Voids Slash Defect Density in GaN LEDs
Jan 26, 2011
Defects in the gallium nitride LED film are drawn to voids and become trapped leaving the portions of the film above the voids with far fewer defects.
Researchers from North Carolina State University have developed a new technique that reduces defects in the gallium nitride (GaN) films used to create LEDs grown on sapphire substrates.
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. So, for a given input of electrical power, the output of light can be increased by a factor of two, which is very big.”,” say the authors of a paper describing the research. This is particularly true for low electrical power input and for LEDs emitting in the ultraviolet range.
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)
The researchers started with a GaN film that was two microns 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).
Using transmission electron and atomic force microscopy, the scientists 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 travelling through the rest of the film.
The voids were embedded near the sapphire substrate, where high densities of dislocations are present.
The network of voids acts as dislocation sink or termination site for the dislocations generated at the GaN/sapphire interface.
“Without voids, the GaN films have approximately 1010
defects per square centimetre,” says Bedair, one of the researchers. “With the voids, they have 107
defects. This technique would add an extra step to the manufacturing process for LEDs, but it would result in higher quality, more efficient LEDs.”
More details of this research are described in the paper “Embedded voids approach for low defect density in epitaxial GaN films,” by P. Frajtag, N. A. El-Masry, N. Nepal and S. M. Bedair in Applied Physics Letters, 98
, 023115 (2011); doi:10.1063/1.3540680)(published online 17 Jan 2011)