Rational growth of highly oriented amorphous silicon nanowire films

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Rational growth of highly oriented amorphous silicon nanowire films Xihong Chen 1 , Yingjie Xing 1 , Jun Xu, Jie Xiang, Dapeng Yu * Department of Physics, School of Physics, State Key Laboratory for Mesoscopic Physics, Electron Microscopy Laboratory, Peking University, Room 211, Building N, Beijing 100871, China Received 10 April 2003; in final form 7 May 2003 Abstract Amorphous silicon nanowire films were rationally synthesized using a simple approach. The films consist of highly oriented nanowires of 30 lm in length and 20–80 nm in diameter. The morphology, microstructure features, and chemical composition of the nanowires were analyzed using electron microscopy and Raman spectroscopy. A novel model concerning solid–liquid–solid phases was proposed to explain the growth mechanism of the nanowires. This approach should be very useful to direct the controlled growth of nanomaterials. Ó 2003 Published by Elsevier Science B.V. 1. Introduction One-dimensional nanomaterials have been a focused research field since the first pioneering work of the discovery of carbon nanotubes [1] and nanowires [2–6]. A diverse variety of semiconduc- tor nanowires, such as silicon, GaAs, GaN, and ZnO nanowires, were synthesized using different approaches. Of those nanowire materials, silicon nanowires (SiNWs) have great scientific and technological importance, and have attracted much research interest [7,8]. For example, the sil- icon nanowires have been used as the building blocks to build nano-scale logic and computa- tional circuits [9], nanodiodes, and random access memory [10]. Controlled growth of the nanowires in their morphology, orientation, for example, is the key to success both for characterization of physical properties and exploration of device ap- plication of the nanowires; however, it is extremely difficult to realize. In this Letter, we will report the rational synthesis of highly oriented amorphous SiNW films on centimetric substrates. The growth mechanism was explained under a novel frame- work of the solid–liquid–solid (SLS) mechanism. 2. Experimental A conventional hot-filament CVD system was modified to grow the silicon nanowires. A thin layer of nickel 5 nm in thickness was deposited by thermal evaporation on a 5 mm  5 mm p-type Si Chemical Physics Letters 374 (2003) 626–630 www.elsevier.com/locate/cplett * Corresponding author. Fax: +8601062759474. E-mail address: yudp@pku.edu.cn (D. Yu). 1 Authors contribute equally to the work. 0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00781-4 (1 1 1) wafer. Such a substrate was heated to about 900 °C for 10 min in the sample stage of the HF- CVD system, while a H 2 gas flow was introduced into the chamber at 20 sccm during the growth to keep an ambient pressure about 1.5 kPa. The original shiny surface of the substrate became gray after cooling down to room temperature. The morphology of the as-grown product was analyzed using an scanning electron microscope (SEM, DB235 FIB,FEI). A Hitachi-9000 NAR high-res- olution transmission electron microscope (TEM) equipped with nano-beam energy dispersive spec- troscopy (EDS) was used to characterize the mi- crostructure and chemical composition of the nanowires. Raman spectrum was measured using a Renishaw 2000 system with a laser source of 514.5 nm. 3. Results and discussions In a brief view of SEM analysis, the whole substrate was found to be covered with a thick layer of wool-like product, as is shown in Fig. 1. The wool-like layer with homogeneous thickness can be easily scratched from the substrate, which is marked with arrow in the SEM image. In the magnified SEM image in the left inset, one piece of the wool-like carpet was scratched from the sub- strate and was folded on top of the film. The right inset shows the details of the edge of the layer, and reveals that the film consists of fine free-standing wires of very high density, and has a thickness of about 30 lm (also the length of the nanowires). The growth rate of nanowires is faster than 100 nm/s. The catalyst nanoparticles were found at the bottom of the a-SiNWs, which is marked with arrow in the right inset, revealing a base growth of the nanowires. Whether a base or a top growth depend on the wetting condition between the cat- alyst droplets and the substrate. If the wetting is very good, the interaction force between the cat- alyst droplets and the substrate can be very strong and the nanoparticles will stay at the substrate as a base growth; otherwise if the wetting is bad, a top growth will work. In the present case, the wetting between Si and Si 2 Ni should be good because the silicon has a very large solubility in