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Full(erene) potential


February 3, 2017

In what could be called a classic “Eureka” moment, UC Santa Barbara materials researchers have discovered a simple yet effective method for mastering the electrical properties of polymer semiconductors. The elegant technique allows for the efficient design and manufacture of organic circuitry (the type found in flexible displays and solar cells, for instance) of varying complexity while using the same semiconductor material throughout.

“It’s a different strategy by which you can take a material and change its properties,” said Guillermo Bazan, a professor of chemistry and materials at UCSB. With the addition of fullerene or copper tetrabenzoporphyrin (CuBP) molecules in strategic places, the charge carriers in semiconducting materials — negative electrons and positive “holes” — may be controlled and inverted for better device performance as well as economical manufacture. The discovery is published in a pair of papers that appear in the journals Advanced Functional Materials and Advanced Electronic Materials.

In the realm of polymer semiconductors, device functionality depends on the movement of the appropriate charge carriers across the material. There have been many advances in the synthesis of high-mobility, high-performance materials, said lead author Michael Ford, graduate student in materials, but the fine control of the electrons and holes is what will allow these sophisticated polymers to reach their full potential.

“There’s been a large effort to make new materials, but a lot of them may not be appropriate in conventional low-power devices,” said Ford. Many of these materials exhibit “ambipolar” conduction, meaning that they transport both negative and positive charges, he explained. So, in situations where only a certain charge is required, the opposite charge is also conducting, which diminishes the utility of the material.

“They’re always ‘on’ so you always have a current running through the device,” Ford said. Conventional means of controlling the movement of charge carriers often involves more complex measures, such as multiple metal evaporation steps or depositing additional layers that are difficult to manage. These actions often require more complicated processing or fabrication, which could in the end defeat the purpose of low-cost flexible electronics.

This new development was actually a classic accidental scientific discovery, according to Ford, who was investigating, simultaneously, the properties of two materials. He observed that the use of fullerene additives limited conduction of one charge carrier (negative electrons), while allowing the other (positive holes) to remain mobile.

“In one experiment, we were just trying to do some extra measurements for a poster, and while making a measurement I noticed it solved the problem that I was having with my other material, which was this problem of never turning off,” Ford said. He decided to employ the fullerene additive from one experiment to address issues in the other and found it could be used to allow only positive charges to move, while adding operational stability.

From there, he and his collaborators worked to control negative charge conduction in the same way. A different additive — one that “likes” holes, CuBP — was introduced and turned off ambipolar transport in the opposite way from the fullerene. Now negative electrons remain mobile and hole transport is limited.

“We had two devices, both using the same polymer semiconductor but with different additives,” Ford explained. “One was a switch for holes, and the other was a switch for electrons. This enabled us to develop a complementary inverter, which is just like the building blocks that make up circuits in modern cell phones and computers.”

“So we have for the first time this ability to take these ambipolar semiconductors and design through solution processing a circuit where in certain parts only the electrons are moving, or only the holes are moving,” Bazan said, “but keep the same semiconductor material.” The additives create “traps” that can be used to master the properties of the semiconductor in a straightforward way, he added.

The potential uses of this method are many, particularly in situations where low-cost, low-power flexible electronics would be helpful, such as printable packaging labels that function as temperature sensors for foods and other sensitive items being shipped long distances.

“It’s this idea where we can have an additive that can be a small fraction of the total and which will allow us to master the electronic properties of the semiconductor,” said Bazan. “Once you have that under control, you can do all sorts of cool things.”

Semiconductors lie at the heart of many of the electronic devices that govern our daily lives. The proper functioning of semiconductor devices relies on their internally generated electric fields. Being able to measure these fields on the nanoscale is crucial for the development of next-generation electronics, but present techniques have been restricted to measurements of the electric field at a semiconductor’s surface. A group of Takayuki Iwasaki, Mutsuko Hatano and colleagues at the Tokyo Institute of Technology, the Japan Science and Technology Agency (JST) and Toshiharu Makino at the National Institute of Advanced Industrial Science and Technology (AIST) has reported a new method for sensing internal electric fields at the interior of operating semiconductor devices. The technique exploits the response of an artificially introduced single electron spin to variations in its surrounding electric field, and enabled the researchers to study a semiconductor diode subject to bias voltages of up to 150 V.

