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H2 plasma jets
Applied Surface Science 270 (2013) 324–330
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Effect of CH4 concentration on the growth behavior, structure, and transparent properties of ultrananocrystalline diamond films synthesized by focused microwave Ar/CH4 /H2 plasma jets Wen-Hsiang Liao a,b , Chii-Ruey Lin a,b,∗ , Da-Hua Wei a,b,∗ a b
Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 16 October 2012 Received in revised form 24 December 2012 Accepted 5 January 2013 Available online 11 January 2013 Keywords: Ultrananocrystalline diamond films Focused microwave plasma jet CH4 concentration Transmission electron microscopy Transmittance
a b s t r a c t The effects of CH4 concentration (0.5–5%) on the growth mechanisms, nanostructures, and optically transparent properties of ultrananocrystalline diamond (UNCD) films grown from focused microwave Ar/CH4 /H2 (argon-rich) plasma jets were systematically studied. The research results indicated that the grain size and surface roughness of the diamond films increased with increasing CH4 concentration in the plasma jet, however, the nondiamond contents in films would not be correspondingly decreased resulting from the dispersed diamond nanocrystallites in the films synthesized at higher CH4 concentration. The reason is due to that the relative emission intensity ratios of the C2 /H␣ and the CH/C2 in the plasma jets were increased and decreased with increasing CH4 concentration, respectively, to lower the etching of nondiamond phase and the renucleation of diamond during synthesis. The studies of transmission electron microscopy demonstrated that, while the CH4 introduction of 1% into the plasma jet produced the UNCD films with a spherical geometry (4–8 nm) and the CH4 introduction of 5% into the plasma jet led to the elongated (∼90 nm in length and ∼35 nm in width) grains in the nanocrystalline diamond (NCD) films with a dendrite-like geometry. The transmittance of diamond films was decreased gradually by films transition from UNCD to NCD, resulting from the enhanced surface roughness and nondiamond contents in films to concurrently increase the light scattering and absorption during photon transmission. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The ultrananocrystalline diamond (UNCD) thin films comprising diamond nanograins dispersed in an amorphous carbon matrix are outstanding materials candidate for the direct fabrication of multifunctional devices and attracting considerably scientific and technological interests [1–3]. Since UNCD thin films possess many exceptional properties stemming from their ultrafine (<10 nm) crystallites and a pure diamond phase, such as high chemical/electrochemical stability [4], wear/corrosion resistance [5,6], optical transparency from deep UV to far infrared [7,8], excellent electron field emission [9–11], and superior capacities for incorporation of n-type dopants and electrical conduction in addition to ultra-smooth surface morphology [12–15]. The regulation of nanometer-scale structures in the UNCD films including the diamond nanograins and grain boundaries (GBs) are crucial to
∗ Corresponding authors at: Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan. E-mail addresses: [email protected] (C.-R. Lin), [email protected] (D.-H. Wei). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.024
the applications for the multifunctionally diamond-based device. In particular, nanosizing of diamond crystal will increase consequently the GBs containing nondiamond carbons in the UNCD films, besides significantly varying the surface characteristic of diamond films [16]. Appropriate modification and control of nanostructures of UNCD films can tailor the physical and chemical properties of the films for developing new applications and technologies. Prior studies have been reported that proper increase in amorphous carbon phase GBs and formation of nanographites in the UNCD films would form the possibly interconnected paths to facilitate the transport of electrons, resulting in marked enhancement of electron field emission properties [17,18]. In the synthesis of nano/ultrananocrystalline diamond (NCD/UNCD) films, various synthesis techniques including the argon-rich Ar/CH4 plasma chemistries [1,15], hydrogen-rich H2 /CH4 plasma chemistries with addition of high CH4 concentrations [19,20], and negative bias-enhanced synthesis were employed for promoting the diamond secondary nucleation [3,21]. Recently, we have demonstrated the improving synthesis of UNCD films using uniquely focused microwave plasma jets [16,22]. The density and activity of plasma species can be significantly improved through excitation of the focused plasma jets as
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Fig. 1. SEM images of the diamond films grown from focused microwave plasma jet with various CH4 concentrations: (a) 0.5%, (b) 1%, (c) 1.5%, (d) 3%, and (e) 5%, respectively. (f) Enlarged SEM image of (e) the diamond films grown with CH4 concentration of 5%.
compared with the other plasma processes, enabling the UNCD films synthesis to achieve high-quality and high-efficiency deposition at relatively low pressure, temperature, consumption of source gases, and output power conditions [8,16,22,23]. Additionally, although increasing the relative proportion of CH4 in a hydrogen-rich H2 /CH4 gas chemistry during synthesis is the most common approach to synthesis the NCD films, so far, the studies on the effect of CH4 concentration on the synthesis and characteristic of the UNCD films using argon-rich plasma chemistries have not been investigated in detail with comprehensive experiments. In this present study, UNCD films were synthesized on N-type Si and transparent quartz substrates by using the unique microwave plasma jet enhanced chemical vapor deposition (MPJCVD) system. We have investigated that the transition in surface morphologies and nanostructures of the UNCD films affected by the CH4 concentration ranged from 0.5% to 5% and the induced variations in the optical transparency of the as-grown diamond films. The detailed transition in structural characteristics of the UNCD films grown at various CH4 concentrations was examined by using transmission electron microscopy (TEM). The in situ optical emission
spectroscopy (OES) was applied to monitor the species composition in the plasma jet as a function of CH4 concentrations. The possible mechanism for the evolution and modification on these structures and properties was discussed based on theses observations and analyses.
