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Helium-dilution effects on thermophysical properties of hydrogenated amorphous carbon thin-films
Diamond & Related Materials 32 (2013) 1–6
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Helium-dilution effects on thermophysical properties of hydrogenated amorphous carbon thin-films Yun Young Kim a, Rozidawati Awang b, Sridhar Krishnaswamy a, Hasan Adli Alwi b,⁎ a b
Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA School of Applied Physics, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 5 April 2012 Received in revised form 21 September 2012 Accepted 27 November 2012 Available online 5 December 2012 Keywords: Amorphous hydrogenated carbon Thermal properties Microstructure Chemical vapor deposition
a b s t r a c t Thermophysical properties of hydrogenated amorphous carbon thin-films were characterized in the present investigation. Samples were deposited in a direct current plasma enhanced chemical vapor deposition (DC PECVD) system from the discharge of methane (CH4) mixed with helium (He). 5 different films were fabricated with He-to-CH4 flow-rate ratios of 1:1, 2:1, 3:1, 4:1 and 5:1 in order to clarify the dependence of thermal diffusivity (α) on the microstructural changes due to the He-dilution effect. Using an ultrafast optical pump-probe technique, time-domain thermoreflectance signals were collected and the thermal property was extracted using the Paddock and Eesley model (1986). Results show strong correlation of α to the changes of sp2 carbon cluster determined by spectroscopic measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Diamond-like carbon (DLC) refers to the family of amorphous carbon materials with distinct sp3 hybridized carbon atoms and hydrogen (H) content. It has unique mechanical, thermal, optical, and chemical properties such as superhardness, thermal stability, antireflectivity, and biocompatibility, and therefore investigations have been actively performed for applications to tribological protective coatings [1], thermal barriers [2], photovoltaic devices [3], and cardiovascular treatment [4]. To fabricate DLC thin-films, several options are available such as ion beam deposition, magnetron sputtering, direct current or radio frequency plasma enhanced chemical vapor deposition (DC or RF PECVD), laser ablation, and filtered cathodic vacuum arcs [5]. Since the material properties can be enhanced and tailored by modifying deposition conditions, their influence on the microstructural, mechanical, and thermophysical characteristics of DLC has been an important issue in this area of research [6]. In this paper, we determined the thermophysical properties of a-C:H thin-films deposited using the direct current plasma enhanced chemical vapor deposition (DC PECVD) technique from the discharge of methane (CH4) mixed with helium (He). The flow-rate ratio of the two gases was parametrically adjusted to clarify the effect of He-dilution on the microscopic structures in the film using optical transmission spectroscopy, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. Subsequent change of thermal diffusivity of the film was also characterized using an ultrafast optical pump-probe technique.
2.1. Sample preparation
⁎ Corresponding author. Tel.: +60 3 8921 5913; fax: +60 3 8921 3777. E-mail address: [email protected] (H.A. Alwi). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2012.11.009
The a-C:H films studied in the present investigation were fabricated in a DC PECVD chamber. The overall schematic of the setup was based on the deposition system introduced in [7], but a DC gas discharge power supply was used in this particular study. Also, the substrate holder was connected to a negative electrode, and the stainless steel gas showerhead was connected to a positive electrode. In this way, DC plasma was generated between the two electrodes when the DC power was on and the process gas was allowed to flow. A series of samples was deposited from the discharge of pure CH4 diluted with He. The flow-rate ratio of He-to-CH4 was varied from 1:1 to 5:1. These films were deposited for 4 h. The deposition temperature and pressure were monitored continuously and maintained at 100 °C and 0.8 mbar, respectively. Total gas was kept constant while the mixing ratio was changed by maintaining the gas pressure. The direct-current power applied across the electrodes was set to 6.7 W. The ionization current and the voltage maintained during the film deposition are given in Table 1. 2.2. Microstructural characterization Optical transmission spectroscopy was performed in order to determine the film thickness, deposition rate, and optical band gap (Tauc gap, Eg) using an Ultra–violet Visible Near-infrared (UV–Vis-NIR) spectrophotometer (Jasco model V-570). Transmission spectra from 190 to 2500 nm wavelengths were collected and analyzed so that the film thickness (d) can be calculated from the absorption edge using
2
Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
Table 1 Deposition conditions. Sample #
He:CH4 flow-rate (sccm)
Ionization current (mA)
DC voltage (V)
1 2 3 4 5
20:20 40:20 60:20 80:20 100:20
9.5 8.9 7.9 7.9 6.1
700 750 850 850 1100
3. Results and discussion 3.1. Microstructural characteristics
the methods in [8,9]. For the optical band gap, the Tauc gap (Eg) was determined from the Tauc relation [10]. The concentration of bonded hydrogen or hydrogen content (H%) in a-C:H films was determined using FTIR spectroscopy (Perkin Elmer 2000 FTIR spectrometer) in a transmission mode. From the integrated intensity of the C–H stretching modes (IC–H) at wavenumbers between 2700 and 3200 cm−1, the absorption band was decomposed into Gaussian components corresponding to the different possible vibration modes of the total C–H stretching bands [11,12]. Similarly, the relative proportions of the H bonded to C-sp3 and C-sp 2 sites from the integrated intensity of the corresponding infrared sub-bands were determined and the sp 2 carbon content in the film was calculated following [13]. To define the quality of the carbon structure, the G peak position, full-width half-maximum (FWHM) of the D peak, and integrated intensity ratio (I(D)/I(G)) were determined from Raman spectroscopy (MicroRaman spectrometer, model Renishaw System 2000). The sp 2 cluster size La was then calculated following the relationship proposed by Cancado et al. [14] given as −10 4 I ðDÞ λ La ðnmÞ ¼ 2:4 10 I ðGÞ
the heat capacity per unit volume, R is the reflectivity, I is the pump-intensity, β is the absorptivity of Al per unit length, and τ is the pump-pulse width of a Gaussian shape. Experimental data were compared to numerical solutions and α was estimated from the best fit.
