Illustrations of the 'beyond CMOS' devices created by Job van Rijn (upper panel) and Anouk Goossens (lower panel). Image: Banerjee group, University of Groningen.
Illustrations of the 'beyond CMOS' devices created by Job van Rijn (upper panel) and Anouk Goossens (lower panel). Image: Banerjee group, University of Groningen.

The development of classic silicon-based computers is approaching its limits. To achieve further miniaturization and reduce energy consumption, different types of materials and architectures are required.

Tamalika Banerjee, professor of spintronics of functional materials in the Zernike Institute for Advanced Materials at the University of Groningen, the Netherlands, is looking at a range of quantum materials to create these new devices. “Our approach is to study these materials and their interfaces, but always with an eye on applications, such as memory or the combination of memory and logic.”

The Banerjee group previously demonstrated how doped strontium titanate can be used to create memristors, which combine memory and logic. Building on this work, they now report, in two papers, the creation of new ‘beyond CMOS’ devices, meaning the complementary metal oxide semiconductors (CMOSs) that are the building blocks of present-day computer chips.

One candidate to replace CMOSs is the magneto-electric spin-orbit (MESO) device, which could be 10 to 30 times more energy efficient. Several materials have been investigated for their suitability in creating such a device. In a paper in Physical Review B, the group describes how strontium manganate (SrMnO3, or SMO) might make a good candidate material for MESO devices.

“It is a multiferroic material that couples spintronics and charge-based effects,” explains Job van Rijn, a PhD student in the Banerjee group and first author of the paper. Spintronics is based on the spin (the magnetic moment) of electrons.

“The magnetic and charge orderings are coupled in this material, so we can switch magnetism with an electric field and polarization with a magnetic field,” says Banerjee. And importantly, these effects are present at temperatures close to room temperature.

Van Rijn is investigating the strong coupling between the two effects. “We know that ferromagnetism and ferroelectricity are tuneable by straining a thin SMO film,” he says. “This straining was done by growing the films on different substrates.”

Van Rijn studies how strain induces ferroelectricity in SMO and how it impacts the magnetic order. He analyzed the domains in the strained films, and noticed that the magnetic interactions are greatly dependent on the crystal structure and, in particular, on the oxygen vacancies, which modify the preferred direction of the magnetic order.

“Spin transport experiments lead us to the conclusion that the magnetic domains play an active role in the devices that are made of this material. Therefore, this study is the first step in establishing the potential use of strontium manganate for novel computing architectures.”

Meanwhile, in a paper in Advanced Electronic Materials, the group reports the development of miniature memristors based on niobium-doped strontium titanate (SrTiO3 or STO). “The number of devices per unit surface area is important,” says Anouk Goossens, another PhD student in the Banerjee group and first author of the paper. “But some memristor types are difficult to downscale.”

In previous work, Goossens showed that it was possible to create ‘logic-in-memory’ devices using STO. Now, she has shown that it is possible to downscale these devices.

A common problem with memristors is that their performance is negatively impacted by miniaturization. Surprisingly, making smaller memristors from STO increases the difference between the high and low resistance ratio.

“We studied the material using scanning transmission electron microscopy and noticed the presence of a large number of oxygen vacancies at the interface between the substrate and the device’s electrode,” says Goossens. “After we applied an electric voltage, we noticed oxygen vacancy movement, which is a key factor in controlling the resistance states.”

The group discovered that the enhanced performance of STO results from edge effects. These can be bad for normal memory, but in STO the increased electric field at the edges actually supports the function of the memristor.

“In our case, the edge is the device,” Goossens explains. “In addition, the exact properties depend on the amount of niobium doping, so the material is tuneable for different purposes.”

Between them, the two papers point the way towards novel computing architectures. Indeed, the STO memristors have already inspired researchers at the University of Groningen Bernoulli Institute for Mathematics, Computer Science and Artificial Intelligence and CogniGron (Groningen Cognitive Systems and Materials Center) to come up with a new design for memory architecture.

“This is exactly what we are working for,” says Banerjee. “We want to understand the physics of materials and the way in which our devices work and then develop applications.”

“We envision several applications and the one we are looking at is a random number generator that works without an algorithm and is therefore impossible to predict,” adds Goosens.

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.