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The Silicon Integration Initiative’s (Si2) Compact Model Coalition (CMC) has approved two integrated circuit design simulation standards that target the fast-growing global market for gallium nitride semiconductors.

The approved standards are the 12th and 13th models currently funded and supported by the CMC, a collaborative group that develops and maintains cost-saving SPICE (Simulation Program with Integrated Circuit Emphasis) models for IC design.

John Ellis, president and CEO, said gallium nitride devices are used in many high-power and high-frequency applications, including satellite communications, radar, cellular, broadband wireless systems, and automotive. “Although it’s currently a small market, gallium nitride devices are expected to show remarkable growth over the coming years.”

To reduce research and developments costs and increase simulation accuracy, the semiconductor industry relies on the CMC to share resources for funding standard SPICE models. Si2 is a research and development joint venture focused on IC design and tool operability standards. “Once the standard models are proven and accepted by CMC, they are incorporated into design tools widely used by the semiconductor industry. The equations at work in the standard model-setting process are developed, refined and maintained by leading universities and national laboratories. The CMC directs and funds the universities to standardize and improve the models,” Ellis explained.

Dr. Ana Villamor, technology and market analyst at Yole Développement (Yole), Lyon, France, said that “2015 and 2016 were exciting years for the gallium nitride power business. We project an explosion of this market with 79% CAGR between 2017 and 2022. Market value will reach US $460 million at the end of the period1. It’s still a small market compared to the impressive US $30 billion silicon power semiconductor market,” Villamor said. “However, its expected growth in the short term is showing the enormous potential of the power gallium nitride technology based on its suitability for high performance and high frequency solutions.”

Yole_GaN_power_device_market_size_split_by_application_M_

Peter Lee, manager at Micron Memory Japan and CMC chair, said that “Gallium nitride devices are playing an increasingly important part in the field of RF and power electronics. With these two advanced models established as the first, worldwide gallium nitride model standards, efficiencies in design will greatly increase by making it possible to take into account accurate device physical behavior in design, and enabling the use of the various simulation tools in the industry with consistent results.”

Click here to download standard models.

 

A scientific team led by the Department of Energy’s Oak Ridge National Laboratory has found a new way to take the local temperature of a material from an area about a billionth of a meter wide, or approximately 100,000 times thinner than a human hair.

This discovery, published in Physical Review Letters, promises to improve the understanding of useful yet unusual physical and chemical behaviors that arise in materials and structures at the nanoscale. The ability to take nanoscale temperatures could help advance microelectronic devices, semiconducting materials and other technologies, whose development depends on mapping the atomic-scale vibrations due to heat.

From left, Andrew Lupini and Juan Carlos Idrobo use ORNL's new monochromated, aberration-corrected scanning transmission electron microscope, a Nion HERMES to take the temperatures of materials at the nanoscale. Credit: Oak Ridge National Laboratory, US Dept. of Energy; photographer Jason Richards

From left, Andrew Lupini and Juan Carlos Idrobo use ORNL’s new monochromated, aberration-corrected scanning transmission electron microscope, a Nion HERMES to take the temperatures of materials at the nanoscale. Credit: Oak Ridge National Laboratory, US Dept. of Energy; photographer Jason Richards

The study used a technique called electron energy gain spectroscopy in a newly purchased, specialized instrument that produces images with both high spatial resolution and great spectral detail. The 13-foot-tall instrument, made by Nion Co., is named HERMES, short for High Energy Resolution Monochromated Electron energy-loss spectroscopy-Scanning transmission electron microscope.

Atoms are always shaking. The higher the temperature, the more the atoms shake. Here, the scientists used the new HERMES instrument to measure the temperature of semiconducting hexagonal boron nitride by directly observing the atomic vibrations that correspond to heat in the material. The team included partners from Nion (developer of HERMES) and Protochips (developer of a heating chip used for the experiment).

“What is most important about this ‘thermometer’ that we have developed is that temperature calibration is not needed,” said physicist Juan Carlos Idrobo of the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL.

Other thermometers require prior calibration. To make temperature graduation marks on a mercury thermometer, for example, the manufacturer needs to know how much mercury expands as the temperature rises.

“ORNL’s HERMES instead gives a direct measurement of temperature at the nanoscale,” said Andrew Lupini of ORNL’s Materials Science and Technology Division. The experimenter needs only to know the energy and intensity of an atomic vibration in a material–both of which are measured during the experiment.

These two features are depicted as peaks, which are used to calculate a ratio between energy gain and energy loss. “From this we get a temperature,” Lupini explained. “We don’t need to know anything about the material beforehand to measure temperature.”

