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The structure of amorphous carbon nitride films using a combined study of NEXAFS, XPS and Raman spectroscopies
Thin Solid Films 482 (2005) 145 – 150 www.elsevier.com/locate/tsf
The structure of amorphous carbon nitride films using a combined study of NEXAFS, XPS and Raman spectroscopies S.S. Roy*, R. McCann, P. Papakonstantinou, P. Maguire, J.A. McLaughlin NIBEC, School of Electrical and Mechanical Engineering, University of Ulster at Jordanstown, Shore Road, Newtownabbey, Co Antrim BT37 OQB, Northern Ireland, United Kingdom Available online 7 January 2005
Abstract The nature of bonding in tetrahedral amorphous carbon nitride (ta-C:N) films deposited by filtered cathodic vacuum arc (FCVA) technique was studied with near edge X-ray absorption fine structure (NEXAFS), X-ray photoelectron and Raman spectroscopies. The interpretation and interrelation of these spectra are discussed. The changes in the local structure were systematically studied as a function of nitrogen content. Deconvolution of the C 1s and N 1s XPS spectra shows that the sp3–C fraction decreases with an increase in nitrogen content. The p* peak at the C K (carbon K) and at the N K (nitrogen K) edges were systematically studied. Comparison of intensities of the p* peak confirms the formation of CN bond at the expense of CC bond. Analysis of NEXAFS spectra at N K edge revealed as the nitrogen concentration in the films increases, the p*/r* intensity ratio increases, indicating that there is an increase of the amount of CjN bond relative to the C–N bonds. Raman parameters, such as G peak width, I D/I G ratio, skewness of the G line ( Q), were critically analysed in terms of N content and sp2 content of the films. We demonstrate that the combined study of normalised Raman, XPS and NEXAFS spectra is very useful in determining the role of nitrogen incorporation in the structure of ta-C films. The hardness values, measured by nanoindentation technique reduced at higher (N7 at.%) N content films. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nitride; Thin film; Cathodic arc; X-ray spectroscopy
1. Introduction During the last decade, tetrahedral amorphous carbon (ta-C) films have been prepared using a variety of techniques, including filtered cathodic vacuum arc (FCVA), pulsed laser deposition and mass selected ion beam deposition [1–3]. The physical properties of ta-C:N films are obviously interrelated to their local structure. The huge complexity in the different hybridization processes in carbon nitride films makes it difficult to work out the actual local structure. Raman spectroscopy [4–6] has been employed to characterize the ta-C:N film structure. Nevertheless, this analysis quite often trusted for qualitative information only. X-ray photoelectron spectroscopy (XPS)
* Corresponding author. Tel.: +44 28 90368997; fax: +44 28 90366863. E-mail address: [email protected] (S.S. Roy). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.132
has been extensively used [7–10] to estimate the composition and chemical bonding states. However, there exist contradictory interpretation and assignment of chemical state especially of the XPS N 1s spectra. A few recent papers [11–18] have been focused to the study of the local bonding structure of carbon nitride films using near edge X-ray absorption fine structure (NEXAFS). There are divided opinions on the assignment of several peaks in C K edge and N K edge NEXAFS spectra. The clarification of the actual bonding environment around C and N atoms is still an active and important issue to the researchers. The XPS and Raman spectroscopies are available in most laboratories. NEXAFS is available only on synchrotronbased facilities; however, it is very sensitive to the local bonding structure. A combination of these spectroscopic techniques could be very useful to obtain powerful information on the bonding environment and to establish interrelation between the analysis techniques.
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In this article, ta-C:N films with various nitrogen atomic concentrations were grown by FCVA techniques. The film structures were systematically probed as a function of nitrogen content by XPS, Raman spectroscopy. Special emphasis is given on NEXAFS spectroscopy. The hardness of the films was measured using nanoindentation techniques.
2. Experimental The ta-C:N films were synthesized in a double bend FCVA system. Carbon plasma is produced from the arc spot on the cathode (99.999% pure graphite). The resulting plasma beam was then passed through an off-plane double bend solenoid to filter out particulates and other neutral species. At the filter exit, the fully ionized plasma, consisting of carbon ions and electrons, streams towards the substrate. Silicon wafers (ptype) were used as substrates and were etched by Argon ions in the vacuum chamber prior to deposition. The vacuum chamber was pumped down to approximately 0.079 mPa, which increased to 13.33–1.33 mPa as more nitrogen was introduced. Films were deposited at room temperature with an arc current of 80 A under floating conditions and the thickness ranged between 64 and 100 nm. Pure ta-C films were prepared for comparison purposes. XPS analysis of the films was carried out with a XSAM 800 (Kratos) spectrometer using a non-monochromated Mg K~ (1253.6 eV) X-ray source and a hemispherical electron energy analyser. Details of the Raman measurements technique can be found in our previous literature [6]. NEXAFS measurements were performed using the synchrotron radiation facility (Beamline 1.1) at Daresbury. The spectra at the C K edge and N K edges were recorded in the total electron yield mode and were normalised to the signal from a gold covered grid, recorded simultaneously. The resolution of the beam line was 0.1 and 0.2 eV at the C K and N K edge, respectively. The experiments were carried out at the magic angle (558) in order to avoid orientation effects on the p* and r* states at the C K and N K edge. In order to obtain the p*/r* ratio, after subtracting the respective ionization jumps at both C K and N K edges, the resulting spectra were decomposed into several Gaussian peaks. Details of the fitting procedure can be found in the literature [11,14]. A nanoindentater XP (Nanoinstruments) was used in the continuous stiffness measurements mode (CSM) to dynamically determine the hardness as a function of the indentation depth.