2. Experimental details The diamond films were grown using the home-made MPJCVD system. N-type Si wafers with a (1 0 0) orientation were initially used as the substrates to grow diamond films with various CH4 concentrations. The transparent quartz substrates were applied to support diamond films for fabricating the highly transparent coatings. Pretreatment on Si and quartz substrates were carried out by ultrasonic agitation in methanol solution containing diamond (4–12 nm) and titanium (80–120 nm) mixed nanoparticles (1:1 wt.%) for 30 min to facilitate the nucleation process. The focused microwave plasma jet was excited in Ar/CH4 /H2 gas chemistries at a pressure of 35 Torr, total gas flow rate of 800 SCCM,
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and microwave power of 700 W. The CH4 concentration was varied from 0.5% to 5% (0.5%, 1%, 1.5%, 3%, and 5%, respectively), while the Ar/Ar + H2 ratio of 90% was kept constant in the synthesis of diamond films. The deposition process was performed without applying bias and heating on the substrates during the synthesis. The surface morphology of the as-grown nanodiamond films was examined using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Chiyoda-ku, Tokyo, Japan). The detailed nanostructure of the films was explored using a field emission transmission electron microscopy (FETEM, Philips Tecnai F30, Best, Netherlands). The atomic bonding characteristic of the nanodiamond films was investigated by visible Raman spectroscopy using a 514.5 nm argon laser beam (Renishaw microRaman, Taichung, Taiwan). The optical transmittance spectra of the nanodiamond films ranging from 350 nm to 950 nm were measured with a UV-A/visible/near-IR spectrophotometer (Mission Peak Optics MP100-M, Fremont, CA, USA). The nanodiamond films grown on quartz substrates at various CH4 concentrations were controlled with a similar thickness around 300 nm for the analysis of transparent properties. The focused microwave plasma jet was monitored during synthesis using the in situ optical emission spectroscopy (OES, B&WTEK BTC112E, Newark, DE, USA) to explore the species composition in the plasma jet with different CH4 concentrations.
3. Results and discussion Plan-view SEM images shown in Fig. 1 demonstrated a significant change in the surface morphology of as-grown films while increased CH4 concentrations from 0.5% to 5% in the plasma jet. The films deposited with CH4 concentration of 0.5% still have few remaining vacant sites and visible pinholes in the films but almost fully covered the Si substrate as shown in Fig. 1(a). For the deposition with CH4 concentration of 1% (Fig. 1(b)), a uniform and ultra-smooth film consisted of very fine grains was synthesized without any visible pinholes. This condition provided a sufficient demand for CH4 concentration to obtain a continuous and dense film from the MPJCVD-enhanced growth. A further increase of CH4 concentration in the plasma jet induced that the atom clusters formed and gradually grew on film’s surface accompanied by a slightly rougher surface characteristic, as shown in Fig. 1(c–e). The enlarged SEM micrograph shown in Fig. 1(f) illustrated that the films grown with CH4 concentration of 5% in the plasma jet would form distinctly elongated clusters on surface with needlelike structures of about 100–200 nm in length. With the increase in CH4 concentration, the grain size of nanodiamond films seems to be increased gradually with increasing the clustered degree and surface roughness. However, the exact change in grain size of the nanodiamond films cannot be definitely distinguish by SEM observations due to the limited resolution, and the details nanostructures were further investigated and discussed below. Fig. 2 shows the visible (wavelength: 514.5 nm) Raman spectra of the diamond films synthesized from the focused microwave plasma jet with various CH4 concentrations from 0.5% to 5%. Raman spectra of as-grown films typically exhibit nanodiamond features [24,25]. The characteristic peak of diamond (sp3 -bonded carbon) at around 1332 cm−1 is overlapped by the D (disordered) band (sp2 bonded carbon) at around 1350 cm−1 while the diamond films were grown with relatively low CH4 concentration (0.5–1.5%). The reason is due to the films consisted of diamond nanocrystallites and the much higher sensitivity of sp2 bonding over sp3 bonding by using visible Raman spectrometer [20,24]. The intensity of characteristic diamond peak is increased and sharp in spectra of the diamond films grown with higher CH4 concentration (3% and 5%), indicating that the size of diamond crystallites was gradually increased
Fig. 2. Visible Raman spectra of the diamond films grown from focused microwave plasma jet with various CH4 concentrations from 0.5% to 5%.
