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Scientists reveal limitation in current blue LED technology

Scientists reveal limitation in current blue LED technology
Scanning transmission electron micrograph of the atomic arrangement in (In, Ga) N monolayer | IMC

For the first time, an international research group has uncovered the core mechanism that limits indium (In) incorporation into indium gallium nitride ((In, Ga) N) thin films – the key material for blue light emitting diodes (LEDs). The increase in In content in InGaN thin films is the common approach to shift the emission of III-nitride based LEDs toward the green and red regions of the optical spectrum necessary for modern RGB LEDs. The new findings answer the long-standing research question: Why does this classic approach fail when we try to gain efficient green and red LEDs based on InGaN?

Despite advances in green LEDs and lasers, researchers were unable to achieve a higher indium content than 30% in the thin films. The reason for this was unclear: Is it a problem to find the right conditions for growth or rather a fundamental effect that can not be overcome? Now an international team from Germany, Poland and China has shed new light on this question and pointed out the mechanism that is responsible for this limitation.

In their work, scientists sought to maximize indium content by growing single atomic layers of InN on GaN. However, regardless of growth conditions, indium concentrations never exceeded 25% – 30% – a clear sign of a fundamentally limited mechanism. The researchers used sophisticated characterization methods, such as the TEM and In-Situ Reflection High Energy Electron Diffraction (RHEED), and discovered that when the indium content reaches about 25%, the atoms within the ( In, Ga) N monolayers are arranged in a regular pattern – a single indium atomic row alternates with two atomic rows of gallium atoms. Comprehensive theoretical calculations revealed that the atomic arrangement is induced by a certain surface reconstruction: indium atoms are connected to four neighboring atoms instead of three as expected. This results in stronger bonds between indium and nitrogen atoms, which on the one hand make it possible to use higher temperatures during growth and on the other hand to give the material a better structural quality. On the other hand, the ordered atomic arrangement limits the indium content to 25%, which can not be overcome under realistic growth conditions. to use higher temperatures during growth and on the other hand to give the material a better structural quality. On the other hand, the ordered atomic arrangement limits the indium content to 25%, which can not be overcome under realistic growth conditions. to use higher temperatures during growth and on the other hand to give the material a better structural quality. On the other hand, the ordered atomic arrangement limits the indium content to 25%, which can not be overcome under realistic growth conditions.

“Obviously, a technological bottleneck impedes all attempts to shift the emission from the green to the yellow and red regions of the spectra,” explains Drs. Tobias Schulz, scientist at the Leibniz Institute for Crystal Growth (ICZ): “For example, the growth of InGaN films on high-quality InGaN pseudo-substrates, which would reduce the strain in the layer.”

The regular arrangement of the atoms, however, may help overcome known limitations of the InGaN material system: localization of charge carriers due to variations in the chemical composition in the layer. The growth of fixed (In, Ga) N alloys with a stable composition at high temperatures can thus improve the optical properties of devices.

The work is the result of a collaboration between the Leibniz Institute for Crystal Growth (Berlin, Germany), the Max Planck Institute for Iron Research (Dusseldorf, Germany), the Paul Drude Institute for Solid State Electronics (Berlin, Germany), the Institute for high-pressure physics (Warsaw, Poland) and the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics (Beijing, China).

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