Thin Solid Films 520 (2011) 74–78

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Silicon nanostructures fabricated by Au and SiH4 co-deposition technique usinghot-wire chemical vapor deposition

Su Kong Chong a,⁎, Boon Tong Goh a, Zarina Aspanut a, Muhamad Rasat Muhamad a, Binni Varghese c,Chorng Haur Sow c, Chang Fu Dee b, Saadah Abdul Rahman a

a Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysiab Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor, Malaysiac Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore

⁎ Corresponding author at: Department of Physics, UnLumpur, Malaysia. Tel.: +60 3 7967 4147; fax: +60 3 7

E-mail address: sukong1985@yahoo.com.my (S.K. Ch

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.06.042

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 November 2010Received in revised form 11 June 2011Accepted 15 June 2011Available online 23 June 2011

Keywords:SiliconNanostructuresHot-wire chemical vapor depositionHigh-resolution transmission electronmicroscopyRaman spectroscopyX-ray diffractionOptical reflectance

In this study, the fabrication of Si nanostructures by Au and SiH4 co-deposition technique using hot-wirechemical vapor deposition was demonstrated. A high deposition rate of 2.7 nm/s and a high density of siliconnanostructures with a diameter of about 140 nm were obtained at Ts of 250 °C. An increase in Ts led to asignificant reduction in the size of the nanostructures. However, coalescence on the nanostructures wasobserved at Ts of 400 °C. The Si nanostructures exhibited a highly crystalline structure, which was induced byAu crystallites. The crystallite size and crystallinity of the Si nanostructures amplified with the increase in Ts.The presence of nanostructures enhanced the surface roughness of the samples and clearly reduced thereflection, especially in the visible region.

iversity of Malaya, 50603 Kuala967 4146.ong).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Hot-wire chemical vapor deposition (HWCVD) has received con-siderable attention for the deposition of Si-based thin film materialssince the turn of the century. One of the most important advantagesof HWCVD is the high deposition rate [1,2], which enables itsinvolvement in low cost and large scale industrial production. Besides,the ion-free decomposition of source gas using the hot-wire (HW)filament avoids the substrate being damaged by the ion bombardment[3]. This allows the direct deposition on indium tin oxide (ITO) coatedglass and plastic substrates. In the HWCVD process, the SiH4 can bedecomposed into Si and H atoms at filament temperatures of around1600 to 1900 °C as discussed in References [4–6]. The Si and H atomsreact with SiH4 parent molecules to produce Si2H4 and SiH3 radicals,which are responsible for the growth process. As there is a con-siderable increase in interest in the fabrication of one dimensionalnanostructures (including nanowires, nanorods, nanocones andnanotubes), HWCVD is an attractive alternative to the synthesis ofSi nanostructures. Liu et al. [7] have demonstrated the growth of Sinanocones by combining the glancing angle deposition and HWCVD.

The fabrication of Si nanowires using HW assisted plasma enhancedchemical vapor deposition was studied in [8]. Meanwhile, thesynthesis of indium-catalyzed Si nanoneedles by using HWCVD wasrecently reported in [9]. However, the fabrication of Si nanostructuresusing HWCVD is not much reported in the literature due to thecommon preference for Si film deposition [10].

Au is a preferable catalyst to induce the growth of Si nanostructuresas it forms a Au/Si alloy with the Si atom, which can effectively reducethe eutectic temperature to less than 400 °C. Moreover, Au is able toinduce crystallization of a-Si to form higher crystalline structures byfollowing the metal induced crystallization process [11–13]. It isresistant to the formation of silicide in contrast to Ni and Pt, andprevents atmospheric oxidation compared to Al and In, thus providinga better quality of Si nanostructures fabrication. The lowmelting pointof Au (~1064 °C) enables it to be coated on substrates by using a simplethermal evaporation technique.

In this research study, we have demonstrated the fabrication ofSi nanostructures at different substrate temperatures, Ts, by the co-deposition of Au and SiH4 using HWCVD. By using this technique, therapidly evaporating Au formed nano-size droplets and induced theformation of Si nanostructures on substrates. The effects of Ts on surfacemorphology, structural and optical properties of the synthesized siliconnanostructures were investigated using field emission scanningelectron microscope (FESEM), atomic force microscopy (AFM), X-ray

Fig. 1. The variations of Rd with Ts of samples prepared by Au and SiH4 co-depositiontechnique using HWCVD.

