IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 6, JUNE 2010
573
Light-Output-Power Enhancement of GaN-Based Light-Emitting Diodes on an n-GaN Layer Using a SiO2 Photonic Quasi-Crystal Overgrowth H. W. Huang, J. K. Huang, K. Y. Lee, C. F. Lin, and H. C. Kuo
Abstract—GaN-based LEDs with a SiO2 oxide PQC pattern on an n-GaN layer by nanoimprint lithography are fabricated and investigated. At a driving current of 20 mA on a TransistorOutline-can package, the light output power of LED III (d = 1.2 µm) was enhanced by a factor of 1.20. The internal-quantumefficiency result offers promising potential to enhance the light output power of commercial light-emitting devices with a SiO2 oxide PQC structure on an n-GaN layer. Index Terms—Gallium nitride (GaN), light-emitting diodes (LEDs), photonic quasi-crystal (PQC).
I. I NTRODUCTION
I
MPRESSIVE recent developments of the high brightness light extraction of GaN-based nitride light-emitting diodes (LEDs) are dominated by both material techniques, such as metal–organic chemical vapor deposition (MOCVD) epitaxial growth, and device fabrication processes. Thus, high-brightness LEDs have been used in various applications, including backlight large- and small-size flat-panel displays, traffic signal lights, and illumination lighting by white-light LEDs [1], [2]. In order to get higher brightness of LEDs, extensive research has been conducted. In the epitaxial growth method, a number of attempts have been made to reduce the dislocation effect using such strategies as the insertion of a microscale epitaxial lateral overgrowth (ELOG) layer over a SiO2 or Six Ny pattern on the GaN thin film [3], [4] as well as the use of a microscale patterned sapphire substrate (PSSs) [5], [6]. Moreover, high-quality GaN-based LEDs have been demonstrated on a microscale PSS by wet or dry etching [5], [6], where the microscale patterns serve as a template for the ELOG of GaN and scattering centers for the guided light. Both the epitaxial crystal quality and the light extraction efficiency were improved by utilizing a microscale PSS. Recently, the MOCVD growth of GaN-based LEDs on PSSs with micro- and nanoscale pyramidal patterns has been reported and compared [7], [8]. The LEDs grown on the nanoscale PSS showed more enhancements in external quantum efficiency (EQE) than those grown on miManuscript received January 21, 2010. Date of publication April 15, 2010; date of current version May 26, 2010. The review of this letter was arranged by Editor P. K.-L. Yu. H. W. Huang, J. K. Huang, K. Y. Lee, and H. C. Kuo are with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail:
[email protected]). C. F. Lin is with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung 400, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2010.2045218
croscale PSS. Furthermore, photonic crystallization is a promising technique due to the great improvement of light extraction efficiency [9]–[11]. Photonic crystal (PhC) structures were usually fabricated by e-beam lithography [12], holographic lithography [13], and nanoimprint lithography [11], [14]. However, the drawback of the e-beam and holographic lithography techniques is low throughput. Therefore, a high-throughput and economic technique, namely, nanoimprint lithography, was recommended to manufacture light-emitting devices with PhC or photonic quasi-crystal (PQC) structures [14]. In this letter, we report the nanoimprinting and epitaxial overgrowth techniques to fabricate GaN-based LEDs using a SiO2 PQC nanopatterned overgrowth on an n-GaN layer for 2-in mass production. As a result, the intensity–current (L–I) measurements demonstrate that the light output power of a LED with a SiO2 PQC overgrowth on an n-GaN layer was higher than that of a conventional LED at 20 mA with standard device processing. II. E XPERIMENTS All GaN-based LED samples are grown by MOCVD with a rotating-disk reactor (Veeco) on a c-axis sapphire (0001) substrate at a growth pressure of 200 mbar. The LED structure with a 100-nm-thick SiO2 PQC patterned on an n-GaN layer structure fabricated with a sapphire substrate consists of a 50-nm-thick GaN nucleation layer grown at 500 ◦ C, a 2-µm undoped GaN buffer, and a 1.0-µm-thick Si-doped GaN buffer layer grown at 1050 ◦ C. The details of the nanoimprint process are described in [14]. Then, the LED structures with a SiO2 PQC overgrowth on an n-GaN layer were designed and regrown on a 2-in wafer. The regrown LED structures consist of a thickness d from 0.3 to 1.2 µm for an n-type Si-doped GaN layer grown at 1050 ◦ C and an unintentionally doped InGaN/GaN multiple-quantum-well (MQW) active region grown at 770 ◦ C. The MQW active region consists of five periods of 3 nm/ 7 nm-thick In0.18 Ga0.82 N/GaN quantum-well and barrier layers, a 50-nm-thick Mg-doped p-AlGaN electron blocking layer grown at 1050 ◦ C, and a 220-nm-thick Mg-doped p-GaN contact layer grown at 1050 ◦ C, which were defined as the SiO2 PQC overgrowth on an n-GaN layer, as shown in Fig. 1. The conventional LED structure consists of a 50-nm-thick GaN nucleation layer grown at 500 ◦ C, a 2-µm undoped GaN buffer, a 1.5-µm-thick Si-doped GaN buffer layer grown at 1050 ◦ C, and afterward, the same epitaxial structure as that of the LEDs with a SiO2 PQC overgrowth on an n-GaN layer.
