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Transparent ultrananocrystalline diamond films on quartz substrate
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Diamond & Related Materials 17 (2008) 476 – 480 www.elsevier.com/locate/diamond
Transparent ultrananocrystalline diamond films on quartz substrate P.T. Joseph a , Nyan-Hwa Tai a , Yi-Chun Chen b , Hsiu-Fung Cheng c,⁎, I-Nan Lin d a
Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu, 300, Taiwan b Department of Physics, National Cheng-Kung University, Tainan, 600, Taiwan c Department of Physics, National Taiwan Normal University, Taipei, 116, Taiwan d Department of Physics, Tamkang University, Tamsui, 251, Taiwan Available online 25 October 2007
Abstract Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported. © 2007 Elsevier B.V. All rights reserved. Keywords: UNCD; MPECVD; Transmittance
1. Introduction Ultrananocrystalline diamond (UNCD) is a unique formation of chemical vapor deposited (CVD) diamond, which can be attained if hydrogen is reinstated by inert gases like Ar or He [1–5] in an archetypal feedstock, 1% CH4 + 99% H2, for microcrystalline diamond (MCD) growth. Although intensive studies of nucleation processes for the growth of diamond have already been done, most focused on the effects on growth of MCD films, which are rather dissimilar from those of UNCD films [5–8]. In CH4/H2 plasma, the substrate temperature is an important parameter in growing high quality diamond. The plasma chemistry for the growth of UNCD is different from the MCD growth process and substrate temperature is not an important parameter. Growth process can be carried out at temperatures as low as 400 °C without significantly altering the qualities of UNCD [5,8]. The ultra-smooth surface characteristic and many other excellent properties of UNCD provide the flexibility of ⁎ Corresponding author. Tel.: +886 2 29331075; fax: +886 2 29326408. E-mail address: [email protected] (H.-F. Cheng). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.10.013
this material make it as a supreme material in contemporary applications, such as protective coatings, infrared windows, MEMS, field emission, SAW devices, and biomedical applications [9–16]. Recent focus of many researchers was mostly on the growth of UNCD on silicon substrate and many effective growth methods are already in practice. The growth of UNCD on substrates other than silicon is interesting, which can be used for applications where silicon substrates are not pertinent or to replace silicon with improved device performance, an example being the application of UNCD in X-ray optics. Nanocrystalline diamond (NCD) grown on substrates other than silicon has already been reported [10–15]. To grow diamond films on quartz substrates, a scratching pretreatment process on the substrate has been widely adopted. However, this method creates strapping defects on the substrate surface [11,12]. Nevertheless, the UNCD growth process on substrates other than silicon still necessitates to be explored. In this present work, we have investigated the effects of UNCD growth process done at low temperature (without substrate heating prior to the growth of UNCD) on transparent quartz substrate. Additionally, we have successfully grown
P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480
NCD and MCD films on quartz substrates by using UNCD as an initial nucleation layer. The detailed microstructure, influence of UNCD for the growth of MCD and NCD, and comparison between the transparent and dielectric properties of grown UNCD, NCD, and MCD are discussed. 2. Experimental methods The UNCD film was grown in microwave plasma enhanced chemical vapor deposition (MPECVD) system (IPLAS Cyrannus). Pretreatment on substrates were performed by ultrasonication in nanodiamond and titanium mixed powders (1:1 wt.%) containing methanol solution for 45 min. The deposition of UNCD on quartz substrate was carried out in methane (CH41%) and argon (Ar) plasma for 3 h (to achieve a thickness of ∼ 450 nm) with a microwave power of 1200 W. The pressure was maintained at 120 Torr and the flow rate was 100 sccm. The growth process was carried out at low temperatures (b465 °C) by without heating the substrate. To grow NCD and MCD films, quartz substrates were initially subjected to grow UNCD for 60 min, then changed the plasma to CH4/H2 medium (CH4 − 2% for NCD and − 1% for MCD growth). The NCD film growth was carried out at 1500 W with pressure of 50 Torr for 22 min and MCD film at 1700 W with pressure of 70 Torr for 17 min to achieve the same thickness as UNCD film. The films were characterized by field emission scanning electron microscopy (FESEM, Joel-6500). Raman spectroscopy measurements were performed at room temperature, using a 514.5 nm laser as the excitation source (Renishaw, Micro-Raman). Transparent properties were measured with a UV-VIS-NIR spectrophotometer (JASCO, V-570). The baseline correction has been done with pretreated quartz substrates prior to the transmittance property measurement of diamond films. The microwave properties of the deposited films were measured by using an evanescent microwave probe (EMP, Ariel-100) method, which can perform nondestructive measurement of the local dielectric properties of the samples. The details about this system and measurement process can be found elsewhere [17–21].
