Open Access
Add to BibSonomyAbstract
A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.
© 2013 Optical Society of America
1. Introduction
Light-emitting diodes (LEDs), currently the most promising solid-state light sources, are revolutionizing an increasing number of applications in traffic signals, lighting, automobiles, communication, imaging and medicine, owing to their energy-efficiency, long-life and environmental benefits [1–4]. One of the crucial problems facing conventional planar gallium nitride (GaN) based LEDs is the low light output efficiency relative to the theoretically predicted maximum value. The output efficiency of LEDs depends on the internal quantum efficiency and the light extraction efficiency. The internal quantum efficiency of GaN-based LEDs has exceeded 80% due to the development of the metal-organic chemical vapor deposition and the improvement of device fabrication techniques [5]. However, the light extraction efficiency of GaN-based LEDs is low due to total internal reflection and a large Fresnel loss, owing to a sharp refractive index difference at the semiconductor-air interface from GaN (n = 2.5) to air (n = 1) [6]. Only light within the critical angle of 23.6° (about 4% of internal photons) can escape out into the air [7,8]. Most photons emitted from the active layers are trapped inside the LED and absorbed, degrading the device performance. Hence, much effort has been focused on improving the light extraction efficiency of GaN-based LEDs, including innovations such as photonic crystals [9,10], surface texturing [11,12], surface plasmons [13,14], flip-chip LEDs [15,16] and patterned sapphire substrates [17,18]. However, these methods involve complex and expensive processes, or the sacrifice of some of the favorable electrical properties of the LED.
Among the surface texturing and photonic crystals methods that have been tried, two-dimensional (2D) micro-hole structures were etched on p-GaAs to enhance the light output efficiency of GaAs-based LEDs by interference lithography and reactive ion etching [19]. A microhole-array pattern or periodic microholes were etched on GaN-based LEDs [20,21]. The periodic microholes have a diffraction grating effect, or a photonic crystal effect, that enhances the fraction of photons escaping. Unfortunately, the p-GaN layer of about 250 nm is too fragile and thin to be etched onto GaN-based LEDs by the reactive ion etching method. Otherwise, the electrical properties of GaN-based LEDs would be damaged.
In our previous work, ZnO nanorods and nanocones were grown on the top surface of low power GaN-based LEDs to improve their light extraction efficiency without detriment to the epitaxial layers [22]. The refractive index of ZnO (~2.0) is between that of GaN (n = 2.5) and air (n = 1), and it is a good graduated refractive index material for GaN-based LEDs [23–25]. We propose that the growth of a ZnO micro-mesh will provide an improvement over the etched 2D micro-hole structure in extracting more light from the LED. To the best of our knowledge, the compound assembly of ZnO microstructures and ZnO nanostructures has not yet been reported. To date, the light transmission performance of the compound ZnO micro-nanostructure has not been studied.
In this paper, we fabricated hybrid ZnO micro-mesh and nanorod arrays (MMNR) by photolithography techniques and a two-step wet chemical growth process to enhance the light extraction efficiency of GaN-based LEDs. The light extraction effect of the ZnO MMNR is greater than that of separate ZnO micro-mesh or ZnO nanorod arrays. The light extraction enhancement is attributed to the observation that the ZnO MMNR can combine the 2D optical diffraction grating effect of the ZnO micro-mesh and the light scattering effect of the ZnO nanorods. The fabrication entails a moderate degree of post processing after the completion of the LED chips, thus avoiding any negative effect on the electrical properties.
