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Thermal diffusivity in diamond, SiCxNy and BCxNy
Diamond and Related Materials 11 (2002) 708–713
Thermal diffusivity in diamond, SiCxNy and BCxNy S. Chattopadhyaya,*, S.C. Chiena, L.C. Chena, K.H. Chenb, H.Y. Leec a
Centre for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan, ROC b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan, ROC c Research Division, Synchrotron Radiation Research Centre, Hsinchu, Taiwan, ROC
Abstract Thermal diffusivity (a) of free standing diamond, amorphous silicon carbon nitride (a-SiCx Ny ) and boron carbon nitride (aBCxNy) thin films on crystalline silicon, has been studied using the travelling wave technique. Thermal diffusivity in all of them was found to depend on the microstructure. For a-SiCx Ny and a-BCx Ny thin films two distinct regimes of high and low carbon contents were observed in which the microstructure changed considerably and that has a profound effect on the thermal diffusivity. The defective C(sp)–N phase plays a key role in determining the film properties. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal conductivity; Microstructure; Photoelectron spectroscopy; Physical vapour deposition
1. Introduction Among the extreme properties of diamond, thermal conductivity w1–3x has been critical to many applications such as heat sinks and tool coating. However, a strong microstructure dependence of hardness and thermal conductivity has been reported w1x. Silicon carbon nitride (SiCxNy) and boron carbon nitride (BCxNy) are also hard and covalent compounds but hetero-atomic in nature where we expect a microstructure dependent hardness and thermal diffusivity. Indeed nanoindentation measurements on the crystalline SiCxNy suggested its hardness and bulk modulus to be in the vicinity of 30 GPa and 322 GPa, respectively w4x, whereas for amorphous SiCxNy (a-SiCxNy) films, the hardness value was approximately 22 GPa w4x and Young’s modulus approximately 245 GPa w5x. The first effort at producing the a-BCxNy material was made by Badzian et al. w6x through the CVD route. Since then BC2N, a semiconductor material with approximately 2 eV band gap w7x, has emerged showing promise as a superhard material w8x, with a hardness *Corresponding author. Tel.: q886-223638035; fax: q886223620200. E-mail address: [email protected] (S. Chattopadhyay).
and bulk modulus of 75 and 282 GPa, respectively. Although optoelectronic and mechanical properties of these materials have been reported, information on their thermal diffusivity has been rather limited. This paper tests the microstructural dependence of the thermal diffusivity (a) in freestanding diamond, a-SiCxNy and a-BCxNy. 2. Experimental Polycrystalline diamonds were deposited on molybdenum substrates by electron assisted hot filament chemical vapour deposition using ethanol and hydrogen as the precursor. After deposition, the molybdenum substrates were removed and freestanding diamond sheets of 0.94–1.8 mm thickness were obtained after mechanical polishing. These polycrystalline diamond sheets had different crystallite size distribution. The nucleation zone (surface in contact with the substrate during growth) and the growth zone (free surface during growth) of any single polycrystalline diamond sheet were also different in terms of their microstructure. Details of the growth parameters, grain size and other properties of these polycrystalline diamond sheets have been reported earlier w9x. The a-SiCxNy samples were deposited via ion beam sputtering (IBS) technique. A 3-cm Kaufmann type ion source was used for sputtering targets of laminated
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 0 6 - 9
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mann-Braun) MB series FTIR spectrometer in the reflection mode. The resolution of the spectrum was 4 cmy1. The Raman spectra were recorded on a Renishaw 2000 Micro Raman spectrometer where an Ar ion laser (514 nm) was used as an excitation source. The full width at half maximum (FWHM) of the diamond line (1332 cmy1) was measured to characterise the microstructure of the polycrystalline diamond samples under investigation. 3. Results
Fig. 1. Variation of the thermal conductivity and thermal diffusivity of the (h) growth side and (j) nucleation side of free standing CVD diamond sheets as a function of the FWHM of the 1332 cmy1 Raman band.
