15-04-2021 | | By Robin Mitchell
Researchers from the University of Sydney have directly observed the nature of ferroelectric fatigue. What is ferroelectric fatigue, why is the discovery important, and how could it help future electronic designs?
A ferroelectric material is one whose electric polarization can be changed using an external electric field. The best way to understand how ferroelectric materials work is to compare them to ferromagnetic materials as these operate in a very similar fashion.
A ferromagnetic material is one whereby the materials' magnetic dipoles can be polarised to create a permanent magnet using an external magnetic field. In the case of a ferroelectric material, the electric dipoles are being separated and polarised instead of the magnetic dipoles.
To see how this property affects the electrical characteristics, we can look at a ferroelectric capacitor which is the storage component in FRAM. If a capacitor using a ferroelectric dielectric is charged, the dielectric separates the positive and negative charges to create a polarisation. However, while the charge on a capacitor reduces over time, the polarisation in the dielectric remains. If the capacitor is charged with the opposite polarisation afterwards, it takes more current to flip the polarised state. This is how FRAM detects stored bits on ferroelectric capacitors, but the act of reading these bits is destructive (as is the case with DRAM). However, the advantage of FRAM is that because the polarisation is permanent (although reversible), it can be used to store data even with power removed.
Ferroelectric materials are used in various components ranging from memory to sensors, but ferroelectric materials suffer from fatigue when used. Simply put, the more charge cycles a ferroelectric material undergoes, the less capable that material becomes. Like FLASH memory, FRAM has a limited number of read/write cycles because of this effect. Each read access requires the bit to be destroyed and re-written, and each write causes the material to partly break down.
Ferroelectric fatigue will, to some degree, contribute to e-waste globally as devices using the technology will only be able to operate for so long. For researchers to extend the life of ferroelectric materials, they need to understand the exact cause of the breakdown, and why it happens. Until recently, the exact nature of damage had not been fully determined or observed, but thanks to a team of researchers from the University of Sydney, this has now changed.
To understand how ferroelectric fatigue occurs, researchers used an electron microscope on a piece of a specially made ferroelectric material to observe how the material changes with each polarization reversal. After 130 cycles, researchers identified small areas in the material that grew in size with each polarisation cycle, and these areas are unable to hold electric polarisation.
Specifically, the researchers stated that charge accumulation at domain walls is the cause for c domains that are less responsive to electric fields. In this case, domain walls refer to areas where the material changes' polarisation changes by 180 degrees (in essence, a boundary between two oppositely polarised areas).
Electron microscopy images show the degradation in action.
Credit: University of Sydney
While the researchers did not propose a method to prevent the damage caused by ferroelectric fatigue, simply imaging the very act taking place provides material scientists with a wealth of data and ideas. Thus, it is up to material scientists to figure out methods for preventing such damage to ferroelectric materials so that future components using the technology can last longer.
The ability to prevent ferroelectric fatigue would have a serious impact on the electronics industry. While ferroelectric materials are not entirely common in the electronics world, they could provide the semiconductor industry with the next generation memories that do not require refresh cycles, have high-speed access, and retain their data even when power is removed. Such materials have also been said to allow for nanometre transistors to take advantage of electron tunnelling (which is currently an issue for transistors).
Either way, ferroelectric materials play an important role in the industry, and identifying why they fail can lead to a world of more reliable electronics.