Pyramid electrode extracts more light

05 Dec 2006

LED emission gets a boost thanks to a pyramid shaped ZnO electrode.

A pyramid shaped electrode can more than double the light extraction efficiency of LEDs, say researchers in the US. Developed by scientists from the University of California Santa Barbara (UCSB), the ZnO wafer bonded LED chip could prove to be a big hit in solid-state lighting applications.

"Our motivation is to develop bright LEDs," Akihiko Murai of USCB's Solid-state Lighting and Display Centre told optics.org. "However, we think that the use of ZnO for LED fabrication could also have an economic advantage."

Wafer bonded LED chip with a hexagonal pyramid shaped ZnO electrode

Shaped electrode

The blue light source is made by direct wafer bonding an n-type ZnO substrate to a gallium nitride LED wafer and then selectively etching the structure to create a truncated pyramid shaped electrode.

According to the group, the pyramid geometry decreases the mean photon path length within the crystal and reduces the impact of loss mechanisms such as internal reflection and absorption. The pyramid shaped chip has a peak wavelength emission of 460 nm and measures approximately 800 microns across its base.

LED team

The group compared its design with a conventional LED made from the same GaN wafer and configured with a thin Ni/Au p-type electrode. Both LEDs had an identical junction area of 0.46 mm².

Using an integrating sphere and spectrometer set-up, the researchers discovered that the output power of their ZnO wafer bonded LED was 2.2 times higher than the conventional device at 20 mA forward current.

Looking ahead, superior results at 1 mA and 5 mA show that further improvements in performance could be possible. The team thinks that light extraction at higher current is being suppressed by the build up of additional heat at the ZnO/GaN interface. The cause is thought to be incomplete wafer bonding and Murai and his colleagues are now looking at ways of optimizing the fabrication process.

Author

James Tyrrell is News Editor of Optics & Laser Europe magazine and a contributor to optics.org.