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Synthesis of thin diamond films from faceted nanosized crystallites
Current Applied Physics 9 (2009) 698–702
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Synthesis of thin diamond films from faceted nanosized crystallites Sobia Allah Rakha *, Shumin Yang, Zhoutong He, Ishaq Ahmed, Dezhang Zhu, Jinlong Gong Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 30 May 2008 Received in revised form 16 June 2008 Accepted 24 June 2008 Available online 1 July 2008 PACS: 81.15Gh 68.55 68.37 81.07
a b s t r a c t Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 1010 cm2) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Hot filament CVD Nanocrystalline Diamond films Electron microscopy
1. Introduction Nowadays nanodiamond thin films are getting particular attention because of their particularities such as smooth surfaces and outstanding field emission properties. Having the outstanding properties of CVD diamond, but ultra smooth surface, nanocrystalline diamond (NCD) is expected to be ideal material for applications in a variety of fields such as optics, electronics, and also biomedicine and biosensors [1]. Up to now, a number of ways have been used to deposit nanocrystalline diamond films using microwave plasma chemical vapor deposition (MPCVD) [2–7]. Including the bias enhance pretreatment followed by MPCVD [8–11]. The hot filament chemical vapor deposition (HFCVD) based deposition technique allows the diamond deposition in a simple way, overcoming some of the limitations of other more sophisticated CVD techniques such as MPCVD. However, for the hot filament chemical vapor deposition (HFCVD), few reports are present for the growth of NCD on Si substrate followed by some conditions including high methane concentration or addition of Ar. In 1999 Zhang et al. reported that diamond nuclei was reduced in size due to addition of nitrogen into the CH4/H2 plasma gas in a hot filament chemical vapor deposition process and density also increases at higher nitrogen concentration [12]. * Corresponding author. Tel.: +86 2159556934. E-mail address: [email protected] (S.A. Rakha). 1567-1739/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.06.010
Lin et al. first discussed the compositional mapping of the Ar– CH4–H2 system for polycrystalline to nanocrystalline diamond film growth by HFCVD [13]. NCD growth using Ar and high concentration of CH4 on substrates other than Si are also reported in the literature [14,15] as well as on Si substrate [16]. Zhang et al. tried to eliminate the function of Ar addition, but the results indicated that high methane concentration or argon addition have effect on the formation of nanocrystallites [17]. So far the methods adopted for the deposition of nanodiamond films in HFCVD using CH4/H2 as reactants resulted in nanograin sized thin films at the cost of high methane concentration or applying particular pretreatments prior to deposition. Details are listed in Table 1 to highlight the difference of present work with previous reports. In this paper, we report on a method for the synthesis of improved quality thin diamond films consist of nano sized grains under the deposition conditions generally used for microcrystalline diamond films. The distance between the filament and substrate is increased while maintaining the substrate temperature to same value. The effects of this large separation on the gas phase chemistry results in reduced grain size. The effect of deposition time on grains size and thickness is discussed. 2. Experimental Diamond films were grown on silicon (Si) wafers by the use of the hot filament chemical vapor deposition (HFCVD) method. Mir-
S.A. Rakha et al. / Current Applied Physics 9 (2009) 698–702
ror polished N-type Si(1 0 0) wafer was used as substrates. Prior to deposition, the substrates were ultrasonically cleaned in acetone, de-ionized water and 5% HF to remove the oxide layer, followed by ultrasonic scratching in an acetone suspension with nano diamond powder to promote the nucleation of crystalline diamond on Si(1 0 0) before deposition. Silicon substrates on the molybdenum holder were placed into the reaction chamber, which has a base pressure of 102 Pa. The substrate temperature was measured by a thermocouple. After the samples were heated to and stabilized at a temperature of about 1023 K, hydrogen gas mixed with 1.5% methane was fed into the chamber at a flow rate of 100 SCCM as a source gas which was thermally activated by tungsten filament. The ratios of gases were controlled by precision mass flow controller. The total pressure of the chamber was kept at 30 torr. The filament temperature of 2200 K heated the substrate to a temperature of 1023 K at a distance of 5 mm from the filament. Under these conditions, a continuous diamond film with typical crystal size of approximately 1 lm and thickness of 2.5 lm is obtained which is always observed to grow. During deposition of nanograin sized diamond films, however, the substrate temperature was maintained at nearly 1023 K at a distance of 15 mm from the filament by increasing the input power heating the filament. The tungsten filaments heated to comparatively higher temperatures were used for the deposition of thin diamond films consist of nanometer grains. The samples of the present investigation were analyzed by scanning electron microscopy (SEM, LEO 1530VP, morphology), high resolution transmission electron microscopy (HRTEM, JEOL JEM-2011 operated at 200 kV). Raman spectra were obtained using a micro Raman spectrometer with Ar+ laser 514 nm line as the excitation source.
