The tiny transistor is a major part of the electronics revolution. Now, PennState materials scientists have discovered a way to give this bit of tech a big boost by using a new technique to incorporate vanadium oxide, a functional oxide, into the electronic devices.
"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. "But there are some materials, like vanadium oxide, that you can add to existing devices to make them perform even better."
Vanadium dioxide is just a specific combination of the elements vanadium and oxygen and has 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-and-off transition is also the basis of computer logic and memory.
The researchers questioned what would happen if they added vanadium oxide close to a device's transistor. Could it boost the transistor's performance? They also wondered what would happen if they added it to the memory cell. Could it improve the stability and energy efficiency to read, write and maintain the information state?
They set out to test these hypotheses.
A 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 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 Penn State team reported its findings in the online journal Nature Communications where it was revealed that the team is the first to achieve growth of thin films of vanadium dioxide on three-inch sapphire wafers with a perfect 1 to 2 ratio of vanadium to oxygen across the entire wafer.
What this means
The material can be used to make hybrid field effect transistors, called hyper-FETs, which could lead to more energy efficient transistors. Using vanadium dioxide can also benefit existing memory technologies, and that is next on the list of conquests for the PennState team.
"The metal-to-insulator property of vanadium dioxide can ideally enhance state-of-the-art non-volatile memories by using the material as an augmentation device, which interestingly can also serve as a selector in some memory architecture," says Sumeet Gupta, Monkowski assistant professor of Electrical Engineering and group leader of the Integrated Circuits and Devices Lab, Penn State.
A selector insures that reading or writing information on a memory chip is done within a single memory cell, without bleeding into neighboring cells. The selector works by changing the resistivity of the cell, which vanadium proves to do extremely well. In addition, the change in resistivity of vanadium oxide can be used to significantly increase the robustness of the read operation.
"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, Ph.D. 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 used in this method was then used to make super-high-frequency switches and important communications technology. These switches show cut-off frequencies an order of magnitude higher than conventional devices.
"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."
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