477
probability of reducing the SiO2 materials from the quartz substrates was greatly reduced. All these factors promote the growth of UNCD [2,22]. We have successfully grown UNCD on substrates at low temperatures with very high nucleation densities (∼1011–1012 particles/cm2). The CH4/Ar-plasma tends to form nano-sized diamond grains, ∼ 3–5 nm (UNCD), due to low activation energy for secondary nucleation of diamond grains, which render the formation of larger diamond grains relatively difficult. To grow
3. Results and discussion 3.1. Surface morphology of UNCD, NCD, and MCD films The growth of UNCD, NCD, and MCD films on quartz substrates were investigated at the outset. The surface morphology of UNCD film deposited on quartz substrate appeared to possess ultra-smooth surface characteristics at the nanoscale (Fig. 1(a)). An easy growth of UNCD was achieved on substrate due to the utilization of Ar-plasma. Such a plasma triggers the dissociation of CH4 species, forming the carbon dimmers (C= C), which can be transformed into sp3-bonded carbon on diamond with relatively lower activation energy, as compared to the transformation of methyl group (CH3–) usually formed in the H2-plasma [1,2]. Moreover, there is markedly smaller proportion of reducing species, such as the neutral and ionized hydrogen species, in CH4/Ar-plasma than those in CH4/H2-plasma, such that the
Fig. 1. FESEM images of (a) UNCD, (b) NCD, and (c) MCD films grown on quartz substrates.
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P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480 Table 1 Optical properties of UNCD, NCD, and MCD films Sample name
Tmax (%)
n
κ (×10− 4) (1/nm)
Eg (eV)
UNCD NCD MCD
91 86 72
2.04 2.18 2.11
5.53 6.64 5.71
2.06 1.83 1.77
Tmax: transmittance maximum; n: refractive index; κ: absorption coefficient; and Eg: energy gap.
Fig. 2. Raman spectra of UNCD, NCD, and MCD films.
ogies of NCD and MCD films, respectively. The NCD and MCD films on quartz substrates were successfully grown by such a new approach, with a UNCD layer as a nucleation layer for the growth of NCD and MCD. Initially UNCD growth was performed for 60 min to grow UNCD layer (using CH4/Ar plasma) on pretreated quartz substrate then changed the plasma medium to CH4/H2 to grow NCD and MCD films. The CH4 and H2 percentage in the plasma influences the grain size and are very apparent from the FESEM images (Fig. 1(b) and (c)). MCD film is showing larger grain size than the NCD and UNCD films. The successful growth of UNCD, NCD, and MCD films are further clarified from the specific features of Raman spectra, which are detailed discussed in the following part.
