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Physica E 9 (2001) 305–309 www.elsevier.nl/locate/physe Controlled growth of oriented amorphous silicon nanowires via a solid–liquid–solid (SLS) mechanism D.P. Yu a; ∗ , Y.J. Xing a; b , Q.L. Hang a , H.F. Yan a ,J.Xu a , Z.H. Xi b , S.Q. Feng a a Department of Physics, Electron Microscopy Laboratory and Mesoscopic Physics National Laboratory, Peking University, Beijing 100871, China b Department of Electronics, Peking University, Beijing, China Received 19 May 2000; received in revised form 19 July 2000; accepted 24 July 2000 Abstract Highly oriented amorphous silicon nanowires (a-SiNWs ) were grown on Si (1 1 1). The length and diameter of oriented SiNWs are almost uniform, which are 1 m and 25 nm, respectively. Dierent from the well-known vapor–liquid–solid (VLS) for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid–liquid–solid mechanism (SLS). This synthesis method is simple and controllable. It may be useful in large-scale synthesis of various nanowires. ? 2001 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 68.65.+g; 73.20.Dx; 78.55.−m Keywords: Si nanowires; Quantum conÿnement eect; Low dimensionality 1. Introduction Intensive research eorts on one-dimensional nano- materials have been carried out since the ÿrst pio- neering work of the discovery of carbon nanotubes by Iijima [1] and other quasi-one-dimensional nanos- tructures such as silicon nanowires (SiNWs) [2,3]. Because bulk Si is an indirect gap material, the SiNWs are useful one-dimensional nanostructure for tailoring its physical properties from indirect to direct band gap. The synthesis of Si whiskers via the vapor–liquid– solid (VLS) growth mechanism was ÿrst described in ∗ Corresponding author. Fax: +86-10-6275-1615. E-mail address: yudp@pku.edu.cn (D.P. Yu). detail by Wagner and co-workers [4,5]. Givargizov developed the growth model and discussed it within a kinetics framework [6]. Yazawa [7] and Westwater [8], and Ozaki [9] produced SiNWs with VLS growth induced by Au metal layer on a Si surface. Recently, Yu et al. reported oven-laser ablation method through simple physical evaporation approach [2,10], to pro- duce very pure ultraÿne freestanding SiNWs. In the previous work for SiNWs synthesis, however, vapor phase with considerable Si concentration was either supplied from laser ablation of a powder target, or directly from silane. In this paper, we report that oriented a-SiNWs can be controllably grown on a silicon substrate via a solid–liquid–solid (SLS) growth mechanism. 1386-9477/01/$ - see front matter ? 2001 Elsevier Science B.V. All rights reserved. PII: S 1386-9477(00)00202-2 306 D.P. Yu et al. / Physica E 9 (2001) 305–309 2. Experimentals Heavily doped (1:5 × 10 −2 =cm) n-type Si (1 1 1) chips wafers were used as substrate. The silicon subs- trate was cleaned ultrasonically in pure petroleum ether and in ethanol in turns for 5 min, and leached in distilled water, then dried. A thin layer of 40 nm nickel was thermally deposited on the substrate. The substrate was placed in a quartz tube which was heated in a tube furnace at 950 ◦ C. Ar (36 sccm) and H 2 (4 sccm) were introduced during growth at an ambient pressure of about 200 Torr. After cooling down to room temperature, a thin layer of gray-colored deposit was found on the surface of the substrate. An Am- ray FEG-1910 scanning electron microscope (SEM), and a Hitachi-9000NAR high-resolution transmis- sion electron microscope (HREM) equipped with energy-dispersive spectrum (EDS) were employed for analysis of the morphology and microstructure of the product. 3. Results and discussions Fig. 1(a) shows plan view of SEM image revealing the general morphology of the SiNWs grown on a large area (10 mm × 10 mm) of 111 Si substrate. The nanowires grew directly on the substrate. It is visible that the deposit consists of pure SiNWs. The growth rate of the nanowires is estimated to be about 30 nm=s. EDS analysis (inset) proved that the nanowires are composed of Si, but there exists a small amount of oxygen in the SiNWs, which was attributed to the surface oxidation sheathing the nanowires. The TEM image shown in Fig. 1(b) reveals that the SiNWs have diameter between 10 and 50 nm, and length up to a few tens micrometers. The highly diusive ring pattern (inset) of selected area electron diraction (SAED) revealed that the SiNWs are completely amorphous (a-SiNWs). To control the orientation of the a-SiNWs, a three-step heating procedure was involved in the growth of the oriented a-SiNWs. Firstly, the system was heated to 800 ◦ C, and a mixture