- Email: [email protected]
IR study of the formation process of polymeric hydrogenated amorphous carbon film
Diamond and Related Materials 11 (2002) 1110–1114
IR study of the formation process of polymeric hydrogenated amorphous carbon film ´ I. Pocsik* ´ M. Veres, M. Koos, Research Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary
Abstract IR spectroscopic studies have been carried out on samples, which were prepared from benzene at relatively high pressure and low self-bias voltage region. The samples show well-resolved spectral features both in the sp3 - and the sp2-hybridized spectral regions. The spectral resolution and separation of bands in the sp2 region, and the good signal-to-noise ratio allows Gaussian decomposition of the distinct bands, even in the more crowded sp3 region. The C–H vibrations show two different neighborhoods, a nearly stress-free, and a highly stressed one. By increasing the deposition voltage, the amplitude of the stress-free region decreases. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Growth; IR; Spectroscopy
1. Introduction The large variety of material properties of different forms of hydrogenated amorphous carbon (a-C:H) has challenged the materials research community for some time. These materials can be prepared by various plasma-enhanced chemical vapor deposition (PE-CVD) methods over a broad range of deposition voltage, gas pressure, composition, and substrate temperature w1–3x. It is well known that, at ambient substrate temperature and few Pascals of gas pressure, we can grow polymeric films at low self-bias voltage up to 200–300 V, diamond-like hard films in the middle region and graphitic films above 600–700 V. Scattering methods, such as X-ray, neutron or electron scattering, provide insufficient information about the structure of amorphous materials. We are able to collect information through the short-range order from the distances of the first few neighbors only, but the resolution is usually not too good. Local vibration methods, such as Raman scattering and infrared (IR) spectroscopy give more useful information about local parameters, through the vibration frequencies. Concerning the structure and growth of a-C:H films, there arise the following questions: what sort of changes *Corresponding author. Tel.: q36-1-392-27-63; fax: q36-1-39222-15. ´ E-mail address: [email protected] (I. Pocsik).
take place in the plasma in the source gas molecules, and in the arrival onto the substrate surface during the deposition process? For such a study, benzene might be the perfect source gas. Being a highly symmetrical molecule, it has many characteristic vibration modes, and many steps from its first ionization to complete decomposition. It is interesting to know how far the molecule decomposes in the plasma, and how much it will be further destroyed during the impingement of the ions into the film during formation. Simplifying the question, what sort of benzene fractions can be recognized as building blocks of the carbon film? Usually, the whole deposition range with homogeneity is discussed, in the manner in which we have been speaking before, but the whole story seems to be more complicated. We will discuss the results of an IR study of a series of samples, prepared from benzene, which show a strong change in a relatively small part of the whole deposition condition region. We used a higher pressure and smaller self-bias voltage region, because this seemed to provide more gentle ionization and scattering in the plasma processes, and as a consequence, good resolution of the IR spectra. Preparing polymeric samples, this result shows some insight into the formation process of a-C:H films. Since the first IR measurements of Dischler et al. w4,5x, intensive research has begun w6–16x to clarify the quite complex IR spectra of a-C:H films. The large
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 0 1 1 - 0
M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
1111
variability of the bond features in these films is complicated by the inherent stress of many cases. Tamor et al.’s paper w9x should be mentioned separately, as they assigned spectra, similar to ours, to slightly damaged pendant (mono-substituted) benzene groups in the sample. The IR monographs w17–21x give a solid background for the assignments, using the data of the large number of molecules with various structures; it is a great help that the previous data have been collected and critically analyzed by Ristein et al. w14x and Heitz et al. w22,23x, who contributed to this infrared absorption investigation with in-situ infrared ellipsometry. In present paper, we analyze the C–H stretching vibration region, the 2750–3250 cmy1 range in the IR spectra of polymeric a-C:H samples. Because of the ‘terminal’ character of the hydrogen atom, this bond is less sensitive to stress and defects than the continuing bonds are. Further results will be published elsewhere. 