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Solid State Communications 151 (2011) 127–129
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Periodic silicon nanocone arrays with controllable dimensions prepared by two-step etching using nanosphere lithography and NH4 OH/H2 O2 solution Mingfei Yang a,∗ , HongYu Yu a,1 , Xiaowei Sun a,2 , Junshuai Li a , Xiaocheng Li a , Lin Ke b , Junhui Hu a , Fei Wang a , Zhihui Jiao a a
School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
Institute of Materials Research & Engineering, A*STAR (Agency for Science, Technology and Research), 3, Research Link Singapore 117602, Singapore
article
info
Article history: Received 27 July 2010 Received in revised form 4 November 2010 Accepted 8 November 2010 by P. Sheng Available online 19 November 2010 Keywords: A. Nanostructures B. Nanofabrication E. Light absorption and reflection
abstract An electroless chemical etching technique using polystyrene nanospheres as a self-assembled mask is developed to fabricate size-controllable, periodic silicon nanopillars (NPs) and subsequent nanocone (NC) arrays. The Si NCs are obtained based on the NPs structure using cost-effective ammonia-related etching chemistry. The diameter, height, and periodicity of the NCs can be systematically controlled. Optical measurement shows a good improvement in the reduction of reflectance properties with Si NCs structures. This method is potentially beneficial to many device applications including super-capacitors, batteries, solar cells, etc. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Two-dimensional silicon nanostructures have attracted significant attention in the past decade, attributed to their unique quantum and optical properties. They have found potential benefits in areas including optical applications [1,2], photovoltaic cells [3–10], and battery fabrications [11–13]. The silicon nanopillars (NPs) and nanocones (NCs) have been actively investigated for their potential applications in enhancing light harvesting of a PV cell [2,14–17]. In particular, by tuning the diameter, height, center-to-center spacing (period), Li et al. reported that the Si NC arrays exhibit superior light absorption as compared to Si NP arrays [14]. It is thus practically important and of great interest to develop low cost and high yield Si NC array fabrication technology with excellent process controllability on manipulating the Si NCs physical dimensions. By using the polystyrene (PS) nanospheres lithography technique, ordered Si NPs are reported by Huang et al. [18]. Periodic Si NC fabrication has thus been demonstrated by using dry etching (RIE) [2,19], or oxidation of Si NPs followed by a oxide-removal step in buffered HF [20]. These processes are able to work effectively,
∗
Corresponding author. E-mail addresses: [email protected] (M. Yang), [email protected] (H. Yu), [email protected] (X. Sun). 1 Tel.: +65 6790 4360. 2 Tel.: +65 6790 5369. 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.11.007
however complicated and high-cost vacuum processes or hightemperature annealing processes are needed. In this paper, we report a novel route of preparing Si NC arrays with controlled size and period. Ordered Si NP arrays are firstly prepared by a PS nanosphere-masked chemical etching process. Si NCs are fabricated subsequently based on the Si NPs using the alkaline substance ammonia (NH4 OH). The chemistry of alkaline ammonia has been well-documented to perform anisotropic silicon etching [21]. With systematically manipulating the etching condition, desired Si NC arrays can be successfully achieved. 2. Experimental Single crystalline p-type (boron heavily doped) silicon (100) substrates with resistivity of ∼0.1 cm were sequentially cleaned in acetone, methanol, and de-ionized (DI) water, each for 20 min, in an ultrasonic bath. After that, a piranha solution was used to modify the cleaned Si sample surface into the hydrophilic phase. Polystyrene (PS) balls dispersed in water (microparticles, GmbH) with a diameter of 0.927 µm were thermally treated and mixed with pure ethanol for the purpose of eliminating island formation during spin coating. After the samples were air dried, a monolayer of PS spheres was spin-coated with rpm 500–1000 for 10 s followed by rpm 4000 for 40 s. The size of the PS sphere was then controllably tuned under reactive ion etching (RIE). A thin layer of Ag (∼15 nm) was coated before hydrofluoric acid (HF) mixed with H2 O2 (HF:H2 O2 ∼ 3:1) was used as an etchant subsequently for
128
M. Yang et al. / Solid State Communications 151 (2011) 127–129
a
3. Results and discussion
f
The formation of Si NPs using PS balls as a mask has been shown elsewhere, e.g. [20]. Fig. 2 illustrates NPs formed in this work. As shown in the figure, the hexagonal arrangement of PS balls can be accurately transferred to the NP pattern. The pillars are further etched via ammonia solution. The reaction is anisotropic as the etch rate in high Si-binding-energy planes are significantly lower [21,22]:
b
c
Si + OH− → Si(OH)4 + 4e− .
