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Binding state transformation in high temperature synthesized CN thin films
Diamond and Related Materials 8 (1999) 614–617
Binding state transformation in high temperature synthesized CN thin films Y.K. Yap *, S. Kida, T. Aoyama, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan Received 27 July 1998; accepted 17 October 1998
Abstract We have obtained tetrahedral carbon nitride (CN ) films by r.f. plasma pulsed laser deposition at 600 °C. As we increase the magnitude of the negative d.c. bias voltage of the Si substrate, predominant formation of tetrahedral CN bonds and suppression of graphite-like CN state are found. By means of such a bias voltage, CN films with adaptable fraction of graphite-like and tetrahedral CN bonds can be tailored according to requirement. All these films are stable to annealing at 800 °C. Likewise, annealing of CN films prepared at room temperature (RT ) revealed a novel transformation of CN binding state. The graphitelike CN bond is ruptured and converted into the aliphatic-like CN state. The tetrahedral CN bond contained inside the RT-prepared samples remained after annealing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: CN films; sp2 Bonding; sp3 Bonding; Thermal stability; Stress; Annealing; Ion-assisted deposition
1. Introduction Amorphous carbon (a-C ) films have been widely investigated in the past two decades for their electrical, chemical and mechanical properties. Their unique combination of properties appeared to be determined by the concentration of sp3-bonded tetrahedral carbon state contained inside the films. Recently, amorphous carbon nitride (CN ) have been recognized to have potential for hard coating [1] and electron field emission [2,3]. Like the a-C films, CN samples are readily prepared at room temperature (RT ), with the addition of nitrogen incorporated into the a-C matrix. The carbon matrix of CN films has a tendency toward graphitization at high temperature, with the incorporated nitrogen being rejected from the films [4,5]. In addition, CN bonds in these films are chemically diverse. The unmanageable binding state of CN films has restricted attempts to control their properties. In analogy with a-C films, the sp3-bonded tetrahedral CN state appeared to be desired. Thus improvement in thermal stability and ways to prepare the desired tetrahedral CN bond are important. We consider that a stable CN phase must be prepared * Corresponding author. Fax: +81 6 879 7708; e-mail: [email protected]
at high temperature. At high temperature, the bonding order of CN films is expected to be enhanced. Volatile binding states such nitrile (–CON ) and other interstitial species are restricted from incorporation. Thus, further structural reorganization can be avoided and thus a stable CN phase ensured. However, nitrogen incorporation of CN films is known to be difficult at high temperature [6 ]. Pure C-to-C bonds like those in a graphite ring are more stable than the polarized CN bonds. The rigid graphite rings prevent significant nitrogen incorporation. Thus, suppression of pure carbon phases such as graphite must be considered. In this work, we attempted to prepared thermally stable tetrahedral CN films at 600 °C. We demonstrate the transformation of the graphite carbon matrix of these films into tetrahedral CN bonds. Likewise, a novel transformation of RT-prepared samples after annealing is also observed. The mechanism involved will be discussed.
2. Experimental details We prepared carbon nitride thin films by r.f. (13.56 MHz) plasma pulsed laser deposition. We used a 10 Hz fifth harmonics generation ( FIHG) of Nd:YAG lasers (213 nm), generated by the CsLiB O crystals, 6 10 for ablation (see for example Ref. [7]). At similar laser
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 33 8 - 0
Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
density, this solid-state laser provides at least 5 times higher laser intensity than that of excimer lasers. We have previously described our deposition system [8]. Briefly, a relatively low laser energy density (0.8 J cm−2) with significantly high laser intensity (#0.27 GW cm−2) is applied to the graphite target, producing a reactive carbon plume at a lower ablation rate. A pure N r.f. plasma was generated at a pressure 2 of 5×10−3 mbar with a constant flow rate of 20 sccm. The r.f.-induced negative d.c. bias voltage is generated on the Si (100) substrate.
