LED 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. “This 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.
The 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.
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.
“Without 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.
NC State’s electrical and computer engineering and material science and engineering departments are part of the university’s College of Engineering.
Note to Editors: The study abstract follows.
“Embedded voids approach for low defect density in epitaxial GaN films”
Authors: P. Frajtag, N.A. El-Masry, S.M. Bedair, North Carolina State University; N. Nepal, North Carolina State University and Naval Research Laboratory
Published: online Jan. 17, Applied Physics Letters
Abstract: We have developed a technique for defect reduction in GaN epitaxial films grown on sapphire substrates. This technique relies on the generation of high densities of embedded microvoids (~108/cm2), a few microns long and less than a micron in diameter. These voids are located near the sapphire substrate, where high densities of dislocations are present. Network of embedded voids offer free surfaces that act as dislocation sinks or termination sites for the dislocations generated at the GaN/sapphire interface. Both transmission electron and atomic force microscopy results confirm the uniform reduction of the dislocation density by two orders of magnitude.
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