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Fig. 1. SEM image showing MCD film grown with CH4–H2 feed gases for 2 h on Si substrate by HFCVD.
3. Results The SEM image of the as grown film deposited for 2 h using CH4/H2 1.5% as feed gases and at a distance of about 5 mm between the substrate and filament in HFCVD (sample 1) is shown in Fig. 1. The film consists of crystal grains with an average size of 1 lm. The SEM image of the as grown diamond film deposited for 2 h using CH4/H2 1.5% as feed gases and at a distance of about 15 mm between the substrate and filament in HFCVD (sample 2) is shown in Fig. 2a. The film consists of crystal grains with an average size of 200 nm or less with a thickness of 2 lm. The nucleation density of diamond observed was about 1010 cm2 s1, comparable to that of by biasing way. Raman spectrum in Fig. 2b confirmed that these grains are diamond crystallites in good crystalline shape. The Raman spectra of the CVD diamond films, in general, consist of sharp diamond peak nearly at 1332 cm1 with a full width at half maximum (FWHM) in the range of 5–16 cm1. The FWHM depends on the film deposition conditions and is commonly used as a measure of the diamond quality [18]. Micro Raman spectrum of this film in Fig. 2b shows peaks at 1133, 1332, 1477, and 1602 cm1. The sharp peak at 1332 cm1 is apparent, indicative of good quality CVD diamond and is characterized by FWHM equal to 9.5 cm1 calculated after baseline subtraction. A peak at 1602 cm1 is referred as G peak, which is a characteristic of sp2 carbon [18]. The peaks at 1133 cm1 and 1477 cm1 are sometimes assigned to nanodiamond phase [20]. However, there is certain ambiguity in these peaks. Ferrari and Robertson have assigned these two peaks trans-polyacetylene segments present at grain boundaries and surfaces of diamond films [19]. However, these two peaks are observed in nanodiamond thin films and can be a convenient probe for nanodiamond thin films.
Fig. 2a. SEM image showing NCD thin film grown with CH4–H2 feed gases for 2 h on Si substrate by HFCVD.
Fig. 2b. Raman spectrum of the corresponding NCD thin film.
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Fig. 3a shows the SEM image of the film deposited for 1 h (sample 3). Nano sized grains with an average size of 100 nm with well facets can be seen in the SEM image with film thickness of 1 lm. The corresponding Raman spectrum in Fig. 3b indicates the characteristic diamond peak at 1332 cm1 and the nanodiamond peak at 1134 cm1. However, the peaks are not as sharp. Fig. 4a shows the SEM image of sample 4 deposited for 30 min (sample 4). Nanosized grains with an average grain size less than 20 nm can be seen and thickness of the film is reduced to 500 nm. Nanocrystalline diamond peak around 1140 cm1 did not appear in the Raman spectrum in Fig. 4b. This is because of the more existence of sp2 bonded carbon at this scale and sp2 bonded carbon is much more sensitive than sp3 (diamond) in Raman spectrum [20]. Figs. 5 and 6 show the high resolution transmission electron microscope image of the film grown for 30 min (sample 4). The spacing of the lattice fringes were measured to be 0.205 nm, matching well with interplanar distance between the diamond {1 1 1} planes. The HRTEM observations show that the diamond grains are about 20 nm, with lot of small sized grains embedded in a matrix of amorphous carbon. In Fig 5, we note there is a grain (indicated by arrow) with clear edges. The enlarged image of this grain is shown in Fig. 6, indicating the size of the grain about
Fig. 4a. SEM image showing NCD thin film grown 30 min on Si substrate.
Fig. 4b. Raman spectrum of the NCD thin film deposited for 30 min. Fig. 3a. SEM images of NCD thin film grown for 1 h on Si substrate.
20 nm. The inset of the Fig. 6 shows the diffraction pattern of the grain. The diffraction pattern is similar to single crystal pattern indicating that the grain is the single crystallite of diamond. 4. Discussion
Fig. 3b. Raman spectrum of the NCD thin film deposited for 1 h.
