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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
Physica E 24 (2004) 328–332 Tiny silicon nano-wires synthesis on silicon wafers Junjie Niu a , Jian Sha a,b , Yujie Ji a , Deren Yang a,Ã a State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 20 April 2004; accepted 10 June 2004 Available online 11 August 2004 Abstract Tiny silicon nano-wires (SiNWs) were synthesized on silicon wafers by the chemical vapor deposition (CVD) technique. The morphology and structure of tiny SiNWs were analyzed by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results indicate that the tiny SiNWs were part-crystalline structure and were about 3 nm in minimal diameter. Based on the line shift of Raman spectra, the structure transform of the tiny SiNWs was discussed. The defect-inducing growth mechanism will probably provide a new method for the minimum of the one-dimensional nano-materials. r 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Cn; 81.05.Ys Keywords: Nano-wires; Synthesis; Silicon 1. Introduction One dimension nano-materials have stimulated much interest for their potential applications in nano-electronics, optics, flat face display, etc. [1–6]. Many techniques are employed to fabricate one-dimension nano-materials, such as laser abla- tion, chemical vapor deposition (CVD), physical vapor deposition, aqua-solution method, sputter deposition, etc. [7–16]. Especially, the research emphases have focused on the growth character- istics [17] diameter minimum [18,19]. Recently Hu et al. fabricated the silicon nanowires with diameters of 10–30 nm sub-grow on the surface of large amorphous SiO 2 nanowires with diameters of 200–400 nm using floating-zone (FZ, 1233–1273 K) melt-vapor method [20]. And the Silicon nano-wires (SiNWs) grown on silicon wafers have also attracted much attention because of their potential compatibility in the miniaturiza- tion of integrated circuit (IC) [21–23]. But the previous methods either cannot get the very thin ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.06.041 Ã Corresponding author. Tel: +86-571-87951667; fax: +86- 571-87952322. E-mail address: mseyang@zju.edu.cn (D. Yang). wires or introduce the other substances such as polymers. Also the process is complex and expensive. In this paper, we synthesized the very thin and long large-scale part-crystalline SiNWs branch with diameters of about 3–8 nm sub-grow on the main well-crystalline SiNWs with diameters of about 40–50 nm growing on silicon wafers by a simple physical vapor deposition. The transmis- sion electron microscopy (TEM) and scanning electron microscopy (SEM) images of the SiNWs displayed the appearance and surface pattern. The X-ray diffraction (XRD) and selected area electric diffraction (SAED) gave an obvious well-crystal- line structure data of the SiNWs. Also the Raman scattering spectra of the SiNWs indicated the optical properties related to temperature depen- dence on laser heating and the small size effect. Furthermore, the growth mechanism was also simply discussed. 2. Experimental Tiny SiNWs were prepared on a heavily doped p-type (1 1 1) silicon wafer with a resistivity of about 0:001 O cm as substrate by means of a CVD process. First, a magnetic sputtering technique was used to deposit gold as a catalyst on the silicon substrate (the thickness of Au film is $ 100–200 nm). The substrate was then placed in a quartz tube furnace, which was pumped down to 10 Pa. When the temperature reached 630 1C, a mixture gas of argon, hydrogen, and silane with the ratio of 100:20:15 was allowed into the chamber. The pressure and temperature in the chamber were kept at 1450 Pa and 630 1C during the deposition. After the deposition, the substrate was removed from the furnace and was investi- gated by means of XRD (Rigaku, D/MAX 2550 PC), a Raman scattering spectroscope (Nicolet Almega) and a SEM (JEOL, JSM-5610LV), respectively. And then, the deposited matters on the substrate were dissolved in an ethanol solution. Finally, the solution was placed dropwise on a copper grid, which was covered with a very thin carbon film, so that the deposited materials could be analyzed with TEM (200 kV, Phillip CM200) equipped with an energy-dispersive X-ray spectro- meter (EDX). 