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Broadband and omnidirectional antireflection of Si nanocone structures cladded by SiN film for Si thin film solar cells
Optics Communications 316 (2014) 37–41
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Broadband and omnidirectional antireflection of Si nanocone structures 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, China Key Labs of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China c Division of Nano-devices and Related Materials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China d Key Laboratory of Thin Films of Jiangsu, Soochow University, Suzhou 215006, China e Analysis and Testing Center, Soochow University, Suzhou 215006, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2013 Accepted 24 November 2013 Available online 6 December 2013
Si nanocone structures cladded by SiN material are fabricated by nanosphere lithography, inductively coupled plasma (ICP) etching and plasma enhanced chemical vapor deposition (PECVD). Their broadband omnidiectional antireflection performance is investigated through measurement and simulation by rigorous coupled-wave analysis (RCWA). Both the measurement and simulation results show that the reflectivity of SiN passivated Si nanocone is lower than that of original Si nanocone structure over broadband and wide view. The average reflectivity data further indicate that such reflectivity seems to be linearly reduced with SiN thickness when it is less than 90 nm. After that, the reflectivity of such composite nanocone structure is less dependent on SiN thickness. The minimum average reflectivity of 1.84% is achieved in Si nanocone structure cladded by 90 nm thick SiN film, which is only 10.3% of the average reflectivity of the same Si nanocone structure. In addition, the comparison further shows that such SiN cladding layer can attenuate the bad influence of random nanosphere mask induced by selfassembly processing on their reflection. Therefore, great antireflection and good passivation performance in such SiN/Si composite nanocone structures are expected, which benefit them to be actually employed in Si thin film solar cells and improve their conversion efficiency finally. & 2013 Elsevier B.V. All rights reserved.
Keywords: Si nanocone structure cladded by SiN Broadband and omnidirectional antireflection Si thin film solar cells
1. Introduction Si thin film solar cells, as one of the promising candidates of cost-effective solar cells, are increasingly paid much attention [1–3]. The passivated and antireflection surface is required for them to achieve high conversion efficiency. Up to now, the most passivation technique employed in solar cells is coating dielectric film, such as SiO2 [4], SiN [5], and Al2O3 [6–8], on their surface to achieve chemical passivation and field passivation effect. Meanwhile, nanotextured surfaces, such as nanopore [9], nanowire [10–11] and nanocone [12–13], with high reflection suppression and lighttrapping performance are attractive in Si thin film solar cells. Therefore, it becomes a trend for Si-based solar cell to form a dielectric film cladded nanostructure surface, in order to further improving their conversion efficiency. The passivation performance of such composite nanostructure surface has been widely n
Corresponding author. Tel.: +865 126 287 2560; fax: +865 126 287 3079. E-mail addresses: [email protected] (X. Wu), [email protected] (R. Zhang). 1 Tel.: þ865 126 511 2066; fax: þ865 126 511 1907. 0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.11.050
investigated [14]. However, the influence of such surface treatment on their surface reflection has rarely concerned, which will further influence their optical absorption and efficiency of solar cells. Recently, we demonstrated the optoelectronic performance of Al2O3/Si composite nanocone structure theoretically and experimentally [15]. We found Al2O3 can not only realize the surface passivation, but also reduces the surface reflection significantly, which is helpful for Si nanocone structure with small aspect ratio and duty cycle to achieve great surface passivation and antireflection performance. In this paper, we focus on the reflection performance of SiN cladded Si nanocone surface experimentally and theoretically. The results indicate that the reflectivity firstly linearly reduces with SiN thickness and then less depends on SiN thickness once the Si nanocone is cladded by SiN film. Therefore, SiN cladding layer is optimized just according to the passivation requirement of Si nanocone structure, but great antireflection and passivation performance can be achieved, which benefit for such composite nanocone structure to be actually employed in Si thin film solar cells and improve their conversion efficiency finally.