In hot filament technique, with the mixture of methane and hydrogen, various species formation depends upon many factors such as the temperature of the hot filament, the system pressure, the composition and the flow rate of the incoming gas, and the extent of various chemical reactions on and near the filament surface. The detailed diamond deposition mechanism depends not only on the transport mechanism of nutrient species from filament to substrate surface but also on the surface chemical reaction kinetics. DebRoy et al. [21] showed that diffusion is the dominant transport mechanism for the various species to transport from the hot filament to the substrate. H atoms and hydrocarbon radicals such as + CH3, CHþ 3 , C2H2, CH3, CH, CH etc., are produced mainly on the filament or in the near filament vicinity. It is suggested that atomic hydrogen and CH+ correlated with the formation of diamond component, whereas the CH relates to the presence of amorphous carbon in the films [22].
S.A. Rakha et al. / Current Applied Physics 9 (2009) 698–702
Fig. 5. Low magnification TEM image showing the mixture of matrix and nanocrystallites. The inset shows the selected area diffraction pattern of the area including the matrix and nanocrystallites.
Fig. 6. High magnification TEM image of a nanodiamond grain (indicated by an arrow Fig. 5).
The methods used for nanodiamond growth in HFCVD in order to get nanograins required special pretreatments, high concentration of methane or Ar gas for deposition on Si substrates or other than Si. Results presented here showed that nanocrystallites can also be obtained by changing separation between substrate and filament and keeping rest of the conditions same as for microcrystalline diamond. Comparison of sample 1 to samples 2–4, experimental conditions including input gas ratios, substrate temperature, pressure and deposition time are same except the substrate to filament dis-
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tance which is more for sample 2 to sample 4. Due to this large separation the size of grains in diamond film is reduced. The other important observation is the reduction in thickness by decreasing grain size i.e. from sample 2–4 (indicated in Figs. 2–4). These two points are discussed separately. Comparison of sample 1 with sample 2 indicates that grain size is decreased from micrometer to nanometer range at low feed gas ratios, without the addition of Ar, N2 or high methane concentration but by increasing separation between substrate and filament (DFS) which in turn increases the filament temperature in comparison to small DFS. We will discuss it by considering that the roles performed by Ar in the formation of nanodiamond can also be played by CH4/H2 gas phase chemistry at high filament temperature. It is clearly evident that two factors are important in the gas phase chemistry, first is atomic hydrogen for the nucleation and stabilization of diamond phase and the second is C2s dimer as the dominant growth species due to which the crystallite size drastically drops. Lin et al. proposed that the effects of Ar in altering the balance of gas phase chemistry from C1-dominated to C2dominated growth precursors is the cause of transformation of well faceted polycrystalline diamond into nanocrystalline diamond [13]. C2 dimer causes high nucleation rate which is promoting nanosized grains. Ar atoms achieve energy from the heater and electrons, which are emitted by hot tungsten filament. By the collision processes energy is transferred to the molecules such as CH4 or H2 for their dissociation. In this way active species like C2 dimer can be formed. Ar promotes secondary nucleation resulted in nanograin sized thick films [23]. Considering the gas phase chemistry in terms of distance between substrate and filament (DFS), large separation between filament and substrate causes two effects, one is the increased filament temperature which now require more input power to raise the substrate temperature to 1023 K and other is long distance traveled by hydrocarbon radicals. The atomic hydrogen and C2 dimer necessary for the diamond deposition can also be obtained when temperature of the filament is high. The change in gas phase chemistry with increasing filament temperature is discussed by many authors [24,25]. The high filament temperature is beneficial to the growth of high quality diamond films as well for the decomposition of molecular hydrogen into atomic hydrogen. It is observed that the concentrations of atomic hydrogen and of CH+ increase with filament temperature, both are related with the formation of diamond component. Moreover, C2H2 and