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Characterization of Tin-catalyzed silicon nanowires synthesized by the hydrogen radical-assisted deposition method Minsung Jeon ⁎ , Hisashi Uchiyama, Koichi Kamisako Department of Electronic and Information Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan abstractarticle info Article history: Received 11 September 2008 Accepted 2 October 2008 Available online 9 October 2008 Keywords: Tin catalyst Silicon nanowires Hydrogen radicals VLS mechanism Phase diagram Tin-catalyzed silicon nanowires (SiNWs) were synthesized at various hydrogen gas flow rates using the hydrogen radical-assisted deposition method. Large quantities of SiNWs with various crystal phases were synthesized and their characteristics were estimated. Tin-capped SiNWs were straightly grown and their structures were changed with increasing hydrogen gas flow rates. Their diameters on the bottom side were increased ranging from approximately 50 to 200 nm and their lengths extended up to ~2 µm with increasing hydrogen gas flow rates. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the synthesis of carbon nanotubes [1], much attempt has been devoted to synthesizing one-dimensional nanostructure materi- als, such as nanowires, nanorods, nanotubes and nanoribbons [2]. These nanomaterials provide a good system to research the depen- dence of electrical, optical and magnetic properties [3–8]. They are also expected to play an important role as both interconnections and functional units in fabrication of e lectronic, optoelectronic and electrochemical devices with low-dimensional structures. In recent years, silicon nanowires (SiNWs) as one-dimensional structure have attracted due to their unique mechanical and semiconducting properties. SiNWs have been synthesized by using various methods, such as chemical vapor deposition (CVD) [9], oxide-assisted [10], template-assisted [11] and laser ablation method [12] via well-known vapor–liquid–solid (VLS) mechanism [9,13]. Moreover, various metal nanoparticles, such as Au, Al, Ga, Ti and Sn [9,14–17], have been studied for synthesizing SiNWs. Among these, tin (Sn) appears to be the favorable catalyst for low temperature synthesis from its phase diagram, because the Sn–Si alloy has relatively low eutectic tempera- ture as 232 °C [18]. The low melting point materials form eutectic with silicon at low temperature and with extremely low content of the elemental semiconductors. We have also reported a simple way to synthesize SiNWs using the low-melting-point metal catalysts, such as In and Ga, by the hydrogen radical-assisted deposition method [19,20]. In particular, the synthesis of SiNWs with Sn catalyst, which has relatively low eutectic temperature with Si, has been reported by a few researchers [17,21]. Moreover, their properties are not well- known yet. In this study, therefore, the SiNWs are synthesized using Sn nanoparticles as catalyst at various growth conditions and their characteristics are investigated. 2. Experimental Tin (Sn) metal thin film as catalyst is evaporated on Corning #1737 glass substrates. Before metal film evaporation, the glass substrates are cleaned in a bath containing acetone, ethanol and deionized water with ultrasonic agitation for 5 min. The substrates are located in vacuum chamber and Sn metal film approximately 100 nm is deposited. The Sn- coated glass substrates are set and heated at 400 °C in the experimental vacuum chamber with a pressure of 2×10 − 5 Torr. Hydrogen (H 2 ) gas is introduced into the 1/2-inch diameter trumpet-like quartz tube, which is surrounded by microwave cavity. Then, the hydrogen radicals generated by 2.45 GHz microwave are irradiated for 1 min onto the samples to fabricate nanosize metal particles. To fabricate nanoparticles, aH 2 gas flow, microwave power and working p ressure are selected to 100 sccm, 40 W and 0.5 Torr, respectively. For synthesizing silicon nanowires (SiNWs), silane (SiH 4 ) gas as Si source is introduced into