the nitrogen flow rate increased from 1 to 30 sccm. In addition to carbon and nitrogen, oxygen was also detected as a minor contaminant in the samples. One reason could be due to the prolonged exposure of the sample to the laboratory environment. The normalised C 1s spectra are shown in Fig. 1 with different N contents. The C 1s peak is deconvoluted by fitting it with Voigt functions. During the fitting procedure, peak position and FWHM of each peaks were fixed in the range as mentioned in Table 1. The area peaks were iterated. The C1 (284.4F0.2 eV) and C2 (285.5F0.2 eV) peaks are attributed to sp2–C coordinated bonds in graphite and sp 3–C bonds in diamond-like amorphous carbon, respectively. The assignments of above binding energies are well established [7–10]. Additional peaks C3 (286.6F0.2 eV) and C4 (287.6F0.2) are related to nitrogen incorporation [3] and oxygen contamination, respectively. A representative curve fitting of C 1s XPS spectra for the film with 12.5 at.% N is shown in the inset of Fig. 1. For the film with N content less than 3 at.%, the shape of the C1s peak was mainly determined by the C2 peak, which means that the sp3 fraction of carbon atoms in the film is very high. As the N content increases, the C1 and C3 peaks become more and more intense whereas the C2 peak contribution lowers down. In addition, it is clear from Fig. 1 that as the N content in the films increases, the C 1s peak becomes broader and the C 1s peak position decreased in particular for the highest N content films. This behavior of the C 1s spectra highlights that the sp3 fraction of carbon atoms decreases with an increase in nitrogen content in the films. The relative importance of all bonding states as obtained from C 1s deconvolution is shown in Table 1, as a function of N content. We can see from Table 1 as the nitrogen at.% increases in the films, the percentage of sp3 C bond decreased mainly due to the formation of CN (sp2–CN and sp3–CN) bonds and a relative increase of sp2– C bonds.
3. Results and discussions 3.1. XPS analysis The carbon and nitrogen content in the ta-C:N films was determined by XPS measurements. The nitrogen content in the films was found to be raised from 0.40 to 12.5 at.% as
Fig. 1. Normalised XPS C 1s core level spectra as function nitrogen content in the films. Inset shows a representative deconvolution.
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Table 1 Approximate binding energy position and relative concentration of the bonding types contributing to the XPS C 1s envelops and Raman parameters as a function of nitrogen at % XPS C 1s
Raman parameters
N at %
CjC 284.4F0.2 eV FWHM 1.2F0.2 eV
C–C 285.5F0.2 eV FWHM 1.5F0.2 eV
CN 286.7F0.2 eV FWHM 1.7F0.2 eV
CjO 287.6F0.2 eV FWHM 1.8F0.2 eV
G width
I D/I G ratio
0.0 0.4 2.80 4.50 6.90 10.6 12.5
17 16 18 20 24 25 25
80 81 76 71 66 62 57
0 2 4 5 8 12 15
3 2 2 4 2 1 3
194.52 193.57 191.86 192.52 189.02 183.4 176.63
0.14 0.13 0.16 0.17 0.26 0.35 0.52
The normalised N 1s spectra are shown in Fig. 2 with different N contents. It is clear from Fig. 2 that the N 1s feature becomes intense as the nitrogen concentration increased in the film. The spectra were deconvoluted using the previous approach. Three components were distinguished in the N 1s spectra at different binding energies. The N1 (398.7F2 eV) and N2 (400.2 F2 eV) peaks are attributed to sp3–CN bonds and sp 2–CN bonds, respectively. The N3 (402.1F2 eV) peak is related to nitrogen bond with oxygen contamination. The above assignment is consistent with majority of previous reports [7–9]. A representative curve fitting of N 1s XPS spectra for the film with 12.5 at.% N is shown in the inset of Fig. 2. The ratio of N1/N2 peak area decreased progressively with the increase of N content in the film, which is indicative of an increase of the percentage of sp2CN bonds. At the lower nitrogen concentration, it is difficult to get accurate bonding configuration as the fitting in N 1s spectra is ambiguous; however, at higher concentration (N10 at.%), we found that the majority of the N atoms are bonded to sp2-bonded carbon. This result is in good agreement with our previous report on nitrogen doped pulsed laser deposited amorphous carbon films [6].
Fig. 2. Normalised XPS N 1s core level spectra as function nitrogen content in the films. Inset shows a representative deconvolution.
Q value 24 28 24 18 8.2 4.3 2.8
3.2. Raman analysis Normalised Raman spectra as a function of nitrogen concentration are presented in Fig. 3. It is clear in Fig. 3 that the shape of the spectra changes and become more asymmetric as N at.% in the film increases. All the spectra were fitted by a Brett–Wigner–Fano (BWF) function [5] in order to make a rough estimation of the sp3/sp2 ratio. In this present study, we fitted the data in the range of 1050–1800 cm 1. According to Gilkes et al. [5], a BWF coupling coefficient ( Q) lower than 10 implies an sp3 fraction greater than 70%. The Q parameter values obtained from BWF fitting are listed in Table 1. Interestingly, all of our films containing less than 5 at.% N showed a Q value lower than 10, implying a sp3 fraction greater than 70%. On the other hand, films containing higher than 5 at.% N, displayed a Q value greater than 10 implying an sp3 fraction less than 70%. This result is in agreement with the previous XPS analysis. The spectra were also fitted with two Gaussian peaks for G and D bands [3]. The Raman parameters obtained from the fittings using 514 nm excitation wavelength are presented in Table 1. We noticed that as the N content in
Fig. 3. Normalised Raman spectra of ta-C:N film as a function of nitrogen content.
