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Combined near edge X-ray absorption fine structure and X-ray photoemission spectroscopies for the study of amorphous carbon thin films
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 545–550
Combined near edge X-ray absorption fine structure and X-ray photoemission spectroscopies for the study of amorphous carbon thin films ´ a,b , *, S. Anders a , X. Zhou a , E.J. Moler a , S.A. Kellar a , Z. Hussain a J. Dıaz a
Advanced Light Source, Lawrence Berkeley National Laboratory, I Cyclotron Road, Berkeley, CA 94720, USA b ´ Universidad de Oviedo, Departamento de Fısica , Avda. Calvo Sotelo s /n, Oviedo-33007, Spain
Abstract Near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies were used to study the chemical bonding in hard amorphous carbon (a-C) thin films. The analysis of their XPS spectra obtained at different photon energy excitations showed that the fraction of sp 3 hybridized atoms increased from the surface to the bulk. About 90% of the carbon atoms became sp 2 hybridized over the depth probed by XPS after annealing the film at 8508C. The intensity of the p* resonance in the NEXAFS spectrum of the hard a-C film annealed at 8508C was significantly smaller than the same resonance in a softer film annealed at 3508C. However, the sp 2 concentration of the former film, measured by XPS, was much higher. This demonstrates that the intensity of the p* resonance in the NEXAFS spectra of a-C films is not strictly proportional to the concentration of sp 2 hybridized atoms, but it can be strongly influenced by other effects, such as the localized character of the particular p* bonding present in the analyzed a-C film. A sharp resonance was observed at the same photon energy as the exciton in diamond, at 289.5 eV, in the NEXAFS spectra of the a-C films. This resonance might indicate that a proportion of carbon atoms in the analyzed films occupied similar sites as the carbon atoms in crystalline diamond. 1999 Elsevier Science B.V. All rights reserved. Keywords: Amorphous carbon; NEXAFS; XPS; Thin films; Amorphous surfaces
1. Introduction Amorphous carbon (a-C) thin films with mechanical properties approaching those of diamond and with wide enough band gaps to be used in electronic devices are currently produced with deposition techniques that do not incorporate hydrogen in the films
*Corresponding author. Fax: 134-8510-2952. ´ E-mail address: [email protected] (J. Dıaz)
[1–3]. Most of these techniques, like laser ablation or cathodic arc deposition, accelerate carbon ions at relatively high kinetic energies before being incorporated in the film. The hardest, densest films are obtained with carbon kinetic energies of about 100 eV [2,4]. These films are characterized by a high internal, compressive stress of about 10 GPa [2,4]. Such a high internal stress might make the first 20 nm thick top layers relax, decreasing their mass density with respect to the bulk [5]. Carbon, in a-C films, can be three- or four-fold coordinated, forming p and s bonds [6]. s bonding is localized at interatomic distances whereas p bonds
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00348-X
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´ et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 545 – 550 J. Dıaz
can be almost as delocalized as metallic bonds. The proportion of p and s bonds in solid carbon, their localization character, and their spatial orientation critically change the mechanical and electronic properties of the solid. Graphite is soft and semi-metallic, whereas diamond is the hardest solid and the best known insulator. Solid amorphous carbon should be a combination of both types of carbon crystalline solid, with the important differences that bonding orientation is random, and sp 2 and sp 3 hybridized atoms are mixed together. This complicates enormously the determination of the atomic structure of a-C. Several experimental techniques have been used to learn about the structure of different kinds of a-C films. Extended X-ray absorption fine structure spectroscopy (EXAFS), performed on sputter-deposited a-C thin films, showed that the analyzed films were a mixture of two phases, a random matrix of carbon atoms, either sp 2 and sp 3 hybridized, and a network of graphite-like bonded atoms [6]. Neutron and electron diffraction of harder a-C proved that the average interatomic distance between carbon atoms in these films is closer to that in diamond [7,8]. Electron spectroscopies, like XPS and NEXAFS, are currently used to determine the proportion of atoms that are sp 2 and sp 3 hybridized. These spectroscopies investigate the chemical environment of the carbon atoms in a-C films. The carbon 1s XPS spectra in a-C can be decomposed in two components [9]. Each component is assumed to be related to the signal coming from atoms that are either sp 2 hybridized or sp 3 hybridized, whose binding energies differ by about 0.9 eV, as in diamond and graphite. Their relative intensities give the proportion of sp 2 and sp 3 hybridized carbon atoms in the analyzed a-C directly [9]. NEXAFS spectroscopy is also used to estimate the concentration of sp 2 hybridized carbon atoms, since the p* resonance related to the unoccupied p bonds is well separated in energy from the rest of the resonances in the carbon K edge spectra [2]. However, in contrast to XPS, the intensity of the resonances in the NEXAFS spectra can be strongly influenced by core hole screening effects and dipole selection rules [10]. The aim of this work is to observe how important these effects are in a-C films, by comparing the proportion of sp 3 hybridized atoms determined by XPS and NEXAFS in two different a-C films.