e
d
Fig. 1. Process flow: (a) blank, cleaned wafer, (b) PS spheres are spin coated, (c) a thin Ag layer is deposited, (d) after HF etching, (e) Ag and PS balls are removed from the NW, and (f) cones have been etched.
NP formation. The remaining silver was removed by boiling aqua regia without damaging the silicon. The samples were cleaned using DI-water before a mixture of ammonium hydroxide and hydrogen peroxide was used to convert the NPs into NCs. Samples were finally rinsed by DI-water and dried for characterization. The schematic process flow is depicted in Fig. 1.
As a result, the etching will be suppressed in the Si ⟨111⟩ plane and a cone shape can be expected. Fig. 3 shows the SEM image of the cones after etching. The Si NC shape can be well obtained and the surfaces of all cones are almost parallel with each other, implying good process control. Finally, NCs with controllable dimensions (with base diameters ranging from 360 to 800 nm) prepared under various RIE conditions are presented in Fig. 4. The longest RIE time, as shown, is 400 s which shrinks the PS sphere to the maximum extent, and generates NPs, as well as subsequent NCs, with base diameter around 360 nm. To obtain NCs with a different period, PS spheres with different dimensions could be applied, because the RIE treatment only modifies the sphere diameter, while maintaining the distance between them. In our experiment, the period is comparable to the diameter of PS spheres, around 950 nm. The height of the cone is also under control via varying of the HF etching time during pillar formation. A longer etching time will generate a ‘‘higher’’ pillar since the etching process is rate-limited. For our observation, 1.5 min etching will create pillars with around 400 nm height, while 8 min etching can lead to pillars with height over 2 µm. Optical characteristics, the reflectance of samples with and without silicon nanopillar and nanocone textured surface were
Fig. 2. Scanning Electronic Microscope (SEM) image of a hexagonally close-packed monolayer of PS ball microspheres with a diameter of 0.927 µm on a p-doped Si ⟨100⟩ substrate, and the image of resultant large-scale silicon nanowires after etching.
Fig. 3. SEM image of Si NC and its forming mechanism.
M. Yang et al. / Solid State Communications 151 (2011) 127–129
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10%. Furthermore, for the sample with the NP structure shows a reflectance at the near infrared range (less than 1100 nm in this case) increasing largely, while the sample with the NC structure shows a reflectance also reduced to well below 10%. 4. Conclusion In conclusion, controlled arrays of Si NCs are fabricated on an Si substrate using electroless chemical etching with PS miconspheres as a self-assembled mask and NH4 OH/H2 O2 solution. The whole process is cost-effective with excellent repeatability, which is practically important for Si NC arrays applications in solar cells, batteries, or super capacitors etc. Optical characterization shows a good improvement in reduction of reflectance properties with NC structures. References
Fig. 4. Different Si NC diameter with corresponding RIE treatment time, and different Si NC height with corresponding etching time.
