Synthesis of nickel catalyzed Si/SiC core–shell nanowires by HWCVD

Boon Tong Goh n, Saadah Abdul RahmanLow Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 12 March 2014Received in revised form1 August 2014Accepted 3 September 2014Communicated by J.M. RedwingAvailable online 16 September 2014

Keywords:A1. Backscattered electronA1. Core–shell nanowiresA3. HWCVDB1. Ni nanoparticlesB2. Si/SiC

a b s t r a c t

Si/SiC core–shell nanowires grown on glass substrates by hot-wire chemical vapor deposition werestudied. Nickel was used as a catalyst to initiate the growth of these core–shell nanowires and thenanowires were grown at different deposition pressures of 0.5 and 1 mbar. The core of the nanowire wasfound to be a single crystalline Si. The shell of the nanowire consisted of Si nano-crystallites embeddedwithin an amorphous SiC matrix which was attributed to a radial growth of columnar structures. The Siand SiC nano-crystallites embedded within an amorphous matrix exhibited room-temperature photo-luminescence emissions in the range of 400 nm–1 μm. A vapor–solid–solid growth mechanism of thesecore–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.

1. Introduction

One dimensional semiconductor nanostructures such as nano-wires and nanorods have increased attention recently due to theirapplications in mesoscopic physics and in the building blocks ofnanoscale devices [1–3]. Recently, Si nanowires have attractedgreat interest among researchers owing to their excellent struc-tural, optical and electrical properties [4,5]. These superior proper-ties have enabled the Si nanowires to achieve excellentperformance in solar cells, lithium ion batteries and thermo-electric devices [6–8]. Incorporation of silicon carbide (SiC) nanos-tructures into the Si nanowires as a core–shell nanowire isexpected to further enhance the mechanical, chemical resistivity,thermal stability and a wide range of optical properties of thecore–shell nanowires.

Hot-wire chemical vapor deposition (HWCVD) is one of themost promising techniques for growing Si based nanowires at lowtemperature 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, struc-tural and optical properties of the grown nanowires at differentpressures were also reported. Finally, the proposed growthmechanism of these core–shell nanowires is briefly described.

2. Experimental methods

Si/SiC core–shell nanowires were synthesized on Ni coated glasssubstrates by a home-built HWCVD system. A Ni film of thicknessabout 3075 nm was thermally evaporated on a heated glasssubstrates in a vacuum condition. The evaporation pressure andsubstrate temperature were monitored at 0.45 mbar and 150 1Crespectively. Prior to the deposition, the Ni films were treated byenergetic atomic hydrogen plasma for 10 min to form Ni nanoparti-cles. The substrate temperature, pressure, hydrogen flow-rate andradio-frequency power were fixed at 450 1C, 0.75 mbar, 100 sccmand 5W respectively. During the deposition, the filament tempera-ture and substrate temperature were fixed at 1900 and 450 1Crespectively. The filament temperature was measured by using apyrometer model Reytek, Raynger 3i. The filament-to-substratedistance was fixed at 2 cm. The SiH4, CH4 and H2 flow-rates werefixed at 1, 2 and 100 sccm respectively. The vacuum base pressureachieved was as low as 5�10�7 mbar for the deposition pressures of0.5 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 8000scanning electron microscope at accelerating voltage of 2 kV. Theenergy dispersive X-ray (EDX) spectrum was collected by OxfordInstrument at accelerating voltage of 15 kV. The working distancesfor imaging and EDX were fixed at 8 and 15 mm respectively.High-resolution transmission electron microscopy (HRTEM) imageof the nanowires was obtained by means of a TEM (JEOL JEM-2100F) with an accelerating voltage of 200 kV. Energy dispersiveX-ray spectroscopy (EDS) elemental mappings of the nanowirewere performed using a scanning TEM (STEM)/high-angle annular

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.09.0040022-0248/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author.E-mail address: boontong77@yahoo.com (B. Tong Goh).

Journal of Crystal Growth 407 (2014) 25–30

dark-field (HAADF) and Oxford EDS detector. The X-ray diffraction(XRD) patterns were recorded in the 2θ range from 101 to 601 at afixed grazing angle of 51 using a SIEMENS D5000 X-ray diffract-ometer. The step time and step size of the scanning were fixed at3 s and 0.021 respectively. The photoluminescence (PL) spectrumof the nanowires at room temperature was recorded using anInVia Raman microscope with a charge-coupled device detectorand a grating of 1600 lines/mm. A HeCd laser with an excitationwavelength and laser power of 325 nm and 5 mW respectivelywere used.

