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Deposition of carbon films containing nitrogen by filtered pulsed cathodic arc discharge method
Diamond and Related Materials 7 (1998) 1190–1195
Deposition of carbon films containing nitrogen by filtered pulsed cathodic arc discharge method A. Stanishevsky a,*, L. Khriachtchev b, I˙. Akula c a University of Maryland, Institute for Plasma Research, College Park, MD 20742, USA b Laboratory of Physical Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland c Plasmoteg Engineering Center of the Belarus Academy of Sciences, Zhodinskaya St. 1/3, 220141 Minsk, Belarus Received 11 November 1997; accepted 3 March 1998
Abstract Nitrogenated carbon films with N/C ratio up to 0.65 have been prepared by a filtered pulsed cathodic arc discharge (PCAD) method at nitrogen pressure in the range 0.1–15 Pa. The influence of the process parameters and nitrogen pressure on the growth rate, chemical composition and quality of C:N films has been studied. The maximum N/C ratio was found at a gas pressure of 1–2 Pa. Raman study of films prepared at nitrogen pressure higher than 2 Pa showed a strong increase of the photoluminescence signal with a maximum at 2.2 eV. © 1998 Elsevier Science S.A. Keywords: Carbon nitride; Pulsed cathodic arc; Photoluminescence; Raman
1. Introduction The deposition of carbon nitride and its hydrogenated variants has been performed by various PVD as well as CVD processes [1–6 ]. Experimentally, most authors reported nitrogen-to-carbon ratios much lower than that in hypothetical C N , and results show that films are 3 4 predominantly amorphous and a rather graphite-like turbostratic structure is observed. There were several attempts to produce C:N films by continuous (CAD) [7–9] and pulsed (PCAD) [10,11] cathodic arc deposition. It is known that the cathodic arc is an efficient ionizer of the ambient gas, and nitrides can easily be formed. A number of experiments were done using this method in various modifications. For example, Merchant et al. [7] used an unfiltered cathodic arc with nitrogen pressures of 2.66 and 6.65 Pa to deposit carbon nitride films. Their results showed that the structure of films is largely graphite-like with both nitrogen and carbon atoms in the graphitic rings. Davis et al. [8] found that the cathodic arc plasma flow produces efficient ionization of the background gas, but their films with 15 at% of nitrogen were already 100% sp2-bonded. They also observed that carbon atoms are * Corresponding author. Fax: +1 301 314 9437; e-mail: [email protected] 0925-9635/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 09 2 5 -9 6 3 5 ( 9 8 ) 0 0 17 4 - 5
substituted by nitrogen into an sp2-bonded structure. Gilkes et al. [9] mentioned 70±10% of sp2 sites in the C:N films deposited by a filtered cathodic arc at nitrogen content about 10 at%. In spite of the number of experiments, N/C ratios of more than 0.54 were not observed. This limit is particularly explained by the formation of volatile C N compounds during the deposition [12]. x y In this work we describe results of the preparation of nitrogenated carbon films by pulsed filtered cathodic arc discharge (PCAD) deposition in the range of nitrogen pressures up to 15 Pa. The aims of this work were to estimate facilities of an industrial pulsed cathodic arc source to operate within the wide range of gas pressures and study the dependence of C:N film composition and properties on the deposition parameters.
2. Experimental Carbon films were deposited by a pulsed cathodic arc discharge method described elsewhere [13]. The deposition has been performed at the following discharge parameters: pulse duration 300 ms, pulse repetition time 0.5 s, peak discharge current 1–4 kA, starting discharge voltage 200–400 V. At these parameters the average deposition rate of impurity-free carbon films was in the range 8–24 nm/min. A curved 90° solenoid of 10 turns
A. Stanishevsky et al. / Diamond and Related Materials 7 (1998) 1190–1195
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Fig. 1. A sketch of the experimental set-up for the deposition of C:N films by PCAD method. The sizes are proportionally reduced from the real values.
with internal diameter of 120 mm was included in the anode discharge circuit to reduce the amount of graphite microparticles in the plasma flow. The discharge circuit was isolated from the ground. The pulsed arc source used was capable of operating with nitrogen pressures up to 15 Pa. A scheme of the experimental set-up is shown in Fig. 1. Polished Si 110 substrates were placed on the grounded holder at 100 mm distance from the solenoid output. The substrates were cleaned with an argon ion-beam source operating at 3 kV accelerating voltage and current density of 1 mA/cm2 during 10–15 min. The substrate temperature during the deposition was measured with a microthermoresistor calibrated in the range 270–350 K. In all experiments the substrate temperature did not exceed 308 K. The film thicknesses were 100–200 nm. C:N films were characterized using known procedures by transmission electron microscopy ( TEM; JEOL-100 and Phillips EM420), Fourier transform infrared (FTIR; BioRad FTS165), X-ray photoelectron ( XPS; ES2401) and secondary ion mass spectroscopy (SIMS; MS1210), electron microprobe (Cameca SK-50) analysis and Raman spectroscopy. A number of Raman spectroscopic measurements were carried out by using a 1 m Jarrell Ash double spectrometer (8 cm−1 resolution) equipped with a cooled low-noise photomultiplier tube. The excitation radiation
of an argon ion laser (typically 200 mW at 514.5 nm) was focused to a round 0.03 mm spot. No visible degradation of the coatings was observed with applied laser power up to 0.1 W [14]. However, micro-Raman studies (ISA U1000) with a laser spot diameter of 2–3 mm result in damage of the film at output power 25 mW.