Fig. 5. Reflection spectra of nanopillar and nanocone textured (diameter around 250 nm, height around 1.2 µm) silicon wafer and blank wafer.
measured by a Lambda 950 UV/VIS/NIR spectrometer from Perkin Elmer. Fig. 5 shows the measurement results. The observation range is from 200 to 1300 nm. One main interest for solar applications is the reduction of reflectance at the visible range, and it has been significantly improved with both silicon nanostructures. For light radiation range of 200–700 nm, the sample with NC structure shows a reflectance less than 5%, while the sample with NP structure shows a reflectance between 5% and
[1] D.F. Dorfner, T. Hurlimann, T. Zabel, L.H. Frandsen, G. Abstreiter, J.J. Finley, Applied Physics Letters 93 (November) (2008). [2] J. Zhu, Z.F. Yu, G.F. Burkhard, C.M. Hsu, S.T. Connor, Y.Q. Xu, Q. Wang, M. McGehee, S.H. Fan, Y. Cui, Nano Letters 9 (January) (2009) 279–282. [3] K.Q. Peng, Y. Xu, Y. Wu, Y.J. Yan, S.T. Lee, J. Zhu, Small 1 (November) (2005) 1062–1067. [4] K.Q. Peng, X. Wang, S.T. Lee, Applied Physics Letters 92 (April) (2008). [5] Y.B. Tang, Z.H. Chen, H.S. Song, C.S. Lee, H.T. Cong, H.M. Cheng, W.J. Zhang, I. Bello, S.T. Lee, Nano Letters 8 (December) (2008) 4191–4195. [6] M.D. Kelzenberg, D.B. Turner-Evans, B.M. Kayes, M.A. Filler, M.C. Putnam, N.S. Lewis, H.A. Atwater, Nano Letters 8 (February) (2008) 710–714. [7] E.C. Garnett, P.D. Yang, Journal of the American Chemical Society 130 (July) (2008) 9224-+. [8] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, Applied Physics Letters 91 (December) (2007). [9] T. Stelzner, M. Pietsch, G. Andra, F. Falk, E. Ose, S. Christiansen, Nanotechnology 19 (July) (2008). [10] V. Sivakov, G. Andra, A. Gawlik, A. Berger, J. Plentz, F. Falk, S.H. Christiansen, Nano Letters 9 (April) (2009) 1549–1554. [11] R. Huang, X. Fan, W.C. Shen, J. Zhu, Applied Physics Letters 95 (September) (2009). [12] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Journal of Physical Chemistry C 113 (July) (2009) 11390–11398. [13] C.K. Chan, R. Ruffo, S.S. Hong, Y. Cui, Journal of Power Sources 189 (April) (2009) 1132–1140. [14] J.S. Li, H.Y. Yu, S.M. Wong, X.C. Li, G. Zhang, P.G.Q. Lo, D.L. Kwong, Applied Physics Letters 95 (December) (2009). [15] L.Y. Cao, J.S. White, J.S. Park, J.A. Schuller, B.M. Clemens, M.L. Brongersma, Nature Materials 8 (August) (2009) 643–647. [16] M.D. Kelzenberg, S.W. Boettcher, J.A. Petykiewicz, D.B. Turner-Evans, M.C. Putnam, E.L. Warren, J.M. Spurgeon, R.M. Briggs, N.S. Lewis, H.A. Atwater, Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications, Nature Materials 9 (March) 239–244. [17] Z.W. Pei, S.T. Chang, C.W. Liu, Y.C. Chen, IEEE Electron Device Letters 30 (December) (2009) 1305–1307. [18] Z.P. Huang, H. Fang, J. Zhu, Advanced Materials 19 (March) (2007) 744–+. [19] C.M. Hsu, S.T. Connor, M.X. Tang, Y. Cui, Applied Physics Letters 93 (September) (2008). [20] W. Li, L. Xu, W.M. Zhao, P. Sun, X.F. Huang, K.J. Chen, Applied Surface Science 253 (September) (2007) 9035–9038. [21] J. Vandenmeerakker, M.H.M. Vanderstraaten, Journal of the Electrochemical Society 137 (April) (1990) 1239–1243. [22] U. Schnakenberg, W. Benecke, B. Lochel, S. Ullerich, P. Lange, Sensors and Actuators A (Physical) 25 (October–January) (1991) 1–7.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Periodic silicon nanocone arrays with controllable dimensions prepared by two-step etching using nanosphere lithography and NH4 OH/H2 O2 solution Mingfei Yang a,∗ , HongYu Yu a,1 , Xiaowei Sun a,2 , Junshuai Li a , Xiaocheng Li a , Lin Ke b , Junhui Hu a , Fei Wang a , Zhihui Jiao a a
School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
Institute of Materials Research & Engineering, A*STAR (Agency for Science, Technology and Research), 3, Research Link Singapore 117602, Singapore
article
info
Article history: Received 27 July 2010 Received in revised form 4 November 2010 Accepted 8 November 2010 by P. Sheng Available online 19 November 2010 Keywords: A. Nanostructures B. Nanofabrication E. Light absorption and reflection
