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Materials Chemistry and Physics 129 (2011) 733– 739

Contents lists available at ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

Investigation of optical properties of core–shell silicon nanowires

Bhabani S. Swaina, Bibhu P. Swainb,∗, Nong M. Hwanga

a National Research Laboratory of Charged Nanoparticles, Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Koreab Research Center for Photovoltaics, National Institute of Advanced Industrials Science and Technology, Tsukuba, Japan

a r t i c l e i n f o

Article history:Received 9 October 2010Received in revised form 6 April 2011Accepted 15 April 2011

Keywords:Core–shell SiNWsReflectivityUV–visFESEM

a b s t r a c t

The ability to control the size, orientation, composition and morphology of silicon nanowires (SiNWs)presents an ideal platform for exploring a wide range of potential technological applications. In this work,we demonstrated the detail study of optical properties of highly disordered core–shell SiNWs that weregrown by atmospheric pressure chemical vapor deposition. The microstructure of SiNWs was charac-terized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy(TEM). The TEM study shows that the SiNWs consists of crystalline core silicon surrounded by thick amor-phous silicon oxide. The total diameter including the outer SiO2 sheath was 60–80 nm. The reflection andabsorption of a-SiO2/c-SiNWs were affected by process parameter like silane flow rate and hydrogendilution. The optical reflection of SiNWs decreased with increasing photon energy across the visible andnear the ultraviolet range, approaching moth’s eye antireflection. Specifically, a minimum reflection of2–3% was observed at 400 nm. The band gap is estimated at ∼1.32 eV by quasi-direct band Tauc’s plot.The sum of localized states at the band edge is ∼0.53 eV. Straight SiNWs have lower reflection than thoseof nanoparticles mixed SiNWs and coil mixed SiNWs. The reflection and absorption of SiO2/SiNWs wereconfirmed to respond strongly to infrared with increasing H2 flow rate.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

To increase the efficiency of photovoltaic cells, it has to minimizethe reflection from the material surface and utilize the photon togenerate electrons and holes. This can be achieved by a suitablemultilayer having a gradient band gap, a perfect device structurewith minimized lattice mismatch, and a heterostructure with 0dimensional (D), 1D, and 2D structures for efficient charge trans-port. Silicon nanowires (SiNWs) have been extensively studied dueto their promising electrical, optoelectronics, mechanical prop-erties with potential applications in solar cells, biosensors andbioelectronics [1–3]. A great effort to minimize the reflection and toenhance the absorption of SiNWs for efficient light harvesting hasbeen made since last decade. The manipulation of light trapping ofSiNWs based on the high surface area and the multiple-reflectionat the surface and interface it is possible to control reflection andtransmission of SiNWs. However, due to the large surface to volumeratio, SiNWs are very sensitive to the adsorption of foreign atomsand the surface passivation modifies the higher occupied molecu-lar orbital (HOMO) and lower unoccupied molecular orbital (LUMO)states, which could also alter the optical and electrical properties ofSiNWs. SiNWs shielded with semiconducting and insulating mate-

∗ Corresponding author. Tel.: +81 298615496; fax: +81 298615497.E-mail addresses: bp.swain@aist.go.jp, bibhuprasad.swain@gmail.com

(B.P. Swain).

rials such as SiO2, a-SiH, SiC or SiN can enhance the light trappingefficiency by anisotropy light propagation. Hence, the controlling ofreflection, transmission and absorption of SiNWs films is importantbefore device fabrication.

In particular, disordered SiNWs further suppressed the Fres-nel reflection over a broad wavelength range at various anglesof incidence and have a gradual transition of effective refractiveindexes was observed across the air-to-wire interface. A cost-effective, optimized technique is needed to precisely control themajor dimensional variables for solar cell applications. However,the zero transmission at short wavelength and the zero absorptionat longer wavelength can be expected for disordered SiNWs.

