- Email: [email protected]
SiC core–shell nanowires by HWCVD
Journal of Crystal Growth 407 (2014) 25–30
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis of nickel catalyzed Si/SiC core–shell nanowires by HWCVD Boon Tong Goh n, Saadah Abdul Rahman Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2014 Received in revised form 1 August 2014 Accepted 3 September 2014 Communicated by J.M. Redwing Available online 16 September 2014
Si/SiC core–shell nanowires grown on glass substrates by hot-wire chemical vapor deposition were studied. Nickel was used as a catalyst to initiate the growth of these core–shell nanowires and the nanowires were grown at different deposition pressures of 0.5 and 1 mbar. The core of the nanowire was found to be a single crystalline Si. The shell of the nanowire consisted of Si nano-crystallites embedded within an amorphous SiC matrix which was attributed to a radial growth of columnar structures. The Si and SiC nano-crystallites embedded within an amorphous matrix exhibited room-temperature photoluminescence emissions in the range of 400 nm–1 μm. A vapor–solid–solid growth mechanism of these core–shell nanowires is proposed. The effects of the deposition pressure on the properties of the core– shell nanowires are also discussed. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Backscattered electron A1. Core–shell nanowires A3. HWCVD B1. Ni nanoparticles B2. Si/SiC
1. Introduction
2. Experimental methods
One dimensional semiconductor nanostructures such as nanowires and nanorods have increased attention recently due to their applications in mesoscopic physics and in the building blocks of nanoscale devices [1–3]. Recently, Si nanowires have attracted great interest among researchers owing to their excellent structural, optical and electrical properties [4,5]. These superior properties have enabled the Si nanowires to achieve excellent performance in solar cells, lithium ion batteries and thermoelectric devices [6–8]. Incorporation of silicon carbide (SiC) nanostructures into the Si nanowires as a core–shell nanowire is expected to further enhance the mechanical, chemical resistivity, thermal stability and a wide range of optical properties of the core–shell nanowires. Hot-wire chemical vapor deposition (HWCVD) is one of the most promising techniques for growing Si based nanowires at low temperature with high deposition rate and large-area deposition [9,10]. In this work, we studied the growth of nickel-catalyzed Si/ SiC core–shell nanowires by HWCVD. The morphological, structural and optical properties of the grown nanowires at different pressures were also reported. Finally, the proposed growth mechanism of these core–shell nanowires is briefly described.
Si/SiC core–shell nanowires were synthesized on Ni coated glass substrates by a home-built HWCVD system. A Ni film of thickness about 3075 nm was thermally evaporated on a heated glass substrates in a vacuum condition. The evaporation pressure and substrate temperature were monitored at 0.45 mbar and 150 1C respectively. Prior to the deposition, the Ni films were treated by energetic atomic hydrogen plasma for 10 min to form Ni nanoparticles. The substrate temperature, pressure, hydrogen flow-rate and radio-frequency power were fixed at 450 1C, 0.75 mbar, 100 sccm and 5 W respectively. During the deposition, the filament temperature and substrate temperature were fixed at 1900 and 450 1C respectively. The filament temperature was measured by using a pyrometer model Reytek, Raynger 3i. The filament-to-substrate distance was fixed at 2 cm. The SiH4, CH4 and H2 flow-rates were fixed at 1, 2 and 100 sccm respectively. The vacuum base pressure achieved was as low as 5 10 7 mbar for the deposition pressures of 0.5 and 1 mbar. The total deposition time was fixed at 5 min. The field-emission scanning electron microscopy (FESEM) images of the nanowires were obtained using a Hitachi SU 8000 scanning electron microscope at accelerating voltage of 2 kV. The energy dispersive X-ray (EDX) spectrum was collected by Oxford Instrument at accelerating voltage of 15 kV. The working distances for imaging and EDX were fixed at 8 and 15 mm respectively. High-resolution transmission electron microscopy (HRTEM) image of the nanowires was obtained by means of a TEM (JEOL JEM2100F) with an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) elemental mappings of the nanowire were performed using a scanning TEM (STEM)/high-angle annular
n
Corresponding author. E-mail address: [email protected] (B. Tong Goh).
http://dx.doi.org/10.1016/j.jcrysgro.2014.09.004 0022-0248/& 2014 Elsevier B.V. All rights reserved.
26
B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
0.5 µm
1µm
2 µm
2 µm
Fig. 1. FESEM secondary electron images of the Si/SiC core–shell nanowires grown by HWCVD at pressure (a) 0.5 and (b) 1 mbar. Insets of the figures (a) and (b) show the backscattered electron images of the nanowires of the respective figure.
dark-field (HAADF) and Oxford EDS detector. The X-ray diffraction (XRD) patterns were recorded in the 2θ range from 101 to 601 at a fixed grazing angle of 51 using a SIEMENS D5000 X-ray diffractometer. The step time and step size of the scanning were fixed at 3 s and 0.021 respectively. The photoluminescence (PL) spectrum of the nanowires at room temperature was recorded using an InVia Raman microscope with a charge-coupled device detector and a grating of 1600 lines/mm. A HeCd laser with an excitation wavelength and laser power of 325 nm and 5 mW respectively were used.
