New LED technology delivers multiple colors from a single layer

In conventional semiconductor LEDs, only a single color can be produced from a single light-emitting layer. As a result, realizing full-color displays has traditionally required stacking multiple light-emitting layers or arranging separate color-emitting devices laterally. GaN-based LEDs have attracted high attention in recent years because integrating each color makes it possible to realize next-generation micro-LED displays. However, a challenge was that the emission color changed depending on the amount of current injection.

A research group including Associate Professor Shuhei Ichikawa of the Graduate School of Engineering at the University of Osaka in collaboration with Professor Yasufumi Fujiwara of the Research Organization of Science and Technology at Ritsumeikan University, successfully demonstrated multicolor LEDs (blue, green, yellow, and red) operating at room temperature using aluminum gallium nitride (AlGaN) doped with the rare-earth element terbium (Tb). Because stable visible multicolor emission can be achieved using a single material, future control of the emission ratio of each color through optical resonators or other post-deposition techniques could enable a new integration approach, eliminating the need for multiple light-emitting layers. The paper was published in Applied Physics Letters and selected as an Editor's Pick.

The research group focused on rare-earth-element-doped semiconductors, which emit light in multiple colors with a single material and whose emission color is unaffected by changes in the surrounding environment. Among them, focusing on Tb ions that emit light in four colors simultaneously, they aimed to realize new visible-light LEDs using AlGaN semiconductors doped with Tb during the crystal growth process.

With the cooperation of Fujiwara, the team fabricated the device using metal-organic vapor phase epitaxy (MOVPE), a method already widely used in the mass production of GaN-based semiconductors. By applying an electric current to the fabricated Tb-doped AlGaN LEDs at room temperature, they demonstrated stable four-color emission originating from the Tb ions.

The study also revealed a mechanism by which energy transfer to Tb ions becomes more efficient as the aluminum content in the host AlGaN semiconductor increases, leading to a dramatic improvement in emission efficiency, measured as external quantum efficiency. They demonstrated that using aluminum nitride (AlN) for the underlayer of the light-emitting layer reduces lattice mismatch with the substrate, yielding high crystal quality. In addition, they found that compressive strain introduced around the Tb ions changes them into a state that emits light more readily, increasing the radiative transition rate and significantly improving both electrical and optical performance. By passing the emitted light through color filters, the researchers successfully extracted individual RGB colors from the same light-emitting region.

Through this, it was shown that stable multicolor emission unaffected by current changes can be obtained from a single semiconductor material. This means departing from complex manufacturing processes of arranging individual RGB chips one by one, increasing the feasibility of monolithic integration to fabricate full-color LEDs all at once on the same substrate.

This is a groundbreaking achievement that not only leads to significant efficiency improvements in manufacturing processes, but also accelerates the practical application of ultra-compact, high-definition full-color displays essential for smart glasses and wearable devices.

Ichikawa stated: "We demonstrated a new visible-light LED capable of generating multiple colors from a single light-emitting layer while maintaining stable emission characteristics regardless of the injection current, both of which were difficult with conventional standard semiconductor LEDs. This research achievement is groundbreaking as it shows a new path toward realizing next-generation micro-LED displays. Moving forward, we hope to establish key technologies, such as further increasing LED efficiency and freely controlling the emission ratio of each color, toward mass production."

Journal Information
Publication: Applied Physics Letters
Title: Ultra-stable multiple emission wavelengths produced by Tb-doped Al_xGa_1−xN-based light-emitting diodes
DOI: 10.1063/5.0331734