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Catalytic growth of α-FeSi2 and silicon nanowires
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Journal of Crystal Growth 280 (2005) 286–291 www.elsevier.com/locate/jcrysgro
Catalytic growth of a-FeSi2 and silicon nanowires Z.Y. Zhanga, X.L. Wua,, L.W. Yanga, G.S. Huanga, G.G. Siub, Paul K. Chub a
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China b Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, People’s Republic of China Received 12 January 2005; accepted 28 February 2005 Available online 7 April 2005 Communicated by M. Uwaha
Abstract Via vaporization of Fe and Si powders mixed with a molar ratio of 1:1, a-FeSi2 nanowires were synthesized on one Si wafer, where the temperature is higher than 950 1C and Fe supply is enough, and meantime Si nanowires were obtained on the other Si wafer, where the temperature is lower than 500 1C and Fe atoms are much less than Si atoms in vapor. The two kinds of nanowires have diameters of 40–80 nm and lengths of 3–10 mm. Their shapes, crystallinity, and compositions are studied in terms of field emission scanning electron microscopy, X-ray diffraction spectra, and energy dispersive X-ray spectroscopy. From the obtained experimental results, we explain the growth mechanism of the two kinds of nanowires using the vapor–liquid–solid growth model. Two important conditions, high temperatures and enough Fe supply, are emphasized for syntheses of the a-FeSi2 nanowires. r 2005 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.15.Lm Keywords: A1. Nanostructures; A2. Vapor–liquid–solid; B1. Nanowires; B2. Silicon compounds
1. Introduction Nanomaterials are the basis of nanoscience and nanotechnology. A distinctive feature for nanostructured material relative to bulk is the wellknown quantum confinement effect owing to the reduced size and/or dimensionality [1,2]. In the Corresponding author. Tel.: 86253593702;
fax: 86253595535. E-mail address: [email protected] (X.L. Wu).
past decade, huge efforts have been devoted to nanomaterials researches, especially one-dimensional (1D) materials such as carbon [3] and silicon [4] nanotubes, Si nanowires (SiNWs) [5,6], ZnO nanowires [7], and metal nanowires [8]. Silicon is an important semiconductor material with its contemporary microelectronic technology being one of the greatest successes of the past century. Nevertheless, there is still room for improvement in the applications of Si-based optoelectronic devices. For instance, Si is an indirect band gap
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.02.061
ARTICLE IN PRESS Z.Y. Zhang et al. / Journal of Crystal Growth 280 (2005) 286–291
(1.1 eV) material with a small exciton binding energy (15 meV) and does not emit visible light. For this reason, a great deal of effort has been made to fabricate low-dimensional nanostructures with strong light emission. Si nanowire is a good 1D nanomaterial for Si-based optoelectronic devices because it can be fabricated easily via vapor–liquid–solid (VLS) growth. In addition, its band gap can be widened by quantum confinement effect to show visible light emission [2]. Si component, FeSi2, is a very interesting material and has been widely researched in thermoelectric application. The phase diagram of FeSi2 shows two kinds of crystal structures: tetragonal metallic phase (a-FeSi2) at high temperature and orthorhombic semiconducting phase (b-FeSi2) at low temperature [9]. b-FeSi2 is a good semiconducting material with direct band gap of 0.85 eV [9] and has large optical absorption coefficient [9] and good physical–chemical stability at high temperature [10]. The metallic phase, resulting from the existence of vacancies in iron sublattice, occurs within a rather wide range of composition, while the semiconducting phase is stoichiometric. b phase will transform into a phase above the temperature of 950 1C [10]. In the previous researches, FeSi2 has been fabricated into low-dimensional nanostructures, such as films [9,11], quantum dots [12], and islands [13], but no FeSi2 nanowires are reported. In this work, we report simultaneous fabrications of semiconductor SiNWs and metallic a-FiSi2 nanowires (a-FSNWs) via VLS growth. We discuss their growth mechanisms in terms of a large number of microstructural observations and spectroscopic analyses. The obtained a-FSNWs are expected to have applications in thermoelectric nanodevices.