The optical properties of SiNWs are tunable by different diam-eters, packing fraction, thickness alignment and length of SiNWs[4,5]. Optical properties of SiNWs are modified by atomic ormolecular adsorption or by surface passivation and alter theHOMO–LUMO level of SiNWs. The interface factor of SiNWs andthe catalytic distribution in wires are known to significantly affectoptical properties [6,7]. The low reflectance can be achieved by ultrahigh surface area, sub-wavelength structure and porosity gradientthroughout the SiNWs, which also implies the change of the refrac-tive index with depth [8]. Tsakalakos et al. investigated the strongbroadband optical absorption of SiNWs films and observed thatpartial reduction in high residual sub-band gap absorption afterhydrogen passivation [1]. Seino et al. investigated the influenceof interface geometry and chemistry on the dielectric func-tions and reflectivity of Si/SiO2 super-lattices using first principle

0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2011.04.046

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Table 1Process parameters of SiNWs were grown with variation of SiH4 flow rate.

SiH4 flow rate H2 flow rate N2 flow rate Microstructure

2 5 1000 Nanoparticles rich SiNWs3 5 1000 Nanoparticles rich SiNWs4 5 1000 Straight SiNWs5 5 1000 Coil mixed straight SiNWs

calculations [9]. Chen et al. reported enhanced light absorptionproperties of SiNWs—arrays-texture for solar cell application [7].Jung et al. reported that a strong antireflective solar cell can befabricated by wet etching process of SiNWs [10]. A more precisenumerical calculation was made by Xiong et al. [5] and estimatedthat a maximum efficiency can be achieved when the filling ratiois around 0.3. Street et al. reported that a-Si coated SiNWs has lessreflectivity those crystalline SiNWs [11].

The SiNWs have thick amorphous silicon oxide surroundedby crystalline core silicon when synthesized under atmosphericpressure condition [12–15]. The a-SiOx/c-SiNWs is expected toefficiently utilize the ultraviolet–visible radiation. Moreover, thethickness and the material quality of the outer oxide sheath canbe controlled by different means such as hydrogen dilution andannealing in a different atmosphere. Therefore, it is necessary tostudy the structural and optical properties of core–shell SiNWs byvariation of process parameter. In this work, we have studied theeffect of thickness and interface morphologies of core silicon on theoptical properties of SiNWs.

2. Experimental

2.1. Sample preparation

Au-catalyzed SiNWs were synthesized by an atmospheric chemical vapor depo-sition (APCVD) reactor. We used Au nanoparticles for the synthesis of SiNWs on theSi wafer. The details of processing methods are reported elsewhere [12]. SiH4 andH2 were used as precursor gases and N2 was used as carrier gas. The furnace andthe substrate temperatures were maintained at 1100 ◦C and 800 ◦C, respectively.

Table 2Process parameter of SiNWs were grown with variation of H2 flow rate.

SiH4 flow rate H2 flow rate N2 flow rate Microstructure

4 5 1000 Straight SiNWs4 10 1000 Coil mixed straight SiNWs4 20 1000 Coil SiNWs4 30 1000 Coil SiNWs

After deposition, N2 gas was supplied at 1000 sccm during cooling of the reactor.The details of processing parameters are tabulated in Tables 1 and 2.

2.2. Characterization

After deposition, the SiNWs samples were put in desiccators to avoid moistureand oxidation from the environment. Prior to studying the microstructure of SiNWs,the samples were coated with Pt by ion beam coater. The microstructure of SiNWswas characterized by FESEM (JEOL, JSM-6700F) operating with a 5-kV acceleratingvoltage. To study the diameter of the inner silicon core and the outer oxide layer,SiNWs were observed by TEM (JEOL-JEM-3000F). The SiNWs were wrapped out fromthe silicon wafer and dissolved in ethanol in a beaker. Henceforth, the beaker wasplaced in an ultrasonic bath in order to avoid the agglomeration of SiNWs. Thentwo drops of solution were placed on the carbon coated copper grid (400 mesh size)for TEM study. The optical properties of SiNWs were analyzed by Uv–Vis–NIR spec-troscopy (Hitachi, U-4000). After background correction, transmission and reflectionspectra were collected from core–shell SiNWs deposited on Si and quartz substrates.

