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21 Nov 2005
Researchers boost the output of blue LEDs by using laser holography to create an array of air holes within the device.
Korean scientists have more than doubled the output power of GaN-based LEDs thanks to a holographic process that etches a two-dimensional photonic crystal (PC) into the device. The researchers say that their holographic method is a major improvement on previous work using electron-beam lithography, as the technique suits large area processing with high throughput. (Appl. Phys. Lett. 87 203508)
Typically, a large portion of the light generated by an LED is wasted because of internal reflection and lateral waveguiding. By integrating a PC lattice within the device structure, LED-makers have the opportunity to control photonic behaviour and improve light output.
Researchers from Seoul National University used a two-beam holography setup equipped with a He-Cd laser (325 nm) to write square-lattice PC patterns with periods of 300, 500 and 700 nm. LED wafers were provided by Samsung's Advanced Institute of Technology.
The team found that its 500 nm lattice design offered the best performance, with the device extracting 2.1 times more light than a conventional planar LED. "The only problem we noticed was a slight increase in operating voltage, but this can be solved in the future," Heonsu Jeon of Seoul National University told Optics.org
Computer simulation experts at Seoul's Korea University are now busy optimizing the structure of the PC-LED, which emits at around 400 nm. The next step for Jeon and his colleagues is to assess the device's commercial viability.
"Adding a photonic crystal pattern should not cause any significant increase in the cost of the LED," he said. "In the future, the laser holography method could [even] be replaced with a phase mask technique to suit a mass production environment."
Author
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.
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