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It’s hardly a character flaw, but organic transistors–the kind envisioned for a host of flexible electronics devices–behave less than ideally, or at least not up to the standards set by their rigid, predictable silicon counterparts. When unrecognized, a new study finds, this disparity can lead to gross overestimates of charge-carrier mobility, a property key to the performance of electronic devices.

If measurements fail to account for these divergent behaviors in so-called “organic field-effect transistors” (OFETs), the resulting estimates of how fast electrons or other charge carriers travel in the devices may be more than 10 times too high, report researchers from the National Institute of Standards and Technology (NIST), Wake Forest University and Penn State University. The team’s measurements implicate an overlooked source of electrical resistance as the root of inaccuracies that can inflate estimates of organic semiconductor performance.

A circuit made from organic thin-film transistors is fabricated on a flexible plastic substrate. A team of NIST, Wake Forest, and Penn State University researchers has identified an overlooked source of electrical resistance that can exert a dominant influence on organic-semiconductor performance. Credit: Patrick Mansell/Penn State

A circuit made from organic thin-film transistors is fabricated on a flexible plastic substrate. A team of NIST, Wake Forest, and Penn State University researchers has identified an overlooked source of electrical resistance that can exert a dominant influence on organic-semiconductor performance. Credit: Patrick Mansell/Penn State

Their article appears in the latest issue of Nature Communications.

Already used in light-emitting diodes, or LEDs, electrically conductive polymers and small molecules are being groomed for applications in flexible displays, flat-panel TVs, sensors, “smart” textiles, solar cells and “Internet of Things” applications. Besides flexibility, a key selling point is that the organic devices–sometimes called “plastic electronics”–can be manufactured in large volumes and far more inexpensively than today’s ubiquitous silicon-based devices.

A key sticking point, however, is the challenge of achieving the high levels of charge-carrier mobility that these applications require. In the semiconductor arena, the general rule is that higher mobility is always better, enabling faster, more responsive devices. So chemists have set out to hurry electrons along. Working from a large palette of organic materials, they have been searching for chemicals–alone or in combination–that will up the speed limit in their experimental devices.

Just as for silicon semiconductors, assessments of performance require measurements of current and voltage. In the basic transistor design, a source electrode injects charge into the transistor channel leading to a drain electrode. In between sits a gate electrode that regulates the current in the channel by applying voltage, functioning much like a valve.

Typically, measurements are analyzed according to a longstanding theory for silicon field-effect transistors. Plug in the current and voltage values and the theory can be used to predict properties that determine how well the transistor will perform in a circuit.

Results are rendered as a series of “transfer curves.” Of particular interest in the new study are curves showing how the drain current changes in response to a change in the gate electrode voltage. For devices with ideal behavior, this relationship provides a good measure of how fast charge carriers move through the channel to the drain.

“Organic semiconductors are more prone to non-ideal behavior because the relatively weak intermolecular interactions that make them attractive for low-temperature processing also limit the ability to engineer efficient contacts as one would for state-of-the-art silicon devices,” says electrical engineer David Gundlach, who leads NIST’s Thin Film Electronics Project. “Since there are so many different organic materials under investigation for electronics applications, we decided to step back and do a measurement check on the conventional wisdom.”

Using what Gundlach describes as the semiconductor industry’s “workhorse” measurement methods, the team scrutinized an OFET made of single-crystal rubrene, an organic semiconductor with a molecule shaped a bit like a microscale insect. Their measurements revealed that electrical resistance at the source electrode–the contact point where current is injected into the OFET– significantly influences the subsequent flow of electrons in the transistor channel, and hence the mobility.

In effect, contact resistance at the source electrode creates the equivalent of a second valve that controls the entry of current into the transistor channel. Unaccounted for in the standard theory, this valve can overwhelm the gate–the de facto¬ regulator between the source and drain in a silicon semiconductor transistor–and become the dominant influence on transistor behavior.

At low gate voltages, this contact resistance at the source can overwhelm device operation. Consequently, model-based estimates of charge-carrier mobility in organic semiconductors may be more than 10 times higher than the actual value, the research team reports.

