Controllable photonic mirror fabricated by the atomic layer deposition on the nanosphere template Shih-Hao Chan,1 Wei-Ting Lin,1 Wen-Hao Cho,1 Chien-Cheng Kuo,1,2 Cheng-Chung Lee,1 and Sheng-Hui Chen1,* 1
Department of Optics and Photonics, National Central University, 300 Chung-Da Rd., Chung-Li 32001, Taiwan
2
Graduate Institute of Energy Engineering, National Central University, 300 Chung-Da Rd., Chung-Li 32001, Taiwan *Corresponding author: [email protected] Received 4 September 2013; revised 15 November 2013; accepted 25 November 2013; posted 27 November 2013 (Doc. ID 196933); published 9 January 2014
In this study, a controllable photonic mirror was fabricated using the atomic layer deposition (ALD) coating technique on a polystyrene (PS) nanosphere template. PS nanospheres were self-assembled on an Al/ glass substrate to form the bottom electrode. A 20 nm ALD Al2 O3 film was then coated onto the surface of the reduced PS nanosphere structure. The PS nanospheres were removed in air at 350°C to form hollow Al2 O3 nanospheres. Then a 30 nm indium tin oxide film was sputtered on the hollow nanosphere structure to form the top electrode. The results show that the incorporation of the photonic mirror could control the reflectance to a value of 0.3% per 0.1 V of bias voltage. © 2014 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.5298) Photonic crystals; (170.0110) Imaging systems. http://dx.doi.org/10.1364/AO.53.00A237
1. Introduction
In recent years, nanostructured metallic, dielectric, or hybrid materials have attracted a lot of attention because of their applications in nanophotonics for the routing and manipulation of light. For example, photonic crystals are artificial dielectric nanostructures with periodic permittivity that affect the propagation of electromagnetic waves [1]. Surface plasmon-based nanophotonics merge electronics and photonics at the nanoscale [2]. Light can be localized and closed to metallic nanoparticles due to the oscillation of electrons. In the past few years, self-assembled polystyrene (PS) and silica sphere templates have been fabricated on different substrates, such as glass, sapphire, and silicon and used for many applications [3–5]. Sun et al. reported on the development of an antireflection structure for broadband wavelengths from 350 to 850 nm fabricated using nanosphere lithography on a silicon substrate [6]. In addition,
1559-128X/14/04A237-05$15.00/0 © 2014 Optical Society of America
Cho et al. utilized the oblique deposition technique and a bottom-up approach to fabricate material with columnar structures on a spherical template for a birefringence photonic crystal [7]. However, it has been found that the high temperature of the electron evaporation process will melt the PS spheres. To solve this problem, an Al2 O3 thin film can be coated onto the surface of the spheres by the atomic layer deposition (ALD) [8] in room temperature. ALD is a thin-film deposition technology that can achieve a thin film with uniform thickness on a high aspect ratio structure. It is a unique process that produces highly conformal films and allows atomic level thickness control. Wang et al. applied the ALD technique for the fabrication of alumina nanotubes using GaQ3 as the template [8]. In this study, we describe the fabrication of a controllable photonic mirror by the ALD technique using PS nanospheres as the template. 2. Experimental Procedure
In this work, the finite-difference time-domain (FDTD) calculation is employed to clarify the relationship between the morphology of the nanospheres 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A237
Fig. 1. (a) FDTD simulation model of hollow nanospheres packed into a honeycomb lattice in the x–y plane. (b) Incident light and simulation detector along the z-direction.
and the electric field as well as the reflectance spectrum of visible light. The photonic mirror is fabricated with a nanosphere template using the ALD technique. Moreover, with the proposed structure, the morphology of the nanospheres can be controlled by the electric field for the reflectance variation. A.
