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One-dimensional silicon nanostructures fabricated by thermal evaporation
Materials Science and Engineering C 26 (2006) 800 – 804 www.elsevier.com/locate/msec
One-dimensional silicon nanostructures fabricated by thermal evaporation Jun Yu a, Jian Sha a,b, Lei Wang a, Qing Yang a, Deren Yang a,* a
State Key Laboratory of Silicon Materials, Zhejiang University, Zheda Lu 38, Hangzhou 310027, People’s Republic of China b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China Available online 18 January 2006
Abstract One-dimensional silicon nanostructures were fabricated by evaporating silicon monoxide (SiO) powders at 1050 -C and collecting the products on gold-coated silicon substrates. The uniform and dense silicon nanowires with the diameter of about 20 – 30 nm and the length of tens of microns were observed. However, the tufts of silicon nanowires were found while the silicon substrates without gold coating were used. Moreover, the chain-like silicon nano-structures with obvious core-shell morphology that displays a homojunction structure composed of cylindrical section and the oscillating section was also found. Finally, the biforked silicon nanowires with the same diameter of branches were detected, which is hard to understand on the current growth mechanism of silicon nanowires. D 2005 Published by Elsevier B.V. Keywords: Thermal evaporation; Silicon; One-dimensional
1. Introduction One-dimensional silicon nanostructures have attracted much attention in recent years for their valuable electrical and optical properties, as well as their potential applications in mesoscopic research and nanodevices. Various methods, such as photolithography [1], excimer laser ablation [2,3], thermal evaporation [4– 8], solution-grown [9], chemical vapor deposition [10 –13], etc., have been developed for the synthesis of one-dimensional silicon nanostructures. Among these methods, thermal evaporation is most suitable for industrial applications because it is of low cost and can produce high-purity and uniformly sized onedimensional nanostructures in bulk quantity. Zhang et al. obtained large-scaled silicon nanowires by simply evaporating SiO powders [4] (or a powder mixture of silicon and SiO2 [16]) without metal catalyst. Further research [14 – 18] showed that the oxide played a crucial role on the nanowire growth. Based on this fact, they proposed an oxideassisted growth (OAG) mechanism that is remarkably different from the traditional vapor –liquid – solid (VLS) mechanism [19]. Furthermore, Si nanosphere chains, another form of onedimensional silicon nanostructure, have also been synthesized [14] by thermal evaporation method. It is believed that in-situ annealing at high temperature converts silicon nanowires to * Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: [email protected] (D. Yang). 0928-4931/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.msec.2005.09.106
nanosphere chains. Based on this comprehension, Peng et al. [20] obtained large quantities of silicon nanochains by longtime annealing the as-grown silicon nanowires at high temperature (1200 –1300 -C). Very recently, Kolb et al. [21] reported VLS grown silicon nanowires by SiO evaporation with metal catalyst. In this paper, we evaporated SiO powders at relatively low temperature (1050 -C) and synthesized several types of onedimensional silicon nanostructures on gold-coated silicon substrates. Besides large quantities of silicon nanowires, nanochains composed of the continuous silicon cores and amorphous oxide shells were fabricated. Homojunction of the silicon nanowires and nanochains were also found and the apparent transition from oscillation to cylinder shape was revealed by a field-emission scanning electron microscopy (FESEM) image. Finally, detailed morphologies of the distinct biforked silicon nanowires were studied. 2. Experimental The experimental apparatus consists of a horizontal tube furnace (75 cm in length), a reacting chamber made of quartz tube (f5.5 150 cm), a rotary pump system, and a gas supply and control system. The ultimate vacuum for this configuration is ¨20 Pa. A polished n-type Si (100) wafer with a resistivity of 0.01 VIcm was cut into 1.5 cm 1.5 cm chips and then pretreated by the following treatments: (1) immersion in