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the film increases, the I D/I G ratio increases monotonically and the G peak width decreases (Table 1). We have not observed clear trend in G peak position. Ferrari et al. [4] have previously reported that FWHM of G band decreases continuously as the order increases. Hence, we may infer from Table 1 that the increase of I D/I G ratio and the reduction of the G linewidth of the ta-C:N films indicates an increase in sp2 content and/or sp2 domain size. Many reports have suggested that the I D/I G intensity ratio and the linewidths are good indications of structural characteristics of amorphous carbon films and our results are in agreement with those [8,16]. The second-order silicon peak which appeared at 960 cm 1 can be used to measure the transparency of the film. One can easily calculate the Si to G peak intensity ratio (I Si/I G) from Fig. 3. The I Si/I G ratio increases with the increase of N content, revealing a reduction in the optical band gap when more N atoms are incorporated in films. This qualitative trend is consistent with Zhang et al. [19]. 3.3. NEXAFS analysis The normalised C K-edge spectrum of ta-CN films, as a function of nitrogen contents, is presented in Fig. 4. All spectra were normalised at the maximum height for comparison purposes. Two main features are identified in the C K-edge spectra: (i) a sharp pre-edge peak (285 eVbEb288 eV) known as p* peak, due to the transitions from the orbital 1s to different p* levels and (ii) a broader resonance at photon energy, EN293 eV corresponding to r* transitions (C–C, CjC, CZC, CZN, CjN, C–N). It is impossible to distinguish r* states as we have observed broad r* features (Fig. 4). The sharp p* peak in carbon nitride films principally originates from CjC and CjN bonds [11–18]. The intensity ratio of p*/r* peak at the C K edge as a function of nitrogen content is shown in Fig. 5.
Fig. 4. Normalised NEXAFS spectra at the C K edge of ta-C:N films as a function of nitrogen content.
Fig. 5. Intensity ratio of the p* to r* peak at the C K edge as a function of nitrogen content.
The p*/r* ratio in the C K edge means the relative contribution from CjC,CjN, CZC (if present) and CZN (if present) to the all possible r* states. This ratio is used to estimate the sp2 domain configurations in amorphous carbon network. In the lower nitrogen content films, p*/ r* ratio did not changed significantly. However, at the higher nitrogen content films, we noticed a clear increase in the p*/r* ratio. It appears that at lower nitrogen content films (b7 at.%), the concentration of CjC and CjN relative to C–C and C–N bonds does not vary much, which in turn keeps the p*/r* ratio unaffected (Fig. 5). In fact, at the lower N content films, both the sp2 cluster size and distribution of sp2 domain in the sp3 matrix changes due to the formation of CN bonds. The increase of p*/r* ratio in higher nitrogen content films (10.6 and 12.5 at.%) is the signature of the formation of graphite like structure. We can see (Fig. 4) that the p* peak width increases at higher nitrogen content films, indicating a fall of sp3 bonding configurations. The tiny peak around 289.5 eV in the films with 12.5 at.% N may originate from the contamination related to either 1sYp*(CH) or 1sYp*(CjO) transitions. The normalised N K-edge NEXAFS spectrum of ta-CN films, as a function of nitrogen contents is depicted in Fig. 6. It is clear from Fig. 6 that as N content increases in the films, the p* features become more intense. Two wellresolved peaks, denoted as P1 and P2, are observed in the energy range 399–403 eV for all films and an additional peak, denoted as P3, was clearly observed for the higher N content films (Fig. 6). These three resonance peaks were previously observed by several authors [11–16] and seem to be typical in a-CN films with moderate nitrogen concentration. The P peaks in the N K edge may indicate p* bonding in three different environments. There is no unanimity in the assignment of three individual peaks [17]. The assignments were mainly based on studying model compounds, such as pyridines, pyrroles and nitrogen molecules. Some authors simply assigned the peaks P1, P2 and P3 to CjN, CZN and substitutional nitrogen in
S.S. Roy et al. / Thin Solid Films 482 (2005) 145–150
Fig. 6. Normalised NEXAFS spectra at the C K edge of ta-C: films as a function of nitrogen content.
graphite, respectively [12,16]. Others state that it is difficult to make assignments due to the complex bonding environments [13,17] in a-C:N films. Some researchers suggested that the peak P1 is related to the constrained CN bonds, the peak P2 is related to the pyridine like double bonds and the P3 to the presence of nitrogen atoms inside the graphite domains [15]. Another opinion is that all these three peaks are from pyridine-like structure due to bonding, antibonding and splitting states [11,18]. In the following section, we will discuss how these peaks change with N content and try to assign the bonding status of each of them. The intensity of each p* peak (P1, P2 and P3) to r* peak is shown in Fig. 7 as a function of nitrogen content in the films. The entire ratio apparently increases with the N at.%, indicating an increase of CjN bonds, graphite-like N substitution bond and CZN bond if present. An increase of nitrogen concentration, hence an increase of nitrogen-related features at the N K edges, is responsible for the increase of these ratios (Fig. 7). We assign the peak
Fig. 7. Intensity ratio of the three p* peak to the r* peak at the N K edge as a function of nitrogen content.
