- Email: [email protected]
NEXAFS study on carbon-based material formed by focused-ion-beam chemical-vapor-deposition
ARTICLE IN PRESS
Radiation Physics and Chemistry 75 (2006) 1850–1854 www.elsevier.com/locate/radphyschem
NEXAFS study on carbon-based material formed by focused-ion-beam chemical-vapor-deposition Kazuhiro Kandaa,b,, Jun-ya Igakia,b, Yuri Katoa,b, Reo Kometania,b, Akihiko Saikuboa,b, Shinji Matsuia,b a
Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan b CREST JST, Kawaguchi, Saitama 332-0012, Japan Accepted 27 July 2005
Abstract The coordination of carbon atoms in the carbon-based material formed by chemical-vapor-deposition of phenanthrene assisted by Ga-focused ion beam was investigated by the measurement of near-edge X-ray absorption fine structure spectra of the carbon K-edge over the excitation energy range 275–320 eV. Novel peak observed at 289.0 eV was assigned to the 1s-s* transition of carbon neighboring to the residue gallium. The material formed by this method was found to be Ga-doped diamond-like carbon, that consists of a high sp3 hybridized carbon. r 2006 Elsevier Ltd. All rights reserved. PACS: 61.10.Ht; 81.05.Uw; 81.15.Gh Keywords: Near-edge X-ray absorption fine structure; Raman spectroscopy; Focused-ion-beam chemical-vapor-deposition; Diamondlike carbon
1. Introduction Focused-ion-beam chemical-vapor-deposition (FIBCVD) has great advantages in the fabrication of a three-dimensional nanostructure (Matsui et al., 2000). In our laboratory, several three-dimensional nanostructures have been demonstrated (e.g. Kometani et al., 2003). In the FIB-CVD method, nanostructure has been fabricated by 30 keV Ga+-focused, ion-beam-assisted chemical-vapor-deposition using phenanthrene as a carbon source. The nanostructural material formed by Corresponding author. Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan. Tel.: +81 791 58 1476; fax: +81 791 58 0242. E-mail address: [email protected] (K. Kanda).
FIB-CVD method was composed of the carbon-based material containing Ga atoms, which is residue of focused ion beam (Morita, et al., 2003) and its Young’s modulus was reported to be E100 GPa (Fujita et al., 2001). The local structure was considered as diamondlike carbon (DLC) from Raman spectroscopy (Matsui et al., 2000), but has not been sufficiently understood. The DLC films are characterized by a different sp2/sp3 ratio, which influences the mechanical and the electronic properties. The coordination of the carbon atoms has been principally determined by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) (Lenardi et al., 1999; Kanda et al., 2002, 2003). The resonance from 1s orbital to p orbitals in sp2 carbon and the resonance from 1s orbital to s orbitals in sp2 and sp3 carbons present a distinguishable difference in energy, and a simple identification of each contribution can be made.
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.07.039
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
In the present study, we prepared the thin film by FIB-CVD method on the Si substrate, and observed carbon K-edge NEXAFS spectrum over the excitation energy range 275–320 eV. Assignment of observed spectrum was performed and the local structure of the DLC film formed by FIB-CVD method was discussed with the reference of commercial DLC formed with ionplating method and isotopic graphite. In addition, Raman spectra of these samples were observed for the confirmation of fundamental structure.
