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Effect of annealing on SiC thin films prepared by pulsed laser deposition
Diamond and Related Materials 8 (1999) 2099–2102 www.elsevier.com/locate/diamond
Effect of annealing on SiC thin films prepared by pulsed laser deposition Jipo Huang a, *, Lianwei Wang a, Jun Wen b, Yuxia Wang b, Chenglu Lin a, ¨ stling c Mikael O a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China b Department of Material Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China c Royal Institute of Technology, Department of Electronics, Electrum 299, SE-16440 Kista-Stockholm, Sweden Accepted 6 May 1999
Abstract Crystalline cubic SiC thin films were successfully fabricated on Si(100) substrates by using laser deposition combined with a vacuum annealing process. The effect of annealing conditions on the structure of the thin films was investigated by X-ray diffraction and Fourier transform infrared spectroscopy. It was demonstrated that amorphous SiC films deposited at 800°C could be transformed into crystalline phase after being annealed in a vacuum and that the annealing temperature played an important role in this transformation, with an optimum annealing temperature of 980°C. Results of X-ray photoelectron spectroscopy revealed the approximate stoichiometry of the SiC films. The characteristic microstructure displayed in a scanning electron microscope image of the films was indicative of epitaxial growth along the (100) plane. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Annealing; Pulsed laser deposition; SiC thin films
1. Introduction Silicon carbide’s excellent physical and electrical properties, such as wide bandgap, high thermal conductivity, high breakdown electric field, high saturated electron drift velocity and resistance to chemical attack, mean that it is a promising material for high-temperature, high-power and high-frequency electronic devices [1,2], as well as for optoelectronic devices [3,4]. Cubic SiC (3C-SiC ) is attractive owing to high electron mobility and high saturation velocity. As a result, there is increasing interest in the growth of high-quality 3C-SiC heteroepitaxial films on silicon. Since Matsunami and co-workers pioneered the growth of single crystalline 3C-SiC films on Si(100) by chemical vapour deposition (CVD) [5], CVD has become the most frequently used method for the growth of SiC thin films [6–9]. In a typical CVD process, reactants consisting of a mixture of SiH , C H and H flow over a silicon substrate 4 3 8 2 * Corresponding author. Tel.: +86-21-625-11070; fax: +86-625-13510. E-mail address: [email protected] (J. Huang)
heated to 1300–1600°C, and then epitaxial growth of SiC films gradually progresses. However, this method suffers from very high substrate temperatures which result in a high hydrogen content and crystalline lattice defects in the SiC films. Although epitaxial 3C-SiC films on silicon have been reported at 750°C [10], the development of other lower-temperature methods is desirable. Recently it has been demonstrated that crystalline SiC films can be grown on silicon substrates by pulsed laser deposition (PLD), a novel and powerful thin-film growth technique at relatively low temperature. Rimai and co-workers [11,12], Balloch et al. [13] and Capano et al. [14] have reported the preparation of 3C-SiC films on silicon; however, the former employed a higher temperature of 1000°C or above, while the crystallinity of the SiC films reported by the latter two groups was poor. In this study, we report on oriented SiC films on Si(100) substrates grown by pulsed laser deposition at 800°C combined with a vacuum annealing process. The purpose is to investigate the effect of annealing on as-deposited SiC films grown by PLD, as well as the possibility of fabricating well-crystallised SiC thin films at relatively low temperature by this method.
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J. Huang et al. / Diamond and Related Materials 8 (1999) 2099–2102
2. Experimental The SiC thin films were deposited inside a stainless steel vacuum chamber system evacuated by a turbomolecular pump to a base pressure of 2×10−7 torr; the working pressure was maintained at 5–6×10−6 torr. Radiation from an ArF excimer laser (l=193 nm, 17 ns pulse duration) was focused to a spot size of 0.1 cm×0.4 cm on a rotating sintered SiC target at an angle of 45°. The repetition of the pulse was 3 Hz and a pulse energy of 150–200 mJ yielded an energy density of 2–4 J cm−2 on the target. The plume of atoms and ions emerged in a cone normal to the target and impinged to the substrate with an area of 2 cm×3 cm. p-Type Si(100) wafer (20–35 V cm, 350 mm thick) substrates cleaned with a standard IC procedure were located about 3–5 cm away from and parallel to the surface of the target. The substrate was resistively heated up to 800°C, which was monitored by a thermocouple. The typical average growth rate of SiC films with the energy density of 2–4 J cm−2 was about ~0.05 nm per pulse. After deposition, the samples were taken out and cut into several slices, and then annealed in a vacuum furnace with at a pressure of 1×10−5 torr and temperature in the range from 900 to 1050°C for 30 min. The crystal structure of the SiC films was characterised by X-ray diffraction ( XRD) using Cu Ka radiation. Fourier transform infrared spectroscopy ( FTIR), X-ray photoelectron spectroscopy ( XPS) and scanning electron microscopy (SEM ) were used to investigate composition and surface morphology.