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P1 to CjN bonds as mentioned by several authors [12,16]. The peak P3 could be due to substitutional nitrogen in graphite domains or due to bound nitrogen atoms in the films. The peak P3 appeared at higher N content films, indicating that graphite-like N substitution is not favourable at lower N concentration. The increase of the intensity ratio of P3/r* indicates the formation of graphite-like structure at higher N content films. Our hardness measurements in the following section also justified the assignment of peak P3. We ruled out the possibility of the peak P2 being attributed to CZN bonding as we found the P2 peak even in film containing 0.4 at.% nitrogen. It is well known that the formation of CZN bond is not favourable in lower N at.% films. In addition, it is clear from Fig. 6 that the relative intensity of peak P2 to P1 decreases at higher N films, which is not acceptable if we assign the peak P2 to CZN bond. It is expected that the concentration of CZN will be more prominent at high N concentrations. It seems the peaks P1 and P2 are interrelated. It should be mentioned here that we have not observed CZN band in our Raman spectra. The origin of the peak P2 is difficult to assign; however, it could be well due to pyridine-like orbital splits [11,18] or N bonded to C with different functionalities [17]. The r* features at C K and N K edge were less informative in all of our ta-C:N films. 3.4. Hardness measurement Fig. 8 depicted the hardness values at a penetration depth of 30 nm for various nitrogen content films. The hardness values of films with less than 7 at.% N are approximately between 31 and 35 Gpa. The possible reason for this is that when a small amount of nitrogen is added to ta-C, the sp2 cluster size changes due to the formation of CjN bonds. The samples containing 10.6 and 12.5 at.% N showed a reduced hardness of approximately 27 Gpa. The further addition of nitrogen leads to the formation of graphite-like
Fig. 8. Hardness of the ta-C:N films as a function of nitrogen content.
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structure (see peak P3 in Fig. 6) along with CjN bonds, which in turn reduced the rigidity of the carbon network. In the NEXAFS spectra, the presence of the P3 peak at high N levels, which originates from the substitutional nitrogen in graphite domains, clearly supports our hardness measurements. These results agree with previous reports [7,20,21] that also observed a decrease in hardness at higher N contents.
4. Conclusions By means of various spectroscopic techniques, we have interpreted the structure and local bonding environment in the ta-C:N films. The nitrogen at.% largely affected the composition, bonding structure and mechanical properties of the deposited films. Deconvolution of the C 1s and N 1s XPS spectra clearly indicate that the sp3–C fraction decreases with an increase in nitrogen content and while the sp2 CN bonds increase. Raman parameters and associated changes in the shape of the spectra point out the formation of CN bonds with the increase of nitrogen content in the films. From a careful analysis of the p* features at the C K and N K edge by NEXAFS spectroscopy, it was revealed that as the nitrogen at.% in the films increases, in general the p*/ r* intensity ratio rises, which in turn indicates that there is an increase of the amount of CjN bonds relative to the C–N bonds. The hardness of the ta-C:N film decreased only when graphite-like nitrogen substitutions are found in the films.
Acknowledgements The NEXAFS experiment was carried out at CCLRC, Daresbury. Special thanks to Dr. Ian Kirkman for helping in NEXAFS measurements.
References [1] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129. [2] T.A. Friedmann, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 (1996) 1643. [3] J.R. Shi, J.P. Wang, A.T.S. Wee, C.B. Yeo, C.T. Cheng, M. Ueda, S. Tomioka, J. Ohsako, J. Appl. Phys. 92 (2002) 5966. [4] A.C. Ferrari, S.E. Rodil, J. Robertson, Phys. Rev., B 67 (2003) 155306 (and reference therein). [5] K.W.R. Gilkes, S. Prawer, K.W. Nugent, J. Robertson, H.S. Sands, Y. Lifshitz, X. Shi, J. Appl. Phys. 87 (2000) 7283. [6] S.S. Roy, P. Papakonstantinou, R. McCann, J. McLaughlin, A. Klini, N. Papadogiannis, Appl. Phys., A 79 (2004) 1009. [7] P. Papakonstantinou, P. Lemoine, J. Phys., Condens. Matter 13 (2001) 2971. [8] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105. [9] N. Hellgren, J. Guo, C. Sathe, A. Agui, J. Nordgren, Y. Luo, H. 2gren, J. Sundgren, Appl. Phys. Lett. 79 (2001) 4348. [10] D. Das, K.H. Chen, S. Chattopadhyay, L.C. Chen, J. Appl. Phys. 91 (2002) 4944. [11] S. Bhattacharyya, M. Lqbbe, F. Richter, J. Appl. Phys. 88 (2000) 5043. [12] I. Shimoyama, G. Wu, T. Sekiguchi, Y. Baba, Phys. Rev., B 62 (2000) R6053. [13] I. Jimenez, W.M. Wong, D.K. Shuh, B.C. Holloway, M.A. Kelly, P. Pianetta, L.J. Terminello, F.J. Himpsel, Appl. Phys. Lett. 74 (1999) 2620. [14] S. Bhattacharyya, M. Lqbbe, P.R. Bressler, D.R.T. Zhan, F. Richter, Diamond Relat. Mater. 11 (2002) 8. [15] M. Mubumbila, B. Bouchet-Fabre, C. Godon, C. Marhic, B. Angleraud, P.Y. Tessier, T. Minea, Diamond Relat. Mater. 13 (2004) 1433. [16] J.M. Ripalda, E. Roman, L. Galan, I. Montero, S. Lizzit, A. Biraldi, G. Comelli, G. Paolucci, A. Goldoni, J. Chem. Phys. 118 (2003) 3748. [17] S.E. Rodil, S. Muhl, Diamond Relat. Mater. 13 (2004) 1521. [18] C. Lenardi, P. Piseri, V. Briois, C.E. Bottani, A.Li. Bassi, P. Milani, J. Appl. Phys. 88 (2000) 5049. [19] X.W. Zhang, N. Ke, W.Y. Cheung, S.P. Wong, Diamond Relat. Mater. 12 (2003) 1. [20] L.K. Cheah, X. Shi, J.R. Shi, E.J. Liu, S.R.P. Silva, J. Non-Cryst. Solids 242 (1998) 40. [21] Y.H. Cheng, B.K. Tay, S.P. Lau, X.L. Qiao, J.G. Chen, Z.H. Sun, C.S. Xie, Appl. Phys., A 75 (2002) 375.