silicon carbide or silicon and graphite. The films were grown on crystalline silicon (c-Si) substrates. Details of the ion beam sputtering reactor are given elsewhere w10x. The boron carbon nitride films were deposited by dual cathode sputtering of boron nitride (BN) and graphite targets. The r.f. power to the BN target and the d.c. power to the graphite target were varied to control the sputtering rate from the respective targets. The deposition temperature was maintained at approximately 400 8C. The a-SiCxNy and a-BCxNy films on crystalline silicon were systematically studied for the thermal diffusivity (a) by the travelling wave technique. The sample was irradiated by an infrared (IR) diode laser array whose output was modulated by a sine wave. The phase of the travelling wave at different distances on the sample was monitored by the deflection of a He–Ne probe laser. From the slope of a linear fit to the phase vs. distance plot, the thermal diffusivity was calculated. Instead of measuring the temperature by an IR microscope as Kosky w2x, a simple probe laser was used to measure the phase difference only. The details of the thermal diffusivity measurement set-up and theory is given elsewhere w11x. For the polycrystalline diamond films, the thermal conductivity was calculated by multiplying the thermal diffusivity with the specific heat and density of natural diamond. The chemical composition of the films were determined by high-resolution scans of Si(2p), B(1s), C(1s) and N(1s) peaks by X-ray photoelectron spectroscopy (XPS). A PHI 1600 system was used for the XPS measurements. Surface acoustic wave (SAW) measurement and X-ray reflectivity were used to determine the density of the films w5,12x. Fourier transform infrared (FTIR) spectra were obtained with a Bomem (Hart-
Both the nucleation and the growth zone of the free standing diamond sheets were studied for their microstructure and thermal diffusivity. Fig. 1 shows the variation of the thermal conductivity and thermal diffusivity with the FWHM of the Raman diamond line. As the FWHM increased signalling a loss of order and increasing heterogeneity, the thermal diffusivityyconductivity decreased. This observation was independent of the nucleation or growth zone of the polycrystalline diamond sheets being measured, indicating a strong microstructural dependence of the thermal conductivity. For a-SiCxNy and a-BCxNy samples the thermal diffusivity (a) were measured as a function of their carbon contents (Fig. 2). The values of a remained at approximately 0.35 cm2 ys for a-SiCxNy with carbon contents lower than 30 at.%, but with increasing carbon contents the thermal diffusivity decreased steadily and reached ;0.15 cm2 ys at a carbon content of )60 at.% (Fig. 2). Similarly, the density for these films also decreased from 3.25 gycm3 at a carbon content of ;25 at.% to less than 2.4 gycm3 at approximately 70 at.% of carbon in the films. For a-BCxNy, the thermal diffusivity values showed a peak of approximately 30 at.% of carbon content (Fig. 3), on both sides of which the thermal diffusivity values
Fig. 2. Variation of the (h) thermal diffusivity a and (j) density as a function of the carbon content in the a-SiCxNy films.
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Fig. 3. Variation of the (n) thermal diffusivity a and (m) density as a function of the carbon content in the a-BCxNy films.
showed a decrease. On the lower carbon content side (-30 at.%) the values were approximately 0.45 cm2 ys for a carbon content of ;15 at.%, but on the higher side ()30 at.%), the thermal diffusivity values showed a steady fall from 0.6 to 0.19 cm2 ys at a carbon content of 60 at.%. Again the density of these films, measured by X-ray reflectivity, varied in a similar fashion with carbon content. That is, the density decreased from a peak value of ;2.02 gycm3 at a carbon content of 25 at.% in the films to less than 1.8 gycm3 at a carbon content of more than 60 at.% (Fig. 3). But the absolute density values in a-BCxNy films were considerably less than those for the a-SiCxNy samples. High resolution X-ray photoelectron spectroscopy (XPS) scans of Si(2p), B(1s), C(1s) and N(1s) peaks of the a-SiCxNy and a-BCxNy films were performed to obtain the composition and bonding characteristics of the network. Fig. 4a,b shows the deconvoluted C(1s) band from XPS for a-SiCxNy films with ;30 and 70 at.% of carbon, respectively. Fig. 4a depicts the 284.6and 287.35-eV peaks corresponding to the C(sp2)–C and C(sp3)–N bonds, respectively w13–15x. In Fig. 4b, along with the 284.6-eV peak two additional subpeaks at 286.54 and 288.9 eV corresponding to C(sp)–N and C–O bonds, respectively w13,14x were also detected. For all the films studied the atomic ratio of silicon to nitrogen was maintained at ;0.75. The C(1s), N(1s) and B(1s) bands confirm the B–C (;284 eV) w16x, C–C, B–N and C–N bonds in the aBCxNy films. Fig. 5 compares the XPS C(1s) spectra of two a-BCxNy films having different carbon contents. The C(1s) spectra were deconvoluted to show the contribution from different bonding arrangements for the low and high carbon a-BCxNy films. Due to the close proximity of the B–C, C–C and C–N bands it was difficult to extract quantitative information on each but the shift in the C(1s) position is clear with increasing
carbon content. Stronger contributions from C(sp)–N (286.53 eV) in the higher carbon containing ()30 at.%) a-BCxNy film is clearly evident w14,15,17x. The presence of the C(sp)–N bonds in the higher carbon regime is also confirmed independently by FTIR spectroscopy. Fig. 6a shows the FTIR spectra for aSiCxNy films with increasing carbon content. At a lower carbon content, the FTIR spectrum of a-SiCxNy showed only a small doublet at 2350 cmy1 and a broad band at 950 cmy1 corresponding to C–O and Si–N w18x, respectively. The increase in carbon content in the network is accompanied by the appearance of the 1250–1700 cmy1, and 2200 cmy1 (C(sp)–N peak) bands in the FTIR spectrum w8,19x. Fig. 6b shows FTIR spectra for a-BCxNy films with increasing carbon contents. The prominent feature in the spectra are the out-of-plane B–N–B bending mode at 780 cmy1, the in-plane B–N stretching at ;1370 cmy1 w20x, the C(sp)–N peak (2200 cmy1), C(sp2)–N peak (1690 cmy1) and the C(sp2)–C peak at 1550 cmy1 w19,21,22x. The C(sp)–N mode evolved and the
Fig. 4. The deconvoluted XPS scans of C(1s) spectrum for the aSiCxNy films with (a) 30 at.% and (b) 60 at.% of carbon.