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Y. Wang et al. / Optics Communications 316 (2014) 37–41
2. Experiment Polystyrene (PS) nanosphere lithography combined with dry etching is employed to fabricate Si nanocone structures on crystalline Si substrate. First, the traditional RCA cleaning method was introduced to remove impurities of Si surface, and then PS spheres with diameter of 600 nm were distributed on its surface as closepacked hexagonal array by spinning coating. The samples with PS monolayer mask were then etched by ICP etching system (Oxford Instrument, Plasmalab System 180) with RF ¼100 W, ICP¼ 400 W, P ¼10 mTorr and action gas of SF6 and Ar, Si nanocone structures were formed as shown in Fig. 1a. Thereafter, 60, 90, 120 nm thick SiN was deposited on such structures respectively through PECVD system (Oxford Instrument, Plasmalab System 100) with a mixture gas of SiH4, N2, NH3 at substrate temperature of 350 1C and RF ¼67 W, P ¼1500 mTorr, in which condition the deposition rate is approximately at 60 nm/min, to get cladded Si nanocone structures. Field emission scanning electron microscopy (Hitachi, S4800) was employed to capture the profile of nanostructures. The reflection spectra of the samples were measured by specular reflectance and integrating sphere accessories of spectrophotometer (PerkinElmer, lambda 750).
Dielectric film deposition on Si nanostructured surface is required to reduce their surface non-radiative recombination and improve their photocarrier collection efficiency. Here, we deposit 60, 90, 120 nm thick SiN conformal cladding layer on the above Si nanocone structure surface, by which, SiN/Si composite nanocone structures are formed as shown in Fig. 1c. Fig. 2 is the optics microscope image of all the above fabricated samples. Their surface color gradually deepens with the thickness of SiN cladding layer, which predicts that their surface reflection decreases with SiN thickness. Fig. 3 shows the SEM pictures of such SiN/Si composite nanocone structures. As it is shown, the duty cycle and aspect ratio of SiN/Si nanocone structure gradually increases with SiN thickness. Such morphology evolution undoubtedly results in their surface reflection variation as shown in Fig. 4 and Fig. 6, which further influences their optical absorption, ultimate efficiency and conversion efficiency of solar cells. The specular reflectivity of these nancone structures was measured by the spectrophotometer at fixed incident angle of 81, 451 and 601 and their results are shown in Fig. 4. Clearly, the reflectivity of SiN/Si composite nanocone structures reduces with the increasing SiN thickness over 290–2000 nm at any incident angle up to 601. Such result is consistent with our optics image evolution and induced by their morphology evolution. Moreover, it
3. Results and discussion 3.1. Measurement results As shown in Fig. 1a, Si nanocone structure with aspect ratio of 0.92 and duty cycle of 0.7 is defined through the above fabrication process. The specular reflectance measured at the incident angle of 81 and total reflectance measured by integrating sphere are shown in Fig. 1b. As a reference, the specular reflectance of bare Si material is also shown in this figure. Compared with bare Si material, the reflectivity reduction over the broadband of 290– 2000 nm can be clearly observed, which indicates that such nanocone structures can effectively reduce the surface reflection. As far as Si nanocone is concerned, when incident wavelength λ o600 nm, the total reflection of up to 30% is higher than its specular reflection due to the high order diffraction induced by its large period of 600 nm; When λ 4600 nm, even though the total reflectivity and the specular reflectivity are nearly same and stable, their values are still more than 5% due to its small aspect ratio and duty cycle as shown in Fig. 1a. Therefore, such pure Si nanocone structure seems to be not suitable for Si solar cells as antireflection film, even though its morphology with slightly increased surface area benefit decreasing the surface non-radiative recombination.
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 composite nanocone sample. Scale bar is 2 cm.
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 through integrating sphere and (c) 90 nm thick SiN cladded Si nanocone structure amplified 30 K.
Y. Wang et al. / Optics Communications 316 (2014) 37–41
39
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.
SiN
Si
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/Si composite nanocone structure.