Si 2 Ni, so the catalyst droplets should stay at the bottom of the nanowires. We repeated the procedure several times under a similar growth conditions and found that the wool-like films were completely repro- ducible. This demonstrates that our method is controllable and easy to scale up.The SEM images in Fig. 2 reveal that the nanowires are highly ori- ented perpendicular to the substrate. As is seen from the cross-sectional view along the edge of the scratched film in Fig. 2a, the orientation of the nanowires is widespread over the whole substrate. Detailed SEM view in Fig. 2b shows that the nanowires appear straight and parallel to each other. Part of the product was scratched off and used to measure the Raman spectrum in a micro-beam mode from different places of the sample. Two peaks around 300 and 516 cm À1 were observed, as is shown in Fig. 3a. It is well known that those two Raman peaks are characteristic of a silicon struc- ture, corresponding to the second-order transverse acoustic phonon mode (2TA), and the first-order transverse optical phonon mode (TO) of silicon, respectively. The Raman result confirmed that the nanowire film is composed of silicon. The possi- bility of the formation amorphous silicon oxide Fig. 1. SEM image revealing a wool-like film on large area. The SEM image in the left inset reveals that one sheet of the film was scratched off the substrate. The film has a thickness about 30 micrometers, and consists of pure nanowires, as shown in the right inset. The bright contrast indicated by an arrow shows the catalyst layer between the substract and nanowire film, which provides evidence for a base growth. X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630 627 nanowires can be excluded by the following dis- cussions. Because the growth was conducted in a steel CVD chamber, it guarantees a very good vacuum status, and the hydrogen gas keeps a reduction atmosphere. On the other hand, the substrate we used is the commercial microelec- tronic wafers having very thin native oxide layer (usually <1 nm), which is not thick enough to grow oxide nanowire layer as thick as 30 lm. The microstructure and chemical composition of the nanowires were also analyzed using TEM. Fig. 3b shows a TEM image of the silicon nano- wires. The diameter of the nanowires ranges from 20 to 80 nm. Selected-area electron diffraction (SAED) reveals that the nanowires are amorphous. The inset in the left shows the corresponding EDS spectrum, which reveals that the nanowires are mainly composed of silicon. The remaining oxygen peak comes from the surface oxidation of the nanowires. Nanosized particles larger than the nanowire diameter were found attached to the end of the nanowires, which is marked with an arrow in the right inset. EDS analysis shows that the nanoparticles were composed of both silicon and nickel. Those nanoparticles can provide evidence for the growth mechanism of the nanowires. It was found that the growth mechanism of the amorphous silicon nanowires (a-SiNWs) is differ- ent from the conventional vapor–liquid–solid (VLS) model [11,12]. The growth circumstances in the present case are completely different from that in laser ablation [2,3], or in the CVD method [8] in which the silicon source is supplied directly from the vapor phase. When the vapor phase plays an important role, the growth of the SiNWs is mostly controlled by the well-known VLS mechanism. In the present circumstance, however, no Si vapor phase is introduced into the system as in CVD growth of SiNWs. Though the eutectic point of Fig. 2. SEM images showing the high orientation of the nanowire films. (a) Edge of the film showing a wide-spread orientation of the nanowires. (b) Details of the highly oriented nanowire film. Fig. 3. (a) Raman spectrum of the nanowire film scratched off from the substrate. Two peaks at 300 and 516 cm À1 , were ob- served which correspond to 2TA, and TO modes of silicon, respectively. (b) TEM image showing the morphology of the silicon nanowires. The inset in the left shows the EDS spectrum and Si and O are visible, where oxygen comes from the surface oxidation of the nanowires. The inset in the right shows a Si–Ni nanoparticle capped at the end of the nanowire. 