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Researchers at UC Berkeley and UC Riverside have developed a new, ultrafast method for electrically controlling magnetism in certain metals, a breakthrough that could lead to greatly increased performance and more energy-efficient computer memory and processing technologies.

In this schematic of a magnetic memory array, an ultrafast electrical pulse switches a magnetic memory bit.

In this schematic of a magnetic memory array, an ultrafast electrical pulse switches a magnetic memory bit.

The findings of the group, led by Berkeley electrical engineering and computer sciences (EECS) professor Jeffrey Bokor, are published in a pair of articles in the journals Science Advances (Vol. 3, No. 49, Nov. 3, 2017) and Applied Physics Letters (Vol. III, No. 4, July 24, 2017).

Computers use different kinds of memory technologies to store data. Long-term memory, typically a hard disk or flash drive, needs to be dense in order to store as much data as possible. But the central processing unit (CPU) — the hardware that enables computers to compute — requires its own memory for short-term storage of information while operations are executed. Random Access Memory (RAM) is one example of such short-term memory.

Reading and writing data to RAM needs to be extremely fast in order to keep up with the CPU’s calculations. Most current RAM technologies are based on charge (electron) retention, and can be written at rates of billions of bits per second (or bits/nanosecond). The downside of these charge-based technologies is that they are volatile, requiring constant power or else they will lose the data.

In recent years, magnetic alternatives to RAM, known as Magnetic Random Access Memory (MRAM), have reached the market. The advantage of magnets is that they retain information even when memory and CPU are powered off, allowing for energy savings. But that efficiency comes at the expense of speed. A major challenge for MRAM has been to speed up the writing of a single bit of information to less than 10 nanoseconds.

“The development of a non-volatile memory that is as fast as charge-based random-access memories could dramatically improve performance and energy efficiency of computing devices,” says Bokor. “That motivated us to look for new ways to control magnetism in materials at much higher speeds than in today’s MRAM.”

“Inspired by recent experiments in the Netherlands on ultrafast magnetic switching using sub-picosecond duration laser pulses, we built special circuits to study how magnetic metals respond to electrical pulses as short as a few trillionths of a second,” or picoseconds, says coauthor Yang Yang (M.S.’13 Ph.D.’17 MSE). “We found that in a magnetic alloy made up of gadolinium and iron, these fast electrical pulses can switch the direction of the magnetism in less than 10 picoseconds. That is orders of magnitude faster than any other MRAM technology.”

“The electrical pulse temporarily increases the energy of the iron atom’s electrons,” says Richard Wilson, currently an assistant professor of mechanical engineering at UC Riverside who began his work on this project as a postdoctoral researcher in EECS at Berkeley.  “This increase in energy causes the magnetism in the each of the iron and gadolinium atoms to exert torque on one another, and eventually leads to a reorientation of the metal’s magnetic poles. It’s a completely new way of using electrical currents to control magnets.”

After their initial demonstration of electrical writing in the special gadolinium-iron alloy, the research team sought ways to expand their method to a broader class of magnetic materials.  “The special magnetic properties of the gadolinium-iron alloy are what makes this work,” says Charles-Henri Lambert, a Berkeley EECS postdoc. “Therefore, finding a way to expand our approach for fast electrical writing to a broader class of magnetic materials was an exciting challenge.”

Addressing that latter challenge was the subject of a second study, published in Applied Physics Letters in July.  “We found that when we stack a single-element magnetic metal such as cobalt on top of the gadolinium-iron alloy, the interaction between the two layers allows us to then manipulate the magnetism of the cobalt on unprecedented time-scales as well,” says Jon Gorchon, a postdoctoral research in the Materials Sciences Division at Lawrence Berkeley Lab and in EECS at UC Berkeley.

“Together, these two discoveries provide a route toward ultrafast magnetic memories that enable  a new generation of high-performance, low power computing processors with high-speed, non-volatile memories right on chip, ” Bokor says.

Additional team members include Akshay Pattabi, a Berkeley EECS Ph.D. candidate, and Berkeley EECS professor Sayeef Salahuddin. The research was supported by grants from the National Science Foundation and the U.S. Department of Energy.