3. Results and discussion 3.1. Thermally stable tetrahedral CN films We deposited CN films at 600 °C at various bias voltages. The related CN binding state is found to be unique compared with the RT-prepared sample. This is indicated from Fourier transform infrared ( FTIR) spectroscopy measurements. For comparison, a typical IR signal of RT-prepared CN films is also shown in Fig. 1. Such an IR signal has a broad absorption band around 1000–1700 cm−1 [9,10]. This band is a combination of various absorption components centered around 1550, 1350 and 1212–1265 cm−1 which representing stretching modes of a pyridine/graphite-like CNN bond
Fig. 1. FTIR spectra of samples prepared at 600 °C at various bias voltage. Predominant formation of sp3-bonded tetrahedral CN phase is observed at a biasing from −120 to −150 V.
615
(G band), an aniline-like disordered C–N bond (D band) [9,10] and a symmetric tetrahedral CN bond [10,11], respectively. A weak shoulder of the FTIR band around 1120 cm−1 is assigned to the aliphatic-like CN bond, by reference to the stretching modes of aliphatic primary and secondary amines (CH –NH and 2 CH –NH–CH ) [10,12]. 2 2 For samples prepared at 600 °C, IR absorption resolved initially into two smaller bands located at 1550 and 1200 cm−1 (sample i), but later transformed into a single band centered at #1265 cm−1 (sample v). The G band seems to be suppressed, together with the enhancement of the tetrahedral CN band. Predominant formation of sp3 C–N bonds at a bias voltage from −120 to −150 V is indicated. A blue shift of the tetrahedral band from 1200 to 1265 cm−1 is observed, which indicates a modification of the binding geometry. This is likely to be induced by the increase of internal film stress arising from the increase of tetrahedral bond density or ion bombardment, as in many cases of cubic boron nitride (c-BN ) films [13]. The nitrogen contents of these samples are measured by X-ray photoelectron spectroscopy ( XPS). As shown in Fig. 2, nitrogen contents of these samples are constant at #4.5 at.% at a biasing up to −60 V. At −100 V, the nitrogen content rose significantly to 15.5 at.% and reached a maximum of 20.2 at.% at a biasing of −120 V. This maximum was maintained up to −135 V but decreased to 16.7 at.% at −150 V. A threshold of nitrogen incorporation occurred at a biasing between −60 and −100 V. In fact, this threshold coincided with a marked decrease in deposition rate, due to the annihilation of graphite matrix as detected by Raman spectroscopy [14]. Briefly, a further increase in bias voltage transformed the residual carbon matrix into the sp3 hybridization. Incorporation of nitrogen under this condition eventually formed a tetrahedral CN film as indicated from the FTIR spectra. Three of these samples were subjected to annealing at 800 °C in vacuum for 1 h. As shown in Fig. 2, these samples are rather stable compared with most
Fig. 2. Dependence of nitrogen content in CN films on the bias voltage at 600 °C. The nitrogen content after annealing are also indicated.
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Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
RT-prepared films [4,5]. Loss of nitrogen content is quite constant at #2.5 at.%, which we believe to be initiated from some weakly bonded sites near the surface. As shown in Fig. 3(a), the as-produced CN binding states at 600 °C remained stable after annealing, even if prepared without biasing (sample i). The FTIR signals of the tetrahedral CN films (sample v) before and after annealing are shown in Fig. 3(b). The tetrahedral band of the annealed sample returned briefly to 1220 cm−1 with a decrease of absorption bandwidth. This may have signified the release of film’s stress by annealing. In summary, a thermally stable CN phase predominated with tetrahedral CN bond was prepared. Various fraction of sp2- and sp3-bonded CN binding states can be tailored, between that of sample i and v. All samples prepared at 600 °C are stable up to at least 800 °C. 3.2. Annealing of RT-prepared CN films Next we discuss the transformation of the CN binding state initiated by the annealing of RT-prepared samples. Fig. 4 shows the nitrogen content for these samples
(a)
Fig. 4. Dependence of nitrogen content of RT-prepared CN films on bias voltage. The nitrogen content after annealing are also indicated.