In 1966, also in Physical Review Letters, H. Boersch, J. Geiger and W. Stickel published a demonstration of electron energy gain spectroscopy, at a larger length scale, and pointed out that the measurement should depend upon the temperature of the sample. Based on that suggestion, the ORNL team hypothesized that it should be possible to measure a nanomaterial’s temperature using an electron microscope with an electron beam that is “monochromated” or filtered to select energies within a narrow range.

To perform electron energy gain and loss spectroscopy experiments, scientists place a sample material in the electron microscope. The microscope’s electron beam goes through the sample, with the majority of electrons barely interacting with the sample. In electron energy loss spectroscopy, the beam loses energy as it passes through the sample, whereas in energy gain spectroscopy, the electrons gain energy from interacting with the sample.

“The new HERMES lets us look at very tiny energy losses and even very small amounts of energy gain by the sample, which are even harder to observe because they are less likely to happen,” Idrobo said. “The key to our experiment is that statistical physical principles tell us that it is more likely to observe energy gain when the sample is heated. That is precisely what allowed us to measure the temperature of the boron nitride. The monochromated electron microscope enables this from nanoscale volumes. The ability to probe such exquisite physical phenomena at these tiny scales is why ORNL purchased the HERMES.”

ORNL scientists are constantly pushing the capabilities of electron microscopes to allow new ways of conducting forefront research. When Nion electron microscope developer Ondrej Krivanek asked Idrobo and Lupini, “Wouldn’t it be fun to try electron energy gain spectroscopy?” they jumped at the chance to be the first to explore this capability of their HERMES instrument.

Nanoscale resolution makes it possible to characterize the local temperature during phase transitions in materials–an impossibility with techniques that do not have the spatial resolution of HERMES spectroscopy. For example, an infrared camera is limited by the wavelength of infrared light to much larger objects.

Whereas in this experiment the scientists tested nanoscale environments at room temperature to about 1300 degrees Celsius (2372 degrees Fahrenheit), the HERMES could be useful for studying devices working across a wide range of temperatures, for example, electronics that operate under ambient conditions to vehicle catalysts that perform over 300 C/600 F.

Scientists at Rice University and the Indian Institute of Science, Bangalore, have discovered a method to make atomically flat gallium that shows promise for nanoscale electronics.

The Rice lab of materials scientist Pulickel Ajayan and colleagues in India created two-dimensional gallenene, a thin film of conductive material that is to gallium what graphene is to carbon.

Extracted into a two-dimensional form, the novel material appears to have an affinity for binding with semiconductors like silicon and could make an efficient metal contact in two-dimensional electronic devices, the researchers said.

The new material was introduced in Science Advances.

Gallium is a metal with a low melting point; unlike graphene and many other 2-D structures, it cannot yet be grown with vapor phase deposition methods. Moreover, gallium also has a tendency to oxidize quickly. And while early samples of graphene were removed from graphite with adhesive tape, the bonds between gallium layers are too strong for such a simple approach.

So the Rice team led by co-authors Vidya Kochat, a former postdoctoral researcher at Rice, and Atanu Samanta, a student at the Indian Institute of Science, used heat instead of force.

Rather than a bottom-up approach, the researchers worked their way down from bulk gallium by heating it to 29.7 degrees Celsius (about 85 degrees Fahrenheit), just below the element’s melting point. That was enough to drip gallium onto a glass slide. As a drop cooled just a bit, the researchers pressed a flat piece of silicon dioxide on top to lift just a few flat layers of gallenene.

They successfully exfoliated gallenene onto other substrates, including gallium nitride, gallium arsenide, silicone and nickel. That allowed them to confirm that particular gallenene-substrate combinations have different electronic properties and to suggest that these properties can be tuned for applications.

“The current work utilizes the weak interfaces of solids and liquids to separate thin 2-D sheets of gallium,” said Chandra Sekhar Tiwary, principal investigator on the project he completed at Rice before becoming an assistant professor at the Indian Institute of Technology in Gandhinagar, India. “The same method can be explored for other metals and compounds with low melting points.”

Gallenene’s plasmonic and other properties are being investigated, according to Ajayan. “Near 2-D metals are difficult to extract, since these are mostly high-strength, nonlayered structures, so gallenene is an exception that could bridge the need for metals in the 2-D world,” he said.

A research team led by UCLA scientists and engineers has developed a method to make new kinds of artificial “superlattices” — materials comprised of alternating layers of ultra-thin “two-dimensional” sheets, which are only one or a few atoms thick. Unlike current state-of-the art superlattices, in which alternating layers have similar atomic structures, and thus similar electronic properties, these alternating layers can have radically different structures, properties and functions, something not previously available.

