Researchers ay Georgia Tech have significantly improved the efficiency of ultra-violet (390nm) thin-film GaN LEDs by adding a zinc oxide microwire to the junction.
"The devices are believed to be the first LEDs whose performance has been enhanced by the creation of an electrical charge in a piezoelectric material," said the university.
The potential created by applying mechanical strain to the wires acts as a local gate voltage which was used to tune the charge transport and enhance carrier injection in the LEDs, increasing the rate at which electrons and holes recombined to generate photons.
"The effect of the piezo potential on the transport behaviour of charge carriers is significant due to its modification of the band structure at the junction. We can enhance the external efficiency of these devices by a factor of more than four times, up to 8%," said Professor Zhong Lin Wang. "From a practical standpoint, this effect could have many impacts for electro-optical processes, including improvements in the energy efficiency of lighting devices."
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," said the university. "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 Tech team is claiming 17x increase in emission intensity from the 4x boosted injection current when compressive strain of 0.093% was applied - conversion efficiency went up "by as much as a factor of 4.25".
17x from 4x is through improved extraction efficiency - which can be awful.
"Although the internal quantum efficiencies of LEDs can be as high as 80%, the external efficiency for a conventional single p-n junction thin-film LED is currently only about 3%," said the Tech.
In the experimental devices, a single zinc oxide wire 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 metal-organic chemical vapour 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.
Transparent polystyrene tape was used to cover the wire. A force was then applied to the tape by an alumina rod connected to a piezo nano-positioning 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 micro-wires, showed a decrease. This difference was due to the reversal in the sign of the piezo potential," said the university.
Source:
