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High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD
High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD R.H. Zhu, J.Y. Miao, J.L. Liu, L.X. Chen, J.C. Guo, C.Y. Hua, T. Ding, H.K. Lian, C.M. Li PII: DOI: Reference:
S0925-9635(14)00188-5 doi: 10.1016/j.diamond.2014.09.007 DIAMAT 6310
To appear in:
Diamond & Related Materials
Received date: Revised date: Accepted date:
9 May 2014 2 September 2014 16 September 2014
Please cite this article as: R.H. Zhu, J.Y. Miao, J.L. Liu, L.X. Chen, J.C. Guo, C.Y. Hua, T. Ding, H.K. Lian, C.M. Li, High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD, Diamond & Related Materials (2014), doi: 10.1016/j.diamond.2014.09.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD
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T. Dingb, H.K. Lianb, C.M. Lia*
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R.H. Zhua, J.Y. Miaob, J.L. Liua, L.X. Chena, J.C. Guoa, C.Y. Huaa， a
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Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, P.R. China b Beijing Laboratory of Space Thermal Technology, Beijing 100094, P.R. China
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Abstract: Free-standing diamond films with 1.68mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness (κ ⊥ ) together with in-plane (κ//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ ⊥ and κ// of free-standing diamond films are up to 1916 and 1739 Wm-1K-1 at room temperature, respectively, showing small anisotropy(9%), and following the relationship κ ~ T-n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed. Key word: DC arc plasma jet CVD; Thermal conductivity; Laser flash technique
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Author, E-mail address: [email protected] (R. H. Zhu) Corresponding author, E-mail address: [email protected] (C. M. Li)
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1 Introduction It has been reported that more than half of failures in today’s electronic systems were due to temperature [1]. So the increasing demand is putting forward to heat dispersion of devices. High-quality CVD diamond films show highest thermal conductivity which is similar to natural IIa diamond[2], coupled with their low dielectic loss, broad optical transparency, making them as key materials for a host of applications, like high-power Nd:YVO4 laser heat conductors [3], megawatt power gyrotrons [4,5], UV LEDs [6], Raman lasers [7], et al. CVD diamond films exhibit some anisotropy in thermal conductivity due to their column growth [8], and in same cases they must be exposed to extreme operating temperatures when used in devices [6, 9, 10]. Various methods have been utilized to determine the temperature-dependent thermal properties of CVD diamond, such as laser flash technique [11], transient thermal grating technique [12], and steady-state heater bar method [13,14], et al, in which laser flash technique is a common and reliable method for through-thickness thermal conductivity diamond over a wide temperature range. In this paper, the thermal conductivity of free-standing diamond films with 1.68 mm thickness prepared using DC arc plasma jet CVD were measured by laser flash technique. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed. 2 Experimental Polycrystalline diamond films were deposited on the substrate of graphite with Ti interlayer by high power DC arc plasma jet CVD operating at gas recycling mode. This system has been depicted in details previously [15]. The main deposition parameters were listed in Table 1. After stable and continuous deposition for more than 350 h, diamond films with thickness of ~2.75 mm were obtained. The as-deposited diamond films were peeled off from substrate, then abraded by using commercial diamond grits and double-sided polished by fast rotating diamond grinding wheel. At last, the thickness of free-standing diamond films was 1.68 mm and surface roughness Ra was less than 10 nm. The optical micrograph and cross-section morphologies of diamond films were observed by optical microscopy (OM) and S-250MK3 scanning electron microscopy. The crystal structure was analyzed by X-ray diffractometer (DMA-RB, 13KW) using Cu Kα radiation. The crystal quality of diamond films was analyzed using NICOLET ALMEGA XR Dispersive Raman spectrum (514nm) and NEXUS 670 Fourier infrared Raman spectrometer. NETZSCH Laser Flash Apparatus 447 and 457 were used to detect the thermal diffusivity (α) of diamond films simultaneously. The introduction of Laser Flash technique has been depicted in details anywhere [10]. The pulse length of Xenon Flash lamp and Nd:YAG laser used in LFA447 and LFA457 was 0.06 ms and 0.33 ms respectively. 1 K variation in the temperature of sample was stability controlled while testing. Each data presented was the average of five replications. The values of
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polycrystalline graphite and copper measured by LFA 447 and 457 showed precision within ±3%. Fig.1 shows the temperature-rising curve on upper surface of diamond after a short-pulsed laser heated on the lower surface. The half-rise time t1/2 is obtained from the curve which is revised using Cowan plus pulse correction model. Then thermal diffusivity and thermal conductivity are calculated using the equation (1) and (2):
