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Direct observation of sp3 bonding in tetrahedral amorphous carbon UV Raman spectroscopy
Journal of Non-Crystalline Solids 227–230 Ž1998. 612–616
Direct observation of sp 3 bonding in tetrahedral amorphous carbon UV Raman spectroscopy K.W.R. Gilkes a , H.S. Sands b, D.N. Batchelder b, W.I. Milne c , J. Robertson
c,)
a
c
School of Physics, UniÕersity of East Anglia, Norwich NR4 7TJ, UK b Department of Physics, UniÕersity of Leeds, Leeds LS2 9JT, UK Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK
Abstract It is shown that Raman spectroscopy using UV excitation at 244 nm directly reveals the vibrational modes of sp 3 sites in tetrahedral amorphous carbon Žta-C.. The spectrum of ta-C consists of two bands around 1100 cmy1 and 1600 cmy1 , which appear to arise from sp 2 sites and sp 3 sites, respectively. Analysis of the spectra shows that the 1100 cmy1 mode arises from a coupling to s states and it shifts from about 1400 cmy1 to 1100 cmy1 with increasing sp 3 content. q 1998 Elsevier Science B.V. All rights reserved. Keywords: sp 3 bonding; Raman spectroscopy; Tetrahedral amorphous carbon
1. Introduction Amorphous carbon films can be produced with a range of sp 3 and sp 2 bonding depending on the preparation conditions. Films with the largest concentration of sp 3 bonding, called diamond-like carbon ŽDLC., are of great interest because of their hardness, low friction and low electron affinity w1x. There is considerable need for a rapid non-destructive method to analyse their bonding. Of methods presently used, nuclear magnetic resonance ŽNMR. can resolve sp 2 and sp 3 sites and can give a quantitative estimate of the fraction of sp 3 bonding w2,3x, but this method requires large or 13 C-enriched samples. The C–H infrared stretching modes have been used
)
Corresponding author. Tel.: q44-1223 3322689; fax: q441223 332662; e-mail: [email protected].
to study bonding w4x, but clearly their use is restricted to sites bonded to hydrogen. Electron energy loss spectroscopy ŽEELS. of the carbon K edge is presently the favoured method to measure bonding in DLC w5,6x. It gives a direct measure of the sp 2 bonding from the size of the p ) peak at 285 eV. However, the sp 3 sites are not probed directly, as they contribute with the sp 2 sites to the step at 290 eV due to s ) states. Furthermore, EELS is a destructive and rather time-consuming technique. Raman is a non-destructive technique for measuring the bonding properties of diamond-like carbon and diamond w7–16x. In principle, Raman should provide a distinction between sp 3 and sp 2 sites in DLC because their Raman frequencies in diamond and graphite of 1550 and 1332 cmy1 are widely separated. However, conventional visible Raman excited by 488 nm or 514 nm photons is limited because the sp 2 sites have a greater cross-section,
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 9 0 - 2
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
and their contribution always dominates the sp 3 sites. Consequently, the visible Raman spectrum of DLC is dominated by the G band at about 1550 cmy1 and a D feature around 1350 cmy1 , both of which are attributed to graphitic sp 2 bonding. These effects are also observed in the spectra of so-called ‘tetrahedral amorphous carbon’ Žta-C. w14x, as this still contains up to 20% sp 2 sites. Thus, the use of visible Raman to characterise DLC has been based on empirical relationships between bonding and shifts or broadening of the G band w15x. In principle, because of the relaxation of the k-selection rule, the Raman spectra of an amorphous solid is proportional to the total vibrational density of states ŽVDOS. multiplied by some cross-section w17x. However, in DLC the problem is that excitation at 514 nm Ž2.4 eV. corresponds to the p – p ) transitions at sp 2 sites and this gives a resonant enhancement of their cross-section w10–12x. This problem can be avoided by using ultraviolet ŽUV. excitation at 244 nm Ž5.1 eV., as this causes the excitation of s states on both sp 2 and sp 3 sites, and this provides a more equally weighted measure of the VDOS w18,19x.