2. Experimental All samples have been prepared by a radio frequency (rf) glow discharge method in our lab in a capacitively coupled deposition chamber. The rf frequency was 2.54 MHz, the benzene vapor pressure was adjusted to 60 and 140 mtorr (8 and 18 Pascal), and the films were deposited onto the polished surface of silicon single crystal wafer. The vacuum tightness of the chamber was checked before every deposition by reaching the 10y5torr pressure, which gives a low enough limit for leakages and other gases during the deposition. The oxide cover of the silicon wafer surface was eliminated by argon plasma before the deposition. One further sample was added to the series, prepared from methane of 60 mtorr pressure at y400 V self-bias. The IR spectra were collected by a Fourier transform spectrometer Bruker IFS-28 attached to a microscope. Avoiding the interference phenomena, we have scratched up the film from a small part of surface of the single crystal silicon sample holder, crunched and piled it up until the film interference disappears. This technique also allows the easy selection of the optimal sample thickness region. The microscope objective determines the pile size needed. 3. Results and discussion The local vibrations, detected by the IR spectrometer, are determined by the structure, and by the specific structural unit of this structure, the cluster. The interatomic bonding conditions are more-or-less optimized within these clusters, and are far from optimal between them. The covalent bonding optimum is influenced by the cluster size, and the molecular arrangement within. The low performance of X-ray and other scattering methods is probably caused by the adding up of the
Fig. 1. The normalized C–H stretching range of the IR spectrum of five a-C:H films, prepared from benzene by 140 mtorr pressure. The different self-bias voltage values are listed.
inter-atomic distance information of different clusters, and also of the inter-cluster boundaries, in investigating amorphous materials. These strongly oscillating structural functions average each other out very easily, and as a consequence, the required structural information is lost. On the other hand, the vibrational methods provide more useful information, because the different vibrations in different clusters do not disturb each other, as they are strongly localized, and the intensity of frequencies is positive only, so addition does not cause much information loss. The IR spectra of these samples show a rich structure of the C–H stretching vibration region, as can be seen on Fig. 1. These samples were prepared at higher than usual gas pressure w140 mtorr (18 Pa) pressure of benzenex and lower than usual self-bias voltage conditions (from y10 to y120 V). Using the 2970 cmy1 value as a rough separating line between the sp3 and sp2 hybridized regions, we can establish that both types of structural elements are present in these samples. These special preparation conditions contributed to the creation of these films, where large sp2 hybridized components survived the deposition process. The low deposition voltage, and the small energy of the impinging ions seems to be crucial in the process, which proves the strong decrease of the intensity of these components by increasing self-bias voltage. The sp3-hybridized part of the spectrum does not change too much in this deposition condition region. The self-bias voltage at 140 mtorr pressure could not be increased further in our deposition system, so the
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M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
Fig. 3. Result of the curve fitting of the top spectrum of Fig. 1 with the sum of 14 Gaussian lines. The best fitting parameters are listed in Table 1. Special care was taken to control the widths of the components.
Fig. 2. The normalized C–H stretching range of the IR spectrum of three a-C:H films. The preparation conditions are listed.
pressure had to be decreased. Fig. 2 compares the spectrum of the 140 mtorr -120 V benzene sample with the spectrum of the 60 mtorr -400 V sample, which shows an interesting similarity to the spectrum of the sample, prepared from 60 mtorr methane at -100 V. In these last two samples, a very small sp2 hybridized component can be recognized. The similarity between the spectra of these two low pressure samples is striking, and until the 140 mtorr samples contain various C–H configurations, these low pressure samples show the dominant sp3 CH2 symmetric and anti-symmetric vibrational modes, showing that most of the C–C bonds are already saturated. These spectra show good resolution, well-separated lines in the aromatic part of the spectra, and a good signal-to-noise ratio, which together provide good data for the numerical separation of the contributions of the chemically different atomic groups. The aromatic part was successfully decomposed using Gaussian lineshapes, because the bands are well separated in this region. The situation is more complicated in the aliphatic part of the spectrum, where the bands are piled up on top of each other. In such a case, free parameter fitting cannot be made, as the measurement does not contain sufficient information for that. To successive fittings, we had to decrease the number of free parameters or strongly limit their variation range. The second case was carried out, the line-width were limited into a range, which well fitted the aromatic regions. The result of this sort of Gaussian fitting of the upper most spectrum of Fig. 1 is displayed in Fig. 3, where the spectrum was