This is an artist's concept of two kinds of monolayer atomic crystal molecular superlattices. On the left, molybdenum disulfide with layers of ammonium molecules, on the right, black phosphorus with layers of ammonium molecules. Credit: UCLA Samueli Engineering

This is an artist’s concept of two kinds of monolayer atomic crystal molecular superlattices. On the left, molybdenum disulfide with layers of ammonium molecules, on the right, black phosphorus with layers of ammonium molecules. Credit: UCLA Samueli Engineering

For example, while one layer of this new kind of superlattice can allow a fast flow of electrons through it, the other type of layer can act as an insulator. This design confines the electronic and optical properties to single active layers, and prevents them from interfering with other insulating layers.

Such superlattices can form the basis for improved and new classes of electronic and optoelectronic devices. Applications include superfast and ultra-efficient semiconductors for transistors in computers and smart devices, and advanced LEDs and lasers.

Compared with the conventional layer-by-layer assembly or growth approach currently used to create 2D superlattices, the new UCLA-led process to manufacture superlattices from 2D materials is much faster and more efficient. Most importantly, the new method easily yields superlattices with tens, hundreds or even thousands of alternating layers, which is not yet possible with other approaches.

This new class of superlattices alternates 2D atomic crystal sheets that are interspaced with molecules of varying shapes and sizes. In effect, this molecular layer becomes the second “sheet” because it is held in place by “van der Waals” forces, weak electrostatic forces to keep otherwise neutral molecules “attached” to each other. These new superlattices are called “monolayer atomic crystal molecular superlattices.”

The study, published in Nature, was led by Xiangfeng Duan, UCLA professor of chemistry and biochemistry, and Yu Huang, UCLA professor of materials science and engineering at the UCLA Samueli School of Engineering.

“Traditional semiconductor superlattices can usually only be made from materials with highly similar lattice symmetry, normally with rather similar electronic structures,” Huang said. “For the first time, we have created stable superlattice structures with radically different layers, yet nearly perfect atomic-molecular arrangements within each layer. This new class of superlattice structures has tailorable electronic properties for potential technological applications and further scientific studies.”

One current method to build a superlattice is to manually stack the ultrathin layers one on top of the other. But this is labor-intensive. In addition, since the flake-like sheets are fragile, it takes a long time to build because many sheets will break during the placement process. The other method is to grow one new layer on top of the other, using a process called “chemical vapor deposition.” But since that means different conditions, such as heat, pressure or chemical environments, are needed to grow each layer, the process could result in altering or breaking the layer underneath. This method is also labor-intensive with low yield rates.

The new method to create monolayer atomic crystal molecular superlattices uses a process called “electrochemical intercalation,” in which a negative voltage is applied. This injects negatively charged electrons into the 2D material. Then, this attracts positively charged ammonium molecules into the spaces between the atomic layers. Those ammonium molecules automatically assemble into new layers in the ordered crystal structure, creating a superlattice.

“Think of a two-dimensional material as a stack of playing cards,” Duan said. “Then imagine that we can cause a large pile of nearby plastic beads to insert themselves, in perfect order, sandwiching between each card. That’s the analogous idea, but with a crystal of 2D material and ammonium molecules.”

The researchers first demonstrated the new technique using black phosphorus as a base 2D atomic crystal material. Using the negative voltage, positively charged ammonium ions were attracted into the base material, and inserted themselves between the layered atomic phosphorous sheets.”

Following that success, the team inserted different types of ammonium molecules with various sizes and symmetries into a series of 2D materials to create a broad class of superlattices. They found that they could tailor the structures of the resulting monolayer atomic crystal molecular superlattices, which had a diverse range of desirable electronic and optical properties.”The resulting materials could be useful for making faster transistors that consume less power, or for creating efficient light-emitting devices,” Duan said.

Semiconductors–a class of materials that can function as both electrical conductor and insulator, depending on the circumstances–are an essential technology for all modern electronic innovations.

Silicon has long been the most famous semiconductor, but in recent years researchers have studied a wider range of materials, including molecules that can be tailored to serve specific electronic needs.

Perhaps appropriately, one of the most cutting-edge electronics–supercomputers–are indispensable research tools for studying complex semiconducting materials at a fundamental level.

Recently, a team of scientists at TU Dresden used the SuperMUC supercomputer at the Leibniz Supercomputing Centre to refine its method for studying organic semiconductors.

