Electrical and Thermal Properties of Nitrogen-Doped Sic Sintered Body

Paper

Electrical and Thermal Properties of Nitrogen-Doped SiC Sintered Body

Yukina TAKI, Mettaya KITIWAN, Hirokazu KATSUI and Takashi GOTO*

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan.

Received December 9, 2017; Revised January 24, 2018; Accepted February 6, 2018

ABSTRACT In this study, the effect of nitrogen (N) doping and microstructural changes on the electrical and thermal

properties of silicon carbide (SiC) were investigated. SiC powder was treated in a N2 atmosphere at 1673, 1973 and 2273 K for 3 h and subsequently sintered by spark plasma sintering (SPS) at 2373 K for 300 s in a vacuum

or in a N2 atmosphere. The a-axis of the N2-treated SiC powders was almost constant, while the c-axis slightly

decreased with an increase in the temperature of N2 treatment. The relative density of the SiC powder sintered

body decreased from 72% to 60% with an increase in the temperature of N2 treatment. The increase in temperature

of N2 treatment caused a decrease in the thermal and electrical conductivities of the SiC. Upon N2 treatment at 3 −1 1673 K and sintering in a N2 atmosphere, SiC exhibited a high electrical conductivity of 1.5 × 10 S m at 1123 K. SiC exhibited n-type conduction, and the highest Seebeck coefficient was −310 μV K−1 at 1073 K. KEY WORDS silicon carbide, nitrogen doping, electrical conductivity, thermal conductivity

1 Introduction doping of N onto a SiC sintered body has not been investigated. Silicon carbide (SiC) has been widely used as a heating element Spark plasma sintering (SPS) would enable the densification of the owing to its high electrical conductivity, high thermal conductivity SiC in a N2 atmosphere without sintering aids. and excellent refractory properties1,2). The electrical and thermal SiC powder manufactured via the Acheson method is n-type conductivities of SiC can be controlled by doping it with various owing to unintentional doping of N from the ambient atmosphere elements2-4). Nitrogen (N) is a common dopant for producing n-type during the production process25,26). Intentional N-doping onto the SiC semiconductors. To produce a highly conductive electronic heater, SiC source powder would be more uniform and effective than 3) high N-doping is required . Many researchers have investigated doping while sintering. The treatment of SiC powder in a N2 the growth of SiC crystals by chemical vapor deposition with the atmosphere before consolidation has not been reported. In this 5-8) addition of N source gas . The carrier concentration (N content) paper, SiC powder was treated in a N2 atmosphere and consolidated 16 19 −3 of 10 –10 cm can be controlled by changing the Si/C ratio, by SPS. Then, the optimized condition of N2 treatment temperature nitrogen flow rate, nitrogen pressure and temperature5-8). to obtained high electrical and thermal properties of the N-doped Nitrogen has been doped in bulk SiC sintered bodies by adding SiC bodies was elucidated. 9-21) nitrides, such as YN, Si3N4, BN, TiN and AlN . High electrical conductivities of ~105 S m−1 at room temperature have been reported13). 2 Experimental procedure The liquid-phase sintering of SiC with rare earth oxides encourages Commercial SiC powder (α-type, 6H, OY-15, Yakushima Denko the incorporation of N into the SiC lattice19-21). However, a SiC Co., Ltd, Tokyo, Japan) with an average particle size of 0.84 μm body containing a liquid phase and/or second phases may degrade was used as the source material. SiC powder was treated in N2 at its mechanical properties at high temperature. Although the sintering 1673, 1973 and 2273 K for 3 h at 0.1 MPa. The SiC powder was of SiC in N2 enhances the electrical conductivity, the densification poured into a graphite die (inner diameter of 10 mm) and sintered using of SiC in a N2 atmosphere without sintering aids is difficult. N SPS equipment (SPS-210LX, Fuji Electronic Industrial, Kawasaki, atoms would dissolve in a SiC lattice and suppress the diffusion of Japan) at 2373 K for 300 s. The heating rate was 100 K min−1 with Si and C22-24). B and AlN are often added as sintering aids for SiC, a pressure of 50 MPa. The sintering was performed in a vacuum 3) while B and Al are p-type dopants . Therefore, the effect of solely or N2 atmosphere. The density of the SiC bodies was measured via the Archimedes method. The relative density was calculated using * Corresponding author, E-mail: [email protected] −3 ** The content of this article had been presented at JSPMIC2017. the theoretical density of SiC (3.21 Mg m ). The crystal phases

