Broadband Optical Antireflection Enhancement by Integrating Antireflective Nanoislands with Silicon Nanoconical-Frustum Arrays

Transcription

1 Broadband Optical Antireflection Enhancement by Integrating Antireflective Nanoislands with Silicon Nanoconical-Frustum Arrays Haesung Park, Dongheok Shin, Gumin Kang, Seunghwa Baek, Kyoungsik Kim, * and Willie J. Padilla By employing antireflective surfaces (ARSs) many researchers have made extensive efforts to overcome the major obstacles of optoelectronic devices, such as low absorption of light in photovoltaic devices [ 1, 2 ] and poor light extraction efficiency out of light-emitting diodes (LEDs), [ 3, 4 ] and so on. Until now, there have been two main approaches to fabricate efficient ARSs: one is coating antireflective (AR) layered films on the surface and the other is fabricating a subwavelength biomimetic moth s eye structure on the surface. AR coatings are suitable for highthroughput industrial production in large area processing [ 5 ] but typically validated to suppress reflection at a specific wavelength and at specific incident angles. On the other hand, the biomimetic moth s eye structure, such as nanocones and nanopillars, enables us to suppress reflectance in the wider ranges of both spectrum and incident angles of light. [ 6, 7 ] For a moth s eye structure, however, it is critical that the lattice constant of the periodic nanostructure is sufficiently smaller than the wavelength of antireflection spectrum so that the array may not be resolved by light and the effective refractive index (RI) can be treated as a mean value between the RI values of air and the bulk semiconductor in proportion to the volume fraction of each material. [ 8, 9 ] The technology to achieve broader-bandwidth ARS is very important for efficient solar energy utilization in tandem photo voltaic cells and for optical display device industries. [ 8, 10 ] Therefore, the broadband ARSs have been demonstrated in many groups by fabricating nanoporous polymer film, [ 11 ] subwavelength aperiodic arrays of nano tips, [ 6 ] hollow pillar-like protuberances, [ 7 ] and randomly oriented sub-50 nm patterning with block copolymers. [ 12 ] Unfortunately, the feature size of sub-300 nm is necessary in order to broaden the optical antireflection spectrum of moth s eye structure into the ultraviolet (UV) and visible light applications, which is important for the performance of solar cells or display devices. Even with current H. Park, D. Shin, G. Kang, S. Baek, Prof. K. Kim School of Mechanical Engineering Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul , Korea kks@yonsei.ac.kr Prof. W. J. Padilla Department of Physics Boston College 140 Commonwealth Avenue, Chestnut Hill, MA 02467, USA DOI: /adma top-down nanofabrication methods such as electron-beam lithography [ 13 ] and interference lithography, [ 14 ] it is not a trivial work to fabricate the size of sub-300 nm because high-cost and time-consuming nano-texturing technologies are required. According to the AM 1.5 solar spectrum, for example, the solar energy in shorter wavelength range from 300 to 500 nm including near-ultraviolet (NUV, nm) spectral range comprises 23% (or 36%) of all the available solar energy with crystalline (or amorphous) silicon semiconductors because of the limited bandgap of 1.12 ev (or 1.75 ev). [ 15 ] On the other hand, the UV sunlight in the spectral range shorter than 300 nm is mostly blocked by the Earth s ozone layer and has almost no influence on the solar energy harvesting on the sea level, as well as the AM 1.5 solar spectrum. Hence, a feasible optical antireflection enhancement up to the NUV region is enough to get sufficiently improved performance of solar cells. In this paper, we propose a novel method to extend the antireflection spectral range shorter than the lattice constant of the nanostructure by combining AR coatings and the moth s eye structure without a complicated process to fabricate a feature size of sub-300 nm. Based on conventional colloidal nanosphere lithography, we demonstrated novel graded-index (GRIN) nanostructures by integrating AR nanoisland coating arrays on top of silicon nano-conical-frustum (NCF) arrays. The NCF introduces the gradient RI in the visible wavelength longer than the lattice constant. The