LED performance improved by zinc oxide microwires

According to the team from the Georgia Institute of Technology, the devices are the first leds in which performance has been enhanced by the creation of an electrical charge in a piezoelectric material using the piezo phototronic effect. The team created an electrical charge – or 'piezoelectric potential' - in the wires by applying mechanical strain to the microwires. This potential was used to tune the charge transport and enhance carrier injection in the leds. The researchers claim 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. Zhong Lin Wang, a Regents professor at the Georgia Tech School of Materials Science and Engineering, pictured, said: "By utilising this effect, we can enhance the external efficiency of these devices by a factor of more than four times, up to 8%. From a practical standpoint, this new effect could have many impacts for electro optical purposes – including improvements in the energy efficiency of lighting devices." Because of the polarisation 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. 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 behaviour 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. 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 team from Georgia Tech says the devices it produced 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, claims the researchers, 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 390nm), 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 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 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 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 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."