larger sized grains, such as submicron sized grains for NCD and micron-sized grains for MCD, CH4/H2-plasma is required. However, such a plasma usually induce the formation of methyl group (CH3–), which need to overcome larger activation energy for the formation of diamond nuclei and, thereafter, requires higher substrate temperature to grow diamond films. The carbon-to-SiO2 interaction is thus inevitable, which impedes markedly the formation of diamond nuclei on quartz substrates. It should be noted that, previous reports suggest that the pretreatment process as long as 8 h is required for the successful growth of MCD on quartz [13]. Interestingly, in our studies, we have carried out the pretreatment process using CH4/Ar-plasma for a short time (60 min), which actually forms a UNCD layer on the quartz substrates. Such a UNCD layer serves as seeding for subsequently growing MCD or NCD films. Additionally, by using this method, no substrate heating was required to grow the NCD or MCD layer, whereas the initial growth temperature higher than 700 °C are needed for the conventional MCD growth process. Fig. 1(b) and (c) show the surface morphol-
The UNCD, NCD, and MCD films deposited on quartz substrates were analyzed by using the non-destructive Raman spectroscopy and the results are shown in Fig. 2. A sharp Raman resonance peak at 1332 cm− 1, characteristic of diamond structure, is clearly observed for MCD films, which manifest that using UNCD as nucleation layer is a successful seeding process for growing the MCD grains on quartz. It should be noted that, it is extremely difficult by using conventional MPECVD process to grow diamond films on quartz substrates, without special pre-coatings to facilitate the formation of diamond grains. The G band is also observed in the MCD films
Fig. 3. Optical transmittance spectra of UNCD, NCD, and MCD films from UV to near IR region.
Fig. 4. EMP frequency response of quartz substrate, UNCD, NCD, and MCD films.
3.2. Raman spectroscopy studies of UNCD, NCD, and MCD films
P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480
479
at around 1550 cm − 1 . The characteristic Raman peak (1332 cm− 1) decreases in relative intensity as the grain size becomes smaller, and finally a broaden peak appear at around 1350 cm− 1 for NCD and UNCD films, which is assigned as D band and a similar trend had been reported by many researchers [8,23]. The peaks at 1150 and 1480 cm− 1 are observed in the NCD films, which are affirmed to be trans-polyacetylene, presented in the grain boundaries of the films [23]. For low temperature grown UNCD film on quartz substrate, a peak is evident at around 1190 cm− 1, which is in accord to the observation in the literature, where Ferrari et. al., reported that, the trans-polyacetylene peak could be shifted with the laser excitation wavelength [23]. Restated, the trans-polyacetylene peak in the NCD film was observed around 1133 cm− 1, which was shifted to 1190 cm− 1 for the UNCD film, possibly due to the differences in the grain size.
response of UNCD, NCD, and MCD are 13.02, 8.16, and 7.05, respectively, i.e., the dielectric constant of UNCD materials is not significantly different from that of NCD and MCD materials. The smallness in the variation of frequency response for these diamond films might be because the effect of quartz substrate on the frequency response is large due to the small thickness of diamond films, as compared with that of the quartz substrates. Nevertheless, this measurement indicates that the diamond films thus grown do not impose large absorption to the microwave. Had the conventional seeding layer, such as tungsten or amorphous carbon layer, was utilized for enhancing the formation of diamond nuclei on quartz substrates, significant amount of absorption to microwave is expected. Restated, the utilization of UNCD can facilitate the formation of diamond nuclei on quartz substrates without inducing deleterious sideeffects, such as the large absorption to microwave.
