Subscribe free to our newsletters via your
. 24/7 Space News .


Subscribe free to our newsletters via your




















CHIP TECH
Uncovering oxygen's role in enhancing red LEDs
by Staff Writers
Bethlehem PA (SPX) Jan 18, 2016


This is 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. Image courtesy B. Mitchell, D. Timmerman, J. Poplawsky, W. Zhu, D. Lee, R. Wakamatsu, J. Takatsu, M. Matsuda, W. Guo, K. Lorenz, E. Alves, A. Koizumi, V. Dierolf and Y. Fujiwara. For a larger version of this image please go here.

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 Tecnico 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.

"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."

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.

Coauthors of the article include Dierolf; Yasufumi Fujiwara, a professor of materials science at Osaka University; and Jonathan D. Poplawsky, a research associate at Oak Ridge National Laboratory who received his Ph.D. from Lehigh in 2012.

A comprehensive study

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, the researchers wrote in Scientific Reports, but oxygen's influence on, and interaction with, RE dopants in GaN is less well understood.

"The presence of oxygen in GaN," the group wrote in their article, which was published online Jan. 4, "...is normally discussed with a purely negative connotation, where possible positive aspects of its influence are not considered.

"For the continued optimization of this material, the positive and negative roles of critical defects, such as oxygen, need 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.

Its investigation, the group wrote, represented the "first comprehensive study of the critical role that oxygen has on Eu in GaN." The group chose to experiment with Eu-doped GaN (GaN:Eu), said Dierolf, 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, 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, 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.

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," the group wrote, "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.

"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."

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.

The other coauthors of the Scientific Reports paper were D. Timmerman, W. Zhu, D. Lee, R. Wakamatsu, J. Takatsu, M. Matsuda, W. Guo, A. Koizumi, and Y. Fujiwara from Osaka University, and K. Lorenz and E. Alves from the Campus Tecnologico e Nuclear of the Instituto Superior Tecnico in Bobadela, Portugal.

.


Related Links
Lehigh University
Computer Chip Architecture, Technology and Manufacture
Nano Technology News From SpaceMart.com






Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle

Previous Report
CHIP TECH
New Chips Ease Operations In Electromagnetic Environs
Washington DC (SPX) Jan 13, 2016
Competition for scarce electromagnetic (EM) spectrum is increasing, driven by a growing military and civilian demand for connected devices. As the spectrum becomes more congested, the Department of Defense (DoD) will need better tools for managing the EM environment and for avoiding interference from competing signals. One recent DARPA-funded advance, an exceptionally high-speed analog-to- ... read more


CHIP TECH
Momentum builds for creation of 'moon villages'

Chang'e-3 landing site named "Guang Han Gong"

South Korea to launch lunar exploration in 2016, land by 2020

Death rumors of Russian lunar program 'greatly exaggerated' - Deputy PM

CHIP TECH
Rover Rounds Martian Dune to Get to the Other Side

Boulders on a Martian Landslide

NASA suspends March launch of InSight mission to Mars

University researchers test prototype spacesuits at Kennedy

CHIP TECH
Strengthening Our Space Technology Future: Snapshots of Success

Six Orion Milestones to Track in 2016

Gadgets get smarter, friendlier at CES show

Congress to NASA: Hurry up on that 'habitation augmentation module'

CHIP TECH
China plans 20 launches in 2016

China's Belt and Road Initiative catches world's imagination: Inmarsat CEO

China launches HD earth observation satellite

Chinese rover analyzes moon rocks: First new 'ground truth' in 40 years

CHIP TECH
British astronaut's first spacewalk set for Jan 15

NASA Delivers New Video Experience On ISS

British astronaut dials wrong number on Xmas call from space

Space Station Receives New Space Tool to Help Locate Ammonia Leaks

CHIP TECH
SpaceX will try to land its reusable rocket on an ocean dock

Maintaining Arianespace's launch services leadership in 2016

Arianespace starts year with record order backlog

Russian Space Forces launched 21 spacecraft in 2015

CHIP TECH
Lab discovery gives glimpse of conditions found on other planets

Nearby star hosts closest alien planet in the 'habitable zone'

ALMA reveals planetary construction sites

Monster planet is 'dancing with the stars'

CHIP TECH
Self-adaptive material heals itself, stays tough

Vietnam army probes mysterious 'space balls'

How seashells get their strength

China chemical giant to acquire Germany's KraussMaffei




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News








The content herein, unless otherwise known to be public domain, are Copyright 1995-2016 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service.