2. Experimental part The details of a method for the fabrication of threedimensional nanostructures by using FIB-CVD method have been described in the previous paper (Matsui et al., 2000). In brief, FIB-CVD DLC thin films were produced using FIB system (Seiko Instrument Inc.; SMI2050) with a Ga+ ion beam operating at 30 keV. The Ga+ ion beam was focused to a spot size of 7 nm at a 2 nA beam current, and it is incident perpendicular to the Si substrate. Phenanthrene (C4H10), as the source material, was evaporated from a heated container and was injected into the vacuum chamber by a gas nozzle at an angle of 451 with respect to the Si surface. A pair of gas nozzles was equipped in order to maintain the gas pressure. Typical pressure of phenanthrene was 5 10 5 Pa during the film growth. The area of produced FIB-CVD DLC was 2 5 mm2. For the reference, isotropic graphite (IG11; Toyo Tanso) and commercial DLC thin film (Nanotec, Inc.) were used. The commercial DLC was coated on Si wafer surface by the ion-plating method, whose Young’s modulus was reported to be 203 GPa. Hereafter, this DLC is called IP DLC. The NEXAFS measurement was performed at the BL8B1 stage of UVSOR in the Institute for Molecular Science. The experimental apparatus and procedures employed in the present study were identical to those in our previous study (Kanda et al., 2003). The synchrotron radiation provided by the 0.75 GeV electron storage ring was dispersed by a constant-deviation constantlength spherical grating monochromator and was irradiated perpendicularly to the sample film surface. The NEXAFS C K-edge spectra were measured in the energy range 275–320 eV with 0.5 eV FWHM resolution. The reading of monochromator was calibrated against the pre-edge resonance at 285.3 eV in the spectrum of graphite (Batson, 1993). The detection of electrons coming from the sample was performed in the total electron yield (TEY) mode. The intensity of the incident photon beams, I0, was measured by monitoring the photocurrent from a gold film. The absorption signal was given by the ratio between the out-coming electron intensity from the sample, Is, and the intensity from the gold film, I0.
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Raman spectroscopy was performed ex situ, at room temperature with back scattering geometry (NRS-2100; JASCO). The spectra were excited with 514.5 nm line of an Ar ion laser, analyzed with triple monochromator, and detected with a liquid nitrogen cooled CCD array detector. The estimated spectral resolution was about 1 cm 1.
3. Results and discussion Raman spectroscopy has given complementary information on the film structure to NEXAFS. Raman spectroscopy is sensitive to the structural changes in DLC films. Fig. 1 shows the Raman spectra of FIB-CVD DLC, IP DLC and graphite. Raman spectra of amorphous carbon materials are characterized by G peak at about 1550–1600 cm 1 and D peak at E1350 cm 1. G peak is assigned to the crystalline graphite arising from the zone-center E2g phonon and D peak is assigned to an A1g zone-edge mode activated because of the finite size of graphitic domains. The observed spectrum of graphite agreed with that in the literature (Tsai and Bogy, 1987). The spectral shape of FIB-CVD DLC resembles with that of IP DLC. To compare the FIB-CVD DLC with IP DLC on a more quantitative basis, we have fitted the spectra with a Gaussian for the D and G peaks. The fitting shows that G peak position is 1549 and 1553 cm 1 and shoulder D peak position is 1396 and 1407 cm 1 in FIB-CVD DLC and IP DLC, respectively. The FWHM is in the range of 160–180 cm 1 for the G peak and in the range of 270–400 cm 1 for the D peak. Thus, the spectral characteristics of FIB-CVD DLC are in good agreements with those of IP DLC. This result supports that fundamental structure of carbon-based material formed with FIB-CVD method is amorphous carbon. Ratio of intensity of D peak to that of G peak, I(D)/ I(G), was known to index of sp2 contents in the DLC films (McCulloch et al., 1997). When I(D)/I(G) is small, it indicates small amount of sp2 content in the DLC film. I(D)/I(G) peak ratio was obtained to be 0.77 and 1.63 for FIB-CVD DLC and IP DLC, respectively. Therefore, the FIB-CVD DLC was expected to be lower sp2 contents than IP DLC. Fig. 2 shows the NEXAFS carbon K-edge spectra of the FIB-CVD DLC, IP DLC and graphite. The shape of the graphite spectrum is in good agreement with that in the literature (Batson, 1993). The NEXAFS spectra of various carbon materials have been investigated previously (Batson, 1993; Jaouen et al., 1995; Lenardi et al., 1999). A pre-edge resonance at 285.3 eV is due to transitions from C 1s level to unoccupied p* orbitals principally originating from sp2 (CQC) sites. The sharp peaks due to transitions from C 1s level to unoccupied s* states at 291.65 and 292.5 eV are observed in the NEXAFS spectrum of the graphite film, while the broad
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
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FIB-CVD DLC
FIB-CVD DLC
G
D
Normalized absorption /arb. units
Raman intensity / arb.units
IP DLC
G D
IP DLC
Graphite Graphite
1200
1400 1600 Raman shift / cm-1
1800 280
290
300
310
Photon energy /eV Fig. 1. Raman spectra of FIB-CVD DLC, IP DLC and graphite.