Hardly ideal behavior, but the aim of the study, the researchers write, is to improve “understanding of the source of the non-ideal behavior and its impact on extracted figures of merit,” especially charge-carrier mobility. This knowledge, they add, can inform efforts to develop accurate, comprehensive measurement methods for benchmarking organic semiconductor performance, as well as guide efforts to optimize contact interfaces.

Demonstrating a strategy that could form the basis for a new class of electronic devices with uniquely tunable properties, researchers at Kyushu University were able to widely vary the emission color and efficiency of organic light-emitting diodes based on exciplexes simply by changing the distance between key molecules in the devices by a few nanometers.

This new way to control electrical properties by slightly changing the device thickness instead of the materials could lead to new kinds of organic electronic devices with switching behavior or light emission that reacts to external factors.

Organic electronic devices such as OLEDs and organic solar cells use thin films of organic molecules for the electrically active materials, making flexible and low-cost devices possible.

A key factor determining the properties of organic devices is the behavior of packets of electrical energy called excitons. An exciton consists of a negative electron attracted to a positive hole, which can be thought of as a missing electron.

In OLEDs, the energy in these excitons is released as light when the electron loses energy and fills the vacancy of the hole. Varying the exciton energy, for example, will change the emission color.

However, excitons are commonly localized on a single organic molecule and tightly bound with binding energies of about 0.5 eV. Thus, entirely new molecules must usually be designed and synthesized to obtain different properties from these Frenkel-type excitons, such as red, green, or blue emission for displays.

Researchers at Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA) instead focused on a different type of exciton called an exciplex, which is formed by a hole and electron located on two different molecules instead of the same molecule.

By manipulating the molecular distance between the electron-donating molecule (donor) and the electron-accepting molecule (acceptor) that carry the exciplex’s hole and electron, respectively, the researchers could modify the properties of these weakly bound excitons.

“What we did is similar to placing sheets of paper between a magnet and a refrigerator,” said Associate Professor Hajime Nakanotani, lead author of the paper reporting these results published online February 26, 2016, in the journal Science Advances.

“By increasing the thickness of an extremely thin layer of organic molecules inserted as a spacer between the donor and acceptor, we could reduce the attraction between the hole and electron in the exciplex and thereby greatly influence the exciplex’s energy, lifetime, and emission color and efficiency.”

Indeed, the changes can be large: by inserting a spacer layer with a thickness of only 5 nm between a donor layer and an acceptor layer in an OLED, the emission color shifted from orange to yellowish green and the light emission efficiency increased 700%.

For this to work, the organic molecule used for the spacer layer must have an excitation energy higher than those of the donor and acceptor, but such materials are already widely available.

While the molecular distance is currently determined by the thickness of the vacuum-deposited spacer layer, the researchers are now looking into other ways to control the distance.

“This gives us a powerful way to greatly vary device properties without redesigning or changing any of the materials,” said Professor Chihaya Adachi, director of OPERA. “In the future, we envision new types of exciton-based devices that respond to external forces like pressure to control the distance and electrical behavior.”

In addition, the researchers found that the exciplexes were still formed when the spacer was 10 nm thick, which is long on a molecular scale.

“This is some of the first evidence that electrons and holes could still interact like this across such a long distance,” commented Professor Adachi, “so this structure may also be a useful tool for studying and understanding the physics of excitons to design better OLEDs and organic solar cells in the future.”

“From both scientific and applications standpoints, we are excited to see where this new path for exciton engineering takes us and hope to establish a new category of exciton-based electronics.”

Researchers from the Moscow Institute of Physics and Technology (MIPT) have for the first time experimentally demonstrated that copper nanophotonic components can operate successfully in photonic devices – it was previously believed that only gold and silver components have the required properties for this. Copper components are not only just as good as components based on noble metals, but, unlike them, they can easily be implemented in integrated circuits using industry-standard fabrication processes.

