Brighter LEDs with low polarization resist ‘efficiency droop’

27 Nov 2024

Nagoya University, Japan, develops higher-output, more efficient LEDs.

Electric blue.

Light-emitting diodes (LEDs) are ubiquitous, from smart phones to home lighting. But today’s LEDs have a major limitation: when the operator tries to make them brighter by increasing their power, they become less efficient.

Now 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. A study describing this development appears in 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,” commented lead researcher Markus Pristovsek.

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 increases light output, but it also requires that a larger chip. As a result, fewer LEDs can be obtained from a wafer, the thin piece of semiconductor material made from InGaN that serves as the base for the fabrication of LED devices. The result of that is higher production costs and greater environmental impact, according to the Nagoya announcement.

Reducing droop

Researchers have been able to 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, the researchers 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,” said Pristovsek. “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.”