Left: Schematic of the structure of the NV center. Middle: Confocal fluorescence image of a single NV center in the device. Right: Schematic of the measurement configuration. Credit: Tokyo Institute of Technology

Left: Schematic of the structure of the NV center. Middle: Confocal fluorescence image of a single NV center in the device. Right: Schematic of the measurement configuration. Credit: Tokyo Institute of Technology

Iwasaki and co-workers applied their method to diamond, a so-called wide-band-gap semiconductor in which the electric fields can become very strong — a property important for low-loss electronic applications. Diamond has the advantage that it easily accommodates nitrogen-vacancy (NV) centers, a type of point defect that arises when two neighboring carbon atoms are removed from the diamond lattice and one of them is replaced by a nitrogen atom. NV centers can be routinely created in diamond by means of ion implantation. A nearby electric field affects an NV center’s energy state, which in turn can be probed by a method called optically detected magnetic resonance (ODMR).

The researchers first fabricated a diamond p-i-n diode (an intrinsic diamond layer sandwiched between an electron- and a hole-doped layer) embedded with NV centers. They then localized an NV center in the bulk of the i-layer, several hundreds of nanometers away from the interface, and recorded its ODMR spectrum for increasing bias voltages. From these spectra, values for the electric field could be obtained using theoretical formulas. The experimental values were then compared with numerical results obtained with a device simulator and found to be in good agreement — confirming the potential of NV centers as local electric-field sensors.

Iwasaki and colleagues explain that the experimentally determined value for the electric field around a given NV center is essentially the field’s component perpendicular to the direction of the NV center — aligned along one of four possible directions in the diamond lattice. They reason that a regular matrix of implanted NV centers should enable reconstructing the electric field with a spatial resolution of about 10nm by combining with super-resolution techniques, which is promising for studying more complex devices in further studies.

The researchers also point out that electric-field sensing is not only relevant for electronic devices, but also for electrochemical applications: the efficiency of electrochemical reactions taking place between a semiconductor and a solution depends on the former’s internal electric field. In addition, Iwasaki and co-workers note that their approach need not be restricted to NV centers in diamond: similar single-electron-spin structures exist in other semiconductors like e.g. silicon carbide.

Background: Wide-band-gap semiconductors

Semiconducting materials feature a so-called band gap: an energy range wherein no accessible energy levels exist. In order for a semiconductor to conduct, electrons must acquire sufficient energy to overcome the band gap; controlling electronic transitions across the band gap forms the basis of semiconducting device action. Typical semiconductors like silicon or gallium arsenide have a band gap of the order of 1 electron volt (eV). Wide-band-gap semiconductors, like diamond or silicon carbide, have a larger band gap — values as high as 3-5 eV are not uncommon.

Due to their large band gap, wide-band-gap semiconductors can operate at temperatures over 300 °C. In addition, they can sustain high voltages and currents. Because of these properties, wide-band-gap semiconductors have many applications, including light-emitting diodes, transducers, alternative-energy devices and high-power components. For further development of these and other future applications, it is essential to be able to characterize wide-band-gap devices in operation. The technique proposed by Iwasaki and colleagues for measuring the electric field generated in a wide-band-gap semiconductor subject to large bias voltages is therefore a crucial step forward.

Nitrogen-vacancy centers

Diamond consists of carbon atoms arranged on a lattice where each atom has four neighbors forming a tetrahedron. The diamond lattice is prone to defects; one such defect is the nitrogen-vacancy (NV) center, which can be thought of as resulting from replacing a carbon atom with a nitrogen atom and removing one neighboring carbon atom. The energy level of an NV center lies in the band gap of diamond but is sensitive to its local environment. In particular, the so-called nuclear hyperfine structure of an NV center depends on its surrounding electric field. This dependence is well understood theoretically, and was exploited by Iwasaki and co-workers: detecting changes in an NV center’s hyperfine structure enabled them to obtain values for the local electric field. A major advantage of this approach is that it allows monitoring the field within the material — not just at the surface, for which methods had already been developed.

Optically-detected magnetic resonance

For probing the nuclear hyperfine structure of an NV center in the bulk of the diamond-based device, Iwasaki and colleagues employed optically detected magnetic resonance (ODMR): by irradiating the sample with laser light, the NV center was optically excited, after which the magnetic resonance spectrum could be recorded. An electric field makes the ODMR resonance split; the experimentally detected split width provides a measure for the electric field.