3. Results and discussion 3.1. Filtered PCAD deposition of C:N films The experiments with 90° solenoid show the better performance of carbon films with less microparticle contamination. It was found that the specific electric resistivity and breakdown voltage of carbon films without any additives at the axis of deposition were 3.4×107 V cm and 1.67×107 V/m, while at distance 20 mm from the axis these parameters were 108 V cm and 6.57×107 V/m, respectively. Such non-uniformity of film properties on the substrate surface has been mentioned for carbon films prepared by PCAD with the straight solenoid in the discharge circuit [13]. It was explained by the influence of the higher film growth rate at the center of deposition. While the specific resistance of films was about the same as for those prepared with a straight solenoid, the breakdown voltage was two to
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d.c. bias voltage applied to the substrate. Besides, PCAD C:N films prepared under high pressures are softer than those without nitrogen by a factor of about three, as was shown by a scratch test method. However, the profile of scratches shows that these films are still quite hard. The brittle delamination of films from the substrate was observed even at P of 7 Pa. N2 3.2. Chemical composition of C:N films
Fig. 2. The influence of the nitrogen pressure on the C:N film deposition rate. The distribution of film thicknesses along the substrate is shown in the inset for two types of solenoid.
four times higher and we connect it with a lesser amount of graphitic microparticles. An increase in the nitrogen pressure results in strong reduction of the deposition rate as shown in Fig. 2. There were minor changes in the film uniformity on the substrate at different gas pressures. The reduction of the deposition rate with increase of nitrogen pressure (P ) N2 is opposite to the result observed by Chhowalla et al. [15]. We suggest it can be due to the difference in arc source design, which influences the cathodic arc behavior in the gas ambient. The same reduction in deposition rate was found by using an argon gas instead of nitrogen, which confirms this suggestion. Because of the very low deposition rate, the quantity of microparticles per square unit of film is increased in comparison even with the straight solenoid, but the particle size is smaller than 0.2 mm. For this reason most studies were carried out with C:N films prepared at the starting discharge voltage of 400 V. The average voltage during the pulse in this case is about 65 V and the average energy of carbon ions is about 70–80 eV [13,16 ]. TEM investigation has shown typical features for a turbostratic graphite-like structure in all ranges of nitrogen pressures used. Films prepared with increasing nitrogen pressure also showed reduction of the optical transparency and electric resistivity. At pressure higher than 7 Pa the transparency of C:N films was about 5–10% better than that of those prepared without nitrogen. It was measured by comparison of the Raman signal from the coated Si substrate. Film resistivity was found to be increased again, but reliable data were not received because of the large amount of microparticles. In a d.c. planar magnetron sputtering experiment, Chen et al. [17] also mentioned that at low nitrogen pressures (0.3–0.4 Pa), C:N films tend to have low resistivity (350–107 V cm), while at comparatively high nitrogen pressures (1.33 Pa), the films were quite resistive (>108 V cm). This trend appeared to be independent of the sputtering power and
The nitrogen concentration calculated from XPS and electron microprobe analysis data did not exceed 40 at%. The dependence of the nitrogen concentration on the pressure is shown in Fig. 3. It seems there is a maximum of the nitrogen concentration (N/C ratio 0.65) at 1–2 Pa. This is in some disagreement with Merchant et al. [7], who found a saturation level of 0.3 for sp2-bonded C:N material. Nevertheless, there is a tendency to have a maximum N/C ratio at 1–2 Pa which is similar to the results of other authors. Comparison of results shows that PCAD deposited C:N films have higher N/C ratio than those prepared by continuous arc. For example, Husein et al. [11] used a magnetically filtered pulsed cathodic arc with operating parameters as follows: arc current 200–250 A, pulse duration 1.5 ms, nitrogen pressure 1.3×10−2 Pa. They found very high N/C ratio of 0.286 at this pressure. Koskinen et al. [10] also used a pulsed cathodic arc source of similar design without focusing solenoid to deposit C:N films. They found nitrogen concentration increasing up to 35% at 0.56 Pa, while at 0.11 Pa it was 19%. A small Raman peak at 2220 cm−1 was mentioned for films with the highest nitrogen content. We found approximately the same amount of nitrogen at 0.56 Pa. This means that the presence of the curved solenoid does not much affect the chemical composition of films if other deposition
Fig. 3. The dependence of the nitrogen concentration on the gas pressure for PCAD C:N films (black squares) in comparison with published data ( Koskinen et al. [10], white circles; Chhowalla et al. [15], black circles; Davis et al. [8], white squares; Hartmann et al. [18], black triangles).
