Memristors combine ferroelectrics and graphene for neuromorphic chips

Memristors combine ferroelectrics and graphene for neuromorphic chips

Technology News |
By Nick Flaherty

To create memristors, switches with a memory of past events, strontium titanium oxide (STO) is often used. This material is a perovskite, whose crystal structure depends on temperature, and can become an incipient ferroelectric at low temperatures. However the ferroelectric behaviour is lost above 105 Kelvin, and the domains and domain walls that accompany these phase transitions are the subject of active research.

“It is in a league of its own,” says Tamalika Banerjee, Professor of Spintronics of Functional Materials at the Zernike Institute for Advanced Materials, University of Groningen.

The oxygen atoms in the crystal appear to be key to its behaviour. “Oxygen vacancies can move through the crystal and these defects are important,” she says. “Furthermore, domain walls are present in the material and they move when a voltage is applied to it.”

Instead Banerjee’s team used graphene.

“The properties of graphene are defined by its purity,” said Banerjee, “whereas the properties of STO arise from imperfections in the crystal structure. We found that combining them leads to new insights and possibilities.”

Placing graphene strips on top of a flake of STO and measuring the conductivity at different temperatures by sweeping a gate voltage between positive and negative values showed the potential. ‘When there is an excess of either electrons or the positive holes, created by the gate voltage, graphene becomes conductive,’ said researcher Si Chen. “But at the point where there are very small amounts of electrons and holes, the Dirac point, conductivity is limited.”

In normal circumstances, the minimum conductivity position does not change with the sweeping direction of the gate voltage. However, in the graphene strips on top of STO, there is a large separation between the minimum conductivity positions for the forward sweep and the backward sweep. The effect is very clear at 4 Kelvin, but less pronounced at 105 Kelvin or at 150 Kelvin. Analysis of the results, along with theoretical studies carried out at Uppsala University, shows that oxygen vacancies near the surface of the STO are responsible.

“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,” said Banerjee. Accumulation of oxygen vacancy channels in the crystal structure of STO explains the shift in the position of the minimum conductivity.

Chen also carried out 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.,” she said. This effect is primarily caused by the accumulation of oxygen vacancies at the STO surface.

The experiments show that the properties of the combined STO/graphene material change through 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,” said Bannerjee.

The study creates new insights into the behaviour of STO memristors. “And the combination with graphene opens up a new path to memristive heterostructures combining ferroelectric materials and 2D materials,” she added.

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