Latest research suggests that materials commonly missed in computer chip design actually play a vital role in information processing, a discovery which may lead to faster and more efficient electronics. Using advanced imaging techniques, a global team led by Penn State researchers found that the fabric that a semiconductor chip device is built on, called the substrate, responds to changes in electricity very like the semiconductor on top of it.
The researchers worked with the semiconductor material, vanadium dioxide, which they said shows great potential as an electronic switch. Additionally they studied how vanadium dioxide interacts with the substrate material titanium dioxide and said they were surprised to find that there appears to be an lively layer within the substrate that behaves similarly to the semiconductor material on top of it when the semiconductor switches between an insulator — not letting electricity flow — and a metal — letting electricity flow. The revelation that substrates can play an lively role in semiconductor processes is critical for designing future materials and devices, said study lead Venkatraman Gopalan, professor of materials science and engineering and of physics at Penn State.
“Latest ideas are needed for smaller and faster electronics with a view to sustain with Moore’s law,” said Gopalan, the corresponding creator of the study in Advanced Materials. “One idea being pursued is materials, similar to vanadium dioxide, that may switch between metal — the one state — and insulator — the zero state — states in a trillionth of a second. That is often known as undergoing metal-insulator transitions.”
The potential of vanadium dioxide as a metal-to-insulator transistor is well-documented and the fabric is taken into account promising for semiconductor technology resulting from its low energy consumption, Gopalan said. Nevertheless, the fabric’s properties are still not fully understood, and until now, it has normally been observed in isolation somewhat than while functioning in an actual device.
Vanadium dioxide has strongly correlated electronic effects, meaning the repulsion between electrons interferes with the device, so can’t be ignored as is currently done in silicon-based electronics. This characteristic can lead to materials with novel functionalities similar to high-temperature superconductivity and enhanced magnetic properties.
“The underlying physics of this material is less understood, and its performance in a tool geometry is even lesser understood,” Gopalan said. “If we will make them work, there shall be a renaissance in electronics. Particularly, neuromorphic computing — where computer systems that take inspiration from the brains of living systems with neurons — could seriously profit by utilizing such devices.”
The team investigated vanadium dioxide in a tool somewhat than in isolation, applying a voltage to it to make it switch from an insulating to a conducting state. They used the Advanced Photon Source (APS) at Argonne National Laboratory, which uses powerful X-ray beams to review the behavior and structure of materials on the atomic level. When mapping the spatial and temporal response of the fabric to the switching event, the researchers observed unexpected changes to the structure of the fabric and substrate.
“What we found was that because the vanadium dioxide film changes to a metal, the entire film channel bulges, which may be very surprising,” Gopalan said. “Normally it’s alleged to shrink. So clearly something else was happening within the film geometry that was missed before.”
The APS X-ray penetrated through the vanadium dioxide film and into the titanium dioxide (TiO2) substrate — which is generally considered an electrically and mechanically passive material — that the skinny film was grown on.
“We found to our great surprise that this substrate may be very much lively, jiving and responding in completely surprising ways because the film switches from an insulator to a metal and back, when the electrical pulses arrive,” Gopalan said. “That is like watching the tail wagging the dog, which stumped us for a protracted while. This surprising and previously missed commentary completely changes how we’d like to view this technology.”
To grasp these findings, the idea and simulation effort — led by Long-Qing Chen, Hamer Professor of Materials Science and Engineering, professor of engineering science and mechanics and of mathematics at Penn State — developed a theoretical framework to clarify the whole strategy of the film and the substrate bulging as an alternative of shrinking. When their model incorporated naturally occurring missing oxygen atoms on this material of two types, charged and uncharged, the experimental results might be satisfactorily explained.
“These neutral oxygen vacancies hold a charge of two electrons, which they will release when the fabric switches from an insulator to a metal,” Gopalan said. “The oxygen emptiness left behind is now charged and swells up, resulting in the observed surprising swelling within the device. This may occur within the substrate. All of those physical processes are beautifully captured within the phase-field theory and modelling performed on this work for the primary time by the postdoc Yin Shi in Professor Chen’s group.”
Gopalan credited the multidisciplinary team’s combined expertise in material growth, synthesis, structure evaluation and synchrotron beamline operation with the brand new understanding. Using a collaborative approach led by Greg Stone, a physical scientist with the U.S. Army and the lead experimental creator, and Yin Chi, postdoctoral scholar at Penn State and the lead theory creator, the researchers disentangled the fabric’s responses and observed them individually using phase field simulations, a simulation that helps scientists understand material changes over time by depicting various states of matter in a virtual setting.
“By bringing these experts together and pooling our understanding of the issue, we were in a position to go far beyond our individual scope of experience and discover something latest,” said Roman Engel-Herbert, director of the Paul Drude Institute of Solid State Electronics in Berlin, Germany, and co-author of the study whose group grew these movies together with Darrell Schlom’s group at Cornell University. “Recognizing the potential of functional materials necessitates an appreciation of their broader context, just as complex scientific challenges can only be solved through widening our individual perspectives.”
The collaboration enabled each a major amount of progress to occur in a brief time period and work to be done in a shorter time period, and brought in a wide range of perspectives from multiple disciplines.
The responses themselves require further investigation, researchers said, but they imagine that understanding them will assist in identifying previously unknown capabilities of vanadium dioxide, including potential yet-to-be discovered phenomena within the TiO2 substrate that was considered passive before this study. The study itself unfolded over 10 years, Gopalan noted, including validating the outcomes.
“That is what it takes to go from interesting science to a working device you possibly can hold within the palm of your hand,” Gopalan said. “Experiments and theory are complex and require large-scale collaborative teams working closely together over an prolonged time period to resolve difficult problems that might have a big impact. We hope and expect that it will speed up the progress towards a brand new generation of electronic devices.”
Prior to his current position, Stone accomplished a postdoctoral fellowship at Penn State. Together with Gopalan, Engel-Herbert, Chen, Schlom, Stone and Chi, other authors of the paper include Matthew Jerry, graduate student, and Vladimir Stoica, research associate professor, each from Penn State; Hanjong Paik from Cornell University; Zhonghou Cai and Haidan Wen from Argonne National Laboratory, and Suman Datta from the Georgia Institute of Technology. The Department of Energy primarily supported this work. The U.S. National Science Foundation supported the film growth for this study.
