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Red LED Optimization Reveals Oxygen’s Role in Optoelectronics

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Investigation of oxygen’s contradictory role in optoelectronics has led to a method to fine-tune the performance of europium (Eu) -doped gallium nitride (GaN) devices, which could provide the red colors in LEDs and other displays.

Oxygen — an element that can enhance optical properties of light devices — is also well-known to simultaneously impede the effectiveness of GaN, an enabling material for LEDs.

GaN is a hard, durable semiconductor, 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.

Now a team of researchers from Lehigh University, 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 reported that small quantities of oxygen promote the uniform incorporation of Eu into the crystal lattices of GaN.
(a) 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. Courtesy of B. Mitchell and D. Timmerman, et al.

The group also demonstrated a method of incorporating Eu uniformly that utilized only the oxygen levels that were naturally present in the GaN. 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, said Lehigh professor Volkmar Dierolf. Exactly why oxygen is needed for Eu incorporation is still unclear, but Dierolf and the team determined that the amount required to facilitation incorporation is roughly 2 percent of the amount of Eu ions. Without oxygen, the Eu clustered up and did not incorporate, the researchers reported.

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.

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, Dierolf said, 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 utilized the relatively small amount of oxygen that is naturally present in GaN grown with organometallic vapor phase epitaxy (OMVPE), the common method of preparing GaN.

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, the researchers 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, rendering the material compatible with current GaN-based devices.

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.

The research was published in Scientific Reports (doi: 10.1038/srep18808).

Photonics Spectra
Mar 2016
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
Research & TechnologyAmericasLehighVolkmar DierolfLEDslight sourcesoptoelectronicsTech Pulse

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