628 X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630 Si 2 Ni is 993 °C, the small-size-melting effect makes it possible for the deposited Ni nanoparticles to react with the Si substrate at temperature above 900 °C, forming Si 2 Ni eutectic liquid droplets. So the source materials comes from directly the sub- strate instead from the vapor phase in the present case. Therefore, we proposed a novel model to explain the growth of the a-SiNWs, which involves the SLS phases in the growth process. In the SLS growth, the catalyst Ni dissolves directly the sili- con substrate to form Si 2 Ni eutectic liquid phase in the following reaction: 2Si ðsÞþNi ðsÞ!NiSi 2 ðlÞð1Þ where s represents the solid phase and l the liquid phase. Because the silicon has a large solubility in the Si 2 Ni liquid phase, more Si atoms will be dissolved continuously into the liquid droplets. When the liquid phase becomes supersaturated, the a-SiNWs will grow out off the liquid droplets, which can be represented as follows: Si ðsÞþNiSi 2 ðlÞ!Si super Ni ðlÞ ! Si 2 Ni ðlÞþSi w ðsÞð2Þ (super represents silicon supersaturation and w represents the finally solidified SiNWs). Since the silicon substrate was placed face up directly on the hot filament to be heated from the back, so the temperature gradient in the substrate must be considerable. The temperature gradient should be the driving force for the silicon substrate to be dissolved to form low temperature Ni–Si eutectic liquid phase, forming silicon nanowires at the cooler side. The SLS mechanism is in some extent an anal- ogy to the VLS model. In the SLS controlled growth of nanowires, the eutectic Ni–Si liquid droplets have to stay at the surface of the silicon substrate in order to grow continuously, and the solidified nanoparticles shall remain at the bottom of the nanowires, as is shown in the right inset in the SEM image in Fig. 1. In this image, there exists a bright-contrasted layer between the substrate and the nanowires film (marked with an arrow), and EDS analysis proved that this bright-con- trasted layer consists of Si and Ni. Two more questions shall be addressed here. First, it is not well understood why the final nanowires are in amorphous state instead of a crystalline phase. The most possible explanation is the unusual high growth rate. The estimated growth rate is about 100 nm/s. Such a high growth rate may explain why the resulting nanowires are amorphous instead of crystalline, because the growth is so rapid that the atoms have no time to stack themselves into crystalline order. Second, we think that the crowding effect between the very dense nanowires plays an important role to keep the nanowires staying oriented. From the SEM images in the previous sections, it is also evident that the density of the a-SiNWs is very high, so the Van der Waals force between nanowires should be large. This interaction force is another important factor to keep the nanowires grow upward and to be oriented towards each other. The a-SiNWs grown on the substrate have re- markable surface/volume ratio, and can show physical/chemical properties completely different from the bulk. In fact, it was recently revealed that the lithium battery using SiNWs as electrode ma- terials showed a capacity as high as eight times that of the ordinary one [13]. Therefore, the a-SiNWs may have potential applications such as rechargeable battery of high capacity with por- table size. The a-SiNWs can also be useful in chemistry, biology, electronics and other fields on consideration of their huge specific surface. Fur- thermore, it is believed that the present rational synthesis method can be used to direct the con- trolled growth of other nanowire structures. 4. Conclusion Highly oriented amorphous silicon nanowire films have been synthesized rationally on large- area silicon substrate via heat treatment of the nickel-coated silicon substrate. The morphology, microstructure, and chemical composition of the nanowires were characterized using electron microscopy and Raman spectroscopy. The growth of the a-SiNWs cannot be explained by the con- ventional VLS mechanism, and a novel SLS model was proposed to explain reasonably the growth of X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630 629 the amorphous silicon nanowires. The as-grown a-SiNWs and the proposed growth model should be useful to direct the controllable growth of other nanowire structures. Acknowledgements This project was financially supported by na- tional Natural Science Foundation of China (NSFC, No. 50025206, 20151002), 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, G.W. Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, Solid State Commun. 105 (1998) 403. [3] M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] X.F. Duan, J.F. Wang, C.M. Lieber, Appl. Phys. Lett. 76 (2000) 1116. [5] W.Q. Han, S.S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287. [6] Y.C. Kong, D.P. Yu, B. 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Rational growth of highly oriented amorphous silicon nanowire films Xihong Chen 1 , Yingjie Xing 1 , Jun Xu, Jie Xiang, Dapeng Yu * Department of Physics,. 2003 Abstract Amorphous silicon nanowire films were rationally synthesized using a simple approach. The films consist of highly oriented nanowires of 30 lm in