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Carbon nitride thin films prepared by nitrogen ion assisted pulsed laser deposition of graphite using KrF excimer laser
Thin Solid Films 339 (1999) 38±43
Carbon nitride thin ®lms prepared by nitrogen ion assisted pulsed laser deposition of graphite using KrF excimer laser Kazuhiro Yamamoto a,*, Yoshinori Koga a, Shuzo Fujiwara a, Fumio Kokai b, Jacob I. Kleiman c, Kyekyoon K. Kim d a
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan Laser Laboratory, Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277, Japan c Institute for Aerospace Studies, University of Toronto, 4925 Dufferin Street, North York, Ontario M3H 5T6, Canada d Department of Electrical and Computer Engineering, University of Illinois, 155 Everitt Laboratory, 1406 West Green Street, Urbana, IL 61801, USA b
Received 23 February 1998; accepted 10 July 1998
Abstract Carbon nitride ®lms were prepared by nitrogen ion assisted pulsed KrF excimer laser deposition of graphite onto Si(100) substrates. The energy of nitrogen ions was changed between 25 and 1500 eV. The transport ratio of carbon atoms to nitrogen ions at the substrate was 1.0. The dependence of the stoichiometry and formed chemical bonds on the nitrogen ion energy was investigated. The nitrogen content in prepared ®lms increased with decreasing the nitrogen ion energy, and showed a constant value of 30 at.% below 200 eV. The peak position of C1s spectra as found by X-ray photoelectron spectroscopy (XPS) analysis shifted to higher binding energy with decreasing nitrogen ion energy. The N1s XPS peak was deconvoluted into three peaks with binding energies BE 398:3, 400.0 and 402.0 eV, which were assigned to sp 3 C±N and sp 2 C±N and N±N bondings, respectively. The ratio of sp 3 to sp 2 bonded nitrogen atoms increased with decreasing ion energy, and showed a maximum value in the energy interval between 50 and 75 eV. The carbon content with the sp 3 C±N bond type was estimated at 12.6 at.% from electron energy loss spectroscopy (EELS) analysis. The nitrogen content with the sp 3 C±N bond type was estimated at 18.0 at.% by XPS. The ratio of carbon to nitrogen atoms with sp 3 bonds was found to be 1.43 in the ®lms grown at nitrogen ion energies of 50 eV, which is close to that of C3N4 compound predicted as a superhard material. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ion assisted pulse laser; KrF excimer laser; Carbon nitride thin ®lm; Graphite
1. Introduction In 1989, Liu and Cohen theoretically predicted the existence of superhard materials formed from carbon and nitrogen, which have a similar structure to b -Si3N4 [1]. According to their calculation, this metastable compound b -C3N4 could have hardness, thermal conductivity, and thermal stability properties superior to those of diamond. Recently, many attempts to form carbon nitride solid have been reported using techniques such as reactive sputtering [2±4], pulsed laser ablation [5,6], ion assisted deposition [7,8], plasma assisted vapor deposition [9], ion beam deposition [10,11], ion implantation [12], pyrolysis [13], vacuum arc [14], and shock wave compression [15]. However, only a few groups have reported the synthesis of b -C3N4 ®lms by using pulsed laser ablation of graphite under an atomic nitrogen beam [5,16], or rf diode sputtering * Corresponding author. Fax: 1 81-298-544474; e-mail: [email protected].
of a graphite target with pure nitrogen [4]. The electron diffraction data of the grown ®lms had shown to be consistent with the b -C3N4 structure [4,5]. Other phases of C3N4 have recently been reported such as a -phase of trigonal structure [17], defect zinc-blend structure [18] and rhombohedral structure [19]. The authors have reported the formation of nanometer size crystals of cubic carbon nitride phase by nitrogen ion implantation into graphite [20]. However, the stoichiometry and the chemical bonding of this carbon nitride solid was not clear. The deposition parameters such as the ¯ux ratio and the kinetic energy of deposition species can be controlled independently during ion assisted deposition. Diamond-like carbon ®lms containing up to 60% sp 3 bonded atoms were produced by the pulsed laser deposition (PLD) of graphite with a KrF excimer laser [21]. In this paper, carbon nitride ®lms were prepared by combining the nitrogen ion assisting technique and the pulsed laser deposition of graphite with the purpose of studying the
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0107 2-4
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 1. Schematic diagram of ion beam assisted pulsed laser deposition apparatus.