“We are the first in the world to present a logic circuit, in this case a transistor, that is controlled by a heat signal instead of an electrical signal,” states Professor Xavier Crispin of the Laboratory of Organic Electronics, Linköping University.

This is the heat driven transistor on Laboratory of organic electronics, Linköping University. Credit: Thor Balkhed

This is the heat driven transistor on Laboratory of organic electronics, Linköping University. Credit: Thor Balkhed

The heat-driven transistor opens the possibility of many new applications such as detecting small temperature differences, and using functional medical dressings in which the healing process can be monitored.

It is also possible to produce circuits controlled by the heat present in infrared light, for use in heat cameras and other applications. The high sensitivity to heat, 100 times greater than traditional thermoelectric materials, means that a single connector from the heat-sensitive electrolyte, which acts as sensor, to the transistor circuit is sufficient. One sensor can be combined with one transistor to create a “smart pixel”.

A matrix of smart pixels can then be used, for example, instead of the sensors that are currently used to detect infrared radiation in heat cameras. With more developments, the new technology can potentially enable a new heat camera in your mobile phone at a low cost, since the materials required are neither expensive, rare nor hazardous.

The heat-driven transistor builds on research that led to a supercapacitor being produced a year ago, charged by the sun’s rays. In the capacitor, heat is converted to electricity, which can then be stored in the capacitor until it is needed.

The researchers at the Laboratory of Organic Electronics had searched among conducting polymers and produced a liquid electrolyte with a 100 times greater ability to convert a temperature gradient to electric voltage than the electrolytes previously used. The liquid electrolyte consists of ions and conducting polymer molecules. The positively charged ions are small and move rapidly, while the negatively charged polymer molecules are large and heavy. When one side is heated, the small ions move rapidly towards the cold side and a voltage difference arises.

“When we had shown that the capacitor worked, we started to look for other applications of the new electrolyte,” says Xavier Crispin.

Dan Zhao, principal research engineer, and Simone Fabiano, senior lecturer, have shown, after many hours in the laboratory, that it is fully possible to build electronic circuits that are controlled by a heat signal.

Research by scientists at Swansea University is helping to meet the challenge of incorporating nanoscale structures into future semiconductor devices that will create new technologies and impact on all aspects of everyday life.

Dr Alex Lord and Professor Steve Wilks from the Centre for Nanohealth led the collaborative research published in Nano Letters. The research team looked at ways to engineer electrical contact technology on minute scales with simple and effective modifications to nanowires that can be used to develop enhanced devices based on the nanomaterials. Well-defined electrical contacts are essential for any electrical circuit and electronic device because they control the flow of electricity that is fundamental to the operational capability.

Specialist research equipment and reseach images. Credit: Scienta Omicron

Specialist research equipment and reseach images. Credit: Scienta Omicron

Everyday materials that are being scaled down to the size of nanometres (one million times smaller than a millimetre on a standard ruler) by scientists on a global scale are seen as the future of electronic devices. The scientific and engineering advances are leading to new technologies such as energy producing clothing to power our personal gadgets and sensors to monitor our health and the surrounding environment.

Over the coming years this will make a massive contribution to the explosion that is the Internet of Things connecting everything from our homes to our cars into a web of communication. All of these new technologies require similar advances in electrical circuits and especially electrical contacts that allow the devices to work correctly with electricity.

Professor Steve Wilks said: “Nanotechnology has delivered new materials and new technologies and the applications of nanotechnology will continue to expand over the coming decades with much of its usefulness stemming from effects that occur at the atomic- or nano-scale. With the advent of nanotechnology, new technologies have emerged such as chemical and biological sensors, quantum computing, energy harvesting, lasers, and environmental and photon-detectors, but there is a pressing need to develop new electrical contact preparation techniques to ensure these devices become an everyday reality.”

“Traditional methods of engineering electrical contacts have been applied to nanomaterials but often neglect the nanoscale effects that nanoscientists have worked so hard to uncover. Currently, there isn’t a design toolbox to make electrical contacts of chosen properties to nanomaterials and in some respects the research is lagging behind our potential application of the enhanced materials.”

The Swansea research team1 used specialist experimental equipment and collaborated with Professor Quentin Ramasse of the SuperSTEM Laboratory, Science and Facilities Technology Council. The scientists were able to physically interact with the nanostructures and measure how the nanoscale modifications affected the electrical performance.2

Their experiments found for the first time, that simple changes to the catalyst edge can turn-on or turn-off the dominant electrical conduction and most importantly reveal a powerful technique that will allow nanoengineers to select the properties of manufacturable nanowire devices.