The structure of amorphous carbon nitride films using a combined study of NEXAFS, XPS and Raman spectroscopies S.S. Roy*, R. McCann, P. Papakonstantinou, P. Maguire, J.A. McLaughlin NIBEC, School of Electrical and Mechanical Engineering, University of Ulster at Jordanstown, Shore Road, Newtownabbey, Co Antrim BT37 OQB, Northern Ireland, United Kingdom Available online 7 January 2005
Abstract The nature of bonding in tetrahedral amorphous carbon nitride (ta-C:N) films deposited by filtered cathodic vacuum arc (FCVA) technique was studied with near edge X-ray absorption fine structure (NEXAFS), X-ray photoelectron and Raman spectroscopies. The interpretation and interrelation of these spectra are discussed. The changes in the local structure were systematically studied as a function of nitrogen content. Deconvolution of the C 1s and N 1s XPS spectra shows that the sp3–C fraction decreases with an increase in nitrogen content. The p* peak at the C K (carbon K) and at the N K (nitrogen K) edges were systematically studied. Comparison of intensities of the p* peak confirms the formation of CN bond at the expense of CC bond. Analysis of NEXAFS spectra at N K edge revealed as the nitrogen concentration in the films increases, the p*/r* intensity ratio increases, indicating that there is an increase of the amount of CjN bond relative to the C–N bonds. Raman parameters, such as G peak width, I D/I G ratio, skewness of the G line ( Q), were critically analysed in terms of N content and sp2 content of the films. We demonstrate that the combined study of normalised Raman, XPS and NEXAFS spectra is very useful in determining the role of nitrogen incorporation in the structure of ta-C films. The hardness values, measured by nanoindentation technique reduced at higher (N7 at.%) N content films. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nitride; Thin film; Cathodic arc; X-ray spectroscopy
1. Introduction During the last decade, tetrahedral amorphous carbon (ta-C) films have been prepared using a variety of techniques, including filtered cathodic vacuum arc (FCVA), pulsed laser deposition and mass selected ion beam deposition [1–3]. The physical properties of ta-C:N films are obviously interrelated to their local structure. The huge complexity in the different hybridization processes in carbon nitride films makes it difficult to work out the actual local structure. Raman spectroscopy [4–6] has been employed to characterize the ta-C:N film structure. Nevertheless, this analysis quite often trusted for qualitative information only. X-ray photoelectron spectroscopy (XPS)
* Corresponding author. Tel.: +44 28 90368997; fax: +44 28 90366863. E-mail address: [email protected] (S.S. Roy). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.132
has been extensively used [7–10] to estimate the composition and chemical bonding states. However, there exist contradictory interpretation and assignment of chemical state especially of the XPS N 1s spectra. A few recent papers [11–18] have been focused to the study of the local bonding structure of carbon nitride films using near edge X-ray absorption fine structure (NEXAFS). There are divided opinions on the assignment of several peaks in C K edge and N K edge NEXAFS spectra. The clarification of the actual bonding environment around C and N atoms is still an active and important issue to the researchers. The XPS and Raman spectroscopies are available in most laboratories. NEXAFS is available only on synchrotronbased facilities; however, it is very sensitive to the local bonding structure. A combination of these spectroscopic techniques could be very useful to obtain powerful information on the bonding environment and to establish interrelation between the analysis techniques.
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S.S. Roy et al. / Thin Solid Films 482 (2005) 145–150
In this article, ta-C:N films with various nitrogen atomic concentrations were grown by FCVA techniques. The film structures were systematically probed as a function of nitrogen content by XPS, Raman spectroscopy. Special emphasis is given on NEXAFS spectroscopy. The hardness of the films was measured using nanoindentation techniques.
2. Experimental The ta-C:N films were synthesized in a double bend FCVA system. Carbon plasma is produced from the arc spot on the cathode (99.999% pure graphite). The resulting plasma beam was then passed through an off-plane double bend solenoid to filter out particulates and other neutral species. At the filter exit, the fully ionized plasma, consisting of carbon ions and electrons, streams towards the substrate. Silicon wafers (ptype) were used as substrates and were etched by Argon ions in the vacuum chamber prior to deposition. The vacuum chamber was pumped down to approximately 0.079 mPa, which increased to 13.33–1.33 mPa as more nitrogen was introduced. Films were deposited at room temperature with an arc current of 80 A under floating conditions and the thickness ranged between 64 and 100 nm. Pure ta-C films were prepared for comparison purposes. XPS analysis of the films was carried out with a XSAM 800 (Kratos) spectrometer using a non-monochromated Mg K~ (1253.6 eV) X-ray source and a hemispherical electron energy analyser. Details of the Raman measurements technique can be found in our previous literature [6]. NEXAFS measurements were performed using the synchrotron radiation facility (Beamline 1.1) at Daresbury. The spectra at the C K edge and N K edges were recorded in the total electron yield mode and were normalised to the signal from a gold covered grid, recorded simultaneously. The resolution of the beam line was 0.1 and 0.2 eV at the C K and N K edge, respectively. The experiments were carried out at the magic angle (558) in order to avoid orientation effects on the p* and r* states at the C K and N K edge. In order to obtain the p*/r* ratio, after subtracting the respective ionization jumps at both C K and N K edges, the resulting spectra were decomposed into several Gaussian peaks. Details of the fitting procedure can be found in the literature [11,14]. A nanoindentater XP (Nanoinstruments) was used in the continuous stiffness measurements mode (CSM) to dynamically determine the hardness as a function of the indentation depth.