decomposed into 14 Gaussian components. The parameters of these lines, the location, half-width and intensity are listed in Table 1. Of course, there are small components on both sides of the spectra, which are needed for a good numerical result, but these components might not necessarily be assigned. Ristein et al. w14x and Heintz et al. w22x have summarized previous results for these modes, and given a comprehensive picture of the C–H stretching vibration modes. These modes represent nine different chemical groups, listed in Table 1. As can be seen, our fitting produced all nine frequencies in that frequency range, whereas the literature usually describes them in isolated molecules. We call attention to the resolution of the sp3 CH and sp3 CH2 asymmetric bands, which we could Table 1 Numerical parameters of the 14 Gaussian lines; the result of the fitting is shown in Fig. 3 Peak location (cmy1)
Peak width (FWHM) (cmy1)
Relative intensity
Assign
2809 2832 2852 2873 2904 2928 2950 2973 3001 3026 3058 3083 3099 3116
31.5 27.4 24.7 29.3 31.9 25.6 25.5 31.0 17.6 20.6 24.6 13.5 19.7 18.0
0.31 0.84 1.05 2.04 2.89 2.26 1.68 2.13 0.80 2.69 2.60 0.63 0.49 0.05
– – sp3CH2 sym sp3CH3 sym sp3CH sp3CH2 asym sp3CH3 asym sp2CH2 sym – sp2CH olef. sp2CH arom. sp2CH2 asym – –
M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
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sample, in order to provide the nicely resolved IR spectra over a 2950 cmy1 wave-number. Other parts of these units are located in a stressed neighborhood, where the aromatic character remains, but the bonding neighborhood disturbs the C–H vibration strongly, producing the broad spectral feature in the aromatic part. The much higher deposition voltage destroys the aromatic or olefin structures. 4. Conclusions
Fig. 4. The composite line-shape, which might cause the non-assignable components of the decomposed spectrum of Fig. 3.
assign to 2904 and 2928 cmy1, and both were assigned to 2920 cmy1 by Ristein et al. w14x. Our fitting provided three further bands: one on the low wave number, one on the high wave-number side, and one in the middle, at 3001 cmy1. This last one cannot be a mistake, as neither of the sidebands were at the two limits. The presence of separate components on these sites cannot be questioned, but no further chemical group was present in the sample, which might vibrate on that frequency. This is such a controversial situation, which can be resolved by some more complex line-shape, than Gaussians. The simplest case is, when we assume a double line structure, a narrow line sitting on top of a substantially broader one, as shown in Fig. 4. The narrow line represents a stress-free structure, the broader one, a stress-full one. The narrow components appear as assigned intensive bands, the broader components appear as less intensive bands at 2809, 2832, 3001, 3099, 3116 cmy1 locations. This double elementary band structure means that these characteristic chemical groups are in two different circumstances. Some part of them is in a quite relaxed neighborhood, where the vibration is nearly free, and the other part is in a strongly strained neighborhood, where the structure of the functional group and its vibrations are strongly distorted by neighbors. This twophase feature also explains this behavior, as can be seen in Fig. 1. By increasing self-bias voltage, only the intensity of the narrow components decreases; the broad background in the aromatic region does not change too much. To eliminate that broad component, we have to increase the self-bias voltage to y400 V, as shown in Fig. 2. This picture explains the formation process in this way: that at low deposition voltages, some percentage of the benzene rings are able to survive the deposition process, and some are opened up to provide olefin chains, but both are in low strain conditions in the
IR spectra were studied of various a-C:H samples, which were prepared from benzene at a relatively high pressure and low self-bias voltage conditions. Well structured IR spectra of low noise were recorded, which allowed the decomposition of the vibration frequencies of the different functional groups. The sp2-hybridized region of the spectra shows well-resolved, sharp peaks, which were also used as line-width standards for the other groups in the curve fitting procedure. The decomposition shows six peaks in the sp2-hybridized region. These sharp peaks in the sp2-hybridized region lose intensity quickly by increasing the self-bias voltage of the deposition condition. Nine different chemical groups were assigned to the C–H stretching region of a-C:H; for the assignment of the other bands, we had to suppose a combined elementary line structure, where the assigned narrow components are sitting on top of much broader components, and these broader components appear as non-assigned smaller intensity components on both sides and at 3001 cmy1. Acknowledgments This work was supported by the Hungarian Science Foundation under contract number OTKA-026073 and OTKA-025540; and by NATO under contract number NATO-SfP-976913. The authors are grateful to K. ´ for measurements. Kamaras References w1x J. Robertson, Adv. Phys. 35 (1986) 317. w2x J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (1987) 2946. w3x R.E. Clausing, Diamond and Diamond-Like Films and Coatings, Plenum, New York, 1991. w4x B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. w5x B. Dischler, A. Bubenzer, P. Koidl, Solid State Comm. 48 (1983) 105. w6x D.R. McKenzie, M.C. McPhedram, N. Savvides, L.C. Botten, Phil. Mag. B 48 (1983) 341. w7x P. Couderc, Y. Catherine, Thin Solid Films 146 (1987) 93. w8x D.C. Green, D.R. McKenzie, P.B. Lukins, in: J.J. Pouch, S.A. Alterovitz (Eds.), Properties and Characterization of Amorphous Carbon Films, Vol’s 52&53, Material Science Forum, 1989, p. 103. w9x M.A. Tamor, C.H. Wu, R.O. Carter, N.E. Lindsay, Appl. Phys. Lett 55 (1989) 1388.