2. Experiment a-FSNWs and SiNWs were formed on some silicon wafers placed in an alumina tube inside a furnace. The vapor source consisting of silicon (99.99%) and iron (Fe) (99.9%) powders mixed together with a molar ratio of 1:1 was placed in an alumina crucible and then the crucible was placed on the head of an alumina boat. A series of the
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cleaned Si (1 0 0) wafers were placed on the alumina boat from the positions near the crucible to the other end of the alumina boat for the collection of the synthesized a-FSNWs and SiNWs. The alumina crucible with vapor source was local at the center of the alumina tube across the heating furnace. The alumina tube was heated to 1350 1C under a carrier gas (99.99% Ar) at a typical total pressure of 500 Torr and a flow rate of 100 sccm. After the temperature of the position at the alumina crucible arrived at 1350 1C, the crucible was held at the same temperature for 2 h. Finally, a-FSNWs and SiNWs (diameters of 40–80 nm and lengths of several mm) were collected on the Si (1 0 0) wafers at distances of 8 and 18 cm near the alumina crucible, respectively. In our experiments, scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) spectra were obtained on an EDAX PV7715/89 ME field emission SEM system. XRD spectra were taken on a Rigaku 3015-type single-crystal diffractometer. All the measurements were performed at room temperature.
3. Results and discussion A dark gray-colored sponge-like product is formed on the Si (1 0 0) wafer at a distance of 8 cm near the alumina crucible. The SEM image and XRD spectrum of the coated Si wafer are shown in Fig. 1. We can see from Fig. 1(a) that many nanowires have been formed. Their lengths are found to be in the range of 5–10 mm. To clarify the composition of the nanowires, we performed the XRD measurement of the Si wafer with nanowires and present the corresponding result in Fig. 1(b). One can see that there exist four XRD peaks related to (0 0 1), (1 1 0), (1 0 2), and (0 0 3) aFeSi2 except for the (1 0 0) XRD peak from the Si substrate. This result indicates that the synthesized nanowires belong to a-FSNWs. We also notice that the intensities of the (0 0 1) and (0 0 3) XRD peaks of a-FeSi2 are obviously higher than the other diffraction peaks, indicating that a-FSNWs grow mainly along the (0 0 1) direction, in good agreement with the orientation of the Si substrate.
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Fig. 1. (a) SEM image of the a-FSNWs obtained at a distance of 8 cm near the alumina crucible; (b) XRD spectrum of the coated Si wafer at a distance of 8 cm near the alumina crucible.
To identify more accurately the diameters of aFSNWs, we carried out the SEM observations of the nanowires with larger magnification and the corresponding result is presented in Fig. 2(a). It can be seen that nanowires generally consist of heads and bodies and have diameters in the range of 40–80 nm. To further identify the composition distribution in whole nanowire, we measured the EDX spectra of different parts of the nanowires. Fig. 2(b) shows the result from the head of the nanowire [see the solid arrow in Fig. 2(a)]. Obviously, this spectrum consists of Si and Fe (the molar ratio of Si/FeE2:1) and O elements. Fig. 2(c) exhibits the EDX spectrum from the bodies of nanowires [see the dotted arrows in Fig. 2(a)]. We can see that the compositions of the bodies are similar to those of the heads. Therefore, the synthesized nanowires are really a-FSNWs.
Fig. 2. (a) SEM image of the a-FSNWs obtained at a distance of 8 cm near the alumina crucible; (b) EDX spectrum of the head of a nanowire (marked by the solid arrow); and (c) EDX spectrum of the body of a nanowire (marked by dotted arrows).
A dark yellow-colored sponge-like product is formed on the Si (1 0 0) wafer at a distance of 18 cm near the alumina crucible. The SEM image and XRD spectrum of the coated Si wafer are shown in Fig. 3. We can see that there are many
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Fig. 3. (a) SEM image of the SiNWs obtained at a distance of 18 cm near the alumina crucible; (b) XRD spectrum of the coated Si wafer at a distance of 18 cm near the alumina crucible.
nanowires on the Si wafer and their lengths are 3–5 mm. We measured the XRD spectrum of the coated Si wafer to obtain the composition of the nanowires. Fig. 3(b) displays the corresponding result. One can see that this XRD spectrum shows four Si peaks from the (1 1 1), (1 0 0), (2 2 0), and (3 1 1) reflections and a broad peak from the surface silicon oxide. The Si (1 0 0) XRD peak originates from the Si-substrate because the nanowires are synthesized on the Si (1 0 0) wafer. In view of the above result, we can conclude that the obtained product is the SiNWs with growth mainly along the (1 1 1) direction [5,6,14,15]. From the enlarged SEM image of the SiNWs [see Fig. 4(a)], we can find the diameters of the SiNWs to be about 40–80 nm. To determine the composition of the nanowires, we have also measured the EDX spectra from different parts of nanowires. Fig. 4(b) shows the EDX spectrum of the junction between
Fig. 4. (a) SEM image of the SiNWs obtained at a distance of 18 cm near the alumina crucible; (b) EDX spectrum of the junction between two nanowires (marked by solid arrows); and (c) EDX spectrum of the body of a nanowire (marked by dotted arrows).
