Broadband and omnidirectional antireflection of Si nanoconestructures cladded by SiN film for Si thin film solar cells

Yanyan Wang a,b,c,d, Biao Shao b,c, Zhen Zhang a,b,c,d, Lanjian Zhuge d,e, Xuemei Wu a,d,1,Ruiying Zhang b,c,n

a Department of Physics, Soochow University, Suzhou 215006, Chinab Key Labs of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, Chinac Division of Nano-devices and Related Materials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, Chinad Key Laboratory of Thin Films of Jiangsu, Soochow University, Suzhou 215006, Chinae Analysis and Testing Center, Soochow University, Suzhou 215006, China

a r t i c l e i n f o

Article history:Received 21 June 2013Accepted 24 November 2013Available online 6 December 2013

Keywords:Si nanocone structure cladded by SiNBroadband and omnidirectionalantireflectionSi thin film solar cells

a b s t r a c t

Si nanocone structures cladded by SiN material are fabricated by nanosphere lithography, inductivelycoupled plasma (ICP) etching and plasma enhanced chemical vapor deposition (PECVD). Their broadbandomnidiectional antireflection performance is investigated through measurement and simulation byrigorous coupled-wave analysis (RCWA). Both the measurement and simulation results show that thereflectivity of SiN passivated Si nanocone is lower than that of original Si nanocone structure overbroadband and wide view. The average reflectivity data further indicate that such reflectivity seems to belinearly reduced with SiN thickness when it is less than 90 nm. After that, the reflectivity of suchcomposite nanocone structure is less dependent on SiN thickness. The minimum average reflectivity of1.84% is achieved in Si nanocone structure cladded by 90 nm thick SiN film, which is only 10.3% of theaverage reflectivity of the same Si nanocone structure. In addition, the comparison further shows thatsuch SiN cladding layer can attenuate the bad influence of random nanosphere mask induced by self-assembly processing on their reflection. Therefore, great antireflection and good passivation performancein such SiN/Si composite nanocone structures are expected, which benefit them to be actually employedin Si thin film solar cells and improve their conversion efficiency finally.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Si thin film solar cells, as one of the promising candidates ofcost-effective solar cells, are increasingly paid much attention [1–3].The passivated and antireflection surface is required for them toachieve high conversion efficiency. Up to now, the most passivationtechnique employed in solar cells is coating dielectric film, such asSiO2 [4], SiN [5], and Al2O3 [6–8], on their surface to achievechemical passivation and field passivation effect. Meanwhile,nanotextured surfaces, such as nanopore [9], nanowire [10–11]and nanocone [12–13], with high reflection suppression and light-trapping performance are attractive in Si thin film solar cells.Therefore, it becomes a trend for Si-based solar cell to form adielectric film cladded nanostructure surface, in order to furtherimproving their conversion efficiency. The passivation perfor-mance of such composite nanostructure surface has been widely

investigated [14]. However, the influence of such surface treat-ment on their surface reflection has rarely concerned, which willfurther influence their optical absorption and efficiency of solarcells.

Recently, we demonstrated the optoelectronic performance ofAl2O3/Si composite nanocone structure theoretically and experi-mentally [15]. We found Al2O3 can not only realize the surfacepassivation, but also reduces the surface reflection significantly,which is helpful for Si nanocone structure with small aspect ratioand duty cycle to achieve great surface passivation and antireflec-tion performance.

In this paper, we focus on the reflection performance of SiNcladded Si nanocone surface experimentally and theoretically. Theresults indicate that the reflectivity firstly linearly reduces withSiN thickness and then less depends on SiN thickness once the Sinanocone is cladded by SiN film. Therefore, SiN cladding layer isoptimized just according to the passivation requirement of Sinanocone structure, but great antireflection and passivation per-formance can be achieved, which benefit for such compositenanocone structure to be actually employed in Si thin film solarcells and improve their conversion efficiency finally.

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journal homepage: www.elsevier.com/locate/optcom

Optics Communications

0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optcom.2013.11.050

n Corresponding author. Tel.: +865 126 287 2560; fax: +865 126 287 3079.E-mail addresses: xmwu@suda.edu.cn (X. Wu),

ryzhang2008@sinano.ac.cn (R. Zhang).1 Tel.: þ865 126 511 2066; fax: þ865 126 511 1907.

Optics Communications 316 (2014) 37–41

2. Experiment

Polystyrene (PS) nanosphere lithography combined with dryetching is employed to fabricate Si nanocone structures on crystal-line Si substrate. First, the traditional RCA cleaning method wasintroduced to remove impurities of Si surface, and then PS sphereswith diameter of 600 nm were distributed on its surface as close-packed hexagonal array by spinning coating. The samples with PSmonolayer mask were then etched by ICP etching system (OxfordInstrument, Plasmalab System 180) with RF¼100 W, ICP¼400 W,P¼10 mTorr and action gas of SF6 and Ar, Si nanocone structureswere formed as shown in Fig. 1a. Thereafter, 60, 90, 120 nm thickSiN was deposited on such structures respectively through PECVDsystem (Oxford Instrument, Plasmalab System 100) with a mixturegas of SiH4, N2, NH3 at substrate temperature of 350 1C andRF¼67 W, P¼1500 mTorr, in which condition the deposition rateis approximately at 60 nm/min, to get cladded Si nanoconestructures.