3. Results

3.1. Microstructure and composition

Fig. 1(a) shows the microstructure of Au nanoparticles afterannealed of 5 nm Au thin films at 700 ◦C in nitrogen atmo-sphere. The Au nanoparticles are appeared to be semispherical andcompletely separated each other on silicon substrate. The size dis-tribution of Au nanoparticles is in the range of 20–80 nm. The planeview low and high resolution FESEM images are shown in Fig. 1(b)and (c), respectively. The SiNWs are long and thick with a smoothouter surface. The diameters of SiNWs in the range 60–80 nm areextended up to 2 �m. The cross-sectional view of deposited SiNWs

Fig. 1. (a) Microstructure of Au nanoparticles, (b) low magnification FESEM image of SiNWs, (c) high magnification FESEM image of SiNWs, (d) cross sectional FESEM imageof random silicon nanowires, (e) EDAX of SiNWs, and (f) TEM image of SiNW.

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Fig. 2. Reflection, transmission and absorption spectra of SiNWs grown on silicon (1 0 0) (a), quartz (b) and R/Rmax as a function of d/� of SiNWs (c).

is shown in Fig. 1(d). The deposition was 2 �m after 120 min ofdeposition. Further energy dispersive X-ray analysis (EDAX) studyanalyzed that the SiNWs contained oxygen and gold as a minor ele-ment. The EDAX image is shown in Fig. 1(e). The atomic percentagesof silicon, oxygen and gold were found to be 60.34, 39.34, and 0.31,respectively. Fig. 1(c) represents the TEM images of SiNWs shownin Fig. 1(f). The TEM study reveals that the SiNWs consist of the coresilicon surrounded by an amorphous oxide sheath. The core siliconthickness was 22 ± 3 nm and the oxide sheath was 27 ± 3 nm. Asexpected before the resulting oxide sheath was believed to resultfrom the fast cooling of the furnace after deposition.

3.2. UV–vis–NIR spectroscopy

3.2.1. SiNWs on Si (1 0 0) and quartz substratesFig. 2(a) shows that reflection, transmission, absorption of

SiNWs grown on the Si wafer substrate is on the spectrum rangeof 300–2000 nm. The reflection of SiNWs is almost constant in the300–900 nm and increases with increasing wavelength in the rangeof 1000–2000 nm. The maximum kink in reflection and transmis-sion spectra and the minimum kink of absorption spectra at 850 nmindicate the optical band of the SiNWs. The reflection spectra inthe range 900–2000 nm increase monotonically with wavelength,indicating diffused scattering at SiO2/Si interface. The absorp-tion spectra of SiO2/SiNWs have just an opposite trend to thatof the reflection spectra. The absorption of SiO2/SiNWs is strongin the 300–850 nm, which is ∼97% of incident photons. Above1000 nm the absorption spectra decrease monotonically from 93to 77% with increasing wavelength. The decrease of the absorp-tion with the increase of wavelength is the collective response ofSiO2/SiNWs interface and packing density of SiNWs. The transmis-sion of SiO2/SiNWs affects only a certain region due to the band gaplimitation of c-Si. Therefore, the transmission of the SiO2/SiNWs on

c-Si is zero at the 300–850 nm range. Fig. 2(b) shows the reflection,transmission and absorption spectra of SiO2/SiNWs on the quartzsubstrate. As the quartz is transparent in ultra-violet, visible andinfrared regions, the actual absorption, reflection and transmissionof SiO2/SiNWs can be easily manipulated from the data. The reflec-tion from SiNWs on quartz is lower than that from SiNWs grownon the Si (1 0 0) substrate. This could be due to different growthdirections of SiNWs on Si (1 0 0) and quartz substrates or due to thefact that quartz is a less reflective substrate than Si. The differentpacking density and distribution of SiNWs on Si (1 0 0) and quartzsubstrates can be another reason for the difference in the reflec-tion. The reflection of SiNWs shows two distinguished regions of(i) 300–900 nm and (ii) 1000–2000 nm. In the region 300–850 nm,SiNWs on quartz show almost a constant reflection of ∼3–4%, whileSiNWs on Si shows decreasing in reflection with increasing pho-ton energy, following moth’s eye antireflection effect [16]. In theshorter wavelength, the Rayleigh scattering is prominently acti-vated, when d � �/�n, where d is the diameter of the nanowires andn is integer. The wavelength larger than the nanowire diameter canbe transparent through and only absorbed by multiple diffuse scat-tering. However, in this longer wavelength region, the low valueof reflection is due to high diffraction of light between SiNWs. Inboth cases, the reflection peak at 850 nm indicates that the inter-band electronic transition of SiNWs on both Si and quartz does notchange much [17].

