Through the use of zinc oxide microwires, the efficiency at which gallium nitride LEDs convert electricity to ultraviolet light has improved significantly. The LEDs are believed to be the first whose performance has been enhanced by the creation of an electrical charge in a piezoelectric material using the piezo-photronic effect. By applying mechanical strain to the microwires, researchers at Georgia Institute of Technology School of Materials Science and Engineering have created a piezoelectric potential in the wires, enabling tuning of the charge transport and enhancement of the carrier injection in the LEDs. “By utilizing this effect, we can enhance the external efficiency of these devices by a factor of more than four times, up to eight percent,” said Zhong Lin Wang, a Regents professor at the school. “From a practical standpoint, this new effect could have many impacts for electro-optical processes, including improvements in the energy efficiency of lighting devices.” Georgia Tech Regents professor Zhong Lin Wang (right) and graduate research assistant Ying Liu study LEDs whose performance has been enhanced through the piezo-phototronic effect. (Images: Gary Meek) Because of the polarization of ions in the crystals of piezoelectric materials such as zinc oxide, mechanically compressing or otherwise straining structures made from the materials creates a piezoelectric potential — an electrical charge. In the gallium nitride LEDs, the researchers used the local piezoelectric potential to tune the charge transport at the p-n junction. The effect was to increase the rate at which electrons and holes recombined to generate photons, enhancing the external efficiency of the device through improved light emission and higher injection current. “The effect of the piezopotential on the transport behavior of charge carriers is significant due to its modification of the band structure at the junction,” Wang said. 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. A LED whose performance has been enhanced through the piezo-phototronic effect is studied in the laboratory of Regents professor Zhong Lin Wang. 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. The devices produced by the Georgia Tech team increased their emission intensity by a factor of 17 and boosted 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 fabricated by the researchers produced emissions at ultraviolet frequencies (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. “This opens up a new field of using the piezoelectric effect to tune optoelectronic devices,” he said. “Improving the efficiency of LED lighting could ultimately be very important, bringing about significant energy savings because so much of the world's energy is used for lighting.” Details of the research were published in Nano Letters. For more information, visit: www.gatech.edu