Eighty years after the theoretical prediction of the force required to overcome the van der Waals’ bonding between layers in a crystal, engineering researchers at Tohoku University have measured it directly. They report their results this week in the Journal of Applied Physics, from AIP Publishing.

In its proof-of-concept, the team also created more durable gallium selenide crystals. The accomplishment could advance the development of terahertz and spintronics technologies, used in a range of applications from medical imaging to quantum computers.

“This is the first time anyone has directly measured the van der Waals bonding force in the layers of a crystal,” Tadao Tanabe, one of the authors, said. “Even high school students know of this force, but in crystals it was very difficult to measure directly.”

Though considered promising for many technologies, the use of gallium selenide crystals has been hampered by the fact that they’re notoriously fragile. To make them stronger, Tanabe’s team, including Department of Materials Science colleague Yutaka Oyama, imagined growing crystals with small amounts of the selenium replaced with the rare element tellurium.

The researchers surmised that tellurium’s larger electron cloud would produce greater van der Waals’ forces between the crystal layers, strengthening the overall structure. Van der Waals’ are weak electric forces that attract atoms to one another through subtle shifts in the atom’s electron configurations.

The team grew and compared three different types of crystals: one pure gallium selenide, one with 0.6 percent tellurium and one with 10.6 percent tellurium. To test the effect on the tellurium on interlayer bonding, the team invented the equivalent of a crystal sandwich opener. Their system is able to measure with exquisite detail the tensile strength, the force required to pull the crystal until it breaks.

“The tensile testing system is very simple in some ways,” Tanabe said. “But it was very difficult to develop a way to identify the exact point at which the crystal broke.”

The crystals tested were about 3 millimeters in width, and only 1/5 of a millimeter thick, about half the thickness of a piece of standard printer paper. Each crystal is comprised of hundreds of individual layers.

The team used special double-sided tape on either side of a crystal to hold it between an anchored stage and a moveable one that could be pulled away slowly, at a rate of 50 millionths of a meter per second. “This enabled us to very precisely measure the interlayer force at which the crystal broke,” Tanabe said.

The researchers found that the interlayer van der Waals bonding in the tellurium-doped crystals was seven times stronger than in pure gallium selenide ones.

With the addition of tellurium, the soft and cleavable gallium selenide crystal becomes rigid by enhancement of the van der Waals’ bonding force, the authors report, paving the way for using this system to improve crystal-based technologies.

Veeco Instruments Inc. (Nasdaq: VECO) announced today the completion of a strategic initiative with ALLOS Semiconductors (ALLOS) to demonstrate 200mm GaN-on-Si wafers for Blue/Green micro-LED production. Veeco teamed up with ALLOS to transfer their proprietary epitaxy technology onto the Propel Single-Wafer MOCVD System to enable micro-LED production on existing silicon production lines.

“With the Propel reactor, we have an MOCVD technology that is capable of high yielding GaN Epitaxy that meets all the requirements for processing micro-LED devices in 200 millimeter silicon production lines,” said Burkhard Slischka, CEO of ALLOS Semiconductors. “Within one month we established our technology on Propel and have achieved crack-free, meltback-free wafers with less than 30 micrometers bow, high crystal quality, superior thickness uniformity and wavelength uniformity of less than one nanometer.  Together with Veeco, ALLOS is looking forward to making this technology more widely available to the micro-LED ecosystem.”

Micro-LED display technology consists of <30×30 square micron red, green, blue (RGB) inorganic LEDs that are transferred to the display backplane to form sub-pixels. Direct emission from these high efficiency LEDs offers lower power consumption compared with OLED and LCD while providing superior brightness and contrast for mobile displays, TV and wearables. The manufacturing of micro-LEDs requires high quality, uniform epitaxial wafers to meet the display yield and cost targets.