band is observed in this energy region of the DLC spectrum. This broad band is assignable to the result of overlapping C 1s-s* transitions at sp2 and sp3 sites. The spectral features of FIB-CVD DLC almost agreed with that of IP DLC. However, novel peak is observed at 289.0 eV in the spectrum of FIB-CVD DLC. This peak has not been observed in the NEXAFS spectra of any carbon materials. FIB-CVD DLC was known to contain the Ga atom, which derived from
Fig. 2. NEXAFS C K-edge spectra of FIB-CVD DLC, IP DLC and graphite.
residue of the focused ion beam (Morita et al., 2003). Therefore, this novel peak is considered to originate from the C atom neighboring the Ga atom. In order to confirm this assignment, we prepared some test samples, which Ga was deposited onto the surface of IP DLC and graphite. Figs. 3 and 4 are depicted the NEXAFS spectra of Ga-deposited IP DLC and graphite, respectively. Top spectrum of each figure is original IP DLC and graphite. Second, third and bottom spectra indicate
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
Graphite
Normalized absorption / arb. units
Normalized absorption / arb. units
IP DLC
1 sec
5 sec
1 sec
5 sec
10 sec
10 sec
280
290
300 Photon energy /eV
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310
280
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300 Photon energy /eV
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Fig. 3. NEXAFS C K-edge spectra of Ga-deposited IP DLC.
Fig. 4. NEXAFS C K-edge spectra of Ga-deposited graphite.
the Ga-deposited samples, whose deposition time is 1, 5 and 10 s, respectively. The absorbance decreases with increasing deposition time, because signal intensity from carbon decreases by the coverage of Ga on the sample surface. The peak at 289.0 eV was observed in the spectra of Ga-deposited IP DLC and graphite. The character of unoccupied orbital is estimated to be s from its peak position (Lenardi et al., 1999). As a result, the peak at 289.0 eV is assignable to C 1s-s* transition originating from C–Ga site. Small peak at 286.4 eV was observed in the spectra of FIB-CVD DLC. The peak at 286.6 eV was assigned to the 1s-p* transition induced by the presence of oxygen by Lenardi et al. (1999). The presence of oxygen on the surface of FIB-CVD DLC was confirmed from XPS study. This peak is ascribable to C 1s-p* transition at CQO site. A pre-edge resonance at 285.3 eV is due to C 1s-p* transition at sp2 site, as described above. This peak is not almost present in the diamond spectrum (Morar et al., 1985), because the diamond consists of only carbon atoms in the sp3 (C–C) sites. Therefore, the peak intensity of this resonance is considered as a good index of sp2 content. The procedure for determination of sp2 content from the NEXAFS measurements was established in the previous studies (Kanda et al., 2002, 2003). The amount of sp2 bonded carbon atoms can be extracted by normalizing the area of the resonance
corresponding to 1s-p* transitions at 285.3 eV with the area of a large section of the spectrum. The estimated relative sp2 content is 0.039 and 0.052 for FIB-CVD DLC and IP DLC, respectively. In other words, the sp3 content of the FIB-CVD DLC is higher than that of the IP DLC. This agrees with the result of Raman spectroscopy described above. As a result, the FIB assisted phenanthrene deposition could be anticipated for the synthesis of the DLC structures with a higher sp3 hybridized carbon than commercial IP DLC.
Acknowledgments This work is supported by a CREST JST Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Joint Studies Program of the Institute for Molecular Science. The authors thank UVSOR staff, especially Dr. Nakamura for the NEXAFS measurement at BL8B1.
References Batson, P.E., 1993. NEXAFS spectra of graphite. Phys. Rev. B 48, 2608–2610.
