Synthesis of silicon nanowires after hydrogen radical treatment

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Synthesis of silicon nanowires after hydrogen radical treatment Minsung Jeon ⁎ , Koichi Kamisako Department of Electronic and Information Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, 184-8588 Tokyo, Japan ABSTRACTARTICLE INFO Article history: Received 16 October 2007 Accepted 16 May 2008 Available online 24 May 20 08 Keywords: Silicon nanowires Hydrogen radicals Metal oxide film Nanoparticles Silicon nanowires (SiNWs) were synthesized through the hydrogen radical-assisted deposition method. Voluminous indium (In) metal catalysts with smooth spherical structures were successfully fabricated from indium oxide films after hydrogen radical treatment for 5 min. Their sizes were widely distributed, ranging from several nm to about 150 nm. Subsequently, the large quantities of SiNWs were synthesized using the hydrogen-radical-assisted deposition method. Their diameters typically ranged from several nm to several hundred nm, and their lengths extended to about 10 μm. The SiNWs were composed of well-crystallized silicon cores and hydrogenated amorphous outer layers. © 20 08 Elsevier B.V. All rights reserved. 1. Introduction Since the initial discovery of carbon nanotubes [1], one-dimen- sional (1D) nanostructures, including the well-known nanotubes and nanowires, have been utilized extensively in the various fields of nanoscale optical and electronic devices [2–5]. In particular, silicon nanowires (SiNWs) are irresistible materials in the semiconductor industry because the bulk properties of silicon are well-known. Several methods, such as the chemical-vapour deposition (CVD) method and the oxide-assisted, laser ablation, and template-based techniques, have been proposed for the synthesis of 1D nanostruc- tures [6–10]. When NWs, however, are synthesized using a vapour– liquid–solid (VLS ) mechanism [11,12], the sizes of the metal nanoparticle catalysts have to be controlled because, to a large extent, the catalyst size determines the growth of NWs. Various methods have been developed for the fabrication of NWs with metal catalysts, such as the dispersion of nano-size metal particles, electroless metal deposition, laser ablation, evaporation, and immobilization of metal colloidal suspensions [13–16]. In this article, hydrogen radical treatment was performed on a glass substrate coated with a metal oxide film to fabricate nano-size particles as catalysts, and their properties were investigated. In addition, SiNWs were synthesized through the hydrogen radical-assisted deposition method [17,18], and their characteristics were analyzed and discussed herein. 2. Experimental The experimental apparatus that was used in this work was a stainless-steel chamber equipped with a 1/2-in diameter trumpet- like quartz discharge tube. A 2.45-GHz microwave was introduced into the discharge tube through a coaxial cable and a microwave cavity surrounding the tube. The hydrogen (H 2 ) gas was decomposed by microwave to generate high-d ensity hydrogen ra dicals in the discharge tube, and hydrogen radicals were released over the sample surface. To synthesize the SiNWs, silane (SiH 4 ) gas was introduced into the chamber as a Si source from a ring-type tube that had many small holes, and it was reacted with the microwave afterglow of H 2 in the discharge tube. This method is referred to herein as the hydrogen radical-assisted deposition method. The fabrication of the metal catalysts from a metal oxide film and the synthesis of SiNWs were performed using this method. Glass substrates coated with an approx- imately 150 nm thick indium oxide film was used. To fabricate the indium particles as metal catalysts, the sub- strates were heated at a temperature of 400 °C in a vacuum chamber with a pressure of 2 ×10 − 5 Torr. Then the substrate was exposed to the hydrogen radicals for 5 min with a gas pressure of 0.4 Torr by introducing H 2 gas with a flow rate of 120 sccm. The indi- um nanopa rticle catalysts were fabricated after hydrogen radical treatment. Their properties, crystal phase pattern, and surface mor- phology were evaluated using an X-ray diffractometer (XRD), with Cu Kα radiation and field-emission scanning electron microscopy (FE-SEM). For synthesizing the SiNWs, SiH 4 gas was introduced as a Si source after the fabrication of the metal nanoparticles. The gas pressure and microwave power were set at 0.4 Torr and 40 W, respectively. The substrate temperature was kept at 400 °C during the growth of the SiNWs. The H 2 and SiH 4 gas flow rates that were introduced to synthesize the SiNWs were 130 sccm and 12 sccm, respectively. The SiNWs were synthesized for 1 h, and their properties were estimated using XRD measurement and FE-SEM microscopy. For further investigation, the characteristics of the synthesized SiNWs were analyzed through selected-area electron diffraction (SAED) and with Materials Letters 62 (2008) 3903–3905 ⁎ Corresponding author. Tel./fax: +81 42 388 7133. E-mail address: joseph@cc.tuat.ac.jp (M. Jeon). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.05.035 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet the use of a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDX). 