FRG: Nanopatterning of Sapphire Substrates for Improved III-Nitride Growth

非技术描述：在诸如激光器和发光二极管（LED）的光学器件的制造中，蓝宝石被用作GaN（和相关半导体材料）生长的底层材料。在该项目的过程中，将开发新型工艺，可用于在蓝宝石晶体表面生产图案化结构，其特征尺寸为纳米级。 图案化特征的存在使得GaN中的晶体完美程度更高，这反过来又导致器件更亮，寿命更长，能效更高。 这样的固态光源可以导致总体成本节约，并且对于许多应用而言可能取代传统的白炽灯泡。 计划在高中和大学一级参与研究的机会，以及通过NSF资助的纳米级非正式科学教育网络的地方和社区推广。技术规格：该项目的目的是实现更高的发光效率的GaN LED通过利用一种新的工艺（AGOG）的蓝宝石衬底的表面图案化。 该工作建立在先前的工作的基础上，先前的工作表明，沉积在蓝宝石表面上的平面金属Al膜（厚度~100 nm）的热处理可以导致外延转换。 AGOG转化过程的一个关键优点是，与目前可用的方法相比，它相对易于对表面化学和形态进行修饰。 目前正在研究几种新的和迄今为止未开发的用于提高蓝光LED器件的光学效率的策略。 其中包括：a）金属涂层的图案化（在转换之前）转换成纳米级台面阵列（宽度50 - 150 nm，高度~100 nm）以增加表面区域的顺应性，并因此降低由GaN和蓝宝石之间的晶格失配引起的失配位错的密度，B）通过利用具有r-平面取向的图案化蓝宝石衬底生长非极性GaN，以及c）通过制造添加氧化铬的台面来将晶格失配降低高达30%。 关于铝的氧化机制和形态稳定性在纳米级的基本问题也正在解决。 显然，随着特征尺寸的减小，在表面或沿着Al/蓝宝石界面发生的扩散过程将变得越来越重要。 这可能涉及与本体行为显著不同的机制和/或转化动力学。

NON-TECHNICAL DESCRIPTION: In the fabrication of optical devices such as lasers and light emitting diodes (LEDs), sapphire is used as the underlying material for the growth of GaN (and related semiconductor materials). Over the course of the project, new types of processes will be developed which can be used to produce a patterned structure on the surface of a sapphire crystal, with feature sizes at the nanometer level. The presence of the patterned features enables a greater degree of crystalline perfection in the GaN, which in turn leads to devices which are brighter, that last longer and are more energy efficient. Such solid state light sources can result in overall cost-savings, and potentially replace traditional incandescent bulbs for many applications. Opportunities for research involvement at the high school and college level are planned, as well as local and community outreach through an NSF-funded Nanoscale Informal Science Education Network. TECHNICAL DETAILS: The aim of the project is to achieve higher luminescence efficiency in GaN LEDs by exploiting a novel process (AGOG) for surface patterning of sapphire substrates. The work builds on prior work which showed that heat-treatment of a planar metallic Al film (thickness ~100 nm) deposited on a sapphire surface can lead to epitaxial conversion. A key advantage of the AGOG conversion procedure is that compared to currently available methods, it is relatively amenable to modification of both surface chemistry and morphology. Several new and hitherto unexplored strategies for enhancing the optical efficiency of blue LED devices are under investigation. These include: a) Patterning of the metallic coating (prior to conversion) into an array of nanoscale mesas (width 50 - 150 nm, height ~100 nm) to increase the compliance of the surface regions, and hence reduce the density of misfit dislocations arising from the lattice mismatch between GaN and sapphire, b) Growth of non-polar GaN by utilizing a patterned sapphire substrate with r-plane orientation, and c) Reducing the lattice mismatch by up to 30% by fabricating mesas with additions of chromia. Fundamental issues with regard to Al oxidation mechanisms and morphological stability at the nanoscale are also being addressed. Clearly diffusional processes which take place at the surface or along the Al/sapphire interface will become increasingly important as the feature size decreases. This may involve mechanisms and/or transformation kinetics which diverge significantly from bulk behavior.