3.3. Dielectric property of UNCD, NCD, and MCD films
4. Conclusions
The transparent properties of UNCD, NCD, and MCD films grown on quartz substrates were measured from UV to near IR region and are shown in Fig. 3. The method of measurement is demonstrated in the inset of the Fig. 3. Above 90% transmittance is observed for UNCD film at around 1530 (nm) (IR region), whereas NCD and MCD films are showing 86% and 72% transmittance, respectively, in the IR region. The transmittance oscillates due to the interference patterns. The detailed optical parameters of the films were estimated from the transmittance spectra and are summarized in Table 1. The calculated refractive index (n = 2.04-2.11) and absorption coefficient (κ) of NCD is slightly higher than other films possibly as a result of the higher non diamond carbon content in the NCD film. The transmittance property of diamond films is mainly influenced by the grain size of the sample. In the case of UNCD films, the grain size (b 10 nm) is much smaller than the wavelength of the incident IR light, and hence the scattering of the light wave by grain boundaries is minimal. In contrast, for MCD films, the grain size (∼ μ) is the same order of magnitude as the IR wavelength, which induces large extent of scattering. Moreover, the increase in grain size results in the increase of surface roughness, which further results in the increase in light scattering and thus diminishes the transparent properties. The non-diamond carbon (graphite/trans-polyacetylene/amorphous carbon) contents may also play a role in the transparent properties. However, our results show that their role is not very apparent in the transparent properties of diamond films. The outstanding transparent nature of UNCD film as compared to NCD or MCD is mainly due to its ultra-nano grain size and ultra-smooth surface characteristics. EMP measurement method is used to analyze the dielectric properties of the different grain sizes of diamond films deposited on quartz substrates in the MHz region. The result of frequency response of UNCD, NCD, MCD, and quartz is shown in Fig. 4. The diamond films are not showing much variation in their frequency responses. A slight shift from the frequency response of quartz due to the diamond films is noticeable. The estimated dielectric constant from the frequency
Successful growth of UNCD, NCD, and MCD films on quartz substrates were carried out and confirmed from the FESEM images and Raman spectroscopy studies. In addition to the low temperature growth process of UNCD film, the new successful method to grow MCD and NCD films by without heating the substrates has been put forward, which uses UNCD as a nucleation layer for the formation of MCD and NCD grains on quartz substrates. This method can also be adopted for other substrates to grow MCD and NCD films. The comparison between the transparent properties of different grain sizes of diamond films revealed the outstanding transparent property of UNCD film as compared to NCD or MCD films. This in turn opens up a great possibility of UNCD film for transparent applications. Furthermore, EMP analysis reveals that none of the diamond films imposes marked absorption to the microwave and the dielectric properties of these films are not significantly different from one another. References [1] D. Zhou, T.G. McCauley, L.C. Qin, A.R. Krauss, D.M. Gruen, J. Appl. Phys. 83 (1998) 540. [2] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [3] W.S. Huang, D.T. Tran, J. Asmussen, T.A. Grotjohn, D. Reinhard, Diam. Relat. Mater. 15 (2006) 341. [4] Y.C. Lee, S.J. Lin, D. Pradhan, I.N. Lin, Diam. Relat. Mater. 15 (2006) 353. [5] T.G. McCauley, D.M. Gruen, A.R. Krauss, Appl. Phys. Lett. 73 (1998) 1646. [6] M. Hiramatsu, K. Kato, C.H. Lau, J.S. Foord, M. Hori, Diam. Relat. Mater. 12 (2003) 365. [7] S.T. Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 123. [8] X. Xiao, J. Birrell, J.E. Gerbi, O. Auciello, J.A. Carlisle, J. Appl. Phys. 96 (2004) 2232. [9] A.V. Sumant, D.S. Grierson, J.E. Gerbi, J. Birrell, U.D. Lanke, O. Auciello, J.A. Carlisle, R.W. Carpik, Adv. Mater. 17 (2005) 1039. [10] Y. Liou, A. Inspektor, R. Weimer, R. Messier, Appl. Phys. Lett. 55 (1989) 631. [11] W.B. Wang, F.X. Lu, Z.X. Cao, J. Appl. Phys. 91 (2002) 10068. [12] S.A. Catledge, Y.K. Vohra, P.B. Mirkarimi, J. Phys., D, Appl. Phys. 38 (2005) 1410.
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[13] D.M. Bhusari, J.R. Yang, T.Y. Wang, S.T. Lin, K.H. Chen, L.C. Chen, Solid State Comm. 107 (1998) 301. [14] R.K. Singh, D.R. Gilbert, J. Laveigne, Appl. Phys. Lett. 69 (1996) 15. [15] M.D. Fries, Y.K. Vohra, Diam. Relat. Mater. 13 (2004) 1740. [16] O. Ternyak, R. Akhvlediani, A. Hoffman, Diam. Relat. Mater. 14 (2005) 323. [17] C. Gao, X.D. Xiang, Rev. Sci. Instrum. 69 (1998) 3846. [18] T. Wei, X.D. Xiang, W.G. Wallace-Freedman, P.G. Schultz, Appl. Phys. Lett. 68 (1996) 3506. [19] D.E. Steinhauer, C.P. Valhacos, F.C. Wellstood, S.M. Anlage, C. Canedy, R. Ramesh, A. Stanishevlesky, J. Melngailis, Appl. Phys. Lett. 75 (1999) 32180.