the experimental chamber from a ring-type copper tube that has many orifices, and it is reacted with hydrogen radicals generated by microwave in the quartz discharge tube. In order to investigate the effect of synthesis conditions, SiNWs are synthesized using the hydrogen radical-assisted deposition method [19] at various hydrogen gas flow rates ranging from 130 to 180 sccm. Detailed other synthesis conditions are summarized in Table 1 . SiNWs are syn thesized for 1 0 min, and t heir characteristics a r e estimated by Field Emission Scanning Electron Microscopy (FE-SEM) and X -ra y diffractometer (XRD). For further investigation, the sy nthesized Materials Letters 63 (2009) 246–248 ⁎ Corresponding author. Tel./fax: +81 42 388 7446. 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.2008.10.005 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet SiNWs are remo ved onto a car bon copper grid. The detailed characteristics of as-synthesized S iNWs are analyzed by a scanning transmission electron microscopy (S- T EM) and an energy dispersive X -ray spectrometer (EDS) . 3. Results and discussion Before synthesis of silicon nanowires (SiNWs ), hydrogen radical treatment is performed on the Sn-coated substrate to fabricate Sn nanoparticles. The hydrogen radical tr eatment is effective to o btain voluminous SiNWs. S ubseq uently, the SiNWs wer e s ynthesized a t various hydrogen (H 2 )gasflow rates. In order t o investig ate the morphological property of as- synthesized SiNWs at varied conditions, a FE-SEM observation was performed after synthesis of SiNWs for 1 0 min at 400 °C. Fig. 1 (a)–(c) shows the low-magnification FE-SEM images of the as-synthesized SiNWs at va rious H 2 gas flow r a tes: (a ) 130 sccm, ( b) 150 sccm a nd (c ) 180 sccm. V o luminous S iNWs were whisker-like ly synthesized at all growth conditions. Their structures were gradually changed w ith i ncr easing H 2 gas flo w rate. When the SiNWs w ere synthesized at H 2 gas flow of 13 0 s ccm, the SiNWs we re smoothly curved as shown in inset of Fig. 1 (a). On the other hand, when the SiNWs w ere s ynthesized at above H 2 flow r ate of 1 30 sccm, the SiNWs were straightly gr own a s shown in insets of Fig. 1 (b) a nd (c). How ever, their si zes became thicker with increasing H 2 gas flo w. The diameters of SiNWs on the bottom side were gradually increased ranging from approximat ely 50 t o 200 nm with increasing H 2 gas flow rates. Moreover , their lengths extended up to ~2 µm. It indicates that the H 2 gas flow affect the growth of SiNWs. Moreover, the high-magnification FE-SEM images explained that th e SiNWs were tapered and that Sn catalysts r emained on the tip of Si NWs (see r ed c ircles i n inset of Fig. 1 (a)). It means that the SiNWs a r e synthesized via vapor–liq uid–solid (VLS) mechanism [9, 13]. The detailed explanation of the VLS mechanism will be followed later . Above mentioned sampl es w ere also e xamined b y XRD measurement. Fig. 1 (d) s hows the XRD p att erns of S iNWs synthesized a t varied H 2 gas flow r at es. The XRD patterns indicate that the syn thesized SiNWs ar e highl y crystallized sil icon with di ffraction peaks of (111), (220)and (3 11). Mor e over, the Snmetal peaks of (200) and (1 0 1) were also w eakly detected at 30.66 and 32.044 deg., r espectively (see black arrows in Fig. 1 ( d)). These peaks were reveal ed because theSncatalystswerelocatedonthetopofSiNWsasshownininsetofFig. 1(a). Such SiNWs grew randomly wi th different crystal orientations. Fig. 2 shows a schematic of the Sn–Si alloy binary phase diagram [18]. SiNWs are typically synthesized via the VLS growth mechanism at temperatures higher than the Sn– Si eutectic temperature. This VLS mechanism has been introduced by Wagner et al. to growth single crystalline silicon [9]. A typical VLS growth starts with the dissolution of gaseous reactants into a nano-size metal liquid droplet, followed by nucleation and growth Table 1 Synthesis conditions for silicon nanowires (SiNWs) H 2 flow [sccm] SiH 4 flow [sccm] M.W.P [W] Press. [Torr] Temp. [°C] Time [min] 130–180 12 40 0.5 400 10 Fig. 1. FE-SEM images of the as-synthesized SiNWs at hydrogen gas flow of (a) 130 sccm, (b) 150 sccm and (c) 180 sccm. All the scale bars represent 2.5 µm. (d) XRD patterns of the as- synthesized SiNWs at various hydrogen gas flow rates. Fig. 2. Schematic of the Sn–Si alloy binary phase diagram. 