seems that these reflection curves regularly red shift with the thickness of SiN, which indicates that such reflection evolution is related with the interference effect of SiN and Si nanocone structure. Further reflection reduction mechanism needs to clarify as reference [16] did. Therefore, the great antireflection performance can be achieved in Si nanocone structures with smaller aspect ratio and duty cycle once they are cladded by SiN material. Meanwhile, such slightly increased surface area and SiN coating benefit decreasing the surface non-radiative recombination and photocarrier collection efficiency. 3.2. Simulation results In order to further understanding and improving the optical behavior of such SiN cladded Si nanocone structures, the
simulation has been conducted by the RCWA method. The simulation model consist of nanocones, which were arranged as nonclose packed hexagonal arrays with the period of 600 nm, height of 470 nm and diameter of 450 nm, as shown in Fig. 5a, then the above defined Si nanocones are coated with 60 nm, 90 nm, 120 nm thick SiN shown in Fig. 5b and c, to finally form the composite nanostructures. Based on the above model, their reflectivities of all above Si nanocone structures and SiN/Si composite nanocone structures have been simulated over the spectral range of 300– 1000 nm and their results are shown in Fig. 6b. As a contrast, the experimental measurement results through the integrating sphere module of spectrophotometer 750 are also shown in Fig. 6a. Compared with the specular reflection curves as shown in Fig. 4, the measured total reflectivity is higher, especially over the short wavelength domain due to the scattering induced by the
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Y. Wang et al. / Optics Communications 316 (2014) 37–41
Fig. 6. (a) Measured and (b) simulated reflection spectra of Si nanocone and SiN/Si composite nanocone, and (c) the average reflectivity variation with SiN thickness.
high order diffraction in either Si nancone structures or SiN/Si composite nanocone structures. Moreover, much reduction of the reflectivity over this wavelength span is observed in Fig. 6a, which indicates SiN film can effectively suppress the scattering induced by the high order diffraction. Further comparison has been made between Fig. 6a and b. Both the measurement and simulation results show that the reflectivity can be reduced over 300– 1000 nm once the Si nanocone surface is covered by SiN dielectric film. Moreover, both exhibits that the reflectivity over the shortwavelength span is sensitive to SiN thickness, which indicates that scattering suppression is sensitive to the SiN thickness. More fluctuation in the simulated reflection spectra over this span indicates scattering resonance in such periodic composite nanocone structure is sensitive to SiN thickness and makes a great contribution to the surface reflection. On the contrast, when the incident wavelength is longer than 600 nm, reflection reduction becomes stable and seems to be insensitive to SiN thickness. Moreover, for Si nanocone structures, the measured reflectivity is much higher than that of the simulation one over the whole broadband, especially when λ o 600 nm, which is due to some random arrangement defects induced by PS nanosphere selfassembly in the real samples. For SiN/Si composite nanocone structures, their difference between the simulated and measured results is much smaller, which indicates that such SiN cladding can not only reduce the reflectivity of the Si nanocone structures, but further attenuates the bad influence of PS random distribution on their reflection during the fabrication processing. In order to evaluate the whole influence of SiN cladded Si nancone structures on their reflection, the average reflectivity over 300–1000 nm is calculated and illustrated in Fig. 6c by the following equation: R λ2 RA ¼
λ1 RðλÞFðλÞdλ R λ2 λ1 FðλÞdλ
where RA is the average reflectivity, λ is the incident wavelength, λ1 is the lower limit of reflection spectral (300 nm) and λ2 for the upper limit (1000 nm), R(λ) is the reflectivity at the incident wavelength, F(λ) is the photo flux from the AM1.5 spectrum [17]. As can be seen, both the simulation and experimental results show that the average reflectivity is almost lineally reduced when SiN thickness is less than 90 nm. After that, the reflectivity seems to be less dependent on the SiN thickness. Therefore, we can thicken SiN cladding layer on such Si nancone structure according to the passivation requirement of Si nanocone structures, meanwhile, extremely low reflection is also achieved in such SiN/Si composite nanocone structures, which benefit such structure to be actually employed in Si thin film solar cells to improve their conversion efficiency. In addition, even though the measured reflectivity is
always higher than the simulated one due to the irregular distribution of PS nanosphere mask during the self-assembly processing, such difference become much less once Si nanocone structures is cladded by SiN material, which indicates that SiN cladding on such Si nanocone structures can further attenuate the disadvantage of fabrication defects and improve their antireflection performance.
