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Physica E 23 (2004) 131 – 134 www.elsevier.com/locate/physe Silicon nanowires fabricated by thermal evaporation of silicon monoxide Junjie Niu a , Jian Sha a; b , Deren Yang a;∗ a State Key Lab 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 9 January 2004; accepted 30 January 2004 Abstract A large-scale crystalline silicon nanowires (SiNWs) with a diameter of ∼30 nm and length of tens of micrometers on Al 2 O 3 templates and silicon wafers were synthesized by the thermal evaporation of silicon monoxide (SiO). The SiNWs were measured by transmission electron microscopy, scanning electron microscopy, X-ray diraction and Raman spectroscopy, respectively. It was pointed out that the SiNWs possessed the well crystalline structure. Therefore, it is considered that SiO could be used as Si sources to produce larger-scale SiNWs and crystalline SiNWs may grow from amorphous nuclei. ? 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Cn; 81.05.Ys Keywords: Nanowires; Silicon; Thermal evaporation 1. Introduction Recently one-dimensional materials such as silicon nanowires (SiNWs) have stimulated much interest be- cause of their dierent electronic and optical charac- teristics compared with bulk materials [1–4]. Many favorable specialities of SiNWs have been reported extensively, including p–n junction [5], chemical sen- sors [6], electrical transport properties [7], and noise characteristics [8], etc. Therefore, both the fabrica- tion of large-scale uniform SiNWs and the under- standing of the growth mechanism of SiNWs are very important for their application. In fact, several dif- ferent methods have been used for producing SiNWs ∗ Corresponding author. Tel.: +86-571-87951667; fax: +86-571-87952322. E-mail address: mseyang@dial.zju.edu.cn (D. Yang). such as laser ablation [9], chemical-vapor-deposition (CVD) [10–12], thermal evaporation Si and SiO 2 at 1200 ◦ C[13,14], and electrochemistry [15]. The dif- ferent growth models including vapor–liquid–solid (VLS) [10], oxygen-assisted [13,16,17], and solid– liquid–solid (SLS) [18] have been reported. In this paper, we successfully synthesized large quantity of uniform crystalline SiNWs on Al 2 O 3 templates and silicon wafers by thermal evaporation of silicon monoxide (SiO) at 1100 ◦ C, respectively. The results of the scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diraction (XRD) experiments indicate that the SiNWs crystallized well. 2. Experimental The samples were prepared in a CVD system as we reported previously [10]. An Al 2 O 3 template and 1386-9477/$ - see front matter ? 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.01.013 132 J. Niu et al. / Physica E 23 (2004) 131 – 134 a p-type (1 1 1) silicon wafer with a resistivity of about 0:001 cm as substrates were placed in a hor- izontal quartz tube furnace, respectively. And SiO particles (purity: 99.99%) were placed in an alumina boat which was placed in the center of the furnace. The furnace was evacuated for several hours to reach a low vacuum of 20 Pa. The temperature of the fur- nace was then raised to 1100 ◦ C at a heating rate of 20 ◦ C=s and held at a constant pressure of 1500 Pa for 6 h. When the temperature reached to 750 ◦ C, the steady mixture gas of 200 sccm argon and 70 sccm hydrogen were sent through the chamber, which acted as the protective gas. The thermal evaporation conditions for SiNWs synthesis were similar to the previously reported work [16,19]. After the reac- tion, the as-grown materials with weak-yellow and brown color on the dierent zones of the substrates were removed out from the furnace and measured by XRD (XRD: Rigaku, D/MAX 2550 PC), SEM (SEM: JEOL, JSM-5610LV), TEM (TEM: 160 kV, JEM200CX) and Raman scattering spectroscopy (Nicolet Almega), respectively, and the chemical composition was detected by energy-dispersive X-ray spectroscopy (EDS) attached to the SEM. 3. Results and discussion Fig. 1 shows the TEM images of the symmetrical SiNWs generated on a silicon wafer and an Al 2 O 3 template, respectively. It can be seen that the di- ameter of those SiNWs is about 30 nm. The insets of Fig. 1(I) and the upper right inset of Fig. 1(II) are the selected area electric diraction (SAED) im- ages, which show that the SiNWs were crystallized well. The top