Vanadium oxide gives electronics a power boost

Researchers have given tiny transistors a power boost by using a new technique that incorporates vanadium oxide — a function oxide — into electronic devices.

"It's tough to replace current transistor technology because semiconductors do such a fantastic job," said Roman Engel-Herbert, assistant professor of materials science and engineering, Penn State University. "But there are some materials, like vanadium oxide, that you can add to existing devices to make them perform even better."

The Penn State researchers knew that vanadium dioxide, which is just a specific combination of the elements vanadium and oxygen, 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. This 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 very difficult to synthesise. 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.

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″ 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. The implementation of vanadium dioxide can also benefit existing memory technologies, a quest that Penn State researchers are actively pursuing.

"To determine the right ratio of vanadium to oxygen, we applied an unconventional approach in which we simultaneously deposit vanadium oxide with varying vanadium-to-oxygen ratios across the sapphire wafer," said Hai-Tian Zhang, PhD student in Engel-Herbert's group.

"Using this 'library' of vanadium-to-oxygen ratios, we can perform flux calculations to determine the optimal combination that would give an ideal 1 to 2 vanadium to oxygen ratio in the film. This new method will allow a rapid identification of the optimal growth condition for industrial applications, avoiding a long and tedious series of trial-and-error experiments."

"We are starting to realise that the class of materials exhibiting these on/off responses can be beneficial in various ways in information technology, such as increasing the robustness and energy efficiency of read/write and compute operations in memory, logic and communication devices," Engel-Herbert said.

"When you can make high-quality vanadium dioxide on a wafer scale, people are going to have many excellent ideas on how it can be used."

Other researchers on this project were graduate students Lei Zhang, Debangshu Mukherjee, Ryan Haislmaier and assistant professor Nasim Alem, all in the Department of Materials Science and Engineering and the Materials Research Institute at Penn State, and Yuan-Xia Zheng, a graduate student in Physics.

The National Science Foundation and the Penn State Center for Nanoscale Science supported this work. Analysis and measurement was performed in the Penn State Materials Characterization Laboratory, a facility of the Materials Research Institute.