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K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
Fujita, J., Ishida, M., Sakamoto, T., Ochiai, Y., Kaito, T., Matsui, S., 2001. Young’ modulus of DLC pillars grown by FIB-CVD. J. Vac. Sci. Technol. B 19, 2834–2837. Jaouen, M., Tourillon, G., Delafond, J., Junqua, N., Hug, G., 1995. NEXAFS spectra of DLC films. Diamond Relat. Mater. 4, 200–206. Kanda, K., Shimizugawa, Y., Haruyama, Y., Yamada, I., Matsui, S., Kitagawa, T., Tsubakino, H., Gejo, T., 2002. NEXAFS spectra of DLC films formed by GCIB method. Jpn. J. Appl. Phys. 41, 4295–4298. Kanda, K., Kitagawa, T., Shimizugawa, Y., Haruyama, Y., Matsui, S., Terasawa, M., Tsubakino, H., Yamada, I., Gejo, T., Kamada, M., 2003. NEXAFS study on DLC films. Nucl. Instr. and Meth. (B) 206, 880–883. Kometani, R., Morita, T., Watanabe, K., Kanda, K., Haruyama, Y., Kaito, T., Fujita, J., Ishida, M., Ochiai, Y., Matsui, S., 2003. Nozzle-nanostructure fabrication by FIB-CVD. Jpn. J. Appl. Phys. 42, 4107–4110.
Lenardi, C., Piseri, P., Briois, V., Bottani, C.E., Li Bassi, A., Milani, P., 1999. NEXAFS spectra of DLC films. J. Appl. Phys. 85, 7159–7167. Matsui, S., Kaito, T., Fujita, J., Komuro, M., Kanda, K., Haruyama, Y., 2000. Three-dimensional nanostructure fabrication by FIB-CVD. J. Vac. Sci. Technol. B 18, 3181–3184. McCulloch, D.G., McKenzie, D.R., Prawer, S., Merchant, A.R., Gerstner, E.G., Kalish, R., 1997. Raman spectra of DLC. Diamond Relat. Mater. 6, 1622–1628. Morar, J.F., Himpsel, F.J., Hollinger, G., Hughes, G., Lordan, J.L., 1985. NEXAFS spectra of diamond. Phys. Rev. Lett. 54, 1960–1963. Morita, T., Kometani, R., Watanabe, K., Kanda, K., Haruyama, Y., Hoshino, T., Kondo, K., Kaito, T., Ichihashi, T., Fujita, J., Ishida, M., Ochiai, Y., Tajima, T., Matsui, S., 2003. TEM observation of pillars formed by FIB-CVD. J. Vac. Sci. Technol. B 21, 2737–2741. Tsai, H-C., Bogy, D.B., 1987. Characterization of diamond like carbon. J. Vac. Sci. Technol. A 5, 3287–3312.
Radiation Physics and Chemistry 75 (2006) 1850–1854 www.elsevier.com/locate/radphyschem
NEXAFS study on carbon-based material formed by focused-ion-beam chemical-vapor-deposition Kazuhiro Kandaa,b,, Jun-ya Igakia,b, Yuri Katoa,b, Reo Kometania,b, Akihiko Saikuboa,b, Shinji Matsuia,b a
Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan b CREST JST, Kawaguchi, Saitama 332-0012, Japan Accepted 27 July 2005
Abstract The coordination of carbon atoms in the carbon-based material formed by chemical-vapor-deposition of phenanthrene assisted by Ga-focused ion beam was investigated by the measurement of near-edge X-ray absorption fine structure spectra of the carbon K-edge over the excitation energy range 275–320 eV. Novel peak observed at 289.0 eV was assigned to the 1s-s* transition of carbon neighboring to the residue gallium. The material formed by this method was found to be Ga-doped diamond-like carbon, that consists of a high sp3 hybridized carbon. r 2006 Elsevier Ltd. All rights reserved. PACS: 61.10.Ht; 81.05.Uw; 81.15.Gh Keywords: Near-edge X-ray absorption fine structure; Raman spectroscopy; Focused-ion-beam chemical-vapor-deposition; Diamondlike carbon
1. Introduction Focused-ion-beam chemical-vapor-deposition (FIBCVD) has great advantages in the fabrication of a three-dimensional nanostructure (Matsui et al., 2000). In our laboratory, several three-dimensional nanostructures have been demonstrated (e.g. Kometani et al., 2003). In the FIB-CVD method, nanostructure has been fabricated by 30 keV Ga+-focused, ion-beam-assisted chemical-vapor-deposition using phenanthrene as a carbon source. The nanostructural material formed by Corresponding author. Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan. Tel.: +81 791 58 1476; fax: +81 791 58 0242. E-mail address: [email protected] (K. Kanda).