(T ) (T ) (T ) C p (T )
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where L is the width of sample along the laser propagation direction. Cp(T) and ρ(T) are the temperature-dependent specific heat capacity and density of CVD diamond films, respectively. According to Ref [16]，specific heat capacity Cp and mass density ρ of CVD diamond films were within 1% of the accepted values for single crystal diamond. Meanwhile, CVD diamond films with different optical properties showed identical Cp(T)[8]. So Cp(T) and ρ(T) values in Ref[16] of natural diamond are used in this study. The testing process is shown in Fig.2. Firstly, the polished free-standing diamond films were laser cut into one disc (Φ12.5 mm×1.68 mm) and six rectangular bars (10 mm×4.57 mm×1.68 mm). The six rectangular bars stacked tightly together to form one rectangular sample (10 mm×10.08 mm×4.57mm) as shown in Fig.2 (a). The upper and lower cross section of this rectangular sample was polished to obtain the flat surface (Ra<100 nm) and uniform width (L//=4.57 mm) in testing process. Secondly, Au coatings (~100 nm) were deposited on the upper and lower sides of the disc and rectangular samples to prevent transmission of laser, and then a very thin graphite layer was followed to increase laser absorptivity. Thirdly, the temperature-dependent measurements of LFA 447 only reach 573K. So the through-thickness and in-plane thermal diffusivity with temperature range from 300K to 550 K were detected by LFA447, meanwhile the in-plane thermal diffusivity with temperature range from 550 K to 1100 K was measured by LFA457. The photo thermal deflection (PTD) method was used to measure the in-plane thermal conductivity of diamond films at room temperature based on the technology of photo thermal deflection spectroscopy. Details of this method have been previously described in Refs [17, 18]. The error in this measurement system is 5%. 3 Results and discussion 3.1 Basic characterizations of free-standing diamond films The optical microscope photo of diamond films using a back light is shown in Fig.3 (a). It can be seen that small amount of dark features with dot and bar shapes, composed of amorphous carbon, nitrogen and pores[19], existed inside the films. Most of them are distributed along the grain boundaries. The cross section
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morphology of polished diamond films is shown in Fig.3 (b) and (c). The columnar growth with grain size increasing from bottom to top is evident. It can be seen from Fig.3 (c) that the columnar grain width of the nucleation side is about 15 μm after polishing. Fig.4 shows X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films. The (111), (220) and (331) peaks are observed in both sides. Based on origin software, I(110)/I(111) is 3.98 and 5.74 for nucleation and growth surface. It seems that (110) preferred orientation was obtained as it grew in early nucleation stage, and became more evident as deposition time increased. Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths are shown in Fig.5. Only the pronounced diamond peak and no signature of non-diamond carbon are found when excited in 514nm. However, a broad band extending from 1000 to 1600cm-1, scattering from non-diamond carbon [20], is observed besides the 1332cm-1 Raman line of diamond when excited in 1064nm. It was reported that the ratio of Raman cross section of diamond and graphite was about 1/50[21]. And the sensitivity of non-diamond carbon can be drastically enhanced with increasing excitation wavelength [22]. It indicates that small amount of non-diamond phase exists in the diamond films. 3.2 Thermal conductivity of free-standing diamond films Fig.6 shows through-thickness thermal conductivity of diamond films between 300K and 550K. It can be seen that the room-temperature thermal conductivity κ⊥ of diamond films reach up to 1916 Wm-1 K-1. It is well know that the main heat carrier in diamond is phonon, which would be scattered by phonon collision, defects and gran boundaries consequently reducing the mean free path l of phonons [23] (eq.3). 1 Among them, l boundary is negligible when phonons propagate along the columns at
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room temperature. Slight difference between κ⊥ and IIa diamond (~2200 Wm-1K-1) 1 [24] would be caused by l defect , which mainly refer to the impurities(N, non-diamond
carbon), lattice mismatch, pores et al, inside the column crystal. The through-thickness thermal conductivity κ ⊥ of diamond films decreases with the increasing temperature, and down to about 1100 W m-1 K-1 at 550K.
l 1 l 1 phonon l 1defect l 1boundary
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The in-plane thermal conductivity κ// of diamond films is 1739 Wm-1K-1 at room temperature, which is similar to the result (1700 Wm-1K-1) measured by photo thermal deflection (PTD) method. A small anisotropy in κ is revealed at 300 K and 350 K. The result of calculations from Fig.6 show that the κ⊥ is higher by 9% than κⅡ at 300 K, which is similar to the results (10 %~15 %)obtained by A.V. Sukhadolau [25]. The gran boundaries would scatter the phonons, and consequently reduce the mean free path l when phonons propagate perpendicular to the columns. Besides, a number of defects would concentrate along boundaries which would further reduce l of phonon.
