High-Density Silicon Nanowires Prepared via a Two-Step TemplateMethodDayong Teng, Luo Wu, Weiwei He, and Changhui Ye*

Anhui Key Laboratory of Nanomaterials and Technology and Key Laboratory of Materials Physics, Institute of Solid State Physics,Chinese Academy of Sciences, Hefei 230031, China

*S Supporting Information

ABSTRACT: High density ordered Si nanowire arrays can be fabricated from aFe2O3 template annealed from polystyrene (PS) microsphere layers via a metal-assisted chemical etching method. The metal mesh films, containing position-and density-defined pores that determine the position and density of theremaining structures after etching, are extremely important for achieving highquality Si nanowires. By adding a structural inversion process, a Au metal meshwith arrays of high density nanopores is devised as a catalyst for metal-assistedchemical etching of silicon. The density of Si nanowires can be increased to twotimes that of the single-layer PS microspheres and further to three times when adouble layer of PS microspheres is introduced. The two-step template methodfor the preparation of high-density Si nanowires shows great potential in thefields of nanofabrication and nanoelectronics.

■ INTRODUCTION

Nanostructures of silicon, the most important material for thecurrent semiconductor industry, are well-documented aspromising building blocks for devices in the fields ofnanoelectronics,1,2 opto-electronics,3 energy conversion,4−9

energy storage,10,11 and bio- and chemical sensors.12,13

Characteristic parameters, such as crystalline orientation,14,15

crystalline quality,16 strain,15,17 orientation relative to thesubstrate,5 and size, affect the properties of Si nanostructures18

and are thus important for their application in devices.Numerous methods have been developed to prepare

nanowires or nanowire arrays.19−21 For the preparation of Sinanostructures, top-down and bottom-up approaches, such asvapor−liquid−solid growth,22 reactive ion etching (RIE),electrochemical etching, or metal-assisted chemical etching(MACE),23 have also been well documented. Among thesemethods, the MACE method has attracted increasing attentionin recent years for the following reasons: (1) it is a simple andcost-effective method for fabricating various Si nanostructureswith the ability to control various parameters; (2) it enables thecontrol of the orientation of Si nanostructures relative to thesubstrate; and (3) it is more versatile and can be used to makehigher surface-to-volume ratio nanostructures. Therefore, thenanostructures fabricated by MACE method have demon-strated their application potentials in fields ranging from solarenergy conversion4−9 to thermal power conversion24 tochemical and biological sensing.25,26

In the MACE method, the Si substrate under noble metalcoverage is etched much faster than Si without noble metalcoverage. MACE combined with polystyrene (PS) microsphereor nanosphere lithography is a popular method presented byHuang et al.27,28 Starting from self-assembly of a monolayer of a

PS microsphere array on the Si substrate, size reduction of thePS microspheres was achieved by an RIE process, transferringthe close-packed PS microspheres into non-close-packed ones.Subsequently, a noble metal film was deposited by thermalevaporation onto the Si substrate, with the non-close-packed PSsphere functioning as a mask, which resulted in a continuouslayer of noble metal with an ordered array of pores. The Sisubstrate covered with the continuous metal film with pores(mesh) was etched in an etchant containing HF and H2O2.During the etching process, the noble metal mesh sankvertically into the Si substrate. The unetched Si protruded fromthe etched surroundings on the mesh, producing a Si nanowire(SiNW) array. The diameters of the pores were determined bythe remaining diameter of the RIE-etched PS microspheres,28

and the length/diameter ratio can be controlled by the chemicaletching duration.In electronic devices, such as optoelectronics for data

processing, the density of SiNWs is one of the determiningfactors for the speed of processing. Efforts have been made toprepare SiNWs with a high density; for example, by usinginterference lithography and applying the mask to the metal-assisted chemical etching, SiNWs with densities of 3.5 × 107/cm2 to 4 × 108/cm2 have been grown by Choi et al.29 Huang etal. reported the growth of SiNWs of ultrasmall diameter byusing an anodic aluminum oxide (AAO) template with smallpore size; however, the density of SiNWs was still limited bythe density of pores in the AAO template which is extremelydifficult to increase beyond the 1010/cm2 level.30 With

