GALLIUM-nitride (GaN) based light-emitting diodes

Transcription

1 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 4, JULY/AUGUST 2015 Enhanced Light Extraction Efficiency of GaN-Based Hybrid Nanorods Light-Emitting Diodes Jhih-Kai Huang, Che-Yu Liu, Tzi-Pei Chen, Hung-Wen Huang, Fang-I Lai, Po-Tsung Lee, Member, IEEE, Chung-Hsiang Lin, Chun-Yen Chang, Tsung Sheng Kao, and Hao-Chung Kuo, Senior Member, IEEE Abstract High light extraction GaN-based light-emitting diodes (LEDs) with a hybrid structure of straight nanorods located in an array of microholes have been successfully demonstrated. Via the nanoimprint lithography and photolithography techniques, high aspect-ratio light-guiding InGaN/GaN nanorods can be fabricated and regularly arranged in microholes, resulting in a great improvement of the light extraction for the GaNbased LED device. The light output power of the hybrid nanorods LED is mw at the driving current standard of 25.4 A/cm 2, an enhancement of 38.7% to the conventional GaN-based LEDs. Furthermore, with a modification of the hybrid structures dimensions and locations, the emitted optical energy can be redistributed to obtain light-emitting devices with homogenueous optical field distributions. Index Terms Light emitting diodes, optoelectronic devices, nanotechnology, lithography. I. INTRODUCTION GALLIUM-nitride (GaN) based light-emitting diodes (LEDs) have attracted intensive interest among scientists and technologists over past twenty years for their promising applications such as monitor backlight, outdoor full-color display, and solid-state lighting [1] [4]. However, the total light output from the GaN-based LEDs is still rather low which may limit their applications in practice. Several key advances have been reported in the fields of III-Nitride LEDs [5], [6], including non-/semi-polar InGaN QWs [7], large overlap InGaN QWs [8], [9], ITO spreading layer [10] and InGaN/GaN LED s reliability [11]. Typically, due to the different refractive indices between the GaN materials and the ambient air, only about 4% of the light emitted from the active regions can escape from the LED surface, while the reflected light from the bottom sapphire wafers is absorbed by the constituent materials after the multiple internal reflections [12]. Such an output quantity is unable Manuscript received September 27, 2014; revised December 30, 2014; accepted January 3, Date of publication January 9, 2015; date of current version February 27, This work was supported in part by the Ministry of Science and Technology, China. J.-K. Huang, C.-Y. Liu, T.-P. Chen, H.-W. Huang, P. T. Lee, T. S. Kao, and H.-C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan ( jkhuang.eo98g@g2.nctu.edu.tw; cheyu.liu0801@gmail.com; joanna @gmail.com; stevinhuang737672@msn.com; potsung@mail. nctu.edu.tw; tskao@nctu.edu.tw; hckuo@faculty.nctu.edu.tw). F.-I. Lai and C. Y. Chang are with the Department of Electrical Engineering, Yuan Ze University, Chung-Li 320, Taiwan ( filai@saturn.yzu.edu.tw; cyc3562@gmail.com). C.-H. Lin is with the Luxtaltek Corporation, Miaoli 350, Taiwan ( sean.lin@luxtaltek.com). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSTQE to meet the increasing demands in today s industries. Thus, how to further promote the light emission efficiency and the output optical power of GaN-based LEDs have become one of the most important topics in the research field [13] [30]. To obtain high output power LED devices, first of all, is to enhance the light extraction efficiency (LEE), which can be promoted by eliminating the light reflections in LED structure and increasing the number of photons escaped from the active regions. Regarding the elimination of light reflections, several advanced methods have been proposed and investigated such as the textured structures on the p-gan surface [13], [14], double-embedded photonic crystals [15], [16], self-assembled microlens arrays [17], [18], concave microstructures via imprinting method [19], [20], embedded air-voids photonic crystals in GaN layers [21], [22], and patterned micro-holes arrays on LEDs surface [23], [24]. These methods have been successfully demonstrated that the LEE could be promoted. Moreover, in terms of increasing