A method developed by a team of researchers from King Abdullah University of Science and Technology (KAUST) and King Abdulaziz City for Science and Technology (KACST) fills the “green-yellow gap” typically seen in white-LED implementations based on multiple phosphors.

KAUST yellow led 01 (cr) Figure 1: 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, the team's 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 9cm x 109cm, with a fill factor of 88%.

KAUST yellow led 02 (cr) Figure 2: 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 29.5A/cm2 (75mA in a 0.5mm x 0.5mm 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 set-up, 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 colour temperature circa 6000K and a colour-rendering index of 87.7.

Author Boon S. Ooi, professor of Electrical Engineering and director of the KACST Technology Innovation Centre for Solid State Lighting, EE Times Europe that the objective of this method is to reduce the blue-intensity of white lights, creating an eye-friendly warmer white, while offering a new way to tune the colour temperature for laser-based SSL.

"We have submitted a U.S. provisional patent on this technology," he added, hinting at some commercial opportunities.