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High-temperature behavior of an amorphous carbon layer on SiC particles
Surface Science 527 (2003) L219–L221 www.elsevier.com/locate/susc
Surface Science Letters
High-temperature behavior of an amorphous carbon layer on SiC particles Yuki Kimura b
a,*
, Yoshio Saito b, Chihiro Kaito
a
a Department of Nanophysics in Frontier Project, Ritsumeikan University, Noji, Kusatsu-shi, Shiga 525-8577, Japan Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
Received 16 September 2002; accepted for publication 2 December 2002
Abstract The dynamic behavior of an amorphous carbon layer of 3 nm thickness on a SiC particle was examined at high temperature using a high-resolution electron microscope equipped with a real-times video recording system. It was found that the surface carbon layer began to solve into the SiC particle at 600 °C. The layer was completely solved into SiC crystal at 800 °C. The lattice image of SiC changed negligibly at high temperature. If the specimen was cooled back to room temperature, the carbon layer on the SiC particle was recovered. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Silicon carbide; Surface diffusion; Carbon
Dynamic behavior on the nano-scale is not merely interesting for its science but also important for future material technology. Impurities on a crystal surface are thought to be evaporated by vacuum heating. In fact, semiconductors are being fabricated through the use of clean techniques and surface control. Here, we report on the unexpected nano-scale behavior of a surface layer upon heating. By the use of a special heating system attached to a high-resolution transmission electron microscope (HRTEM), dynamic phenomena, such as the crystallization process, structural changes, evap-
* Corresponding author. Tel.: +81-77-561-2709; fax: +81-77561-3994. E-mail address: [email protected] (Y. Kimura).
oration process, external changes and transformation process, can be directly observed in lattice images [1–3]. In the present experiment, b-SiC particles with an amorphous carbon layer were produced by the advanced gas evaporation method [4]. Fig. 1 shows a typical HRTEM image of an obtained particle with the size of about 50 nm. The crossed lattice in the image and their angles of (1 1 1) indicate the formation of b-SiC crystal. The crystal surfaces of each SiC particle were covered with an amorphous phase of 3 nm thickness. The amorphous layer was determined to be a graphitic carbon layer based on the following: the 0.34 nm fringes of microcrystallites with sizes of 2 nm order in the enlarged image in Fig. 1; the characteristic broad absorption peak at 230 nm in the ultraviolet spectrum of
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00011-6
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Y. Kimura et al. / Surface Science 527 (2003) L219–L221
Fig. 1. Typical HRTEM image of the b-SiC particle. The closed lattice in the image of the (1 1 1) lattice at 70° shows the growth of b-SiC. The surface of the particles was covered with a uniform amorphous carbon layer of 3 nm thickness. The enlarged image shows the existence of graphite microcrystallites.
the collected particles [5]; and the comparison of the diffractograms of the amorphous layer and SiC crystal. This sample was set on the special heating stage of the HRTEM under a vacuum of 3 10 8 Torr and the structural change of the particles was photographed using the real-time video recording system in the HRTEM mode [3]. Because of the cooling decontamination system around the heating stage and the high vacuum, contamination of the samples during observation is negligible. As shown in the video images of the particle, in spite of the evaporation temperature of carbon being above 3000 °C, the carbon layer becomes 1/4 thinner upon heating at 600 °C, as seen in Fig. 2(a). After the heating temperature was increased to 800 °C, the carbon layer disappeared completely, as shown in Fig. 2(b). When we cooled the specimen back to room temperature, the surface carbon layer about 3 nm thick was recovered, as shown in Fig. 2(c). Moreover, the particle is heated again, the amorphous carbon layer diffused once more into the crystal, i.e. this process is cyclic. By
Fig. 2. HRTEM image obtained during heating. The same particles were observed and photographed. The 3 nm carbon layer shown in Fig. 1 becomes thinner at 600 °C and disappears at 800 °C. When the grains were cooled to room temperature, the 3 nm carbon layer was recovered. This indicates that the carbon layer diffused into the SiC crystal. The dotted line shows the SiC crystal surface.
examining the entire process, we were able to observe the carbon layer diffusing into the SiC crystal. If we look only at the results shown in Figs. 1
SU N IE SC RS CE E A TT R F LE CE
Y. Kimura et al. / Surface Science 527 (2003) L219–L221
and 2(c) the disappearance of the carbon layer cannot be recognized. Since the carbon layer has uniform thickness, the number of carbon atoms has been estimated from the density and volume of the amorphous carbon layer. The chemical composition of silicon carbide with the diffused carbon at 800 °C becomes SiC1:73 . Since the lattice image of SiC is clearly seen in the experiment, the diffused carbon may exist at the vacancy sites of the antifluorite structure. In the present experiment, it can be concluded that the dynamics of the surface layer below the melting point temperature will be important in the future material technology and fabrication of nano-semiconductors.
L221
Acknowledgements This work was supported in part by grants from JSPS Research fellowships for Young Scientists. References [1] T. Kobayashi, Y. Kimura, H. Suzuki, T. Sato, T. Tanigaki, Y. Saito, C. Kaito, J. Crystal Growth 243 (1) (2002) 143. [2] H. Mori, M. Komatsu, K. Takeda, H. Fujita, Philos. Mag. Lett. 63 (3) (1991) 173. [3] S. Kimura, C. Kaito, S. Wada, Antarct. Meteorite. Res. 13 (2000) 145. [4] C. Kaito, Jpn. J. Appl. Phys. 24 (1985) 261. [5] S. Wada, A.T. Tokunaga, C. Kaito, S. Kimura, Astron. Astrophys. 339 (1998) L61.