the number of emitted photons, several groups have made their great efforts to produce GaN nanorods in the GaN-based LEDs structure [25] [30]. These nanorods not only provide a large sidewall-surface area as the pathways for the photons escape, but also function as light guiding pillars to extract the photons in the longitudinal direction. In this paper, we intend to take the both advantages by exploiting a designed hybrid structure of InGaN/GaN multi quantum wells (MQWs) nanorods within an array of microholes, simultaneously reducing the light reflections and increasing the escaped photons to enhance the light extraction in the new type LED device. Meanwhile, via the nano-imprint [31] and photolithography technologies, well-arranged InGaN/GaN nanorods can be generated in microholes as proposed, giving the opportunities in reliable mass production in future lighting industry. II. EXPERIMENT To illustrate the fabrication process of InGaN/GaN nanorods in microholes, a schematic diagram to describe the layer structure of a GaN-based LED wafer and how the nanorods generated in microholes is represented in Fig. 1. The LED wafer was prepared via the metal organic chemical vapor deposition (MOCVD) method and from the bottom to the top, sequentially consisted a 50 nm GaN nucleation layer, a 2 μm undoped GaN buffer layer, a 3 μm Si-doped n-gan layer, 10 pairs In 0.21 Ga 0.79 N/GaN MQWs with a central wavelength of 460 nm anda0.2μm Mg-doped p-gan layer grow on a patterned sapphire substrate. After the growth of the LED wafer, the nano-imprint lithography (NIL) and photolithography technique were applied to X 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See standards/publications/rights/index.html for more information.

2 HUANG et al.: ENHANCED LIGHT EXTRACTION EFFICIENCY OF GaN-BASED HYBRID NANORODS LIGHT-EMITTING DIODES Fig. 2. The (a) tilt-view and (b) cross-section SEM images of nanorods in microholes arrays. The diameter of the micro-holes is around 15 μm, while the height and the diameter of nanorods are 1.2 and 0.45 μm, respectively. Fig. 1. Schematic diagrams. (a) Layer structure of a hybrid nanorods GaNbased LED with nanorods located in an array of micro-holes. (b) (e) Brief fabrication flow chart of the nanorods fabrications in microholes arrays. synthesis the GaN nanorods within microholes arrays. Fig. 1(b) illustrates the flow chart of the nano-rods inside the micro-holes fabrications process. First, a 400 nm-thick SiO2 layer was deposited on the surface of the prepared LED wafer by a plasmaenhanced chemical vapor deposition and then an imprint-resist (IR) layer of 360 nm was coated onto the prepared SiO2 layer by spin coating at a rotational speed of 3000 rpm. By placing and releasing a nano-imprint mold on the IR layer, a 12-fold photonic quasi-periodic crystal pattern would be transferred to the IR layer, forming SiO2 dielectric nano-discs at 400 nm in diameter on the wafer surface. A photo-resist (PR) layer with a thickness of 2 μm was coated afterwards on the dielectric layer by a spin coater operated at a rotational speed of 3000 rpm. A standard photolithography method was utilized to fabricate a pattern of micro-holes arrays on the PR layer. Then, the LED wafer with a micro-patterned PR layer and a nano-patterned dielectric layer was etched by an inductively coupled plasma reactive ion etching (ICP-RIE) system with mixed process gases of Cl2 and BCl3 (Cl2 /BCl3 = 20/10 sccm) at a bias power of 80 W and ICP power of 100 W. After the dry etching process, the residual PR layer and dielectric layer were removed via a wet-bench system. Finally, a hybrid structure of uniformly arranged nanorods located in microholes arrays was formed on the entire LED wafer by using NIL and standard lithography technique. To complete the LED chip, a 0.24 μm indium tin oxide (ITO) thin film as a contact layer was e-beam evaporated on the top of the above hybrid nanorods LED wafer, while 1.4 μm p-pad and n-pad electrodes were produced onto the ITO layer and n-gan layer surface, respectively. Finally, the high light extraction hybrid nanorods LED have been demonstrated. By different current densities driving, the intensity of the output light emission from the LED chip can