ACCEPTED MANUSCRIPT The difference between κ⊥ and κ// decreases to 4 % as temperature rises to 350K, and vanishes when temperature rises to 400K. The grain boundary scattering mechanisms ( l
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negligible when phonons propagate perpendicular to the columns of diamond film above 400K. The in-plane thermal conductivity of diamond films between 300 K and 1100 K is shown in Fig.7. For comparison, the data for high-quality single crystal diamond taken from Refs [23, 27, 28] are also shown in Fig.7. The curves with its horizontal axis (Temperature) in reciprocal type confirm the relationship κ ~ T-n held well for diamond films as temperature rises. The n value, which depends on κ at room temperature [26], is 0.91 and 0.72 for κ⊥ and κ// of diamond films. The in-plane and through-thickness thermal conductivity of free-standing diamond films above 500K shows similar value compared to that of single crystal diamond reported in Refs [27, 28]. As is known in Refs [29-31], the phonon collision scattering can limit the phonons free path and plays a dominant role from room temperature, and its influence is further increased when temperature rises, while the boundary and defect scattering hardly have any effect on mean free path. It also indicates that the diamond films can display similar heat transfer result compared to high-quality single crystal diamond when used at and above 500K.
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Polycrystalline diamond films with (110) preferred orientation were obtained after stable and continuous deposition for more than 350 h using DC arc plasma jet CVD with gas recycling mode. The thickness of free-standing diamond films after polishing was 1.68mm. The results examined by optical photo and IR Raman indicate that small amount of non-diamond phase exists in the diamond films. The through-thickness thermal conductivity κ⊥ is 1916 Wm-1K-1 at room temperature, slightly smaller than that of IIa diamond caused by scattering processes from non-diamond impurities inside the column crystal. The in-plane thermal conductivity κ// is 1739 Wm-1K-1 at 300K, showing small anisotropy (9%) compared to κ⊥ owing to grain boundaries scattering mechanisms in phonon transfer processes at room temperature. This anisotropy in κ values disappears when temperature reaches to 400K. The thermal conductivity of diamond films follows the relationship κ ~ T-n as temperature rises, and exhibits similar values compared to that of high-quality single crystal diamond above 500K, where the grain boundaries and defects scattering mechanisms can be neglected. Acknowledgements This work was sponsored by the National Natural Science Foundation of China
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(NSFC) (No.51272024), and the Ph.D. Programs Foundation of Ministry of Education of China (No.20110006110011). The authors deeply appreciated their financial supports.
