27 January 2011

Voids cut defects 2–3 orders of magnitude in GaN-on-sapphire

Researchers from North Carolina State University have developed a new technique that reduces defects in gallium nitride (GaN) epitaxial films grown on sapphire substrates, enabling the creation of more efficient light-emitting diodes (‘Embedded voids approach for low defect density in epitaxial GaN films’, Appl. Phys. Lett. 98, 023115 (2011), 17 January).

Normally, dislocations generated at the GaN/sapphire interface run through the crystalline structure of the GaN films until they reach the surface. The researchers started with a GaN film that was 2 microns thick and embedded half of that thickness with large voids that were 1–2 microns long and 0.25 microns in diameter near the sapphire substrate (where high densities of dislocations are present). Generating a high-density network of embedded microvoids (~108/cm2) in the film effectively created a ‘surface’ in the middle of the material, preventing the defects from traveling through the rest of the film. It was found that the defects were drawn to the voids (which act as dislocation sinks or termination sites for the dislocations, which therefore became trapped), leaving the portions of the film above the voids with far fewer defects.

Both transmission electron and atomic force microscopy results confirm that the technique reduces the dislocation density uniformly by 2–3 orders of magnitude. “Without voids, the GaN films have approximately 1010 defects per square centimeter. With the voids, they have 107 defects,” says electrical and computer engineering professor Salah Bedair (co-author with materials science professor Nadia El-Masr, Ph.D. student Pavel Frajtag, and former post-doctoral researcher N. Nepal, now working at the US Naval Research Laboratory).

“This improves the quality of the material that emits light,” Bedair adds. “So, for a given input of electrical power, the output of light can be increased by a factor of two.” This is particularly true for low electrical power input and for LEDs emitting in the ultraviolet range.

“This technique would add an extra step to the manufacturing process for LEDs, but it would result in higher-quality, more efficient LEDs,” says Bedair.

The research was funded by the US Army Research Office.

Tags: GaN Sapphire substrates GaN-on-sapphire

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