FDTD Simulation
The electric field of the photonic mirror was simulated using the FDTD method in order to design an optimal structure for the periodic nanosphere array. Figure 1(a) illustrates the FDTD model for this system. The hollow nanospheres with 20 nm thick Al2 O3 insulated shells were organized in a closely packed hexagonal monolayer structure on a 50 nm thick Al thin film. Outside the insulated shells, a
30 nm thick ITO film covered the grooves of the hollow nanospheres. The hollow nanospheres had the inside diameter and the period of 400 and 540 nm, respectively. The simulated wavelength range was in the visible light region (400–800 nm). The incident light and simulation detector were oriented along the z-direction as shown in Fig. 1(b). Figures 2(a)–2(c) show the hollow nanospheres with diameters of 400, 395, and 390 nm, respectively. A normal incident plane wave was propagated in the visible wavelength region. To calculate the electric field distributions of the hollow nanospheres in the near-field, the FDTD method was applied using the simulation model in Fig. 1. The electric field distributions of x–y plane for Figs. 2(a)–2(c) are shown in Figs. 2(d)–2(f), respectively. The electric fields show a tendency to be distracted with decreasing diameter. We also found that the photonic structure can control the reflectance through the various diameters of the hollow nanospheres. Similar to Fig. 2, Fig. 3 shows the simulation results for the electric-field distributions in the x–z plane for the various photonic structures. The electric-field intensity distributions in Figs. 3(d)–3(f) show that the electric fields were focused on the top of the nanospheres. Calculation of the integral electric field over the photonic structure shows the electric field is
Fig. 2. Calculation results of the nanospheres (x–y plane) with various sizes: (a) 400 nm, (b) 395 nm, and (c) 390 nm, and their electric field distributions: (d), (e), and (f). A238
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Fig. 3. Calculation results for the nanospheres (x–z plane) with various sizes: (a) 400 nm, (b) 395 nm, and (c) 390 nm and the electric field distributions for (d), (e), and (f) corresponding to (a), (b), and (c).
strongly concentrated in the Al2 O3 ∕ITO layer in this structure, which will result in a decrease in the reflectance value with a decrease in the size of the nanospheres, as shown in Fig. 4. Furthermore, the electric field coupling varied with each wavelength depending on the photonic structure. Moreover, this makes the effective-index alter and shift.
Fig. 4. Simulated reflectance of the nanospheres.
B. Photonic Mirror Procedure
Figure 5 shows the schematic illustration of the photonic mirror fabrication process. A 50 nm aluminum film was coated utilizing a sputtering method onto a B270 glass substrate (5 cm2 ) as a back electrode. A monolayer of 540 nm diameter PS nanospheres was spun onto the substrate. Then, the diameter of the nanospheres was reduced from 540 to 400 nm in an oxygen plasma chamber for a reaction time of 180 s. The reduced nanosphere array was introduced into the ALD reaction chamber for the Al2 O3 film growth, following the ALD process detailed in [8]. Al2 O3 is an extremely good material for high-K applications such as insulators. Besides, its fabrication procedure is a mature technique in an ALD system where a trimethylaluminum (TMA) precursor can be reacted with H2 O to form an Al2 O3 film with excellent uniformity. The PS nanosphere structure was removed in an atmosphere at 350°C to form the Al2 O3 hollow nanosphere template. A 30 nm thick ITO film was deposited on the top of the hollow nanospheres to be the front electrode by the pulsed-direct current sputtering method at room temperature. The sputtering chamber was pumped down to a base pressure of 8 × 10−6 Torr 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A239
Fig. 5. Flow chart of photonic mirror fabrication by the ALD technique on a PS nanosphere template.