Dr Lord said: “The experiments had a simple premise but were challenging to optimise and allow atomic-scale imaging of the interfaces. However, it was essential to this study and will allow many more materials to be investigated in a similar way.”

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

“In the near future this work can help enhance current nanotechnology devices such as biosensors and also lead to new technologies such as Transient Electronics that are devices that diminish and vanish without a trace which is an essential property when they are applied as diagnostic tools inside the human body.”

Silicon crystals are the semiconductors most commonly used to make transistors, which are critical electronic components used to carry out logic operations in computing. However, as faster and more powerful processors are created, silicon has reached a performance limit: the faster it conducts electricity, the hotter it gets, leading to overheating.

Graphene, made of a single-atom-thick sheet of carbon, stays much cooler and can conduct much faster, but it must be into smaller pieces, called nanoribbons, in order to act as a semiconductor. Despite much progress in the fabrication and characterization of nanoribbons, cleanly transferring them onto surfaces used for chip manufacturing has been a significant challenge.

A recent study conducted by researchers at the Beckman Institute for Advanced Science and Technology at the University of Illinois and the Department of Chemistry at the University of Nebraska-Lincoln has demonstrated the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. The paper, “Solution-Synthesized Chevron Graphene Nanoribbons Exfoliated onto H:Si(100),” was published in Nano Letters.

Researchers have made the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. Credit: Adrian Radocea, Beckman Institute for Advanced Science and Technology

Researchers have made the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. Credit: Adrian Radocea, Beckman Institute for Advanced Science and Technology

Graphene nanoribbons measure only several nanometers across, beyond the limits of conventional chip top-down patterning used in chip manufacturing. As a result, when carved from larger pieces of graphene by various nanofabrication approaches, graphene nanoribbons are neither uniform nor narrow enough to exhibit the desired semiconductor properties.

“When you’re going from the top-down, it’s very hard to get control over the width. It turns out that if the width modulates by just an atom or two, the properties change significantly,” said Adrian Radocea, a doctoral student in Beckman’s Nanoelectronics and Nanomaterials Group.

As a result, the nanoribbons must be made from “the bottom up,” from smaller molecules to create atomically precise nanoribbons with highly uniform electronic properties.

“It’s like molecular building blocks: kind of like snapping Legos together to building something,” said Radocea. “They lock in place, and you end up with the exact control over the ribbon width.”

The “bottom-up” approach was first shown for graphene nanoribbons by Cai et al. in a 2010 Nature paper demonstrating the growth of atomically precise graphene nanoribbons on metallic substrates. In 2014, the research group of Alexander Sinitskii at the University of Nebraska-Lincoln developed an alternative approach for making atomically precise graphene nanoribbons in solution.

“The previously demonstrated synthesis on metallic substrates yields graphene nanoribbons of very high quality, but their number is rather small, as the growth it limited to the precious metal’s surface,” said Sinitskii, associate professor of chemistry at University of Nebraska-Lincoln and an author of the study. “It is difficult to scale this synthesis up. In contrast, when nanoribbons are synthesized in the unrestricted three-dimensional solution environment, they can be produced in large quantities.”

The difficulty in cleanly transferring nanoribbons stems from the high sensitivity to environmental contaminants. Both solution-synthesized and surface-grown nanoribbons are exposed to chemicals during the transfer process that can affect the performance of graphene nanoribbon devices. To overcome this challenge, the interdisciplinary team used a dry transfer in an ultra-high vacuum environment.

A fiberglass applicator coated in graphene nanoribbon powder was heated to remove contaminants and solvent residue and then pressed onto a freshly prepared hydrogen-passivated silicon surface. The nanoribbons were studied in great detail with ultra-high vacuum scanning tunneling microscope developed by Joseph Lyding, professor of electrical and computer engineering at Illinois and an author of the study. The researchers obtained atomic-scale images and electronic measurements of the graphene nanoribbons that were critical for confirming their electronic properties and understanding the influence of the substrate.

Computational expertise available at Beckman, Radocea explained, was instrumental in understanding the experimental results. “I was still collecting more data trying to figure out what was going on. Once the modeling results came in and we started looking at the data differently, it all made sense.”

