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H2 ambient
Thin Solid Films 473 (2005) 24 – 30 www.elsevier.com/locate/tsf
Growth of nanocrystalline diamond films in CCl4/H2 ambient Jih-Jen Wua,*, Chen-Hao Kua, Te-Chi Wonga, Chien-Ting Wub, Kuei-Hsien Chenb, Li-Chyong Chenc a
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan c Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
Received 28 October 2003; received in revised form 3 May 2004; accepted 28 June 2004 Available online 10 August 2004
Abstract We investigated the growth characteristics of the nanocrystalline diamond films using CCl4/H2 as gas sources in a hot-filament chemical vapor deposition (CVD) reactor. Successful growth of nanocrystalline diamond at typical growth condition of 1.5–2.5% CCl4 and 550–730 8C substrate temperature has been demonstrated. Glancing angle X-ray diffraction (XRD) clearly indicated the formation of diamond in the films. Typical root-mean-square surface roughness of 10–15 nm and an optimal root-mean-square surface roughness of 6 nm have been achieved. Transmission electron microscopy (TEM) analyses indicated that nanocrystalline diamond film with an average grain size in the range of 10–20 nm was deposited from 2.5% CCl4/H2 at 610 8C. Effects of different source gas composition and substrate temperature on the grain nucleation and grain growth processes, whereby the grain size of the nanocrystalline film could be controlled, were discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond; Chlorine; Chemical vapor deposition; Nanostructures
1. Introduction Owing to their tremendous industrial applications, diamond films have become one of the most extensively investigated materials since 1980s [1]. Polycrystalline diamond films have been successfully synthesized from hydrocarbon–hydrogen gas mixtures using chemical vapor deposition (CVD) methods [1,2]. The highly rough surface of the polycrystalline diamond films with grain sizes of the order of microns, however, is the main restriction of the diamond films on most optical and tribological applications [3]. Nanocrystalline diamond films, which possess smooth surface and high sp3 content, have therefore been recognized as the promising candidates for those applications [4]. Nanocrystalline diamond film growths using CH4/Ar or C60/ Ar plasma have been demonstrated, which exhibit a
* Corresponding author. Tel.: +886 62757575x62694; fax: +886 62344496. E-mail address: [email protected] (J.-J. Wu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.152
nanocrystalline structure with a size distribution around 3– 5 nm and a typical rms surface roughness of 10–20 nm [3– 7]. The sp2 phase content in the nanocrystalline diamond films can be as low as 2–5% at grain boundaries, which is correlated with the high hardness (up to 88 GPa) of the film [3]. Most nanocrystalline diamond films were deposited in conventional methane–hydrogen plasma by increasing the primary nucleation density on the substrates or increasing the concentration of methane in the source gas [8–11]. It is shown that the grain size of the nanocrystalline diamond film can be varied ranging from 4 nm to a few hundreds of nanometers via the methane fraction in the source gas and the substrate pretreatment. Optical transmittance as high as 80–84% in the near-IR and surface roughness as low as 8 nm have been achieved by this technique [10]. Since that high nucleation density is a key for nanocrystalline diamond growth, we proposed to grow nanocrystalline diamond using CCl4/H2 as the source gas in a hot-filament CVD reactor. This is based on our experience that chloromethane reactants possess potential in facilitating diamond film growth at low temperature [12–15]. In
J.-J. Wu et al. / Thin Solid Films 473 (2005) 24–30 Table 1 Typical film deposition conditions in this study Filament temperature Total pressure Total flow rate CCl4 concentration Substrate temperature Distance between substrate and filament Deposition time
2000 8C 667 Pa 100 sccm 1.5–2.5% 550–730 8C 5–6 mm 1h
comparison with methane, chloromethane enhances the diamond film growth rate at low temperature and low pressure. Atomic Cl produced in the gas phase enhances diamond growth rate via increasing both the concentrations of carbon radicals in the gas and the active sites on the surface. Moreover, chloromethane facilitates the low temperature growth mainly by increasing the growth rate of nuclei and by protecting the residual seeds from being etched by H atoms [15]. Therefore, CCl4 is employed in this work to enhance the concentrations of carbon radicals and to protect the nuclei from being etched during the growth of nanocrystalline diamond.
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evaluate the diamond film structure. The microstructure of the diamond films was studied using transmission electron microscopy (TEM, JEOL, JEM-4000EX).