3. Results and discussion XRD analysis of the SiC films deposited on Si(100) substrates at 800°C for 20 min could not detect any characteristic SiC diffraction peaks, indicating that the films were amorphous. The lack of crystallinity in the SiC films appeared to be related to the low growth temperature. Fig. 1 shows the XRD patterns of SiC films annealed in a vacuum at different temperatures for 30 min. Only one peak around 41.4° was observed for the whole range of annealing temperature but its intensity varied as a function of the annealing temperature, suggesting that there was an optimum annealing temperature in order to obtain well-crystallised films. This optimum temperature was 980°C in our experiment. The low intensity of peaks for 920°C could be explained by the initial formation of crystalline phase. For the case of annealing at 1020°C, the low peak intensity might be caused by a decrease of the crystalline phase as a consequence of volatilisation into the ambient vacuum. Owing to the higher formation temperature of 6H-SiC than for 3C-SiC, the peak at 41.4° seemed to correspond to 3C-SiC(200) rather than 6H-SiC(104).
Fig. 1. XRD patterns of SiC films deposited at 800°C and annealed in a vacuum at different temperatures for 30 min.
To investigate the effect of annealing temperature further, FTIR transmittance spectra of the SiC films annealed in a vacuum at various temperatures were measured and are shown in Fig. 2. In addition to the adsorption peaks centred at 609 and 1070 cm−1, which correspond to the Si(100) substrate and SiO , respec2 tively, an adsorption peak around 800 cm−1 corresponding to SiMC bonds was also observed. The intensities varied with the annealing temperature and reached a maximum value at 980°C, thus revealing an optimum annealing effect at 980°C. Clearly, this result was consistent with the result of XRD. Figs. 3a and b present the XPS spectra of Si 2p and C 1s regions of the SiC film annealed at 980°C. As is evident from Fig. 3, the Si 2p and C 1s spectra exhibit one single component at 100.2 eV and 283.3 eV, respectively, with no components at 99.2 eV (corresponding to pure silicon) and 282.6 eV (corresponding to graphite) being identified. This result also indicated the formation of crystalline SiC, and the measured binding energies of the Si 2p and C 1s states agree well with previous reports
Fig. 2. FTIR transmittance spectra of SiC films deposited at 800° and annealed in vacuum at different temperatures for 30 min.
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sample annealed in a vacuum at 980°C for 30 min. Rectangular morphology and an oriented arrangement could be observed, similar to that observed for the carbonisation layer initially formed during chemical vapour deposition of SiC films on silicon, and is indicative of the cubic nature of the (100) growth plane and the presence of crystalline films [12,17]. The SEM pictures of samples annealed at 920, 950 and 1020°C also presented similar square patterns, but poorer morphology. Observations indicated that these defects were pits which formed via the diffusion of voids on the silicon substrate during annealing; the voids were most likely bound on the four sides of the (111) plane before the final pyramidal shapes came into being.
4. Conclusion Pulsed excimer laser deposition combined with a vacuum annealing process has been developed to prepare stoichiometric, crystalline SiC thin films on Si(100) substrates. The amorphous SiC films deposited at 800°C could be transformed into crystalline phase after vacuum annealing at temperatures of 920–1020°C for 30 min. The sample annealed at 980°C displayed the optimum crystallinity and epitaxial growth along the (100) plane.
Fig. 3. XPS spectra of the SiC film deposited at 800°C and annealed in vacuum at 980°C for 30 min: (a) Si 2p; (b) C 1s.
[15,16 ]. The atomic ratio of C/Si using the ratio of peak areas and the empirical sensitivity factors [16 ] (0.87 for silicon and 1.0 for carbon) was 1.16, indicating that the films were almost stoichiometric. Fig. 4 shows a scanning electron micrograph of the
Fig. 4. SEM micrograph of the SiC film deposited at 800°C and annealed in vacuum at 980°C for 30 min.
Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant No. 69776003 and the Shanghai Youth Foundation under Grant No. 97QD14034.