“In contrast to competing MOCVD platforms, Propel offers leading-edge uniformity and simultaneously achieves excellent film quality as a result of the wide process window afforded by Veeco’s TurboDisc® technology,” said Peo Hansson, Ph.D., Senior Vice President and General Manager of Veeco MOCVD Operations. “Combining Veeco’s leading MOCVD expertise with ALLOS’ GaN-on-Silicon epi-wafer technology enables our customers to develop micro-LEDs cost effectively for new applications in new markets.”

In the late 18th century, Ernst Chladni, a scientist and musician, discovered that the vibrations of a rigid plate could be visualized by covering it with a thin layer of sand and drawing a bow across its edge. With the bow movement, the sand bounces and shifts, collecting along the nodal lines of the vibration. Chladni’s discovery of these patterns earned him the nickname, “father of acoustics.” His discovery is still used in the design and construction of acoustic instruments, such as guitars and violins.

Recently, investigators have discovered a similar effect with much smaller vibrating objects excited by light waves. When laser light is used to drive the motion of a thin, rigid membrane, it plays the role of the bow in Chladni’s original experiment and the membrane vibrates in resonance with the light. The resulting patterns can be visualized through an array of quantum dots (QDs), where these tiny structures emit light at a frequency that responds to movement. The advance is reported this week in a cover article of Applied Physics Letters, by AIP Publishing.

Background: Image of a Chladni plate's mode of vibration visualized by grains of sand collected at the nodes. Left-top: Cross-sectional scanning tunneling microscopy image of an indium arsenide quantum dot. Left-bottom: Variation of quantum dot emission line frequencies as a function of time due to vibrations of the photonic crystal membrane. Right: Scanning electron micrograph of a photonic crystal membrane, displaced according to one of the vibrational modes, with red and blue representing positive and negative displacement, respectively. Credit: Sam Carter and co-authors

Background: Image of a Chladni plate’s mode of vibration visualized by grains of sand collected at the nodes. Left-top: Cross-sectional scanning tunneling microscopy image of an indium arsenide quantum dot. Left-bottom: Variation of quantum dot emission line frequencies as a function of time due to vibrations of the photonic crystal membrane. Right: Scanning electron micrograph of a photonic crystal membrane, displaced according to one of the vibrational modes, with red and blue representing positive and negative displacement, respectively. Credit: Sam Carter and co-authors

In addition to being a modern take on an old phenomenon, the new discovery could lead to the development of sensing devices as well as methods for controlling the emission characteristics of QDs. Since the light frequency emitted by the QDs is correlated with the movement of the underlying membrane, new devices for sensing motion, such as accelerometers, can be envisioned. A reverse application is also possible since the motion of the underlying membrane can be used to control the frequency of light emitted by the QDs.

The tiny devices in the work reported here consist of a 180-nanometer thick slice of semiconductor, suspended like a trampoline above a solid substrate. An array of QDs, analogous to the sand in the acoustic example, are embedded in the slice, whose thickness is less than one-tenth of one percent that of a human hair.

A second probe laser is used to visualize the resulting resonances. The QDs absorb the probe light and emit a second light pulse in response, which is picked up by a detector and routed to a display. The resulting patterns are remarkably like those visualized in Chladni’s original acoustic experiment, even though the new device is driven entirely by light.

One possible application of this discovery, according to Sam Carter of the Naval Research Lab who is one of the paper’s authors, is to sense subtle forces produced by nearby dense objects. “Concealed nuclear materials could be detectable,” he said, “since dense materials like lead are used to shield the devices.”

The highly dense shielding needed for nuclear materials causes small gravitational anomalies and tiny movements that might be detectable by a device based on the principle discovered here. The investigators plan to continue their work by looking at electronic spin. It is hoped that techniques to measure the effect on spin will increase the sensitivity of the devices.

The phenomenon that forms interference patterns on television displays when a camera focuses on a pattern like a person wearing stripes has inspired a new way to conceptualize electronic devices. Researchers at the University of Illinois are showing how the atomic-scale version of this phenomenon may hold the secrets to help advance electronics design to the limits of size and speed.

