Using Oxygen to enhance red LEDs
a) Eu distribution of delta structure samples with alternating 10nm GaN layers and 4nm GaN:Eu layers. (b) Zoomed in view of the sample structure aligning with atomic percentage of Eu and oxygen.
An international research group has shown that quantity and location of oxygen in GaN can be fine-tuned to improve the optical performance of europium-doped GaN LED 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 uses only the oxygen levels that are inevitably present in the GaN anyway. Eu, a rare earth element, is added to GaN as a dopant to provide highly efficient red colour 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.
"If the oxygen is not there, the Eu clusters up and does not incorporate. When the oxygen is present at about 2 percent, oxygen passivation takes place, allowing the Eu to incorporate into the GaN without clustering."
Preliminary LED devices containing a single 300-nanometer active GaN:Eu layer have been demonstrated in recent years, the group reported, but have not yet achieved commercial viability, in part because of the incompatibility of oxygen with GaN.
To overcome that hurdle, said Dierolf, the researchers decided that instead of growing one thick, homogeneous layer of GaN:Eu they would grow several thinner layers of alternating doped and undoped regions. This approach, they found, uses the relatively small amount of oxygen that is naturally present in GaN grown with MOVPE.
"Instead of growing a thick layer of Eu-doped GaN," said Dierolf, "we grew a layer that alternated doped and undoped regions. Through the diffusion of the europium ion, oxygen from the undoped regions was used to incorporate the Eu into the GaN. The europium then diffused into the undoped regions."
To determine the optimal amount of oxygen needed to circumvent the oxygen-GaN incompatibility, the researchers also conducted experiments on GaN grown with an Eu "precursor" containing oxygen and on GaN intentionally doped with argon-diluted oxygen.
They found that the MOVPE- grown GaN contained significantly less oxygen than the other samples, with a concentration over two orders of magnitude lower than those found in the samples grown with the oxygen-containing Eu precursor.
The group plans next to grow GaN quantum well structures and determine if they enable Eu to incorporate even more favorably and effectively into GaN. Toward that end, Dierolf and Nelson Tansu, professor of electrical and computer engineering and director of Lehigh's Center for Photonics and Nanoelectronics, have been awarded a Collaborative Research Opportunity (CORE) grant from Lehigh.
'Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications' by B. Mitchell et al; Scientific Reports 6, Article number: 18808 (2016)