FIB-CVD method was composed of the carbon-based material containing Ga atoms, which is residue of focused ion beam (Morita, et al., 2003) and its Young’s modulus was reported to be E100 GPa (Fujita et al., 2001). The local structure was considered as diamondlike carbon (DLC) from Raman spectroscopy (Matsui et al., 2000), but has not been sufficiently understood. The DLC films are characterized by a different sp2/sp3 ratio, which influences the mechanical and the electronic properties. The coordination of the carbon atoms has been principally determined by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) (Lenardi et al., 1999; Kanda et al., 2002, 2003). The resonance from 1s orbital to p orbitals in sp2 carbon and the resonance from 1s orbital to s orbitals in sp2 and sp3 carbons present a distinguishable difference in energy, and a simple identification of each contribution can be made.
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.07.039
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
In the present study, we prepared the thin film by FIB-CVD method on the Si substrate, and observed carbon K-edge NEXAFS spectrum over the excitation energy range 275–320 eV. Assignment of observed spectrum was performed and the local structure of the DLC film formed by FIB-CVD method was discussed with the reference of commercial DLC formed with ionplating method and isotopic graphite. In addition, Raman spectra of these samples were observed for the confirmation of fundamental structure.
2. Experimental part The details of a method for the fabrication of threedimensional nanostructures by using FIB-CVD method have been described in the previous paper (Matsui et al., 2000). In brief, FIB-CVD DLC thin films were produced using FIB system (Seiko Instrument Inc.; SMI2050) with a Ga+ ion beam operating at 30 keV. The Ga+ ion beam was focused to a spot size of 7 nm at a 2 nA beam current, and it is incident perpendicular to the Si substrate. Phenanthrene (C4H10), as the source material, was evaporated from a heated container and was injected into the vacuum chamber by a gas nozzle at an angle of 451 with respect to the Si surface. A pair of gas nozzles was equipped in order to maintain the gas pressure. Typical pressure of phenanthrene was 5 10 5 Pa during the film growth. The area of produced FIB-CVD DLC was 2 5 mm2. For the reference, isotropic graphite (IG11; Toyo Tanso) and commercial DLC thin film (Nanotec, Inc.) were used. The commercial DLC was coated on Si wafer surface by the ion-plating method, whose Young’s modulus was reported to be 203 GPa. Hereafter, this DLC is called IP DLC. The NEXAFS measurement was performed at the BL8B1 stage of UVSOR in the Institute for Molecular Science. The experimental apparatus and procedures employed in the present study were identical to those in our previous study (Kanda et al., 2003). The synchrotron radiation provided by the 0.75 GeV electron storage ring was dispersed by a constant-deviation constantlength spherical grating monochromator and was irradiated perpendicularly to the sample film surface. The NEXAFS C K-edge spectra were measured in the energy range 275–320 eV with 0.5 eV FWHM resolution. The reading of monochromator was calibrated against the pre-edge resonance at 285.3 eV in the spectrum of graphite (Batson, 1993). The detection of electrons coming from the sample was performed in the total electron yield (TEY) mode. The intensity of the incident photon beams, I0, was measured by monitoring the photocurrent from a gold film. The absorption signal was given by the ratio between the out-coming electron intensity from the sample, Is, and the intensity from the gold film, I0.
1851
Raman spectroscopy was performed ex situ, at room temperature with back scattering geometry (NRS-2100; JASCO). The spectra were excited with 514.5 nm line of an Ar ion laser, analyzed with triple monochromator, and detected with a liquid nitrogen cooled CCD array detector. The estimated spectral resolution was about 1 cm 1.
