Zhang et al. Nanoscale Research Letters (2015) 10:9 DOI 10.1186/s11671-014-0718-x

NANO EXPRESS Open Access

Highly-ordered silicon inverted nanocone arrayswith broadband light antireflectanceDong Zhang1, Weina Ren2, Zhichao Zhu2, Haifeng Zhang2, Bo Liu2, Wangzhou Shi1, Xiaomei Qin1*

and Chuanwei Cheng2*

Abstract

In this work, highly-ordered silicon inverted nanocone arrays are fabricated by integration of nanosphere lithographywith reactive ion etching (RIE) method. The optical characteristics of as-prepared Si inverted nanocone arrays areinvestigated both by experiments and simulations. It is found that the Si nanocone arrays present excellentbroadband light antireflectance properties, which are attributed to the gradient in the effective refractive index ofnanocones and enhanced light trapping owing to optical diffraction. The inverted Si nanocone arrays might finda variety of applications in solar cells and photodetectors.

Keywords: Inverted nanocone arrays; Antireflection; Nanosphere lithography; Si

BackgroundPhotovoltaic is a promising technology for generatingelectrical power from the sun on a large scale. The siliconsolar cell is presently dominating the solar cell market,owing to the abundance of raw materials, near ideal bandgap, and mature fabrication process [1]. However, themajor issue with the planar Si is the high light reflectanceloss on the interface between the air and Si. Due to thehigh refractive index (n = 3.4) of Si, more than 30% of inci-dent sunlight is scattered or reflected from the Si surface,which has limited the efficient utilization of sunlight. Oneof the traditional methods to reduce the reflection loss onthe surface and enhance the light absorption is to use ananti-reflection (AR) layer [2,3], such as Si3N4, SiO2, etc.However, such AR layer works best only for light with in-dividual wavelength and special incident angle. Texturiza-tion of the surface is another efficient way to realize lighttrapping and absorption enhancement [4-7]. In the pastfew years, various nanostructures including nanowires[8-10], nanopillars [11], nonopyramids [12,13], and nano-cones [14,15] were explored for nanostructured thinfilm photovoltaic devices with excellent light trappingabilities. All the nanostructures mentioned above can

* Correspondence: xmqin@shnu.edu.cn; cwcheng@tongji.edu.cn1Department of Physics, Shanghai Normal University, No.100 Guilin Road,Shanghai 200234, PR China2Shanghai Key Laboratory of Special Artificial Microstructure Materials andTechnology and School of Physics Science and Engineering, TongjiUniversity, 1239 Siping Road, Shanghai 200092, PR China

© 2015 Zhang et al.; licensee Springer. This is aAttribution License (http://creativecommons.orin any medium, provided the original work is p

be categorized as ‘positive’ structures with respect tothe substrates, that is, the structures protrude out fromthe substrates into free space. In contrast to the deepresearch on the ‘positive’ structures, the development of‘negative’ nanostructure, for instance, nanoholes, is stillfar behind due to the limited fabrication methods.In this letter, we report the fabrication of highly-

ordered Si inverted nanocone arrays with desired diame-ters and pitches by a combination of colloid lithographyand reactive ion etching route. The photon-trappingprocess in the nanocone arrays were studied by experi-mentally and theoretically investigating their optical ab-sorption properties. It was found that strong diffractionof light can enhance the photon-harvesting ability, espe-cially when the diameters of the holes are matched withthe optical wavelength.

MethodsSi inverted nanocone arrays fabricationFirst, the planar Si (MTI, China) wafer was ultrasonicatedin deionized (DI) water, acetone, and methanol for 5 min.Then, the substrate was heated in boiling piranha solution(H2SO4/H2O2 with a volume ratio of 4:1) for 10 min to re-move organic residues. After each cleaning step, the waferwas washed with DI water. After the standard RACprocess mentioned above, the surface of the substrate ishydrophobic. A monolayer of closed-packed polystyrene(PS) spheres was fabricated on the Si surface via a self-

n Open Access article distributed under the terms of the Creative Commonsg/licenses/by/4.0), which permits unrestricted use, distribution, and reproductionroperly credited.