be acquired. A clear tilt-view field emission scanning electron microscope (Hitachi S-4800) image of the fabricated nanorods array within a microhole was shown in the Fig. 2(a). Fig. 2(a) and (b) show the tilt-view and cross-section SEM images of the hybrid structures of nanorods in microholes, respectively. In Fig. 2(a), the image shows an array of nanorods in a microhole on a LED chip, while the diameter of the micro-hole is around 15 μm. As the cross-section SEM image represents in Fig. 2(b), via the fabrication process, high aspect-ratio straight nanorods can be fabricated with a precise control of different pitches, dimensions, and depths etc. In this case, the chip size of whole LED devices is 300 μm 300 μm, while the height and the diameter of the nanorods is around 1.2 μm and 450 nm, respectively. III. RESULTS AND DISCUSSION The intensity current voltage (L I V) performance of the hybrid nanorods LEDs was investigated by conducting the electroluminescence (EL) measurements using an integrating sphere system. Fig. 3 shows the L I V characteristics of the nanorods

3 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 4, JULY/AUGUST 2015 Fig. 3. The intensity current voltage (L I V) characteristics of the conventional (black circles), microholes (red triangles) and hybrid nanorods (blue squares) LEDs. LED chips, accompanied with the comparison of the conventional and microholes LEDs. Here, the height of the MQWs nanorods is 1.2 μm. In the I V characteristics, we found that the forward voltages among the three types LED structures are approximate to each other at the same current density, indicating that the fabrication process of the constituent microholes and nanorods does not destroy the electric properties of the LED components. The similar forward voltage of the three types of LED chips is also due to the limited total area of the digging microholes. In terms of the L-I curve, at the current density of 25.4 A/cm2, which is corresponding to a driving current of 20 ma for the conventional LED, the hybrid nanorods LED chip exhibits an enhanced optical power of mw, while the conventional and microholes LEDs give the optical power at and mw, respectively. Compared with the conventional GaN-based LED device, the hybrid nanorods LEDs exhibit an output light enhancement of 38.7%. In the hybrid structures, the constituent microholes provide an increase of the light scattering from the LED chips and the exposed MQWs from the sidewall area. Also, the reduction in total active region by the etched microholes is considered to increase the current density, promoting the photons generation at per unit area. According to the work done by Hsueh et. al. and Lai et. al. [23], [24], the micro-holes structure acts as scattering media to improve the LEE. Regarding the high aspect-ratio nanorods, they function as light-guiding pillars to support more light extraction paths for photons escaping from the LED devices. In addition, the patterned ITO layer was designed to cover the most area of p-gan surface except the hybrid region of microholes and nanorods, preventing the current leaking from the sidewall. As a result, applying the hybrid structures of nanorods in a microholes array to the LED devices, light extraction performance can be enhanced, increasing the output optical energy. The hybrid structures not only can be employed in current LED devices, but also may be expandable to the photovoltaic components with flexible wavelength and material choices, giving the opportunities in reliable mass production in future industry. The wall plug efficiency (WPE) η of the LEDs operated at 20 ma can be calculated as follows: η= Pphoton Pphoton = Pelec I V (1) Fig. 4. The microscope images of the (a) microholes and (b) hybrid nanorods GaN-based LEDs. The injected current is at 5 ma. At a high injected current of 20 ma, (c) and (d) represent the 3-D beam view images of the microholes and hybrid nanorods LEDs, respectively. where Pphoton and Pelec indicate the average light output power and the input electrical power, respectively. The value of the average input electrical power can be acquired from the input current I and the operation voltage V. According to this equation, the WPE of