quarter-wavelength ( λ /4) singlelayer AR nanoisland coatings on top of the frustum arrays are designed to suppress the reflectance at a specific NUV wavelength shorter than the lattice constant. Finally, the antireflection property of our design is significantly enhanced compared to that of sharp-tipped nanocone structures, so that the average reflectance in the NUV spectral range ( nm) decreased from 9.2% to 3.8%. To our best knowledge, this is the first method to extend antireflection spectrum including the NUV region without any extra process to fabricate feature size below 300 nm. Because our structure does not rigorously require either smaller lattice constant than the NUV wavelength or sharp-tipped nanocone structure, it provides extra flexibility and tolerance to the fabrication of GRIN nanostructure to achieve broadband antireflection. It is also desirable that our design results in less increase of surface area of nanostructure on silicon substrate, since the higher surface recombination caused by the larger surface area typically degrades the performance of photovoltaic cell due to the reductions of both open circuit voltage and fill factor. [ 2, 13 ] In addition, the residual polystyrene (PS) layer with intermediate 5796

2 Figure 1. Schematic diagram shows the detailed fabrication process of AR nanoislands in silicon NCF arrays: (a) deposition of nanosphere crystal on silicon substrate, (b) simultaneous removal of Si substrate and nanosphere mask during the RIE process, and (c) fi nally fabricated AR nanoislands on NCF. (d) Experimental reflection data from the interface between planar c-si and air, and the real ( n ) and imaginary ( k ) parts of c-si. [ 16 ] (e) FDTD simulation results with various thicknesses of AR nanoislands on top of a NCF. The inset represents the geometry of the structure (d top = 300 nm, d base = 500 nm, h = 500 nm). RI value between the RI s of air and silicon [ 16 ] may behave as a buffer layer which relieves the large difference of RI s for impedance matching effect in overall range of wavelength. Colloidal nanosphere lithography is a simple, low-cost, and time-efficient bottom-up subwavelength scale nanofabrication method. [ 1, 3, 17, 18 ] Figures 1 (a) to 1 (c) illustrate the detailed process of our fabricating AR NCF arrays integrated with AR nanoislands using colloidal lithography. Firstly, hexagonal close-packed monolayer nanosphere crystals are prepared on substrate as etching mask via self-assembly from floating on air-water interface. (see Figure 1 (a)) Then, the substrates are etched by reactive ion etching (RIE) using self-assembled mono layer nanospheres crystals as etching mask for patterning of periodic nanostructure. (see Figure 1 (b)) Owing to the physicochemical etching nature of the RIE with directional bombardment of accelerated ion and isotropic chemical etch of highly reactive radicals from inert plasma gases, the sphere masks are also etched away during RIE process and the traverse diameter of sphere decreases gradually. [ 19 ] As the traverse diameter of sphere decreases, the etched area of underlying substrate increases accordingly. Since the top diameter of the obtained structure is nearly the same as the traverse diameter of sphere mask, the shape of produced GRIN structure will be transformed from cylinder to conical frustum. [ 8 ] In our experiment, by controlling the mixture plasma gases (C 4 F 8, SF 6, O2 ) and processing time appropriately, we were able to fabricate the wanted thickness of nearly planar PS bead mask on top of NCF before the sphere is totally etched away. These plane-like residuals of PS bead mask are designed to behave as AR nanoislands coating to enhance the antireflection at specific NUV wavelength. The addition of O 2 gas in the mixture of C 4 F 8, SF 6 gases in a single-step deep RIE process [ 14 ] allowed us to regulate the thickness of nanoislands AR coating and to manipulate the sidewall profile of finally fabricated GRIN nanostructure from conical frustum to cone at the same time. [ 17 ] After the sphere mask is totally etched, the textured profile starts to change from conical frustum to cone. [ 8 ] The antireflection performances of GRIN structure surfaces in the UV wavelength are limited by the feasibility of the fabrication of lattice constant