in the films as the CH4 concentration increased. Simultaneously, the broadening of G (graphitic) band at around 1560 cm−1 and the increase in peak intensity of trans-polyacetylene (t-PA) bonding at around 1480 cm−1 with increasing CH4 concentration in the plasma jet indicated the increase in t-PA structures of carbon in the films, also implying that the grain size might be increased, but the nondiamond contents in films would not be consequently decreased during synthesis at 3% and 5% CH4 concentrations. Moreover, a peak centered at around 1140 cm−1 in spectra was observed, which also represented the t-PA bonding in nanodiamond films. In particular, a peak centered at around 1190 cm−1 in the spectra was found to gradually decrease with increasing CH4 concentration. This is opposite to the variation of t-PA bands at approximately 1140 and 1480 cm−1 in the spectra. The phenomenon probably originated from the difference in the crystallite size of nanodiamond films between UNCD and NCD [8]. Fig. 3(a) shows the cross-sectional TEM image of diamond film grown by the focused microwave plasma jet with CH4 concentration of 1%, which reveals that the films grown on Si substrates consisted of ultrafine crystallites uniformly and densely dispersed in an amorphous carbon matrix. Fig. 3(b) shows the enlarged TEM image of the diamond film grown with CH4 concentration of 1%, which illustrates distinctly that the grain size is approximately 4–8 nm with a spherical geometry. A corresponding high-resolution TEM image (Fig. 3(b), inset) demonstrates an individual nanograin (∼6 nm) in the UNCD films with a spacing of ˚ which can be further confirmed with the interplanar dis2.06 A, tance between {1 1 1} planes of diamond [3]. The cross-sectional TEM image shown in Fig. 3(c) illustrates that the film grown with CH4 concentration of 5% consisted of diamond crystallites with larger size dispersed in an amorphous carbon matrix as compared with ultra-nanosized grains at a CH4 concentration of 1%. The enlarged TEM image (Fig. 3(d)) clearly reveals that the film grown at 5% CH4 concentration actually consisted of elongated diamond grains with about 90 nm in length and 35 nm in width. However, the distribution of diamond grains with a lower density in the films
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Fig. 3. Low magnification and enlarged TEM cross-sectional images of the diamond films grown from focused microwave plasma jet at ((a) and (b)) 1% and ((c) and (d)) 5% CH4 concentrations. The inset in (b) shows the corresponding high-resolution TEM image of a single diamond nanograin.
synthesized at 5% CH4 concentration was also observed, besides forming the larger diamond grains. The TEM studies confirmed that the grain size and nondiamond contents in the diamond films concurrently increased with increasing CH4 concentration to 5% in the plasma jet and consisted with the previous analyses of SEM and Raman spectra. To explore the growth mechanism resulting from the increase in CH4 concentration that induced such changes on morphological, structural, and bonding characteristics of diamond films synthesized by microwave plasma jets, the in situ OES analysis of the focused plasma jet was carried out, as shown in Fig. 4. Fig. 4(a) reveals that C2 -species increased gradually and obviously with increasing CH4 concentration in the plasma jet. The plasma jets were predominated by Ar emissions (over 700 nm) at all depositions with various CH4 concentrations. The CH-species seems to be increased in the spectra with increasing CH4 concentration. A further inspection through evaluation of relative intensity ratios of C2 /H␣ and CH/C2 versus CH4 concentration is plotted in Fig. 4(b). The CH/C2 ratio is seen to decrease significantly in the plasma jets at CH4 concentrations from 0.5% to 1.5% compared with the maintaining an increase with a linear correlation between C2 /H␣ ratio and CH4 concentration. Reduced hydrogen atom (H␣ and H ) concentration during synthesis could suppress the etching and transformation of nondiamond phase [26]. However, the
hydrocarbons (CH-species) were insufficient in the plasma jets at relatively high CH4 concentrations that would not efficiently adhere on the surfaces of diamond clusters during synthesis. The C2 -species could thus directly attach to the surfaces of diamond clusters to form sp3 bonds, resulting in the elongated (anisotropic) diamond grains in the films [27]. This could account for the increase and elongation of diamond grain with a dendrite-like geometry during synthesis at higher CH4 concentration. Moreover, excess dissociation of C2 -species from the plasma jet would also facilitate the formation of sp2 -bonded carbon in films, resulting from the deficiency of the hydrogen atom reaction during synthesis at relatively higher CH4 concentration for transforming and removing the nondiamond phase. Therefore, the grain size and nondiamond contents in the diamond films were simultaneously increased with increasing CH4 concentration in the plasma jet. A schematic diagram of the possible grain growth mechanism of diamond films grown from focused microwave plasma jet with various CH4 introductions is shown in Fig. 5. Nanodiamond nucleation and growth on the Si substrate resulted from deposition of carbon atoms from the continuous plasma jet dissociation of C2 -species (Fig. 5(a)). For the deposition at relatively low CH4 concentration of 1% (Fig. 5(b)), C2 -species cannot directly attach to the surfaces of as-synthesized diamond nanograins owing to the full coverage of CH-species on each nanograin. Therefore, the diamond films
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Fig. 4. (a) In situ OES spectra of the focused microwave plasma jets with the CH4 introduction ranging from 0.5% to 5%. (b) The C2 /H␣ and CH/C2 intensity ratios are as a function of CH4 concentration in the plasma jet.
Fig. 5. Schematic diagram of grain-growth evolution is for various CH4 introductions into the plasma jet. (a) Deposition of nanodiamonds on Si substrate resulted from the continuous plasma jet dissociation. (b) Formation of equi-axed UNCD grains densely dispersed in an amorphous carbon matrix for synthesis with relatively low CH4 introduction. (c) Formation of elongated NCD grains non-densely dispersed in films for synthesis with relatively high CH4 introduction.