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and lattice parameters were examined by X-ray diffraction (CuKα, 0.077 and 0.111 nm, respectively23). The substitution of the N atom Ultima IV, Rigaku Corp., Tokyo, Japan) using a high-purity mostly occurs at the C site in the SiC lattice29). The small size of silicon powder as the internal standard. The microstructure was the N atom substitution causes a decrease in the lattice parameter observed using a scanning electron microscope (SEM; S-3400N, of SiC. The effect of N incorporation on the lattice parameter of Hitachi High-Technologies Corp., Tokyo, Japan). The electrical 3C (cubic)-SiC has been reported23,29). An increase in the N content conductivity (σ) was measured using the DC four probe method in resulted in a decrease in the lattice constant of 3C-SiC29). The vacuum at 298–1123 K. The Seebeck coefficient S( ) was measured annealing of 3C-SiC in a N2 atmosphere at 1873–2073 K caused in a He atmosphere at 298–973 K by changing the temperature a decrease in the lattice parameter, while it was almost constant at gradient (ZEM-3, ULVACRIKO, Kanagawa, Japan). The thermal 2073–2173 K23). This suggests that the N content in SiC may be conductivity (κ) was measured via the laser flash method (TC-7000, close to the solubility limit. The N-doping in 6H (hexagonal)-SiC ULVAC-RIKO, Kanagawa, Japan) in a vacuum at 298–973 K. led to a decrease in the a-axis, while it caused an increase in the c-axis30). The N atom has been assumed to interstitially dissolve in 3 Results and discussion the hexagonal layer of 6H-SiC, thus elongating the c-axis30).

Fig. 1 shows the images of the untreated SiC powder and N2- Fig. 3 shows the effect of an increase in the temperature of treated SiC powder at different temperatures. The untreated SiC N2 treatment of SiC powder on the relative density of the SiC body powder had a yellow-green color. After N2 treatment, the color of sintered at 2373 K for 300 s. The relative densities of the untreated the SiC powder changed from grey to dark green with an increase SiC sintered in a vacuum and N2 atmosphere were 81.6% and 72.6%, 27,28) in temperature, thus suggesting that N-doping in SiC occurred . respectively. This result implies that sintering in a N2 atmosphere Pochaczka et al.22) reported that the N content adsorbed in SiC tends retarded the densification of the SiC body. The relative density of to increase with an increase in N2 partial pressure and temperature. the N2-treated SiC specimens sintered in a vacuum also exhibited

Fig. 2 shows the SEM images of the untreated SiC powder and higher values than those sintered in a N2 atmosphere. However,

N2-treated SiC powder at different temperatures. The untreated relative densities of N2-treated SiC specimens sintered in a vacuum

SiC had an average particle size of 0.84 μm. The N2-treated SiC and in a N2 atmosphere decreased from 71.5% to 59.9% and powder at 1673 K exhibited a slightly larger particle size that of from 69.7% to 59.6%, respectively, with an increase in the the untreated SiC powder. The particle size of the SiC powder increased with an increase in temperature. The average particle sizes of the SiC powders were 0.96 and 2.24 μm at 1973 and 2273 K, respectively.

The crystal phase of the N2-treated powders was identified as

6H-SiC (ICSD #01-072-0018). The a-axis of the N2-treated powder was constant for all powders (a = 0.3090 nm), while the c-axis of the N2-treated SiC powder at 1973 and 2273 K (c = 1.5118– 1.5119) was slightly lower than that of the untreated powder (c = 1.5121 nm). The covalent atomic radii of N, C and Si are 0.075,

Fig. 1 Pictures of (a) the untreated SiC powder, and N2-treated SiC powders Fig. 3 Effect of the temperature of the N2-treated SiC powder on the at (b) 1673 K, (c) 1973 K and (d) 2273 K. relative density of SiC bodies sintered at 2373 K for 300 s.