2. Experiments
2.1 Growth of ZnO MMNR
We fabricated the ZnO MMNR by photolithography techniques and a two-step wet chemical growth process [26,27]. The fabrication procedure for ZnO MMNR on silica glass is illustrated in Fig. 1.A transparent ZnO seed layer with a thickness of 100 nm was uniformly sputtered on a 25 mm × 25 mm silica glass slide with a magnetron sputtering apparatus. Then a square arrays micro-cylinders photoresist-1 mask with 3 μm diameter and 2.5 μm separation was fabricated on the ZnO seed layer by photolithography. The glass was immersed in a solution consisting of 0.15 M of zinc nitrate (Zn(NO3)2) and 0.15 M hexamethylenetetramine (C6H12N4) dissolved in de-ionized water. The solution was placed in a preheated oven and maintained at 90 °C for about 2.5 hours. In this way ZnO was selectively grown on the exposed ZnO seed layer. After reaction, the glass was rinsed with de-ionized water five times. A ZnO micro-mesh was obtained after removing the micro-cylinders photoresist-1 with photoresist remover. Then another ZnO seed layer with a thickness of 100 nm was sputtered onto the entire area. To grow ZnO nanorods, the glass was immersed into a source solution consisting of 0.05M Zn(NO3)2 and 0.05M C6H12N4 dissolved in de-ionized water, and then maintained at 95 °C for 3 hours [28]. Finally, the glass was rinsed with de-ionized water five times to remove any residual salts, and dried with high-purity nitrogen at room temperature.
For comparison purposes, ZnO micro-mesh and ZnO nanorod arrays were prepared separately on silica glass substrates. Additionally, the ZnO MMNR was fabricated on the ITO layer of a conventional planar GaN-based LED to investigate the enhancement of the light extraction efficiency following the same method.
2.2 Incident angle-resolved light transmission
To investigate the light transmission properties, a blank glass substrate and glass substrates with a ZnO micro-mesh, ZnO nanorods and ZnO MMNR were adhered to the flat face of a specially designed transparent hemi-cylindrical silica glass block. The gap between glass sample and the glass block was filled with silicon resin of high refractive index (n = 1.50), so light was transmitted through the glass block with little loss [29]. In this way, we were able to directly observe and measure the light transmission characteristics from the medium into air as a function of the incident angle. The critical angle for total internal reflection of silica glass is θcrit = 43.0°, as determined from its refractive index of 1.46. During measurement, the hemi-cylindrical silica glass block with the sample attached was placed at the center of a rotatable stage. A 473 nm laser beam from a semiconductor blue laser with variable incident angle θ = passed through the circular-edge of the hemi-cylinder and impinged on the center of the silica glass where the ZnO structure was attached. A white screen or an integrating sphere with a 1 inch aperture located behind the glass slide captured the total transmitted light. A schematic diagram of incident angle-resolved light transmission measuring system is shown in Fig. 2.
The integrating sphere was connected to a highly sensitive charge coupled device (CCD) through an optical fiber. A 473 nm blue laser beam with variable incident angle θ passes through the circular-edge of the hemi-cylinder and impinges on the center of the glass with the ZnO structures attached with an angular resolution of 2°.
Field-emission scanning electron microscopy (FE-SEM, Hitachi S4800) and atomic force microscopy (AFM, Nanoscope MultiMode V instrument/Bruker Systems) were used to study the morphology of the ZnO structures. Light output measurement and the electrical characteristics of the GaN-based LEDs were measured using an on-wafer testing configuration (IPT 6000 LED chip/wafer probing system). The system is comprised of an optical parameter analyzer and integrating half-sphere system mounted above the LED chip. Far-field radiation patterns of all LEDs were measured by an LED goniophotometer (LED626, EVERFINE Corporation). The local optical field distribution patterns of the GaN-based LEDs were investigated by confocal scanning electroluminescence microscopy (CSEM, NT-MDT) with a spatial resolution of 200 nm, which is an effective experimental tool for measuring optical characteristics such as local light output and light propagation [30].
3. Results and discussion
3.1 Interface morphology
To study the relationship between the interface structure and light extraction performance, atomic force microscopy (AFM) was used to examine the surface morphology of the photoresist-1 micro-cylinders mask, the ZnO micro-mesh, and the ZnO MMNR. Figure 3(a) shows the three-dimensional (3D) AFM morphology of the photoresist-1 micro-cylinders with a diameter of 3 μm, spacing of 2.5 μm and height of 2 μm. As shown in Fig. 3(b), the ZnO micro-mesh with a hole depth of about 1.4 μm is formed as the negative structure of the photoresist-1 micro-cylinders mask. Figure 3(c) shows the 3D AFM image of the ZnO MMNR with a rough morphology and a hole depth of about 1.5 μm. The detail of the ZnO micro-mesh with tetragonally arranged holes is clearly shown in Fig. 3(d). As shown in Fig. 3(e), the ZnO nanorods are well aligned on the surface of the ZnO MMNR. ZnO nanorods with a length of 1 μm and diameter of 100 nm on the mesh are clearly shown in the inset of Fig. 3(e).
