Georgia Tech Regents professor Zhong Lin Wang (right) and graduate research assistant Ying Liu study light-emitting diodes whose performance has been enhanced through the piezo-phototronic effect. Photo: Gary Meek |
Researchers have used zinc oxide microwires to significantly
improve the efficiency at which gallium nitride light-emitting diodes (LED)
convert electricity to ultraviolet light. 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 using the piezo-phototronic effect.
By applying mechanical strain to the microwires, researchers
at the Georgia Institute of Technology 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, known as piezo-phototronics, represents another
example of how materials that have both piezoelectric and semiconducting
properties can be controlled mechanically.
“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,” says 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.”
Details of the research were reported in Nano Letters.
The research was sponsored by the Defense Advanced Research Projects Agency
(DARPA) and the U.S. Department of Energy (DOE). In addition to Wang, the
research team mainly included Qing Yang, a visiting scientist at Georgia Tech
from the Department of Optical Engineering at Zhejiang
University in China.
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 piezo potential on the transport behavior of charge carriers is
significant due to its modification of the band structure at the junction,”
Wang explains.
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.
A light-emitting diode (LED) whose performance has been enhanced through the piezo-phototronic effect is studied in the laboratory of Regents professor Zhong Lin Wang. Photo: Gary Meek |
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% 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 wavelengths (about 390 nm), but Wang believes the wavelengths
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 says.
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.
Transparent polystyrene tape was used to cover the nanowire. A 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.
High-efficiency ultraviolet emitters are needed for
applications in chemical, biological, aerospace, military, and medical
technologies. Although the internal quantum efficiencies of these LEDs can be
as high as 80%, the external efficiency for a conventional single p-n junction
thin-film LED is currently only about three percent.
Beyond LEDs, Wang believes 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 opto-electronic devices,” Wang says. “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.”