2018 年 8 月 510 Yukina TAKI, Mettaya KITIWAN, Hirokazu KATSUI and Takashi GOTO

temperature of N2 treatment. The microstructures of the surface of those of specimens sintered in a vacuum. the SiC bodies are shown in Fig. 4. The SiC bodies had a porous Fig. 6 shows the temperature dependence of the electrical microstructure corresponding to the relative density. The decrease conductivities (σ) of untreated SiC and N2-treated SiC sintered in in relative density is mainly due to the increase in SiC particle size a vacuum and in a N2 atmosphere. The untreated SiC exhibited after N2 treatment at high temperature. The dissolved N in the SiC extrinsic conduction below 800 K, and the activation energy of σ inhibits solid-state sintering. The N2-treated SiC that was sintered was 0.08 eV. With increasing temperature in the range 800–1150 K, in a vacuum had a higher density compared with that of sintered in the activation energy increased to 0.54 eV, which was less than half 32) N2, possibly due to the slight desorption of N. the band gap of 6H-SiC (3.02 eV) . For the N2-treated SiC sintered Fig. 5 shows the temperature dependence of the Seebeck in a vacuum, the specimen treated at 1673 K exhibited the highest 3 2 −1 coefficient of untreated SiC, 2N -treated SiC sintered in a vacuum σ in the range 1.4 × 10 to 1.1 × 10 S m . With an increase in 1 and N2-treated SiC sintered in a N2 atmosphere. All SiC bodies temperature of N2 treatment, the σ decreased to ranges of 6.6 × 10 showed negative Seebeck coefficients, thus implying n-type to 1.7 × 10−1 S m−1 and 1.9 × 101 to 2.5 × 10−3 S m−1 for specimens conduction. The Seebeck coefficients of the N-doped SiC bodies treated at 1973 and 2273 K, respectively. The SiC bodies sintered increased with an increase in temperature. The highest values at in a N2 atmosphere yielded a high σ. The specimens treated at 1073 K were −310 μV K−1 for specimens treated at 1973 K and 1673 and 1973 K exhibited similarly high σ values of ~1.5 × 103 sintered in a vacuum and −250 μV K−1 for those treated at 1673 K to ~1.7 × 102 S m−1, while that treated at 2273 K decreased to the 1 −2 −1 and sintered in N2. The Seebeck coefficient generally decreases range of 4.4 × 10 to 7.5 × 10 S m . The decrease in σ for the 31) when the carrier concentration increases . Therefore, N2-treated specimens sintered in a vacuum may be due to desorption of N from 26) SiC specimens sintered in a N2 atmosphere which has higher N SiC. Wang et al. reported that the N concentration in SiC powder content showed the lower Seebeck coefficient values compare to synthesised in an Ar atmosphere decreased with an increase in

specimens sintered in a N2 atmosphere: (e) untreated SiC and N2-treated SiC at (f) 1673 K, (g) 1973 K and (h) 2273 K.

Fig. 5 Temperature dependence of the Seebeck coefficient of untreated SiC and 2N -treated SiC sintered (a) in a vacuum and (b) in a N2 atmosphere.

「粉体および粉末冶金」第 65 巻第 8 号 Electrical and Thermal Properties of Nitrogen-Doped SiC Sintered Body 511

Fig. 6 Temperature dependence of the electrical conductivities of untreated SiC and N2-treated SiC sintered (a) in a vacuum and (b) in a N2 atmosphere.

Fig. 7 Temperature dependence of the thermal conductivities of untreated SiC and N2-treated SiC sintered (a) in a vacuum and (b) in a N2 atmosphere.

temperature. The specimens treated at 1973 and 2273 K sintered N2 atmosphere using untreated SiC and N2-treated SiC. The κ of in a vacuum had a high porosity and a high surface area, which the sintered bodies decreased with an increase in the temperature may lead to the release of N from the SiC body. Although the SiC of N2 treatment. The SiC powder treated at high temperature bodies sintered in a N2 atmosphere have a lower density, they show resulted in particle growth and caused a low density of SiC bodies. the higher σ than those of sintered in a vacuum. The high N content The κ of the SiC bodies decreased with an increase in porosity 34,35) of SiC sintered in a N2 atmosphere has a dominant influence on the which corresponding to those reported in literatures . At 298 K, electrical conductivity. the highest κ of 152 W m−1 K−1 was obtained from the SiC body It is also observed that the σ of the SiC bodies of untreated and sintered in a vacuum using untreated SiC. The thermal conductivities