“This is a kind of revolution – using copper will solve one of the main problems in nanophotonics,” say the authors of the paper. The results have been published in the scientific journal Nano Letters.

The discovery, which is revolutionary for photonics and the computers of the future, was made by researchers from the Laboratory of Nanooptics and Plasmonics at MIPT’s Centre of Nanoscale Optoelectronics. They have succeeded, for the first time, in producing copper nanophotonic components, whose characteristics are just as good as that of gold components. It is interesting to note that the scientists fabricated the copper components using the process compatible with the industry-standard manufacturing technologies that are used today to produce modern integrated circuits. This means that in the very near future copper nanophotonic components will form a basis for the development of energy-efficient light sources, ultra-sensitive sensors, as well as high-performance optoelectronic processors with several thousand cores.

The discovery was made under what is known as nanophotonics – a branch of research which aims, among other things, to replace existing components in data processing devices with more modern components by using photons instead of electrons. However, while the main component in modern electronics, the transistor, can be scaled down in size to a few nanometres, the diffraction of light limits the minimum dimensions of photonic components to the size of about the light wavelength (~1 micrometre). Despite the fundamental nature of this so-called diffraction limit, one can overcome it by using metal-dielectric structures to create truly nanoscale photonic components. Firstly, most metals show a negative permittivity at optical frequencies, and light cannot propagate through them, penetrating to a depth of only 25 nanometres. Secondly, light may be converted into surface plasmon polaritons, surface waves propagating along the surface of a metal. This makes it possible to switch from conventional 3D photonics to 2D surface plasmon photonics, which is known as plasmonics. This gives a possibility to control light at the scale of the order of 100 nanometres, i.e. far beyond the diffraction limit.

It was previously believed that only two metals – gold and silver – could be used to build efficient nanophotonic metal-dielectric nanostructures and it was also thought that all other metals could not be an alternative to these two materials, since they exhibit strong absorption. However, in practice, creating components using gold and silver is not possible because both metals, as they are noble, do not enter into chemical reactions and therefore it is extremely difficult, expensive and in many cases simply impossible to use them to create nanostructures – the basis of modern photonics.

Researchers from MIPT’s Laboratory of Nanooptics and Plasmonics have found a solution to the problem. Based on a generalization of the theory for so-called plasmonic metals, in 2012 they found that copper, as an optical material, is not only able to compete with gold, but it can also be a better alternative. Unlike gold, copper can be easily structured using wet or dry etching. This gives a possibility to make nanoscale components that are easily integrated into silicon photonic or electronic integrated circuits. It took more than two years for the researchers to purchase the required equipment, develop the fabrication process, produce samples, conduct several independent measurements, and confirm this hypothesis experimentally.

“As a result, we succeeded in fabricating copper chips with optical properties that are in no way inferior to gold-based chips,” says the research leader Dmitry Fedyanin. “Furthermore, we managed to do this in a fabrication process compatible with the CMOS technology, which is the basis for all modern integrated circuits, including microprocessors. It’s a kind of revolution in nano photonics.”

The researchers note that the optical properties of thin polycrystalline copper films are determined by their internal structure, and the ability to control this structure, achieve and consistently reproduce the required parameters in technological cycles is the most difficult task. However, they have managed to solve this problem demonstrating that it is possible not only to achieve the required properties with copper, but also that this can be done in nanoscale components, which can be integrated both with silicon nanoelectronics and silicon nanophotonics.

“We conducted ellipsometry of the copper films and then confirmed these results using near-field scanning optical microscopy of the nanostructures. This proves that the properties of copper are not impaired during the whole process of manufacturing nanoscale plasmonic components,” says Dmitry Fedyanin.

These studies provide a foundation for the practical use of copper nanophotonic and plasmonic components, which in the very near future will be used to create LEDs, nanolasers, highly sensitive sensors and transducers for mobile devices, and high performance optoelectronic processors with several tens of thousand cores for graphics cards, personal computers, and supercomputers.