by a cryopump. During the deposition, argon gas was injected as the working gas and the working pressure was 1 × 10−3 Torr. The distance between the target and the substrate was 7.5 cm, and the sputtering power 200 W. The Al2 O3 ALD film growth was accomplished by two self-limiting surface reactions: AlOH AlCH3 3 → AlOAlCH3 2 CH4 ; AlCH3 H2 O → AlOH CH4 :
(1)
(2)
A cycle in the ALD process has four steps as shown in Fig. 6. AlCH3 3 TMA was fed into the chamber as the first reactant A and then N 2 gas was used as a purging gas to remove the excessive reactant and by-products. Next, deionized water was fed into the chamber as the second reactant B and N 2 gas was again used as a purging gas. The pulse time of the precursors was 0.2 s. The N 2 gas was
Fig. 6. ALD growth steps of the Al2 O3 film. A240
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Fig. 7. SEM images of (a) self-assembled PS nanosphere lattice with a honeycomb crystal structure with (b) the diameter reduced by oxygen plasma. (c) Cross-section of the photonic structure after ALD Al2 O3 growth and (d) formation of a hollow structure after PS nanosphere removal in air at 350°C.
sandwiched between the two pulse steps for 5 s. The growth rate was about ∼0.1 nm per cycle and the total ALD process was 200 cycles to form a 20 nm Al2 O3 film. However, although it has been found in many studies that uniformity in thickness of the Al2 O3 film appears at an operating temperature of over 80°C, we found that the thickness could be adjusted with the value of the working pressure in our ALD system, which was 1 Torr. The morphology of the photonic mirror was measured by scanning electron microscopy (SEM). The optical reflectance of the photonic mirror was measured by a Hitachi U4100 spectrometer in the wavelength range from 400 to 800 nm. The bias was provided by a power supply to the top electrode and the back electrode, the positive and negative, respectively. 3. Results and Discussion
Figure 7(a) shows the experimental results where the PS nanospheres were self-assembled to a honeycomb crystal structure on the Al film substrate. PS nanospheres can be readily self-assembled to single layer or multilayers using the spin-coating method. However, there are many dislocation crystal domains remaining, the sufficient area is provided for measurement. Figure 7(b) shows an SEM image of the reduced PS nanosphere arrays produced by chemical etching with the assistance of oxygen plasma. The diameter of the reduced PS nanosphere is related to the duration of the etching time. We found that the nanospheres were shrunk gradually, so they are not spheres after the etching process. Figure 7(c) shows a cross-sectional SEM image of the nanospheres covered with the ALD Al2 O3 film. After removal of the PS nanosphere by heating the substrate to 350°C in air, the hollow nanosphere Al2 O3 shells were formed, as shown in Fig. 7(d). The Al2 O3 ALD growth occurred during alternating exposures
4. Conclusion
In summary, the ALD technique was used to successfully fabricate a nanosphere shell template for a controllable photonic mirror. The size of the nanospheres could be controlled by the etching time in oxygen plasma. The reflectance shows that the orderly nanosphere array possesses the physical features of a photonic crystal in visible light. We conjecture that the period of the nanospheres is the dominant factor for the reflectance and stop band. The minimal variation of reflectance is 0.3% after treatment with a 0.1 V bias voltage. This is a promising method for 3D photonic mirror fabrication, and we would like to try to take advantage of the positive features of this method to design a modulator in future. Fig. 8. Variation of the reflectance spectra of the photonic mirror with the application of bias voltages from 0.1 to 5 V. The inserted image shows the wavelength range from 620 to 670 nm.
to TMA and H2 O and the surface reactions are very efficient and self-limiting. The main driver for the ALD Al2 O3 film is the formation of a strong Al-O bond. The sample was then transferred to the sputtering chamber for the deposition of the top electrode. The reflectance spectra of the samples during bias voltage treatment are shown in Fig. 8. In comparison, the simulation results from the FDTD method demonstrated that the spectra altered and shifted through the reduced spheres, and these variations were caused by a change in the effective-index. To achieve this purpose, we applied a bias voltage to the top and bottom electrode of the photonic mirror to generate an electric field for shrinking the spheres. In the insert image of Fig. 8, it can be seen that the spectra varied with a small ratio of 0.3%, and we found that the breakdown voltage for this photonic mirror is 5 V. Besides, the quantity of reflectivity variation of the photonic mirror in the wavelength is completely similar, which is because the arrangement of the nanospheres is not very regular. The disorder influence in the radii and the positions of the nanospheres is significant; even if there is only a few percent variations [9]. On the other hand, we can control the value of the reflectance by a bias voltage treatment.