Table 1 Compositions of silicon (Si), carbon (C), oxygen (O) and nickel (Ni) in percentage, of the nanowires measured by EDX elemental analysis. Pressure (mbar)
Nanowire
Atomic % Si
C
O
Ni
0.5
Tip nanowire Stem nanowire Deposited layer
63.9 63 60.5
2.8 5.5 3.2
15.3 16.3 15.4
11.8 10.9 12.1
1
Tip nanowire Stem nanowire Deposited layer
54.6 49.8 43.7
12.7 25 28.9
8.9 7.8 8.0
19.4 15 16.8
3. Results and discussion Fig. 1 shows FESEM secondary electron images of the Si/SiC core–shell nanowires grown by HWCVD at pressures of (a) 0.5 and (b) 1 mbar. High density of tapered nanowires with a vertical alignment was grown at 0.5 mbar. The average length and diameter of these nanowires were about 587 and 51 nm respectively. The increase of pressure to 1 mbar significantly decreased the length and diameter of the nanowires. These nanowires were surrounded by agglomerated grains on the root of the nanowires. Higher pressure enhances the gas-phase reaction which leads to the formation of the agglomerated grains. Backscattered electron (BSE) images of these nanowires are shown in the inset of the each figure. Generally, the BSE image was used to illustrate the distribution of heavier elements that were present on the surface of the nanostructures. The high density of metals gave a significant contrast compared to the matrix in the BSE image. Apparently bright particles on the tips of the nanowires revealed the presence of Ni nanoparticles which act as a catalyst for the growth of the nanowires. The Ni nanoparticles were also found distributed on the stems of nanowires and the agglomerated grains. This could be due to the diffusion of Ni into the nanowires, thus allowing formation of Ni nanoparticles during the growth of the nanowires. The EDX elemental analysis of the nanowires prepared at different pressures are tabulated in Table 1. The presence of 11.8% and 19.4% of Ni on the tip of the nanowires prepared at 0.5 and 1 mbar respectively indicated that the Ni catalyzed the growth of the nanowires. The nanowires prepared at both pressures contained about 50% or higher Si, revealing that the nanowires could be Si-rich Si/SiC core–shell nanowires. The C content showed a significant increase with increased pressure at the tip and stem of the nanowires. The increasing of the C content was due to an enhancement of the gas-phase reactions at higher pressure. As reported by Wu et al. [11], increase in deposition pressure increases the generation of H radicals which enhances the decomposition of CH4 thus producing more C-rich radicals to
the growth surface. More than 10% of Ni found on the stem of the nanowires for both pressures confirmed the diffusion of the Ni into the nanowires. This can lead to a radial growth of SiC shell that can be attributed to the tapering of nanowires and agglomerated grains. The detected O could be related to the formation of SiOx on the surface of the nanowires, which occurred generally during nanowire growth using the CVD technique. In addition, presence of C in the deposited layer near the nanowires revealed the absorption of C into the accumulated Si layer during the growth of the nanowires which will be discussed later in more detail. The microstructure of these nanowires was further investigated by TEM as shown in Fig. 2. A single nanowire of the sample prepared at pressure 0.5 mbar was selected for this TEM measurement. Fig. 2(a) indicates the presence of Ni on the tip of the nanowire surrounded by amorphous SiOx layer. HRTEM scans at the near end of the nanowire sidewall revealed a single crystalline structure of the Si nanowire with core diameter of 7 nm [Fig. 2(b)]. The estimated lattice spacing was about 0.19 nm corresponding to Si (220) crystallographic plane. The growth direction of [110] was further revealed by a fast Fourier transform (FFT) as shown in Fig. 2(c). The nanowires growth in [110] direction by HWCVD was reported previously due to the natural preference orientation of the thin nanowires with a diameter o50 nm [12]. Radial growth of columnar structures were observed in the stem of the nanowires and these columnar structures attributed to the tapering nanowires. The HRTEM images of the columnar structures [Fig. 2 (d) and (e)] revealed that these columnar structures consisted of a mixture of Si nano-crystallites embedded within an amorphous matrix. The estimated crystallite size was approximately 6 72 nm. The preferred orientation of the Si nano-crystallite followed the Si (111) plane. These nanocolumns deposited on the side walls of the nanowires possess an enhancement of light absorption ability which can be used for photovoltaic applications.
B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
220
(a)
27
100 nm
a c b
Si (220)
(d) 0.19 nm
5 nm
3 nm
d
e
0.31 nm
Si (111)
10 nm 10 nm
Fig. 2. HRTEM image of the Si/SiC core–shell nanowires grown by HWCVD at pressure of 0.5 mbar. Insets of (a), (b), (d) and (e) show HRTEM images of the tip, core, stem and columnar growth of the nanowires as labeled in the figure. Inset (c) shows the FFT of the core of the nanowire as shown in inset (b).
100 nm
Si
O
C
Ni
Fig. 3. (a) Dark-field STEM image of the Si/SiC core–shell nanowires grown by HWCVD at pressure of 0.5 mbar. The dotted box in (a) shows the scanning area for the elemental maps on the single nanowire from stem to tip; (b)–(e) represent each EDS element map of the core–shell nanowire. (For interpretation of the reference to color in this figure, the reader is referred to the web version of this article.)
The compositions of the nanowires were further investigated by STEM/EDS elemental mappings using a HAADF detector in the TEM, as shown in Fig. 3. Fig. 3(a) depicts a dark-field STEM image of a single nanowire. The dotted box indicates a scan area for the EDS elemental mappings. The compositions of the nanowire were demonstrated by the EDS maps as shown in Fig. 3(b-e) correspond to the Si, O, C and Ni Kα maps respectively. The nanowire mainly consists of Si, O and C. Clearly, the presence of Ni nanoparticle (blue) at the tip of the nanowire supports the Ni catalyzed growth of these core–shell nanowires by HWCVD. High density of Si can be observed at the stem and it is distributed uniformly along the nanowire from the stem to the tip. Less amount of O appeared at the stem of the nanowires and also amount of O decreased from the stem to the tip. This indicated that oxidation occurred at the initial stage of nucleation during the growth of the nanowires.
