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www.elsevier.com/locate/ph y se Physica E 24 (2004) 278–281 Sulfide-assisted growth of silicon nano-wires by thermal evaporation of sulfur powders 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 23 March 2004; accepted 11 May 2004 Available online 10 July 2004 Abstract Silicon nanowires (SiNWs) with a diameter of B20 nm were synthesized by the thermal evaporation of sulfur powders on silicon wafers. The source of the SiNWs came from the silicon substrates. It is considered that the generated SiS compound assisted the formation of SiNWs. Finally, the Raman shift of SiNWs was discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Cn; 81.05.Ys Keywords: Silicon; Nanowires; Sulfide assisted 1. Introduction Recently the one-dimensional nano-materials, especially silicon nanowires (SiNWs), have stimu- lated much interest because of their different properties in comparison with the corresponding bulk materials [1–6]. Several synthesized methods for SiNWs have been reported, including laser ablation [7], chemical-vapor-deposition (CVD) via vapor–liquid–solid (VLS) mechanism [8–13], ther- mal evaporation via oxygen-assisted [14–17] and solid–liquid–solid (SLS) mechanisms [18–20], and electronic-chemical method [21]. In this paper, the thermal evaporation of sulfur powders on silicon wafers used to grow SiNWs is reported. Compared with the other approach, this process was simple and the source of SiNWs was from the silicon wafer substrates but not from the silane gas [11] and silicon oxide [14,17]. It is also found that sulfide played an important role in the formation of SiNWs, therefore, a sulfide-assisted growth mechanism was suggested. In the experiments, the samples were checked by filed emission scanning electron microscopy (FESEM), transmis- sion electron microscopy (TEM), and X-ray diffraction (XRD), respectively. Finally, the Ra- man spectroscopy was also used to investigate the SiNWs. ARTICLE IN PRESS *Corresponding author. Tel./fax: +86-571-879-523-22. E-mail address: mseyang@zju.edu.cn (D. Yang). 1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.05.002 2. Experimental SiNWs were produced on p-type (1 1 1) silicon wafers with a resistivity of about 0.001 O cm by means of a low-vacuum CVD system. First, several pieces of silicon wafers and plenty of sulfur powders were placed in a semi-sealed alumina boat which was put at the center of a horizontal quartz tube furnace. Then the furnace was evacuated to reach 30 Pa by a mechanical pump. The tempera- ture of the system was then raised to 900 Cata heating rate of 25 C min À1 and continually up to 1250 C at a heating rate of 10 C min À1 , and held at 1250 C for 30 min at a constant pressure of 30 Pa. After reaction, the weak black and yellow substrates with the as-grown materials were removed from the furnace and characterized by FESEM (FEI, Sirion), TEM (JEOL, JEM200CX), XRD (Rigaku, D/MAX-rA), and Raman scatter- ing spectroscopy (Nicolet Almega), respectively. The possible chemical composition of the as- grown materials on the wafers was investigated by using energy-dispersive X-ray spectroscopy (EDX) attached to the FESEM. 3. Results and discussion The FESEM morphology of the SiNWs with a diameter of B20 nm (that of a very few SiNWs is over 50 nm) is shown in Fig. 1. The EDX (the inset of Fig. 1) taken from the corresponding nanowires indicates that the nanowires were silicon. How- ever, oxygen could also be detected. The oxygen is considered to have come mainly from the surface oxidation of nanowires. This suggests that the nanowires were composed of silicon and silicon oxide as sheath. Actually, some SiNWs could be oxidized to be silicon oxide nanowires because of the low-vacuum system and high temperature, as found in our previous work [6]. The TEM image of a number of curved SiNWs is shown in Fig. 2.It can be seen that the average diameter of the SiNWs was B20 nm, while the minimum one was less than 10 nm. The structure of the SiNWs is indicated by the selected area electric diffraction (SAED) image in the top right of Fig. 2. The weak electric diffraction