[20] Y. Lu, T. Wei, F. Duewer, Y. Lu, N.B. Ming, P.G. Schultz, X.D. Xiang, Science 276 (1997) 2004. [21] P.T. Joseph, Y.C. Chen, Y.H. Chu, H.F. Cheng, I.N. Lin, Integr. Ferroelectr. 77 (2005) 45. [22] Y. Zhang, L. Xie, Z. Zhou, J. Sun, S.T. Lee, J. Cryst. Growth 169 (1996) 722. [23] A.C. Ferrari, J. Robertson, Phys. Rev., B 63 (2001) 121405.
Diamond & Related Materials 17 (2008) 476 – 480 www.elsevier.com/locate/diamond
Transparent ultrananocrystalline diamond films on quartz substrate P.T. Joseph a , Nyan-Hwa Tai a , Yi-Chun Chen b , Hsiu-Fung Cheng c,⁎, I-Nan Lin d a
Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu, 300, Taiwan b Department of Physics, National Cheng-Kung University, Tainan, 600, Taiwan c Department of Physics, National Taiwan Normal University, Taipei, 116, Taiwan d Department of Physics, Tamkang University, Tamsui, 251, Taiwan Available online 25 October 2007
Abstract Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported. © 2007 Elsevier B.V. All rights reserved. Keywords: UNCD; MPECVD; Transmittance
1. Introduction Ultrananocrystalline diamond (UNCD) is a unique formation of chemical vapor deposited (CVD) diamond, which can be attained if hydrogen is reinstated by inert gases like Ar or He [1–5] in an archetypal feedstock, 1% CH4 + 99% H2, for microcrystalline diamond (MCD) growth. Although intensive studies of nucleation processes for the growth of diamond have already been done, most focused on the effects on growth of MCD films, which are rather dissimilar from those of UNCD films [5–8]. In CH4/H2 plasma, the substrate temperature is an important parameter in growing high quality diamond. The plasma chemistry for the growth of UNCD is different from the MCD growth process and substrate temperature is not an important parameter. Growth process can be carried out at temperatures as low as 400 °C without significantly altering the qualities of UNCD [5,8]. The ultra-smooth surface characteristic and many other excellent properties of UNCD provide the flexibility of ⁎ Corresponding author. Tel.: +886 2 29331075; fax: +886 2 29326408. E-mail address: [email protected] (H.-F. Cheng). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.10.013
this material make it as a supreme material in contemporary applications, such as protective coatings, infrared windows, MEMS, field emission, SAW devices, and biomedical applications [9–16]. Recent focus of many researchers was mostly on the growth of UNCD on silicon substrate and many effective growth methods are already in practice. The growth of UNCD on substrates other than silicon is interesting, which can be used for applications where silicon substrates are not pertinent or to replace silicon with improved device performance, an example being the application of UNCD in X-ray optics. Nanocrystalline diamond (NCD) grown on substrates other than silicon has already been reported [10–15]. To grow diamond films on quartz substrates, a scratching pretreatment process on the substrate has been widely adopted. However, this method creates strapping defects on the substrate surface [11,12]. Nevertheless, the UNCD growth process on substrates other than silicon still necessitates to be explored. In this present work, we have investigated the effects of UNCD growth process done at low temperature (without substrate heating prior to the growth of UNCD) on transparent quartz substrate. Additionally, we have successfully grown
P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480