613
3. Results Fig. 1 shows the visible Raman spectra of the evaporated a-C and ta-C films deposited at a number of bias voltages, for reference. The spectrum of evaporated a-C is dominated by the G band at 1570 cmy1 , plus a smaller band at the D position around 1350 cmy1 , as expected for graphitic bonding. The G band is attributed to the modes of similar to the zone-centre E 2g mode of graphite, while the D band is attributed to disorder-activated aromatic modes of A 1 symmetry. The spectra of ta-C are dominated by the broad G band at 1600 cmy1 , and there is essentially no shoulder at the D position, as seen by other workers w15x. The 960 cmy1 band is due to the Si substrate. Fig. 2 shows the first-order UV Raman spectra of the ta-C and a-C films. The spectra differ from that in Fig. 1. They consist of two bands centred around 1100 cmy1 and 1650 cmy1 , together with a plateau that extends down to 500 cmy1 . The small band at 1551 cmy1 is due to atmospheric oxygen. The key differences in the visible Raman spectra are that the G band has moved up from 1600 cmy1 to 1650 cmy1 , and is now narrower; any shoulder around the
2. Method The UV Raman spectra were produced by the 244 nm line of an intracavity frequency-doubled Ar ion laser ŽCoherent Innova 300.. A laser output power of 15 mW was used to give an incident power of 1 mW. The spectra were collected on a UV-enhanced charge coupled digital ŽCCD. camera using a microRaman system ŽRenishaw. modified for UV use, which had a spectral resolution of about 4 cmy1 . The visible Raman spectra were excited by the 514 nm line of an Ar ion laser ŽCoherent Innova 90. using a double monochromator ŽSPEX. coupled to a CCD detector array. The ta-C films were grown using the filtered cathodic vacuum arc system w6x, which can produce films with a range of sp 3 content by varying the ion energy of the plasma beam. The sp 3 contents were measured by carbon K edge EELS, and were found to reach a maximum of 84% for ion energies of 90–120 eV w20x. An evaporated a-C film is also studied for comparison; this form of a-C is essentially fully sp 2 bonded.
Fig. 1. Visible Ž514 nm. Raman spectra of evaporated a-C and ta-C films, deposited at a range of bias voltages. The ion energies are given by the bias plus 20 V w20x.
614
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
Fig. 2. UV Ž244 nm. excited Raman spectra of evaporated a-C and ta-C films.
D position at 1350 cmy1 has gone, and a new band around 1100 cmy1 has appeared. The latter band declines in intensity, and perhaps moves to large wave numbers in films with smaller sp 3 content. The spectrum of evaporated a-C consists of a single band at 1600 cmy1 , together with a smaller feature around 1300 cmy1 in the region of the D mode.
consist of a broad, approximately Gaussian band around 1100 cmy1 , which moves upwards and acquires a more complex shape as the sp 3 content decreases. The spectra in fact resemble the calculated VDOS of continuous random networks as found by Beeman et al. and subsequent groups w22–25x. This similarity suggests that the UV Raman spectra can be separated into a contribution around 1650 cmy1 due to a coupling to the p states, and a contribution which resembles the VDOS due to a coupling to the s states. Merkulov et al. w19x modeled this contribution as a function of sp 3 content. In this interpretation, the ŽG band. p contribution is still resonantly enhanced. To correlate the spectra more directly to the bonding, we fitted the central region of the s peak by a Gaussian. Fig. 4 shows that the position of this band decreases with sp 3 content as measured by EELS w20x. The correlation is seen to be reasonable, but this is expected to improve with experience, and should form the basis for an analytic method. The separation of the UV Raman spectrum into s and p contributions, in which the s component resembles the VDOS, should also allow the development of a realistic theory of the Raman effect in a-C. In contrast, the popular habit of modelling the spectrum as a few Gaussian peaks w16x is then more
4. Discussion We attribute the 1100 cmy1 band in the UV spectra to sp 3 bonding for the following reasons. First, its amplitude increases with increasing sp 3 content. Secondly, this feature has already been seen in low-energy EELS and inelastic neutron scattering spectra of ta-C w14,21x. Thirdly, the peak lies at the maximum of the calculated VDOS of a sp 3 bonded network w22x. Finally, features around this position have already been attributed to ‘nanocrystalline diamond’ by Nemanich et al. w8,9x in CVD diamond films. The UV spectra can be understood in more detail by assuming that the G band has a Lorentzian shape, and subtracting it from the spectra, to give the results shown in Fig. 3. It is seen that the ta-C spectra now
Fig. 3. UV Raman spectra after the subtraction of a Lorenztian peak fitted to the upper G peak.