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conditions are the same. It should be mentioned that Hartmann et al. [18] found only 18 at% of nitrogen in PCAD C:N films deposited at 0.2 Pa. FTIR spectra of PCAD C:N films show in our case little absorption in the range 2100–2200 cm−1 and 1100–1600 cm−1 with a maximum at about 1200 cm−1. The difference between spectra of films prepared in the studied range of gas pressures was not significant. Merchant et al. [7] mentioned 2200 and 2325 cm−1 peaks in IR spectra attributed to the presence of a small amount of sp1-bonded nitrogen and carbon (CNN and CNC bonds) within the amorphous structure with N/C ratio 0.25 and 0.35. Vorlicek et al. [19] also found that the presence of nitrogen is reflected by the weak feature peaking at 2226 cm−1 in Raman spectra which was related to C–N triple bond stretching. Strong absorption in the 1200–1600 cm−1 region is presented if the N atoms are bonded into the carbon sp2 network. The absorption on C–N single bonds occurs at 1230 cm−1 and nC–N at 1272 cm−1 [20]. Unfortunately, broad and weak absorption in FTIR spectra does not allow us to define bonding in our films. PCAD C:N films show very little absorption on stretched C–H bonds around 2900 cm−1. This absorption is approximately the same for C:N films prepared at any nitrogen pressure used, even though the hydrogen content should grow with increase of P because of a possibility of a backstream N2 from an oil diffusion pump or desorbtion of H O vapor 2 of the chamber walls [15]. Even at the highest nitrogen pressures no significant amount of hydrogen was found by SIMS. There was no sign of absorption on N–H bonds at 3400 cm−1. XPS and microprobe analysis have shown that films prepared at the highest nitrogen pressures of 7–15 Pa contain impurities of oxygen up to 5 at%, Cu (0.1–0.2%) and Fe (<0.1%). This occurs because of some sputtering of the arc source and solenoid parts at the higher P , N2 and the film deposition rate is very low. At lower pressures the amount of metal impurities was below the sensitivity of the measuring technique. XPS spectra of C:N films show C1s electron binding energy level at 284.6 eV with a shoulder at 286.8 eV. The ratio of fits for the maxima at 286.8 and 284.6 eV corresponds roughly to the N/C ratio. We assign the peak at 286.8 eV to C–N single bonds [21]. The N1s energy level has maxima at 397.5, 398.9 (main), 399.3 and 400.3 eV. The peaks at 398.9 and 400.3 may correspond to N bonded to sp3- and sp2-hybridized carbon atoms, respectively, as proposed by Chhowalla et al. [15] and Hammer et al. [22]. However, the maxima at 397.5, 398.9 and 399.3 can also arise because of different local atomic arrangement in distorted sp2-bonded carbon–nitrogen clusters and the appearance of a polymeric component. The character of chemical bonding does not change much with the increase of P , but some modification of XPS N2 spectra occurs.
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What do the above results show? First, we consider PCAD C:N films practically hydrogen-free. Second, growth of films takes place at a substrate temperature of about 308 K and at specific conditions of the periodical plasma flow. Under these conditions the nitrogen and some volatile C N molecules can be absorbed at x y the film surface during the pause between pulses [23]. The next pulse produces partial sputtering of a surface layer and subplantation [24] of both carbon and nitrogen atoms. It results in a higher N/C ratio than in CAD films. The experiments on the low energy implantation of N+ ions in carbon [21,23,25] show a similar N/C ratio. We assume that the temperature of the substrate surface plays a crucial role in the formation of C:N films by PCAD. The difference in the presented results and those of Hartmann et al. [18] can be explained on this basis. The substrate temperature of about 373 K and the longer pulse duration in the latest case can reduce the absorption of volatile molecules and result in lower N/C ratio. With the P increase, carbon ions N2 lose their energy because of recombination of N mole2 cules. The lower energy of incident species reduces the probability of subplantation and a softer material is formed. The N/C ratio falls to a value of 0.3, which is reasonable for C N-like polymeric material. The increase 2 in film electric resistivity and transparency partially confirms this. 3.3. Raman analysis of C:N films Films prepared with increased pressure of nitrogen show Raman spectra mostly typical for amorphous graphite-like films. However, photoluminescence (PL) of films was found to exceed the Raman signal with nitrogen pressure increase, as shown in Fig. 4 and Table 1. In this table, the parameter I /I reflects the 1800 1570
Fig. 4. Raman spectra of C:N films prepared at a nitrogen pressure of 2 Pa (CN7), 7 Pa (CN8) and 15 Pa (CN9).
