Nov 23 2020
Researchers across the globe have been studying new materials to develop neuromorphic computers, with a design based on the human brain.
The combination with graphene opens up a new path to memristive heterostructures combining ferroelectric materials and 2D materials. Image Credit: Banerjee lab, University of Groningen.
A memristive device is a vital component of neuromorphic computers and its resistance depends on the history of the device—quite similar to the human neurons, the response of which depends on previous input.
Materials scientists at the University of Groningen studied the behavior of strontium titanium oxide, a platform material for research on memristors, and used the 2D material graphene to investigate it. The study outcomes were reported in the ACS Applied Materials and Interfaces journal on November 11th, 2020.
Computers are huge calculators with numerous switches with a value of 0 or 1. They use several of these binary systems to perform calculations very fast. But, in other respects, computers are not so efficient.
When compared to a standard microprocessor, the human brain uses less energy to recognize faces or perform other complex tasks. This is because the brain is composed of neurons that can take several values apart from 0 and 1 and since the output of the neurons depends on previous input.
Oxygen Vacancies
Strontium titanium oxide (STO) is often used to develop memristors—switches with a memory of past events. This material is a perovskite, the crystal structure of which relies on temperature and can turn into an incipient ferroelectric at low temperatures. Above temperatures of 105 K, the material loses its ferroelectric behavior.
The domains and domain walls associated with these phase transitions are being studied actively. However, it is still not evident why the material acts the way it does. “It is in a league of its own,” stated Tamalika Banerjee, Professor of Spintronics of Functional Materials at the Zernike Institute for Advanced Materials, University of Groningen.
Its behavior seems to depend more on the oxygen atoms in the crystal.
Oxygen vacancies can move through the crystal and these defects are important. Furthermore, domain walls are present in the material and these move when a voltage is applied to it.
Tamalika Banerjee, Professor of Spintronics of Functional Materials, Zernike Institute for Advanced Materials, University of Groningen
Although several research works have endeavored to determine how this happens, looking within this material is not so easy. But Banerjee’s research group successfully used another material that is in a league of its own—graphene, the two-dimensional carbon sheet.
Conductivity
“The properties of graphene are defined by its purity,” added Banerjee, “whereas the properties of STO arise from imperfections in the crystal structure. We found that combining them leads to new insights and possibilities.”
A major part of this study was performed by Si Chen, Banerjee’s PhD student. Chen positioned graphene strips over a flake of STO and quantified the conductivity at various temperatures by sweeping a gate voltage between positive and negative values.
When there is an excess of either electrons or the positive holes, created by the gate voltage, graphene becomes conductive. But at the point where there are very small amounts of electrons and holes, the Dirac point, conductivity is limited.
Si Chen, PhD Student, Banerjee’s Lab, Zernike Institute for Advanced Materials, University of Groningen
Under normal conditions, there is no change in the minimum conductivity position with the sweeping direction of the gate voltage. But in the case of the graphene strips placed over the STO, there is a huge separation between the minimum conductivity positions for the forward and backward sweeps.
The effect is quite evident at 4 K, but less clear at 105 K or 150 K. Analysis of the findings, together with theoretical studies performed at Uppsala University, reveals that oxygen vacancies close to the surface of the STO are behind this effect.
Memory
According to Banerjee, “The phase transitions below 105 Kelvin stretch the crystal structure, creating dipoles. We show that oxygen vacancies accumulate at the domain walls and that these walls offer the channel for the movement of oxygen vacancies. These channels are responsible for memristive behaviour in STO.”
The shift in the position of the minimum conductivity is explained by the accumulation of oxygen vacancy channels in the crystal structure of STO.
Chen also performed another experiment: “We kept the STO gate voltage at -80 V and measured the resistance in the graphene for almost half an hour. In this period, we observed a change in resistance, indicating a shift from hole to electron conductivity.” This effect is mainly due to the accumulation of oxygen vacancies at the STO surface.
On the whole, the experiments demonstrate that the properties of the combined STO/graphene material vary due to the movement of both electrons and ions, each at different time scales.
By harvesting one or the other, we can use the different response times to create memristive effects, which can be compared to short-term or long-term memory effects. And the combination with graphene opens up a new path to memristive heterostructures combining ferroelectric materials and 2D materials.
Tamalika Banerjee, Professor of Spintronics of Functional Materials, Zernike Institute for Advanced Materials, University of Groningen
The study offers a new understanding of the behavior of STO memristors.
Journal Reference:
Chen, S., et al. (2020) Unveiling Temperature-Induced Structural Domains and Movement of Oxygen Vacancies in SrTiO3 with Graphene. ACS Applied Materials & Interfaces. doi.org/10.1021/acsami.0c15458.