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The deposition routes used for the preparation of aSiCxNy and a-BCxNy films produced various defects such as dangling bonds, voids as well as bonds with lower coordination numbers. The presence of defects in the films could translate into a reduction in the thermal diffusivity. Indeed, strong effect of the bonding configuration of the a-SiCxNy and a-BCxNy networks, as examined by the FTIR and XPS spectra analyses, on the thermal diffusivity was observed. In the binary SiNx film, silicon atoms are bonded tetrahedrally to nitrogen. A small introduction of carbon in the network still keeps the coordination number to a maximum. The C(1s) core level spectrum indicates the predominant C_C bonds (284.6 eV) and C(sp3)–N bonds for the film with low carbon content (Fig. 4a). However, exceeding a carbon concentration of ;30 at.%, the excess carbon atoms introduced tend to bond primarily to nitrogen as C(sp2)–N, and C(sp)–N in the film network as evident from the shift in the C(1s) spectra towards higher binding energies (286.54 eV).
Fig. 5. The deconvoluted XPS scans of C(1s) spectrum for the aBCxNy films with (a) 10 at.% and (b) 60 at.% of carbon.
B–N–B mode diminished as the carbon content increased. 4. Discussion From earlier results w1x and the present experiments on the free standing diamond sheets it was evident that microstructure has a strong effect on the thermal conductivity of this class of covalent materials. Hence as the FWHM of the Raman diamond line increased, which indicated a larger disorder in the carbon–carbon bonding, the thermal conductivity decreased (Fig. 1). This was independent of the growth or nucleation zones studied. The relatively lower thermal diffusivity values for the nucleation zones of the diamond sheets in comparison to the growth zone is due to the smaller crystallite sizes and greater heterogeneity in the network in the nucleation zones w9x. The velocity of surface acoustic waves (SAWs) on these diamond sheets was also found to have a strong dependence on its microstructure w9x. Hence a possible correlation of thermal diffusivity and velocity of SAWs was envisaged.
Fig. 6. The FTIR spectra of the (a) a-SiCx Ny films with 0, 30 and 70 at.% of carbon and (b) a-BCxNy films with 10, 30 and 60 at.% of carbon.
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The FTIR spectra for the series of a-SiCxNy films also reveals the evolution of the C(sp)–N (2200 cmy1) phase w19x, as the carbon content in the film increases (Fig. 6a). This, in addition to other modes at 950 and 1250–1700 cmy1, confirms the different bonding environments in the a-SiCxNy network. XPS analyses of the a-BCxNy films indicates more than two types of atomic environments around the carbon and boron atoms. The C(1s) spectra showed the presence of B–C, C–C and C–N bonds. The B(1s) spectra (not presented) confirmed the presence of B–N, B–C and some B–O bonding. With the increase in carbon in the film within 25 at.%, B–C fractions increased which may explain the slight increase in the thermal diffusivity and density in the films (Fig. 3). Interestingly, the growth of B–C bonding was suppressed with the increase in carbon in the film beyond ;30 at.%. Instead, only a growing influence of C(sp)–N was observed (Fig. 5). This indicates a preferential bonding of carbon with nitrogen in the films at higher carbon concentrations. The broad band between 1150 and 1770 cmy1 of the FTIR spectra for the a-BCxNy films is indicative of the various vibration modes (Fig. 6b). Increasing carbon content in the films facilitates the growth of a defective phase promoted mostly by the C(sp)–N (2200 cmy1) and a simultaneous decrease in the B–N–B (780 cmy1) phase. The increase in the lower coordinated C(sp)–N phase and the preferential carbon to nitrogen bonding with increase in carbon in the films was consistent with the XPS results presented earlier. The common understanding that emerges from the XPS and FTIR studies of a-SiCxNy and a-BCxNy films is the growth of a lower coordinated C(sp)–N at high carbon concentrations that produces a less dense and more defective material. This defective phase holds the key for the thermal diffusivity and density in these material systems. The variation of the density with increasing carbon content (Figs. 2 and 3) in a-SiCxNy and a-BCxNy supports an earlier report of increasing voids with carbon in a-SiC:H w23x. For a-SiCxNy, with carbon content in the range of 25 at.%, the film retained its fourfold coordination and hence the density does not decrease. In the case of a-BCxNy films, B–C bonding was observed in the low carbon (-25 at.%) regime. Beyond 30 at.% of carbon, increased C(sp)–N led to a substantial decrease of density in both a-SiCxNy and a-BCxNy films. Theoretical molecular dynamic studies and experiments agree to the higher value of the density (3.45 gy cm3) and Young’s modulus (265 GPa) for the binary SiNx film where the constituent atoms share maximum coordination w5x. The maximum coordination is nearly maintained up to a carbon concentration of approximately 30 at.%. But when the carbon content is in excess of ;30 at.%, the C(sp)–N bonds start evolving and the values of density in a-SiCxNy start decreasing.
