Category Archives: Process Materials

SiFive, a provider of commercial RISC-V processor IP, today announced Shafy Eltoukhy as vice president of operations. Eltoukhy, a veteran of Microsemi and Intel, will lead SiFive’s DesignShare activities and ensure the smooth rollout of new Core IP, SoCs and services.

Over the course of his career, Eltoukhy has been awarded 24 patents, and is the author of more than 20 technical articles. He brings this expertise to SiFive with a goal to help the company expand its DesignShare program, which gives any SoC designer, inventor or maker the ability to harness the power of custom silicon with little to no upfront risk. Since its inception, companies including Analog Bits, eMemory, FlexLogix, Rambus, Think Silicon and UltraSoC have joined the DesignShare ecosystem. He will also help to coordinate SiFive’s fast-growing hardware and software engineering teams with key partners as the company launches new products and services.

“SiFive’s mission – to lower the barriers for innovation in the silicon industry – immediately resonated with me when presented with the opportunity to work with the team,” Eltoukhy said. “Knowing firsthand the challenges involved in bringing a new SoC to market, I immediately recognized SiFive’s ability to resolve the issues that customers and designers have faced for decades. I am thrilled to join the SiFive team and am honored to have the opportunity to help revolutionize the semiconductor industry.”

Eltoukhy brings three decades of experience to his role at SiFive, having most recently served as vice president and business unit manager for the analog mixed signal division at Microsemi. Earlier in his career, Eltoukhy was vice president of operations and technology at Open-Silicon, where he released to production over 150 ASIC and complex SoC products. He also served as Vice President of Technology at Lightspeed Semiconductor, where he joined the founding team that invented structured ASIC technology with a goal to simplify ASIC design cycle and reduce development cost. As Director of Technology Development at Actel Corporation (now Microsemi), he participated in the early development of the first and second generation of Antifuse FPGA products. He has also held senior engineering positions at semiconductor pioneers Intel and Fairchild. Eltoukhy holds a doctorate in electrical engineering from the University of Waterloo as well as master’s and bachelor’s degrees from Cairo University.

“We are thrilled to have someone with Shafy’s credentials join the SiFive executive team,” said Naveed Sherwani, CEO of SiFive. “His experience as a founder and leader of numerous startups is invaluable to SiFive as we strive to breathe new life into a stagnant industry. His perspective will benefit not only to SiFive but the semiconductor market writ large as we work to simplify the design process.”

Researchers have identified a mechanism that triggers shape-memory phenomena in organic crystals used in plastic electronics. Shape-shifting structural materials are made with metal alloys, but the new generation of economical printable plastic electronics is poised to benefit from this phenomenon, too. Shape-memory materials science and plastic electronics technology, when merged, could open the door to advancements in low-power electronics, medical electronics devices and multifunctional shape-memory materials.

The findings are published in the journal Nature Communications and confirm the shape-memory phenomenon in two organic semiconductors materials.

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Devices like the expandable stents that open and unblock clogged human blood vessels use shape-memory technology. Heat, light and electrical signals, or mechanic forces pass information through the devices telling them to expand, contract, bend and morph back into their original form – and can do so repeatedly, like a snake constricting to swallow its dinner. This effect works well with metals, but remains elusive in synthetic organic materials because of the complexity of the molecules used to create them.

“The shape-memory phenomenon is common in nature, but we are not really sure about nature’s design rules at the molecular level,” said professor of chemical and biomolecular engineering and co-author of the study, Ying Diao. “Nature uses organic compounds that are very different from the metal alloys used in shape-memory materials on the market today,” Diao said. “In naturally occurring shape-memory materials, the molecules transform cooperatively, meaning that they all move together during shape change. Otherwise, these materials would shatter and the shape change would not be reversible and ultrafast.”

The discovery of the shape-memory mechanism in synthetic organic material was quite serendipitous, Diao said. The team accidentally created large organic crystals and was curious to find out how they would transform given heat.

“We looked at the single crystals under a microscope and found that the transformation process is dramatically different than we expected,” said graduate student and co-author Hyunjoong Chung. “We saw concerted movement of a whole layer of molecules sweeping through the crystal that seem to drive the shape-memory effect – something that is rarely observed in organic crystals and is therefore largely unexplored.”

