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.
(January 12, 2016) 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 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 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.