3.2.2. Optical band gap of SiNWsAlthough SiO2 has a direct band gap, the oxide sheath does not

contribute to optical absorption in SiO2/SiNWs because the opti-cal band gap of SiO2 (>8 eV) is out of visible and infrared regions.Therefore, the absorption of SiO2/SiNWs can be studied only forcore crystalline Si and SiO2/Si interface. Furthermore, the diameterdependence optical parameter and the spectral dependence of the

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Fig. 3. Absorption coefficient of SiNWs deposited on quartz substrates (a), square of absorption coefficient as a function of photon energy (b) and Tauc’s plot of SiNWsdeposited on Si (100).

absorption coefficient are more complex to analysis. If we assumeSiNWs is spherical symmetry of its energy bands, this relation couldbe rewritten in a simpler way in terms of the absorption function.

(˛h�)n = constant (h� − Eg) where n = 2, 2/3 depends on directand forbidden direct band gap, h� is the photon energy of the light.

Fig. 3(a) shows the absorption coefficient, (cm−1) as a func-tion of increasing of photon energy for SiNWs deposited on quartzsubstrate. The absorption coefficient of SiO2/SiNWs is calculated by

(cm−1) = −1d

[ln

T

100 − R

]where d is effective thickness of SiNWs thin films in �m, T and Rare transmission and reflection in percentage. The absorption coef-ficient of SiNWs is relatively very high > 105 cm−1. We know isdirectly proportional to extinction coefficient k of materials. Threedistinguish maxima were observed at 0.88, 2.33, 3.7 eV in Fig. 3(a).The broad peak at 0.88 eV corresponds to phonon assist absorptionfrom core SiNWs or interface absorption from SiNWs. The broadpeak at 2.33 and 3.7 eV corresponds to direct transition at � pointsand � 25 − � 15 or (1 1 0) planes, respectively [11]. Fig. 3(b) showssquare of absorption coefficient as a function of increasing withphoton energy. The ˛2 increased exponentially above 2.75 eV, how-ever it increases linearly above 3.5 eV. The extrapolation of straightpart of 2.75–3.5 eV and intercept to energy axis at 2.25 eV corre-spond to optical band gap of SiNWs. The optical band gap Eg, isenergy difference between exponential part of valence and con-duction band. In the linear region, the slope of intercept at 3.2–4 eVregions is at Eg′ corresponding to band to band transition of SiNWsThe difference of energy between Eg′ and Eg can be assigned asthe localized states width (∼0.53 eV) of SiNWs. Tauc’s rigid matrixelement assumption is an important parameter to calculate band

in SiNWs. Fig. 3(c) shows the variation (˛h�)1/2 as a function ofincrease of photon energy. At lower regions (0.6 < E < 1.2 eV) of theTauc’s plot is associated with defect assist absorption betweenSiO2/SiNWs interface. The linier plot intercept of Tauc’s band gapto energy axis is 1.32 eV. This Tauc’s band gap agrees the band gapcalculated by Zhao et al. [18].

3.2.3. Effect of SiH4 flow rateWe used SiH4 gas as silicon source and grew a series of

SiNWs with varying SiH4 flow rate. The increase in SiH4 flowrate results in more Si participating in the chemical decom-position/dissociation process in the reactor. The microstructurechange of SiNWs deposited at varying SiH4 flow rate has alreadybeen reported [12,13]. The FESEM images show that both siliconnanoparticles and SiNWs were deposited at 2 sccm SiH4 flow rate.At 4 sccm SiH4 flow rate, straight and smooth SiNWs were syn-thesized. Nevertheless, when the SiH4 flow rate was increasedto 5 sccm, the shape of the SiNWs were changed to coiled type.Fig. 4(a) shows reflection spectra of silicon nanoparticles mixedSiNWs, straight nanowires and coils mixed with straight SiNWs.For comparison, we recorded the reflection spectra of a bare Si(1 0 0) wafer and also for a gold nanoparticles coated Si (1 0 0)wafer (see Fig. 1(a)). The reflection from the bare silicon waferis about 25% in the visible and infrared regions. The reflectionspectra indicated two distinguished regions for Si,(a) reflection atthe shorter wavelength (300 nm < � < 1250 nm), shows a curvaturewith a minimum at 750 nm and the reflection at the longer wave-length (1250 nm < � < 2000 nm), is almost constant. In the regionsfrom 365 to 1165 nm, the reflection is deep for Si (1 0 0), Au-coatedSi and SiNWs. The reflection deep decreases for SiNWs and theminimum are observed for straight SiNWs. The peak at 350 nm is

