Uncovering oxygen’s role in enhancing red LEDs
The research 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 researchers reported that small quantities of oxygen promote the uniform incorporation of Eu into the crystal
lattices of GaN. The research 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 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, suggested 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 two 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 two percent, oxygen passivation takes place, allowing the Eu to incorporate into the GaN without clustering.”
The article is titled ‘Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications’. The lead author, Brandon
Mitchell, received his Ph.D. from Lehigh in 2014 and is now an assistant professor of physics and astronomy at the University of Mount Union and a visiting professor
at Osaka University.
Gallium nitride, a hard and durable semiconductor, is valued in solid state lighting because it emits light in the visible spectrum and because its wide band gap makes GaN electronic devices more powerful and energy-efficient than devices made of silicon and other semiconductors.
The adverse effect of oxygen on GaN’s properties has been much discussed in the scientific literature but oxygen’s influence on, and interaction with, RE dopants in GaN is less well understood.
The researchers believed that for the continued optimization of the GaN, the positive and negative roles of critical defects, such as oxygen, needed to be explored.
The group 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.
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.
The reseachers 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 realizing red LED light.
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 favorable optical properties".
“However, for the high performance and reliability of GaN-based devices, the minimization of oxygen is essential. It is clear that these two requirements are not
mutually compatible.”
Preliminary LED devices containing a single 300-nanometer active GaN:Eu layer have been demonstrated in recent years 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. The approach utilizes the relatively small amount of oxygen that is naturally present in GaN grown with organo-metallic vapor phase epitaxy (OMVPE), the common method of preparing GaN.
“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 utilized 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.
The researchers found that the OMVPE- grown GaN contained less oxygen than the other samples.
The concentration of the oxygen [in the OMVPE- grown GaN] is more than 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 optimization 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," reported the researchers. “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 optimization of these systems as well as GaN:Eu.”
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