sufficiently less than the incident light s short wavelength. The performance of GRIN ARS on silicon substrate may also be degenerated in the NUV spectral range by the increased values of both real (n) and imaginary (k) parts of RI, according to the Fresnel reflection at the interface of air (n air ) and opaque dielectric media (n media + i k media ), R = {(n media n air ) 2 + k media 2 }/{(n media + n air ) 2 + k media 2 } [ 20 ] For example, as you can see in Figure 1 (d), for planar bulk crystalline silicon (c-si) substrate, two peaks of real and imaginary parts of RI around 280 nm and 370 nm bring about two peaks of reflectance. To investigate the effect of AR nanoislands coating, using FDTD simulation, we numerically calculated the reflectance of AR-coated silicon NCF when the thickness of each planar nanoisland is given as 0 nm, 50 nm, and 100 nm. The simulation results of reflectance are shown in Figure 1 (e) when the top diameter and base diameter are fixed as 300 nm and 500 nm with a height of 500 nm, respectively. Consistently with a conventional λ /4 AR coating, the suppression peak of reflectance spectra shifts to blue side if the thickness of nanoisland AR coating decreases. Finally, broadband antireflection including the NUV spectral range is achieved when the thickness is 50 nm. These data confirm that the reflectance at specific wavelength is able to be efficiently suppressed with a single layer nanoisland AR coating on conical frustum. Besides, the thicker buffer layer of PS bead with the intermediate RI reduces reflection in visible wavelength region ( > 400 nm) because of the smooth RI profile for the impedance matching effect. The AR nanoislands in silicon NCF arrays are fabricated in three major steps: nanospheres self-assembly on water-air interface, deposition of monolayer on a silicon substrate, and singlestep deep reactive ion etching (SDRIE) with SF 6, C 4 F 8, and O 2 gases to tailor the shape of NCF structure. We purchased PS spheres solutions of 10 wt.% (5050A, diameter = 500 nm and 5036A, diameter = 360 nm) from Thermo-scientific Inc. After careful application of PS sphere mixture solution with ethanol onto de-ionized water (resistivity = 18.0 M Ω ) in a Petri dish, the formation of two dimensional (2D) colloidal nanosphere mask starts on air-water interface. The relatively low mass density of amphiphilic ethanol molecules allows PS beads to float and disperse over a large area (up to several cm 2 ) on the air-water interface. [ 19 ] The trapped 2D colloidal crystals can be used as a nanospheres mask for fabrication of GRIN nanostructure. The proper ethanol content in the sphere mixture solution grants an 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 5797

3 Figure 2. (a) An optical photograph and (b) an SEM image (normal view) of 2D PS nanospheres monolayer mask pattern transferred on Si substrate (20 20 mm 2 ). (c) A SEM image of PS nanoislands on top of Si NCF arrays while NCF arrays are fabricated via ICP etching. (d) Captured side wall profi le image with a technique for creating cross-sections of the milled structures using a dual-beam FIB system. (e) A photograph of bare silicon and surface textured silicon. optimal balance between attractive and repulsive forces between spheres such as van der Waals, capillary, and so on. [ 21 ] After successful formation of monolayer colloidal crystal on air-water interface, pre-cleaned c-si substrate (20 20 mm 2 ) with RCA solution (NH 4 OH (25%): H 2 O 2 (30%): H 2 O = 1: 1: 5, kept for 15 min at 80 C) is introduced in the Petri dish. Then, the water is carefully drained from the dish and nanospheres monolayer mask is deposited onto the Si substrate. [ 21 ] Figures 2 (a) and 2 (b) show optical photograph and scanning electron microscope (SEM) image of 2D PS beads crystals on Si substrate, respectively. When PS nanospheres successfully formed a hexagonal close-packed 2D lattice, beautiful iridescent color diffracted from the crystal is visible to the naked eye. Using PS nanospheres crystals as etching mask, the AR nanoislands-coated NCF arrays were fabricated by applying a SDRIE process [ 14 ] in an inductive coupled plasma (ICP) etcher (Oxford Instrument, ICP380). Deep reactive ion etching (DRIE or Bosch process ) is an attractive fabrication method because of high