dependence of the stoichiometry and chemical bondings on the nitrogen energy. 2. Experimental The deposition of thin carbon ®lms was performed in a vacuum chamber. A schematic diagram of the deposition chamber is shown in Fig. 1. The chamber is equipped with a Kaufman type 3 cm ion source (Commonwealth Scienti®c) that was used for ion irradiation of the substrate. The target for laser ablation (a highly oriented pyrolytic graphite (HOPG)) was rotated at 150 rpm during the deposition. Nitrogen ions were irradiating the substrate during carbon deposition. The incident angle of nitrogen ions was
39
normal to the substrate, and that of the carbon species was 458 to the substrate normal. Si(100) wafers were used as substrates. No heating of the substrates was employed. The base vacuum pressure in the deposition chamber was below 6:7 £ 1025 Pa and the nitrogen gas pressure during ®lm deposition was 6:7 £ 1022 Pa. A KrF excimer pulsed laser (wavelength 248 nm) was used for ablation of the graphite target. The pulse width of the KrF laser was 23 ns. The laser was focused into the vacuum chamber at an incident angle of 458 using a spherical lens. The laser spot size was 4:4 £ 1023 mm 2. The laser repetition rate was 30 Hz and the laser ¯uence was 3.5 J/cm 2. The nitrogen ion beam is mixture of N 1 and N1 2 , and the ratio between two types of ions could not be measured. This ratio may vary with the ion source operation parameters such as the discharge voltage and the gas ¯ow rate. The discharge voltage of 40 V and the gas ¯ow rate of 5 sccm was ®xed in this study. Acceleration voltage of nitrogen ions was changed between 200 and 1500 V. For the low energy irradiation below 200 V, a decelerator, positioned in front of the ion source, was used. An electrical bias ranging from 0 to 200 eV was supplied to the decelerator. During the use of the decelerator, the ion energy from the ion source was kept at 200 eV. The ion current was measured using a Faraday cage. A thickness monitor was used to measure the carbon ¯ux. The transport ratio of carbon ¯ux to nitrogen ions (atoms to ions ratio) was kept at a constant value of 1.0. The composition of carbon nitride ®lms was determined using X-ray photoelectron spectroscopy (XPS) (Perkin Elmer, model PHI 5600). The monochromated Al Ka Xray radiation was used. The spectrometer was calibrated using Au 4f7=2 84:0 eV, Ag 4d5=2 368:3 eV and Cu 2p3=2 932:7 eV. Before all measurements, the line position C 1s 284:5 eV of HOPG was used as a binding energy reference. The characterization of the carbon ®lm was carried out by transmission electron microscopy (TEM) (model Zeiss CEM 902). The microscope was equipped with a Castaing-Henry-type energy ®lter. Electron energy loss spectroscopic (EELS) analysis was performed at an accelerating voltage of 80 kV. The deposited carbon ®lms were separated from the Si substrate by dipping them into a solution of HF and HNO3. The ®lms were washed in water and transferred to a copper grid for TEM/ EELS analysis. 3. Results and discussion
Fig. 2. Electron energy loss spectrum in the carbon K-edge region of prepared ®lm without nitrogen ion irradiation.
The chemical bonding of carbon ®lms prepared by pulsed laser deposition (PLD) was found to be dependent on the laser wavelength and the laser ¯uence [21,22]. Hence the fraction of sp 3 bonds in carbon ®lm prepared by KrF laser deposition without the nitrogen ion irradiation was investigated ®rst using EELS analysis. A typical EELS spectrum is shown in Fig. 2. The peak at 295 eV can be attributed to the transition from 1s to the s * state and the shoulder at 284 eV
40
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 3. Dependence of the nitrogen content of prepared ®lms on the nitrogen ion energy.