Veeco Instruments Inc. announced today the launch of the new TurboDisc K475i Arsenic Phosphide (As/P) Metal Organic Chemical Vapor Deposition (MOCVD) System for the production of red, orange, yellow (R/O/Y) light emitting diodes (LEDs), as well as multi-junction III-V solar cells, laser diodes and transistors.

“Veeco continues to drive innovation with MOCVD technology that enables us to lower manufacturing costs and increase production with systems that are reliable, flexible and easy to use,” said Shuangxiang Zhang, General Manager of Yangzhou Changelight Co., Ltd.

According to research firm Strategies Unlimited, R/O/Y LED demand is expected to grow at a 10 percent compound annual rate through 2023. This demand for red, orange and yellow LEDs is being driven by signage, automotive, display and general lighting applications, as well as the emergence of new applications such as wearable smart devices.

Incorporating proprietary TurboDisc and Uniform FlowFlange MOCVD technologies, the new K475i system enables Veeco customers to reduce LED cost per wafer by up to 20 percent compared to alternative systems through higher productivity, best-in-class yields and reduced operating expenses.

Veeco’s proprietary Uniform FlowFlange technology produces films with very high uniformity and improved within-wafer and wafer-to-wafer repeatability resulting in the industry’s lowest cost of ownership. This patented technology provides ease-of-tuning for fast process optimization and fast tool recovery time after maintenance enabling the highest productivity for applications such as lighting, display, solar, laser diodes, pseudomorphic high electron mobility transistors (pHEMTs) and heterojunction bipolar transistors (HBTs).

Graphene, the two-dimensional powerhouse, packs extreme durability, electrical conductivity, and transparency into a one-atom-thick sheet of carbon. Despite being heralded as a breakthrough “wonder material,” graphene has been slow to leap into commercial and industrial products and processes.

Now, scientists have developed a simple and powerful method for creating resilient, customized, and high-performing graphene: layering it on top of common glass. This scalable and inexpensive process helps pave the way for a new class of microelectronic and optoelectronic devices–everything from efficient solar cells to touch screens.

The collaboration–led by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute–published their results February 12, 2016, in the journal Scientific Reports.

“We believe that this work could significantly advance the development of truly scalable graphene technologies,” said study coauthor Matthew Eisaman, a physicist at Brookhaven Lab and professor at SBU.

The scientists built the proof-of-concept graphene devices on substrates made of soda-lime glass–the most common glass found in windows, bottles, and many other products. In an unexpected twist, the sodium atoms in the glass had a powerful effect on the electronic properties of the graphene.

“The sodium inside the soda-lime glass creates high electron density in the graphene, which is essential to many processes and has been challenging to achieve,” said coauthor Nanditha Dissanayake of Voxtel, Inc., but formerly of Brookhaven Lab. “We actually discovered this efficient and robust solution during the pursuit of something a bit more complex. Such surprises are part of the beauty of science.”

Crucially, the effect remained strong even when the devices were exposed to air for several weeks–a clear improvement over competing techniques.

The experimental work was done primarily at Brookhaven’s Sustainable Energy Technologies Department and the Center for Functional Nanomaterials (CFN), which is a DOE Office of Science User Facility.

The graphene tweaks in question revolve around a process called doping, where the electronic properties are optimized for use in devices. This adjustment involves increasing either the number of electrons or the electron-free “holes” in a material to strike the perfect balance for different applications. For successful real-world devices, it is also very important that the local number of electrons transferred to the graphene does not degrade over time.

“The graphene doping process typically involves the introduction of external chemicals, which not only increases complexity, but it can also make the material more vulnerable to degradation,” Eisaman said. “Fortunately, we found a shortcut that overcame those obstacles.”

The team initially set out to optimize a solar cell containing graphene stacked on a high-performance copper indium gallium diselenide (CIGS) semiconductor, which in turn was stacked on an industrial soda-lime glass substrate.

The scientists then conducted preliminary tests of the novel system to provide a baseline for testing the effects of subsequent doping. But these tests exposed something strange: the graphene was already optimally doped without the introduction of any additional chemicals.