References [1] G. Mu¨ller, G. Kro¨tz, E. Niemann, Sensors and Actuators A 43 (1994) 259. [2] D.M. Brown, E. Downey, M. Grezzo, J. Kretchmer, V. Krishnamrthy, W. Hennessy, G. Michon, Solid State Electronics 59 (1996) 1531. [3] J.W. Palmour, J.A. Edmond, H.S. Kong, C.H. Carter Jr., Physica B 185 (1993) 461. [4] S. Sheng, M.G. Spencer, X. Tang, P. Zhou, K. Wongchoitgul, C. Taylor, G.L. Harris, Mater. Sci. Eng. B 46 (1997) 147. [5] H. Matsunami, S. Nishino, H. Ono, IEEE. Trans. Electron Devices 28 (1981) 1235. [6 ] S. Nishino, H. Suhara, H. Ono, H. Matsunami, J. Appl. Phys. 61 (1987) 4889. [7] Y. Gao, J.H. Edgar, J. Chaudhuri, S.N. Cheema, M.V. Sidorov, D.N. Braski, J. Crystal Gorwth 191 (1998) 439. [8] W. Wesch, Nucl. Instrum. Methods Phys. Res. B 116 (1996) 305. [9] J.A. Cooper Jr., Mater. Sci. Eng. B 44 (1997) 387. [10] I. Golecki, F. Reidinger, J. Marti, Appl. Phys. Lett. 60 (1992) 1703. [11] L. Rimai, R. Ager, W.H. Weber, J. Hangas, Appl. Phys. Lett. 59 (1991) 2266.
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[12] L. Rimai, R. Ager, W.H. Weber, J. Hangas, A. Samman, W. Zhu, J. Appl. Phys. 77 (1995) 6601. [13] M. Balloch, R.J. Tench, W.J. Sielhause, M.J. Allen, A.L. Conor, D.R. Olander, Appl. Phys. Lett. 57 (1990) 1540. [14] M.A. Capano, S.D. Walck, P.T. Murray, Appl. Phys. Lett. 64 (1994) 3413.
[15] M.Y. Chen, P.T. Murray, J. Mater. Sci. 35 (1990) 4929. [16 ] A. Watanabe, M. Mukaida, T. Tsunoda, Y. Imai, Thin Solid Films 300 (1997) 95. [17] H.J. Kim, R.F. Davis, J. Electrochem. Soc. 134 (1987) 2269.
Effect of annealing on SiC thin films prepared by pulsed laser deposition Jipo Huang a, *, Lianwei Wang a, Jun Wen b, Yuxia Wang b, Chenglu Lin a, ¨ stling c Mikael O a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China b Department of Material Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China c Royal Institute of Technology, Department of Electronics, Electrum 299, SE-16440 Kista-Stockholm, Sweden Accepted 6 May 1999
Abstract Crystalline cubic SiC thin films were successfully fabricated on Si(100) substrates by using laser deposition combined with a vacuum annealing process. The effect of annealing conditions on the structure of the thin films was investigated by X-ray diffraction and Fourier transform infrared spectroscopy. It was demonstrated that amorphous SiC films deposited at 800°C could be transformed into crystalline phase after being annealed in a vacuum and that the annealing temperature played an important role in this transformation, with an optimum annealing temperature of 980°C. Results of X-ray photoelectron spectroscopy revealed the approximate stoichiometry of the SiC films. The characteristic microstructure displayed in a scanning electron microscope image of the films was indicative of epitaxial growth along the (100) plane. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Annealing; Pulsed laser deposition; SiC thin films
1. Introduction Silicon carbide’s excellent physical and electrical properties, such as wide bandgap, high thermal conductivity, high breakdown electric field, high saturated electron drift velocity and resistance to chemical attack, mean that it is a promising material for high-temperature, high-power and high-frequency electronic devices [1,2], as well as for optoelectronic devices [3,4]. Cubic SiC (3C-SiC ) is attractive owing to high electron mobility and high saturation velocity. As a result, there is increasing interest in the growth of high-quality 3C-SiC heteroepitaxial films on silicon. Since Matsunami and co-workers pioneered the growth of single crystalline 3C-SiC films on Si(100) by chemical vapour deposition (CVD) [5], CVD has become the most frequently used method for the growth of SiC thin films [6–9]. In a typical CVD process, reactants consisting of a mixture of SiH , C H and H flow over a silicon substrate 4 3 8 2 * Corresponding author. Tel.: +86-21-625-11070; fax: +86-625-13510. E-mail address: [email protected] (J. Huang)
heated to 1300–1600°C, and then epitaxial growth of SiC films gradually progresses. However, this method suffers from very high substrate temperatures which result in a high hydrogen content and crystalline lattice defects in the SiC films. Although epitaxial 3C-SiC films on silicon have been reported at 750°C [10], the development of other lower-temperature methods is desirable. Recently it has been demonstrated that crystalline SiC films can be grown on silicon substrates by pulsed laser deposition (PLD), a novel and powerful thin-film growth technique at relatively low temperature. Rimai and co-workers [11,12], Balloch et al. [13] and Capano et al. [14] have reported the preparation of 3C-SiC films on silicon; however, the former employed a higher temperature of 1000°C or above, while the crystallinity of the SiC films reported by the latter two groups was poor. In this study, we report on oriented SiC films on Si(100) substrates grown by pulsed laser deposition at 800°C combined with a vacuum annealing process. The purpose is to investigate the effect of annealing on as-deposited SiC films grown by PLD, as well as the possibility of fabricating well-crystallised SiC thin films at relatively low temperature by this method.