two nanowires [see the solid arrows in Fig. 4(a)]. The composition of this junction is comprised of Si and Fe (the molar ratio of Si/FeE5:1) and O elements. Previously, it has been known that the
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Si (311)
Si (100) Si (111)
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FeSi (210) Si (220) FeSi (211)
they occur at a distance of 18 cm near the alumina crucible. This indicates that the temperature and the Fe content are important factors for the syntheses of different kinds of nanowires. To clarify the VLS growth process in our experiments, we obtained the XRD spectra and SEM images of the Si wafers at a distance ranging from 8 to 18 cm near the alumina crucible. Similar nanowires can be observed in the obtained SEM images (not shown). Fig. 5 shows the X-ray diffraction spectrum of the coated Si wafer at a distance of 13 cm near the alumina crucible. It is obvious from Fig. 5 that four XRD peaks arise from the (1 1 1), (1 0 0), (2 2 0), and (3 1 1) reflections of Si. Two other XRD peaks are related to a-FeSi2 and the remaining two peaks are connected to FeSi. So we can infer that the nanowires on the Si wafers at a distance ranging from 8 to 18 cm near the alumina crucible are neither a-FSNWs nor SiNWs. They should be the nanowires with complicated Si and Fe compositions. a-FeSi2 can exist stably when the temperature is higher than 950 1C [10] and FeSi is easy to obtain when the temperature is in the range of 500–1410 1C [16,17]. SiNWs are synthesized at a lower temperature (p500 1C) no matter what the catalyst is, Fe [15] or SiO2 [15,18]. In the XRD spectrum of Fig. 3(b), we cannot observe any XRD peaks related to iron silicides, indicating that Fe atoms are much less than Si atoms in vapor at a distance of 8 cm near the alumina crucible. Most
α−FeSi2 (001)
junction (a growth point) has Fe element generally in the form of FeSi2 if Fe is employed as a catalyst to synthesize SiNWs [6,15]. The EDX spectrum of the body [see the dotted arrows in Fig. 4(a)] of the nanowire is shown in Fig. 4(c). This spectrum is comprised of Si and O and no Fe element is found. By considering the above XRD and EDX results, we can conclude that the obtained nanowires are Si nanowires with the FeSi2 as a junction. Here we should mention that, since EDX spectral accuracy is generally higher than the XRD spectral one, we can obtain the signal from the Fe element in Fig. 4(b). No Fe element is observed in Fig. 3(b). In the previous researches on growth of SiNWs with Fe catalyst, the XRD spectrum also shows no FeSi2 composition, but the EDX analysis shows the existence of the Fe element on the head of the nanowire [14,15]. Therefore, the XRD and EDX results are consistent. Below, we consider two important conditions for the syntheses of a-FSNWs: (1) enough Fe supply; (2) high temperatures. In the previous experiments for growing the SiNWs, the Fe element was generally employed as a catalyst. The content of the Fe powder is much less than that of the Si powder in the vapor source, with the molar ratio of Fe:Si being from 1:18 to 1:400 [6,14,15]. Thus, there was no enough Fe to synthesize FSNWs and only FeSi2 heads attaching to the nanowires were formed. In our current experiments, we increase the content of the Fe element in the vapor source to the molar ratio of Fe:Si ¼ 1:1 so that the Fe supply is enough. The temperature at the vapor source position is evaluated to be 1350 1C, which is sufficient for the vaporization of Fe. The position of synthesizing a-FSNWs is at a distance of 8 cm near the alumina crucible, where the temperature is kept rather high (X950 1C). Thus, Fe will react with Si to form the a-FeSi2 under high temperature. From the SEM image of Fig. 2(a), we can see that there are FeSi2 heads attaching to nanowires. This is similar to the VLS growth process of SiNWs. The FeSi2 heads are firstly formed and then FSNWs begin to grow from the FeSi2 heads. Comparing Fig. 1(b) with Fig. 3(b), we can see that the SiNWs have not been synthesized on the Si wafers at a distance of 8 cm near the alumina crucible, but
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Fig. 5. XRD spectrum of the coated Si wafer at a distance of 13 cm near the alumina crucible.