2. Experimental Hard a-C films were prepared by cathodic arc deposition. The silicon substrates were biased to 2100 V during deposition. It is known from previous experiments that these films have an internal compressive stress of about 10 GPa, a hardness of 60 GPa, and are semiconductors. Softer films, with an internal stress of 4 GPa, were also prepared by cathodic arc deposition, biasing the substrate to 22000 V during deposition. More details of the deposition method are described elsewhere [11]. Synchrotron radiation experiments were performed at beam line 9.3.2 at the Advanced Light Source [12]. XPS spectra were obtained at three different photon energies: 304, 330 and 500 eV. Spectra at 330 eV are supposed to be most sensitive to the surface, whereas those taken at 304 and 500 eV are more sensitive to the bulk. XPS spectra were calibrated in energy with the carbon 1s spectra of a clean graphite crystal set at 284.4 eV [13]. NEXAFS spectra at the carbon K edge were calibrated in energy to the s* graphite exciton at 291.65 eV [14]. They were normalized to the intensity of the beam measured on a clean silicon surface. All NEXAFS spectra were taken in total electron yield mode by measuring the current from the sample to ground. The incidence angle of the beam was set at 54.78, the ‘magic angle’, to avoid preferential bond orientation effects [10]. Photon energy resolution was 0.2 eV at the carbon K edge, and 0.5 eV at 500 eV. All of the analyzed samples were exposed to air before being introduced into the chamber. Oxygen contamination was monitored by XPS, and it was less than 6% before annealing. The surface was cleaned by annealing the samples at 3508C for 30 min for the hard a-C film, and 15 min for the soft a-C film. Oxygen contamination decreased to less than 2% for both samples, as checked by XPS. The samples were annealed at the desired temperatures for 15 min, and cooled down to room temperature by liquid nitrogen before measuring their XPS and NEXAFS spectra.
3. Results and discussion Fig. 1 shows the proportion of sp 3 hybridized atoms at different annealing temperatures obtained
´ et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 545 – 550 J. Dıaz
Fig. 1. Plot of the concentration of sp 3 hybridized carbon atoms in the hard a-C film vs. annealing temperature on the left abscise axis. Full circles correspond to an excitation photon energy of 330 eV, full squares to 304 eV, and full triangles to 500 eV. On the right abscise axis is plotted the p* resonance intensity from the NEXAFS spectra of the hard a-C film vs. annealing temperature.
from the analysis of the carbon 1s XPS spectra taken at three different excitation photon energies. The carbon 1s spectra were fitted to two Gaussians convoluted with Doniac-Sunjic functions [9]. The background was simulated with the integration in energy of the spectra itself [16]. The additional secondary electron background present in the XPS spectra taken at 304 eV was recorded at a photon energy of 290 eV [15]. The only unchanged parameter in the fits was the Lorentzian width, fixed to 214 meV [13]. The Gaussian width and asymmetry parameter were set identical for both components. The FWHM of the Gaussians was about 1 eV for all photon energies and annealing temperatures. This width was caused mostly by bonding disorder of the carbon atoms since the instrument energy resolution was lower. The asymmetry parameter in the DoniacSunjic function was less than 0.1, the value measured in graphite, and was almost constant throughout the experiment. The proportion of sp 3 hybridized atoms was obtained from the area of the component with the highest binding energy, divided by the total area of the carbon 1s peak. The sp 3 proportion decreased with annealing temperature in a similar way as that previously reported [9]. Total graphitization of the surface at the
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annealing temperature of 9008C was confirmed since the photoemission spectra of the valence band, at a photon energy of 122 eV, corresponded well with that measured at the same photon energy by Bianconi et al. [17] in polycrystalline graphite. The sp 3 proportion differed depending on the photon excitation energy. The highest sp 3 proportion was measured at 500 eV. It was lower at 304 eV. The lowest proportion was found at 330 eV. We attribute such a difference in the sp 3 proportions to a gradient in the concentration of sp 3 hybridized atoms that increases from the surface into the bulk. According to the universal electron escape curve [10,16], the electron escape length is minimum for a photon energy of 330 eV, i.e., for the electron kinetic energy of 45 eV. The present observations suggest that the escape length of electrons with a kinetic energy of about 