本論文旨在利用非等向性蝕刻乾蝕刻製程，製備二階高度形貌於GaN-基藍光垂直結構發光二極體(vertical structure light emitting diode, VLED)之n-GaN表面，藉由降低距離接觸電極(contact pad)較遠區域電流路徑之垂直電阻，降低不同電流路徑之壓降差，提升電流擴散能力，克服傳統VLED電流擁擠效應之缺點。此外，本論文亦提出不同二階高度形貌之n-GaN表面及理想電極圖案設計於研究中，提升垂直結構發光二極體之電流分布及發光效率。
本論文架構主要分為兩部分。第一部分以Crosslight APSYS半導體模擬軟體進行VLED之光電特性模擬，除計算不同深度及不同蝕刻形貌之階梯狀n-GaN層對電流分布之影響，亦加入不同設計之理想電極進行模擬，由模擬中取得最佳電流擴散結果及實際製程可行性。第二部分為實際元件之製程及光電量測。實驗結果顯示，具備二階高度形貌n-GaN表面之VLED皆具有使光強度分布趨於均勻之改善效果。於二階蝕刻深度為200 nm之條件，可獲最高之改善量。實驗結果顯示，操作電流350 mA，選擇大面積二階高度結構之VLED，其光析出效率改善21.2 %，光電轉換效率增加23.4 %；僅電極下方具備二階高度結構之VLED，則擁有最高光析出效率改善33.3 %，光電轉換效率增加36.5 %；理想電極圖案設計搭大面積二階高度結構之VLED其光析出效率改善30.9 %，光電轉換效率增加37.1 %。
本研究使用模擬軟體及實際實驗於理論及製程上皆驗證二階高度結構應用在VLED之效果，利用不同二階高度結構及理想電極圖案設計皆確實能改善電流分佈並提升VLED光輸出效率。

An efficient improving current spreading scheme through the use of two-step etching on n-GaN surface at selected region and n-electrode pattern design is proposed for the fabrication of vertical-structured GaN-based high power light-emitting diodes (VLEDs). The present design allows the patterned n-electrodes having their vertical distances to the active region of VLED decreases with increasing their lateral distances to the contact pad. It could balance the difference in voltage drops on the n-GaN layer as encountered in regular-VLEDs. The feasibility of the proposed scheme was verified theoretically and experimentally. As compared to regular-VLEDs with flat n-GaN structure and fence-shaped electrode, the proposed VLEDs with a two-step etching under patterned n-electrodes with the etching depth of 200 nm shows a typical increase in light output power (Lop) by 33.3% at 350 mA.

[1] M. Fujita, “Silicon photonics: Nanocavity brightens silicon,” Nature Photon., vol. 7, pp. 264-265, 2013.
[2] 郭浩中，賴芳儀，郭守義，“LED原理與應用”，五南圖書出版公司，2009。
[3] A. Zukauskas, “Introduction to Solid-State Lighting,” John Wiley & Sons, New York, 2002.
[4] 史光國，“半導體發光二極體及固體照明”，全華科技圖書股份有限公司，2005。
[5] S. Nakamura, T. Mukai, and M.Senoh, “High brightness InGaN/AlGaN double heterostructure blue green light emitting diodes,” J. Appl. Phys., vol. 76, pp. 8180-8191, 1994.
[6] http://www.itri.org.tw/chi/eol/
[7] M. G. Craford, presented at the Nanoscience and Solid State Lighting Department of Energy Nanosummit, June 2004, Washington, D.C.
[8] Industrial Materials Magazine, no. 236, pp. 150-154, 2006.
[9] McKinsey & Company, “Lighting the way,” First Edition, 2011.
[10] S. Montanari, “III-V compound semiconductor material systems,” 2005.
[11] S. Yoshida, S. Misawa and S. Gonda, “Improvements on the electrical and luminescent properties of reactive molecular beam epitaxially grown GaN films by using AlN-coated sapphire substrates,” Appl. Phys. Lett., vol. 42, pp. 427-429, 1983.
[12] 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 (0 < x ≦ 0.4) films grown on sapphire substrate by MOVPE,” J. Cryst. Growth, vol. 98, pp. 209-219, 1989.
[13] S. Nakamura, “GaN growth using GaN buffer layer,” Jpn. J. Appl. Phys, vol. 30, pp. 1705-1707, 1991.