deposited at various bias voltages, as measured by XPS. Three of these samples were selected for annealing as in the previous case. The nitrogen contents of these films after annealing are indicated in the same figure. About 50–65% of the initial nitrogen content is dissociated from these samples during annealing, regardless of the biasing condition. The related CN bonds were investigated by using the FTIR spectroscopy. All these films have a typical FTIR signal before annealing. Transformation of the CN binding state of these samples occurred in similar manner. The related FTIR signal is shown in Fig. 5. The positions representing graphite-like, tetrahedral and aliphatic-like CN binding states are indicated as i, ii and iii, respectively. As shown, a reduction in absorption at position i is observed after annealing, while the absorption at position ii remained. An extra absorption at position iii is initiated by annealing. Such a transformation is unique and has never been observed before. We consider that annealing of these films results in structural reorganization of the related carbon matrix. The graphite-like CN bond is the most unstable state compared with the graphite ring. During annealing,
(b)
Fig. 3. Comparison of FTIR spectra of CN films before and after annealing at 800 °C, as prepared at 600 °C with (a) zero biasing and (b) a biasing of −150 V.
Fig. 5. Typical FTIR spectra of samples prepared at RT before and after annealing at 800 °C. Absorption at positions i, ii and iii is initiated by graphite-like, tetrahedral and aliphatic-like CN bonds, respectively.
Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
rearrangement of carbon atoms into a thermodynamically stable graphite ring forced the nitrogen atoms out of this CN structure [5]. In some cases, a graphite-like CN ring could be broken into an sp2-bonded aliphaticlike CN structure. This explained the reduction in the IR band intensity at position i and the development of a clear IR absorption at position iii. The tetrahedral bond seems to be robust with respect to annealing. This is verified by the conservation of the IR band at position ii. In summary, the graphite-like CN bonds of RT-prepared films have a tendency to graphitize or to resolve into an aliphatic-like CN structure. The annealed samples are expected to be thermally stable and consist mainly of tetrahedral and aliphatic-like CN binding sates.
4. Conclusion In conclusion, we found some novel transformation of binding states. The graphite matrix of CN films was found to be transformed into the sp3-bonded tetrahedral CN state at 600 °C, with the suppression of graphitelike CN bond formation. CN films with a different fraction of graphite-like and tetrahedral CN bonds can be tailored by means of the bias voltage. Annealing was shown to release the internal stress of the tetrahedral CN films, similar to the behavior of c-BN films. Likewise, annealing of RT-prepared samples ruptured
617
the graphite-like CN bond, which was either graphitized of converted to the aliphatic-like CN binding state. The tetrahedral phase of the RT-prepared samples was conserved after annealing.
References [1] S. Neuville, A. Matthews, MRS Bull. 22 (9) (1997) 22. [2] S.R.P. Silva, G.A.J. Amaratunga, J.R. Barnes, Appl. Phys. Lett. 71 (1997) 1477. [3] E.J. Chi, J.Y. Shim, H.K. Baik, S.M. Lee, Appl. Phys. Lett. 71 (1997) 324. [4] A. Hoffman, I. Gouzman, R. Brener, Appl. Phys. Lett. 64 (1994) 845. [5] D.G. McCulloch, A.R. Merchant, Thin Solid Films 290291 (1996) 99. [6 ] M. Tabbal, P. Merel, S. Moisa, M. Chaker, A. Ricard, M. Moisan, Appl. Phys. Lett. 69 (1996) 1698. [7] Y.K. Yap, K. Deki, N. Kitatochi, Y. Mori, T. Sasaki, Opt. Lett. 23 (1998) 1016 and references therein [8] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Super carbon, in: S. Fujiwara et al. ( Eds.), Proc. Symp. I, IUMRS-ICA-97, MYU, Tokyo, 1998, p. 101. [9] X.-A. Zhao, C.W. Ong, Y.C. Tsang, T.W. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 (1996) 2652 and references therein [10] Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. [11] M.R. Wixom, J. Am. Ceram. Soc. 73 (1990) 1973. [12] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 37 (1998) L746. [13] D. Litvinov, R. Clarke, Appl. Phys. Lett. 71 (1997) 1969 and Ref. [11] therein [14] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915.
