《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_SiC CVD_A study of the effect of excessive free carbon on mean whisker diameters grown by chemical vapor deposition

SUHACE lirilNnlnGr ELSEVIER Surface Coatings Technology 192(2005)247-2 a study of the effect of excessive free carbon on mean whisker diameters grown by chemical vapor deposition D.CLim, Y.J. Lee, D.J. Choi* Department of Ceramic Engineering, Yonsei University. 134 Shinchon-dong, Sudaemmun-ku, Seoul 120-749, South Korea Received 3 November 2003; accepted 30 April 2004 Available online 2 July 2004 Abstract Silicon carbide whiskers were grown by chemical vapor deposition without a metallic catalyst at a temperature ranging between 1000 and 1100C, and at a constant pressure of 5 Torr with input gas ratios, a [H2/MTS (methy trichlorosilane), of 30 to 50. The mean diameter of the whiskers changed as temperature and input gas ratios were varied. To determine why the diameter of the whiskers changed, we investigated the effect of these two parameters, temperature and gas ratio, on the stoichiometry of the deposit, both from thermodynamical calculations and from XPS measurements on the as grown whiskers. We demonstrated that the amount of free carbon increases as the temperature increases and the input gas ratio decreases. We showed the correction between the amount of free carbon and the mean whisker diameter. C 2004 Elsevier B V. All rights reserved. Keywords: Silicon carbide: Whisker: CVD: Excessive carbon 1. Introduction o far, whiskers have been grown using a metallic catal act as an impurity. This Silicon carbide has a low density and thermal expansion degradation in the mechanical and electrical properties of coefficient, a high melting point, strength and hardness, due the whiskers [7-9. In our study, we synthesized silicon to its covalent bonding characteristics. Therefore, its me- carbide whiskers by chemical vapor deposition without chanical and physical properties are excellent and now it is one of the most important structural ceramic materials [1] Together with these properties, it also has a wide band gap 100.0 and high electron mobility, so it is used as a semiconductor SiC material, and it can be used at high temperature, high power, high frequency and in severe environments [2]. Further- more, the electron field emission properties of silicon bide nanorods have also been demonstrated in recent studies [3-5] Since the discovery of carbon nanotubes, interest in the 5 fabrication and characterization of one-dimensional crys- talline materials has increased because of their superior characteristics for structural. electronic and functional plications. Silicon carbide whiskers possess not only C(Graphite) well known physical and electronic properties, but also special properties which can be obtained from geometric figures 6 T 4 Corresponding author. Fax: +82-2-365-5882. Fig. 1. Thermodynamic yield of SiC in MTS-H2 system as a function of E-mail address: drchoidj@ yonsei ac kr(DJ. Choi). 0257-8972/S-see front matter e 2004 Elsevier B.V. All rights reserved doi:10.1016.surcoat2004.04.081

A study of the effect of excessive free carbon on mean whisker diameters grown by chemical vapor deposition D.C. Lim, Y.J. Lee, D.J. Choi* Department of Ceramic Engineering, Yonsei University, 134 Shinchon-dong, Sudaemun-ku, Seoul 120-749, South Korea Received 3 November 2003; accepted 30 April 2004 Available online 2 July 2004 Abstract Silicon carbide whiskers were grown by chemical vapor deposition without a metallic catalyst at a temperature ranging between 1000 and 1100 jC, and at a constant pressure of 5 Torr with input gas ratios, a [H2/MTS (methyltrichlorosilane)], of 30 to 50. The mean diameter of the whiskers changed as temperature and input gas ratios were varied. To determine why the diameter of the whiskers changed, we investigated the effect of these two parameters, temperature and gas ratio, on the stoichiometry of the deposit, both from thermodynamical calculations and from XPS measurements on the as grown whiskers. We demonstrated that the amount of free carbon increases as the temperature increases and the input gas ratio decreases. We showed the correction between the amount of free carbon and the mean whisker diameter. D 2004 Elsevier B.V. All rights reserved. Keywords: Silicon carbide; Whisker; CVD; Excessive carbon 1. Introduction Silicon carbide has a low density and thermal expansion coefficient, a high melting point, strength and hardness, due to its covalent bonding characteristics. Therefore, its me￾chanical and physical properties are excellent and now it is one of the most important structural ceramic materials [1]. Together with these properties, it also has a wide band gap and high electron mobility, so it is used as a semiconductor material, and it can be used at high temperature, high power, high frequency and in severe environments [2]. Further￾more, the electron field emission properties of silicon carbide nanorods have also been demonstrated in recent studies [3– 5]. Since the discovery of carbon nanotubes, interest in the fabrication and characterization of one-dimensional crys￾talline materials has increased because of their superior characteristics for structural, electronic and functional applications. Silicon carbide whiskers possess not only well known physical and electronic properties, but also special properties which can be obtained from geometric figures [6]. So far, whiskers have been grown using a metallic catalyst which can act as an impurity. This can cause degradation in the mechanical and electrical properties of the whiskers [7– 9]. In our study, we synthesized silicon carbide whiskers by chemical vapor deposition without a 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.081 * Corresponding author. Fax: +82-2-365-5882. E-mail address: drchoidj@yonsei.ac.kr (D.J. Choi). www.elsevier.com/locate/surfcoat Fig. 1. Thermodynamic yield of SiC in MTS-H2 system as a function of deposition temperature. Surface & Coatings Technology 192 (2005) 247 – 251