0741-3106/$26.00 © 2010 IEEE
574
IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 6, JUNE 2010
Fig. 1. Schematic diagram of GaN-based LEDs with a SiO2 oxide PQC pattern on an n-GaN layer. Fig. 3. (a) Current–voltage (I–V ) characteristics of conventional LED, LED I (d = 0.3 µm), LED II (d = 0.6 µm), and LED III (d = 1.2 µm), respectively (the inset data were the measured EL intensity–current). (b) Light-outputpower–current (L–I) and WPE characteristics of LED with/without a SiO2 oxide PQC structure on an n-GaN layer, respectively.
gular lattice and eightfold PQC [16]. The recursive tiling of offspring dodecagons packed with random ensembles of squares and triangles in dilated parent cells forms the lattice. Additionally, in this case, the PQC rod dimension (D) and pattern pitch (a) we used were approximately 435 and 750 nm, respectively, and the rod diameter was fixed to a ratio D/a = 0.58. III. R ESULTS
Fig. 2. (a) OM of a LED die with SiO2 oxide PQC pattern on n-GaN layer and the tilted plane-view OM image between the ITO and the n-GaN region. (b) Cross-sectional SEM image of LED with SiO2 oxide PQC pattern on n-GaN. (c) Top-view AFM image of SiO2 oxide PQC pattern on n-GaN.
All LED samples are fabricated using the following standard processes with a mesa area of 265 µm × 265 µm. The left-side image of Fig. 2(a) is an optical micrograph (OM) of a LED die with a SiO2 oxide PQC pattern on an n-GaN layer (with a LED chip area of 300 µm × 300 µm). The right-side image of Fig. 2(a) is the top-view OM image between the ITO transparent contact layer and the n-side surface regions. In the rightside image of Fig. 2(a), the chip surface of the GaN-based LED with a SiO2 oxide PQC pattern was distinct between the ITO and the n-GaN region. The cross-sectional scanning-electronmicroscopy (SEM) photograph of the LED with a SiO2 oxide PQC pattern on an n-GaN layer is shown in Fig. 2(b). The SEM image in Fig. 2(b) shows that the thickness of the SiO2 layer was approximately 100 nm and that the height of the etching depth of the mesa is approximately 0.94 µm. Fig. 2(c) shows a top-view image of a 12-fold PQC based on a square-triangular lattice. The “PQC” is unusual in the respect that, on first sight, it appears random; however, a closer inspection reveals it to possess a long-range order but short-range disorder [15]. We choose the 12-fold PQC pattern because the enhancement of surface emission was obtained with a PhC with a dodecagonal symmetric quasi-crystal lattice than a regular PhC with a trian-
Fig. 3(a) shows a typical current–voltage (I–V ) characteristic. It is found that the measured forward voltages under an injection current of 20 mA at room temperature for conventional LED, LED I (d = 0.3 µm), LED II (d = 0.6 µm), and LED III (d = 1.2 µm) are 3.11, 3.14, 3.10, and 3.09 V, respectively. Therefore, there is no influence on this type of devices by incorporating the PQC structure. The light output is detected by calibrating an integrating sphere with a Si photodiode on the package device, so that light emitted in all directions from the LED can be collected. The intensity–current (L–I) characteristics of the LEDs with and without a PQC structure are shown in Fig. 3(b). At an injection current of 20 mA and peak wavelength of 460 nm for a Transistor-Outline (TO)can package, the light output powers of conventional LED, LED I (d = 0.3 µm), LED II (d = 0.6 µm), and LED III (d = 1.2 µm) on a TO-can are given as 11.6, 13.0, 13.5, and 13.9 mW, respectively. Hence, the enhancement percentages of LEDs I (d = 0.3 µm), II (d = 0.6 µm), and III (d = 1.2 µm) are 12%, 16%, and 20%, respectively, compared with that of conventional LED. The higher enhancements on LED III (d = 1.2 µm) were the reflected light from the oxide PQC layer onto the top direction and higher epitaxial crystal quality [17] to increase light output power. In addition, the corresponding wall-plug efficiencies (WPEs) of conventional LED, LED I (d = 0.3 µm), LED II (d = 0.6 µm), and LED III (d = 1.2 µm) are 19%, 21%, 22%, and 23%, respectively, which address a substantial improvement by the SiO2 oxide PQC pattern on an n-GaN layer as well as at a driving current of 20 mA. Fig. 4 shows the temperature-dependent photoluminescence (PL) used to determine the internal quantum efficiency (IQE) of the InGaN/GaN MQW structures. At low temperatures, nonradiative recombination is close to minimum and radiative recombination processes are dominant. With increasing temperature, nonradiative recombination processes get activated and play a
HUANG et al.: LIGHT-OUTPUT-POWER ENHANCEMENT OF GaN-BASED LEDs
Fig. 4. Arrhenius plots of integrated PL intensities for GaN-based LED with/without a SiO2 oxide PQC structure on the n-GaN layer.
