增進LED光萃取效率之GaN膜粗化蝕刻技術開發

Development on improving extraction efficiency of GaN LEDs by surface roughing technique of wet etching

彭榆鈞(Peng Yu-Jun)

指導教授 ： 楊啟榮

國立臺灣師範大學/科技學院/機電科技研究所/碩士(2006年)

摘要

氮化鎵發光二極體(GaN LED)光萃取效率不佳，是其一直存在的問題，而對於提升光萃取效率，通常最簡易也最為有效的技術，莫過於粗化技術。GaN中之光子，因GaN的高折射係數( n=2.5 )，依施乃耳定律，臨界角僅有23.6°，光子也只能在此臨界角所形成的光逃脫角錐內，才能順利的穿過邊界，達到空氣之中。藉由粗化技術，可破壞臨界角的限制，光子因此不易產生全反射且易於脫離原本介質，進而提升其光萃取效率。GaN主要有兩種極性方向-鎵面與氮面，在鎵面上非常穩定且不易蝕刻，故可藉由雷射掀離法將薄膜掀離，並轉換到另一基材上，經由此技術後，薄膜較不穩定的氮面便會翻轉向上，藉此便可輕易的藉由溼式蝕刻技術，製作出大面積的六角錐柱。
本研究主要目的，是利用溼式蝕刻技術，與添加劑YR、YC、YP、T-X與Y-A的使用，藉此改變氮面上的表面能，進而改變錐柱的形態與控制錐柱的角度。本研究也已成功的藉由YR添加劑的使用，在YR與50 wt.%.的KOH溶液為10 ml : 1 liter比例上，將原本六角錐柱，改變成圓錐之形態。角度方面，也可以藉各種添加劑比例上的配合，將角錐控制在65-120°的範圍內。本研究也將此新式的蝕刻技術與一般的粗化技術相比較，在350 mA的驅動下，105°的圓錐形態，比一般的六角錐粗化，其光輸出功率高出了7 %左右，此原因即為此種新式的蝕刻技術所製造之圓錐，可使側向的光子路徑皆為法線方向，因此提升其光萃取效率。以上的結果顯示出，在表面形態上，本研究所提出之蝕刻技術，以優於一般傳統的六角錐柱之蝕刻。

關鍵字

氮化鎵 ； 表面粗化 ； 雷射掀離法 ； 氮面

並列摘要

Light extraction efficiency is always a problem for GaN LED. And surface roughing is simple and useful technology for raising light extraction efficiency. Because of GaN with high refraction coefficient (n=2.5), by Snell’s law, it can create a critical angle of 23.6° and just allow photons escaping from original medium in this escape cone made by critical angle. But we can overcome this rule to raise light extraction efficiency by surface roughing technology. There are two polarization direction for GaN, those are: Ga-face and n-face. Ga-face is too stable to etch for creating rough surface, but n-face is easy to make it. So we can transform sapphire to another substrate and take GaN film up-side down by laser lift off (LLO). By LLO, we can etch the surface easily to fabricate hexagonal cones of large area on n-face.
In this study, we will change surface energy of n-face to change shape and control angle of hexagonal cones by additives, and those additives are YR, YC, YP, T-X and Y-A. By YR, we can change hexagonal cones to cones. And we also can control angle of cone by mixed additives. In this study, we have ability to control angle what the range is 65°~120° by this novel etching technology. Comparison between conventional LED with hexagonal cones of 70° and our LED with cones of 105°, power efficiency of ours is higher than conventional LED with hexagonal cones about 7%
.

並列關鍵字

GaN ； surface roughing ； laser lift-off ； n-face

參考文獻

1. T. Mukai, M. Yamda and S. Nakamura,”Characteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting Diodes", Japanese Journal of Applied Physics, Vol. 38, NO. 7A, pp. 3976–3981(1999).

2. H. Amano, M. Kito, K. Hiramatsu, K. Hiramatsu and I. Akasaki,”P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI)", Japanese Journal of Applied Physics, Vol. 28, NO. 12, pp. 2112–2114(1989).

4. S. Nakamura, T. Mukai and M. Senoh,”Candela-class high-brightness InGaN/AIGaN double-heterostructure blue-light-emitting”, Applied Physics Letters, Vol. 64, NO. 13, pp. 1687–1689(1994)

5. S. Nakamura, M. Senoh, N. Iwasa, and S. I. Nagahama" High-power InGaN single-quantum-well-structure blue and violet light-emitting diodes” ,Applied Physics Letters , Vol. 67, NO. 13, pp. 1868–1870(1995)

8. 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”, Journal of Applied Physics, Vol. 93, NO. 11, pp. 9383–9385(2003).