A two-dimensional materials transistor technology that could restart Moore’s law

Transistor Schematics of the new transistor: the insulator in red and blue, and the semiconductor above (Credit: TU Wien)

An important breakthrough in transistor technology has been achieved at TU Wien: With the help of novel insulators, high-quality transistors can be produced using two-dimensional materials.

For decades, the transistors on our microchips have become smaller, faster and cheaper. Approximately every two years the number of transistors on commercial chips has doubled – this phenomenon became known as “Moore’s Law”. But for several years now, Moore’s law does not hold any more. The miniaturization has reached a natural limit, as completely new problems arise when a length scale of only a few nanometers is approached.

Now, however, the next big miniaturization step could soon become possible – with so-called “two-dimensional (2D) materials” that may consist of only a single atomic layer. With the help of a novel insulator made of calcium fluoride, scientists at TU Wien (Vienna) have created an ultra-thin transistor, which has excellent electrical properties and, in contrast to previous technologies, can be miniaturized to an extremely small size. The new technology has now been presented in the journal “Nature Electronics”.

Ultra-Thin Semiconductors and Insulators

Research on semiconductor materials needed to fabricate transistors has seen significant progress in recent years. Today, ultra-thin semiconductors can be made of 2D materials, consisting of only a few atomic layers. “But this is not enough to build an extremely small transistor,” says Professor Tibor Grasser from the Institute of Microelectronics at TU Wien. “In addition to the ultra-thin semiconductor, we also need an ultra-thin insulator.”

This is due to the fundamental design structure of a transistor: current can flow from one side of the transistor to the other, but only if a voltage is applied in the middle, creating an electric field. The electrode providing this field must be electrically insulated from the semiconductor itself. “There have already been transistor experiments with ultra-thin semiconductors, but until now they were coupled with ordinary insulators,” says Tibor Grasser. “There is not much benefit in reducing the thickness of the semiconductor when it still has to be combined with a thick layer of insulator material. There is no way of miniaturizing such a transistor any further. Also, at very small length scales the insulator surface turned out to disturb the electronic properties of the semiconductor.”

Therefore, Yury Illarionov, a postdoc in Tibor Grasser’s team, tried a novel approach. He used ultra-thin 2D-materials not only for the semiconductor part of the transistor, but also for the insulating part. By selecting ultra-thin insulating materials such as ionic crystals, a transistor with a size of only a few nanometers can be built. The electronic properties are improved because ionic crystals can have a perfectly regular surface, without a single atom protruding from the surface, which could disturb the electric field. “Conventional materials have covalent bonds in the third dimension – atoms that couple to the neighboring materials above and below,” explains Tibor Grasser. “This is not the case in 2D materials and ionic crystals, and so they do not interfere with the electrical properties of the semiconductor.”

The Prototype is a World Champion

To produce the new ultra-thin transistor, calcium fluoride was selected as the insulating material. The calcium fluoride layer was produced at the Ioffe Institute in St. Petersburg, where the first author of the publication, Yury Illarionov, is originally from before joining the team in Vienna. The transistor itself was then manufactured by Prof. Thomas Müller’s team at the Institute of Photonics at TU Wien and analyzed at the Institute for Microelectronics.

The very first prototype already surpassed all expectations: “For years, we have received quite a number of different transistors to investigate their technical properties – but we have never seen anything like our transistor with the calcium fluoride insulator,” says Tibor Grasser. “The prototype with its superior electrical properties outshines all previous models.”

Now the team wants to find out which combinations of insulators and semiconductors work best. It may take a few more years before the technology can be used for commercially available computer chips as the manufacturing processes for the material layers still need to be improved. “In general, however, there is no doubt that transistors made of 2D materials are a highly interesting option for the future,” says Tibor Grasser. “From a scientific point of view, it is clear that the fluorides we have just tested are currently the best solution for the insulator problem. Now, only a few technical questions remain to be answered. ”

This new kind of smaller and faster transistor should enable the computer industry to take the next big step. This way, Moore’s law of exponentially increasing computer power could soon come to life again.

Learn more: Ultrathin Transistors for Faster Computer Chips

 

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Changeable 2D materials to transform electronics, optics, computing, and a host of other technologies

Artist’s rendering of a 2D material undergoing phase change using a transistor-scale platform developed in the lab of Stephen Wu, assistant professor of electrical and computer engineering and of physics. (University of Rochester illustration / Michael Osadciw)

Two-dimensional (2D) materials—as thin as a single layer of atoms—have intrigued scientists with their flexibility, elasticity, and unique electronic properties since first being discovered in materials such as graphene in 2004. Some of these materials can be especially susceptible to changes in their material properties as they are stretched and pulled. Under applied strain, they have been predicted to undergo phase transitions as disparate as superconducting in one moment to nonconducting the next, or optically opaque in one moment to transparent in the next.