silicon etching rate (several μ m per min) and high aspect ratio ( > 10) of subwavelength nanostructure. [ 18 ] However, during the cyclic etching/deposition step of DRIE, naturally generated undesirable periodic ripples on the sidewall of nanostructure make it unsuitable for fabricating AR tapered nanostructure. [ 22 ] Recently, a SDRIE technique is introduced to fabricate tapered silicon photonic crystals without ripples on sidewall. [ 14 ] Besides conventional SF 6 and C 4 F 8 plasma gases, we added O 2 reactive plasma gas content and this enabled us to regulate the smooth sidewall profile of NCF array and the thickness of residual PS AR nanoislands simultaneously. As shown in Figure 2 (c), the residual PS nanoislands on top of NCF arrays are manipulated by controlling the O 2 gas content and etching time in SDRIE process. The side wall profiles of our nanostructures are investigated by a technique for creating cross-sections of the milled structures using a dual-beam Focused Ion Beam (FIB, FEI Nova 200 Nanolab) system, which is a combination of a gallium ion (Ga + ) milling FIB and a high resolution SEM system, as shown in Figure 2 (d). [ 23 ] Figure 2 (e) is a photograph of bare silicon and surface textured silicon samples. In order to characterize the reflectance of our samples, we performed absolute hemispherical measurement using UV-VIS-NIR spectrophotometer (UV3600, Shimadzu Scientific Instruments) with a 60 mm-diameter integrating sphere (MPC-3100) by scanning a monochromator coupled to a halogen lamp. The reflected beam including specular and diffuse reflections from the sample are scattered and collected in an integrating sphere and measured by a photomultiplier tube detector. [ 17 ] To experimentally demonstrate the effect of AR nanoisland, we compared the reflectance of NCF arrays of 500 nm lattice constant with and without AR nanoislands on 250 μm thick Si substrates. We used the PS bead residuals as AR coatings and controlled the thickness of PS residual nanoislands on top of NCF by manipulating O 2 content during the SDRIE process. Figures 3 (a) and 3 (b) are the bird view (52 ) SEM images of samples (a) and (b), which have elliptic AR nanoislands with center thickness of 90 nm and 110 nm on NCF arrays, respectively. Each NCF array of samples (a) and (b) has the top diameter (d top ) as d top = 250 nm and d top = 270 nm with similar height (h) of h = 450 nm. Samples (a ) and (b ) are NCF arrays samples which we got after we rinsed the AR nano islands from samples (a) and (b), respectively. Figure 3. Top view SEM images (a to c) of Si GRIN nanostructures with the same lattice constant of 500 nm. Two ARS s integrating PS beads AR nanoislands with Si NCF arrays with similar heights of 450 nm and each top diameter (d top ) as (a) d top = 250 nm and (b) d top = 270 nm. (a ) and (b ) are two NCF structures obtained after rinsing samples (a) and (b). (c) A nanocone structure with height (h) and base diameter (d base ) as h = 500 nm, d base = 420 nm. (d) and (d ) are the cross-sectional profi les of samples (a) and (a ). (e) Overall reflectance spectra including both specular and diffuse reflections of samples from (a) to (c). 5798

4 To eliminate the complicated effects of morphological difference between the fabricated NCF samples, we measured the reflectance of NCF arrays with AR nanoislands, and then, after removing only the AR nanoislands, observed the reflectance again for the same sample. These procedures enable us to compare reflectance of samples with and without AR nanoislands along with exactly the same morphologies of NCF geometry. From the reflectance spectra (see Figure 3 (e)), in the range of 300 nm to 400 nm, average reflectance of the samples with AR nanoislands are significantly suppressed from 20.4% (19.4%) to 9.3% (14.2%), when the center thickness of elliptic AR layer is 90 nm (110 nm). The lattice constant (500 nm) is larger than the wavelength of this NUV region, hence, these antireflection enhancements are originated from the destructive interferences using λ /4-thick AR layer. The antireflection of sample (a) at 370 nm region is better than those of sample (b) and a typical nanocone structure (see Figure 3 (c)) because the effective optical thickness of elliptic AR nanoislands is closer to the optimal λ /4 AR