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[17] C. Gu, Z. Jin, X. Lu, G. Zou, J. Zhang, R. Fang. The deposition of diamond film with high thermal conductivity[J]. Thin Solid Films, 311(1997) 124-127. [18] M Bertolotti, G.L Liakhou, A Ferrari, V.G Ralchenko, A.A Smolin, E.D Obraztsova, K.G Korotoushenko, S.M Pimenov, V.I Konov. Measurements of thermal conductivity of diamond films by photothermal deflection technique[J]. Journal of applied physics, 75(1994) 7795-7798. [19] J. Yang, X. Duan, F. Lu, C. Li, T. Zuo, F. Wang, The influence of dark feature on optical and thermal property of DC Arc Plasma Jet CVD diamond films. Diamond and related materials, 14 (2005) 1583-1587. [20] E. Wörner, C. Wild, W. Müller-Sebert, R. Locher, P. Koidl. Infrared Raman scattering as a sensitive probe for the thermal conductivity of chemical vapor deposited diamond films. Applied physics letters, 68(1996)1482-1484. [21] A D Papadopoulos, E Anastassakis. Optical properties of diamond. Physical review. B, Condensed matter, 43(1991) 5090-5097. [22] J Wagner, C Wild, P Koidl. Resonance effects in Raman scattering from polycrystalline diamond film. Applied physics letters, 59(1991) 779-781. [23] Onn D G, Witek A, Qiu Y Z, Anthony, T. R., Banholzer, W. F.. Some aspects of the thermal conductivity of isotopically enriched diamond single crystals. Physical review letters, 68(1992)2806-2809. [24] M.A. Angadi, T. Watanabe, A. Bodapati, X. Xiao, O. Auciello, J.A. Carlisle, J.A. Eastman, P. Keblinski, P.K. Schelling, S.R. Phillpot. Thermal transport and grain boundary conductance in ultrananocrystalline diamond thin films. Journal of applied physics, 99(2006)114301. [25] A. Sukhadolau, E. Ivakin, V. Ralchenko, A. Khomich, A. Vlasov, A. Popovich, Thermal conductivity of CVD diamond at elevated temperatures. Diamond and related materials, 14 (2005) 589-593. [26] D. Morelli, C. Beetz, T. Perry, Thermal conductivity of synthetic diamond films. Journal of applied physics, 64 (1988) 3063-3066. [27] A. Witek, Some aspects of thermal conductivity of isotopically pure diamond—a comparison with nitrides. Diamond and related materials, 7 (1998) 962-964. [28] J.R. Olson, R.O. Pohl, J.W. Vandersande, A. Zoltan, T.R. Anthony, W.F. Banholzer. Thermal conductivity of diamond between 170 and 1200 K and the isotope effect[J]. Physical Review B, 47(1993) 14850. [29] Graebner J E, Jin S, Herb J A, Gardinier C F. Local thermal conductivity in chemical vapor deposited diamond. Journal of applied physics,76(1994) 1552-1556. [30] W Masierak, K Fabisiak, M Kaczmarski, M Kozanecki. Simple model for the thermal conductivity estimation on the basis of Raman and ESR spectroscopy measurements. Optica Applicata, 36(2006)225-234. [31] G A Slack. Thermal conductivity of pure and impure silicon, silicon carbide, and diamond. Journal of Applied Physics, 35(2004) 3460-3466.
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Fig.1 The temperature-rising curve on upper surface of diamond films after a short-pulsed laser heated on the lower surface. Fig.2 The temperature-dependent thermal conductivity testing process of free-standing diamond films. Fig.3 Optical microscope photo (a) and SEM Cross-section micrographs (b, c) of diamond films. Fig.4 X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films. Fig.5 Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths. Fig.6 The through-thickness and in-plane thermal conductivity of diamond films between 300K and 550K. Fig.7 The in-plane thermal conductivity of diamond films between 300K and 1100K. The literature data for single crystal diamond (circles) at T<500K refer to synthetic diamond enriched with 12C isotope.
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Table 1 The main deposition parameters of diamond films by DC arc plasma jet CVD
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Fig.1 The temperature-rising curve on upper surface of diamond films after a short-pulsed laser heated on the lower surface
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Fig.2 The temperature-dependent thermal conductivity testing process of free-standing diamond films
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Fig.3 Optical microscope photo (a) and SEM Cross-section micrographs (b) (c) of diamond films
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Fig.4 X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films
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Fig.5 Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths
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Fig.6 The through-thickness and in-plane thermal conductivity of diamond films between 300K and 550K
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Fig.7 The in-plane thermal conductivity of diamond films between 300K and 1100K. The literature data for single crystal diamond (circles) at T<500K refer to synthetic diamond enriched with 12C isotope.
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Table1 The main deposition parameters of diamond films by DC arc plasma jet CVD Temperature/°C Power/kW Pc/kPa If/A Feed gas flow H2/slm Ar/slm CH4/sccm 810-860 20 2.9-3.1 1.1 7.5 3.5 120 Note: Pc is chamber pressure
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Prime Novelty Statement: The through-thickness and in-plane thermal conductivity of thick diamond films prepared by DC arc plasma jet CVD was measured by laser flash technique using the way of simply changing the placing orientation of diamond films along the laser transmission direction while testing. The temperature-dependent effect of impurities and grain boundaries on thermal conductivity of diamond films is discussed.
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Highlights: 1. The temperature-dependent through-thickness and in-plane thermal conductivity of diamond films was measured by laser flash technique. 2. The anisotropy in κ// and κ⊥ values is 9% at room temperature, and disappeared when temperature reach to 400K. 3. The thermal conductivity of diamond films exhibits similar value compared to that of high-quality single crystal diamond above 500K.