Binding state transformation in high temperature synthesized CN thin films Y.K. Yap *, S. Kida, T. Aoyama, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan Received 27 July 1998; accepted 17 October 1998
Abstract We have obtained tetrahedral carbon nitride (CN ) films by r.f. plasma pulsed laser deposition at 600 °C. As we increase the magnitude of the negative d.c. bias voltage of the Si substrate, predominant formation of tetrahedral CN bonds and suppression of graphite-like CN state are found. By means of such a bias voltage, CN films with adaptable fraction of graphite-like and tetrahedral CN bonds can be tailored according to requirement. All these films are stable to annealing at 800 °C. Likewise, annealing of CN films prepared at room temperature (RT ) revealed a novel transformation of CN binding state. The graphitelike CN bond is ruptured and converted into the aliphatic-like CN state. The tetrahedral CN bond contained inside the RT-prepared samples remained after annealing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: CN films; sp2 Bonding; sp3 Bonding; Thermal stability; Stress; Annealing; Ion-assisted deposition
1. Introduction Amorphous carbon (a-C ) films have been widely investigated in the past two decades for their electrical, chemical and mechanical properties. Their unique combination of properties appeared to be determined by the concentration of sp3-bonded tetrahedral carbon state contained inside the films. Recently, amorphous carbon nitride (CN ) have been recognized to have potential for hard coating [1] and electron field emission [2,3]. Like the a-C films, CN samples are readily prepared at room temperature (RT ), with the addition of nitrogen incorporated into the a-C matrix. The carbon matrix of CN films has a tendency toward graphitization at high temperature, with the incorporated nitrogen being rejected from the films [4,5]. In addition, CN bonds in these films are chemically diverse. The unmanageable binding state of CN films has restricted attempts to control their properties. In analogy with a-C films, the sp3-bonded tetrahedral CN state appeared to be desired. Thus improvement in thermal stability and ways to prepare the desired tetrahedral CN bond are important. We consider that a stable CN phase must be prepared * Corresponding author. Fax: +81 6 879 7708; e-mail: [email protected]
at high temperature. At high temperature, the bonding order of CN films is expected to be enhanced. Volatile binding states such nitrile (–CON ) and other interstitial species are restricted from incorporation. Thus, further structural reorganization can be avoided and thus a stable CN phase ensured. However, nitrogen incorporation of CN films is known to be difficult at high temperature [6 ]. Pure C-to-C bonds like those in a graphite ring are more stable than the polarized CN bonds. The rigid graphite rings prevent significant nitrogen incorporation. Thus, suppression of pure carbon phases such as graphite must be considered. In this work, we attempted to prepared thermally stable tetrahedral CN films at 600 °C. We demonstrate the transformation of the graphite carbon matrix of these films into tetrahedral CN bonds. Likewise, a novel transformation of RT-prepared samples after annealing is also observed. The mechanism involved will be discussed.
2. Experimental details We prepared carbon nitride thin films by r.f. (13.56 MHz) plasma pulsed laser deposition. We used a 10 Hz fifth harmonics generation ( FIHG) of Nd:YAG lasers (213 nm), generated by the CsLiB O crystals, 6 10 for ablation (see for example Ref. [7]). At similar laser
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 33 8 - 0
Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
density, this solid-state laser provides at least 5 times higher laser intensity than that of excimer lasers. We have previously described our deposition system [8]. Briefly, a relatively low laser energy density (0.8 J cm−2) with significantly high laser intensity (#0.27 GW cm−2) is applied to the graphite target, producing a reactive carbon plume at a lower ablation rate. A pure N r.f. plasma was generated at a pressure 2 of 5×10−3 mbar with a constant flow rate of 20 sccm. The r.f.-induced negative d.c. bias voltage is generated on the Si (100) substrate.