Fig. 1 shows the optical transmission spectra of a-C:H films in the wavelength region of 200 to 2500 nm and in the wavelength region close to the absorption edge. At most two broad interference fringes are observed, indicating that these films are thin and have low refractive indices. The absorption edges of these films also shift towards the longer wavelength as He-to-CH4 flow-rate ratio increases, indicating that the optical band gap Eg also decreases. Film thickness and optical band gaps obtained from the transmission spectra were presented in Table 2. The deposition rate was calculated in Table 2, and it increases very slowly but linearly from ~1.1 nm/min to ~1.2 nm/min. The deposition rate of the a-C:H films depends on the reactions of hydrocarbon radicals (mostly long-lived CH3 radicals) and hydrocarbon ion bombardment on the growing surface. The high density and long life time (6× 105 s) of excited He (23He) atoms in the
!−1 ð1Þ
where λ = 514.5 nm is the photon excitation wavelength. 2.3. Thermoreflectance measurement A 30-nm thick layer of aluminum (Al) layer was deposited on the a-C:H films after the spectroscopic characterization of microstructures. Samples were installed in an e-beam evaporator (Edwards Auto 500) operating at a DC voltage of 4.7 kV and a current level of 25 mA. The deposition rate and chamber pressure were maintained at 0.15 nm/s and 3 × 10 −6 Torr, respectively. An ultrafast optical pump-probe technique was employed to obtain the time-resolved thermoreflectance signals. Detailed experimental setup is introduced elsewhere [15], so it is only briefly described here. A femtosecond laser system (Millenia Pro and Tsunami, Spectra-physics) emits 120-fs wide pulses at the wavelength of 780 nm and a repetition rate of 80 MHz. The beam was divided into two paths with an intensity ratio of 10 to 1. The pump-beam, which has the higher intensity, was chopped at 100 kHz using an acoustooptic modulator and focused onto the sample to induce temperature change. The optical path length of the probe-beam was adjusted using a retroreflector and computer-controlled stage so that the transient change of thermoreflectance can be monitored using a photodetector and a lock-in amplifier. A total of 250 data points were collected for a maximum time-delay of 1 ns. The heat flow model proposed by Paddock and Eesley [16] was employed to extract thermal diffusivity (α) of samples. The onedimensional heat conduction equation is as follows: 2
∂T ðz; t Þ ∂2 T ðz; t Þ Ið1−RÞβe−βz e−ðt=τÞ þ ¼α C ∂t ∂z2
ð2Þ
where T is the film temperature, z is the distance to the depth direction, t is the time-delay with respect to the application of pump-pulse, C is
Fig. 1. The optical transmission spectra of DC PECVD He-diluted a-C:H films, (a) in the full wavelength range of 190 to 250 nm, and (b) in the wavelength region close to the absorption edge.
Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
3
Table 2 Film characteristics. Sample #
Optical band gap, Eg (eV)
Film thickness (nm)
Deposition rate (nm/min)
1 2 3 4 5
3.12 2.97 2.79 2.73 2.63
270 280 295 300 300
1.13 1.17 1.23 1.25 1.25
plasma of He diluted CH4 actively produce additional CHn+ ions through Penning ionization with an increase in He-dilution of CH4 [17]. The presence of higher concentration of CHn+ ions in the He-diluted plasma, increases the hydrocarbon ion bombardment effects at low He-dilution. At higher He-dilution, the increase in the number of He atoms creates shielding effect which has the effect of reducing the bombardment effects. Eg decreases gradually from ~ 3.1 eV to ~ 2.6 eV with an increase of He-to-CH4 flow-rate ratio. Generally, a decrease in the Eg can be due to an increase in sp 2 fraction [18], a decrease in H content [19], or an increase in sp 2 cluster size [20] in the a-C: H films. In this case, it is due to the fact that He-dilution of CH4 produces higher hydrocarbon ion bombardment effects on the growth surface of the film, thus promoting the formation of sp 3 bonding sites compared to sp 2 bonding sites (Fig. 4). Also, the bombardment onto the growing surface of film breaks C–H bonds and therefore reduces the H content in the film structure (Fig. 3). Fig. 2a depicts the normalized absorption band centered at 2900 cm −1 obtained from the FTIR spectroscopy. This absorption band is related to the presence of H bonds in the form of sp 3-CH and sp 3-CH2 groups in the a-C:H films [21]. These absorption bands are normalized by film thickness and are then deconvoluted as in Fig. 2b to the various Gaussian components representing CHn absorption bands to determine the total integrated intensity under the band. This value is then used to determine bonded H content in the film, shown in Fig. 3 and the evaluation of sp 2 C content in the film (Fig. 4). The IR spectra (Fig. 2a) indicate that most H atoms are bonded to sp 3 carbon atoms since absorption bands below 2955 cm −1 are more dominant. From these FTIR spectra, the effects of He-dilution on hydrogen content and sp 2 C content in the films were analyzed. In Fig. 3, the hydrogen content of the films is plotted as a function of He-to-CH4 flow-rate ratio. The increase of He-to-CH4 flow-rate ratio up to 3:1 significantly reduces the