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Fig. 6. Optical transmittance spectra and AFM images. (a) Optical transmittance spectra of the diamond films grown from focused microwave plasma jet at various CH4 concentrations (1%, 3%, and 5%, respectively) with a similar thickness of approximately 300 nm. (b) The corresponding AFM images of the diamond films grown at 1% and 5% CH4 concentrations.
synthesized on Si substrates with an ultra-high level of renucleation and consisted of UNCD grains uniformly and densely dispersed in an amorphous carbon matrix. For the deposition at relatively high CH4 concentration of 5% (Fig. 5(c)), CH-species are deficiently served as the coverage coating on the surfaces of as-synthesized diamond grains during synthesis and thus the C2 -species directly combine with the surfaces of diamond grains to form sp3 bonds, resulting in the formation of elongated NCD grains in the films. Also, excess C2 species landing on surfaces of as-grown films will simultaneously promote the deposition of sp2 -bonded carbon, resulting from that the deficiency of the hydrogen atoms act as etching species for removing the sp2 bonding phase during synthesis at relatively higher CH4 introduction. A simultaneous increase of grain size and nondiamond contents in diamond films accompanying by a rougher surface morphology was caused by relatively low CH/C2 and high C2 /H␣ ratios in the plasma jets during synthesis with higher CH4 introduction. The transmittance spectra of the diamond films with a similar thickness of approximately 300 nm grown from the focused microwave plasma jet on quartz substrates at 1%, 3%, and 5% CH4 concentrations are presented in Fig. 6. The method of measurement is illustrated in the top left inset of Fig. 6. The optical transmittance of the diamond films is oscillated in spectra due to the interference effects during the photon transmission through the films [7]. Above 93% transmittance is achieved for UNCD film grown at 1% CH4 concentration at approximately 785 nm (near-IR region), whereas NCD films grown respectively at 3% and 5% CH4 concentrations are both showing about 90% maximum transmittance in the near-IR region. The transparent property of diamond films is dominated by the surface smoothness and diamond quality (sp3 -bonded carbon content) of the films [7,16,28]. The film grown at 1% CH4 concentration consisted of dense and pure diamond ultra-nanocrystallites (4–8 nm), which supported a high degree of diamond purity throughout the whole film and exhibited a ultra-smooth surface with the root-mean-square (rms) roughness of 6.8 nm (Fig. 6(b)) to minimized the light absorption and scattering, respectively, resulting in the superior optical transparency obtained from the synthesis at low CH4 concentration. The highly transparent UNCD films can meet the requirements on
the diamond-based transparent and protective coating for optical components, while the modified ones with higher nondiamond contents in films may provide the development of the composite nanodiamond films for optoelectronic devices (e.g. electron field emitting sources) in the future [17,18]. The optical transmittance analysis complemented the SEM, TEM, AFM, visible Raman, and OES studies to demonstrate the transition in nanometer-scale structures of the UNCD films with simultaneous increase of grain size and nondiamond contents by increasing CH4 introductions into focused microwave plasma jets and complete the investigation of the relationships between the growth conditions, structures, and material properties of the nanodiamond films grown with different CH4 concentrations. 4. Conclusions The modification and regulation on nanometer-scale structures and characterizations of the UNCD films due to the increase of CH4 introduction into the focused microwave Ar/CH4 /H2 plasma jet have been systematically studied. We report an approach to concurrently increase the grain size and nondiamond contents in nanodiamond films grown with argon-rich plasma chemistries by regulating the CH4 concentrations in a systematic way. Based on the TEM and visible Raman studies of the films, it has been demonstrated that the transition from UNCD to NCD films was controlled by focused microwave plasma jets with 0.5% to 5% CH4 introduction, however, the nondiamond contents in films would not be consequently decreased due to the nondense distribution of diamond crystallites within the films grown at higher CH4 concentration. The in situ OES analysis suggested that the structural modification was formed by competitions between the attachment and the removing of hydrocarbons onto the diamond clusters and the transforming and the landing of nondiamond carbons in the nanodiamond films. The simultaneous increase of grain size and nondiamond contents in diamond films was reconfirmed by transmittance studies with a gradually decreased transparency. The increase in light scattering and absorption during photon transmission through the films was caused by the rougher surface characteristics and higher nondiamond contents in films, respectively.