CH3 increased sharply above a filament temperature of 2200 K and the molar ratios calculated by Goodwin et al. 1–3, respectively [24]. But CH3 recombines at low substrate temperature of nearly 1000 K. The recombination of CH3 to CH4 or C2H2 at low substrate temperatures occurs either through homogeneous chemistry occurring near the substrate, or heterogeneous chemistry on the substrate. At high TF (TF > 2200 K), in comparison to CH3, C2H2 or C2H6 shows more consistency and also a conversion of methane (CH4) to acetylene (C2H2) increases sharply above a filament temperature of 2200 K [25]. It implies that at moderate substrate temperature TS 1023 K methyl is consumed and acetylene is dominant due to which occurred high nucleation density (1010 cm2). C2 is produced by the fragmentation of C2H2 or C2H6 which causes the size to drop. At comparatively low TF as in sample 1 the atomic hydrogen is enough but C2 is not enough and that is the reason which resulted micro grain sized diamond films as in sample1 at low TF. Considering with reference to separation between the substrate and filament and assuming that changes in the concentration of reaction species with filament temperature are not significant. For sample 1, DFS is small (5 mm) in comparison to sample 2–4 (15 mm). Atomic hydrogen is more in the region near to filament [26] and hence more possibility of secondary nucleation suppres-
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sion and resulted in the growth of diamond films consisting of large grain size. The important hydrocarbon species CH3 has less possibility of consumption in this region as mean free path has reduced by decreasing DFS, whereas in case of comparatively large DFS, mean free path is large and more recombination reactions took place from CH3 to C2H2 or C2H6. The other observation is the reduction in the thickness by decreasing grain size i.e. from sample 2–4. Increasing the filament temperature (TF) above 2200 K high concentration of C2 dimer (due to recombination of CH3) is obtained which resulted in nanosized grains. But high TF cannot overcome secondary nucleation suppression caused by atomic hydrogen as it is also increased with increasing TF. C2 concentration is saturated with time as neither the concentration of CH4 is increased nor Ar is added in place of hydrogen in input gases but atomic hydrogen increases as in the input gases ratio more than 98% is hydrogen gas. Renucleation is not promoted by atomic hydrogen and the nanograins present in the film acts as nuclei and start growing in size with the increase in deposition time as is observed from sample 2 to 4. That is the reason of increased grain size with increasing thickness. Synthesizing diamond films with nano sized grains using low concentration of methane can improve film quality indicated by FWHM in Raman spectra because it is observed that high methane concentration can be a cause of poor quality film [27]. 5. Conclusion In summary, diamond nanocrystallites were obtained using HFCVD technique. The grain sizes were found in the range of 20– 200 nm (for thin films). Increasing the separation between filament and substrate changes the gas phase chemistry and increases the concentration of C2 dimer, due to more consistency of acetylene than methyl and more consumption of methyl, and C2 dimer is necessary for the deposition of nanodiamond crystallites. C2 dimer causes high nucleation rate which is promoting nanosized grains. With the increase in deposition time, atomic hydrogen suppress secondary nucleation ruling out nanosized grains formation and allowing their growth only resulted in micrograin sized diamond film. This work helps in understanding the deposition of thin
diamond films with grain sizes in nanometer range at low concentration of methane which results in improved quality films and its drawback being thickness dependent size for CH4/H2 as reactants gases. References [1] J.A. Carlisle, O. Auciello, Electrochem. Soc. Interf. 12 (2003) 28. [2] C. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A. Erdemir, C. Bindal, G. Fensake, Thin Solid Film 270 (1995) 154. [3] D. Zhou, A.R. Krauss, L.C. Qin, T.G. McCauley, D.M. Gruen, J. Appl. Phys. 83 (1998) 540. [4] D.M. Gruen, S. Liu, A.R. Krauss, J. Luo, X. Pan, Appl. Phys. Lett. 64 (1994) 1502. [5] A.N. Goyette, Y. Matsuda, L.W. Anderson, J.E. Lawler, J. Vac. Sci. Technol. A 16 (1998) 337. [6] A.N. Goyette, J.E. Lawler, L.W. Anderson, D.M. Gruen, T.G. McCauley, D. Zhou, A.R. Krauss, Plasma Source Sci. Technol. 7 (1998) 149. [7] Z. Remes, A. Kromka, M. Vanecek, A. Grinevich, H. Hartmannova, S. Kmoch, Diam. Relat. Mater. 