Fig. 3 3D AFM morphology of (a) photoresist-1 micro-cylinders, (b) ZnO micro-mesh, and (c) ZnO MMNR. SEM images of (d) ZnO micro-mesh and (e) ZnO MMNR.
3.2 Light transmission properties
To visualize the transmission characteristics of the ZnO structures, a white screen was placed behind the glass sample to capture the diffraction pattern of the transmitted beam. Figures 4(a)–4(d) exhibit sharp differences among the blank glass, glass with the ZnO micro-mesh, glass with ZnO nanorods and glass with ZnO MMNR when illuminated by a 473 nm blue incident laser beam at the same 50° angle of incidence (larger than the critical angle θcrit = 43.0°). Obviously, the images illustrate total internal reflection occurring on the blank glass sample as shown in Fig. 4(a). In Fig. 4(b), the light transmitted into the air from the glass with the ZnO micro-mesh is shown. The transmitted 2D diffraction spots pattern from the ZnO micro-mesh is clearly shown on the screen, and the diffraction is clearly dominant beyond the critical angle. Figure 4(c) shows light transmitted into the air from the glass with the ZnO nanorods. The light intensity gradually drops off in concentric circles from the center outward. Figure 4(d) shows that the diffraction pattern of the ZnO MMNR is a superposition of the 2D diffraction pattern of the ZnO micro-mesh and the gradually decreasing intensity pattern of the ZnO nanorods. The most photons are transmitted into air from the glass with the ZnO MMNR.
Fig. 4 Light diffraction patterns of (a) blank glass, (b) glass with ZnO micro-mesh, (c) glass with ZnO nanorods and (d) glass with ZnO MMNR.
To quantitatively measure the incident angle-resolved light transmission of the ZnO structures, an integrating sphere with a 1 inch light collecting aperture was located close to the glass slide to capture the total transmitted light through the ZnO structures. As shown in Fig. 5, the incident angle-resolved light transmittance of the blank glass, the glass with the ZnO micro-mesh, the glass with the ZnO nanorods and the glass with the ZnO MMNR were all measured in comparison with each other.
Fig. 5 Experimental light transmittance curves vs. incident angle for the blank glass, glass with ZnO micro-mesh, glass with ZnO nanorods and glass with ZnO MMNR.
Clearly, higher light transmittance is obtained in the large range beyond the critical angle with the glass having the ZnO micro-mesh, the glass with the ZnO nanorods and the glass with the ZnO MMNR. The light transmittance enhancement of the glass with the ZnO micro-mesh is due to the 2D diffraction grating effect. Light transmittance enhancement of the glass with the ZnO nanorods is due to the light scattering effect of the nanostructures. Moreover, light transmittance enhancement of the glass with the ZnO MMNR is greater than that of the ZnO micro-mesh and the ZnO nanorods beyond the critical angle. This result is attributed to the concept that the ZnO MMNR can combine the 2D diffraction grating effect of the ZnO micro-mesh and the light scattering effect of the ZnO nanorods, as illustrated in Fig. 4(d).
3.3 Application to improving light extraction in GaN-based LEDs
Due to its excellent light transmission properties, the ZnO MMNR has an important potential application in improving the light extraction efficiency in high power GaN-based blue LEDs. We have grown a ZnO MMNR on the ITO layer of a 2 inch GaN-based LED wafer with complete planar LED structures (with a size of 45 × 45 mil2 and an emission wavelength of about 450 nm). The GaN-based LEDs were fabricated by a conventional process [31]. The GaN-based LED epitaxial layers were grown on the c-plane of sapphire by metal-organic chemical vapor deposition. The GaN-based LED chip structure consists of sapphire (400 μm), undoped GaN (2 μm), n-GaN (2 μm), five periods of InGaN/GaN multiple-quantum-wells (MQW), p-GaN (250 nm), indium tin oxide (200 nm), and p and n Cr/Au electrodes.