Members of Beckman’s Computational Multiscale Nanosystems Group, Tao Sun, a doctoral student, and Narayana Aluru, professor of mechanical science and engineering, provided expertise in computational modeling via density functional theory to investigate the properties of the nanoribbons.

“Density functional theory calculations provided a deeper understanding of the electronic properties of the integrated system and the interactions between graphene nanoribbons and the silicon substrate,” said Sun. “It was exciting that the computational results could help explain and confirm the experimental results and provided a coherent story.”

“Atomically precise graphene nanoribbons (APGNRs) are serious candidates for the post-silicon era when conventional silicon transistor scaling fails,” said Lyding. “This demonstrates the first important step toward integrating APGNRs with technologically relevant silicon substrates.”

“I find the project very exciting because you are building things with atomic level control, so you try to put every atom exactly where you want it to go,” said Radocea. “There aren’t many materials out there where you can say you have that ability. Nanoribbons are exciting because there is a real need and a real application.”

Research managed by SUNY Polytechnic Institute (SUNY Poly) and conducted by a number of collaborating institutions has led to findings that have been named a top ten 2016 breakthrough in physics by Physics World. The publication recently named the SUNY Poly-led Institute for Nanoelectronics Discovery and Exploration’s (INDEX) “Theme I” work on the negative refraction of electrons in graphene p-n junctions as “a top ten breakthrough,” as it supports the physics for p-n junctions in graphene, which could lead to more powerful and energy efficient computing capabilities in the future.

“SUNY Poly’s position as a world class research institution is unmatched, and our faculty and students should be proud to be a part of that success,” said Dr. Bahgat Sammakia, Interim President of SUNY Polytechnic Institute. “It’s an incredible honor to have research managed by the talented people here at SUNY Poly recognized among the top ten physics breakthroughs of this past year, and I salute the SUNY Poly INDEX team and the researchers at partnering institutions who, collectively, enabled this fascinating research.”

As part of the research, scientists created a p-n junction, a building block of many modern day semiconductor-based electronic devices, in graphene, a two-dimensional honeycomb-shaped form of carbon that is incredibly strong and conductive. By ensuring that the p-n junction interface was smooth, the researchers minimized reflections, which enabled them to measure the negative refraction of electrons, an accomplishment that could one day form the basis of a new type of electronic switch, potentially replacing the transistor, which is currently the basis of computers worldwide. While this research shows that this new type of switch is possible, it could still take many years for any practical applications to result.

“We are excited that this great work of physics has been recognized by Physics World, and as part of the SUNY Poly team, we are thrilled to have solidified INDEX’s funding and look forward to continuing this important work, ” said SUNY Poly Vice President for Research Dr. Michael Liehr. “This acknowledgement is a testament not only to SUNY Poly’s ability to lead collaborations that can have significant research impact, but also to working collaboratively as research partners with other leading institutions such as Columbia University.”

The research that led to the notable findings was specifically conducted at Columbia University, the University of Virginia, and Harvard University, and was managed by SUNY Poly; Cornell University, the National Institute for Materials Science in Japan, and IBM were also recognized by Physics World for their teams’ contributions.

“This work is significant for proving the fundamental physics of the graphene p-n junction, and we are excited that the research of ‘Theme I’ of INDEX has resulted in this recognition,” SUNY Poly Interim Dean of the College of Nanoscale Science and Empire Innovation Professor of Nanoscale Science Dr. Alain Diebold said. “This is a credit to researchers Cory Dean and Jim Hone of Columbia University, who fabricated and measured the test structures using a method called magnetic steering, as well as Avik Ghosh of the University of Virginia, who modeled and simulated the data enabling the interpretation and helping to design new test structures. SUNY Poly was proud to play an enabling role.”

The research was conducted under the SUNY Poly-led umbrella of INDEX, which is one of three active centers in the Semiconductor Research Corporation’s Nanoelectronics Initiative leveraging faculty and students across ten universities. INDEX has three research areas, or themes: graphene p-n junction devices, spintronic devices, and fabrication – with a goal to develop a new switch to replace the transistor. Currently, Dr. Alain Diebold serves as INDEX’s Director, following the tenure of Dr. Michael Liehr, who had previously served as director at the Nanoelectronics Research Institute-funded center. In addition, INDEX is a Semiconductor Research Corporation (SRC) program sponsored by the Nano-Electronics Research Corporation (NERC) and the National Institute of Standards and Technology (NIST).

