Improvement of Crystalline Quality and Light Extraction Efficiency in GaN-based Light Emitting Diodes by Nanoimprint Lithography

• Main Authors: Chang, Ruey-Wen, 張瑞文
• Other Authors: Kuo, Hao-Chung
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/53715351940697594543

碩士 === 國立交通大學 === 光電工程研究所 === 101 === In this research, the high performance GaN-based light-emitting diodes (LEDs) growth on SiO2 crown-shaped pattern substrates and cubic airvoids by Nanoimprint Lithography (NIL) were demonstrated. In the first part, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using Nanoimprint Lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggested that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation revealed that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS. At the previous research, we transferred the cone-shaped pattern into sapphire substrate. Based on the studies of SiO2-LEDs, we started to transfer the imprint pattern from sapphire substrate to undoped-GaN. To improve the light extraction efficiency of nitride-based LEDs, we expect the cubic airvoids structure in undoped-GaN can reflect more downward light from MQW. Finally, the cubic airvoids template has been demonstrated to satisfy our purpose. In the second part, we successfully transferred the cubic patterns into uGaN layer to fabricate a cubic airvoids patterned template by using Nanoimprint Lithography (NIL). Submicro-scale airvoids were clearly observed in undoped-GaN by transmission electron microscopy (TEM). The difference reflective index between air voids (n=1) and GaN (n=2.45) can increase the light extraction efficiency due to additional total reflection. To quantify the internal quantum efficiency (IQE), power dependent photoluminescence measurement (PDPL) at room temperature and low temperature has been used. And the finite-difference time domain (FDTD) simulation was also be used to calculate the light extraction efficiency. From the PL measurement and FDTD simulation, we estimate the IQE and LEE enhancement are 20.6% and 20.3%.