The variation of density for a-BCxNy films with increasing carbon content shows the same tendency (Fig. 3). There is a drop in the density value with carbon concentration exceeding ;25 at.% in the films due to the evolution of the C(sp)–N phase that introduces voidlike defects in the film. Having discussed the bonding configuration and density of the films and their variation with the carbon concentration, the variation of the thermal diffusivity is also explained. The presence of voids and a defective phase in the high carbon regime of a-SiCxNy and aBCxNy films, as confirmed by XPS, FTIR spectroscopy and density measurements, led to a discontinuous atomic network. The decrease in thermal diffusivity becomes obvious since the connectivity of the network was crucial for the transport of vibrational energy. The increasing anharmonicity of the network at high carbon concentrations resulted in a reduced phonon mean free path and a reduced thermal diffusivity. 5. Conclusions Microsturucture dependent thermal diffusivity was found for diamond, amorphous silicon carbon nitride and amorphous boron carbon nitride thin films. For free standing diamond films the thermal diffusivity decreased with increasing Raman 1332 cmy1 line width. In amorphous silicon carbon nitride and amorphous boron carbon nitride thin films with increasing carbon content above ;30 at.%, the thermal diffusivity and density was found to decrease. The evolution of a soft C(sp)–N phase was responsible for the growth of a less dense and discontinuous network that was unable to transport vibrational energy and hence the thermal diffusivity values were reduced in high carbon regimes for both materials. Acknowledgments This work was carried out under a project funded by the National Science Council, Taiwan, under Contract Nos. NSC89-2112-M002-085, and NSC89-2119-M001011. We acknowledge the help of Prof. P. Hess and G. Lehmann, of the Institute of Physical Chemistry, University of Heidelberg, Germany, for the density measurements of the a-SiCxNy films and many helpful discussions. One of the author (S.C) acknowledges the award of a post-doctoral fellowship by the National Science Council, Taiwan. References w1x J.E. Graebner, J.A. Mucha, L. Seibles, G.W. Kammlott, J. Appl. Phys. 71 (1992) 3143. w2x P.G. Kosky, Rev. Sci. Instrum. 64 (1992) 1071. w3x K.M. Leung, A.C. Cheung, B.C. Liu, et al., Diamond Relat. Mater. 8 (1999) 1607.
S. Chattopadhyay et al. / Diamond and Related Materials 11 (2002) 708–713 w4x L.C. Chen, K.H. Chen, S.L. Wei, et al., Thin Solid Films 355356 (1999) 112. w5x G. Lehmann, P. Hess, J.J. Wu, et al., Phys. Rev. B 64 (2001) 165305. w6x A.R. Badzian, T. Niemyski, S. Appenheimer, E. Olkusnik, in: F.A. Glaski (Ed.), Proc. of the 3rd. Int. Conf. on Chemical Vapour Deposition, American Nuclear Society, Hinsdale, IL, 1972, p. 747. w7x M.O. Watanabe, S. Itoh, T. Sasaki, K. Mizushima, Phys. Rev. Lett. 77 (1996) 187. w8x V.L. Solozhenko, D. Andrault, G. Fiquet, M. Mezouar, D.C. Rubie, Appl. Phys. Lett. 78 (2001) 1385. w9x G. Lehmann, M. Schreck, L. Hou, J. Lambers, P. Hess, Diamond Relat. Mater. 10 (2001) 686. w10x J.J. Wu, T.R. Lu, C.T. Wu, et al., Diamond Relat. Mater. 8 (1999) 605. w11x D.M. Bhusari, C.W. Teng, K.H. Chen, S.L. Wei, L.C. Chen, Rev. Sci. Instrum. 68 (1997) 4180. w12x S. Chattopadhyay, L.C. Chen, C.T. Wu, et al., Appl. Phys. Lett. 79 (2001) 332. w13x D. Marton, K.J. Boyd, A.H. Al-Bayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (1994) 118.
