Research reveals how blue LEDs emit light

13 Sep 2006

A new understanding of the mechanism behind blue LED luminescence could lead to the development of revolutionary new families of light emitters.

Researchers based in Japan say they now understand why indium gallium nitride (InGaN) semiconductor materials, used for commercial blue LEDs, emit bright light despite their poor crystal quality (Nature Materials 10.1038/nmat1726).

"If the structural defect density is higher than 10⁵ cm^-2 for conventional LEDs, no light comes out from the material," lead author, Shigefusa Chichibu, who is based at Tsukuba University, told optics.org. "However, InGaN alloys can emit bright light even though the defect concentration is as high as 10⁹ cm^-2, which is a million times higher than in conventional LED films."

The team concluded that the charge carrying holes in In-containing alloys are preferentially trapped or localized by atomic arrangements consisting of just three indium atoms alternating with nitrogen in a chain (In-N-In-N-In) or in a tetrahedron. The trapped holes then form localized excitons (hole--electron pairs) to emit the light, which was found to be responsible for the material's brightness.

On this basis, the researchers suggest "the enterprising use" of such atomic arrangements to develop highly efficient light emitters using defective crystals and even amorphous glass-like materials. Group-III-nitride semiconductors have already shown potential as light sources for displays, optical storage and lighting.

The team used time-resolved photoluminescence and slow positron annihilation measurements to determine the relationship between the lifetimes of light emission and the defect concentration in (Al,In,Ga)N semiconductors.

"We found that positron diffusion lengths in InGaN alloys are very short. This was strange, because usually this means that the material contains high density point defects and may not emit the light," said Chichibu.

The positron beams were used to simulate the behavior of holes in the material, since positrons have a positive charge identical to holes. Data from positron diffusion lengths therefore provided an insight into the size or density of point defects.

One of the paper's authors, Shuji Nakamura, first developed blue LEDs from nitride materials in 1993 and was awarded the prestigious Millennium Technology prize for 2006 in Finland last week.