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w10x N. Ohtani, M. Katsuno, T. Futagi, Y. Ohta, H. Mimura, K. Kavamura, Jpn. J. Appl. Phys. 30 (1991) L1539. w11x Y. Bounouh, M.L. Theye, A. Dehbi-Alaoui, A. Matthews, J.P. Stokuert, Phys. Rev. B 51 (1995) 9597. w12x R. Stief, J. Schafer, J. Ristein, L. Ley, W. Bayer, J. Non-Cryst. Solids 198–200 (1996) 636. w13x E.A. Konshina, V.A. Tolmatchev, A.L. Vangonen, L.A. Fatkulina, J. Opt. Technol. 64 (1997) 476. w14x J. Ristein, R.T. Stief, L. Ley, J. Appl. Phys. 84 (1998) 3836. w15x D.P. Manage, J.M. Perz, F. Gaspari, E. Sagnes, S. Zukotynski, J. Non-Cryst. Solids 270 (2000) 247. w16x Y. Hayshi, K. Hagimoto, H. Ebisu, et al., Jpn. J. Appl. Phys. 39 (2000) 4088.
w17x C.N.R. Rao, Chemical Applications of Infrared Spectroscopy, Academic, Boston, 1963. w18x N.B. Colthup, L.H. Daly, S.E. Wiberley, Infrared and Raman Spectroscopy, Academic Press, New York, 1975. w19x L.J. Bellamy, The Infra-Red Spectra of Complex Molecules, Chapman and Hall, London, 1975. w20x J.W. Robinson, Practical Handbook of Spectroscopy, CRC, Boston, 1991. w21x G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1994. w22x T. Heintz, B. Drevillon, C. Godet, J.E. Bouree, ´ Phys. Rev. B 58 (1998) 13957. w23x T. Heintz, B. Drevillon, C. Godet, J.E. Bouree, ´ Carbon 37 (1999) 771.
IR study of the formation process of polymeric hydrogenated amorphous carbon film ´ I. Pocsik* ´ M. Veres, M. Koos, Research Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary
Abstract IR spectroscopic studies have been carried out on samples, which were prepared from benzene at relatively high pressure and low self-bias voltage region. The samples show well-resolved spectral features both in the sp3 - and the sp2-hybridized spectral regions. The spectral resolution and separation of bands in the sp2 region, and the good signal-to-noise ratio allows Gaussian decomposition of the distinct bands, even in the more crowded sp3 region. The C–H vibrations show two different neighborhoods, a nearly stress-free, and a highly stressed one. By increasing the deposition voltage, the amplitude of the stress-free region decreases. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Growth; IR; Spectroscopy
1. Introduction The large variety of material properties of different forms of hydrogenated amorphous carbon (a-C:H) has challenged the materials research community for some time. These materials can be prepared by various plasma-enhanced chemical vapor deposition (PE-CVD) methods over a broad range of deposition voltage, gas pressure, composition, and substrate temperature w1–3x. It is well known that, at ambient substrate temperature and few Pascals of gas pressure, we can grow polymeric films at low self-bias voltage up to 200–300 V, diamond-like hard films in the middle region and graphitic films above 600–700 V. Scattering methods, such as X-ray, neutron or electron scattering, provide insufficient information about the structure of amorphous materials. We are able to collect information through the short-range order from the distances of the first few neighbors only, but the resolution is usually not too good. Local vibration methods, such as Raman scattering and infrared (IR) spectroscopy give more useful information about local parameters, through the vibration frequencies. Concerning the structure and growth of a-C:H films, there arise the following questions: what sort of changes *Corresponding author. Tel.: q36-1-392-27-63; fax: q36-1-39222-15. ´ E-mail address: [email protected] (I. Pocsik).