key role at room temperature [18]. The IQE of LED samples with and without a SiO2 oxide PQC pattern on an n-GaN layer were calculated from IQE = PL300 K /PL90 K , where PL300 K is the PL intensity at room temperature and PL90 K is the PL intensity at the lowest temperature in our system. Assuming that the IQE equals unity at 90 K, we obtain the IQEs of LED I (d = 0.3 µm), LED II (d = 0.6 µm), and LED III (d = 1.2 µm) as 35%, 40%, and 41%, which are higher than that of the conventional LED (34%) at room temperature. The integrated intensity of the PL peak at 462 nm of MQW with a SiO2 PQC [LED III (d = 1.2 µm)] was increased by 59%, compared with that of an MQW without a SiO2 PQC at room temperature. The increase in IQE with a SiO2 PQC can be attributed to the reduction of defects, for example, screwand edge-type threading dislocations in the n-GaN and MQW layers. Based on the observed increases of 59% for EQE and 20% for IQE, the increase in light extraction efficiency was estimated to be 33% for the LED with a SiO2 PQC. The increase of 33% is attributed to the enhanced light extraction from the LED by the SiO2 PQC structure [19]. IV. S UMMARY GaN-based LEDs with a SiO2 oxide PQC pattern on an n-GaN layer by nanoimprint lithography have been fabricated and investigated. At a driving current of 20 mA on a TO-can package, the light output powers of LEDs I (d = 0.3 µm), II (d = 0.6 µm), and III (d = 1.2 µm) were enhanced by factors of 1.12, 1.16, and 1.20. The WPE of InGaN/GaN LEDs is increased by 10%–21% by the SiO2 oxide PQC pattern on the n-GaN layer. This work offers promising potential to improve IQE performance and increase the light output powers of commercial light-emitting devices. R EFERENCES [1] 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. 271–277, Mar./Apr. 2002. [2] E. F. Schubert, Light-Emitting Diodes. Cambridge, U.K.: Cambridge Univ. Press, 2003.
