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Hydrogen-free CVD diamond synthesis
Superlattices and Microstructures 40 (2006) 519–525 www.elsevier.com/locate/superlattices
Hydrogen-free CVD diamond synthesis Shinji Hiraga, Shouhei Shimada, Yoshiki Takagi ∗ , Kiyoshi Kuribayashi, Tsuyoshi Hayashi Teikyo University of Science and Technology, 2525 Yatsusawa, Uenohara-city, Yamanashi-pref, Japan Received 25 September 2006; accepted 27 September 2006 Available online 15 November 2006
Abstract A wide band gap semiconductor, diamond, has recently emerged as an important and promising material for a wide field of optoelectronic and electronic applications. In traditional CVD diamond synthesis, we thought that hydrogen radicals were inevitable. But, in this work, we are trying to synthesize diamond particles without hydrogen, in a process we call “hydrogen-free diamond synthesis”. Graphite rods were used as heaters and, at the same time, carbon sources. In argon or helium atmospheres, completely free from hydrogen, diamond particles were synthesized and confirmed with SEM photographs and Raman spectra for the first time. OES (Optical Emission Spectroscopy) results will be presented. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Diamond; CVD; Ar; He; Hydrogen; Graphite; OES
1. Introduction In traditional CVD diamond synthesis, hydrogen radicals were thought to be inevitable. In this work, we are trying to synthesize diamond particles without inflammable hydrogen gas for easier and safer experiments, a process we call “hydrogen-free diamond synthesis”. 2. Experimental Fig. 1 is a schematic figure of the experimental apparatus. The substrate was set perpendicularly to the graphite rod. The temperature of substrate was measured by a thermo-couple ∗ Corresponding author. Tel.: +81 554 63 4422x2542; fax: +81 554 63 4431.
E-mail address: [email protected] (Y. Takagi). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.030
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S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 1. Schematic figure of experimental apparatus.
Fig. 2. Model of CVD mechanism with hydrogen.
attached to the back surface of substrate. The temperature of the graphite rod was measured through the silica glass window on the chamber wall by a two-colored type pyrometer (modelIR-AQ 2CS, CHINO corporation Japan). With this apparatus, we confirmed diamond synthesis in a pure hydrogen atmosphere. In this work, the graphite rods were used as heaters and, at the same time, carbon sources. Fig. 2 showed model of the mechanism for diamond synthesis with hydrogen with present apparatus. With this model, hydrogen works in every important stages of diamond synthesis. The details of this model was explained in elsewhere [1]. We tried diamond synthesis with this apparatus under argon, free from hydrogen, atmosphere; the graphite rod temperature was 2350 ◦ C. And, next, we tried diamond synthesis with this apparatus under helium atmosphere; the graphite rod temperature was 2350 ◦ C.
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Fig. 3. SEM photograph (×15 000) of particles synthesized under argon.
Fig. 4. Raman spectrum of particles synthesized under argon.
3. Results and discussion Fig. 3 shows SEM photograph of particles synthesized in argon gas. About 1 µm diameter particles were confirmed as diamond with the Raman spectrum, Fig. 4. The peak was on 1333 cm−1 , and FWHM was 12.27 cm−1 . Next, Fig. 5 shows particles synthesized in helium gas. About 0.4–0.8 µm diameter particles were confirmed as diamond with the Raman spectrum, Fig. 6. The peak was on 1334 cm−1 , and FWHM was 9.55 cm−1 . 4. OES (optical emission spectroscopy) It is unclear whether hydrogen impurities are included in the gas cylinders. Thus, OES (Optical Emission Spectroscopy) was measured to identify gaseous species in the reaction. The graphite rod emitted strongly by thermal energy. The OES spectrum was observed and analyzed for the plasma CVD method. In the present work, we tried to identify gaseous species and to analyze the mechanism of the graphite rod heating method. Fig. 7 shows the OES spectra of plasma CVD and the graphite rod heating method. The OES result by the plasma CVD method was compared with the OES result by graphite rod heating method under argon, helium and hydrogen experiments, respectively. We searched for peaks of many important species for
522
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 5. SEM photograph (×15 000) of partices synthesized under helium.