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Acknowledgments This work was financially supported by the main research projects of the National Science Council of R.O.C. under grant nos. NSC 101-2221-E-027-009 and NSC 101-2221-E-027-042, respectively. References [1] D.M. Gruen, Annual Review of Materials Science 29 (1999) 211–259. [2] J. Birrell, J.A. Carlisle, O. Auciello, D.M. Gruen, J.M. Gibson, Applied Physics Letters 81 (2002) 2235–2237. [3] Y. Lifshitz, X.M. Meng, S.T. Lee, R. Akhveldiany, A. Hoffman, Physical Review Letters 93 (2004) 056101-1–056101-4. [4] X. Xiao, J. Wang, C. Liu, J.A. Carlisle, B. Mech, R. Greenberg, D. Guven, R. Freda, M.S. Humayun, J. Weiland, O. Auciello, Journal of Biomedical Materials Research Part B 77 (2006) 273–281. [5] K.-H. Kim, N. Moldovan, C. Ke, H.D. Espinosa, X. Xiao, J.A. Carlisle, O. Auciello, Small 1 (2005) 866–874. [6] A.V. Sumant, D.S. Grierson, J.E. Gerbi, J.A. Carlisle, O. Auciello, R.W. Carpick, Physical Review B 76 (2007) 235429-1–235429-11. [7] A. Kriele, O.A. Williams, M. Wolfer, D. Brink, W. Müller-Sebert, C.E. Nebel, Applied Physics Letters 95 (2009) 031905-1–031905-3. [8] W.H. Liao, D.H. Wei, C.R. Lin, Nanoscale Research Letters 7 (2012) 82-1–82-8. [9] W. Zhu, G.P. Kochanski, S. Jin, Science 282 (1998) 1471–1473. [10] N. Jiang, K. Eguchi, S. Noguchi, T. Inaoka, Y. Shintani, Journal of Crystal Growth 236 (2002) 577–582. [11] J.M. Garguilo, F.A.M. Koeck, R.J. Nemanich, X.C. Xiao, J.A. Carlisle, O. Auciello, Physical Review B 72 (2005) 165404-1–165404-6. [12] S. Bhattacharyya, O. Auciello, J. Birrell, J.A. Carlisle, L.A. Curtiss, A.N. Goyette, D.M. Gruen, A.R. Krauss, J. Schlueter, A. Sumant, P. Zapol, Applied Physics Letters 79 (2001) 1441–1443.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of CH4 concentration on the growth behavior, structure, and transparent properties of ultrananocrystalline diamond films synthesized by focused microwave Ar/CH4 /H2 plasma jets Wen-Hsiang Liao a,b , Chii-Ruey Lin a,b,∗ , Da-Hua Wei a,b,∗ a b
Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 16 October 2012 Received in revised form 24 December 2012 Accepted 5 January 2013 Available online 11 January 2013 Keywords: Ultrananocrystalline diamond films Focused microwave plasma jet CH4 concentration Transmission electron microscopy Transmittance
a b s t r a c t The effects of CH4 concentration (0.5–5%) on the growth mechanisms, nanostructures, and optically transparent properties of ultrananocrystalline diamond (UNCD) films grown from focused microwave Ar/CH4 /H2 (argon-rich) plasma jets were systematically studied. The research results indicated that the grain size and surface roughness of the diamond films increased with increasing CH4 concentration in the plasma jet, however, the nondiamond contents in films would not be correspondingly decreased resulting from the dispersed diamond nanocrystallites in the films synthesized at higher CH4 concentration. The reason is due to that the relative emission intensity ratios of the C2 /H␣ and the CH/C2 in the plasma jets were increased and decreased with increasing CH4 concentration, respectively, to lower the etching of nondiamond phase and the renucleation of diamond during synthesis. The studies of transmission electron microscopy demonstrated that, while the CH4 introduction of 1% into the plasma jet produced the UNCD films with a spherical geometry (4–8 nm) and the CH4 introduction of 5% into the plasma jet led to the elongated (∼90 nm in length and ∼35 nm in width) grains in the nanocrystalline diamond (NCD) films with a dendrite-like geometry. The transmittance of diamond films was decreased gradually by films transition from UNCD to NCD, resulting from the enhanced surface roughness and nondiamond contents in films to concurrently increase the light scattering and absorption during photon transmission. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The ultrananocrystalline diamond (UNCD) thin films comprising diamond nanograins dispersed in an amorphous carbon matrix are outstanding materials candidate for the direct fabrication of multifunctional devices and attracting considerably scientific and technological interests [1–3]. Since UNCD thin films possess many exceptional properties stemming from their ultrafine (<10 nm) crystallites and a pure diamond phase, such as high chemical/electrochemical stability [4], wear/corrosion resistance [5,6], optical transparency from deep UV to far infrared [7,8], excellent electron field emission [9–11], and superior capacities for incorporation of n-type dopants and electrical conduction in addition to ultra-smooth surface morphology [12–15]. The regulation of nanometer-scale structures in the UNCD films including the diamond nanograins and grain boundaries (GBs) are crucial to
∗ Corresponding authors at: Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan. E-mail addresses: [email protected] (C.-R. Lin), [email protected] (D.-H. Wei). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.024
the applications for the multifunctionally diamond-based device. In particular, nanosizing of diamond crystal will increase consequently the GBs containing nondiamond carbons in the UNCD films, besides significantly varying the surface characteristic of diamond films [16]. Appropriate modification and control of nanostructures of UNCD films can tailor the physical and chemical properties of the films for developing new applications and technologies. Prior studies have been reported that proper increase in amorphous carbon phase GBs and formation of nanographites in the UNCD films would form the possibly interconnected paths to facilitate the transport of electrons, resulting in marked enhancement of electron field emission properties [17,18]. In the synthesis of nano/ultrananocrystalline diamond (NCD/UNCD) films, various synthesis techniques including the argon-rich Ar/CH4 plasma chemistries [1,15], hydrogen-rich H2 /CH4 plasma chemistries with addition of high CH4 concentrations [19,20], and negative bias-enhanced synthesis were employed for promoting the diamond secondary nucleation [3,21]. Recently, we have demonstrated the improving synthesis of UNCD films using uniquely focused microwave plasma jets [16,22]. The density and activity of plasma species can be significantly improved through excitation of the focused plasma jets as
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Fig. 1. SEM images of the diamond films grown from focused microwave plasma jet with various CH4 concentrations: (a) 0.5%, (b) 1%, (c) 1.5%, (d) 3%, and (e) 5%, respectively. (f) Enlarged SEM image of (e) the diamond films grown with CH4 concentration of 5%.