3. Results The SEM images of the diamond deposits grown for 1 h at a substrate temperature of 550 8C and a distance between filament and substrate of 6 mm using 2.5% CCl4 and 2.5% CH4 are shown in Fig. 1a and b, respectively. An almost continuous film with grain sizes of several tens of nanometer was deposited by 2.5% CCl4 while sporadic deposition was formed using 2.5% CH4. Fig. 1c shows that a continuous film still cannot be formed even when the CH4 concentration was increased to 6.5%, revealing that CCl4 facilitates the formation of the nanocrystalline diamond films in comparison with CH4.
2. Experimental details The deposition of diamond films was carried out in a hotfilament CVD reactor. The hot-filament CVD system consists of a process chamber equipped with a tungsten filament and a substrate stage that were employed for activation of gas-phase reactions and for the independent control of the substrate temperature, respectively. A new filament was always precarburized in 2.0% CH4/H2 for 4 h before its first deposition run. The temperature of W filament was measured by an optical pyrometer through a window of the chamber. A thermocouple placed beside the substrate was used to monitor the substrate temperature during film growth. A mixture of CCl4/H2 was used as the reactant gas. To achieve high nucleation densities of diamond films, the Si (100) substrates were pretreated for 30 min in an ultrasonic bath of toluene with 10 Am diamond powder suspension after standard chemical cleaning with acetone and deionized water. The deposition conditions are listed in Table 1. Film thickness and surface morphology studies were conducted by scanning electron microscopy (SEM, Hitachi, S-4200). The surface roughness of the diamond film was measured using atomic force microscopy (AFM, DI, Ns3a-MMAFM). A silicon tip was employed to perform the tapping modek measurement. The tip velocity, the integral gain, the proportional gain, the amplitude set point, the drive frequency and the drive amplitude for the tapping mode operations are 5 Am s 1, 0.8, 0.8, 0.8 V, 78 kHz, and 80 mV, respectively. Raman spectroscopy (Renishaw system 2000 mirco-Raman spectrometer with a 514.5 nm Ar+ laser) and glancing incident angle X-ray diffraction (XRD, Rigaku, D/MAX2500) were employed to
Fig. 1. SEM images of the diamond films deposited for 1 h at a substrate temperature of 550 8C and a distance of filament to substrate of 6 mm using (a) 2.5% CCl4, (b) 2.5% CH4 and (c) 6.5% CH4, respectively.
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The following studies were conducted at a distance between filament and substrate of 5 mm to synthesize the diamond films for investigation of the growth behaviors of the nanocrystalline diamond films using CCl4. Fig. 2a–f shows the SEM images of the diamond films deposited using three CCl4 concentrations and two different substrate temperatures, 610 and 730 8C, for 1 h. They reveal that the grain sizes of the films grown at 610 8C are reduced at higher CCl4 concentrations. Moreover, in comparison with that grown at 610 8C, diamond film with smaller grain sizes was deposited at 730 8C under the lower CCl4 concentration. The surface roughness of the films measured by AFM show significant reduction of root-mean-square (rms) roughness from 26 to 6 nm as the CCl4 concentration is increased from 1.5% to 2.0% at a temperature of 610 8C. Their AFM images are illustrated in Fig. 3a and b, respectively. The surfaces of the other films deposited using CCl4/H2 in the present work possess an rms roughness in the range of 10–15 nm.
Glancing angle XRD was employed to examine the structures of the films. As shown in Fig. 4, (111) and (220) diamond diffraction peaks appear in the spectra (a)–(e). A (111) diffraction peak is present ambiguously in the spectrum of the film deposited at 730 8C using 2.5% CCl4. Grain sizes of the diamond films estimated from the full width at half maximum (FWHM) of the (111) diffraction peak by the Scherrer formula [16] are shown in Table 2. Results confirm the trends that the grain sizes of the films grown at 610 8C decrease with increasing CCl4 concentration and that diamond film with smaller grain sizes is deposited at 730 8C using 1.5% CCl 4 in comparison with that grown at 610 8C. Therefore, the growth of diamond films with smaller grain size can be achieved by adjusting the CCl4 concentration and substrate temperature. The Raman spectra of the films are shown in Fig. 5. The spectrum of the film deposited from 1.5% CCl4 and 610 8C shows a Raman characteristic band of diamond structure at 1332 cm 1 and a broad Raman scattering in the
Fig. 2. SEM images of the diamond films deposited at various conditions: (a) 1.5% CCl4, 610 8C, (b) 2.0% CCl4, 610 8C, (c) 2.5% CCl4, 610 8C, (d) 1.5% CCl4, 730 8C, (e) 2.0% CCl4, 730 8C and (f) 2.5% CCl4, 730 8C.