16 (2007) 671. [8] T. Sharda, M. Umeno, T. Soga, T. Jimbo, Appl. Phys. Lett. 77 (2000) 4304. [9] Y.C. Lee, D. Pradhan, S.J. Lin, C.T. Chia, H.F. Cheng, I.N. Lin, Diam. Relat. Mater. 14 (2005) 2055. [10] Y.C. Chen, C.Y. Chen, N.H. Tai, Y.C. Lee, S.J. Lin, Diam. Relat. Mater. 15 (2006) 324. [11] Y.K. Liu, P.L. Tso, I.N. Lin, Y. Tzeng, Y.C. Chen, Diam. Relat. Mater. 15 (2006) 234. [12] G.F. Zhang, D.S. Geng, Z.J. Yang, Surf. Coat. Technol. 122 (1999) 268. [13] T. Lin, G.Y. Yu, A.T.S. Wee, Z.X. Shen, K.P. Loh, Appl. Phys. Lett. 77 (2000) 2692. [14] C.S. Abreu, M. Amaral, A.J.S. Fernandes, F.J. Oliveira, R.F. Silva, J.R. Gomes, Diam. Relat. Mater. 15 (2006) 739. [15] Y.P. Ma, F.H. Sun, H.G. Xue, Z.M. Zhang, M. Chen, Diam. Relat. Mater. 16 (2007) 481. [16] Y.F. Zhang, F. Zhanga, Q.J. Gao, X.F. Peng, Z.D. Lin, Diam. Relat. Mater. 10 (2001) 1523. [17] F. Zhang, Y.F. Zhang, Q.J. Gao, S.L. Zhang, X.F. Peng, Z.D. Lin, Chin. Phys. Lett. 17 (2000) 376. [18] Y.M. LeGrice, R.J. Nemanich, J.T. Glass, Y.H. Lee, R.A. Rudder, R.J. Markunas, Mater. Res. Symp. Proc. 62 (1990) 219. [19] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) 121405R. [20] R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Shroder, J. Vac. Sci. Technol., A 6 (1988) 1783. [21] T. DebRoy, K. Tankala, W.A. Yaarbrough, J. Appl. Phys. 68 (1990) 2424. [22] J. Cui, R. Fang, J. Appl. Phys. 81 (1997) 2856. [23] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [24] E.J. Corat, D.G. Goodwin, J. Appl. Phys. 74 (1993) 2021. [25] F.G. Celii, J.E. Butler, Appl. Phys. Lett. 54 (1989) 1031. [26] K.H. Chen, M.C. Chuang, C.M. Penney, W.F. Banholzer, J. Appl. Phys. 71 (1992) 1485. [27] A. Gicquel, K. Hussouni, Francois, J. Achard, Curr. Appl. Phys. 1 (2001) 479.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Synthesis of thin diamond films from faceted nanosized crystallites Sobia Allah Rakha *, Shumin Yang, Zhoutong He, Ishaq Ahmed, Dezhang Zhu, Jinlong Gong Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 30 May 2008 Received in revised form 16 June 2008 Accepted 24 June 2008 Available online 1 July 2008 PACS: 81.15Gh 68.55 68.37 81.07
a b s t r a c t Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 1010 cm2) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Hot filament CVD Nanocrystalline Diamond films Electron microscopy
1. Introduction Nowadays nanodiamond thin films are getting particular attention because of their particularities such as smooth surfaces and outstanding field emission properties. Having the outstanding properties of CVD diamond, but ultra smooth surface, nanocrystalline diamond (NCD) is expected to be ideal material for applications in a variety of fields such as optics, electronics, and also biomedicine and biosensors [1]. Up to now, a number of ways have been used to deposit nanocrystalline diamond films using microwave plasma chemical vapor deposition (MPCVD) [2–7]. Including the bias enhance pretreatment followed by MPCVD [8–11]. The hot filament chemical vapor deposition (HFCVD) based deposition technique allows the diamond deposition in a simple way, overcoming some of the limitations of other more sophisticated CVD techniques such as MPCVD. However, for the hot filament chemical vapor deposition (HFCVD), few reports are present for the growth of NCD on Si substrate followed by some conditions including high methane concentration or addition of Ar. In 1999 Zhang et al. reported that diamond nuclei was reduced in size due to addition of nitrogen into the CH4/H2 plasma gas in a hot filament chemical vapor deposition process and density also increases at higher nitrogen concentration [12]. * Corresponding author. Tel.: +86 2159556934. E-mail address: [email protected] (S.A. Rakha). 1567-1739/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.06.010
Lin et al. first discussed the compositional mapping of the Ar– CH4–H2 system for polycrystalline to nanocrystalline diamond film growth by HFCVD [13]. NCD growth using Ar and high concentration of CH4 on substrates other than Si are also reported in the literature [14,15] as well as on Si substrate [16]. Zhang et al. tried to eliminate the function of Ar addition, but the results indicated that high methane concentration or argon addition have effect on the formation of nanocrystallites [17]. So far the methods adopted for the deposition of nanodiamond films in HFCVD using CH4/H2 as reactants resulted in nanograin sized thin films at the cost of high methane concentration or applying particular pretreatments prior to deposition. Details are listed in Table 1 to highlight the difference of present work with previous reports. In this paper, we report on a method for the synthesis of improved quality thin diamond films consist of nano sized grains under the deposition conditions generally used for microcrystalline diamond films. The distance between the filament and substrate is increased while maintaining the substrate temperature to same value. The effects of this large separation on the gas phase chemistry results in reduced grain size. The effect of deposition time on grains size and thickness is discussed. 2. Experimental Diamond films were grown on silicon (Si) wafers by the use of the hot filament chemical vapor deposition (HFCVD) method. Mir-