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 1 6 3- 6
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J. Huang et al. / Diamond and Related Materials 8 (1999) 2099–2102
2. Experimental The SiC thin films were deposited inside a stainless steel vacuum chamber system evacuated by a turbomolecular pump to a base pressure of 2×10−7 torr; the working pressure was maintained at 5–6×10−6 torr. Radiation from an ArF excimer laser (l=193 nm, 17 ns pulse duration) was focused to a spot size of 0.1 cm×0.4 cm on a rotating sintered SiC target at an angle of 45°. The repetition of the pulse was 3 Hz and a pulse energy of 150–200 mJ yielded an energy density of 2–4 J cm−2 on the target. The plume of atoms and ions emerged in a cone normal to the target and impinged to the substrate with an area of 2 cm×3 cm. p-Type Si(100) wafer (20–35 V cm, 350 mm thick) substrates cleaned with a standard IC procedure were located about 3–5 cm away from and parallel to the surface of the target. The substrate was resistively heated up to 800°C, which was monitored by a thermocouple. The typical average growth rate of SiC films with the energy density of 2–4 J cm−2 was about ~0.05 nm per pulse. After deposition, the samples were taken out and cut into several slices, and then annealed in a vacuum furnace with at a pressure of 1×10−5 torr and temperature in the range from 900 to 1050°C for 30 min. The crystal structure of the SiC films was characterised by X-ray diffraction ( XRD) using Cu Ka radiation. Fourier transform infrared spectroscopy ( FTIR), X-ray photoelectron spectroscopy ( XPS) and scanning electron microscopy (SEM ) were used to investigate composition and surface morphology.
3. Results and discussion XRD analysis of the SiC films deposited on Si(100) substrates at 800°C for 20 min could not detect any characteristic SiC diffraction peaks, indicating that the films were amorphous. The lack of crystallinity in the SiC films appeared to be related to the low growth temperature. Fig. 1 shows the XRD patterns of SiC films annealed in a vacuum at different temperatures for 30 min. Only one peak around 41.4° was observed for the whole range of annealing temperature but its intensity varied as a function of the annealing temperature, suggesting that there was an optimum annealing temperature in order to obtain well-crystallised films. This optimum temperature was 980°C in our experiment. The low intensity of peaks for 920°C could be explained by the initial formation of crystalline phase. For the case of annealing at 1020°C, the low peak intensity might be caused by a decrease of the crystalline phase as a consequence of volatilisation into the ambient vacuum. Owing to the higher formation temperature of 6H-SiC than for 3C-SiC, the peak at 41.4° seemed to correspond to 3C-SiC(200) rather than 6H-SiC(104).
Fig. 1. XRD patterns of SiC films deposited at 800°C and annealed in a vacuum at different temperatures for 30 min.
To investigate the effect of annealing temperature further, FTIR transmittance spectra of the SiC films annealed in a vacuum at various temperatures were measured and are shown in Fig. 2. In addition to the adsorption peaks centred at 609 and 1070 cm−1, which correspond to the Si(100) substrate and SiO , respec2 tively, an adsorption peak around 800 cm−1 corresponding to SiMC bonds was also observed. The intensities varied with the annealing temperature and reached a maximum value at 980°C, thus revealing an optimum annealing effect at 980°C. Clearly, this result was consistent with the result of XRD. Figs. 3a and b present the XPS spectra of Si 2p and C 1s regions of the SiC film annealed at 980°C. As is evident from Fig. 3, the Si 2p and C 1s spectra exhibit one single component at 100.2 eV and 283.3 eV, respectively, with no components at 99.2 eV (corresponding to pure silicon) and 282.6 eV (corresponding to graphite) being identified. This result also indicated the formation of crystalline SiC, and the measured binding energies of the Si 2p and C 1s states agree well with previous reports
Fig. 2. FTIR transmittance spectra of SiC films deposited at 800° and annealed in vacuum at different temperatures for 30 min.
