For decades, the energy demands of an exponentially growing number of computations in computers and other devices has been kept in check by the increasingly efficient, and ever-more compact CMOS (silicon-based) microchips. This effect is related to the famous ‘Moore's Law.' However, as fundamental physics limits are approached on the designs of silicon-based microchips, Moore's Law is ending, and future enhancements to efficiency are limited.
Ultra-low energy electronics such as topological transistors could allow computing to continue to grow in speed and complexity as we near the end of possible improvements in conventional, silicon electronics.
Now, a study conducted at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has shown, for the first time, electronic switching in a unique ultrathin material at room temperature can conduct a charge with almost no loss. The researchers showed this switching when they exposed the material to a low-current electric field.
Researchers at Berkeley Lab's Advanced Light Source used an X-ray technique known as ARPES to produce the images at the top of this article showing the electronic ranges of energy in an ultrathin material. (Credit: Berkeley Lab, Monash University)
Researchers at Monash University in Australia led the team that included
Berkeley Lab scientists, which grew the material and studied it. Scientists from the Australian Synchrotron, National University of Singapore, Singapore University of Technology and Design, University of Illinois at Urbana-Champaign, and YALE-NUS College in Singapore also took part in the study.
The scientists used scanning tunneling microscopy and angle-resolved photoelectron spectroscopy to confirm that the films of Na3Bi are two-dimensional topological insulators. They conducted the latest phase of the experiment at the Advanced Light Source (ALS), a facility at Berkeley Lab.
The material, sodium bismuthide (Na3Bi), is one of just two materials that are known to be a "topological Dirac semimetal," with unique electronic properties that can be tuned to act in different ways. Its topological properties were first verified in earlier experiments at ALS.
Topological materials are thought to be ideal for next-generation transistors and other electronics because of their ability to reduce power consumption and energy loss in devices. Furthermore, these properties occur at room temperature, unlike superconductors that demand extremely low temperatures. Additionally, the robust material continues to have these properties even when the material has structural defects and is subject to stress.
The researchers found that the material can easily change from an electrically conducting state to an insulating or non-conducting state. This ability to change readily bodes well for its future transistor applications, according to Sung-Kwan Mo, a staff scientist at the ALS who worked on the latest study. The study is detailed in the journal Nature.
In another critical phase of the latest study, the team from Monash University devised a way to grow the material in extremely thin layers down to a single-layer, honeycomb pattern of sodium and bismuth atoms, and to control the depth of each layer they fabricate.
"If you want to make a device, you want to make it thin," Mo said. "This study proves that it can be done for Na3Bi, and its electrical properties can easily be controlled with low voltage. We are a step closer to a topological transistor."
Michael Fuhrer, a physicist at Monash University who worked on the study, said, "This discovery is a step in the direction of topological transistors that could transform the world of computation."
In the latest study, scientists grew the material, measuring just several millimeters on a side, on a silicon wafer using molecular beam epitaxy. They used a special system at Berkley Lab, that lets researchers grow samples and then conduct tests under the same vacuum conditions to prevent contamination.
This beamline uses a specialized X-ray technique known as angle-resolved photoemission spectroscopy, or ARPES, which gives information about how electrons travel through materials. In typical topological materials, electrons tend to flow around the edges, and the remainder of the material behaves as an insulator, preventing this flow.
The Australian portion of the team performed some X-ray experiments on similar samples at the Australian Synchrotron to confirm that the Na3Bi was free-standing and did not chemically interact with the silicon wafer upon which it was grown. The scientists also looked at samples with a scanning tunneling microscope at Monash University that helped to validate other measurements.
"In these edge paths, electrons can only travel in one direction," said Mark Edmonds, a physicist at Monash University who led the study. "And this means there can be no ‘back-scattering,' which is what causes electrical resistance in conventional electrical conductors."
In this instance, researchers found that the material became fully conductive when exposed to the electric field and could also be transformed into an insulator across the entire material when exposed to a slightly higher electric field.
Mo noted that the electrically driven switching is a crucial step in achieving applications for materials. Other research efforts have attempted processes such as chemical doping or mechanical strain that are more difficult to control and to perform the switching operation.
The research team is also testing other samples that can be switched on and off in a similar fashion to guide the development of ultralow-energy electronics, Edmonds said.
“Topological insulators are novel materials that behave as electrical insulators in their interior, but can carry a current along their edges,
"In these edge paths, electrons can only travel in one direction," said Dr. Edmonds. "And this means there can be no ‘back-scattering,' which is what causes electrical resistance in conventional electrical conductors."
"Ultra-low energy topological electronics are a potential answer to the increasing challenge of energy wasted in modern computing," said study author Professor Michael Fuhrer, from the Monash School of Physics and Astronomy, and Director of the ARC Centre for Future Low-Energy Electronics Technologies (FLEET).
"Information and Communications Technology (ICT) already consumes 8% of global electricity, and that's doubling every decade," he said.
The study “Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi” was published in Nature and is a major advance towards that goal of a functioning topological transistor.
How it works: Topological materials and topological transistors
Topological insulators are novel materials that behave as electrical insulators in their interior, but can carry a current along their edges.
"In these edge paths, electrons can only travel in one direction," said study lead Dr Mark Edmonds, a lecturer from the School of Physics and Astronomy at Monash University. "And this means there can be no ‘back-scattering,' which is what causes electrical resistance in conventional electrical conductors."
"Unlike conventional electrical conductors, such topological edge paths can carry electrical current with near-zero dissipation of energy, meaning that topological transistors could burn much less energy than conventional electronics. They could also potentially switch must faster," Dr. Edmonds added.
"Each time a transistor switches, a tiny amount of energy is burnt, but there are trillions of transistors in the world, all switching billions of times per second, so this energy adds up very quickly," Dr. Edmonds said.
"The information technology revolution has improved our lives, and we want it to continue," Professor Fuhrer said. "But for computation to continue to grow, to keep up with changing demands, we need more-efficient electronics."
"We need a new type of transistor that burns less energy when it switches," Professor Fuhrer added.
The work was supported by the U.S. Department of Energy's Office of Science, the Australian Research Council's Centers of Excellence and DECRA Fellowship programs, the International Synchrotron Access Program, and the Monash Center for Atomically Thin Materials Research.
Collins, J. L., Tadich, A., Wu, W., et al.Electric-field-tuned topological phase transition in ultrathin Na3Bi. Nature. volume 564, pages 390-394 (2018).