3. Results and discussion 3.1. Thermally stable tetrahedral CN films We deposited CN films at 600 °C at various bias voltages. The related CN binding state is found to be unique compared with the RT-prepared sample. This is indicated from Fourier transform infrared ( FTIR) spectroscopy measurements. For comparison, a typical IR signal of RT-prepared CN films is also shown in Fig. 1. Such an IR signal has a broad absorption band around 1000–1700 cm−1 [9,10]. This band is a combination of various absorption components centered around 1550, 1350 and 1212–1265 cm−1 which representing stretching modes of a pyridine/graphite-like CNN bond
Fig. 1. FTIR spectra of samples prepared at 600 °C at various bias voltage. Predominant formation of sp3-bonded tetrahedral CN phase is observed at a biasing from −120 to −150 V.
615
(G band), an aniline-like disordered C–N bond (D band) [9,10] and a symmetric tetrahedral CN bond [10,11], respectively. A weak shoulder of the FTIR band around 1120 cm−1 is assigned to the aliphatic-like CN bond, by reference to the stretching modes of aliphatic primary and secondary amines (CH –NH and 2 CH –NH–CH ) [10,12]. 2 2 For samples prepared at 600 °C, IR absorption resolved initially into two smaller bands located at 1550 and 1200 cm−1 (sample i), but later transformed into a single band centered at #1265 cm−1 (sample v). The G band seems to be suppressed, together with the enhancement of the tetrahedral CN band. Predominant formation of sp3 C–N bonds at a bias voltage from −120 to −150 V is indicated. A blue shift of the tetrahedral band from 1200 to 1265 cm−1 is observed, which indicates a modification of the binding geometry. This is likely to be induced by the increase of internal film stress arising from the increase of tetrahedral bond density or ion bombardment, as in many cases of cubic boron nitride (c-BN ) films [13]. The nitrogen contents of these samples are measured by X-ray photoelectron spectroscopy ( XPS). As shown in Fig. 2, nitrogen contents of these samples are constant at #4.5 at.% at a biasing up to −60 V. At −100 V, the nitrogen content rose significantly to 15.5 at.% and reached a maximum of 20.2 at.% at a biasing of −120 V. This maximum was maintained up to −135 V but decreased to 16.7 at.% at −150 V. A threshold of nitrogen incorporation occurred at a biasing between −60 and −100 V. In fact, this threshold coincided with a marked decrease in deposition rate, due to the annihilation of graphite matrix as detected by Raman spectroscopy [14]. Briefly, a further increase in bias voltage transformed the residual carbon matrix into the sp3 hybridization. Incorporation of nitrogen under this condition eventually formed a tetrahedral CN film as indicated from the FTIR spectra. Three of these samples were subjected to annealing at 800 °C in vacuum for 1 h. As shown in Fig. 2, these samples are rather stable compared with most
Fig. 2. Dependence of nitrogen content in CN films on the bias voltage at 600 °C. The nitrogen content after annealing are also indicated.
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Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
RT-prepared films [4,5]. Loss of nitrogen content is quite constant at #2.5 at.%, which we believe to be initiated from some weakly bonded sites near the surface. As shown in Fig. 3(a), the as-produced CN binding states at 600 °C remained stable after annealing, even if prepared without biasing (sample i). The FTIR signals of the tetrahedral CN films (sample v) before and after annealing are shown in Fig. 3(b). The tetrahedral band of the annealed sample returned briefly to 1220 cm−1 with a decrease of absorption bandwidth. This may have signified the release of film’s stress by annealing. In summary, a thermally stable CN phase predominated with tetrahedral CN bond was prepared. Various fraction of sp2- and sp3-bonded CN binding states can be tailored, between that of sample i and v. All samples prepared at 600 °C are stable up to at least 800 °C. 3.2. Annealing of RT-prepared CN films Next we discuss the transformation of the CN binding state initiated by the annealing of RT-prepared samples. Fig. 4 shows the nitrogen content for these samples
(a)
Fig. 4. Dependence of nitrogen content of RT-prepared CN films on bias voltage. The nitrogen content after annealing are also indicated.
