Nitride-Based MicroLED Emits Pure Red Light

A microLED developed at KAUST is able to efficiently emit pure red light and could help in the quest to develop full-color displays based on a single semiconductor.

MicroLEDs are a favored technology for next-generation displays, given their (small) size and energy efficiency. However, each LED is only able to emit light over a narrow range of colors. Devices can be created that combine many different LEDs that each emit a different color. Full-color displays can be created by combining red, green, and blue (RGB) microLEDs.

The microLED was developed in KAUST's nanofabrication core lab. Courtesy of Anastasia Serin, KAUST.

The problem, however is that an LED’s emission color is determined by the material properties of the semiconductor. Nitride semiconductors, for example, can be used to make blue and green micro-LEDs, whereas phosphide semiconductors are used to make red light. Combining different semiconductors for this purpose makes the construction of RGB microLEDs difficult and expensive. In the case of phosphide, efficiency is also reduced significantly as the chip size shrinks.

“[Indium gallium nitride]-based blue microLEDs are the best choice for blue color displays, because they have demonstrated remarkable performances, even when the device dimension decreases to 10 μm or less,” the researchers wrote in Optics Letters.

Red light-emitting indium gallium nitride can be created by increasing the material’s indium content, though this tends to lower the efficiency of the resulting LED due to a mismatch between the separation of atoms in the GaN and InGaN, which creates atomic-level imperfections. Further, damage to the sidewalls of an InGaN microLED induced during the fabrications process also decreases efficiency.

“But we have a chemical treatment to remove the damage and retain the high crustal quality of the InGaN and GaN sidewall interface,” said Zhe Zhuang, a postdoctoral research fellow in the KAUST electrical and computer engineering program.

Zhang and his team created and characterized a series of square devices with a side length of 98 or 47 μm. Their 47-μm-long devices emitted light at a peak wavelength of 626 nm and were shown to have an external quantum efficiency — the number of photons emitted from the LED per electron injected into the device — of up to 0.87%. The red microLED showed color purity that was very close to the 630-nm primary red color defined by the industrial standard Rec. 2020.

“The next step is to increase the efficiency of the red microLED with even smaller chip sizes, maybe below 20 μm,” Zhuang said. “Then we hope to integrate RGB nitride-based LEDs for full-color displays.”

The research was published in Optics Letters (www.doi.org/10.1364/ol.422579).