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Nanotechnological approaches have been widely explored for improved light output in InGaN/GaN light emitting diodes (LEDs). In this talk, I will introduce the application of SiO2 nanoparticle (NP) embedded in GaN nanopillar template for high optical extraction, of Ag/SiO2 core/shell NP for a localized surface plasmon (LSP) LED, and of porous GaN templates made by combined wet chemical etchings.
With the SiO2 NPs placed between the GaN nanopillars, subsequent overgrowth of GaN layer started only on the exposed tips of the nanopillars and rapidly switched to the lateral growth mode. This resulted in a high quality GaN layer sitting on the nanopillars and the layer of pores formed over the SiO2 nanoparticles. For LEDs grown on top of such template, ~3 fold increase in optical output was observed compared to reference samples. The effect is attributed mainly to the improved light extraction efficiency due to additional scattering in the nanopillars-SiO2-pores portion of the structure, also to the increased internal quantum efficiency caused by a decreased dislocation density and relaxed strain due to the GaN nanopillars.
Practical approaches to making LSP-LEDs will be discussed. The stability problem of NPs could be solved by developing the technology of Ag/SiO2 core/shell NPs prepared by sol-gel method. The NPs places the LSP resonance peak near 450 nm matching it to the blue MQW LEDs. It is demonstrated that strong PL intensity enhancement can be achieved for MQW structures with Ag/SiO2 NPs. These studies seem to indicate that the extent of efficient spatial coupling covers the range of about 40 nm, in agreement with earlier calculations for Ag NPs. Measurements of the LSP-related PL enhancement stability over time due to Ag/SiO2 core/shell NPs is better than for Ag NPs.
Free-standing GaN LED structure with high crystalline quality was fabricated by combining electrochemical and photoelectrochemical etching followed by regrowth of LED structure and subsequent mechanical detachment from a substrate. The structural quality and composition of the regrown LED film thus produced was similar to standard LED, but the PL and EL intensity of the LED structures on the etched template were several times higher than for standard LED. The performance enhancement was attributable to additional light scattering and improved crystalline quality as a result of the combined etching scheme.
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