We study theoretically and experimentally the hemi-urchin shaped zinc oxide (ZnO) nanostructures for broadband and wide-angle antireflection coatings. The antireflective characteristics of hemi-urchin shaped ZnO nanostructures, which can be formed by integrating one-dimensional (1D) nanostructures (i.e., nanorods) on the periodic 2D structural architecture, are investigated. The optimization process is performed using a rigorous coupled-wave analysis method in terms of the order of taper of Si subwavelength gratings (SWGs) as a 2D structural architecture, the geometry of Si SWGs, and the height/size of ZnO nanorods. To simply test an experimental feasibility, a hemi-urchin shaped ZnO nanostructure is fabricated by hydrothermally growing ZnO nanorods on the periodic Si SWG structure. The angle-dependent reflectance of the hemi-urchin shaped ZnO nanostructures on the Si SWG is compared with that of the vertically aligned ZnO nanorod arrays on the Si substrate. The optimized hemi-urchin shaped ZnO nanostructure can significantly improve the antireflective property by suppressing the surface reflection over a broad spectrum and a wide range of angles of light incidence, indicating a reasonable agreement with the experimental results.
©2010 Optical Society of America
CorrectionsYeong Hwan Ko and Jae Su Yu, "Design of hemi-urchin shaped ZnO nanostructures for broadband and wide-angle antireflection coatings: erratum," Opt. Express 22, 25193-25193 (2014)
The broadband and omni-directional antireflection coatings have attracted much attention because of their promising potentials in various fields such as optical, optoelectronic, and photovoltaic applications [1–4]. Although multilayer stacks of films were traditionally used for antireflection coatings in a specific wavelength range, they have some technical problems such as material selection, thermal mismatch, and accurate thickness control. To achieve efficient antireflective property, many different types of structures including vertically aligned one-dimensional (1D) nanostructures [5–8], gratings [9,10], photonic crystals [11–13], and porous structures  have been reported over the several past years. As a new approach in advanced nanotechnology, many efforts are currently focused on subwavelength grating (SWG) structures, which have a period smaller than the wavelength of light, bioinspired by the moth eye structure [15–19]. The biomimetic moth eye structure is superior to other structures for suppressing Fresnel reflection due to the gradient refractive index profile. Unfortunately, this structure also suffers from the limited spectral range and the restricted angular range of light incidence. To find the optimal 3D geometric profile of SWG structures, the antireflective properties of conical, hemispherical, parabolic, and corrugated shapes have been demonstrated for improving the antireflection performance [20–23].
On the other hand, the fabrication of urchin shaped nanostructures by integrating closely 1D nanostructures on a 2D or 3D architecture has recently attracted intensive interest due to their fascinating properties for various applications including dye sensitized solar cells (DSSCs), collective sensors, and nanoscale electronic devices [24–27]. However, most fabricated urchin shaped nanostructures are often arbitrarily distributed with non-uniform morphology, which is not suitable for practical applications. Furthermore, the theoretical analysis on the optical properties of urchin shaped structures has been still not studied. To obtain the desirable urchin shaped nanostructures for optoelectronic and photovoltaic applications, the size and height of nanostructures should be controlled on periodic or highly ordered structural architectures through theoretical analysis on antireflective characteristics. The investigation of ZnO-based urchin shaped nanostructures would be very interesting because ZnO is a promising wide bandgap semiconductor for a wide range of device applications.
In this work, we investigated the antireflective properties of the hemi-urchin shaped ZnO nanostructures formed using the Si SWG structures as the periodic 2D template architecture over the broadband and wide-angle ranges for optoelectronic devices such as Si-based solar cells. Theoretical analysis and optimization of the nanostructures were performed using the rigorous coupled-wave analysis (RCWA) method. The experimental results were also presented for a simple structure fabricated by the hydrothermal growth.