3. Results and discussion

Fig. 1 shows FESEM secondary electron images of the Si/SiCcore–shell nanowires grown by HWCVD at pressures of (a) 0.5and (b) 1 mbar. High density of tapered nanowires with a verticalalignment was grown at 0.5 mbar. The average length anddiameter of these nanowires were about 587 and 51 nm respec-tively. The increase of pressure to 1 mbar significantly decreasedthe length and diameter of the nanowires. These nanowires weresurrounded by agglomerated grains on the root of the nanowires.Higher pressure enhances the gas-phase reaction which leads tothe formation of the agglomerated grains. Backscattered electron(BSE) images of these nanowires are shown in the inset of the eachfigure. Generally, the BSE image was used to illustrate thedistribution of heavier elements that were present on the surfaceof the nanostructures. The high density of metals gave a significantcontrast compared to the matrix in the BSE image. Apparentlybright particles on the tips of the nanowires revealed the presenceof Ni nanoparticles which act as a catalyst for the growth of thenanowires. The Ni nanoparticles were also found distributed onthe stems of nanowires and the agglomerated grains. This could bedue to the diffusion of Ni into the nanowires, thus allowingformation of Ni nanoparticles during the growth of the nanowires.

The EDX elemental analysis of the nanowires prepared atdifferent pressures are tabulated in Table 1. The presence of11.8% and 19.4% of Ni on the tip of the nanowires prepared at0.5 and 1 mbar respectively indicated that the Ni catalyzed thegrowth of the nanowires. The nanowires prepared at both pres-sures contained about 50% or higher Si, revealing that thenanowires could be Si-rich Si/SiC core–shell nanowires. The Ccontent showed a significant increase with increased pressure atthe tip and stem of the nanowires. The increasing of the C contentwas due to an enhancement of the gas-phase reactions at higherpressure. As reported by Wu et al. [11], increase in depositionpressure increases the generation of H radicals which enhancesthe decomposition of CH4 thus producing more C-rich radicals to

the growth surface. More than 10% of Ni found on the stem of thenanowires for both pressures confirmed the diffusion of the Niinto the nanowires. This can lead to a radial growth of SiC shellthat can be attributed to the tapering of nanowires and agglom-erated grains. The detected O could be related to the formation ofSiOx on the surface of the nanowires, which occurred generallyduring nanowire growth using the CVD technique. In addition,presence of C in the deposited layer near the nanowires revealedthe absorption of C into the accumulated Si layer during thegrowth of the nanowires which will be discussed later in moredetail.

The microstructure of these nanowires was further investigatedby TEM as shown in Fig. 2. A single nanowire of the sampleprepared at pressure 0.5 mbar was selected for this TEM measure-ment. Fig. 2(a) indicates the presence of Ni on the tip of thenanowire surrounded by amorphous SiOx layer. HRTEM scans atthe near end of the nanowire sidewall revealed a single crystallinestructure of the Si nanowire with core diameter of 7 nm [Fig. 2(b)].The estimated lattice spacing was about 0.19 nm corresponding toSi (220) crystallographic plane. The growth direction of [110] wasfurther revealed by a fast Fourier transform (FFT) as shown inFig. 2(c). The nanowires growth in [110] direction by HWCVD wasreported previously due to the natural preference orientation ofthe thin nanowires with a diameter o50 nm [12]. Radial growthof columnar structures were observed in the stem of the nano-wires and these columnar structures attributed to the taperingnanowires. The HRTEM images of the columnar structures [Fig. 2(d) and (e)] revealed that these columnar structures consisted of amixture of Si nano-crystallites embedded within an amorphousmatrix. The estimated crystallite size was approximately 672 nm.The preferred orientation of the Si nano-crystallite followed the Si(111) plane. These nanocolumns deposited on the side walls of thenanowires possess an enhancement of light absorption abilitywhich can be used for photovoltaic applications.

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 thebackscattered electron images of the nanowires of the respective figure.

Table 1Compositions of silicon (Si), carbon (C), oxygen (O) and nickel (Ni) in percentage, ofthe nanowires measured by EDX elemental analysis.

Pressure (mbar) Nanowire Atomic %

Si C O Ni

0.5 Tip nanowire 63.9 2.8 15.3 11.8Stem nanowire 63 5.5 16.3 10.9Deposited layer 60.5 3.2 15.4 12.1

1 Tip nanowire 54.6 12.7 8.9 19.4Stem nanowire 49.8 25 7.8 15Deposited layer 43.7 28.9 8.0 16.8

B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–3026

The compositions of the nanowires were further investigatedby STEM/EDS elemental mappings using a HAADF detector in theTEM, as shown in Fig. 3. Fig. 3(a) depicts a dark-field STEM imageof a single nanowire. The dotted box indicates a scan area for theEDS elemental mappings. The compositions of the nanowire weredemonstrated by the EDS maps as shown in Fig. 3(b-e) correspondto the Si, O, C and Ni Kα maps respectively. The nanowire mainlyconsists of Si, O and C. Clearly, the presence of Ni nanoparticle(blue) at the tip of the nanowire supports the Ni catalyzed growthof these core–shell nanowires by HWCVD. High density of Si canbe observed at the stem and it is distributed uniformly along thenanowire from the stem to the tip. Less amount of O appeared atthe stem of the nanowires and also amount of O decreased fromthe stem to the tip. This indicated that oxidation occurred at theinitial stage of nucleation during the growth of the nanowires.

