Brighter and more efficient LEDs that don't droop. Credit: Nagoya University

Light-emitting diodes (LEDs) are everywhere in modern life, powering everything from smartphones to home lighting solutions. However, the current generation of LEDs has a significant drawback – when we increase their power for added brightness, efficiency takes a hit.

Now, a team of researchers at Nagoya University in Japan has introduced an innovative approach that promises to make LEDs brighter while maintaining their efficiency. Their research aims to reduce the cost and environmental impact of LED production while improving performance in applications such as visible light communication and virtual reality (VR) glasses.

“The innovation of this work is a better understanding of the effects of polarization, an intrinsic property of the gallium nitride/indium gallium nitride (GaN/InGaN) layer structure that is needed for light generation,” lead researcher Markus Pristovsek said.

InGaN LEDs are considered the world’s most efficient source of light, although they generally function at low power levels. To achieve greater brightness, it is essential to raise their power. However, increasing the power supplied to the LED leads to a reduction in efficiency, a phenomenon referred to as efficiency droop.

To overcome this hurdle, expanding the LED area can result in more light output. However, this also requires a larger chip, leading to fewer LEDs being produced from a single wafer – the thin, flat semiconductor base made of InGaN used to fabricate LED devices. The outcome is higher manufacturing costs and a larger environmental footprint.

Researchers are actively working on mitigating efficiency droop by tilting InGaN layers and adjusting the wafer orientations, which significantly alters the crystal properties. One crucial property affected by this technique is ‘polarization.’ Although tilted orientations with low polarization have been under study for over 15 years, InGaN LEDs produced with these angles have consistently demonstrated less than 50% efficiency compared to standard high-polarization LEDs.

A study conducted by Pristovsek and Nan Hu at the Center for Integrated Research of Future Electronics (CIRFE) at Nagoya University revealed that lower polarization is advantageous only when it aligns with the orientation of standard LEDs. Building on this insight, the researchers grew LEDs on an economical sapphire substrate in the (10-13) orientation, which features reduced polarization while maintaining a direction akin to conventional LEDs.

These (10-13) LEDs demonstrate improved efficiency at higher power levels. This discovery indicates new possibilities for manufacturers to create advanced LED technologies, such as more efficient and brighter micro-LED screens for mobile devices and large televisions. Increased current density capacity could also lead to novel applications in automotive and specialized industrial lighting, while quicker switching speeds could be useful in visible-light communication technology and VR eyewear.

“Future research is unlikely to find a better orientation, particularly on the cost-efficient sapphire substrates, because only two tilted directions can be fit to it,” Pristovsek said. “However, there are other ways to make (10-13) LEDs with fewer defects on sapphire and maybe even silicon. But the other orientations achieved on sapphire or silicon so far are worse because they are either inherently rough, they increase the amount of polarization, or they have the wrong sign of polarization.”

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