利用碳纳米管改善激光剥离工艺及垂直结构LED的研究

Sapphire, as the most favorable substrate for III-nitride materials, suffered its inferior electrical and thermal conductance. Laser lift off (LLO), as the-state-of-the-art methodology to exfoliate GaN from sapphire was, however, exhibiting high threshold energy, severe crystalline degradation, large residual stress, and defective simulation theory. Recently, carbon nanotube (CNT) arrays were utilized by us to fabricate Carbon nanotube Patterned Sapphire Substrate (CPSS). High-quality, stress tunable GaN materials and enhanced performed LEDs were epitaxied on CPSS by MOCVD. Moreover, lower threshold energy and little residual stress were observed in GaN/CPSS LLO process. Therefore, in this project, CNT assisted LLO is announced to realize low threshold, little crystal damage and low residual stress process. Particularly, the physical mechanism of CNT assisted LLO and the effects of different CNT properties and patterns on LLO will be researched. The vertical structure LED based on LLO of CPSS/GaN would also be fabricated and characterized. Furthermore, a three-dimensional LLO simulation mode including stress calculation modules and anisotropy will be built in this project. The promising results and achievements will be positive and helpful for high-performed VCSEL, vertical LED, flexible LED and free standing GaN substrate.

蓝宝石作为三族氮化物最常用的外延衬底，其导电、导热性差的缺点制约着三族氮化物材料和器件的发展。目前将氮化镓(GaN)从蓝宝石上分离最常用的激光剥离工艺存在阈值高、残余应力大、理论模型不完善等问题。近期，我们采用碳纳米管(CNT)阵列铺设在蓝宝石衬底上，制备了CNT掩膜蓝宝石衬底（CPSS），并在CPSS上实现了高质量、应力可控、LED性能提升的材料、器件的MOCVD外延。同时，初期的激光剥离实验发现，CNT阵列可以有效地降低剥离阈值，减少损伤。因此，本项目提出一种利用CNT调节GaN与蓝宝石界面，实现低阈值、小损伤、低残余应力的激光剥离工艺。项目具体实施中，研究CNT在激光剥离中起的作用，不同CNT分布和CNT特性对剥离阈值、晶体损伤、残余应力的影响，制作高性能垂直结构LED，同时完善激光剥离模型。本项目的研究成果将有助于高性能VCSEL、垂直结构LED、柔性LED及自支撑GaN的研究。

目前三族氮化物材料的外延衬底主要以蓝宝石衬底为主。但蓝宝石衬底的导电导热性极差，这使得大功率垂直结构三族氮化物器件必须通过激光剥离将蓝宝石衬底去除。但激光剥离工艺采用高能量密度激光对GaN/蓝宝石界面进行照射，在界面层产生高温，会带来有源层的损伤。针对上述问题，本项目提出基于CNT图形化蓝宝石衬底（CPSS）上氮化物的激光剥离技术，重点研究了引入CNT后，GaN材料激光剥离的阈值变化、晶格损伤、蓝宝石/CNT/GaN界面的热场分布、基于CPSS的正装、垂直结构LED制作、表征和分析。. 本项目的实施过程中，我们按照项目计划开展了不同层数CNT的CPSS上GaN材料外延、激光剥离、垂直结构LED制作、模拟计算等方面的工作。本项目取得了以下成果：1）得到了不同CNT层数掩膜下的高质量氮化物MOCVD外延，分析了不同层数CNT上正装LED的辐射复合、非辐射复合、光提取效率等参数；2）利用CNT将GaN激光剥离阈值降低40%，晶体质量损伤小于5%，残余应力小于<80MPa；3）利用不同层数CNT制备了近紫外垂直结构LED器件，分析了由于CNT层数不同造成的垂直结构LED器件性能参数影响；4）模拟计算了蓝宝石/CNT/GaN界面激光剥离时热分布，分析得到CNT的高吸收率、高热导率和低比热容是造成激光剥离阈值降低的原因；5）在项目执行期间，以第一作者或通讯作者发表SCI学术论文9篇，培养研究生6名。