Now, University of Rochester researchers have combined 2D materials with oxide materials in a new way, using a transistor-scale device platform, to fully explore the capabilities of these changeable 2D materials to transform electronics, optics, computing, and a host of other technologies.

“We’re opening up a new direction of study,” says Stephen Wu, assistant professor of electrical and computer engineering and physics. “There’s a huge number of 2D materials with different properties—and if you stretch them, they will do all sorts of things.”

The platform developed in Wu’s lab, configured much like traditional transistors, allows a small flake of a 2D material to be deposited onto a ferroelectric material. Voltage applied to the ferroelectric—which acts like a transistor’s third terminal, or gate—strains the 2D material by the piezoelectric effect, causing it to stretch. That, in turn, triggers a phase change that can completely alter the way the material behaves. When the voltage is turned off, the material retains its phase until an opposite polarity voltage is applied, causing the material to revert to its original phase.

“The ultimate goal of two-dimensional straintronics is to take all of the things that you couldn’t control before, like the topological, superconducting, magnetic, and optical properties of these materials, and now be able to control them, just by stretching the material on a chip,” Wu says.

“If you do this with topological materials you could impact quantum computers, or if you do it with superconducting materials you can impact superconducting electronics.”

Maxing out Moore’s Law

In a paper in Nature Nanotechnology, Wu and his students describe using a thin film of two-dimensional molybdenum ditelluride (MoTe2) in the device platform. When stretched and unstretched, the MoTe2 changes from a low conductivity semiconductor material to a highly conductive semimetallic material and back again.

“It operates just like a field effect transistor. You just have to put a voltage on that third terminal, and the MoTe2 will stretch a little bit in one direction and become something that’s conducting. Then you stretch it back in another direction, and all of a sudden you have something that has low conductivity,” Wu says.

The process works at room temperature, he adds, and, remarkably, “requires only a small amount of strain—we’re stretching the MoTe2 by only 0.4 percent to see these changes.”

Moore’s Law famously predicts that the number of transistors in a dense, integrated circuit will double about every two years.

Yet technology is nearing the limits at which traditional transistors can be scaled down in size. So, as we reach the limits of Moore’s Law, the technology developed in Wu’s lab could have far-reaching implications in moving past these limitations in the quest for ever faster, more enhanced computing power.

Wu’s platform has the potential to perform the same functions as a transistor with far less power consumption since power is not needed to retain the conductivity state. Moreover, it minimizes the leakage of electrical current due to the steep slope at which the device changes conductivity with applied gate voltage. Both of these issues—high power consumption and leakage of electrical current—have constrained the performance of traditional transistors at the nanoscale.

“This is the first demonstration,” Wu adds. “Now it’s up to researchers to figure out how far it goes.”

No strain, no gain

One advantage of Wu’s platform is that it is configured much like a traditional transistor, making it easier to eventually adapt into current electronics. However, more work is needed before the platform reaches that stage. Currently, the device can operate only 70 to 100 times in the lab before device failure. While the endurance of other non-volatile memories, like flash, are much higher, they also operate much slower than the ultimate potential of the strain-based devices being developed in Wu’s lab.

“Do I think it’s a challenge that can be overcome? Absolutely,” says Wu, who will be working on the problem with Hesam Askari, an assistant professor of mechanical engineering at Rochester, also a coauthor on the paper. “It’s a materials engineering problem that we can solve as we move forward in our understanding how this concept works.”

They will also explore how much strain can be applied to various two-dimensional materials without causing them to break. Determining the ultimate limit of the concept will help guide researchers to other phase-change materials as the technology moves forward.

Learn more: Researchers ‘stretch’ the ability of 2D materials to change technology

 

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Space update: A number of 2D materials can not only withstand being sent into space, but potentially thrive in the harsh conditions

Tobias Vogl Credit: Lannon Harley ANU

A new study from The Australian National University (ANU) has found a number of 2D materials can not only withstand being sent into space, but potentially thrive in the harsh conditions.

It could influence the type of materials used to build everything from satellite electronics to solar cells and batteries – making future space missions more accessible, and cheaper to launch.

PhD candidate and lead author Tobias Vogl was particularly interested in whether the 2D materials could withstand intense radiation.