J. Yu et al. / Materials Science and Engineering C 26 (2006) 800 – 804
NH3IH2O/H2O2/H2O (1:1:5 v/v) at 80 -C for 10 min; (2) immersion in HCl/H2O2/H2O (1:1:6 v/v) at 80 -C for 10 min; (3) etching in HF solution (5%) for 5 min. Finally, the chips were thoroughly rinsed with deionized water and dried in air. Au films with the thickness of about 5 nm as the catalyst were deposited on a moiety of these silicon chips by radio frequency (RF) magnetron sputtering system. These silicon chips with or without Au coating were used as substrates to collect growth products. SiO powders were placed in the center of the chamber, where the temperature was controlled by furnace. After the reacting chamber was pumped down to 20 Pa, a flow mixture of 150 sccm hydrogen and 350 sccm argon was introduced as the carrier gas to transport the SiO vapor downstream. Simultaneously, the chamber was heated to 1050 -C at a heating rate of 15 -C and held totally for 2 h in the flowing H2/Ar carrier gas. After deposition, the furnace was cooled down and the products were characterized by a field-emission scanning electron microscope (FESEM, FEI, Sirion). The possible chemical composition of the as-grown materials on the wafers was investigated by using energy-dispersive X-ray spectroscopy (EDX) attached to the FESEM. For transmission electron microscope (TEM, JEM200CX, JEOL) examination, samples were ultrasonically dispersed in alcohol for 30 min and transferred to a Cu grid coated with a holey carbon film. 3. Results and discussion Fig. 1a shows the FESEM image of the silicon nanowires synthesized on the gold-coated substrate. The uniform and dense nanowires with a length of tens of micrometers can be found on the silicon substrate. TEM image shown in Fig. 1b reveals that the silicon nanowires have smooth surface morphology and good monodispersity in diameter (about 20 –30 nm). A typical selected-area electron diffraction pattern (SEAD, upper right inset in Fig. 1b) taken from a nanowire could be labeled for (111), (220) and (311) lattice planes of silicon crystal. In our experiment, bare silicon substrates for comparison were placed abreast with the gold-coated substrates. However, silicon nanowires on the bare substrates presented apparently different morphology comparing to those
801
Fig. 2. FESEM image of the tufts of silicon nanowires on a bare silicon substrate.
on the gold-coated substrates. Fig. 2 shows the FESEM image displaying the tufts of silicon nanowires in relatively lower quantity. The substrate is partially covered and exposed to air even in some sections. This distinct difference indicates that there exist different growth mechanisms for the two types of substrates. According to the previous reports [4,14 – 18], it is readily deduced that silicon nanowires on bare substrates grow via OAG mechanism. The relatively low production might be due to the short growing time (2 h) and low evaporating temperature (1050 -C). But if neglecting the effect of Au catalyst, it will be hard to comprehend that using gold-coated substrates results to a large yield of silicon nanowires. Fig. 3a shows a high-magnified FESEM image revealing the local morphology of the silicon nanowires on a gold-coated substrate. It can be clearly seen that a silicon nanowire (pointed by a arrow) is capped with a little bigger particle (pointed by h arrow), which looks very similar to the typical metal-catalyzed nanowire morphology. Based on this observation, we suppose that there is an Au-catalyzed VLS process on gold-coated substrates. Fig. 3b and c display the EDX spectra from the wire stem and top catalyst particle, respectively. It is apparently seen that the content of Au in the catalyst particle is much higher than that in the silicon wire stem (Au in the silicon wire stem is due to the measurement needed). Considering the precision of EDX attached to FESEM, we deduce that the catalyst particle
Fig. 1. (a) FESEM image showing silicon nanowires on gold-coated silicon substrate, fabricated by evaporating SiO powders at 1050 -C. (b) TEM image revealing that the silicon nanowires have smooth surface morphology and good monodispersity in diameter, the inset shows a SAED pattern corresponding to a nanowire.