H content in a-C:H films due to the increase of hydrocarbon ion bombardment onto the film surface. At higher flow-rate ratio, the hydrocarbon ion bombardment is reduced by the shielding effect of He atoms. Being smaller than the CHn+ ions, the excited H atoms from the discharge have higher probability of reaching growth surface which additionally increases H content at flow-rate ratio of 4. At flow-rate ratio of 5, the shielding effect on H atoms becomes more effective which decreases the H content. Fig. 4 shows the variation of sp2 C content with He-to-CH4 flow-rate ratio. The sp2 carbon content decreases overall with higher flow-rate ratio, and this indicates that hydrocarbon ion bombardment and shielding effect play major roles in the formation of hydrogenated sp2 C bonds in the films. Low He-to-CH4 flow-rate ratio results in less frequent collisions between hydrocarbon ions and diluent molecules but more frequent collisions with CH4 molecules and therefore producing more CH3 radicals, which are the main growth precursors for a-C:H film [22]. This incorporates more C atoms into the film structure, thus the sp2 C content is high. Higher dilution results in a decrease of sp2 C content in the film because the concentration of CH3 radicals at growth surface is reduced by the shielding effect of He atoms, thus reducing the incorporation of C atoms into film structure. He-to-CH4 flow-rate ratio of 4 significantly increases the sp2 C content in the film due to the shielding effects of growth surface from hydrocarbon ion bombardments and the smaller H atoms in the plasma are able to diffuse through the shielding. Higher dilution of
Fig. 2. (a) Normalized absorption coefficient spectra of C–H stretching vibrational bands at wavenumber 2900 cm−1. (b) A typical decomposition of partial IR absorption spectrum for sample #2. The overlapping fitted curve and the experimental spectrum are shown.
Fig. 3. Bonded hydrogen content variation in the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
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Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
Fig. 4. Hydrogen-bonded sp2 carbon content variation in the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
He-to-CH4 flow-rate ratio of 5:1 significantly decreases the sp2 C content in the film due to the higher shielding effect on H atoms and hydrocarbon ions. Raman spectra are shown in Fig. 5a; the G-peak and D-peak at around 1560 and 1360 cm −1 deconvoluted using a double Gaussian fit. Fig. 5b shows the deconvolution of Raman spectrum to the G-band and D-band for Sample #2 as an example. The variations of G band peak position, full-width at half-maximum (FWHM) of the G band, I(D)/I(G), FWHM of the D band and sp 2 C cluster size of a-C:H films are shown in Fig. 6. The G peak position (Fig. 6a) shifts towards 1530 cm −1 with an increase in He-to-CH4 flow-ratio to 2 and 3, indicating that the sp 2 C content in the film (Fig. 4) is significantly reduced. This is supported by the decrease in I(D)/I(G) values (Fig. 6c) at these gas flow-rate ratios. The increase in G band width (Fig. 6b) at these flow-rate ratios indicates inhomogeneous distribution of bond angle disorder in these films, resulting in large sp2 clusters (Fig. 6e). At low He-dilution with flow-rate ratio up to 3:1, the increase in frequency of CHn+ ion bombardment effects as a result of higher He dilution significantly reduces the sp 2 C content in the film (Fig. 4), increases sp2 C cluster size (Fig. 6e) and results in inhomogeneous distribution of bond angle disorder (Fig. 6b). At higher flow-rate ratio, the quality of the film is influenced by the ion bombardment and shielding effect. 3.2. Thermal diffusivity of a-C:H films Fig. 7 shows the time-domain thermoreflectance signals from a-C: H samples of different He-to-CH4 flow-rate ratios. The decay rates of plots for Samples #1, #2, and #3 are faster than those of Samples #4 and #5, indicating faster heat diffusion into the sample layer. Thermal diffusivity values were extracted and plotted in Fig. 8. They fall in the range of 1.5 × 10 −2–4.5 × 10 −2 mm 2/s, and the variation shows a strong correlation to the microstructural changes in the sp 2 cluster. Referring to Fig. 8, the increase of thermal diffusivity at the flow-rate ratios of 2:1 and 3:1 is influenced by the reduction of the sp 2 C content in the film (Fig. 4), due to inhomogeneous distribution of bond angle disorder in these films (Fig. 6b), resulting in large sp 2 clusters, as supported in Fig. 6e. The sudden drop of thermal diffusivity in Sample #4 is attributed to the shielding effects of growth surface from ion bombardment. Since the smaller H atoms are able to diffuse through the shielding produced by He atoms, the sp2 C content is enhanced in the film structure. On the other hand, at the He-to-CH4 flow-rate ratio of 5:1, the sp 2 C content in the film decreases as a result of the higher shielding effect of the
Fig. 5. (a) Raman spectra of the a-C:H films with varying He:CH4 flow-rate ratios. (b) A typical decomposition of partial Raman spectrum into the D-band (1350 cm−1) and G-band (1580 cm−1) for sample #2. The overlapping fitted curve and the experimental spectrum are shown.