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of the Fe atoms have been synthesized into iron silicides such as FeSi2 and FeSi before the vapor arrives. Therefore, we can suggest a VLS growth process: (1) a-FeSi2 are synthesized at the position with enough Fe supply and high temperature (X950 1C) and then attached to the Si wafer. Following the formation of FeSi2 heads, aFSNWs start to grow from the formed FeSi2 heads. (2) When the content of Fe element in the vapor decreases and meantime the temperature decreases to below 950 1C, other sorts of iron silicide, such as FeSi, are formed. They partially carry to the position with lower temperature to form the heads of iron silicide on Si wafers. Since there is enough Fe content, the nanowires with complicated Si and Fe compositions begin growth from the heads of iron silicides. (3) When the temperature descends to lower than 500 1C and Fe atoms are much less than Si atoms in vapor, the FeSi2 heads are firstly formed and then the SiNWs start to grow from the formed FeSi2 heads.
4. Conclusion We have synthesized a-FSNWs on one Si wafer, where the temperature is higher than 950 1C and Fe supply is enough, and meantime obtained SiNWs on the other Si wafer, where the temperature is lower than 500 1C and Fe atoms are much less than Si atoms in vapor. The two kinds of nanowires have diameters of 40–80 nm and lengths of 3–10 mm. Their crystallinity and compositions are characterized in terms of XRD and EDX spectral measurements. From the obtained experimental results, we explain the growth mechanism of the two kinds of nanowires using VLS growth model. We have emphasized that two important conditions, high temperatures and enough Fe supply, are necessary for the syntheses of aFSNWs. The obtained a-FSNWs can be expected to have applications in thermoelectric nanodevices.
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Acknowledgement This work was supported by the Grants (nos. 10225416 and 60476038) from the National Natural Science Foundations of China and the LPEMST as well as the special funds for Major State Basic Research Project No. G001CB3095 of China. Partial support was also from the Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grants (CERG) #CityU 1137/03E and CityU 1120/04E, and City University of Hong Kong Strategic Research Grant (SRG) #7001642. References [1] A.P. Alivisators, Science 271 (1996) 933. [2] S. Bhattacharya, D. Banerjee, K.W. Adu, S. Samui, S. Bhattacharyya, Appl. Phys. Lett. 85 (2004) 2008. [3] S. Iijima, Nature 354 (1991) 56. [4] J. Sha, J.J. Niu, X.Y. Ma, J. Xu, X.B. Zhang, Q. Yang, D. Yang, Adv. Mater. 14 (2002) 1219. [5] D.D.D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. [6] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [7] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [8] S.R.C. Vivekchand, G. Gundiah, A. Govindaraj, C.N.R. Rao, Adv. Mater. 16 (2004) 1842. [9] C.A. Dimitriadis, J.H. Werner, S. Logothetidis, M. Stutzmann, J. Weber, R. Nesper, J. Appl. Phys. 68 (1990) 1726. [10] U. Starke, W. Weiss, M. Kutschera, R. Bandorf, K. Heinz, J. Appl. Phys. 91 (2002) 6154. [11] T. Yoshitake, M. Yatabe, M. Itakura, N. Kuwano, Y. Tomokiyo, Appl. Phys. Lett. 83 (2003) 3057. [12] L. Do´zsa, G. Molna´r, Zs.J. Horva´th, A.L. To´th, J. Gyulai, V. Raineri, F. Giannazzo, Appl. Surf. Sci. 234 (2004) 60. [13] N.G. Galkin, V.O. Polyarnyi, A.S. Gouralnik, Thin Solid Films 464 (2004) 199. [14] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [15] Y.H. Yang, S.J. Wu, H.S. Chiu, P.I. Lin, Y.T. Chen, J. Phys. Chem. B 108 (2004) 846. [16] T. Yoshitake, T. Nagamoto, K. Nagayama, Thin Solid Films 381 (2001) 236. [17] R.N. Wang, J.Y. Feng, J. Crystal Growth 244 (2002) 206. [18] R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003) 635.