2. Modeling and simulation results
2.1. Theoretical modeling and optimization of Si SWGs
For hemi-urchin shaped ZnO nanostructures on Si SWGs, the optimization was progressed by a sequential design of (i) the order of taper (OT) of Si SWGs, (ii) the geometry of Si SWGs, and (iii) the height and size of 1D ZnO nanostructures (i.e., nanorods). First, the reflection characteristics of various shaped Si SWG structures were calculated in the wide wavelength region of 300-3000 nm and then an optimized shape of Si SWG structure was determined. For 3D numerical modeling of SWG structures, the objective geometry was represented in the Cartesian coordinate system by the scalar-valued function of three variables, f(x, y, z). Under an assumption of the rotational symmetry about the z axis of the SWG structure, the various shaped geometries are expressed by definition of the OT in the equation of the tapered cone followed as:Figure 1(a) shows the general geometry of Si SWGs expressed by the equation of the tapered cone. The 3D geometry was built up by the determination of r with varying the z value from 0 to HSWG. Figure 1(b) shows the geometries of Si SWGs at OT = 0.4, 1, 1.4, 2, and 3 in hexagonally close-packed arrays. For OT < 1, the equation of the tapered cone provides needle-shaped geometries. In the case of OT = 1, the geometry is a conical shape, and it turns to the parabolic shapes at OT ≥ 2. The effective refractive index gradient of Si SWG structures was calculated from the volume fraction of Si using the effective medium theory .
The calculated effective refractive index profiles of the Si SWGs with RSWG = 150 nm and HSWG = 600 nm at OT = 0.4, 1, 1.4, 2, and 3 are shown in Fig. 1(c). For OT = 0.4 and 3, the effective refractive index of the SWGs was exponentially and logarithmically decreased from the Si substrate to air, respectively. For OT = 1.4 and 2, however, the Si SWGs exhibited nearly linear gradient profiles of effective refractive index, which could efficiently reduce the Fresnel reflection .
Figure 2(a) shows the contour plot of the calculated reflectance as a function of the OT for the Si SWG with RSWG = 150 nm and HSWG = 600 nm. To investigate the reflection property over a wide range of wavelengths, the theoretical calculations were carried out by using RCWA method at wavelengths of 300-3000 nm. For the Si SWG structure at OT < 1, the reflectance region with values of > 10% extended towards the shorter wavelength with the decrease of QT. In fact, the highly tapered needle-shaped Si SWG structure cannot act as an efficient broadband antireflection coating layer because of a sharp change in the effective refractive index between the Si substrate and air as shown in Fig. 1(c). When the OT is between 1.35 and 2.2, the low reflectance values of < 3% were achieved in the wide wavelength range of 300-2500 nm. For the Si SWG at OT > 3, the reflectance was increased at wavelength regions of 800-1100 nm and 1200-1400 nm, especially, rapidly increased for wavelengths above 2000 nm. Figure 2(b) shows the calculated reflectance spectra of the Si SWGs at OT = 0.4, 1, 1.4, 2, and 3. For the Si SWG structure at OT = 0.4, the low reflectance values of < 5% were obtained in the wavelength range of 300-1100 nm, but it was dramatically increased at wavelengths above 1100 nm. For OT = 3, the Si SWG could produce very low reflectance values of < 1% in the short wavelength range of 300-620 nm, while the reflectance was fluctuated with relatively high values in the long wavelength of 750-2000 nm. Particularly, for OT = 1.4, the Si SWG structure exhibited a very low average reflectance value of ~1.31% in the wavelength range of 300-3000 nm. It is clear that the geometric shape of the Si SWG which provides a nearly linear profile of effective refractive index enhances the broadband antireflection property. For Si SWG structures, therefore, an OT value was chosen as 1.4 to achieve a relatively low surface reflection over a wide wavelength range. After determining the optimal SWG shape, the reflectance characteristics as a function of the height of Si SWG structure were explored.
Figure 3(a) shows the contour plot of the calculated reflectance as a function of the height of the Si SWG structure with RSWG = 150 nm for OT = 1.4. As the height was increased, the reflectance was efficiently reduced over a wider range of wavelengths. For heights above 600 nm, the reflectance values of < 2.5% at wavelengths of 300-2500 nm were obtained. Evidently, the higher height allows an improved effective refractive index gradient for suppressing the surface reflection because it can relax the change of refractive index between the Si substrate and air. However, the lower height is desirable for actual device applications. Thus, we determined the optimal Si SWG structure with RSWG = 150 nm and HSWG = 600 nm i.e., aspect ratio of 2. Figure 3(b) shows the contour plot of the calculated reflectance as a function of the angle of light incidence for the corresponding Si SWG structure. For incident angles above 70 degree, the reflectance was increased to values of > 10% in the whole wavelength region. The optimized Si SWG structure exhibited the low average reflectance values of < ~3% in the wide wavelength range of 300-1750 nm and angular range of 0-60 degree.