This unexpected observation led the team to want to explore the merger between shape-memory materials science and the field of organic electronics, the researchers said. “Today’s electronics are dependent on transistors to switch on and off, which is a very energy-intensive process,” Diao said. “If we can use the shape-memory effect in plastic semiconductors to modulate electronic properties in a cooperative manner, it would require very low energy input, potentially contributing to advancements in low-power and more efficient electronics.”

The team is currently using heat to demonstrate the shape-memory effect, but are experimenting with light waves, electrical fields and mechanical force for future demonstrations. They are also exploring the molecular origin of the shape-memory mechanism by tweaking the molecular structure of their materials. “We have already found that changing just one atom in a molecule can significantly alter the phenomenon,” Chung said.

The researchers are very excited about the molecular cooperativity aspect discovered with this research and its potential application to the recent Nobel Prize-winning concept of molecular machines, Diao said. “These molecules can change conformation cooperatively at the molecular level, and the small molecular structure change is amplified over millions of molecules to actuate large motion at the macroscopic scale.”

By Jay Chittooran, Manager, Public Policy, SEMI

International trade is one of the best tools to spur growth and create high-skill and high-paying jobs. Over 40 million American jobs rely on trade, and this is particularly true in the semiconductor supply chain. Over the past three decades, the semiconductor industry has averaged nearly double-digit growth rates in revenue and, by 2030, the semiconductor supply chain is forecast to reach $1 trillion. Trade paves the way for this growth.

Unfortunately, despite its importance to the industry, trade has been transformed from an economic issue into a political one, raising many new trade challenges to companies throughout the semiconductor industry.

GHz-ChinaChina’s investments in the industry will continue to anchor the country as a major force in the semiconductor supply chain. China’s outsized spending has spawned concern among other countries about the implications of these investments. According to SEMI’s World Fab Forecast, 20 fabs are being built in China – and construction on 14 more is rumored to begin in the near term – compared to the 10 fabs under construction in the rest of the world. China is clearly outpacing the pack.

The Trump Administration has levied intense criticism of China, citing unfair trade practices, especially related to intellectual property issues. The U.S. Trade Representative has launched a Section 301 investigation into whether China’s practice of forced technology transfer has discriminated against U.S. consumers. Even as the probe unfolds, expectations are growing that the United States will take action against China, raising fears of not only possible retaliation in time but rising animosity between two trading partners that rely deeply on each other.

A number of other open investigations also cloud the future. The Administration launched two separate Section 232 investigations into steel and aluminum industry practices by China, claiming Chinese overproduction of both items are a threat to national security. The findings from these investigations will be submitted to the President, who, in the coming weeks, will decide an appropriate response, which could include imposing tariffs and quotas.

Another high priority area is Korea. While U.S. threats to withdraw from the U.S.-Korea Free Trade Agreement (KORUS) reached a fever pitch in August, rhetoric has since tempered. Informal discussions between the countries on how best to amend the trade deal are ongoing. The number of KORUS implementation issues aside, continued engagement with Korea – instead of scrapping a comprehensive, bilateral trade deal – will be critically important for the industry.

Lastly, negotiations to modernize the North American Free Trade Agreement (NAFTA) will continue this year. The United States wants to conclude talks by the end of March, but with the deadline fast approaching and the promise of resolution waning, tensions are running high. Notably, the outcome of the NAFTA talks will inform and set the tone for other trade action.

What’s more, a number of other actions on trade will take place this year. As we wrote recently, Congress has moved to reform the Committee on Foreign Investment in the United States (CFIUS), a government body designed to review sales and transfer of ownership of U.S. companies to foreign entities. Efforts have also started to revise the export control regime – a key component to improving global market access and making international trade more equitable.

SEMI will continue its work on behalf of its members around the globe to open up new markets and lessen the burden of regulations on cross-border trade and commerce. In addition, SEMI will continue to educate policymakers on the critical importance of unobstructed trade in continuing to push the rapid advance of semiconductors and the emerging technologies they enable into the future. If you are interested in more information on trade, or how to be involved in SEMI’s public policy program, please contact Jay Chittooran, Manager, Public Policy, at [email protected].

Entegris, Inc. (NASDAQ: ENTG), a developer of specialty chemicals and advanced materials solutions for the microelectronics industry, announced today that it acquired Particle Sizing Systems, LLC (PSS), a company focused on particle sizing instrumentation for liquid applications in both semiconductor and life science industries.