Journal of Crystal Growth 280 (2005) 286–291 www.elsevier.com/locate/jcrysgro
Catalytic growth of a-FeSi2 and silicon nanowires Z.Y. Zhanga, X.L. Wua,, L.W. Yanga, G.S. Huanga, G.G. Siub, Paul K. Chub a
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China b Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, People’s Republic of China Received 12 January 2005; accepted 28 February 2005 Available online 7 April 2005 Communicated by M. Uwaha
Abstract Via vaporization of Fe and Si powders mixed with a molar ratio of 1:1, a-FeSi2 nanowires were synthesized on one Si wafer, where the temperature is higher than 950 1C and Fe supply is enough, and meantime Si nanowires were obtained on the other Si wafer, where the temperature is lower than 500 1C and Fe atoms are much less than Si atoms in vapor. The two kinds of nanowires have diameters of 40–80 nm and lengths of 3–10 mm. Their shapes, crystallinity, and compositions are studied in terms of field emission scanning electron microscopy, X-ray diffraction spectra, and energy dispersive X-ray spectroscopy. From the obtained experimental results, we explain the growth mechanism of the two kinds of nanowires using the vapor–liquid–solid growth model. Two important conditions, high temperatures and enough Fe supply, are emphasized for syntheses of the a-FeSi2 nanowires. r 2005 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.15.Lm Keywords: A1. Nanostructures; A2. Vapor–liquid–solid; B1. Nanowires; B2. Silicon compounds
1. Introduction Nanomaterials are the basis of nanoscience and nanotechnology. A distinctive feature for nanostructured material relative to bulk is the wellknown quantum confinement effect owing to the reduced size and/or dimensionality [1,2]. In the Corresponding author. Tel.: 86253593702;
fax: 86253595535. E-mail address: [email protected] (X.L. Wu).
past decade, huge efforts have been devoted to nanomaterials researches, especially one-dimensional (1D) materials such as carbon [3] and silicon [4] nanotubes, Si nanowires (SiNWs) [5,6], ZnO nanowires [7], and metal nanowires [8]. Silicon is an important semiconductor material with its contemporary microelectronic technology being one of the greatest successes of the past century. Nevertheless, there is still room for improvement in the applications of Si-based optoelectronic devices. For instance, Si is an indirect band gap
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.02.061
ARTICLE IN PRESS Z.Y. Zhang et al. / Journal of Crystal Growth 280 (2005) 286–291
(1.1 eV) material with a small exciton binding energy (15 meV) and does not emit visible light. For this reason, a great deal of effort has been made to fabricate low-dimensional nanostructures with strong light emission. Si nanowire is a good 1D nanomaterial for Si-based optoelectronic devices because it can be fabricated easily via vapor–liquid–solid (VLS) growth. In addition, its band gap can be widened by quantum confinement effect to show visible light emission [2]. Si component, FeSi2, is a very interesting material and has been widely researched in thermoelectric application. The phase diagram of FeSi2 shows two kinds of crystal structures: tetragonal metallic phase (a-FeSi2) at high temperature and orthorhombic semiconducting phase (b-FeSi2) at low temperature [9]. b-FeSi2 is a good semiconducting material with direct band gap of 0.85 eV [9] and has large optical absorption coefficient [9] and good physical–chemical stability at high temperature [10]. The metallic phase, resulting from the existence of vacancies in iron sublattice, occurs within a rather wide range of composition, while the semiconducting phase is stoichiometric. b phase will transform into a phase above the temperature of 950 1C [10]. In the previous researches, FeSi2 has been fabricated into low-dimensional nanostructures, such as films [9,11], quantum dots [12], and islands [13], but no FeSi2 nanowires are reported. In this work, we report simultaneous fabrications of semiconductor SiNWs and metallic a-FiSi2 nanowires (a-FSNWs) via VLS growth. We discuss their growth mechanisms in terms of a large number of microstructural observations and spectroscopic analyses. The obtained a-FSNWs are expected to have applications in thermoelectric nanodevices.
