The new technique reduces the number of defects in those films by two to three orders of magnitude — increasing the output of light by a factor of two for a given amount of power. (Image courtesy of Lukasz Tylec)
diodes (LEDs) are an increasingly popular technology for use in
energy-efficient lighting. Researchers from North Carolina State
University have now developed a new technique that reduces defects in
the gallium nitride (GaN) films used to create LEDs, making them more
lighting relies on GaN thin films to create the diode structure that
produces light. The new technique reduces the number of defects in those
films by two to three orders of magnitude.
improves the quality of the material that emits light,” says Dr. Salah
Bedair, a professor of electrical and computer engineering at NC State
and co-author, with NC State materials science professor Nadia El-Masry,
of a paper describing the research. “So, for a given input of
electrical power, the output of light can be increased by a factor of
two – which is very big.” This is particularly true for low electrical
power input and for LEDs emitting in the ultraviolet range.
researchers started with a GaN film that was two microns, or two
millionths of a meter, thick and embedded half of that thickness with
large voids – empty spaces that were one to two microns long and 0.25
microns in diameter. The researchers found that defects in the film were
drawn to the voids and became trapped – leaving the portions of the
film above the voids with far fewer defects.
are slight dislocations in the crystalline structure of the GaN films.
These dislocations run through the material until they reach the
surface. By placing voids in the film, the researchers effectively
placed a “surface” in the middle of the material, preventing the defects
from traveling through the rest of the film.
The voids make an impressive difference.
voids, the GaN films have approximately 10[to the 10th power] defects
per square centimeter,” Bedair says. “With the voids, they have 10[to
the 7th power] defects. This technique would add an extra step to the
manufacturing process for LEDs, but it would result in higher quality,
more efficient LEDs.”
The paper, “Embedded voids approach for low defect density in epitaxial GaN films,” was published online Jan. 17 by Applied Physics Letters.
The paper was co-authored by Bedair; Pavel Frajtag, a Ph.D. student at
NC State; Dr. Nadia El-Masry, a professor of material science and
engineering at NC State; and Dr. N. Nepal, a former post-doctoral
researcher at NC State now working at the Naval Research Laboratory. The
research was funded by the U.S. Army Research Office.