The C map showed the presence of C in the scanning area of thenanowire; however it is not clearly presented in this case, whichcould be due to the background of the C film of the copper grid.

Fig. 4 shows the XRD patterns of the nanowires grown byHWCVD at pressures of 0.5 and 1 mbar. The XRD pattern of thenanowires mainly consists of Si and Ni2Si diffraction peaks. The Sidiffraction peaks are located at 28.41, 47.31 and 56.11 correspond-ing to c-Si with the orientations of (111), (220) and (311) planesrespectively. The appearance of a small peak located at 35.8 1indicates a formation of nano-crystallites of 3C–SiC embeddedwithin an amorphous matrix. Presence of NiSi2 diffraction peakscould be due to the diffusion of Ni into the nanowires formingNiSi2 during the growth of the nanowires. The Ni2Si diffractionpeaks located at 31.21, 451, 45.71 and 51.8 1 are associated withcrystalline Ni2Si orientations of (102), (202), (013) and (004)

e

a

d

(a)

(d)

c

220

5 nm

10 nm

b

3 nm

0.19 nm

10 nm

100 nm

0.31 nm

Si (111)

Si (220)

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 andcolumnar 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

O

C Ni

Si

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 theelemental 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 inthis figure, the reader is referred to the web version of this article.)

B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–30 27

planes respectively, according to JCPDS card number 00-065-1507.The decrease of the Si diffraction peaks of (111) and (311) planeswith an increase in pressure revealed the increasing of the amor-phous columnar structures surrounding the nanowires. These colum-nar structures mainly consisted of Si nano-crystallites embeddedwithin an amorphous matrix. The estimated crystalline grain sizewas around 672 nm, which was about the same as measuredby HRTEM.

The characterization results showed that those Si/SiC core–shell nanowires can be synthesized by a simple vapor depositionmethod. The synthesis process involved a series of chemicalreactions, in which the atomic H and Ni nanoparticles might playan important role. In order to ensure the exact growth mechanismof these nanowires, the substrate surface temperature during thedeposition was measured at 785 1C. The difference temperature of335 1C between the substrate temperature at the bottom of thesubstrate and the substrate surface temperature can be under-stood by the presence of hydrogen assisted heat transfer from hotfilament during the process. The deposition temperature involvedwas considerably lower than the eutectic temperature of Ni–Si(993 1C) and Ni–C (1550 1C) [13,14], although the surface tem-perature was 785 1C. We suggested that the growth process ofthese core–shell nanowires follows a vapor–solid–solid (VSS)growth mechanism [15,16]. A schematic diagram of the proposedgrowth mechanism for these core–shell nanowires is shown inFig. 5. In the reactions (a and b), the Ni film was deposited andtreated by energetic atomic hydrogen forming Ni nanoparticles ata substrate temperature of 450 1C. A similar type of metallicnanoparticle formation had been observed in the works reportedby Alet et al. [17] and Nagsen et al. [18]. The atomic hydrogentreatment also played a role in removing the oxide layer on the Nifilm before the formation of Ni nanoparticles.

In reaction (c), the decomposition of SiH4 and CH4 in the highdilution of H2 by hot filament at temperature above 1800 1Csupplied large amounts of Si and C source impinging on the

surface of the Ni nanoparticles. At the initial stage, the depositionof Si on Ni nanoparticle surface led to Ni diffusion into theaccumulated Si layer. Diffusion of Ni into the accumulated layerinitiated first NiSi formation layer [19]. Because of its relativelyhigh diffusivity and being a dominant moving species of Ni in theSi layer, it created a nucleation site and induced a precipitation ofthe Si nanowire. Diffusion of Ni may continue in the growth ofnanowire with the deposition of Si. Some amount of Si contributedto the lengthening of the nanowire and the feedback mechanismto continue the process. According to ternary Ni–Si–C phasediagram [20], such low reaction temperature of 785 1C on thesubstrate surface was not sufficient to achieve a nucleation stagefor precipitation of SiC nanowires. There was almost no diffusionof SiC in various Ni silicides as compared to Si in Ni2Si attemperature of 850 1C (diffusivities of Si in Ni2Si and SiC in Ni

20 25 30 35 40 45 50 55 600

5

10

15

20

5

10

15

20

Δ

Δ

#

#

#

*

**

*

**#

##

Δ*

Inte

nsity

(arb

. uni

ts)

2θ

# -- Si -- Ni2Si -- SiC

Fig. 4. XRD spectra of the Si/SiC core–shell nanowires grown by HWCVD atpressure (a) 0.5 and (b) 1 mbar.