1 µm

a b

c d

1 µm 1 µm

1 µm

Fig. 2. Top view of FESEM images of samples prepared on ITO glass substrate at Ts of(a) 200, (b) 250, (c) 350, and (d) 400 °C. The inset in (d) shows the coalescence ofnanostructures to form a film structure.

75S.K. Chong et al. / Thin Solid Films 520 (2011) 74–78

diffraction (XRD), micro-Raman scattering spectroscopy, high resolu-tion transmission electronmicroscope (HRTEM) and ultra-violet visiblenear-infrared (UV–VIS–NIR) spectrophotometer.

2. Experimental details

Si nanostructures were fabricated onto ITO coated glass substratewith a surface resistivity of 8–12Ω/sq using a home-built HWCVDsystem [14]. Substrate cleaning procedures were performed by rinsingITO coated glass in a diluted ethanol solution, followed by sonicationin an ultrasonic bath for 20 min. This was followed by subsequentrinsing in acetone, ethanol and deionized (DI) water and lastly dryingunder nitrogen purging. A tungsten filament with 99.95% puritywas employed as the HW filament for the Au evaporation and SiH4

decomposition. It was shaped to a coil of 25 turns with a length of30 mm and a diameter of 3 mm. The HW filament was pre-heated atabout 1500 °C for 10 min in a constant H2 flow rate of 100 sccm. 4 mmof Au wires with a purity of 99.999%, which was used as the catalyst,was tightly hung onto the pre-heated filament coils, and then placedat a distance of 4 cm from the substrates. The chamber was evacuatedto about 2×10−3 Pa as the base pressure, followed by heating tothe selected Ts. Next, 5 sccm of SiH4 gas were introduced into thechamber, resulting in a chamber pressure of 16 Pa. This low pressureis required to create enough mean free paths for the precursor. Theevaporation of Au and decomposition of SiH4 were performedsimultaneously by heating the HW filament to about 1900 °C for3 min deposition time. Four sets of samples were prepared at differentTs of 200, 250, 350 and 400 °C under the same deposition parameters.A Maltec-T type K thermocouple was attached to the bottom side ofthe substrate to monitor the Ts. During each of the depositions, anadditional substrate heating of ~5–10 °C was observed due to thethermal irradiation from the HW.

The film thickness, d, was measured using a KKA TENCOR P-6profilometry. The deposition rate, Rd, of samples was calculated from

the relation as: Rd =dt, where t is the deposition time, which was

fixed at 3 min for all depositions. The surface morphology of sampleswas characterized by a FEI-Quanta 200 FESEM and a VEECO D3000AFM. The FESEM scanning was performed at an operating voltage of20 kV. AFM scanning was controlled by a Nanoscope IIIa scanningprobe microscope controller, operated in contact mode using a Si3N4

tip. HRTEM images of the nanostructure were obtained from a JEOL-JEM-2010F HRTEM operating at 200 kV. Samples were scraped fromthe substrates using tweezers and immersed in an ethanol solution,then transferred to a copper supporting grid for TEMmeasurements. ASIEMENS D5000 X-ray diffractometer and a Horiba Jobin Yvon 800 UVMicro-Raman Spectrometer were employed to investigate thestructural property and crystallinity of the deposited samples. TheX-ray radiation was produced by CuKα at a wavelength of 1.5418 Åand the XRD patterns that were obtained at 2θ ranged from 10 to 80°.The micro-Raman spectrometer operated using a laser source of Ar+

with selected excitation wavelength and laser power of 514.5 nm and20 mW, respectively. A JASCO V570 UV–VIS–NIR spectrophotometerwas employed for optical reflectance measurements at a range of 200to 2000 nm with a Deuterium discharge tube (200 to 350 nm) and aTungsten iodine lamp (330 to 2000 nm). Samples were measured at ascanning area of 0.5 cm2 and an incident beam at an angle of 5° of thearc.