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Table 1 Results of the chemical and Raman analysis of the C:N films prepared by PCAD Sample #
U (V ) d
Deposition rate (nm/pulse)
P (Pa)
I
/I 1800 1570
CN1 CN2 CN3 CN4 CN5 CN6 CN7 CN8 CN9
300 200 300 400 200 300 400 400 400
0.1 0.014 0.053 0.085 0.01 0.028 0.055 0.02 0.011
0.19 0.56 0.56 0.56 2.0 2.0 2.0 7.0 15.0
0.19 0.38 0.26 0.29 0.80 0.52 0.35 * **
S /S D G
G-line position
N/C ratio
2.3 2.8 2.5 2.8 2.6 2.6 3.3
1562 1565 1565 1568 1580 1572 1572
0.28
0.52
0.65 0.33 0.28
* Strong luminescence, weak Raman. ** Strong luminescence.
increase of the PL signal, S /S is the ratio of areas D G under D and G lines. The analysis of Raman data shows a tendency for the photoluminescence signal to grow with an increase of nitrogen pressure and decrease of deposition rate. Koskinen et al. [10] did not observe luminescence of C:N films prepared at the P <0.56 Pa. This signal grows significantly only when N the2 nitrogen pressure is more than 2 Pa. The intensity of the PL signal has an approximate ratio 4:2:1 for C:N films prepared at 15, 7 and 2 Pa, respectively. The photoluminescence signal of films prepared at a nitrogen pressure of 7 and 15 Pa has a broad maximum centered at approximately 2.2 eV (570 nm). The excitation of Raman spectra with 514.5 and 488 nm wavelengths does not show visible changes in the maximum position and profile of the luminescence spectrum. However, the excitation with 488 nm gives a luminescence signal two orders of magnitude higher than that with 514.5 nm. All films show less stability to the incident laser beam than films without nitrogen. The luminescence of PCAD nitrogenated carbon films is similar to that observed for C:H films, but was not mentioned, to our knowledge, for C:N films with low hydrogen concentration. The hydrogen concentration in C:N films is not increased significantly with higher ambient pressure. Thus, we anticipate that growth of the luminescence signal is connected with the changes of the short-range order structure of films. The average cluster size is 0.5–0.7 nm in nitrogen-free PCAD carbon films [13] and grows insignificantly with increase of nitrogen pressure. We found a very weak cathodoluminescence signal with a maximum at 450–550 nm in the impurity-free carbon film, but no PL signal was found at the excitation with laser beam. That film was found to have more than 75% of sp3 sites. C:N films may have more non-uniform quasi-amorphous structure than impurity-free films with some C–N polymeric-like ordered areas and quite rigid highly disordered C–C matrix. This situation probably occurs at P >5–7 Pa. N2 The efficiency of photoluminescence strongly depends on the configuration and separation of sp2-bonded clusters because of the competition between radiative and
non-radiative processes and a different optical gap. The model of luminescence for C:H films proposed by Robertson [26 ] can be partially applied to the nitrogencontaining material. The similarity of PL in C:N and C:H films is in the cluster structure which is responsible for PL [27] but the origin of photoluminescence may not be the same. At the same time we do not exclude the influence of impurities.
4. Conclusion Filtered pulsed cathodic arc deposition of C:N films is possible in a wide range of pulse source operation parameters. The increase of nitrogen pressure, however, reduces the deposition rate and quality of films, which limits the application of given equipment. The maximum N/C ratio of 0.65 was found at a nitrogen pressure of 1–2 Pa. The pulsed character and energetic parameters of the carbon plasma flow as well as the substrate temperature seem to be responsible for such an amount of nitrogen in the films. The remarkable growth of the photoluminescence signal with a maximum at 2.2 eV was found to dominate in Raman spectra of films prepared at nitrogen pressures of more than 5 Pa. PCAD C:N films show photoluminescence very similar to that observed for C:H material. The changes in type of chemical bonding between C and N atoms are not significant in the studied range of gas pressures. We conclude that the appearance of photoluminescence signal is connected with changes of the local arrangement of C and N atoms in mostly sp2-bonded clustered network. The influence of a polymeric C N component and metal impurities on the x y photoluminescence of C:N films is also under consideration.
Acknowledgement The authors are thankful to Professor A. Badzian for help with the microprobe analysis and useful discussions
A. Stanishevsky et al. / Diamond and Related Materials 7 (1998) 1190–1195
on this work, and Dr. W. Otano for help with FTIR. One of us (A.S.) appreciates the partial support of this work by a CAST Grant from the National Research Council and by the University of Maryland, Materials Science Research and Engineering Center (MRSEC ).
