| Sep 11, 2024 |
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(Nanowerk News) Phase separation, when molecules part like oil and water, works alongside oxygen diffusion to help memristors – electrical components that store information using electrical resistance – retain information even after the power is shut off, according to a University of Michigan led study published in Matter (“Thermodynamic origin of nonvolatility in resistive memory”).
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Up to this point, explanations have not fully grasped how memristors retain information without a power source, known as nonvolatile memory, because models and experiments do not match up.
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“While experiments have shown devices can retain information for over 10 years, the models used in the community show that information can only be retained for a few hours,” said Jingxian Li, U-M doctoral graduate of materials science and engineering and first author of the study.
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To better understand the underlying phenomenon driving nonvolatile memristor memory, the researchers focused on a device known as resistive random access memory or RRAM, an alternative to the volatile RAM used in classical computing, and are particularly promising for energy-efficient artificial intelligence applications.
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The specific 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 different 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”.
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It was once thought that RRAM retains information over time because oxygen is too slow to diffuse back. However, a series of experiments revealed that previous models have neglected the role of phase separation.
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“In these devices, oxygen ions prefer to be away from the filament and will never diffuse back, even after an indefinite period of time. This process is analogous to how a mixture of water and oil will not mix, no matter how much time we wait, because they have lower energy in a de-mixed state,” said Yiyang Li, U-M assistant professor of materials science and engineering and senior author of the study.
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To test retention time, the researchers sped up experiments by increasing the temperature. One hour at 250 –C is equivalent to about 100 years at 85 °C – the typical temperature of a computer chip.
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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 within the one micron wide RRAM device.
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“We were surprised that we could find the filament in the device. It’s like finding a needle in a haystack,” Li said.
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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 size-based difference cannot be explained by diffusion alone.
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Together, experimental results and models incorporating thermodynamic principles showed the formation and stability of conductive filaments depend on phase separation.
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The research team leveraged phase separation to extend memory retention from one day to well over 10 years in a rad-hard memory chip – a memory device built to withstand radiation exposure for use in space exploration.
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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 known as e-skin, this material could be used to provide sensory feedback to prosthetic limbs, create new wearable fitness trackers or help robots develop tactile sensing for delicate tasks.
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“We hope that our findings can inspire new ways to use phase separation to create information storage devices,” Li said.
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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.
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The device was built in the Lurie Nanofabrication Facility and studied at the Michigan Center for Materials Characterization.
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