the hybrid nanorods LED device is estimated around 29%, while the conventional and microholes LEDs give the value at 24.8% and 27.7%, respectively. Thus, even though the light emitting area is decreased due to the reduced active region, we still can acquire a better energy conversion efficiency on the hybfied nanorods LEDs by a compensation of the increased current density and the enhanced LEE. Fig. 4(a) and (b) respectively show the optical microscope images of a microholes LED and a hybrid nanorods LED at the driving current of 5 ma. In this low current setting, the light extraction performance between these two structured LEDs can be compared and analyzed. Here, a neutral density filter of light intensity was applied in the microscope system operated at a dark condition. In Fig. 4(a), light from the constituent microholes is much weaker than that from the surrounding MQWs regions, indicating that photon extraction from the microholes LED device is limited at low current injection. In terms of the LED device with the hybrid structure of nanorods in microholes, at the same injected current, luminescence excited from the microholes region is enhanced with the existence of nanorods. Since the current spreading cannot form a complete path at the locations of nanorods, photons from the microholes in the hybrid nanorods LEDs are not generated from the MQWs in the InGaN/GaN nanorods. Light from the bright micorholes in the hybrid nanorods LEDs result from the superior light extraction between the nanords. The high aspect-ratio InGaN/GaN nanorods function as light guiding pillars in the hybrid nanorods LED device, providing strong light extraction in a large area. Regarding the LEDs operated at high driving current, the light intensity distributions on the surfaces of the microholes and hybrid nanorods LEDs were acquired using a beam view

4 HUANG et al.: ENHANCED LIGHT EXTRACTION EFFICIENCY OF GaN-BASED HYBRID NANORODS LIGHT-EMITTING DIODES images measurement as the results demonstrated in Fig. 4(c) and (d), respectively. The driving current is at 20 ma. In the comparison of these two types of LEDs, although the light intensity from the microholes is enhanced by the light scattering and the exposed MQWs, the stronger light intensity is still obtained in the same locations but with the existence of nanorods in the hybrid nanorods LED. Such a result corresponds to the L I V measurements as shown in Fig. 3, indicating that the nanorods in microholes exhibit the ability to extract more photons and act as nano-pillars to guide light propagating at center. The light intensity distributions on the surface of the structured LEDs also provide us the opportunities to the development of the future lighting components. Moreover, from the measured results shown in Fig. 4(a) and (c), we found that only at high injected current, more photons could be extracted from the microholes structure. Therefore, with the nanorods incorporated in the LED device, the LEE is greatly promoted, especially at lower operation power. By designing the dimensions and density of the hybrid nanorods, the light intensity distribution on a hybrid nanorods LED chip can be controlled, coordinating the other promising applications. For example, phosphors such as unevenness of the coating, we can design the phosphor-rich region has a strong light extraction from the nanorods, achieving higher conversion efficiency. In order to investigate how the InGaN/GaN nanorods function as light-guiding pillars to enhance the light extraction in the LED devices, 3D finite difference time domain simulations using the FullWAVE program were conducted to calculate the electric field distributions and the far-field light enhancement with the existence of nanorods [21], [32]. GaN nanorods of 450 nm in diameter were arranged in a unit-cell area of 2.8 μm 8 μm as it performed in the experiments. The height of the nanorods was set at 1.2 μm, while the pitch and the spacing were at 750 and 300 nm, respectively. In the simulations, a z-direction electric dipole with the radiation wavelength of 460 nm was employed as a light source, placed 0.4 μm below the bottom of the nanorods. The realistic material parameters and Joule loss factors