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Fig. 4. Reflection spectra (a) and absorption spectra (b) of bare silicon, Au coated Si (1 0 0) and R/Rmax as a function of d/� (c) nano-particle mixed SiNWs, only straight SiNWsand coil mixed SiNWs.

due to the infrared regions for the reflection for bare Si (1 0 0) andAu coated Si (1 0 0) substrates. Four reflection maxima, at 400 nm,500 nm, 720 nm and 900 nm were observed for different band c-Sistructures. The 5 nm Au coated Si shows the high reflectivity ∼45%of incident light. The reflection spectra of Au coated Si have thebehavior similar to bare Si. The expected reflection maximum at400 nm and other reflection maxima are vanished for Au coatedSi and SiNWs grown on the Si surface. Moreover, the attributeof reflection of Au coated Si is matched with the result reportedby Convertino et al. [19]. Reflections of SiNWs mixed with siliconnanoparticles; straight SiNWs and coil mixed SiNWs are less than15% at the shorter wavelength while they monotonically increasewith increasing wavelength in addition to a kink at 920 nm. Thestraight SiNWs have relatively low reflection compared to coil andnanoparticles mixed SiNWs. With a close look, one can find thatthe reflection of coil SiNWs is convex while that of straight SiNWsis concave. Fig. 3(b) shows the absorbance spectra of 300–2000 nmof above samples, where most of the above band-gap photons areconcentrated in silicon-based materials. In the shorter wavelengthrange (300 nm < � < 1000 nm), the permittivity of silicon shows dis-persion. As shown in the figure, the photons with wavelength justabove the 1100 nm, the optical absorption is very limited for Siand Au coated Si. As the wavelength decreases, the absorption ofnanowires rises and reaches a plateau region shared by all lengths.The absorbance for nanowires in the plateau region is higher thanthat of the optically denser thin film, while in the low-energyphoton regime, the absorption in the film is more efficient. Theabsorption from Si (1 0 0) indicates ∼80%, being ∼20% higher than

that of Stelzer et al. [20] while Au coated SiNWs has absorption 20%less than that of the substrate. The straight SiNWs shows the max-imum absorption, 97%, of total photons. The absorption of SiNWsfrom 300 to 1140 nm remained constant due to SiNWs depositedon Si. The absorption of SiO2/SiNWs is similar to that of SiNWs/mc-Si/glass [20]. In the absorbance at the 1000–2000 nm of abovesamples, the absorbance is 33% for the Si (100) substrate while itincreases to 67% for the Au-coated Si substrate. This may be due toAu diffusion through the Si (100) surface, which increases the deepstates of the Si surface. The absorption of as-deposited SiNWs isdecreased at the infrared region due to Au states in SiNWs and SiO2becomes broader than that of Si. Galkin et al. [21] investigated thehighly disordered surface (root mean roughness of 150–250 nm)and observed that the porous silicon had a sharp reduction in thetransmission (up to a factor of 20) in the range 1240–2000 nm.The reduction of absorption in SiNWs is due to scattering andirreversible loss of light in multiple reflections within the porousregions [21].

3.2.4. Effect of H2 dilutionH2 dilution is an important parameter to modify SiO2/SiNWs

network. H2 dilution increases the crystallinity of core SiNWs andreduces oxygen content in a-SiO2 networks. At same the time thick-nesses of the core silicon and the amorphous SiOx are also modifiedwith increasing H2 dilution. Moreover, at the higher H2 dilution theSiO2 layer is etched. In our previous work H2 dilution increased coilrich SiNWs. Fig. 4(a) shows reflection spectra of SiNWs as a func-tion of H2 flow rate [14,15]. In comparison with SiNWs deposited

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Fig. 5. Reflection spectra (a), absorption spectra (b), and R/Rmax as a function of d/� (c) of SiNWs with variation of H2 flow rate.