Illustration of a doped organic semiconductor based on fullerene C60 molecules (green). The benzimidazoline dopant (purple) donates an electron to the C60 molecules in its surrounding (dark green). These electrons can then propagate through the semiconductor material (light green). Credit: S. Hutsch/F. Ortmann, TU Dresden

Illustration of a doped organic semiconductor based on fullerene C60 molecules (green). The benzimidazoline dopant (purple) donates an electron to the C60 molecules in its surrounding (dark green). These electrons can then propagate through the semiconductor material (light green). Credit: S. Hutsch/F. Ortmann, TU Dresden

Specifically, the team uses an approach called semiconductor doping, a process in which impurities are intentionally introduced into a material to give it specific semiconducting properties. It recently published its results in Nature Materials.

“New kinds of semiconductors, organic semiconductors, are starting to get used in new device concepts,” said team leader Dr. Frank Ortmann. “Some of these are already on the market, but some are still limited by their inefficiency. We are researching doping mechanisms–a key technology for tuning semiconductors’ properties–to understand these semiconductors’ limitations and respective efficiencies.”

Quantum impurities

When someone changes a material’s physical properties, he or she also changes its electronic properties and, therefore, the role it can play in electronic devices. Small changes in material makeup can lead to big changes in a material’s characteristics–in certain cases one slight atomic alteration can lead to a 1000-fold change in electrical conductivity.

While changes in material properties may be big, the underlying forces–exerting themselves on atoms and molecules and governing their interactions–are generally weak and short-range (meaning the molecules and the atoms of which they are composed must be close together). To understand changes in properties, therefore, researchers have to accurately compute atomic and molecular interactions as well as the densities of electrons and how they are transferred among molecules.

Introducing specific atoms or molecules to a material can change its conducting properties on a hyperlocal level. This allows a transistor made from doped material to serve a variety of roles in electronics, including routing currents to perform operations based on complex circuits or amplifying current to help produce sound in a guitar amplifier or radio.

Quantum laws govern interatomic and intermolecular interactions, in essence holding material together, and, in turn, structuring the world as we know it. In the team’s work, these complex interactions need to be calculated for individual atomic interactions, including interactions among semiconductor “host” molecules and dopant molecules on a larger scale.

The team uses density functional theory (DFT)–a computational method that can model electronic densities and properties during a chemical interaction–to efficiently predict the variety of complex interactions. It then collaborates with experimentalists from TU Dresden and the Institute for Molecular Science in Okazaki, Japan to compare its simulations to spectroscopy experiments.

“Electrical conductivity can come from many dopants and is a property that emerges on a much larger length scale than just interatomic forces,” Ortmann said. “Simulating this process needs more sophisticated transport models, which can only be implemented on high-performance computing (HPC) architectures.”

Goal!

To test its computational approach, the team simulated materials that already had good experimental datasets as well as industrial applications. The researchers first focused on C60, also known as Buckminsterfullerene.

Buckminsterfullerene is used in several applications, including solar cells. The molecule’s structure is very similar to that of a soccer ball–a spherical arrangement of carbon atoms arranged in pentagonal and hexagonal patterns the size of less than one nanometer. In addition, the researches simulated zinc phthalocyanine (ZnPc), another molecule that is used in photovoltaics, but unlike C60, has a flat shape and contains a metallic atom (zinc).

As its dopant the team first used a well-studied molecule called 2-Cyc-DMBI (2-cyclohexyl-dimethylbenzimidazoline). 2-Cyc-DMBI is considered an n-dopant, meaning that it can provide its surplus electrons to the semiconductor to increase its conductivity. N-dopants are relatively rare, as few molecules are “willing” to give away an electron. In most cases, molecules that do so become unstable and degrade during chemical reactions, which in this context can lead to an electronic device failure. 2-Cyc-DMBI dopants are the exception, because they can be sufficiently weakly attractive for electrons–allowing them to move over long distances–while also remaining stable after donating them.

The team got good agreement between its simulations and experimental observations of the same molecule-dopant interactions. This indicates that they can rely on simulation to guide predictions as they relate to the doping process of semiconductors. They are now working on more complex molecules and dopants using the same methods.

Despite these advances, the team recognizes that next-generation supercomputers such as SuperMUC-NG–announced in December 2017 and set to be installed in 2018–will help the researchers expand the scope of their simulations, leading to ever bigger efficiency gains in a variety of electronic applications.

“We need to push the accuracy of our simulations to the maximum,” Ortmann said. “This would help us extend the range of applicability and allow us to more precisely simulate a broader set of materials or larger systems of more atoms.”