Surface Science Letters
High-temperature behavior of an amorphous carbon layer on SiC particles Yuki Kimura b
a,*
, Yoshio Saito b, Chihiro Kaito
a
a Department of Nanophysics in Frontier Project, Ritsumeikan University, Noji, Kusatsu-shi, Shiga 525-8577, Japan Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
Received 16 September 2002; accepted for publication 2 December 2002
Abstract The dynamic behavior of an amorphous carbon layer of 3 nm thickness on a SiC particle was examined at high temperature using a high-resolution electron microscope equipped with a real-times video recording system. It was found that the surface carbon layer began to solve into the SiC particle at 600 °C. The layer was completely solved into SiC crystal at 800 °C. The lattice image of SiC changed negligibly at high temperature. If the specimen was cooled back to room temperature, the carbon layer on the SiC particle was recovered. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Silicon carbide; Surface diffusion; Carbon
Dynamic behavior on the nano-scale is not merely interesting for its science but also important for future material technology. Impurities on a crystal surface are thought to be evaporated by vacuum heating. In fact, semiconductors are being fabricated through the use of clean techniques and surface control. Here, we report on the unexpected nano-scale behavior of a surface layer upon heating. By the use of a special heating system attached to a high-resolution transmission electron microscope (HRTEM), dynamic phenomena, such as the crystallization process, structural changes, evap-
* Corresponding author. Tel.: +81-77-561-2709; fax: +81-77561-3994. E-mail address: [email protected] (Y. Kimura).
oration process, external changes and transformation process, can be directly observed in lattice images [1–3]. In the present experiment, b-SiC particles with an amorphous carbon layer were produced by the advanced gas evaporation method [4]. Fig. 1 shows a typical HRTEM image of an obtained particle with the size of about 50 nm. The crossed lattice in the image and their angles of (1 1 1) indicate the formation of b-SiC crystal. The crystal surfaces of each SiC particle were covered with an amorphous phase of 3 nm thickness. The amorphous layer was determined to be a graphitic carbon layer based on the following: the 0.34 nm fringes of microcrystallites with sizes of 2 nm order in the enlarged image in Fig. 1; the characteristic broad absorption peak at 230 nm in the ultraviolet spectrum of
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00011-6
CE RF A LE C E TT S E R C IE S N
SU
L220
Y. Kimura et al. / Surface Science 527 (2003) L219–L221
Fig. 1. Typical HRTEM image of the b-SiC particle. The closed lattice in the image of the (1 1 1) lattice at 70° shows the growth of b-SiC. The surface of the particles was covered with a uniform amorphous carbon layer of 3 nm thickness. The enlarged image shows the existence of graphite microcrystallites.
the collected particles [5]; and the comparison of the diffractograms of the amorphous layer and SiC crystal. This sample was set on the special heating stage of the HRTEM under a vacuum of 3 10 8 Torr and the structural change of the particles was photographed using the real-time video recording system in the HRTEM mode [3]. Because of the cooling decontamination system around the heating stage and the high vacuum, contamination of the samples during observation is negligible. As shown in the video images of the particle, in spite of the evaporation temperature of carbon being above 3000 °C, the carbon layer becomes 1/4 thinner upon heating at 600 °C, as seen in Fig. 2(a). After the heating temperature was increased to 800 °C, the carbon layer disappeared completely, as shown in Fig. 2(b). When we cooled the specimen back to room temperature, the surface carbon layer about 3 nm thick was recovered, as shown in Fig. 2(c). Moreover, the particle is heated again, the amorphous carbon layer diffused once more into the crystal, i.e. this process is cyclic. By
Fig. 2. HRTEM image obtained during heating. The same particles were observed and photographed. The 3 nm carbon layer shown in Fig. 1 becomes thinner at 600 °C and disappears at 800 °C. When the grains were cooled to room temperature, the 3 nm carbon layer was recovered. This indicates that the carbon layer diffused into the SiC crystal. The dotted line shows the SiC crystal surface.
examining the entire process, we were able to observe the carbon layer diffusing into the SiC crystal. If we look only at the results shown in Figs. 1
SU N IE SC RS CE E A TT R F LE CE
Y. Kimura et al. / Surface Science 527 (2003) L219–L221
and 2(c) the disappearance of the carbon layer cannot be recognized. Since the carbon layer has uniform thickness, the number of carbon atoms has been estimated from the density and volume of the amorphous carbon layer. The chemical composition of silicon carbide with the diffused carbon at 800 °C becomes SiC1:73 . Since the lattice image of SiC is clearly seen in the experiment, the diffused carbon may exist at the vacancy sites of the antifluorite structure. In the present experiment, it can be concluded that the dynamics of the surface layer below the melting point temperature will be important in the future material technology and fabrication of nano-semiconductors.
L221
Acknowledgements This work was supported in part by grants from JSPS Research fellowships for Young Scientists. References [1] T. Kobayashi, Y. Kimura, H. Suzuki, T. Sato, T. Tanigaki, Y. Saito, C. Kaito, J. Crystal Growth 243 (1) (2002) 143. [2] H. Mori, M. Komatsu, K. Takeda, H. Fujita, Philos. Mag. Lett. 63 (3) (1991) 173. [3] S. Kimura, C. Kaito, S. Wada, Antarct. Meteorite. Res. 13 (2000) 145. [4] C. Kaito, Jpn. J. Appl. Phys. 24 (1985) 261. [5] S. Wada, A.T. Tokunaga, C. Kaito, S. Kimura, Astron. Astrophys. 339 (1998) L61.