“The space environment is obviously very different to what we have here on Earth. So we exposed a variety of 2D materials to radiation levels comparable to what we expect in space,” Mr Vogl said.

“We found most of these devices coped really well. We were looking at electrical and optical properties and basically didn’t see much difference at all.”

During a satellite’s orbit around the earth, it is subject to heating, cooling, and radiation. While there’s been plenty of work done demonstrating the robustness of 2D materials when it comes to temperature fluctuations, the impact of radiation has largely been unknown – until now.

The ANU team carried out a number of simulations to model space environments for potential orbits. This was used to expose 2D materials to the expected radiation levels. They found one material actually improved when subjected to intense gamma radiation.

“A material getting stronger after irradiation with gamma rays – it reminds me of the hulk,” Mr Vogl said.

“We’re talking about radiation levels above what we would see in space – but we actually saw the material become better, or brighter.”

Mr Vogl says this specific material could potentially be used to detect radiation levels in other harsh environments, like near nuclear reactor sites.

“The applications of these 2D materials will be quite versatile, from satellite structures reinforced with graphene – which is five-times stiffer than steel – to lighter and more efficient solar cells, which will help when it comes to actually getting the experiment into space.”

Among the tested devices were atomically thin transistors. Transistors are a crucial component for every electronic circuit. The study also tested quantum light sources, which could be used to form what Mr Vogl describes as the “backbone” of the future quantum internet.

“They could be used for satellite-based long-distance quantum cryptography networks. This quantum internet would be hacking proof, which is more important than ever in this age of rising cyberattacks and data breaches.”

“Australia is already a world leader in the field of quantum technology,” senior author Professor Ping Koy Lam said.

“In light of the recent establishment of the Australian Space Agency, and ANU’s own Institute for Space, this work shows that we can also compete internationally in using quantum technology to enhance space instrumentations.”

Learn more: ANU research set to shake-up space missions

 

 

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A new generation of unique ceramics could act as a sensor for structures

Ceramics with networked nanosheets of graphene and white graphene would have the unique ability to alter their electrical properties when strained, according to a researcher at Rice University. The surprising ability could lead to new types of structural sensors. Illustration by Rouzbeh Shahsavari

Rice-led simulations show unique ceramic could act as a sensor for structures

A ceramic that becomes more electrically conductive under elastic strain and less conductive under plastic strain could lead to a new generation of sensors embedded into structures like buildings, bridges and aircraft able to monitor their own health.

The electrical disparity fostered by the two types of strain was not obvious until Rice University’s Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering and of materials science and nanoengineering, and his colleagues modeled a novel two-dimensional compound, graphene-boron-nitride (GBN).

Under elastic strain, the internal structure of a material stretched like a rubber band does not change. But the same material under plastic strain — caused in this case by stretching it far enough beyond elasticity to deform — distorts its crystalline lattice. GBN, it turns out, shows different electrical properties in each case, making it a worthy candidate as a structural sensor.

Shahsavari had already determined that hexagonal-boron nitride – aka white graphene – can improve the properties of ceramics. He and his colleagues have now discovered that adding graphene makes them even stronger and more versatile, along with their surprising electrical properties.

The magic lies in the ability of two-dimensional, carbon-based graphene and white graphene to bond with each other in a variety of ways, depending on their relative concentrations. Though graphene and white graphene naturally avoid water, causing them to clump, the combined nanosheets easily disperse in a slurry during the ceramic’s manufacture.

The resulting ceramics, according to the authors’ theoretical models, would become tunable semiconductors with enhanced elasticity, strength and ductility.

The research led by Shahsavari and Asghar Habibnejad Korayem, an assistant professor of structural engineering at Iran University of Science and Technology and a research fellow at Monash University in Melbourne, Australia, appears in the American Chemical Society journal Applied Materials and Interfaces.

Graphene is a well-studied form of carbon known for its lack of a band gap – the region an electron has to leap to make a material conductive. With no band gap, graphene is a metallic conductor. White graphene, with its wide band gap, is an insulator. So the greater the ratio of graphene in the 2D compound, the more conductive the material will be.

Mixed into the ceramic in a high enough concentration, the 2D compound dubbed GBN would form a network as conductive as the amount of carbon in the matrix allows. That gives the overall composite a tunable band gap that could lend itself to a variety of electrical applications.

“Fusing 2D materials like graphene and boron nitride in ceramics and cements enables new compositions and properties we can’t achieve with either graphene or boron nitride by themselves,” Shahsavari said.