The C map showed the presence of C in the scanning area of the nanowire; however it is not clearly presented in this case, which could be due to the background of the C film of the copper grid. Fig. 4 shows the XRD patterns of the nanowires grown by HWCVD at pressures of 0.5 and 1 mbar. The XRD pattern of the nanowires mainly consists of Si and Ni2Si diffraction peaks. The Si diffraction peaks are located at 28.41, 47.31 and 56.11 corresponding to c-Si with the orientations of (111), (220) and (311) planes respectively. The appearance of a small peak located at 35.8 1 indicates a formation of nano-crystallites of 3C–SiC embedded within an amorphous matrix. Presence of NiSi2 diffraction peaks could be due to the diffusion of Ni into the nanowires forming NiSi2 during the growth of the nanowires. The Ni2Si diffraction peaks located at 31.21, 451, 45.71 and 51.8 1 are associated with crystalline Ni2Si orientations of (102), (202), (013) and (004)
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B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
20 #
15
Intensity (arb. units)
10
#
*
# -- Si -- Ni2Si Δ -- SiC
*
Ni SiO2
* Δ
5
*
#
Ni NPs
20 15
#
#
*
*
10
Δ
*
5 0 20
VSS #
Si
25
30
35
40 2θ
45
50
55
60 C H
Fig. 4. XRD spectra of the Si/SiC core–shell nanowires grown by HWCVD at pressure (a) 0.5 and (b) 1 mbar.
VS planes respectively, according to JCPDS card number 00-065-1507. The decrease of the Si diffraction peaks of (111) and (311) planes with an increase in pressure revealed the increasing of the amorphous columnar structures surrounding the nanowires. These columnar structures mainly consisted of Si nano-crystallites embedded within an amorphous matrix. The estimated crystalline grain size was around 6 72 nm, which was about the same as measured by HRTEM. The characterization results showed that those Si/SiC core– shell nanowires can be synthesized by a simple vapor deposition method. The synthesis process involved a series of chemical reactions, in which the atomic H and Ni nanoparticles might play an important role. In order to ensure the exact growth mechanism of these nanowires, the substrate surface temperature during the deposition was measured at 785 1C. The difference temperature of 335 1C between the substrate temperature at the bottom of the substrate and the substrate surface temperature can be understood by the presence of hydrogen assisted heat transfer from hot filament during the process. The deposition temperature involved was considerably lower than the eutectic temperature of Ni–Si (993 1C) and Ni–C (1550 1C) [13,14], although the surface temperature was 785 1C. We suggested that the growth process of these core–shell nanowires follows a vapor–solid–solid (VSS) growth mechanism [15,16]. A schematic diagram of the proposed growth mechanism for these core–shell nanowires is shown in Fig. 5. In the reactions (a and b), the Ni film was deposited and treated by energetic atomic hydrogen forming Ni nanoparticles at a substrate temperature of 450 1C. A similar type of metallic nanoparticle formation had been observed in the works reported by Alet et al. [17] and Nagsen et al. [18]. The atomic hydrogen treatment also played a role in removing the oxide layer on the Ni film before the formation of Ni nanoparticles. In reaction (c), the decomposition of SiH4 and CH4 in the high dilution of H2 by hot filament at temperature above 1800 1C supplied large amounts of Si and C source impinging on the
VS
Si core SiC shell
Fig. 5. Schematic diagram of the proposal growth mechanism for the Si/SiC core– shell nanowires.
surface of the Ni nanoparticles. At the initial stage, the deposition of Si on Ni nanoparticle surface led to Ni diffusion into the accumulated Si layer. Diffusion of Ni into the accumulated layer initiated first NiSi formation layer [19]. Because of its relatively high diffusivity and being a dominant moving species of Ni in the Si layer, it created a nucleation site and induced a precipitation of the Si nanowire. Diffusion of Ni may continue in the growth of nanowire with the deposition of Si. Some amount of Si contributed to the lengthening of the nanowire and the feedback mechanism to continue the process. According to ternary Ni–Si–C phase diagram [20], such low reaction temperature of 785 1C on the substrate surface was not sufficient to achieve a nucleation stage for precipitation of SiC nanowires. There was almost no diffusion of SiC in various Ni silicides as compared to Si in Ni2Si at temperature of 850 1C (diffusivities of Si in Ni2Si and SiC in Ni
PL emission intensity (arb. units)
B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
amorphous matrix or to the size of the nanowires. The various PL emissions in the visible region of the nanowires have revealed that these PL emissions strongly depend on the quantum confinement effects either by the size of the nano-crystallites embedded in the nanowires or the morphology of the nanowires.