Fig. 6. Raman spectrum of particles synthesized under helium.
diamond synthesis, such as Hα (656 nm), Hβ (486 nm), CH+ (397.3 nm, 444.4 nm, 479.4 nm), C2 (382.7 nm, 385.0 nm, 435.3 nm, 667.7 nm), C3 (379.3 nm, 387.8 nm, 403.0 nm, 404.8 nm), CI (493.3 nm) and C− 2 (486.5 nm) [2,3]. In Figs. 7–11 Hα (656 nm), C2 (385.0 nm) and C3 (404.8 nm) peaks were confirmed on all methods. But, the peak of CH was absent in argon and helium atmospheres. All peaks were summarized in Table 1. In Table 1, the abbreviation ‘sg’ means the strongest peak in the group (e.g. Hα peak in the hydrogen group), ‘vs’ means the very strong peak (e.g. CH+ peak in the hydro-carbon group), ‘ms’ means the medium strong peak (e.g. the C2 peak in the carbon group), ‘no’ means the not observed peak (e.g. the Hβ peak in the hydrogen group). 5. Conclusions Many researchers believed that hydrogen radicals were inevitable. But, in this work, we synthesized diamond particles without hydrogen, in argon or helium atmospheres. The existence of the Hα peak in OES results for argon and helium atmospheres suggests that hydrogen existed as an impurity in the argon and helium cylinders. On the other hand, CH peaks were all absent in argon and helium atmospheres. So, the very small amount of hydrogen did not get included in the diamond synthesis through CH species. C2 , C3 peaks were observed on all OES results. In a
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 7. OES spectra of plasma CVD and graphite rod heating method.
Fig. 8. Detailed OES spectrum of wavelength range 370–395 nm for Fig. 7.
Fig. 9. Detailed OES spectrum of wavelength range 395–420 nm for Fig. 7.
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Fig. 10. Detailed OES spectrum of wavelength range 470–495 nm for Fig. 7.
Fig. 11. Detailed OES spectrum of wavelength range 645–670 nm for Fig. 7.
Table 1 Summarized list of all OES results (sg, the strongest in the group; vs, very strong; ms, medium strong; no, not observed) Method
Atmosphere
H-group
CH-group
C-group
Hα
Hβ
Hγ
CH+
CH
C1
C2
C− 2
C3
Plasma CVD
H2
sg
vs
vs
sg
vs
vs
sg
vs
sg
Graphite rod heating
H2 Ar He
ms ms ms
no no no
no no no
vs no no
no no no
no no no
vs ms ms
vs no no
vs vs vs
word, with all these results we obtained through OES spectra, we can declare that the diamond has been synthesized directly from a graphite rod through vaporized carbon elements. In the present work, diamond was synthesized with a PVD (physical vapor deposition) method and not a CVD method by graphite rod heating in an inert gas atmosphere.
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
525
References [1] M. Uede, Y. Takagi, J. Mater. Res. 16 (11) (2001) 3069. [2] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, fourth ed., Chapman and Hall, London, 1976. [3] Website database for OES of NIST (National Institute of Standards and Technology, USA), Physical Laboratory. http://physics.nist.gov/cgi-bin/ASD/lines1.pl.
Hydrogen-free CVD diamond synthesis Shinji Hiraga, Shouhei Shimada, Yoshiki Takagi ∗ , Kiyoshi Kuribayashi, Tsuyoshi Hayashi Teikyo University of Science and Technology, 2525 Yatsusawa, Uenohara-city, Yamanashi-pref, Japan Received 25 September 2006; accepted 27 September 2006 Available online 15 November 2006
Abstract A wide band gap semiconductor, diamond, has recently emerged as an important and promising material for a wide field of optoelectronic and electronic applications. In traditional CVD diamond synthesis, we thought that hydrogen radicals were inevitable. But, in this work, we are trying to synthesize diamond particles without hydrogen, in a process we call “hydrogen-free diamond synthesis”. Graphite rods were used as heaters and, at the same time, carbon sources. In argon or helium atmospheres, completely free from hydrogen, diamond particles were synthesized and confirmed with SEM photographs and Raman spectra for the first time. OES (Optical Emission Spectroscopy) results will be presented. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Diamond; CVD; Ar; He; Hydrogen; Graphite; OES
1. Introduction In traditional CVD diamond synthesis, hydrogen radicals were thought to be inevitable. In this work, we are trying to synthesize diamond particles without inflammable hydrogen gas for easier and safer experiments, a process we call “hydrogen-free diamond synthesis”. 2. Experimental Fig. 1 is a schematic figure of the experimental apparatus. The substrate was set perpendicularly to the graphite rod. The temperature of substrate was measured by a thermo-couple ∗ Corresponding author. Tel.: +81 554 63 4422x2542; fax: +81 554 63 4431.
E-mail address: [email protected] (Y. Takagi). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.030
520
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 1. Schematic figure of experimental apparatus.
Fig. 2. Model of CVD mechanism with hydrogen.
attached to the back surface of substrate. The temperature of the graphite rod was measured through the silica glass window on the chamber wall by a two-colored type pyrometer (modelIR-AQ 2CS, CHINO corporation Japan). With this apparatus, we confirmed diamond synthesis in a pure hydrogen atmosphere. In this work, the graphite rods were used as heaters and, at the same time, carbon sources. Fig. 2 showed model of the mechanism for diamond synthesis with hydrogen with present apparatus. With this model, hydrogen works in every important stages of diamond synthesis. The details of this model was explained in elsewhere [1]. We tried diamond synthesis with this apparatus under argon, free from hydrogen, atmosphere; the graphite rod temperature was 2350 ◦ C. And, next, we tried diamond synthesis with this apparatus under helium atmosphere; the graphite rod temperature was 2350 ◦ C.