3. Results and discussion

The variation of Rd as a function of Ts is illustrated in Fig. 1. Gen-erally, the samples prepared by Au and SiH4 co-deposition using theHWCVD technique show a high RdN2.0 nm/s. This is due to the highefficiency of gas utilization [15] that created large amounts of de-posited silicon atoms on the substrate. The high decomposition prob-

ability of the SiH4 molecules with the HW filament surface alsocontributed to the high utilization of SiH4. The Rd decreased fromabout 2.7 to 2.2 nm/s with the increase of Ts from 200 to 400 °C. In theHWCVD process, the decrease of Rd that occur concomitant with theincrease in Ts is usually attributed to the increase of the surfacemobility of Si atoms [16]. This could reduce the probability of theincorporation of the Si adatoms into the film structure, thus reducingthe thickness of the deposited film. Meanwhile, the etching process byH atoms can also lead to the reduction of Rd[17]. The Si adatoms couldbe extracted from those weak Si\Si bonds by the H atoms that arereaching the growing surface. This process is predicted more often athigher Ts due to the higher mobility of the Si atoms. In this case, moreordered structures with an improvement in crystallinity of the Si filmsare expected to be obtained at higher Ts.

An overview of the morphology of the samples prepared atdifferent Ts is presented in the FESEM images as shown in Fig. 2. Thenanostructures of Si were evenly distributed on ITO coated glasssubstrates for all samples. The worm-like nanostructures with roughsurfaces were observed throughout the sample prepared at Ts of200 °C. The increase in Ts to 250 °C produced a smoother surface of thenanocolumnar structures, which appeared to be denser and more

100 nm 5 nm

c

ba

0.31 nm

0.23 nm Au(111)

Si(111)

5 nm

Fig. 4. (a) TEM image of the silicon nanostructures prepared at Ts of 250 °C. (b) and(c) are the HRTEM images of the top and stem surface of the nanostructures.

76 S.K. Chong et al. / Thin Solid Films 520 (2011) 74–78

uniform, compared to the sample prepared at 200 °C. At Ts of 350 °C,the size of the nanostructures was reduced and the shape becameirregular, with nanopieces forming on the surface of the films. Afurther increase of Ts to 400 °C resulted in the coalescence of thenanostructures to form a film-like structure. The grain sizes of thenanostructures as estimated from the FESEM images and the AFMmeasurements are plotted in Fig. 3. The grain sizes of the nano-structures as estimated from the FESEM images are about 179±45,147±42 and 86±34 nm for samples prepared at Ts of 200, 250 and350 °C respectively. The coalescence of the nanostructures of thesamples prepared at 400 °C limited their estimation of diameter. Fromthe AFMmeasurement, the grain sizes of the worm-like and columnarnanostructures prepared at 200 and 250 °C are determined to be 130and 128 nm, respectively. The grain sizes significantly decreasedto about 69 and 53 nm for samples prepared at 350 and 400 °C,respectively. The grain sizes of the nanostructures determined fromthe AFM are comparatively smaller than the sizes estimated from theFESEM images. The differences in the magnitude of the grain sizes ofthese nanostructures as measured by FESEM and AFM were mainlydue to the different sensitivities of these equipments. However, bothmeasurements show a decrease in the size of the nanostructures withan increase in Ts. The reduction in size accompanied by the coa-lescence of the nanostructures indicates a tendency for the formationof ordered film structures rather than the columnar structures withthe deposition at higher Ts.

The structural properties of the nanostructures grown were ex-amined by TEM. Fig. 4(a) shows the TEM image of the nanostructuresprepared at Ts of 250 °C. The nanostructures revealed a columnarshape with smaller whiskers dispersed out from the structures. Thetops of the nanostructures were capped by a darker regime. TheHRTEM image [Fig. 4(b)] of the darker regime shows prominentatomic fringes, indicating a highly crystalline structure. The latticespacing of 0.23 nm is consistent with the (111) plane of the Au crystal.The HRTEM observation on the stem of the nanostructures [Fig. 4(c)]shows a mixture of the crystalline and the amorphous phases, how-ever, the crystalline structure is greatly dominant. The crystal latticespacing of 0.31 nm is equivalent to the Si (111) crystallographicorientation.