compared with the other plasma processes, enabling the UNCD films synthesis to achieve high-quality and high-efficiency deposition at relatively low pressure, temperature, consumption of source gases, and output power conditions [8,16,22,23]. Additionally, although increasing the relative proportion of CH4 in a hydrogen-rich H2 /CH4 gas chemistry during synthesis is the most common approach to synthesis the NCD films, so far, the studies on the effect of CH4 concentration on the synthesis and characteristic of the UNCD films using argon-rich plasma chemistries have not been investigated in detail with comprehensive experiments. In this present study, UNCD films were synthesized on N-type Si and transparent quartz substrates by using the unique microwave plasma jet enhanced chemical vapor deposition (MPJCVD) system. We have investigated that the transition in surface morphologies and nanostructures of the UNCD films affected by the CH4 concentration ranged from 0.5% to 5% and the induced variations in the optical transparency of the as-grown diamond films. The detailed transition in structural characteristics of the UNCD films grown at various CH4 concentrations was examined by using transmission electron microscopy (TEM). The in situ optical emission
spectroscopy (OES) was applied to monitor the species composition in the plasma jet as a function of CH4 concentrations. The possible mechanism for the evolution and modification on these structures and properties was discussed based on theses observations and analyses.
2. Experimental details The diamond films were grown using the home-made MPJCVD system. N-type Si wafers with a (1 0 0) orientation were initially used as the substrates to grow diamond films with various CH4 concentrations. The transparent quartz substrates were applied to support diamond films for fabricating the highly transparent coatings. Pretreatment on Si and quartz substrates were carried out by ultrasonic agitation in methanol solution containing diamond (4–12 nm) and titanium (80–120 nm) mixed nanoparticles (1:1 wt.%) for 30 min to facilitate the nucleation process. The focused microwave plasma jet was excited in Ar/CH4 /H2 gas chemistries at a pressure of 35 Torr, total gas flow rate of 800 SCCM,
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and microwave power of 700 W. The CH4 concentration was varied from 0.5% to 5% (0.5%, 1%, 1.5%, 3%, and 5%, respectively), while the Ar/Ar + H2 ratio of 90% was kept constant in the synthesis of diamond films. The deposition process was performed without applying bias and heating on the substrates during the synthesis. The surface morphology of the as-grown nanodiamond films was examined using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Chiyoda-ku, Tokyo, Japan). The detailed nanostructure of the films was explored using a field emission transmission electron microscopy (FETEM, Philips Tecnai F30, Best, Netherlands). The atomic bonding characteristic of the nanodiamond films was investigated by visible Raman spectroscopy using a 514.5 nm argon laser beam (Renishaw microRaman, Taichung, Taiwan). The optical transmittance spectra of the nanodiamond films ranging from 350 nm to 950 nm were measured with a UV-A/visible/near-IR spectrophotometer (Mission Peak Optics MP100-M, Fremont, CA, USA). The nanodiamond films grown on quartz substrates at various CH4 concentrations were controlled with a similar thickness around 300 nm for the analysis of transparent properties. The focused microwave plasma jet was monitored during synthesis using the in situ optical emission spectroscopy (OES, B&WTEK BTC112E, Newark, DE, USA) to explore the species composition in the plasma jet with different CH4 concentrations.
3. Results and discussion Plan-view SEM images shown in Fig. 1 demonstrated a significant change in the surface morphology of as-grown films while increased CH4 concentrations from 0.5% to 5% in the plasma jet. The films deposited with CH4 concentration of 0.5% still have few remaining vacant sites and visible pinholes in the films but almost fully covered the Si substrate as shown in Fig. 1(a). For the deposition with CH4 concentration of 1% (Fig. 1(b)), a uniform and ultra-smooth film consisted of very fine grains was synthesized without any visible pinholes. This condition provided a sufficient demand for CH4 concentration to obtain a continuous and dense film from the MPJCVD-enhanced growth. A further increase of CH4 concentration in the plasma jet induced that the atom clusters formed and gradually grew on film’s surface accompanied by a slightly rougher surface characteristic, as shown in Fig. 1(c–e). The enlarged SEM micrograph shown in Fig. 1(f) illustrated that the films grown with CH4 concentration of 5% in the plasma jet would form distinctly elongated clusters on surface with needlelike structures of about 100–200 nm in length. With the increase in CH4 concentration, the grain size of nanodiamond films seems to be increased gradually with increasing the clustered degree and surface roughness. However, the exact change in grain size of the nanodiamond films cannot be definitely distinguish by SEM observations due to the limited resolution, and the details nanostructures were further investigated and discussed below. Fig. 2 shows the visible (wavelength: 514.5 nm) Raman spectra of the diamond films synthesized from the focused microwave plasma jet with various CH4 concentrations from 0.5% to 5%. Raman spectra of as-grown films typically exhibit nanodiamond features [24,25]. The characteristic peak of diamond (sp3 -bonded carbon) at around 1332 cm−1 is overlapped by the D (disordered) band (sp2 bonded carbon) at around 1350 cm−1 while the diamond films were grown with relatively low CH4 concentration (0.5–1.5%). The reason is due to the films consisted of diamond nanocrystallites and the much higher sensitivity of sp2 bonding over sp3 bonding by using visible Raman spectrometer [20,24]. The intensity of characteristic diamond peak is increased and sharp in spectra of the diamond films grown with higher CH4 concentration (3% and 5%), indicating that the size of diamond crystallites was gradually increased
Fig. 2. Visible Raman spectra of the diamond films grown from focused microwave plasma jet with various CH4 concentrations from 0.5% to 5%.