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Table 2 The FWHM of the diffraction peak (111) and the grain sizes of the diamond films deposited at various conditions Deposition conditions 2.0% 1.5% 2.5% 2.0% 1.5%
Fig. 3. AFM images of the films deposited at 610 8C using (a) 1.5% and (b) 2.0% CCl4.
range of 1400–1600 cm 1 corresponding to sp2 phase carbon in the grain boundaries. Since the grain size of the film deposited from 1.5% CCl4 is already in the order of
Fig. 4. (a) Glancing angle XRD spectra of the diamond films deposited at various conditions: (I) 1.5% CCl4, 610 8C, (II) 2.0% CCl4, 610 8C, (III) 2.5% CCl4, 610 8C, (IV) 1.5% CCl4, 730 8C, (V) 2.0% CCl4, 730 8C and (VI) 2.5% CCl4, 730 8C. (b) The grain sizes of the diamond films estimated from the FWHM of the diffraction peak (111) in (a) by the Scherrer formula.
CCl4, CCl4, CCl4, CCl4, CCl4,
730 730 610 610 610
8C 8C 8C 8C 8C
FWHM (deg.)
Grain size (nm)
0.50666 0.54146 0.54602 0.51034 0.41885
18.8 17.6 17.5 18.7 22.8
submicrometer, as shown in Fig. 2a, its diamond band at 1332 cm 1 is not as sharp as that of diamond film with larger grain size. The diamond band at 1332 cm 1 is further broadened and the Raman scattering intensity in the range of 1400–1600 cm 1 is pronounced as the CCl4 concentration and the substrate temperature increase. Meanwhile, the extra Raman band at 1140 cm 1 is present obviously in the spectra of those films. The microstructures of the nanocrystalline diamond films were further studied by TEM. Fig. 6a and c shows the plan-view TEM bright-field images and the corresponding selected area diffraction (SAD) patterns of the nanocrystalline diamond films deposited from 2.5% CCl4 at 610 and 730 8C, respectively. The corresponding dark-field images are shown in Fig. 6b and d, which provides direct evidence of the grain size. All rings in the SAD patterns can be indexed to diamond structure. The dark field images, which were obtained by selecting a fraction of the inner (111) ring in the SAD patterns, show that the grain sizes of the nanocrystalline diamond films deposited from 2.5% CCl4 are in the range of 10–20 nm and reduce slightly at higher substrate temperature. Fig. 7 shows a high-resolution TEM image of the nanocrystalline diamond film deposited at 610 8C, which also confirms the 10–20 nm grain size in the film. The thicknesses of the films deposited at various CCl4 concentrations and substrate temperatures for 1 h are shown
Fig. 5. Raman spectra of the diamond films deposited at various conditions: (a) 1.5% CCl4, 610 8C, (b) 2.0% CCl4, 610 8C, (c) 2.5% CCl4, 610 8C, (d) 1.5% CCl4, 730 8C, (e) 2.0% CCl4, 730 8C and (f) 2.5% CCl4, 730 8C.
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Fig. 6. (a, c) Bright-field images with selective area electron diffraction patterns and (b, d) dark-filed images of the nanocrystalline diamond films deposited using 2.5% CCl4 at 610 and 730 8C, respectively.
Fig. 7. High-resolution image of the nanocrystalline diamond film deposited at 610 8C using 2.5% CCl4.
Fig. 8. The thicknesses the films deposited at various CCl4 concentrations and substrate temperatures for 1 h.
J.-J. Wu et al. / Thin Solid Films 473 (2005) 24–30
in Fig. 8. It reveals that the deposition rate is lowered at higher CCl4 concentration at the same substrate temperature. Moreover, the deposition rate increases at higher substrate temperature for 1.5% and 2.0% CCl4. For 2.5% CCl4, however, the deposition rate decreases when the growth temperature increases. The opposite trend of the deposition rate with temperature in the cases of high and low CCl4 concentrations imply that mechanisms of nanocrystalline diamond films grown from high and low CCl4 concentration might be different.
