NW-LED fills the green-yellow gap in white light

October 27, 2016 // By Julien Happich
An international team of researchers from King Abdullah University of Science and Technology (KAUST) and King Abdulaziz City for Science and Technology (KACST) have come up with a novel way to fill the “green-yellow gap” typically seen in white-LED implementations based on multiple phosphors.

SEM image of the device structure based
on a two temperature-step growth method
(inset, TEM of Qdisks embedded in a nanowire).

Published in the ACS Photonics journal, their paper "True Yellow Light-Emitting Diodes as Phosphor for Tunable Color-Rendering Index Laser-Based White Light" describes a nanowire-based LED emitting at 588nm, grown on a low-cost, CMOS-foundry-compatible Ti-thin-film/Si substrate platform. A dense layer of nanowires is grown, reaching a surface density of 9x109 cm2, with a fill factor of 88%.

Device structure showing multiple nanowires
grown side-by-side.

And each nanowire p-i-n LED structure embeds an active region made of five stacks of 3nm thick InGaN quantum disks (Qdisk) separated by a 10nm quantum barrier.

Operating the yellow NW LED alone, they observed a peak emission of 588nm at 29.5 A/cm2 (75mA in a 0.5x0.5mm2 device) and a low turn-on voltage of about 2.5V, with an internal quantum efficiency of 39%, without “efficiency droop” up to an injection current density of 29.5A/cm2.

Then they demonstrated the benefit of mixing such a yellow light with the light of red, green, and blue laser diodes. In their setup, the yellow NW LED was illuminated by RGB beams, like a phosphor, and used as a light-scattering plane in a reflective configuration, adding its own yellow emission to the white mix.

The result was a correlated color temperature circa 6000 K and a color-rendering index of 87.7.