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[12] D.G. McCulloch, A.R. Merchant, Thin Solid Films 290291 (1996) 99. [13] A. Stanishevsky, E. Tochitsky, J. CVD 4 (1996) 292. [14] A. Stanishevsky, L. Khriachtchev, Diamond Relat. Mater. 5 (1996) 1355. [15] M. Chhowalla, I. Alexandrou, C. Kiely, G.A.J. Amaratunga, R. Aharonov, R.F. Fontana, Thin Solid Films 290291 (1996) 103. [16 ] A.I. Maslov, G.K. Dmitriev, Yu.D. Chistyakov, Instrum. Exp. Technol. 28 (1985) 662. [17] M.Y. Chen, D. Li, X. Lin, V.P. Dravid, Y.-W. Chung, M.-S. Wong, W.D. Sproul, J. Vac. Sci. Technol. A 11 (1993) 521. [18] J. Hartmann, P. Siemroth, B. Schultrich, B. Rauschenbach, J. Vac. Sci. Technol. A 15 (1997) 2983. [19] V. Vorlicek, P. Siroky, J. Sobota, V. Perina, V. Zelezny, J. Hrdina, Diamond Relat. Mater. 5 (1996) 570. [20] R. Hauert, G. Spescha, B. Keller, F. Birchler, J. Mayer, E. Wintermantel, in: A. Feldman, Y. Tzeng, W.A. Yarbrough, M. Yoshikawa, M. Murakawa ( Eds.), Application of Diamond Films and Related Materials, Third International Conference, 1995, p. 749. [21] F.L. Freire Jr.,, C.A. Achete, G. Mariotto, Nucl. Instrum. Meth. Phys. Res. B 99 (1995) 606. [22] P. Hammer, M.A. Baker, C. Lenardi, W. Gissler, J. Vac. Sci. Technol. A 15 (1997) 107. [23] S.S. Todorov, D. Marton, K.J. Boyd, A.H. Al-Bayati, J.W. Rabalais, J. Vac. Sci. Technol. A 12 (1994) 3192. [24] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, Phys. Rev. Lett. 62 (1989) 1290. [25] P. Hammer, M.A. Baker, C. Lenardi, W. Gissler, Thin Solid Films 290291 (1996) 107. [26 ] J. Robertson, Phys. Rev. B 53 (1996) 16302. [27] Rusli, G.A.J. Amaratunga, S.R.P. Silva, Thin Solid Films 270 (1995) 160.
Deposition of carbon films containing nitrogen by filtered pulsed cathodic arc discharge method A. Stanishevsky a,*, L. Khriachtchev b, I˙. Akula c a University of Maryland, Institute for Plasma Research, College Park, MD 20742, USA b Laboratory of Physical Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland c Plasmoteg Engineering Center of the Belarus Academy of Sciences, Zhodinskaya St. 1/3, 220141 Minsk, Belarus Received 11 November 1997; accepted 3 March 1998
Abstract Nitrogenated carbon films with N/C ratio up to 0.65 have been prepared by a filtered pulsed cathodic arc discharge (PCAD) method at nitrogen pressure in the range 0.1–15 Pa. The influence of the process parameters and nitrogen pressure on the growth rate, chemical composition and quality of C:N films has been studied. The maximum N/C ratio was found at a gas pressure of 1–2 Pa. Raman study of films prepared at nitrogen pressure higher than 2 Pa showed a strong increase of the photoluminescence signal with a maximum at 2.2 eV. © 1998 Elsevier Science S.A. Keywords: Carbon nitride; Pulsed cathodic arc; Photoluminescence; Raman
1. Introduction The deposition of carbon nitride and its hydrogenated variants has been performed by various PVD as well as CVD processes [1–6 ]. Experimentally, most authors reported nitrogen-to-carbon ratios much lower than that in hypothetical C N , and results show that films are 3 4 predominantly amorphous and a rather graphite-like turbostratic structure is observed. There were several attempts to produce C:N films by continuous (CAD) [7–9] and pulsed (PCAD) [10,11] cathodic arc deposition. It is known that the cathodic arc is an efficient ionizer of the ambient gas, and nitrides can easily be formed. A number of experiments were done using this method in various modifications. For example, Merchant et al. [7] used an unfiltered cathodic arc with nitrogen pressures of 2.66 and 6.65 Pa to deposit carbon nitride films. Their results showed that the structure of films is largely graphite-like with both nitrogen and carbon atoms in the graphitic rings. Davis et al. [8] found that the cathodic arc plasma flow produces efficient ionization of the background gas, but their films with 15 at% of nitrogen were already 100% sp2-bonded. They also observed that carbon atoms are * Corresponding author. Fax: +1 301 314 9437; e-mail: [email protected] 0925-9635/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 09 2 5 -9 6 3 5 ( 9 8 ) 0 0 17 4 - 5
substituted by nitrogen into an sp2-bonded structure. Gilkes et al. [9] mentioned 70±10% of sp2 sites in the C:N films deposited by a filtered cathodic arc at nitrogen content about 10 at%. In spite of the number of experiments, N/C ratios of more than 0.54 were not observed. This limit is particularly explained by the formation of volatile C N compounds during the deposition [12]. x y In this work we describe results of the preparation of nitrogenated carbon films by pulsed filtered cathodic arc discharge (PCAD) deposition in the range of nitrogen pressures up to 15 Pa. The aims of this work were to estimate facilities of an industrial pulsed cathodic arc source to operate within the wide range of gas pressures and study the dependence of C:N film composition and properties on the deposition parameters.