spots proved that the SiNWs did not crystallize well. This might be due to the fast growth rate of the SiNWs. The thorough analysis of crystal nature is indicated by the XRD data as shown in Fig. 3. The sharp peaks of Si (1 1 1), Si (3 1 1), Si (4 0 0), and Si (3 3 1) indicates that the SiNWs were crystalline. Some crystal Al peaks and low-intensity peaks of SiS 2 (3 0 1) and SiS 2 (2 1 3) are simultaneously observed. Al peaks ARTICLE IN PRESS Fig. 1. FESEM image of the SiNWs on a silicon wafer. The lower left inset is the EDX taken from the corresponding sample. Fig. 2. TEM image of the SiNWs. The top right inset is the SAED taken from one of the SiNWs. On top left is the magnified TEM image of a SiNW with tip. J. Niu et al. / Physica E 24 (2004) 278–281 279 come from the sample preparation process for the XRD analysis, while SiS 2 comes mainly from the decomposition of SiS. The discussion of the growth mechanism is displayed below. As the function of silicon oxide in the oxygen- assisted mechanism [15,16], in our experiments, the silicon sulfide (SiS) also played a key role in assisting the growth of SiNWs. Therefore, a sulfide-assisted model for SiNW growth is sug- gested here. The reaction procedure mainly took two steps. One was that the sulfur reacted with the silicon substrate and generated SiS compound at the lower temperature (B900 C). The next step was that the SiS decomposed to be Si and SiS 2 at higher temperature (B1000 C). During the de- composition, SiNWs grew up from the generated Si as source. The reaction equations are shown below. S þ O 2 ¼ SO 2 m ð1Þ S þ Si ¼ SiS ðB900 CÞð2Þ 2SiSm ¼ Si þ SiS 2 ðB1000 CÞð3Þ and SiS 2 þ 2H 2 O ¼ SiO 2 þ2H 2 Sm: ð4Þ When the temperature reached B900 C, Eq. (2) happened and the SiS film was produced. Con- tinually, the SiS is decomposed into Si and SiS 2 at higher temperature (B1000 C), which could be confirmed by the XRD spectrum in Fig. 3. Therefore, it is believed that the SiNWs generated from the SiS acted as nucleation centers which were located at the tip of the SiNWs, as shown in the top left of Fig. 2. Thus, the tip should contain SiS 2 . But there was no S signal in the EDX spectrum (the inset of Fig. 1) and only weak SiS 2 peaks are displayed in XRD data (Fig. 3). This is because the SiS 2 could sublimate and disappear when the temperature was higher than 1090 C. Furthermore, SiS 2 was also easy to react with H 2 O, which exists in air, as illustrated in Eq. (4). Surely, not only SiS but also other sulfides such as zinc sulfide, ferric sulfide, etc, which form silicon sulfides by the reaction with silicon, can also be used to assist the growth of SiNWs. In principle, all of silicon compounds, such as silicon sulfide shown in this paper and silicon oxide [17], can assist the formation of SiNWs. It is called the silicon compound-assisted mechanism for SiNWs growth. The SiNWs and bulk silicon in comparison were also checked using Raman spectroscopy (Fig. 4). It is clear that the 510.5 cm À1 peak of the SiNWs (Fig. 4(a)) shows a B 10 cm À1 downshift compared with the 520.3 cm À1 peak of bulk silicon (Fig. 4(b)). Usually, the peak of 510.5 cm À1 was regarded to be the first-order transverse optical phonon mode (TO). The downshift might be ARTICLE IN PRESS 30 40 50 60 70 80 0 500 1000 1500 2000 Al(111) Al(200) SiS 2 (213) SiS 2 (301) Al(220) Al(311) Si(331) Si(400) Si(311) Si(111) Intensity (CPS) 2θ (degrees) Fig. 3. XRD spectrum of the as-grown SiNWs. 400 450 500 550 600 650 700 750 0 1000 2000 3000 4000 b a 510.5cm -1 520.3cm -1 Intensity Raman Shift (cm -1 ) Fig. 4. Raman spectra of SiNWs (a) and bulk silicon (b). J. Niu et al. / Physica E 24 (2004) 278–281280 associated with the quantum confinement effect and laser heating effect [22,23]. 4. Conclusion In summary, silicon nanowires (SiNWs) with a diameter of B20 nm were successfully synthesized on silicon wafers by thermal evaporation of sulfur powders. It is considered that the decomposition of SiS resulted in the formation of SiNWs. Furthermore, a sulfide-assisted growth model of SiNWs was suggested. 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The source of the