2. Experiment a-FSNWs and SiNWs were formed on some silicon wafers placed in an alumina tube inside a furnace. The vapor source consisting of silicon (99.99%) and iron (Fe) (99.9%) powders mixed together with a molar ratio of 1:1 was placed in an alumina crucible and then the crucible was placed on the head of an alumina boat. A series of the
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cleaned Si (1 0 0) wafers were placed on the alumina boat from the positions near the crucible to the other end of the alumina boat for the collection of the synthesized a-FSNWs and SiNWs. The alumina crucible with vapor source was local at the center of the alumina tube across the heating furnace. The alumina tube was heated to 1350 1C under a carrier gas (99.99% Ar) at a typical total pressure of 500 Torr and a flow rate of 100 sccm. After the temperature of the position at the alumina crucible arrived at 1350 1C, the crucible was held at the same temperature for 2 h. Finally, a-FSNWs and SiNWs (diameters of 40–80 nm and lengths of several mm) were collected on the Si (1 0 0) wafers at distances of 8 and 18 cm near the alumina crucible, respectively. In our experiments, scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) spectra were obtained on an EDAX PV7715/89 ME field emission SEM system. XRD spectra were taken on a Rigaku 3015-type single-crystal diffractometer. All the measurements were performed at room temperature.
3. Results and discussion A dark gray-colored sponge-like product is formed on the Si (1 0 0) wafer at a distance of 8 cm near the alumina crucible. The SEM image and XRD spectrum of the coated Si wafer are shown in Fig. 1. We can see from Fig. 1(a) that many nanowires have been formed. Their lengths are found to be in the range of 5–10 mm. To clarify the composition of the nanowires, we performed the XRD measurement of the Si wafer with nanowires and present the corresponding result in Fig. 1(b). One can see that there exist four XRD peaks related to (0 0 1), (1 1 0), (1 0 2), and (0 0 3) aFeSi2 except for the (1 0 0) XRD peak from the Si substrate. This result indicates that the synthesized nanowires belong to a-FSNWs. We also notice that the intensities of the (0 0 1) and (0 0 3) XRD peaks of a-FeSi2 are obviously higher than the other diffraction peaks, indicating that a-FSNWs grow mainly along the (0 0 1) direction, in good agreement with the orientation of the Si substrate.
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Fig. 1. (a) SEM image of the a-FSNWs obtained at a distance of 8 cm near the alumina crucible; (b) XRD spectrum of the coated Si wafer at a distance of 8 cm near the alumina crucible.
To identify more accurately the diameters of aFSNWs, we carried out the SEM observations of the nanowires with larger magnification and the corresponding result is presented in Fig. 2(a). It can be seen that nanowires generally consist of heads and bodies and have diameters in the range of 40–80 nm. To further identify the composition distribution in whole nanowire, we measured the EDX spectra of different parts of the nanowires. Fig. 2(b) shows the result from the head of the nanowire [see the solid arrow in Fig. 2(a)]. Obviously, this spectrum consists of Si and Fe (the molar ratio of Si/FeE2:1) and O elements. Fig. 2(c) exhibits the EDX spectrum from the bodies of nanowires [see the dotted arrows in Fig. 2(a)]. We can see that the compositions of the bodies are similar to those of the heads. Therefore, the synthesized nanowires are really a-FSNWs.
Fig. 2. (a) SEM image of the a-FSNWs obtained at a distance of 8 cm near the alumina crucible; (b) EDX spectrum of the head of a nanowire (marked by the solid arrow); and (c) EDX spectrum of the body of a nanowire (marked by dotted arrows).
A dark yellow-colored sponge-like product is formed on the Si (1 0 0) wafer at a distance of 18 cm near the alumina crucible. The SEM image and XRD spectrum of the coated Si wafer are shown in Fig. 3. We can see that there are many
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Fig. 3. (a) SEM image of the SiNWs obtained at a distance of 18 cm near the alumina crucible; (b) XRD spectrum of the coated Si wafer at a distance of 18 cm near the alumina crucible.
nanowires on the Si wafer and their lengths are 3–5 mm. We measured the XRD spectrum of the coated Si wafer to obtain the composition of the nanowires. Fig. 3(b) displays the corresponding result. One can see that this XRD spectrum shows four Si peaks from the (1 1 1), (1 0 0), (2 2 0), and (3 1 1) reflections and a broad peak from the surface silicon oxide. The Si (1 0 0) XRD peak originates from the Si-substrate because the nanowires are synthesized on the Si (1 0 0) wafer. In view of the above result, we can conclude that the obtained product is the SiNWs with growth mainly along the (1 1 1) direction [5,6,14,15]. From the enlarged SEM image of the SiNWs [see Fig. 4(a)], we can find the diameters of the SiNWs to be about 40–80 nm. To determine the composition of the nanowires, we have also measured the EDX spectra from different parts of nanowires. Fig. 4(b) shows the EDX spectrum of the junction between
Fig. 4. (a) SEM image of the SiNWs obtained at a distance of 18 cm near the alumina crucible; (b) EDX spectrum of the junction between two nanowires (marked by solid arrows); and (c) EDX spectrum of the body of a nanowire (marked by dotted arrows).