The authors thank the National Central University, the National Applied Research Laboratories, and the National Science Council of Taiwan for the financial support of this research under Contract Nos. NSC 102-2221-E-008-068 and 101-3113-P-008009. References 1. J. D. Joannopoulos, P. R. Villeneuve, and S. H. Fan, “Photonic crystals: putting a new twist on light,” Nature 386, 143–149 (1997). 2. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311, 189–193 (2006). 3. R. P. Vanduyne, J. C. Hulteen, and D. A. Treichel, “Atomic-force microscopy and surface-enhanced Raman-spectroscopy. I. Ag island films and Ag film over polymer nanosphere surfaces supported on glass,” J. Chem. Phys. 99, 2101–2115 (1993). 4. T. H. Chang, P. H. Wu, S. H. Chen, C. H. Chan, C. C. Lee, C. C. Chen, and Y. K. Su, “Efficiency enhancement in GaAs solar cells using self-assembled microspheres,” Opt. Express, 17, 6519–6524 (2009). 5. B. Fuhrmann, H. S. Leipner, H. R. Hoche, L. Schubert, P. Werner, and U. Gosele, “Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy,” Nano Lett. 5, 2524–2527 (2005). 6. C. H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92, 061112 (2008). 7. W. H. Cho, C. T. Lee, C. C. Yu, C. C. Kei, D. R. Liu, and C. C. Lee, “Microstructure and optical properties of Al2O3 prepared by oblique deposition using microsphere shell templates,” Appl. Opt. 50, C246–C249 (2011). 8. C. C. Wang, C. C. Kei, Y. W. Yu, and T. P. Perng, “Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition,” Nano Lett. 7, 1566–1569 (2007). 9. T. H. Chang, S. H. Chen, C. H. Chan, Y. W. Yeh, S. L. Ku, C. C. Lee, and C. C. Chen, “Fabrication of three-dimensional photonic crystals using autocloning layers on the self-assembled microspheres,” Opt. Eng. 48, 073401 (2009).
1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A241
Department of Optics and Photonics, National Central University, 300 Chung-Da Rd., Chung-Li 32001, Taiwan
2
Graduate Institute of Energy Engineering, National Central University, 300 Chung-Da Rd., Chung-Li 32001, Taiwan *Corresponding author: [email protected] Received 4 September 2013; revised 15 November 2013; accepted 25 November 2013; posted 27 November 2013 (Doc. ID 196933); published 9 January 2014
In this study, a controllable photonic mirror was fabricated using the atomic layer deposition (ALD) coating technique on a polystyrene (PS) nanosphere template. PS nanospheres were self-assembled on an Al/ glass substrate to form the bottom electrode. A 20 nm ALD Al2 O3 film was then coated onto the surface of the reduced PS nanosphere structure. The PS nanospheres were removed in air at 350°C to form hollow Al2 O3 nanospheres. Then a 30 nm indium tin oxide film was sputtered on the hollow nanosphere structure to form the top electrode. The results show that the incorporation of the photonic mirror could control the reflectance to a value of 0.3% per 0.1 V of bias voltage. © 2014 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.5298) Photonic crystals; (170.0110) Imaging systems. http://dx.doi.org/10.1364/AO.53.00A237
1. Introduction
In recent years, nanostructured metallic, dielectric, or hybrid materials have attracted a lot of attention because of their applications in nanophotonics for the routing and manipulation of light. For example, photonic crystals are artificial dielectric nanostructures with periodic permittivity that affect the propagation of electromagnetic waves [1]. Surface plasmon-based nanophotonics merge electronics and photonics at the nanoscale [2]. Light can be localized and closed to metallic nanoparticles due to the oscillation of electrons. In the past few years, self-assembled polystyrene (PS) and silica sphere templates have been fabricated on different substrates, such as glass, sapphire, and silicon and used for many applications [3–5]. Sun et al. reported on the development of an antireflection structure for broadband wavelengths from 350 to 850 nm fabricated using nanosphere lithography on a silicon substrate [6]. In addition,
1559-128X/14/04A237-05$15.00/0 © 2014 Optical Society of America