“To our surprise, the graphene and CIGS layers already formed a good solar cell junction!” Dissanayake said. “After much investigation, and the later isolation of graphene on the glass, we discovered that the sodium in the substrate automatically created high electron density within our multi-layered graphene.”

Pinpointing the mechanism by which sodium acts as a dopant involved a painstaking exploration of the system and its performance under different conditions, including making devices and measuring the doping strength on a wide range of substrates, both with and without sodium.

“Developing and characterizing the devices required complex nanofabrication, delicate transfer of the atomically thin graphene onto rough substrates, detailed structural and electro-optical characterization, and also the ability to grow the CIGS semiconductor,” Dissanayake said. “Fortunately, we had both the expertise and state-of-the-art instrumentation on hand to meet all those challenges, as well as generous funding.”

The bulk of the experimental work was conducted at Brookhaven Lab using techniques developed in-house, including advanced lithography. For the high-resolution electron microscopy measurements, CFN staff scientists and study coauthors Kim Kisslinger and Lihua Zhang lent their expertise. Coauthors Harry Efstathiadis and Daniel Dwyer–both at the College of Nanoscale Science and Engineering at SUNY Polytechnic Institute–led the effort to grow and characterize the high-quality CIGS films.

“Now that we have demonstrated the basic concept, we want to focus next on demonstrating fine control over the doping strength and spatial patterning,” Eisaman said.

The scientists now need to probe more deeply into the fundamentals of the doping mechanism and more carefully study material’s resilience during exposure to real-world operating conditions. The initial results, however, suggest that the glass-graphene method is much more resistant to degradation than many other doping techniques.

“The potential applications for graphene touch many parts of everyone’s daily life, from consumer electronics to energy technologies,” Eisaman said. “It’s too early to tell exactly what impact our results will have, but this is an important step toward possibly making some of these applications truly affordable and scalable.”

For example, graphene’s high conductivity and transparency make it a very promising candidate as a transparent, conductive electrode to replace the relatively brittle and expensive indium tin oxide (ITO) in applications such as solar cells, organic light emitting diodes (OLEDs), flat panel displays, and touch screens. In order to replace ITO, scalable and low-cost methods must be developed to control graphene’s resistance to the flow of electrical current by controlling the doping strength. This new glass-graphene system could rise to that challenge, the researchers say.

Today, computer chips are built by stacking layers of different materials and etching patterns into them.

But in the latest issue of Advanced Materials, MIT researchers and their colleagues report the first chip-fabrication technique that enables significantly different materials to be deposited in the same layer. They also report that, using the technique, they have built chips with working versions of all the circuit components necessary to produce a general-purpose computer.

The layers of material in the researchers’ experimental chip are extremely thin — between one and three atoms thick. Consequently, this work could abet efforts to manufacture thin, flexible, transparent computing devices, which could be laminated onto other materials. “The methodology is universal for many kinds of structures,” says Xi Ling, a postdoc in the Research Laboratory of Electronics and one of the paper’s first authors. “This offers us tremendous potential with numerous candidate materials for ultrathin circuit design.”

The technique also has implications for the development of the ultralow-power, high-speed computing devices known as tunneling transistors and, potentially, for the integration of optical components into computer chips.

“It’s a brand new structure, so we should expect some new physics there,” says Yuxuan Lin, a graduate student in electrical engineering and computer science and the paper’s other first author.

Ling and Lin are joined on the paper by Mildred Dresselhaus, an Institute Professor emerita of physics and electrical engineering; Jing Kong, an ITT Career Development Professor of Electrical Engineering; Tomás Palacios, an associate professor of electrical engineering; and by another 10 MIT researchers and two more from Brookhaven National Laboratory and Taiwan’s National Tsing-Hua University.

Strange bedfellows

Computer chips are built from crystalline solids, materials whose atoms are arranged in a regular geometrical pattern known as a crystal lattice. Previously, only materials with closely matched lattices have been deposited laterally in the same layer of a chip. The researchers’ experimental chip, however, uses two materials with very different lattice sizes: molybdenum disulfide and graphene, which is a single-atom-thick layer of carbon.