Received: December 25, 2013Revised: February 10, 2014Published: February 11, 2014

Article

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microsphere or nanosphere lithography, SiNWs with a densityof 4.6 × 108/cm2 has been reported by Wang’s group.31

However, the disadvantage of this method is that the density ofSiNWs cannot be adjusted in a large range because the densityof the PS microsphere layer was limited since currently it isextremely challenging to assemble PS microspheres withdiameters smaller than 200 nm into large-area ordered arrays.32

Therefore, the preparation method toward a high-density ofSiNWs is still a research field which deserves more researchefforts.In this paper, we used the MACE method to prepare SiNW

arrays of high density via a new PS microspheres-based two-step template method. Through controlling the plasma etchingtreatment of PS microspheres and adjusting the content ofFe(NO3)3 solvent during the first template fabrication,33,34 weachieved Si NWs of two times the density of the monolayer PSmicrospheres. Moreover, by using a bilayer PS microspheretemplate, we obtained Si NWs of three times the density of thePS microspheres in one layer.

■ EXPERIMENTAL DETAILSMaterials and Chemicals. Si wafers were of single crystalline p-

type (100) (ρ = 1−10 Ω·cm; Jingyifang Electronics Co.). Polystyrenemicrospheres (5 wt %, Alfa Aesar) of different diameters (2000 and500 nm) were used without further treatment. HF (GR reagent, 40 wt%), H2O2 (AR reagent, 30 wt %), Fe(NO3)3-9H2O (GR reagent, 98.5wt %), HCl (AR reagent, 36−38 wt %), and HNO3 (AR reagent, 65−68 wt %) were used as received (Jinchuang New Materials Co.). Aquaregia (concentrated nitric acid mixed with concentrated hydrochloricacid of 1:3 in volume ratio) was formulated in our lab.Preparation of Clean Silicon Substrates and PS Microsphere

Templates. Si substrates were cut into small pieces (1 × 1 cm2) fromSi wafers. To create a hydrophilic surface, the wafers wereultrasonically cleaned in ethanol, acetone, and deionized water for20 min each, and then in the piranha solution (3:1, v/v, 98%H2SO4/30%H2O2) at room temperature for 8 h. PS microsphere templateswere assembled according to literature method.35,36 A certain volume(5 μL) of PS microsphere solution was then spin-coated on the Sisubstrates (coated area is around 1 cm in diameter) at a certain speed(500 rpm) to make a monolayer PS microsphere. We increased thevolume of the PS microsphere solvent to 10 μL (coated area is stillaround 1 cm in diameter) and used a slower spin-coating speed (400rpm) to make bilayer PS microspheres (when the diameter of coatedarea increased from 1 to 2 cm, the volume of PS microsphere solutionused must increase to 35−40 μL, and the spin-coating speed must bereduced to 300−350 rpm). The size of PS microspheres was thenreduced by using Ar plasma etching at an input power of 100 W(PDC-32G-2) for a certain duration.Preparation of Fe2O3 Template. PS microspheres-coated Si

substrates were immersed in Fe(NO3)3 solution of a certainconcentration for 5 s and then dried naturally, followed by annealingin a tube furnace at 400 °C for 1 h under atmosphere. The experimentparameters are listed in Table 1.

Preparation of SiNW Arrays. Fe2O3 porous template was coatedwith gold film by using a sputter coating machine (K550X). The Sisubstrate covered with the Au/Fe2O3/PS film was etched in an etchantcontaining HF and H2O2 (4.6 mol/L of HF and 0.4 mol/L of H2O2)at 25 °C for 2 min. Finally the samples were washed in aqua regiasolvent for 5 min to remove Fe2O3 template and Au film.

Characterization. Morphologies of PS microsphere arrays, theFe2O3 template, and the different densities of SiNW arrays werecharacterized by using a Sirion 200 field emission scanning electronmicroscope (FESEM). The improvement in antireflection perform-ance of Si substrates with different densities of SiNWs wascharacterized with a SolidSpec 3700DUV UV−vis spectrophotometer.

■ RESULTS AND DISCUSSIONNoble-Metal-Assisted Chemical Etching Reactions.