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J. Yu et al. / Materials Science and Engineering C 26 (2006) 800 – 804
Fig. 3. (a) A high-magnified FESEM image of the silicon nanowires grown on a gold-coated substrate. The corresponding EDX spectra from (b) the silicon nanowire stem (pointed by a arrow) and (c) catalyst particle (pointed by h arrow).
is some kind of gold silicide, which results in the onedimensional growth of VLS process. Finally, based on the above investigation, we concluded that OAG and Au catalyzed VLS growth mechanisms coexist for the silicon nanowires grown on the gold-coated substrate. Because Au –Si eutectic temperature is lower, Au – Si alloy droplets as nucleation sites could cover the whole substrates. Therefore, VLS process is relatively more dominant than the OAG process under our experimental conditions. Besides the nanowires, small quantity of chain-like silicon nanostructures grown on the gold-coated substrates was also found. As illustrated in Fig. 4a, the fluctuations of the diameter are observed in the part of a silicon nanowire. The smooth cylindrical section (pointed by h arrow) and the oscillating one (pointed by an arrow) interconnect and form a homojunction nanostructure. Fig. 4b shows a high-magnified FESEM image revealing the detailed morphology of the transition from oscillation to smooth. Previous reports [14,20] considered that silicon nanochains originated from nanowires annealed for a long time at high temperatures. However, neither the growth time nor the temperatures could be reached in our experiment. Therefore, the homojunction structure can not be explained by the mechanism mentioned above. Considering the metal catalyst involved, we suggest that the chain-like structure is
also derived from VLS process. As is well known, the diameter of nanowires via VLS mechanism is determined by the diameter of liquid catalyst droplets. If the diameter of droplets changes periodically during growth, chain-like nanostructures will be produced. In fact, that the diameter of droplets changes periodically has been suggested on the formation of silicon whiskers [22]. Details of a chain-like silicon nanostructure are also characterized by a TEM image and a SAED pattern (Fig. 5a). A typical core-shell morphology including the dark core and the relative lighter sheath is illustrated in the TEM image. Different from the nanochains reported in Ref. [20], which are consisted of separate Si nanospheres connected by amorphous Si-oxide bars, the chain-like structures obtained in our experiment have a continuous silicon structure along the whole length. This difference confirms that the nanochains in our experiments originated from the droplet of self-oscillation, not due to the annealing process. The SAED pattern showed in the upper right inset of Fig. 5a reveals the monocrystalline phase nature of the silicon core. We also note that the oscillation amplitude of the nanochains is mainly shared by the variety of Si-oxide shell. The continuous crystallized silicon core has only a little fluctuation. This morphology is very interesting to be further researched, and we speculate that it is due to the superfluous silicon oxide generated by the disproportionation
Fig. 4. FESEM images of chain-like silicon nanostructures: (a) low-magnified morphology of a homojunction composed of the oscillating section (pointed by a arrow) and cylindrical section (pointed by h arrow); (b) high-magnified morphology showing the transition from oscillation to smooth.
J. Yu et al. / Materials Science and Engineering C 26 (2006) 800 – 804
803
Fig. 5. TEM images of different silicon nanostructures: (a) chain-like structures, the inset shows a corresponding SAED pattern revealing the monocrystalline phase nature of the silicon core; (b, c) distinct biforked silicon nanowires (the arrows point the crotch).
of SiO vapor. Furthermore, a distinct biforked silicon nanostructure is displayed in Fig. 5b and c. It is clearly seen that the nanowire developed several branches at different section and the branches can also have branches, which is similar to those named secondary growth. Hu et al. [23] reported a VLS secondary growth case in which thin nanowires sub-grew on the surface of thick nanowires. But the diameter of those thin nanowires is almost one tenth that of thick nanowires. Pan et al. [6] fabricated an octopus-like structure by evaporating the SiO powders at 1350 -C and collecting the products at 1250 –1320 -C. Two to five arms are excluded from a large crystalline Si sphere (1 –2 Am), and the authors proposed that Si spheres as catalyst that is similar to metal particles in traditional VLS growth. Comparing to the above two branched nanostructures, the biforked nanowires have no obvious particles on the crotches (pointed by arrows in Fig. 5b and c) and all the branches showed in the TEM images are of almost the same diameter. Moreover, we can find that there is no apparent difference between the trunks and the branch. It is much complicated to understand its growth mechanism, which might suggest an unknown process to be further discovered.