575
[3] K. Iida, T. Kawashima, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, and A. Bandoh, “Epitaxial lateral overgrowth of Alx Ga1−x N (x > 0.2) on sapphire and its application to UV-B-light-emitting devices,” J. Cryst. Growth, vol. 298, pp. 265–267, Jan. 2007. [4] K. Hoshino, T. Murata, M. Araki, and K. Tadatomo, “Fabrication of GaNbased light emitting diodes using direct heteroepitaxial lateral overgrowth on patterned sapphire substrates with thick SiO2 masks,” Phys. Stat. Sol. (C), vol. 5, no. 9, pp. 3060–3062, Jul. 2008. [5] D. S. Wuu, W. K. Wang, K. S. Wen, S. C. Huang, S. H. Lin, R. H. Horng, Y. S. Yu, and M. H. Pan, “Fabrication of pyramidal patterned sapphire substrates for high-efficiency InGaN-based light emitting diodes,” J. Electrochem. Soc., vol. 153, no. 8, pp. G765–G770, Jun. 2006. [6] T. S. Oh, S. H. Kim, T. K. Kim, Y. S. Lee, H. Jeong, G. M. Yang, and E. K. Suh, “GaN-based light-emitting diodes on micro-lens patterned sapphire substrate,” Jpn. J. Appl. Phys., vol. 47, no. 7, pp. 5333–5336, Jul. 2008. [7] Y. K. Su, J. J. Chen, C. L. Lin, S. M. Chen, W. L. Li, and C. C. Kao, “GaNbased light-emitting diodes grown on photonic crystal-patterned sapphire substrates by nanosphere lithography,” Jpn. J. Appl. Phys., vol. 47, no. 8, pp. 6706–6708, Aug. 2008. [8] H. Gao, F. Yan, Y. Zhang, J. Li, Y. Zeng, and G. Wang, “Enhancement of the light output power of InGaN/GaN light-emitting diodes grown on pyramidal patterned sapphire substrates in the micro- and nanoscale,” J. Appl. Phys., vol. 103, no. 1, pp. 014 314-1–014 314-5, Jan. 2008. [9] J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J. R. Wendt, J. A. Simmons, and M. M. Sigalas, “InGaN/GaN quantumwell heterostructure light-emitting diodes employing photonic crystal structures,” Appl. Phys. Lett., vol. 84, no. 19, pp. 3885–3887, May 2004. [10] A. David, T. Fujii, R. Sharma, K. McGroddy, S. Nakamura, S. P. DenBaars, E. L. Hu, and C. Weisbuch, “Photonic-crystal GaN lightemitting diodes with tailored guided modes distribution,” Appl. Phys. Lett., vol. 88, no. 6, pp. 061 124-1–061 124-3, 2006. [11] K. J. Byeon, S. Y. Hwang, and H. Lee, “Fabrication of two-dimensional photonic crystal patterns on GaN-based light-emitting diodes using thermally curable monomer-based nanoimprint lithography,” Appl. Phys. Lett., vol. 91, no. 9, pp. 091 106-1–091 106-3, Aug. 2007. [12] C. H. Lin, H. H. Yen, C. F. Lai, H. W. Huang, C. H. Chao, H. C. Kuo, T. C. Lu, S. C. Wang, and K. M. Leung, “Enhanced vertical extraction efficiency from a thin-film InGaN–GaN light-emitting diode using a 2-D photonic crystal and an omnidirectional reflector,” IEEE Photon. Technol. Lett., vol. 20, no. 10, pp. 836–838, May 2008. [13] D. H. Kim, C. O. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q.-H. Park, “Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns,” Appl. Phys. Lett., vol. 87, no. 20, pp. 203 508-1–203 508-3, Nov. 2005. [14] H. W. Huang, C. H. Lin, K. Y. Lee, C. C. Yu, J. K. Huang, B. D. Lee, H. C. Kuo, K. M. Leung, and S. C. Wang, “Enhanced light output power of GaN-based vertical-injection light-emitting diodes with a 12-fold photonic quasi-crystal by nano-imprint lithography,” Semicond. Sci. Technol., vol. 24, no. 8, pp. 085 008-1–085 008-5, Aug. 2009. [15] M. D. B. Charlton, M. E. Zoorob, and T. Lee, “Photonic quasi-crystal LEDs: Design, modeling, and optimization,” Proc. SPIE, vol. 6486, pp. 648 60R-1–648 60R-10, Mar. 2007. [16] Z. S. Zhang, B. Zhang, J. Xu, K. Xu, Z. J. Yang, Z. X. Qin, T. J. Yu, and D. P. Yu, “Effects of symmetry of GaN-based two-dimensional photonic crystal with quasicrystal lattices on enhancement of surface light extraction,” Appl. Phys. Lett., vol. 88, no. 17, pp. 171 103-1–171 103-3, Apr. 2006. [17] H. Matsubara, S. Yoshimoto, H. Saito, Y. Jianglin, Y. Tanaka, and S. Noda, “GaN photonic-crystal surface-emitting laser at blue-violet wavelengths,” Science, vol. 319, no. 5862, pp. 445–447, Jan. 2008. [18] D. Fuhrmann, U. Rossow, C. Netzel, H. Bremers, G. Ade, P. Hinze, and A. Hangleiter, “Optimizing the internal quantum efficiency of GaInN SQW structures for green light emitters,” Phys. Stat. Sol. (C), vol. 3, no. 6, pp. 1966–1969, 2006. [19] C. H. Chiu, H. H. Yen, C. L. Chao, Z. Y. Li, P. Yu, H. C. Kuo, T. C. Lu, S. C. Wang, K. M. Lau, and S. J. Cheng, “Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO2 nanorod-array patterned sapphire template,” Appl. Phys. Lett., vol. 93, no. 8, pp. 081 1081–081 108-3, Aug. 2008.