The mechanism of the galvanic process can be summarizedby two half-cell reactions:31,37−39

Cathode reaction at the metal

+ → ++ +H O 2H 2H O 2h2 2 2 (RI 1)

+ →+ −2H 2e H2 (RI 2)

Anode reaction at Si

+ + → ++ +Si 2H O 4h SiO 4H2 2 (RII 1)

+ → +SiO 6HF H SiF 2H O2 2 6 2 (RII 2)

Overall reaction

+ + → + +Si H O 6HF 2H O H SiF H2 2 2 2 6 2 (RIII)

In the process, the metal layer adhering to the Si surfaces hasa greater electronegativity than Si, and thus electrons areattracted from Si to metal, making the metal layer negativelycharged. Subsequently, O− ions from H2O2 capture electronspreferentially from the negatively charged metal layers andbecome O2‑ ions in RI. This charge transfer causes the localoxidation of the Si underneath the metal patterns. Theproduced SiO2 is then continuously etched away by HF,leading to the penetration of metal into Si substrates in RII. Asa result, the pattern defined by PS microspheres is transferredto the substrates, and eventually SiNWs are obtained whenmetal penetration reaches a certain depth in RIII and theprocess can be visually described in Figure 1.In our work, we used two materials, Fe2O3 and Au, to

catalyze the chemical etching reactions as shown in Figure 2. Aswe explained in the MACE reaction, the normal process can bedescribed as a flowchart (a−c) in Figure 2 a,nd in our method,

Table 1. Experiment Parameters: Layers of PS Microsphereon Si Substrate, Plasma Etching Time, Fe(NO3)3Concentration, Annealing Condition, and Pore Densitywithin the Fe2O3 Template

layer of PSmicrosphere

plasmaetchingduration(min)

Fe(NO3)3concentration,

mmol/L

annealingdurationtime/

temperature

pore density/PS

microspheredensity

single 9 75 1 h/400 °C one timesingle 4 50 1 h/400 °C two timesdouble 9 75 1 h/400 °C three times

Figure 1. Two-step metal-assisted chemical etching process: reaction(RI) took place at metal/etching solution and reaction (RII) tookplace at metal/Si interface.

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Au/Fe2O3 contact in the system belongs to Schottky junction,and Fe2O3/Si contact belongs to a typical P−N junction. In theSchottky junction, Au has a higher work junction value thanFe2O3. Because of the electric potential difference, electronswere attracted and flowed from Fe2O3 into Au; the Fermienergy level of Fe2O3 decreased due to the loss of electrons andthe energy barrier decreased accordingly. In the Fe2O3/Sisystem, which is a typical P−N junction, Si acted as a holereceiver and holes flowed from Fe2O3 into Si. During this Au/

Fe2O3/Si sandwich system, Fe2O3 acted as a shuttle andprovided the system directional charge transportation. Becauseof this system, the etching rate of Si in Au/Fe2O3/Si was higherthan normal Au/Si, and this is the key point to our followingresults. It is worth emphasizing that Fe2O3 is not only anelectron transport acceleration layer but also a pore position-and density-defined template layer (we will discuss this later).

Narrow Space to Improve SiNWs Density withDecreasing the Size of PS microspheres. We tried two

Figure 2. (I) MACE method with Au/Si system (a−c) and MACE method with Au/Fe2O3/Si system (d−f). (II) Energy band diagram for MACE-Au/Si system. (III) Energy band diagram for MACE-Au/Fe2O3/Si system.

Figure 3. Left panels: (a) 500 and (d) 2000 nm diameter PS microsphere arrays on Si substrate; middle panels: (b) 500 and (e) 2000 nm PSmicrosphere arrays after plasma etching treatment; right panels: (c and f) SiNWs prepared based on normal MACE method.