to the transition from 1s to the p * state. The presence of the p * peak indicates the existence of sp 2 or sp bonds between carbon atoms in the structure. Selected area electron diffraction analysis of these ®lms shows only a halo, thus con®rming amorphous structure of the ®lm. The absence of an ordered crystalline structure results in broadening of the s * peak. The fraction of sp 3 bonds was calculated from the EELS spectra according to the method of Berger et al. [23]. This method is based on the comparison between the integrals under the p * peak, normalized by the integrated counts over a de®ned energy width in the carbon K-edge spectrum, and that of a standard material consisting of atoms with 100% sp 2 hybridized bonds. In this study, the HOPG was used as the standard material. The amount of carbon atoms with sp 3 hybridized bonds calculated from EELS spectra of these ®lms was estimated at 42%. The carbon nitride thin ®lms prepared by nitrogen ion assisted PLD were investigated next. The ®lm composition was determined by XPS after Ar 1 ion etching at an energy of 500 eV. The nitrogen content in the CN ®lms after Ar 1 ion etching was found to be a few percent lower than before etching. The dependence of nitrogen content in the ®lm on the nitrogen ion energy is shown in Fig. 3. The nitrogen content is 12 at.% at the nitrogen ion energy of 1500 eV, and increases with decreasing the nitrogen ion energy. The nitrogen content stabilizes at 30 at.% at ion energy below 200 eV. It is interesting to note that a similar behavior of N concentration upon substrate temperature was observed in CNx ®lms deposited by reactive magnetron sputter deposition on Si(001) substrates [24]. It was found that for substrate temperature, Ts, below 4008C, the N content was constant at approximately 25%, whereas for higher temperature it started to decrease. It is thought that at ion energies higher than 200 eV the content of nitrogen is reduced by
nitrogen ions impacting the growing CN ®lm. In general, the content of nitrogen in the growing ®lm can be reduced by a number of processes. It is well known that a threshold for physical sputtering exists for many materials below which there will be no sputtering and above which the sputtering process can be increased dramatically [25]. Another mechanism that can be responsible for removal of atoms from the growing surface is associated with selective desorption of atoms through formation of volatiles that can take place at certain surface energy conditions. The decrease in the concentration of nitrogen content in the present work can, probably, be attributed to the onset of physical sputtering of nitrogen by increasingly more energetic ions, while the selective desorption can explain the decrease in N content upon substrate temperature [24]. The C1s XPS spectra of CN thin ®lms prepared under nitrogen ion irradiation with energies between 25 and 1500 eV are shown in Fig. 4. The peak position of C1s spectrum at ion energy of 1500 eV is 284.8 eV. This value is close to the BE of 284.5 eV of graphite. However, the peak position was found to shift to higher energies and the full width at half-maximum (FWHM) to broaden with decreasing the ion energy. The BE of carbon in C±N bond is at 287.7 eV for sp 3-hybridization and at 285.9 eV for sp 2hybridization [11]. Therefore, the content of carbon atoms bonded to nitrogen by sp 3 hybridized bonds increases with lowering nitrogen ion energy. It is dif®cult to estimate the C±N fraction with sp 3 bonds from the peak deconvolution analysis of C1s spectrum due to the presence of other types of carbon bonds like the sp 2 bonds of carbon at 284.5 eV, the sp 3 bonds of carbon at 285.5 eV and C±O at 288.0 eV [11,22]. The N1s region of the XPS spectra of CN thin ®lms discussed above is shown in Fig. 5. Two main contributions are visible in the N1s spectra. By the Fourier transform infrared (FTIR) analysis of these specimens, a CvN stretching mode at 1500±1750 cm 21 and a C±N stretching mode at 1200±1450 cm 21 were observed, but the signal at 2200 cm 21 as a CuN stretching mode was not apparent [26]. It is thus thought that most nitrogen bonds are sp 3 C±N bond and sp 2 C±N bond. The ®rst at 398.3 eV corresponds to sp 3 C±N bond. The second at 400 eV is associated with sp 2 C±N bond. Both these peaks were also observed by Narayan et al. [16]. The contribution of N±O or N±N bonds at 402 eV seems to be very small. The peak intensity of sp 3 C±N bond at the nitrogen ion energy of 1500 eV is weaker than that of sp 2 C±N bond. This is well in accord with the above observations on the C1s spectra at 1500 eV. A deconvolution analysis of N1s spectra was carried out using a Gaussian curve ®tting procedure and subtracting the inelastic scattering background. The ratio of areas under the 398.3 eV and 400.0 eV peaks re¯ects the fraction of sp 3 to sp 2 bonds in the total content of nitrogen and can be used for their quantitative evaluation. The dependence of nitrogen atoms content with sp 3 and sp 2 hybridization on nitrogen ion energy is shown in Fig. 6. At the ion energy of 1500 eV
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 4. X-ray photoelectron spectra in the carbon 1s region of prepared ®lms depending on the nitrogen ion energy.
the nitrogen content with the sp 3 C±N bond is 5 at.% and is lower than that with the sp 2 C±N bond of 6.5 at.%. Both fractions increase with decreasing ion energy. However, the nitrogen content with sp 3 C±N bond increases more rapidly than that with the sp 2 C±N bond. At the energy of 750 eV both fractions show the same value of 8 at.% and the sp 3 bonding starts to dominate below 750 eV. The sp 3 C±N bond peaks at 18 at.% in the ion energy interval from 50 to 75 eV, and then decreases at lower energies. On the other hand, the sp 2 hybridized nitrogen atoms fraction peaks at 13 at.% at ion energy of 150 eV. Then it decreases with decreasing ion energy ranging in the interval from 150 to 50 eV, and increases again at still lower ion energies. Since the distance between the graphite target and substrate was kept at 70 mm and the nitrogen gas pressure was 6:7 £ 1022 Pa during deposition, it is conceivable to assume that the probability of the ablated carbon species
41
Fig. 5. X-ray photoelectron spectra in the carbon 1s region of prepared ®lms depending on the nitrogen ion energy.
