrogen doped diamond-like carbon films are broad and
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Fig. 1. AFM surface morphology of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2.
asymmetric at the higher binding energy. It suggests that there are several bonding configurations related to carbon atoms in the deposited films. Using Gaussian function fitting after the inelastic background subtraction, the C 1s spectra were deconvolved into five
sub-peaks corresponding to the different bonding state of C atoms. Fig. 6(a) shows the typical deconvolved XPS C 1s spectra of the nitrogen doped diamond-like carbon films deposited under 30 vol.% N2, and those deconvolutions are in good agreement with previous
Fig. 2. The Raman spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
Fig. 3. The ID/IG ratio of the nitrogen doped diamond-like carbon films as a function of N2 volume percentage.
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Fig. 4. The N/C atomic ratio and N atom content in the nitrogen doped diamond-like carbon films as a function of N2 volume percentage.
reported results [22,12]. In C 1s spectra, the peaks at binding energies of 284.3 and 285.3 eV are contributed to the sp2C–C and sp3C–C bond atoms, respectively. The two peaks at 286.3 and 287.5 eV correspond to sp2-hybridization bonding structures of C atoms bonded to N atoms (sp2C–N) and sp3hybridization bonding structures of C atoms bonded to N atoms (sp3C–N), respectively. The peak with the highest binding energy value in C 1s spectra was assigned to the bonding states of C with O. It is possible produced by surface oxidation during the exposure to the atmosphere. The N 1s spectra of the films deposited under various N2 volume percentages are shown in Fig. 7.
Fig. 5. The XPS C 1s spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
Fig. 6. (a) Deconvolved with Gaussian function XPS C 1s spectra of nitrogen doped diamond-like carbon film deposited under 30 vol.% N2 and (b) deconvolved with Gaussian function XPS N 1s spectra of nitrogen doped diamond-like carbon film deposited under 30 vol.% N2.
Fig. 7. The XPS N 1s spectra of the nitrogen doped diamond-like carbon films deposited under different N2 volume percentage.
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N 1s spectra have a broad peak corresponding to some bonding configurations related to N atoms. These spectra were deconvolved into three sub-peaks with Gaussian function that correspond to different chemical bonding states. Fig. 6(b) shows the typical deconvolved XPS N 1s spectra of the nitrogen doped diamond-like carbon film deposited under 30 vol.% N2. According to the published data [24,27], the peak at the binding energy of 398.3 eV is attributed to N atoms bonded to sp3-coordinated C atoms (similar to b-C3N4), and the second peak at the binding energy of 400 eV is attributed to substitutional N atoms in an sp2 graphite-like configuration bonded to sp2-coordinated C atoms (N–sp2C bond). Whereas the small peak located at higher binding energy side can be assigned to N atoms bonded to the O atoms, which related to incorporation of oxygen into the sample due to exposure to the atmosphere. Marton et al. [23] as well as Lu and Komvopoulos [12] have found that the total sp3 carbon bonding included sp3 coordinated C atoms bonded to N atoms in a configuration resembling that of b-C3N4 (sp3C–N) and C atoms with sp3-hybridized C atoms (sp3C–C), whereas the total sp2 carbon bonding included N atoms bonded to sp2-coordinated C atoms (sp2C–N) and C atoms with sp2-hybridized C atoms (sp2C–C). Based on the assignment and deconvolution of different carbon bonding states, the contents of sp2 and sp3 bonds in the deposited film were calculated. Fig. 8 shows the total sp2 and sp3 bonds contents as a function of N2 volume percentage. As the N2 volume
2
3
Fig. 8. The total sp and sp bonds content as a function of N2 volume percentage.
percentage increases from 30 to 90, the total sp3 content decreases from 29.9 to 21.0%, while the total sp2 bonds content increases from 61.0 to 76.6%. Those results indicate that the incorporation of nitrogen atom into the film promotes the development of sp2 bond. Compared with the result of the diamond-like carbon film deposited in pure Ar, the N incorporation into the diamond-like carbon films obviously affects the total sp2 and sp3 bonds contents in the film. Fig. 9(a) and (b) shows the variation of every bonding content as a function of N2 volume percentage. From Fig. 9(a), it can be seen that the total sp3 bonding content decreases with increasing the N2 volume percentage mainly due to the sp3C–C bonding content decreasing, although the content of sp3C–N bonding slightly increases with increasing of N2 volume percentage. In Fig. 9(b), it can be seen that
Fig. 9. (a) The content of total sp3, sp3C–C, sp3C–N as a function of N2 volume percentage and (b) the content of total sp2, sp2C–C, sp2C–N as a function of N2 volume percentage.
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4. Conclusion
Fig. 10. The relationship between the N–sp3C and N–sp2C content as a function of N2 volume percentage.
the total sp2 bonding content in the deposited film increases with increasing the N2 volume percentage. Compared with the sp2C–C content, the content of sp2C–N bonding slightly affects the total sp2 bonding content in the range of N2 volume percentage from 30 to 90. Consequently, the structure is affected by the N atomic content, and this is major reason for the variation of total sp3 and sp2 bonding contents. Fig. 10 shows the relationship between the N–sp3C and N–sp2C contents as a function of N2 volume percentage according to the peak intensity of the N 1s spectra. The result is in agreement with those from C 1s spectra. With increasing the N2 volume percentage, the N–sp2C bonding content decreases, while the N–sp3C bonding content increases. The decrease of N–sp2C atom content and the increase of N–sp3C atom content are related to the total amount of N atoms in nitrogen doped diamond-like carbon films. It is obvious that the N–sp2C bonds dominate in the deposited films in the low N2 volume percentage. With N2 volume percentage increasing, the N–sp3C bonds begin to dominate in the films. The content ratio of the N–sp3C to N–sp2C increases monotonously with increasing N2 volume percentage. A similar tendency was also observed for the films deposited by the reactive dc magnetron sputtering method [28]. From the variation tendency, it can be suggested that the nitrogen atoms tend to be easily bound to sp2hybridized carbon atoms at the lower nitrogen atom content, and then nitrogen atoms will be easily bound to sp3-hybridized carbon atoms when more nitrogen atoms are incorporated.