2.2. Design optimization of hemi-urchin shaped ZnO nanostructures
In order to design the hemi-urchin shaped ZnO nanostructures for broadband and omni-directional antireflection coatings, the effect of the morphology of ZnO nanorods integrated on the optimized Si SWG structure on their reflection property was studied. It is noted that the size of the ZnO nanorods fabricated experimentally by the hydrothermal method can be generally controlled as ~30-100 nm depending on its growth conditions. Also, the sputtered AZO films might be suitable as a seed layer on the patterned substrate during the hydrothermal growth because the sputtering deposition technique can provide very high step coverage. In the theoretical modeling of hemi-urchin shaped ZnO nanostructures, we assume that the ZnO nanorods are grown uniformly along the c-axis orientation of wurzite crystal ZnO films of 100 nm on the optimized Si SWG structure. Moreover, the ZnO seed layer is expected to enhance the antireflective property because it leads to the more graded change of the effective refractive index between the Si SWG structure and air. The objective geometry of the hemi-urchin shaped ZnO nanostructure on the ZnO seed layer/optimized Si SWG in the hexagonal periodic arrangement is shown in Fig. 4(a) . The radius and height of Si SWG structure are 150 nm and 600 nm, respectively, for OT = 1.4.
By changing the height and size of the ZnO nanosrods (HZnO and DZnO), the reflectance characteristics of hemi-urchin shaped ZnO nanostructures were investigated. Figure 4(b) shows the contour plot of the calculated reflectance of hemi-urchin shaped ZnO nanostructures as a function of the height of ZnO nanorods with DZnO = 50 nm. Below 150 nm, the reflectance was not much affected by the HZnO though it was slightly improved in the long wavelength region of > 2500 nm. As the HZnO was increased, for HZnO > 150 nm, the overall reflectance was increased with increased oscillations in the wavelength range of 300-1500 nm. This reason may be that the antireflective property would be degraded by the rapid change of the effective refractive index due to the overlap between the taller ZnO nanorods. Figure 4(c) shows the average reflectance of hemi-urchin shaped ZnO nanostructures at wavelengths of 300-3000 nm as a function of the size of ZnO nanorods with HZnO = 140 nm. The contour plot of the calculated reflectance of the corresponding hemi-urchin shaped ZnO nanostructures as a function of the DZnO is shown in the inset of Fig. 4(c). For DZnO < 60 nm, the very low reflectance was kept at wavelengths below 1500 nm, but it began to slightly increase with oscillations for larger size. Consequently, the average reflectance value at wavelengths of 300-3000 nm was slowly decreased as the DZnO was increased, but it was rapidly increased at DZnO > 70 nm. For the hemi-urchin shaped ZnO nanostructure with DZnO = 50 nm, a very low average reflectance value of ~0.54% was obtained in the wavelength range of 300-3000 nm.
For comparison, the angle-dependent reflectance spectra of the closely packed ZnO nanorods with DZnO = 50 nm and HZnO = 600 nm on the ZnO seed layer/Si substrate were calculated. Simply, the nanorods were assumed to be periodic. Figure 5 shows the contour plot of the calculated reflectance of (a) the optimized hemi-urchin shaped ZnO nanostructure on the ZnO seed layer/Si SWG structure and (b) the hexagonally aligned ZnO nanorods with a period of 60 nm on the ZnO seed layer/Si substrate as a function of the angle of light incidence. The period of ZnO nanorods was determined to achieve a relatively low reflectance. For hemi-urchin shaped ZnO nanostructure, the reflectance was kept at values of < 3% at wavelengths of approximately 300-3000 nm and 300-2250 nm up to the incident angle of 40 degree and 60 degree, respectively. In the case of the ZnO nanorods, the reflectance was relatively high with large oscillations at normal incidence. At wavelengths above 2000 nm, high reflectance values of > 15% were observed up to the incident angle of 50 degree. The average reflectance value of the hemi-urchin shaped ZnO nanostructure was about 12.75% at wavelengths of 300-3000 nm over the whole angular range of 0-90 degree, whereas that of the ZnO nanorods was about 37.59%. Furthermore, the optimized hemi-urchin shaped ZnO nanostructure has lower average reflectance value than that of the optimized Si SWG structure (i.e., ~15.1%) and it exhibited less angle-dependent reflectance over a wide wavelength range as shown in Fig. 3(b). This confirms that the integrated ZnO nanorods as well as the sputtered ZnO seed layer improve the antireflective properties over a wide range of wavelengths and incident angles. It is clear that the 1D ZnO nanorods integrated on the Si SWG structure act as efficient broadband and omni-directional antireflection coatings.