S.A. Rakha et al. / Current Applied Physics 9 (2009) 698–702
ror polished N-type Si(1 0 0) wafer was used as substrates. Prior to deposition, the substrates were ultrasonically cleaned in acetone, de-ionized water and 5% HF to remove the oxide layer, followed by ultrasonic scratching in an acetone suspension with nano diamond powder to promote the nucleation of crystalline diamond on Si(1 0 0) before deposition. Silicon substrates on the molybdenum holder were placed into the reaction chamber, which has a base pressure of 102 Pa. The substrate temperature was measured by a thermocouple. After the samples were heated to and stabilized at a temperature of about 1023 K, hydrogen gas mixed with 1.5% methane was fed into the chamber at a flow rate of 100 SCCM as a source gas which was thermally activated by tungsten filament. The ratios of gases were controlled by precision mass flow controller. The total pressure of the chamber was kept at 30 torr. The filament temperature of 2200 K heated the substrate to a temperature of 1023 K at a distance of 5 mm from the filament. Under these conditions, a continuous diamond film with typical crystal size of approximately 1 lm and thickness of 2.5 lm is obtained which is always observed to grow. During deposition of nanograin sized diamond films, however, the substrate temperature was maintained at nearly 1023 K at a distance of 15 mm from the filament by increasing the input power heating the filament. The tungsten filaments heated to comparatively higher temperatures were used for the deposition of thin diamond films consist of nanometer grains. The samples of the present investigation were analyzed by scanning electron microscopy (SEM, LEO 1530VP, morphology), high resolution transmission electron microscopy (HRTEM, JEOL JEM-2011 operated at 200 kV). Raman spectra were obtained using a micro Raman spectrometer with Ar+ laser 514 nm line as the excitation source.
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Fig. 1. SEM image showing MCD film grown with CH4–H2 feed gases for 2 h on Si substrate by HFCVD.
3. Results The SEM image of the as grown film deposited for 2 h using CH4/H2 1.5% as feed gases and at a distance of about 5 mm between the substrate and filament in HFCVD (sample 1) is shown in Fig. 1. The film consists of crystal grains with an average size of 1 lm. The SEM image of the as grown diamond film deposited for 2 h using CH4/H2 1.5% as feed gases and at a distance of about 15 mm between the substrate and filament in HFCVD (sample 2) is shown in Fig. 2a. The film consists of crystal grains with an average size of 200 nm or less with a thickness of 2 lm. The nucleation density of diamond observed was about 1010 cm2 s1, comparable to that of by biasing way. Raman spectrum in Fig. 2b confirmed that these grains are diamond crystallites in good crystalline shape. The Raman spectra of the CVD diamond films, in general, consist of sharp diamond peak nearly at 1332 cm1 with a full width at half maximum (FWHM) in the range of 5–16 cm1. The FWHM depends on the film deposition conditions and is commonly used as a measure of the diamond quality [18]. Micro Raman spectrum of this film in Fig. 2b shows peaks at 1133, 1332, 1477, and 1602 cm1. The sharp peak at 1332 cm1 is apparent, indicative of good quality CVD diamond and is characterized by FWHM equal to 9.5 cm1 calculated after baseline subtraction. A peak at 1602 cm1 is referred as G peak, which is a characteristic of sp2 carbon [18]. The peaks at 1133 cm1 and 1477 cm1 are sometimes assigned to nanodiamond phase [20]. However, there is certain ambiguity in these peaks. Ferrari and Robertson have assigned these two peaks trans-polyacetylene segments present at grain boundaries and surfaces of diamond films [19]. However, these two peaks are observed in nanodiamond thin films and can be a convenient probe for nanodiamond thin films.
Fig. 2a. SEM image showing NCD thin film grown with CH4–H2 feed gases for 2 h on Si substrate by HFCVD.
Fig. 2b. Raman spectrum of the corresponding NCD thin film.