Nitrogen doped diamond-like carbon films have been prepared on Si (1 0 0) substrates by arc ion plating under different N2 volume percentage. The N atomic content, N/C atomic ratio and bonding structure of the deposited films determined by XPS depend on the N2 volume percentage. Raman and XPS results indicate that the total sp3 bond content decreases with the increasing of N atom content in the nitrogen doped diamond-like films. The nitrogen atom content and N/C atomic ratio in the deposited films increase with increasing the volume percentage of N2 in gas mixture, and exhibit the maximum value of 12.7 at.% and 0.162 under the 90 vol.% N2, respectively. The XPS C 1s spectra are asymmetric and have tails at the higher energy side, which suggests there are several types of bonds of carbon atoms. The peaks at binding energies of 284.3 and 285.3 eV are attributed to sp2C–C and sp3C–C bonding state C atoms, respectively. The two carbon peaks at 286.2 and 287.5 eV in C 1s spectrum correspond to sp2C–N and sp3C–N bonding state C atoms. The incorporation of N results in two different N–C bonding state, including N–sp3C bond and N–sp2C bond. As the N2 volume percentage increases, the N–sp3C bonds content increases, whereas the N–sp2C bonds content decreases.
References [1] A.A. Ogwu, R.W. Lamberton, S. Morley, P. Maguire, J. McLaughlin, Physica B 269 (1999) 335. [2] K.R. Lee, M.G. Kim, S.J. Cho, K.Y. Eun, T.Y. Seong, Thin Solid Films 308–309 (1997) 263. [3] S. Bhattacharyya, C. Cardinaud, G. Turban, J. Appl. Phys. 83 (1998) 4491. [4] J. Schwan, V. Batori, S. Ulrich, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 84 (1998) 2071. [5] F.L. Freire Jr., D.F. Franceschini, Thin Solid Films 293 (1997) 236. [6] T.W. Mountsier, J.A. Samuels, Thin Solid Films 332 (1998) 362. [7] M. Wang, K. Schmidt, K. Reichelt, H. Dimigen, H. Hu¨ bsch, J. Mater. Res. 7 (1992) 667. [8] W.J. Meng, B.A. Gillispie, J. Appl. Phys. 84 (1998) 4314. [9] R. Hauert, A. Glisenti, S. Metin, J. Goitia, J.H. Kaufman, et al. Thin Solid Films 268 (1995) 22. [10] M.K. Arora, A.H. Lettington, D.R. Waterman, Diamond Relat. Mater. 8 (1999) 623.
302
Y.S. Zou et al. / Applied Surface Science 241 (2005) 295–302
[11] B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 (1998) 2491. [12] W. Lu, K. Komvopoulos, J. Appl. Phys. 85 (1999) 2642. [13] C. De Martino, F. Demichelis, G. Fusco, A. Tagliaferro, Diamond Relat. Mater. 5 (1996) 461. [14] X.-A. Zhao, C.W. Ong, Y.C. Tsang, Y.W. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 (1995) 2652. [15] J. Narayan, J. Reddy, N. Biunno, S.M. Kanetkar, P. Tiwari, N. Parikh, Mater. Sci. Eng. B 26 (1994) 49. [16] D.-H. Lee, H. Lee, B. Park, J. Mater. Res. 12 (1997) 2057. [17] N. Laidani, A. Miotello, A. Glisenti, C. Bettani, J. Perriere, J. Phys.: Condens. Mat. 9 (1997) 1743. [18] M. Zhang, Y. Nakayama, J. Appl. Phys. 82 (1997) 4912. [19] Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. [20] Z.M. Zhou, L.F. Xia, Surf. Coat. Technol. 172 (2003) 102.
[21] A. Liu, M.L. Cohen, Science 245 (1989) 841. [22] H.-S. Jung, H.-H. Park, Thin Solid Films 377–378 (2000) 320. [23] D. Marton, K.J. Boyd, A.H. Albayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (1994) 118. [24] H. Sjo¨ stro¨ m, L. Hultman, J.-E. Sundgren, S.V. Hainsworth, T.F. Page, G.S.A.M. Theunissen, J. Vac. Sci. Technol. A 14 (1996) 56. [25] A.C. Ferrari, B. Kleinsorge, G. Adamopoulos, J. Robertson, W.I. Milne, V. Stolojan, L.M. Brown, A. LiBassi, B.K. Tanner, J. Non-Cryst. Solids 266–269 (2000) 765. [26] Y. Aoi, K. Ono, E. Kamijo, J. Appl. Phys. 86 (1999) 2318. [27] C.W. Ong, X.A. Zhao, Y.C. Tsang, C.L. Choy, P.W. Chan, J. Mater. Sci. 32 (1997) 2347. [28] N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, J.-E. Sundgren, Phys. Rev. B 59 (1999) 5162.