colliding with the nitrogen ions is very low. Hence, it seems that the sp 3 C±N bonds are formed mostly on the substrate surface upon nitrogen ion impact in the ion energy interval between 50 and 75 eV. The decrease of the sp 3 bond fraction and the maximum in sp 2 bond fraction observed in the ion energy range 100±175 eV (Fig. 6) suggests that the sp 3 bonds are transformed to the sp 2 type by ion impact. Due to the broadening of the C1s XPS peak it was not possible to estimate the sp 3 hybridized fraction of carbon atoms, as was done for the nitrogen atoms above. The broadening is due to contributions from species like sp 2 C±N, sp 3 C±N, sp 2 C±C, sp 3 C±C and C±O bonded atoms. To estimate the fraction of sp 3 C±N bonded atoms, EELS analysis was carried out. EELS spectra of carbon and nitrogen K-edge region of the C±N ®lm prepared at the ion energy of 50 eV are shown in Fig. 7. The transition from
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K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 6. Dependence of the nitrogen content with sp 3 C±N bond (closed circle) and with sp 2 C±N bond (open circle) on the nitrogen ion energy.
1s to the p * state observed at 284 eV for the carbon K-edge and 398 eV for the nitrogen K-edge were indicative of the presence of sp 2 C±N bonded atoms in the C±N ®lm. The fraction of carbon atoms with sp 3 hybridization, which
includes sp 3-hybridized carbon species bonded as C±C and C±N, was calculated from the carbon K-edge EELS spectrum using the method described above, and was found to constitute 18% of all carbon atoms. A conclusion can therefore be reached that the sp 3 hybridized fraction of C atoms decreases from 42%, as found in experiments without nitrogen ion irradiation, to 18% upon ion irradiation during deposition. It seems that most of sp 3 C±C bonds are transformed to the sp 2 C±N bonds by nitrogen ion impact with the energy of 50 eV. Assuming that all sp 3 bonded carbon species are of C±N type (i.e. ignoring the formation of C±C sp 3 bonds) in the C±N ®lm, and for a total carbon content in the ®lm estimated as 70 at.%, the content of carbon atoms with sp 3 C±N bonds can be estimated at 12.6 at.% for ®lms grown at 50 eV nitrogen irradiation conditions. From the XPS analysis the content of sp 3 bonded nitrogen was estimated at 18.0 at.%. The nitrogen to carbon concentration ratio of the sp 3 bonded phase in the C±N ®lm is therefore 1.43. This ratio is close to the C3N4 stoichiometry of 1.33. From the above discussion it can be concluded that an amorphous, sp 3 bonded phase of stoichiometry close to C3N4 was produced in the present work at nitrogen ion irradiation conditions in the interval of energies 50 to 75 eV. 4. Conclusion Amorphous carbon nitride ®lms with stoichiometry close to C3N4 were prepared by nitrogen ion assisted pulsed laser deposition of graphite using a KrF excimer laser. The dependence of the nitrogen content in prepared ®lms on the nitrogen ion energy was investigated. The nitrogen content in the ®lm was found to be constant at 30 at.% in the range of ion energies between 25 and 200 eV. Above 200 eV, the content decreases with increasing ion energy due to physical sputtering by energetic nitrogen ions upon impact. At energies between 100 and 200 eV the fraction of N atoms with sp 2 hybridized bonds increases, showing a maximum at 175 eV. The increase of sp 2 and decrease of sp 3 hybridized atoms can be explained by a transformation process due to ion impact of the growing ®lm. The most favorable conditions for formation of sp 3 hybridized bonds were found at the nitrogen ion energy interval 50±75 eV.
References
Fig. 7. Electron energy loss spectrum in the carbon and nitrogen K-edge regions of the ®lm prepared at the nitrogen ion energy of 50 eV.