DGIST announced that Professor Kyung-in Jang’s research team succeeded in developing a technology that can control various color changes by coating several nanometers of semiconducting materials on a metal substrate through joint research with a research team led by professor Young-min Song of GIST.

Professor Kyung-in Jang’s research team has succeeded in changing the unique color of metals such as gold, silver, aluminum, etc. with strong thin-film interference effect caused by light reflected on the surface of the metals and semiconducting materials by coating an ultra-thin layer of several nanometers (1 nanometer is one one-billionth of a meter) of semiconductor substances on the metals.

There have been previous studies that show that color changes depend on the thickness of ultra-thin film of semiconducting materials such as germanium coated on a gold substrate; however, there have been some difficulties due to the rapid change of colors and with color darkening techniques.

The research team coated a thin germanium film of 5 to 25 nanometers on a gold substrate by utilizing oblique angle deposition (OAD). As a result, they succeeded in producing various colors such as yellow, orange, blue, and purple at will according to the thickness and deposition angle of the germanium coating.

It was confirmed that the range of color expression expanded and the purity of color was enhanced by making a porous structure with a large number of fine holes that have a significant presence in the germanium layer. By applying the oblique angle deposition method, the variation and purity of colors were also varied according to the thickness change of the germanium film in nanometers.

Professor Kyung-in Jang from DGIST’s Department of Robotics Engineering said, “The result of this research is the development of a simple method of applying various colors to existing electronic devices and currently we have succeeded in expressing single colors, but we may also be able to coat patterns such as symbols and pictures. In the future, I think it can be used in coating visual designs on flexible devices such as solar cells, wearable devices, and displays that are used for various purposes including building exterior walls. It can also be applied in camouflage by coating things with the same pattern or color as the surrounding objects.”

Meanwhile, this research outcome was published on December 9, 2016 in the online edition of Nanoscale, an international academic journal in the field of nanotechnology, and the research was supported by the basic research project (collective research) of the National Research Foundation of Korea.

Based on a study of the optical properties of novel ultrathin semiconductors, researchers of Ludwig-Maximilians-Universitaet (LMU) in Munich have developed a method for rapid and efficient characterization of these materials.

Chemical compounds based on elements that belong to the so-called transition metals can be processed to yield atomically thin two-dimensional crystals consisting of a monolayer of the composite in question. The resulting materials are semiconductors with surprising optical properties. In cooperation with American colleagues, a team of LMU physicists led by Alexander Högele has now explored the properties of thin-film semiconductors made up of transition metal dichalcogenides (TMDs). The researchers report their findings in the journal Nature Nanotechnology.

These semiconductors exhibit remarkably strong interaction with light and therefore have great potential for applications in the field of opto-electronics. In particular, the electrons in these materials can be excited with polarized light. “Circularly polarized light generates charge carriers that exhibit either left- or right-handed circular motion. The associated angular momentum is quantized and described by the so-called valley index which can be detected as valley polarization,” Högele explains. In accord with the laws of quantum mechanics, the valley index can be used just like quantum mechanical spin to encode information for many applications including quantum computing.

However, recent studies of the valley index in TMD semiconductors have led to controversial results. Different groups worldwide have reported inconsistent values for the degree of valley polarization. With the aid of their newly developed polarimetric method and using monolayers of the semiconducting TMD molybdenum disulfide as a model system, the LMU researchers have now clarified the reasons for these discrepancies: “Response to polarized light turns out to be very sensitive to the quality of the crystals, and can thus vary significantly within the same crystal,” Högele says. “The interplay between crystal quality and valley polarization will allow us to measure rapidly and efficiently those properties of the sample that are relevant for applications based on the valley quantum degree of freedom.”

Moreover, the new method can be applied to other monolayer semiconductors and systems made up of several different materials. In the future, this will enable the functionalities of devices based on atomically thin semiconductors — such as novel types of LEDs — to be characterized swiftly and economically.

A team of researchers at the University of Illinois at Urbana-Champaign has advanced gallium nitride (GaN)-on-silicon transistor technology by optimizing the composition of the semiconductor layers that make up the device. Working with industry partners Veeco and IBM, the team created the high electron mobility transistor (HEMT) structure on a 200 mm silicon substrate with a process that will scale to larger industry-standard wafer sizes.