4. Discussion The deposition of the crystalline diamond films on the Si substrates consists of both grain nucleation and grain growth processes while the nucleation process should be enhanced to be the dominant process for the formation of the nanocrystalline diamond films. We suggest that CCl4 facilitates the formation of the nanocrystalline diamond films, as demonstrated in this study, mainly by enhancing the nucleation process of the nanocrystalline diamond film formation. Because of the weaker CUCl bond than CUH bond, CCl4 can be dissociated readily around the filament and higher carbon radicals are thus formed in gas phase in comparison with CH4. Besides, atomic Cl can be formed in this system either from the dissociation of chloromethane or through the Cl and H exchange reaction [15], H+HCl=Cl+H2. Atomic Cl has been suggested to enhance the concentration of carbon radical sites (Cd) on the diamond surface through a faster surface hydrogen abstraction reaction, Cl+Cd–H=Cd+HCl, than that by atomic H [15],
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Fig. 9. The surface dehydrochlorination reaction.
suggested to be attributed to preferential grain nucleation rather than grain growth for film thickening in the higher CCl4 concentration case. This is confirmed by the fact that the nanocrystalline diamond films deposited in higher CCl4 concentration show smaller average grain size. Furthermore, when the surface temperature increases, the amount of carbon radicals adsorbed on surface decrease which results in the reduction of nucleation rate. Therefore, the deposition rate of the nanocrystalline diamond film decreases as the substrate temperature increases in the case of 2.5% CCl4 which is the condition of preferring grain nucleation for the nanocrystalline diamond films formation. In contrast, the increase of deposition rate with the substrate temperature is mainly due to the enhancement of both grain growth and grain nucleation processes for diamond film thickening in the cases of 1.5% and 2.0% CCl4. The concentration of the surface active sites for grain growth can be increased with substrate temperatures by sufficient [H] or [Cl] from gas phase. On the other hand, the surface dehydrochlorination reaction rate for grain nucleation is also enhanced by increase of the substrate temperature. Therefore, the grain size is reduced and the film thickness is increased when the substrate temperature is increased in these cases. The grain size control of the nanocrystalline diamond films can be achieved by adjusting the ratio of the grain nucleation rate and the grain growth rate in terms of varying the CCl4 concentration in the source gas and the substrate temperature.
H+Cd–H=Cd+H2. 5. Conclusion Both increases of carbon radicals in gas phase and carbon radical sites on the diamond surface would result in an increase of the amount of carbon radicals adsorbed on the surface. Without a corresponding increase of [H] or [Cl] to remove the surface hydrogen, the growth of the diamond grain would not proceed because carbon active sites on the diamond surface cannot be formed and carbon radicals from gas phase cannot be adsorbed on the diamond surface further. In this case, however, surface dehydrochlorination reaction would occur to form a new CUC bond and even an sp2 hybridized bond, as shown in Fig. 9, which might favor the nucleation of nanocrystalline diamond grain [12–15]. The reduction of deposition rate by increasing CCl4 concentration under the same deposition condition is
In summary, a new route toward the growth of nanocrystalline diamond films is reported. Nanocrystalline diamond films with an average grain size in the range of 10–20 nm have been successfully grown in 2.5% CCl4/H2 ambient using hot-filament CVD. The growth characteristics including substrate temperature effect and CCl4 concentration effect were reported. An optimal rms surface roughness of 6 nm has been demonstrated in this process. Moreover, the CCl4 concentration in the source gas and the substrate temperature can be used to control the grain size of the nanocrystalline diamond films. We suggest that CCl4 facilitates the formation of the nanocrystalline diamond films, as demonstrated in this study, mainly by enhancing
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the nucleation process of the nanocrystalline diamond film formation.
Acknowledgements All authors would like to gratefully acknowledge the financial support from the National Science Council and Academia Sinica in Taiwan.
References [1] T.D. Moustakas, in: K.E. Spear, J. Dismukes (Eds.), Synthetic Diamond: Emerging CVD Science and Technology, John Wiley & Sons, New York, 1994, p. 148. [2] J.E. Butler, D.G. Goodwin, in: M.H. Nazare, A.J. Neves (Eds.), Properties, Growth, and Applications of Diamond, The Institution of Electrical Engineers, London, United Kingdom, 2001, p. 262. [3] A.R. Krauss, O. Auciello, D.M. Gruen, A. Jayatissa, A. Sumant, J. Tucek, D.C. Mancini, N. Moldovan, A. Erdemir, D. Ersoy, M.N. Gardos, H.G. Busmann, E.M. Meyer, M.Q. Ding, Diamond Relat. Mater. 10 (2001) 1952.