two nanowires [see the solid arrows in Fig. 4(a)]. The composition of this junction is comprised of Si and Fe (the molar ratio of Si/FeE5:1) and O elements. Previously, it has been known that the
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Si (311)
Si (100) Si (111)
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FeSi (210) Si (220) FeSi (211)
they occur at a distance of 18 cm near the alumina crucible. This indicates that the temperature and the Fe content are important factors for the syntheses of different kinds of nanowires. To clarify the VLS growth process in our experiments, we obtained the XRD spectra and SEM images of the Si wafers at a distance ranging from 8 to 18 cm near the alumina crucible. Similar nanowires can be observed in the obtained SEM images (not shown). Fig. 5 shows the X-ray diffraction spectrum of the coated Si wafer at a distance of 13 cm near the alumina crucible. It is obvious from Fig. 5 that four XRD peaks arise from the (1 1 1), (1 0 0), (2 2 0), and (3 1 1) reflections of Si. Two other XRD peaks are related to a-FeSi2 and the remaining two peaks are connected to FeSi. So we can infer that the nanowires on the Si wafers at a distance ranging from 8 to 18 cm near the alumina crucible are neither a-FSNWs nor SiNWs. They should be the nanowires with complicated Si and Fe compositions. a-FeSi2 can exist stably when the temperature is higher than 950 1C [10] and FeSi is easy to obtain when the temperature is in the range of 500–1410 1C [16,17]. SiNWs are synthesized at a lower temperature (p500 1C) no matter what the catalyst is, Fe [15] or SiO2 [15,18]. In the XRD spectrum of Fig. 3(b), we cannot observe any XRD peaks related to iron silicides, indicating that Fe atoms are much less than Si atoms in vapor at a distance of 8 cm near the alumina crucible. Most
α−FeSi2 (001)
junction (a growth point) has Fe element generally in the form of FeSi2 if Fe is employed as a catalyst to synthesize SiNWs [6,15]. The EDX spectrum of the body [see the dotted arrows in Fig. 4(a)] of the nanowire is shown in Fig. 4(c). This spectrum is comprised of Si and O and no Fe element is found. By considering the above XRD and EDX results, we can conclude that the obtained nanowires are Si nanowires with the FeSi2 as a junction. Here we should mention that, since EDX spectral accuracy is generally higher than the XRD spectral one, we can obtain the signal from the Fe element in Fig. 4(b). No Fe element is observed in Fig. 3(b). In the previous researches on growth of SiNWs with Fe catalyst, the XRD spectrum also shows no FeSi2 composition, but the EDX analysis shows the existence of the Fe element on the head of the nanowire [14,15]. Therefore, the XRD and EDX results are consistent. Below, we consider two important conditions for the syntheses of a-FSNWs: (1) enough Fe supply; (2) high temperatures. In the previous experiments for growing the SiNWs, the Fe element was generally employed as a catalyst. The content of the Fe powder is much less than that of the Si powder in the vapor source, with the molar ratio of Fe:Si being from 1:18 to 1:400 [6,14,15]. Thus, there was no enough Fe to synthesize FSNWs and only FeSi2 heads attaching to the nanowires were formed. In our current experiments, we increase the content of the Fe element in the vapor source to the molar ratio of Fe:Si ¼ 1:1 so that the Fe supply is enough. The temperature at the vapor source position is evaluated to be 1350 1C, which is sufficient for the vaporization of Fe. The position of synthesizing a-FSNWs is at a distance of 8 cm near the alumina crucible, where the temperature is kept rather high (X950 1C). Thus, Fe will react with Si to form the a-FeSi2 under high temperature. From the SEM image of Fig. 2(a), we can see that there are FeSi2 heads attaching to nanowires. This is similar to the VLS growth process of SiNWs. The FeSi2 heads are firstly formed and then FSNWs begin to grow from the FeSi2 heads. Comparing Fig. 1(b) with Fig. 3(b), we can see that the SiNWs have not been synthesized on the Si wafers at a distance of 8 cm near the alumina crucible, but
Intensity (arb. units)
290
55
60
Fig. 5. XRD spectrum of the coated Si wafer at a distance of 13 cm near the alumina crucible.