This acquisition reflects Entegris’ value creation strategy by leveraging its global technology platform and customer relationships. The total purchase price of the acquisition was approximately $37 million in cash, subject to customary working capital adjustments. Entegris expects this transaction to be accretive to 2018 earnings.

Digital transformation continues to create a high demand for sophisticated cloud computing infrastructures that require the most advanced logic and memory chips available. However, advanced-node manufacturers already challenged by a continuously shrinking process window and high fab costs struggle to maintain yield  and  eliminate losses  associated with CMP performance.

In advanced-node CMP applications, scratch defects are often caused by the agglomeration of slurry abrasive particles that have the potential to become a key factor in process yield performance. With the technology from PSS, Entegris is enabling customers to perform particle size analysis online and in real time, directly in the fluid stream process.  Automating the monitoring process can lead to the application of more effective solutions like proper filter selection and system maintenance. This ability to intervene with these solutions prevents costly yield excursions.

“To stay competitive, our advanced-node customers need tools that allow them to shorten process times while maintaining accuracy and consistency in order to meet the high-quality standards of the manufacturers they partner with,” says Todd Edlund, Chief Operating Officer, Entegris. “PSS technology is unique in that it measures every particle in the slurry, making it more accurate than commonly used methods that employ averaging techniques. As a result, this technology eliminates the need for manual sampling and intervention, which is less efficient and runs a higher risk of slurry excursions.”

The historic flood of merger and acquisition agreements that swept through the semiconductor industry in 2015 and 2016 slowed significantly in 2017, but the total value of M&A deals reached in the year was still more than twice the annual average in the first half of this decade, according to IC Insights’ new 2018 McClean Report, which becomes available this month.  Subscribers to The McClean Report can attend one of the upcoming half-day seminars (January 23 in Scottsdale, AZ; January 25 in Sunnyvale, CA; and January 30 in Boston, MA) that discuss the highlights of the report free of charge.

In 2017, about two dozen acquisition agreements were reached for semiconductor companies, business units, product lines, and related assets with a combined value of $27.7 billion compared to the record-high $107.3 billion set in 2015 and the $99.8 billion total in 2016 (Figure 1).  Prior to the explosion of semiconductor acquisitions that erupted several years ago, M&A agreements in the chip industry had a total annual average value of about $12.6 billion between 2010 and 2015.

Figure 1

Figure 1

Two large acquisition agreements accounted for 87% of the M&A total in 2017, and without them, the year would have been subpar in terms of the typical annual value of announced transactions.  The falloff in the value of semiconductor acquisition agreements in 2017 suggests that the feverish pace of M&A deals is finally cooling off.  M&A mania erupted in 2015 when semiconductor acquisitions accelerated because a growing number of companies began buying other chip businesses to offset slow growth rates in major end-use applications (such as smartphones, PCs, and tablets) and to expand their reach into huge new market opportunities, like the Internet of Things (IoT), wearable systems, and highly “intelligent” embedded electronics, including the growing amount of automated driver-assist capabilities in new cars and fully autonomous vehicles in the not-so-distant future.

With the number of acquisition targets shrinking and the task of merging operations together growing, industry consolidation through M&A transactions decelerated in 2017.  Regulatory reviews of planned mergers by government agencies in Europe, the U.S., and China have also slowed the pace of large semiconductor acquisitions.

One of the big differences between semiconductor M&A in 2017 and the two prior years was that far fewer megadeals were announced.  In 2017, only two acquisition agreements exceeded $1 billion in value (the $18 billion deal for Toshiba’s memory business and Marvell’s planned $6 billion purchase of Cavium).  Ten semiconductor acquisition agreements in 2015 exceeded $1 billion and seven in 2016 were valued over $1 billion.  The two large acquisition agreements in 2017 pushed the average value of semiconductor M&A pacts to $1.3 billion.  Without those megadeals, the average would have been just $185 million last year. The average value of 22 semiconductor acquisition agreements struck in 2015 was $4.9 billion.  In 2016, the average for 29 M&A agreements was $3.4 billion, based on data compiled by IC Insights.

Scientists used spiraling X-rays at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) to observe, for the first time, a property that gives handedness to swirling electric patterns – dubbed polar vortices – in a synthetically layered material.

This property, also known as chirality, potentially opens up a new way to store data by controlling the left- or right-handedness in the material’s array in much the same way magnetic materials are manipulated to store data as ones or zeros in a computer’s memory.