gold-coated silicon substrates, bare substrates at the same place were partially covered and produced a small quantity of silicon nanowires. This phenomenon indicates that there is a coexistence of VLS and OAG process on gold-coated substrates. Furthermore, silicon nanochains having a continuous silicon core and silicon oxide shell were observed. The obvious transition from oscillation to smooth in the silicon nanochains is believed due to the instability of VLS growth process. Finally, the biforked silicon nanowires with all the branches of almost the same diameter were found, which is difficult to be understood on the base of OAG and VLS mechanism. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 50272057 and 60225010) and the key project of Education Ministry of China. The authors would like to thank Mr. Youwen Wang, at the Analysis and Measurement Center of Zhejiang University, for his great help. References
4. Conclusion Employing SiO powders as precursors and the mixture of Ar/H2 as carrier gas, one-dimensional silicon nanostructures with different morphologies were fabricated. Comparing the
[1] H.I. Liu, N.I. Maluf, R.F.W. Pease, J. Vac. Sci. Technol., B 10 (1992) 2846. [2] A.M. Morles, C.M. Lieber, Science 279 (1998) 208. [3] 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.
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[4] Y.F. Zhang, Y.H. Tang, C. Lam, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Cryst. Growth 212 (2000) 115. [5] D.D.D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. [6] Z.W. Pan, Z.R. Dai, L. Xu, S.T. Lee, Z.L. Wang, J. Phys. Chem., B 105 (2001) 2507. [7] D.P. Yu, Y.J. Xing, Q.L. Hang, H.F. Yan, J. Xu, Z.H. Xi, S.Q. Feng, Physica, E, Low-Dimens. Syst. Nanostruct. 9 (2001) 305. [8] J.J.N., J. Sha, D.R. Yang, Physica, E, Low-Dimens. Syst. Nanostruct. 23 (2004) 131. [9] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [10] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Technol., B 15 (1997) 554. [11] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J.F. Wang, C.M. Lieber, Appl. Phys. Lett. 78 (2001) 2214. [12] X.Y. Zhang, L.D. Zhang, G.W. Meng, G.H. Li, N.Y.J. Phillipp, F. Phillipp, Adv. Mater. 13 (2001) 1238. [13] J. Sha, J. Niu, X.Y. Ma, J. Xu, X.B. Zhang, Q. Yang, D.R. Yang, Adv. Mater. 14 (2002) 1219.
[14] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev., B 54 (1998) R16024. [15] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Cryst. Growth 197 (1999) 136. [16] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T. Lee, Chem. Phys. Lett. 299 (1999) 237. [17] Z. Zhang, X.H. Fan, L. Xu, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 337 (2001) 18. [18] H.Y. Ping, Z.W. Pan, L. Xu, X.H. Fan, N. Wang, C.S. Lee, S.T. Lee, Adv. Mater. 13 (2001) 317. [19] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [20] H.Y. Peng, N. Wang, W.S. Shi, Y.F. Zhang, C.S. Lee, S.T. Lee, J. Appl. Phys. 89 (2001) 727. [21] F.M. Kolb, H. Hofmeister, R. Scholz, M. Zacharias, U. Go¨sele, D.D. Ma, S.T. Lee, J. Electrochem. Soc. 151 (2004) G472. [22] E.I. Givargizov, J. Cryst. Growth 20 (1973) 217. [23] Q.L. Hu, G.Q. Li, H. Suzuki, H. Araki, N. Ishikawa, W. Yang, T. Noda, J. Cryst. Growth 246 (2002) 64.
