Zinc oxide microwires boost LED performance

Ashley N. Paddock, ashley.paddock@photonics.com

Microwires made of zinc oxide can enhance LED performance, improving the efficiency at which LEDs convert electricity into ultraviolet light. LEDs may be the first to be enhanced by the creation of an electrical charge in a piezoelectric material using the piezo-phototronic effect.

A mechanical strain to the microwires creates a piezoelectric potential – an electrical charge – in the wires, enabling tuning of the charge transport and enhancing the carrier injection in the LEDs, said researchers at Georgia Institute of Technology School of Materials Science and Engineering. This could mean big things for many electro-optical processes, including advances in the energy efficiency of lighting devices.

Georgia Tech professor Zhong Lin Wang (right) and graduate research assistant Ying Liu study their enhanced LEDs. Courtesy of Gary Meek.

“Our discovery is groundbreaking research not only for exploring the piezo-phototronic effect through three-way coupling among mechanical, electronical and optical properties, but also can largely improve the efficiency and performance of LEDs and the design of a large range of optoelectronic devices based on ZnO and GaN with the use of their piezoelectric property,” said Zhong Lin Wang, a Regents professor at Georgia Tech.

Traditional LED designs use structures such as quantum wells to trap electrons and holes, which must remain close together long enough to recombine. The longer that electrons and holes can be retained in proximity to one another, the higher the ultimate efficiency of the LED device.

Mechanically compressing or otherwise straining structures made from piezoelectric materials such as zinc oxide creates a piezoelectric potential because of the polarization of ions within the crystals. In the gallium nitride LEDs, the researchers used the local piezoelectric potential to tune the charge transport at the p-n junction. The zinc oxide wires form the “n” component of a p-n junction, with the gallium nitride thin film providing the “p” component. Free carriers were trapped at this interface region in a channel created by the piezoelectric charge formed by compressing the wires.

Adding mechanical strain onto zinc oxide microwires enhances LED performance.

The effect was to increase the rate at which electrons and holes recombine to generate photons, enhancing the external efficiency of the device through improved light emission and a higher injection current.

The new devices increased their emission intensity by a factor of 17 and boosted the injection current by a factor of four when compressive strain of 0.093 percent was applied to the zinc oxide wire. That improved conversion efficiency by as much as a factor of 4.25. The LEDs produced emissions at ultraviolet wavelengths (about 390 nm), but Wang believes that the frequencies can be extended into the visible light range for a variety of optoelectronic devices.

Beyond LEDs, he also believes that the approach pioneered in this study can be applied to other optical devices that are controlled by electrical fields.

“We are now extending this research to thin-film-based LEDs and arrays of nanowire-based LEDs,” Wang said. “We anticipate to use the piezo-phototronic effect to enhance the efficiency of GaN-based LEDs so it can make great contributions to energy saving and solid-state lighting.”

The research was published online in Nano Letters (doi: 10.1021/nl202619d).