200 eV is higher than the escape length of electrons with a kinetic energy of 20 eV, in agreement with the electron escape curve. The higher graphitization of the surface compared to the bulk can be explained by the lower coordination number and internal stress of the atoms at the surface. If stress is the driving force for atoms to be four-coordinated [2,4], the present observations are consistent with a higher relaxation of the strain of the film at the surface compared to the bulk [4]. Examination of the annealed films at 9008C showed that neither their Raman nor their mechanical properties were changed [18], and it demonstrated that the changes measured by XPS and NEXAFS in the film were concentrated, at least in the first 5 nm from the top layer. Fig. 2 shows the NEXAFS spectra of the a-C samples at different annealing temperatures. They are compared with the NEXAFS spectra of diamond, graphite and soft amorphous carbon. The onset for the diamond absorption resonances is at a photon energy of 289.5 eV. The onset for graphite and a-C absorption resonances is at about 284 eV. The resonances between 284 and 289.5 eV are related to p antiboding states. Atoms in diamond are exclusively s-bonded, and this is the reason why no resonances are present in the energy interval between 284 and 289.5 eV. The most significant changes in the hard a-C NEXAFS spectra with annealing temperature are observed at the p* resonance at 284.6 eV, and at the
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Fig. 2. NEXAFS spectra of the hard a-C film at different annealing temperatures. The top NEXAFS spectra were taken on crystalline diamond and graphite. The two spectra at the beginning and at the end of the series were taken on a softer a-C thin film.
structure at about 305 eV. As the annealing temperature is increased, the p* resonance gets wider, and a broad structure at 305 eV grows in intensity. These changes can be understood as a progressive polymerization of the hard a-C films with increasing temperature. The changes in the NEXAFS spectra are depicted in more detail in Fig. 3(a). We associate the origin of the structure at 305 eV as a result of the p* resonance splitting caused by s*–s* bond interaction in the chains and / or rings formed during annealing of the film [10]. Such a broad resonance has been observed previously in sputter-deposited and hydrogenated a-C films after annealing [6,19], and also in non-annealed laserdeposited a-C films with low sp 3 concentration [20]. It is interesting to note that the intensity of the 305 eV resonance is lower in the spectrum of the softer a-C film [Figs. 2 and 3(a)]. That means that poly-
Fig. 3. (a) NEXAFS spectra of the hard a-C sample annealed at 350, 600 and 8508C; (b) Comparison between the NEXAFS spectra of the hard a-C sample annealed at 8508C and the soft a-C sample annealed at 3508C; (c) Comparison of the NEXAFS spectra of the non-annealed hard a-C diamond; (d) NEXAFS spectra of the non-annealed soft a-C and diamond
merization is lower in the 3508C annealed soft a-C film than in the 9008C annealed hard a-C film. The increase in width of the p* resonance also indicates a progressive polymerization of the film with temperature. If the p* resonance gets wider, this means that new p* states are being created at different energies than in the non-annealed film. The higher energy of these new states suggests that they should be associated to p bonds in the polymerized carbon formed during annealing, since they have a more delocalized character than the p bonds in a disordered matrix [10]. This can be explained in the following way. Core hole effects can strongly affect the position in energy of the excitations [10]. For instance, these effects are responsible for a down-
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ward shift in energy of the p* resonance in graphite [21,22]. In our samples, the p* resonance is even more shifted to lower binding energy, 0.7 eV with respect to graphite, whereas the position in energy of the carbon 1s photoemission peak is the same in both materials [9]. This confirms that the higher the delocalization of the p bonding, the lower is the downward shift of the p* resonance. The p* resonance also increased with annealing temperature (Fig. 1). The changes in intensity of the p* resonance at each annealing temperature were measured as the percentage of increase with respect to its intensity in the non-annealed sample. Spectra were normalized to the maximum intensity of the spectra, at the s* resonance at 297 eV. Such a normalization of the spectra yielded a higher rate of change in the p* resonance than if the spectra were normalized to the tail of the spectra, since graphitization eliminates s bonds and creates new p bonds. The intensity of