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Fig. 3a shows the SEM image of the film deposited for 1 h (sample 3). Nano sized grains with an average size of 100 nm with well facets can be seen in the SEM image with film thickness of 1 lm. The corresponding Raman spectrum in Fig. 3b indicates the characteristic diamond peak at 1332 cm1 and the nanodiamond peak at 1134 cm1. However, the peaks are not as sharp. Fig. 4a shows the SEM image of sample 4 deposited for 30 min (sample 4). Nanosized grains with an average grain size less than 20 nm can be seen and thickness of the film is reduced to 500 nm. Nanocrystalline diamond peak around 1140 cm1 did not appear in the Raman spectrum in Fig. 4b. This is because of the more existence of sp2 bonded carbon at this scale and sp2 bonded carbon is much more sensitive than sp3 (diamond) in Raman spectrum [20]. Figs. 5 and 6 show the high resolution transmission electron microscope image of the film grown for 30 min (sample 4). The spacing of the lattice fringes were measured to be 0.205 nm, matching well with interplanar distance between the diamond {1 1 1} planes. The HRTEM observations show that the diamond grains are about 20 nm, with lot of small sized grains embedded in a matrix of amorphous carbon. In Fig 5, we note there is a grain (indicated by arrow) with clear edges. The enlarged image of this grain is shown in Fig. 6, indicating the size of the grain about
Fig. 4a. SEM image showing NCD thin film grown 30 min on Si substrate.
Fig. 4b. Raman spectrum of the NCD thin film deposited for 30 min. Fig. 3a. SEM images of NCD thin film grown for 1 h on Si substrate.
20 nm. The inset of the Fig. 6 shows the diffraction pattern of the grain. The diffraction pattern is similar to single crystal pattern indicating that the grain is the single crystallite of diamond. 4. Discussion
Fig. 3b. Raman spectrum of the NCD thin film deposited for 1 h.
In hot filament technique, with the mixture of methane and hydrogen, various species formation depends upon many factors such as the temperature of the hot filament, the system pressure, the composition and the flow rate of the incoming gas, and the extent of various chemical reactions on and near the filament surface. The detailed diamond deposition mechanism depends not only on the transport mechanism of nutrient species from filament to substrate surface but also on the surface chemical reaction kinetics. DebRoy et al. [21] showed that diffusion is the dominant transport mechanism for the various species to transport from the hot filament to the substrate. H atoms and hydrocarbon radicals such as + CH3, CHþ 3 , C2H2, CH3, CH, CH etc., are produced mainly on the filament or in the near filament vicinity. It is suggested that atomic hydrogen and CH+ correlated with the formation of diamond component, whereas the CH relates to the presence of amorphous carbon in the films [22].
S.A. Rakha et al. / Current Applied Physics 9 (2009) 698–702
Fig. 5. Low magnification TEM image showing the mixture of matrix and nanocrystallites. The inset shows the selected area diffraction pattern of the area including the matrix and nanocrystallites.
Fig. 6. High magnification TEM image of a nanodiamond grain (indicated by an arrow Fig. 5).
The methods used for nanodiamond growth in HFCVD in order to get nanograins required special pretreatments, high concentration of methane or Ar gas for deposition on Si substrates or other than Si. Results presented here showed that nanocrystallites can also be obtained by changing separation between substrate and filament and keeping rest of the conditions same as for microcrystalline diamond. Comparison of sample 1 to samples 2–4, experimental conditions including input gas ratios, substrate temperature, pressure and deposition time are same except the substrate to filament dis-
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tance which is more for sample 2 to sample 4. Due to this large separation the size of grains in diamond film is reduced. The other important observation is the reduction in thickness by decreasing grain size i.e. from sample 2–4 (indicated in Figs. 2–4). These two points are discussed separately. Comparison of sample 1 with sample 2 indicates that grain size is decreased from micrometer to nanometer range at low feed gas ratios, without the addition of Ar, N2 or high methane concentration but by increasing separation between substrate and filament (DFS) which in turn increases the filament temperature in comparison to small DFS. We will discuss it by considering that the roles performed by Ar in the formation of nanodiamond can also be played by CH4/H2 gas phase chemistry at high filament temperature. It is clearly evident that two factors are important in the gas phase chemistry, first is atomic hydrogen for the nucleation and stabilization of diamond phase and the second is C2s dimer as the dominant growth species due to which the crystallite size drastically drops. Lin et al. proposed that the effects of Ar in altering the balance of gas phase chemistry from C1-dominated to C2dominated growth precursors is the cause of transformation of well faceted