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
615
sp 2 groups, which have higher vibrational frequencies than graphitic groups due to their shorter bond lengths w27x.
5. Conclusion
Fig. 4. Correlation of the s peak position with the sp 3 content of ta-C films from EELS.
We suggest that UV Raman provides a more equal excitation of both graphitic and olefinic sp 2 bonding than does visible Raman, because 244 nm can excite all p states, not just the lower lying transitions of the more graphitic sp 2 groups. Thus, UV Raman also provides an opportunity to probe the configuration of sp 2 sites.
References useful for the visible Raman spectrum, and this seems to succeed from the rather singular dependence of the Raman cross-section on the vibrational eigenvectors for the zone centre G mode and the zone boundary D mode. It is perhaps surprising that separate contributions from s and p states can be seen in Raman, as the high disorder of diamond-like carbon is known to mix up the s and p states, according to EELS of the valence excitations w26x. We assume this separation occurs because the sp 2 –sp 2 modes have a higher frequency, and form localized modes on these bonds above the broad mass of sp 3 –sp 3 and sp 3 –sp 2 modes, according to participation ratio analyses w25x. It is interesting that the G band shifts from 1600 to 1664 cmy1 with increasing sp 3 content. The position 1664 cmy1 is much higher than its 1550 cmy1 value in visible Raman ŽFig. 1. and requires comment. The G and D bands are known to rise with excitation frequency in visible Raman, because of their resonant character w10–12x. However, the G position generally saturates at high excitation frequencies at around 1600 cmy1 , the band limit of graphitic carbon w11,12x, as seen for evaporated a-C in Fig. 1. A wave number of 1664 cmy1 clearly exceeds this limit and is significant. The shift may be attributed to the large distortions of sp 2 sites in ta-C, seen in the broad calculated VDOSs w25x. However, this effect should apply equally to the UV and visible spectra. We alternatively attribute the G shift in UV to a larger contribution from olefinic Žchain.