Ortmann also noted that while current-generation systems allowed the team to gain insights in specific situations and prove its concept, there is still room to get better. “We are often limited by system memory or CPU power,” he said. “The system size and simulation’s accuracy are essentially competing for computing power, which is why it is important to have access to better supercomputers. Supercomputers are perfectly suited to deliver answers to these problems in a realistic amount of time.”

Phonons, which are packets of vibrational waves that propagate in solids, play a key role in condensed matter and are involved in various physical properties of materials. In nanotechnology, for example, they affect light emission and charge transport of nanodevices. As the main source of energy dissipation in solid-state systems, phonons are the ultimate bottleneck that limits the operation of functional nanomaterials.In an article recently published in Nature Communications, an INRS team of researchers led by Professor Luca Razzari and their European collaborators show that it is possible to modify the phonon response of a nanomaterial by exploiting the zero-point energy (i.e., the lowest possible – “vacuum” – energy in a quantum system) of a terahertz nano-cavity. The researchers were able to reshape the nanomaterial phonon response by generating new light-matter hybrid states. They did this by inserting some tens of semiconducting (specifically, cadmium sulfide) nanocrystals inside plasmonic nanocavities specifically designed to resonate at terahertz frequencies, i.e., in correspondence of the phonon modes of the nanocrystals.

“We have thus provided clear evidence of the creation of a new hybrid nanosystem with phonon properties that no longer belong to the original nanomaterial,” the authors said.

This discovery holds promise for applications in nanophotonics and nanoelectronics, opening up new possibilities for engineering the optical phonon response of functional nanomaterials. It also offers an innovative platform for the realization of a new generation of quantum transducers and terahertz light sources.

SUNY Polytechnic Institute (SUNY Poly) today announced that its advanced semiconductor-based research and development efforts at its Albany NanoTech Complex have successfully received ISO 9001:2015 certification from TÜV SÜD AMERICA INC. for its effective quality management system. This certification acknowledges that SUNY Poly’s Center for Semiconductor Research (CSR) consistently provides products and services meeting the stringent and ever-improving requirements of the internationally recognized ISO 9001 designation, especially as it relates to excellent customer focus, strong top management, and a process-driven approach for the fabrication of test structures on 300mm semiconductor wafers, the platform upon which computer chips are made.

“By earning the ISO 9001:2015 certification, SUNY Poly’s technological and process management capabilities are further validated. It demonstrates the strength of SUNY’s research and development facility and capacity that renders SUNY a reliable, world-class partner for high-tech industry and contributes to New York State’s thriving innovation ecosystem,” said SUNY Interim Provost and Vice Chancellor for Research and Economic Development Grace Wang.”

“This certification showcases not only what SUNY Poly’s advanced facilities and nano-focused know-how are capable of, it is also another indication of how our institution aims to constantly improve via the implementation of its quality management systems with an eye toward continual progress,” said SUNY Poly Interim President Dr. Bahgat Sammakia. “This is one more way in which our globally recognized partners and potential future partners will know that they can work with SUNY Poly on advanced projects with extreme confidence.”

There are more than one million companies and organizations in over 170 countries certified to ISO 9001, but it is relatively rare for a research and educational institution to obtain this certification, with SUNY Poly’s Albany NanoTech Complex sharing the high-level distinction with well-regarded facilities such as the MIT Lincoln Laboratory, for example.

SUNY Poly’s CSR is a 300mm silicon wafer fabrication facility which provides researchers and partners with an industry-compliant and state-of-the-art fully integrated research, development, and prototyping line where companies of all sizes, as well as universities, national laboratories, and other researchers are able to gain access to advanced tool sets. The ISO 9001:2015 quality management system certification will offer current and future research partners even greater assurance of SUNY Poly’s ability to consistently provide high quality products and services as SUNY Poly seeks continual improvement in this area.  In addition, it could help lead to the facilities being designated as a U.S. Department of Defense Trusted Foundry, allowing it to work with any other trusted foundry to develop next-generation semiconductor wafer technologies.

“This third-party certification and detailed audit process are a strong signal to SUNY Poly’s research partners that our facilities, our externally-focused production capacity, as well as our management of services related to the fabrication of test structures on 300mm wafers, follow the strictest, most reliable standards, and we look forward to refining and improving the processes we employ to continually increase SUNY Poly’s fabrication competencies,” said SUNY Poly VP for Research Dr. Michael Liehr.