of H 2 (36 sccm)= Ar (4 sccm) was introduced to the tube. The tem- perature was then raised from 800 ◦ Cto950 ◦ C and pure Ar (100 sccm) was used as a carrier gas in this step. The ambient pressure of the tube was Fig. 1. (a) SEM micrograph showing the general morphology of the SiNWs grown via a SLS growth mechanism. The inset shows EDS spectrum with the peak corresponding to Si, (b) TEM image revealing that the SiNWs have smooth morphology and average diameter around 40 nm. The SAED pattern shown in inset reveals characteristic diusive ring pattern, showing that the nanowires are completely amorphous. kept near 750 Torr by adjusting the exit valve, then the tube was evacuated. This procedure was repeated three times. Finally, the temperature was held at 950 ◦ C at the pressure of about 200 Torr for 1 hr. A mixture of H 2 (4 sccm) and Ar (36 sccm) was introduced to the tube. A low-magniÿed SEM image of the oriented a-SiNWs is shown in Fig. 2(a), representing a gen- eral planar view of the oriented a-SiNWs. It is visible that the nanowires were grown on centimeter-sized substrate. An interesting phenomenon is that the nanowire ÿlm was found chapped in a network of white-contrasted lines. The inclined view at a crossover point of the white in Fig. 2(b) revealed the white lines are in fact V-shaped chaps. From this im- age it is visible that the ÿlm consists of pure SiNWs. D.P. Yu et al. / Physica E 9 (2001) 305–309 307 Fig. 2. (a) SEM image of oriented SiNWs on the substrate (top view, low magniÿcation), regions except white lines are composed of SiNWs perpendicular to the substrate. The length of SiNWs is about 1 m, (b) SEM image of a cross part of the grooves. SiNWs on the edge of grooves fall on to the substrate, (c) TEM image of SiNWs with average diameter around 25 nm, which were scrapped from the substrate. The SAED pattern shown in inset reveals characteristic ring pattern, showing that the nanowires are completely amorphous (a-SiNWs). Parts of a-SiNWs on the edge of chaps fall on the substrate. Fig. 2(c) is the TEM image of ultraÿne SiNW, which were scratched from the substrate. It shows that the diameter of a-SiNWs is about 25 nm. The highly diusive ring pattern (inset) in select area electron diraction (SAED) revealed that SiNWs are completely amorphous. Though the reason why the resultant nanowires are amorphous instead of being crystalline is not yet very clear, the authors think that amorphous state was closely related to the fast growth speed in the present condition. It was found that the growth of the amorphous SiNWs here is dierent from the VLS mechanism for conventional whiskers [4–6], revealing a dier- ent growth mechanism. In the case of oven-laser abla- tion approach [1,2], silicon source for SiNWs growth was supplied from the vapor phase in which atomic Si species were ablated o by the laser beam, and the growth of the SiNWs is controlled by the well-known VLS mechanism. The central idea of the VLS growth of SiNWs is that, the catalysts (usually Ni, or Fe as impurity) act as a liquid-forming agent, which reacts with the vapor phase, and forms the NiSi 2 eutectic liq- uid droplets. The vapor phase is rich in Si atoms. With the further absorption of Si atoms into the droplets from the vapor phase, the droplets become supersat- urated, resulting in the precipitation of SiNWs from the droplets. In the present circumstance, however, the Si con- centration in the vapor phase is negligible at the growth temperature, because the speciÿc surface= volume ratio of bulk Si substrate is extremely low. On the other hand, the Si substrate was covered by a thin layer of Ni. Therefore, the only possible silicon source comes from the bulk silicon substrate, because no extra Si source was introduced in the vapor phase. From the binary Ni–Si diagram, it is visible that the eutectic point of Si 2 Ni is 993 ◦ C. However, due to the melting eect of small-size grains, the eutectic compound NiSi 2 can begin to form at a temperature lower than 993 ◦ C. As we proved, the deposited Ni ÿlm can react with the Si substrate at temperature above 930 ◦ C, and forms Si 2 Ni eutectic liquid alloy droplets. Because of the relatively high solubility of Si in Si 2 Ni eutectic alloy, more Si atoms will diuse through the solid (the substrate) –liquid inter- face into the liquid-phase (the Ni Si 2 droplets). A second liquid–solid (nanowire) interface will form when the liquid phase becomes supersaturated due to thermal or compositional uctuations, resulting in the growth of SiNWs. Because this growth process involves solid–liquid–solid phases, it