were obtained from a well-established data in Ref. [33], [34]. A simulation model of the LED structure without the nanorods was also prepared to have a comparison of the light extraction performance. The calculated electric field distributions are shown in Fig. 5(a) and (b), which are represented the conventional and nanorods LEDs, respectively. As can be seen in the figures, more concentrated optical energy in the light propagation direction can be obtained in the nanorods LED structure, indicating that the photons emitted from the MQWs are guided by the nanorods and extracted in the longitudinal direction. In the simulation, we performed the established model at periodic boundary condition with a unit cell of 0.75 μm. By setting a time-dependent energy detector above 1.2 μm of the simulation model, the corresponding normalized light output as a function of the simulation time is calculated and shown in Fig. 5(c), giving the relative light extraction enhancement between the nanorods and conventional LED devices. Via the calculation of the ratio between the steady-state light outputs of the conventional (black line) and nanorods (red line) LEDs, around 22% output power enhancement can be obtained in the LED devices Fig. 5. Simulation restuls. (a) and (b) are the calculated electric field distributions of conventional and nanorods LEDs, respectively. (c) The comparison of the normalized light output betwee the conventional and nanorods LEDs. (d) and (e) show the intensity mappings of the light projection in the far-field region of the conventional and nanorods LEDs, respectively. with the existence of the InGaN/GaN nanorods, corresponding to the above L I V measurements in Fig. 4. Furthermore, such a light extraction enhancement can also be observed in the calculated far-field mapping as shown in Fig. 5(d) and (e). Regarding the conventional LED structure, a flat surface on the top of the chips, emitted light will diverge in the far-field region, meaning that less optical energy per unit area can be acquired. With the nanorods constructed in the LED devices, light will be guided and propagated to the farther region with an enhancement of 26.9%. To investigate the light extraction performance in the hybrid nanorods LEDs at different nanorods aspect ratios, LED

5 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 4, JULY/AUGUST 2015 REFERENCES Fig. 6. The intensity current voltage (L I V) characteristics of the hybrid nanorods LEDs with nanorods of 540 nm (pink), 840 nm (green) and 1.2 μm (blue) in height. The diameter of the constituent nanorods is fixed at 450 nm in these three nanorods LEDs. devices with nanorods of 540 nm, 840 nm and 1.2 μmwereprepared with the same diameter of 450 nm. Different heights of the nanorods can be fabricated by manipulating the etching time in the ICP-RIE process. The L I V characteristics of these three nanorods LEDs are shown in Fig. 6. At the standard driving current of 20 ma, the forward voltages are approximately the same in these three LED chips. As long as the height of the etched nanorods do not exceed to the n-gan layer, the electrical property would not be damaged and the I V characteristics would exhibit an electric stability in these three kinds of nanorods LEDs. Regarding the light output power, the high aspect-ratio nanorods offer a larger sidewall area, increasing more opportunities for light escape at the GaN/air interface. Thus, high aspect-ratio nanorods at the height of 1.2 μm can provide the light-emitting devices with a better performance at high operating current, giving the feasible applications especially in the large power chip with the working power above 1 W. IV. CONCLUSION High light extraction GaN-based LEDs with a hybrid structure of high aspect-ratio nanorods in microholes arrays have been successfully fabricated, showing an enhanced light output power of mw at an operating current density of 25.4 A/cm 2 from the nanorods LED devices. Not only the constituent microholes but also the well-arranged nano-rods can further increase the LEE in the nanorods LEDs. The high aspect-ratio straight nanorods function as light guiding pillars, providing strong light extraction in a large area. Furthermore, the higher