polycrystalline diamond into nanocrystalline diamond [13]. C2 dimer causes high nucleation rate which is promoting nanosized grains. Ar atoms achieve energy from the heater and electrons, which are emitted by hot tungsten filament. By the collision processes energy is transferred to the molecules such as CH4 or H2 for their dissociation. In this way active species like C2 dimer can be formed. Ar promotes secondary nucleation resulted in nanograin sized thick films [23]. Considering the gas phase chemistry in terms of distance between substrate and filament (DFS), large separation between filament and substrate causes two effects, one is the increased filament temperature which now require more input power to raise the substrate temperature to 1023 K and other is long distance traveled by hydrocarbon radicals. The atomic hydrogen and C2 dimer necessary for the diamond deposition can also be obtained when temperature of the filament is high. The change in gas phase chemistry with increasing filament temperature is discussed by many authors [24,25]. The high filament temperature is beneficial to the growth of high quality diamond films as well for the decomposition of molecular hydrogen into atomic hydrogen. It is observed that the concentrations of atomic hydrogen and of CH+ increase with filament temperature, both are related with the formation of diamond component. Moreover, C2H2 and CH3 increased sharply above a filament temperature of 2200 K and the molar ratios calculated by Goodwin et al. 1–3, respectively [24]. But CH3 recombines at low substrate temperature of nearly 1000 K. The recombination of CH3 to CH4 or C2H2 at low substrate temperatures occurs either through homogeneous chemistry occurring near the substrate, or heterogeneous chemistry on the substrate. At high TF (TF > 2200 K), in comparison to CH3, C2H2 or C2H6 shows more consistency and also a conversion of methane (CH4) to acetylene (C2H2) increases sharply above a filament temperature of 2200 K [25]. It implies that at moderate substrate temperature TS 1023 K methyl is consumed and acetylene is dominant due to which occurred high nucleation density (1010 cm2). C2 is produced by the fragmentation of C2H2 or C2H6 which causes the size to drop. At comparatively low TF as in sample 1 the atomic hydrogen is enough but C2 is not enough and that is the reason which resulted micro grain sized diamond films as in sample1 at low TF. Considering with reference to separation between the substrate and filament and assuming that changes in the concentration of reaction species with filament temperature are not significant. For sample 1, DFS is small (5 mm) in comparison to sample 2–4 (15 mm). Atomic hydrogen is more in the region near to filament [26] and hence more possibility of secondary nucleation suppres-
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sion and resulted in the growth of diamond films consisting of large grain size. The important hydrocarbon species CH3 has less possibility of consumption in this region as mean free path has reduced by decreasing DFS, whereas in case of comparatively large DFS, mean free path is large and more recombination reactions took place from CH3 to C2H2 or C2H6. The other observation is the reduction in the thickness by decreasing grain size i.e. from sample 2–4. Increasing the filament temperature (TF) above 2200 K high concentration of C2 dimer (due to recombination of CH3) is obtained which resulted in nanosized grains. But high TF cannot overcome secondary nucleation suppression caused by atomic hydrogen as it is also increased with increasing TF. C2 concentration is saturated with time as neither the concentration of CH4 is increased nor Ar is added in place of hydrogen in input gases but atomic hydrogen increases as in the input gases ratio more than 98% is hydrogen gas. Renucleation is not promoted by atomic hydrogen and the nanograins present in the film acts as nuclei and start growing in size with the increase in deposition time as is observed from sample 2 to 4. That is the reason of increased grain size with increasing thickness. Synthesizing diamond films with nano sized grains using low concentration of methane can improve film quality indicated by FWHM in Raman spectra because it is observed that high methane concentration can be a cause of poor quality film [27]. 5. Conclusion In summary, diamond nanocrystallites were obtained using HFCVD technique. The grain sizes were found in the range of 20– 200 nm (for thin films). Increasing the separation between filament and substrate changes the gas phase chemistry and increases the concentration of C2 dimer, due to more consistency of acetylene than methyl and more consumption of methyl, and C2 dimer is necessary for the deposition of nanodiamond crystallites. C2 dimer causes high nucleation rate which is promoting nanosized grains. With the increase in deposition time, atomic hydrogen suppress secondary nucleation ruling out nanosized grains formation and allowing their growth only resulted in micrograin sized diamond film. This work helps in understanding the deposition of thin
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