One-dimensional silicon nanostructures fabricated by thermal evaporation Jun Yu a, Jian Sha a,b, Lei Wang a, Qing Yang a, Deren Yang a,* a
State Key Laboratory of Silicon Materials, Zhejiang University, Zheda Lu 38, Hangzhou 310027, People’s Republic of China b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China Available online 18 January 2006
Abstract One-dimensional silicon nanostructures were fabricated by evaporating silicon monoxide (SiO) powders at 1050 -C and collecting the products on gold-coated silicon substrates. The uniform and dense silicon nanowires with the diameter of about 20 – 30 nm and the length of tens of microns were observed. However, the tufts of silicon nanowires were found while the silicon substrates without gold coating were used. Moreover, the chain-like silicon nano-structures with obvious core-shell morphology that displays a homojunction structure composed of cylindrical section and the oscillating section was also found. Finally, the biforked silicon nanowires with the same diameter of branches were detected, which is hard to understand on the current growth mechanism of silicon nanowires. D 2005 Published by Elsevier B.V. Keywords: Thermal evaporation; Silicon; One-dimensional
1. Introduction One-dimensional silicon nanostructures have attracted much attention in recent years for their valuable electrical and optical properties, as well as their potential applications in mesoscopic research and nanodevices. Various methods, such as photolithography [1], excimer laser ablation [2,3], thermal evaporation [4– 8], solution-grown [9], chemical vapor deposition [10 –13], etc., have been developed for the synthesis of one-dimensional silicon nanostructures. Among these methods, thermal evaporation is most suitable for industrial applications because it is of low cost and can produce high-purity and uniformly sized onedimensional nanostructures in bulk quantity. Zhang et al. obtained large-scaled silicon nanowires by simply evaporating SiO powders [4] (or a powder mixture of silicon and SiO2 [16]) without metal catalyst. Further research [14 – 18] showed that the oxide played a crucial role on the nanowire growth. Based on this fact, they proposed an oxideassisted growth (OAG) mechanism that is remarkably different from the traditional vapor –liquid – solid (VLS) mechanism [19]. Furthermore, Si nanosphere chains, another form of onedimensional silicon nanostructure, have also been synthesized [14] by thermal evaporation method. It is believed that in-situ annealing at high temperature converts silicon nanowires to * Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: [email protected] (D. Yang). 0928-4931/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.msec.2005.09.106
nanosphere chains. Based on this comprehension, Peng et al. [20] obtained large quantities of silicon nanochains by longtime annealing the as-grown silicon nanowires at high temperature (1200 –1300 -C). Very recently, Kolb et al. [21] reported VLS grown silicon nanowires by SiO evaporation with metal catalyst. In this paper, we evaporated SiO powders at relatively low temperature (1050 -C) and synthesized several types of onedimensional silicon nanostructures on gold-coated silicon substrates. Besides large quantities of silicon nanowires, nanochains composed of the continuous silicon cores and amorphous oxide shells were fabricated. Homojunction of the silicon nanowires and nanochains were also found and the apparent transition from oscillation to cylinder shape was revealed by a field-emission scanning electron microscopy (FESEM) image. Finally, detailed morphologies of the distinct biforked silicon nanowires were studied. 2. Experimental The experimental apparatus consists of a horizontal tube furnace (75 cm in length), a reacting chamber made of quartz tube (f5.5 150 cm), a rotary pump system, and a gas supply and control system. The ultimate vacuum for this configuration is ¨20 Pa. A polished n-type Si (100) wafer with a resistivity of 0.01 VIcm was cut into 1.5 cm 1.5 cm chips and then pretreated by the following treatments: (1) immersion in
J. Yu et al. / Materials Science and Engineering C 26 (2006) 800 – 804
NH3IH2O/H2O2/H2O (1:1:5 v/v) at 80 -C for 10 min; (2) immersion in HCl/H2O2/H2O (1:1:6 v/v) at 80 -C for 10 min; (3) etching in HF solution (5%) for 5 min. Finally, the chips were thoroughly rinsed with deionized water and dried in air. Au films with the thickness of about 5 nm as the catalyst were deposited on a moiety of these silicon chips by radio frequency (RF) magnetron sputtering system. These silicon chips with or without Au coating were used as substrates to collect growth products. SiO powders were placed in the center of the chamber, where the temperature was controlled by furnace. After the reacting chamber was pumped down to 20 Pa, a flow mixture of 150 sccm hydrogen and 350 sccm argon was introduced as the carrier gas to transport the SiO vapor downstream. Simultaneously, the chamber was heated to 1050 -C at a heating rate of 15 -C and held totally for 2 h in the flowing H2/Ar carrier gas. After deposition, the furnace was cooled down and the products were characterized by a field-emission scanning electron microscope (FESEM, FEI, Sirion). The possible chemical composition of the as-grown materials on the wafers was investigated by using energy-dispersive X-ray spectroscopy (EDX) attached to the FESEM. For transmission electron microscope (TEM, JEM200CX, JEOL) examination, samples were ultrasonically dispersed in alcohol for 30 min and transferred to a Cu grid coated with a holey carbon film. 3. Results and discussion Fig. 1a shows the FESEM image of the silicon nanowires synthesized on the gold-coated substrate. The uniform and dense nanowires with a length of tens of micrometers can be found on the silicon substrate. TEM image shown in Fig. 1b reveals that the silicon nanowires have smooth surface morphology and good monodispersity in diameter (about 20 –30 nm). A typical selected-area electron diffraction pattern (SEAD, upper right inset in Fig. 1b) taken from a nanowire could be labeled for (111), (220) and (311) lattice planes of silicon crystal. In our experiment, bare silicon substrates for comparison were placed abreast with the gold-coated substrates. However, silicon nanowires on the bare substrates presented apparently different morphology comparing to those
801
Fig. 2. FESEM image of the tufts of silicon nanowires on a bare silicon substrate.