4. Conclusion In summary, the antireflection performance of Si nanocone structures fabricated by nanosphere lithography, ICP etching, and SiN/Si composite nanocone structures fabricated by further PECVD deposition is investigated experimentally and theoretically. The reflectivity of more than 10% is observed in Si nanocone structures due to their smaller aspect ratio and duty cycle, and mask defects induced by the PS nanosphere lithography. Once such Si nanocone structures are cladded by SiN, the reflection reduction can be observed over broadband and wide view. Such suppression mechanism has been analyzed according to the comparsion of their total reflectivity measurement results and simulation results. Moreover, such comparison further indicates that SiN cladding layer on such Si nanocone structures can attenuate the disadvantage of random distribution defects during PS nanosphere self-assembly processing on their antireflection performance. In addition, the almost linearly reduction of the average reflectivity with SiN thickness has been observed when its thickness is less than 90 nm. After that, their antireflection performance is less dependent on SiN thickness. Therefore, great antireflection and passivation performance should be expected in such SiN cladded Si nancone structures when such structure is optimized according to the passivation requirement of Si nanocone structures. All the above performance of SiN cladded Si nanocone structure predicts that such composite nanocone structure could be actually employed in Si thin film solar cells and improves their conversion efficiency finally. Acknowledgments This work is funded by the National Natural Science Foundation (Nos. 51202284, 11175126 and 11075114); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Jiangsu Province Project (No. BE2009056) and the Suzhou City Project (No. SG201020). References [1] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (5428) (1999) 692. [2] J. Yang, A. Banerjee, S. Guha, Appl. Phys. Lett. 70 (22) (1997) 2975.
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[3] J. Zhu, C.M. Hsu, Z.F. Yu, S.H. Fan, Y. Cui, Nano Lett. 10 (6) (2010) 1979. [4] R.B.M. Girisch, R.P. Mertens, R.F. De Keersmaecker, IEEE Trans. Electron. Dev. 35 (2) (1988) 203. [5] J. Schmidt, M. Kerr, A. Cuevas, Semicond. Sci. Tech. 16 (3) (2001) 164. [6] M.K. Lee, C.F. Yen, Phys. Status Solidi A 209 (11) (2012) 2147. [7] G. Dingemans, W.M.M. Kessels, J. Vac. Sci. Technol. A 30 (4) (2012) 040802. [8] J. Schmidt, A. Merkle, Prog. Photovolt. 16 (6) (2008) 461. [9] S.E. Han, G. Chen, Nano Lett. 10 (3) (2010) 1012. [10] E. Garnett, P.D. Yang, Nano Lett. 10 (3) (2010) 1082. [11] Y.J. Hwang, C. Hahn, B. Liu, P.D. Yang, ACS Nano 6 (6) (2012) 5060. [12] R.Y. Zhang, B. Shao, J.R. Dong, J.C. Zhang, H. Yang, J. Appl. Phys. 110 (11) (2011) 113105.
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[13] P.H. Wangyang, Q.K. Wang, K.X. Hu, X. Wan, K. Huang, Opt. Commun. 294 (2013) 395. [14] M. Otto, M. Kroll, T. Kasebier, R. Salzer, A. Tunnermann, R.B. Wehrspohn, Appl. Phys. Lett. 100 (19) (2012) 191603. [15] R.Y. Zhang, Y.Y. Wang, J. Zhu, Z. Zhang, Y. Zhang, K. Huang, B. Shao, J.R. Dong, H. Yang, Adv. Energy Mater, under review. [16] R.Y. Zhang, Z. Zhang, B. Shao, J.R. Dong, H. Yang, J. Appl. Phys. D 46 (14) (2013) 145104. [17] F. Zhan, J.F. He, X.J. Shang, M.F. Li, H.Q. Ni, Y.Q. Xu, Z.C Niu, Chin. Phys. B 21 (3) (2012) 037802.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Broadband and omnidirectional antireflection of Si nanocone structures 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, China Key Labs of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China c Division of Nano-devices and Related Materials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China d Key Laboratory of Thin Films of Jiangsu, Soochow University, Suzhou 215006, China e Analysis and Testing Center, Soochow University, Suzhou 215006, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2013 Accepted 24 November 2013 Available online 6 December 2013
Si nanocone structures cladded by SiN material are fabricated by nanosphere lithography, inductively coupled plasma (ICP) etching and plasma enhanced chemical vapor deposition (PECVD). Their broadband omnidiectional antireflection performance is investigated through measurement and simulation by rigorous coupled-wave analysis (RCWA). Both the measurement and simulation results show that the reflectivity of SiN passivated Si nanocone is lower than that of original Si nanocone structure over broadband and wide view. The average reflectivity data further indicate that such reflectivity seems to be linearly reduced with SiN thickness when it is less than 90 nm. After that, the reflectivity of such composite nanocone structure is less dependent on SiN thickness. The minimum average reflectivity of 1.84% is achieved in Si nanocone structure cladded by 90 nm thick SiN film, which is only 10.3% of the average reflectivity of the same Si nanocone structure. In addition, the comparison further shows that such SiN cladding layer can attenuate the bad influence of random nanosphere mask induced by selfassembly processing on their reflection. Therefore, great antireflection and good passivation performance in such SiN/Si composite nanocone structures are expected, which benefit them to be actually employed in Si thin film solar cells and improve their conversion efficiency finally. & 2013 Elsevier B.V. All rights reserved.