w1x J. Robertson, Prog. Solid State Chem. 21 Ž1991. 199. w2x S. Kaplan, F. Jansen, M. Machonkin, Appl. Phys. Lett. 47 Ž1985. 750. w3x M.A. Tamor, W.C. Vassell, K.R. Carduner, Appl. Phys. Lett. 58 Ž1991. 592. w4x B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 Ž1983. 105. w5x J. Fink et al., Solid State Commun. 47 Ž1983. 687. w6x P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 47 Ž1993. 4777. w7x N. Wada, P.J. Gaczi, S.A. Solin, J. Non-Cryst. Solids 35 Ž1980. 543. w8x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Scroder, J. Vac. Sci. Technol. A 6 Ž1988. 1783. w9x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Scroder, Phys. Rev. B 41 Ž1990. 3738. w10x J. Wagner, M. Ramsteiner, C. Wild, P. Koidl, Phys. Rev. B 40 Ž1989. 1817. w11x M.A. Tamor, J.A. Haire, C.H. Wu, K.C. Hass, Appl. Phys. Lett. 54 Ž1989. 123. w12x M. Yoshikawa, N. Nagai, M. Matsuki, H. Fukada, G. Katagiri, H. Ishida, A. Ishitani, I. Nagai, Phys. Rev. B 46 Ž1992. 7169. w13x F. Li, J.S. Lannin, Appl. Phys. Lett. 61 Ž1992. 2116. w14x W.S. Bacsa, J.S. Lannin, D.L. Pappas, J.J. Cuomo, Phys. Rev. B 47 Ž1993. 10931. w15x S. Prawer et al., Diamond Rel. Mater. 5 Ž1996. 433. w16x M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 Ž1994. 3823. w17x D. Beeman, R. Alben, Adv. Phys., 1981. w18x K. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Appl. Phys. Lett. 70 Ž1997. 1980. w19x V.I. Merkulov, J.S. Lannin, C.H. Munro, S.A. Asher, W.I. Milne, Phys. Rev. Lett. 78 Ž1997. 4869. w20x M. Chhowalla, J. Robertson, S.R.P. Silva, C.A. Davis, G. Amaratunga, W.I. Milne, J. Appl. Phys. 80 Ž1997. 231.
616
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
w21x G.P. Lopinski, V.I. Merkulov, J.S. Lannin, Appl. Phys. Lett. 69 Ž1996. 3348. w22x D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 Ž1984. 870. w23x C.Z. Wang, K.M. Ho, Phys. Rev. Lett. 71 Ž1993. 1184. w24x D.A. Drabold, P.A. Fedders, P. Stumm, Phys. Rev. B 49 Ž1994. 16415.
w25x T. Kohler, T. Frauenheim, G. Jungnickel, Phys. Rev. B 52 Ž1995. 11837. w26x C. Gao, Y.Y. Wang, A.L. Ritter, J.R. Dennison, Phys. Rev. Lett. 62 Ž1989. 945. w27x G. Herzberg, Spectra of Diatomic Molecules, Van Nostrand, 1950.
Direct observation of sp 3 bonding in tetrahedral amorphous carbon UV Raman spectroscopy K.W.R. Gilkes a , H.S. Sands b, D.N. Batchelder b, W.I. Milne c , J. Robertson
c,)
a
c
School of Physics, UniÕersity of East Anglia, Norwich NR4 7TJ, UK b Department of Physics, UniÕersity of Leeds, Leeds LS2 9JT, UK Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK
Abstract It is shown that Raman spectroscopy using UV excitation at 244 nm directly reveals the vibrational modes of sp 3 sites in tetrahedral amorphous carbon Žta-C.. The spectrum of ta-C consists of two bands around 1100 cmy1 and 1600 cmy1 , which appear to arise from sp 2 sites and sp 3 sites, respectively. Analysis of the spectra shows that the 1100 cmy1 mode arises from a coupling to s states and it shifts from about 1400 cmy1 to 1100 cmy1 with increasing sp 3 content. q 1998 Elsevier Science B.V. All rights reserved. Keywords: sp 3 bonding; Raman spectroscopy; Tetrahedral amorphous carbon
1. Introduction Amorphous carbon films can be produced with a range of sp 3 and sp 2 bonding depending on the preparation conditions. Films with the largest concentration of sp 3 bonding, called diamond-like carbon ŽDLC., are of great interest because of their hardness, low friction and low electron affinity w1x. There is considerable need for a rapid non-destructive method to analyse their bonding. Of methods presently used, nuclear magnetic resonance ŽNMR. can resolve sp 2 and sp 3 sites and can give a quantitative estimate of the fraction of sp 3 bonding w2,3x, but this method requires large or 13 C-enriched samples. The C–H infrared stretching modes have been used
)
Corresponding author. Tel.: q44-1223 3322689; fax: q441223 332662; e-mail: [email protected].