In their new study, mechanical science and engineering professor Harley Johnson his co-authors recast a detail previously seen as a defect in nanomaterial design to a concept that could reshape the way engineers design electronics. The team, which also includes mechanical science and engineering graduate student Brian McGuigan and French collaborators Pascal Pochet and Johann Coraux, published its findings in the journal Applied Materials Today.

On display screens, moire patterns occur when the pixelation is at almost the same scale as a photographed pattern, Johnson said, or when two thin layers of a material with a periodic structure, like sheer fabrics and window screens, are placed on top of each other slightly askew.

At the macro scale, moires are optical phenomena that do not form tangible objects. However, when these patterns occur at the atomic level, arrangements of electrons are locked into place by atomic forces to form nanoscale wires capable of transmitting electricity, the researchers said.

“Two-dimensional materials – thin films engineered to be of single-atom thickness – create moire patterns when stacked on top of each other and are skewed, stretched, compressed or twisted,” Johnson said. “The moire emerges as atoms form linear areas of high electron density. The resulting lines create what is essentially an extremely thin wire.”

For decades, physicists observed microscope images of atomic arrangements of 2-D thin films and recognized them as periodic arrays of small defects known as dislocations, but Johnson’s group is the first to note that these are also common moire patterns.

“A moire pattern is simply an array of dislocations, and an array of dislocations is a moire pattern – it goes both ways,” Johnson said. This realization opened the door to what Johnson’s group refers to as moire engineering – what could lead to a new way to manufacture the smallest, lightest and fastest electronics.

By manipulating the orientation of stacked layers of 2-D thin films like graphene, wires of single-atom thickness can be assembled, building the foundation to write nanocircuitry. A wire of single-atom thickness is the limit of thinness. The thinner the wire, the faster electrons can travel, meaning this technology has the potential to produce the quickest transmitting wires and circuits possible, the researchers said.

“There is always the question of how to connect to a circuit that small,” Johnson said. “There is still a lot of work to be done in finding ways to stitch together 2-D materials in a way that could produce a device.”

In the meantime, Johnson’s group is focusing on types of devices that can be made using moire engineering.

“Being able to engineer the moire pattern itself is a path to new lightweight and less-intrusive devices that could have applications in the biomedical and space industries,” he said. “The possibilities are limited only by the imagination of engineers.”

Research by scientists at Swansea University has shown that improvements in nanowire structures will allow for the manufacture of more stable and durable nanotechnology for use in semiconductor devices in the future.

Dr. Alex Lord and Professor Steve Wilks from the Centre for NanoHealth led the collaborative research published in Nano Letters. The research team defined the limits of electrical contact technology to nanowires at atomic scales with world-leading instrumentation and global collaborations that can be used to develop enhanced devices based on the nanomaterials. Well-defined, stable and predictable electrical contacts are essential for any electrical circuit and electronic device because they control the flow of electricity that is fundamental to the operational capability.

Their experiments found for the first time, that atomic changes to the metal catalyst particle edge can entirely alter electrical conduction and most importantly reveal physical evidence of the effects of a long standing problem for electrical contacts known as barrier inhomogeneity. The study revealed the electrical and physical limits of the materials that will allow nanoengineers to select the properties of manufacturable nanowire devices.

One-of-a-kind multi-probe LT Nanoprobe at Swansea University used to obtain the electrical measurements of nanowires that were correlated to atomic resolution imaging. Credit: Swansea University

One-of-a-kind multi-probe LT Nanoprobe at Swansea University used to obtain the electrical measurements of nanowires that were correlated to atomic resolution imaging. Credit: Swansea University

Dr Lord, recently appointed as a Senior Sêr Cymru II Fellow part-funded by the European Regional Development Fund through the Welsh Government, said: “The experiments had a simple premise but were challenging to optimise and allow atomic-scale imaging of the interfaces. However, it was essential to this study and will allow many more materials to be investigated in a similar way.

“This research now gives us an understanding of these new effects and will allow engineers in the future to reliably produce electrical contacts to these nanomaterials which is essential for the materials to be used in the technologies of tomorrow.

“The new concepts shown here provide interesting possibilities for bridged nanowire devices such as transient electronics and reactive circuit breakers that respond to changes in electrical signals or environmental factors and provide instantaneous reactions to electrical overload.”