[14] H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, “P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI),” Jpn. J. Appl. Phys, vol. 28, pp. 2112-2114, 1989.
[15] S. Nakamura, T. Mukai, M. Senoh and N. Iwasa, “Thermal annealing effects on p-type Mg-doped GaN films,” Jpn. J. Appl. Phys, vol. 31, pp. 139-142, 1992.
[16] J. Park, J. K. Oh, K. W. Kwon, Y. H. Kim, S. S. Jo, J. K. Lee, and S. W. Ryu, "Improved light output of photonic crystal light-emitting diode fabricated by anodized aluminum oxide nano-patterns," IEEE Photonics Technol. Lett., vol. 20, pp. 321-323, 2008.
[17] G. Verzellesi, D. Saguatti, M. Meneghini, F. Bertazzi, M. Goano, G. Meneghesso, and E. Zanoni, “Efficiency droop in InGaN/GaN blue light-emitting diodes:Physical mechanisms and remedies,” J. Appl. Phys., vol. 114, pp. 071101-1-071101-14, 2013.
[18] J. J. Chen, Y. K. Su, C. L. Lin, S. M. Chen, W. L. Li and C. C. Kao, "Enhanced output power of GaN-based LEDs with nano-patterned sapphire substrates," IEEE Photonics Technol. Lett., vol. 20, pp. 1193-1195, 2005.
[19] Y. K. Su, J. J. Chen, C. L. Lin, S. M. Chen, W. L. Li and C. C. Kao, "GaN-based light-emitting diodes grown on photonic crystal-patterned sapphire substrates by nano-sphere lithography," Jpn. J. Appl. Phys., vol. 47, no. 8, pp. 6706-6708, 2008.
[20] C. H. Chan, C. H. Hou, S. Z. Tseng, T. J. Chen, H. T. Chien, F. L. Hsiao, C. C. Lee, Y. L. Tsai and C. C. Chen, "Improved output power of GaN-based light-emitting diodes grown on a nano-patterned sapphire substrate," App. Phys. Lett., vol. 95, pp. 011110-011112, 2009.
[21] J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, G. Christenson,Y.-C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Götz, N. F. Gardner, R. S. Kern and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” App. Phys. Lett., vol. 78, no. 22, pp. 3379-3381, 2001.
[22] 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," App. Phys. Lett., vol. 87, pp. 203508-203510, 2005.
[23] H. K. Cho, 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 Yong-Hee Lee, "Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes," Opt. Express, vol. 14, no. 19, pp. 8654-8660, 2006.
[24] H. K. Cho, S. K. Kim, D. K. Bae, B. C. Kang, J. S. Lee, and Y. H. Lee, "Laser liftoff GaN thin-film photonic crystal GaN-based light-emitting diodes," IEEE Photonics Technol. Lett., vol. 20, pp. 2096-2098, 2008.
[25] R. Windish, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G. H. Döhler, B. Dutta, M. Kuijk, G. Borghs and P. Heremans, "Impact of texture-enhanced transmission of high-efficiency surface-textured light-emitting diodes," App. Phys. Lett., vol. 79, pp. 2315-2317, 2001.
[26] H. W. Hunag, J. T. Chu, C. C. Kao, T. H. Hseuh, T. C. Lu, H. C. Kuo, S. C. Wang and C. C. Yu, "Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface," Nanotechnology, vol. 16, pp. 1844-1849, 2005.
[27] S. J. Chang, C. F. Shen, W. S. Chen, C. T. Kuo, T. K. Ko, S. C. Shei and J. K. Sheu, "Nitride-based light emitting diodes with indium tin oxide electrode patterned by imprint lithography," App. Phys. Lett., vol. 91, pp. 013504-013506, 2007.
[28] W. C. Lee, K. M. Uang, D. M. Kuo, J. C. Chou, T. M. Chen, H. Y. Kuo, and S. J. Wang, "Use of highly reflective ohmic contact and surface KrF laser roughening to improve light output of vertical GaN-based light-emitting diodes," IEEE Photonics Technol. Lett., pp. 141-142, 2008.
[29] W. C. Lee, S. J. Wang, K. M. Uang, T. M. Chen, D. M. Kuo, P. R. Wang and P. H. Wang, "Enhanced light output of GaN-based vertical-structured light-emitting diodes with two-step surface roughening using KrF laser and chemical wet etching," IEEE Photonics Technol. Lett., vol. 22, pp. 1318-1320, 2010.
