- Email: [email protected]
Raman studies of tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc
Surface and Coatings Technology 105 (1998) 155–158
Raman studies of tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc B.K. Tay *, X. Shi, H.S. Tan, H.S. Yang, Z. Sun School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 Received 10 October 1997; accepted 14 February 1998
Abstract Raman spectra of tetrahedral amorphous carbon (ta-C ) films have been obtained as a function of impinging carbon ion energy. In order to analyze the spectra quantitatively, the Raman spectra were fitted using a least-squares computer program. The relative Raman intensity is found to decrease with increasing sp3/sp2 bonding ratio in the films. In particular, the parameters from the fits show a strong correlation between the relative intensity ratio and the sp3 fraction. © 1998 Elsevier Science S.A. Keywords: Cathodic vacuum arc; Raman studies; Tetrahedral amorphous carbon films
1. Introduction
2. Experimental details
Hydrogen-free tetrahedral amorphous carbon (ta-C ) films produced by the Filtered Cathodic Vacuum Arc (FCVA) technique have attracted considerable interest over the last 10 years [1–8]. About 90% of the carbon atoms in the film are tetrahedrally bonded, which is the same bonding structure as diamond [3,6 ]. This bonding gives ta-C many excellent properties such as a high micro-hardness (>60 GPa, about three times that of TiN ), a high Young’s modulus (>600 GPa) and a smooth morphology ( RMS roughness about 0.5 nm) [8,9]. The properties of ta-C films have been strongly correlated to the energy of the incident carbon ions during deposition [7,8]. This paper reports Raman spectroscopy studies of ta-C films deposited using ion energies from 55 to 195 eV. The objectives of the study were to establish whether any correlation exists between the Raman spectra profile and some macroscopic film properties. Raman spectroscopy is a comparatively simple and non-destructive technique that involves no special specimen preparation. Hence, a Raman signature characteristic of ta-C films of high sp3 content could be used to determine the optimal growth conditions of ta-C films suited for particular applications.
Ta-C films were deposited at various ion energies from 55 eV to 195 eV on 100 Si substrate. The ion energy is controlled by changing the substrate bias, and the relation between the substrate bias and the ion energy is carefully established by Langmuir and Faraday cup measurements. The details of the deposition system have been published elsewhere [8]. Raman spectra were measured at room temperature on a Spectra-Physics Stabilite 2011 spectrometer using the 514.5-nm lines of Ar+ laser as excitation source. The Raman spectra were acquired over the range of 800–2000 cm−1 at 2 cm−1 resolution. A low input power was used in order to minimize any possible beam heating effects. The thickness of the films, measured by spectral reflectometry to ˚ , was typically 70 nm. The ellipsometric within ±50 A angles were measured with a UVISEL spectroscopic phase-modulated ellipsometer in the wide spectral range of 250–900 nm. The optical band gap of the films was determined by fitting the ellipsometric measurements to a Forouchi Bloomer model [10] shown to be appropriate for amorphous diamond-like carbon films [11,12].
3. Results and discussions * Corresponding author. Tel: +65 799 6127; Fax: +65 799 792 0415; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 47 5 - 7
Fig. 1 shows the Raman spectra between 800 cm−1 and 2000 cm−1 for ta-C films deposited on silicon sub-
156
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
Fig. 2. Curve fitting using two Gaussian line shape. The solid line shows the fit, and the broken lines show the individual G and D band components of the fit.
