"It's tough to replace current transistor technology because semiconductors do such a fantastic job," explained Roman Engel-Herbert, assistant professor of materials science and engineering. "But there are some materials, like vanadium oxide, that you can add to existing devices to make them perform even better."
The researchers knew that vanadium dioxide had an unusual property called the metal-to-insulator transition. In the metal state, electrons move freely, while in the
insulator state, electrons cannot flow. The on/off transition, inherent to vanadium dioxide, is also the basis of computer logic and memory.
The researchers thought that if they could add vanadium oxide close to a device's transistor it could boost the transistor's performance. Also, by adding it to the
memory cell, it could improve the stability and energy efficiency to read, write and maintain the information state. The major challenge they faced was that vanadium
dioxide of sufficiently high quality had never been grown in a thin film form on the scale required to be of use to industry - the wafer scale.
Although vanadium dioxide, the targeted compound, looks simple, it is difficult to synthesize. In order to create a sharp metal-to-insulator transition, the ratio of
vanadium to oxygen needs to be precisely controlled. When the ratio is exactly right, the material will show more than four orders-of-magnitude change in resistance, enough for a sufficiently strong on/off response.
A schematic of the crystal structures in VO2, showing the motion of the vanadium (black arrows) with respect to the oxygen ions across the metal-insulator transition. VO2 acts like an insulator at low temperatures but like a metal at near room temperature. Image: Lawrence Berkeley National Laboratory
The Penn State team reports in the online journal Nature Communications that they are the first to achieve growth of thin films of vanadium dioxide on 3-inch sapphire wafers with a perfect 1 to 2 ratio of vanadium to oxygen across the entire wafer. The material can be used to make hybrid field effect transistors, called hyper-FETs, which could lead to more energy efficient transistors. Earlier this year, also in Nature Communications, a research group led by Suman Datta, professor of electrical and electronic engineering, Penn State, showed that the addition of vanadium dioxide provided steep and reversible switching at room temperature, reducing the effects of self-heating and lowering the energy requirements of the transistor.