to study bonding w4x, but clearly their use is restricted to sites bonded to hydrogen. Electron energy loss spectroscopy ŽEELS. of the carbon K edge is presently the favoured method to measure bonding in DLC w5,6x. It gives a direct measure of the sp 2 bonding from the size of the p ) peak at 285 eV. However, the sp 3 sites are not probed directly, as they contribute with the sp 2 sites to the step at 290 eV due to s ) states. Furthermore, EELS is a destructive and rather time-consuming technique. Raman is a non-destructive technique for measuring the bonding properties of diamond-like carbon and diamond w7–16x. In principle, Raman should provide a distinction between sp 3 and sp 2 sites in DLC because their Raman frequencies in diamond and graphite of 1550 and 1332 cmy1 are widely separated. However, conventional visible Raman excited by 488 nm or 514 nm photons is limited because the sp 2 sites have a greater cross-section,
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 9 0 - 2
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
and their contribution always dominates the sp 3 sites. Consequently, the visible Raman spectrum of DLC is dominated by the G band at about 1550 cmy1 and a D feature around 1350 cmy1 , both of which are attributed to graphitic sp 2 bonding. These effects are also observed in the spectra of so-called ‘tetrahedral amorphous carbon’ Žta-C. w14x, as this still contains up to 20% sp 2 sites. Thus, the use of visible Raman to characterise DLC has been based on empirical relationships between bonding and shifts or broadening of the G band w15x. In principle, because of the relaxation of the k-selection rule, the Raman spectra of an amorphous solid is proportional to the total vibrational density of states ŽVDOS. multiplied by some cross-section w17x. However, in DLC the problem is that excitation at 514 nm Ž2.4 eV. corresponds to the p – p ) transitions at sp 2 sites and this gives a resonant enhancement of their cross-section w10–12x. This problem can be avoided by using ultraviolet ŽUV. excitation at 244 nm Ž5.1 eV., as this causes the excitation of s states on both sp 2 and sp 3 sites, and this provides a more equally weighted measure of the VDOS w18,19x.
613
3. Results Fig. 1 shows the visible Raman spectra of the evaporated a-C and ta-C films deposited at a number of bias voltages, for reference. The spectrum of evaporated a-C is dominated by the G band at 1570 cmy1 , plus a smaller band at the D position around 1350 cmy1 , as expected for graphitic bonding. The G band is attributed to the modes of similar to the zone-centre E 2g mode of graphite, while the D band is attributed to disorder-activated aromatic modes of A 1 symmetry. The spectra of ta-C are dominated by the broad G band at 1600 cmy1 , and there is essentially no shoulder at the D position, as seen by other workers w15x. The 960 cmy1 band is due to the Si substrate. Fig. 2 shows the first-order UV Raman spectra of the ta-C and a-C films. The spectra differ from that in Fig. 1. They consist of two bands centred around 1100 cmy1 and 1650 cmy1 , together with a plateau that extends down to 500 cmy1 . The small band at 1551 cmy1 is due to atmospheric oxygen. The key differences in the visible Raman spectra are that the G band has moved up from 1600 cmy1 to 1650 cmy1 , and is now narrower; any shoulder around the
2. Method The UV Raman spectra were produced by the 244 nm line of an intracavity frequency-doubled Ar ion laser ŽCoherent Innova 300.. A laser output power of 15 mW was used to give an incident power of 1 mW. The spectra were collected on a UV-enhanced charge coupled digital ŽCCD. camera using a microRaman system ŽRenishaw. modified for UV use, which had a spectral resolution of about 4 cmy1 . The visible Raman spectra were excited by the 514 nm line of an Ar ion laser ŽCoherent Innova 90. using a double monochromator ŽSPEX. coupled to a CCD detector array. The ta-C films were grown using the filtered cathodic vacuum arc system w6x, which can produce films with a range of sp 3 content by varying the ion energy of the plasma beam. The sp 3 contents were measured by carbon K edge EELS, and were found to reach a maximum of 84% for ion energies of 90–120 eV w20x. An evaporated a-C film is also studied for comparison; this form of a-C is essentially fully sp 2 bonded.