SUNY Poly’s ISO 9001:2015 certification is also significant because it opens the doors to the potential to work with certain commercial organizations that require the use of the formal quality management system. While the certification primarily concerns SUNY Poly CSR’s test structures program, which uses advanced CMOS processing for commercial customers, research leaders anticipate expanding its scope to also cover highly advanced silicon carbide (SiC) power electronics-centered research capabilities and processes, as well as photonics efforts, such as those related to the American Institute for Manufacturing Integrated Photonics (AIM Photonics), an industry-driven public-private partnership spearheaded by the Department of Defense, SUNY Poly, and New York State with numerous top universities from around the nation and high-tech industry partners. SUNY Poly plans to seek annual recertification.

Enabling further advancements in metrology, HEIDENHAIN CORPORATION recently donated some equipment to UNC Charlotte’s Center for Precision Metrology (CPM), a world premier university metrology lab.

As part of the UNC Charlotte William States Lee College of Engineering, the Metrology Lab is central to the education and research efforts in the areas of precision engineering and metrology, and includes a wide variety of high-end measurement instruments. Providing measurement research support to the University community and local industry, and already equipped with a HEIDENHAIN KGM grid plate, the HEIDENHAIN donation of a new EIB interface box with cabling and ACCOM software is allowing important upgrades to be realized to the system.

“At UNC Charlotte, the HEIDENHAIN KGM grid encoder is used to demonstrate the measurement of dynamic machine tool errors to the graduate class in Machine Tool Metrology utilizing the ISO230 standard series,” explained CPM Chief Engineer Dr. Jimmie Miller. “As far as R&D with this equipment, other plans involve its utilization by directly connecting to research machine encoders to assess the machine multi-axis position and control. This will enable us to move the assessment metrology loop outside of the control loop for a faster non-interfering independent evaluation.

“The generous support of companies like HEIDENHAIN supporting education and R&D allows us to continue to maintain the CPM capabilities at the state-of-the-art level by utilizing today’s top technologies such as the donated HEIDENHAIN interface equipment and software,” stated Dr. Miller.

Interface electronics from HEIDENHAIN adapt the encoder signals to the interface of the subsequent electronics. They are used when the subsequent electronics cannot directly process the output signals from HEIDENHAIN encoders, or if additional interpolation of the signals is necessary.  Because of their high IP 65 degree of protection, interface electronics with a box design are well suited for a rough industrial environment, for example where machine tools operate. The inputs and outputs are equipped with robust M23 and M12 connecting elements. The stable cast-metal housing offers protection against physical damage as well as against electrical interference.

 

University of Groningen physicists have managed to alter the flow of spin waves through a magnet, using only an electrical current. This is a huge step towards the spin transistor that is needed to construct spintronic devices. These promise to be much more energy efficient than conventional electronics. The results were published on 2 March in Physical Review Letters.

Spin is a quantum mechanical property of electrons. Simply put, it makes electrons behave like small magnetic compass needles which can point up or down. This can be used to transfer or store information, creating spintronic devices that promise several advantages over normal microelectronics.

In a conventional computer, separate devices are needed for data storage (often using a magnetic process) and data processing (electronic transistors). Spintronics could integrate both in one device, so it would no longer be necessary to move information between storage and processing units. Furthermore, spins can be stored in a non-volatile way, which means that their storage requires no energy, in contrast to normal RAM memory. All this means that spintronics could potentially make faster and more energy-efficient computers.

Wave

To realize this, many steps have to be taken and a lot of fundamental knowledge has to be obtained. The Physics of Nano Devices group of physics professor Bart van Wees at the University of Groningen’s Zernike Institute of Advanced Materials is at the forefront of this field. In their latest paper, they present a spin transistor based on magnons. Magnons, or spin waves, are a type of wave that only occurs in magnetic materials. ‘You can view magnons as a wave, or a particle, like electrons’, explains Ludo Cornelissen, PhD student in the Van Wees group and first author of the paper.

In their experiments, Cornelissen and Van Wees generate magnons in materials that are magnetic, but also electrically insulating. Electrons can’t travel through the magnet, but the spin waves can – just like a wave in a stadium moves while the spectators all stay in place. Cornelissen used a strip of platinum to inject magnons into a magnet made of yttrium iron garnet (YIG). ‘When an electron current travels through the strip, electrons are scattered by the interaction with the heavy atoms, a process that is called the spin Hall effect. The scattering depends on the spin of these electrons, so electrons with spin up and spin down are separated.’

Spin flip

At the interface of platinum and YIG, the electrons bounce back as they can’t enter the magnet. ‘When this happens, their spin flips from up to down, or vice versa. However, this causes a parallel spin flip inside the YIG, which creates a magnon.’ The magnons travel through the material and can be detected with a second platinum strip.