view of the SiNWs on the dierent substrates observed by SEM is shown in Fig. 2. The plenty of the SiNWs with the length of tens of micrometers were observed. The corresponding EDS in the insets indicate that the SiNWs consists of high-intensity Si and small quantity of oxygen. Fig. 3 shows the XRD spectrum of the SiNWs grown on the Al 2 O 3 template, which displays high-intensity peaks of the Si (1 1 1); (220); (311); (400) and (3 3 1), indicating that the SiNWs were the well crystalline structure [20]. Some SiO 2 and -Al 2 O 3 came from the decomposition of SiO and the Al 2 O 3 substrate, respectively. Fig. 1. TEM images of the SiNWs produced by thermal evaporation of SiO on a silicon wafer (I, the upper right inset is the SAED image taken from the corresponding SiNWs), and on an Al 2 O 3 template (II, the upper right and lower right insets are the SAED images taken from the middle position (a) and end position (c) of the SiNW). Lee et al. contributed that silicon oxide played an important role on the formation of SiNWs, so called oxygen-assisted mechanism [13,16]. In our experi- ments, SiO powders were used as Si sources. During annealing at 1100 ◦ C, SiO evaporated ÿrstly and was transported by the gas to the lower-temperature region (850–1000 ◦ C) to decompose on substrates. The reac- tion is as follows: 2SiO(↑)=Si+SiO 2 : J. Niu et al. / Physica E 23 (2004) 131 – 134 133 Fig. 2. (a) SEM images of the SiNWs on a silicon wafer, and (b) on an Al 2 O 3 template. The insets of (a) and (b) are the EDS taken from the corresponding SiNWs. 20 30 40 50 60 70 80 500 1000 1500 2000 2500 3000 SiO 2 Al 2 O 3 Si ¦Ã-Al 2 O 3 Al(220) Si(331) Si(400) Si(311) Si(220) Si(111) Intensity 2 Theta (degree) Fig. 3. XRD spectrum of the SiNWs grown on an Al 2 O 3 template. Therefore, in the XRD spectrum (Fig. 3), SiO 2 could also be detected. When Si atoms precipitate, the atoms will easily centralize to form nuclei on low-energy places of sil- icon wafers such as defects. With the increase of Si atoms, the nuclei will grow up to wires. And because of oxide reaction and growth energy, some growth di- rections of SiNWs will be limited; therefore, the 111 and 112 orientations of the SiNWs may be the main directions, as reported in the previous work [10,14]. The lower left inset of Fig. 1(II) is the SAED image of the tip of the nanowires, indicating that is amor- phous. The tip should be the nuclear of the SiNW. 134 J. Niu et al. / Physica E 23 (2004) 131 – 134 400 600 800 1000 0 500 1000 1500 2000 2500 3000 3500 4000 960cm -1 960cm -1 517.75cm -1 517.67cm -1 b a Intensity Raman Shift(cm -1 ) Fig. 4. (a) Raman spectra of the SiNWs on a silicon wafer and (b) on an Al 2 O 3 template. Furthermore, the upside of the wire was crystalline, as illustrated in the upper right inset of Fig. 1(II). It is considered that crystalline SiNWs grew from amor- phous nuclei. Fig. 4 shows the Raman scattering spectra of the SiNWs grown on a silicon wafer (a) and an Al 2 O 3 template (b), which reveals that there are the same peaks at 517 and 960 cm −1 for both the SiNWs. It can be seen from the spectra that the peaks with high intensity are good symmetric and narrow, which could be due to the uniform diameter. Usually, those peaks are regarded to be the ÿrst-order transverse optical photon mode (TO) which is caused by the diameter decrease of the SiNWs [21]. 4. Conclusion A large-scale crystalline SiNWs on silicon wafers and Al 2 O 3 templates were fabricated by the thermal evaporation of silicon monoxide (SiO), respectively. The SiNWs were about ∼30 nm in diameter and tens of micrometers in length. It was also found that the SiNWs crystallized well. Finally, SiO is considered to be Si sources to produce SiNWs. Acknowledgements This work was supported by the National Natu- ral Science Foundation of China (No.50272057 and 60225010). 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Masurnoto, T. Matsumoto, T. Futagi, H. Mimura, Phys. Rev. B 48 (1993) 2827. . 134 www.elsevier.com/locate/physe Silicon nanowires fabricated by thermal evaporation of silicon monoxide Junjie Niu a , Jian Sha a; b , Deren Yang a;∗ a State Key Lab of Silicon. synthesized large quantity of uniform crystalline SiNWs on Al 2 O 3 templates and silicon wafers by thermal evaporation of silicon monoxide (SiO) at 1100 ◦ C,