Field emission scanning electron microscopy (Hitachi, S4800)was employed to capture the profile of nanostructures. Thereflection spectra of the samples were measured by specularreflectance and integrating sphere accessories of spectrophot-ometer (PerkinElmer, lambda 750).

3. Results and discussion

3.1. Measurement results

As shown in Fig. 1a, Si nanocone structure with aspect ratio of0.92 and duty cycle of 0.7 is defined through the above fabricationprocess. The specular reflectance measured at the incident angle of81 and total reflectance measured by integrating sphere are shownin Fig. 1b. As a reference, the specular reflectance of bare Simaterial is also shown in this figure. Compared with bare Simaterial, the reflectivity reduction over the broadband of 290–2000 nm can be clearly observed, which indicates that suchnanocone structures can effectively reduce the surface reflection.As far as Si nanocone is concerned, when incident wavelengthλo600 nm, the total reflection of up to 30% is higher than itsspecular reflection due to the high order diffraction induced by itslarge period of 600 nm; When λ4600 nm, even though the totalreflectivity and the specular reflectivity are nearly same and stable,their values are still more than 5% due to its small aspect ratio andduty cycle as shown in Fig. 1a. Therefore, such pure Si nanoconestructure seems to be not suitable for Si solar cells as antireflectionfilm, even though its morphology with slightly increased surfacearea benefit decreasing the surface non-radiative recombination.

Dielectric film deposition on Si nanostructured surface isrequired to reduce their surface non-radiative recombination andimprove their photocarrier collection efficiency. Here, we deposit60, 90, 120 nm thick SiN conformal cladding layer on the above Sinanocone structure surface, by which, SiN/Si composite nanoconestructures are formed as shown in Fig. 1c. Fig. 2 is the opticsmicroscope image of all the above fabricated samples. Theirsurface color gradually deepens with the thickness of SiN claddinglayer, which predicts that their surface reflection decreases withSiN thickness. Fig. 3 shows the SEM pictures of such SiN/Sicomposite nanocone structures. As it is shown, the duty cycleand aspect ratio of SiN/Si nanocone structure gradually increaseswith SiN thickness. Such morphology evolution undoubtedlyresults in their surface reflection variation as shown in Fig. 4 andFig. 6, which further influences their optical absorption, ultimateefficiency and conversion efficiency of solar cells.

The specular reflectivity of these nancone structures wasmeasured by the spectrophotometer at fixed incident angle of 81,451 and 601 and their results are shown in Fig. 4. Clearly, thereflectivity of SiN/Si composite nanocone structures reduces withthe increasing SiN thickness over 290–2000 nm at any incidentangle up to 601. Such result is consistent with our optics imageevolution and induced by their morphology evolution. Moreover, it

SiN

Si

Fig. 1. (a) Si nanocone structures amplified 40 K, (b) reflection spectra of Si and Si nanocone structures measured at 81 and of Si nanocone structures measured throughintegrating sphere and (c) 90 nm thick SiN cladded Si nanocone structure amplified 30 K.

Fig. 2. Photograph of (a) Si nanocone, (b) SiN (60 nm)/Si composite nanocone,(c) SiN (90 nm)/Si composite nanocone, and (d) SiN (120 nm)/Si compositenanocone sample. Scale bar is 2 cm.

Y. Wang et al. / Optics Communications 316 (2014) 37–4138

seems that these reflection curves regularly red shift with thethickness of SiN, which indicates that such reflection evolution isrelated with the interference effect of SiN and Si nanoconestructure. Further reflection reduction mechanism needs to clarifyas reference [16] did. Therefore, the great antireflection perfor-mance can be achieved in Si nanocone structures with smalleraspect ratio and duty cycle once they are cladded by SiN material.Meanwhile, such slightly increased surface area and SiN coatingbenefit decreasing the surface non-radiative recombination andphotocarrier collection efficiency.

3.2. Simulation results

In order to further understanding and improving the opticalbehavior of such SiN cladded Si nanocone structures, the

simulation has been conducted by the RCWA method. The simula-tion model consist of nanocones, which were arranged as non-close packed hexagonal arrays with the period of 600 nm, heightof 470 nm and diameter of 450 nm, as shown in Fig. 5a, then theabove defined Si nanocones are coated with 60 nm, 90 nm, 120 nmthick SiN shown in Fig. 5b and c, to finally form the compositenanostructures. Based on the above model, their reflectivities of allabove Si nanocone structures and SiN/Si composite nanoconestructures have been simulated over the spectral range of 300–1000 nm and their results are shown in Fig. 6b. As a contrast, theexperimental measurement results through the integrating spheremodule of spectrophotometer 750 are also shown in Fig. 6a.

Compared with the specular reflection curves as shown inFig. 4, the measured total reflectivity is higher, especially over theshort wavelength domain due to the scattering induced by the

Fig. 3. SEM cross-section pictures of (a) SiN (60 nm)/Si composite nanocone structure amplified 90 K, (b) SiN (90 nm)/Si composite nanocone structure amplified 90 K, and(c) SiN (120 nm)/Si composite nanocone structure amplified 90 K.

