Pure Red LEDs Fulfill a Primary Goal

This MO-CVD machine is the key technology needed to achieve bright red LEDs. (Source: KAUST)

Making pure red LEDs from nitride crystals is a goal that has so far frustrated engineers. However, these LEDs are vital for building the next generation of energy-effi­cient micro-LED displays to follow OLED displays and for creating lighting with color tuning. Now, for the first time, a team of elec­trical engineers at KAUST has succeeded in making these LEDs.

“Electrical engineers can already make bright LEDs using varying materials to produce different colors. But to improve display techno­logies, engineers must inte­grate the three primary color LEDs, red, green and blue, onto one chip,” explains Daisuke Iida, an electrical engineer at KAUST. This means they need to find one material that is suitable for manu­facturing all three colors. The material should be able to produce each color with high intensity, and ideally, it should have a high-power output, but use rela­tively little battery voltage.

The best candidates for generating all three colors are a family of nitride semiconductors. These crystals containing nitrogen in theory can be used to create LEDs that produce light with wave­lengths between ultraviolet and infrared, which includes the entire visible spectrum. Engineers usually use gallium nitride to make blue and green LEDs, but they have struggled to make bright red LEDs with this crystal. “Red vision has been almost impos­sible – other groups have only really succeeded in making orange, not apple red,” says group leader, Kazuhiro Ohkawa. “Now, we have developed a crystal growth system to realize pure red LEDs.”

Replacing a large portion of the gallium with the element indium gives the desired red, but it is hard to do because indium easily eva­porates from the crystal. So Iida, Ohkawa and colleagues created a reactor with extra indium vapor above the crystal’s surface, a process known as metal­organic vapor-phase deposition. This added pressure prevents the indium in the crystal from escaping. “This gives us a higher indium concen­tration at the surface,” says Ohkawa. “That’s our secret!”

But there was another hurdle to overcome. Indium is made of larger atoms than gallium, so when it is intro­duced, it creates defects in the crystal, degrading the quality of output light. The team’s trick was to also add aluminum, which has small atoms. “The intro­duction of the small atoms reduces the strain on the crystal, resulting in fewer crystal defects,” says Iida. “Another advan­tage is that the LEDs operate at about half the voltage of its compe­titors,” says Ohkawa. “This will give you a longer lifetime for batteries.” (Source: KAUST)

Reference: D. Iida et al.: Demonstration of low forward voltage InGaN-based red LEDs, Appl. Phys. Exp. 13, 031001 (2020); DOI: 10.35848/1882-0786/ab7168

Link: Computer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science and Technology KAUST, Thuwal, Saudi Arabia

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