Eliminating 'efficiency droop' for brighter LEDs
A team of researchers at Nagoya University in Japan have found a way to make light-emitting diodes (LEDs) brighter while maintaining their efficiency. The research findings could 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 polarisation, an intrinsic property of the gallium nitride/indium gallium nitride (GaN/InGaN) layer structure that is needed for light generation,” said Markus Pristovsek, lead researcher.
InGaN LEDs are efficient, though they typically operate at low power levels. To obtain a 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 more light but also requires a larger chip. As a result, there are fewer LEDs sourced 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 ‘polarisation’. Despite tilted orientations with low polarisation being researched for years, InGaN LEDs made using these orientations have exhibited less than half the efficiency of standard high-polarisation LEDs.
The researchers at Nagoya University found that a lower polarisation is only helpful 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 10–13 orientation (an orientation with lower polarisation but in a direction similar to that of standard LEDs); these LEDs showed greater efficiency at higher power.
The findings suggest 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 speciality 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. 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 polarisation, or they have the wrong sign of polarisation,” Pristovsek said.
The study was published in the journal Laser & Photonics Review.
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