growth surface by He atoms, which decrease the energy of excited H atoms and ions reaching the growth surface. This significantly reduces the sp 2 C content. Accordingly, the thermal diffusivity of Sample #5 is higher than that of Sample #4. The conversion of thermal diffusivity α data into thermal conductivity κ using κ = ρCpα leads to 0.027–0.080 W/mK. A density ρ of 1.8 g/cm 3 was based on X-ray reflectivity measurements by Shamsa et al. [23] on their DLCH a-C:H films. The average value of specific heat capacity Cp of 0.986 J/gK was taken from DSC measurements by Hakovirta et al. [24], Moelle et al. [25] and Arlein et al. [26] on hydrogenated amorphous carbon. Our samples with thermal conductivity ranges from 0.027 to 0.080 W/mK are very much lower compared to samples DLCH by Shamsa et al. (0.566–0.69 W/mK). The low thermal conductivity of our samples suggests that our samples have higher structural disorder of the sp 3 phase as discussed by Shamsa et al. 4. Conclusion Thermophysical properties of DC PECVD-grown hydrogenated amorphous carbon thin-films from helium-diluted methane were characterized using optical transmission spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and ultrafast optical pump-probe technique. The effect of He-dilution on the microstructrual
Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
5
Fig. 6. Plots of (a) G peak position, (b) full-width at half-maximum (FWHM) of G band, (c) I(D)/I(G) integrated intensity ratio, (d) FWHM of D band, (e) sp2 cluster size of the DC PECVD a-C:H films. (Lines are drawn as guide to the eyes).
changes was clarified, and the subsequent influence on the thermal diffusivity of the films was studied. Results show thermal diffusivity values of 1.5× 10−2–4.5 × 10−2 mm2/s, and the variation is strongly correlated to the changes of sp2 C structures in the film.
Prime novelty statement Thermal diffusivity of hydrogenated amorphous carbon thin films was measured using ultrafast optical pump-probe technique. We
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Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
Acknowledgments HAA thanks MOSTI, Malaysia for the financial support during the visit to Northwestern University, and MOHE, Malaysia for research grant UKM-ST-07-FRGS0020-2010. RA thanks UKM for research grants UKM-GGPM-NBT-092-2010 and FRGS/1/2011/SG/UKM/02/20. The samples were fabricated during RA's Ph.D. study with Prof. S.A. Rahman at the University of Malaya. SK acknowledges support from the US National Science Foundation research grant OISE-0730259. References
Fig. 7. Responses of thermoreflectance change on the Al-coated a-C:H films.
Fig. 8. Thermal diffusivity variation of the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
discovered that the thermal diffusivity was influenced strongly by the sp 2 C clusters which were governed by the effect of helium dilution of CH4 discharge.
[1] C.F. Hong, J.P. Tu, R.L. Li, D.G. Liu, H.L. Sun, S.X. Mao, R. Peng, J. Phys. D: Appl. Phys. 42 (2009) 215303. [2] Y. Zhang, Z.B. He, P. Li, J.C. Yan, J. Wang, G.-C. Liu, Acta Phys. Sin. 60 (2011) 126501. [3] D.S. da Silva, A.D.S. Côrtes, M.H. Oliveira Jr., E.F. Motta, G.A. Viana, P.R. Mei, F.C. Marques, J. Appl. Phys. 110 (2011) 043510. [4] R. Hang, M. Zhang, S. Ma, P.K. Chu, J. Biomed. Mater. Res. Part A 100A (2012) 496–506. [5] J.K. Luo, Y.Q. Fu, H.R. Le, J.A. Williams, S.M. Spearing, W.I. Milne, J. Micromech. Microeng. 17 (2007) S147–S163. [6] C. Corbella, S. Portal-Marco, M. Rubio-Roy, E. Bertran, G. Oncins, M.A. Vallvé, J. Ignès-Mullol, J.L. Andújar, J. Phys. D: Appl. Phys. 44 (2011) 395301. [7] Y.Y. Kim, H.A. Alwi, R. Awang, S. Krishnaswamy, Jpn. J. Appl. Phys. 50 (2011) 125602. [8] J. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E: Sci. Instrum. 9 (1976) 1002–1004. [9] E.A. Davis, N. Piggins, S.C. Bayliss, J. Phys. C: Solid State Phys. 20 (1987) 4415–4427. [10] S.R. Arekat, J. Non-Cryst. Solids 224 (1998) 201–204. [11] J. Bellamy, in: The Infrared Spectra of Complex Molecules, vol. 2, Chapman and Hall, London, 1975. [12] B. Dischler, R.E. Sah, P. Koidl, W. Fluhr, A. Wokaun, in: Proc. 7th Int. Symp. Plasma Chemistry, 1985, p. 45. [13] B. Dischler, in: P. Koidl, P. Oelhaven (Eds.), Les Editions de Physique, Paris, Symp. Proc. EMRS, vol. 17, 1987. [14] L.G. Cancado, K. Takai, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, A. Jorio, L.N. Coelho, R. Magalhaes-Paniago, M.A. Pimenta, Appl. Phys. Lett. 88 (2006) 163106. [15] Y.Y. Kim, H.A. Alwi, Q. Huang, R. Abd-Shukor, C.F. Tsai, H. Wang, K.W. Kim, D.G. Naugle, S. Krishnaswamy, Physica C 470 (2010) 365–368. [16] C.A. Paddock, G.L. Eesley, J. Appl. Phys. 60 (1986) 285–291. [17] N. Mutsukura, K. Miyatani, Diamond Relat. Mater. 