2. Experimental Carbon films were deposited by a pulsed cathodic arc discharge method described elsewhere [13]. The deposition has been performed at the following discharge parameters: pulse duration 300 ms, pulse repetition time 0.5 s, peak discharge current 1–4 kA, starting discharge voltage 200–400 V. At these parameters the average deposition rate of impurity-free carbon films was in the range 8–24 nm/min. A curved 90° solenoid of 10 turns
A. Stanishevsky et al. / Diamond and Related Materials 7 (1998) 1190–1195
1191
Fig. 1. A sketch of the experimental set-up for the deposition of C:N films by PCAD method. The sizes are proportionally reduced from the real values.
with internal diameter of 120 mm was included in the anode discharge circuit to reduce the amount of graphite microparticles in the plasma flow. The discharge circuit was isolated from the ground. The pulsed arc source used was capable of operating with nitrogen pressures up to 15 Pa. A scheme of the experimental set-up is shown in Fig. 1. Polished Si 110 substrates were placed on the grounded holder at 100 mm distance from the solenoid output. The substrates were cleaned with an argon ion-beam source operating at 3 kV accelerating voltage and current density of 1 mA/cm2 during 10–15 min. The substrate temperature during the deposition was measured with a microthermoresistor calibrated in the range 270–350 K. In all experiments the substrate temperature did not exceed 308 K. The film thicknesses were 100–200 nm. C:N films were characterized using known procedures by transmission electron microscopy ( TEM; JEOL-100 and Phillips EM420), Fourier transform infrared (FTIR; BioRad FTS165), X-ray photoelectron ( XPS; ES2401) and secondary ion mass spectroscopy (SIMS; MS1210), electron microprobe (Cameca SK-50) analysis and Raman spectroscopy. A number of Raman spectroscopic measurements were carried out by using a 1 m Jarrell Ash double spectrometer (8 cm−1 resolution) equipped with a cooled low-noise photomultiplier tube. The excitation radiation
of an argon ion laser (typically 200 mW at 514.5 nm) was focused to a round 0.03 mm spot. No visible degradation of the coatings was observed with applied laser power up to 0.1 W [14]. However, micro-Raman studies (ISA U1000) with a laser spot diameter of 2–3 mm result in damage of the film at output power 25 mW.
3. Results and discussion 3.1. Filtered PCAD deposition of C:N films The experiments with 90° solenoid show the better performance of carbon films with less microparticle contamination. It was found that the specific electric resistivity and breakdown voltage of carbon films without any additives at the axis of deposition were 3.4×107 V cm and 1.67×107 V/m, while at distance 20 mm from the axis these parameters were 108 V cm and 6.57×107 V/m, respectively. Such non-uniformity of film properties on the substrate surface has been mentioned for carbon films prepared by PCAD with the straight solenoid in the discharge circuit [13]. It was explained by the influence of the higher film growth rate at the center of deposition. While the specific resistance of films was about the same as for those prepared with a straight solenoid, the breakdown voltage was two to
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d.c. bias voltage applied to the substrate. Besides, PCAD C:N films prepared under high pressures are softer than those without nitrogen by a factor of about three, as was shown by a scratch test method. However, the profile of scratches shows that these films are still quite hard. The brittle delamination of films from the substrate was observed even at P of 7 Pa. N2 3.2. Chemical composition of C:N films
Fig. 2. The influence of the nitrogen pressure on the C:N film deposition rate. The distribution of film thicknesses along the substrate is shown in the inset for two types of solenoid.
four times higher and we connect it with a lesser amount of graphitic microparticles. An increase in the nitrogen pressure results in strong reduction of the deposition rate as shown in Fig. 2. There were minor changes in the film uniformity on the substrate at different gas pressures. The reduction of the deposition rate with increase of nitrogen pressure (P ) N2 is opposite to the result observed by Chhowalla et al. [15]. We suggest it can be due to the difference in arc source design, which influences the cathodic arc behavior in the gas ambient. The same reduction in deposition rate was found by using an argon gas instead of nitrogen, which confirms this suggestion. Because of the very low deposition rate, the quantity of microparticles per square unit of film is increased in comparison even with the straight solenoid, but the particle size is smaller than 0.2 mm. For this reason most studies were carried out with C:N films prepared at the starting discharge voltage of 400 V. The average voltage during the pulse in this case is about 65 V and the average energy of carbon ions is about 70–80 eV [13,16 ]. TEM investigation has shown typical features for a turbostratic graphite-like structure in all ranges of nitrogen pressures used. Films prepared with increasing nitrogen pressure also showed reduction of the optical transparency and electric resistivity. At pressure higher than 7 Pa the transparency of C:N films was about 5–10% better than that of those prepared without nitrogen. It was measured by comparison of the Raman signal from the coated Si substrate. Film resistivity was found to be increased again, but reliable data were not received because of the large amount of microparticles. In a d.c. planar magnetron sputtering experiment, Chen et al. [17] also mentioned that at low nitrogen pressures (0.3–0.4 Pa), C:N films tend to have low resistivity (350–107 V cm), while at comparatively high nitrogen pressures (1.33 Pa), the films were quite resistive (>108 V cm). This trend appeared to be independent of the sputtering power and
The nitrogen concentration calculated from XPS and electron microprobe analysis data did not exceed 40 at%. The dependence of the nitrogen concentration on the pressure is shown in Fig. 3. It seems there is a maximum of the nitrogen concentration (N/C ratio 0.65) at 1–2 Pa. This is in some disagreement with Merchant et al. [7], who found a saturation level of 0.3 for sp2-bonded C:N material. Nevertheless, there is a tendency to have a maximum N/C ratio at 1–2 Pa which is similar to the results of other authors. Comparison of results shows that PCAD deposited C:N films have higher N/C ratio than those prepared by continuous arc. For example, Husein et al. [11] used a magnetically filtered pulsed cathodic arc with operating parameters as follows: arc current 200–250 A, pulse duration 1.5 ms, nitrogen pressure 1.3×10−2 Pa. They found very high N/C ratio of 0.286 at this pressure. Koskinen et al. [10] also used a pulsed cathodic arc source of similar design without focusing solenoid to deposit C:N films. They found nitrogen concentration increasing up to 35% at 0.56 Pa, while at 0.11 Pa it was 19%. A small Raman peak at 2220 cm−1 was mentioned for films with the highest nitrogen content. We found approximately the same amount of nitrogen at 0.56 Pa. This means that the presence of the curved solenoid does not much affect the chemical composition of films if other deposition
Fig. 3. The dependence of the nitrogen concentration on the gas pressure for PCAD C:N films (black squares) in comparison with published data ( Koskinen et al. [10], white circles; Chhowalla et al. [15], black circles; Davis et al. [8], white squares; Hartmann et al. [18], black triangles).