Researchers said the behavior also could be explored for coupling to magnetic or optical (light-based) devices, which could allow better control via electrical switching.

Chirality is present in many forms and at many scales, from the spiral-staircase design of our own DNA to the spin and drift of spiral galaxies; it can even determine whether a molecule acts as a medicine or a poison in our bodies.

A molecular compound known as d-glucose, for example, which is an essential ingredient for human life as a form of sugar, exhibits right-handedness. Its left-handed counterpart, l-glucose, though, is not useful in human biology.

“Chirality hadn’t been seen before in this electric structure,” said Elke Arenholz, a senior staff scientist at Berkeley Lab’s Advanced Light Source (ALS), which is home to the X-rays that were key to the study. The study was published online this week in the journal Proceedings of the National Academy of Sciences.

The experiments can distinguish between left-handed chirality and right-handed chirality in the samples’ vortices. “This offers new opportunities for fundamentally new science, with the potential to open up applications,” she said.

“Imagine that one could convert a right-handed form of a molecule to its left-handed form by applying an electric field, or artificially engineer a material with a particular chirality,” said Ramamoorthy Ramesh, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and associate laboratory director of the Lab’s Energy Technologies Area, who co-led the latest study.

Ramesh, who is also a professor of materials science and physics at UC Berkeley, custom-made the novel materials at UC Berkeley.

Padraic Shafer, a research scientist at the ALS and the lead author of the study, worked with Arenholz to carry out the X-ray experiments that revealed the chirality of the material.

The samples included a layer of lead titanate (PbTiO3) and a layer of strontium titanate (SrTiO3) sandwiched together in an alternating pattern to form a material known as a superlattice. The materials have also been studied for their tunable electrical properties that make them candidates for components in precise sensors and for other uses.

Neither of the two compounds show any handedness by themselves, but when they were combined into the precisely layered superlattice, they developed the swirling vortex structures that exhibited chirality.

“Chirality may have additional functionality,” Shafer said, when compared to devices that use magnetic fields to rearrange the magnetic structure of the material.

The electronic patterns in the material that were studied at the ALS were first revealed using a powerful electron microscope at Berkeley Lab’s National Center for Electron Microscopy, a part of the Lab’s Molecular Foundry, though it took a specialized X-ray technique to identify their chirality.

“The X-ray measurements had to be performed in extreme geometries that can’t be done by most experimental equipment,” Shafer said, using a technique known as resonant soft X-ray diffraction that probes periodic nanometer-scale details in their electronic structure and properties.

Spiraling forms of X-rays, known as circularly polarized X-rays, allowed researchers to measure both left-handed and right-handed chirality in the samples.

Arenholz, who is also a faculty member of the UC Berkeley Department of Materials Science & Engineering, added, “It took a lot of time to understand the results, and a lot of modeling and discussions.” Theorists at the University of Cantabria in Spain and their network of computational experts performed calculations of the vortex structures that aided in the interpretation of the X-ray data.

The same science team is pursuing studies of other types and combinations of materials to test the effects on chirality and other properties.

“There is a wide class of materials that could be substituted,” Shafer said, “and there is the hope that the layers could be replaced with even higher functionality materials.”

Researchers also plan to test whether there are new ways to control the chirality in these layered materials, such as by combining materials that have electrically switchable properties with those that exhibit magnetically switchable properties.

“Since we know so much about magnetic structures,” Arenholz said, “we could think of using this well-known connection with magnetism to implement this newly discovered property into devices.”

A nanostructured gate dielectric may have addressed the most significant obstacle to expanding the use of organic semiconductors for thin-film transistors. The structure, composed of a fluoropolymer layer followed by a nanolaminate made from two metal oxide materials, serves as gate dielectric and simultaneously protects the organic semiconductor – which had previously been vulnerable to damage from the ambient environment – and enables the transistors to operate with unprecedented stability.

Image shows organic-thin film transistors with a nanostructured gate dielectric under continuous testing on a probe station. (Credit: Rob Felt, Georgia Tech)

Image shows organic-thin film transistors with a nanostructured gate dielectric under continuous testing on a probe station. (Credit: Rob Felt, Georgia Tech)

The new structure gives thin-film transistors stability comparable to those made with inorganic materials, allowing them to operate in ambient conditions – even underwater. Organic thin-film transistors can be made inexpensively at low temperature on a variety of flexible substrates using techniques such as inkjet printing, potentially opening new applications that take advantage of simple, additive fabrication processes.

