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

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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.