A. Stanishevsky et al. / Diamond and Related Materials 7 (1998) 1190–1195
conditions are the same. It should be mentioned that Hartmann et al. [18] found only 18 at% of nitrogen in PCAD C:N films deposited at 0.2 Pa. FTIR spectra of PCAD C:N films show in our case little absorption in the range 2100–2200 cm−1 and 1100–1600 cm−1 with a maximum at about 1200 cm−1. The difference between spectra of films prepared in the studied range of gas pressures was not significant. Merchant et al. [7] mentioned 2200 and 2325 cm−1 peaks in IR spectra attributed to the presence of a small amount of sp1-bonded nitrogen and carbon (CNN and CNC bonds) within the amorphous structure with N/C ratio 0.25 and 0.35. Vorlicek et al. [19] also found that the presence of nitrogen is reflected by the weak feature peaking at 2226 cm−1 in Raman spectra which was related to C–N triple bond stretching. Strong absorption in the 1200–1600 cm−1 region is presented if the N atoms are bonded into the carbon sp2 network. The absorption on C–N single bonds occurs at 1230 cm−1 and nC–N at 1272 cm−1 [20]. Unfortunately, broad and weak absorption in FTIR spectra does not allow us to define bonding in our films. PCAD C:N films show very little absorption on stretched C–H bonds around 2900 cm−1. This absorption is approximately the same for C:N films prepared at any nitrogen pressure used, even though the hydrogen content should grow with increase of P because of a possibility of a backstream N2 from an oil diffusion pump or desorbtion of H O vapor 2 of the chamber walls [15]. Even at the highest nitrogen pressures no significant amount of hydrogen was found by SIMS. There was no sign of absorption on N–H bonds at 3400 cm−1. XPS and microprobe analysis have shown that films prepared at the highest nitrogen pressures of 7–15 Pa contain impurities of oxygen up to 5 at%, Cu (0.1–0.2%) and Fe (<0.1%). This occurs because of some sputtering of the arc source and solenoid parts at the higher P , N2 and the film deposition rate is very low. At lower pressures the amount of metal impurities was below the sensitivity of the measuring technique. XPS spectra of C:N films show C1s electron binding energy level at 284.6 eV with a shoulder at 286.8 eV. The ratio of fits for the maxima at 286.8 and 284.6 eV corresponds roughly to the N/C ratio. We assign the peak at 286.8 eV to C–N single bonds [21]. The N1s energy level has maxima at 397.5, 398.9 (main), 399.3 and 400.3 eV. The peaks at 398.9 and 400.3 may correspond to N bonded to sp3- and sp2-hybridized carbon atoms, respectively, as proposed by Chhowalla et al. [15] and Hammer et al. [22]. However, the maxima at 397.5, 398.9 and 399.3 can also arise because of different local atomic arrangement in distorted sp2-bonded carbon–nitrogen clusters and the appearance of a polymeric component. The character of chemical bonding does not change much with the increase of P , but some modification of XPS N2 spectra occurs.
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What do the above results show? First, we consider PCAD C:N films practically hydrogen-free. Second, growth of films takes place at a substrate temperature of about 308 K and at specific conditions of the periodical plasma flow. Under these conditions the nitrogen and some volatile C N molecules can be absorbed at x y the film surface during the pause between pulses [23]. The next pulse produces partial sputtering of a surface layer and subplantation [24] of both carbon and nitrogen atoms. It results in a higher N/C ratio than in CAD films. The experiments on the low energy implantation of N+ ions in carbon [21,23,25] show a similar N/C ratio. We assume that the temperature of the substrate surface plays a crucial role in the formation of C:N films by PCAD. The difference in the presented results and those of Hartmann et al. [18] can be explained on this basis. The substrate temperature of about 373 K and the longer pulse duration in the latest case can reduce the absorption of volatile molecules and result in lower N/C ratio. With the P increase, carbon ions N2 lose their energy because of recombination of N mole2 cules. The lower energy of incident species reduces the probability of subplantation and a softer material is formed. The N/C ratio falls to a value of 0.3, which is reasonable for C N-like polymeric material. The increase 2 in film electric resistivity and transparency partially confirms this. 3.3. Raman analysis of C:N films Films prepared with increased pressure of nitrogen show Raman spectra mostly typical for amorphous graphite-like films. However, photoluminescence (PL) of films was found to exceed the Raman signal with nitrogen pressure increase, as shown in Fig. 4 and Table 1. In this table, the parameter I /I reflects the 1800 1570
Fig. 4. Raman spectra of C:N films prepared at a nitrogen pressure of 2 Pa (CN7), 7 Pa (CN8) and 15 Pa (CN9).