3. Fabrication and characterization
To simply perform an experimental test, a hemi-urchin shaped ZnO nanostructure was fabricated though the formed ZnO nanorods show a somewhat disordered structure. The integration of hemi-urchin ZnO nanostructures on Si SWGs can be implemented by a simple hydrothermal method using the sputtered Al-doped ZnO (AZO) seed layer. Additionally, this AZO layer can serve as a transparent conducting oxide electrode for photovoltaic and optoelectronic devices. Figure 6(a) shows the schematic illustration and scanning electron micrograph (SEM) images for fabricating the hemi-urchin ZnO nanostructures on the AZO seed layer/Si SWGs. In order to obtain the periodic template of 2D architecture, the Si SWG structure was fabricated by the combination technology of the interference lithography of two laser beams using a 363.8 nm Ar ion laser and the inductively coupled plasma (ICP) etching. The etching was performed with 25 W RF power in 5 sccm SiCl4 plasma at 10 mTorr. The detailed fabrication procedure of Si SWGs was given in Ref. 17. In the hydrothermal growth of ZnO nanorods on Si SWGs, the 100 nm-thick AZO seed layer was deposited by using RF magnetron sputtering system at room temperature. The 5N purity AZO target containing 2 wt.% Al2O3 was used. Then, the AZO deposited Si SWG sample was dipped into an aqueous solution composed of zinc nitrate hydrate (Zn(NO3)26H2O) and hexamethylenetetramine (HMT, C6H12N4) at 80 °C. By adjusting the zinc nitrate concentration, growth temperature, and growth time, the height and size of ZnO nanorods can be controlled .
Figure 6(b) shows the measured specular reflectance spectra as a function of the angle of light incidence for (i) hemi-urchin shaped ZnO nanostructures on the AZO seed layer/Si SWG and (ii) ZnO nanorod arrays on the AZO seed layer/Si substrate. The cross-sectional SEM images of the corresponding structures are shown in the insets of Fig. 6(b). The average height and size of the integrated ZnO nanorods on the AZO seed layer (100 nm)/Si SWGs with the height of 300 nm and the period of 300 nm were 240 nm and 54 nm, respectively. The average height and size of the fabricated ZnO nanorods on the AZO seed layer/Si substrate were 550 nm and 44 nm, respectively. The angle-dependent reflectance was measured by using UV-VIS-IR spectrophotometer with a Cary variable angle specular reflectance accessory in the specular mode. In specular mode, the lowest angle close to normal incidence (i.e., near-normal incidence) is 8 degree so that the incident beam is not blocked by the detector. The reflectance of the hemi-urchin shaped ZnO nanostructure was very low at near-normal incidence, indicating an average value of 1.62% at wavelengths of 300-2000 nm. At incident angles of 8-70 degree, there was not much increase in the reflectance. For ZnO nanorod arrays, high reflectance values were observed with strong oscillations below the band edge at near-normal incidence, as shown in Fig. 6(b). This phenomenon is caused by the interference between the multiple internal reflections at the air/AZO and AZO/Si interfaces . As the incident angle was increased, the oscillations were kept up to 50 degree and then were slightly reduced at 60-70 degree. It is noticeable that the measured reflectance has a similar tendency to the calculated result. The experimental results indicate that the hemi-urchin shaped ZnO nanostructures show relatively low reflectance values over a wide range of wavelengths and angles of light incidence, which is a great potential to provide the improved broadband and omni-directional antireflective property.
The hemi-urchin shaped ZnO nanostructures on Si SWGs, which can be implemented by a simple hydrothermal growth using the sputtered ZnO seed layer, were designed and optimized for broadband and omni-directional antireflection. The Si SWG structure was optimized by varying its shape using the RCWA simulation. The optimal geometry of hemi-urchin shaped ZnO nanostructures was obtained by minimizing the reflectance at 300-3000 nm over a wide range of incident angles of 0-90 degree. The average reflectance value of the hemi-urchin shaped ZnO nanostructure was lower than that of the optimized Si SWG structure. It is found that the 1D ZnO nanorods as well as the ZnO seed layer help to suppress the surface reflection over a wide range of wavelengths and angles of light incidence. The simple fabrication of the hemi-urchin shaped ZnO nanostructure on the AZO seed layer/Si SWGs exhibited a significant improvement in the broadband and omni-directional antireflective property compared to the ZnO nanorod arrays, indicating a similar tendency to the simulated result. These results suggest that the optimized hemi-urchin shaped ZnO nanostructure through further experimental optimization improve the performance in photovoltaic and optoelectronic device applications.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0025071).
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