‘We described this spin transport through a magnet some time ago. Now, we’ve taken the next step: we wanted to influence the transport.’ This was done using a third platinum strip between injector and detector. By applying a positive or negative current, it is possible to either inject additional magnons in the conduction channel or drain magnons from it. ‘That makes our set up analogous to a field effect transistor. In such a transistor, an electric field of a gate electrode reduces or increases the number of free electrons in the channel, thus shutting down or boosting the current.’

Cornelissen and his colleagues show that adding magnons increases the spin current, while draining them causes a significant reduction. ‘Although we were not yet able to switch off the magnon current completely, this device does act as a transistor’, says Cornelissen. Theoretical modelling shows that reducing the thickness of the device can increase the depletion of magnons enough to stop the magnon current completely.

Superconductivity

But there is another interesting option, explains Cornelissen’s supervisor Bart van Wees: ‘In a thinner device, it could be possible to increase the amount of magnons in the channel to a level where they would form a Bose-Einstein condensate.’ This is the phenomenon that is responsible for superconductivity. And it occurs at room temperature, contrary to normal superconductivity, which only occurs at very low temperatures.

The study shows that a YIG spin transistor can be made, and that in the long run this material could even produce a spin superconductor. The beauty of the system is that spin injection and control of spin currents is achieved with a simple DC current, making these spintronic devices compatible with normal electronics. ‘Our next step is to see if we can realize this promise’, concludes Van Wees.

As the world of advanced manufacturing enters the sub-nanometer scale era, it is clear that ALD, MLD and SAM represent viable options for delivering the required few-atoms-thick layers required with uniformity, conformality, and purity.

BY BARRY ARKLES, JONATHAN GOFF, Gelest Inc., Morrisville PA; ALAIN E. KALOYEROS, SUNY Polytechnic Institute, Albany, NY

Device and system technologies across several industries are on the verge of entering the sub-nanometer scale regime. This regime requires processing techniques that enable exceptional atomic level control of the thickness, uniformity, and morphology of the exceedingly thin (as thin as a few atomic layers) film structures required to form such devices and systems.[1]

In this context, atomic layer deposition (ALD) has emerged as one of the most viable contenders to deliver these requirements. This is evidenced by the flurry of research and devel- opment activities that explore the applicability of ALD to a variety of material systems,[2,3] as well as the limited introduction of ALD TaN in full-scale manufacturing of nanoscale integrated circuitry (IC) structures.[4] Both the success and inherent limitations of ALD associated with repeated dual-atom interactions have stimulated great interest in additional self-limiting deposition processes, particularly Molecular Layer Deposition (MLD) and Self- Assembled Monolayers (SAM). MLD and SAM are being explored both as replacements and extensions of ALD as well as surface modification techniques prior to ALD.[5]

ALD is a thin film growth technique in which a substrate is exposed to alternate pulses of source precursors, with intermediate purge steps typically consisting of an inert gas to evacuate any remaining precursor after reaction with the substrate surface. ALD differs from chemical vapor deposition (CVD) in that the evacuation steps ensure that the different precursors are never present in the reaction zone at the same time. Instead, the precursor doses are applied as successive, non-overlapping gaseous injections. Each does is followed by an inert gas purge that serves to remove both byproducts and unreacted precursor from the reaction zone.

The fundamental premise of ALD is based on self-limiting surface reactions, wherein each individual precursor-substrate interaction is instantaneously terminated once all surface reactive sites have been depleted through exposure to the precursor. For the growth of binary materials, each ALD cycle consists of two precursor and two purge pulses, with the thickness of the resulting binary layer per cycle (typically about a monolayer) being determined by the precursor-surface reaction mode. The low growth rates associated with each ALD cycle enable precise control of ultimate film thickness via the application of repeated ALD cycles. Concurrently, the self-limiting ALD reaction mechanisms allow excellent conformality in ultra-high-aspect-ratio nanoscale structures and geometries.[6]

A depiction of an individual ALD cycle is shown in FIGURE 1. In Fig. 1(a), a first precursor A is introduced in the reaction zone above the substrate surface.

Screen Shot 2018-03-01 at 3.03.03 PM

Precursor A then adsorbs intact or reacts (partially) with the substrate surface to form a first monolayer, as shown in Fig. 1(b), with any excess precursor and potential byproducts being evacuated from the reaction zone through a subsequent purge step. In Fig. 1(d), a second precursor Y is injected into the reaction zone and is made to react with the first monolayer to form a binary atomic layer on the substrate surface, as displayed in Fig. 1(e). Again, all excess precursors and reaction byproducts are flushed out with a second purge step 1(f). The entire process is performed repeatedly to achieve the targeted binary film thickness.

