Tag Archives: letter-pulse-tech

Texas Instruments (TI) (NASDAQ: TXN) today introduced the first 3-channel high-side linear automotive light-emitting diode (LED) controller without internal MOSFETs which gives designers greater flexibility for their lighting designs. The TPS92830-Q1’s novel architecture enables higher power and better thermal dissipation than conventional LED controllers, and are particularly beneficial for automotive LED lighting applications that require high performance and reliability.

Conventional LED drivers integrate the MOSFET, which limits designers’ ability to customize features. With that type of driver, designers often must make significant design modifications to achieve the desired system performance. The TPS92830-Q1 LED controller’s flexible on-board features give designers the freedom to select the best MOSFET for their system requirements. With this new approach, designers can more quickly and efficiently optimize their lighting power designs for automotive system requirements and desired dimming features.

Key features and benefits

  • Flexibility: The on-chip pulse-width modulation (PWM) generator or PWM input enables flexible dimming. Designers can use either the analog control or PWM to manage an output current of more than 150 mA per channel, to power automotive rear combination lamps and daytime running lights.
  • Improved thermal dissipation: By pairing the LED controller with an external MOSFET, the designer can achieve the required high power output while distributing the power across the controller and MOSFET to avoid system overheating. By retaining linear architecture, the TPS92830-Q1 provides improved electromagnetic interference (EMI) and electromagnetic compatibility (EMC) performance.
  • Greater system reliability: Advanced protection and built-in open and short detection features help designers meet original equipment manufacturer (OEM) system reliability requirements. The output current derating feature protects the external MOSFET under high voltage conditions to ensure system reliability.

The TPS92830-Q1 expands TI’s extensive portfolio of LED drivers, design tools and technical resources that help designers implement innovative automotive lighting features.

Smartphones and computers wouldn’t be nearly as useful without room for lots of apps, music and videos.

Devices tend to store that information in two ways: through electric fields (think of a flash drive) or through magnetic fields (like a computer’s spinning hard disk). Each method has advantages and disadvantages. However, in the future, our electronics could benefit from the best of each.

“There’s an interesting concept,” says Chang-Beom Eom, the Theodore H. Geballe Professor and Harvey D. Spangler Distinguished Professor of Materials Science and Engineering at the University of Wisconsin-Madison. “Can you cross-couple these two different ways to store information? Could we use an electric field to change the magnetic properties? Then you can have a low-power, multifunctional device. We call this a ‘magnetoelectric’ device.”

In research published recently in the journal Nature Communications, Eom and his collaborators describe not only their unique process for making a high-quality magnetoelectric material, but exactly how and why it works.

Physics graduate student Julian Irwin checks equipment in the lab of materials science and engineering Professor Chang-Beom Eom, where researchers have produced a material that could exhibit the best qualities of both solid-state and spinning disk digital storage. Credit: Sarah Page/UW-Madison College of Engineering

Physics graduate student Julian Irwin checks equipment in the lab of materials science and engineering Professor Chang-Beom Eom, where researchers have produced a material that could exhibit the best qualities of both solid-state and spinning disk digital storage. Credit: Sarah Page/UW-Madison College of Engineering

Magnetoelectric materials — which have both magnetic and electrical functionalities, or “orders” — already exist. Switching one functionality induces a change in the other.

“It’s called cross-coupling,” says Eom. “Yet, how they cross-couple is not clearly understood.”

Gaining that understanding, he says, requires studying how the magnetic properties change when an electric field is applied. Up to now, this has been difficult due to the complicated structure of most magnetoelectric materials.

In the past, says Eom, people studied magnetoelectric properties using very “complex” materials, or those that lack uniformity. In his approach, Eom simplified not only the research, but the material itself.

Drawing on his expertise in material growth, he developed a unique process, using atomic “steps,” to guide the growth of a homogenous, single-crystal thin film of bismuth ferrite. Atop that, he added cobalt, which is magnetic; on the bottom, he placed an electrode made of strontium ruthenate.

