I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, and N. Sawaki, “Effects of AlN buffer layer on crystallographic structure and on electrical and optical-properties of GaN and Ga1-xAlxN films grown on sapphire substrate by MOVPE,” Journal of Crystal Growth, Vol. 98, pp. 209, 1989.
 S Nakamura, “GaN growth using GaN buffer layer,” Japanese Journal of Applied Physics Part 2-Letters, Vol. 30, pp. L1705, 1991.
 S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High-power InGaN single-quantum-well-structure blue and violet light-emitting-diodes,” Applied Physics Letters, Vol. 67, pp. 1868, 1995.
 E. Fred Schubert, “Light-Emitting Diodes” 2nd. , Cambridge Univ. Press, New York, 2006.
 Katsushi Akita, Takashi Kyono, Yusuke Yoshizumi, Hiroyuki Kitabayashi, and Koji Katayama, “Improvement of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates,” Journal of Applied Physics, Vol. 101, pp. 033104, 2007.
 M.R. Krames, G.E. Höfler, E.I. Chen, I.H. Tan, P. Grillot, N.F. Gardner, H.C. Chui, J.W. Huang, S.A. Stockman, F.A. Kish, and M.G.Craford, “High power truncated inverted pyramid AlxGa12x.0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency,” Applied Physics Letters, Vol. 75, pp. 2365, 1999.
 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 nanoimprinted photonic crystal GaN-based blue light emitting diodes,” Optics Express, Vol. 14, pp. 19, 2006.
 T. Fujii, Y.Gao, R.Sharma, E.L. Hu, S.P. DenBaars, and S. Nakamura, ”Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Applied Physics Letters, Vol. 84, pp. 6, 2004.
 Y.J. Lee, J. M. Hwang, T. C. Hsu, M. H. Hsieh, M.J. Jou, B.J. Lee, T.C. Lu, H.C. Kuo,anc S C. Wang, ”Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates,” IEEE, Vol. 18, pp. 10, 2006.
 O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb,P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN–GaN light-emitting diodes” Applied Physics Letters, vol. 89, pp.071109, 2006.
 H. W. Huang, H. C. Kuo, Senior Member, IEEE, C. F. Lai, C. E. Lee, C. W. Chiu, T. C.Lu, S. C. Wang, and Life Member, “High-Performance GaN-Based Vertical-Injection Light-Emitting Diodes With TiO2–SiO2 Omnidirectional Reflector and n-GaN Roughness,” IEEE Photonics Technology Letters, vol. 19, No.8,
 Dong-Ho Kim, Chi-O Cho, Yeong-Geun Roh, Heonsu Jeon, Yoon Soo Park, Jaehee Cho, Jin Seo Im, Cheolsoo Sone, Yongjo Park, Won Jun Choi and Q-Han Park, “Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns,” Applied Physics Letters, vol. 87, pp. 203508, 2005.
 Shao-Hua Huang, Ray-Hua Horng, Kuo-Sheng Wen, Yi-Feng Lin, Kuo-Wei Yen, and Dong-Sing Wuu, “Improved Light Extraction of Nitride-Based Flip-Chip Light-Emitting Diodes Via Sapphire Shaping and Texturing,” IEEE Photonics Technology Letters, vol. 18, No.10, 2006.
 C. L. Lin, P. H. Chen, Chia-Hua Chan, C. C. Lee, Chii-Chang Chen, Jeng-Yang Chang, and C. Y. Liu, “Light enhancement by the formation of an Al oxide honeycomb nanostructure on the n-GaN surface of thin-GaN light-emitting diodes,” Applied Physics Letters, vol. 90, pp.242106, 2007.
 Tsung-Xian Lee, Chao-Ying Lin, Shih-Hsin Ma and Ching-Cherng
Sun,“Analysis of position-dependent light extraction of GaN-based LEDs,” Optics Express, vol. 13, No. 11, pp.4175, 2005.
 Ching-Cherng Sun, Tsung-Xian Lee, Shih-Hsin Ma, Ya-Luan Lee, and Shih-Ming Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Optics Letters, Vol. 31, No. 14, pp. 2193, 2006.
 Chia-Hsin Chao, S. L. Chuang, and Tzong-Lin Wu, “Theoretical demonstration of enhancement of light extraction of flip-chip GaN light-emitting diodes with photonic crystals,” Applied Physics Letters, vol. 89, pp.091116, 2006.
 C E Lee, Y C Lee, H C Kuo, M R Tsai, T C Lu, and S C Wang,“ High brightness GaN-based flip-chip light-emitting diodes by adopting geometric sapphire shaping structure,” Semiconductor Science and Technology, Vol. 23, pp. 025015, 2008.