at 5 sccm H2 flow rate, the optical reflection increases from 7 to20% for the SiNWs sample prepared at 10 sccm H2 flow rate. Theincrease in the reflection of SiNWs can be explained in terms ofthe microstructure. For a higher H2 flows rate, however, the opti-cal reflection decreases with H2 flow rate. The reason why thereflection of SiNWs decreases with increasing H2 flow rate can beexplained by three different points of view: (1) microstructure (2)thickness of a-SiO2/c-SiNWs and (3) the oxide and sub oxide speciespresent in the outer a-SiO2 matrix. The microstructure directlyreflects the properties of SiNWs, and consequently alters the opti-cal and electrical properties. Hence the increase in the reflection ofthe SiNWs sample prepared at 10 sccm H2 flow rate results fromthe microstructure. Furthermore, the reflection decreases whenthe H2 flow rate increases to 20 and 30 sccm. These results can beexplained in terms of reasons 2 and 3 mentioned above (Fig. 5).

4. Discussion

The optical absorption and reflection of SiNWs are broadlydivided into two parts: absorption above the band gap (Eg

(�) < E(�)), where the photon can absorb by band to band transition.The defect absorption (Eg(�) > E(�)), which is the inter-band transi-tion from between localized states at (a) the shallow states, (b) deepstates, (c) from valence or conduction bands to localized states and(d) interface states between SiNWs/SiO2. The optical absorptionfrom SiO2 covered SiNWs has modified the surface states and regu-late surface dangling bonds, creates novel quantum confinementseffects in the core SiNWs and involved new surface states in theinterface between SiO2/SiNWs. 0.8 eV broad bands in the absorp-

tion coefficient and Tauc’s plot indicates, a large number of surfacestates involved in SiO2/SiNWs states. The strong absorption coef-ficient (105 cm−1), indicated the role of Au is not dominated. Thenanowire nature permits a direct or quasi-direct band gap. Hence,the ˛2 vs. E plot or (˛E)1/2 vs. E plot are suitable for band gap calcu-lation of SiNWs. The optical absorption of SiNWs covered with SiO2due to quantum confinements Eopt = Eo + A/dn, where Eg is the bandgap of c-Si. The quasi-direct optical band gap observed at 1.32 eVmeasured by Tauc’s assumption good way to estimate the opticalband gap. The optical absorption does not change so much due todiameter of SiNWs varies 60–80 nm, not the region of quantum con-finement. Absorption of 2.33 eV is the optical absorption of outerSiNWs. This reflects the SiNWs is not uniform from radial direction.

The SiH4 flow rate in the CVD process affects the silicon contentand the deposition rate. At 2 sccm SiH4 flow rate, the silicon con-tent is not sufficient for SiNWs to undergo nucleation and growth.Hence, the SiNWs density on the Si (100) is small and a largenumber of silicon nanoparticles are grown on the substrate. Thereflections from silicon nanoparticles are sufficiently lower thanthat from the base Si substrate. As the SiH4 flow rate increasesto 4 sccm, a large number of straight SiNWs were observed. Themulti-reflection from the SiNWs surface, and the re-absorption bySiO2/SiNWs increased and consequently the reflection from SiNWswas reduced to a minimum value. H2 dilution is a reducing pro-cess, which alter the crystallinity and decreasing deposition rate athigher flow rate. In case of H2 dilution in SiNWs increased the sili-con content of outer SiO2 shield layer. Moreover, the microstructureof SiNWs and dielectric constant were strongly affected by coiling,which modified the reflection and absorption [14].

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5. Conclusions

We demonstrated the detail study of optical properties ofhighly disordered core–shell SiNWs grown by APCVD. The effectsof the silane flow rate indicated that straight SiNWs have thesmaller reflection than nano-Si mixed SiNWs and coil mixed SiNWs.SiO2/SiNWs was confirmed to have a strong infrared response withincreasing H2 flow rate. In core–shell SiNWs, the optical reflectionspectra decreased with increasing photon energy across the visibleand near optical range approaching moth’s eye antireflection.

Acknowledgements

The financial support from BK21 program, South Korea isacknowledged. This work also supported by the National ResearchFoundation of Korea (NRF) grant funded by the Korea government(MEST) (No. M10600000159-06J0000-15910).

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