4000
3000
2000
4. Conclusion
1000
0 400
29
500
600
700
800
900
1000
Wavelength (nm) Fig. 6. A typical PL spectrum of the Si/SiC core–shell nanowires grown by HWCVD.
silicides are 5.7–5.9 10 14 and 0.04 10 14 m2/s respectively). Therefore the dissolved C atoms were absorbed with excess of Si, mainly Si dangling bonds, and formed SiC clusters on the surface of the accumulated Si layer as shown in reaction (d). These SiC clusters could lead to the formation of SiC nano-crystallites embedded within an amorphous matrix. The SiC on the accumulated Si layer subsequently grew as a shell following the precipitation of the Si nanowires as shown in reaction (e). Moreover, the Ni nanoparticles on the surface of the nanowire also created some growth sites for the SiC formation and followed the radial growth. Finally, the growth of these core–shell nanowires would stop with the reduction of the temperature or with insufficient NiSi. Fig. 6 shows a typical PL emission spectrum of the nanowires prepared by HWCVD. These nanowires exhibited a broad PL emission spectrum in the range between 400 nm and to 1 μm which covered the whole visible part and near infrared regions. This PL emission mainly consisted of five emission bands centered at around 450, 600, 700, 800 and 850 nm. The PL emission band at around 450 nm consisted of two small emission bands located at 415 and 445 nm as shown in the inset of the figure. The emission band centered at around 700 nm was reported to be originated from the quantum confinement effect of the Si nano-crystallites embedded within an amorphous matrix [21]. The emission band located at 600 nm was generally referred to the emission due to the oxygen related defects and/or to surface and interface effects [22,23]. The formation of Si nano-crystallites at the nc-Si/SiO2 interface created an intermediate state for electron–hole radiative recombination that led to the strong emission in the visible region. According to the quantum confinement effect model that has been described by Trwoga et al.[24] the Si nano-crystallites with a diameter less than 10 nm embedded within an amorphous matrix widened the band gap, thus resulting in band gap larger than the band gap of bulk crystal silicon (1.12 eV at room temperature), thus producing PL emission in the visible region. The origin of the PL emission due to the quantum confinement effect follows the 1:39 quantum confinement effect model as [25] EPL ¼ EO þð3:73=d Þ, where EPL is PL energy, EO is the room temperature band gap of bulk c-Si of 1.12 eV and d is the crystallite size of the Si nanocrystallite. The two emission bands at 800 and 850 nm could be due to the localized state transitions of a-Si nanoclusters [26]. The appearance of two small emission bands at 415 and 445 nm indicated the emission of SiC nanostructures [27–29]. These emission bands are comparable to the PL emissions from SiC nanowires [27,28] or crystalline SiC nanoparticles [29]. However, the emissions from these nanowires were obviously blueshifted to lower wavelength as compared with the band gap of bulk 3C–SiC of 2.39 eV (520 nm). This phenomenon was reported as due to the quantum size effect of SiC nano-crystallites embedded within an
Ni-catalyzed Si/SiC core–shell nanowires have been grown by HWCVD on glass substrates at pressure of 0.5 and 1 mbar. The nanowires showed high density tapered nanowires with a vertical alignment at 0.5 mbar. These nanowires consisted of single crystalline Si and amorphous SiC attributed to core and shell of the nanowires respectively. Radial growth of columnar structures formed the shell of the nanowires and thus led to the tapering of the nanowires. The columnar structures consisted of a mixture of Si nano-crystallites embedded within an amorphous matrix. An increase in pressure is attributed to the enhancement of radial growth and agglomerated grains formation. A VSS growth mechanism has been proposed to explain the growth of these core–shell nanowires by the HWCVD using Ni nanoparticles as a catalyst. The Si and SiC nano-crystallites embedded within an amorphous matrix exhibited various room-temperature PL emissions in the visible region.
Acknowledgment This work was supported by the Ministry of Higher Education of Malaysia, for Exploratory Research Grant Scheme (ERGS) of ER003-2013A and the University of Malaya Research Grant (UMRG) Program of RP007B-13AFR. References [1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [2] R.X. Yan, D. Gargas, P.D. Yang, Nat. Photonics 3 (2009) 569. [3] Y. Wan, J. Sha, B. Chen, Y. Fang, Z. Wang, Y. Wang, Recent Pat. Nanotechnol. 3 (2009) 1. [4] J. Bae, H. Kim, X.-M. Zhang, C.H. Dang, Y. Zhang, Y.J. Choi, A. Nurmikko, Z.L. Wang, Nanotechnology 21 (2010) 095502. [5] E. Garnett, P. Yang, Nano Lett 10 (2010) 1082. [6] X. Wang, K.L. Pey, C.H. Yip, E.A. Fitzgerald, D.A. Antoniadis, J. Appl. Phys. 108 (2010) 124303. [7] L.-F. Cui, R. Ruffo, C.K. Chan, H. Peng, Y. Cui, Nano Lett. 9 (2009) 491. [8] Y. Wu, R. Fan, P. Yang, Nano Lett. 2 (2002) 83. [9] S.K. Chong, B.T. Goh, Z. Aspanut, M.R. Muhamad, C.F. Dee, S.A. Rahman, Mater. Lett. 65 (2011) 2452. [10] S.K. Chong, B.T. Goh, C.F. Dee, S.A. Rahman, Mater. Chem. Phys 135 (2012) 635. [11] Tianru Wu, Honglie Shen, Bin. Cheng, Yuanyuan, Bing Liu, Jiancang Shen, Appl. Surf. Sci. 258 (2011) 999. [12] S.K. Chong, C.F. Dee, N. Yahya, S.A. Rahman, J. Nanopart. Res. 15 (2013) 1571. [13] P. Nash, A. Nash, Bull. Alloy Phase Diagr. 8 (1) (1987) 6. [14] L.L. Oden, N.A. Gokcen, Metall. Mater. Trans. A 28A (1997) 2453. [15] S. Kodambaka, J. Tersoff, M.C. Reuter, F.M. Ross, Science 316 (2007) 729. [16] S. Hofmann, R. Sharma, C.T. Wirth, F. Cervantes-Sodi, C. Ducati, T. Kasama, R.E. Dunin-Borkowski, J. Drucker, P. Bennett, J. Robertson, Nat. Mater. 7 (2008) 372. [17] P.J. Alet, L. Eude, S. PalacinP. Roca i Cabarrocas, Phys. Status Solidi A 205 (2008) 1429. [18] Nagsen P. Meshram, Alka KumbharR.O. Dusane, Thin Solid Films 519 (2011) 4609. [19] Jessica L. Lensch-Falk, Eric R. Hemesath, Daniel E. Pere, Lincoln J. Lauhon, J. Mater. Chem. 19 (2009) 849. [20] Jan H. Gulpen, Alexander A. Kodentsov, Frans J.J. van Loo, Z. Metallkd. 86 (8) (1995) 530–539. [21] X.Y. Chen, Y.F. Lu, L.J. Tang, Y.H. Wu, B.J. Cho, X.J. Xu, J.R. Dong, W.D. Song, J. Appl. Phys. 97 (2005) 014913. [22] L.N. Dinh, L.L. Chase, M. Balooch, W.J. Siekhaus, F. Wooten, Phys. Rev. B 54 (1996) 5029. [23] T. Inokuma, Y. Wakayama, T. Muramoto, R. Aoki, Y. Kurata, S. Hasegawa, J. Appl. Phys. 83 (1998) 2228. [24] P.F. Trwoga, A.J. Kenyon, C.W. Pitt, J. Appl. Phys. 83 (1998) 3789.