abstract An electroless chemical etching technique using polystyrene nanospheres as a self-assembled mask is developed to fabricate size-controllable, periodic silicon nanopillars (NPs) and subsequent nanocone (NC) arrays. The Si NCs are obtained based on the NPs structure using cost-effective ammonia-related etching chemistry. The diameter, height, and periodicity of the NCs can be systematically controlled. Optical measurement shows a good improvement in the reduction of reflectance properties with Si NCs structures. This method is potentially beneficial to many device applications including super-capacitors, batteries, solar cells, etc. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Two-dimensional silicon nanostructures have attracted significant attention in the past decade, attributed to their unique quantum and optical properties. They have found potential benefits in areas including optical applications [1,2], photovoltaic cells [3–10], and battery fabrications [11–13]. The silicon nanopillars (NPs) and nanocones (NCs) have been actively investigated for their potential applications in enhancing light harvesting of a PV cell [2,14–17]. In particular, by tuning the diameter, height, center-to-center spacing (period), Li et al. reported that the Si NC arrays exhibit superior light absorption as compared to Si NP arrays [14]. It is thus practically important and of great interest to develop low cost and high yield Si NC array fabrication technology with excellent process controllability on manipulating the Si NCs physical dimensions. By using the polystyrene (PS) nanospheres lithography technique, ordered Si NPs are reported by Huang et al. [18]. Periodic Si NC fabrication has thus been demonstrated by using dry etching (RIE) [2,19], or oxidation of Si NPs followed by a oxide-removal step in buffered HF [20]. These processes are able to work effectively,
∗
Corresponding author. E-mail addresses: [email protected] (M. Yang), [email protected] (H. Yu), [email protected] (X. Sun). 1 Tel.: +65 6790 4360. 2 Tel.: +65 6790 5369. 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.11.007
however complicated and high-cost vacuum processes or hightemperature annealing processes are needed. In this paper, we report a novel route of preparing Si NC arrays with controlled size and period. Ordered Si NP arrays are firstly prepared by a PS nanosphere-masked chemical etching process. Si NCs are fabricated subsequently based on the Si NPs using the alkaline substance ammonia (NH4 OH). The chemistry of alkaline ammonia has been well-documented to perform anisotropic silicon etching [21]. With systematically manipulating the etching condition, desired Si NC arrays can be successfully achieved. 2. Experimental Single crystalline p-type (boron heavily doped) silicon (100) substrates with resistivity of ∼0.1 cm were sequentially cleaned in acetone, methanol, and de-ionized (DI) water, each for 20 min, in an ultrasonic bath. After that, a piranha solution was used to modify the cleaned Si sample surface into the hydrophilic phase. Polystyrene (PS) balls dispersed in water (microparticles, GmbH) with a diameter of 0.927 µm were thermally treated and mixed with pure ethanol for the purpose of eliminating island formation during spin coating. After the samples were air dried, a monolayer of PS spheres was spin-coated with rpm 500–1000 for 10 s followed by rpm 4000 for 40 s. The size of the PS sphere was then controllably tuned under reactive ion etching (RIE). A thin layer of Ag (∼15 nm) was coated before hydrofluoric acid (HF) mixed with H2 O2 (HF:H2 O2 ∼ 3:1) was used as an etchant subsequently for
128
M. Yang et al. / Solid State Communications 151 (2011) 127–129
a
3. Results and discussion
f
The formation of Si NPs using PS balls as a mask has been shown elsewhere, e.g. [20]. Fig. 2 illustrates NPs formed in this work. As shown in the figure, the hexagonal arrangement of PS balls can be accurately transferred to the NP pattern. The pillars are further etched via ammonia solution. The reaction is anisotropic as the etch rate in high Si-binding-energy planes are significantly lower [21,22]:
b
c
Si + OH− → Si(OH)4 + 4e− .