Figure 6 shows a schematic of a GaN-based LED with ZnO MMNR (MMNR-LED). First, a protective photoresist-2 mask was employed to expose the ITO layer, with the rest of the area of the LED-chip protected from damage by means of a photoresist lift-off process [32]. Then, ZnO MMNR was grown on the ITO of GaN-based LEDs following the procedure as illustrated in Fig. 1. After the fabrication of the ZnO MMNR, the protective photoresist-2 was removed by acetone. For comparison purposes, LEDs with ZnO micro-mesh and LEDs with ZnO nanorods were also fabricated. The morphology of ZnO micro-mesh, ZnO nanorods and ZnO MMNR on the ITO of GaN-based LEDs is the same with that on the silica glasses due to the same fabrication process.
Figure 7(a) shows the typical light output power versus forward current (LOP–I) curves of the blank LED, LED with ZnO micro-mesh, LED with ZnO nanorods and LED with ZnO MMNR. Every point on the curves is the average value from the measurement of five LED chips with uniform ZnO structures. The light output power of the four types of LED at forward currents of 350 mA and 700 mA are listed in Table 1.The light output power at 350 mA of the blank LED, LED with ZnO micro-mesh, LED with ZnO nanorods and LED with ZnO MMNR is 115.2, 179.4, 180.8 and 225.7 mW, respectively. The light output powers of the LEDs with ZnO micro-mesh, LEDs with ZnO nanorods and LEDs with ZnO MMNR are all greater than that of the blank LEDs by 55.7%, 57.0% and 95.9% at 350 mA, respectively. These results indicate that the ZnO micro-mesh, ZnO nanorods and ZnO MMNR can dramatically increase the light output power of GaN-based LEDs. In addition, the light output power of the LEDs with ZnO MMNR is much greater than that of the LEDs with ZnO micro-mesh and the LEDs with ZnO nanorods at the same injection current within the measurement region. The light extraction efficiency of the LEDs with ZnO MMNR is higher than that of the LEDs with ZnO micro-mesh and the LEDs with ZnO nanorods. The ZnO MMNR shows greater enhancement than either the ZnO micro-mesh or the ZnO nanorods.
Fig. 7 (a) LOP-I curves and (b) I-V curves of the blank LED, LED with ZnO micro-mesh (MM-LED), LED with ZnO nanorods (NR-LED) and LED with ZnO MMNR (MMNR-LED). Inset shows EL spectra at 350 mA.
Table 1. Light output power (LOP) of blank LED, LED with ZnO micro-mesh (MM-LED), LED with ZnO nanorods (NR-LED) and LED with ZnO MMNR (MMNR-LED) at 350 and 700 mA.
As shown in Fig. 7(b), the current-voltage (I-V) curves of the blank LED, LED with ZnO micro-mesh, LED with ZnO nanorods and LED with ZnO MMNR nearly overlap. No obvious difference in forward voltage is observed. Apparently these data indicate that the growth of ZnO micro-mesh, ZnO nanorods and ZnO MMNR on the LEDs cause no degradation of the electrical properties of the GaN-based LEDs.
The inset in Fig. 7(b) shows the electroluminescence (EL) spectra of the blank LED, LED with ZnO micro-mesh, LED with ZnO nanorods and LED with ZnO MMNR at 350 mA. There is no significant shift in the EL peak position at about 450 nm for any of the four types of LED, with the same full width at half maximum of about 19 nm. Apparently, the EL intensity of the LED with ZnO micro-mesh and the LED with ZnO nanorods is larger than that of the blank LED at 350 mA. Moreover, the EL intensity of the LED with ZnO MMNR is larger than that of the LED with ZnO micro-mesh or that of the LED with ZnO nanorods. This result indicates that the light extraction efficiency of the LEDs with ZnO MMNR is greater than that of the LEDs with ZnO micro-mesh or that of the LEDs with ZnO nanorods.