Moreover, the researchers’ fabrication technique generalizes to any material that, like molybdenum disulfide, combines elements from group six of the periodic table, such as chromium, molybdenum, and tungsten, and elements from group 16, such as sulfur, selenium, and tellurium. Many of these compounds are semiconductors — the type of material that underlies transistor design — and exhibit useful behavior in extremely thin layers.

Graphene, which the researchers chose as their second material, has many remarkable properties. It’s the strongest known material, but it also has the highest known electron mobility, a measure of how rapidly electrons move through it. As such, it’s an excellent candidate for use in thin-film electronics or, indeed, in any nanoscale electronic devices.

To assemble their laterally integrated circuits, the researchers first deposit a layer of graphene on a silicon substrate. Then they etch it away in the regions where they wish to deposit the molybdenum disulfide.

Next, at one end of the substrate, they place a solid bar of a material known as PTAS.

They heat the PTAS and flow a gas across it and across the substrate. The gas carries PTAS molecules with it, and they stick to the exposed silicon but not to the graphene. Wherever the PTAS molecules stick, they catalyze a reaction with another gas that causes a layer of molybdenum disulfide to form.

In previous work, the researchers characterized a range of materials that promote the formation of crystals of other compounds, any of which could be plugged into the process.

A research team at Umeå University in Sweden has showed, for the first time, that a very efficient vertical charge transport in semiconducting polymers is possible by controlled chain and crystallite orientation. These pioneering results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices and were recently published in the journal Advanced Materials.

Conjugated semiconducting polymers (plastic) possess exceptional optical and electronic properties, which make them highly attractive in the production of organic opto-electronic devices, such as for instance photovoltaic solar cells (OPV), light emitting diodes (OLED) and lasers.

Polythiophene polymers, such as poly(3-hexylthiophene), P3HT, have been among the most studied semiconducting polymers due to their strong optical absorbance and ease of processing into a thin film from solution. In both OPVs and OLEDs, charges must be transported in the out of plane (vertical) direction inside the polymer film.

However, until now the vertical charge carrier mobility of organic semiconductors, i.e. the ability of charges to move inside the material, has been too low to produce fast charge transport in electronic devices. Faster charge transport can occur along the polymer chain backbone. However, a method to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now.

In the present work, a team of chemists and materials scientists, led by Professor David R. Barbero at Umeå University, has found a new method to align chains vertically and to produce efficient transport of electric charges through the chain backbone. In this new study, moreover, high charge transport and high mobility were obtained without any chemical doping, which is often used to artificially enhance charge transport in polymers.

“The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film. The mobility measured was approximately one thousand times higher than previously reported in the same organic semiconductor,” says David Barbero.

In what way will these results affect the field of organic electronics?

“We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device. Organic-based devices have traditionally been slower and less efficient than inorganic ones (e.g. made of silicon), in part due to the low mobility of organic (plastic) semiconductors. Typically, plastic semiconductors, which are only semi-crystalline, have hole mobilities about 10,000 times lower than doped silicon, which is used in many electronic devices. Now we show it is possible to obtain much higher mobility, and much closer to that of silicon, by controlled vertical chain alignment, and without doping,” says David Barbero.

The charge transport was measured using nanoscopic electrical measurements, and gave a mobility averaging 3.1 cm2/V.s, which is the highest mobility ever measured in P3HT, and which comes close to a theoretical estimation of the maximum mobility in P3HT. Crystallinity and molecular packing characterisation of the polymer was performed by synchrotron X-ray diffraction at Stanford University’s National Accelerator (SLAC) and confirmed that the high mobilities measured were due to the re-orientation of the polymer chains and crystallites, leading to fast charge transport along the polymer backbones.

These results, published in Advanced Materials, may open up the route to produce more efficient organic electronic devices with vertical charge transport (e.g. OPV, OLED, lasers etc.), by a simple and inexpensive method, and without requiring chemical modification of the polymer.