D. C Lim et al. Surface Coatings Technology 192(2005)247-251 Heating was performed under a H2 atmosphere. The nput gas ratios, o(H2/MTS), ranged between 30 and 50, and deposition temperature varied from 1000 to 1150C 0.006 1200°c We investigated the effect of these two parameters on the toichiometry of the deposit, both from thermodynamical calculations and from XPS measurements on the as grown whiskers. To examine the microstructure and surface mor- E phology of deposits, we used scanning electron microscopy (SEM, Hitachi S-2700/FESEM, Hitachi S-4200). 900°c 3. Results and discussion 0.002 We calculated the thermodynamic equilibrium composi tion by using SOLGASMIX-PV. Results are shown in Figs I and 2 ig. 1 shows the thermodynamic yield of Sic in the Fig.2. Equilibrium mole fraction of carbon as a function of input gas ratio z. MTS-H2 system at a stationary pressure of 5 Torr, an input atures. Below 1050C, the thermodynamic yields mper- gas ratio(a) of 30 and with different deposition temper catalyst, and we investigated theoretical and experimental and carbon changed little. However, above 1050C, the factors which can affect mean whisker diameter thermodynamic yield of Sic decreased, while that of carbon as the temperatu ing probability of excess free carbon in Sic deposits at high 2. Experimental deposition temperature can be higher than 1050C. Fig. 2 shows the equilibrium mole fraction of carbon as a Details of the deposition system were described in a function of input gas ratio, a, at a constant pressure of 5 previous work [10]. We used MTS (methyltrichlorosilane; Torr. As the input gas ratio increased from 30 to 50, the CH3 SiCl3, Acros Organics, USA)as the precursor and high equilibrium mole fraction of carbon decreased. From this, purity H2 as the carrier and diluent gas. Isotropic graphite, we can surmise that the existing probability of excess free which was carbonized at 1000C for 1 h, was used as a carbon in SiC deposits can be lower when the input gas ratio e ek.x1ee 3 (c)×"6um Fig 3. SEM images of whiskers grown at different deposition temperatures(a 30 ).(a)1000C. (b)1050C,(c)1100C

catalyst, and we investigated theoretical and experimental factors which can affect mean whisker diameter. 2. Experimental Details of the deposition system were described in a previous work [10]. We used MTS (methyltrichlorosilane; CH3SiCl3, Acros Organics, USA) as the precursor and high purity H2 as the carrier and diluent gas. Isotropic graphite, which was carbonized at 1000 jC for 1 h, was used as a substrate. Heating was performed under a H2 atmosphere. The input gas ratios, a (H2/MTS), ranged between 30 and 50, and deposition temperature varied from 1000 to 1150 jC. We investigated the effect of these two parameters on the stoichiometry of the deposit, both from thermodynamical calculations and from XPS measurements on the as grown whiskers. To examine the microstructure and surface mor￾phology of deposits, we used scanning electron microscopy (SEM, Hitachi S-2700/FESEM, Hitachi S-4200). 3. Results and discussion We calculated the thermodynamic equilibrium composi￾tion by using SOLGASMIX-PV. Results are shown in Figs. 1 and 2. Fig. 1 shows the thermodynamic yield of SiC in the MTS-H2 system at a stationary pressure of 5 Torr, an input gas ratio (a) of 30 and with different deposition temper￾atures. Below 1050 jC, the thermodynamic yields of SiC and carbon changed little. However, above 1050 jC, the thermodynamic yield of SiC decreased, while that of carbon increased as the temperature increased. Therefore, the exist￾ing probability of excess free carbon in SiC deposits at high deposition temperature can be higher than 1050 jC. Fig. 2 shows the equilibrium mole fraction of carbon as a function of input gas ratio, a, at a constant pressure of 5 Torr. As the input gas ratio increased from 30 to 50, the equilibrium mole fraction of carbon decreased. From this, we can surmise that the existing probability of excess free carbon in SiC deposits can be lower when the input gas ratio is higher. Fig. 2. Equilibrium mole fraction of carbon as a function of input gas ratio a. Fig. 3. SEM images of whiskers grown at different deposition temperatures (a 30 ). (a) 1000 jC, (b) 1050 jC, (c) 1100 jC. 248 D.C. Lim et al. / Surface & Coatings Technology 192 (2005) 247–251