take place in the plasma in the source gas molecules, and in the arrival onto the substrate surface during the deposition process? For such a study, benzene might be the perfect source gas. Being a highly symmetrical molecule, it has many characteristic vibration modes, and many steps from its first ionization to complete decomposition. It is interesting to know how far the molecule decomposes in the plasma, and how much it will be further destroyed during the impingement of the ions into the film during formation. Simplifying the question, what sort of benzene fractions can be recognized as building blocks of the carbon film? Usually, the whole deposition range with homogeneity is discussed, in the manner in which we have been speaking before, but the whole story seems to be more complicated. We will discuss the results of an IR study of a series of samples, prepared from benzene, which show a strong change in a relatively small part of the whole deposition condition region. We used a higher pressure and smaller self-bias voltage region, because this seemed to provide more gentle ionization and scattering in the plasma processes, and as a consequence, good resolution of the IR spectra. Preparing polymeric samples, this result shows some insight into the formation process of a-C:H films. Since the first IR measurements of Dischler et al. w4,5x, intensive research has begun w6–16x to clarify the quite complex IR spectra of a-C:H films. The large
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 0 1 1 - 0
M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
1111
variability of the bond features in these films is complicated by the inherent stress of many cases. Tamor et al.’s paper w9x should be mentioned separately, as they assigned spectra, similar to ours, to slightly damaged pendant (mono-substituted) benzene groups in the sample. The IR monographs w17–21x give a solid background for the assignments, using the data of the large number of molecules with various structures; it is a great help that the previous data have been collected and critically analyzed by Ristein et al. w14x and Heitz et al. w22,23x, who contributed to this infrared absorption investigation with in-situ infrared ellipsometry. In present paper, we analyze the C–H stretching vibration region, the 2750–3250 cmy1 range in the IR spectra of polymeric a-C:H samples. Because of the ‘terminal’ character of the hydrogen atom, this bond is less sensitive to stress and defects than the continuing bonds are. Further results will be published elsewhere. 2. Experimental All samples have been prepared by a radio frequency (rf) glow discharge method in our lab in a capacitively coupled deposition chamber. The rf frequency was 2.54 MHz, the benzene vapor pressure was adjusted to 60 and 140 mtorr (8 and 18 Pascal), and the films were deposited onto the polished surface of silicon single crystal wafer. The vacuum tightness of the chamber was checked before every deposition by reaching the 10y5torr pressure, which gives a low enough limit for leakages and other gases during the deposition. The oxide cover of the silicon wafer surface was eliminated by argon plasma before the deposition. One further sample was added to the series, prepared from methane of 60 mtorr pressure at y400 V self-bias. The IR spectra were collected by a Fourier transform spectrometer Bruker IFS-28 attached to a microscope. Avoiding the interference phenomena, we have scratched up the film from a small part of surface of the single crystal silicon sample holder, crunched and piled it up until the film interference disappears. This technique also allows the easy selection of the optimal sample thickness region. The microscope objective determines the pile size needed. 3. Results and discussion The local vibrations, detected by the IR spectrometer, are determined by the structure, and by the specific structural unit of this structure, the cluster. The interatomic bonding conditions are more-or-less optimized within these clusters, and are far from optimal between them. The covalent bonding optimum is influenced by the cluster size, and the molecular arrangement within. The low performance of X-ray and other scattering methods is probably caused by the adding up of the
Fig. 1. The normalized C–H stretching range of the IR spectrum of five a-C:H films, prepared from benzene by 140 mtorr pressure. The different self-bias voltage values are listed.