S0925-9635(14)00188-5 doi: 10.1016/j.diamond.2014.09.007 DIAMAT 6310
To appear in:
Diamond & Related Materials
Received date: Revised date: Accepted date:
9 May 2014 2 September 2014 16 September 2014
Please cite this article as: R.H. Zhu, J.Y. Miao, J.L. Liu, L.X. Chen, J.C. Guo, C.Y. Hua, T. Ding, H.K. Lian, C.M. Li, High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD, Diamond & Related Materials (2014), doi: 10.1016/j.diamond.2014.09.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD
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T. Dingb, H.K. Lianb, C.M. Lia*
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R.H. Zhua, J.Y. Miaob, J.L. Liua, L.X. Chena, J.C. Guoa, C.Y. Huaa， a
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Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, P.R. China b Beijing Laboratory of Space Thermal Technology, Beijing 100094, P.R. China
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Abstract: Free-standing diamond films with 1.68mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness (κ ⊥ ) together with in-plane (κ//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ ⊥ and κ// of free-standing diamond films are up to 1916 and 1739 Wm-1K-1 at room temperature, respectively, showing small anisotropy(9%), and following the relationship κ ~ T-n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed. Key word: DC arc plasma jet CVD; Thermal conductivity; Laser flash technique
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Author, E-mail address: [email protected] (R. H. Zhu) Corresponding author, E-mail address: [email protected] (C. M. Li)
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1 Introduction It has been reported that more than half of failures in today’s electronic systems were due to temperature [1]. So the increasing demand is putting forward to heat dispersion of devices. High-quality CVD diamond films show highest thermal conductivity which is similar to natural IIa diamond[2], coupled with their low dielectic loss, broad optical transparency, making them as key materials for a host of applications, like high-power Nd:YVO4 laser heat conductors [3], megawatt power gyrotrons [4,5], UV LEDs [6], Raman lasers [7], et al. CVD diamond films exhibit some anisotropy in thermal conductivity due to their column growth [8], and in same cases they must be exposed to extreme operating temperatures when used in devices [6, 9, 10]. Various methods have been utilized to determine the temperature-dependent thermal properties of CVD diamond, such as laser flash technique [11], transient thermal grating technique [12], and steady-state heater bar method [13,14], et al, in which laser flash technique is a common and reliable method for through-thickness thermal conductivity diamond over a wide temperature range. In this paper, the thermal conductivity of free-standing diamond films with 1.68 mm thickness prepared using DC arc plasma jet CVD were measured by laser flash technique. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed. 2 Experimental Polycrystalline diamond films were deposited on the substrate of graphite with Ti interlayer by high power DC arc plasma jet CVD operating at gas recycling mode. This system has been depicted in details previously [15]. The main deposition parameters were listed in Table 1. After stable and continuous deposition for more than 350 h, diamond films with thickness of ~2.75 mm were obtained. The as-deposited diamond films were peeled off from substrate, then abraded by using commercial diamond grits and double-sided polished by fast rotating diamond grinding wheel. At last, the thickness of free-standing diamond films was 1.68 mm and surface roughness Ra was less than 10 nm. The optical micrograph and cross-section morphologies of diamond films were observed by optical microscopy (OM) and S-250MK3 scanning electron microscopy. The crystal structure was analyzed by X-ray diffractometer (DMA-RB, 13KW) using Cu Kα radiation. The crystal quality of diamond films was analyzed using NICOLET ALMEGA XR Dispersive Raman spectrum (514nm) and NEXUS 670 Fourier infrared Raman spectrometer. NETZSCH Laser Flash Apparatus 447 and 457 were used to detect the thermal diffusivity (α) of diamond films simultaneously. The introduction of Laser Flash technique has been depicted in details anywhere [10]. The pulse length of Xenon Flash lamp and Nd:YAG laser used in LFA447 and LFA457 was 0.06 ms and 0.33 ms respectively. 1 K variation in the temperature of sample was stability controlled while testing. Each data presented was the average of five replications. The values of
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polycrystalline graphite and copper measured by LFA 447 and 457 showed precision within ±3%. Fig.1 shows the temperature-rising curve on upper surface of diamond after a short-pulsed laser heated on the lower surface. The half-rise time t1/2 is obtained from the curve which is revised using Cowan plus pulse correction model. Then thermal diffusivity and thermal conductivity are calculated using the equation (1) and (2):
(T ) (T ) (T ) C p (T )
(2)
