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

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 9x10⁹cm⁻², 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.