Novel memories: Modeling negative capacitance; nonvolatility of RRAM; MTJ degradation
Researchers from Lawrence Berkeley National Laboratory developed an open-source 3D simulation framework capable of modeling the atomistic origins of negative capacitance in ferroelectric thin films at the device level.
When a material has negative capacitance, it can store a greater amount of electrical charge at lower voltages. The team believes the FerroX framework will aid in the design of energy-efficient ferroelectric memory.
“There’s a lot of trial and error in the making of new materials. It’s like making a new recipe. Researchers typically have to work days and nights in the lab to change that recipe. But with our modeling tool, FerroX, you can use your own computer to target specific parameters that can affect the performance of the negative capacitance effect,” said Zhi (Jackie) Yao, a research scientist in Berkeley Lab’s Applied Mathematics & Computational Research Division, in a press release.
The framework enabled the researchers to develop 3D phase-field simulations of a ferroelectric thin film, in which they could vary the phase composition at will and study the impacts on the film’s electronic properties. By using the tool, they found that the negative capacitance effect can be enhanced by optimizing the domain structure – reducing the size of the ferroelectric grains and arranging them to have a particular direction of ferroelectric polarization. [1]
Researchers from the University of Michigan, Ford Research, Oak Ridge National Laboratory, Sandia National Laboratories, University at Albany, and Arizona State University investigated how memristors are able to retain information without a power source.
“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, in a press release.
The team focused on filament-type valence change memory (VCM), a type of resistive random access memory (RRAM). VCM sandwiches an insulating tantalum oxide layer between two platinum electrodes, explained Patsy DeLacey of U-M in a press release. “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’.”
“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, in a press release.
The team used atomic force microscopy to image the approximately 5nm wide filaments under increased temperatures that mimicked a much longer period of time. Different sized filaments yielded different retention behavior, with smaller ones dissolving over time while longer ones strengthened. They determined that the formation and stability of conductive filaments depend on phase separation. By leveraging phase separation, the researchers were able to extend memory retention from one day to over 10 years in a rad-hard memory chip. [2]
Researchers from the University of Minnesota Twin Cities investigated how a device based on spintronic magnetic tunnel junctions (MTJs), such as MRAM, degrades over time.
Using real-time transmission electron microscopy, they looked at the nanopillars within an MTJ memory device. When a continuous current was run through the device, the layers of the device got pinched and caused the device to malfunction. Once the device forms a “pinhole” (the pinch), it is in the early stages of degradation. As the researchers added more and more current to the device, it melted down and completely burned out.
“What was unusual with this discovery is that we observed this burn out at a much lower temperature than what previous research thought was possible,” said Andre Mkhoyan, professor and chair in the University of Minnesota Department of Chemical Engineering and Material Sciences, in a statement. “The temperature was almost half of the temperature that had been expected before.”
The team attributes this to the way that materials at the atomic scale can have very different properties, including melting temperature. They hope the work can be used to improve the design of computer memory units to increase longevity and efficiency. [3]
[1] P. Kumar, M. Hoffmann, A. Nonaka, S. Salahuddin, Z. J. Yao, 3D Ferroelectric Phase Field Simulations of Polycrystalline Multi-Phase Hafnia and Zirconia Based Ultra-Thin Films. Adv. Electron. Mater. 2024, 2400085. https://doi.org/10.1002/aelm.202400085
[2] Jingxian Li, Anirudh Appachar, Sabrina L. Peczonczyk, Elisa T. Harrison, Anton V. Ievlev, Ryan Hood, Dongjae Shin, Sangmin Yoo, Brianna Roest, Kai Sun, Karsten Beckmann, Olya Popova, Tony Chiang, William S. Wahby, Robin B. Jacobs-Godrim, Matthew J. Marinella, Petro Maksymovych, John T. Heron, Nathaniel Cady, Wei D. Lu, Suhas Kumar, A. Alec Talin, Wenhao Sun, Yiyang Li. Thermodynamic origin of nonvolatility in resistive memory. Matter, 2024. http://dx.doi.org/10.1016/j.matt.2024.07.018
[3] Hwanhui Yun, Deyuan Lyu, Yang Lv, Brandon R. Zink, Pravin Khanal, Bowei Zhou, Wei-Gang Wang, Jian-Ping Wang, K. Andre Mkhoyan. Uncovering Atomic Migrations Behind Magnetic Tunnel Junction Breakdown. ACS Nano, 2024. http://dx.doi.org/10.1021/acsnano.4c08023
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