Research reveals how blue LEDs emit light

Researchers 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). This according to a report which appeared originally in optics.org.

"If the structural defect density is higher than 105 /cm2 for conventional LEDs, no light comes out from the material," lead author, Shigefusa Chichibu, who is based at Tsukuba University in Japan, told optics.org. "However, InGaN alloys can emit bright light even though the defect concentration is as high as 109 /cm2, which is a million times higher than in conventional LED films." Other authors on the paper are from the University of California at Santa Barbara.

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.

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.