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 explained.
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.
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 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 research team produced emissions at ultraviolet frequencies (about 390 nanometers), but Wang believes the frequencies can be extended into the visible light range for a variety of optoelectronic devices. "These devices are important for today's focus on green and renewable energy technology," he said.
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 junc
|Contact: John Toon|
Georgia Institute of Technology Research News