January 12, 2016

Uncovering oxygen’s role in enhancing red LEDs

A reconstructed atom probe tomography image (a) shows the europium (Eu) distribution
of the delta structure (DS) samples with alternating 10-nanometer gallium nitride (GaN)
layers and 4-nm GaN:Eu layers. A zoomed in view (b) of the DS sample structure aligns
with a plot of the atomic percentage of Eu and oxygen as a function of space.
The background signal of Eu is also indicated for reference.

(January 12, 2016)  Oxygen is indispensable to animal and plant life, but its presence in the wrong places can feed a fire and cause iron to rust.

In the fabrication of solid state lighting devices, scientists are learning, oxygen also plays a two-edged role. While oxygen can impede the effectiveness of gallium nitride (GaN), an enabling material for LEDs, small amounts of oxygen in some cases are needed to enhance the devices’ optical properties. GaN doped with europium (Eu), which could provide the red color in LEDs and other displays, is one such case.

Last week, an international group of researchers shed light on this seeming contradiction and reported that the quantity and location of oxygen in GaN can be fine-tuned to improve the optical performance of Eu-doped GaN devices. The group includes researchers from Lehigh, Osaka University in Japan, the Instituto Superior Técnico in Portugal, the University of Mount Union in Ohio, and Oak Ridge National Laboratory in Tennessee.

Writing in Scientific Reports, a Nature publication, the group said that small quantities of oxygen promote the uniform incorporation of Eu into the crystal lattices of GaN. The group also demonstrated a method of incorporating Eu uniformly that utilizes only the oxygen levels that are inevitably present in the GaN anyway. Eu, a rare earth (RE) element, is added to GaN as a “dopant” to provide highly efficient red color emission, which is still a challenge for GaN-based optoelectronic devices.

The devices’ ability to emit light is dependent on the relative homogeneity of Eu incorporation, said Volkmar Dierolf, professor and chair of Lehigh’s physics department.

“Some details, such as why the oxygen is needed for Eu incorporation, are still unclear,” said Dierolf, “but we have determined that the amount required is roughly 2 percent of the amount of Eu ions. For every 100 Eu ions, you need two oxygen atoms to facilitate the incorporation of Eu to GaN.

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