The bismuth ferrite material was important because it made it much easier for Eom to study the fundamental magnetoelectric cross-coupling.

“We found that in our work, because of our single domain, we could actually see what was going on using multiple probing, or imaging, techniques,” he says. “The mechanism is intrinsic. It’s reproducible — and that means you can make a device without any degradation, in a predictable way.”

To image the changing electric and magnetic properties switching in real time, Eom and his colleagues used the powerful synchrotron light sources at Argonne National Laboratory outside Chicago, and in Switzerland and the United Kingdom.

“When you switch it, the electrical field switches the electric polarization. If it’s ‘downward,’ it switches ‘upward,'” he says. “The coupling to the magnetic layer then changes its properties: a magnetoelectric storage device.”

That change in direction enables researchers to take the next steps needed to add programmable integrated circuits — the building blocks that are the foundation of our electronics — to the material.

While the homogenous material enabled Eom to answer important scientific questions about how magnetoelectric cross-coupling happens, it also could enable manufacturers to improve their electronics.

“Now we can design a much more effective, efficient and low-power device,” he says.

Researchers at the University of Liverpool have made a discovery that could improve the conductivity of a type of glass coating which is used on items such as touch screens, solar cells and energy efficient windows.

Coatings are applied to the glass of these items to make them electrically conductive whilst also allowing light through. Fluorine doped tin dioxide is one of the materials used in commercial low cost glass coatings as it is able to simultaneously allow light through and conduct electrical charge but it turns out that tin dioxide has as yet untapped potential for improved performance.

Compensating acceptor fluorine interstitials (light green) dramatically reduce electronic performance of tin dioxide transparent conducting glass coatings doped with fluorine atoms (dark green). Credit: University of Liverpool

Compensating acceptor fluorine interstitials (light green) dramatically reduce electronic performance of tin dioxide transparent conducting glass coatings doped with fluorine atoms (dark green). Credit: University of Liverpool

In a paper published in the journal Advanced Functional Materials, physicists identify the factor that has been limiting the conductivity of fluorine doped tin dioxide, which should be highly conductive because fluorine atoms substituted on oxygen lattice sites are each expected to give an additional free electron for conduction.

The scientists report, using a combination of experimental and theoretical data, that for every two fluorine atoms that give an additional free electron, another one occupies a normally unoccupied lattice position in the tin dioxide crystal structure.

Each so-called “interstitial” fluorine atom captures one of the free electrons and thereby becomes negatively charged. This reduces the electron density by half and also results in increased scattering of the remaining free electrons. These combine to limit the conductivity of fluorine doped tin dioxide compared with what would otherwise be possible.

PhD student Jack Swallow, from the University’s Department of Physics and the Stephenson Institute for Renewable Energy, said: “Identifying the factor that has been limiting the conductivity of fluorine doped tin dioxide is an important discovery and could lead to coatings with improved transparency and up to five times higher conductivity, reducing cost and enhancing performance in a myriad of applications from touch screens, LEDs, photovoltaic cells and energy efficient windows.”

The researchers now intend to address the challenge of finding alternative novel dopants that avoid these inherent drawbacks.

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future: While graphene – a one atom thin, honeycomb-shaped carbon layer – is a conductive material, it can become a semiconductor in the form of nanoribbons. This means that it has a sufficiently large energy or band gap in which no electron states can exist: it can be turned on and off – and thus may become a key component of nanotransistors.

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The smallest details in the atomic structure of these graphene bands, however, have massive effects on the size of the energy gap and thus on how well-suited nanoribbons are as components of transistors. On the one hand, the gap depends on the width of the graphene ribbons, while on the other hand it depends on the structure of the edges. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals, i.e. they are conductive, they become semiconductors with the armchair edge.

This poses a major challenge for the production of nanoribbons: If the ribbons are cut from a layer of graphene or made by cutting carbon nanotubes, the edges may be irregular and thus the graphene ribbons may not exhibit the desired electrical properties.