Fig. 1. Raman spectra of ta-C films deposited at different ion energies.
strates at different ion energies from 55 eV, 115 eV and 195 eV, respectively. The spectra have been displaced vertically for clarity, but otherwise are displayed using the same vertical scale. The spectra exhibit a broad Raman intensity distribution in the range 1400–1700 cm−1 centered at 1550 cm−1, which agrees with the results reported by other researchers [5,6 ]. This confirms that the films are amorphous. The figure also shows that the absolute intensity of the spectrum decreases as the ion energy increases from 55 eV to 115 eV, and thereafter increases when the ion energy increases further to 155 eV. The Raman spectrum is largely insensitive to the sp3 bonded component of the films [13]. Furthermore, the vibrational density of states (DOS) of sp3-bonded material shows a cut-off at about 1350 cm−1 [14]. Therefore, the Raman peak centered at about 1500 cm−1 cannot be due to sp3-bonded materials [13]. Hence, the Raman spectrum monitors the state of the sp2-bonded materials within the sp3 matrix. This is consistent with the qualitative trend shown in Fig. 1, which indicates that the absolute intensity of the spectrum decreases as the sp2 content decreases. This interpretation is similar to that reported by Prawer [13] who measured the Raman spectra of diamond-like carbon films prepared by a mass-selected ion beam and also observed that a decrease of the absolute intensity is related to a decrease of the sp2 content in the films. Also noticeable in all the spectra shown in Fig. 1 is the existence of a peak at ~965 cm−1, which is the secondorder phonon scattering from the silicon substrate. The appearance of this peak is a measure of the transparency
Fig. 3. G-line width as a function of G-line position.
of the film near the laser wavelength of 514 nm [15]. The transparency of these films is a result of the high fraction of carbon atoms with tetrahedral coordination. In order to analyze the spectra quantitatively, the Raman spectra were fitted to two Gaussian peaks using PeakFit, a least-squares computer program. For consistency with the published literature, we define these two peaks as the disorder (‘‘D’’) and graphite (‘‘G’’) peaks. This notation arises from the Raman spectra of nanocrystalline graphite, which generally shows two peaks. One peak (~1590 cm−1) is due to the in-plane Raman mode of graphite, and the other (~1360 cm−1) is attributed to small domain-size graphitic regions. [16 ]. The fit for one of the ta-C films deposited at ion energy of 175 eV is shown in Fig. 2. The thick solid line is the
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
Fig. 4. Intensity ratio I /I as a function of G-line position. D G
Gaussian fit to the data points. The decomposed bands are shown as broken lines. Fig. 3 shows the G line width (the Gaussian width) as a function of G line peak position (W ) for various types of amorphous carbon G films deposited under different ion energy range. The i-C were cathodic-arc deposited carbon films by MultiArc Scientific Corp. [17], and the a-C were amorphous carbon formed by Ar-ion sputtering of graphite by the Research Laboratory, Ford Motor Company [17]. The result shows that each type of film was clustered together at a different location structurally, one being distinct from another. The G linewidth is determined by several factors, namely the cluster size, cluster size distribution and stress in the carbon film, whereas the G line peak position is affected by factors such as the cluster size and the hydrogen content [18]. From Fig. 3, it can be seen that a narrowing of the G band observed is probably indicative of a lower compressive stress present in our films as compared to the i-C films. The lower G line peak position of the a-C may be due to the hydrogen content in these films. Fig. 4 illustrates the relative intensity ratio I /I as a function of G line peak D G position. It has been shown that the relative intensity ratio I /I can be used as a parameter for sp3 content D G [17,19]. A smaller value of I /I ratio will correspond D G to a higher sp3 content. The figure shows that our ta-C films have a low intensity ratio, I /I , indicating that D G our films have a high sp3 content, similar to the i-C films. The shift of the G line peak position, W within G an individual cluster is relatively small as compared to the large value of W itself. This suggests that the shift G of W is not a good parameter describing the properties G of the same type of films. Fig. 5 shows the I /I ratio D G as a function of ion energy. As can be seen, the I /I D G ratio decreases from 0.16 at an ion energy of 55 eV and reaches a minimum of 0.09 at an ion energy around 115 eV. Thereafter, the ratio increases to 0.24 at 195 eV.