Keywords: Si nanocone structure cladded by SiN Broadband and omnidirectional antireflection Si thin film solar cells
1. Introduction Si thin film solar cells, as one of the promising candidates of cost-effective solar cells, are increasingly paid much attention [1–3]. The passivated and antireflection surface is required for them to achieve high conversion efficiency. Up to now, the most passivation technique employed in solar cells is coating dielectric film, such as SiO2 [4], SiN [5], and Al2O3 [6–8], on their surface to achieve chemical passivation and field passivation effect. Meanwhile, nanotextured surfaces, such as nanopore [9], nanowire [10–11] and nanocone [12–13], with high reflection suppression and lighttrapping performance are attractive in Si thin film solar cells. Therefore, it becomes a trend for Si-based solar cell to form a dielectric film cladded nanostructure surface, in order to further improving their conversion efficiency. The passivation performance of such composite nanostructure surface has been widely n
Corresponding author. Tel.: +865 126 287 2560; fax: +865 126 287 3079. E-mail addresses: [email protected] (X. Wu), [email protected] (R. Zhang). 1 Tel.: þ865 126 511 2066; fax: þ865 126 511 1907. 0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.11.050
investigated [14]. However, the influence of such surface treatment on their surface reflection has rarely concerned, which will further influence their optical absorption and efficiency of solar cells. Recently, we demonstrated the optoelectronic performance of Al2O3/Si composite nanocone structure theoretically and experimentally [15]. We found Al2O3 can not only realize the surface passivation, but also reduces the surface reflection significantly, which is helpful for Si nanocone structure with small aspect ratio and duty cycle to achieve great surface passivation and antireflection performance. In this paper, we focus on the reflection performance of SiN cladded Si nanocone surface experimentally and theoretically. The results indicate that the reflectivity firstly linearly reduces with SiN thickness and then less depends on SiN thickness once the Si nanocone is cladded by SiN film. Therefore, SiN cladding layer is optimized just according to the passivation requirement of Si nanocone structure, but great antireflection and passivation performance can be achieved, which benefit for such composite nanocone structure to be actually employed in Si thin film solar cells and improve their conversion efficiency finally.
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Y. Wang et al. / Optics Communications 316 (2014) 37–41
2. Experiment Polystyrene (PS) nanosphere lithography combined with dry etching is employed to fabricate Si nanocone structures on crystalline Si substrate. First, the traditional RCA cleaning method was introduced to remove impurities of Si surface, and then PS spheres with diameter of 600 nm were distributed on its surface as closepacked hexagonal array by spinning coating. The samples with PS monolayer mask were then etched by ICP etching system (Oxford Instrument, Plasmalab System 180) with RF ¼100 W, ICP¼ 400 W, P ¼10 mTorr and action gas of SF6 and Ar, Si nanocone structures were formed as shown in Fig. 1a. Thereafter, 60, 90, 120 nm thick SiN was deposited on such structures respectively through PECVD system (Oxford Instrument, Plasmalab System 100) with a mixture gas of SiH4, N2, NH3 at substrate temperature of 350 1C and RF ¼67 W, P ¼1500 mTorr, in which condition the deposition rate is approximately at 60 nm/min, to get cladded Si nanocone structures. Field emission scanning electron microscopy (Hitachi, S4800) was employed to capture the profile of nanostructures. The reflection spectra of the samples were measured by specular reflectance and integrating sphere accessories of spectrophotometer (PerkinElmer, lambda 750).