is named as a SLS growth, which is in fact an analogy of the VLS mechanism. The growth process of the a-SiNWs via an SLS model is depicted schematically in Fig. 3. Cross-sectional SEM analysis of the sample pro- vided direct evidence to support the a-SiNW growth 308 D.P. Yu et al. / Physica E 9 (2001) 305–309 Fig. 3. Schematic depiction of the SiNW growth via the SLS mechanism: (a) deposition of a thin layer of Ni on the Si (111) substrate; (b) formation of the Si–Ni eutectic liquid droplets; (c) the continuous diusion of Si atoms through the substrate–liquid (S–L) interface; (d) ÿnal state of the SiNW growth. The smooth surface of the original substrate becomes rough at the end of the SiNW growth. via a SLS mechanism. Fig. 4(a) shows an SEM image of oriented a-SiNWs grown on the substrate (cross-sectional view). The a-SiNWs grew densely and are all perpendicular to the substrate. The length of the SiNWs is about 1 m. It is visible that be- tween the SiNWs and the substrate there is a layer of nano-sized particles which proved to consist of Ni and Si. Fig. 4(b) shows a low-magniÿed cross-sectional SEM image. It is visible that a layer of oriented a-SiNWs was grown on the substrate. EDS analysis between the Si substrate and the a-SiNW layer further conÿrmed that there is a thin layer of Si–Ni alloy, which is indicated with a white arrow. We found that the a-SiNWs grow from base, which manifests itself by the fact that the solidiÿed Si–Ni nano particles were visible between the surface of the substrate and the a-SiNW ÿlm, instead of being attached at the free tip of the SiNWs. The SiNWs are interesting to evaluate the quantum conÿnement eect related to materials of low dimen- sionality [10,11]. The a-SiNWs grown on substrate have remarkable surface= volume ratio, possibly show- ing physical–chemical properties completely dierent from the bulk. From this point of view, it is speculated that the a-SiNWs may have potential applications such as rechargeable battery of high capacity with portable size, which is closely related to the surface eects. In fact, it was recently revealed that the lithium battery using SiNWs as electrode materials showed a capacity as high as 8 times than that of the ordinary one [12]. Fig. 4. (a) Low and (b) magniÿed cross-sectional SEM images of the SiNWs grown on Si (111) substrate, which is controlled by a SLS mechanism. The length of the SiNWs is about 1 m. The Si–Ni particles are visible attached to the Si substrate surface. D.P. Yu et al. / Physica E 9 (2001) 305–309 309 By optimizing dopants, it is believed that the a-SiNW ÿlm thus prepared will ÿnd applications in future nanotechnology. 4. Summary remarks In summary, oriented silicon nanowires have been grown using Si substrate as Si source via a solid– liquid–solid mechanism by heating. They have uni- form length and diameter. The growth is explained by solid–liquid–solid model. This synthesis method of oriented SiNWs is simple and controllable. It can also be used to synthesize other nanowires. Acknowledgements This project was ÿnancially supported by national Natural Science Foundation of China (NSFC), and by the Research Fund for the Doctoral Program of higher Education (RFDP), China. References [1] S. Iijima, Nature 354 (1991) 56. [2] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou, Z .G. Bai, Z. Zhang, S.Q. Feng, Solid State Commun. 105 (1998) 403. [3] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, H.T. Zhou, G.C. Xiong, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [4] R.S. Wager, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [5] R.S. Wager, W.C. Ellis, Trans. Metall. Soc. AIME 233 (1965) 1053. [6] E.I. Givargizov, J. Crystal Growth 31 (1975) 20. [7] M. Yazawa, M. Koguchi, A. Muto, M. Ozawa, K. Hiruma, Appl. Phys. Lett. 61 (1992) 2051. [8] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Technol. B 15 (1997) 554. [9] N. Ozaki, Y. Ohno, S. Takeda, Appl. Phys. Lett. 73 (1998) 3700. [10] D.P. Yu, Z.G. Bai, J.J. Wang, Y.H. Zou, W. Qian, J.S. Fu, H.Z. Zhang, Y. Ding, G.C. Xiong, S.Q. Feng, Phys. Rev. B 59 (1999) 2498. [11] J. Hu, M. Ouyang, P. Yang, C.M. Lieber, Nature 399 (1999) 48. [12] G.W. Zhou, H. Li, H.P. Sun, D.P. Yu, Y.Q. Wang, L.Q. Chen, Ze Zhang, Appl. Phys. Lett. 75 (1999) 2447. . Physica E 9 (2001) 305–309 www.elsevier.nl/locate/physe Controlled growth of oriented amorphous silicon nanowires via a solid–liquid–solid (SLS) mechanism D.P are completely amorphous. Though the reason why the resultant nanowires are amorphous instead of being crystalline is not yet very clear, the authors think that amorphous