on the gold-coated substrates. Fig. 2 shows the FESEM image displaying the tufts of silicon nanowires in relatively lower quantity. The substrate is partially covered and exposed to air even in some sections. This distinct difference indicates that there exist different growth mechanisms for the two types of substrates. According to the previous reports [4,14 – 18], it is readily deduced that silicon nanowires on bare substrates grow via OAG mechanism. The relatively low production might be due to the short growing time (2 h) and low evaporating temperature (1050 -C). But if neglecting the effect of Au catalyst, it will be hard to comprehend that using gold-coated substrates results to a large yield of silicon nanowires. Fig. 3a shows a high-magnified FESEM image revealing the local morphology of the silicon nanowires on a gold-coated substrate. It can be clearly seen that a silicon nanowire (pointed by a arrow) is capped with a little bigger particle (pointed by h arrow), which looks very similar to the typical metal-catalyzed nanowire morphology. Based on this observation, we suppose that there is an Au-catalyzed VLS process on gold-coated substrates. Fig. 3b and c display the EDX spectra from the wire stem and top catalyst particle, respectively. It is apparently seen that the content of Au in the catalyst particle is much higher than that in the silicon wire stem (Au in the silicon wire stem is due to the measurement needed). Considering the precision of EDX attached to FESEM, we deduce that the catalyst particle
Fig. 1. (a) FESEM image showing silicon nanowires on gold-coated silicon substrate, fabricated by evaporating SiO powders at 1050 -C. (b) TEM image revealing that the silicon nanowires have smooth surface morphology and good monodispersity in diameter, the inset shows a SAED pattern corresponding to a nanowire.
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Fig. 3. (a) A high-magnified FESEM image of the silicon nanowires grown on a gold-coated substrate. The corresponding EDX spectra from (b) the silicon nanowire stem (pointed by a arrow) and (c) catalyst particle (pointed by h arrow).