D. C Lim et al. Surface Coatings Technology 192(2005)247-25 (b) Binding Energy(e Binding Energy(ev) 106104102 Binding Energy (ev) Fig 4. XPS deconvolution of narrow scan spectra of SiC whiskers. (a)CIs, (b) Si2p at 1000C, a=30, (c)Cls, (d) Si2p at 1100C, a=30 忘e为 3m(b) 2um Ni: 2um Fig. 5. SEM images of the whiskers grown at different input gas ratios at 1000C.(a)a=30, (b)a=40, (c)a=50

Fig. 4. XPS deconvolution of narrow scan spectra of SiC whiskers. (a) C1s, (b) Si2p at 1000 jC, a = 30, (c) C1s, (d) Si2p at 1100 jC, a = 30. Fig. 5. SEM images of the whiskers grown at different input gas ratios at 1000 jC. (a) a = 30, (b) a = 40, (c) a = 50. D.C. Lim et al. / Surface & Coatings Technology 192 (2005) 247–251 249

D. C Lim et al. Surface Coatings Technology 192(2005)247-251 Fig 3 shows the SEM images of the whiskers grown at the normalized areas that were calculated from the peak area different deposition temperatures and a constant input gas per atomic sensitivity factor were compared [14], the ratio of ratio of a, 30. The mean whisker diameter of whiskers relative composition of C/Si was 1.04 at 1000C and 1.50 at grown at 1000 and 1100C were 140 and 810 nm, 1100C. This shows that the XPS results were identical with respectively. When the deposition temperature increased, the theoretical results, as mentioned previously. For the the mean whisker diameter increased [11]. The excess free reasons mentioned above, the excess free carbon verified carbon existing in SiC deposits can produce higher surface through thermodynamic computation is most likely related to energy sites, and the deposition process can be activated by the mean diameter of the whiskers these sites to lower the surface energy. For this reason, it is Fig. 5 shows SEM images of whiskers grown at different considered that the mean diameter of whiskers which have input gas ratios, 30, 40 and 50, and with a stationary more excess carbon over 1050C can be thicker than those temperature of 1000C. The mean diameter of whiskers which have less excess carbon below 1050C grown at 30 and 50 was 140 and 80 nm, respectively. As we We examined the composition of whiskers by using XPS indicated earlier, the mean diameter of whiskers grown at 50 and the results are shown in Fig 4. Deconvolution of the si is thinner than that at 30. due to the low content of excess and CIs peaks, according to Gaussian-Lorentzian distribu- free carbon. Complete results of mean whisker diameter tion, showed Si-C and C-Si peaks that have binding were plotted in Fig. 6. As we expected, the mean whisker energies of 100.6 and 282.8, respectively [12, 13]. When diameter increased as the temperature increased and de- creased as input gas ratio increased. We suppose that this is because of the existence of excess free carbon in whiskers As shown in Fig. 6, the size gap of mean whisker diameters between 1000 and 1100C is larger than that of mean whisker diameters at the input gas ratios of between 30 and 50. Therefore, the gap of the relative composition ratio ofC Si in the range of the deposition temperature can be larger of the b400 4. Conclusions We synthesized silicon carbide whiskers without a metal lic catalyst, and examined mean whisker diameter changes according to deposition temperatures and input gas ratios Also we investigated their excess free carbon contents. theoretically and experimentally. As we examined theoreti 1000 cally, the excessive free carbon contents of whiskers can be Temperature(°c) increased as the deposition temperatures increase and input gas ratios decrease. The XPS measurements confirm these trends. The ratio of the relative composition of C/Si incr from 1.04 to 1.50 as the temperature increased from 1000 to 1100C. The mean diameter of whiskers changed from 140 一■-1100c to 810 nm as the temperature increased from 1000 to 1100C, and from 140 to 80 nm as the input gas ratio increased from 30 to 50. The excess free carbon can be related to the mean whisker diameter. when the excess free carbon content was high, the mean whisker diameter was thick, and when it was low. the mean whisker diameter was thin 100 This research, under the contract project code MS-03- 211-01, has been supported by the Intelligent Microsystem Center(imc:http://www.microsystem.re.kr),whichcarries out one of the 21st century's Frontier R&D Projects Fig. 6. Temperature and input gas ratio dependencies of mean whisker sponsored by the Korea Ministry of Science and econo