Dielectric film deposition on Si nanostructured surface is required to reduce their surface non-radiative recombination and improve their photocarrier collection efficiency. Here, we deposit 60, 90, 120 nm thick SiN conformal cladding layer on the above Si nanocone structure surface, by which, SiN/Si composite nanocone structures are formed as shown in Fig. 1c. Fig. 2 is the optics microscope image of all the above fabricated samples. Their surface color gradually deepens with the thickness of SiN cladding layer, which predicts that their surface reflection decreases with SiN thickness. Fig. 3 shows the SEM pictures of such SiN/Si composite nanocone structures. As it is shown, the duty cycle and aspect ratio of SiN/Si nanocone structure gradually increases with SiN thickness. Such morphology evolution undoubtedly results in their surface reflection variation as shown in Fig. 4 and Fig. 6, which further influences their optical absorption, ultimate efficiency and conversion efficiency of solar cells. The specular reflectivity of these nancone structures was measured by the spectrophotometer at fixed incident angle of 81, 451 and 601 and their results are shown in Fig. 4. Clearly, the reflectivity of SiN/Si composite nanocone structures reduces with the increasing SiN thickness over 290–2000 nm at any incident angle up to 601. Such result is consistent with our optics image evolution and induced by their morphology evolution. Moreover, it
3. Results and discussion 3.1. Measurement results As shown in Fig. 1a, Si nanocone structure with aspect ratio of 0.92 and duty cycle of 0.7 is defined through the above fabrication process. The specular reflectance measured at the incident angle of 81 and total reflectance measured by integrating sphere are shown in Fig. 1b. As a reference, the specular reflectance of bare Si material is also shown in this figure. Compared with bare Si material, the reflectivity reduction over the broadband of 290– 2000 nm can be clearly observed, which indicates that such nanocone structures can effectively reduce the surface reflection. As far as Si nanocone is concerned, when incident wavelength λ o600 nm, the total reflection of up to 30% is higher than its specular reflection due to the high order diffraction induced by its large period of 600 nm; When λ 4600 nm, even though the total reflectivity and the specular reflectivity are nearly same and stable, their values are still more than 5% due to its small aspect ratio and duty cycle as shown in Fig. 1a. Therefore, such pure Si nanocone structure seems to be not suitable for Si solar cells as antireflection film, even though its morphology with slightly increased surface area benefit decreasing the surface non-radiative recombination.
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 composite nanocone sample. Scale bar is 2 cm.
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 through integrating sphere and (c) 90 nm thick SiN cladded Si nanocone structure amplified 30 K.
Y. Wang et al. / Optics Communications 316 (2014) 37–41
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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.
SiN
Si
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/Si composite nanocone structure.
seems that these reflection curves regularly red shift with the thickness of SiN, which indicates that such reflection evolution is related with the interference effect of SiN and Si nanocone structure. Further reflection reduction mechanism needs to clarify as reference [16] did. Therefore, the great antireflection performance can be achieved in Si nanocone structures with smaller aspect ratio and duty cycle once they are cladded by SiN material. Meanwhile, such slightly increased surface area and SiN coating benefit decreasing the surface non-radiative recombination and photocarrier collection efficiency. 3.2. Simulation results In order to further understanding and improving the optical behavior of such SiN cladded Si nanocone structures, the
simulation has been conducted by the RCWA method. The simulation model consist of nanocones, which were arranged as nonclose packed hexagonal arrays with the period of 600 nm, height of 470 nm and diameter of 450 nm, as shown in Fig. 5a, then the above defined Si nanocones are coated with 60 nm, 90 nm, 120 nm thick SiN shown in Fig. 5b and c, to finally form the composite nanostructures. Based on the above model, their reflectivities of all above Si nanocone structures and SiN/Si composite nanocone structures have been simulated over the spectral range of 300– 1000 nm and their results are shown in Fig. 6b. As a contrast, the experimental measurement results through the integrating sphere module of spectrophotometer 750 are also shown in Fig. 6a. Compared with the specular reflection curves as shown in Fig. 4, the measured total reflectivity is higher, especially over the short wavelength domain due to the scattering induced by the
40
Y. Wang et al. / Optics Communications 316 (2014) 37–41
Fig. 6. (a) Measured and (b) simulated reflection spectra of Si nanocone and SiN/Si composite nanocone, and (c) the average reflectivity variation with SiN thickness.