N2-treated SiC at 1673 and 1973 K exhibited a less temperature of all SiC bodies decreased with an increase in temperature, which dependence. This behaviour is favourable for the application of implies that phonons are responsible for the thermal conduction. SiC to heating elements. The activation energies of σ of these According to the Wiedemann–Franz law, the thermal conductivities SiC bodies were ~0.03 eV below 800 K and ~0.20 eV at 800– by free electrons is less than 0.07% for all specimens. 1150 K. The ionization energies of the N donor in 6H-SiC are The thermal conductivity of a pure 6H-SiC single crystal was 0.07–0.15 eV at N contents of ~1018 cm−3, which slightly decrease found to be 490 W m−1 K−1 at 300 K4). However, in polycrystalline with an increase in the N content33). Karmann et al.7) observed a SiC, the phonons scatter at the grain boundary and reduce the small temperature dependence for the Hall constant and electrical thermal conductivity. The variation in the thermal conductivities conductivity of the 6H-SiC film with a high N content of ~4.0 × of the SiC polycrystal has been reported to be in the range of 19 −3 −1 −1 10 cm and an ionization energy of 0.045 eV. The N2-treated SiC 60–270 W m K depending on the sintering additives, such as B, 19 −3 36) powder prepared in this study may have the N content of 10 cm . Al, Al2O3, BeO and Al2O3–Y2O3 . The 6H-SiC single crystal with Fig. 7 shows the temperature dependence of the thermal high N-doping at ~1019 cm−3 exhibited a κ as low as 60 W m−1 K−1 conductivities (κ) of the SiC bodies sintered in a vacuum and in a at 300 K4).

2018 年 8 月 512 Yukina TAKI, Mettaya KITIWAN, Hirokazu KATSUI and Takashi GOTO

4 Summary 12) K. J. Kim, K. Y. Lim, Y. W. Kim: J. Am. Ceram. Soc., 97 N-doped SiC (6H type) powders were prepared via heat treatment (2014) 2943-2949. at 1673, 1973 and 2273 K and subsequently sintered via SPS at 13) K. J. Kim, K. M. Kim, Y. W. Kim: J. Eur. Ceram. Soc., 34

