Army-Funded Research Makes Breakthrough in Non-Volatile Memory TechnologyJuly 27, 2020 by Jake Hertz
Using van der Waals materials, USC researchers have made a breakthrough in non-volatile memory based on ferroelectric tunnel junctions.
Researchers are always looking for new methods of creating non-volatile memory to better store data even when power is removed. In theory, a machine that utilizes only non-volatile memory, as opposed to a combination of the volatile and non-volatile, requires no booting up.
Unfortunately, volatile memory tends to be much faster and inexpensive thanks to the use of transistors, making it better suited for RAM than its non-volatile counterpart.
Standard volatile DRAM cell. Image used courtesy of Michigan State University
One technology that researchers have turned to for fast and inexpensive non-volatile memory is that of the ferroelectric tunnel junction (FTJ).
Ferroelectric RAM (FRAM)
Ferroelectric memory is a form of memory where information is stored in ferroelectric polarizations. Put simply, ferroelectric materials have a spontaneous electrical polarization that can be reversed by the application of an external electric field.
As the ferroelectric polarization of these materials reverses, it either facilitates or inhibits the flow of current. This on/off characteristic is how digital information—1 or 0—is stored. This technology is commonly referred to as ferroelectric RAM (FRAM).
FRAM cell. Screenshot used courtesy of Microchip
A FRAM cell will utilize a ferroelectric capacitor, which stores the polarization, and a pass transistor to read out the state. This is very similar to a DRAM cell, except it doesn’t require a refresh, making it a non-volatile technology.
The Ferroelectric Tunnel Junction
In order to miniaturize this technology such that it can be compatible with current CMOS nodes, scientists look to the ferroelectric tunnel junction. In an FTJ, two metal electrodes are separated by a thin ferroelectric layer, and the on/off state is determined by their tunneling electroresistance (TER). Here, the TER is affected by the potential difference across the ferroelectric barrier, along with a transmission and attenuation coefficient across the interface.
FTJ layout. Image used courtesy of the University of Nebraska-Lincoln
The FTJ offers very low power consumption and fast write speed, and thus are promising for developing memory and computing applications.
However, one of the major setbacks for FTJ technology is a lack of reliability. Historically, FTJs have a small barrier height modulation of about 0.1 eV, according to recent research. This means that there is a small detectable difference between different states of an FTJ, making it difficult to distinguish.
Merging FTJ with van der Waals Materials
A new Army-funded study at the University of Southern California has come up with a way to eliminate this reliability issue.
Researchers at the University of Southern California Viterbi School of Engineering have married FTJ technology with van der Waals materials and have found some impressive results. van der Waals materials are those with strong in-plane covalent bonding and weak interlayer interactions.
Depiction of the research team's new memory device. Image used courtesy of the USC Viterbi School of Engineering
By merging these two technologies, researchers were able to create an FTJ with a barrier height modulation of 1 eV.
Ferroelectric Memory Immune to Data Corruption
With this research, published in Nature, scientists have finally been able to create ferroelectric memory that is not subject to data corruption. Increasing the barrier height modulation allows for more easily distinguished states, making the technology more reliable.
Han Wang, professor of ECE at USC, says, “These materials allow us to build devices that can potentially be scaled to atomic-scale thickness. This means the voltage required to read, write, and erase data can be drastically reduced which in turn can make the memory electronics much more energy efficient.”
This improvement in FRAM technology could be the key to longer battery life and increased upload speeds in future generations of computers.