thickness. Figure 3 (d) is the cross-sectional profile of sample (a) and the center thickness and diameter of AR nanoislands are 90 nm and 250 nm, respectively. Because the AR nanoislands have elliptically curved structures, the center optical path length of AR layer is observed to be larger than λ /4. Figure 3 (d ) is the cross-sectional profile of sample (a ), which we got after rinsing the AR nanoislands from sample (a). In order to enhance antireflection in NUV region, we fabricated nanocones or NCF arrays with smaller lattice constant of 360 nm on 250 μ m thick Si substrates. Figures 4 (a) to 4 (d) and Figures 4 (a ) to 4 (d ) are the bird view and the top view SEM images of four samples, respectively. After a proper SDRIE process with PS crystals as etching mask, we fabricated the tips of samples (a) and (b) as sharp as possible because the smooth RI profile results in improvement of antireflection due to the impedance matching between air and silicon. For samples (a) and (b), the heights (h) and base diameters (d base ) are h = 300 nm, d base = 230 nm and h = 370 nm, d base = 200 nm, respectively. It is reported that the influence of the distance between arrays, i.e. packing density, is small compared to those of height and lattice constant. [ 8, 24 ] Figure 4 (e) is the reflectance spectra of four samples. Although we can enhance antireflection using a sharp-tipped GRIN nanostructure in the range of 450 nm to 900 nm, the average reflectance in the NUV spectral range ( nm) is 9.2% even with wellmade nanocone structures because of following reasons: The antireflection enhancement appears mainly in the wavelength region longer than the lattice constant [ 9 ] and the increased RI of silicon in Figure 1 (d) induces significant Fresnel reflection in the NUV region. Hence, in spite of well achieved sharpness of our silicon nanocones, the sample (a) and (b) still show the reflectance peak at NUV range especially at 370 nm, in which the usage is important as energy sources in photovoltaic application. In the other two samples of Figures 4 (c) and 4 (d), we controlled the thickness of PS residual nanoislands on top of NCF of different taper angles by manipulating O 2 content during the SDRIE process. An additional 10 sccm of O 2 content made steeper taper angles of sharp-tipped nanocone (sample (b)) and larger top diameter of NCF (sample (d)) compared to the cases of nanocone (sample (a)) and NCF (sample (c)) without O 2 content. In the case of AR nanoislands coated Si NCF arrays, sample (c) and (d) have top diameters of 90 nm and 190 nm with similar height of 320 nm, respectively. In comparison with the average reflectance ( 9.2%) in the range of 300 nm to 400 nm of sharp-tipped nanocone structures of (a) and (b), the average reflectance of samples (c) and (d) are significantly suppressed to 4.8% and 3.8%, respectively. The antireflections of samples (c) and (d) in longer wavelength than 450 nm are similarly efficient as well. There are two major factors for these enhancements of broadband antireflection via nanoislands structure. Firstly, λ /4-wavelength AR coatings of nanoislands at specific NUV spectral range significantly suppress the reflection in the range from 300 to 400 nm. Secondly, because the RI (n PS = 1.6 at 500 nm, for example) of PS nanoislands is intermediate value Figure 4. The bird view (a to d) and the top view (a to d ) SEM images of four Si GRIN nanostructures with the same lattice constant of 360 nm. Two sharp-tipped nanocone structures with height (h) and base diameter (d base ) as (a) h = 300 nm, d base = 230 nm and (b) h = 370 nm, d base = 200 nm, respectively. Two ARS s integrating PS beads AR nanoislands with Si NCF arrays with similar height of 320 nm and each top diameter (d top ) as (c) d top = 90 nm and (d) d top = 190 nm. (d ) The cross-sectional profi le of sample (d). (e) Overall reflectance spectra including both specular and diffuse reflections of four samples from (a) to (d) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 5799