the p* resonance was taken as the area under the spectra between the energies of 280 and 287 eV. At 7008C, the change in intensity was 22% and became 55% at 8008C. These values were small by comparison with the soft a-C sample. The change of the p* resonance in this film when annealed at 3508C for 15 min was 40%. Moreover, the p* resonance is more intense in this film than in the annealed hard a-C film [Fig. 3(b)]. The concentration of sp 3 hybridized atoms in the hard a-C annealed at 9008C was about 10% at the surface detected by XPS. The sp 3 concentration in a softer a-C film annealed at 3508C was 30%, as calculated from the analysis of the XPS carbon 1s spectra. Given the higher proportion of sp 2 hybridized atoms in the annealed hard a-C film, the density of electronic states related to p bonds should be higher in the annealed hard a-C film than in the softer film. This should be reflected in their NEXAFS spectra. If the intensity of the NEXAFS resonances depends only on the density of unoccupied states, the p* resonance should be more intense in the spectrum of the hard a-C annealed at 9008C than in the spectrum of the softer a-C. Fig. 3(b) shows that what is observed is the contrary to what was expected. This contradictory result can be explained by considering the effects of the core hole localization. The intensity of the p* resonance is largely de-
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termined by the density of states of the p* orbital on the excited atom [10]. The higher the localization of the p bond, the stronger the p* resonance is. NEXAFS data show that annealing causes the polymerization of the a-C films. XPS demonstrates that the polymerization is done by the rehybridization of sp 3 orbitals to sp 2 orbitals. Polymerization is lower in the soft a-C film, which suggests that p bonding is more localized in the soft a-C film than in the hard a-C film. For this reason, the p* resonance is more intense in the soft a-C film than in the hard a-C film. Finally, the NEXAFS spectra of both hard and soft a-C films have a sharp resonance at 289.5 eV. This can be observed in Figs. 3(c) and 3(d) where the NEXAFS spectra of the hard and soft a-C films obtained before annealing are shown in more detail. This resonance is also present at the onset of the absorption spectra of diamond, and it is excitonic. An explanation for the origin of this excitonic resonance in diamond has been proposed recently [14]. According to it, the position in energy and sharp width of the diamond exciton requires the carbon atom to occupy a four-fold symmetric position, like the atoms in diamond. Otherwise, the exciton resonance would broaden. As a consequence, there should be carbon atoms in the films whose bond distance and orientation are similar to those in diamond. Although an excitation at 289.5 eV has only been observed to date in crystalline or polycrystalline diamond, this resonance is in a region of the spectra where other excitations due to bonds between carbon and oxygen are present. The surfaces of the analyzed samples were contaminated with oxygen. Confirmation of the diamond exciton in the NEXAFS spectrum of a-C films should be done on cleaner amorphous films.
4. Conclusions We have studied the chemical bond in hard, semiconductor amorphous carbon by NEXAFS and XPS spectroscopies. Graphitization of these films by annealing starts at the surface. The concentration of sp 3 hybridized atoms is observed to increase from the surface to the bulk. The comparison between the NEXAFS spectra of annealed soft and hard a-C films
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demonstrated that the intensity of the p* resonance is affected by the delocalization of the p bonds. A sharp resonance at the diamond excitonic energy was observed in a-C thin films. If this excitation could be identified with the diamond exciton, it would be a proof that there are carbon atoms bonded in the same way as in crystalline diamond.
Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Sciences, of the U.S. Department of Energy under contract No. DEACO3-765F00098.
References [1] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Z.H. Wang, E. Kravtchinskaia, D. Segal, P.B. Lukins, P.D. Swift, P.J. Martin, G. Amaratunga, P.H. Gaskell, A. Saeed, Diamond Relat. Mater. 1 (1991) 51–59. [2] 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) 4777–4782. [3] D.L. Pappas, K.L. Saenger, J. Bruley, W. Krakow, T. Gu, R.W. Collins, J. Appl. Phys. 71 (1992) 5675–5684. [4] J.W. Ager III, S. Anders, A. Anders, I.G. Brown, Appl. Phys. Lett. 66 (1995) 3444–3446. [5] G.M. Pharr, D.L. Callahan, S.D. McAdams, T.Y. Tsui, S. Anders, A. Anders, J.W. Ager III, I.G. Brown, C.S. Bhatia, S.R.P. Silva, J. Robertson, Appl. Phys. Lett. 6 (1996) 779– 781.