Can Bayram, an assistant professor of electrical and computer engineering (ECE), and his team have created the GaN HEMT structure on a silicon platform because it is compatible with existing CMOS manufacturing processes and is less expensive than other substrate options like sapphire and silicon carbide.

However, silicon does have its challenges. Namely, the lattice constant, or space between silicon atoms, doesn’t match up with the atomic structure of the GaN grown on top of it.

“When you grow the GaN on top, there’s a lot of strain between the layers, so we grew buffer layers [between the silicon and GaN] to help change the lattice constant into the proper size,” explained ECE undergraduate researcher Josh Perozek, lead author of the group’s paper, “Investigation of structural, optical, and electrical characteristics of an AlGaN/GaN high electron mobility transistor structure across a 200mm Si(1 1 1) substrate,” in the Journal of Physics D: Applied Physics.

Without these buffer layers, cracks or other defects will form in the GaN material, which would prevent the transistor from operating properly. Specifically, these defects — threading dislocations or holes where atoms should be–ruin the properties of the 2-dimensional electron gas channel in the device. This channel is critical to the HEMTs ability to conduct current and function at high frequencies.

“The single most important thing for these GaN [HEMT] devices is to have high 2D electron gas concentration,” said Bayram, about the accumulation of electrons in a channel at the interface between the silicon and the various GaN-based layers above it.

“The problem is you have to control the strain balance among all those layers–from substrate all the way up to the channel — so as to maximize the density of the of the conducting electrons in order to get the fastest transistor with the highest possible power density.”

After studying three different buffer layer configurations, Bayram’s team discovered that thicker buffer layers made of graded AlGaN reduce threading dislocation, and stacking those layers reduces stress. With this type of configuration, the team achieved an electron mobility of 1,800 cm2/V-sec.

“The less strain there is on the GaN layer, the higher the mobility will be, which ultimately corresponds to higher transistor operating frequencies,” said Hsuan-Ping Lee, an ECE graduate student researcher leading the scaling of these devices for 5G applications.

According to Bayram, the next step for his team is to fabricate fully functional high-frequency GaN HEMTs on a silicon platform for use in the 5G wireless data networks.

When it’s fully deployed, the 5G network will enable faster data rates for the world’s 8 billion mobile phones, and will provide better connectivity and performance for Internet of Things (IoT) devices and driverless cars.

Germanium may not be a household name like silicon, its group-mate on the periodic table, but it has great potential for use in next-generation electronics and energy technology.

Of particular interest are forms of germanium that can be synthesized in the lab under extreme pressure conditions. However, one of the most-promising forms of germanium for practical applications, called ST12, has only been created in tiny sample sizes–too small to definitively confirm its properties.

“Attempts to experimentally or theoretically pin down ST12-germanium’s characteristics produced extremely varied results, especially in terms of its electrical conductivity,” said Carnegie’s Zhisheng Zhao, the first author on a new paper about this form of germanium.

The study’s research team, led by Carnegie’s Timothy Strobel, was able to create ST12-germanium in a large enough sample size to confirm its characteristics and useful properties. Their work is published by Nature Communications.

“This work will be of interest to a broad range of readers in the field of materials science, physics, chemistry, and engineering,” explained Carnegie’s Haidong Zhang, the co-leading author.

ST12-germanium has a tetragonal structure–the nameST12 means “simple tetragonal with 12 atoms.”(See illustration below.) It was created by putting germanium under about 138 times normal atmospheric pressure (14 gigapascals) and then decompressing it slowly at room temperature.

The millimeter-sized samples of ST12-germanium that the team created were large enough that they could be studied using a variety of spectroscopic techniques in order to confirm its long-debated characteristics.

Like the most common, diamond-cubic form of germanium, they found that ST12 is a semiconductor with a so-called indirect band gap. Metallic substances conduct electrical current easily, whereas insulating materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can be moved to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the “band gap.” While direct band gap materials can effectively absorb and emit light, indirect band gap materials cannot.

“Our team was able to quantify ST12’s optical band gap–where visible light energy can be absorbed by the material–as well as its electrical and thermal properties, which will help define its potential for practical applications,” Strobel said. “Our findings indicate that due to the size of its band gap, ST12-germanium may be a better material for infrared detection and imaging technology than the diamond-cubic form of the element already being used for these purposes.”