Creating a semiconductor with nine atoms

Empa researchers in collaboration with the Max Planck Institute for Polymer Research in Mainz and the University of California at Berkeley have now succeeded in growing ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. The specially prepared molecules are evaporated in an ultra-high vacuum for this purpose. After several process steps, they are combined like puzzle pieces on a gold base to form the desired nanoribbons of about one nanometer in width and up to 50 nanometers in length.

These structures, which can only be seen with a scanning tunneling microscope, now have a relatively large and, above all, precisely defined energy gap. This enabled the researchers to go one step further and integrate the graphene ribbons into nanotransistors. Initially, however, the first attempts were not very successful: Measurements showed that the difference in the current flow between the “ON” state (i.e. with applied voltage) and the “OFF” state (without applied voltage) was far too small. The problem was the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, it needed to be 50 nanometers thick, which in turn influenced the behavior of the electrons.

However, the researchers subsequently succeeded in massively reducing this layer by using hafnium oxide(HfO2) instead of silicon oxide as the dielectric material. The layer is therefore now only 1.5 nanometers thin and the “on”-current is orders of magnitudes higher.

Another problem was the incorporation of graphene ribbons into the transistor. In the future, the ribbons should no longer be located criss-cross on the transistor substrate, but rather aligned exactly along the transistor channel. This would significantly reduce the currently high level of non-functioning nanotransistors.

High-power white LEDs face the same problem that Michigan Stadium faces on game day — too many people in too small of a space. Of course, there are no people inside of an LED. But there are many electrons that need to avoid each other and minimize their collisions to keep the LED efficiency high. Using predictive atomistic calculations and high-performance supercomputers at the NERSC computing facility, researchers Logan Williams and Emmanouil Kioupakis at the University of Michigan found that incorporating the element boron into the widely used InGaN (indium-gallium nitride) material can keep electrons from becoming too crowded in LEDs, making the material more efficient at producing light.

This is the crystal structure of a BInGaN alloy. Using atomistic calculations and high-performance supercomputers at the NERSC facility, Logan Williams and Emmanouil Kioupakis at the University of Michigan predicted that incorporating boron into the InGaN active region of nitride LEDs reduces or even eliminates the lattice mismatch with the underlying GaN layers while keeping the emission wavelength approximately the same. The lattice matching enables the growth of thicker active regions and increases the efficiency of LEDs at high power. Credit: Michael Waters and Logan Williams

This is the crystal structure of a BInGaN alloy. Using atomistic calculations and high-performance supercomputers at the NERSC facility, Logan Williams and Emmanouil Kioupakis at the University of Michigan predicted that incorporating boron into the InGaN active region of nitride LEDs reduces or even eliminates the lattice mismatch with the underlying GaN layers while keeping the emission wavelength approximately the same. The lattice matching enables the growth of thicker active regions and increases the efficiency of LEDs at high power. Credit: Michael Waters and Logan Williams

Modern LEDs are made of layers of different semiconductor materials grown on top of one another. The simplest LED has three such layers. One layer is made with extra electrons put into the material. Another layer is made with too few electrons, the empty spaces where electrons would be are called holes. Then there is a thin middle layer sandwiched between the other two that determines what wavelength of light is emitted by the LED. When an electrical current is applied, the electrons and holes move into the middle layer where they can combine together to produce light. But if we squeeze too many electrons in the middle layer to increase the amount of light coming out of the LED, then the electrons may collide with each other rather than combine with holes to produce light. These collisions convert the electron energy to heat in a process called Auger recombination and lower the efficiency of the LED.

A way around this problem is to make more room in the middle layer for electrons (and holes) to move around. A thicker layer spreads out the electrons over a wider space, making it easier for them to avoid each other and reduce the energy lost to their collisions. But making this middle LED layer thicker isn’t as simple as it sounds.