157
Fig. 5. Intensity ratio I /I as a function of carbon ion energy. D G
Fig. 6. sp3 fraction as a function of intensity ratio I /I . D G
This shows that a carbon film with a high sp3 content is achievable at an ion energy around 115 eV, which is consistent with previous measurements of the stress, hardness and density [8,9]. Fig. 6 shows a relatively linear correlation of the I /I ratio with the sp3 fraction D G content present in the carbon films. The sp3 fraction content was determined by EELS measurements obtained in a previous study [8]. Fig. 7 is the optical bandgap of our ta-C films as a function of the I /I D G ratio. The optical bandgap of the a-C films is low, in the range of 0.7–1.2 eV, and the corresponding I /I D G ratio is higher, in the range of 0.5–1.5. This indicates that a-C films are not as transparent as the ta-C films, which have a higher optical bandgap and a lower I /I ratio. Fig. 7 also shows that the optical bandgap D G is inversely proportional to the I /I ratio and hence D G the sp3 content. The optical bandgap of our ta-C films decreases from 2.6 eV to 2.2 eV when the I /I ratio D G changes from 0.10 to 0.24.
158
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
References
Fig. 7. Optical bandgap as a function of intensity ratio (I /I ). The D G insert shows the optical bandgap as a function of I /I of our ta-C D G films in detail.
4. Conclusion In summary, we have investigated the Raman scattering of ta-C films. The relative Raman intensity is found to decrease with increasing sp3/sp2 bonding ratio in the films. The I /I ratio decreases from 0.16 at ion energy D G of 55 eV and reaches a minimum of 0.09 at an ion energy of around 115 eV. Thereafter, the ratio increases to 0.24 at 195 eV, indicating that the optimal ion energy is at 115 eV. A relatively linear correlation of the I /I D G ratio with the sp3 fraction content present in the carbon films was obtained. The optical bandgap was found to be inversely proportional to the I /I ratio and hence D G the sp3 content. These show that the I /I ratio by D G Raman spectroscopy can be used as a parameter for the sp3 content.
[1] P.J. Martin, S.W. Filipczuk, R.P. Netterfield, J.S. Field, D.F. Whitnall, D.R. McKenzie, J. Mater. Sci. Lett. 7 (1988) 410. [2] P.J. Martin, R.P. Netterfield, T.J. Kinder, L. Descotes, Surf. Coat. Technol. 49 (1991) 239. [3] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [4] S. Falabella, D.B. Boercker, D.M. Sanders, Thin Film Solids 236 (1993) 82. [5] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 48 (1993) 4777. [6 ] V.S. Veerasamy, G.A.J. Amaratunga, W.I. Wilne, P. Hewitt, P.J. Fallon, D.R. McKenzie, C.A. Davis, Diamond Relat. Mater. 2 (1993) 782. [7] Y. Lifshitz, G.D. Lempert, S. Rotter, Diamond Relat. Mater. 3 (1994) 542. [8] S. Xu, B.K. Tay, H.S. Tan, L. Zhong, Y.Q. Tu, S.R.P. Silva, W.I. Milne, J. Appl. Phys. 79 (1996) 7239. [9] X. Shi, B.K. Tay, D.I. Flynn, Z. Sun, Tribological Properties of Tetrahedral Carbon Films Prepared by Filtered Cathodic Vacuum Arc Deposition, MRS Proc., USA, 1997, Vol. 436, p. 293–298. [10] A.R. Forouhi, I. Bloomer, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. [11] S.A. Alterovitz, N. Savvides, F.W. Smith, J.A. Woollam, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. [12] B.K. Tay, X. Shi, L.K. Cheah, D.I. Flynn, Optical Properties of tetrahedral amorphous carbon films determined by spectroscopic ellipsometry, Thin Solid Films 308–309 (1997) 268–272. [13] S. Prawer, K.W. Nugent, Y. Lifshitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond Relat. Mater. 5 (1996) 433. [14] D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 (1984) 870. [15] T.A. Friedman, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 (1996) 1643. [16 ] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 1126 (1969) 53. [17] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. [18] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 (1996) 440. [19] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, J. Appl. Phys. 64 (1988) 6464.