Fig. 4. Measured reflection spectra of Si nanocone and SiN/Si composite nanocone over 290–2000 nm at (a) 81, (b) 451, and (c) 601 angle of incidence.

Si

SiN

Fig. 5. (a) The schematic of Si nanocone structure, (b) the schematic of SiN/Si composite nanocone structure, and (c) simulation model of cross-section view of SiN/Sicomposite nanocone structure.

Y. Wang et al. / Optics Communications 316 (2014) 37–41 39

high order diffraction in either Si nancone structures or SiN/Sicomposite nanocone structures. Moreover, much reduction of thereflectivity over this wavelength span is observed in Fig. 6a, whichindicates SiN film can effectively suppress the scattering inducedby the high order diffraction. Further comparison has been madebetween Fig. 6a and b. Both the measurement and simulationresults show that the reflectivity can be reduced over 300–1000 nm once the Si nanocone surface is covered by SiN dielectricfilm. Moreover, both exhibits that the reflectivity over the short-wavelength span is sensitive to SiN thickness, which indicates thatscattering suppression is sensitive to the SiN thickness. Morefluctuation in the simulated reflection spectra over this spanindicates scattering resonance in such periodic composite nano-cone structure is sensitive to SiN thickness and makes a greatcontribution to the surface reflection. On the contrast, when theincident wavelength is longer than 600 nm, reflection reductionbecomes stable and seems to be insensitive to SiN thickness.Moreover, for Si nanocone structures, the measured reflectivity ismuch higher than that of the simulation one over the wholebroadband, especially when λo600 nm, which is due to somerandom arrangement defects induced by PS nanosphere self-assembly in the real samples. For SiN/Si composite nanoconestructures, their difference between the simulated and measuredresults is much smaller, which indicates that such SiN cladding cannot only reduce the reflectivity of the Si nanocone structures, butfurther attenuates the bad influence of PS random distribution ontheir reflection during the fabrication processing.

In order to evaluate the whole influence of SiN cladded Sinancone structures on their reflection, the average reflectivity over300–1000 nm is calculated and illustrated in Fig. 6c by thefollowing equation:

RA ¼R λ2λ1

RðλÞFðλÞdλR λ2λ1

FðλÞdλ

where RA is the average reflectivity, λ is the incident wavelength, λ1is the lower limit of reflection spectral (300 nm) and λ2 for theupper limit (1000 nm), R(λ) is the reflectivity at the incidentwavelength, F(λ) is the photo flux from the AM1.5 spectrum [17].As can be seen, both the simulation and experimental results showthat the average reflectivity is almost lineally reduced when SiNthickness is less than 90 nm. After that, the reflectivity seems to beless dependent on the SiN thickness. Therefore, we can thicken SiNcladding layer on such Si nancone structure according to thepassivation requirement of Si nanocone structures, meanwhile,extremely low reflection is also achieved in such SiN/Si compositenanocone structures, which benefit such structure to be actuallyemployed in Si thin film solar cells to improve their conversionefficiency. In addition, even though the measured reflectivity is

always higher than the simulated one due to the irregular distribu-tion of PS nanosphere mask during the self-assembly processing,such difference become much less once Si nanocone structures iscladded by SiN material, which indicates that SiN cladding on suchSi nanocone structures can further attenuate the disadvantage offabrication defects and improve their antireflection performance.

4. Conclusion

In summary, the antireflection performance of Si nanoconestructures fabricated by nanosphere lithography, ICP etching, andSiN/Si composite nanocone structures fabricated by further PECVDdeposition is investigated experimentally and theoretically. Thereflectivity of more than 10% is observed in Si nanocone structuresdue to their smaller aspect ratio and duty cycle, and mask defectsinduced by the PS nanosphere lithography. Once such Si nanoconestructures are cladded by SiN, the reflection reduction can beobserved over broadband and wide view. Such suppressionmechanism has been analyzed according to the comparsion oftheir total reflectivity measurement results and simulation results.Moreover, such comparison further indicates that SiN claddinglayer on such Si nanocone structures can attenuate the disadvantageof random distribution defects during PS nanosphere self-assemblyprocessing on their antireflection performance. In addition, thealmost linearly reduction of the average reflectivity with SiN thick-ness has been observed when its thickness is less than 90 nm. Afterthat, their antireflection performance is less dependent on SiNthickness. Therefore, great antireflection and passivation perfor-mance should be expected in such SiN cladded Si nancone structureswhen such structure is optimized according to the passivationrequirement of Si nanocone structures. All the above performanceof SiN cladded Si nanocone structure predicts that such compositenanocone structure could be actually employed in Si thin film solarcells and improves their conversion efficiency finally.

Acknowledgments

This work is funded by the National Natural Science Foundation(Nos. 51202284, 11175126 and 11075114); the Scientific ResearchFoundation for the Returned Overseas Chinese Scholars, StateEducation Ministry; the Jiangsu Province Project (No. BE2009056)and the Suzhou City Project (No. SG201020).

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