As shown in Fig. 8, the far-field angular radiation patterns of a blank LED, an LED with ZnO micro-mesh, an LED with ZnO nanorods and an LED with ZnO MMNR were measured at 350 mA. The light output intensity of the LED with ZnO nanorods is enhanced compared with the blank LED and the greatest light intensity is in the vertical direction. The plot shows that the light output from the top surface of the LED is enhanced. We attribute this enhancement to the light scattering effect of the ZnO nanorods. The light output enhancement of the LED with ZnO micro-mesh within the range of −30° to −60° and 30° to 60° is relatively greater than in other ranges. The light intensity enhancement of the LED with ZnO MMNR over the whole range is relatively homogeneous. In addition, the light output enhancement of the LED with ZnO MMNR is greater than that of the LED with ZnO micro-mesh or that of the LED with ZnO nanorods.
Fig. 8 Far-field radiation patterns of blank LED, LED with ZnO micro-mesh (MM-LED), LED with ZnO nanorods (NR-LED) and LED with ZnO MMNR (MMNR-LED).
The light extraction effects of ZnO structures were visualized by macroscopic light emitting photographs of the four types of LEDs at 10 mA in Fig. 9. Figure 9(a) shows a LED chip set on the LED base plate. Figures 9(b)–9(e) are the blank LED, LED with ZnO micro-mesh, LED with ZnO nanorods, and LED with ZnO MMNR, respectively. LED with ZnO micro-mesh and LED with ZnO nanorods are brighter than the blank LED. LED with ZnO MMNR is the brightest in the four types of LEDs.
Fig. 9 (a) A LED chip set on the LED base plate. The light emitting photographs of (b) blank LED, (c) LED with ZnO micro-mesh, (d) LED with ZnO nanorods and (e) LED with ZnO MMNR at 10 mA.
The four types of LED chips were capped with hemispherical epoxy caps and encapsulated with silicone resin. The light output interface was changed. Photons would propagate from GaN to the sequent media of ITO, ZnO structures, silicone resin and hemispherical-caps, then into the air. ZnO MMNR, silicone resin and hemispherical epoxy caps jointly influenced the light extraction of the GaN-LEDs. Table 2 shows the light output power of encapsulated four types of LEDs at 350 mA. Each kind of LEDs was selected from samples with uniform morphology of ZnO structures. The light output power of encapsulated LEDs with ZnO micro-mesh, LEDs with ZnO nanorods are greater than encapsulated blank LEDs. What’s more, it can be clearly seen form Table 2 that the light output power of encapsulated MMNR-LEDs are the greater than all other samples.
Table 2. Light output power (LOP) of encapsulated blank LEDs, LEDs with ZnO micro-mesh (MM-LEDs), LEDs with ZnO nanorods (NR-LEDs) and LEDs with ZnO MMNR (MMNR-LEDs) at 350 mA.
The local optical field distribution pattern of the ZnO micro-mesh and ZnO MMNR was investigated by using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism is analysed in-depth. Figures 10(a) and 10(c) show the CSEM images from LED surfaces with ZnO micro-mesh and ZnO MMNR, respectively. During scanning, the LEDs were kept at a static current of 10 mA using a Keithley 2400S source meter. Light from the LED with ZnO micro-mesh exhibits a tetragonally patterned optical field. The local optical field intensity from the hole sidewalls of the ZnO micro-mesh is evidently greater than that from the top of the wall of the ZnO micro-mesh and the hole interior. As shown in Fig. 10(b), the cross sectional intensity of the green line in Fig. 10(a) also exhibits a periodic optical field with a peak intensity around the hole sidewalls of the ZnO micro-mesh. For the first time light extraction enhancement from the hole sidewalls of the ZnO micro-mesh has been observed. This technique directly demonstrates that the ZnO micro-mesh leads to light extraction enhancement in LEDs. The microscopic light extraction mechanism of the two-dimensional diffraction grating effect of the ZnO micro-mesh is visualized as light extraction from the hole sidewalls. Figure 10(c) shows the tetragonal pattern of the optical field generated by combining the effects of the ZnO micro-mesh and the ZnO nanorods. The optical field intensity from the walls of the ZnO MMNR is greater than from other areas. As shown in Fig. 10(d), the periodic optical field cross sectional intensity distribution, as marked by the blue line in Fig. 10(c), shows that the light intensity from the walls of the ZnO micro-mesh is greater than that from other areas.
Fig. 10 CSEM images of LEDs with (a) ZnO micro-mesh and (c) ZnO MMNR; (b) The cross sectional intensity of green line in CSEM images (a) and (d) cross sectional intensity of blue line in CSEM images (c).