“We have now proven a geometry that yields lifetime performance that for the first time establish that organic circuits can be as stable as devices produced with conventional inorganic technologies,” said Bernard Kippelen, the Joseph M. Pettit professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE) and director of Georgia Tech’s Center for Organic Photonics and Electronics (COPE). “This could be the tipping point for organic thin-film transistors, addressing long-standing concerns about the stability of organic-based printable devices.”

The research was reported January 12 in the journal Science Advances. The research is the culmination of 15 years of development within COPE and was supported by sponsors including the Office of Naval Research, the Air Force Office of Scientific Research, and the National Nuclear Security Administration.

Transistors comprise three electrodes. The source and drain electrodes pass current to create the “on” state, but only when a voltage is applied to the gate electrode, which is separated from the organic semiconductor material by a thin dielectric layer. A unique aspect of the architecture developed at Georgia Tech is that this dielectric layer uses two components, a fluoropolymer and a metal-oxide layer.

“When we first developed this architecture, this metal oxide layer was aluminum oxide, which is susceptible to damage from humidity,” said Canek Fuentes-Hernandez, a senior research scientist and coauthor of the paper. “Working in collaboration with Georgia Tech Professor Samuel Graham, we developed complex nanolaminate barriers which could be produced at temperatures below 110 degrees Celsius and that when used as gate dielectric, enabled transistors to sustain being immersed in water near its boiling point.”

The new Georgia Tech architecture uses alternating layers of aluminum oxide and hafnium oxide – five layers of one, then five layers of the other, repeated 30 times atop the fluoropolymer – to make the dielectric. The oxide layers are produced with atomic layer deposition (ALD). The nanolaminate, which ends up being about 50 nanometers thick, is virtually immune to the effects of humidity.

“While we knew this architecture yielded good barrier properties, we were blown away by how stably transistors operated with the new architecture,” said Fuentes-Hernandez. “The performance of these transistors remained virtually unchanged even when we operated them for hundreds of hours and at elevated temperatures of 75 degrees Celsius. This was by far the most stable organic-based transistor we had ever fabricated.”

For the laboratory demonstration, the researchers used a glass substrate, but many other flexible materials – including polymers and even paper – could also be used.

In the lab, the researchers used standard ALD growth techniques to produce the nanolaminate. But newer processes referred to as spatial ALD – utilizing multiple heads with nozzles delivering the precursors – could accelerate production and allow the devices to be scaled up in size. “ALD has now reached a level of maturity at which it has become a scalable industrial process, and we think this will allow a new phase in the development of organic thin-film transistors,” Kippelen said.

An obvious application is for the transistors that control pixels in organic light-emitting displays (OLEDs) used in such devices as the iPhone X and Samsung phones. These pixels are now controlled by transistors fabricated with conventional inorganic semiconductors, but with the additional stability provided by the new nanolaminate, they could perhaps be made with printable organic thin-film transistors instead.

Internet of things (IoT) devices could also benefit from fabrication enabled by the new technology, allowing production with inkjet printers and other low-cost printing and coating processes. The nanolaminate technique could also allow development of inexpensive paper-based devices, such as smart tickets, that would use antennas, displays and memory fabricated on paper through low-cost processes.

But the most dramatic applications could be in very large flexible displays that could be rolled up when not in use.

“We will get better image quality, larger size and better resolution,” Kippelen said. “As these screens become larger, the rigid form factor of conventional displays will be a limitation. Low processing temperature carbon-based technology will allow the screen to be rolled up, making it easy to carry around and less susceptible to damage.

For their demonstration, Kippelen’s team – which also includes Xiaojia Jia, Cheng-Yin Wang and Youngrak Park – used a model organic semiconductor. The material has well-known properties, but with carrier mobility values of 1.6 cm2/Vs isn’t the fastest available. As a next step, they researchers would like to test their process on newer organic semiconductors that provide higher charge mobility. They also plan to continue testing the nanolaminate under different bending conditions, across longer time periods, and in other device platforms such as photodetectors.

Though the carbon-based electronics are expanding their device capabilities, traditional materials like silicon have nothing to fear.

