H2 gas mixture

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Diamond & Related Materials 17 (2008) 1660 – 1665 www.elsevier.com/locate/diamond

Synthesis of carbon coated β-SiC nanofibers by microwave plasma assisted chemical vapour deposition in CH4/H2 gas mixture S. Rizk a , M.B. Assouar a,⁎, C. Gatel b , M. Belmahi a , J. Lambert c , J. Bougdira a Laboratoire de Physique des Milieux Ionisés et Applications, Nancy University — CNRS, Boulevard des Aiguillettes — BP 239, F-54506 Vandoeuvre-lès-Nancy Cédex, France Centre d'Elaboration de Matériaux et d'Etudes Structurales, CNRS UPR 801, 29, rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France c Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564 CNRS — Université Henri Poincaré, Nancy I 405 rue de Vandoeuvre, F-54600 Villers les Nancy, France a

b

Available online 13 February 2008

Abstract A simple and direct synthesis method was used to grow silicon carbide nanofibers in CH4/H2 mixture on silicon substrates covered by Fe thin film catalyst using microwave plasma assisted chemical vapour deposition. The silicon source is the substrate itself. The samples have been characterized by field emission scanning electron microscopy and transmission electron microscopy combined with electron energydispersive X-ray spectroscopy. These characterizations revealed that fibrous nanostructures having stacking faults planes perpendicular to the growth direction coexist with fibrous with no stacking fault. These two types have a core–shell structure, with a diameter of 25–65 nm, and were both assigned to β-SiC. Selected area electron diffraction pattern shows that the faulted SiC structures exhibit streaks indicating defects (irregular layers) while the other ones have a single crystal pattern. One can also observe that the SiC nanofibers grow along (111) orientation. The formation of SiC nanofibers can be explained by the diffusion of Fe in the silicon substrate due to the high temperature during the process which is around 900 °C. The combination of the etching of the surface by atomic hydrogen and the interaction with carbon radicals and carbon atoms allows then the growth of SiC nanofibers. © 2008 Elsevier B.V. All rights reserved. Keywords: Silicon carbide (SiC); Nanofibers; Plasma CVD; Catalytic processes; HRTEM

1. Introduction Silicon carbide (SiC) is an important semiconductor which can be operated at high powers, high temperatures, and high frequencies. It is well known that nanostructures with sharp tips are promising materials for applications as the cold cathode field emission devices [1–4]. Many kinds of nanomaterials have attracted attention as electrons emitters for field emission display applications, including carbon nanotubes, silicon and silicon carbide nanowires [2]. In particular, one-dimensional nanostructures of SiC such as nanofibers have high aspect ⁎ Corresponding author. LPMIA — Université Nancy I, Boulevard des Aiguillettes, BP 239, F-54506 Vandoeuvre-lès-Nancy, France. Tel.: +33 3 83 68 49 05; fax: +33 3 83 68 49 33. E-mail address: [email protected] (M.B. Assouar). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.108

ratios, which are suitable for field emitters due to their field enhancement effect. Several techniques have been applied to synthesize SiC nanostructures, including sol–gel [1], carbothermal reduction of SiO2, decomposition of organic silicon compounds, laser ablation [5] and chemical vapour deposition (CVD) [6]. Concerning this last method, they are few reports on the growth of SiC nanofibers using microwave plasma assisted chemical vapour deposition (MPACVD). In this work, we utilized a microwave plasma CVD apparatus under CH4/H2 gas mixture to synthesize fibrous nanostructures of SiC on silicon substrate covered by iron thin film without any silicon gas sources. The silicon source is the substrate itself. First characterizations have been done using field emission scanning electronic microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) analyses and suggest SiC nanofibers synthesis. Furthermore,