Because LED semiconductor materials are crystals, the atoms that make them up must be arranged in specific regular distances apart from each other. That regular spacing of atoms in crystals is called the lattice parameter. When crystalline materials are grown in layers on top of one another, their lattice parameters must be similar so that the regular arrangements of atoms match where the materials are joined. Otherwise the material gets deformed to match the layer underneath it. Small deformations aren’t a problem, but if the top material is grown too thick and the deformation becomes too strong then atoms become misaligned so much that they reduce the LED efficiency. The most popular materials for blue and white LEDs today are InGaN surrounded by layers of GaN. Unfortunately, the lattice parameter of InGaN does not match GaN. This makes growing thicker InGaN layers to reduce electron collisions challenging.

Williams and Kioupakis found that by including boron in this middle InGaN layer, its lattice parameter becomes much more similar to GaN, even becoming exactly the same for some concentrations of boron. In addition, even though an entirely new element is included in the material, the wavelength of light emitted by the BInGaN material is very close to that of InGaN and can be tuned to different colors throughout the visible spectrum. This makes BInGaN suitable to be grown in thicker layers, reducing electron collisions and increasing the efficiency of the visible LEDs.

Although this material is promising to produce more efficient LEDs, it is important that it can be realized in the laboratory. Williams and Kioupakis have also shown that BInGaN could be grown on GaN using the existing growth techniques for InGaN, allowing quick testing and use of this material for LEDs. Still, the primary challenge of applying this work will be to fine tune how best to get boron incorporated into InGaN at sufficiently high amounts. But this research provides an exciting avenue for experimentalists to explore making new LEDs that are powerful, efficient, and affordable at the same time.

Semiconductor test equipment supplier Advantest Corporation (TSE:6857) has developed the M4171 handler to meet the mobile electronics market’s needs for cost-efficient thermal control testing of ICs with high power dissipation during device characterization and pre-production bring up.  This portable, single-site handler automates device loading and unloading, thermal conditioning and binning in engineering labs, where most testing today involves manual device handling. It also features an active thermal control (ATC) capability typically available only on larger footprint, more costly production-volume handlers.

The M4171 can be used to remotely conduct device handling and thermal control from anywhere around the world through a network connection.  In addition to requiring fewer operators and lowering labor costs, this handler maximizes system utilization among working groups in different locations.

The combination of automated device handling, wide-temperature ATC capabilities from -45° C to 125° C and remote operation make the M4171 unique.  It can run multi-mode test processes (Single Insertion Multiple Temperature), automated testing, automatic ID testing, output tray re-testing and manual testing, both pre-defined and user defined.

The Tri Temp Technology on the M4171 enables the users to operate over a broad range of temperatures which greatly increases any lab’s efficiency.  The system uses direct device-surface contact, which enables quick temperature switching for fast ramp up and ramp down and improves cycle temperature testing by over 40 percent compared to manual thermal-control solutions.

The M4171 handler is compatible with the V93000 and T2000 platforms as well as other testers.  Other features include a 2D code reader, a device rotator and a high contact force option.  Operation is simple with an intuitive, easy-to-use GUI that includes pre-defined functions.

“By bringing cost-efficient automated testing into the lab and enabling our customers to get higher utilization from their installed base, we are providing substantial productivity advantages,” said Toshio Goto, executive officer and manager of the Device Handling business unit at Advantest.  “As our first single-site ATC handler, the M4171 is opening new market opportunities for us in device characterization within labs and benchtop environments.”

 

Kateeva, a developer of inkjet deposition equipment solutions for OLED display manufacturing, today formally introduced a suite of YIELDjet inkjet equipment for red, green and blue (RGB) pixel deposition to enable the development and pilot production of large-size OLED displays, including televisions (TVs). The new YIELDjet family, which consists of the EXPLORE and EXPLORE PRO systems, provides display manufacturers with an industry-proven inkjet deposition platform to help bring the next generation of OLED TVs and other large-size displays to market. This year so far, Kateeva has shipped four systems from the EXPLORE family. The company expects to ship three additional systems by the second quarter of 2018.