“When it comes to high speeds, crystalline materials like silicon or gallium nitride will certainly have a bright and very long future,” said Kippelen. “But for many future printed applications, a combination of the latest organic semiconductor with higher charge mobility and the nanostructured gate dielectric will provide a very powerful device technology.”

A recent paper published in NANO showed the gas-solid reaction method provides a full coverage of the perovskite film and avoids damage from the organic solvent, which is beneficial for light capture and electrons transportation, resulting in a faster response time and stability for perovskite photodetectors.

A schematic illustration of hybrid perovskite photoconductivity visible region detector with high speed and high stability. The gas-solid reaction in replace of the traditional solution methods provides a non-solvent environment during the reaction process, constructs a high crystallization and a full coverage film to increase the light capture and transportation, as well as enhance a good stability in the humidity condition, leading to a high response performance for the photodetector. Credit: Dr. Guoqing Tong

A schematic illustration of hybrid perovskite photoconductivity visible region detector with high speed and high stability. The gas-solid reaction in replace of the traditional solution methods provides a non-solvent environment during the reaction process, constructs a high crystallization and a full coverage film to increase the light capture and transportation, as well as enhance a good stability in the humidity condition, leading to a high response performance for the photodetector. Credit: Dr. Guoqing Tong

Pervoskite materials have long been considered candidates in the semiconductor manufacturing due to their characteristics of high light absorption, carrier mobility and wider light spectrum. They are widely applied in solar cells, light-emitted devices and photodetectors. However, the organic solvent in the traditional solution method will damage the perovskite film and form unstable phases during the synthesis process, which makes the perovskite film decompose quickly in wet conditions, limiting the practical application of perovskite devices. Considering the significant influence of the solvent, a team of researchers from Dongchang college of Liaocheng University and Hefei University of Technology proposed a new gas-solid process to fabricate the perovskite film. This non-solvent approach provides high crystallization and full coverage film in lower vacuum and low temperature systems.

The researchers investigated the morphology, light absorption and the crystal phases of the perovskite film at the different annealing temperature after gas-reaction to obtain the high-quality perovskite film. The devices exhibited high responsivity and detectivity of 5.87AW-1 and 1012 Jones. The response time of the device is estimated to be 248 μs/207 μs, which is faster than most previous reports via the solution method. Remarkably, the responsivity and detectivity are estimated to be 0.26 AW-1, 2.13×1010 Jones after lasting exposure in air (25oC, RH~40%) for up to two months. This improvement of the stability of the devices demonstrates that the well-controlled vapor deposition method allows a thorough removal of the residual solvents (i.e. DMF, DMSO et. al) and thus effectively promotes a high-quality crystallization of perovskite grains, reducing the metastable phases among the thin films.

This work was financed by Science and Technology Plan Project of Shandong Higher Education Institutions, NSFC and Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology of China.

A discovery by an international team of researchers from Princeton University, the Georgia Institute of Technology and Humboldt University in Berlin points the way to more widespread use of an advanced technology generally known as organic electronics.

The research, published in the journal Nature Materials, focused on organic semiconductors, a class of materials prized for their applications in emerging technologies such as flexible electronics, solar energy conversion, and high-quality color displays for smartphones and televisions. In the short term, the advancement could particularly help with organic light-emitting diodes that operate at high energy to emit colors such as green and blue.

Researchers used ultraviolet light to excite molecules in a semiconductor, triggering reactions that split up and activated a dopant. Credit: Princeton University / Jing Wang and Xin Lin

Researchers used ultraviolet light to excite molecules in a semiconductor, triggering reactions that split up and activated a dopant. Credit: Princeton University / Jing Wang and Xin Lin

“Organic semiconductors are ideal materials for the fabrication of mechanically flexible devices with energy-saving, low-temperature processes,” said Xin Lin, a doctoral student and a member of the Princeton research team. “One of their major disadvantages has been their relatively poor electrical conductivity. In some applications, this can lead to difficulties and inefficient devices. We are working to improve the electrical properties of organic semiconductors.”

Semiconductors, typically made of silicon, are the foundation of modern electronics because engineers can take advantage of their unique properties to control electrical currents. Among many applications, semiconductor devices are used for computing, signal amplification, and switching. They are used in energy-saving devices such as light-emitting diodes and devices that convert energy such as solar cells.