247M. Jeon et al. / Materials Letters 63 (2009) 246–248 of crystalline wires. Here, the Sn nanoparticles and decomposed SiH 4 are typical metal catalyst and silicon source used to synthesize SiNWs. The fabricated Snnanoparticles after hydrogen radical treatment provide energetically favored Sn–Si sites for the adsorption of incoming vapor silicon sources. The adsorbed Si atoms diffused into the liquid Sn nanoparticles, which results in the formation of a Sn–Si alloy. The continued adsorption of the Si sources into the Sn–Si alloy liquid droplets leads to the supersaturation of the Sn–Si eutectic in a broad region above 230 °C, which results in the growth of a solid Si nucleus. TheSnnanoparticlesarepushedawayfromthesubstratebycontinuousSisourcefeeding in the liquid–solid interface and it is lifted upward by the growing SiNWs. As a result, large quantities of SiNWs are synthesized. The detailed compositions of as-synthesized SiNWs via VLS mechanism were examined by S-TEM and EDS measurement. Fig. 3(a)–(c) shows a high-magnification SEM image of as-synthesized Si nanowire and corresponding EDS analysis. The S-TEM micrograph in inset of Fig. 3(a) represents the Si nanowire capped by a catalyst nanoparticle. The EDS measurement was operated during the S-TEM observation. The EDS analysis carried out on the nanowire stem and metal nanoparticle. As can be seen in Fig. 3(b), the Si nanowire comprises only Si element. The other peaks, such as C and Cu, were also detected. It is attributed to the effect of the TEM grid. Fig. 3(c) shows the EDS spectra taken for the nanoparticle shown in inset of Fig. 3(a). It indicates that the nanoparticle comprises Sn and Si elements. The Sn nanoparticle located on the top of Si nanowire implies that a Sn catalyst assisted VLS mechanism is typically related in the growth of SiNWs. These results denote that the as-synthesized SiNWs by ourexperimental method are pure crystalline silicon without oxygen and other impurities. 4. Conclusion Tin-catalyzed silicon nanowires (SiNWs) were synthesized atvarious hydrogen gas flow rates using the hydrogen radical-assisted deposition method. Voluminous SiNWs, which have various crystal phases, were whisker-likely synthesized at all growth condition. Their structures were gradually changed with increasing hydrogen gas flow rate. The diameters of SiNWs on the bottom side were increased ranging from approximately 50 to 200 nm and their lengths extended up to ~2 µm. It indicates that the SiNWs can be controlled by the introduced hydrogen gas flow rates. References [1] Iijima S. Nature 1991;354:56. [2] Wang ZL. 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Appl Phys Lett 2000;76:562. [17] Chen ZH, Jie JS, Luo LB, Wang H, Lee CS, Lee ST. Nanotechnology 2007;18:345502. [18] Olesinski RW, Abbaschian GJ. Bull Alloy Phase Diagr 1984;5:273. [19] Jeon MS, Kamisako K. Mater Lett 2008;62:3903. [20] Jeon MS,Tomitsuka Y,Kamisako K. J Ind Eng Chem 2008. doi:10.1016/j.jiec.2008.06.004. [21] Shao M, Hui H, Li M, Ban H, Wang M, Jiang J. Chem Commun 2007;8:793. Fig. 3. (a) S-TEM micrograph of the silicon nanowire capped by a Sn catalyst nanoparticle. (b) and (c) represent the corresponding EDS spectra taken from the nanowire stem and the catalyst nanoparticle, respectively. 248 M. Jeon et al. / Materials Letters 63 (2009) 246–248 . Characterization of Tin-catalyzed silicon nanowires synthesized by the hydrogen radical-assisted deposition method Minsung Jeon ⁎ ,. catalyst Silicon nanowires Hydrogen radicals VLS mechanism Phase diagram Tin-catalyzed silicon nanowires (SiNWs) were synthesized at various hydrogen gas