[4] D.M. Gruen, in: M.H. Nazare, A.J. Neves (Eds.), Properties, Growth, and Applications of Diamond, The Institution of Electrical Engineers, London, United Kingdom, 2001, p. 313. [5] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [6] D.M. Gruen, MRS Bull. 26 (2001) 771. [7] S. Jiao, A. Sumant, M.A. Kirk, D.M. Gruen, A.R. Krauss, O. Auciello, J. Appl. Phys. 90 (2001) 118. [8] D.M. Bhusari, J.R. Yang, T.Y. Wang, K.H. Chen, S.T. Lin, L.C. Chen, Mater. Lett. 36 (1998) 279. [9] D.M. Bhusari, J.R. Yang, T.Y. Wang, K.H. Chen, S.T. Lin, L.C. Chen, J. Mater. Res 13 (1998) 1769. [10] L.C. Chen, P.D. Kichambare, K.H. Chen, J.J. Wu, J.R. Yang, S.T. Lin, J. Appl. Phys. 89 (2001) 753. [11] A. Heiman, I. Gouzman, S.H. Christiansen, H.P. Strunk, G. Comtet, L. Hellner, G. Dujardin, R. Edrei, A. Hoffman, J. Appl. Phys. 89 (2001) 2622. [12] F.C.N. Hong, G.T. Liang, J.J. Wu, D. Chang, J.C. Hsieh, Appl. Phys. Lett. 63 (1993) 3149. [13] F.C.N. Hong, J.C. Hsieh, J.J. Wu, G.T. Liang, J.H. Hwang, Diamond Relat. Mater. 2 (1993) 365. [14] Jih-Jen Wu, Shih-Hsien Yeh, Chin-Ta Su, Franklin Chau-Nan Hong, Appl. Phys. Lett. 68 (1996) 3254. [15] Jih-Jen Wu, Franklin Chau-Nan Hong, J. Mater. Res. 13 (1998) 2498. [16] B.D. Cullity, Element of X-ray Diffraction, Addison-Wesley Publishing, Reading, Massachusetts, USA, 1978, p. 102.
Growth of nanocrystalline diamond films in CCl4/H2 ambient Jih-Jen Wua,*, Chen-Hao Kua, Te-Chi Wonga, Chien-Ting Wub, Kuei-Hsien Chenb, Li-Chyong Chenc a
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan c Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
Received 28 October 2003; received in revised form 3 May 2004; accepted 28 June 2004 Available online 10 August 2004
Abstract We investigated the growth characteristics of the nanocrystalline diamond films using CCl4/H2 as gas sources in a hot-filament chemical vapor deposition (CVD) reactor. Successful growth of nanocrystalline diamond at typical growth condition of 1.5–2.5% CCl4 and 550–730 8C substrate temperature has been demonstrated. Glancing angle X-ray diffraction (XRD) clearly indicated the formation of diamond in the films. Typical root-mean-square surface roughness of 10–15 nm and an optimal root-mean-square surface roughness of 6 nm have been achieved. Transmission electron microscopy (TEM) analyses indicated that nanocrystalline diamond film with an average grain size in the range of 10–20 nm was deposited from 2.5% CCl4/H2 at 610 8C. Effects of different source gas composition and substrate temperature on the grain nucleation and grain growth processes, whereby the grain size of the nanocrystalline film could be controlled, were discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond; Chlorine; Chemical vapor deposition; Nanostructures
1. Introduction Owing to their tremendous industrial applications, diamond films have become one of the most extensively investigated materials since 1980s [1]. Polycrystalline diamond films have been successfully synthesized from hydrocarbon–hydrogen gas mixtures using chemical vapor deposition (CVD) methods [1,2]. The highly rough surface of the polycrystalline diamond films with grain sizes of the order of microns, however, is the main restriction of the diamond films on most optical and tribological applications [3]. Nanocrystalline diamond films, which possess smooth surface and high sp3 content, have therefore been recognized as the promising candidates for those applications [4]. Nanocrystalline diamond film growths using CH4/Ar or C60/ Ar plasma have been demonstrated, which exhibit a
* Corresponding author. Tel.: +886 62757575x62694; fax: +886 62344496. E-mail address: [email protected] (J.-J. Wu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.152
nanocrystalline structure with a size distribution around 3– 5 nm and a typical rms surface roughness of 10–20 nm [3– 7]. The sp2 phase content in the nanocrystalline diamond films can be as low as 2–5% at grain boundaries, which is correlated with the high hardness (up to 88 GPa) of the film [3]. Most nanocrystalline diamond films were deposited in conventional methane–hydrogen plasma by increasing the primary nucleation density on the substrates or increasing the concentration of methane in the source gas [8–11]. It is shown that the grain size of the nanocrystalline diamond film can be varied ranging from 4 nm to a few hundreds of nanometers via the methane fraction in the source gas and the substrate pretreatment. Optical transmittance as high as 80–84% in the near-IR and surface roughness as low as 8 nm have been achieved by this technique [10]. Since that high nucleation density is a key for nanocrystalline diamond growth, we proposed to grow nanocrystalline diamond using CCl4/H2 as the source gas in a hot-filament CVD reactor. This is based on our experience that chloromethane reactants possess potential in facilitating diamond film growth at low temperature [12–15]. In
J.-J. Wu et al. / Thin Solid Films 473 (2005) 24–30 Table 1 Typical film deposition conditions in this study Filament temperature Total pressure Total flow rate CCl4 concentration Substrate temperature Distance between substrate and filament Deposition time
2000 8C 667 Pa 100 sccm 1.5–2.5% 550–730 8C 5–6 mm 1h
comparison with methane, chloromethane enhances the diamond film growth rate at low temperature and low pressure. Atomic Cl produced in the gas phase enhances diamond growth rate via increasing both the concentrations of carbon radicals in the gas and the active sites on the surface. Moreover, chloromethane facilitates the low temperature growth mainly by increasing the growth rate of nuclei and by protecting the residual seeds from being etched by H atoms [15]. Therefore, CCl4 is employed in this work to enhance the concentrations of carbon radicals and to protect the nuclei from being etched during the growth of nanocrystalline diamond.