the nanorods in the LED chips, the better light extraction performance can be obtained at high injections currents without causing an electrical damage, giving the feasible applications especially in the large power chip with the working power above 1 W. The novel nanorods light-emitting devices can be fabricated by utilizing the NIL and the standard photolithography, giving the opportunities in reliable mass production in future lighting industry. [1] T. Mukai, M. Yamada, and S. Nakamura Characteristics of InGaN-based UV/blue/green/amber/red light-emitting diodes, Jpn. J. Appl. Phys., vol. 38, pp , [2] M. Koike, N. Shibata, H. Kato, and Y. Takahashi, Development of high efficiency GaN-based multiquantum-well light-emitting diodes and their applications, IEEE J. Sel. Topics Quantum Electron., vol. 8, no. 2, pp , Mar./Apr [3] M. R. Krames et al., Status and future of high-power light-emitting diodes for solid-state lighting, J. Display Technol., vol. 3, no. 2, pp , Jun [4] E. F. Schubert,Light-Emitting Diodes. Cambridge, U.K.: Cambridge Univ. Press, [5] M. H. Crawford, Leds for solid-state lighting performance challenges and recent advances, IEEE J. Sel. Top. Quantum Electron., vol. 15,no.4, pp , Jul./Aug [6] N. Tansu et al., III-nitride photonics, IEEE Photon. J., vol. 2, no. 2, pp , Apr [7] D. F. Feezell, J. S. Speck, S. P. DenBaars, and S. Nakamura, Semipolar (20 2 1) ingan/gan light-emitting diodes for high-efficiency solid-state lighting, J. Display Technol., vol. 9, pp , [8] R. A. Arif, Y. K. Ee, and N. Tansu, Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes, Appl. Phys. Lett., vol. 91, art. no , [9] H. Zhao et al. Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells, Opt. Exp., vol. 19, pp. A991 A1007, [10] M. V. Bogdanov, Effect of ITO spreading layer on performance of blue light-emitting diodes, Phys. Status Solidi, vol. 8, pp , [11] Z. L. Li, P. T. PLai, and H. W. Choi, A reliability study on green ingan gan light-emitting diodes, IEEE Photon. Technol. Lett., vol. 21, no. 19, pp , Oct [12] M. Y. Broditsk and E. Yablonovitch, Light-emitting diode extraction efficiency, Proc. SPIE, vol. 3002, art. no. 119, [13] C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-gan surface, J. Appl. Phys., vol. 93, art. no. 9385, [14] H. W. Huang et al., Improvement of InGaN GaN light-emitting diode performance with a nano-roughened p-gan surface, IEEE Photon. Technol. Lett., vol. 17, no. 5, p , May [15] J. J. Wierer, A. David, and M. M. Megens, III-nitride photonic-crystal light-emitting diodes with high extraction efficiency, Nat. Photon., vol. 3, pp , [16] J. Jewell et al., Double embedded photonic crystals for extraction of guided light in light-emitting diodes, Appl. Phys. Lett., vol. 100, art. no , [17] X. H. Li et al., Light extraction efficiency and radiation patterns of IIInitride light-emitting diodes with colloidal microlens arrays with various aspect ratios, IEEE Photon. J., vol. 3, no. 3, pp , Jun [18] P. Zhu, G. Liu, J. Zhang, and N. Tansu, FDTD analysis on extraction efficiency of GaN light-emitting diodes with microsphere arrays, J. Display Technol., vol. 9, pp , [19] Y. K. Ee et al. Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures, Opt. Exp., vol. 17, pp , [20] W. H. Koo et al., Light extraction of organic light emitting diodes by defective hexagonal-close-packed array, Adv. Funct. Mater., vol. 22, pp , [21] C. H. Chiu et al., High efficiency GaN-based light-emitting diodes with embedded air voids/sio 2 nanomasks, Nanotechnology, vol. 23, art. no , [22] E. Matioli et al., High extraction efficiency light-emitting diodes based on embedded air-gap photonic-crystals, Appl. Phys. Lett., vol. 96, art. no , [23] T. H. Hsueh et al., Enhancement in light output of InGaN-Based microhole array light-emitting diodes, IEEE Photon. Technol. Lett., vol.17, no. 6, pp , Jun [24] F. I. Lai et al., Extraction efficiency enhancement of GaN-based lightemitting diodes by microhole array and roughened surface oxide, IEEE Electron Dev. Lett., vol. 30, no. 5, pp , May [25] H. M. Kim et al., High-brightness light emitting diodes using dislocationfree Indium Gallium Nitride/Gallium Nitride multiquantum-well nanorod arrays, Nano Lett., vol. 4, art. no. 1059, 2004.