As shown in Fig. 11(a), only if the incident angle is larger than the critical angle of total internal reflection (θc = 23.6° for GaN), photons can escape from the device. The light escape angle is increased by θ1, which is attributed to additional light escaping from the hole sidewalls of the ZnO micro-mesh [33, 34]. This mechanism is shown in Fig. 11(b) and demonstrated in Fig. 10(a). Though some of the light propagates from the tops and sides of the walls of the ZnO micro-mesh, many photons are trapped in the ZnO micro-mesh because of the mesh-like wave guide structure. When ZnO nanorods were grown on the surface of the ZnO micro-mesh, the light escape path changed. As shown in Fig. 11(c), the ZnO nanorods increase the surface roughness of the whole interface and extract more of the light trapped in the ZnO micro-mesh from the top surface of the walls and the bottom of the holes through diffuse scattering. As shown in Fig. 10, the light intensity from the top surfaces of the walls and the bottoms of the holes of the ZnO MMNR is enhanced. Light escaping from the top surface of the walls of the ZnO MMNR is dominant and light from the hole sidewalls is relatively dwarfed. The escape angle was increased by θ2 through light scattering by the ZnO nanorods from the top surface of the whole interface of the ZnO MMNR. In short, the ZnO micro-mesh can guide the light output from the sidewalls and increase the light extraction efficiency of the LEDs. ZnO nanorods can further increase light scattering and enhance the light extraction efficiency of the LEDs. The ZnO MMNR integrates the advantages of the two individual structures and extracts more light from the LEDs. The 2D diffraction grating effect of the ZnO micro-mesh in macro-presentation is equivalent to light outputting from the hole sidewalls of the ZnO micro-mesh in micro-presentation. The ZnO MMNR can combine the 2D diffraction grating effect of the ZnO micro-mesh and the light scattering effect of the ZnO nanorods, and has a greater advantage in extracting light than the micro-mesh or the nanorods by themselves.
Fig. 11 Light escape mechanism of (a) blank-LED, (b) LED with ZnO micro-mesh and (c) LED with ZnO MMNR.
4. Conclusions
In summary, we have fabricated the ZnO MMNR on the surface of high power GaN-based LEDs to enhance the light extraction efficiency. The ZnO MMNR is fabricated by photolithography and a two-step wet chemical growth process. The light output power of a GaN-based LED with ZnO MMNR compared to an unmodified LED is improved by 95%. The light extraction efficiency of the LED with ZnO MMNR is greater than that of the LED with ZnO micro-mesh and the LED with ZnO nanorods. The light transmission enhancement is attributed to the concept that the ZnO MMNR can combine 2D diffraction grating effect of the ZnO micro-mesh and the light scattering effect of the ZnO nanorods, and has a greater advantage in extracting light than the ZnO micro-mesh or the ZnO nanorods by themselves. This mechanism is visualized and demonstrated by light diffraction patterns and confocal scanning electroluminescence microscopy images. Light transmission of the ZnO MMNR beyond the critical angle is tremendously enhanced in the incident angle-resolved light transmission measurements. GaN-based LEDs with the ZnO MMNR show excellent optical and electrical performance. This simple, low-cost and damage-free ZnO MMNR growth process is valuable in fabricating high-brightness solid-state lighting. We anticipate that this work will open new avenues in photoelectric research and in other fields by integrating microstructures and nanostructures.
Acknowledgments
This work was supported by National Basic Research Program of China (2009CB930503, 2011CB301904), NSFC (Contract No. 51021062), and IIFSDU (2012JC007). Thanks to Prof. Robert Ivan Boughton JR and Xianlei Li for linguistic advice.