inter-atomic distance information of different clusters, and also of the inter-cluster boundaries, in investigating amorphous materials. These strongly oscillating structural functions average each other out very easily, and as a consequence, the required structural information is lost. On the other hand, the vibrational methods provide more useful information, because the different vibrations in different clusters do not disturb each other, as they are strongly localized, and the intensity of frequencies is positive only, so addition does not cause much information loss. The IR spectra of these samples show a rich structure of the C–H stretching vibration region, as can be seen on Fig. 1. These samples were prepared at higher than usual gas pressure w140 mtorr (18 Pa) pressure of benzenex and lower than usual self-bias voltage conditions (from y10 to y120 V). Using the 2970 cmy1 value as a rough separating line between the sp3 and sp2 hybridized regions, we can establish that both types of structural elements are present in these samples. These special preparation conditions contributed to the creation of these films, where large sp2 hybridized components survived the deposition process. The low deposition voltage, and the small energy of the impinging ions seems to be crucial in the process, which proves the strong decrease of the intensity of these components by increasing self-bias voltage. The sp3-hybridized part of the spectrum does not change too much in this deposition condition region. The self-bias voltage at 140 mtorr pressure could not be increased further in our deposition system, so the
1112
M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
Fig. 3. Result of the curve fitting of the top spectrum of Fig. 1 with the sum of 14 Gaussian lines. The best fitting parameters are listed in Table 1. Special care was taken to control the widths of the components.
Fig. 2. The normalized C–H stretching range of the IR spectrum of three a-C:H films. The preparation conditions are listed.
pressure had to be decreased. Fig. 2 compares the spectrum of the 140 mtorr -120 V benzene sample with the spectrum of the 60 mtorr -400 V sample, which shows an interesting similarity to the spectrum of the sample, prepared from 60 mtorr methane at -100 V. In these last two samples, a very small sp2 hybridized component can be recognized. The similarity between the spectra of these two low pressure samples is striking, and until the 140 mtorr samples contain various C–H configurations, these low pressure samples show the dominant sp3 CH2 symmetric and anti-symmetric vibrational modes, showing that most of the C–C bonds are already saturated. These spectra show good resolution, well-separated lines in the aromatic part of the spectra, and a good signal-to-noise ratio, which together provide good data for the numerical separation of the contributions of the chemically different atomic groups. The aromatic part was successfully decomposed using Gaussian lineshapes, because the bands are well separated in this region. The situation is more complicated in the aliphatic part of the spectrum, where the bands are piled up on top of each other. In such a case, free parameter fitting cannot be made, as the measurement does not contain sufficient information for that. To successive fittings, we had to decrease the number of free parameters or strongly limit their variation range. The second case was carried out, the line-width were limited into a range, which well fitted the aromatic regions. The result of this sort of Gaussian fitting of the upper most spectrum of Fig. 1 is displayed in Fig. 3, where the spectrum was
decomposed into 14 Gaussian components. The parameters of these lines, the location, half-width and intensity are listed in Table 1. Of course, there are small components on both sides of the spectra, which are needed for a good numerical result, but these components might not necessarily be assigned. Ristein et al. w14x and Heintz et al. w22x have summarized previous results for these modes, and given a comprehensive picture of the C–H stretching vibration modes. These modes represent nine different chemical groups, listed in Table 1. As can be seen, our fitting produced all nine frequencies in that frequency range, whereas the literature usually describes them in isolated molecules. We call attention to the resolution of the sp3 CH and sp3 CH2 asymmetric bands, which we could Table 1 Numerical parameters of the 14 Gaussian lines; the result of the fitting is shown in Fig. 3 Peak location (cmy1)
Peak width (FWHM) (cmy1)
Relative intensity
Assign
2809 2832 2852 2873 2904 2928 2950 2973 3001 3026 3058 3083 3099 3116
31.5 27.4 24.7 29.3 31.9 25.6 25.5 31.0 17.6 20.6 24.6 13.5 19.7 18.0
0.31 0.84 1.05 2.04 2.89 2.26 1.68 2.13 0.80 2.69 2.60 0.63 0.49 0.05
– – sp3CH2 sym sp3CH3 sym sp3CH sp3CH2 asym sp3CH3 asym sp2CH2 sym – sp2CH olef. sp2CH arom. sp2CH2 asym – –
M. Veres et al. / Diamond and Related Materials 11 (2002) 1110–1114
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sample, in order to provide the nicely resolved IR spectra over a 2950 cmy1 wave-number. Other parts of these units are located in a stressed neighborhood, where the aromatic character remains, but the bonding neighborhood disturbs the C–H vibration strongly, producing the broad spectral feature in the aromatic part. The much higher deposition voltage destroys the aromatic or olefin structures. 4. Conclusions
Fig. 4. The composite line-shape, which might cause the non-assignable components of the decomposed spectrum of Fig. 3.