Zhang et al. Nanoscale Research Letters (2015) 10:9 Page 2 of 6

assembly route. For this experiment, PS beads with diam-eter of 500 and 1,000 nm have been used. A drop of thecolloidal dispersion was put onto the P-type Si wafers, andthe water was allowed to evaporate slowly under ambientconditions. The sphere diameter was subsequently re-duced slightly via reactive ion etching (RIE) in oxygen(O2) plasma. The gas flow was 40 sccm, the power was40 W, the chamber pressure was 9.8 Pa, and the etchingtime was 300 s. After that, a 50-nm nickel (Ni) layer wasdeposited onto the samples by magnetron sputteringmethod. The power was 70 W and sputtering time was3,600 s. After the deposition, the PS spheres were dis-solved in toluene for 5 ~ 6 h. Then, the samples werewashed with DI water. Afterward, the samples wereetched by RIE using the mixture of SF6 and O2. The gasflow of SF6 and O2 was 70 and 10 sccm, respectively, thepower was 150 W, and the chamber pressure was 9.8 Pa.Since the selective properties of RIE with different gas, themost removed part is only Si but not Ni. The depth of thenanohole could be adjusted by well controlling the etchingtime. Finally, the remained Ni layer was removed by soak-ing the sample in HCl solution (HCl:DI = 1:3) for 15 min.Then, the samples were rinsed with DI water and driedwith nitrogen.

CharacterizationThe morphologies of the Si inverted nanocone arrays werecharacterized by scanning electron microscope (SEM,Hitachi S-4800, Hitachi, Tokyo, Japan). The hemisphericalreflectance of the samples was measured by an UV-visspectrometer (Zolix Instruments Co., Ltd, Beijing, China).The numerical simulations for the reflectance spectra andthe spatial distributions of electric field intensity wereperformed based on a rigorous coupled wave analysis(RCWA) method and three-dimensional finite-difference-time-domain (FDTD) method.

Figure 1 Scheme of the fabrication procedures of Si inverted nanocone(c) PS spheres removal, and (d) RIE etching.

Results and discussionThe fabrication procedures of the periodical Si invertednanocone arrays are illustrated in Figure 1. First, amonolayer of closed-packed polystyrene spheres is fabri-cated on the hydrophilic P-type Si surface which wascleaned by a standard RAC process. The size of PSsphere was reduced by RIE with O2, forming a 2D non-close-packed PS template, which was used as a nanopat-terned mask, as shown in Figure 1a. Second, 50 nm ofNi film was deposited on the PS spheres and the intersti-tials. By removing the PS sphere template, a periodicalNi nanohole arrays was obtained, as shown in Figure 1c.The Ni film with periodical nanohole arrays was used asa hard mask for RIE in the SF6/O2 plasma. The Si nano-hole's depth can be controlled by adjusting the etchingtime, while the diameters and pitches of the nanoholeswere defined by the PS nanosphere patterns, as shownin Figure 1d. The remained Ni mask layer can be re-moved by soaking the samples in HCl solution.Figure 2a shows the SEM image of the etched PS

sphere pattern arrays. The initial diameter of the sphereswas 500 nm. It can be seen that the PS spheres are peri-odically arranged on the surface in a large scale area.After 4 min of O2 plasma etching, the size of PS sphereswas reduced to a specific diameter of about 390 nm. Byusing the Ni nanoholes as etching mask, highly-orderedSi nanohole arrays are obtained, as demonstrated inFigure 2b. The diameters and adjacent distances of theSi nanoholes are defined by the PS spheres. From thecross-sectional view in Figure 2c, the Si nanoholes showinverted cone shape. The depth of the holes is around1 μm after 20 min of etching with 200 W of RIE power.The SEM images of PS spheres with diameters of1,000 nm and Si inverted nanocone arrays with 1,000 nmpitches are provided in the Additional file (see Additionalfile 1: Figure S1 and S2). Figure 2d shows the schematic ofthe proposed inverted nanocone arrays. The lattice