4 (1995) 342–345. [18] C.W. Chen, J. Roberson, J. Non-Cryst. Solids 227–230 (1998) 602–606. [19] E. Tomasella, C. Meunier, S. Mikhailov, Surf. Coat. Technol. 141 (2001) 286–296. [20] S. Tóth, M. Veres, M. Fule, M. Koos, Diamond Relat. Mater. 15 (2006) 967–971. [21] G. Lazar, K. Zellama, I. Vascan, M. Stamate, I. Lazar, I. Rusu, J. Optoelectron. Adv. Mater. 7 (2005) 647–652. [22] N. Mutsukura, S. Inoue, Y. Machi, J. Appl. Phys. 72 (1992) 43–53. [23] M. Shamsa, W.L. Liu, A.A. Balandin, C. Casiraghi, W.I. Milne, A.C. Ferrari, Appl. Phys. Lett. 89 (2006) 161921. [24] M. Hakovirta, J.E. Vuorinen, X.M. He, M. Nastasi, R.B. Schwarz, Appl. Phys. Lett. 77 (15) (2000) 2340–2342. [25] C. Moelle, M. Werner, F. Szucs, D. Wittorf, M. Sellschopp, J. Von Borany, H.-J. Fecht, C. Johnston, Diamond Relat. Mater. 7 (1998) 499–503. [26] J.L. Arlein, S.E.M. Palaich, B.C. Daly, P. Subramonium, G.A. Antonelli, J. Appl. Phys. 104 (2008) 033508.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Helium-dilution effects on thermophysical properties of hydrogenated amorphous carbon thin-films Yun Young Kim a, Rozidawati Awang b, Sridhar Krishnaswamy a, Hasan Adli Alwi b,⁎ a b
Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA School of Applied Physics, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 5 April 2012 Received in revised form 21 September 2012 Accepted 27 November 2012 Available online 5 December 2012 Keywords: Amorphous hydrogenated carbon Thermal properties Microstructure Chemical vapor deposition
a b s t r a c t Thermophysical properties of hydrogenated amorphous carbon thin-films were characterized in the present investigation. Samples were deposited in a direct current plasma enhanced chemical vapor deposition (DC PECVD) system from the discharge of methane (CH4) mixed with helium (He). 5 different films were fabricated with He-to-CH4 flow-rate ratios of 1:1, 2:1, 3:1, 4:1 and 5:1 in order to clarify the dependence of thermal diffusivity (α) on the microstructural changes due to the He-dilution effect. Using an ultrafast optical pump-probe technique, time-domain thermoreflectance signals were collected and the thermal property was extracted using the Paddock and Eesley model (1986). Results show strong correlation of α to the changes of sp2 carbon cluster determined by spectroscopic measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Diamond-like carbon (DLC) refers to the family of amorphous carbon materials with distinct sp3 hybridized carbon atoms and hydrogen (H) content. It has unique mechanical, thermal, optical, and chemical properties such as superhardness, thermal stability, antireflectivity, and biocompatibility, and therefore investigations have been actively performed for applications to tribological protective coatings [1], thermal barriers [2], photovoltaic devices [3], and cardiovascular treatment [4]. To fabricate DLC thin-films, several options are available such as ion beam deposition, magnetron sputtering, direct current or radio frequency plasma enhanced chemical vapor deposition (DC or RF PECVD), laser ablation, and filtered cathodic vacuum arcs [5]. Since the material properties can be enhanced and tailored by modifying deposition conditions, their influence on the microstructural, mechanical, and thermophysical characteristics of DLC has been an important issue in this area of research [6]. In this paper, we determined the thermophysical properties of a-C:H thin-films deposited using the direct current plasma enhanced chemical vapor deposition (DC PECVD) technique from the discharge of methane (CH4) mixed with helium (He). The flow-rate ratio of the two gases was parametrically adjusted to clarify the effect of He-dilution on the microscopic structures in the film using optical transmission spectroscopy, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. Subsequent change of thermal diffusivity of the film was also characterized using an ultrafast optical pump-probe technique.
