Zinc oxide microwires were used by Georgia Tech researchers to improve the efficiency at which gallium nitride light-emitting diodes (LEDs) convert electricity to ultraviolet light. By applying mechanical strain to the microwires, the researchers created a piezoelectric potential in the wires, and that potential was used to tune the charge transport and enhance carrier injection in the LEDs. This control of an optoelectronic device with piezoelectric potential — or piezo-phototronics — represents another example of how materials that have both piezoelectric and semi conducting properties can be controlled mechanically.

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Georgia Tech Regents professor Zhong Lin Wang (right) and graduate research assistant Ying Liu study the improved LEDs. (Georgia Tech/Gary Meek)
“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 in the Georgia Tech School of Materials Science and Engineering. “From a practical standpoint, this new effect could have many impacts for electro-optical processes — including improvements in the energy efficiency of lighting devices.”

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An LED's performance has been enhanced through the piezo-phototronic effect. (Georgia Tech/Gary Meek)
Because of the polarization of ions in the crystals of piezoelectric materials like zinc oxide, mechanically straining structures made from the materials creates a piezoelectric potential. 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 piezo potential 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 and the gallium nitride thin film provides the “p” component. Free carriers were trapped at this interface region in a channel created by the piezoelectric charge formed by compressing the wires. 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 efficiency of the LED device will ultimately be.

The devices from 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 newly fabricated LEDs produced emissions at ultraviolet wavelengths (about 390 nm).

In the experimental devices, a single zinc oxide micro/ nanowire LED was fabricated by manipulating a wire on a trenched substrate. A magnesium-doped gallium nitride film was grown epitaxially on a sapphire substrate by metalorganic chemical vapor deposition, and was used to form a p-n junction with the zinc oxide wire. A sapphire substrate was used as the cathode that was placed side-by-side with the gallium nitride substrate with a well-controlled gap. The wire was placed across the gap in close contact with the gallium nitride, and transparent polystyrene tape was used to cover the nanowire. Force was then applied to the tape by an alumina rod connected to a piezo nanopositioning stage — creating the strain in the wire.

The researchers then studied the change in light emission produced by varying the amount of strain in 20 different devices. Half of the devices showed enhanced efficiency, while the others — fabricated with the opposite orientation of the microwires — showed a decrease. This difference was due to the reversal in the sign of the piezo potential because of the switch of the microwire orientation from +c to –c. Although the internal quantum efficiencies of these LEDs can be as high as 80 percent, the external efficiency for a conventional single p-n junction thin-film LED is currently only about three percent.

For more information, visit www.gatech.edu.