[30] C. Y. Cho, N. Y. Kim, J. W. Kang, Y. C. Leem, S. H. Hong, W. Lim, S. T. Kim and S. J. Park, "Improved extraction efficiency in blue light-emitting diodes by SiO2-coated ZnO nanorod arrays," Appl. Phys. Express., vol. 6, no. 4, pp. 042102-1-042102-3, 2013.
[31] J. Piprek, “Efficiency droop in nitride-based light-emitting diodes,” Phys. Status Solidi A, vol. 207, No. 10, pp. 2217-2225, 2010.
[32] H. Kim, S. J. Park, and H. Hwang, “Design and fabrication of highly efficient GaN-based light-emitting diodes,” IEEE Trans. Electron Devices, vol. 49, no. 10, pp. 1715-1722, 2002.
[33] X.A. Cao, E.B. Stokes, P. Sandvik, N. Taskar, J. Kretchmer and D. Walker, “Optimization of current spreading metal layer for GaN/InGaN-based light emitting diodes,” Solid-State Electron., vol. 46 pp, 1235-1239, 2002.
[34] C. Huh, J. M. Lee, D. J. Kim, and S. J. Park, “Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer,” J. Appl. Phys., vol. 92, no. 5, pp, 2248-2250, 2002.
[35] A. Ebong, S. Arthur, E. Downey, X.A. Cao, S. LeBoeuf and D.W. Merfeld, “Device and circuit modeling of GaN/InGaN light emitting diodes (LEDs) for optimum current spreading,” Solid-State Electron., vol. 47, pp. 1817-1823, 2003.
[36] J. T. Chu, C. C. Kao, H. W. Huang, W. D. Liang, C. F. Chu,T. C. Lu, H. C. Kuo and S. C. Wang, “Effects of different n-electrode patterns on optical characteristics of large-area p-side-down InGaN light-emitting diodes fabricated by laser lift-off,” Jpn. J. Appl. Phys, vol. 44, no. 11, pp. 7910-7912, 2005.
[37] T. M. Chen, S. J. Wang, K. M. Uang, S. L. Chen, W. C. Tsai, W. C. Lee, and C. C. Tsai, “Use of anisotropic laser etching to the top n-GaN layer to alleviate current-crowding effect in vertical-structured GaN-based light-emitting diodes,” App. Phys. Lett., vol. 90, pp. 041115-1-041115-3, 2007.
[38] T. M. Chen, S. J. Wang, K. M. Uang, H. Y. Kuo, C. C. Tsai, W. C. Lee, and H. Kuan, “Current spreading and blocking designs for improving light output power from the vertical-structured GaN-based light-emitting diodes,” IEEE Photonics Technol. Lett., vol. 20, no. 9, pp. 703-705, 2008.
[39] Y. B. Tao, S. Y. Wang, Z. Z. Chen, Z. Gong, E. Y. Xie, Y. J. Chen, Y. F. Zhang, J. McKendry, D. Massoubre, E. D. Gu, B. R. Rae, R. K. Henderson, and G. Y. Zhang, “Size effect on efficiency droop of blue light emitting diode,” Phys. Status Solidi C, vol. 9, No. 3-4, pp. 616-619, 2012.
[40] N. L. Ploch, H. Rodriguez, C. Stölmacker, M. Hoppe, M. Lapeyrade, J. Stellmach, F. Mehnke, T. Wernicke, A. Knauer, V. Kueller, M. Weyers, S. Einfeldt, and M. Kneissl, “Effective thermal management in ultraviolet light-emitting diodes with micro-LED arrays,” IEEE Trans. Electron Devices, vol. 60, no. 2, pp. 782-786, 2013.
[41] G. H. B. Thompson, “Physics of Semiconductor Laser Devices,” John Wiley & Sons, New York, 1980.
[42] D. W. Kim, H. Y. Lee, and G. Y. Yeon, “A study of transparent contact to vertical GaN-based light-emitting diodes,” J. Appl. Phys. Lett., vol. 98, no. 5, pp. 053102-1-053102-4, 2005.
[43] 曾伯霜, “JSM-6700F HR-FESEM Operation Manual,” Department of Chemical Engineering National Cheng Kung University.
[44] APSYS by Crosslight Software, Inc., Burnaby, Canada, 2007. Available: http://www.crosslight.com