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where L is the width of sample along the laser propagation direction. Cp(T) and ρ(T) are the temperature-dependent specific heat capacity and density of CVD diamond films, respectively. According to Ref [16]，specific heat capacity Cp and mass density ρ of CVD diamond films were within 1% of the accepted values for single crystal diamond. Meanwhile, CVD diamond films with different optical properties showed identical Cp(T)[8]. So Cp(T) and ρ(T) values in Ref[16] of natural diamond are used in this study. The testing process is shown in Fig.2. Firstly, the polished free-standing diamond films were laser cut into one disc (Φ12.5 mm×1.68 mm) and six rectangular bars (10 mm×4.57 mm×1.68 mm). The six rectangular bars stacked tightly together to form one rectangular sample (10 mm×10.08 mm×4.57mm) as shown in Fig.2 (a). The upper and lower cross section of this rectangular sample was polished to obtain the flat surface (Ra<100 nm) and uniform width (L//=4.57 mm) in testing process. Secondly, Au coatings (~100 nm) were deposited on the upper and lower sides of the disc and rectangular samples to prevent transmission of laser, and then a very thin graphite layer was followed to increase laser absorptivity. Thirdly, the temperature-dependent measurements of LFA 447 only reach 573K. So the through-thickness and in-plane thermal diffusivity with temperature range from 300K to 550 K were detected by LFA447, meanwhile the in-plane thermal diffusivity with temperature range from 550 K to 1100 K was measured by LFA457. The photo thermal deflection (PTD) method was used to measure the in-plane thermal conductivity of diamond films at room temperature based on the technology of photo thermal deflection spectroscopy. Details of this method have been previously described in Refs [17, 18]. The error in this measurement system is 5%. 3 Results and discussion 3.1 Basic characterizations of free-standing diamond films The optical microscope photo of diamond films using a back light is shown in Fig.3 (a). It can be seen that small amount of dark features with dot and bar shapes, composed of amorphous carbon, nitrogen and pores[19], existed inside the films. Most of them are distributed along the grain boundaries. The cross section
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morphology of polished diamond films is shown in Fig.3 (b) and (c). The columnar growth with grain size increasing from bottom to top is evident. It can be seen from Fig.3 (c) that the columnar grain width of the nucleation side is about 15 μm after polishing. Fig.4 shows X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films. The (111), (220) and (331) peaks are observed in both sides. Based on origin software, I(110)/I(111) is 3.98 and 5.74 for nucleation and growth surface. It seems that (110) preferred orientation was obtained as it grew in early nucleation stage, and became more evident as deposition time increased. Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths are shown in Fig.5. Only the pronounced diamond peak and no signature of non-diamond carbon are found when excited in 514nm. However, a broad band extending from 1000 to 1600cm-1, scattering from non-diamond carbon [20], is observed besides the 1332cm-1 Raman line of diamond when excited in 1064nm. It was reported that the ratio of Raman cross section of diamond and graphite was about 1/50[21]. And the sensitivity of non-diamond carbon can be drastically enhanced with increasing excitation wavelength [22]. It indicates that small amount of non-diamond phase exists in the diamond films. 3.2 Thermal conductivity of free-standing diamond films Fig.6 shows through-thickness thermal conductivity of diamond films between 300K and 550K. It can be seen that the room-temperature thermal conductivity κ⊥ of diamond films reach up to 1916 Wm-1 K-1. It is well know that the main heat carrier in diamond is phonon, which would be scattered by phonon collision, defects and gran boundaries consequently reducing the mean free path l of phonons [23] (eq.3). 1 Among them, l boundary is negligible when phonons propagate along the columns at
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room temperature. Slight difference between κ⊥ and IIa diamond (~2200 Wm-1K-1) 1 [24] would be caused by l defect , which mainly refer to the impurities(N, non-diamond
carbon), lattice mismatch, pores et al, inside the column crystal. The through-thickness thermal conductivity κ ⊥ of diamond films decreases with the increasing temperature, and down to about 1100 W m-1 K-1 at 550K.
l 1 l 1 phonon l 1defect l 1boundary
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The in-plane thermal conductivity κ// of diamond films is 1739 Wm-1K-1 at room temperature, which is similar to the result (1700 Wm-1K-1) measured by photo thermal deflection (PTD) method. A small anisotropy in κ is revealed at 300 K and 350 K. The result of calculations from Fig.6 show that the κ⊥ is higher by 9% than κⅡ at 300 K, which is similar to the results (10 %~15 %)obtained by A.V. Sukhadolau [25]. The gran boundaries would scatter the phonons, and consequently reduce the mean free path l when phonons propagate perpendicular to the columns. Besides, a number of defects would concentrate along boundaries which would further reduce l of phonon.