2.1. Sample preparation
⁎ Corresponding author. Tel.: +60 3 8921 5913; fax: +60 3 8921 3777. E-mail address: [email protected] (H.A. Alwi). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2012.11.009
The a-C:H films studied in the present investigation were fabricated in a DC PECVD chamber. The overall schematic of the setup was based on the deposition system introduced in [7], but a DC gas discharge power supply was used in this particular study. Also, the substrate holder was connected to a negative electrode, and the stainless steel gas showerhead was connected to a positive electrode. In this way, DC plasma was generated between the two electrodes when the DC power was on and the process gas was allowed to flow. A series of samples was deposited from the discharge of pure CH4 diluted with He. The flow-rate ratio of He-to-CH4 was varied from 1:1 to 5:1. These films were deposited for 4 h. The deposition temperature and pressure were monitored continuously and maintained at 100 °C and 0.8 mbar, respectively. Total gas was kept constant while the mixing ratio was changed by maintaining the gas pressure. The direct-current power applied across the electrodes was set to 6.7 W. The ionization current and the voltage maintained during the film deposition are given in Table 1. 2.2. Microstructural characterization Optical transmission spectroscopy was performed in order to determine the film thickness, deposition rate, and optical band gap (Tauc gap, Eg) using an Ultra–violet Visible Near-infrared (UV–Vis-NIR) spectrophotometer (Jasco model V-570). Transmission spectra from 190 to 2500 nm wavelengths were collected and analyzed so that the film thickness (d) can be calculated from the absorption edge using
2
Y.Y. Kim et al. / Diamond & Related Materials 32 (2013) 1–6
Table 1 Deposition conditions. Sample #
He:CH4 flow-rate (sccm)
Ionization current (mA)
DC voltage (V)
1 2 3 4 5
20:20 40:20 60:20 80:20 100:20
9.5 8.9 7.9 7.9 6.1
700 750 850 850 1100
3. Results and discussion 3.1. Microstructural characteristics
the methods in [8,9]. For the optical band gap, the Tauc gap (Eg) was determined from the Tauc relation [10]. The concentration of bonded hydrogen or hydrogen content (H%) in a-C:H films was determined using FTIR spectroscopy (Perkin Elmer 2000 FTIR spectrometer) in a transmission mode. From the integrated intensity of the C–H stretching modes (IC–H) at wavenumbers between 2700 and 3200 cm−1, the absorption band was decomposed into Gaussian components corresponding to the different possible vibration modes of the total C–H stretching bands [11,12]. Similarly, the relative proportions of the H bonded to C-sp3 and C-sp 2 sites from the integrated intensity of the corresponding infrared sub-bands were determined and the sp 2 carbon content in the film was calculated following [13]. To define the quality of the carbon structure, the G peak position, full-width half-maximum (FWHM) of the D peak, and integrated intensity ratio (I(D)/I(G)) were determined from Raman spectroscopy (MicroRaman spectrometer, model Renishaw System 2000). The sp 2 cluster size La was then calculated following the relationship proposed by Cancado et al. [14] given as −10 4 I ðDÞ λ La ðnmÞ ¼ 2:4 10 I ðGÞ
the heat capacity per unit volume, R is the reflectivity, I is the pump-intensity, β is the absorptivity of Al per unit length, and τ is the pump-pulse width of a Gaussian shape. Experimental data were compared to numerical solutions and α was estimated from the best fit.
Fig. 1 shows the optical transmission spectra of a-C:H films in the wavelength region of 200 to 2500 nm and in the wavelength region close to the absorption edge. At most two broad interference fringes are observed, indicating that these films are thin and have low refractive indices. The absorption edges of these films also shift towards the longer wavelength as He-to-CH4 flow-rate ratio increases, indicating that the optical band gap Eg also decreases. Film thickness and optical band gaps obtained from the transmission spectra were presented in Table 2. The deposition rate was calculated in Table 2, and it increases very slowly but linearly from ~1.1 nm/min to ~1.2 nm/min. The deposition rate of the a-C:H films depends on the reactions of hydrocarbon radicals (mostly long-lived CH3 radicals) and hydrocarbon ion bombardment on the growing surface. The high density and long life time (6× 105 s) of excited He (23He) atoms in the
!−1 ð1Þ
where λ = 514.5 nm is the photon excitation wavelength. 2.3. Thermoreflectance measurement A 30-nm thick layer of aluminum (Al) layer was deposited on the a-C:H films after the spectroscopic characterization of microstructures. Samples were installed in an e-beam evaporator (Edwards Auto 500) operating at a DC voltage of 4.7 kV and a current level of 25 mA. The deposition rate and chamber pressure were maintained at 0.15 nm/s and 3 × 10 −6 Torr, respectively. An ultrafast optical pump-probe technique was employed to obtain the time-resolved thermoreflectance signals. Detailed experimental setup is introduced elsewhere [15], so it is only briefly described here. A femtosecond laser system (Millenia Pro and Tsunami, Spectra-physics) emits 120-fs wide pulses at the wavelength of 780 nm and a repetition rate of 80 MHz. The beam was divided into two paths with an intensity ratio of 10 to 1. The pump-beam, which has the higher intensity, was chopped at 100 kHz using an acoustooptic modulator and focused onto the sample to induce temperature change. The optical path length of the probe-beam was adjusted using a retroreflector and computer-controlled stage so that the transient change of thermoreflectance can be monitored using a photodetector and a lock-in amplifier. A total of 250 data points were collected for a maximum time-delay of 1 ns. The heat flow model proposed by Paddock and Eesley [16] was employed to extract thermal diffusivity (α) of samples. The onedimensional heat conduction equation is as follows: 2
∂T ðz; t Þ ∂2 T ðz; t Þ Ið1−RÞβe−βz e−ðt=τÞ þ ¼α C ∂t ∂z2
ð2Þ
where T is the film temperature, z is the distance to the depth direction, t is the time-delay with respect to the application of pump-pulse, C is
Fig. 1. The optical transmission spectra of DC PECVD He-diluted a-C:H films, (a) in the full wavelength range of 190 to 250 nm, and (b) in the wavelength region close to the absorption edge.