The Swansea research team used specialist experimental equipment at the Centre for NanoHealth and collaborated with Professor Quentin Ramasse of the SuperSTEM Laboratory, Science and Facilities Technology Council1-3 and Dr Frances Ross of the IBM Thomas J. Watson Research Center, USA.3 The scientists were able to physically interact with the nanostructures and measure how atomic changes in the materials affected the electrical performance.

Dr. Frances Ross, IBM, USA, added: “”This research shows the importance of global collaboration, particularly in allowing unique instrumentation to be used to obtain fundamental results that allow nanoscience to deliver the next generation of technologies.”

Nanotechnology is the scaling down of everyday materials by scientists to the size of nanometres (one million times smaller than a millimetre on a standard ruler) and is seen as the future of electronic devices. Progressions in scientific and engineering advances are resulting in new technologies such as computer components for smart devices and sensors to monitor our health and the surrounding environment.

Nanotechnology is having a major influence on the Internet of Things which connects everything from our homes to our cars into a web of communication. All of these new technologies require similar advances in electrical circuits and especially electrical contacts that allow the devices to work correctly with electricity.

Graphene – a one-atom-thick layer of the stuff in pencils – is a better conductor than copper and is very promising for electronic devices, but with one catch: Electrons that move through it can’t be stopped.

Until now, that is. Scientists at Rutgers University-New Brunswick have learned how to tame the unruly electrons in graphene, paving the way for the ultra-fast transport of electrons with low loss of energy in novel systems. Their study was published online in Nature Nanotechnology.

“This shows we can electrically control the electrons in graphene,” said Eva Y. Andrei, Board of Governors professor in Rutgers’ Department of Physics and Astronomy in the School of Arts and Sciences and the study’s senior author. “In the past, we couldn’t do it. This is the reason people thought that one could not make devices like transistors that require switching with graphene, because their electrons run wild.”

Now it may become possible to realize a graphene nano-scale transistor, Andrei said. Thus far, graphene electronics components include ultra-fast amplifiers, supercapacitors and ultra-low resistivity wires. The addition of a graphene transistor would be an important step towards an all-graphene electronics platform. Other graphene-based applications include ultra-sensitive chemical and biological sensors, filters for desalination and water purification. Graphene is also being developed in flat flexible screens, and paintable and printable electronic circuits.

Graphene is a nano-thin layer of the carbon-based graphite that pencils write with. It is far stronger than steel and a great conductor. But when electrons move through it, they do so in straight lines and their high velocity does not change. “If they hit a barrier, they can’t turn back, so they have to go through it,” Andrei said. “People have been looking at how to control or tame these electrons.”

Her team managed to tame these wild electrons by sending voltage through a high-tech microscope with an extremely sharp tip, also the size of one atom. They created what resembles an optical system by sending voltage through a scanning tunneling microscope, which offers 3-D views of surfaces at the atomic scale. The microscope’s sharp tip creates a force field that traps electrons in graphene or modifies their trajectories, similar to the effect a lens has on light rays. Electrons can easily be trapped and released, providing an efficient on-off switching mechanism, according to Andrei.

“You can trap electrons without making holes in the graphene,” she said. “If you change the voltage, you can release the electrons. So you can catch them and let them go at will.”

The next step would be to scale up by putting extremely thin wires, called nanowires, on top of graphene and controlling the electrons with voltages, she said.

To make continuous, strong and conductive carbon nanotube fibers, it’s best to start with long nanotubes, according to scientists at Rice University.

The Rice lab of chemist and chemical engineer Matteo Pasquali, which demonstrated its pioneering method to spin carbon nanotube into fibers in 2013, has advanced the art of making nanotube-based materials with two new papers in the American Chemical Society’s ACS Applied Materials and Interfaces.

The first paper characterized 19 batches of nanotubes produced by as many manufacturers to determine which nanotube characteristics yield the most conductive and strongest fibers for use in large-scale aerospace, consumer electronics and textile applications.