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Table 1 Results of the chemical and Raman analysis of the C:N films prepared by PCAD Sample #
U (V ) d
Deposition rate (nm/pulse)
P (Pa)
I
/I 1800 1570
CN1 CN2 CN3 CN4 CN5 CN6 CN7 CN8 CN9
300 200 300 400 200 300 400 400 400
0.1 0.014 0.053 0.085 0.01 0.028 0.055 0.02 0.011
0.19 0.56 0.56 0.56 2.0 2.0 2.0 7.0 15.0
0.19 0.38 0.26 0.29 0.80 0.52 0.35 * **
S /S D G
G-line position
N/C ratio
2.3 2.8 2.5 2.8 2.6 2.6 3.3
1562 1565 1565 1568 1580 1572 1572
0.28
0.52
0.65 0.33 0.28
* Strong luminescence, weak Raman. ** Strong luminescence.
increase of the PL signal, S /S is the ratio of areas D G under D and G lines. The analysis of Raman data shows a tendency for the photoluminescence signal to grow with an increase of nitrogen pressure and decrease of deposition rate. Koskinen et al. [10] did not observe luminescence of C:N films prepared at the P <0.56 Pa. This signal grows significantly only when N the2 nitrogen pressure is more than 2 Pa. The intensity of the PL signal has an approximate ratio 4:2:1 for C:N films prepared at 15, 7 and 2 Pa, respectively. The photoluminescence signal of films prepared at a nitrogen pressure of 7 and 15 Pa has a broad maximum centered at approximately 2.2 eV (570 nm). The excitation of Raman spectra with 514.5 and 488 nm wavelengths does not show visible changes in the maximum position and profile of the luminescence spectrum. However, the excitation with 488 nm gives a luminescence signal two orders of magnitude higher than that with 514.5 nm. All films show less stability to the incident laser beam than films without nitrogen. The luminescence of PCAD nitrogenated carbon films is similar to that observed for C:H films, but was not mentioned, to our knowledge, for C:N films with low hydrogen concentration. The hydrogen concentration in C:N films is not increased significantly with higher ambient pressure. Thus, we anticipate that growth of the luminescence signal is connected with the changes of the short-range order structure of films. The average cluster size is 0.5–0.7 nm in nitrogen-free PCAD carbon films [13] and grows insignificantly with increase of nitrogen pressure. We found a very weak cathodoluminescence signal with a maximum at 450–550 nm in the impurity-free carbon film, but no PL signal was found at the excitation with laser beam. That film was found to have more than 75% of sp3 sites. C:N films may have more non-uniform quasi-amorphous structure than impurity-free films with some C–N polymeric-like ordered areas and quite rigid highly disordered C–C matrix. This situation probably occurs at P >5–7 Pa. N2 The efficiency of photoluminescence strongly depends on the configuration and separation of sp2-bonded clusters because of the competition between radiative and
non-radiative processes and a different optical gap. The model of luminescence for C:H films proposed by Robertson [26 ] can be partially applied to the nitrogencontaining material. The similarity of PL in C:N and C:H films is in the cluster structure which is responsible for PL [27] but the origin of photoluminescence may not be the same. At the same time we do not exclude the influence of impurities.
4. Conclusion Filtered pulsed cathodic arc deposition of C:N films is possible in a wide range of pulse source operation parameters. The increase of nitrogen pressure, however, reduces the deposition rate and quality of films, which limits the application of given equipment. The maximum N/C ratio of 0.65 was found at a nitrogen pressure of 1–2 Pa. The pulsed character and energetic parameters of the carbon plasma flow as well as the substrate temperature seem to be responsible for such an amount of nitrogen in the films. The remarkable growth of the photoluminescence signal with a maximum at 2.2 eV was found to dominate in Raman spectra of films prepared at nitrogen pressures of more than 5 Pa. PCAD C:N films show photoluminescence very similar to that observed for C:H material. The changes in type of chemical bonding between C and N atoms are not significant in the studied range of gas pressures. We conclude that the appearance of photoluminescence signal is connected with changes of the local arrangement of C and N atoms in mostly sp2-bonded clustered network. The influence of a polymeric C N component and metal impurities on the x y photoluminescence of C:N films is also under consideration.
Acknowledgement The authors are thankful to Professor A. Badzian for help with the microprobe analysis and useful discussions
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on this work, and Dr. W. Otano for help with FTIR. One of us (A.S.) appreciates the partial support of this work by a CAST Grant from the National Research Council and by the University of Maryland, Materials Science Research and Engineering Center (MRSEC ).
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