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w14x S. Bhattacharyya, C. Cardinaud, G. Turban, J. Appl. Phys. 83 (1998) 4491. w15x B.C. Holloway, O. Kraft, D.K. Shuh, et al., Appl. Phys. Lett. 74 (1999) 3290. w16x M.O. Watanabe, S. Itoh, K. Mizushima, T. Sasaki, Appl. Phys. Lett. 68 (1996) 2962. w17x H. Kunzli, P. Gantenbein, R. Steiner, P. Oelhafen, Anal. Chem. 346 (1993) 41. w18x W.F.A. Besling, A. Goossens, B. Meester, J. Schoonman, J. Appl. Phys. 83 (1998) 544. w19x J. Yu, E.G. Wang, J. Ahn, S.F. Yoon, et al., J. Appl. Phys. 87 (2000) 4022. w20x A. Tempez, N. Badi, A. Bensaoula, J. Kulik, J. Vac. Sci. Technol. A 16 (1998) 2896. w21x C.-W. Ong, K-F. Chan, X-A. Zhao, C. Loong Choy, Surf. Coat. Technol. 115 (1999) 145. w22x Y.F. Lu, Z.M. Ren, W.D. Song, et al., J. Appl. Phys. 84 (1998) 2909. w23x S. Chattopadhyay, D. Das, A.K. Barua, D.L. Williamson, S.T. Kshirsagar, Jpn. J. Appl. Phys. 37 (1998) 5480.
Thermal diffusivity in diamond, SiCxNy and BCxNy S. Chattopadhyaya,*, S.C. Chiena, L.C. Chena, K.H. Chenb, H.Y. Leec a
Centre for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan, ROC b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan, ROC c Research Division, Synchrotron Radiation Research Centre, Hsinchu, Taiwan, ROC
Abstract Thermal diffusivity (a) of free standing diamond, amorphous silicon carbon nitride (a-SiCx Ny ) and boron carbon nitride (aBCxNy) thin films on crystalline silicon, has been studied using the travelling wave technique. Thermal diffusivity in all of them was found to depend on the microstructure. For a-SiCx Ny and a-BCx Ny thin films two distinct regimes of high and low carbon contents were observed in which the microstructure changed considerably and that has a profound effect on the thermal diffusivity. The defective C(sp)–N phase plays a key role in determining the film properties. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal conductivity; Microstructure; Photoelectron spectroscopy; Physical vapour deposition
1. Introduction Among the extreme properties of diamond, thermal conductivity w1–3x has been critical to many applications such as heat sinks and tool coating. However, a strong microstructure dependence of hardness and thermal conductivity has been reported w1x. Silicon carbon nitride (SiCxNy) and boron carbon nitride (BCxNy) are also hard and covalent compounds but hetero-atomic in nature where we expect a microstructure dependent hardness and thermal diffusivity. Indeed nanoindentation measurements on the crystalline SiCxNy suggested its hardness and bulk modulus to be in the vicinity of 30 GPa and 322 GPa, respectively w4x, whereas for amorphous SiCxNy (a-SiCxNy) films, the hardness value was approximately 22 GPa w4x and Young’s modulus approximately 245 GPa w5x. The first effort at producing the a-BCxNy material was made by Badzian et al. w6x through the CVD route. Since then BC2N, a semiconductor material with approximately 2 eV band gap w7x, has emerged showing promise as a superhard material w8x, with a hardness *Corresponding author. Tel.: q886-223638035; fax: q886223620200. E-mail address: [email protected] (S. Chattopadhyay).
and bulk modulus of 75 and 282 GPa, respectively. Although optoelectronic and mechanical properties of these materials have been reported, information on their thermal diffusivity has been rather limited. This paper tests the microstructural dependence of the thermal diffusivity (a) in freestanding diamond, a-SiCxNy and a-BCxNy. 2. Experimental Polycrystalline diamonds were deposited on molybdenum substrates by electron assisted hot filament chemical vapour deposition using ethanol and hydrogen as the precursor. After deposition, the molybdenum substrates were removed and freestanding diamond sheets of 0.94–1.8 mm thickness were obtained after mechanical polishing. These polycrystalline diamond sheets had different crystallite size distribution. The nucleation zone (surface in contact with the substrate during growth) and the growth zone (free surface during growth) of any single polycrystalline diamond sheet were also different in terms of their microstructure. Details of the growth parameters, grain size and other properties of these polycrystalline diamond sheets have been reported earlier w9x. The a-SiCxNy samples were deposited via ion beam sputtering (IBS) technique. A 3-cm Kaufmann type ion source was used for sputtering targets of laminated
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 0 6 - 9
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mann-Braun) MB series FTIR spectrometer in the reflection mode. The resolution of the spectrum was 4 cmy1. The Raman spectra were recorded on a Renishaw 2000 Micro Raman spectrometer where an Ar ion laser (514 nm) was used as an excitation source. The full width at half maximum (FWHM) of the diamond line (1332 cmy1) was measured to characterise the microstructure of the polycrystalline diamond samples under investigation. 3. Results
Fig. 1. Variation of the thermal conductivity and thermal diffusivity of the (h) growth side and (j) nucleation side of free standing CVD diamond sheets as a function of the FWHM of the 1332 cmy1 Raman band.