Fig. 3 shows the SEM images of the whiskers grown at different deposition temperatures and a constant input gas ratio of a, 30. The mean whisker diameter of whiskers grown at 1000 and 1100 jC were 140 and 810 nm, respectively. When the deposition temperature increased, the mean whisker diameter increased [11]. The excess free carbon existing in SiC deposits can produce higher surface energy sites, and the deposition process can be activated by these sites to lower the surface energy. For this reason, it is considered that the mean diameter of whiskers which have more excess carbon over 1050 jC can be thicker than those which have less excess carbon below 1050 jC. We examined the composition of whiskers by using XPS and the results are shown in Fig. 4. Deconvolution of the Si2p and C1s peaks, according to Gaussian –Lorentzian distribu￾tion, showed Si –C and C –Si peaks that have binding energies of 100.6 and 282.8, respectively [12,13]. When the normalized areas that were calculated from the peak area per atomic sensitivity factor were compared [14], the ratio of relative composition of C/Si was 1.04 at 1000 jC and 1.50 at 1100 jC. This shows that the XPS results were identical with the theoretical results, as mentioned previously. For the reasons mentioned above, the excess free carbon verified through thermodynamic computation is most likely related to the mean diameter of the whiskers. Fig. 5 shows SEM images of whiskers grown at different input gas ratios, 30, 40 and 50, and with a stationary temperature of 1000 jC. The mean diameter of whiskers grown at 30 and 50 was 140 and 80 nm, respectively. As we indicated earlier, the mean diameter of whiskers grown at 50 is thinner than that at 30, due to the low content of excess free carbon. Complete results of mean whisker diameter were plotted in Fig. 6. As we expected, the mean whisker diameter increased as the temperature increased and de￾creased as input gas ratio increased. We suppose that this is because of the existence of excess free carbon in whiskers. As shown in Fig. 6, the size gap of mean whisker diameters between 1000 and 1100 jC is larger than that of mean whisker diameters at the input gas ratios of between 30 and 50. Therefore, the gap of the relative composition ratio of C/ Si in the range of the deposition temperature can be larger than that in the range of the input gas ratio. 4. Conclusions We synthesized silicon carbide whiskers without a metal￾lic catalyst, and examined mean whisker diameter changes according to deposition temperatures and input gas ratios. Also we investigated their excess free carbon contents, theoretically and experimentally. As we examined theoreti￾cally, the excessive free carbon contents of whiskers can be increased as the deposition temperatures increase and input gas ratios decrease. The XPS measurements confirm these trends. The ratio of the relative composition of C/Si increased from 1.04 to 1.50 as the temperature increased from 1000 to 1100 jC. The mean diameter of whiskers changed from 140 to 810 nm as the temperature increased from 1000 to 1100 jC, and from 140 to 80 nm as the input gas ratio increased from 30 to 50. The excess free carbon can be related to the mean whisker diameter. when the excess free carbon content was high, the mean whisker diameter was thick, and when it was low, the mean whisker diameter was thin. Acknowledgements This research, under the contract project code MS-03- 211-01, has been supported by the Intelligent Microsystem Center (IMC: http://www.microsystem.re.kr), which carries out one of the 21st century’s Frontier R&D Projects sponsored by the Korea Ministry of Science and Technology. Fig. 6. Temperature and input gas ratio dependencies of mean whisker diameter. 250 D.C. Lim et al. / Surface & Coatings Technology 192 (2005) 247–251