arrays. (a) Fabrication of PS spheres monolayer, (b) Ni film deposition,

Figure 2 SEM images and schematic drawing. (a) SEM image of PS spheres on planar Si after etching with initial diameters of 500 nm. SEMimages of ordered Si inverted nanocone arrays with spacing 500 nm, depth 800 nm (b) top-view, (c) cross-sectional view. (d) Schematic drawingof Si inverted nanocone arrays (depth h, diameter d).

Zhang et al. Nanoscale Research Letters (2015) 10:9 Page 3 of 6

constant (period) of the hexagonal lattice is indicatedas a. The diameter of the air hole at the top surface is d(d = a), and the depth is h.To investigate the anti-reflection properties of the Si

inverted nanocone arrays, the diffuse reflectance spectrumwere measured via a UV-vis spectrometer. For comparison,

Figure 3 Total hemispherical optical reflectance of planar Si and Si in

both the planar Si and ordered Si inverted nanocone arrayswith different sizes were measured in a wavelength rangeof 300 to 1,100 nm. As shown in Figure 3, the Si invertednanocone arrays can greatly suppress the reflection withwavelength above the Si bang gap (1.12 eV). The reflect-ance intensity of Si inverted nanocone arrays is less than

verted nanocone arrays. With different 500 and 1,000 nm pitches.

Figure 4 (See legend on next page.)

Zhang et al. Nanoscale Research Letters (2015) 10:9 Page 4 of 6

(See figure on previous page.)Figure 4 Simulated reflectance spectrum and |E|2 cross-sectional distribution of Si inverted nanocone arrays. (a) The simulated reflectancespectrum of the Si inverted nanocone arrays with different 500 and 1,000 nm pitches. Simulated |E|2 cross-sectional distribution of Si invertednanocone arrays: (b1 and b2) 500 pitch at 806.723 and 564.706 nm wavelength, respectively; (c1 and c2) 1,000 pitch at 1,200 and 1,043 nmwavelength, respectively.

Zhang et al. Nanoscale Research Letters (2015) 10:9 Page 5 of 6

7% over broad range of 400 to 1,000 nm, much less thanthat of the planar Si (above 20%). For a closer observationof the reflectance spectrum of Si inverted nanocone arrayswith 500 and 1,000 nm lattice in Figure 3, it can be seenthat the Si inverted nanocone arrays yield different valleysof reflectance with the change of the sizes. Particularly,the Si inverted nanocone arrays with 500-nm lattice showsthe lowest reflection around 542 nm wavelength, and the1,000-nm lattice one demonstrates 4.7% reflection at ap-proximately 1,020 nm wavelength, i.e., the Si invertednanocone arrays can provide enhanced light trapping abil-ity when the incident light wavelength is close to the sizesof the Si inverted nanocone.The excellent broad antireflection properties of as-

fabricated Si inverted nanocone arrays can be attributedthe following two reasons. First, the gradient of effectiveindex in the Si inverted nanocone arrays causes the inci-dent light to be reflected at different depths from theinterface of air and Si, as result of suppression of broad-band reflectance by destructive interferences [16]. Theeffective index (neff ) gradient of the Si inverted nano-cones can be estimated by the following equation [17]:

neff ¼ f nqSi þ 1−fð Þnqairh i1=q

where q is 2/3, nSi and nair

are the refractive indices of the Si and air, respectively,and f is the fill factor. Second, the periodical invertednanocones might provide additional light trapping effectdue to the optical diffraction.In order to further verify the periodicity and sizes ef-