2373 K in a vacuum or in a N2 atmosphere. A high temperature of (2014) 1149-1154.

N2 treatment caused particle growth and prevented densification of 14) C. H. Pai, K. Koumoto, H. J. Yanagida: J. Ceram. Soc. Jpn., the sintered body. As a result, the electrical and thermal properties 97 (1989) 1170-1175. decreased due to a porous microstructure. With the optimum N2- 15) K. Y. Lim, Y. W. Kim, K. J. Kim, J. H. Yu: J. Ceram. Soc. treatment temperature at 1673 K, the SiC sintered bodies contained Jpn., 119 (2011) 965-967. high N content with a less porosity thus resulted in the high 16) T. Y. Cho, Y. W. Kim, K. J. Kim: J. Eur. Ceram. Soc., 36 (2016) electrical and thermal conductivities. The temperature dependence 2659-2665. of the electrical conductivities of SiC bodies was less for the 17) K. Y. Lim, Y. W. Kim, K. J. Kim: Ceram. Int., 40 (2014) 8885- specimens sintered in a N2 atmosphere. The highest electrical 8890. conductivity of ~2 × 103 S m−1 at 1123 K was obtained for SiC 18) A. Kondo, H. Kitahama: J. Ceram. Soc. Jpn., 107 (1999) 757- bodies sintered in N2 using untreated SiC and N2-treated SiC at 1673 761. and 1973 K. The highest thermal conductivity of 152 W m−1 K−1 was 19) Y. W. Kim, T. Y. Cho, K. J. Kim: J. Eur. Ceram. Soc., 35 (2015) obtained from the SiC body sintered in a vacuum using untreated 4137-4142. SiC. 20) K. J. Kim, K. Y. Lim, Y. W. Kim: J. Eur. Ceram. Soc., 32 (2012) 4401-4406. Acknowledgement 21) Y. W. Kim, K. Y. Lim, K. J. Kim: J. Eur. Ceram. Soc., 32 This study was partially supported by the JSPS KAKENHI, (2012) 4427-4434. Grant Numbers 16K14089 and 16H04211, a cooperative program 22) S. Prochazka, C. A. Johnson, R. A. Giddings: Proceedings of (Proposal No. 17G0413) of the CRDAM-IMR, Tohoku University, the International Symposium of Factors in Densification and Japan and the Creation of Life Innovation Materials for Interdisciplinary Sintering of Oxide and Non-oxide Ceramic, S. Somiya and S. and International Researcher Development, Ministry of Education, Sato ed., Hakone, Japan, 1987, Gakujutsu Bunker Fukyu-kai Culture, Sports, Science and Technology, Japan. (1979) 366-381. 23) W. S. Seo, C. H. Pai, K. Koumoto, H. Yanagida: J. Ceram. References Soc. Jpn., 99 (1991) 443-447. 1) K. Pelissier, T. Chartier, J. M. Laurent: Ceram. Int., 24 (1998) 24) M. S. Datta, A. K. Bandyopadhyay, B. Chaudhuri: Trans. 371-377. Indian Ceram. Soc., 63 (2004) 105-108. 2) S. Somiya, Y. Inomata, ed: Silicon carbide ceramics-1: 25) Y. Takeda, K. Nakamura, K. Maeda, Y. Matushita: J. Ceram. Fundamental and solid reaction, Elsevier Applied Science, Assoc. Jpn., 95 (1987) 860-863. London (1991) 26) H. Wang, C. F. Yan, H. Y. Kong, J. J. Chen, J. Xin, E. W. Shi, J. 3) W. J. Choyke, G. Pensl: MRS Bull., 22 (1997) 25-29. H. Yang: J. Electron. Mater., 42 (2013) 1037-1041. 4) G. A. Slack: J. Appl. Phys., 35 (1964) 3460-3466. 27) W. E. Nelson, F. A. Halden, A. Rosengreen: J. Appl. Phys., 37 5) U. Forsberg, O. Danielsson, A. Henry, M. Linnarsson, E. (1966) 333-336. Janzén: J. Cryst. Growth, 236 (2002) 101-112. 28) L. Patrick, W. J. Choyke: Phys. Rev., 186 (1969) 775-777. 6) T. Kimoto, A. Itoh, H. Matsunami: Appl. Phys. Lett., 67 (1995) 29) G. A. Slack, R. I. Scace: J. Chem. Phys., 42 (1965) 805-806. 2385-2387. 30) M. Stockmeier, R. Müller, S. A. Sakwe, P. J. Wellmann, A. 7) S. Karmann, W. Suttrop, A. Schöner, M. Schadt, C. Magerl: J. Appl. Phys., 105 (2009) 33511-33514. Haberstroh, F. Engelbrecht, R. Helbig, G. Pensl, R. A. Stein, S. 31) G. J. Snyder, E. S. Toberer: Nat. Mater., 7 (2008) 105-114. Leibenzeder: J. Appl. Phys., 72 (1992) 5437-5442. 32) J. B. Casady, R. W. Johnson: Solid-State Electron., 39 (1996) 8) A. Suzuki, A. Uemoto, M. Shigeta, K. Furukawa, S. Nakajima: 1409-1422. Appl. Phys. Lett., 49 (1986) 450-452. 33) G. Pensl, W. J. Choyke: Phys. B Condens. Matter, 185 (1993) 9) K. J. Kim, K. Y. Lim, Y. W. Kim: J. Am. Ceram. Soc., 94 264-283. (2011) 3216-3219. 34) C. F. Cai, J. P. Liu, C. W. Nan, X. M. Min: J. Mater. Sci. Lett., 10) H. Kitagawa, N. Kado, Y. Noda: Mater. Trans., 43 (2002) 16 (1997) 1876-1878. 3239-3241. 35) E. Volz, A. Roosen, W. Hartung, A. Winnacker: J. Eur. Ceram. 11) H. J. Yeom, Y. W. Kim, K. J. Kim: J. Eur. Ceram. Soc., 35 Soc., 21 (2001) 2089-2093. (2015) 77-86. 36) K. Watari: J. Ceram. Soc. Jpn., 109 (2001) S7-S16.

「粉体および粉末冶金」第 65 巻第 8 号