Cho et al. utilized the oblique deposition technique and a bottom-up approach to fabricate material with columnar structures on a spherical template for a birefringence photonic crystal [7]. However, it has been found that the high temperature of the electron evaporation process will melt the PS spheres. To solve this problem, an Al2 O3 thin film can be coated onto the surface of the spheres by the atomic layer deposition (ALD) [8] in room temperature. ALD is a thin-film deposition technology that can achieve a thin film with uniform thickness on a high aspect ratio structure. It is a unique process that produces highly conformal films and allows atomic level thickness control. Wang et al. applied the ALD technique for the fabrication of alumina nanotubes using GaQ3 as the template [8]. In this study, we describe the fabrication of a controllable photonic mirror by the ALD technique using PS nanospheres as the template. 2. Experimental Procedure
In this work, the finite-difference time-domain (FDTD) calculation is employed to clarify the relationship between the morphology of the nanospheres 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A237
Fig. 1. (a) FDTD simulation model of hollow nanospheres packed into a honeycomb lattice in the x–y plane. (b) Incident light and simulation detector along the z-direction.
and the electric field as well as the reflectance spectrum of visible light. The photonic mirror is fabricated with a nanosphere template using the ALD technique. Moreover, with the proposed structure, the morphology of the nanospheres can be controlled by the electric field for the reflectance variation. A.
FDTD Simulation
The electric field of the photonic mirror was simulated using the FDTD method in order to design an optimal structure for the periodic nanosphere array. Figure 1(a) illustrates the FDTD model for this system. The hollow nanospheres with 20 nm thick Al2 O3 insulated shells were organized in a closely packed hexagonal monolayer structure on a 50 nm thick Al thin film. Outside the insulated shells, a
30 nm thick ITO film covered the grooves of the hollow nanospheres. The hollow nanospheres had the inside diameter and the period of 400 and 540 nm, respectively. The simulated wavelength range was in the visible light region (400–800 nm). The incident light and simulation detector were oriented along the z-direction as shown in Fig. 1(b). Figures 2(a)–2(c) show the hollow nanospheres with diameters of 400, 395, and 390 nm, respectively. A normal incident plane wave was propagated in the visible wavelength region. To calculate the electric field distributions of the hollow nanospheres in the near-field, the FDTD method was applied using the simulation model in Fig. 1. The electric field distributions of x–y plane for Figs. 2(a)–2(c) are shown in Figs. 2(d)–2(f), respectively. The electric fields show a tendency to be distracted with decreasing diameter. We also found that the photonic structure can control the reflectance through the various diameters of the hollow nanospheres. Similar to Fig. 2, Fig. 3 shows the simulation results for the electric-field distributions in the x–z plane for the various photonic structures. The electric-field intensity distributions in Figs. 3(d)–3(f) show that the electric fields were focused on the top of the nanospheres. Calculation of the integral electric field over the photonic structure shows the electric field is
Fig. 2. Calculation results of the nanospheres (x–y plane) with various sizes: (a) 400 nm, (b) 395 nm, and (c) 390 nm, and their electric field distributions: (d), (e), and (f). A238
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Fig. 3. Calculation results for the nanospheres (x–z plane) with various sizes: (a) 400 nm, (b) 395 nm, and (c) 390 nm and the electric field distributions for (d), (e), and (f) corresponding to (a), (b), and (c).
strongly concentrated in the Al2 O3 ∕ITO layer in this structure, which will result in a decrease in the reflectance value with a decrease in the size of the nanospheres, as shown in Fig. 4. Furthermore, the electric field coupling varied with each wavelength depending on the photonic structure. Moreover, this makes the effective-index alter and shift.
Fig. 4. Simulated reflectance of the nanospheres.
