Brighter and more efficient LEDs with low polarization resist 'efficiency droop'

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Light-emitting diodes (LEDs) are everywhere in modern life, from smartphones to home lighting. But today's LEDs have a major limitation: when you try to make them brighter by increasing their power, they become less efficient.

A team of researchers at Nagoya University in Japan has now found a way to make LEDs brighter while maintaining their efficiency. Their research promises 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 study appears in the journal Laser & Photonics Review.

"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 represent the most efficient light source globally, although they typically operate at low power levels. To obtain brighter light, it is necessary to increase their power. However, an increase in power supplied to the LED results in a decrease in its efficiency, a phenomenon known as efficiency droop.

One way to overcome efficiency droop is to increase the area of the LED, which gives you more light, but it also means that you need a larger chip. As a result, you get fewer LEDs from a wafer—the thin, flat piece of semiconductor material made from InGaN that serves as the base for the fabrication of LED devices. The result is higher production costs and greater environmental impact.

Researchers can reduce the efficiency droop by tilting the InGaN layers and cutting the wafer into different orientations, which alter the resulting crystal's properties. The most important property altered in this manner is known as "polarization." Despite tilted orientations with low polarization being researched for over 15 years, InGaN LEDs made using these orientations have consistently exhibited less than half the efficiency of standard high-polarization LEDs.

The study by Pristovsek and Nan Hu at the Center for Integrated Research of Future Electronics (CIRFE) at Nagoya University found that a lower polarization is helpful only if it points in the same direction as that of standard LEDs. Using their findings, they grew LEDs on a cheap sapphire substrate in the so-called (101̅3) orientation, an orientation with lower polarization but in a direction similar to that of standard LEDs. These (101̅3) LEDs show greater efficiency at higher power.

This finding suggests innovative ways for manufacturers to develop next-generation LED technologies, such as more efficient and brighter micro-LED displays for mobile devices and large-screen TVs. Higher current density capability could also enable new applications in automotive and specialty industrial lighting, while faster switching speeds could find applications in visible-light communication technologies and VR glasses.

"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 (101̅3) 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."

More information: Markus Pristovsek et al, How to Make Semi‐Polar InGaN Light Emitting Diodes with High Internal Quantum Efficiency: The Importance of the Internal Field, Laser & Photonics Reviews (2024). DOI: 10.1002/lpor.202400529

Provided by Nagoya University