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[25] A.I. Hochbaum, D. Gargas, Y.J. Hwang, P. Yang, Nano Lett. 9 (2009) 3550. [26] X.X. Wang, J.G. Zhang, L. Ding, B.W. Cheng, W.K. Ge, J.Z. Yu, Q.M. Wang, Phys. Rev. B 72 (2005) 195313. [27] C.H. Liang, G.W. Meng, L.D. Zhang, Y.C. Wu, Z. Cui, Chem. Phys. Lett. 329 (2000) 323.
[28] D.H. Wang, D. Xu, Q. Wang, Y.J. Hao, G.Q. Jin, X.Y. Guo, K.N. Tu, Nanotechnology 19 (2008) 215602. [29] G.C. Xi, S.J. Yu, R. Zhang, M. Zhang, D.K. Ma, Y.T. Qian, J. Phys. Chem. B 109 (2005) 13200.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis of nickel catalyzed Si/SiC core–shell nanowires by HWCVD Boon Tong Goh n, Saadah Abdul Rahman Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2014 Received in revised form 1 August 2014 Accepted 3 September 2014 Communicated by J.M. Redwing Available online 16 September 2014
Si/SiC core–shell nanowires grown on glass substrates by hot-wire chemical vapor deposition were studied. Nickel was used as a catalyst to initiate the growth of these core–shell nanowires and the nanowires were grown at different deposition pressures of 0.5 and 1 mbar. The core of the nanowire was found to be a single crystalline Si. The shell of the nanowire consisted of Si nano-crystallites embedded within an amorphous SiC matrix which was attributed to a radial growth of columnar structures. The Si and SiC nano-crystallites embedded within an amorphous matrix exhibited room-temperature photoluminescence emissions in the range of 400 nm–1 μm. A vapor–solid–solid growth mechanism of these core–shell nanowires is proposed. The effects of the deposition pressure on the properties of the core– shell nanowires are also discussed. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Backscattered electron A1. Core–shell nanowires A3. HWCVD B1. Ni nanoparticles B2. Si/SiC
1. Introduction
2. Experimental methods
One dimensional semiconductor nanostructures such as nanowires and nanorods have increased attention recently due to their applications in mesoscopic physics and in the building blocks of nanoscale devices [1–3]. Recently, Si nanowires have attracted great interest among researchers owing to their excellent structural, optical and electrical properties [4,5]. These superior properties have enabled the Si nanowires to achieve excellent performance in solar cells, lithium ion batteries and thermoelectric devices [6–8]. Incorporation of silicon carbide (SiC) nanostructures into the Si nanowires as a core–shell nanowire is expected to further enhance the mechanical, chemical resistivity, thermal stability and a wide range of optical properties of the core–shell nanowires. Hot-wire chemical vapor deposition (HWCVD) is one of the most promising techniques for growing Si based nanowires at low temperature with high deposition rate and large-area deposition [9,10]. In this work, we studied the growth of nickel-catalyzed Si/ SiC core–shell nanowires by HWCVD. The morphological, structural and optical properties of the grown nanowires at different pressures were also reported. Finally, the proposed growth mechanism of these core–shell nanowires is briefly described.
