A cubic III nitride active layer with an internal quantum efficiency of 32%

Color mixing is the process of combining two or more colors: red and green make yellow, blue and red make purple, red and green, and blue make white. This color mixing process forms the basis of the future of solid-state lighting. While currently white light is achieved by phosphor downconversion, LED color mixing actually has a higher theoretical maximum efficiency, which is necessary to meet the DOE’s 2035 energy efficiency goals.

Despite the potential effectiveness of mixed-color LED sources, there is one significant challenge: green. The “green gap” is described as the lack of suitable green LEDs. Current green LEDs are made from state-of-the-art III hexagonal nitride, but only meet one-third of the efficiency goals outlined in the DOE’s 2035 roadmap.

In a new study, researchers at the University of Illinois at Urbana-Champaign have found a potential path to closing the green gap and reported a green-emitting cubic III nitride active layer with internal quantum efficiency (IQE ) by 32%, or more than Efficiency 6 times higher than that reported in the literature for conventional cubic active layers.

“The ultimate goal is to triple the efficiency of current white light-emitting diodes. And to do that, we need to fill the green gap in the spectrum, which is no easy task. It takes innovation. And “We’re showing innovation from materials using cubic nitrides,” says Can Bayram, professor of electrical and computer engineering, who led this work alongside graduate student Jaekwon Lee.

The results of this research were recently published in Applied physics letters as the issue’s cover story.

Today, the most efficient white LEDs use blue light-emitting diodes with a rare earth phosphor coating that converts blue light to yellow, green, and/or red, resulting in white illumination. This process is called phosphorus downconversion. Phosphors are luminescent materials that can absorb and convert high-energy photons (like blue light) into lower-energy, longer-wavelength light (like green, yellow, and red, respectively).

This process of downward conversion of phosphorus, however, has limitations. The downconversion process is inherently inefficient because high-energy photons must lose energy (in the form of heat) to be converted to photons of other energies. Currently, the white LEDs used in SSL generate seven times more heat than the light output. Additionally, phosphors are chemically unstable and add significant raw material and packaging costs (by approximately 20%) to the LED device.

Despite the increase in efficiency of blue LEDs in recent years, SSL using phosphors only has a maximum theoretical luminous efficiency of 255 lumens/watt (lm/W), whereas color mixing of LEDs can achieve a theoretical maximum luminous efficiency of 408 lm/W.

However, many established approaches for green LEDs suffer from an “efficiency drop” at high current densities. It has been difficult to achieve high-efficiency green emission with traditional hexagonal III nitride, even when increasing the indium content (an expensive element required for green emissions), resulting in higher defect densities and drop efficiency. This presents a fundamental challenge to the widespread adoption of SSL.

“We found a way to synthesize high-quality, low-defect-density, single-phase cubic gallium nitride using an aspect-ratio phase-trapping technique invented by the Bayram group,” says Lee. In aspect ratio phase trapping, defects, along with unwanted hexagonal phase, are “trapped” inside the grooves so that the surface of the active layer is perfect cubic phase material. The cubic and hexagonal phases refer to the way the atoms of materials organize themselves.

Here, researchers have developed a cubic III-nitride system that can enable highly efficient, sag-free green LEDs with an IQE of 32% and an indium content of just 16%. This is the highest WQI reported for cubic wells, with approximately 30% less indium than the amount needed in a traditional hexagonal well.

Bayram says the green gap can be closed using cubic III nitride, as the benefits of these materials for SSL are well documented both theoretically and experimentally. The actual effectiveness of cubic devices has been hampered by the quality and purity of the cubic phase, but the new aspect ratio phase trapping technique used in this research achieves pure, high-precision cubic III nitride. quality.