The EXPLORE family broadens Kateeva’s product line and deepens the company’s penetration of the OLED display sector. The YIELDjet FLEX system already leads the inkjet deposition market for OLED mobile displays, with multiple systems deployed in mass production for OLED thin film encapsulation (TFE). The YIELDjet EXPLORE and EXPLORE PRO tools contain the same demonstrated core technologies found in the YIELDjet platform, with system designs that are optimized for rapid development of RGB pixel printing. Both tools, for instance, feature Kateeva’s unique nitrogen printing capability, which provides an oxygen- and- moisture-free enclosure for inkjet deposition. This capability is known to greatly increase OLED device lifetime.

The new products aim to help customers compress their in-house development- to- pilot-production cycle for printed RGB OLED displays, including TVs. To achieve this, the systems are designed for flexibility and scalability. The EXPLORE processes small panels (up to 200 mm square) for initial development, while the EXPLORE PRO targets mid-size panels (up to 55-in. display) for development through pilot production. As many as nine inks can be loaded into each tool at the same time. This enables accelerated evaluation of multiple materials during critical phases of process development.

The products offer an alternative to the traditional RGB pixel deposition approach of vacuum thermal evaporation (VTE) with a fine metal mask (FMM). Instead, printing is used to form the active layers within the pixels that generate the red, green and blue light emitted from the OLED device. Manufacturers are interested in using inkjet printing to overcome the scalability limitations of VTE with FMM.

VTE with FMM is currently used for small displays to fabricate patterned RGB active layers. However, the approach has not been successfully scaled to enable production of large displays such as those required for premium TVs. White OLED (WOLED) TV works around the issue by using VTE to form an un-patterned white OLED layer. This eliminates the need for FMM and creates the red, green, and blue light using three separate color filters (similar to the structure of a liquid crystal display). Although WOLED TVs are considered the best on the market, RGB OLED TVs fabricated using inkjet deposition can potentially offer superior performance. Moreover, manufacturing costs could be 20 percent lower, according to a recent analysis.

The potential of inkjet-fabricated RGB OLED TVs, coupled with the enabling capabilities of the YIELDjet EXPLORE products, have generated excitement among OLED display manufacturers, according to Kateeva’s President and COO, Dr. Conor Madigan. “There is increasing enthusiasm among our customers to develop RGB OLED TVs and we believe our new systems will help them accelerate their initiatives,” he said. “These companies are innovating rapidly and pioneering novel processes to mass-produce differentiated displays. Our products let them utilize Kateeva’s unique technologies as part of their inkjet RGB pixel printing programs. We are excited to work with them to move this approach closer to mass production.”

The YIELDjet Inkjet Advantage

Kateeva’s inkjet solution for RGB pixel deposition R&D utilizes core disruptive features found in the company’s YIELDjet platform. This OLED production equipment solution has already helped display manufacturers transition to flexible OLED mass production with high yields and low costs. Now, the same features, coupled with additional innovations for RGB pixel printing, promise to enable a similar transition to RGB OLED TV mass production by addressing customers’ yield and productivity priorities. Key YIELDjet technical features and advantages include:

  • Pure process environment: Trace amounts of oxygen and moisture, as well as large particles, can degrade OLED device performance and reduce yield. The same impurities are known to degrade OLED device lifetime. Processing in a clean, high-purity environment, therefore, is a central requirement for OLED front-plane manufacturing equipment. The YIELDjet solution features a specially designed nitrogen-purged enclosure that delivers an ultra-pure printing environment and enables fast recovery after maintenance. The result is longer OLED lifetime, higher yields, and higher uptime.
  • Superior uniformity: Non-uniform deposition of the printed layer can create “mura”. Mura, which refers to visibly noticeable non-uniformities in the finished display, will reduce yield. Print non-uniformity can be caused by inherent variations in the nozzles contained in the print array. The YIELDjet platform addresses the issue by combining two proprietary technologies—ultra-fast print head monitoring and Smart Mixing™ software. A remote drop inspection (RDI) system measures the drop characteristics for every nozzle in the print array on a continuous basis so that the state of the print array is known at all times. The nozzle data is used to calibrate the proprietary Smart Mixing software, which determines the optimized nozzle mixing for each sub-pixel during the print. The result is a system that delivers displays that are free of print mura in mass production.
  • High resolution: To achieve the resolution required for a product like an 8K TV, a key printing imperative is ink drop placement accuracy. This requires high stage accuracy. To enable high stage accuracy for all glass sizes, Kateeva pioneered the use of a “floating stage” for inkjet printers. With this capability, the glass floats on a thin cushion of nitrogen, which flows from a specially designed stationary stage. As the glass is scanned at high speed over the nitrogen cushion, proprietary stage-error correction technology is deployed to ensure the high accuracies needed for RGB pixel printing.

In addition to RGB pixel printing, the EXPLORE tools can be configured to process OLED TFE. This allows customers who are interested in both applications to conduct R&D or pilot production with the same EXPLORE or EXPLORE PRO tool.

Crosstalk and noise can become a major source of reliability problems of CNT based VLSI interconnects in the near future. Downscaling of component size in integrated circuits (ICs) to nanometer scale coupled with high density integration makes it challenging for researchers to maintain signal integrity in ICs. There are high chances of occurrence of crosstalk between adjacent wires. This crosstalk in turn, will increase the peak noise in the transient signals that pass through the interconnects. As multiple occurrences of crosstalk happen, the noise propagates through multiple stages of wires and the problem worsens to logic failure.

But thanks to semiconducting CNTs, which till now have found applications in the fabrication of futuristic field effect transistors, when placed around an interconnect, can reduce crosstalk to a large extent. Basically, semiconducting CNTs are non-conducting, have small dielectric constant, medium to large band gaps and hence can act as insulating shields to electric fields.

As semiconducting CNTs are one dimensional nanowires, they have very high anisotropic properties along their axis as well as their radius. The dielectric polarizability, which is the measure of number of polarizable bonds in a material, is found to be very smaller along the CNT radius compared to its axis. So, semiconducting CNTs are less polarizable along their radius which further suggests that they have small dielectric constants. The famous Clausius-Mossotti relation can be used to derive the dielectric constant from the dielectric polarizability. Further, this relation also tells that the dielectric constant of a CNT increases with its radius. So, obviously small diameter semiconducting CNTs are the ideal candidates as the low-k dielectric medium between two CNT interconnects.

The contact geometry is modified in such a way that more metal atoms are present at the centre where metallic CNTs are present. The contact has lesser number of metal atoms at the periphery where semiconducting CNTs are present. This helps in building a Schottky barrier at the contact semiconducting CNT interface and hence, inhibits any carrier movement.

Finally, experimental results show that the radial dielectric constant can be as low as 2.82 if (2,2) CNTs are used as shields. The coupling capacitance between adjacent wires is dependent on the interconnect thickness as well as the semiconducting CNT shield thickness. Crosstalk between CNT wires can be reduced by 28% if semiconducting CNTs are used. The crosstalk induced peak noise was also found to be 25% lesser for semiconducting CNT shielded interconnects at different input voltages of 0.8V, 0.5V and 0.3V.

For the first time, physicists have developed a technique that can peer deep beneath the surface of a material to identify the energies and momenta of electrons there.

The energy and momentum of these electrons, known as a material’s “band structure,” are key properties that describe how electrons move through a material. Ultimately, the band structure determines a material’s electrical and optical properties.

The team, at MIT and Princeton University, has used the technique to probe a semiconducting sheet of gallium arsenide, and has mapped out the energy and momentum of electrons throughout the material. The results are published today in the journal Science.

By visualizing the band structure, not just at the surface but throughout a material, scientists may be able to identify better, faster semiconductor materials. They may also be able to observe the strange electron interactions that can give rise to superconductivity within certain exotic materials.