3. Results and discussion Top view SEM image of the SiNWs grew on the silicon substrate is shown in Fig. 1. A large quantity of long and thick SiNWs (trunk SiNWs) were observed on the surface of the silicon wafer. The TEM images (Fig. 2) indicate a plenty of tiny SiNWs attached on the top of a thick SiNW. They are about 3–8 and 40–50 nm in diameter, respec- tively. And the holistic SiNWs have a well crystalline structure (the Si (1 1 1) direction is indicated in the upper left of the Fig. 2). The TEM image in Fig. 3a shows a large scale tiny SiNWs. And the chemical characterizations of the SiNWs using EDX show that they are composed of silicon with neglectable traces of oxygen (Fig. 3b). It can be seen that Si, Cu and O with smaller quantity were mainly detected. Obviously the intensity of the Si is the strongest among these peaks. Cu peak came from Cu grid for TEM analysis, and O peak mainly from the weak surface oxidation of nano- wire or adsorption of oxygen on the nano-wire because of the impure fixed gases and low vacuum system. This means that the very tiny wires were mainly composed of silicon core and a small quantity of silicon oxide sheath. Detailed analysis ARTICLE IN PRESS Fig. 1. Top view SEM image of the SiNWs grew on the silicon substrate (the part of Au nanoparticles was pointed by white arrows). J. Niu et al. / Physica E 24 (2004) 328–332 329 on the lattice images of the tiny SiNWs gives an inter-planar spacing of 0.314 nm completely corresponding to that of Si (1 1 1) planes by the HRTEM in Fig. 3a. It was seen that the tiny SiNWs have a double-layer nature with a crystal- line silicon core and an amorphous SiO 2 outer shell just like the Hu reported [20]. And the XRD data in Fig. 4 showed that the Si peaks were very strong with the Si (1 1 1), Si (2 2 0), and Si (3 1 1). The intensity indicated the Si (1 1 1) is the possible leading growth direction of the SiNWs. The Au peaks in the figure come from the catalyst film for the preparation of the sample (the images of Au tips attaching to the SiNWs are shown in Fig. 1 and 2). The remarkable crystalline structure of the SiNWs was confirmed clearly by above XRD data. Here we briefly develop the defect-inducing growth mechanism of the silicon wires based on the previous reports [20,24]. According the vapor–li- quid–solid (VLS) mechanism, the catalyst can induce the deposition atoms to form a droplet and grow nano-wires. The mostimportant is the size and state of the catalyst which would play a key effect on the quantity and diameter size of the wires. In our experiments, the size of most catalysts is very small and uniform. This con- tributes the small catalyst particles to easily form the nucleating point with the deposited atoms and come into a wire. Once the trunk SiNWs formed, high surface density of the SiNWs and the unstable growth surroundings lead to a plenty of faults and defects such as oxygen vacancies produce. The properties of the silicon wafer also express some effects in the form of these defects in our experimentation. The existence of these defects provides a good new nuclear center and will form a wire to continue to grow with the more silicon atoms joining in at low temperature. The new nano-droplet composed of defects and silicon atoms is easier to reach supersaturation to grow ARTICLE IN PRESS Fig. 2. TEM image of the SiNWs. The diameter of the trunk SiNW is about 40 nm. The diameters of plenty of tiny SiNWs, which attach on the tip, is about 3 nm. The upper left inset is the SAED image of the same SiNWs. Fig. 3. HRTEM image of tiny SiNWs (the inset is the TEM image of the large scale of tiny SiNWs) (a) and the EDX spectrum of the SiNWs (b). In the EDX spectrum, the Si, Cu and O with smaller quantity were detected. Obviously the intensity of the Si is the strongest among these peaks. Cu peak came from Cu grid, and O peak mainly from the possible weak surface oxidation of SiNWs during the synthesis or the adsorption of oxygen on the SiNWs due to the survived oxygen in TEM system. 30 40 50 60 70 10 20 30 40 50 60 70 Au SiNWs Au(220) Au(200) Au(111) Si(311) Si(220) Si(111) Intensity 2 Theta ( de g ree ) Fig. 4. XRD data of the SiNWs. J. Niu et al. / Physica E 24 (2004) 328–332330 SiNW because of the lower energy compared with the Au–Si droplet. Here the growth procedure can be regarded as the developed VLS model with the defects as catalyst but the metal. As the inter- planar spacing of Si (1 1 1) planes needs low-energy to form the crystalline array at a relatively low temperature, finally the Si (1 1 1) dominates the main growth direction just as the SAED and XRD data showed. Here one case must be mentioned, because the form of the tiny SiNWs was effected by the properties of the defects, new nucleation density, and other surrounding conditions, the quantity of the tiny SiNWs is lesser compared with the thick SiNWs. We carried out Raman spectrum at room temperature at an excitation wavelength of 532 nm. In Fig. 5, the characteristic peaks of SiNWs at 494 and 480 cm À1 with a clearly broad downshift of nearly 35 cm À1 compared with the bulk silicon of 523 cm À1 . Such a feature could be due to different temperature dependence on laser heating and the small size effect of SiNWs [25,26]. The peak of 480 cm À1 and asymmetry of the peak should contribute to the thin SiO x sheath out of the silicon nano-wire and the defects which are contained among the silicon nano-wire. 4. Conclusions In conclusion, we have synthesized large-scale, very tiny, and very long SiNWs on the substrate of p-Si (1 1 1) wafer using the simple approach of CVD method at 630 1C. 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