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characterizations using high resolution transmission electron microscopy (HRTEM) combined with energy-dispersive X-ray spectroscopy (EDXS) and selected area electron diffraction (SAED) reveal that the resultant nanostructures are assigned to β-SiC nanofibers with good crystallinity. The growth mechanism of SiC nanofibers is proposed in the discussion section. 2. Experimental set-up The SiC growth reactor used in this study is composed of a cylindrical quartz tube (50 mm in diameter, 350 mm in length) which intersects a rectangular waveguide, the dimensions of which have been chosen to drive the TE10 mode of a 2.45 GHz microwave provided by a 0–1200 W power generator. The CH4/H2 plasma is generated in the cylindrical tube. The temperature is measured by means of an infrared bicolour pyrometer. The gas mixture composition is ensured by mass flowmeters that are computer controlled in order to maintain both the CH4/H2 ratio and the total pressure at constant values [7,8]. The substrates used in this study are Si (100) covered by a Fe catalyst thin film which we have deposited by a high vacuum thermal deposition system. The thickness of Fe thin film was of about 10 nm. Prior to the growth of SiC nanofibers, the Fe/Si substrates have been pre-treated by hydrogen plasma during 15 min. The

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discharge parameters are 300 W microwave power, the concentration of CH4 in CH4/H2 gas mixture was fixed at 10%, the total gas pressure was varied from 15 mbar to 60 mbar, the temperature of growth was around 900 °C and the growth duration was kept to 45 min. Surface morphology of the resultant fibrous structures was investigated using FESEM, while the structural and compositional characterizations were carried out using TEM and HRTEM combined with EDXS and SAED. 3. Results and discussion 3.1. Catalyst and pre-treatment In order to control the catalyst deposition step before pretreatment and growth, the thickness and structure of the catalyst iron layer deposited on a silicon substrate was analysed by TEM. EDXS spectrum indicates that the layer is made of iron and contains a bit of oxygen [9]. In a second step, the 10 nm Fe/Si sample has undergone plasma hydrogen pre-treatment. The nature and structure of this pre-treated sample were analysed by TEM, EDXS, and SAED. The corresponding TEM and EDXS results, not shown here, [see reference 9] reveal the presence of a 100 nm layer of silicon and iron. We suggest with the analysis of the SAED pattern we obtained, that the iron

Fig. 1. FESEM micrographs showing the effect of CH4/H2 gas pressure on the density and length of the SiC nanofibers. (a) 15 mbar, (b) 25 mbar, (c) 45 mbar, (d) 60 mbar.

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diffuses into silicon and leads to the formation of β-FeSi2 iron silicide observed also by other authors [10,11]. 3.2. Growth of SiC Before proceeding to the growth of SiC nanofibers, each sample has undergone plasma hydrogen pre-treatment for 15 min to nanostructure the catalyst. After the pre-treatment step and without stopping the plasma, we introduced the CH4/H2 gas mixture into the chamber, and changed plasma parameters. We first investigated the effect of the total gas pressure on the nanofibers density, length and diameter. The total pressure was varied from 15 to 60 mbar. Microwave power (300 W), mass flow rates (50 sccm CH4/H2) and synthesis time (45 min) were kept constant during the growth. FESEM micrographs of the different samples shown on Fig. 1 reveal that the density and length of the nanofibers increase with increasing total CH4/H2 gas pressure from 15 to 25 mbar, and decrease for higher pressures. The fibers diameters range approximately between 20 and 80 nm for all the samples, and the same morphology was observed all over the substrate. We note that the same effect was observed when the plasma microwave power varies from 200 to 400 W for a fixed pressure of 25 mbar. We conclude that there is a particular range of pressure or microwave power for which favourable growth occurs. When increasing total CH4/H2 gas pressure, the total number of reactive species is increased in the plasma. Consequently, more excited species are involved causing an increase in density and length of our SiC nanofibers. At high plasma pressures, the gas becomes more energetic and can alter or etch the fibers that can explain the decrease of the fibers density. XPS measurements were carried out on our samples to obtain composition information. A typical survey spectrum of one of our SiC samples is presented in Fig. 2(a). It shows Si2p, C1s and O1s peaks of high intensity. High-resolution XPS spectra of Si2p