fects on the light trapping, FDTD simulations were per-formed on these Si inverted nanocone arrays with thehexagonal lattice model, resulting in the simulated re-flection spectra as shown in Figure 4a. As expected, thesimulated reflection spectra showed a quite consistenttrend over all the wavelengths with the experimentalones as described in Figure 3. Obviously, the 500-nmperiodicity Si inverted nanocone arrays show strong lightcapturing capability for approximately 500 nm wave-length light. Similar simulation result also occurred forthe sample with 1,000-nm periodicity. Hence, the period-icity could lead to improved light capturing when the sizesmatch with the input wavelength. This phenomenoncould be explained by the grating theory [18,19]. The dif-fraction of light in the periodic grating structure can in-crease the optical path length of photons, leading toincreased absorption probability, which follows the gratingequation [20]: dsin θ =mλ, where d is the grating lattice

constant, that is the lattice constant of the Si invertednanocone arrays, θ is the diffraction angle, m is the dif-fraction order, and λ is the incident light wavelength.When d is approaching λ, thus the mth order diffractedlight will be propagating in plane inside the Si invertednanocone array structures, as result of maximizing thelight absorption probability.In order to understand the electromagnetic (EM) wave

coupling and propagation in the Si inverted nanoconearray structures, the cross-sectional electric field inten-sity (|E|2) distribution of the EM wave at different wave-lengths was calculated in Figure 4b,c. In these foursimulations, the EM plane waves propagate from top tobottom. The color index at the specific location reflectsthe magnitude of |E|2 at that point. For the 500-nm lat-tice Si inverted nanocone arrays (as shown in Figure 4(b1 and b2)), the majority energy of EM wave at564.706 nm are limited inside of the inverted nanoconearrays, while most of the EM wave at 806.723 nm arereflected. This observation can be attributed to the factthat the diameter of nanocone-hole is only 500 nm,which is much smaller than wavelength (λ = 806 nm).The |E|2 distributions of the 1,000-nm sample at 1,200and 1,043 nm are shown in Figure 4c1 and c2, respect-ively. It can be found that most of the energy of EMwaves around 1,000 nm are confined inside. The |E|2

distributions in the Si nanocone arrays with differentpitches are well agreed with the optical diffraction the-ory, when the d is approaching λ, the highest order dif-fracted EM wave is propagating in plane inside thestructures that can significantly improve the light coup-ling efficiency into Si inverted nanocone arrays.

ConclusionsIn summary, we have presented a simple and scalablemethod for the fabrication of highly-ordered invertednanocone arrays with desirable diameters, depth, andpitches on crystalline silicon surface with colloid photo-lithography and RIE process. Compared to the planar Si,the inverted nanocone arrays structures exhibit out-standing broad anti-reflection properties in a widespectrum range due to the gradient in the effective re-fractive index of nanocones and enhanced light trappingowing to optical diffraction. These inverted nanocone ar-rays might find application in nanostructured photovol-taic devices and photodetectors.

Zhang et al. Nanoscale Research Letters (2015) 10:9 Page 6 of 6

Additional file

Additional file 1: Figure S1. Typical SEM images of PS spheres onplanar Si after etching with initial diameters of 1,000 nm. Figure S2. SEMimages of ordered Si inverted nanocone arrays with spacing of 1,000 nm(a) top-view, (b) cross-sectional view.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsDZ, XMQ, and CWC concieved the project. DZ, WNR, and HFZ carried out allthe experiments. ZCZ and BL performed the FDTD simulations. DZ, WZS,XMQ, and CWC analyzed the experimental data. All the authors contributedto the writing the manuscript. All authors read and approved the finalmanuscript.

AcknowledgementsThis work was financially supported by the 973 Program (Grant No.2013CB632701), the National Natural Science Foundation of China (Grant No.51202163), and the Innovation Program of Shanghai Municipal EducationCommission (Grant Nos. 09YZ151 and 13ZZ025).

Received: 26 November 2014 Accepted: 26 December 2014

References1. Bergmann RB. Crystalline Si thin-film solar cells: a review. Appl Phys A.

