Developed at KAUST (King Abdullah University of Science and Technology), the proton-mediated approach shows potential for high-performance neuromorphic computing chips. The team’s findings are detailed in Science Advances.
Ferroelectrics, such as indium selenide, switch polarity when placed in an electric field, which makes them suited for memory technologies. In addition to requiring low operating voltages, the resulting memory devices display favourable maximum read/write endurance and write speeds, but their storage capacity is low. This is because existing methods can only trigger a few ferroelectric phases, and capturing these phases is experimentally challenging, said Xin He, who co-led the study under the guidance of Fei Xue and Xixiang Zhang.
The method devised at KAUST relies on the protonation of indium selenide to generate a multitude of ferroelectric phases. The researchers incorporated the ferroelectric material in a transistor consisting of a silicon-supported stacked heterostructure for evaluation.
They deposited a multilayered indium selenide film on the heterostructure, which comprised an aluminium oxide insulating sheet sandwiched between a platinum layer at the bottom and porous silica at the top. While the platinum layer served as electrodes for the applied voltage, the porous silica acted as an electrolyte and supplied protons to the ferroelectric film.
The researchers gradually injected or removed protons from the ferroelectric film by changing the applied voltage. This reversibly produced several ferroelectric phases with various degrees of protonation, which KAUST said is crucial for implementing multilevel memory devices with substantial storage capacity.
Higher positive applied voltages increased protonation and negative voltages of higher amplitudes depleted protonation levels to a greater extent.
Protonation levels also varied depending on the proximity of the film layer to silica. They reached maximum values in the bottom layer, which was in contact with silica, and decreased in stages to achieve minimum amounts in the top layer.
The proton-induced ferroelectric phases returned to their initial state when the applied voltage was turned off. “We observed this unusual phenomenon because protons diffused out of the material and into the silica,” Xue said in a statement.
By manufacturing a film that displayed a smooth and continuous interface with silica, the team obtained a high proton-injection efficiency device that operates below 0.4V.
“Our biggest challenge was to reduce the operating voltage, but we realised that the proton-injection efficiency over the interface governed operating voltages and could be tuned accordingly,” said Xue. “We are committed to developing ferroelectric neuromorphic computing chips that consume less energy and operate faster.”
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