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evaluate the diamond film structure. The microstructure of the diamond films was studied using transmission electron microscopy (TEM, JEOL, JEM-4000EX).
3. Results The SEM images of the diamond deposits grown for 1 h at a substrate temperature of 550 8C and a distance between filament and substrate of 6 mm using 2.5% CCl4 and 2.5% CH4 are shown in Fig. 1a and b, respectively. An almost continuous film with grain sizes of several tens of nanometer was deposited by 2.5% CCl4 while sporadic deposition was formed using 2.5% CH4. Fig. 1c shows that a continuous film still cannot be formed even when the CH4 concentration was increased to 6.5%, revealing that CCl4 facilitates the formation of the nanocrystalline diamond films in comparison with CH4.
2. Experimental details The deposition of diamond films was carried out in a hotfilament CVD reactor. The hot-filament CVD system consists of a process chamber equipped with a tungsten filament and a substrate stage that were employed for activation of gas-phase reactions and for the independent control of the substrate temperature, respectively. A new filament was always precarburized in 2.0% CH4/H2 for 4 h before its first deposition run. The temperature of W filament was measured by an optical pyrometer through a window of the chamber. A thermocouple placed beside the substrate was used to monitor the substrate temperature during film growth. A mixture of CCl4/H2 was used as the reactant gas. To achieve high nucleation densities of diamond films, the Si (100) substrates were pretreated for 30 min in an ultrasonic bath of toluene with 10 Am diamond powder suspension after standard chemical cleaning with acetone and deionized water. The deposition conditions are listed in Table 1. Film thickness and surface morphology studies were conducted by scanning electron microscopy (SEM, Hitachi, S-4200). The surface roughness of the diamond film was measured using atomic force microscopy (AFM, DI, Ns3a-MMAFM). A silicon tip was employed to perform the tapping modek measurement. The tip velocity, the integral gain, the proportional gain, the amplitude set point, the drive frequency and the drive amplitude for the tapping mode operations are 5 Am s 1, 0.8, 0.8, 0.8 V, 78 kHz, and 80 mV, respectively. Raman spectroscopy (Renishaw system 2000 mirco-Raman spectrometer with a 514.5 nm Ar+ laser) and glancing incident angle X-ray diffraction (XRD, Rigaku, D/MAX2500) were employed to
Fig. 1. SEM images of the diamond films deposited for 1 h at a substrate temperature of 550 8C and a distance of filament to substrate of 6 mm using (a) 2.5% CCl4, (b) 2.5% CH4 and (c) 6.5% CH4, respectively.
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The following studies were conducted at a distance between filament and substrate of 5 mm to synthesize the diamond films for investigation of the growth behaviors of the nanocrystalline diamond films using CCl4. Fig. 2a–f shows the SEM images of the diamond films deposited using three CCl4 concentrations and two different substrate temperatures, 610 and 730 8C, for 1 h. They reveal that the grain sizes of the films grown at 610 8C are reduced at higher CCl4 concentrations. Moreover, in comparison with that grown at 610 8C, diamond film with smaller grain sizes was deposited at 730 8C under the lower CCl4 concentration. The surface roughness of the films measured by AFM show significant reduction of root-mean-square (rms) roughness from 26 to 6 nm as the CCl4 concentration is increased from 1.5% to 2.0% at a temperature of 610 8C. Their AFM images are illustrated in Fig. 3a and b, respectively. The surfaces of the other films deposited using CCl4/H2 in the present work possess an rms roughness in the range of 10–15 nm.