3. Results and discussion Raman spectroscopy has given complementary information on the film structure to NEXAFS. Raman spectroscopy is sensitive to the structural changes in DLC films. Fig. 1 shows the Raman spectra of FIB-CVD DLC, IP DLC and graphite. Raman spectra of amorphous carbon materials are characterized by G peak at about 1550–1600 cm 1 and D peak at E1350 cm 1. G peak is assigned to the crystalline graphite arising from the zone-center E2g phonon and D peak is assigned to an A1g zone-edge mode activated because of the finite size of graphitic domains. The observed spectrum of graphite agreed with that in the literature (Tsai and Bogy, 1987). The spectral shape of FIB-CVD DLC resembles with that of IP DLC. To compare the FIB-CVD DLC with IP DLC on a more quantitative basis, we have fitted the spectra with a Gaussian for the D and G peaks. The fitting shows that G peak position is 1549 and 1553 cm 1 and shoulder D peak position is 1396 and 1407 cm 1 in FIB-CVD DLC and IP DLC, respectively. The FWHM is in the range of 160–180 cm 1 for the G peak and in the range of 270–400 cm 1 for the D peak. Thus, the spectral characteristics of FIB-CVD DLC are in good agreements with those of IP DLC. This result supports that fundamental structure of carbon-based material formed with FIB-CVD method is amorphous carbon. Ratio of intensity of D peak to that of G peak, I(D)/ I(G), was known to index of sp2 contents in the DLC films (McCulloch et al., 1997). When I(D)/I(G) is small, it indicates small amount of sp2 content in the DLC film. I(D)/I(G) peak ratio was obtained to be 0.77 and 1.63 for FIB-CVD DLC and IP DLC, respectively. Therefore, the FIB-CVD DLC was expected to be lower sp2 contents than IP DLC. Fig. 2 shows the NEXAFS carbon K-edge spectra of the FIB-CVD DLC, IP DLC and graphite. The shape of the graphite spectrum is in good agreement with that in the literature (Batson, 1993). The NEXAFS spectra of various carbon materials have been investigated previously (Batson, 1993; Jaouen et al., 1995; Lenardi et al., 1999). A pre-edge resonance at 285.3 eV is due to transitions from C 1s level to unoccupied p* orbitals principally originating from sp2 (CQC) sites. The sharp peaks due to transitions from C 1s level to unoccupied s* states at 291.65 and 292.5 eV are observed in the NEXAFS spectrum of the graphite film, while the broad
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
1852
FIB-CVD DLC
FIB-CVD DLC
G
D
Normalized absorption /arb. units
Raman intensity / arb.units
IP DLC
G D
IP DLC
Graphite Graphite
1200
1400 1600 Raman shift / cm-1
1800 280
290
300
310
Photon energy /eV Fig. 1. Raman spectra of FIB-CVD DLC, IP DLC and graphite.
band is observed in this energy region of the DLC spectrum. This broad band is assignable to the result of overlapping C 1s-s* transitions at sp2 and sp3 sites. The spectral features of FIB-CVD DLC almost agreed with that of IP DLC. However, novel peak is observed at 289.0 eV in the spectrum of FIB-CVD DLC. This peak has not been observed in the NEXAFS spectra of any carbon materials. FIB-CVD DLC was known to contain the Ga atom, which derived from
Fig. 2. NEXAFS C K-edge spectra of FIB-CVD DLC, IP DLC and graphite.
residue of the focused ion beam (Morita et al., 2003). Therefore, this novel peak is considered to originate from the C atom neighboring the Ga atom. In order to confirm this assignment, we prepared some test samples, which Ga was deposited onto the surface of IP DLC and graphite. Figs. 3 and 4 are depicted the NEXAFS spectra of Ga-deposited IP DLC and graphite, respectively. Top spectrum of each figure is original IP DLC and graphite. Second, third and bottom spectra indicate
ARTICLE IN PRESS K. Kanda et al. / Radiation Physics and Chemistry 75 (2006) 1850–1854
Graphite
Normalized absorption / arb. units
Normalized absorption / arb. units
IP DLC
1 sec
5 sec
1 sec
5 sec
10 sec
10 sec
280
290
300 Photon energy /eV
1853
310
280
290
300 Photon energy /eV
310
Fig. 3. NEXAFS C K-edge spectra of Ga-deposited IP DLC.
