Zinc-oxide microwires improve the efficiency of UV LEDs

The devices are believed to be the first LEDs that have had their performance enhanced by using the so-called piezo-phototronic effect.

‘By utilising this effect, we can enhance the external efficiency of these devices by a factor of more than four times, up to eight per cent,’ said Prof Zhong Lin Wang of the Georgia Institute of Technology’s 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.’

High-efficiency UV 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 per cent, the external efficiency for a conventional single p-n junction thin-film LED is currently only about three per cent.

By applying mechanical strain to the microwires, the researchers the created a piezoelectric potential in the wires, and that potential was used to tune the charge transport and enhance carrier injection in the LEDs.

Due to 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 — 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 behaviour of charge carriers is significant due to its modification of the band structure at the junction,’ Wang said. ‘This opens up a new field of using the piezoelectric effect to tune opto-electronic devices.’