is some kind of gold silicide, which results in the onedimensional growth of VLS process. Finally, based on the above investigation, we concluded that OAG and Au catalyzed VLS growth mechanisms coexist for the silicon nanowires grown on the gold-coated substrate. Because Au –Si eutectic temperature is lower, Au – Si alloy droplets as nucleation sites could cover the whole substrates. Therefore, VLS process is relatively more dominant than the OAG process under our experimental conditions. Besides the nanowires, small quantity of chain-like silicon nanostructures grown on the gold-coated substrates was also found. As illustrated in Fig. 4a, the fluctuations of the diameter are observed in the part of a silicon nanowire. The smooth cylindrical section (pointed by h arrow) and the oscillating one (pointed by an arrow) interconnect and form a homojunction nanostructure. Fig. 4b shows a high-magnified FESEM image revealing the detailed morphology of the transition from oscillation to smooth. Previous reports [14,20] considered that silicon nanochains originated from nanowires annealed for a long time at high temperatures. However, neither the growth time nor the temperatures could be reached in our experiment. Therefore, the homojunction structure can not be explained by the mechanism mentioned above. Considering the metal catalyst involved, we suggest that the chain-like structure is
also derived from VLS process. As is well known, the diameter of nanowires via VLS mechanism is determined by the diameter of liquid catalyst droplets. If the diameter of droplets changes periodically during growth, chain-like nanostructures will be produced. In fact, that the diameter of droplets changes periodically has been suggested on the formation of silicon whiskers [22]. Details of a chain-like silicon nanostructure are also characterized by a TEM image and a SAED pattern (Fig. 5a). A typical core-shell morphology including the dark core and the relative lighter sheath is illustrated in the TEM image. Different from the nanochains reported in Ref. [20], which are consisted of separate Si nanospheres connected by amorphous Si-oxide bars, the chain-like structures obtained in our experiment have a continuous silicon structure along the whole length. This difference confirms that the nanochains in our experiments originated from the droplet of self-oscillation, not due to the annealing process. The SAED pattern showed in the upper right inset of Fig. 5a reveals the monocrystalline phase nature of the silicon core. We also note that the oscillation amplitude of the nanochains is mainly shared by the variety of Si-oxide shell. The continuous crystallized silicon core has only a little fluctuation. This morphology is very interesting to be further researched, and we speculate that it is due to the superfluous silicon oxide generated by the disproportionation
Fig. 4. FESEM images of chain-like silicon nanostructures: (a) low-magnified morphology of a homojunction composed of the oscillating section (pointed by a arrow) and cylindrical section (pointed by h arrow); (b) high-magnified morphology showing the transition from oscillation to smooth.
J. Yu et al. / Materials Science and Engineering C 26 (2006) 800 – 804
803
Fig. 5. TEM images of different silicon nanostructures: (a) chain-like structures, the inset shows a corresponding SAED pattern revealing the monocrystalline phase nature of the silicon core; (b, c) distinct biforked silicon nanowires (the arrows point the crotch).
of SiO vapor. Furthermore, a distinct biforked silicon nanostructure is displayed in Fig. 5b and c. It is clearly seen that the nanowire developed several branches at different section and the branches can also have branches, which is similar to those named secondary growth. Hu et al. [23] reported a VLS secondary growth case in which thin nanowires sub-grew on the surface of thick nanowires. But the diameter of those thin nanowires is almost one tenth that of thick nanowires. Pan et al. [6] fabricated an octopus-like structure by evaporating the SiO powders at 1350 -C and collecting the products at 1250 –1320 -C. Two to five arms are excluded from a large crystalline Si sphere (1 –2 Am), and the authors proposed that Si spheres as catalyst that is similar to metal particles in traditional VLS growth. Comparing to the above two branched nanostructures, the biforked nanowires have no obvious particles on the crotches (pointed by arrows in Fig. 5b and c) and all the branches showed in the TEM images are of almost the same diameter. Moreover, we can find that there is no apparent difference between the trunks and the branch. It is much complicated to understand its growth mechanism, which might suggest an unknown process to be further discovered.
gold-coated silicon substrates, bare substrates at the same place were partially covered and produced a small quantity of silicon nanowires. This phenomenon indicates that there is a coexistence of VLS and OAG process on gold-coated substrates. Furthermore, silicon nanochains having a continuous silicon core and silicon oxide shell were observed. The obvious transition from oscillation to smooth in the silicon nanochains is believed due to the instability of VLS growth process. Finally, the biforked silicon nanowires with all the branches of almost the same diameter were found, which is difficult to be understood on the base of OAG and VLS mechanism. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 50272057 and 60225010) and the key project of Education Ministry of China. The authors would like to thank Mr. Youwen Wang, at the Analysis and Measurement Center of Zhejiang University, for his great help. References
4. Conclusion Employing SiO powders as precursors and the mixture of Ar/H2 as carrier gas, one-dimensional silicon nanostructures with different morphologies were fabricated. Comparing the
[1] H.I. Liu, N.I. Maluf, R.F.W. Pease, J. Vac. Sci. Technol., B 10 (1992) 2846. [2] A.M. Morles, C.M. Lieber, Science 279 (1998) 208. [3] 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.
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