Making green LEDs more efficient and brighter

August 12, 2016 // By Julien Happich
Researchers at the University of Illinois at Urbana Champaign have developed a new method for making brighter and more efficient green LEDs, by growing gallium nitride (GaN) cubic crystals directly onto a silicon substrate.

Typically, GaN forms in one of two crystal structures: hexagonal or cubic. Hexagonal GaN is thermodynamically stable and is by far the more conventional form of the semiconductor, however, it is prone to a phenomenon known as polarization, where an internal electric field separates the negatively charged electrons and positively charged holes, preventing them from combining, which, in turn, diminishes the light output efficiency.

Until now, the only way researchers were able to make cubic GaN was to use molecular beam epitaxy, a very expensive and slow crystal growth method when compared to the widely used metal-organic chemical vapour deposition (MOCVD) method that the researchers used.

Can Bayram, an assistant professor of electrical and computer engineering at Illinois, and his graduate student Richard Liu made the cubic GaN by using lithography and isotropic etching to create a U-shaped groove on Si (100). This non-conducting layer essentially served as a boundary that shapes the hexagonal material into cubic form.


Hexagonal-to-cubic phase transformation. The scale bars represent 100 nm in all images. (a) Cross sectional and (b) Top-view SEM images of cubic GaN grown on U-grooved Si(100). (c) Cross sectional and (d) Top-view EBSD images of cubic GaN grown on U-grooved Si(100), showing cubic GaN in blue, and hexagonal GaN in red.

"Our cubic GaN does not have an internal electric field that separates the charge carriers—the holes and electrons," explained Liu. "So, they can overlap and when that happens, the electrons and holes combine faster to produce light."

According to the researchers, the new cubic GaN fabrication method may lead to LEDs free from the "droop" phenomenon, where light-emission efficiency declines as more current is being injected.

Their work was published in the Applied Physics Letters in a paper titled "Maximizing Cubic Phase Gallium Nitride Surface Coverage on Nano-patterned Silicon (100)".

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