ARTICLE IN PRESS Z.Y. Zhang et al. / Journal of Crystal Growth 280 (2005) 286–291
of the Fe atoms have been synthesized into iron silicides such as FeSi2 and FeSi before the vapor arrives. Therefore, we can suggest a VLS growth process: (1) a-FeSi2 are synthesized at the position with enough Fe supply and high temperature (X950 1C) and then attached to the Si wafer. Following the formation of FeSi2 heads, aFSNWs start to grow from the formed FeSi2 heads. (2) When the content of Fe element in the vapor decreases and meantime the temperature decreases to below 950 1C, other sorts of iron silicide, such as FeSi, are formed. They partially carry to the position with lower temperature to form the heads of iron silicide on Si wafers. Since there is enough Fe content, the nanowires with complicated Si and Fe compositions begin growth from the heads of iron silicides. (3) When the temperature descends to lower than 500 1C and Fe atoms are much less than Si atoms in vapor, the FeSi2 heads are firstly formed and then the SiNWs start to grow from the formed FeSi2 heads.
4. Conclusion We have synthesized a-FSNWs on one Si wafer, where the temperature is higher than 950 1C and Fe supply is enough, and meantime obtained SiNWs on the other Si wafer, where the temperature is lower than 500 1C and Fe atoms are much less than Si atoms in vapor. The two kinds of nanowires have diameters of 40–80 nm and lengths of 3–10 mm. Their crystallinity and compositions are characterized in terms of XRD and EDX spectral measurements. From the obtained experimental results, we explain the growth mechanism of the two kinds of nanowires using VLS growth model. We have emphasized that two important conditions, high temperatures and enough Fe supply, are necessary for the syntheses of aFSNWs. The obtained a-FSNWs can be expected to have applications in thermoelectric nanodevices.
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Acknowledgement This work was supported by the Grants (nos. 10225416 and 60476038) from the National Natural Science Foundations of China and the LPEMST as well as the special funds for Major State Basic Research Project No. G001CB3095 of China. Partial support was also from the Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grants (CERG) #CityU 1137/03E and CityU 1120/04E, and City University of Hong Kong Strategic Research Grant (SRG) #7001642. References [1] A.P. Alivisators, Science 271 (1996) 933. [2] S. Bhattacharya, D. Banerjee, K.W. Adu, S. Samui, S. Bhattacharyya, Appl. Phys. Lett. 85 (2004) 2008. [3] S. Iijima, Nature 354 (1991) 56. [4] J. Sha, J.J. Niu, X.Y. Ma, J. Xu, X.B. Zhang, Q. Yang, D. Yang, Adv. Mater. 14 (2002) 1219. [5] D.D.D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. [6] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [7] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [8] S.R.C. Vivekchand, G. Gundiah, A. Govindaraj, C.N.R. Rao, Adv. Mater. 16 (2004) 1842. [9] C.A. Dimitriadis, J.H. Werner, S. Logothetidis, M. Stutzmann, J. Weber, R. Nesper, J. Appl. Phys. 68 (1990) 1726. [10] U. Starke, W. Weiss, M. Kutschera, R. Bandorf, K. Heinz, J. Appl. Phys. 91 (2002) 6154. [11] T. Yoshitake, M. Yatabe, M. Itakura, N. Kuwano, Y. Tomokiyo, Appl. Phys. Lett. 83 (2003) 3057. [12] L. Do´zsa, G. Molna´r, Zs.J. Horva´th, A.L. To´th, J. Gyulai, V. Raineri, F. Giannazzo, Appl. Surf. Sci. 234 (2004) 60. [13] N.G. Galkin, V.O. Polyarnyi, A.S. Gouralnik, Thin Solid Films 464 (2004) 199. [14] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [15] Y.H. Yang, S.J. Wu, H.S. Chiu, P.I. Lin, Y.T. Chen, J. Phys. Chem. B 108 (2004) 846. [16] T. Yoshitake, T. Nagamoto, K. Nagayama, Thin Solid Films 381 (2001) 236. [17] R.N. Wang, J.Y. Feng, J. Crystal Growth 244 (2002) 206. [18] R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003) 635.