in the films as the CH4 concentration increased. Simultaneously, the broadening of G (graphitic) band at around 1560 cm−1 and the increase in peak intensity of trans-polyacetylene (t-PA) bonding at around 1480 cm−1 with increasing CH4 concentration in the plasma jet indicated the increase in t-PA structures of carbon in the films, also implying that the grain size might be increased, but the nondiamond contents in films would not be consequently decreased during synthesis at 3% and 5% CH4 concentrations. Moreover, a peak centered at around 1140 cm−1 in spectra was observed, which also represented the t-PA bonding in nanodiamond films. In particular, a peak centered at around 1190 cm−1 in the spectra was found to gradually decrease with increasing CH4 concentration. This is opposite to the variation of t-PA bands at approximately 1140 and 1480 cm−1 in the spectra. The phenomenon probably originated from the difference in the crystallite size of nanodiamond films between UNCD and NCD [8]. Fig. 3(a) shows the cross-sectional TEM image of diamond film grown by the focused microwave plasma jet with CH4 concentration of 1%, which reveals that the films grown on Si substrates consisted of ultrafine crystallites uniformly and densely dispersed in an amorphous carbon matrix. Fig. 3(b) shows the enlarged TEM image of the diamond film grown with CH4 concentration of 1%, which illustrates distinctly that the grain size is approximately 4–8 nm with a spherical geometry. A corresponding high-resolution TEM image (Fig. 3(b), inset) demonstrates an individual nanograin (∼6 nm) in the UNCD films with a spacing of ˚ which can be further confirmed with the interplanar dis2.06 A, tance between {1 1 1} planes of diamond [3]. The cross-sectional TEM image shown in Fig. 3(c) illustrates that the film grown with CH4 concentration of 5% consisted of diamond crystallites with larger size dispersed in an amorphous carbon matrix as compared with ultra-nanosized grains at a CH4 concentration of 1%. The enlarged TEM image (Fig. 3(d)) clearly reveals that the film grown at 5% CH4 concentration actually consisted of elongated diamond grains with about 90 nm in length and 35 nm in width. However, the distribution of diamond grains with a lower density in the films
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Fig. 3. Low magnification and enlarged TEM cross-sectional images of the diamond films grown from focused microwave plasma jet at ((a) and (b)) 1% and ((c) and (d)) 5% CH4 concentrations. The inset in (b) shows the corresponding high-resolution TEM image of a single diamond nanograin.
synthesized at 5% CH4 concentration was also observed, besides forming the larger diamond grains. The TEM studies confirmed that the grain size and nondiamond contents in the diamond films concurrently increased with increasing CH4 concentration to 5% in the plasma jet and consisted with the previous analyses of SEM and Raman spectra. To explore the growth mechanism resulting from the increase in CH4 concentration that induced such changes on morphological, structural, and bonding characteristics of diamond films synthesized by microwave plasma jets, the in situ OES analysis of the focused plasma jet was carried out, as shown in Fig. 4. Fig. 4(a) reveals that C2 -species increased gradually and obviously with increasing CH4 concentration in the plasma jet. The plasma jets were predominated by Ar emissions (over 700 nm) at all depositions with various CH4 concentrations. The CH-species seems to be increased in the spectra with increasing CH4 concentration. A further inspection through evaluation of relative intensity ratios of C2 /H␣ and CH/C2 versus CH4 concentration is plotted in Fig. 4(b). The CH/C2 ratio is seen to decrease significantly in the plasma jets at CH4 concentrations from 0.5% to 1.5% compared with the maintaining an increase with a linear correlation between C2 /H␣ ratio and CH4 concentration. Reduced hydrogen atom (H␣ and H ) concentration during synthesis could suppress the etching and transformation of nondiamond phase [26]. However, the
hydrocarbons (CH-species) were insufficient in the plasma jets at relatively high CH4 concentrations that would not efficiently adhere on the surfaces of diamond clusters during synthesis. The C2 -species could thus directly attach to the surfaces of diamond clusters to form sp3 bonds, resulting in the elongated (anisotropic) diamond grains in the films [27]. This could account for the increase and elongation of diamond grain with a dendrite-like geometry during synthesis at higher CH4 concentration. Moreover, excess dissociation of C2 -species from the plasma jet would also facilitate the formation of sp2 -bonded carbon in films, resulting from the deficiency of the hydrogen atom reaction during synthesis at relatively higher CH4 concentration for transforming and removing the nondiamond phase. Therefore, the grain size and nondiamond contents in the diamond films were simultaneously increased with increasing CH4 concentration in the plasma jet. A schematic diagram of the possible grain growth mechanism of diamond films grown from focused microwave plasma jet with various CH4 introductions is shown in Fig. 5. Nanodiamond nucleation and growth on the Si substrate resulted from deposition of carbon atoms from the continuous plasma jet dissociation of C2 -species (Fig. 5(a)). For the deposition at relatively low CH4 concentration of 1% (Fig. 5(b)), C2 -species cannot directly attach to the surfaces of as-synthesized diamond nanograins owing to the full coverage of CH-species on each nanograin. Therefore, the diamond films
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Fig. 4. (a) In situ OES spectra of the focused microwave plasma jets with the CH4 introduction ranging from 0.5% to 5%. (b) The C2 /H␣ and CH/C2 intensity ratios are as a function of CH4 concentration in the plasma jet.