silicon carbide or silicon and graphite. The films were grown on crystalline silicon (c-Si) substrates. Details of the ion beam sputtering reactor are given elsewhere w10x. The boron carbon nitride films were deposited by dual cathode sputtering of boron nitride (BN) and graphite targets. The r.f. power to the BN target and the d.c. power to the graphite target were varied to control the sputtering rate from the respective targets. The deposition temperature was maintained at approximately 400 8C. The a-SiCxNy and a-BCxNy films on crystalline silicon were systematically studied for the thermal diffusivity (a) by the travelling wave technique. The sample was irradiated by an infrared (IR) diode laser array whose output was modulated by a sine wave. The phase of the travelling wave at different distances on the sample was monitored by the deflection of a He–Ne probe laser. From the slope of a linear fit to the phase vs. distance plot, the thermal diffusivity was calculated. Instead of measuring the temperature by an IR microscope as Kosky w2x, a simple probe laser was used to measure the phase difference only. The details of the thermal diffusivity measurement set-up and theory is given elsewhere w11x. For the polycrystalline diamond films, the thermal conductivity was calculated by multiplying the thermal diffusivity with the specific heat and density of natural diamond. The chemical composition of the films were determined by high-resolution scans of Si(2p), B(1s), C(1s) and N(1s) peaks by X-ray photoelectron spectroscopy (XPS). A PHI 1600 system was used for the XPS measurements. Surface acoustic wave (SAW) measurement and X-ray reflectivity were used to determine the density of the films w5,12x. Fourier transform infrared (FTIR) spectra were obtained with a Bomem (Hart-
Both the nucleation and the growth zone of the free standing diamond sheets were studied for their microstructure and thermal diffusivity. Fig. 1 shows the variation of the thermal conductivity and thermal diffusivity with the FWHM of the Raman diamond line. As the FWHM increased signalling a loss of order and increasing heterogeneity, the thermal diffusivityyconductivity decreased. This observation was independent of the nucleation or growth zone of the polycrystalline diamond sheets being measured, indicating a strong microstructural dependence of the thermal conductivity. For a-SiCxNy and a-BCxNy samples the thermal diffusivity (a) were measured as a function of their carbon contents (Fig. 2). The values of a remained at approximately 0.35 cm2 ys for a-SiCxNy with carbon contents lower than 30 at.%, but with increasing carbon contents the thermal diffusivity decreased steadily and reached ;0.15 cm2 ys at a carbon content of )60 at.% (Fig. 2). Similarly, the density for these films also decreased from 3.25 gycm3 at a carbon content of ;25 at.% to less than 2.4 gycm3 at approximately 70 at.% of carbon in the films. For a-BCxNy, the thermal diffusivity values showed a peak of approximately 30 at.% of carbon content (Fig. 3), on both sides of which the thermal diffusivity values
Fig. 2. Variation of the (h) thermal diffusivity a and (j) density as a function of the carbon content in the a-SiCxNy films.
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Fig. 3. Variation of the (n) thermal diffusivity a and (m) density as a function of the carbon content in the a-BCxNy films.