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negligible when phonons propagate perpendicular to the columns of diamond film above 400K. The in-plane thermal conductivity of diamond films between 300 K and 1100 K is shown in Fig.7. For comparison, the data for high-quality single crystal diamond taken from Refs [23, 27, 28] are also shown in Fig.7. The curves with its horizontal axis (Temperature) in reciprocal type confirm the relationship κ ~ T-n held well for diamond films as temperature rises. The n value, which depends on κ at room temperature [26], is 0.91 and 0.72 for κ⊥ and κ// of diamond films. The in-plane and through-thickness thermal conductivity of free-standing diamond films above 500K shows similar value compared to that of single crystal diamond reported in Refs [27, 28]. As is known in Refs [29-31], the phonon collision scattering can limit the phonons free path and plays a dominant role from room temperature, and its influence is further increased when temperature rises, while the boundary and defect scattering hardly have any effect on mean free path. It also indicates that the diamond films can display similar heat transfer result compared to high-quality single crystal diamond when used at and above 500K.
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Polycrystalline diamond films with (110) preferred orientation were obtained after stable and continuous deposition for more than 350 h using DC arc plasma jet CVD with gas recycling mode. The thickness of free-standing diamond films after polishing was 1.68mm. The results examined by optical photo and IR Raman indicate that small amount of non-diamond phase exists in the diamond films. The through-thickness thermal conductivity κ⊥ is 1916 Wm-1K-1 at room temperature, slightly smaller than that of IIa diamond caused by scattering processes from non-diamond impurities inside the column crystal. The in-plane thermal conductivity κ// is 1739 Wm-1K-1 at 300K, showing small anisotropy (9%) compared to κ⊥ owing to grain boundaries scattering mechanisms in phonon transfer processes at room temperature. This anisotropy in κ values disappears when temperature reaches to 400K. The thermal conductivity of diamond films follows the relationship κ ~ T-n as temperature rises, and exhibits similar values compared to that of high-quality single crystal diamond above 500K, where the grain boundaries and defects scattering mechanisms can be neglected. Acknowledgements This work was sponsored by the National Natural Science Foundation of China
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(NSFC) (No.51272024), and the Ph.D. Programs Foundation of Ministry of Education of China (No.20110006110011). The authors deeply appreciated their financial supports.
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[17] C. Gu, Z. Jin, X. Lu, G. Zou, J. Zhang, R. Fang. The deposition of diamond film with high thermal conductivity[J]. Thin Solid Films, 311(1997) 124-127. [18] M Bertolotti, G.L Liakhou, A Ferrari, V.G Ralchenko, A.A Smolin, E.D Obraztsova, K.G Korotoushenko, S.M Pimenov, V.I Konov. Measurements of thermal conductivity of diamond films by photothermal deflection technique[J]. Journal of applied physics, 75(1994) 7795-7798. [19] J. Yang, X. Duan, F. Lu, C. Li, T. Zuo, F. Wang, The influence of dark feature on optical and thermal property of DC Arc Plasma Jet CVD diamond films. Diamond and related materials, 14 (2005) 1583-1587. [20] E. Wörner, C. Wild, W. Müller-Sebert, R. Locher, P. Koidl. Infrared Raman scattering as a sensitive probe for the thermal conductivity of chemical vapor deposited diamond films. Applied physics letters, 68(1996)1482-1484. [21] A D Papadopoulos, E Anastassakis. Optical properties of diamond. Physical review. B, Condensed matter, 43(1991) 5090-5097. [22] J Wagner, C Wild, P Koidl. Resonance effects in Raman scattering from polycrystalline diamond film. Applied physics letters, 59(1991) 779-781. [23] Onn D G, Witek A, Qiu Y Z, Anthony, T. R., Banholzer, W. F.. Some aspects of the thermal conductivity of isotopically enriched diamond single crystals. Physical review letters, 68(1992)2806-2809. [24] M.A. Angadi, T. Watanabe, A. Bodapati, X. Xiao, O. Auciello, J.A. Carlisle, J.A. Eastman, P. Keblinski, P.K. Schelling, S.R. Phillpot. Thermal transport and grain boundary conductance in ultrananocrystalline diamond thin films. Journal of applied physics, 