Glancing angle XRD was employed to examine the structures of the films. As shown in Fig. 4, (111) and (220) diamond diffraction peaks appear in the spectra (a)–(e). A (111) diffraction peak is present ambiguously in the spectrum of the film deposited at 730 8C using 2.5% CCl4. Grain sizes of the diamond films estimated from the full width at half maximum (FWHM) of the (111) diffraction peak by the Scherrer formula [16] are shown in Table 2. Results confirm the trends that the grain sizes of the films grown at 610 8C decrease with increasing CCl4 concentration and that diamond film with smaller grain sizes is deposited at 730 8C using 1.5% CCl 4 in comparison with that grown at 610 8C. Therefore, the growth of diamond films with smaller grain size can be achieved by adjusting the CCl4 concentration and substrate temperature. The Raman spectra of the films are shown in Fig. 5. The spectrum of the film deposited from 1.5% CCl4 and 610 8C shows a Raman characteristic band of diamond structure at 1332 cm 1 and a broad Raman scattering in the
Fig. 2. SEM images of the diamond films deposited at various conditions: (a) 1.5% CCl4, 610 8C, (b) 2.0% CCl4, 610 8C, (c) 2.5% CCl4, 610 8C, (d) 1.5% CCl4, 730 8C, (e) 2.0% CCl4, 730 8C and (f) 2.5% CCl4, 730 8C.
J.-J. Wu et al. / Thin Solid Films 473 (2005) 24–30
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Table 2 The FWHM of the diffraction peak (111) and the grain sizes of the diamond films deposited at various conditions Deposition conditions 2.0% 1.5% 2.5% 2.0% 1.5%
Fig. 3. AFM images of the films deposited at 610 8C using (a) 1.5% and (b) 2.0% CCl4.
range of 1400–1600 cm 1 corresponding to sp2 phase carbon in the grain boundaries. Since the grain size of the film deposited from 1.5% CCl4 is already in the order of
Fig. 4. (a) Glancing angle XRD spectra of the diamond films deposited at various conditions: (I) 1.5% CCl4, 610 8C, (II) 2.0% CCl4, 610 8C, (III) 2.5% CCl4, 610 8C, (IV) 1.5% CCl4, 730 8C, (V) 2.0% CCl4, 730 8C and (VI) 2.5% CCl4, 730 8C. (b) The grain sizes of the diamond films estimated from the FWHM of the diffraction peak (111) in (a) by the Scherrer formula.
CCl4, CCl4, CCl4, CCl4, CCl4,
730 730 610 610 610
8C 8C 8C 8C 8C
FWHM (deg.)
Grain size (nm)
0.50666 0.54146 0.54602 0.51034 0.41885
18.8 17.6 17.5 18.7 22.8
submicrometer, as shown in Fig. 2a, its diamond band at 1332 cm 1 is not as sharp as that of diamond film with larger grain size. The diamond band at 1332 cm 1 is further broadened and the Raman scattering intensity in the range of 1400–1600 cm 1 is pronounced as the CCl4 concentration and the substrate temperature increase. Meanwhile, the extra Raman band at 1140 cm 1 is present obviously in the spectra of those films. The microstructures of the nanocrystalline diamond films were further studied by TEM. Fig. 6a and c shows the plan-view TEM bright-field images and the corresponding selected area diffraction (SAD) patterns of the nanocrystalline diamond films deposited from 2.5% CCl4 at 610 and 730 8C, respectively. The corresponding dark-field images are shown in Fig. 6b and d, which provides direct evidence of the grain size. All rings in the SAD patterns can be indexed to diamond structure. The dark field images, which were obtained by selecting a fraction of the inner (111) ring in the SAD patterns, show that the grain sizes of the nanocrystalline diamond films deposited from 2.5% CCl4 are in the range of 10–20 nm and reduce slightly at higher substrate temperature. Fig. 7 shows a high-resolution TEM image of the nanocrystalline diamond film deposited at 610 8C, which also confirms the 10–20 nm grain size in the film. The thicknesses of the films deposited at various CCl4 concentrations and substrate temperatures for 1 h are shown
Fig. 5. Raman spectra of the diamond films deposited at various conditions: (a) 1.5% CCl4, 610 8C, (b) 2.0% CCl4, 610 8C, (c) 2.5% CCl4, 610 8C, (d) 1.5% CCl4, 730 8C, (e) 2.0% CCl4, 730 8C and (f) 2.5% CCl4, 730 8C.
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Fig. 6. (a, c) Bright-field images with selective area electron diffraction patterns and (b, d) dark-filed images of the nanocrystalline diamond films deposited using 2.5% CCl4 at 610 and 730 8C, respectively.