¨ C.J. Robinson, W. Jark, Phys. Rev. B 38 [6] G. Comelli, J. Stohr, (1988) 7511–7519. [7] P. Gaskell, A. Saeed, P. Chieux, D.R. McKenzie, Phys. Rev. Lett. 67 (1991) 1286. [8] K.W. Gilkes, P.H. Gaskell, J. Robertson, Phys. Rev. B 51 (1995) 12303. ´ [9] J. Dıaz, G. Paolicelli, S. Ferrer, F. Comin, Phys. Rev. B 54 (1996) 8064–8069. ¨ [10] J. Stohr, NEXAFS spectroscopy, Springer, New York, 1992. [11] S. Anders, A. Anders, I.G. Brown, B. Wei, K. Komvopoulos, J.W. Ager III, K.M. Yu, Surf. Coat. Technol. 68 / 69 (1994) 388. [12] Z. Hussain, W.R.A. Huff, S.A. Kellar, E.J. Moler, P.A. Heimann, W. McKinney, H.A. Padmore, C.S. Fadley, D.A. Shirley, J. Electron Spectrosc. Relat. Phenomena 80 (1996) 401–404. [13] C.T. Chen, F. Sette, Phys. Scr. T 31 (1990) 119. [14] Y. Ma, P. Skytt, N. Wassdahl, P. Glans, D.C. Mancini, J. Guo, J. Nordgren, Phys. Rev. Lett. 71 (1993) 3725. [15] J.F. Morar, F.J. Himpsel, G. Hollinger, J.L. Jordan, G. Hughes, F.R. McFeely, Phys. Rev. B 33 (1986) 1340–1345. ¨ [16] S. Huffner, Photoemission spectroscopy: principles and applications, Springer Series in Solid State Sciences, Vol. 82, Springer-Verlag, 1996. ¨ R.Z. Bachrach, Phys. Rev. B [17] A. Bianconi, S.B.M. Hagstrom, 16 (1977) 5543–5548. ´ J.W. Ager III, R.Y. Lo, D.B. Bogy, Appl. [18] S. Anders, J. Dıaz, Phys. Lett. 71 (1997) 3367–3369. [19] D. Wesner, S. Krummacher, R. Carr, T.K. Sham, M. Strongin, W. Eberhardt, S.L. Weng, G. Williams, M. Howells, F.W. Smith, Phys. Rev. B 28 (1983) 2152–2156. ´ ´ M.F. Lopez, ´ [20] A. Gutierrez, J. Dıaz, Appl. Phys. A 61 (1995) 111–114. [21] E.J. Mele, J.J. Ritsko, Phys. Rev. Lett. 43 (1979) 68. ¨ [22] P.A. Bruhwiler, A.J. Maxwell, C. Puglia, A. Nilsson, S. Andersson, N. Martensson, Phys. Rev. Lett. 74 (1995) 614.
Combined near edge X-ray absorption fine structure and X-ray photoemission spectroscopies for the study of amorphous carbon thin films ´ a,b , *, S. Anders a , X. Zhou a , E.J. Moler a , S.A. Kellar a , Z. Hussain a J. Dıaz a
Advanced Light Source, Lawrence Berkeley National Laboratory, I Cyclotron Road, Berkeley, CA 94720, USA b ´ Universidad de Oviedo, Departamento de Fısica , Avda. Calvo Sotelo s /n, Oviedo-33007, Spain
Abstract Near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies were used to study the chemical bonding in hard amorphous carbon (a-C) thin films. The analysis of their XPS spectra obtained at different photon energy excitations showed that the fraction of sp 3 hybridized atoms increased from the surface to the bulk. About 90% of the carbon atoms became sp 2 hybridized over the depth probed by XPS after annealing the film at 8508C. The intensity of the p* resonance in the NEXAFS spectrum of the hard a-C film annealed at 8508C was significantly smaller than the same resonance in a softer film annealed at 3508C. However, the sp 2 concentration of the former film, measured by XPS, was much higher. This demonstrates that the intensity of the p* resonance in the NEXAFS spectra of a-C films is not strictly proportional to the concentration of sp 2 hybridized atoms, but it can be strongly influenced by other effects, such as the localized character of the particular p* bonding present in the analyzed a-C film. A sharp resonance was observed at the same photon energy as the exciton in diamond, at 289.5 eV, in the NEXAFS spectra of the a-C films. This resonance might indicate that a proportion of carbon atoms in the analyzed films occupied similar sites as the carbon atoms in crystalline diamond. 1999 Elsevier Science B.V. All rights reserved. Keywords: Amorphous carbon; NEXAFS; XPS; Thin films; Amorphous surfaces
1. Introduction Amorphous carbon (a-C) thin films with mechanical properties approaching those of diamond and with wide enough band gaps to be used in electronic devices are currently produced with deposition techniques that do not incorporate hydrogen in the films
*Corresponding author. Fax: 134-8510-2952. ´ E-mail address: [email protected] (J. Dıaz)
[1–3]. Most of these techniques, like laser ablation or cathodic arc deposition, accelerate carbon ions at relatively high kinetic energies before being incorporated in the film. The hardest, densest films are obtained with carbon kinetic energies of about 100 eV [2,4]. These films are characterized by a high internal, compressive stress of about 10 GPa [2,4]. Such a high internal stress might make the first 20 nm thick top layers relax, decreasing their mass density with respect to the bulk [5]. Carbon, in a-C films, can be three- or four-fold coordinated, forming p and s bonds [6]. s bonding is localized at interatomic distances whereas p bonds