Fig. 4. NEXAFS C K-edge spectra of Ga-deposited graphite.
the Ga-deposited samples, whose deposition time is 1, 5 and 10 s, respectively. The absorbance decreases with increasing deposition time, because signal intensity from carbon decreases by the coverage of Ga on the sample surface. The peak at 289.0 eV was observed in the spectra of Ga-deposited IP DLC and graphite. The character of unoccupied orbital is estimated to be s from its peak position (Lenardi et al., 1999). As a result, the peak at 289.0 eV is assignable to C 1s-s* transition originating from C–Ga site. Small peak at 286.4 eV was observed in the spectra of FIB-CVD DLC. The peak at 286.6 eV was assigned to the 1s-p* transition induced by the presence of oxygen by Lenardi et al. (1999). The presence of oxygen on the surface of FIB-CVD DLC was confirmed from XPS study. This peak is ascribable to C 1s-p* transition at CQO site. A pre-edge resonance at 285.3 eV is due to C 1s-p* transition at sp2 site, as described above. This peak is not almost present in the diamond spectrum (Morar et al., 1985), because the diamond consists of only carbon atoms in the sp3 (C–C) sites. Therefore, the peak intensity of this resonance is considered as a good index of sp2 content. The procedure for determination of sp2 content from the NEXAFS measurements was established in the previous studies (Kanda et al., 2002, 2003). The amount of sp2 bonded carbon atoms can be extracted by normalizing the area of the resonance
corresponding to 1s-p* transitions at 285.3 eV with the area of a large section of the spectrum. The estimated relative sp2 content is 0.039 and 0.052 for FIB-CVD DLC and IP DLC, respectively. In other words, the sp3 content of the FIB-CVD DLC is higher than that of the IP DLC. This agrees with the result of Raman spectroscopy described above. As a result, the FIB assisted phenanthrene deposition could be anticipated for the synthesis of the DLC structures with a higher sp3 hybridized carbon than commercial IP DLC.
Acknowledgments This work is supported by a CREST JST Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Joint Studies Program of the Institute for Molecular Science. The authors thank UVSOR staff, especially Dr. Nakamura for the NEXAFS measurement at BL8B1.
References Batson, P.E., 1993. NEXAFS spectra of graphite. Phys. Rev. B 48, 2608–2610.
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Fujita, J., Ishida, M., Sakamoto, T., Ochiai, Y., Kaito, T., Matsui, S., 2001. Young’ modulus of DLC pillars grown by FIB-CVD. J. Vac. Sci. Technol. B 19, 2834–2837. Jaouen, M., Tourillon, G., Delafond, J., Junqua, N., Hug, G., 1995. NEXAFS spectra of DLC films. Diamond Relat. Mater. 4, 200–206. Kanda, K., Shimizugawa, Y., Haruyama, Y., Yamada, I., Matsui, S., Kitagawa, T., Tsubakino, H., Gejo, T., 2002. NEXAFS spectra of DLC films formed by GCIB method. Jpn. J. Appl. Phys. 41, 4295–4298. Kanda, K., Kitagawa, T., Shimizugawa, Y., Haruyama, Y., Matsui, S., Terasawa, M., Tsubakino, H., Yamada, I., Gejo, T., Kamada, M., 2003. NEXAFS study on DLC films. Nucl. Instr. and Meth. (B) 206, 880–883. Kometani, R., Morita, T., Watanabe, K., Kanda, K., Haruyama, Y., Kaito, T., Fujita, J., Ishida, M., Ochiai, Y., Matsui, S., 2003. Nozzle-nanostructure fabrication by FIB-CVD. Jpn. J. Appl. Phys. 42, 4107–4110.
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