D. C Lim et al. Surface Coatings Technology 192(2005)247-25 References [ D. Chattopadhyay, I. Galeska, F. Papadimitrakopoulos, Carbon 40 [1 J.G. Lee, I.B. Cutler, Am. Ceram Soc. Bull. 54(1975)195 [9] H.S. Youn, H. Ryu, T-H. Cho, W.-K. Choi, Int. J. Hydrogen Energy 2]A Fissel, B Schroter, W. Richter, Appl. Phys. Lett. 66(23)(1995) 27(2002)9 3182. 10] D. C. Lim, D.J. Choi, J Ceram. Proc. Res. 3(3)(2002)205. 3 X.T. Zhou, N. Wang, F.C. K. Au, H L. Lai, H.Y. Peng, I. Bello, C S [11 D.C. Lim, H.S. Ahn, D.J. Choi, C.H. Wang, H. Tomokage, Surf. Coat. Lee, S.T. Lee, Mater. Sci. Eng. A286(2000)119 Tech.168(2003)37 [4]XT. Zhou, H.Y. Peng, E.C.K. Au, L.S. Liao, N. Wang, I. Bello, C S [12] J M. Grow, R.A. Levy, M. Bhaskaran, H.J. Boeglin, R. Shalvoy Lee, S.T. Lee, Chem. Phys. Lett. 318(2000)58 J. Electrochem. Soc. 140(10)(1993)30 5]XT Zhou, N. Wang, H L Lai, H.Y. Peng, I. Bello, N.B. Wong, C s [13] C.C. Liu, C. Lee, K L Cheng, H C Cheng, T.R. Yew, J Electrochem Lee, S.T. Lee, Appl. Phys. Lett. 74(26)(1999)3942. Soc.142(12)(1995)4279 6 Z Pan, H.-L. Lai, F.C. K. Au, X Duan, w. Zhou, w. Shi, N. Wang [14] D. Briggs, M.P. Seah, Practical Surface Analysis, 1, wiley, New York C.-S. Lee N.-B. Wong, S.-T. Lee, S. Xie, Adv Mater. 12(16)(2000) 7 E. Dujardin, C. Meny, P. Panissod, J.-P. Dintzinger, N. Yao, T w Ebbesen, Solid State Commun. 114(2000)543

References [1] J.G. Lee, I.B. Cutler, Am. Ceram. Soc. Bull. 54 (1975) 195. [2] A. Fissel, B. Schro¨ter, W. Richter, Appl. Phys. Lett. 66 (23) (1995) 3182. [3] X.T. Zhou, N. Wang, F.C.K. Au, H.L. Lai, H.Y. Peng, I. Bello, C.S. Lee, S.T. Lee, Mater. Sci. Eng. A286 (2000) 119. [4] X.T. Zhou, H.Y. Peng, F.C.K. Au, L.S. Liao, N. Wang, I. Bello, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 318 (2000) 58. [5] X.T. Zhou, N. Wang, H.L. Lai, H.Y. Peng, I. Bello, N.B. Wong, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 74 (26) (1999) 3942. [6] Z. Pan, H.-L. Lai, F.C.K. Au, X. Duan, W. Zhou, W. Shi, N. Wang, C.-S. Lee, N.-B. Wong, S.-T. Lee, S. Xie, Adv. Mater. 12 (16) (2000) 1186. [7] E. Dujardin, C. Meny, P. Panissod, J.-P. Dintzinger, N. Yao, T.W. Ebbesen, Solid State Commun. 114 (2000) 543. [8] D. Chattopadhyay, I. Galeska, F. Papadimitrakopoulos, Carbon 40 (2002) 985. [9] H.S. Youn, H. Ryu, T.-H. Cho, W.-K. Choi, Int. J. Hydrogen Energy 27 (2002) 937. [10] D.C. Lim, D.J. Choi, J. Ceram. Proc. Res. 3 (3) (2002) 205. [11] D.C. Lim, H.S. Ahn, D.J. Choi, C.H. Wang, H. Tomokage, Surf. Coat. Tech. 168 (2003) 37. [12] J.M. Grow, R.A. Levy, M. Bhaskaran, H.J. Boeglin, R. Shalvoy, J. Electrochem. Soc. 140 (10) (1993) 3001. [13] C.C. Liu, C. Lee, K.L. Cheng, H.C. Cheng, T.R. Yew, J. Electrochem. Soc. 142 (12) (1995) 4279. [14] D. Briggs, M.P. Seah, Practical Surface Analysis, 1, Wiley, New York, 1990. D.C. Lim et al. / Surface & Coatings Technology 192 (2005) 247–251 251