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00348-X
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can be almost as delocalized as metallic bonds. The proportion of p and s bonds in solid carbon, their localization character, and their spatial orientation critically change the mechanical and electronic properties of the solid. Graphite is soft and semi-metallic, whereas diamond is the hardest solid and the best known insulator. Solid amorphous carbon should be a combination of both types of carbon crystalline solid, with the important differences that bonding orientation is random, and sp 2 and sp 3 hybridized atoms are mixed together. This complicates enormously the determination of the atomic structure of a-C. Several experimental techniques have been used to learn about the structure of different kinds of a-C films. Extended X-ray absorption fine structure spectroscopy (EXAFS), performed on sputter-deposited a-C thin films, showed that the analyzed films were a mixture of two phases, a random matrix of carbon atoms, either sp 2 and sp 3 hybridized, and a network of graphite-like bonded atoms [6]. Neutron and electron diffraction of harder a-C proved that the average interatomic distance between carbon atoms in these films is closer to that in diamond [7,8]. Electron spectroscopies, like XPS and NEXAFS, are currently used to determine the proportion of atoms that are sp 2 and sp 3 hybridized. These spectroscopies investigate the chemical environment of the carbon atoms in a-C films. The carbon 1s XPS spectra in a-C can be decomposed in two components [9]. Each component is assumed to be related to the signal coming from atoms that are either sp 2 hybridized or sp 3 hybridized, whose binding energies differ by about 0.9 eV, as in diamond and graphite. Their relative intensities give the proportion of sp 2 and sp 3 hybridized carbon atoms in the analyzed a-C directly [9]. NEXAFS spectroscopy is also used to estimate the concentration of sp 2 hybridized carbon atoms, since the p* resonance related to the unoccupied p bonds is well separated in energy from the rest of the resonances in the carbon K edge spectra [2]. However, in contrast to XPS, the intensity of the resonances in the NEXAFS spectra can be strongly influenced by core hole screening effects and dipole selection rules [10]. The aim of this work is to observe how important these effects are in a-C films, by comparing the proportion of sp 3 hybridized atoms determined by XPS and NEXAFS in two different a-C films.
2. Experimental Hard a-C films were prepared by cathodic arc deposition. The silicon substrates were biased to 2100 V during deposition. It is known from previous experiments that these films have an internal compressive stress of about 10 GPa, a hardness of 60 GPa, and are semiconductors. Softer films, with an internal stress of 4 GPa, were also prepared by cathodic arc deposition, biasing the substrate to 22000 V during deposition. More details of the deposition method are described elsewhere [11]. Synchrotron radiation experiments were performed at beam line 9.3.2 at the Advanced Light Source [12]. XPS spectra were obtained at three different photon energies: 304, 330 and 500 eV. Spectra at 330 eV are supposed to be most sensitive to the surface, whereas those taken at 304 and 500 eV are more sensitive to the bulk. XPS spectra were calibrated in energy with the carbon 1s spectra of a clean graphite crystal set at 284.4 eV [13]. NEXAFS spectra at the carbon K edge were calibrated in energy to the s* graphite exciton at 291.65 eV [14]. They were normalized to the intensity of the beam measured on a clean silicon surface. All NEXAFS spectra were taken in total electron yield mode by measuring the current from the sample to ground. The incidence angle of the beam was set at 54.78, the ‘magic angle’, to avoid preferential bond orientation effects [10]. Photon energy resolution was 0.2 eV at the carbon K edge, and 0.5 eV at 500 eV. All of the analyzed samples were exposed to air before being introduced into the chamber. Oxygen contamination was monitored by XPS, and it was less than 6% before annealing. The surface was cleaned by annealing the samples at 3508C for 30 min for the hard a-C film, and 15 min for the soft a-C film. Oxygen contamination decreased to less than 2% for both samples, as checked by XPS. The samples were annealed at the desired temperatures for 15 min, and cooled down to room temperature by liquid nitrogen before measuring their XPS and NEXAFS spectra.
