Graphene and Ammonia Combine for Novel Ferroelectric Tunnel Junction

Illustration: University of Nebraska-Lincoln

The next generation of high-speed and high-capacity random-access memory (RAM) has seen a number of competitive approaches take center stage of late, from spin-transfer torque (STT) Magnetic RAM to resistive RAM.

Now, researchers at the University of Nebraska at Lincoln have shown a way forward that could offer a general improvement to RAM. They’ve made improvements to the ferroelectric tunnel junction by combining graphene with ammonia so that it is capable of switching on and off the flow of electrons more completely. The result is a distinct improvement in the reliability of RAM devices and the ability to read data without having to rewrite it.

“This is one of the most important differences between previous technology that has already been commercialized and this emergent ferroelectric technology,” said Alexei Gruverman, a physics professor who co-authored the study, in a press release.

The researchers, who published their findings in the journal Nature Communications, tackled the ferroelectric tunnel junction (FTJ), which consists of a layer of material so thin that electrons can tunnel through it. This ferroelectric layer is positioned between two electrodes so that when an electric field is applied to them, it can reverse the direction of the junction’s polarization. This reversal of polarization serves to change the alignment of positive and negative charges, which in turn is used to represent zero and one in binary computing.

The new wrinkle the Nebraska researchers brought to this setup was making the electrodes out of graphene for the first time. While graphene is primarily useful in electronic applications for its conductivity, the Nebraska-Lincoln team was more interested in its ability to accommodate just about any molecule. In this case, they used an ammonia molecule, which sat between the electrodes and the ferroelectric layer.

Typically, the polarity of a junction determines its resistance to the tunneling current; in one direction, the current is allowed to flow and in the other direction the current is greatly reduced. The researchers determined that the graphene-ammonia combination made the difference between these “on” and “off” states even more dramatic.

Ferroelectric materials do have the advantage of being non-volatile, in that they can maintain their polarization even in the absence of an external power source and thereby keep their stored information. Nonetheless, the space between the positive and negative charges is so small in these junctions that it becomes difficult to maintain the polarization.

“In all memory devices, there is a gradual relaxation, or decrease, of this polarization,” says Gruverman in the press release. “The thinner the ferroelectric layer is, the more difficult it is to keep these polarization charges separate, as there is a stronger driving force in the material that tries to get rid of it.”

This most recent research promises a way to eliminate that weakness for ferrolectric materials.



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Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY