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Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
A study in the growth mechanism of silicon nanowires with or without metal catalyst Jun-Jie Niu ⁎ , Jian-Nong Wang School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 11 April 2007; accepted 22 June 2007 Available online 28 June 2007 Abstract The growth mechanism of silicon nanowires synthesized with or without a metal catalyst via chemical-vapor-deposition (CVD) is discussed by using a developed vapor–liquid–solid and novel sulfide-assisted growth models, respectively. The metal catalyst plays an important role on the catalytic growth. However, the growth of silicon nanowires with sulfide is chiefly affected by the compound decomposition, gas stream, and temperature difference. Silicon nanowires fabricated with metal can be self-organized while a large scale of samples can be achieved with metal- free catalyst. The growth mechanism comparison between metal- and non-metal assisted methods for synthesizing silicon nanowires will supply a beneficial help in deepening the understanding of crystal procedure and improving the sample quality. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Crystal growth 1. Introduction Silicon nanowires (SiNWs), as a candidate material for nano- electronic devices, are being intensively studied [1,2]. This is because of the feasibility of integrating SiNWs as functional building blocks into the existing CMOS technology [3]. Specifically, SiNWs can be applied in the fields including ultra sensitive bio-sensor, field effect transistors (FETs), and single - electron detector [4]. The fabrication of SiNWs involves metal-assisted or metal- free growth. The metal-assisted growth mainly follow s the vapor–liquid–solid (VLS) mechanism, which was first de- scribed by Wagner and Ellis in 1964 and developed by Givargizov in 1975 [5,6]. The metal catalyst used was usually Au although other metals were also used [7,8]. The synthesis methods mainly include laser ablation [9], chemical-vapor- deposition (CVD) [10,11], thermal evaporation [12,13], and growth from organic solution [14]. Amongst these methods, laser ablation is widely adopted. However, this method does not allow the local ized growth on a patterned substrate for further processing [8]. Recently, a solid–liquid–solid (SLS) process derived from the VLS mode was also reported for obtaining SiNWs [15]. The growth based on an oxide, which was proposed by Lee and coworkers [16], is a typical sample of synthesis of SiNWs without a metal catalyst [17]. In this case, the SiO x vapor decomposes into Si atoms, which act as the nuclear of SiNWs covered by shells of silicon oxide. Metal-assisted growth by CVD technique is a simple and efficient route for synthesizing controllable and even self- organized SiNWs at a low temperature [11,18]. However, the metal-free growth based on sulfide is believed to have a potential for large-scale production. The growth mechanisms, however, are still poorly understood under both circumstances. This study thus attempts to improve the understanding on the basis of new experimental observations. 2. Experimental A CVD system was used for the fabrication of SiNWs (Fig. 1). For metal-assisted growth, the transition metal such as Au or Ni was used as a catalyst. The substrate was put in a quartz tube furnace which was then pumped down to ∼ 20 Pa and heated to 600–900 °C. The mixed gases of argon, hydrogen, and silane with a desired flow rati o were flowed. For sulfide-assisted growth, silicon wafer was used both as the silicon source and A vailable online at www.sciencedirect.com Materials Letters 62 (2008) 767 – 771 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 21 62932050. E-mail address: jjniu@sjtu.edu.cn (J J. Niu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.06.056 sample collector. The ZnS or S powder was put in one side and would be flowed to the wafer. The reaction temperature was set at about 1100 °C. The samples were analyzed by scanning electron microscopy (SEM, JSM-5610LV) and transmission electron microscopy (TEM, JEM2100F). 3. Results and discussion 3.1. Metal-assisted growth of SiNWs A basic aspect of the VLS mechanism is the metal particle acting as a catalyst for the anisotropic growth of SiNW with a crystalline structure. A catalyst particle provides a site for absorption of vapor- phase silicon atoms. The continuous absorption induces the supersat- uration of the formed liquid alloy with silicon atoms, which leads to nucleation and growth of a SiNW [19]. Therefore, the diameter and location of the SiNW are determined by the features of the catalyst particle [20]. Thus, it is important to control the catalyst size distribution and the delicate catalyst positioning. The metal catalyst can be generated by thermal evaporation, sputtering, or electrochemical methods. The particle size can be modified by varying the reaction parameters. In particular, the catalyst distribution can be well-organized by using a nano-channel-alumina (NCA) technique [11]. Otherwise, the pressure of reaction channel and temperature are also important for the growth of SiNWs. In the present CVD system (Fig. 1), the pressure can be adjusted by a gas-controlled valve. Normally, a high pressure can lead to a high yield of the sample. If a very low pressure was used, plenty of Si atoms will be flowed away and have no time to contact Fig. 1. Diagrammatic sketch of the CVD system. Fig. 2. A) Binary phase diagram of Au and Si. B) A mode of VLS mechanism. 768 J J. Niu, J N. Wang / Materials Letters 62 (2008) 767–771 with the catalyst. This results in formation of only a few of SiNWs. The reaction temperature is usually decided with the consideration of the catalyst type. According to the binary phase diagram of a metal and Si, the temperature for SiNW formation must be above the eutectic point as liquid droplets can form under this condition. Here we give an example of using Au as the catalyst. As seen from Fig. 2A, if the reaction temperature is increased to be above the eutectic point of 363 °C (such as T1), a liquid alloy can form and will assist the growth. Considering the production output, a temperature of 620 °C was used for the case of Au and 900 °C for the case of Ni to fabricate SiNWs [11,21]. In general, the chemical reaction equations can be written as the following: SiH 4 ¼ Si þ 2H 2 ð1Þ Si þ O x ¼ SiO x ðxb2Þð2Þ (3) Si is proffered by the decomposition of the precursor of SiH 4 (Eq. (1)). When dropping Si atoms come to contact with the metal particle located on the substrate (Fig. 2B a), a liquid metal–Si alloy will be formed. With more and more Si atoms added, the Si content in the droplet will reach a saturated value (in the Au–Si system, this value is ∼ 25% at a point in Fig. 2A). If the supply of Si atoms is continued, the liquid alloy will be supersaturated with Si atoms and excessive Si atoms will then precipitate. The precipitated atoms will grow freely with a crystalline structure (Fig. 2B b). Since the orientation of b111N in Si lattice has the lowest energy, this orientation dominates the growth direction and the final extending direction of SiNW (Fig. 3A). In particular, when the nanowire becomes longer, the droplet at the tip will be pushed randomly and thus sometimes two or more will combine to form a bigger one, leading to an intercrossing structure. Fig. 3B clearly shows the original growth stage of SiNWs. As can be seen from the figure, if Si atoms cannot be added continuously into the droplet, the growth will be terminated and a short SiNW is obtained. On the con- trary, if enough Si atoms are provided, a long SiNW with random direction will be observed (Fig. 3C). In this case, an excellent crystalline nature of Si (111) is observed as shown in Fig. 3A. In addition, the as- formed SiNW is easily oxidized with an oxide shell of 1–3nm according to the Eq. (2) (Fig. 3A). 3.2. Sulfide-assisted growth of SiNWs If the growth with a metal catalyst can cause contamination, the synthesis without metal will be useful for obtaining clean samples with high quality. The oxide-assisted growth is an effective approach for large-scale production of high-quality SiNWs without metal [16,17]. During the reaction, SiO decomposes into Si and SiO 2 . The pre- cipitated Si atoms under the assistance of outer SiO 2 shell will form a nuclear and thus grow up to a wire. It is clear that silicon oxide plays a key role on the formation of SiNW. Similar with this mode, we have Fig. 3. High resolution TEM image (A), SEM image of original growth stage (B), and final stage (C) of SiNWs synthesized via VLS mechanism. 769J J. Niu, J N. Wang / Materials Letters 62 (2008) 767–771 developed a novel sulfide-assisted mechanism to synthesize SiNWs in large quantity [22]. Relative to SiO decomposition, compound SiS also plays a significant role on the growth of SiNW. Furthermore, the gas flow and temperature difference have simultaneously important effects. We now analyze the detailed growth process in the following sections. Firstly, the whole reaction equations can be summarized to: 2ZnS þ O 2 ¼ 2ZnO þ 2S↑ðN∼200-CÞ; ð4Þ S þ Si ¼ SiSðN∼900 -C Þ ; ð5Þ 2SiS↑ ¼ Si þ SiS 2 ð∼950-C–1080-CÞ; ð6Þ and SiS 2 þ 2H 2 O ¼ SiO 2 þ 2H 2 S↑: ð7Þ The S source can be originated from ZnS or direct S powders [22].As presented in Fig. 4A, the S vapor formed at low temperature zone (b ∼ 900 °C) will be carried to a higher temperature zone (∼ 900 °C b Tb∼ 950 °C). In this region, the S vapor will encounter silicon wafer which is used as a substrate and form plenty of SiS particles relative to the Eq. (5). When the temperature is continually increased to ∼ 950–1080 °C, the new-formed SiS compound will sublimate and decompose into Si and SiS 2 (see Eq. (6) and zone A in Fig. 4A). At the beginning, the Si/SiS 2 is present as a phase of quasi-liquid droplet. Thus, a large quantity of Si and SiS 2 atoms are quickly flowed away by the protected gas to an area with lower temperature (B in Fig. 4A). When temperature decreases, the Si atoms are easier to reach saturation in quasi-liquid Si/SiS 2 and precipitate along the droplet to form a nuclear. As a result, the precipitated Si atoms develop to be a SiNW with crystal directions along the energy lowest theory (B in Fig. 4A). Amongst this method, the reactions Eqs. (5) and (6) are very fast and thus induce a quick growth of numbers of SiNWs with a badly crystalline structure (Fig. 4B). Otherwise, the Si is easy to be oxidized in low-vacuum system and the generated SiS 2 is very feasible to react with H 2 O (Eq. (7)). The above factors easily cause a low-crystal quality of as-received SiNWs even amorphous SiO 2 nanowires. 3.3. Advantage and disadvantage of with and without metal growth The mentioned metal-assisted and free-metal methods are both simple and convenient to synthesize SiNWs. The VLS mechanism with metal catalyst has disadvantages of c ontamination, low quantity, and complicated process. Furthermore, if the Si source is used by silane, t he vacuum requirement for equipment is high. However, this process can be reacted at a r elatively l ow tempera ture. And growth rate can be controlled even can be self-organized with the assis tance of template. Thus i t provides an operable route to combine with present nano-techniques for real applications. The fabrication with sulfide-assisted mechanism cannot be easily controlled and the structure of SiNWs is badly crystalline even amorphous. Further- more, t he growth temperature is relatively h igher. However, by using this quick-growth technique, we can receive large scales of sample with a very simple device. If we can control the growth velocity and improve the crystal nature, the process may have more applications in the future. 4. Conclusions The catalyst features take important actions on metal-assisted growth. Otherwise, the compound decomposition, gas stream, and temperature difference play key roles on sulfide-assisted synthesis. The growth with metal catalyst can be controlled while the sulfide-assisted growth can obtain large scales of samples. Acknowledgements This wor k was supported by the Shanghai-Applied Materials Research and Development Fund (No. 06SA06) and Youth Teacher Fund of Shanghai Jiaotong University (A2306B). 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