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SCIENTISTS at Pennsylvania State University have found a way to incorporate vanadium dioxide into existing semiconductor devices, providing a performance boost.
Vanadium dioxide is part of a family of materials called functional oxides. It has an unusual property called the metal-to-insulator transition, where the material changes between the electrically-conductive metal state, and the insulator state which prevents the flow of electrons.
This on/off transition is inherent to vanadium dioxide, but is also the basis of computer logic and memory: a similar mechanism is at work in semiconductors.
From earlier research, the scientists knew that adding vanadium dioxide to conventional transistors would provide steep and reversible switching at room temperature, reducing the effects of self-heating and lowering the energy requirements of the transistor.
They also theorised that adding the material to the memory cell would improve the stability and energy efficiency associated with read, write and memory operations.
"The metal-to-insulator property of vanadium dioxide can ideally enhance state-of-the-art non-volatile memories by using it as an augmentation device, which, interestingly, can also serve as a selector in some memory architecture," said Sumeet Gupta, Monkowski assistant professor of electrical engineering and group leader of the Integrated Circuits and Devices Lab at Penn State.
However, vanadium dioxide of sufficiently high quality had never been grown in the thin film wafer scale, which is needed to allow industrial use. Vanadium dioxide is also hard to synthesise: the ratio of vanadium to oxygen must be precisely controlled in order to create a sharp metal-to-insulator transition that shows a more than four-order-of-magnitude change in electrical resistance.
The researchers made a breakthrough, as documented in their paper in Nature Communications titled "Wafer scale growth of VO2 thin films using a combinatorial approach".
They simultaneously deposited vanadium oxide with varying vanadium-to-oxygen ratios across the sapphire wafer. They then performed flux calculations to determine the optimal combination that would give an ideal 1:2 vanadium/oxygen ratio in the film. This allowed them to quickly identify the optimal growth condition for industrial applications, without going through a long series of trial-and-error experiments.
As a result, the researchers managed to grow thin films of vanadium dioxide on three-inch sapphire wafers with a perfect 1:2 ratio of vanadium to oxygen across the entire wafer.
This means that vanadium dioxide can now be used to make hybrid field effect transistors, called hyper-FETs, which could lead to more energy efficient transistors. The vanadium dioxide thin film material grown with this method has also been used to make super high frequency switches.