Fig. 5 shows the XRD patterns of the Si nanostructures prepared atdifferent Ts. The XRD pattern of the ITO coated glass substrate isinserted as a reference. The high crystalline peaks of the ITO revealedfrom the XRD pattern of the Si nanostructures samples are attributedto the ITO layer on the substrate since the penetration power of X-rayis N1 μm. The Si and Au diffraction peaks within the 2θ ranged from10 to 80°, corresponding to different crystallographic orientations as

m m

Fig. 3. The variation in diameters estimated from FESEM images in an area of22×25 μm2 and grain sizes calculated from AFM for nanostructures prepared atdifferent Ts. The inset is the two-dimensional AFM images of the nanostructuresprepared at Ts of 250 and 350 °C.

indexed in the figure. The sharp peaks of Si and Au indicate that thehighly crystalline structures of Si are induced by Au crystallites. Thecrystalline Si followed the preferential growth of Au crystallitestowards the (111) orientation. The crystalline sizes of Si (DSi), and Au(DAu) of the deposited nanostructures were calculated from theFWHM of the diffraction peaks of the Si and Au orientation using

Scherrer's formula [18] as: crystallite size, D =0:89λβ cos θ

, where λ is the

wavelength of CuKα at 1.5418 Å, β is the FWHMof the diffraction peakand θ is the angle of the diffraction peak. The calculated DSi and DAu atdifferent crystallographic orientations are illustrated in Table 1.Generally, the DSi in the (111) and (311) orientations increasefrom 30.0 to 102.6 nm and from 11.7 to 44.2 nm, respectively, withthe increase in Ts from 200 to 400 °C. However, DSi in the (220)orientation is maximized (52.2 nm) at the Ts of 250 °C and graduallydecreases to 35.8 nm at Ts of 400 °C. This indicates that the (220)crystallites might not perform well at high temperature deposition.The DAu shows an increase at Ts≤350 °C and a decrease at Ts of 400 °Cfor all crystalline orientations. The increase of DAu and DSi with theincrease in Ts is due to the higher surface heating effect, resulting inthe formation of larger crystallites. At Ts of 400 °C, the formation of theAu/Si alloy with a eutectic point of 363 °C led to the reduction in thecrystallite size of Au.

The crystallinity of samples was further investigated by using amicro-Raman scattering spectroscopy. The Lorentz fit of the Raman

Fig. 5. The XRD patterns of ITO coated glass substrate and silicon nanostructuresprepared at different Ts.

Table 1The variations of crystallite size of Si at different orientation: DSi(111), DSi(220) and DSi(311),and crystallite size of Au: DAu(111), DAu(200), DAu(220) and DAu(311) of Au induced siliconnanostructures prepared at different Ts. The error bar is selected from the largestcalculated error for the accuracy of the measurements.

Ts (°C) Crystallite size of Si (±3.0 nm) Crystallite size of Au (±1.5 nm)

DSi(111) DSi(220) DSi(311) DAu(111) DAu(200) DAu(220) DAu(311)

200 30.0 30.2 11.7 31.6 23.5 21.5 17.3250 44.0 52.2 18.3 41.9 30.0 26.3 22.2350 58.8 43.5 22.5 54.0 43.2 35.7 31.3400 102.6 35.8 41.2 37.8 26.2 22.5 21.2

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scattering transverse optic (TO)-phononmode of crystalline Si locatedat ~520 cm−1 for Si nanostructures prepared at different Ts is shownin Fig. 6(a). A single peak located at 517–520 cm−1 is fitted well for Sinanostructures prepared at 200 and 350 °C whereas a broadening at500 cm−1 corresponding to a grain boundary component was ob-served for Si nanostructures prepared at 250 and 400 °C. Fig. 6(b)shows the variation of full width half maximum (FWHM) andintegrated intensity of the fitted Raman crystalline peak of Si withTs. A decrease in FWHM from 9.3±0.4 to 6.2±0.4 cm−1 and increasein integrated intensity from 714.1±26.3 to 1698.6±113.6 cm−1 wasobserved with the increase in Ts from 200 to 400 °C. The FWHM of theSi peak was correlated to the crystallinity of the Si films [19], wherethe reduction in FWHM was attributed to the improvement of the