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Table 2 Film characteristics. Sample #
Optical band gap, Eg (eV)
Film thickness (nm)
Deposition rate (nm/min)
1 2 3 4 5
3.12 2.97 2.79 2.73 2.63
270 280 295 300 300
1.13 1.17 1.23 1.25 1.25
plasma of He diluted CH4 actively produce additional CHn+ ions through Penning ionization with an increase in He-dilution of CH4 [17]. The presence of higher concentration of CHn+ ions in the He-diluted plasma, increases the hydrocarbon ion bombardment effects at low He-dilution. At higher He-dilution, the increase in the number of He atoms creates shielding effect which has the effect of reducing the bombardment effects. Eg decreases gradually from ~ 3.1 eV to ~ 2.6 eV with an increase of He-to-CH4 flow-rate ratio. Generally, a decrease in the Eg can be due to an increase in sp 2 fraction [18], a decrease in H content [19], or an increase in sp 2 cluster size [20] in the a-C: H films. In this case, it is due to the fact that He-dilution of CH4 produces higher hydrocarbon ion bombardment effects on the growth surface of the film, thus promoting the formation of sp 3 bonding sites compared to sp 2 bonding sites (Fig. 4). Also, the bombardment onto the growing surface of film breaks C–H bonds and therefore reduces the H content in the film structure (Fig. 3). Fig. 2a depicts the normalized absorption band centered at 2900 cm −1 obtained from the FTIR spectroscopy. This absorption band is related to the presence of H bonds in the form of sp 3-CH and sp 3-CH2 groups in the a-C:H films [21]. These absorption bands are normalized by film thickness and are then deconvoluted as in Fig. 2b to the various Gaussian components representing CHn absorption bands to determine the total integrated intensity under the band. This value is then used to determine bonded H content in the film, shown in Fig. 3 and the evaluation of sp 2 C content in the film (Fig. 4). The IR spectra (Fig. 2a) indicate that most H atoms are bonded to sp 3 carbon atoms since absorption bands below 2955 cm −1 are more dominant. From these FTIR spectra, the effects of He-dilution on hydrogen content and sp 2 C content in the films were analyzed. In Fig. 3, the hydrogen content of the films is plotted as a function of He-to-CH4 flow-rate ratio. The increase of He-to-CH4 flow-rate ratio up to 3:1 significantly reduces the H content in a-C:H films due to the increase of hydrocarbon ion bombardment onto the film surface. At higher flow-rate ratio, the hydrocarbon ion bombardment is reduced by the shielding effect of He atoms. Being smaller than the CHn+ ions, the excited H atoms from the discharge have higher probability of reaching growth surface which additionally increases H content at flow-rate ratio of 4. At flow-rate ratio of 5, the shielding effect on H atoms becomes more effective which decreases the H content. Fig. 4 shows the variation of sp2 C content with He-to-CH4 flow-rate ratio. The sp2 carbon content decreases overall with higher flow-rate ratio, and this indicates that hydrocarbon ion bombardment and shielding effect play major roles in the formation of hydrogenated sp2 C bonds in the films. Low He-to-CH4 flow-rate ratio results in less frequent collisions between hydrocarbon ions and diluent molecules but more frequent collisions with CH4 molecules and therefore producing more CH3 radicals, which are the main growth precursors for a-C:H film [22]. This incorporates more C atoms into the film structure, thus the sp2 C content is high. Higher dilution results in a decrease of sp2 C content in the film because the concentration of CH3 radicals at growth surface is reduced by the shielding effect of He atoms, thus reducing the incorporation of C atoms into film structure. He-to-CH4 flow-rate ratio of 4 significantly increases the sp2 C content in the film due to the shielding effects of growth surface from hydrocarbon ion bombardments and the smaller H atoms in the plasma are able to diffuse through the shielding. Higher dilution of
Fig. 2. (a) Normalized absorption coefficient spectra of C–H stretching vibrational bands at wavenumber 2900 cm−1. (b) A typical decomposition of partial IR absorption spectrum for sample #2. The overlapping fitted curve and the experimental spectrum are shown.