References and links
1. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]
2. N. Holonyak, “Is the light emitting diode (LED) an ultimate lamp?” Am. J. Phys. 68(9), 864–866 (2000). [CrossRef]
3. S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3(4), 180–182 (2009). [CrossRef]
4. S. Noda and M. Fujita, “Light-emitting diodes: photonic crystal efficiency boost,” Nat. Photonics 3(3), 129–130 (2009). [CrossRef]
5. T. Nishida, H. Saito, and N. Kobayashi, “Efficient and high-power AlGaN-based ultraviolet light-emitting diode grown on bulk GaN,” Appl. Phys. Lett. 79(6), 711–712 (2001). [CrossRef]
6. B.-U. Ye, B. J. Kim, Y. H. Song, J. H. Son, H. K. Yu, M. H. Kim, J.-L. Lee, and J. M. Baik, “Enhancing light emission of nanostructured vertical light-emitting diodes by minimizing total internal reflection,” Adv. Funct. Mater. 22(3), 632–639 (2012). [CrossRef]
7. E. F. Schubert, Light-Emitting Diodes, 2nd ed. (Cambridge University, 2006).
8. A. I. Zhmakin, “Enhancement of light extraction from light emitting diodes,” Phys. Rep. 498(4–5), 189–241 (2011). [CrossRef]
9. J. J. Wierer, A. David, and M. M. Megens, “III-nitride photonic-crystal light-emitting diodes with high extraction efficiency,” Nat. Photonics 3(3), 163–169 (2009). [CrossRef]
10. H. K. Cho, J. Jang, J.-H. Choi, J. Choi, J. Kim, J. S. Lee, B. Lee, Y. H. Choe, K.-D. Lee, S. H. Kim, K. Lee, S.-K. Kim, and Y.-H. Lee, “Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes,” Opt. Express 14(19), 8654–8660 (2006). [CrossRef] [PubMed]
11. J. H. Kang, J. H. Ryu, H. K. Kim, H. Y. Kim, N. Han, Y. J. Park, P. Uthirakumar, and C.-H. Hong, “Comparison of various surface textured layer in InGaN LEDs for high light extraction efficiency,” Opt. Express 19(4), 3637–3647 (2011). [CrossRef] [PubMed]
12. T. Wei, Q. Kong, J. Wang, J. Li, Y. Zeng, G. Wang, J. Li, Y. Liao, and F. Yi, “Improving light extraction of InGaN-based light emitting diodes with a roughened p-GaN surface using CsCl nano-islands,” Opt. Express 19(2), 1065–1071 (2011). [CrossRef] [PubMed]
13. A. J. Huber, B. Deutsch, L. Novotny, and R. Hillenbrand, “Focusing of surface phonon polaritons,” Appl. Phys. Lett. 92(20), 203104 (2008). [CrossRef]
14. C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007). [CrossRef] [PubMed]
15. P. Zhao and H. Zhao, “Analysis of light extraction efficiency enhancement for thin-film-flip-chip InGaN quantum wells light-emitting diodes with GaN micro-domes,” Opt. Express 20(S5), A765–A776 (2012). [CrossRef] [PubMed]
16. Y. C. Yang, J.-K. Sheu, M.-L. Lee, C. H. Yen, W.-C. Lai, S. J. Hon, and T. K. Ko, “Vertical InGaN light-emitting diode with a retained patterned sapphire layer,” Opt. Express 20(S6), A1019–A1025 (2012). [CrossRef]
17. X.-H. Huang, J.-P. Liu, J.-J. Kong, H. Yang, and H.-B. Wang, “High-efficiency InGaN-based LEDs grown on patterned sapphire substrates,” Opt. Express 19(S4), A949–A955 (2011). [CrossRef] [PubMed]
18. Y.-C. Lee, C.-H. Ni, and C.-Y. Chen, “Enhancing light extraction mechanisms of InGaN-based light-emitting diodes through the integration of imprinting microstructures, patterned sapphire substrates, and surface roughness,” Opt. Express 18(S4), A489–A498 (2010). [CrossRef] [PubMed]
19. D. Pudiš, L. Šušlik, J. Škriniarová, J. Kováč, I. Martinček, J. Kováč Jr, Š. Haščík, I. Kubicová, J. Novák, and M. Veselý, “Light extraction from a light emitting diode with photonic structure in the surface layer investigated by NSOM,” Opt. Laser Technol. 43(5), 917–921 (2011). [CrossRef]