the nitrogen flow rate increased from 1 to 30 sccm. In addition to carbon and nitrogen, oxygen was also detected as a minor contaminant in the samples. One reason could be due to the prolonged exposure of the sample to the laboratory environment. The normalised C 1s spectra are shown in Fig. 1 with different N contents. The C 1s peak is deconvoluted by fitting it with Voigt functions. During the fitting procedure, peak position and FWHM of each peaks were fixed in the range as mentioned in Table 1. The area peaks were iterated. The C1 (284.4F0.2 eV) and C2 (285.5F0.2 eV) peaks are attributed to sp2–C coordinated bonds in graphite and sp 3–C bonds in diamond-like amorphous carbon, respectively. The assignments of above binding energies are well established [7–10]. Additional peaks C3 (286.6F0.2 eV) and C4 (287.6F0.2) are related to nitrogen incorporation [3] and oxygen contamination, respectively. A representative curve fitting of C 1s XPS spectra for the film with 12.5 at.% N is shown in the inset of Fig. 1. For the film with N content less than 3 at.%, the shape of the C1s peak was mainly determined by the C2 peak, which means that the sp3 fraction of carbon atoms in the film is very high. As the N content increases, the C1 and C3 peaks become more and more intense whereas the C2 peak contribution lowers down. In addition, it is clear from Fig. 1 that as the N content in the films increases, the C 1s peak becomes broader and the C 1s peak position decreased in particular for the highest N content films. This behavior of the C 1s spectra highlights that the sp3 fraction of carbon atoms decreases with an increase in nitrogen content in the films. The relative importance of all bonding states as obtained from C 1s deconvolution is shown in Table 1, as a function of N content. We can see from Table 1 as the nitrogen at.% increases in the films, the percentage of sp3 C bond decreased mainly due to the formation of CN (sp2–CN and sp3–CN) bonds and a relative increase of sp2– C bonds.
3. Results and discussions 3.1. XPS analysis The carbon and nitrogen content in the ta-C:N films was determined by XPS measurements. The nitrogen content in the films was found to be raised from 0.40 to 12.5 at.% as
Fig. 1. Normalised XPS C 1s core level spectra as function nitrogen content in the films. Inset shows a representative deconvolution.
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Table 1 Approximate binding energy position and relative concentration of the bonding types contributing to the XPS C 1s envelops and Raman parameters as a function of nitrogen at % XPS C 1s
Raman parameters
N at %
CjC 284.4F0.2 eV FWHM 1.2F0.2 eV
C–C 285.5F0.2 eV FWHM 1.5F0.2 eV
CN 286.7F0.2 eV FWHM 1.7F0.2 eV
CjO 287.6F0.2 eV FWHM 1.8F0.2 eV
G width
I D/I G ratio
0.0 0.4 2.80 4.50 6.90 10.6 12.5
17 16 18 20 24 25 25
80 81 76 71 66 62 57
0 2 4 5 8 12 15
3 2 2 4 2 1 3
194.52 193.57 191.86 192.52 189.02 183.4 176.63
0.14 0.13 0.16 0.17 0.26 0.35 0.52
The normalised N 1s spectra are shown in Fig. 2 with different N contents. It is clear from Fig. 2 that the N 1s feature becomes intense as the nitrogen concentration increased in the film. The spectra were deconvoluted using the previous approach. Three components were distinguished in the N 1s spectra at different binding energies. The N1 (398.7F2 eV) and N2 (400.2 F2 eV) peaks are attributed to sp3–CN bonds and sp 2–CN bonds, respectively. The N3 (402.1F2 eV) peak is related to nitrogen bond with oxygen contamination. The above assignment is consistent with majority of previous reports [7–9]. A representative curve fitting of N 1s XPS spectra for the film with 12.5 at.% N is shown in the inset of Fig. 2. The ratio of N1/N2 peak area decreased progressively with the increase of N content in the film, which is indicative of an increase of the percentage of sp2CN bonds. At the lower nitrogen concentration, it is difficult to get accurate bonding configuration as the fitting in N 1s spectra is ambiguous; however, at higher concentration (N10 at.%), we found that the majority of the N atoms are bonded to sp2-bonded carbon. This result is in good agreement with our previous report on nitrogen doped pulsed laser deposited amorphous carbon films [6].
Fig. 2. Normalised XPS N 1s core level spectra as function nitrogen content in the films. Inset shows a representative deconvolution.
Q value 24 28 24 18 8.2 4.3 2.8
3.2. Raman analysis Normalised Raman spectra as a function of nitrogen concentration are presented in Fig. 3. It is clear in Fig. 3 that the shape of the spectra changes and become more asymmetric as N at.% in the film increases. All the spectra were fitted by a Brett–Wigner–Fano (BWF) function [5] in order to make a rough estimation of the sp3/sp2 ratio. In this present study, we fitted the data in the range of 1050–1800 cm 1. According to Gilkes et al. [5], a BWF coupling coefficient ( Q) lower than 10 implies an sp3 fraction greater than 70%. The Q parameter values obtained from BWF fitting are listed in Table 1. Interestingly, all of our films containing less than 5 at.% N showed a Q value lower than 10, implying a sp3 fraction greater than 70%. On the other hand, films containing higher than 5 at.% N, displayed a Q value greater than 10 implying an sp3 fraction less than 70%. This result is in agreement with the previous XPS analysis. The spectra were also fitted with two Gaussian peaks for G and D bands [3]. The Raman parameters obtained from the fittings using 514 nm excitation wavelength are presented in Table 1. We noticed that as the N content in
Fig. 3. Normalised Raman spectra of ta-C:N film as a function of nitrogen content.