Si/SiC core–shell nanowires were synthesized on Ni coated glass substrates by a home-built HWCVD system. A Ni film of thickness about 3075 nm was thermally evaporated on a heated glass substrates in a vacuum condition. The evaporation pressure and substrate temperature were monitored at 0.45 mbar and 150 1C respectively. Prior to the deposition, the Ni films were treated by energetic atomic hydrogen plasma for 10 min to form Ni nanoparticles. The substrate temperature, pressure, hydrogen flow-rate and radio-frequency power were fixed at 450 1C, 0.75 mbar, 100 sccm and 5 W respectively. During the deposition, the filament temperature and substrate temperature were fixed at 1900 and 450 1C respectively. The filament temperature was measured by using a pyrometer model Reytek, Raynger 3i. The filament-to-substrate distance was fixed at 2 cm. The SiH4, CH4 and H2 flow-rates were fixed at 1, 2 and 100 sccm respectively. The vacuum base pressure achieved was as low as 5 10 7 mbar for the deposition pressures of 0.5 and 1 mbar. The total deposition time was fixed at 5 min. The field-emission scanning electron microscopy (FESEM) images of the nanowires were obtained using a Hitachi SU 8000 scanning electron microscope at accelerating voltage of 2 kV. The energy dispersive X-ray (EDX) spectrum was collected by Oxford Instrument at accelerating voltage of 15 kV. The working distances for imaging and EDX were fixed at 8 and 15 mm respectively. High-resolution transmission electron microscopy (HRTEM) image of the nanowires was obtained by means of a TEM (JEOL JEM2100F) with an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) elemental mappings of the nanowire were performed using a scanning TEM (STEM)/high-angle annular
n
Corresponding author. E-mail address: [email protected] (B. Tong Goh).
http://dx.doi.org/10.1016/j.jcrysgro.2014.09.004 0022-0248/& 2014 Elsevier B.V. All rights reserved.
26
B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
0.5 µm
1µm
2 µm
2 µm
Fig. 1. FESEM secondary electron images of the Si/SiC core–shell nanowires grown by HWCVD at pressure (a) 0.5 and (b) 1 mbar. Insets of the figures (a) and (b) show the backscattered electron images of the nanowires of the respective figure.
dark-field (HAADF) and Oxford EDS detector. The X-ray diffraction (XRD) patterns were recorded in the 2θ range from 101 to 601 at a fixed grazing angle of 51 using a SIEMENS D5000 X-ray diffractometer. The step time and step size of the scanning were fixed at 3 s and 0.021 respectively. The photoluminescence (PL) spectrum of the nanowires at room temperature was recorded using an InVia Raman microscope with a charge-coupled device detector and a grating of 1600 lines/mm. A HeCd laser with an excitation wavelength and laser power of 325 nm and 5 mW respectively were used.
Table 1 Compositions of silicon (Si), carbon (C), oxygen (O) and nickel (Ni) in percentage, of the nanowires measured by EDX elemental analysis. Pressure (mbar)
Nanowire
Atomic % Si
C
O
Ni
0.5
Tip nanowire Stem nanowire Deposited layer
63.9 63 60.5
2.8 5.5 3.2
15.3 16.3 15.4
11.8 10.9 12.1
1
Tip nanowire Stem nanowire Deposited layer
54.6 49.8 43.7
12.7 25 28.9
8.9 7.8 8.0
19.4 15 16.8
3. Results and discussion Fig. 1 shows FESEM secondary electron images of the Si/SiC core–shell nanowires grown by HWCVD at pressures of (a) 0.5 and (b) 1 mbar. High density of tapered nanowires with a vertical alignment was grown at 0.5 mbar. The average length and diameter of these nanowires were about 587 and 51 nm respectively. The increase of pressure to 1 mbar significantly decreased the length and diameter of the nanowires. These nanowires were surrounded by agglomerated grains on the root of the nanowires. Higher pressure enhances the gas-phase reaction which leads to the formation of the agglomerated grains. Backscattered electron (BSE) images of these nanowires are shown in the inset of the each figure. Generally, the BSE image was used to illustrate the distribution of heavier elements that were present on the surface of the nanostructures. The high density of metals gave a significant contrast compared to the matrix in the BSE image. Apparently bright particles on the tips of the nanowires revealed the presence of Ni nanoparticles which act as a catalyst for the growth of the nanowires. The Ni nanoparticles were also found distributed on the stems of nanowires and the agglomerated grains. This could be due to the diffusion of Ni into the nanowires, thus allowing formation of Ni nanoparticles during the growth of the nanowires. The EDX elemental analysis of the nanowires prepared at different pressures are tabulated in Table 1. The presence of 11.8% and 19.4% of Ni on the tip of the nanowires prepared at 0.5 and 1 mbar respectively indicated that the Ni catalyzed the growth of the nanowires. The nanowires prepared at both pressures contained about 50% or higher Si, revealing that the nanowires could be Si-rich Si/SiC core–shell nanowires. The C content showed a significant increase with increased pressure at the tip and stem of the nanowires. The increasing of the C content was due to an enhancement of the gas-phase reactions at higher pressure. As reported by Wu et al. [11], increase in deposition pressure increases the generation of H radicals which enhances the decomposition of CH4 thus producing more C-rich radicals to
the growth surface. More than 10% of Ni found on the stem of the nanowires for both pressures confirmed the diffusion of the Ni into the nanowires. This can lead to a radial growth of SiC shell that can be attributed to the tapering of nanowires and agglomerated grains. The detected O could be related to the formation of SiOx on the surface of the nanowires, which occurred generally during nanowire growth using the CVD technique. In addition, presence of C in the deposited layer near the nanowires revealed the absorption of C into the accumulated Si layer during the growth of the nanowires which will be discussed later in more detail. The microstructure of these nanowires was further investigated by TEM as shown in Fig. 2. A single nanowire of the sample prepared at pressure 0.5 mbar was selected for this TEM measurement. Fig. 2(a) indicates the presence of Ni on the tip of the nanowire surrounded by amorphous SiOx layer. HRTEM scans at the near end of the nanowire sidewall revealed a single crystalline structure of the Si nanowire with core diameter of 7 nm [Fig. 2(b)]. The estimated lattice spacing was about 0.19 nm corresponding to Si (220) crystallographic plane. The growth direction of [110] was further revealed by a fast Fourier transform (FFT) as shown in Fig. 2(c). The nanowires growth in [110] direction by HWCVD was reported previously due to the natural preference orientation of the thin nanowires with a diameter o50 nm [12]. Radial growth of columnar structures were observed in the stem of the nanowires and these columnar structures attributed to the tapering nanowires. The HRTEM images of the columnar structures [Fig. 2 (d) and (e)] revealed that these columnar structures consisted of a mixture of Si nano-crystallites embedded within an amorphous matrix. The estimated crystallite size was approximately 6 72 nm. The preferred orientation of the Si nano-crystallite followed the Si (111) plane. These nanocolumns deposited on the side walls of the nanowires possess an enhancement of light absorption ability which can be used for photovoltaic applications.