B. Photonic Mirror Procedure
Figure 5 shows the schematic illustration of the photonic mirror fabrication process. A 50 nm aluminum film was coated utilizing a sputtering method onto a B270 glass substrate (5 cm2 ) as a back electrode. A monolayer of 540 nm diameter PS nanospheres was spun onto the substrate. Then, the diameter of the nanospheres was reduced from 540 to 400 nm in an oxygen plasma chamber for a reaction time of 180 s. The reduced nanosphere array was introduced into the ALD reaction chamber for the Al2 O3 film growth, following the ALD process detailed in [8]. Al2 O3 is an extremely good material for high-K applications such as insulators. Besides, its fabrication procedure is a mature technique in an ALD system where a trimethylaluminum (TMA) precursor can be reacted with H2 O to form an Al2 O3 film with excellent uniformity. The PS nanosphere structure was removed in an atmosphere at 350°C to form the Al2 O3 hollow nanosphere template. A 30 nm thick ITO film was deposited on the top of the hollow nanospheres to be the front electrode by the pulsed-direct current sputtering method at room temperature. The sputtering chamber was pumped down to a base pressure of 8 × 10−6 Torr 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A239
Fig. 5. Flow chart of photonic mirror fabrication by the ALD technique on a PS nanosphere template.
by a cryopump. During the deposition, argon gas was injected as the working gas and the working pressure was 1 × 10−3 Torr. The distance between the target and the substrate was 7.5 cm, and the sputtering power 200 W. The Al2 O3 ALD film growth was accomplished by two self-limiting surface reactions: AlOH AlCH3 3 → AlOAlCH3 2 CH4 ; AlCH3 H2 O → AlOH CH4 :
(1)
(2)
A cycle in the ALD process has four steps as shown in Fig. 6. AlCH3 3 TMA was fed into the chamber as the first reactant A and then N 2 gas was used as a purging gas to remove the excessive reactant and by-products. Next, deionized water was fed into the chamber as the second reactant B and N 2 gas was again used as a purging gas. The pulse time of the precursors was 0.2 s. The N 2 gas was
Fig. 6. ALD growth steps of the Al2 O3 film. A240
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Fig. 7. SEM images of (a) self-assembled PS nanosphere lattice with a honeycomb crystal structure with (b) the diameter reduced by oxygen plasma. (c) Cross-section of the photonic structure after ALD Al2 O3 growth and (d) formation of a hollow structure after PS nanosphere removal in air at 350°C.
sandwiched between the two pulse steps for 5 s. The growth rate was about ∼0.1 nm per cycle and the total ALD process was 200 cycles to form a 20 nm Al2 O3 film. However, although it has been found in many studies that uniformity in thickness of the Al2 O3 film appears at an operating temperature of over 80°C, we found that the thickness could be adjusted with the value of the working pressure in our ALD system, which was 1 Torr. The morphology of the photonic mirror was measured by scanning electron microscopy (SEM). The optical reflectance of the photonic mirror was measured by a Hitachi U4100 spectrometer in the wavelength range from 400 to 800 nm. The bias was provided by a power supply to the top electrode and the back electrode, the positive and negative, respectively. 3. Results and Discussion
Figure 7(a) shows the experimental results where the PS nanospheres were self-assembled to a honeycomb crystal structure on the Al film substrate. PS nanospheres can be readily self-assembled to single layer or multilayers using the spin-coating method. However, there are many dislocation crystal domains remaining, the sufficient area is provided for measurement. Figure 7(b) shows an SEM image of the reduced PS nanosphere arrays produced by chemical etching with the assistance of oxygen plasma. The diameter of the reduced PS nanosphere is related to the duration of the etching time. We found that the nanospheres were shrunk gradually, so they are not spheres after the etching process. Figure 7(c) shows a cross-sectional SEM image of the nanospheres covered with the ALD Al2 O3 film. After removal of the PS nanosphere by heating the substrate to 350°C in air, the hollow nanosphere Al2 O3 shells were formed, as shown in Fig. 7(d). The Al2 O3 ALD growth occurred during alternating exposures
4. Conclusion
In summary, the ALD technique was used to successfully fabricate a nanosphere shell template for a controllable photonic mirror. The size of the nanospheres could be controlled by the etching time in oxygen plasma. The reflectance shows that the orderly nanosphere array possesses the physical features of a photonic crystal in visible light. We conjecture that the period of the nanospheres is the dominant factor for the reflectance and stop band. The minimal variation of reflectance is 0.3% after treatment with a 0.1 V bias voltage. This is a promising method for 3D photonic mirror fabrication, and we would like to try to take advantage of the positive features of this method to design a modulator in future. Fig. 8. Variation of the reflectance spectra of the photonic mirror with the application of bias voltages from 0.1 to 5 V. The inserted image shows the wavelength range from 620 to 670 nm.