Raman studies of tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc B.K. Tay *, X. Shi, H.S. Tan, H.S. Yang, Z. Sun School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 Received 10 October 1997; accepted 14 February 1998
Abstract Raman spectra of tetrahedral amorphous carbon (ta-C ) films have been obtained as a function of impinging carbon ion energy. In order to analyze the spectra quantitatively, the Raman spectra were fitted using a least-squares computer program. The relative Raman intensity is found to decrease with increasing sp3/sp2 bonding ratio in the films. In particular, the parameters from the fits show a strong correlation between the relative intensity ratio and the sp3 fraction. © 1998 Elsevier Science S.A. Keywords: Cathodic vacuum arc; Raman studies; Tetrahedral amorphous carbon films
1. Introduction
2. Experimental details
Hydrogen-free tetrahedral amorphous carbon (ta-C ) films produced by the Filtered Cathodic Vacuum Arc (FCVA) technique have attracted considerable interest over the last 10 years [1–8]. About 90% of the carbon atoms in the film are tetrahedrally bonded, which is the same bonding structure as diamond [3,6 ]. This bonding gives ta-C many excellent properties such as a high micro-hardness (>60 GPa, about three times that of TiN ), a high Young’s modulus (>600 GPa) and a smooth morphology ( RMS roughness about 0.5 nm) [8,9]. The properties of ta-C films have been strongly correlated to the energy of the incident carbon ions during deposition [7,8]. This paper reports Raman spectroscopy studies of ta-C films deposited using ion energies from 55 to 195 eV. The objectives of the study were to establish whether any correlation exists between the Raman spectra profile and some macroscopic film properties. Raman spectroscopy is a comparatively simple and non-destructive technique that involves no special specimen preparation. Hence, a Raman signature characteristic of ta-C films of high sp3 content could be used to determine the optimal growth conditions of ta-C films suited for particular applications.
Ta-C films were deposited at various ion energies from 55 eV to 195 eV on 100 Si substrate. The ion energy is controlled by changing the substrate bias, and the relation between the substrate bias and the ion energy is carefully established by Langmuir and Faraday cup measurements. The details of the deposition system have been published elsewhere [8]. Raman spectra were measured at room temperature on a Spectra-Physics Stabilite 2011 spectrometer using the 514.5-nm lines of Ar+ laser as excitation source. The Raman spectra were acquired over the range of 800–2000 cm−1 at 2 cm−1 resolution. A low input power was used in order to minimize any possible beam heating effects. The thickness of the films, measured by spectral reflectometry to ˚ , was typically 70 nm. The ellipsometric within ±50 A angles were measured with a UVISEL spectroscopic phase-modulated ellipsometer in the wide spectral range of 250–900 nm. The optical band gap of the films was determined by fitting the ellipsometric measurements to a Forouchi Bloomer model [10] shown to be appropriate for amorphous diamond-like carbon films [11,12].
3. Results and discussions * Corresponding author. Tel: +65 799 6127; Fax: +65 799 792 0415; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 47 5 - 7
Fig. 1 shows the Raman spectra between 800 cm−1 and 2000 cm−1 for ta-C films deposited on silicon sub-
156
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
Fig. 2. Curve fitting using two Gaussian line shape. The solid line shows the fit, and the broken lines show the individual G and D band components of the fit.
Fig. 1. Raman spectra of ta-C films deposited at different ion energies.