99(2006)114301. [25] A. Sukhadolau, E. Ivakin, V. Ralchenko, A. Khomich, A. Vlasov, A. Popovich, Thermal conductivity of CVD diamond at elevated temperatures. Diamond and related materials, 14 (2005) 589-593. [26] D. Morelli, C. Beetz, T. Perry, Thermal conductivity of synthetic diamond films. Journal of applied physics, 64 (1988) 3063-3066. [27] A. Witek, Some aspects of thermal conductivity of isotopically pure diamond—a comparison with nitrides. Diamond and related materials, 7 (1998) 962-964. [28] J.R. Olson, R.O. Pohl, J.W. Vandersande, A. Zoltan, T.R. Anthony, W.F. Banholzer. Thermal conductivity of diamond between 170 and 1200 K and the isotope effect[J]. Physical Review B, 47(1993) 14850. [29] Graebner J E, Jin S, Herb J A, Gardinier C F. Local thermal conductivity in chemical vapor deposited diamond. Journal of applied physics,76(1994) 1552-1556. [30] W Masierak, K Fabisiak, M Kaczmarski, M Kozanecki. Simple model for the thermal conductivity estimation on the basis of Raman and ESR spectroscopy measurements. Optica Applicata, 36(2006)225-234. [31] G A Slack. Thermal conductivity of pure and impure silicon, silicon carbide, and diamond. Journal of Applied Physics, 35(2004) 3460-3466.
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Fig.1 The temperature-rising curve on upper surface of diamond films after a short-pulsed laser heated on the lower surface. Fig.2 The temperature-dependent thermal conductivity testing process of free-standing diamond films. Fig.3 Optical microscope photo (a) and SEM Cross-section micrographs (b, c) of diamond films. Fig.4 X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films. Fig.5 Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths. Fig.6 The through-thickness and in-plane thermal conductivity of diamond films between 300K and 550K. Fig.7 The in-plane thermal conductivity of diamond films between 300K and 1100K. The literature data for single crystal diamond (circles) at T<500K refer to synthetic diamond enriched with 12C isotope.
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Table 1 The main deposition parameters of diamond films by DC arc plasma jet CVD
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Fig.1 The temperature-rising curve on upper surface of diamond films after a short-pulsed laser heated on the lower surface
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Fig.2 The temperature-dependent thermal conductivity testing process of free-standing diamond films
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Fig.3 Optical microscope photo (a) and SEM Cross-section micrographs (b) (c) of diamond films
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Fig.4 X-ray diffraction (XRD) pattern of nucleation and growth surfaces of diamond films
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Fig.5 Raman spectra of diamond films excited using 514 nm and 1064 nm wavelengths
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Fig.6 The through-thickness and in-plane thermal conductivity of diamond films between 300K and 550K
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Fig.7 The in-plane thermal conductivity of diamond films between 300K and 1100K. The literature data for single crystal diamond (circles) at T<500K refer to synthetic diamond enriched with 12C isotope.
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Table1 The main deposition parameters of diamond films by DC arc plasma jet CVD Temperature/°C Power/kW Pc/kPa If/A Feed gas flow H2/slm Ar/slm CH4/sccm 810-860 20 2.9-3.1 1.1 7.5 3.5 120 Note: Pc is chamber pressure
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Prime Novelty Statement: The through-thickness and in-plane thermal conductivity of thick diamond films prepared by DC arc plasma jet CVD was measured by laser flash technique using the way of simply changing the placing orientation of diamond films along the laser transmission direction while testing. The temperature-dependent effect of impurities and grain boundaries on thermal conductivity of diamond films is discussed.
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Highlights: 1. The temperature-dependent through-thickness and in-plane thermal conductivity of diamond films was measured by laser flash technique. 2. The anisotropy in κ// and κ⊥ values is 9% at room temperature, and disappeared when temperature reach to 400K. 3. The thermal conductivity of diamond films exhibits similar value compared to that of high-quality single crystal diamond above 500K.