and C1s were then obtained for the same sample. For analysis, every spectrum had its background removed by the Shirley subtraction method and was fitted to a Gaussian–Lorentzian function. Fig. 2(b) and (c) shows the peaks corresponding to Si2p core level and C1s core level, respectively. The spectrum of Si2p can be decomposed into two components. The strongest component is located at 100.96 eV and can be attributed to SiC. The second one is located at 103 eV and can be attributed to SiO2 contamination. For the case of C1s, it consists of three Gaussian–Lorentzian components centred at 283.22, 284.44, and 285.29 eV. The peak at 283.22 eV corresponds to C1s binding energy of SiC. The other two peaks can be assigned to the carbon of the outer layer (284.4 eV for C1s in graphite) and the adsorbed CO2 on sample surface. Quantification analysis of the peaks we obtained presents a Si and C atomic ratio of 1:1.3 indicating that the formed SiC nanofibers could be a candidate material for field emitter due to its carbon-rich stoichiometry [12]. To investigate in more details the composition and morphology of the obtained SiC nanofibers, TEM combined with EDXS analysis was performed on our samples. Concerning TEM analyses, nanofibers are picked up by a diamond point and put on a grid for observation. TEM micrographs performed on the same sample reveal the existence of two sorts of SiC nanofibers: one presenting defects and the other without defects. Fig. 3(a) shows a typical TEM micrograph of the first sort. The single SiC nanofiber has alternately bright and dark strips. The core part of the fiber appears to have numerous stacking faults. Stacking faults are known to exist in silicon carbide nanostructures independently on the deposition method [13–15]. A similar morphology was observed by C.H. Liang et al. [16] and Kang et al. [12]. In order to get more information on this SiC structure, SAED pattern was performed on the fiber. The corresponding pattern shown in the inset image in Fig. 3(a) exhibits featureless streaks and spots of irregular layers of a

Fig. 2. XPS spectra of the grown SiC nanofibers. (a) XPS survey spectrum (b) XPS spectra of C1s (c) XPS spectra of Si2p.

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Fig. 3. TEM micrograph of a β-SiC nanofibers growing along [111] direction. (a) Shows a nanofiber with stacking faults with its corresponding selected area electron diffraction pattern (b). (c) Shows a SiC nanofiber without defects with its corresponding selected area electron diffraction pattern (d).

β-SiC single crystalline phase and demonstrates the [111] orientation of the nanofibers. The streaks are representative of a disordered layer structure. The same pattern was also observed by Liang et al. [16], Hao et al. [1], and Seo et al. [14]. Furthermore, since the streaks are always perpendicular to the stacking faults planes {111}, therefore the fiber's main growth direction is parallel to [111] direction [17–19]. The surface energy of the {111} planes in the β-SiC structures is much smaller than those of the other crystal planes; therefore, it is generally accepted that β-SiC nanofibers can grow easily in the [111] direction, to decrease the formation energy, and, hence, stacking faults can be inserted easily in the {111} planes [14,18]. Fig. 3(b) presents the TEM micrograph of the defects free SiC nanofiber. The core part of the fiber has a relatively smooth surface and is composed of a single crystalline structure of a diameter of 60 nm. The corresponding pattern shown in the inset image in Fig. 3(b) presents the diffraction spots which imply a single crystalline phase. The diffraction pattern indexation confirms the cubic β-SiC nature of the fiber growing along the [111] direction. Similar structures were observed by [20]. We note that both types present a core–shell structure, and are enveloped with an outer sheath which nature will be determined

later on in this paper. Their average diameters range from 25 to 65 nm. Concerning the chemical composition of the nanoparticles in the top of the SiC fibers showed in Fig. 3(b), the EDXS carried out on these nanoparticles reveals the presence of iron, silicon, and carbon. The Fig. 4 exhibits the EDXS spectrum related to the nanoparticles. The inset of the Fig. 4 shows the structure of the entire nanofiber presenting stacking faults as shown in Fig. 3(a). As we have showed in Fig. 3, we have obtained two kinds of β-SiC nanofibers. The existence of both kinds depends on the species energy existing in the plasma. The ratio of the two kinds of obtained nanofibers was not determined at this step of the study. EDXS analyses were performed on both types of the synthesized SiC nanofibers. No main differences were found between the faulted and non-faulted structure with regard of the composition. The corresponding spectrum reveals that they are composed mainly of silicon and carbon [9]. One can observe that the oxygen component hardly appeared in our spectrum. In previous reported papers, the outer layer that wraps the SiC nanofiber, can be made of amorphous silicon oxide SiOx [16,21–24] or amorphous carbon [12,25] or graphite [26]. Since none of our sources used to grow the SiC nanofibers contain