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[18] A.Y. Liu, R.M. Wentzcovitch, Phys. Rev. B 50 (1994) 10362. [19] J. Ortega, O.F. Sankey, Phys. Rev. B 51 (1995) 2624. [20] K. Yamamoto, Y. Koga, K. Yase, S. Fujiwara, M. Kubota, Jpn. J. Appl. Phys. 36 (1997) L230. [21] K. Yamamoto, Y. Koga, S. Fujiwara, F. Kokai, Jpn. J. Appl. Phys. 36 (1997) L1333. [22] K. Yamamoto, Y. Koga, S. Fujiwara, F. Kokai, R.B. Heimann, Appl. Phys. A 66 (1998) 115. [23] S.D. Berger, D.R. McKenzie, P.J. Martin, Phil. Mag. Lett. 57 (1988) 2361. [24] H. SjoÈstroÈm, I. Ivanov, M. Johansson, L. Hultman, J.-E. Sundgren, S.V. Hainsworth, T.F. Page, L.R. Wallenberg, Thin Solid Films 246 (1994) 103. [25] W. Ekstein, C. Garcia-Rosales, J. Roth, W. Ottenberger, Sputtering Data, Max-Plank-Institut fur Plasmaphysik, 8046 Garching Bei Munchen, IPP9/82, 1993. [26] X.A. Zhao, C.W. Ong, Y.C. Tsang, Y.W. Wong, P.W. Chang, C.L. Choy, Appl. Phys. Lett. 66 (1995) 2652.
Carbon nitride thin ®lms prepared by nitrogen ion assisted pulsed laser deposition of graphite using KrF excimer laser Kazuhiro Yamamoto a,*, Yoshinori Koga a, Shuzo Fujiwara a, Fumio Kokai b, Jacob I. Kleiman c, Kyekyoon K. Kim d a
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan Laser Laboratory, Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277, Japan c Institute for Aerospace Studies, University of Toronto, 4925 Dufferin Street, North York, Ontario M3H 5T6, Canada d Department of Electrical and Computer Engineering, University of Illinois, 155 Everitt Laboratory, 1406 West Green Street, Urbana, IL 61801, USA b
Received 23 February 1998; accepted 10 July 1998
Abstract Carbon nitride ®lms were prepared by nitrogen ion assisted pulsed KrF excimer laser deposition of graphite onto Si(100) substrates. The energy of nitrogen ions was changed between 25 and 1500 eV. The transport ratio of carbon atoms to nitrogen ions at the substrate was 1.0. The dependence of the stoichiometry and formed chemical bonds on the nitrogen ion energy was investigated. The nitrogen content in prepared ®lms increased with decreasing the nitrogen ion energy, and showed a constant value of 30 at.% below 200 eV. The peak position of C1s spectra as found by X-ray photoelectron spectroscopy (XPS) analysis shifted to higher binding energy with decreasing nitrogen ion energy. The N1s XPS peak was deconvoluted into three peaks with binding energies BE 398:3, 400.0 and 402.0 eV, which were assigned to sp 3 C±N and sp 2 C±N and N±N bondings, respectively. The ratio of sp 3 to sp 2 bonded nitrogen atoms increased with decreasing ion energy, and showed a maximum value in the energy interval between 50 and 75 eV. The carbon content with the sp 3 C±N bond type was estimated at 12.6 at.% from electron energy loss spectroscopy (EELS) analysis. The nitrogen content with the sp 3 C±N bond type was estimated at 18.0 at.% by XPS. The ratio of carbon to nitrogen atoms with sp 3 bonds was found to be 1.43 in the ®lms grown at nitrogen ion energies of 50 eV, which is close to that of C3N4 compound predicted as a superhard material. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ion assisted pulse laser; KrF excimer laser; Carbon nitride thin ®lm; Graphite
1. Introduction In 1989, Liu and Cohen theoretically predicted the existence of superhard materials formed from carbon and nitrogen, which have a similar structure to b -Si3N4 [1]. According to their calculation, this metastable compound b -C3N4 could have hardness, thermal conductivity, and thermal stability properties superior to those of diamond. Recently, many attempts to form carbon nitride solid have been reported using techniques such as reactive sputtering [2±4], pulsed laser ablation [5,6], ion assisted deposition [7,8], plasma assisted vapor deposition [9], ion beam deposition [10,11], ion implantation [12], pyrolysis [13], vacuum arc [14], and shock wave compression [15]. However, only a few groups have reported the synthesis of b -C3N4 ®lms by using pulsed laser ablation of graphite under an atomic nitrogen beam [5,16], or rf diode sputtering * Corresponding author. Fax: 1 81-298-544474; e-mail: [email protected].
of a graphite target with pure nitrogen [4]. The electron diffraction data of the grown ®lms had shown to be consistent with the b -C3N4 structure [4,5]. Other phases of C3N4 have recently been reported such as a -phase of trigonal structure [17], defect zinc-blend structure [18] and rhombohedral structure [19]. The authors have reported the formation of nanometer size crystals of cubic carbon nitride phase by nitrogen ion implantation into graphite [20]. However, the stoichiometry and the chemical bonding of this carbon nitride solid was not clear. The deposition parameters such as the ¯ux ratio and the kinetic energy of deposition species can be controlled independently during ion assisted deposition. Diamond-like carbon ®lms containing up to 60% sp 3 bonded atoms were produced by the pulsed laser deposition (PLD) of graphite with a KrF excimer laser [21]. In this paper, carbon nitride ®lms were prepared by combining the nitrogen ion assisting technique and the pulsed laser deposition of graphite with the purpose of studying the
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0107 2-4
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 1. Schematic diagram of ion beam assisted pulsed laser deposition apparatus.