In some applications, a direct or remote plasma is used as an intermediate treatment step between the two precursor-surface interactions. This treatment has been reported to increase the probability of surface adsorption by boosting the number of active surface sites and lowering the reaction activation energy. As a result, such treatment has led to increased growth rates and reduce processing temperatures.[7]

A number of benefits have been cited for the use of ALD, including high purity films, absence of particle contami- nation and pin-holes, precise control of thickness at the atomic level, excellent thickness uniformity and step coverage in complex via and trench topographies, and the ability to grow an extensive array of binary material systems. However, issues with surface roughness and large surface grain morphology have also been reported. Another limitation of ALD is the fact that it is primarily restricted to single or binary material systems. Finally, extremely slow growth rates continue to be a challenge, which could potentially restrict ALD’s applicability to exceptionally ultrathin films and coatings.

These concerns have spurred a renewed interest in other molecular level processing technologies that share the self-limiting surface reaction characteristics of ALD. Chief among them are MLD and SAM. MLD refers principally to ALD-like processes that also involve successive precursor-surface reactions in which the various precursors never cross paths in the reaction zone. [8] However, while ALD is employed to grow inorganic material systems, MLD is mainly used to deposit organic molecular films. It should be noted that this definition of MLD, although the most common, is not yet universally accepted. An alternative characterization refers to MLD as a process for the growth of organic molecular components that may contain inorganic fragments, yet it does not exhibit the self-limiting growth features of ALD or its uniformity of film thickness and step coverage.[2]

A depiction illustrating a typical MLD cycle, according to the most common definition, is shown in FIGURE 2. In Fig. 2(a), a precursor is introduced in the reaction zone above the substrate surface. Precursor C adsorbs to the substrate surface and is confined by physisorption (Fig. 2(b)). The precursor then undergoes a quick chemisorption reaction with a significant number of active surface sites, leading to the self-limiting formation of molecular attachments in specific assemblies or regularly recurring structures, as displayed in Fig. 2(c). These structures form at significantly lower process temperatures compared to traditional deposition techniques.

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To date, MLD has been successfully applied to grow exceptionally thin films for applications as organic, inorganic, and hybrid organic-inorganic dielectrics and polymers for IC applications; [1,9] nanoprobes for in-vitro imaging and interrogation of biological cells; [10] photoluminescent devices; [7] and lithium-ion battery electrodes.[11]

SAM is a deposition technique that involves the spontaneous adherence of organized organic structures on a substrate surface. Such adherence takes place through adsorption from the vapor or liquid phase through relatively weak interactions with the substrate surface. Initially, the structures are adsorbed on the surface by physisorption through, for instance, van der Waals forces or polar interactions. Subsequently, the self-assembled monolayers become slowly confined by a chemisorption process, as depicted in FIGURE 3.

Screen Shot 2018-03-01 at 3.03.18 PM

The ability of SAM to grow layers as thin as a single molecule through chemisorption-driven interactions with the substrate has triggered enthusiasm for its potential use in the formation of “near-zero-thickness” activation or barrier layers. It has also sparked interest in its appli- cability to area-selective or area-specific deposition. Molecules can be directed to exhibit preferential reactions with specific segments of the underlying substrate rather than others to facilitate or obstruct subsequent material growth. This feature makes SAM desirable for incorpo- ration in area-selective ALD (AS-ALD) or CVD (AS-CVD), where the SAM-formed layer would serve as a foundation or blueprint to drive AS-ALD or AS-CVD. [12,13]

To date, SAM has been effectively employed to form organic layers as thin as a single molecule for applications as organic, inorganic, and hybrid organic-inorganic dielec- trics; polymers for IC applications; [13,14] encapsulation and barrier layers for IC metallization; [15] photoluminescent devices; [5] molecular and organic electronics; [16] and liquid crystal displays.[17]

As the world of advanced manufacturing enters the sub-nanometer scale era, it is clear that ALD, MLD and SAM represent viable options for delivering the required few-atoms-thick layers required with uniformity, conformality, and purity. By delivering the constituents of the material systems individually and sequentially into the processing environment, and precisely controlling the resulting chemical reactions with the substrate surface, these techniques enable excellent command of processing parameters and superb management of the target specifications of the resulting films. In order to determine whether one or more ultimately make it into full-scale manufacturing, a great deal of additional R&D is required in the areas of understanding and establishing libraries of fundamental interactions, mechanisms of source chemistries with various substrate surfaces, engineering viable solutions for surface smoothness and rough morphology, and developing protocols to enhance growth rates and overall throughput.

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