Fig. 7. High-resolution image of the nanocrystalline diamond film deposited at 610 8C using 2.5% CCl4.
Fig. 8. The thicknesses the films deposited at various CCl4 concentrations and substrate temperatures for 1 h.
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in Fig. 8. It reveals that the deposition rate is lowered at higher CCl4 concentration at the same substrate temperature. Moreover, the deposition rate increases at higher substrate temperature for 1.5% and 2.0% CCl4. For 2.5% CCl4, however, the deposition rate decreases when the growth temperature increases. The opposite trend of the deposition rate with temperature in the cases of high and low CCl4 concentrations imply that mechanisms of nanocrystalline diamond films grown from high and low CCl4 concentration might be different.
4. Discussion The deposition of the crystalline diamond films on the Si substrates consists of both grain nucleation and grain growth processes while the nucleation process should be enhanced to be the dominant process for the formation of the nanocrystalline diamond films. We suggest that CCl4 facilitates the formation of the nanocrystalline diamond films, as demonstrated in this study, mainly by enhancing the nucleation process of the nanocrystalline diamond film formation. Because of the weaker CUCl bond than CUH bond, CCl4 can be dissociated readily around the filament and higher carbon radicals are thus formed in gas phase in comparison with CH4. Besides, atomic Cl can be formed in this system either from the dissociation of chloromethane or through the Cl and H exchange reaction [15], H+HCl=Cl+H2. Atomic Cl has been suggested to enhance the concentration of carbon radical sites (Cd) on the diamond surface through a faster surface hydrogen abstraction reaction, Cl+Cd–H=Cd+HCl, than that by atomic H [15],
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Fig. 9. The surface dehydrochlorination reaction.
suggested to be attributed to preferential grain nucleation rather than grain growth for film thickening in the higher CCl4 concentration case. This is confirmed by the fact that the nanocrystalline diamond films deposited in higher CCl4 concentration show smaller average grain size. Furthermore, when the surface temperature increases, the amount of carbon radicals adsorbed on surface decrease which results in the reduction of nucleation rate. Therefore, the deposition rate of the nanocrystalline diamond film decreases as the substrate temperature increases in the case of 2.5% CCl4 which is the condition of preferring grain nucleation for the nanocrystalline diamond films formation. In contrast, the increase of deposition rate with the substrate temperature is mainly due to the enhancement of both grain growth and grain nucleation processes for diamond film thickening in the cases of 1.5% and 2.0% CCl4. The concentration of the surface active sites for grain growth can be increased with substrate temperatures by sufficient [H] or [Cl] from gas phase. On the other hand, the surface dehydrochlorination reaction rate for grain nucleation is also enhanced by increase of the substrate temperature. Therefore, the grain size is reduced and the film thickness is increased when the substrate temperature is increased in these cases. The grain size control of the nanocrystalline diamond films can be achieved by adjusting the ratio of the grain nucleation rate and the grain growth rate in terms of varying the CCl4 concentration in the source gas and the substrate temperature.
H+Cd–H=Cd+H2. 5. Conclusion Both increases of carbon radicals in gas phase and carbon radical sites on the diamond surface would result in an increase of the amount of carbon radicals adsorbed on the surface. Without a corresponding increase of [H] or [Cl] to remove the surface hydrogen, the growth of the diamond grain would not proceed because carbon active sites on the diamond surface cannot be formed and carbon radicals from gas phase cannot be adsorbed on the diamond surface further. In this case, however, surface dehydrochlorination reaction would occur to form a new CUC bond and even an sp2 hybridized bond, as shown in Fig. 9, which might favor the nucleation of nanocrystalline diamond grain [12–15]. The reduction of deposition rate by increasing CCl4 concentration under the same deposition condition is
In summary, a new route toward the growth of nanocrystalline diamond films is reported. Nanocrystalline diamond films with an average grain size in the range of 10–20 nm have been successfully grown in 2.5% CCl4/H2 ambient using hot-filament CVD. The growth characteristics including substrate temperature effect and CCl4 concentration effect were reported. An optimal rms surface roughness of 6 nm has been demonstrated in this process. Moreover, the CCl4 concentration in the source gas and the substrate temperature can be used to control the grain size of the nanocrystalline diamond films. We suggest that CCl4 facilitates the formation of the nanocrystalline diamond films, as demonstrated in this study, mainly by enhancing
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the nucleation process of the nanocrystalline diamond film formation.
Acknowledgements All authors would like to gratefully acknowledge the financial support from the National Science Council and Academia Sinica in Taiwan.
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