Scientists at Arizona State University have reported creating a GaN-based memory device that can operate at very high temperatures. The results were published in IEEE Electronic Device Letters (March 2019).

The research is funded by NASA's Hot Operating Temperature Technology (HOTTech) program to support future missions to Mercury and Venus. Temperatures on the surface of Mercury can reach 430degC while those on Venus can climb to 462degC.

The team observed threshold switching and memory behaviours in epitaxially regrown GaN-on-GaN vertical p-n diodes. The mechanism seems to be linked with the conductive path formed by traps in the insulating layer at the regrowth interface after soft breakdown. They say that the device can reliably switch more than 1000 cycles at both room temperature and 300degC with a small fluctuation on the set and reset voltage.

The set voltage increased with the increasing temperature due to the enhanced thermal de-trapping effect that made it harder to form conductive path at high temperatures. The device showed memory behaviours when the reset voltage was higher than 4.4 V. They think this is the first stage towards developing GaN-based memory devices.

In an article in IEEE Spectrum magazine, Yuji Zhao, a scientist at Arizona State University who worked on the project, explained that the device's performance was down to the etching and regrowth processes used during fabrication. After several layers of GaN were deposited, some areas were etched away with plasma, then regrown. That created an interface layer with vacancy sites that are missing nitrogen atoms, said Zhao. “The interface layer is critical for the memory effect,” he says. The researchers believe that the nitrogen vacancies are responsible for capturing and releasing electrons, giving rise to high- and low-resistance states - or 0 and 1 states - in the device.

Above 350 degC, the device lost its memory effect. But its performance returned after the device was brought back to room temperature, says Zhao. “This device is actually very robust,” he says.

Zhao and his team are now testing another version of the device for stability up to 500degC, and for long-term stability.

'Threshold Switching and Memory Behaviors of Epitaxially Regrown GaN-on-GaN Vertical p-n Diodes With High Temperature Stability' by Kai Fu et al; IEEE Electronic Device Letters (March 2019)

https://powerelectronicsworld.net/article/106786/First_steps_to_high_temperature_GaN_memory