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
521
Fig. 3. SEM photograph (×15 000) of particles synthesized under argon.
Fig. 4. Raman spectrum of particles synthesized under argon.
3. Results and discussion Fig. 3 shows SEM photograph of particles synthesized in argon gas. About 1 µm diameter particles were confirmed as diamond with the Raman spectrum, Fig. 4. The peak was on 1333 cm−1 , and FWHM was 12.27 cm−1 . Next, Fig. 5 shows particles synthesized in helium gas. About 0.4–0.8 µm diameter particles were confirmed as diamond with the Raman spectrum, Fig. 6. The peak was on 1334 cm−1 , and FWHM was 9.55 cm−1 . 4. OES (optical emission spectroscopy) It is unclear whether hydrogen impurities are included in the gas cylinders. Thus, OES (Optical Emission Spectroscopy) was measured to identify gaseous species in the reaction. The graphite rod emitted strongly by thermal energy. The OES spectrum was observed and analyzed for the plasma CVD method. In the present work, we tried to identify gaseous species and to analyze the mechanism of the graphite rod heating method. Fig. 7 shows the OES spectra of plasma CVD and the graphite rod heating method. The OES result by the plasma CVD method was compared with the OES result by graphite rod heating method under argon, helium and hydrogen experiments, respectively. We searched for peaks of many important species for
522
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 5. SEM photograph (×15 000) of partices synthesized under helium.
Fig. 6. Raman spectrum of particles synthesized under helium.
diamond synthesis, such as Hα (656 nm), Hβ (486 nm), CH+ (397.3 nm, 444.4 nm, 479.4 nm), C2 (382.7 nm, 385.0 nm, 435.3 nm, 667.7 nm), C3 (379.3 nm, 387.8 nm, 403.0 nm, 404.8 nm), CI (493.3 nm) and C− 2 (486.5 nm) [2,3]. In Figs. 7–11 Hα (656 nm), C2 (385.0 nm) and C3 (404.8 nm) peaks were confirmed on all methods. But, the peak of CH was absent in argon and helium atmospheres. All peaks were summarized in Table 1. In Table 1, the abbreviation ‘sg’ means the strongest peak in the group (e.g. Hα peak in the hydrogen group), ‘vs’ means the very strong peak (e.g. CH+ peak in the hydro-carbon group), ‘ms’ means the medium strong peak (e.g. the C2 peak in the carbon group), ‘no’ means the not observed peak (e.g. the Hβ peak in the hydrogen group). 5. Conclusions Many researchers believed that hydrogen radicals were inevitable. But, in this work, we synthesized diamond particles without hydrogen, in argon or helium atmospheres. The existence of the Hα peak in OES results for argon and helium atmospheres suggests that hydrogen existed as an impurity in the argon and helium cylinders. On the other hand, CH peaks were all absent in argon and helium atmospheres. So, the very small amount of hydrogen did not get included in the diamond synthesis through CH species. C2 , C3 peaks were observed on all OES results. In a
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 7. OES spectra of plasma CVD and graphite rod heating method.
Fig. 8. Detailed OES spectrum of wavelength range 370–395 nm for Fig. 7.
Fig. 9. Detailed OES spectrum of wavelength range 395–420 nm for Fig. 7.
523
524
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
Fig. 10. Detailed OES spectrum of wavelength range 470–495 nm for Fig. 7.
Fig. 11. Detailed OES spectrum of wavelength range 645–670 nm for Fig. 7.
Table 1 Summarized list of all OES results (sg, the strongest in the group; vs, very strong; ms, medium strong; no, not observed) Method
Atmosphere
H-group
CH-group
C-group
Hα
Hβ
Hγ
CH+
CH
C1
C2
C− 2
C3
Plasma CVD
H2
sg
vs
vs
sg
vs
vs
sg
vs
sg
Graphite rod heating
H2 Ar He
ms ms ms
no no no
no no no
vs no no
no no no
no no no
vs ms ms
vs no no
vs vs vs
word, with all these results we obtained through OES spectra, we can declare that the diamond has been synthesized directly from a graphite rod through vaporized carbon elements. In the present work, diamond was synthesized with a PVD (physical vapor deposition) method and not a CVD method by graphite rod heating in an inert gas atmosphere.
S. Hiraga et al. / Superlattices and Microstructures 40 (2006) 519–525
525
References [1] M. Uede, Y. Takagi, J. Mater. Res. 16 (11) (2001) 3069. [2] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, fourth ed., Chapman and Hall, London, 1976. [3] Website database for OES of NIST (National Institute of Standards and Technology, USA), Physical Laboratory. http://physics.nist.gov/cgi-bin/ASD/lines1.pl.