e
d
Fig. 1. Process flow: (a) blank, cleaned wafer, (b) PS spheres are spin coated, (c) a thin Ag layer is deposited, (d) after HF etching, (e) Ag and PS balls are removed from the NW, and (f) cones have been etched.
NP formation. The remaining silver was removed by boiling aqua regia without damaging the silicon. The samples were cleaned using DI-water before a mixture of ammonium hydroxide and hydrogen peroxide was used to convert the NPs into NCs. Samples were finally rinsed by DI-water and dried for characterization. The schematic process flow is depicted in Fig. 1.
As a result, the etching will be suppressed in the Si ⟨111⟩ plane and a cone shape can be expected. Fig. 3 shows the SEM image of the cones after etching. The Si NC shape can be well obtained and the surfaces of all cones are almost parallel with each other, implying good process control. Finally, NCs with controllable dimensions (with base diameters ranging from 360 to 800 nm) prepared under various RIE conditions are presented in Fig. 4. The longest RIE time, as shown, is 400 s which shrinks the PS sphere to the maximum extent, and generates NPs, as well as subsequent NCs, with base diameter around 360 nm. To obtain NCs with a different period, PS spheres with different dimensions could be applied, because the RIE treatment only modifies the sphere diameter, while maintaining the distance between them. In our experiment, the period is comparable to the diameter of PS spheres, around 950 nm. The height of the cone is also under control via varying of the HF etching time during pillar formation. A longer etching time will generate a ‘‘higher’’ pillar since the etching process is rate-limited. For our observation, 1.5 min etching will create pillars with around 400 nm height, while 8 min etching can lead to pillars with height over 2 µm. Optical characteristics, the reflectance of samples with and without silicon nanopillar and nanocone textured surface were
Fig. 2. Scanning Electronic Microscope (SEM) image of a hexagonally close-packed monolayer of PS ball microspheres with a diameter of 0.927 µm on a p-doped Si ⟨100⟩ substrate, and the image of resultant large-scale silicon nanowires after etching.
Fig. 3. SEM image of Si NC and its forming mechanism.
M. Yang et al. / Solid State Communications 151 (2011) 127–129
129
10%. Furthermore, for the sample with the NP structure shows a reflectance at the near infrared range (less than 1100 nm in this case) increasing largely, while the sample with the NC structure shows a reflectance also reduced to well below 10%. 4. Conclusion In conclusion, controlled arrays of Si NCs are fabricated on an Si substrate using electroless chemical etching with PS miconspheres as a self-assembled mask and NH4 OH/H2 O2 solution. The whole process is cost-effective with excellent repeatability, which is practically important for Si NC arrays applications in solar cells, batteries, or super capacitors etc. Optical characterization shows a good improvement in reduction of reflectance properties with NC structures. References
Fig. 4. Different Si NC diameter with corresponding RIE treatment time, and different Si NC height with corresponding etching time.