strates at different ion energies from 55 eV, 115 eV and 195 eV, respectively. The spectra have been displaced vertically for clarity, but otherwise are displayed using the same vertical scale. The spectra exhibit a broad Raman intensity distribution in the range 1400–1700 cm−1 centered at 1550 cm−1, which agrees with the results reported by other researchers [5,6 ]. This confirms that the films are amorphous. The figure also shows that the absolute intensity of the spectrum decreases as the ion energy increases from 55 eV to 115 eV, and thereafter increases when the ion energy increases further to 155 eV. The Raman spectrum is largely insensitive to the sp3 bonded component of the films [13]. Furthermore, the vibrational density of states (DOS) of sp3-bonded material shows a cut-off at about 1350 cm−1 [14]. Therefore, the Raman peak centered at about 1500 cm−1 cannot be due to sp3-bonded materials [13]. Hence, the Raman spectrum monitors the state of the sp2-bonded materials within the sp3 matrix. This is consistent with the qualitative trend shown in Fig. 1, which indicates that the absolute intensity of the spectrum decreases as the sp2 content decreases. This interpretation is similar to that reported by Prawer [13] who measured the Raman spectra of diamond-like carbon films prepared by a mass-selected ion beam and also observed that a decrease of the absolute intensity is related to a decrease of the sp2 content in the films. Also noticeable in all the spectra shown in Fig. 1 is the existence of a peak at ~965 cm−1, which is the secondorder phonon scattering from the silicon substrate. The appearance of this peak is a measure of the transparency
Fig. 3. G-line width as a function of G-line position.
of the film near the laser wavelength of 514 nm [15]. The transparency of these films is a result of the high fraction of carbon atoms with tetrahedral coordination. In order to analyze the spectra quantitatively, the Raman spectra were fitted to two Gaussian peaks using PeakFit, a least-squares computer program. For consistency with the published literature, we define these two peaks as the disorder (‘‘D’’) and graphite (‘‘G’’) peaks. This notation arises from the Raman spectra of nanocrystalline graphite, which generally shows two peaks. One peak (~1590 cm−1) is due to the in-plane Raman mode of graphite, and the other (~1360 cm−1) is attributed to small domain-size graphitic regions. [16 ]. The fit for one of the ta-C films deposited at ion energy of 175 eV is shown in Fig. 2. The thick solid line is the
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
Fig. 4. Intensity ratio I /I as a function of G-line position. D G
Gaussian fit to the data points. The decomposed bands are shown as broken lines. Fig. 3 shows the G line width (the Gaussian width) as a function of G line peak position (W ) for various types of amorphous carbon G films deposited under different ion energy range. The i-C were cathodic-arc deposited carbon films by MultiArc Scientific Corp. [17], and the a-C were amorphous carbon formed by Ar-ion sputtering of graphite by the Research Laboratory, Ford Motor Company [17]. The result shows that each type of film was clustered together at a different location structurally, one being distinct from another. The G linewidth is determined by several factors, namely the cluster size, cluster size distribution and stress in the carbon film, whereas the G line peak position is affected by factors such as the cluster size and the hydrogen content [18]. From Fig. 3, it can be seen that a narrowing of the G band observed is probably indicative of a lower compressive stress present in our films as compared to the i-C films. The lower G line peak position of the a-C may be due to the hydrogen content in these films. Fig. 4 illustrates the relative intensity ratio I /I as a function of G line peak D G position. It has been shown that the relative intensity ratio I /I can be used as a parameter for sp3 content D G [17,19]. A smaller value of I /I ratio will correspond D G to a higher sp3 content. The figure shows that our ta-C films have a low intensity ratio, I /I , indicating that D G our films have a high sp3 content, similar to the i-C films. The shift of the G line peak position, W within G an individual cluster is relatively small as compared to the large value of W itself. This suggests that the shift G of W is not a good parameter describing the properties G of the same type of films. Fig. 5 shows the I /I ratio D G as a function of ion energy. As can be seen, the I /I D G ratio decreases from 0.16 at an ion energy of 55 eV and reaches a minimum of 0.09 at an ion energy around 115 eV. Thereafter, the ratio increases to 0.24 at 195 eV.