1999;69:187–94.2. Yoo J, Dhungel SK, Yi J. Properties of plasma enhanced chemical vapor

deposited silicon nitride for the application in multicrystalline silicon solarcells. Thin Solid Films. 2007;515:5000–3.

3. Li M, Shen H, Zhuang L, Chen D, Liang X. SiO2 antireflection coatingsfabricated by electron-beam evaporation for black monocrystalline siliconsolar cells. Int J Photoenergy. 2014;2014:67043.

4. Haase C, Stiebig H. Thin-film silicon solar cells with efficient periodic lighttrapping texture. Appl Phys Lett. 2007;91:061116.

5. Shir D, Yoon J, Chanda D, Ryu JH, Rogers JA. Performance of ultrathinsilicon solar microcells with nanostructures of relief formed by soft imprintlithography for broad band absorption enhancement. Nano Lett.2010;10:3041–6.

6. Lu Y, Lal A. High-efficiency ordered silicon nano-conical-frustum array solarcells by self-powered parallel electron lithography. Nano Lett. 2010;10:4651–6.

7. Yu Z, Raman A, Fan S. Fundamental limit of nanophotonic light trapping insolar cells. Proc Natl Acad Sci U S A. 2010;107:17491–6.

8. Kelzenberg MD, Boettcher SW, Petykiewicz JA, Turner-Evans DB, PutnamMC, Warren EL, et al. Enhanced absorption and carrier collection in Si wirearrays for photovoltaic applications. Nat Mater. 2010;9:239–44.

9. Tsakalakos L, Balch J, Fronheiser J, Korevaar BA, Sulima O, Rand J. Siliconnanowire solar cells. Appl Phys Lett. 2007;91:233117.

10. Hu L, Chen G. Analysis of optical absorption in silicon nanowire arrays forphotovoltaic applications. Nano Lett. 2007;7:3249–52.

11. Mariani G, Scofield AC, Hung CH, Huffaker DL. GaAs nanopillar-array solarcells employing in situ surface passivation. Nat Commun. 2013;4:1497.

12. Han SE, Chen G. Toward the lambertian limit of light trapping in thinnanostructured silicon solar cells. Nano Lett. 2010;10:4692–6.

13. Chen HL, Chuang SY, Lin CH, Lin YH. Using colloidal lithography to fabricateand optimize sub-wavelength pyramidal and honeycomb structures in solarcells. Opt Express. 2007;15:14793–803.

14. Hsu CM, Connor ST, Tang MX, Cui Y. Wafer-scale silicon nanopillars andnanocones by Langmuir–Blodgett assembly and etching. Appl Phys Lett.2008;93:133109.

15. Zhu J, Yu Z, Burkhard GF, Hsu CM, Connor ST, Xu Y, et al. Optical absorptionenhancement in amorphous silicon nanowire and nanocone arrays. NanoLett. 2008;9:279–82.

16. Lin YR, Lai KY, Wang HP, He JH. Slope-tunable Si nanorod arrays with enhancedantireflection and self-cleaning properties. Nanoscale. 2010;2:2765–8.

17. Stavenga DG, Foletti S, Palasantzas G, Arikawa K. Light on the moth-eye cornealnipple array of butterflies. Proc R Soc B. 2006;273:661–7.

18. Adachi MM, Labelle AJ, Thon SM, Lan X, Hoogland S, Sargent EH.Broadband solar absorption enhancement via periodic nanostructuring ofelectrodes. Sci Rep. 2013;3:2928.

19. Leung SF, Zhang Q, Xiu F, Yu D, Ho JC, Li D, et al. Light management withnanostructures for optoelectronic devices. J Phys Chem Lett. 2014;5:1479–95.

20. Hutley MC. In diffraction gratings (techniques of physics). London:Academic Press; 1982.

Submit your manuscript to a journal and benefi t from:

7 Convenient online submission

7 Rigorous peer review

7 Immediate publication on acceptance

7 Open access: articles freely available online

7 High visibility within the fi eld

7 Retaining the copyright to your article

Submit your next manuscript at 7 springeropen.com