Fig. 5. Reflection spectra of nanopillar and nanocone textured (diameter around 250 nm, height around 1.2 µm) silicon wafer and blank wafer.
measured by a Lambda 950 UV/VIS/NIR spectrometer from Perkin Elmer. Fig. 5 shows the measurement results. The observation range is from 200 to 1300 nm. One main interest for solar applications is the reduction of reflectance at the visible range, and it has been significantly improved with both silicon nanostructures. For light radiation range of 200–700 nm, the sample with NC structure shows a reflectance less than 5%, while the sample with NP structure shows a reflectance between 5% and
[1] D.F. Dorfner, T. Hurlimann, T. Zabel, L.H. Frandsen, G. Abstreiter, J.J. Finley, Applied Physics Letters 93 (November) (2008). [2] J. Zhu, Z.F. Yu, G.F. Burkhard, C.M. Hsu, S.T. Connor, Y.Q. Xu, Q. Wang, M. McGehee, S.H. Fan, Y. Cui, Nano Letters 9 (January) (2009) 279–282. [3] K.Q. Peng, Y. Xu, Y. Wu, Y.J. Yan, S.T. Lee, J. Zhu, Small 1 (November) (2005) 1062–1067. [4] K.Q. Peng, X. Wang, S.T. Lee, Applied Physics Letters 92 (April) (2008). [5] Y.B. Tang, Z.H. Chen, H.S. Song, C.S. Lee, H.T. Cong, H.M. Cheng, W.J. Zhang, I. Bello, S.T. Lee, Nano Letters 8 (December) (2008) 4191–4195. [6] M.D. Kelzenberg, D.B. Turner-Evans, B.M. Kayes, M.A. Filler, M.C. Putnam, N.S. Lewis, H.A. Atwater, Nano Letters 8 (February) (2008) 710–714. [7] E.C. Garnett, P.D. Yang, Journal of the American Chemical Society 130 (July) (2008) 9224-+. [8] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, Applied Physics Letters 91 (December) (2007). [9] T. Stelzner, M. Pietsch, G. Andra, F. Falk, E. Ose, S. Christiansen, Nanotechnology 19 (July) (2008). [10] V. Sivakov, G. Andra, A. Gawlik, A. Berger, J. Plentz, F. Falk, S.H. Christiansen, Nano Letters 9 (April) (2009) 1549–1554. [11] R. Huang, X. Fan, W.C. Shen, J. Zhu, Applied Physics Letters 95 (September) (2009). [12] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Journal of Physical Chemistry C 113 (July) (2009) 11390–11398. [13] C.K. Chan, R. Ruffo, S.S. Hong, Y. Cui, Journal of Power Sources 189 (April) (2009) 1132–1140. [14] J.S. Li, H.Y. Yu, S.M. Wong, X.C. Li, G. Zhang, P.G.Q. Lo, D.L. Kwong, Applied Physics Letters 95 (December) (2009). [15] L.Y. Cao, J.S. White, J.S. Park, J.A. Schuller, B.M. Clemens, M.L. Brongersma, Nature Materials 8 (August) (2009) 643–647. [16] M.D. Kelzenberg, S.W. Boettcher, J.A. Petykiewicz, D.B. Turner-Evans, M.C. Putnam, E.L. Warren, J.M. Spurgeon, R.M. Briggs, N.S. Lewis, H.A. Atwater, Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications, Nature Materials 9 (March) 239–244. [17] Z.W. Pei, S.T. Chang, C.W. Liu, Y.C. Chen, IEEE Electron Device Letters 30 (December) (2009) 1305–1307. [18] Z.P. Huang, H. Fang, J. Zhu, Advanced Materials 19 (March) (2007) 744–+. [19] C.M. Hsu, S.T. Connor, M.X. Tang, Y. Cui, Applied Physics Letters 93 (September) (2008). [20] W. Li, L. Xu, W.M. Zhao, P. Sun, X.F. Huang, K.J. Chen, Applied Surface Science 253 (September) (2007) 9035–9038. [21] J. Vandenmeerakker, M.H.M. Vanderstraaten, Journal of the Electrochemical Society 137 (April) (1990) 1239–1243. [22] U. Schnakenberg, W. Benecke, B. Lochel, S. Ullerich, P. Lange, Sensors and Actuators A (Physical) 25 (October–January) (1991) 1–7.