The researchers determined the nanotubes’ aspect ratio — length versus width — is a critical factor, as is the overall purity of the batch. They found the tubes’ diameters, number of walls and crystalline quality are not as important to the product properties.

Pasquali said that while the aspect ratio of nanotubes was known to have an influence on fiber properties, this is the first systematic work to establish the relationship across a broad range of nanotube samples. Researchers found that longer nanotubes could be processed as well as shorter ones, and that mechanical strength and electrical conductivity increased in lockstep.

The best fibers had an average tensile strength of 2.4 gigapascals (GPa) and electrical conductivity of 8.5 megasiemens per meter, about 15 percent of the conductivity of copper. Increasing nanotube length during synthesis will provide a path toward further property improvements, Pasquali said.

The second paper focused on purifying fibers produced by the floating catalyst method for use in films and aerogels. This process is fast, efficient and cost-effective on a medium scale and can yield the direct spinning of high-quality nanotube fibers; however, it leaves behind impurities, including metallic catalyst particles and bits of leftover carbon, allows less control of fiber structure and limits opportunities to scale up, Pasquali said.

“That’s where these two papers converge,” he said. “There are basically two ways to make nanotube fibers. In one, you make the nanotubes and then you spin them into fibers, which is what we’ve developed at Rice. In the other, developed at the University of Cambridge, you make nanotubes in a reactor and tune the reactor such that, at the end, you can pull the nanotubes out directly as fibers.

“It’s clear those direct-spun fibers include longer nanotubes, so there’s an interest in getting the tubes included in those fibers as a source of material for our spinning method,” Pasquali said. “This work is a first step toward that goal.”

The reactor process developed a decade ago by materials scientist Alan Windle at the University of Cambridge produces the requisite long nanotubes and fibers in one step, but the fibers must be purified, Pasquali said. Researchers at Rice and the National University of Singapore (NUS) have developed a simple oxidative method to clean the fibers and make them usable for a broader range of applications.

The labs purified fiber samples in an oven, first burning out carbon impurities in air at 500 degrees Celsius (932 degrees Fahrenheit) and then immersing them in hydrochloric acid to dissolve iron catalyst impurities.

Impurities in the resulting fibers were reduced to 5 percent of the material, which made them soluble in acids. The researchers then used the nanotube solution to make conductive, transparent thin films.

“There is great potential for these disparate techniques to be combined to produce superior fibers and the technology scaled up for industrial use,” said co-author Hai Minh Duong, an NUS assistant professor of mechanical engineering. “The floating catalyst method can produce various types of nanotubes with good morphology control fairly quickly. The nanotube filaments can be collected directly from their aerogel formed in the reactor. These nanotube filaments can then be purified and twisted into fibers using the wetting technique developed by the Pasquali group.”

Pasquali noted the collaboration between Rice and Singapore represents convergence of another kind. “This may well be the first time someone from the Cambridge fiber spinning line (Duong was a postdoctoral researcher in Windle’s lab) and the Rice fiber spinning line have converged,” he said. “We’re working together to try out materials made in the Cambridge process and adapting them to the Rice process.”

A new method that precisely measures the mysterious behavior and magnetic properties of electrons flowing across the surface of quantum materials could open a path to next-generation electronics.

Found at the heart of electronic devices, silicon-based semiconductors rely on the controlled electrical current responsible for powering electronics. These semiconductors can only access the electrons’ charge for energy, but electrons do more than carry a charge. They also have intrinsic angular momentum known as spin, which is a feature of quantum materials that, while elusive, can be manipulated to enhance electronic devices.

A team of scientists, led by An-Ping Li at the Department of Energy’s Oak Ridge National Laboratory, has developed an innovative microscopy technique to detect the spin of electrons in topological insulators, a new kind of quantum material that could be used in applications such as spintronics and quantum computing.