“Electrons are constantly zipping around in a material, and they have a certain momentum and energy,” says Raymond Ashoori, professor of physics at MIT and a co-author on the paper. “These are fundamental properties which can tell us what kind of electrical devices we can make. A lot of the important electronics in the world exist under the surface, in these systems that we haven’t been able to probe deeply until now. So we’re very excited — the possibilities here are pretty vast.”

Ashoori’s co-authors are postdoc Joonho Jang and graduate student Heun Mo Yoo, along with Loren Pfeffer, Ken West, and Kirk Baldwin, of Princeton University.

Pictures beneath the surface

To date, scientists have only been able to measure the energy and momentum of electrons at a material’s surface. To do so, they have used angle-resolved photoemission spectroscopy, or ARPES, a standard technique that employs light to excite electrons and make them jump out from a material’s surface. The ejected electrons are captured, and their energy and momentum are measured in a detector. Scientists can then use these measurements to calculate the energy and momentum of electrons within the rest of the material.

“[ARPES] is wonderful and has worked great for surfaces,” Ashoori says. “The problem is, there is no direct way of seeing these band structures within materials.”

In addition, ARPES cannot be used to visualize electron behavior in insulators — materials within which electric current does not flow freely. ARPES also does not work in a magnetic field, which can greatly alter electronic properties inside a material.

The technique developed by Ashoori’s team takes up where ARPES leaves off and enables scientists to observe electron energies and momenta beneath the surfaces of materials, including in insulators and under a magnetic field.

“These electronic systems by their nature exist underneath the surface, and we really want to understand them,” Ashoori says. “Now we are able to get these pictures which have never been created before.”

Tunneling through

The team’s technique is called momentum and energy resolved tunneling spectroscopy, or MERTS, and is based on quantum mechanical tunneling, a process by which electrons can traverse energetic barriers by simply appearing on the other side — a phenomenon that never occurs in the macroscopic, classical world which we inhabit. However, at the quantum scale of individual atoms and electrons, bizarre effects such as tunneling can occasionally take place.

“It would be like you’re on a bike in a valley, and if you can’t pedal, you’d just roll back and forth. You would never get over the hill to the next valley,” Ashoori says. “But with quantum mechanics, maybe once out of every few thousand or million times, you would just appear on the other side. That doesn’t happen classically.”

Ashoori and his colleagues employed tunneling to probe a two-dimensional sheet of gallium arsenide. Instead of shining light to release electrons out of a material, as scientists do with ARPES, the team decided to use tunneling to send electrons in.

The team set up a two-dimensional electron system known as a quantum well. The system consists of two layers of gallium arsenide, separated by a thin barrier made from another material, aluminum gallium arsenide. Ordinarily in such a system, electrons in gallium arsenide are repelled by aluminum gallium arsenide, and would not go through the barrier layer.

“However, in quantum mechanics, every once in a while, an electron just pops through,” Jang says.

The researchers applied electrical pulses to eject electrons from the first layer of gallium arsenide and into the second layer. Each time a packet of electrons tunneled through the barrier, the team was able to measure a current using remote electrodes. They also tuned the electrons’ momentum and energy by applying a magnetic field perpendicular to the tunneling direction. They reasoned that those electrons that were able to tunnel through to the second layer of gallium arsenide did so because their momenta and energies coincided with those of electronic states in that layer. In other words, the momentum and energy of the electrons tunneling into gallium arsenide were the same as those of the electrons residing within the material.

By tuning electron pulses and recording those electrons that went through to the other side, the researchers were able to map the energy and momentum of electrons within the material. Despite existing in a solid and being surrounded by atoms, these electrons can sometimes behave just like free electrons, albeit with an “effective mass” that may be different than the free electron mass. This is the case for electrons in gallium arsenide, and the resulting distribution has the shape of a parabola. Measurement of this parabola gives a direct measure of the electron’s effective mass in the material.