NCD and MCD films on quartz substrates by using UNCD as an initial nucleation layer. The detailed microstructure, influence of UNCD for the growth of MCD and NCD, and comparison between the transparent and dielectric properties of grown UNCD, NCD, and MCD are discussed. 2. Experimental methods The UNCD film was grown in microwave plasma enhanced chemical vapor deposition (MPECVD) system (IPLAS Cyrannus). Pretreatment on substrates were performed by ultrasonication in nanodiamond and titanium mixed powders (1:1 wt.%) containing methanol solution for 45 min. The deposition of UNCD on quartz substrate was carried out in methane (CH41%) and argon (Ar) plasma for 3 h (to achieve a thickness of ∼ 450 nm) with a microwave power of 1200 W. The pressure was maintained at 120 Torr and the flow rate was 100 sccm. The growth process was carried out at low temperatures (b465 °C) by without heating the substrate. To grow NCD and MCD films, quartz substrates were initially subjected to grow UNCD for 60 min, then changed the plasma to CH4/H2 medium (CH4 − 2% for NCD and − 1% for MCD growth). The NCD film growth was carried out at 1500 W with pressure of 50 Torr for 22 min and MCD film at 1700 W with pressure of 70 Torr for 17 min to achieve the same thickness as UNCD film. The films were characterized by field emission scanning electron microscopy (FESEM, Joel-6500). Raman spectroscopy measurements were performed at room temperature, using a 514.5 nm laser as the excitation source (Renishaw, Micro-Raman). Transparent properties were measured with a UV-VIS-NIR spectrophotometer (JASCO, V-570). The baseline correction has been done with pretreated quartz substrates prior to the transmittance property measurement of diamond films. The microwave properties of the deposited films were measured by using an evanescent microwave probe (EMP, Ariel-100) method, which can perform nondestructive measurement of the local dielectric properties of the samples. The details about this system and measurement process can be found elsewhere [17–21].
477
probability of reducing the SiO2 materials from the quartz substrates was greatly reduced. All these factors promote the growth of UNCD [2,22]. We have successfully grown UNCD on substrates at low temperatures with very high nucleation densities (∼1011–1012 particles/cm2). The CH4/Ar-plasma tends to form nano-sized diamond grains, ∼ 3–5 nm (UNCD), due to low activation energy for secondary nucleation of diamond grains, which render the formation of larger diamond grains relatively difficult. To grow
3. Results and discussion 3.1. Surface morphology of UNCD, NCD, and MCD films The growth of UNCD, NCD, and MCD films on quartz substrates were investigated at the outset. The surface morphology of UNCD film deposited on quartz substrate appeared to possess ultra-smooth surface characteristics at the nanoscale (Fig. 1(a)). An easy growth of UNCD was achieved on substrate due to the utilization of Ar-plasma. Such a plasma triggers the dissociation of CH4 species, forming the carbon dimmers (C= C), which can be transformed into sp3-bonded carbon on diamond with relatively lower activation energy, as compared to the transformation of methyl group (CH3–) usually formed in the H2-plasma [1,2]. Moreover, there is markedly smaller proportion of reducing species, such as the neutral and ionized hydrogen species, in CH4/Ar-plasma than those in CH4/H2-plasma, such that the
Fig. 1. FESEM images of (a) UNCD, (b) NCD, and (c) MCD films grown on quartz substrates.
478
P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480 Table 1 Optical properties of UNCD, NCD, and MCD films Sample name
Tmax (%)
n
κ (×10− 4) (1/nm)
Eg (eV)
UNCD NCD MCD
91 86 72
2.04 2.18 2.11
5.53 6.64 5.71
2.06 1.83 1.77
Tmax: transmittance maximum; n: refractive index; κ: absorption coefficient; and Eg: energy gap.
Fig. 2. Raman spectra of UNCD, NCD, and MCD films.
ogies of NCD and MCD films, respectively. The NCD and MCD films on quartz substrates were successfully grown by such a new approach, with a UNCD layer as a nucleation layer for the growth of NCD and MCD. Initially UNCD growth was performed for 60 min to grow UNCD layer (using CH4/Ar plasma) on pretreated quartz substrate then changed the plasma medium to CH4/H2 to grow NCD and MCD films. The CH4 and H2 percentage in the plasma influences the grain size and are very apparent from the FESEM images (Fig. 1(b) and (c)). MCD film is showing larger grain size than the NCD and UNCD films. The successful growth of UNCD, NCD, and MCD films are further clarified from the specific features of Raman spectra, which are detailed discussed in the following part.