Fig. 3. Bonded hydrogen content variation in the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
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Fig. 4. Hydrogen-bonded sp2 carbon content variation in the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
He-to-CH4 flow-rate ratio of 5:1 significantly decreases the sp2 C content in the film due to the higher shielding effect on H atoms and hydrocarbon ions. Raman spectra are shown in Fig. 5a; the G-peak and D-peak at around 1560 and 1360 cm −1 deconvoluted using a double Gaussian fit. Fig. 5b shows the deconvolution of Raman spectrum to the G-band and D-band for Sample #2 as an example. The variations of G band peak position, full-width at half-maximum (FWHM) of the G band, I(D)/I(G), FWHM of the D band and sp 2 C cluster size of a-C:H films are shown in Fig. 6. The G peak position (Fig. 6a) shifts towards 1530 cm −1 with an increase in He-to-CH4 flow-ratio to 2 and 3, indicating that the sp 2 C content in the film (Fig. 4) is significantly reduced. This is supported by the decrease in I(D)/I(G) values (Fig. 6c) at these gas flow-rate ratios. The increase in G band width (Fig. 6b) at these flow-rate ratios indicates inhomogeneous distribution of bond angle disorder in these films, resulting in large sp2 clusters (Fig. 6e). At low He-dilution with flow-rate ratio up to 3:1, the increase in frequency of CHn+ ion bombardment effects as a result of higher He dilution significantly reduces the sp 2 C content in the film (Fig. 4), increases sp2 C cluster size (Fig. 6e) and results in inhomogeneous distribution of bond angle disorder (Fig. 6b). At higher flow-rate ratio, the quality of the film is influenced by the ion bombardment and shielding effect. 3.2. Thermal diffusivity of a-C:H films Fig. 7 shows the time-domain thermoreflectance signals from a-C: H samples of different He-to-CH4 flow-rate ratios. The decay rates of plots for Samples #1, #2, and #3 are faster than those of Samples #4 and #5, indicating faster heat diffusion into the sample layer. Thermal diffusivity values were extracted and plotted in Fig. 8. They fall in the range of 1.5 × 10 −2–4.5 × 10 −2 mm 2/s, and the variation shows a strong correlation to the microstructural changes in the sp 2 cluster. Referring to Fig. 8, the increase of thermal diffusivity at the flow-rate ratios of 2:1 and 3:1 is influenced by the reduction of the sp 2 C content in the film (Fig. 4), due to inhomogeneous distribution of bond angle disorder in these films (Fig. 6b), resulting in large sp 2 clusters, as supported in Fig. 6e. The sudden drop of thermal diffusivity in Sample #4 is attributed to the shielding effects of growth surface from ion bombardment. Since the smaller H atoms are able to diffuse through the shielding produced by He atoms, the sp2 C content is enhanced in the film structure. On the other hand, at the He-to-CH4 flow-rate ratio of 5:1, the sp 2 C content in the film decreases as a result of the higher shielding effect of the
Fig. 5. (a) Raman spectra of the a-C:H films with varying He:CH4 flow-rate ratios. (b) A typical decomposition of partial Raman spectrum into the D-band (1350 cm−1) and G-band (1580 cm−1) for sample #2. The overlapping fitted curve and the experimental spectrum are shown.
growth surface by He atoms, which decrease the energy of excited H atoms and ions reaching the growth surface. This significantly reduces the sp 2 C content. Accordingly, the thermal diffusivity of Sample #5 is higher than that of Sample #4. The conversion of thermal diffusivity α data into thermal conductivity κ using κ = ρCpα leads to 0.027–0.080 W/mK. A density ρ of 1.8 g/cm 3 was based on X-ray reflectivity measurements by Shamsa et al. [23] on their DLCH a-C:H films. The average value of specific heat capacity Cp of 0.986 J/gK was taken from DSC measurements by Hakovirta et al. [24], Moelle et al. [25] and Arlein et al. [26] on hydrogenated amorphous carbon. Our samples with thermal conductivity ranges from 0.027 to 0.080 W/mK are very much lower compared to samples DLCH by Shamsa et al. (0.566–0.69 W/mK). The low thermal conductivity of our samples suggests that our samples have higher structural disorder of the sp 3 phase as discussed by Shamsa et al. 4. Conclusion Thermophysical properties of DC PECVD-grown hydrogenated amorphous carbon thin-films from helium-diluted methane were characterized using optical transmission spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and ultrafast optical pump-probe technique. The effect of He-dilution on the microstructrual
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Fig. 6. Plots of (a) G peak position, (b) full-width at half-maximum (FWHM) of G band, (c) I(D)/I(G) integrated intensity ratio, (d) FWHM of D band, (e) sp2 cluster size of the DC PECVD a-C:H films. (Lines are drawn as guide to the eyes).
changes was clarified, and the subsequent influence on the thermal diffusivity of the films was studied. Results show thermal diffusivity values of 1.5× 10−2–4.5 × 10−2 mm2/s, and the variation is strongly correlated to the changes of sp2 C structures in the film.
Prime novelty statement Thermal diffusivity of hydrogenated amorphous carbon thin films was measured using ultrafast optical pump-probe technique. We
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Acknowledgments HAA thanks MOSTI, Malaysia for the financial support during the visit to Northwestern University, and MOHE, Malaysia for research grant UKM-ST-07-FRGS0020-2010. RA thanks UKM for research grants UKM-GGPM-NBT-092-2010 and FRGS/1/2011/SG/UKM/02/20. The samples were fabricated during RA's Ph.D. study with Prof. S.A. Rahman at the University of Malaya. SK acknowledges support from the US National Science Foundation research grant OISE-0730259. References
Fig. 7. Responses of thermoreflectance change on the Al-coated a-C:H films.
Fig. 8. Thermal diffusivity variation of the film with respect to the He:CH4 flow-rate ratio. (Lines are drawn as guide to the eyes).
discovered that the thermal diffusivity was influenced strongly by the sp 2 C clusters which were governed by the effect of helium dilution of CH4 discharge.
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