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sample annealed in a vacuum at 980°C for 30 min. Rectangular morphology and an oriented arrangement could be observed, similar to that observed for the carbonisation layer initially formed during chemical vapour deposition of SiC films on silicon, and is indicative of the cubic nature of the (100) growth plane and the presence of crystalline films [12,17]. The SEM pictures of samples annealed at 920, 950 and 1020°C also presented similar square patterns, but poorer morphology. Observations indicated that these defects were pits which formed via the diffusion of voids on the silicon substrate during annealing; the voids were most likely bound on the four sides of the (111) plane before the final pyramidal shapes came into being.
4. Conclusion Pulsed excimer laser deposition combined with a vacuum annealing process has been developed to prepare stoichiometric, crystalline SiC thin films on Si(100) substrates. The amorphous SiC films deposited at 800°C could be transformed into crystalline phase after vacuum annealing at temperatures of 920–1020°C for 30 min. The sample annealed at 980°C displayed the optimum crystallinity and epitaxial growth along the (100) plane.
Fig. 3. XPS spectra of the SiC film deposited at 800°C and annealed in vacuum at 980°C for 30 min: (a) Si 2p; (b) C 1s.
[15,16 ]. The atomic ratio of C/Si using the ratio of peak areas and the empirical sensitivity factors [16 ] (0.87 for silicon and 1.0 for carbon) was 1.16, indicating that the films were almost stoichiometric. Fig. 4 shows a scanning electron micrograph of the
Fig. 4. SEM micrograph of the SiC film deposited at 800°C and annealed in vacuum at 980°C for 30 min.
Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant No. 69776003 and the Shanghai Youth Foundation under Grant No. 97QD14034.
References [1] G. Mu¨ller, G. Kro¨tz, E. Niemann, Sensors and Actuators A 43 (1994) 259. [2] D.M. Brown, E. Downey, M. Grezzo, J. Kretchmer, V. Krishnamrthy, W. Hennessy, G. Michon, Solid State Electronics 59 (1996) 1531. [3] J.W. Palmour, J.A. Edmond, H.S. Kong, C.H. Carter Jr., Physica B 185 (1993) 461. [4] S. Sheng, M.G. Spencer, X. Tang, P. Zhou, K. Wongchoitgul, C. Taylor, G.L. Harris, Mater. Sci. Eng. B 46 (1997) 147. [5] H. Matsunami, S. Nishino, H. Ono, IEEE. Trans. Electron Devices 28 (1981) 1235. [6 ] S. Nishino, H. Suhara, H. Ono, H. Matsunami, J. Appl. Phys. 61 (1987) 4889. [7] Y. Gao, J.H. Edgar, J. Chaudhuri, S.N. Cheema, M.V. Sidorov, D.N. Braski, J. Crystal Gorwth 191 (1998) 439. [8] W. Wesch, Nucl. Instrum. Methods Phys. Res. B 116 (1996) 305. [9] J.A. Cooper Jr., Mater. Sci. Eng. B 44 (1997) 387. [10] I. Golecki, F. Reidinger, J. Marti, Appl. Phys. Lett. 60 (1992) 1703. [11] L. Rimai, R. Ager, W.H. Weber, J. Hangas, Appl. Phys. Lett. 59 (1991) 2266.
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[12] L. Rimai, R. Ager, W.H. Weber, J. Hangas, A. Samman, W. Zhu, J. Appl. Phys. 77 (1995) 6601. [13] M. Balloch, R.J. Tench, W.J. Sielhause, M.J. Allen, A.L. Conor, D.R. Olander, Appl. Phys. Lett. 57 (1990) 1540. [14] M.A. Capano, S.D. Walck, P.T. Murray, Appl. Phys. Lett. 64 (1994) 3413.
[15] M.Y. Chen, P.T. Murray, J. Mater. Sci. 35 (1990) 4929. [16 ] A. Watanabe, M. Mukaida, T. Tsunoda, Y. Imai, Thin Solid Films 300 (1997) 95. [17] H.J. Kim, R.F. Davis, J. Electrochem. Soc. 134 (1987) 2269.