A new microscopy method developed by an ORNL-led team has four movable probing tips, is sensitive to the spin of moving electrons and produces high-resolution results. Using this approach, they observed the spin behavior of electrons on the surface of a quantum material. Credit: Saban Hus and An-Ping Li/Oak Ridge National Laboratory, U.S. Dept. of Energy

A new microscopy method developed by an ORNL-led team has four movable probing tips, is sensitive to the spin of moving electrons and produces high-resolution results. Using this approach, they observed the spin behavior of electrons on the surface of a quantum material. Credit: Saban Hus and An-Ping Li/Oak Ridge National Laboratory, U.S. Dept. of Energy

“The spin current, namely the total angular momentum of moving electrons, is a behavior in topological insulators that could not be accounted for until a spin-sensitive method was developed,” Li said.

Electronic devices continue to evolve rapidly and require more power packed into smaller components. This prompts the need for less costly, energy-efficient alternatives to charge-based electronics. A topological insulator carries electrical current along its surface, while deeper within the bulk material, it acts as an insulator. Electrons flowing across the material’s surface exhibit uniform spin directions, unlike in a semiconductor where electrons spin in varying directions.

“Charge-based devices are less energy efficient than spin-based ones,” said Li. “For spins to be useful, we need to control both their flow and orientation.”

To detect and better understand this quirky particle behavior, the team needed a method sensitive to the spin of moving electrons. Their new microscopy approach was tested on a single crystal of Bi2Te2Se, a material containing bismuth, tellurium and selenium. It measured how much voltage was produced along the material’s surface as the flow of electrons moved between specific points while sensing the voltage for each electron’s spin.

The new method builds on a four-probe scanning tunneling microscope–an instrument that can pinpoint a material’s atomic activity with four movable probing tips–by adding a component to observe the spin behavior of electrons on the material’s surface. This approach not only includes spin sensitivity measurements. It also confines the current to a small area on the surface, which helps to keep electrons from escaping beneath the surface, providing high-resolution results.

“We successfully detected a voltage generated by the electron’s spin current,” said Li, who coauthored a paper published by Physical Review Letters that explains the method. “This work provides clear evidence of the spin current in topological insulators and opens a new avenue to study other quantum materials that could ultimately be applied in next-generation electronic devices.”

A research collaboration between Osaka University and the Nara Institute of Science and Technology for the first time used scanning tunneling microscopy (STM) to create images of atomically flat side-surfaces of 3D silicon crystals. This work helps semiconductor manufacturers continue to innovate while producing smaller, faster, and more energy-efficient computer chips for computers and smartphones.

Spatial-derivative STM images with 200x200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Spatial-derivative STM images with 200×200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Our computers and smartphones each are loaded with millions of tiny transistors. The processing speed of these devices has increased dramatically over time as the number of transistors that can fit on a single computer chip continues to increase. Based on Moore’s Law, the number of transistors per chip will double about every 2 years, and in this area it seems to be holding up. To keep up this pace of rapid innovation, computer manufacturers are continually on the lookout for new methods to make each transistor ever smaller.

Current microprocessors are made by adding patterns of circuits to flat silicon wafers. A novel way to cram more transistors in the same space is to fabricate 3D-structures. Fin-type field effect transistors (FETs) are named as such because they have fin-like silicon structures that extend into the air, off the surface of the chip. However, this new method requires a silicon crystal with a perfectly flat top and side-surfaces, instead of just the top surface, as with current devices. Designing the next generation of chips will require new knowledge of the atomic structures of the side-surfaces.

Now, researchers at Osaka University and the Nara Institute of Science and Technology report that they have used STM to image the side-surface of a silicon crystal for the first time. STM is a powerful technique that allows the locations of the individual silicon atoms to be seen. By passing a sharp tip very close to the sample, electrons can jump across the gap and create an electrical current. The microscope monitored this current, and determined the location of the atoms in the sample.

“Our study is a big first step toward the atomically resolved evaluation of transistors designed to have 3D-shapes,” study coauthor Azusa Hattori says.

To make the side-surfaces as smooth as possible, the researchers first treated the crystals with a process called reactive ion etching. Coauthor Hidekazu Tanaka says, “Our ability to directly look at the side-surfaces using STM proves that we can make artificial 3D structures with near-perfect atomic surface ordering.”