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the film increases, the I D/I G ratio increases monotonically and the G peak width decreases (Table 1). We have not observed clear trend in G peak position. Ferrari et al. [4] have previously reported that FWHM of G band decreases continuously as the order increases. Hence, we may infer from Table 1 that the increase of I D/I G ratio and the reduction of the G linewidth of the ta-C:N films indicates an increase in sp2 content and/or sp2 domain size. Many reports have suggested that the I D/I G intensity ratio and the linewidths are good indications of structural characteristics of amorphous carbon films and our results are in agreement with those [8,16]. The second-order silicon peak which appeared at 960 cm 1 can be used to measure the transparency of the film. One can easily calculate the Si to G peak intensity ratio (I Si/I G) from Fig. 3. The I Si/I G ratio increases with the increase of N content, revealing a reduction in the optical band gap when more N atoms are incorporated in films. This qualitative trend is consistent with Zhang et al. [19]. 3.3. NEXAFS analysis The normalised C K-edge spectrum of ta-CN films, as a function of nitrogen contents, is presented in Fig. 4. All spectra were normalised at the maximum height for comparison purposes. Two main features are identified in the C K-edge spectra: (i) a sharp pre-edge peak (285 eVbEb288 eV) known as p* peak, due to the transitions from the orbital 1s to different p* levels and (ii) a broader resonance at photon energy, EN293 eV corresponding to r* transitions (C–C, CjC, CZC, CZN, CjN, C–N). It is impossible to distinguish r* states as we have observed broad r* features (Fig. 4). The sharp p* peak in carbon nitride films principally originates from CjC and CjN bonds [11–18]. The intensity ratio of p*/r* peak at the C K edge as a function of nitrogen content is shown in Fig. 5.
Fig. 4. Normalised NEXAFS spectra at the C K edge of ta-C:N films as a function of nitrogen content.
Fig. 5. Intensity ratio of the p* to r* peak at the C K edge as a function of nitrogen content.
The p*/r* ratio in the C K edge means the relative contribution from CjC,CjN, CZC (if present) and CZN (if present) to the all possible r* states. This ratio is used to estimate the sp2 domain configurations in amorphous carbon network. In the lower nitrogen content films, p*/ r* ratio did not changed significantly. However, at the higher nitrogen content films, we noticed a clear increase in the p*/r* ratio. It appears that at lower nitrogen content films (b7 at.%), the concentration of CjC and CjN relative to C–C and C–N bonds does not vary much, which in turn keeps the p*/r* ratio unaffected (Fig. 5). In fact, at the lower N content films, both the sp2 cluster size and distribution of sp2 domain in the sp3 matrix changes due to the formation of CN bonds. The increase of p*/r* ratio in higher nitrogen content films (10.6 and 12.5 at.%) is the signature of the formation of graphite like structure. We can see (Fig. 4) that the p* peak width increases at higher nitrogen content films, indicating a fall of sp3 bonding configurations. The tiny peak around 289.5 eV in the films with 12.5 at.% N may originate from the contamination related to either 1sYp*(CH) or 1sYp*(CjO) transitions. The normalised N K-edge NEXAFS spectrum of ta-CN films, as a function of nitrogen contents is depicted in Fig. 6. It is clear from Fig. 6 that as N content increases in the films, the p* features become more intense. Two wellresolved peaks, denoted as P1 and P2, are observed in the energy range 399–403 eV for all films and an additional peak, denoted as P3, was clearly observed for the higher N content films (Fig. 6). These three resonance peaks were previously observed by several authors [11–16] and seem to be typical in a-CN films with moderate nitrogen concentration. The P peaks in the N K edge may indicate p* bonding in three different environments. There is no unanimity in the assignment of three individual peaks [17]. The assignments were mainly based on studying model compounds, such as pyridines, pyrroles and nitrogen molecules. Some authors simply assigned the peaks P1, P2 and P3 to CjN, CZN and substitutional nitrogen in
S.S. Roy et al. / Thin Solid Films 482 (2005) 145–150
Fig. 6. Normalised NEXAFS spectra at the C K edge of ta-C: films as a function of nitrogen content.
graphite, respectively [12,16]. Others state that it is difficult to make assignments due to the complex bonding environments [13,17] in a-C:N films. Some researchers suggested that the peak P1 is related to the constrained CN bonds, the peak P2 is related to the pyridine like double bonds and the P3 to the presence of nitrogen atoms inside the graphite domains [15]. Another opinion is that all these three peaks are from pyridine-like structure due to bonding, antibonding and splitting states [11,18]. In the following section, we will discuss how these peaks change with N content and try to assign the bonding status of each of them. The intensity of each p* peak (P1, P2 and P3) to r* peak is shown in Fig. 7 as a function of nitrogen content in the films. The entire ratio apparently increases with the N at.%, indicating an increase of CjN bonds, graphite-like N substitution bond and CZN bond if present. An increase of nitrogen concentration, hence an increase of nitrogen-related features at the N K edges, is responsible for the increase of these ratios (Fig. 7). We assign the peak
Fig. 7. Intensity ratio of the three p* peak to the r* peak at the N K edge as a function of nitrogen content.