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kinds of PS microspheres, 2000 and 500 nm in diameter, tocompare the SiNWs density within a fixed Si substrate as shownin Figure 3. We compared the images a−c to d−f in Figure 3and found that with the decrease of the size of PS microspheresfrom 2000 to 500 nm, the density of SiNWs increased from 2.8× 107/cm2 to 4.6 × 108/cm2. The density of SiNWs is the sameto that of PS microspheres. It is difficult to assemble PSmicrosphere with diameter smaller than 200 nm into large-areaordered arrays, therefore, we have to go beyond the simple PSnanolithography to improve the density of SiNWs. Si nanowiresprepared with the etching method are highly crystalline (FigureS1). Energy dispersive X-ray spectroscopy (EDS) analysisindicated clearly the distribution of Au, Fe, and Si followingeach step of the fabrication processes (Figure S2).Preparation of One and Two Times the Density of

SiNWs. Experiments were started from Si substrate coated withPS microsphere (500 nm diameter) monolayer achieved bynormal spin-coating technique as in Figure 4a, and then wewent through two flowcharts as mentioned in Figure 4.In flowchart 1, with longer plasma etching time, PS

microspheres became smaller in Figure 4f compared to thosein Figure 4b. Due to the longer plasma treatment, the contactarea between PS microspheres to the Si substrate was muchbigger than the situation in flowchart 2 because of the highermelting level of PS microspheres. Additionally, the higherconcentration of Fe(NO3)3 solution applied as mentioned inTable 1 resulted in the connection of Fe2O3 being strongenough to survive the annealing process as shown in Figure 4g.Under this circumstance, only the contact area betweenseriously melted PS microspheres and the Si substrate becamepores in the Fe2O3 template in Figure 4g. After sputtering alayer of 35 nm (see Figure 4h) gold film and being etched in

HF and H2O2 for a certain duration, one time the density(namely, the density of PS microspheres) SiNWs in Figure 4iwith a SiNW density of 4.6 × 108/cm2 were obtained.The situation was totally different in flowchart 1, where a

shorter plasma etching time and lower Fe(NO3)3 concentrationwere used. Because of the short plasma etching time, PSmicrosphere sizes were decreased only slightly and the contactbetween PS microspheres and Si substrate was changed fromprevious nanoplane contact to point contact as shown in Figure4b, and this point contact between PS microspheres and Sisubstrate can be easily broken and covered by Fe2O3 filmduring a later annealing process because of the weak bondingstrength (therefore, the Fe2O3 film was continuous). Weobserved that the gaps between the neighboring PS micro-spheres were narrow and the connection of the Fe2O3 templateamong the gaps can be easily broken during the annealingprocess because a lower concentration of Fe(NO3)3 wasapplied. We identified clearly that additional pores were formedamong previous PS microspheres because of the hightemperature annealing process in Figure 4c. These pores inthe Fe2O3 template after a high temperature annealing processwill “grow” SiNWs after MACE as we explained. We calculatedthe density of SiNWs as follows: each pore was shared by threePS microspheres, and each PS microspheres have six porearound as in shown in Figure 4d, equivalent to two SiNWs forone PS microsphere and SiNW density of 9.2 × 108/cm2 wasachieved (Figure 4e).

Preparation of Three Times the Density of SiNWs. Theexperiment flowchart and steps to prepare three times thedensity SiNWs were similar to previous experiments as wediscussed. The key point was to form the three times thedensity pores on Fe2O3 template before MACE process. We

Figure 4. Flowchart 1: preparation steps for one time density SiNWs from a → f → g → h → i. Flowchart 2: preparation steps for two times thedensity of SiNWs from a → b → c → d → e. (a) One PS microsphere layer on the Si substrate; size decreasing of PS microspheres after (b) 4 minand (f) 9 min plasma etching; (c) two times and (g) one time the density pores in Fe2O3 template were formed by immersing the samples inFe(NO3)3 solution and then annealed at high temperature; (d and h) Au film was sputtered on the Fe2O3 template; (e) two times and (i) one timethe density SiNW arrays were obtained after standard MACE.

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Figure 5. Schematic illustration for three times the density of SiNWs preparation: (a) Bilayer PS microspheres on Si substrate; (b) size decreasing ofPS microspheres after plasma etching; (c) Fe2O3 template formation after immersing the samples in Fe(NO3)3 solution and annealing at hightemperature; (d) Au film was sputtered on the Fe2O3 template; (e) SiNW arrays were obtained after standard MACE; (f) three times the density ofSiNWs after aqua regia treatment to remove the Au/Fe2O3 template.