NiSiO2

Ni NPs

VS

VS

SiCshell

Si core

Si

C

H

VSS

Fig. 5. Schematic diagram of the proposal growth mechanism for the Si/SiC core–shell nanowires.

B. Tong Goh, S. Abdul Rahman / Journal of Crystal Growth 407 (2014) 25–3028

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 surfaceof the accumulated Si layer as shown in reaction (d). These SiCclusters could lead to the formation of SiC nano-crystallitesembedded within an amorphous matrix. The SiC on the accumu-lated Si layer subsequently grew as a shell following the precipita-tion of the Si nanowires as shown in reaction (e). Moreover, the Ninanoparticles on the surface of the nanowire also created somegrowth sites for the SiC formation and followed the radial growth.Finally, the growth of these core–shell nanowires would stop withthe reduction of the temperature or with insufficient NiSi.

Fig. 6 shows a typical PL emission spectrum of the nanowiresprepared by HWCVD. These nanowires exhibited a broad PLemission spectrum in the range between 400 nm and to 1 μmwhich covered the whole visible part and near infrared regions.This PL emission mainly consisted of five emission bands centeredat around 450, 600, 700, 800 and 850 nm. The PL emission band ataround 450 nm consisted of two small emission bands located at415 and 445 nm as shown in the inset of the figure. The emissionband centered at around 700 nm was reported to be originatedfrom the quantum confinement effect of the Si nano-crystallitesembedded within an amorphous matrix [21]. The emission bandlocated at 600 nm was generally referred to the emission due tothe 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 radiativerecombination that led to the strong emission in the visible region.According to the quantum confinement effect model that has beendescribed by Trwoga et al.[24] the Si nano-crystallites with adiameter less than 10 nm embedded within an amorphous matrixwidened the band gap, thus resulting in band gap larger than theband gap of bulk crystal silicon (1.12 eV at room temperature),thus producing PL emission in the visible region. The origin of thePL emission due to the quantum confinement effect follows thequantum confinement effect model as [25] EPL ¼ EOþð3:73=d1:39Þ,where EPL is PL energy, EO is the room temperature band gap ofbulk c-Si of 1.12 eV and d is the crystallite size of the Si nano-crystallite. The two emission bands at 800 and 850 nm could bedue to the localized state transitions of a-Si nanoclusters [26]. Theappearance of two small emission bands at 415 and 445 nmindicated the emission of SiC nanostructures [27–29]. Theseemission bands are comparable to the PL emissions from SiCnanowires [27,28] or crystalline SiC nanoparticles [29]. However,the emissions from these nanowires were obviously blueshifted tolower wavelength as compared with the band gap of bulk 3C–SiCof 2.39 eV (520 nm). This phenomenon was reported as due to thequantum size effect of SiC nano-crystallites embedded within an

amorphous matrix or to the size of the nanowires. The various PLemissions in the visible region of the nanowires have revealed thatthese PL emissions strongly depend on the quantum confinementeffects either by the size of the nano-crystallites embedded in thenanowires or the morphology of the nanowires.

4. Conclusion

Ni-catalyzed Si/SiC core–shell nanowires have been grown byHWCVD on glass substrates at pressure of 0.5 and 1 mbar. Thenanowires showed high density tapered nanowires with a verticalalignment at 0.5 mbar. These nanowires consisted of single crys-talline Si and amorphous SiC attributed to core and shell of thenanowires respectively. Radial growth of columnar structuresformed the shell of the nanowires and thus led to the taperingof the nanowires. The columnar structures consisted of a mixtureof Si nano-crystallites embedded within an amorphous matrix. Anincrease in pressure is attributed to the enhancement of radialgrowth and agglomerated grains formation. A VSS growthmechanism has been proposed to explain the growth of thesecore–shell nanowires by the HWCVD using Ni nanoparticles as acatalyst. The Si and SiC nano-crystallites embedded within anamorphous matrix exhibited various room-temperature PL emis-sions in the visible region.

Acknowledgment

This work was supported by the Ministry of Higher Educationof Malaysia, for Exploratory Research Grant Scheme (ERGS) ofER003-2013A and the University of Malaya Research Grant(UMRG) Program of RP007B-13AFR.

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400 500 600 700 800 900 10000

1000

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3000

4000PL

em

issi

on in

tens

ity (a

rb. u

nits

)

Wavelength (nm)

Fig. 6. A typical PL spectrum of the Si/SiC core–shell nanowires grown by HWCVD.

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