Fig. 6. (a) The deconvolution of Raman scattering of TO-phonon mode of c-Si located atabout 520 cm−1 for silicon nanostructures prepared at different Ts. (b) The variation ofFWHM and integrated intensity obtained from deconvoluted TO-phonon mode of c-Sipeak with Ts.

crystalline structure exhibited by the films. Moreover, the narrowingin the width was accompanied by an increase in the integrated in-tensity of the TO band of crystalline Si. This indicates an enhancementin crystallinity of Si nanostructures due to the increase in Ts, which isconsistent with the XRD results.

Reflectance spectra of single crystal silicon (c-Si) film and Sinanostructures are shown in Fig. 7. A prominent suppression inoptical reflectivity of the Si nanostructures as compared to the c-Sifilmwas observed. Two possible reasons could lead to the reduction inreflection; these are the thickness of the film [20] and the presence ofnanostructures [21]. The increase in thickness of the Si film canimprove the photon absorption efficiency, thus reducing the opticalreflection. In our case, the samples were prepared at a short de-position time of 3 min to obtain a thin layer of film. The measured d ofthe samples varied from 400±9 to 480±12 nm in accordance withTs. The typical low d obtained was not able to effectively absorb theincident photons and reduce the reflection [22]. Thus, the low re-flection results are mainly due to the presence of the Si nanostruc-tures. The anti-reflection properties exhibited by nanostructures overa broad range of wavelengths are known as the “moth eye” phe-nomenon, which is explained by effective medium models [23]. Theinhomogeneous nanostructures suspended in air create a compositemedium of lower effective refractive index, which prevents thereflection of light from it. Moreover, the light scattering within thelarge surface area of the sub-wavelength size nanostructures sig-nificantly enhances the optical re-absorption ability. This agrees withthe relatively high absorption coefficient, α of the nanostructures(α~104 cm−1 for c-Si in visible region) as depicted in the inset ofFig. 8. The α of the samples was calculated from the relation as [24]:

α = 1d ln

100−RT

� �, where the d is the film thickness in unit of

centimeter, R and T are the reflectance and transmission, respectively, inpercentage. Furthermore, a decrease in α with an increase in Ts wasobserved,which is stronglydependent on the surfacemorphology of thenanostructures. This is correlated to the surface roughness, Rq of thenanostructures, as the rougher surface creates more sub-interaction oflight within the nanostructures and enhances the α. The plot of Rqagainst Ts in Fig. 8 shows the decrease of Rq of the Si nanostructureswiththe increase in Ts, which again agreed with the assumption above. Thelarger sized nanostructures for the film prepared at low Ts increased thesurface roughness, and this contributed to the increased absorption.

4. Conclusion

The high density of the Au induced Si nanostructures wasfabricated at a relatively low Ts of 250 °C by Au and SiH4 co-deposition

Fig. 7. Optical reflectance spectra of single crystal silicon (c-Si) and siliconnanostructures prepared by Au and SiH4 co-deposition technique using HWCVD atdifferent Ts.

Fig. 8. The variations of Rq of silicon nanostructures with Ts. The inset is the calculatedspectra of absorption coefficient, α in visible region of silicon nanostructures preparedat different Ts.

78 S.K. Chong et al. / Thin Solid Films 520 (2011) 74–78

technique using HWCVD. The Rd of 2.2 to 2.7 nm/s was achieved forthe Si nanostructures fabrication. We found that sample growth athigh Ts using this technique might suppress the catalytic activity of Auto induce nanostructures and result in a tendency of silicon filmformation. The Au catalyst not only induced the growth of nano-structures, it also induced the crystallization of a-Si to form highlycrystalline structures of Si. High Rq of nanostructures suppresses thereflectivity and acts as a stronger absorber.

Acknowledgment

This work was supported by the Ministry of Higher Education underthe Fundamental Research Grant Scheme (FRGS) of FP008/2008C,

University of Malaya Research Grant (UMRG) of RG061/09AFR andUniversity ofMalaya Postgraduate Research Fund(PPP) of PS310/2009B.

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