Figure 6. Schematic illustration and SEM images for three times the pore formation on the Fe2O3 template: (a) bilayer PS microspheres on Sisubstrate, inset is the enlarged view; (b) size decreasing of PS microspheres after plasma etching; (c) Fe2O3 template formed after annealing at hightemperature; (d) deposition of Au layer; (e) HF/H2O2 chemical etching; (f) three times the density of SiNWs, inset is the cross-sectional view (witha long HF/H2O2 etching time).

Figure 7. Fe2O3 template structures based on the solvents with different Fe(NO3)3 concentrations: (a) 50, (b) 75, and (c) 100 mmol/L.

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explain the formation processes in Figure 5 and show SEMimages of the structures in Figure 6.As we realized that the key point was to form three times the

density of pores based on the Fe2O3 template before theMACE process, we used bilayer PS microspheres as shown inFigures 5a and 6a. We successfully achieved the structure byusing larger PS content and lower spin-coating speed asmentioned in Table 1. In the experiments, with enough plasmaetching time and proper Fe(NO3)3 concentration, we achievedthe structure in Figure 5b, matching well with the image inFigure 6b. Because of this superposed structure, we successfullyprepared three times the density of pores on the Fe2O3template after high temperature annealing treatment whichcan be easily understood based on Figures 5c and 6c. Once thethree pores appeared within one PS microsphere size area, wewent ahead with our standard MACE process as described inFigure 5d−f that characterized by the SEM images in Figure6d−f with a high SiNWs density value of 1.38 × 109/cm2.In this experiment, we realized that the concentration of

Fe(NO3)3 in the solution was one of the important parametersto the final result, and we performed experiments with low andhigh concentrations of Fe(NO3)3 solution and obtained theresults in Figure 7. If the concentration of Fe(NO3)3 was toolow, the connection of the Fe2O3 template will be brokenduring the annealing process in Figure 7a, and the result is badtoo with higher Fe(NO3)3 concentration in Figure 7c. Based ona series of experiments, we obtained the proper Fe(NO3)3concentration for our system as 75 mmol/L as shown in Figure7b.After successfully achieving three kinds of samples with

different SiNWs density, we identified the difference among thesamples by testing the antireflection properties because manypotential applications relied on this performance. We obtainedthe curves in Figure 8. By introducing a surface nanostructure,

the total reflectance decreases to 3% in the visible rangecompare to >40% of nonstructured flat Si sample as shown inFigure 8. Therefore, SiNWs on a Si substrate do bring a bigimprovement in optical performance to the Si wafer whichmight have an influence on optical devices, such as opticalsensors which prefer low reflection during testing.40

■ CONCLUSION

In this work, we developed a method to fabricate high densitySiNWs by using the MACE process based on a two-steptemplate method. We successfully manufactured high-densitySiNWs, with a density of 9.2 × 108/cm2 (two times the densityof PS microspheres (4.6 × 108/cm2)), by controlling theplasma treatment time to the PS microspheres and theconcentration of Fe(NO3)3 solution. We further achievedthree times the density (1.4 × 109/cm2) of SiNWs by usingbilayer PS microspheres on Si substrate based on the propertime for plasma treatment and the right quantity of Fe(NO3)3solution. Combinative usage of Fe2O3/Au, which acts as anelectron transport acceleration layer and pore position- anddensity-defined template layer, was another highlight of ourresearch, and it played an important part in our process. Finally,the idea of the SiNWs density increasing causes a dramaticimprovement in the antireflection performance. The ease,reliability, and versatility of this two-step template method mayshow great potential in well-controlled high-density SiNWsproduction for future electronic device applications.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional figures (TEM images and EDS analyses) showingthe crystallinity and elemental distribution information of Sinanowires. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: chye@issp.ac.cn. Telephone: +86-551-65595629. Fax:+86-551-65591434.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by thte National Basic ResearchProgram of China (973 Program, Grant No. 2011CB302103),National Natural Science Foundation of China (GrantNo.11274308), and the Hundred Talent Program of theChinese Academy of Sciences.

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dx.doi.org/10.1021/la404903c | Langmuir 2014, 30, 2259−22652265