Micro-imprinted TiO2 nanoparticles on top of nitride-based LEDs

February 06, 2018 // By Julien Happich
In cooperation with a team from the Royal institute of Technology of Stockholm (KTH), researchers from CEA Leti found a cheap trick to limit the total internal reflection in high index III-V layers and boost the light extraction of blue nitride-based LEDs: micro-imprinted titanium oxide nanoparticles.

In a paper titled "Nanoparticle-based microstructures for light extraction enhancement in nitride-based LEDs" presented during SPIE Photonics West in San Francisco, the researchers took the well-known approach of adding a thin textured layer over the light emitting devices. But instead of using a solid layer to be etched or a polymer to be cured solid, they used a cheap colloidal solution of TiO 2 nanoparticles to be softly compacted under various embossing patterns from a master mold, ensuring a close contact with the light emitting surface, without damaging the LED structure.

The scientists first created a 1x1cm 2 patterned Polydimethylsiloxane (PDMS) stamp by polymerizing PDMS directly on a patterned substrate. Once peeled off, the stamp could be pressed onto a drop of water-based solution of the nanoparticles, directly on top of the nitride-based LEDs, at room temperature (then heated at 100ºC to remove the solvent).

Very much like casting castles of wet sand with a mold but at the nanoscale, the researchers experimented with various textures already reported in literature for their light extraction capabilities when made from bulk material.

Those include nanopillars at a 600nm period, micropyramids or microcones at a 3µm period and micromesas or truncated micropyramids at a 5µm period. Using these cheap nanoparticles, the paper reports 1.4x to 2.1x optical power enhancements depending on the chosen shapes, the microcones being the most successful.

The authors also conclude that this economical and low-temperature approach could also be well suited for the light extraction management of other optoelectronic devices, such as solar cells or mid-infrared devices.


Optical power (arbitrary units) versus electrical current measured for 5 un-embossed (black) and 6 microcone-embossed (red doted lines) 820x820 μm 2 blue emitting vertical thin-film LEDs.

 

CEA Leti - www.leti-cea.com

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