3. Results and discussion Fig. 1 shows the proportion of sp 3 hybridized atoms at different annealing temperatures obtained
´ et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 545 – 550 J. Dıaz
Fig. 1. Plot of the concentration of sp 3 hybridized carbon atoms in the hard a-C film vs. annealing temperature on the left abscise axis. Full circles correspond to an excitation photon energy of 330 eV, full squares to 304 eV, and full triangles to 500 eV. On the right abscise axis is plotted the p* resonance intensity from the NEXAFS spectra of the hard a-C film vs. annealing temperature.
from the analysis of the carbon 1s XPS spectra taken at three different excitation photon energies. The carbon 1s spectra were fitted to two Gaussians convoluted with Doniac-Sunjic functions [9]. The background was simulated with the integration in energy of the spectra itself [16]. The additional secondary electron background present in the XPS spectra taken at 304 eV was recorded at a photon energy of 290 eV [15]. The only unchanged parameter in the fits was the Lorentzian width, fixed to 214 meV [13]. The Gaussian width and asymmetry parameter were set identical for both components. The FWHM of the Gaussians was about 1 eV for all photon energies and annealing temperatures. This width was caused mostly by bonding disorder of the carbon atoms since the instrument energy resolution was lower. The asymmetry parameter in the DoniacSunjic function was less than 0.1, the value measured in graphite, and was almost constant throughout the experiment. The proportion of sp 3 hybridized atoms was obtained from the area of the component with the highest binding energy, divided by the total area of the carbon 1s peak. The sp 3 proportion decreased with annealing temperature in a similar way as that previously reported [9]. Total graphitization of the surface at the
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annealing temperature of 9008C was confirmed since the photoemission spectra of the valence band, at a photon energy of 122 eV, corresponded well with that measured at the same photon energy by Bianconi et al. [17] in polycrystalline graphite. The sp 3 proportion differed depending on the photon excitation energy. The highest sp 3 proportion was measured at 500 eV. It was lower at 304 eV. The lowest proportion was found at 330 eV. We attribute such a difference in the sp 3 proportions to a gradient in the concentration of sp 3 hybridized atoms that increases from the surface into the bulk. According to the universal electron escape curve [10,16], the electron escape length is minimum for a photon energy of 330 eV, i.e., for the electron kinetic energy of 45 eV. The present observations suggest that the escape length of electrons with a kinetic energy of about 200 eV is higher than the escape length of electrons with a kinetic energy of 20 eV, in agreement with the electron escape curve. The higher graphitization of the surface compared to the bulk can be explained by the lower coordination number and internal stress of the atoms at the surface. If stress is the driving force for atoms to be four-coordinated [2,4], the present observations are consistent with a higher relaxation of the strain of the film at the surface compared to the bulk [4]. Examination of the annealed films at 9008C showed that neither their Raman nor their mechanical properties were changed [18], and it demonstrated that the changes measured by XPS and NEXAFS in the film were concentrated, at least in the first 5 nm from the top layer. Fig. 2 shows the NEXAFS spectra of the a-C samples at different annealing temperatures. They are compared with the NEXAFS spectra of diamond, graphite and soft amorphous carbon. The onset for the diamond absorption resonances is at a photon energy of 289.5 eV. The onset for graphite and a-C absorption resonances is at about 284 eV. The resonances between 284 and 289.5 eV are related to p antiboding states. Atoms in diamond are exclusively s-bonded, and this is the reason why no resonances are present in the energy interval between 284 and 289.5 eV. The most significant changes in the hard a-C NEXAFS spectra with annealing temperature are observed at the p* resonance at 284.6 eV, and at the
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Fig. 2. NEXAFS spectra of the hard a-C film at different annealing temperatures. The top NEXAFS spectra were taken on crystalline diamond and graphite. The two spectra at the beginning and at the end of the series were taken on a softer a-C thin film.
structure at about 305 eV. As the annealing temperature is increased, the p* resonance gets wider, and a broad structure at 305 eV grows in intensity. These changes can be understood as a progressive polymerization of the hard a-C films with increasing temperature. The changes in the NEXAFS spectra are depicted in more detail in Fig. 3(a). We associate the origin of the structure at 305 eV as a result of the p* resonance splitting caused by s*–s* bond interaction in the chains and / or rings formed during annealing of the film [10]. Such a broad resonance has been observed previously in sputter-deposited and hydrogenated a-C films after annealing [6,19], and also in non-annealed laserdeposited a-C films with low sp 3 concentration [20]. It is interesting to note that the intensity of the 305 eV resonance is lower in the spectrum of the softer a-C film [Figs. 2 and 3(a)]. That means that poly-
Fig. 3. (a) NEXAFS spectra of the hard a-C sample annealed at 350, 600 and 8508C; (b) Comparison between the NEXAFS spectra of the hard a-C sample annealed at 8508C and the soft a-C sample annealed at 3508C; (c) Comparison of the NEXAFS spectra of the non-annealed hard a-C diamond; (d) NEXAFS spectra of the non-annealed soft a-C and diamond