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P1 to CjN bonds as mentioned by several authors [12,16]. The peak P3 could be due to substitutional nitrogen in graphite domains or due to bound nitrogen atoms in the films. The peak P3 appeared at higher N content films, indicating that graphite-like N substitution is not favourable at lower N concentration. The increase of the intensity ratio of P3/r* indicates the formation of graphite-like structure at higher N content films. Our hardness measurements in the following section also justified the assignment of peak P3. We ruled out the possibility of the peak P2 being attributed to CZN bonding as we found the P2 peak even in film containing 0.4 at.% nitrogen. It is well known that the formation of CZN bond is not favourable in lower N at.% films. In addition, it is clear from Fig. 6 that the relative intensity of peak P2 to P1 decreases at higher N films, which is not acceptable if we assign the peak P2 to CZN bond. It is expected that the concentration of CZN will be more prominent at high N concentrations. It seems the peaks P1 and P2 are interrelated. It should be mentioned here that we have not observed CZN band in our Raman spectra. The origin of the peak P2 is difficult to assign; however, it could be well due to pyridine-like orbital splits [11,18] or N bonded to C with different functionalities [17]. The r* features at C K and N K edge were less informative in all of our ta-C:N films. 3.4. Hardness measurement Fig. 8 depicted the hardness values at a penetration depth of 30 nm for various nitrogen content films. The hardness values of films with less than 7 at.% N are approximately between 31 and 35 Gpa. The possible reason for this is that when a small amount of nitrogen is added to ta-C, the sp2 cluster size changes due to the formation of CjN bonds. The samples containing 10.6 and 12.5 at.% N showed a reduced hardness of approximately 27 Gpa. The further addition of nitrogen leads to the formation of graphite-like
Fig. 8. Hardness of the ta-C:N films as a function of nitrogen content.
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structure (see peak P3 in Fig. 6) along with CjN bonds, which in turn reduced the rigidity of the carbon network. In the NEXAFS spectra, the presence of the P3 peak at high N levels, which originates from the substitutional nitrogen in graphite domains, clearly supports our hardness measurements. These results agree with previous reports [7,20,21] that also observed a decrease in hardness at higher N contents.
4. Conclusions By means of various spectroscopic techniques, we have interpreted the structure and local bonding environment in the ta-C:N films. The nitrogen at.% largely affected the composition, bonding structure and mechanical properties of the deposited films. Deconvolution of the C 1s and N 1s XPS spectra clearly indicate that the sp3–C fraction decreases with an increase in nitrogen content and while the sp2 CN bonds increase. Raman parameters and associated changes in the shape of the spectra point out the formation of CN bonds with the increase of nitrogen content in the films. From a careful analysis of the p* features at the C K and N K edge by NEXAFS spectroscopy, it was revealed that as the nitrogen at.% in the films increases, in general the p*/ r* intensity ratio rises, which in turn indicates that there is an increase of the amount of CjN bonds relative to the C–N bonds. The hardness of the ta-C:N film decreased only when graphite-like nitrogen substitutions are found in the films.
Acknowledgements The NEXAFS experiment was carried out at CCLRC, Daresbury. Special thanks to Dr. Ian Kirkman for helping in NEXAFS measurements.
References [1] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129. [2] T.A. Friedmann, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 (1996) 1643. [3] J.R. Shi, J.P. Wang, A.T.S. Wee, C.B. Yeo, C.T. Cheng, M. Ueda, S. Tomioka, J. Ohsako, J. Appl. Phys. 92 (2002) 5966. [4] A.C. Ferrari, S.E. Rodil, J. Robertson, Phys. Rev., B 67 (2003) 155306 (and reference therein). [5] K.W.R. Gilkes, S. Prawer, K.W. Nugent, J. Robertson, H.S. Sands, Y. Lifshitz, X. Shi, J. Appl. Phys. 87 (2000) 7283. [6] S.S. Roy, P. Papakonstantinou, R. McCann, J. McLaughlin, A. Klini, N. Papadogiannis, Appl. Phys., A 79 (2004) 1009. [7] P. Papakonstantinou, P. Lemoine, J. Phys., Condens. Matter 13 (2001) 2971. [8] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105. [9] N. Hellgren, J. Guo, C. Sathe, A. Agui, J. Nordgren, Y. Luo, H. 2gren, J. Sundgren, Appl. Phys. Lett. 79 (2001) 4348. [10] D. Das, K.H. Chen, S. Chattopadhyay, L.C. Chen, J. Appl. Phys. 91 (2002) 4944. [11] S. Bhattacharyya, M. Lqbbe, F. Richter, J. Appl. Phys. 88 (2000) 5043. [12] I. Shimoyama, G. Wu, T. Sekiguchi, Y. Baba, Phys. Rev., B 62 (2000) R6053. [13] I. Jimenez, W.M. Wong, D.K. Shuh, B.C. Holloway, M.A. Kelly, P. Pianetta, L.J. Terminello, F.J. Himpsel, Appl. Phys. Lett. 74 (1999) 2620. [14] S. Bhattacharyya, M. Lqbbe, P.R. Bressler, D.R.T. Zhan, F. Richter, Diamond Relat. Mater. 11 (2002) 8. [15] M. Mubumbila, B. Bouchet-Fabre, C. Godon, C. Marhic, B. Angleraud, P.Y. Tessier, T. Minea, Diamond Relat. Mater. 13 (2004) 1433. [16] J.M. Ripalda, E. Roman, L. Galan, I. Montero, S. Lizzit, A. Biraldi, G. Comelli, G. Paolucci, A. Goldoni, J. Chem. Phys. 118 (2003) 3748. [17] S.E. Rodil, S. Muhl, Diamond Relat. Mater. 13 (2004) 1521. [18] C. Lenardi, P. Piseri, V. Briois, C.E. Bottani, A.Li. Bassi, P. Milani, J. Appl. Phys. 88 (2000) 5049. [19] X.W. Zhang, N. Ke, W.Y. Cheung, S.P. Wong, Diamond Relat. Mater. 12 (2003) 1. [20] L.K. Cheah, X. Shi, J.R. Shi, E.J. Liu, S.R.P. Silva, J. Non-Cryst. Solids 242 (1998) 40. [21] Y.H. Cheng, B.K. Tay, S.P. Lau, X.L. Qiao, J.G. Chen, Z.H. Sun, C.S. Xie, Appl. Phys., A 75 (2002) 375.