Fig. 1. Visible Ž514 nm. Raman spectra of evaporated a-C and ta-C films, deposited at a range of bias voltages. The ion energies are given by the bias plus 20 V w20x.
614
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
Fig. 2. UV Ž244 nm. excited Raman spectra of evaporated a-C and ta-C films.
D position at 1350 cmy1 has gone, and a new band around 1100 cmy1 has appeared. The latter band declines in intensity, and perhaps moves to large wave numbers in films with smaller sp 3 content. The spectrum of evaporated a-C consists of a single band at 1600 cmy1 , together with a smaller feature around 1300 cmy1 in the region of the D mode.
consist of a broad, approximately Gaussian band around 1100 cmy1 , which moves upwards and acquires a more complex shape as the sp 3 content decreases. The spectra in fact resemble the calculated VDOS of continuous random networks as found by Beeman et al. and subsequent groups w22–25x. This similarity suggests that the UV Raman spectra can be separated into a contribution around 1650 cmy1 due to a coupling to the p states, and a contribution which resembles the VDOS due to a coupling to the s states. Merkulov et al. w19x modeled this contribution as a function of sp 3 content. In this interpretation, the ŽG band. p contribution is still resonantly enhanced. To correlate the spectra more directly to the bonding, we fitted the central region of the s peak by a Gaussian. Fig. 4 shows that the position of this band decreases with sp 3 content as measured by EELS w20x. The correlation is seen to be reasonable, but this is expected to improve with experience, and should form the basis for an analytic method. The separation of the UV Raman spectrum into s and p contributions, in which the s component resembles the VDOS, should also allow the development of a realistic theory of the Raman effect in a-C. In contrast, the popular habit of modelling the spectrum as a few Gaussian peaks w16x is then more
4. Discussion We attribute the 1100 cmy1 band in the UV spectra to sp 3 bonding for the following reasons. First, its amplitude increases with increasing sp 3 content. Secondly, this feature has already been seen in low-energy EELS and inelastic neutron scattering spectra of ta-C w14,21x. Thirdly, the peak lies at the maximum of the calculated VDOS of a sp 3 bonded network w22x. Finally, features around this position have already been attributed to ‘nanocrystalline diamond’ by Nemanich et al. w8,9x in CVD diamond films. The UV spectra can be understood in more detail by assuming that the G band has a Lorentzian shape, and subtracting it from the spectra, to give the results shown in Fig. 3. It is seen that the ta-C spectra now
Fig. 3. UV Raman spectra after the subtraction of a Lorenztian peak fitted to the upper G peak.
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
615
sp 2 groups, which have higher vibrational frequencies than graphitic groups due to their shorter bond lengths w27x.
5. Conclusion
Fig. 4. Correlation of the s peak position with the sp 3 content of ta-C films from EELS.
We suggest that UV Raman provides a more equal excitation of both graphitic and olefinic sp 2 bonding than does visible Raman, because 244 nm can excite all p states, not just the lower lying transitions of the more graphitic sp 2 groups. Thus, UV Raman also provides an opportunity to probe the configuration of sp 2 sites.
