Oxygen enhances optical properties of LEDs

Researchers have shed light on oxygen’s role in enhancing red LEDs and reported that the quantity and location of oxygen in gallium nitride (GaN) can be fine-tuned to improve the optical performance of europium (Eu)-doped GaN devices.

While oxygen can impede the effectiveness of gallium nitride (GaN), an enabling material for LEDs, 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 have found 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 (RE) 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, Volkmar Dierolf, professor and chair of Lehigh’s physics department, said.

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

“If the oxygen is not there, the Eu clusters up and does not incorporate. When the oxygen is present at about 2%, oxygen passivation takes place, allowing the Eu to incorporate into the GaN without clustering.”

The researchers used several imaging techniques, including Rutherford Backscattering, Atomic Probe Tomography and Combined Excitation Emission Spectroscopy, to obtain an atomic-level view of the diffusion and local concentrations of oxygen and Eu in the GaN crystal lattice.

The group chose to experiment with Eu-doped GaN (GaN:Eu) because europium emits bright light in the red portion of the electromagnetic spectrum, a promising quality given the difficulty scientists have encountered in realising red LED light, Dierolf said.

The group said its results “strongly indicate that for single layers of GaN:Eu, significant concentrations of oxygen are required to ensure uniform Eu incorporation and favourable optical properties”.

“However, for the high performance and reliability of GaN-based devices, the minimisation of oxygen is essential. It is clear that these two requirements are not mutually compatible.” Preliminary LED devices containing a single 300 nm 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, 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, Dierolf said. This approach uses the relatively small amount of oxygen that is naturally present in GaN grown with organometallic vapour phase epitaxy (OMVPE), the common method of preparing GaN.

“Instead of growing a thick layer of Eu-doped GaN, we grew a layer that alternated doped and undoped regions. Through the diffusion of the europium ion, oxygen from the undoped regions was utilised to incorporate the Eu into the GaN. The europium then diffused into the undoped regions,” Dierolf said.

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 OMVPE- grown GaN contained significantly less oxygen than the other samples.

“The concentration of this oxygen [in the OMVPE- grown GaN] is over two orders of magnitude lower than those [concentrations] found in the samples grown with the oxygen-containing Eu...precursor, rendering the material compatible with current GaN-based devices.

“We have demonstrated that the oxygen concentration in GaN:Eu materials can be reduced to a device-compatible level. Periodic optimisation of the concentration ratio between the normally occurring oxygen found in GaN and the Eu ions resulted in uniform Eu incorporation, without sacrificing emission intensity.

“These results appear to coincide with observations in other RE-doped GaN materials. Adoption of the methods discussed in this article could have a profound influence on the future optimisation of these systems as well as GaN:Eu.”

The group plans next to grow GaN quantum well structures and determine if they enable Eu to incorporate even more favourably 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.