Phase separation, when molecules part like oil and water, works alongside oxygen diffusion to assist memristors — electrical components that store information using electrical resistance — retain information even after the ability is shut off, in keeping with a University of Michigan led study recently published in Matter.
Up thus far, explanations haven’t fully grasped how memristors retain information with out a power source, generally known as nonvolatile memory, because models and experiments don’t match up.
“While experiments have shown devices can retain information for over 10 years, the models used locally show that information can only be retained for just a few hours,” said Jingxian Li, U-M doctoral graduate of materials science and engineering and first creator of the study.
To raised understand the underlying phenomenon driving nonvolatile memristor memory, the researchers focused on a tool generally known as resistive random access memory or RRAM, a substitute for the volatile RAM utilized in classical computing, and are particularly promising for energy-efficient artificial intelligence applications.
The precise RRAM studied, a filament-type valence change memory (VCM), sandwiches an insulating tantalum oxide layer between two platinum electrodes. When a certain voltage is applied to the platinum electrodes, a conductive filament forms a tantalum ion bridge passing through the insulator to the electrodes, which allows electricity to flow, putting the cell in a low resistance state representing a “1” in binary code. If a unique voltage is applied, the filament is dissolved as returning oxygen atoms react with the tantalum ions, “rusting” the conductive bridge and returning to a high resistance state, representing a binary code of “0.”
It was once thought that RRAM retains information over time because oxygen is just too slow to diffuse back. Nevertheless, a series of experiments revealed that previous models have neglected the role of phase separation.
“In these devices, oxygen ions prefer to be away from the filament and won’t ever diffuse back, even after an indefinite time frame. This process is analogous to how a combination of water and oil won’t mix, irrespective of how much time we wait, because they’ve lower energy in a de-mixed state,” said Yiyang Li, U-M assistant professor of materials science and engineering and senior creator of the study.
To check retention time, the researchers sped up experiments by increasing the temperature. One hour at 250°C is akin to about 100 years at 85°C — the standard temperature of a pc chip.
Using the extremely high-resolution imaging of atomic force microscopy, the researchers imaged filaments, which measure only about five nanometers or 20 atoms wide, forming inside the one micron wide RRAM device.
“We were surprised that we could find the filament within the device. It’s like finding a needle in a haystack,” Li said.
The research team found that different sized filaments yielded different retention behavior. Filaments smaller than about 5 nanometers dissolved over time, whereas filaments larger than 5 nanometers strengthened over time. The dimensions-based difference can’t be explained by diffusion alone.
Together, experimental results and models incorporating thermodynamic principles showed the formation and stability of conductive filaments rely upon phase separation.
The research team leveraged phase separation to increase memory retention from in the future to well over 10 years in a rad-hard memory chip — a memory device built to resist radiation exposure to be used in space exploration.
Other applications include in-memory computing for more energy efficient AI applications or memory devices for electronic skin — a stretchable electronic interface designed to mimic the sensory capabilities of human skin. Also generally known as e-skin, this material might be used to supply sensory feedback to prosthetic limbs, create latest wearable fitness trackers or help robots develop tactile sensing for delicate tasks.
“We hope that our findings can encourage latest ways to make use of phase separation to create information storage devices,” Li said.
Researchers at Ford Research, Dearborn; Oak Ridge National Laboratory; University at Albany; NY CREATES; Sandia National Laboratories; and Arizona State University, Tempe contributed to this study.
The device was in-built the Lurie Nanofabrication Facility and studied on the Michigan Center for Materials Characterization. The work on the University of Michigan was primarily funded by the National Science Foundation (ECCS-2106225).