3. Results and discussion For the fabrication of the metal nanocrystal catalysts, an indium oxide (In 2 O 3 ) film was used as a metal oxide film-coated glass substrate, and its properties were analyzed through XRD and FE-SEM measurement. Fig. 1 shows the XRD pattern of the metal oxide film (a) before and (b) after hydrogen radical treatment. The FE-SEM image after hydrogen radical treatment is shown in Fig. 1(c). The typical XRD peaks of the as- deposited In 2 O 3 film shown in Fig. 1(a) reveal the high relative intensity of the (222) peak compared with the other peaks. On the other hand, hydrogen radical treatment was performed on the substrate for 5 min at 400 °C to fabricate the metal nanocrystal catalysts. As for the XRD results after hydrogen radical treatment, as shown in Fig. 1(b), only the crystal indium (In) diffraction peaks of (101), (002), (110), (112), and (200) were observed. In particular, the indium peak of (101) was strongly revealed. The hydrogen radicals could react with the metal oxide film on the substrate surface to fabricate the metal nanocrystal catalysts. As a result, only the metal indium nanocrystals remain on the substrate after hydrogen radical treatment. To estimate the morphologies of the as- fabricated indium nanoparticles after hydrogen radical treatment, FE-SEM observation was performed. Fig. 1(c) shows the large quantity of nano-size indium crystals. The fabricated nanocrystals are shown as having smooth spherical structures, and their sizes are shown to range from several nm to 150 nm. Based on the above results, it was realized that hydrogen radical treatment is an essential process that must be performed when metal nanocrystal catalysts are fabricated from metal oxide films. After the fabrication of the indium nanocrystal catalysts, the SiNWs were synthesized using the hydrogen radical-assisted reaction deposition method for 1 h, at the same temper- ature. The morphologies of the synthesized SiNWs were observed through FE-SEM measurement. Fig. 2 shows the low-magnification top-view FE-SEM micrograph of the as- synthesized SiNWs. The voluminous quantities of SiNWs were grown for 1 h at 400 °C. The detailed FE-SEM analysis revealed that the SiNWs were tapered and that the In catalyst remained on top of the SiNWs (Fig. 2 inset). The as-synthesized SiNWs were approximately several nm to several hundred nm in diameter, and their lengths extended to about 10 μm. Furthermore, to confirm the crystal structure of the SiNWs, TEM analysis was carried out. The results of the high-resolution TEM (HR-TEM) analysis support the structural distinction made above. Fig. 3 shows the HR-TEM micrograph and SAED pattern of an as-synthesized SiNW. The lattice fringes correspond to the (-1-11) and (1-11) planes. The crystalline SiNW was revealed to be sheathed by an approximately 10 nm-thick amorphous outer layer. The SAED pattern of one individual SiNW is shown in the inset of Fig. 3. The set of diffraction spots visible in the SAED pattern reveal that this particular SiNW contains a twin. Moreover, it can be indexed as the diffraction along the [110] zone axis of crystalline Si, and it suggests that SiNW growth does occur along the [111] direction. The TEM micrograph in Fig. 4(a) reveals that the nanocrystal catalysts are located at the end of the SiNW. The EDX measurement made on the nanoparticles (Fig. 4(b)) indicates that the particle is composed of Si and In. Here, the detected O and Cu are the effect of the TEM copper grid. The indium nanoparticle located on top of the synthesized SiNW suggests that an indium catalyst-assisted VLS mechanism is basically involved in the growth of SiNWs [18]. Fig. 4(c) reveals the EDX measurement of the Fig. 1. XRD patterns before and after hydrogen radical treatment. (a) Peaks of the as- deposited indium oxide film as a metal oxide film. (b) Indium peaks after hydrogen radical treatment for 5 min at 400 °C. (c) FE-SEM image of the as-fabricated indium nanocrystals as metal catalysts after hydrogen radical treatment. Fig. 2. Top-view FE-SEM image of the SiNWs. The inset shows a high-magnification FE-SEM image of the SiNWs. Fig. 3. HR-TEM micrograph of the as-synthesized SiNW taken along the [110] direction and the SAED diffraction spots (inset). 3904 M. Jeon, K. Kamisako / Materials Letters 62 (2008) 3903–3905 SiNW stem. The SiNW is composed only of the Si element. These results confirm that SiNWs are crystalline and that their sheathed outer layer is a hydrogenated amorphous layer. 4. Conclusion In this work, the formation of metal catalysts from a metal oxide film and the synthesis of SiNW were presented using the hydrogen radical-assisted deposition method. After hydrogen radical treatment for 5 min at 400 °C, the indium metal catalysts were successfully fabricated from the indium oxide films. They were shown to have smooth spherical structures and to have sizes ranging from several nm to 150 nm. Subsequently, the SiNWs were synthesized, and large quantities of them were whisker-likely grown. The diameters of the as-synthesized SiNWs were approximately several nm to several hundred nm, and their lengths extended to about 10 μm. The SiNW was crystalline, and it was sheathed with a hydrogenated amorphous outer layer that was about 10 nm thick. These results suggest that the hydrogen radical-assisted deposition method, including the radical pre-treatment process, is a candidate method. Moreover, the results indicate that a voluminous quantity of SiNWs, not including oxygen, can be synthesized in a simple way. References [1] Iijima S. Nature 1991;354:56. [2] Alivisatos AP. Science 1996;271:933. [3] Hu J, Ouyang M, Yang P, Lieber CM. Nature 1999;399:48. [4] Au FK, Wong KW, Tang YH, Zhang YF, Lee ST. Appl Phys Lett 1999;75:1700. [5] Cui Y, Lieber CM. Science 2001;291:851. [6] Westwater J, Gosain DP, Tomiya S, Usui S, Ruda HE. J Vac Sci Technol B 1997;15:554. [7] Cui Y, Lauhon LJ, Gudiksen MS, Wang J, Lieber CM. Appl Phys Lett 2001;78:2214. 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Kamisako / Materials Letters 62 (2008) 3903–3905 . Synthesis of silicon nanowires after hydrogen radical treatment Minsung Jeon ⁎ , Koichi Kamisako Department of Electronic and Information. shows the XRD pattern of the metal oxide film (a) before and (b) after hydrogen radical treatment. The FE-SEM image after hydrogen radical treatment is shown