Boost for Transistors Grows on Sapphire

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Incorporating vanadium dioxide into electronic devices could boost the performance of transistors. A team of researchers has now synthesized the material in a form that would make this possible.

“It’s tough to replace current transistor technology because semiconductors do such a fantastic job,” says Roman Engel-Herbert, assistant professor of materials science and engineering at Penn State. “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, 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 Wafer Scale

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 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.

The team reports in 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 at 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.

The Right Ratio

“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,” says Hai-Tian Zhang, a 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.”

The vanadium dioxide thin-film material grown with this method was used to make super-high-frequency switches, a technology important in communications. These switches show cut-off frequencies an order of magnitude higher than conventional devices. This work will be reported at the IEEE International Electron Device Meeting in December.

“We are starting to realize 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 says. “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.”

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