Exotic, unseen phenomena

The researchers used their technique to visualize electron behavior in gallium arsenide under various conditions. In several experimental runs, they observed “kinks” in the resulting parabola, which they interpreted as vibrations within the material.

“Gallium and arsenic atoms like to vibrate at certain frequencies or energies in this material,” Ashoori says. “When we have electrons at around those energies, they can excite those vibrations. And we could see that for the first time, in the little kinks that appeared in the spectrum.”

They also ran the experiments under a second, perpendicular magnetic field and were able to observe changes in electron behavior at given field strengths.

“In a perpendicular field, the parabolas or energies become discrete jumps, as a magnetic field makes electrons go around in circles inside this sheet,” Ashoori says.

“This has never been seen before.”

The researchers also found that, under certain magnetic field strengths, the ordinary parabola resembled two stacked donuts.

“It was really a shock to us,” Ashoori says.

They realized that the abnormal distribution was a result of electrons interacting with vibrating ions within the material.

“In certain conditions, we found we can make electrons and ions interact so strongly, with the same energy, that they look like some sort of composite particles: a particle plus a vibration together,” Jang says.

Further elaborating, Ashoori explains that “it’s like a plane, traveling along at a certain speed, then hitting the sonic barrier. Now there’s this composite thing of the plane and the sonic boom. And we can see this sort of sonic boom — we’re hitting this vibrational frequency, and there’s some jolt happening there.”

The team hopes to use its technique to explore even more exotic, unseen phenomena below the material surface.

“Electrons are predicted to do funny things like cluster into little bubbles or stripes,” Ashoori says. “These are things we hope to see with our tunneling technique. And I think we have the power to do that.”

A team of Hokkaido University researchers has developed a novel material synthesis method called proton-driven ion introduction (PDII) which utilizes a phenomenon similar to “ion billiards.” The new method could pave the way for creating numerous new materials, thus drastically advancing materials sciences.

The synthesis method is based on a liquid-free process that allows for intercalation – insertion of guest ions into a host material – and ion substitution with those in the host material by driving ions with protons. This study, led by Assistant Professor Masaya Fujioka and Professor Junji Nishii at the university’s Research Institute for Electric Science, was published in the Journal of the American Chemical Society on November 16th.

Conventionally, intercalation and ion substitution have been conducted in an ion solution, but the process is regarded as cumbersome and problematic. In a liquid-based process, solvent molecules can be inserted into the host materials along with guest ions, degrading the crystal quality. It is also difficult to homogeneously introduce ions into host materials, and some host materials are not suitable when used with liquids.

In the PDII method, a high voltage of several kilovolts is applied to a needle-shaped anode placed in atmospheric hydrogen to generate protons via the electrolytic disassociation of hydrogen. The protons migrate along the electric field and are shot into the supply source of the desired ions – similar to balls in billiards – and the ions are driven out of the source to keep it electrically neutral. Ions forced out of the source are introduced, or intercalated, into a nanometer-level gap in the host material.

In this study, by using different materials as ion supply sources, the team succeeded in homogenously introducing lithium ions (Li+), sodium ions (Na+), potassium ions (K+), copper ions (Cu+) and silver ions (Ag+) into nanometer-level gaps in tantalum (IV) sulfide (TaS2), a layered material, while maintaining its crystallinity. Furthermore, the team successfully substituted Na+ of Na3V2(PO4)3 with K+, producing a thermodynamically metastable material, which cannot be obtained using the conventional solid-state reaction method.

“At present, we have shown that hydrogen ions (H+), Li+, Na+, K+, Cu+ and Ag+ can be used to introduce ions in our method, and we expect a larger variety of ions will be usable. By combining them with various host materials, our method could enable the production of numerous new materials,” says Masaya Fujioka. “In particular, if a method to introduce negatively charged ions and multivalent ions is established, it will spur the development of new functional materials in the solid ion battery and electronics fields.”