dependence of the stoichiometry and chemical bondings on the nitrogen energy. 2. Experimental The deposition of thin carbon ®lms was performed in a vacuum chamber. A schematic diagram of the deposition chamber is shown in Fig. 1. The chamber is equipped with a Kaufman type 3 cm ion source (Commonwealth Scienti®c) that was used for ion irradiation of the substrate. The target for laser ablation (a highly oriented pyrolytic graphite (HOPG)) was rotated at 150 rpm during the deposition. Nitrogen ions were irradiating the substrate during carbon deposition. The incident angle of nitrogen ions was
39
normal to the substrate, and that of the carbon species was 458 to the substrate normal. Si(100) wafers were used as substrates. No heating of the substrates was employed. The base vacuum pressure in the deposition chamber was below 6:7 £ 1025 Pa and the nitrogen gas pressure during ®lm deposition was 6:7 £ 1022 Pa. A KrF excimer pulsed laser (wavelength 248 nm) was used for ablation of the graphite target. The pulse width of the KrF laser was 23 ns. The laser was focused into the vacuum chamber at an incident angle of 458 using a spherical lens. The laser spot size was 4:4 £ 1023 mm 2. The laser repetition rate was 30 Hz and the laser ¯uence was 3.5 J/cm 2. The nitrogen ion beam is mixture of N 1 and N1 2 , and the ratio between two types of ions could not be measured. This ratio may vary with the ion source operation parameters such as the discharge voltage and the gas ¯ow rate. The discharge voltage of 40 V and the gas ¯ow rate of 5 sccm was ®xed in this study. Acceleration voltage of nitrogen ions was changed between 200 and 1500 V. For the low energy irradiation below 200 V, a decelerator, positioned in front of the ion source, was used. An electrical bias ranging from 0 to 200 eV was supplied to the decelerator. During the use of the decelerator, the ion energy from the ion source was kept at 200 eV. The ion current was measured using a Faraday cage. A thickness monitor was used to measure the carbon ¯ux. The transport ratio of carbon ¯ux to nitrogen ions (atoms to ions ratio) was kept at a constant value of 1.0. The composition of carbon nitride ®lms was determined using X-ray photoelectron spectroscopy (XPS) (Perkin Elmer, model PHI 5600). The monochromated Al Ka Xray radiation was used. The spectrometer was calibrated using Au 4f7=2 84:0 eV, Ag 4d5=2 368:3 eV and Cu 2p3=2 932:7 eV. Before all measurements, the line position C 1s 284:5 eV of HOPG was used as a binding energy reference. The characterization of the carbon ®lm was carried out by transmission electron microscopy (TEM) (model Zeiss CEM 902). The microscope was equipped with a Castaing-Henry-type energy ®lter. Electron energy loss spectroscopic (EELS) analysis was performed at an accelerating voltage of 80 kV. The deposited carbon ®lms were separated from the Si substrate by dipping them into a solution of HF and HNO3. The ®lms were washed in water and transferred to a copper grid for TEM/ EELS analysis. 3. Results and discussion
Fig. 2. Electron energy loss spectrum in the carbon K-edge region of prepared ®lm without nitrogen ion irradiation.
The chemical bonding of carbon ®lms prepared by pulsed laser deposition (PLD) was found to be dependent on the laser wavelength and the laser ¯uence [21,22]. Hence the fraction of sp 3 bonds in carbon ®lm prepared by KrF laser deposition without the nitrogen ion irradiation was investigated ®rst using EELS analysis. A typical EELS spectrum is shown in Fig. 2. The peak at 295 eV can be attributed to the transition from 1s to the s * state and the shoulder at 284 eV
40
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 3. Dependence of the nitrogen content of prepared ®lms on the nitrogen ion energy.
