Non-volatile memory is an essential component for any device that retrieves stored information after being switched off and on. Certain oxide materials switch between high and low resistive states in response to an applied electric field, providing a binary system for ReRAM non-volatile information storage.

ReRAM devices are readily down-sized and consume little power, giving them an advantage over other non-volatile memory technologies, such as ferroelectric (FeRAM), magnetic (MRAM) and phase-change (PCRAM) systems. However, incorporating both flexibility and transparency has remained a challenge.

The most common transparent conducting semiconductor is indium tin oxide, but this is famously brittle. While IZO is more flexible, its conductivity is lower and more difficult to control.

“My group focuses on IZO, alongside nanowires and other transparent conducting oxides,” says Seung-Won Yeom, the first author of this latest work and a PhD student supervised by Byeong-Kwon Ju at Korea University and Jae Won Shim at Dongguk University. “We make it by sputtering, and then optimize the parameters – power, pressure and gas – so our IZO is good quality.”

Using their optimized IZO for the electrode material, Yeom’s group demonstrated ReRAM devices with 80% transparency. Set and reset processes were achieved at just 0.1 V and retained for more than 10⁵ seconds – over a day. In addition, the memory properties were unaffected when the researchers bent the device through a radius of curvature as tight as 10 nm.

The researchers behind the transparent flexible memory

Optimization through thick and thin

“About three months ago I thought of using IZO in resistive memory to make good, transparent and flexible memory,” says Yeom. He and his colleagues used optimized IZO for the electrodes and 40 nm thin films of Al₂O₃ for the resistive switching material.

Al₂O₃ has a large bandgap and breakdown electric field, high permittivity and good thermal stability, making it a good material for resistive switching. Although bulk Al₂O₃ is brittle and opaque, films with thicknesses below 100 nm are transparent and flexible.

“The thickness of the IZO is also important to optimize the conductivity and transparency,” says Yeom. After several experiments, the researchers concluded 250 nm was the optimum IZO thickness for the electrodes.

Yeom suggests the Al₂O₃ IZO non-volatile system may be useful for flexible solar cells and electronic displays, since there is currently a lack of transparent flexible memory technology compatible with these devices. In future work he hopes to use gold nanoparticles in an oxide for low-voltage operation to produce memory devices with further reduced power consumption.

Full details of the work are reported in Nanotechnology 27 07LT01. This letter also features in Nanotechnology Select.

For more on non-volatile memory based on nanostructures visit the Nanotechnology special issue.