larger sized grains, such as submicron sized grains for NCD and micron-sized grains for MCD, CH4/H2-plasma is required. However, such a plasma usually induce the formation of methyl group (CH3–), which need to overcome larger activation energy for the formation of diamond nuclei and, thereafter, requires higher substrate temperature to grow diamond films. The carbon-to-SiO2 interaction is thus inevitable, which impedes markedly the formation of diamond nuclei on quartz substrates. It should be noted that, previous reports suggest that the pretreatment process as long as 8 h is required for the successful growth of MCD on quartz [13]. Interestingly, in our studies, we have carried out the pretreatment process using CH4/Ar-plasma for a short time (60 min), which actually forms a UNCD layer on the quartz substrates. Such a UNCD layer serves as seeding for subsequently growing MCD or NCD films. Additionally, by using this method, no substrate heating was required to grow the NCD or MCD layer, whereas the initial growth temperature higher than 700 °C are needed for the conventional MCD growth process. Fig. 1(b) and (c) show the surface morphol-
The UNCD, NCD, and MCD films deposited on quartz substrates were analyzed by using the non-destructive Raman spectroscopy and the results are shown in Fig. 2. A sharp Raman resonance peak at 1332 cm− 1, characteristic of diamond structure, is clearly observed for MCD films, which manifest that using UNCD as nucleation layer is a successful seeding process for growing the MCD grains on quartz. It should be noted that, it is extremely difficult by using conventional MPECVD process to grow diamond films on quartz substrates, without special pre-coatings to facilitate the formation of diamond grains. The G band is also observed in the MCD films
Fig. 3. Optical transmittance spectra of UNCD, NCD, and MCD films from UV to near IR region.
Fig. 4. EMP frequency response of quartz substrate, UNCD, NCD, and MCD films.
3.2. Raman spectroscopy studies of UNCD, NCD, and MCD films
P.T. Joseph et al. / Diamond & Related Materials 17 (2008) 476–480
479
at around 1550 cm − 1 . The characteristic Raman peak (1332 cm− 1) decreases in relative intensity as the grain size becomes smaller, and finally a broaden peak appear at around 1350 cm− 1 for NCD and UNCD films, which is assigned as D band and a similar trend had been reported by many researchers [8,23]. The peaks at 1150 and 1480 cm− 1 are observed in the NCD films, which are affirmed to be trans-polyacetylene, presented in the grain boundaries of the films [23]. For low temperature grown UNCD film on quartz substrate, a peak is evident at around 1190 cm− 1, which is in accord to the observation in the literature, where Ferrari et. al., reported that, the trans-polyacetylene peak could be shifted with the laser excitation wavelength [23]. Restated, the trans-polyacetylene peak in the NCD film was observed around 1133 cm− 1, which was shifted to 1190 cm− 1 for the UNCD film, possibly due to the differences in the grain size.
response of UNCD, NCD, and MCD are 13.02, 8.16, and 7.05, respectively, i.e., the dielectric constant of UNCD materials is not significantly different from that of NCD and MCD materials. The smallness in the variation of frequency response for these diamond films might be because the effect of quartz substrate on the frequency response is large due to the small thickness of diamond films, as compared with that of the quartz substrates. Nevertheless, this measurement indicates that the diamond films thus grown do not impose large absorption to the microwave. Had the conventional seeding layer, such as tungsten or amorphous carbon layer, was utilized for enhancing the formation of diamond nuclei on quartz substrates, significant amount of absorption to microwave is expected. Restated, the utilization of UNCD can facilitate the formation of diamond nuclei on quartz substrates without inducing deleterious sideeffects, such as the large absorption to microwave.