The team used density functional theory calculations to model variations of the 2D compound mixed with tobermorite, a calcium silicate hydrate material commonly used as cement for concrete. They determined the oxygen-boron bonds formed in the ceramic would turn it into a p-type semiconductor.

Tobermorite by itself has a large band gap of about 4.5 electron volts, but the researchers calculated that when mixed with GBN nanosheets of equal parts graphene and white graphene, that gap would shrink to 0.624 electron volts.

When strained in the elastic regime, the ceramic’s band gap dropped, making the material more conductive, but when stretched beyond elasticity — that is, in the plastic regime — it became less conductive. That switch, the researchers said, makes it a promising material for self-sensing and structural health monitoring applications.

The researchers suggested other 2D sheets with molybdenum disulfide, niobium diselenide or layered double hydroxides may provide similar opportunities for the bottom-up design of tunable, multifunctional composites. “This would provide a fundamental platform for cement and concrete reinforcement at their smallest possible dimension,” Shahsavari said.

Learn more: Nano-infused ceramic could report on its own health

 

 

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Could energy from Wi-Fi signals be turned into electricity that could power electronics?

Researchers from MIT and elsewhere have designed the first fully flexible, battery-free “rectenna” — a device that converts energy from Wi-Fi signals into electricity — that could be used to power flexible and wearable electronics, medical devices, and sensors for the “internet of things.”
Image: Christine Daniloff

Device made from flexible, inexpensive materials could power large-area electronics, wearables, medical devices, and more.

Imagine a world where smartphones, laptops, wearables, and other electronics are powered without batteries. Researchers from MIT and elsewhere have taken a step in that direction, with the first fully flexible device that can convert energy from Wi-Fi signals into electricity that could power electronics.

Devices that convert AC electromagnetic waves into DC electricity are known as “rectennas.” The researchers demonstrate a new kind of rectenna, described in a study appearing in Nature today, that uses a flexible radio-frequency (RF) antenna that captures electromagnetic waves — including those carrying Wi-Fi — as AC waveforms.

The antenna is then connected to a novel device made out of a two-dimensional semiconductor just a few atoms thick. The AC signal travels into the semiconductor, which converts it into a DC voltage that could be used to power electronic circuits or recharge batteries.

In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power. Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas.

“What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics?” says paper co-author Tomás Palacios, a professor in the Department of Electrical Engineering and Computer Science and director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories. “We have come up with a new way to power the electronics systems of the future — by harvesting Wi-Fi energy in a way that’s easily integrated in large areas — to bring intelligence to every object around us.”

Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and sensors for the “internet of things.” Flexible smartphones, for instance, are a hot new market for major tech firms. In experiments, the researchers’ device can produce about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). That’s more than enough power to light up an LED or drive silicon chips.

Another possible application is powering the data communications of implantable medical devices, says co-author Jesús Grajal, a researcher at the Technical University of Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics.

“Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die,” Grajal says. “It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.”

All rectennas rely on a component known as a “rectifier,” which converts the AC input signal into DC power. Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive. Researchers have been trying to fix these problems for a long time. But the few flexible rectennas reported so far operate at low frequencies and can’t capture and convert signals in gigahertz frequencies, where most of the relevant cell phone and Wi-Fi signals are.

To build their rectifier, the researchers used a novel 2-D material called molybdenum disulfide (MoS2), which at three atoms thick is one of the thinnest semiconductors in the world. In doing so, the team leveraged a singular behavior of MoS2: When exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material. The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal.

“By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” says first author and EECS postdoc Xu Zhang, who will soon join Carnegie Mellon University as an assistant professor.

Parasitic capacitance is an unavoidable situation in electronics where certain materials store a little electrical charge, which slows down the circuit. Lower capacitance, therefore, means increased rectifier speeds and higher operating frequencies. The parasitic capacitance of the researchers’ Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, so it is much faster at signal conversion and allows it to capture and convert up to 10 gigahertz of wireless signals.

“Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” Zhang says.

The reported work provides blueprints for other flexible Wi-Fi-to-electricity devices with substantial output and efficiency. The maximum output efficiency for the current device stands at 40 percent, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the MoS2 rectifier is about 30 percent. For reference, today’s rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 percent.

There are 15 other paper co-authors from MIT, Technical University of Madrid, the Army Research Laboratory, Charles III University of Madrid, Boston University, and the University of Southern California.

Learn more: Converting Wi-Fi signals to electricity with new 2-D materials

 

 

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