merization is lower in the 3508C annealed soft a-C film than in the 9008C annealed hard a-C film. The increase in width of the p* resonance also indicates a progressive polymerization of the film with temperature. If the p* resonance gets wider, this means that new p* states are being created at different energies than in the non-annealed film. The higher energy of these new states suggests that they should be associated to p bonds in the polymerized carbon formed during annealing, since they have a more delocalized character than the p bonds in a disordered matrix [10]. This can be explained in the following way. Core hole effects can strongly affect the position in energy of the excitations [10]. For instance, these effects are responsible for a down-
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ward shift in energy of the p* resonance in graphite [21,22]. In our samples, the p* resonance is even more shifted to lower binding energy, 0.7 eV with respect to graphite, whereas the position in energy of the carbon 1s photoemission peak is the same in both materials [9]. This confirms that the higher the delocalization of the p bonding, the lower is the downward shift of the p* resonance. The p* resonance also increased with annealing temperature (Fig. 1). The changes in intensity of the p* resonance at each annealing temperature were measured as the percentage of increase with respect to its intensity in the non-annealed sample. Spectra were normalized to the maximum intensity of the spectra, at the s* resonance at 297 eV. Such a normalization of the spectra yielded a higher rate of change in the p* resonance than if the spectra were normalized to the tail of the spectra, since graphitization eliminates s bonds and creates new p bonds. The intensity of the p* resonance was taken as the area under the spectra between the energies of 280 and 287 eV. At 7008C, the change in intensity was 22% and became 55% at 8008C. These values were small by comparison with the soft a-C sample. The change of the p* resonance in this film when annealed at 3508C for 15 min was 40%. Moreover, the p* resonance is more intense in this film than in the annealed hard a-C film [Fig. 3(b)]. The concentration of sp 3 hybridized atoms in the hard a-C annealed at 9008C was about 10% at the surface detected by XPS. The sp 3 concentration in a softer a-C film annealed at 3508C was 30%, as calculated from the analysis of the XPS carbon 1s spectra. Given the higher proportion of sp 2 hybridized atoms in the annealed hard a-C film, the density of electronic states related to p bonds should be higher in the annealed hard a-C film than in the softer film. This should be reflected in their NEXAFS spectra. If the intensity of the NEXAFS resonances depends only on the density of unoccupied states, the p* resonance should be more intense in the spectrum of the hard a-C annealed at 9008C than in the spectrum of the softer a-C. Fig. 3(b) shows that what is observed is the contrary to what was expected. This contradictory result can be explained by considering the effects of the core hole localization. The intensity of the p* resonance is largely de-
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termined by the density of states of the p* orbital on the excited atom [10]. The higher the localization of the p bond, the stronger the p* resonance is. NEXAFS data show that annealing causes the polymerization of the a-C films. XPS demonstrates that the polymerization is done by the rehybridization of sp 3 orbitals to sp 2 orbitals. Polymerization is lower in the soft a-C film, which suggests that p bonding is more localized in the soft a-C film than in the hard a-C film. For this reason, the p* resonance is more intense in the soft a-C film than in the hard a-C film. Finally, the NEXAFS spectra of both hard and soft a-C films have a sharp resonance at 289.5 eV. This can be observed in Figs. 3(c) and 3(d) where the NEXAFS spectra of the hard and soft a-C films obtained before annealing are shown in more detail. This resonance is also present at the onset of the absorption spectra of diamond, and it is excitonic. An explanation for the origin of this excitonic resonance in diamond has been proposed recently [14]. According to it, the position in energy and sharp width of the diamond exciton requires the carbon atom to occupy a four-fold symmetric position, like the atoms in diamond. Otherwise, the exciton resonance would broaden. As a consequence, there should be carbon atoms in the films whose bond distance and orientation are similar to those in diamond. Although an excitation at 289.5 eV has only been observed to date in crystalline or polycrystalline diamond, this resonance is in a region of the spectra where other excitations due to bonds between carbon and oxygen are present. The surfaces of the analyzed samples were contaminated with oxygen. Confirmation of the diamond exciton in the NEXAFS spectrum of a-C films should be done on cleaner amorphous films.
4. Conclusions We have studied the chemical bond in hard, semiconductor amorphous carbon by NEXAFS and XPS spectroscopies. Graphitization of these films by annealing starts at the surface. The concentration of sp 3 hybridized atoms is observed to increase from the surface to the bulk. The comparison between the NEXAFS spectra of annealed soft and hard a-C films
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demonstrated that the intensity of the p* resonance is affected by the delocalization of the p bonds. A sharp resonance at the diamond excitonic energy was observed in a-C thin films. If this excitation could be identified with the diamond exciton, it would be a proof that there are carbon atoms bonded in the same way as in crystalline diamond.
Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Sciences, of the U.S. Department of Energy under contract No. DEACO3-765F00098.
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