Making tiny switches do enormous jobs in a more efficient way than current technology allows is one of the goals of a research team led by Cornell engineering professor Huili (Grace) Xing.

Xing and her group – which includes her husband, Debdeep Jena, also an engineering professor at Cornell – have created gallium nitride (GaN) power diodes capable of serving as the building blocks for future GaN power switches. The group built a GaN power-switching device, approximately one-fifth the width of a human hair, that could support 2,000 volts of electricity.

With silicon-based semiconductors rapidly approaching their performance limits in electronics, GaN is seen as the next generation in power control and conversion. Applications span nearly all electronics products and electricity distribution infrastructure.

“With some of these new materials, it’s actually conceivable now to shrink medium-scale power-distribution systems onto a chip,” Jena said. “Looking into the future, this is one of the goals, and it’s not a moonshot. It’s possible, but the materials have to be right, the design has to be right.”

The team’s work was published Dec. 15 in the journal Applied Physics Letters, a publication of the American Institute of Physics. The group includes researchers from Cornell, the University of Notre Dame – from where Xing and Jena arrived at Cornell last year – and the semiconductor company IQE.

Xing said the key to her team’s discovery was building the device on a GaN base layer that contained relatively few energy-sapping defects, in comparison to traditional silicon-based substrates.

“We’re going to take the defects, some of them anyway, out of the equation,” said Xing, the Richard Lundquist Sesquicentennial Professor of Electrical and Computer Engineering and a professor of materials science and engineering. “Nothing can be 100 percent [free of defects], but we’re talking about improvements along an order of magnitude of up to 10,000 times.”

The team used a couple of indicators to determine the defect level in the GaN diode, including “diode ideality factor” as measured by the Shockley-Read-Hall recombination lifetime. The SRH lifetime is the average time it takes positively and negatively charged particles to move around before recombining at defects, which creates inefficiency.

The team’s work yielded near-ideal performance in all aspects, spawning hope for the future of GaN power diodes.

“Our results are an important step toward understanding the intrinsic properties and the true potential of GaN,” said Zongyang Hu, a Cornell postdoctoral associate and the paper’s co-lead author.

While much of energy-related research and development is focused on alternative energy sources, such as wind and solar, the Xing team’s efforts in power transmission are just as important, Jena said.

“Power generation gets a lot of press, and it should,” he said. “But once the power is generated, the amount of power that is lost because of inefficiencies is mind-bogglingly large. This problem is about conservation rather than generating power, which is really the same thing.

“And the scale of losses today actually far surpasses the total of renewable energies combined,” he said. “And it’s a clear and present solution; it’s not like we have to discover something fundamental.”

The team’s work is supported in part by the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) “SWITCHES” program. SWITCHES stands for Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High-Efficiency Systems.

“Leading one of these projects, we at Cornell – in collaboration with our industrial partners – have established an integrated plan to develop three terminal GaN power transistors, package them, and insert them into circuits and products,” Xing said.

The team’s paper is titled “Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown.” Cornell collaborators included Kazuki Nomoto and Vladimir Protasenko, research associates in the School of Electrical and Computer Engineering, and graduate students Bo Song and Mingda Zhu. The team also included Jena’s Ph.D. student Meng Qi at the University of Notre Dame, and engineers Ming Pan and Xiang Gao of IQE.

In the nanoworld, tiny particles of gold can operate like snow blowers, churning through surface layers of an important class of semiconductors to dig unerringly straight paths. The surprising trenching capability, reported by scientists from the National Institute of Standards and Technology (NIST) and IBM, is an important addition to the toolkit of nature-supplied ‘self-assembly’ methods that researchers aim to harness for making useful devices.

Foreseeable applications include integrating lasers, sensors, wave guides and other optical components into so-called lab-on-a-chip devices now used for disease diagnosis, screening experimental materials and drugs, DNA forensics and more. Easy to control, the new gold-catalyzed process for creating patterns of channels with nanoscale dimensions could help to spawn entirely new technologies fashioned from ensembles of ultra-small structures.