showed a decrease. On the lower carbon content side (-30 at.%) the values were approximately 0.45 cm2 ys for a carbon content of ;15 at.%, but on the higher side ()30 at.%), the thermal diffusivity values showed a steady fall from 0.6 to 0.19 cm2 ys at a carbon content of 60 at.%. Again the density of these films, measured by X-ray reflectivity, varied in a similar fashion with carbon content. That is, the density decreased from a peak value of ;2.02 gycm3 at a carbon content of 25 at.% in the films to less than 1.8 gycm3 at a carbon content of more than 60 at.% (Fig. 3). But the absolute density values in a-BCxNy films were considerably less than those for the a-SiCxNy samples. High resolution X-ray photoelectron spectroscopy (XPS) scans of Si(2p), B(1s), C(1s) and N(1s) peaks of the a-SiCxNy and a-BCxNy films were performed to obtain the composition and bonding characteristics of the network. Fig. 4a,b shows the deconvoluted C(1s) band from XPS for a-SiCxNy films with ;30 and 70 at.% of carbon, respectively. Fig. 4a depicts the 284.6and 287.35-eV peaks corresponding to the C(sp2)–C and C(sp3)–N bonds, respectively w13–15x. In Fig. 4b, along with the 284.6-eV peak two additional subpeaks at 286.54 and 288.9 eV corresponding to C(sp)–N and C–O bonds, respectively w13,14x were also detected. For all the films studied the atomic ratio of silicon to nitrogen was maintained at ;0.75. The C(1s), N(1s) and B(1s) bands confirm the B–C (;284 eV) w16x, C–C, B–N and C–N bonds in the aBCxNy films. Fig. 5 compares the XPS C(1s) spectra of two a-BCxNy films having different carbon contents. The C(1s) spectra were deconvoluted to show the contribution from different bonding arrangements for the low and high carbon a-BCxNy films. Due to the close proximity of the B–C, C–C and C–N bands it was difficult to extract quantitative information on each but the shift in the C(1s) position is clear with increasing
carbon content. Stronger contributions from C(sp)–N (286.53 eV) in the higher carbon containing ()30 at.%) a-BCxNy film is clearly evident w14,15,17x. The presence of the C(sp)–N bonds in the higher carbon regime is also confirmed independently by FTIR spectroscopy. Fig. 6a shows the FTIR spectra for aSiCxNy films with increasing carbon content. At a lower carbon content, the FTIR spectrum of a-SiCxNy showed only a small doublet at 2350 cmy1 and a broad band at 950 cmy1 corresponding to C–O and Si–N w18x, respectively. The increase in carbon content in the network is accompanied by the appearance of the 1250–1700 cmy1, and 2200 cmy1 (C(sp)–N peak) bands in the FTIR spectrum w8,19x. Fig. 6b shows FTIR spectra for a-BCxNy films with increasing carbon contents. The prominent feature in the spectra are the out-of-plane B–N–B bending mode at 780 cmy1, the in-plane B–N stretching at ;1370 cmy1 w20x, the C(sp)–N peak (2200 cmy1), C(sp2)–N peak (1690 cmy1) and the C(sp2)–C peak at 1550 cmy1 w19,21,22x. The C(sp)–N mode evolved and the
Fig. 4. The deconvoluted XPS scans of C(1s) spectrum for the aSiCxNy films with (a) 30 at.% and (b) 60 at.% of carbon.
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The deposition routes used for the preparation of aSiCxNy and a-BCxNy films produced various defects such as dangling bonds, voids as well as bonds with lower coordination numbers. The presence of defects in the films could translate into a reduction in the thermal diffusivity. Indeed, strong effect of the bonding configuration of the a-SiCxNy and a-BCxNy networks, as examined by the FTIR and XPS spectra analyses, on the thermal diffusivity was observed. In the binary SiNx film, silicon atoms are bonded tetrahedrally to nitrogen. A small introduction of carbon in the network still keeps the coordination number to a maximum. The C(1s) core level spectrum indicates the predominant C_C bonds (284.6 eV) and C(sp3)–N bonds for the film with low carbon content (Fig. 4a). However, exceeding a carbon concentration of ;30 at.%, the excess carbon atoms introduced tend to bond primarily to nitrogen as C(sp2)–N, and C(sp)–N in the film network as evident from the shift in the C(1s) spectra towards higher binding energies (286.54 eV).
Fig. 5. The deconvoluted XPS scans of C(1s) spectrum for the aBCxNy films with (a) 10 at.% and (b) 60 at.% of carbon.
B–N–B mode diminished as the carbon content increased. 4. Discussion From earlier results w1x and the present experiments on the free standing diamond sheets it was evident that microstructure has a strong effect on the thermal conductivity of this class of covalent materials. Hence as the FWHM of the Raman diamond line increased, which indicated a larger disorder in the carbon–carbon bonding, the thermal conductivity decreased (Fig. 1). This was independent of the growth or nucleation zones studied. The relatively lower thermal diffusivity values for the nucleation zones of the diamond sheets in comparison to the growth zone is due to the smaller crystallite sizes and greater heterogeneity in the network in the nucleation zones w9x. The velocity of surface acoustic waves (SAWs) on these diamond sheets was also found to have a strong dependence on its microstructure w9x. Hence a possible correlation of thermal diffusivity and velocity of SAWs was envisaged.
Fig. 6. The FTIR spectra of the (a) a-SiCx Ny films with 0, 30 and 70 at.% of carbon and (b) a-BCxNy films with 10, 30 and 60 at.% of carbon.