20. C.-F. Lin, C.-M. Lin, K.-T. Chen, M.-S. Lin, and J.-J. Dai, “Green light-emitting diodes with a photoelectrochemically treated microhole-array pattern,” Electrochem. Solid State Lett. 13(3), H90–H93 (2010). [CrossRef]
21. S. H. Kim, H. H. Park, Y. H. Song, H. J. Park, J. B. Kim, S. R. Jeon, H. Jeong, M. S. Jeong, and G. M. Yang, “An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes,” Opt. Express 21(6), 7125–7130 (2013). [CrossRef] [PubMed]
22. Z. Yin, X. Liu, Y. Wu, X. Hao, and X. Xu, “Enhancement of light extraction in GaN-based light-emitting diodes using rough beveled ZnO nanocone arrays,” Opt. Express 20(2), 1013–1021 (2012). [CrossRef] [PubMed]
23. J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie, and H. Shen, “Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency,” Appl. Phys. Lett. 90(20), 203515 (2007). [CrossRef]
24. K. K. Kim, S. D. Lee, H. Kim, J. C. Park, S. N. Lee, Y. Park, S. J. Park, and S. W. Kim, “Enhanced light extraction efficiency of GaN-based light-emitting diodes with ZnO nanorod arrays grown using aqueous solution,” Appl. Phys. Lett. 94(7), 071118 (2009). [CrossRef]
25. K. Dai, C. B. Soh, S. J. Chua, L. Wang, and D. Huang, “Influence of the alignment of ZnO nanorod arrays on light extraction enhancement of GaN-based light-emitting diodes,” J. Appl. Phys. 109(8), 083110 (2011). [CrossRef]
26. B. S. Kang, S. J. Pearton, and F. Ren, “Low temperature (<100 °C) patterned growth of ZnO nanorod arrays on Si,” Appl. Phys. Lett. 90(8), 083104 (2007). [CrossRef]
27. D. Yuan, R. Guo, Y. Wei, W. Wu, Y. Ding, Z. L. Wang, and S. Das, “Heteroepitaxial patterned growth of vertically aligned and periodically distributed ZnO nanowires on GaN using laser interference ablation,” Adv. Funct. Mater. 20(20), 3484–3489 (2010). [CrossRef]
28. S. J. An, J. H. Chae, G. C. Yi, and G. H. Park, “Enhanced light output of GaN-based light-emitting diodes with ZnO nanorod arrays,” Appl. Phys. Lett. 92(12), 121108 (2008). [CrossRef]
29. X.-X. Fu, X.-N. Kang, B. Zhang, C. Xiong, X.-Z. Jiang, D.-S. Xu, W.-M. Du, and G.-Y. Zhang, “Light transmission from the large-area highly ordered epoxy conical pillar arrays and application to GaN-based light emitting diodes,” J. Mater. Chem. 21(26), 9576–9581 (2011). [CrossRef]
30. K. S. Kim, S. M. Kim, H. Jeong, M. S. Jeong, and G. Y. Jung, “Enhancement of light extraction through the wave-guiding effect of ZnO sub-microrods in InGaN blue light-emitting diodes,” Adv. Funct. Mater. 20(7), 1076–1082 (2010). [CrossRef]
31. X. L. Nguyen, T. N. N. Nguyen, V. T. Chau, and M. C. Dang, “The fabrication of GaN-based light emitting diodes (LEDs),” Adv. Nat. Sci. Nanosci. Nanotechnol. 1(2), 025015 (2010). [CrossRef]
32. S. Xu, Y. Shen, Y. Ding, and Z. L. Wang, “Growth and transfer of monolithic horizontal ZnO nanowire superstructures onto flexible substrates,” Adv. Funct. Mater. 20(9), 1493–1497 (2010). [CrossRef]
33. A. N. Noemaun, F. W. Mont, G.-B. Lin, J. Cho, E. F. Schubert, G. B. Kim, C. Sone, and J. K. Kim, “Optically functional surface composed of patterned graded-refractive-index coatings to enhance light-extraction of GaInN light-emitting diodes,” J. Appl. Phys. 110(5), 054510 (2011). [CrossRef]
34. M. K. Lee, C. L. Ho, and P. C. Chen, “Light extraction efficiency enhancement of GaN blue LED by liquid-phase-deposited ZnO rods,” IEEE Photonics Technol. Lett. 20(4), 252–254 (2008). [CrossRef]