high order diffraction in either Si nancone structures or SiN/Si composite nanocone structures. Moreover, much reduction of the reflectivity over this wavelength span is observed in Fig. 6a, which indicates SiN film can effectively suppress the scattering induced by the high order diffraction. Further comparison has been made between Fig. 6a and b. Both the measurement and simulation results show that the reflectivity can be reduced over 300– 1000 nm once the Si nanocone surface is covered by SiN dielectric film. Moreover, both exhibits that the reflectivity over the shortwavelength span is sensitive to SiN thickness, which indicates that scattering suppression is sensitive to the SiN thickness. More fluctuation in the simulated reflection spectra over this span indicates scattering resonance in such periodic composite nanocone structure is sensitive to SiN thickness and makes a great contribution to the surface reflection. On the contrast, when the incident wavelength is longer than 600 nm, reflection reduction becomes stable and seems to be insensitive to SiN thickness. Moreover, for Si nanocone structures, the measured reflectivity is much higher than that of the simulation one over the whole broadband, especially when λ o 600 nm, which is due to some random arrangement defects induced by PS nanosphere selfassembly in the real samples. For SiN/Si composite nanocone structures, their difference between the simulated and measured results is much smaller, which indicates that such SiN cladding can not only reduce the reflectivity of the Si nanocone structures, but further attenuates the bad influence of PS random distribution on their reflection during the fabrication processing. In order to evaluate the whole influence of SiN cladded Si nancone structures on their reflection, the average reflectivity over 300–1000 nm is calculated and illustrated in Fig. 6c by the following equation: R λ2 RA ¼
λ1 RðλÞFðλÞdλ R λ2 λ1 FðλÞdλ
where RA is the average reflectivity, λ is the incident wavelength, λ1 is the lower limit of reflection spectral (300 nm) and λ2 for the upper limit (1000 nm), R(λ) is the reflectivity at the incident wavelength, F(λ) is the photo flux from the AM1.5 spectrum [17]. As can be seen, both the simulation and experimental results show that the average reflectivity is almost lineally reduced when SiN thickness is less than 90 nm. After that, the reflectivity seems to be less dependent on the SiN thickness. Therefore, we can thicken SiN cladding layer on such Si nancone structure according to the passivation requirement of Si nanocone structures, meanwhile, extremely low reflection is also achieved in such SiN/Si composite nanocone structures, which benefit such structure to be actually employed in Si thin film solar cells to improve their conversion efficiency. In addition, even though the measured reflectivity is
always higher than the simulated one due to the irregular distribution of PS nanosphere mask during the self-assembly processing, such difference become much less once Si nanocone structures is cladded by SiN material, which indicates that SiN cladding on such Si nanocone structures can further attenuate the disadvantage of fabrication defects and improve their antireflection performance.
4. Conclusion In summary, the antireflection performance of Si nanocone structures fabricated by nanosphere lithography, ICP etching, and SiN/Si composite nanocone structures fabricated by further PECVD deposition is investigated experimentally and theoretically. The reflectivity of more than 10% is observed in Si nanocone structures due to their smaller aspect ratio and duty cycle, and mask defects induced by the PS nanosphere lithography. Once such Si nanocone structures are cladded by SiN, the reflection reduction can be observed over broadband and wide view. Such suppression mechanism has been analyzed according to the comparsion of their total reflectivity measurement results and simulation results. Moreover, such comparison further indicates that SiN cladding layer on such Si nanocone structures can attenuate the disadvantage of random distribution defects during PS nanosphere self-assembly processing on their antireflection performance. In addition, the almost linearly reduction of the average reflectivity with SiN thickness has been observed when its thickness is less than 90 nm. After that, their antireflection performance is less dependent on SiN thickness. Therefore, great antireflection and passivation performance should be expected in such SiN cladded Si nancone structures when such structure is optimized according to the passivation requirement of Si nanocone structures. All the above performance of SiN cladded Si nanocone structure predicts that such composite nanocone structure could be actually employed in Si thin film solar cells and improves their conversion efficiency finally. Acknowledgments This work is funded by the National Natural Science Foundation (Nos. 51202284, 11175126 and 11075114); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Jiangsu Province Project (No. BE2009056) and the Suzhou City Project (No. SG201020). References [1] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (5428) (1999) 692. [2] J. Yang, A. Banerjee, S. Guha, Appl. Phys. Lett. 70 (22) (1997) 2975.
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