157
Fig. 5. Intensity ratio I /I as a function of carbon ion energy. D G
Fig. 6. sp3 fraction as a function of intensity ratio I /I . D G
This shows that a carbon film with a high sp3 content is achievable at an ion energy around 115 eV, which is consistent with previous measurements of the stress, hardness and density [8,9]. Fig. 6 shows a relatively linear correlation of the I /I ratio with the sp3 fraction D G content present in the carbon films. The sp3 fraction content was determined by EELS measurements obtained in a previous study [8]. Fig. 7 is the optical bandgap of our ta-C films as a function of the I /I D G ratio. The optical bandgap of the a-C films is low, in the range of 0.7–1.2 eV, and the corresponding I /I D G ratio is higher, in the range of 0.5–1.5. This indicates that a-C films are not as transparent as the ta-C films, which have a higher optical bandgap and a lower I /I ratio. Fig. 7 also shows that the optical bandgap D G is inversely proportional to the I /I ratio and hence D G the sp3 content. The optical bandgap of our ta-C films decreases from 2.6 eV to 2.2 eV when the I /I ratio D G changes from 0.10 to 0.24.
158
B.K. Tay et al. / Surface and Coatings Technology 105 (1998) 155–158
References
Fig. 7. Optical bandgap as a function of intensity ratio (I /I ). The D G insert shows the optical bandgap as a function of I /I of our ta-C D G films in detail.
4. Conclusion In summary, we have investigated the Raman scattering of ta-C films. The relative Raman intensity is found to decrease with increasing sp3/sp2 bonding ratio in the films. The I /I ratio decreases from 0.16 at ion energy D G of 55 eV and reaches a minimum of 0.09 at an ion energy of around 115 eV. Thereafter, the ratio increases to 0.24 at 195 eV, indicating that the optimal ion energy is at 115 eV. A relatively linear correlation of the I /I D G ratio with the sp3 fraction content present in the carbon films was obtained. The optical bandgap was found to be inversely proportional to the I /I ratio and hence D G the sp3 content. These show that the I /I ratio by D G Raman spectroscopy can be used as a parameter for the sp3 content.
[1] P.J. Martin, S.W. Filipczuk, R.P. Netterfield, J.S. Field, D.F. Whitnall, D.R. McKenzie, J. Mater. Sci. Lett. 7 (1988) 410. [2] P.J. Martin, R.P. Netterfield, T.J. Kinder, L. Descotes, Surf. Coat. Technol. 49 (1991) 239. [3] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [4] S. Falabella, D.B. Boercker, D.M. Sanders, Thin Film Solids 236 (1993) 82. [5] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 48 (1993) 4777. [6 ] V.S. Veerasamy, G.A.J. Amaratunga, W.I. Wilne, P. Hewitt, P.J. Fallon, D.R. McKenzie, C.A. Davis, Diamond Relat. Mater. 2 (1993) 782. [7] Y. Lifshitz, G.D. Lempert, S. Rotter, Diamond Relat. Mater. 3 (1994) 542. [8] S. Xu, B.K. Tay, H.S. Tan, L. Zhong, Y.Q. Tu, S.R.P. Silva, W.I. Milne, J. Appl. Phys. 79 (1996) 7239. [9] X. Shi, B.K. Tay, D.I. Flynn, Z. Sun, Tribological Properties of Tetrahedral Carbon Films Prepared by Filtered Cathodic Vacuum Arc Deposition, MRS Proc., USA, 1997, Vol. 436, p. 293–298. [10] A.R. Forouhi, I. Bloomer, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. [11] S.A. Alterovitz, N. Savvides, F.W. Smith, J.A. Woollam, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. [12] B.K. Tay, X. Shi, L.K. Cheah, D.I. Flynn, Optical Properties of tetrahedral amorphous carbon films determined by spectroscopic ellipsometry, Thin Solid Films 308–309 (1997) 268–272. [13] S. Prawer, K.W. Nugent, Y. Lifshitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond Relat. Mater. 5 (1996) 433. [14] D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 (1984) 870. [15] T.A. Friedman, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 (1996) 1643. [16 ] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 1126 (1969) 53. [17] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. [18] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 (1996) 440. [19] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, J. Appl. Phys. 64 (1988) 6464.