to TMA and H2 O and the surface reactions are very efficient and self-limiting. The main driver for the ALD Al2 O3 film is the formation of a strong Al-O bond. The sample was then transferred to the sputtering chamber for the deposition of the top electrode. The reflectance spectra of the samples during bias voltage treatment are shown in Fig. 8. In comparison, the simulation results from the FDTD method demonstrated that the spectra altered and shifted through the reduced spheres, and these variations were caused by a change in the effective-index. To achieve this purpose, we applied a bias voltage to the top and bottom electrode of the photonic mirror to generate an electric field for shrinking the spheres. In the insert image of Fig. 8, it can be seen that the spectra varied with a small ratio of 0.3%, and we found that the breakdown voltage for this photonic mirror is 5 V. Besides, the quantity of reflectivity variation of the photonic mirror in the wavelength is completely similar, which is because the arrangement of the nanospheres is not very regular. The disorder influence in the radii and the positions of the nanospheres is significant; even if there is only a few percent variations [9]. On the other hand, we can control the value of the reflectance by a bias voltage treatment.
The authors thank the National Central University, the National Applied Research Laboratories, and the National Science Council of Taiwan for the financial support of this research under Contract Nos. NSC 102-2221-E-008-068 and 101-3113-P-008009. References 1. J. D. Joannopoulos, P. R. Villeneuve, and S. H. Fan, “Photonic crystals: putting a new twist on light,” Nature 386, 143–149 (1997). 2. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311, 189–193 (2006). 3. R. P. Vanduyne, J. C. Hulteen, and D. A. Treichel, “Atomic-force microscopy and surface-enhanced Raman-spectroscopy. I. Ag island films and Ag film over polymer nanosphere surfaces supported on glass,” J. Chem. Phys. 99, 2101–2115 (1993). 4. T. H. Chang, P. H. Wu, S. H. Chen, C. H. Chan, C. C. Lee, C. C. Chen, and Y. K. Su, “Efficiency enhancement in GaAs solar cells using self-assembled microspheres,” Opt. Express, 17, 6519–6524 (2009). 5. B. Fuhrmann, H. S. Leipner, H. R. Hoche, L. Schubert, P. Werner, and U. Gosele, “Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy,” Nano Lett. 5, 2524–2527 (2005). 6. C. H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92, 061112 (2008). 7. W. H. Cho, C. T. Lee, C. C. Yu, C. C. Kei, D. R. Liu, and C. C. Lee, “Microstructure and optical properties of Al2O3 prepared by oblique deposition using microsphere shell templates,” Appl. Opt. 50, C246–C249 (2011). 8. C. C. Wang, C. C. Kei, Y. W. Yu, and T. P. Perng, “Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition,” Nano Lett. 7, 1566–1569 (2007). 9. T. H. Chang, S. H. Chen, C. H. Chan, Y. W. Yeh, S. L. Ku, C. C. Lee, and C. C. Chen, “Fabrication of three-dimensional photonic crystals using autocloning layers on the self-assembled microspheres,” Opt. Eng. 48, 073401 (2009).
1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A241