assign to 2904 and 2928 cmy1, and both were assigned to 2920 cmy1 by Ristein et al. w14x. Our fitting provided three further bands: one on the low wave number, one on the high wave-number side, and one in the middle, at 3001 cmy1. This last one cannot be a mistake, as neither of the sidebands were at the two limits. The presence of separate components on these sites cannot be questioned, but no further chemical group was present in the sample, which might vibrate on that frequency. This is such a controversial situation, which can be resolved by some more complex line-shape, than Gaussians. The simplest case is, when we assume a double line structure, a narrow line sitting on top of a substantially broader one, as shown in Fig. 4. The narrow line represents a stress-free structure, the broader one, a stress-full one. The narrow components appear as assigned intensive bands, the broader components appear as less intensive bands at 2809, 2832, 3001, 3099, 3116 cmy1 locations. This double elementary band structure means that these characteristic chemical groups are in two different circumstances. Some part of them is in a quite relaxed neighborhood, where the vibration is nearly free, and the other part is in a strongly strained neighborhood, where the structure of the functional group and its vibrations are strongly distorted by neighbors. This twophase feature also explains this behavior, as can be seen in Fig. 1. By increasing self-bias voltage, only the intensity of the narrow components decreases; the broad background in the aromatic region does not change too much. To eliminate that broad component, we have to increase the self-bias voltage to y400 V, as shown in Fig. 2. This picture explains the formation process in this way: that at low deposition voltages, some percentage of the benzene rings are able to survive the deposition process, and some are opened up to provide olefin chains, but both are in low strain conditions in the
IR spectra were studied of various a-C:H samples, which were prepared from benzene at a relatively high pressure and low self-bias voltage conditions. Well structured IR spectra of low noise were recorded, which allowed the decomposition of the vibration frequencies of the different functional groups. The sp2-hybridized region of the spectra shows well-resolved, sharp peaks, which were also used as line-width standards for the other groups in the curve fitting procedure. The decomposition shows six peaks in the sp2-hybridized region. These sharp peaks in the sp2-hybridized region lose intensity quickly by increasing the self-bias voltage of the deposition condition. Nine different chemical groups were assigned to the C–H stretching region of a-C:H; for the assignment of the other bands, we had to suppose a combined elementary line structure, where the assigned narrow components are sitting on top of much broader components, and these broader components appear as non-assigned smaller intensity components on both sides and at 3001 cmy1. Acknowledgments This work was supported by the Hungarian Science Foundation under contract number OTKA-026073 and OTKA-025540; and by NATO under contract number NATO-SfP-976913. The authors are grateful to K. ´ for measurements. Kamaras References w1x J. Robertson, Adv. Phys. 35 (1986) 317. w2x J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (1987) 2946. w3x R.E. Clausing, Diamond and Diamond-Like Films and Coatings, Plenum, New York, 1991. w4x B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. w5x B. Dischler, A. Bubenzer, P. Koidl, Solid State Comm. 48 (1983) 105. w6x D.R. McKenzie, M.C. McPhedram, N. Savvides, L.C. Botten, Phil. Mag. B 48 (1983) 341. w7x P. Couderc, Y. Catherine, Thin Solid Films 146 (1987) 93. w8x D.C. Green, D.R. McKenzie, P.B. Lukins, in: J.J. Pouch, S.A. Alterovitz (Eds.), Properties and Characterization of Amorphous Carbon Films, Vol’s 52&53, Material Science Forum, 1989, p. 103. w9x M.A. Tamor, C.H. Wu, R.O. Carter, N.E. Lindsay, Appl. Phys. Lett 55 (1989) 1388.
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