Preliminary research results that began as lemons — a contaminant-caused failure that impeded the expected formation of nanowires — eventually turned into lemonade when scanning electron microscope images revealed long, straight channels.

“We were disappointed, at first,” says NIST research chemist Babak Nikoobakht. “Then we figured out that water was the contaminant in the process — a problem that turned out to be a good thing.”

That’s because, as determined in subsequent experiments, the addition of water vapor served to transform gold nanoparticles into channel diggers, rather than the expected wire makers. Beginning with studies on the semiconductor indium phosphide, the team teased out the chemical mechanisms and necessary conditions underpinning the surface-etching process.

First, they patterned the surface of the semiconductor by selectively coating it with a gold layer only a few nanometers thick. Upon heating, the film breaks up into tiny particles that become droplets. The underlying indium phosphide dissolves into the gold nanoparticles above, creating a gold alloy. Then, heated water vapor is introduced into the system. At temperatures below 300 degrees Celsius (572 degrees Fahrenheit), the tiny gold-alloy particles, now swathed with water molecules, etch nanoscale pits into the indium phosphide.

But at 440 degrees Celsius (824 degrees Fahrenheit) and above, long V-shaped nanochannels formed. The channels followed straight paths dictated by the regularly repeating lattice of atoms in the crystalline semiconductor. During the process, indium and phosphorous atoms interact with oxygen atoms in the water molecules on the surface of the gold alloy droplet. The oxidized indium and phosphorous evaporate, and the droplet advances, picking up more semiconductor atoms to oxidize as it goes.

The result is a series of crystalline groves. The dimensions of the grooves correspond to the size of droplet, which can be controlled.

In effect, the droplet is the chemical equivalent of the auger on a snow blower that, instead of snow, burrows through the top portion of the semiconductor and ejects evaporated bits, Nikoobakht explains.

The team observed the same phenomena in gallium phosphide and indium arsenide, two more examples of semiconductors formed by combining elements from the third and fifth columns of the periodic table. Compound semiconductors in this class are used to make LEDs, and for communications, high-speed electronics and many other applications. Nikoobakht believes that, with adjustments, the etching process might also work for creating patterns of channels on silicon and other materials.

Controllable, fast and flexible, the “bottom up” channel-fabrication process shows promise for use on industrial scales, the researchers suggest. In their article, the teams describe how they used the process to etch patterns of hollow channels like those used to direct the flow of liquids, such as a blood sample, in a microfluidic device, or lab on a chip.

An international team of researchers including Professor Federico Rosei and members of his group at INRS has developed a new strategy for fabricating atomically controlled carbon nanostructures used in molecular carbon-based electronics. An article just published in the prestigious journal Nature Communications presents their findings: the complete electronic structure of a conjugated organic polymer, and the influence of the substrate on its electronic properties.

The researchers combined two procedures previously developed in Professor Rosei’s lab–molecular self-assembly and chain polymerization–to produce a network of long-range poly(para-phenylene) (PPP) nanowires on a copper (Cu) surface. Using advanced technologies such as scanning tunneling microscopy and photoelectron spectroscopy as well as theoretical models, they were able to describe the morphology and electronic structure of these nanostructures.

“We provide a complete description of the band structure and also highlight the strong interaction between the polymer and the substrate, which explains both the decreased bandgap and the metallic nature of the new chains. Even with this hybridization, the PPP bands display a quasi one-dimensional dispersion in conductive polymeric nanowires,” said Professor Federico Rosei, one of the authors of the study.

Although further research is needed to fully describe the electronic properties of these nanostructures, the polymer’s dispersion provides a spectroscopic record of the polymerization process of certain types of molecules on gold, silver, copper, and other surfaces. It’s a promising approach for similar semiconductor studies–an essential step in the development of actual devices.

The results of the study could be used in designing organic nanostructures, with significant potential applications in nanoelectronics, including photovoltaic devices, field-effect transistors, light-emitting diodes, and sensors.