deposited at various bias voltages, as measured by XPS. Three of these samples were selected for annealing as in the previous case. The nitrogen contents of these films after annealing are indicated in the same figure. About 50–65% of the initial nitrogen content is dissociated from these samples during annealing, regardless of the biasing condition. The related CN bonds were investigated by using the FTIR spectroscopy. All these films have a typical FTIR signal before annealing. Transformation of the CN binding state of these samples occurred in similar manner. The related FTIR signal is shown in Fig. 5. The positions representing graphite-like, tetrahedral and aliphatic-like CN binding states are indicated as i, ii and iii, respectively. As shown, a reduction in absorption at position i is observed after annealing, while the absorption at position ii remained. An extra absorption at position iii is initiated by annealing. Such a transformation is unique and has never been observed before. We consider that annealing of these films results in structural reorganization of the related carbon matrix. The graphite-like CN bond is the most unstable state compared with the graphite ring. During annealing,
(b)
Fig. 3. Comparison of FTIR spectra of CN films before and after annealing at 800 °C, as prepared at 600 °C with (a) zero biasing and (b) a biasing of −150 V.
Fig. 5. Typical FTIR spectra of samples prepared at RT before and after annealing at 800 °C. Absorption at positions i, ii and iii is initiated by graphite-like, tetrahedral and aliphatic-like CN bonds, respectively.
Y.K. Yap et al. / Diamond and Related Materials 8 (1999) 614–617
rearrangement of carbon atoms into a thermodynamically stable graphite ring forced the nitrogen atoms out of this CN structure [5]. In some cases, a graphite-like CN ring could be broken into an sp2-bonded aliphaticlike CN structure. This explained the reduction in the IR band intensity at position i and the development of a clear IR absorption at position iii. The tetrahedral bond seems to be robust with respect to annealing. This is verified by the conservation of the IR band at position ii. In summary, the graphite-like CN bonds of RT-prepared films have a tendency to graphitize or to resolve into an aliphatic-like CN structure. The annealed samples are expected to be thermally stable and consist mainly of tetrahedral and aliphatic-like CN binding sates.
4. Conclusion In conclusion, we found some novel transformation of binding states. The graphite matrix of CN films was found to be transformed into the sp3-bonded tetrahedral CN state at 600 °C, with the suppression of graphitelike CN bond formation. CN films with a different fraction of graphite-like and tetrahedral CN bonds can be tailored by means of the bias voltage. Annealing was shown to release the internal stress of the tetrahedral CN films, similar to the behavior of c-BN films. Likewise, annealing of RT-prepared samples ruptured
617
the graphite-like CN bond, which was either graphitized of converted to the aliphatic-like CN binding state. The tetrahedral phase of the RT-prepared samples was conserved after annealing.
References [1] S. Neuville, A. Matthews, MRS Bull. 22 (9) (1997) 22. [2] S.R.P. Silva, G.A.J. Amaratunga, J.R. Barnes, Appl. Phys. Lett. 71 (1997) 1477. [3] E.J. Chi, J.Y. Shim, H.K. Baik, S.M. Lee, Appl. Phys. Lett. 71 (1997) 324. [4] A. Hoffman, I. Gouzman, R. Brener, Appl. Phys. Lett. 64 (1994) 845. [5] D.G. McCulloch, A.R. Merchant, Thin Solid Films 290291 (1996) 99. [6 ] M. Tabbal, P. Merel, S. Moisa, M. Chaker, A. Ricard, M. Moisan, Appl. Phys. Lett. 69 (1996) 1698. [7] Y.K. Yap, K. Deki, N. Kitatochi, Y. Mori, T. Sasaki, Opt. Lett. 23 (1998) 1016 and references therein [8] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Super carbon, in: S. Fujiwara et al. ( Eds.), Proc. Symp. I, IUMRS-ICA-97, MYU, Tokyo, 1998, p. 101. [9] X.-A. Zhao, C.W. Ong, Y.C. Tsang, T.W. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 (1996) 2652 and references therein [10] Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. [11] M.R. Wixom, J. Am. Ceram. Soc. 73 (1990) 1973. [12] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 37 (1998) L746. [13] D. Litvinov, R. Clarke, Appl. Phys. Lett. 71 (1997) 1969 and Ref. [11] therein [14] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915.