Red LEDs for Next-Generation Displays

Optimizing the geometry, fabrication and electrical contacts is vital to maximizing the efficiency of the LED. (Source: Zhuag et al.)

In efforts to optimize the per­formance of light-emitting diodes, KAUST researchers are looking at every aspect of the design, fabrication and operation of these devices. Now, they have succeeded in fabri­cating red LEDs, based on the naturally blue-emitting semi­conductor indium gallium nitride, that are as stable as those based on indium gallium phosphide.

LEDs are optical sources made from semi­conductors that offer improve­ments on conventional visible-light sources in terms of energy saving, smaller size and longer lifetimes. LEDs can emit across the spectrum, from the ultra­violet to blue, green, red and into the infrared. And arrays of tiny RGB devices – micro-LEDs – can be used to make vivid-color displays, which could underpin the next generation of monitors and tele­visions. A major challenge facing the development of microLEDs is to integrate red, green and blue light into a single LED chip.

Current RGB LEDs are made by combining two kinds of materials: red-light LEDs are made of indium gallium phosphide (InGaP), while blue and green LEDs comprise indium gallium nitride (InGaN) semi­conductors. Inte­grating two material systems is difficult. “Creating RGB displays requires the mass transfer of the separate blue, green and red LEDs together,” says Zhe Zhuang. An easier solution would be to create different-colored LEDs all on a single semi­conductor chip.

Since InGaP semi­conductors are unable to emit blue or green light, the only solution to making monolithic RGB micro-LEDs is to use InGaN. This material has the potential to shift its emission from blue to green, yellow and red by intro­ducing more indium into the mix. And InGaN red LEDs have been predicted to have better per­formance than the current InGaP ones. Zhuang, Daisuke Iida, Kazuhiro Ohkawa and their colleagues have succeeded in growing high-quality indium-rich InGaN to fabricate red LEDs using special nano­fabrication faci­lities.

The team also developed excellent transparent elec­trical contacts using a thin film of indium-tin-oxide (ITO), which allows for a current to pass through their InGaN-based amber and red LEDs. “We have optimized the fabri­cation of the ITO film to realize low electrical resistance and high trans­mittance”. The team demons­trated that these charac­teristics signi­ficantly improved the perfor­mance of InGaN red LEDs. They also carefully studied InGaN red LEDs of different sizes and at various tempera­tures. Changes in tempera­ture affect the output light power and cause different color impres­sions, making them crucial for practical device performance.

“A critical disad­vantage of InGaP red LEDs is that they are not stable when operated at high tempera­tures,” explains Zhuang. “Therefore, we created InGaN red LEDs of different designs to realize very stable red-light InGaN sources at high tempera­tures.” They have developed an InGaN red LED structure where the output power is more stable than that of InGaP red LEDs. Also, its emission color shift at high tempera­tures was less than half of that of  those made with InGaP. (Source: KAUST)

Reference: Z. Zhuang et al.: Effects of size on the electrical and optical properties of InGaN-based red light-emitting diodes, Appl. Phys. Lett. 116, 173501 (2020); DOI: 10.1063/5.0006910

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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