6 HUANG et al.: ENHANCED LIGHT EXTRACTION EFFICIENCY OF GaN-BASED HYBRID NANORODS LIGHT-EMITTING DIODES [26] S. Keller et al., Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN/GaN multi-quantum wells, J. Appl. Phys., vol. 100, art. no , [27] Y. J. Lee et al., High output power density from GaN-based twodimensional nanorod light-emitting diode arrays, Appl. Phys. Lett., vol. 94, pp , [28] J. Zhu et al., The fabrication of GaN-based nanopillar light-emitting diodes, J. Appl. Phys., vol. 108, art. no , [29] D. W. Jeon et al., Nanopillar InGaN/GaN light emitting diodes integrated with homogeneous multilayer graphene electrodes, J. Mater. Chem., vol. 21, pp , [30] S. H. Kim et al., An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes, Opt. Exp., vol. 21, pp , [31] H. W. Huang et al., Enhanced light output from a nitride-based power chip of green light-emitting diodes with nano-rough surface using nanoimprint lithography, Nanotechnology, vol. 19, art. no , [32] M. A. Tsai et al., Efficiency enhancement and beam shaping of GaN InGaN vertical-injection light-emitting diodes via high-aspect-ratio nanorod arrays, IEEE Photon. Technol. Lett., vol. 21, no. 4, pp , Feb [33] O. Madelung, M. Schultz, and H. Weiss. Physics of Group IV Elements and III-V Compounds. Landolt-Börnstein New Series, Group III 17, no. Pt A. Berlin, Germany: Springer, [34] S. Strite and H. Morkoç, GaN, AlN, and InN: A review, J. Vac. Sci. Technol. B, vol, 10, pp , Hung-Wen Huang received the M.S. and Ph.D. degrees in electrooptical engineering from National Chiao Tung University, Hsinchu, Taiwan, in 2003 and 2007, respectively. In 2009, he joined the Research and Design Division in TSMC Solid State Lighting Corporation. His current research interests include III-nitride semiconductor light-emitting diodes and applications. Fang-I Lai received the M.S. and the Ph.D. degrees from the Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu City, Taiwan, in 2001 and Since 2007, she has been with Yuan Ze University as a Faculty Member of the Department of Electrical Engineering. Her current research interests include vertical cavity surface emitting lasers, blue and UV lasers and LEDs, quantum confined optoelectronic structures, and nanostructure applications. Jhih-Kai Huang received the B.S. degree from the National Central University, Taoyuan, Taiwan, in 2007, and the M.S. degree in Electro-Optical Engineering from National Chiao Tung University, Hsinchu, Taiwan, in He is currently working toward the Ph.D. degree in the Institute of Electro- Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan. His research interests include GaN-based device fabrication, nanoimprint technology, and nanostructure processes. Che-Yu Liu received the B.S. degree in electrical engineering from National Central University, Taoyuan, Taiwan, in 2010, and the M.S. degree in photonics from National Chiao Tung University, Hsinchu, Taiwan, in 2012, respectively. He is currently working toward the Ph.D. degree in the Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan. His research interests include the epitaxy of III V compound semiconductor materials by MOCVD and analysis for GaN-based light-emitting diodes. Po-Tsung Lee (M 06) received the B.S. degree in physics from National Taiwan University, Taipei, Taiwan, in 1997, and the M.S. and Ph.D. degrees from the University of Southern California, Los Angeles, CA, USA, in 1998, and 2003, respectively. During the Ph.D. study, she was engaged in photonic crystal microcavity lasers. In 2003, she joined the Institute of Electro- Optical Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan, as an Assistant Professor. In 2007, she became an Associate Professor in the Department of Photonics, NCTU. Her recent research interests include semiconductor photonic crystal active and passive devices and their applications, metallic nanostructures with localized surface plasmon resonances, and siliconbased solar cell technologies. She received the University of Southern California Women in Science and Engineering Award in and the Outstanding Young Electrical Engineer Award from the Chinese Institute of Electrical Engineering in Tzi-Pei Chen received the B.S. degree in physics from National Chung Hsing University, Taichung, Taiwan, in Her research areas include photoluminescence measurement and materials analysis, optical simulation, and characterization for high-power light-emitted diodes. Chung-Hsiang Lin received the B.S. and M.S. degrees in physics from National Taiwan University, Taipei City, Taiwan. He received the M.S. degree in electrical and computer engineering and the Ph.D. degree in physics from Polytechnic Institute of New York University, Brooklyn, NY, USA. He is the President of the New Business Unit of Luxtaltek Corporation, Miaoli, Taiwan, and serves as an Adjunct Professor at the Institute of Electro-Optical Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan. He has more than ten years of experience in the LED industry, specifically photonic crystal modeling and nanofabrication on optoelectronic devices. He has more than 30 professional publications related to photonic crystal devices. Prior to joining Luxtaltek, he held several research positions including one as a Visiting Scholar with the Jet Propulsion Laboratory.