Fig. 5. Schematic diagram of grain-growth evolution is for various CH4 introductions into the plasma jet. (a) Deposition of nanodiamonds on Si substrate resulted from the continuous plasma jet dissociation. (b) Formation of equi-axed UNCD grains densely dispersed in an amorphous carbon matrix for synthesis with relatively low CH4 introduction. (c) Formation of elongated NCD grains non-densely dispersed in films for synthesis with relatively high CH4 introduction.
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Fig. 6. Optical transmittance spectra and AFM images. (a) Optical transmittance spectra of the diamond films grown from focused microwave plasma jet at various CH4 concentrations (1%, 3%, and 5%, respectively) with a similar thickness of approximately 300 nm. (b) The corresponding AFM images of the diamond films grown at 1% and 5% CH4 concentrations.
synthesized on Si substrates with an ultra-high level of renucleation and consisted of UNCD grains uniformly and densely dispersed in an amorphous carbon matrix. For the deposition at relatively high CH4 concentration of 5% (Fig. 5(c)), CH-species are deficiently served as the coverage coating on the surfaces of as-synthesized diamond grains during synthesis and thus the C2 -species directly combine with the surfaces of diamond grains to form sp3 bonds, resulting in the formation of elongated NCD grains in the films. Also, excess C2 species landing on surfaces of as-grown films will simultaneously promote the deposition of sp2 -bonded carbon, resulting from that the deficiency of the hydrogen atoms act as etching species for removing the sp2 bonding phase during synthesis at relatively higher CH4 introduction. A simultaneous increase of grain size and nondiamond contents in diamond films accompanying by a rougher surface morphology was caused by relatively low CH/C2 and high C2 /H␣ ratios in the plasma jets during synthesis with higher CH4 introduction. The transmittance spectra of the diamond films with a similar thickness of approximately 300 nm grown from the focused microwave plasma jet on quartz substrates at 1%, 3%, and 5% CH4 concentrations are presented in Fig. 6. The method of measurement is illustrated in the top left inset of Fig. 6. The optical transmittance of the diamond films is oscillated in spectra due to the interference effects during the photon transmission through the films [7]. Above 93% transmittance is achieved for UNCD film grown at 1% CH4 concentration at approximately 785 nm (near-IR region), whereas NCD films grown respectively at 3% and 5% CH4 concentrations are both showing about 90% maximum transmittance in the near-IR region. The transparent property of diamond films is dominated by the surface smoothness and diamond quality (sp3 -bonded carbon content) of the films [7,16,28]. The film grown at 1% CH4 concentration consisted of dense and pure diamond ultra-nanocrystallites (4–8 nm), which supported a high degree of diamond purity throughout the whole film and exhibited a ultra-smooth surface with the root-mean-square (rms) roughness of 6.8 nm (Fig. 6(b)) to minimized the light absorption and scattering, respectively, resulting in the superior optical transparency obtained from the synthesis at low CH4 concentration. The highly transparent UNCD films can meet the requirements on
the diamond-based transparent and protective coating for optical components, while the modified ones with higher nondiamond contents in films may provide the development of the composite nanodiamond films for optoelectronic devices (e.g. electron field emitting sources) in the future [17,18]. The optical transmittance analysis complemented the SEM, TEM, AFM, visible Raman, and OES studies to demonstrate the transition in nanometer-scale structures of the UNCD films with simultaneous increase of grain size and nondiamond contents by increasing CH4 introductions into focused microwave plasma jets and complete the investigation of the relationships between the growth conditions, structures, and material properties of the nanodiamond films grown with different CH4 concentrations. 4. Conclusions The modification and regulation on nanometer-scale structures and characterizations of the UNCD films due to the increase of CH4 introduction into the focused microwave Ar/CH4 /H2 plasma jet have been systematically studied. We report an approach to concurrently increase the grain size and nondiamond contents in nanodiamond films grown with argon-rich plasma chemistries by regulating the CH4 concentrations in a systematic way. Based on the TEM and visible Raman studies of the films, it has been demonstrated that the transition from UNCD to NCD films was controlled by focused microwave plasma jets with 0.5% to 5% CH4 introduction, however, the nondiamond contents in films would not be consequently decreased due to the nondense distribution of diamond crystallites within the films grown at higher CH4 concentration. The in situ OES analysis suggested that the structural modification was formed by competitions between the attachment and the removing of hydrocarbons onto the diamond clusters and the transforming and the landing of nondiamond carbons in the nanodiamond films. The simultaneous increase of grain size and nondiamond contents in diamond films was reconfirmed by transmittance studies with a gradually decreased transparency. The increase in light scattering and absorption during photon transmission through the films was caused by the rougher surface characteristics and higher nondiamond contents in films, respectively.
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