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220
(a)
27
100 nm
a c b
Si (220)
(d) 0.19 nm
5 nm
3 nm
d
e
0.31 nm
Si (111)
10 nm 10 nm
Fig. 2. HRTEM image of the Si/SiC core–shell nanowires grown by HWCVD at pressure of 0.5 mbar. Insets of (a), (b), (d) and (e) show HRTEM images of the tip, core, stem and columnar growth of the nanowires as labeled in the figure. Inset (c) shows the FFT of the core of the nanowire as shown in inset (b).
100 nm
Si
O
C
Ni
Fig. 3. (a) Dark-field STEM image of the Si/SiC core–shell nanowires grown by HWCVD at pressure of 0.5 mbar. The dotted box in (a) shows the scanning area for the elemental maps on the single nanowire from stem to tip; (b)–(e) represent each EDS element map of the core–shell nanowire. (For interpretation of the reference to color in this figure, the reader is referred to the web version of this article.)
The compositions of the nanowires were further investigated by STEM/EDS elemental mappings using a HAADF detector in the TEM, as shown in Fig. 3. Fig. 3(a) depicts a dark-field STEM image of a single nanowire. The dotted box indicates a scan area for the EDS elemental mappings. The compositions of the nanowire were demonstrated by the EDS maps as shown in Fig. 3(b-e) correspond to the Si, O, C and Ni Kα maps respectively. The nanowire mainly consists of Si, O and C. Clearly, the presence of Ni nanoparticle (blue) at the tip of the nanowire supports the Ni catalyzed growth of these core–shell nanowires by HWCVD. High density of Si can be observed at the stem and it is distributed uniformly along the nanowire from the stem to the tip. Less amount of O appeared at the stem of the nanowires and also amount of O decreased from the stem to the tip. This indicated that oxidation occurred at the initial stage of nucleation during the growth of the nanowires.
The C map showed the presence of C in the scanning area of the nanowire; however it is not clearly presented in this case, which could be due to the background of the C film of the copper grid. Fig. 4 shows the XRD patterns of the nanowires grown by HWCVD at pressures of 0.5 and 1 mbar. The XRD pattern of the nanowires mainly consists of Si and Ni2Si diffraction peaks. The Si diffraction peaks are located at 28.41, 47.31 and 56.11 corresponding to c-Si with the orientations of (111), (220) and (311) planes respectively. The appearance of a small peak located at 35.8 1 indicates a formation of nano-crystallites of 3C–SiC embedded within an amorphous matrix. Presence of NiSi2 diffraction peaks could be due to the diffusion of Ni into the nanowires forming NiSi2 during the growth of the nanowires. The Ni2Si diffraction peaks located at 31.21, 451, 45.71 and 51.8 1 are associated with crystalline Ni2Si orientations of (102), (202), (013) and (004)
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20 #
15
Intensity (arb. units)
10
#
*
# -- Si -- Ni2Si Δ -- SiC
*
Ni SiO2
* Δ
5
*
#
Ni NPs
20 15
#
#
*
*
10
Δ
*
5 0 20
VSS #
Si
25
30
35
40 2θ
45
50
55
60 C H
Fig. 4. XRD spectra of the Si/SiC core–shell nanowires grown by HWCVD at pressure (a) 0.5 and (b) 1 mbar.
VS planes respectively, according to JCPDS card number 00-065-1507. The decrease of the Si diffraction peaks of (111) and (311) planes with an increase in pressure revealed the increasing of the amorphous columnar structures surrounding the nanowires. These columnar structures mainly consisted of Si nano-crystallites embedded within an amorphous matrix. The estimated crystalline grain size was around 6 72 nm, which was about the same as measured by HRTEM. The characterization results showed that those Si/SiC core– shell nanowires can be synthesized by a simple vapor deposition method. The synthesis process involved a series of chemical reactions, in which the atomic H and Ni nanoparticles might play an important role. In order to ensure the exact growth mechanism of these nanowires, the substrate surface temperature during the deposition was measured at 785 1C. The difference temperature of 335 1C between the substrate temperature at the bottom of the substrate and the substrate surface temperature can be understood by the presence of hydrogen assisted heat transfer from hot filament during the process. The deposition temperature involved was considerably lower than the eutectic temperature of Ni–Si (993 1C) and Ni–C (1550 1C) [13,14], although the surface temperature was 785 1C. We suggested that the growth process of these core–shell nanowires follows a vapor–solid–solid (VSS) growth mechanism [15,16]. A schematic diagram of the proposed growth mechanism for these core–shell nanowires is shown in Fig. 5. In the reactions (a and b), the Ni film was deposited and treated by energetic atomic hydrogen forming Ni nanoparticles at a substrate temperature of 450 1C. A similar type of metallic nanoparticle formation had been observed in the works reported by Alet et al. [17] and Nagsen et al. [18]. The atomic hydrogen treatment also played a role in removing the oxide layer on the Ni film before the formation of Ni nanoparticles. In reaction (c), the decomposition of SiH4 and CH4 in the high dilution of H2 by hot filament at temperature above 1800 1C supplied large amounts of Si and C source impinging on the
VS
Si core SiC shell
Fig. 5. Schematic diagram of the proposal growth mechanism for the Si/SiC core– shell nanowires.