References useful for the visible Raman spectrum, and this seems to succeed from the rather singular dependence of the Raman cross-section on the vibrational eigenvectors for the zone centre G mode and the zone boundary D mode. It is perhaps surprising that separate contributions from s and p states can be seen in Raman, as the high disorder of diamond-like carbon is known to mix up the s and p states, according to EELS of the valence excitations w26x. We assume this separation occurs because the sp 2 –sp 2 modes have a higher frequency, and form localized modes on these bonds above the broad mass of sp 3 –sp 3 and sp 3 –sp 2 modes, according to participation ratio analyses w25x. It is interesting that the G band shifts from 1600 to 1664 cmy1 with increasing sp 3 content. The position 1664 cmy1 is much higher than its 1550 cmy1 value in visible Raman ŽFig. 1. and requires comment. The G and D bands are known to rise with excitation frequency in visible Raman, because of their resonant character w10–12x. However, the G position generally saturates at high excitation frequencies at around 1600 cmy1 , the band limit of graphitic carbon w11,12x, as seen for evaporated a-C in Fig. 1. A wave number of 1664 cmy1 clearly exceeds this limit and is significant. The shift may be attributed to the large distortions of sp 2 sites in ta-C, seen in the broad calculated VDOSs w25x. However, this effect should apply equally to the UV and visible spectra. We alternatively attribute the G shift in UV to a larger contribution from olefinic Žchain.
w1x J. Robertson, Prog. Solid State Chem. 21 Ž1991. 199. w2x S. Kaplan, F. Jansen, M. Machonkin, Appl. Phys. Lett. 47 Ž1985. 750. w3x M.A. Tamor, W.C. Vassell, K.R. Carduner, Appl. Phys. Lett. 58 Ž1991. 592. w4x B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 Ž1983. 105. w5x J. Fink et al., Solid State Commun. 47 Ž1983. 687. w6x P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 47 Ž1993. 4777. w7x N. Wada, P.J. Gaczi, S.A. Solin, J. Non-Cryst. Solids 35 Ž1980. 543. w8x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Scroder, J. Vac. Sci. Technol. A 6 Ž1988. 1783. w9x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Scroder, Phys. Rev. B 41 Ž1990. 3738. w10x J. Wagner, M. Ramsteiner, C. Wild, P. Koidl, Phys. Rev. B 40 Ž1989. 1817. w11x M.A. Tamor, J.A. Haire, C.H. Wu, K.C. Hass, Appl. Phys. Lett. 54 Ž1989. 123. w12x M. Yoshikawa, N. Nagai, M. Matsuki, H. Fukada, G. Katagiri, H. Ishida, A. Ishitani, I. Nagai, Phys. Rev. B 46 Ž1992. 7169. w13x F. Li, J.S. Lannin, Appl. Phys. Lett. 61 Ž1992. 2116. w14x W.S. Bacsa, J.S. Lannin, D.L. Pappas, J.J. Cuomo, Phys. Rev. B 47 Ž1993. 10931. w15x S. Prawer et al., Diamond Rel. Mater. 5 Ž1996. 433. w16x M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 Ž1994. 3823. w17x D. Beeman, R. Alben, Adv. Phys., 1981. w18x K. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Appl. Phys. Lett. 70 Ž1997. 1980. w19x V.I. Merkulov, J.S. Lannin, C.H. Munro, S.A. Asher, W.I. Milne, Phys. Rev. Lett. 78 Ž1997. 4869. w20x M. Chhowalla, J. Robertson, S.R.P. Silva, C.A. Davis, G. Amaratunga, W.I. Milne, J. Appl. Phys. 80 Ž1997. 231.
616
K.W.R. Gilkes et al.r Journal of Non-Crystalline Solids 227–230 (1998) 612–616
w21x G.P. Lopinski, V.I. Merkulov, J.S. Lannin, Appl. Phys. Lett. 69 Ž1996. 3348. w22x D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 Ž1984. 870. w23x C.Z. Wang, K.M. Ho, Phys. Rev. Lett. 71 Ž1993. 1184. w24x D.A. Drabold, P.A. Fedders, P. Stumm, Phys. Rev. B 49 Ž1994. 16415.
w25x T. Kohler, T. Frauenheim, G. Jungnickel, Phys. Rev. B 52 Ž1995. 11837. w26x C. Gao, Y.Y. Wang, A.L. Ritter, J.R. Dennison, Phys. Rev. Lett. 62 Ž1989. 945. w27x G. Herzberg, Spectra of Diatomic Molecules, Van Nostrand, 1950.