to the transition from 1s to the p * state. The presence of the p * peak indicates the existence of sp 2 or sp bonds between carbon atoms in the structure. Selected area electron diffraction analysis of these ®lms shows only a halo, thus con®rming amorphous structure of the ®lm. The absence of an ordered crystalline structure results in broadening of the s * peak. The fraction of sp 3 bonds was calculated from the EELS spectra according to the method of Berger et al. [23]. This method is based on the comparison between the integrals under the p * peak, normalized by the integrated counts over a de®ned energy width in the carbon K-edge spectrum, and that of a standard material consisting of atoms with 100% sp 2 hybridized bonds. In this study, the HOPG was used as the standard material. The amount of carbon atoms with sp 3 hybridized bonds calculated from EELS spectra of these ®lms was estimated at 42%. The carbon nitride thin ®lms prepared by nitrogen ion assisted PLD were investigated next. The ®lm composition was determined by XPS after Ar 1 ion etching at an energy of 500 eV. The nitrogen content in the CN ®lms after Ar 1 ion etching was found to be a few percent lower than before etching. The dependence of nitrogen content in the ®lm on the nitrogen ion energy is shown in Fig. 3. The nitrogen content is 12 at.% at the nitrogen ion energy of 1500 eV, and increases with decreasing the nitrogen ion energy. The nitrogen content stabilizes at 30 at.% at ion energy below 200 eV. It is interesting to note that a similar behavior of N concentration upon substrate temperature was observed in CNx ®lms deposited by reactive magnetron sputter deposition on Si(001) substrates [24]. It was found that for substrate temperature, Ts, below 4008C, the N content was constant at approximately 25%, whereas for higher temperature it started to decrease. It is thought that at ion energies higher than 200 eV the content of nitrogen is reduced by
nitrogen ions impacting the growing CN ®lm. In general, the content of nitrogen in the growing ®lm can be reduced by a number of processes. It is well known that a threshold for physical sputtering exists for many materials below which there will be no sputtering and above which the sputtering process can be increased dramatically [25]. Another mechanism that can be responsible for removal of atoms from the growing surface is associated with selective desorption of atoms through formation of volatiles that can take place at certain surface energy conditions. The decrease in the concentration of nitrogen content in the present work can, probably, be attributed to the onset of physical sputtering of nitrogen by increasingly more energetic ions, while the selective desorption can explain the decrease in N content upon substrate temperature [24]. The C1s XPS spectra of CN thin ®lms prepared under nitrogen ion irradiation with energies between 25 and 1500 eV are shown in Fig. 4. The peak position of C1s spectrum at ion energy of 1500 eV is 284.8 eV. This value is close to the BE of 284.5 eV of graphite. However, the peak position was found to shift to higher energies and the full width at half-maximum (FWHM) to broaden with decreasing the ion energy. The BE of carbon in C±N bond is at 287.7 eV for sp 3-hybridization and at 285.9 eV for sp 2hybridization [11]. Therefore, the content of carbon atoms bonded to nitrogen by sp 3 hybridized bonds increases with lowering nitrogen ion energy. It is dif®cult to estimate the C±N fraction with sp 3 bonds from the peak deconvolution analysis of C1s spectrum due to the presence of other types of carbon bonds like the sp 2 bonds of carbon at 284.5 eV, the sp 3 bonds of carbon at 285.5 eV and C±O at 288.0 eV [11,22]. The N1s region of the XPS spectra of CN thin ®lms discussed above is shown in Fig. 5. Two main contributions are visible in the N1s spectra. By the Fourier transform infrared (FTIR) analysis of these specimens, a CvN stretching mode at 1500±1750 cm 21 and a C±N stretching mode at 1200±1450 cm 21 were observed, but the signal at 2200 cm 21 as a CuN stretching mode was not apparent [26]. It is thus thought that most nitrogen bonds are sp 3 C±N bond and sp 2 C±N bond. The ®rst at 398.3 eV corresponds to sp 3 C±N bond. The second at 400 eV is associated with sp 2 C±N bond. Both these peaks were also observed by Narayan et al. [16]. The contribution of N±O or N±N bonds at 402 eV seems to be very small. The peak intensity of sp 3 C±N bond at the nitrogen ion energy of 1500 eV is weaker than that of sp 2 C±N bond. This is well in accord with the above observations on the C1s spectra at 1500 eV. A deconvolution analysis of N1s spectra was carried out using a Gaussian curve ®tting procedure and subtracting the inelastic scattering background. The ratio of areas under the 398.3 eV and 400.0 eV peaks re¯ects the fraction of sp 3 to sp 2 bonds in the total content of nitrogen and can be used for their quantitative evaluation. The dependence of nitrogen atoms content with sp 3 and sp 2 hybridization on nitrogen ion energy is shown in Fig. 6. At the ion energy of 1500 eV
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 4. X-ray photoelectron spectra in the carbon 1s region of prepared ®lms depending on the nitrogen ion energy.
