标题: | 以奈米压印技术制作高磊晶品质与光萃取效率氮化镓发光二极体 Improvement of Crystalline Quality and Light Extraction Efficiency in GaN-based Light Emitting Diodes by Nanoimprint Lithography |
作者: | 张瑞文 Chang, Ruey-Wen 郭浩中 程育人 Kuo, Hao-Chung Cheng, Yuh-Jen 光电工程研究所 |
关键字: | 发光二极体;奈米压印;Light Emitting Diodes;Nanoimprint Lithography |
公开日期: | 2012 |
摘要: | 在本研究中,我们利用 Nanoimprint Lithography (NIL) 制作二氧化矽圆锥状图型基板以及内嵌中空空气孔洞模板,并成功成长成氮化镓系列之发光二极体。 在二氧化矽圆锥状图型基板上成长的发光二极体 (SiO2-LED) 研究中,从式电子显微镜 (Transmission electron microscopy, TEM) 拍摄的图片中我们可以明显的指出二氧化矽圆锥状图型侧壁有横向的堆叠错误(stacking faults),这些横向的堆叠错误(stacking faults)能够有效的抑制直向的缺陷(threading dislocation)。此二氧化矽圆锥状图型有藉由光散射效应进而增加光萃取效率之可能性,这是由于二氧化矽本身和氮化镓磊晶层有着极大的折射率差异,导致光从氮化镓欲进入空气时有着极小的全反射角,进而使的光能够反射而使萃取效率提升。这也在TracePro的模拟中得到验证。 再来我们对光强度、电流对电压作图来研究其电学特性可以发现成长在蓝宝石圆锥型图形化基板 (cone-shaped patterned sapphire substrate, CPSS) 和成长在二氧化矽圆锥状图形化模板之发光二极体相对于成长于一般蓝宝石基板之发光二极体 (C-LED)在二十毫安培操作电流下,光强度有着百分之四十一以及百分之六十三之提升。 在前一个部分,我们成功的使用奈米压印技术在蓝宝石基板上制作出二氧化矽圆锥状图型。在这个研究的基础下,接着我们想将奈米压印技术应用在未掺杂氮化镓磊晶层,且为了更进一步提升光萃取效率,使用了长方体空气孔洞的结构,来达到我们的需求。 在发光二极体成长于长方体空气孔洞模板的研究中,我们可以从穿隧式电子显微镜 (TEM) 中明显的观察到次微米级的空气孔洞氮化镓磊晶层之中。空气孔洞氮化镓磊晶层的折射率差可以有效的提高光萃取效率。进一步地,我们利用室低温的变强度光学量测 (PDPL) 来定义内部量子效率来验证磊晶品质的提升。藉着Finite-difference time domain (FDTD) 模拟来进一步的验证光萃取的效率。从光学量测和光学模拟的结果中,我们得知内部量子效率和光萃取效率分别有了 20.6 % 和 20.3 %的提升。最后我们对光强度、电流对电压作图来研究其电学特性,可以发现成长成长在长方体空气孔洞模板之发光二极体相对于成长于一般蓝宝石基板之发光二极体 (C-LED)在二十毫安培操作电流下,光强度有着百分之四十五之提升。 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%. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT070050527 http://hdl.handle.net/11536/72614 |
显示于类别: | Thesis |