5 between those of air ( n air = 1) and silicon (n Si = 4.29 at 500 nm) in the overall UV, visible, and near-infrared (NIR) regions, the effective RI profile of GRIN structure may be better impedancematched. In the overall spectrum from the NUV to NIR, the antireflection of the larger area AR nanoislands in NCF with steeper frustum of sample (d) is better than the smaller area AR nanoislands in NCF close to an ideal nanocone of sample (c). This signifies that AR coating effect plays a dominant role in the antireflection enhancement of the NUV region and the smooth impedance matching effect of PS nanoislands makes the antireflection in the visible and NIR region as good as sharp-tipped nanocone structures. Figure 4 (d ) is the crosssectional profile of sample (d) and the thickness and dia meter of AR nanoislands are observed as 60 nm and 190 nm, respectively. As the RI of PS bead is 1.7 around 370 nm, the effective optical path length of AR nanoislands coating consistently corresponds to λ /4. In conclusion, we demonstrated novel GRIN nanostructures for improved broadband optical antireflection by integrating AR nanoisland coating arrays with silicon NCF arrays fabricated by SDRIE with a close-packed PS nanosphere mask. The average reflectance of our structure in the NUV spectral range ( nm) is significantly reduced to 3.8%, while that of sharp-tipped nanocone structures is as much as 9.2%. This is a feasible optimized integration method of two major approaches for ARS: AR coating and biomimetic moth s eye structure, which does not need any complicated process to fabricate feature sizes below 300 nm to achieve high energy harvesting in photovoltaic cells. To apply this novel method in photovoltaic cells, further research is required to obtain more suitable dielectric materials for p-n junction forming procedures. [ 25, 26 ] Experimental Section Fabrication of Nano-Conical-Frustum Arrays via SDRIE : The SDRIE process can be simply understood as the DRIE process without timemultiplexing, in which we simultaneously carry out silicon etching and side-wall passivation in a flow of SF 6 and C 4 F 8. In our experiments an inductive coupled plasma (ICP) etcher (Oxford Instrument, ICP, 380) was used. The samples were fabricated by fi xed mixing of SF 6, C 4 F 8 gases(45 and 35 sccm), and variable O 2 gas content(0, 5,10 sccm) at a r.f. power of 150 W and process pressure of 50 mtorr. Details of FIB Cross-sectioning Method : After site-specifi c FIB-induced deposition of a platinum (Pt) layer over the region of interest to prevent deformation while milling the cross-section, a large rectangular hole with a Ga + ion was milled. Then, precise line-by-line scan milling is applied to fi nd the cross-section through the center of the NCF arrays. Finally, the side wall profi les of the nanostructure are captured by SEM at a fi xed angle of 52. Absolute Hemispherical Measurements : Wavelength-dependent measurements from 250 nm to 900 nm were carried out by scanning a monochromator coupled to a halogen lamp. The reflected and transmitted light from the sample are scattered and collected in an integrating sphere and measured by a photomultiplier tube detector. The use of an integrating sphere enables us to thoroughly measure overall reflectance spectra R( λ ) of our samples with textured surfaces including both specular and diffuse reflections. [ 17 ] For the measurement of the reflection spectrum, the fabricated samples were mounted at the backside of the integrating sphere with an oblique incidence angle (8 ) with respect to the normal incident light beam. This way the collection and detection of the reflected light from the sample was achieved. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work has been supported by the Low Observable Technology Research Center program of the Defense Acquisition Program Administration and Agency for Defense Development, basic research program of Agency for Defense Development (ADD ), and the National Research Foundation of Korea grants funded by the Ministry of Education, Science and Technology (NRF , ). Received: September 3, 2011 Published online: November 24, 2011 [1 ] 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 Lett. 2009, 9, 279. [2 ] E. Garnett, P. D. Yang, Nano Lett. 2010, 10, [3 ] C. B. Soh, B. Wang, S. J. Chua, V. K. X. Lin, R. J. N. Tan, S. Tripathy, Nanotechnology 2008, 19, [4 ] J. K. Kim, S. Chhajed, M. F. Schubert, E. F. Schubert, A. J. Fischer, M. H. Crawford, J. Cho, H. Kim, C. Sone, Adv. Mater. 2008, 20, 801. [5 ] W. 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