7 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 4, JULY/AUGUST 2015 Chun-Yen Chang was born in Feng-Shan, Taiwan. He received the B.S. degree in electrical engineering from the National Cheng Kung University (NCKU), Tainan, Taiwan, in 1960, and the M.S. and Ph.D. degrees from the National Chiao Tung University (NCTU), Hsinchu, Taiwan, in 1962 and 1969, respectively. He has profoundly contributed to the areas of microelectronics, microwave, and optoelectronics, including the invention of the method of low-pressure MOCVD using triethylgallium to fabricate LED, laser, and microwave devices. He pioneered works on Zn incorporation (1968), nitridation (1984), and fluorine incorporation (1984) in SiO2 for ULSIs, as well as in the charge transfer in semiconductor oxide semiconductor systems (1968), carrier transport across metal semiconductor barriers (1970), and the theory of metal semiconductor contact resistivity (1971). In 1963, he joined the NCTU to serve as an Instructor, establishing a high vacuum laboratory. In 1964, he and his colleagues established the nation s first and state-of-the-art Semiconductor Research Center, NCTU, with a facility for silicon planar device processing, where they made the nation s first Si planar transistor in April 1965 and, subsequently, the first IC and MOSFET in August 1966, which strongly forms the foundation of Taiwan s hi-tech development. From 1977 to 1987, he single-handedly established a strong electrical engineering and computer science program at the NCKU, where GaAs, α-si, and poly-si research was established in Taiwan for the first time. He consecutively served as the Dean of Research ( ), the Dean of Engineering ( ), and the Dean of Electrical Engineering and Computer Science ( ). Simultaneously, from 1990 to 1997, he served as the Founding President of the National Nano Device Laboratories, Hsinchu. Since August 1, 1998, he has been the President with the Institute of Electronics, NCTU. In 2002, to establish a strong system design capability, he initiated the National program of system on chip, which is based on a strong Taiwanese semiconductor foundry. He is a Member of Academia Sinica (1996) and a Foreign Associate of the National Academy of Engineering, U.S. (2000). He received the 1987 IEEE Fellow Award, the 2000 Third Millennium Medal, and the 2007 Nikkei Asia Prize for Science category in Japan and regarded as the father of Taiwan semiconductor industries. Tsung Sheng Kao received the B.S. degree in physics from National Central University, Taoyuan, Taiwan in 2001, the M.S. degree in physics from National Taiwan University, Taipei City, Taiwan in 2004, and the Ph.D. degree from the Optoelectronics Research Centre, University of Southampton, Southampton, U. K., in From 2013 to 2014, he was a Postdoctoral Research Fellow with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore. Since early 2014, he has been an Assistant Research Fellow with the Department of Photonics and the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan. His primary research interests include nanooptics, superresolution imaging technology, controlled light localization on the nanolandscapes, nonlinear optical responses of hybrid nanosystems, light enhancements in plasmonic optoelectronic devices, and surface plasmon resonance for bio-sensing. Hao-Chung Kuo (S 98 M 99 SM 06) received the B.S. degree in physics from the National Taiwan University, Taipei, Taiwan, in 1990, the M.S. degree in electrical and computer engineering from Rutgers University, Camden, NJ, USA, in 1995, and the Ph.D. degree in electrical and computer engineering from the University of Illinois at Urbana-Champaign, Urbana, IL, USA, in He has an extensive professional career both in research and industrial research institutions. From 1995 to 1997, he was a Research Consultant with Lucent Technologies, Bell Labs, Holmdel, NJ. From 1999 to 2001, he was an R&D Engineer with the Fiber-Optics Division, Agilent Technologies. From 2001 to 2002, he was the R&D Manager with LuxNet Corporation. Since September 2002, he has been a Member of the faculty at the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan. He has authored or coauthored more than 60 publications. His current research interests include the epitaxy, design, fabrication, and measurement of high-speed InPand GaAs-based vertical-cavity surface-emitting lasers, as well as GaN-based lighting-emitting devices and nanostructures.