the nitrogen content with the sp 3 C±N bond is 5 at.% and is lower than that with the sp 2 C±N bond of 6.5 at.%. Both fractions increase with decreasing ion energy. However, the nitrogen content with sp 3 C±N bond increases more rapidly than that with the sp 2 C±N bond. At the energy of 750 eV both fractions show the same value of 8 at.% and the sp 3 bonding starts to dominate below 750 eV. The sp 3 C±N bond peaks at 18 at.% in the ion energy interval from 50 to 75 eV, and then decreases at lower energies. On the other hand, the sp 2 hybridized nitrogen atoms fraction peaks at 13 at.% at ion energy of 150 eV. Then it decreases with decreasing ion energy ranging in the interval from 150 to 50 eV, and increases again at still lower ion energies. Since the distance between the graphite target and substrate was kept at 70 mm and the nitrogen gas pressure was 6:7 £ 1022 Pa during deposition, it is conceivable to assume that the probability of the ablated carbon species
41
Fig. 5. X-ray photoelectron spectra in the carbon 1s region of prepared ®lms depending on the nitrogen ion energy.
colliding with the nitrogen ions is very low. Hence, it seems that the sp 3 C±N bonds are formed mostly on the substrate surface upon nitrogen ion impact in the ion energy interval between 50 and 75 eV. The decrease of the sp 3 bond fraction and the maximum in sp 2 bond fraction observed in the ion energy range 100±175 eV (Fig. 6) suggests that the sp 3 bonds are transformed to the sp 2 type by ion impact. Due to the broadening of the C1s XPS peak it was not possible to estimate the sp 3 hybridized fraction of carbon atoms, as was done for the nitrogen atoms above. The broadening is due to contributions from species like sp 2 C±N, sp 3 C±N, sp 2 C±C, sp 3 C±C and C±O bonded atoms. To estimate the fraction of sp 3 C±N bonded atoms, EELS analysis was carried out. EELS spectra of carbon and nitrogen K-edge region of the C±N ®lm prepared at the ion energy of 50 eV are shown in Fig. 7. The transition from
42
K. Yamamoto et al. / Thin Solid Films 339 (1999) 38±43
Fig. 6. Dependence of the nitrogen content with sp 3 C±N bond (closed circle) and with sp 2 C±N bond (open circle) on the nitrogen ion energy.
1s to the p * state observed at 284 eV for the carbon K-edge and 398 eV for the nitrogen K-edge were indicative of the presence of sp 2 C±N bonded atoms in the C±N ®lm. The fraction of carbon atoms with sp 3 hybridization, which
includes sp 3-hybridized carbon species bonded as C±C and C±N, was calculated from the carbon K-edge EELS spectrum using the method described above, and was found to constitute 18% of all carbon atoms. A conclusion can therefore be reached that the sp 3 hybridized fraction of C atoms decreases from 42%, as found in experiments without nitrogen ion irradiation, to 18% upon ion irradiation during deposition. It seems that most of sp 3 C±C bonds are transformed to the sp 2 C±N bonds by nitrogen ion impact with the energy of 50 eV. Assuming that all sp 3 bonded carbon species are of C±N type (i.e. ignoring the formation of C±C sp 3 bonds) in the C±N ®lm, and for a total carbon content in the ®lm estimated as 70 at.%, the content of carbon atoms with sp 3 C±N bonds can be estimated at 12.6 at.% for ®lms grown at 50 eV nitrogen irradiation conditions. From the XPS analysis the content of sp 3 bonded nitrogen was estimated at 18.0 at.%. The nitrogen to carbon concentration ratio of the sp 3 bonded phase in the C±N ®lm is therefore 1.43. This ratio is close to the C3N4 stoichiometry of 1.33. From the above discussion it can be concluded that an amorphous, sp 3 bonded phase of stoichiometry close to C3N4 was produced in the present work at nitrogen ion irradiation conditions in the interval of energies 50 to 75 eV. 4. Conclusion Amorphous carbon nitride ®lms with stoichiometry close to C3N4 were prepared by nitrogen ion assisted pulsed laser deposition of graphite using a KrF excimer laser. The dependence of the nitrogen content in prepared ®lms on the nitrogen ion energy was investigated. The nitrogen content in the ®lm was found to be constant at 30 at.% in the range of ion energies between 25 and 200 eV. Above 200 eV, the content decreases with increasing ion energy due to physical sputtering by energetic nitrogen ions upon impact. At energies between 100 and 200 eV the fraction of N atoms with sp 2 hybridized bonds increases, showing a maximum at 175 eV. The increase of sp 2 and decrease of sp 3 hybridized atoms can be explained by a transformation process due to ion impact of the growing ®lm. The most favorable conditions for formation of sp 3 hybridized bonds were found at the nitrogen ion energy interval 50±75 eV.
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Fig. 7. Electron energy loss spectrum in the carbon and nitrogen K-edge regions of the ®lm prepared at the nitrogen ion energy of 50 eV.
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