3.3. Dielectric property of UNCD, NCD, and MCD films
4. Conclusions
The transparent properties of UNCD, NCD, and MCD films grown on quartz substrates were measured from UV to near IR region and are shown in Fig. 3. The method of measurement is demonstrated in the inset of the Fig. 3. Above 90% transmittance is observed for UNCD film at around 1530 (nm) (IR region), whereas NCD and MCD films are showing 86% and 72% transmittance, respectively, in the IR region. The transmittance oscillates due to the interference patterns. The detailed optical parameters of the films were estimated from the transmittance spectra and are summarized in Table 1. The calculated refractive index (n = 2.04-2.11) and absorption coefficient (κ) of NCD is slightly higher than other films possibly as a result of the higher non diamond carbon content in the NCD film. The transmittance property of diamond films is mainly influenced by the grain size of the sample. In the case of UNCD films, the grain size (b 10 nm) is much smaller than the wavelength of the incident IR light, and hence the scattering of the light wave by grain boundaries is minimal. In contrast, for MCD films, the grain size (∼ μ) is the same order of magnitude as the IR wavelength, which induces large extent of scattering. Moreover, the increase in grain size results in the increase of surface roughness, which further results in the increase in light scattering and thus diminishes the transparent properties. The non-diamond carbon (graphite/trans-polyacetylene/amorphous carbon) contents may also play a role in the transparent properties. However, our results show that their role is not very apparent in the transparent properties of diamond films. The outstanding transparent nature of UNCD film as compared to NCD or MCD is mainly due to its ultra-nano grain size and ultra-smooth surface characteristics. EMP measurement method is used to analyze the dielectric properties of the different grain sizes of diamond films deposited on quartz substrates in the MHz region. The result of frequency response of UNCD, NCD, MCD, and quartz is shown in Fig. 4. The diamond films are not showing much variation in their frequency responses. A slight shift from the frequency response of quartz due to the diamond films is noticeable. The estimated dielectric constant from the frequency
Successful growth of UNCD, NCD, and MCD films on quartz substrates were carried out and confirmed from the FESEM images and Raman spectroscopy studies. In addition to the low temperature growth process of UNCD film, the new successful method to grow MCD and NCD films by without heating the substrates has been put forward, which uses UNCD as a nucleation layer for the formation of MCD and NCD grains on quartz substrates. This method can also be adopted for other substrates to grow MCD and NCD films. The comparison between the transparent properties of different grain sizes of diamond films revealed the outstanding transparent property of UNCD film as compared to NCD or MCD films. This in turn opens up a great possibility of UNCD film for transparent applications. Furthermore, EMP analysis reveals that none of the diamond films imposes marked absorption to the microwave and the dielectric properties of these films are not significantly different from one another. References [1] D. Zhou, T.G. McCauley, L.C. Qin, A.R. Krauss, D.M. Gruen, J. Appl. Phys. 83 (1998) 540. [2] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [3] W.S. Huang, D.T. Tran, J. Asmussen, T.A. Grotjohn, D. Reinhard, Diam. Relat. Mater. 15 (2006) 341. [4] Y.C. Lee, S.J. Lin, D. Pradhan, I.N. Lin, Diam. Relat. Mater. 15 (2006) 353. [5] T.G. McCauley, D.M. Gruen, A.R. Krauss, Appl. Phys. Lett. 73 (1998) 1646. [6] M. Hiramatsu, K. Kato, C.H. Lau, J.S. Foord, M. Hori, Diam. Relat. Mater. 12 (2003) 365. [7] S.T. Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 123. [8] X. Xiao, J. Birrell, J.E. Gerbi, O. Auciello, J.A. Carlisle, J. Appl. Phys. 96 (2004) 2232. [9] A.V. Sumant, D.S. Grierson, J.E. Gerbi, J. Birrell, U.D. Lanke, O. Auciello, J.A. Carlisle, R.W. Carpik, Adv. Mater. 17 (2005) 1039. [10] Y. Liou, A. Inspektor, R. Weimer, R. Messier, Appl. Phys. Lett. 55 (1989) 631. [11] W.B. Wang, F.X. Lu, Z.X. Cao, J. Appl. Phys. 91 (2002) 10068. [12] S.A. Catledge, Y.K. Vohra, P.B. Mirkarimi, J. Phys., D, Appl. Phys. 38 (2005) 1410.
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