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Fig. 4. EDXS spectrum of the attached spherical nanoparticles. The inset shows the nanoparticle at the tip of the SiC nanofiber.

oxygen the possibility of forming SiOx in our case is too small. The atomic percentage ratio of Si and C is 10.6:88 indicating rather a carbon layer formation than any other phase. The internal structure of the synthesised nanofibers was investigated using HRTEM. Fig. 5(a) and (b) presents the typical HRTEM micrographs of the faulted and non-faulted SiC nanofibers respectively. From Fig. 5(a) we can see the partially disordered structure, where the numerous stacking faults are perpendicular to the growth direction. Fig. 5(b) shows the single crystalline smooth core of the nanofiber. The inset of the Fig. 5 (b), shows the periodic lattice structure and the d-spacing of the lattice fringes is 2.5 which corresponds to the (111) plane spacing of cubic β-SiC. All HRTEM micrographs reveal the core–shell structure. Based on TEM micrographs, we reported in our previous work [9], that the outer layer was an amorphous layer. From the inset of Fig. 5(a) we can see that the layer is not amorphous but crystalline. The lattice spacing was measured to be 3.34 corresponding to the d-spacing of the graphite. So we have synthesised β-SiC nanofibers coated with graphite sheets. On the different FESEM, and HRTEM micrographs we note the presence of spherical nanoparticles at the tip of our nanofibers [9]. These particles act as catalysts so during the CH4/H2 plasma, atomic hydrogen species play a role, via etching the substrate, in incorporating Si and C atoms into the nanoparticle. The incorporated Si and C atoms diffuse and precipitate promoting the growth of β-SiC nanofibers and its outer layer following the Vapor–Liquid–Solid (VLS) growth mechanism as proposed by other authors [6,21]. 4. Conclusion Fig. 5. HRTEM micrograph of a β-SiC nanofiber (a) showing the stacking faults structure. The inset shows the graphite sheets that constitute the shell. (b) β-SiC nanofiber without stacking faults. The inset shows the periodic lattice structure of the corresponding fiber.

Under CH4/H2 plasma using MPACVD technique, we succeeded to synthesise cubic β-SiC nanofibers directly on Si substrate covered by a Fe catalyst layer. The originality of this

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growth is the use of the silicon substrate as the provider of the Si component for the SiC nanofibers synthesis and the use of a relatively low temperature synthesis technique. The formation of these nanofibers can be explained by precipitation of SiC from supersaturated Fe nanoparticles containing Si and C based on VLS mechanism. HRTEM, SAED and EDXS analyses show that the nanofibers have a core–shell structure. The core structure is composed of single crystalline β-SiC and can present stacking faults. The shell structure is found to be made of graphite sheets. References [1] Y.J. Hao, G.Q. Jin, X.D. Han, X.Y. Guo, Mater. Lett. 60 (2006) 1334. [2] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassel, H. Dai, Science 283 (1999) 512. [3] J. Li, W. Lei, X. Zhang, X. Zhou, Q. Wang, Appl. Surf. Sci. 220 (2003) 96. [4] Y.B. Li, Y. Bando, D. Golberg, Appl. Phys. Lett. 84 (2004) 3603. [5] W. Shi, Y. Zheng, H. Peng, N. Wang, C. Sing Lee, S-T. Lee, J. Am. Ceram. Soc. 83/12 (2000) 3228. [6] S.I. Honda, Y.G. Baek, T. Ikuno, H. Kohara, M. Katayama, K. Oura, T. Hirao, Appl. Surf. Sci. 212–213 (2003) 378. [7] M. Belmahi, F. Bénédic, J. Bougdira, H. Chatei, M. Rémy, P. Alnot, Surf. Coat. Technol. 106 (1998) 53. [8] H. Chatei, M. Belmahi, M.B. Assouar, L. Le Brizoual, P. Bourson, J. Bougdira, Diamond Relat. Mater. 15 (2006) 1041.

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