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The FTIR spectra for the series of a-SiCxNy films also reveals the evolution of the C(sp)–N (2200 cmy1) phase w19x, as the carbon content in the film increases (Fig. 6a). This, in addition to other modes at 950 and 1250–1700 cmy1, confirms the different bonding environments in the a-SiCxNy network. XPS analyses of the a-BCxNy films indicates more than two types of atomic environments around the carbon and boron atoms. The C(1s) spectra showed the presence of B–C, C–C and C–N bonds. The B(1s) spectra (not presented) confirmed the presence of B–N, B–C and some B–O bonding. With the increase in carbon in the film within 25 at.%, B–C fractions increased which may explain the slight increase in the thermal diffusivity and density in the films (Fig. 3). Interestingly, the growth of B–C bonding was suppressed with the increase in carbon in the film beyond ;30 at.%. Instead, only a growing influence of C(sp)–N was observed (Fig. 5). This indicates a preferential bonding of carbon with nitrogen in the films at higher carbon concentrations. The broad band between 1150 and 1770 cmy1 of the FTIR spectra for the a-BCxNy films is indicative of the various vibration modes (Fig. 6b). Increasing carbon content in the films facilitates the growth of a defective phase promoted mostly by the C(sp)–N (2200 cmy1) and a simultaneous decrease in the B–N–B (780 cmy1) phase. The increase in the lower coordinated C(sp)–N phase and the preferential carbon to nitrogen bonding with increase in carbon in the films was consistent with the XPS results presented earlier. The common understanding that emerges from the XPS and FTIR studies of a-SiCxNy and a-BCxNy films is the growth of a lower coordinated C(sp)–N at high carbon concentrations that produces a less dense and more defective material. This defective phase holds the key for the thermal diffusivity and density in these material systems. The variation of the density with increasing carbon content (Figs. 2 and 3) in a-SiCxNy and a-BCxNy supports an earlier report of increasing voids with carbon in a-SiC:H w23x. For a-SiCxNy, with carbon content in the range of 25 at.%, the film retained its fourfold coordination and hence the density does not decrease. In the case of a-BCxNy films, B–C bonding was observed in the low carbon (-25 at.%) regime. Beyond 30 at.% of carbon, increased C(sp)–N led to a substantial decrease of density in both a-SiCxNy and a-BCxNy films. Theoretical molecular dynamic studies and experiments agree to the higher value of the density (3.45 gy cm3) and Young’s modulus (265 GPa) for the binary SiNx film where the constituent atoms share maximum coordination w5x. The maximum coordination is nearly maintained up to a carbon concentration of approximately 30 at.%. But when the carbon content is in excess of ;30 at.%, the C(sp)–N bonds start evolving and the values of density in a-SiCxNy start decreasing.
The variation of density for a-BCxNy films with increasing carbon content shows the same tendency (Fig. 3). There is a drop in the density value with carbon concentration exceeding ;25 at.% in the films due to the evolution of the C(sp)–N phase that introduces voidlike defects in the film. Having discussed the bonding configuration and density of the films and their variation with the carbon concentration, the variation of the thermal diffusivity is also explained. The presence of voids and a defective phase in the high carbon regime of a-SiCxNy and aBCxNy films, as confirmed by XPS, FTIR spectroscopy and density measurements, led to a discontinuous atomic network. The decrease in thermal diffusivity becomes obvious since the connectivity of the network was crucial for the transport of vibrational energy. The increasing anharmonicity of the network at high carbon concentrations resulted in a reduced phonon mean free path and a reduced thermal diffusivity. 5. Conclusions Microsturucture dependent thermal diffusivity was found for diamond, amorphous silicon carbon nitride and amorphous boron carbon nitride thin films. For free standing diamond films the thermal diffusivity decreased with increasing Raman 1332 cmy1 line width. In amorphous silicon carbon nitride and amorphous boron carbon nitride thin films with increasing carbon content above ;30 at.%, the thermal diffusivity and density was found to decrease. The evolution of a soft C(sp)–N phase was responsible for the growth of a less dense and discontinuous network that was unable to transport vibrational energy and hence the thermal diffusivity values were reduced in high carbon regimes for both materials. Acknowledgments This work was carried out under a project funded by the National Science Council, Taiwan, under Contract Nos. NSC89-2112-M002-085, and NSC89-2119-M001011. We acknowledge the help of Prof. P. Hess and G. Lehmann, of the Institute of Physical Chemistry, University of Heidelberg, Germany, for the density measurements of the a-SiCxNy films and many helpful discussions. One of the author (S.C) acknowledges the award of a post-doctoral fellowship by the National Science Council, Taiwan. References w1x J.E. Graebner, J.A. Mucha, L. Seibles, G.W. Kammlott, J. Appl. Phys. 71 (1992) 3143. w2x P.G. Kosky, Rev. Sci. Instrum. 64 (1992) 1071. w3x K.M. Leung, A.C. Cheung, B.C. Liu, et al., Diamond Relat. Mater. 8 (1999) 1607.
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