Effects of Different Patterned Sapphire Substrate Surface Morphology on Performance of GaN-based LEDs

• Main Authors: Chen, Chien-Chih, 陳建誌
• Other Authors: Wu, Yew-Chung
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/rx8j87

博士 === 國立交通大學 === 材料科學與工程學系所 === 103 === 　　High-brightness GaN-based light-emitting diodes (LEDs) have been widely used in a variety of applications due to their advantages of high efficiency, long life, small size and environmental protection. For the purpose of next-generation application of solid-state lighting, LEDs with higher optoelectronic characteristics of LEDs are required. Many techniques such as surface roughing, metal mirror reflect layer, epitaxial lateral overgrowth (ELOG), and patterned sapphire substrate (PSS) have been developed to achieve these objectives. Currently, the PSS technique has attracted much attention because it can improve both internal quantum efficiency (IQE) and light extraction efficiency (LEE). 　　There are two methods for fabricating PSSs: (1) dry etching and (2) wet etching. In dry etching, ion bombardment damages PSS surface and increases the threading dislocation density in GaN epitaxial layers. On the other hand, wet etching does not damage PSS surface. Moreover, wet etching is a less expensive method in terms of equipment and process costs. 　　In wet etching, sapphire substrate covered with SiO2 hard mask is usually etched with a mixed solution of hot H2SO4 and H3PO4. When using wet-etched PSS, it was found that GaN quality and performance of LEDs improved with decrease in bottom c-plane areas of PSS. This was because the threading dislocation density decreased with bottom c-plane areas. However, further decrease in bottom c-plane areas made epitaxy of GaN film very difficult. In this dissertation, it was found that epitaxy difficulty was due to the appearance of 6C facets {4 -1 -3 18} and the disappearance of bottom c-plane. Besides, it was also found that most of the growth of zinc-blende GaN was initiated from the ridge of patterns. 　　In addition, reducing the pattern size to nanoscale can further enhance the optoelectronic characteristics and GaN crystal quality of LEDs. In this dissertation, nano-sized PSS was fabricated by using anodic aluminum oxide (AAO). After growth of the GaN layer, air voids were formed at the GaN/sapphire interface. It was found that the crystal quality of GaN-based LEDs grown on nanoporous patterned sapphire substrate (NPSS) was better than that on micron-sized patterned sapphire substrate (MPSS) and on conventional sapphire substrate. However, the light output power of GaN-based LEDs grown on NPSS was smaller than that on MPSS because the light extraction efficiency (LEE) of MPSS was much larger than that of NPSS. 　　In appendix of this dissertation, a new manufacturing method for Ni-metal-induced lateral crystallization polycrystalline silicon thin-film transistors (NILC poly-Si TFTs) using fluorinated silicate glass (FSG) was proposed. NILC is one of effect methods for manufacturing poly-Si TFTs because it could crystallize at a temperature below 600oC. Unfortunately, poly-Si/oxide interfaces and grain boundaries trap Ni and NiSi2 (Ni-related defects), which increases trap states density and degrades electrical performance of TFTs. It was found FSG-TFTs exhibited high electrical characteristics, good thermal stability and good electrical reliability compared with typical NILC-TFTs. This was because F atoms could passivate dangling bonds and strain bonds in active layer resulting in lower trap states.