surface of the Ni nanoparticles. At the initial stage, the deposition of Si on Ni nanoparticle surface led to Ni diffusion into the accumulated Si layer. Diffusion of Ni into the accumulated layer initiated first NiSi formation layer [19]. Because of its relatively high diffusivity and being a dominant moving species of Ni in the Si layer, it created a nucleation site and induced a precipitation of the Si nanowire. Diffusion of Ni may continue in the growth of nanowire with the deposition of Si. Some amount of Si contributed to the lengthening of the nanowire and the feedback mechanism to continue the process. According to ternary Ni–Si–C phase diagram [20], such low reaction temperature of 785 1C on the substrate surface was not sufficient to achieve a nucleation stage for precipitation of SiC nanowires. There was almost no diffusion of SiC in various Ni silicides as compared to Si in Ni2Si at temperature of 850 1C (diffusivities of Si in Ni2Si and SiC in Ni
PL emission intensity (arb. units)
B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30
amorphous matrix or to the size of the nanowires. The various PL emissions in the visible region of the nanowires have revealed that these PL emissions strongly depend on the quantum confinement effects either by the size of the nano-crystallites embedded in the nanowires or the morphology of the nanowires.
4000
3000
2000
4. Conclusion
1000
0 400
29
500
600
700
800
900
1000
Wavelength (nm) Fig. 6. A typical PL spectrum of the Si/SiC core–shell nanowires grown by HWCVD.
silicides are 5.7–5.9 10 14 and 0.04 10 14 m2/s respectively). Therefore the dissolved C atoms were absorbed with excess of Si, mainly Si dangling bonds, and formed SiC clusters on the surface of the accumulated Si layer as shown in reaction (d). These SiC clusters could lead to the formation of SiC nano-crystallites embedded within an amorphous matrix. The SiC on the accumulated Si layer subsequently grew as a shell following the precipitation of the Si nanowires as shown in reaction (e). Moreover, the Ni nanoparticles on the surface of the nanowire also created some growth sites for the SiC formation and followed the radial growth. Finally, the growth of these core–shell nanowires would stop with the reduction of the temperature or with insufficient NiSi. Fig. 6 shows a typical PL emission spectrum of the nanowires prepared by HWCVD. These nanowires exhibited a broad PL emission spectrum in the range between 400 nm and to 1 μm which covered the whole visible part and near infrared regions. This PL emission mainly consisted of five emission bands centered at around 450, 600, 700, 800 and 850 nm. The PL emission band at around 450 nm consisted of two small emission bands located at 415 and 445 nm as shown in the inset of the figure. The emission band centered at around 700 nm was reported to be originated from the quantum confinement effect of the Si nano-crystallites embedded within an amorphous matrix [21]. The emission band located at 600 nm was generally referred to the emission due to the oxygen related defects and/or to surface and interface effects [22,23]. The formation of Si nano-crystallites at the nc-Si/SiO2 interface created an intermediate state for electron–hole radiative recombination that led to the strong emission in the visible region. According to the quantum confinement effect model that has been described by Trwoga et al.[24] the Si nano-crystallites with a diameter less than 10 nm embedded within an amorphous matrix widened the band gap, thus resulting in band gap larger than the band gap of bulk crystal silicon (1.12 eV at room temperature), thus producing PL emission in the visible region. The origin of the PL emission due to the quantum confinement effect follows the 1:39 quantum confinement effect model as [25] EPL ¼ EO þð3:73=d Þ, where EPL is PL energy, EO is the room temperature band gap of bulk c-Si of 1.12 eV and d is the crystallite size of the Si nanocrystallite. The two emission bands at 800 and 850 nm could be due to the localized state transitions of a-Si nanoclusters [26]. The appearance of two small emission bands at 415 and 445 nm indicated the emission of SiC nanostructures [27–29]. These emission bands are comparable to the PL emissions from SiC nanowires [27,28] or crystalline SiC nanoparticles [29]. However, the emissions from these nanowires were obviously blueshifted to lower wavelength as compared with the band gap of bulk 3C–SiC of 2.39 eV (520 nm). This phenomenon was reported as due to the quantum size effect of SiC nano-crystallites embedded within an
Ni-catalyzed Si/SiC core–shell nanowires have been grown by HWCVD on glass substrates at pressure of 0.5 and 1 mbar. The nanowires showed high density tapered nanowires with a vertical alignment at 0.5 mbar. These nanowires consisted of single crystalline Si and amorphous SiC attributed to core and shell of the nanowires respectively. Radial growth of columnar structures formed the shell of the nanowires and thus led to the tapering of the nanowires. The columnar structures consisted of a mixture of Si nano-crystallites embedded within an amorphous matrix. An increase in pressure is attributed to the enhancement of radial growth and agglomerated grains formation. A VSS growth mechanism has been proposed to explain the growth of these core–shell nanowires by the HWCVD using Ni nanoparticles as a catalyst. The Si and SiC nano-crystallites embedded within an amorphous matrix exhibited various room-temperature PL emissions in the visible region.
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