Essential to these functionalities is a process called doping, in which the semiconductor’s chemical makeup is modified by adding a small amount of chemicals or impurities. By carefully choosing the type and amount of dopant, researchers can alter semiconductors’ electronic structure and electrical behavior in a variety of ways.

In their Nature Materials paper, the researchers have described a new approach for greatly increasing the conductivity of organic semiconductors, formed of carbon-based molecules rather than silicon atoms. The dopant, a ruthenium-containing compound, was a reducing agent, which means it added electrons to the organic semiconductor as part of the doping process. The addition of the electrons was the key to increasing the semiconductor’s conductivity. The compound belongs to a newly-introduced class of dopants called dimeric organometallic dopants. Unlike many other powerful reducing agents, these dopants are stable when exposed to air but still work as strong electron donors both in solution and solid state.

Georgia Tech’s Seth Marder, a Regents Professor in the School of Chemistry and Biochemistry, and Stephen Barlow, a research scientist in the school, led the development of the new dopant. They called the ruthenium compound a “hyper-reducing dopant.”

They said it was unusual, not only in its combination of electron donation strength and air stability but also in its ability to work with a class of organic semiconductors that have previously been very difficult to dope. In studies conducted at Princeton, the researchers found that the new dopant increased the conductivity of these semiconductors by about a million times.

The ruthenium compound was a dimer, meaning it consisted of two identical molecules, or monomers, connected by a chemical bond.  As is, the compound proved relatively stable and, when added to these difficult-to-dope semiconductors, it did not react and remained in its equilibrium state. That posed a problem because to increase the conductivity of the organic semiconductor, the ruthenium dimer needed to split and release its two identical monomers.

Princeton’s Lin, the study’s lead author, said the researchers looked for different ways to break up the ruthenium dimer and activate the doping. Eventually, he and Berthold Wegner, a visiting graduate student from the group of Norbert Koch at Humboldt University, took a hint from how photosynthetic systems work. They irradiated the system with ultraviolet light, which excited molecules in the semiconductor and initiated the reaction. Under exposure to the light, the dimers were able to dope the semiconductor, leading to a roughly 100,000 times increase in the conductivity.

After that, the researchers made an interesting observation.

“Once the light was turned off, one might naively expect the reverse reaction to occur and the increased conductivity to disappear,” said Georgia Tech’s Marder, who is also associate director of the Center for Organic Photonics and Electronics (COPE) at Georgia Tech. “However, this was not the case.”

The researchers found that the ruthenium monomers remained isolated in the semiconductor, increasing conductivity, even though thermodynamics should have returned the molecules to their original configuration as dimers. Antoine Kahn, a Princeton professor who led the research team, said the physical layout of the molecules inside the doped semiconductor provides a likely answer to this puzzle. The hypothesis is that the monomers are scattered in the semiconductor in such a way that it was very difficult for them to return to their original configuration and re-form the ruthenium dimer. To recombine, he said, the monomers would have to have faced in the correct orientation, but in the mixture, they remained askew. So, even though thermodynamics showed that dimers should reform, most never snapped back together.

“The question is why aren’t these things moving back together into equilibrium,” said Kahn, who is Stephen C. Macaleer ’63 Professor in Engineering and Applied Science. “The answer is they are kinetically trapped.”

In fact, the researchers observed the doped semiconductor for over a year and found very little decrease in the electrical conductivity. Also, by observing the material in light-emitting diodes fabricated by the group of Barry Rand, an assistant professor of electrical engineering at Princeton and the Andlinger Center for Energy and the Environment, the researchers discovered that doping was continuously re-activated by the light produced by the device.

“The light activates the system more, which leads to more light production and more activation until the system is fully activated, said Marder, who is Georgia Power Chair in Energy Efficiency. “This alone is a novel and surprising observation.”

The paper was co-authored by Kyung Min Lee, Michael A. Fusella, and Fengyu Zhang, of Princeton, and Karttikay Moudgil of Georgia Tech. Research was funded by the National Science Foundation (grants DMR-1506097, DMR-1305247), the Department of Energy’s Energy Efficiency & Renewable Energy Solid-State Lighting program (award DE-EE0006672) and the DoE’s Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (award DE-SC0012458), the Deutsche Forschungsgemeinschaft (project SFB 951) and the Helmholtz Energy-Alliance Hybrid Photovoltaics project.

An international team of researchers from ETH Zurich, IBM Research Zurich, Empa and four American research institutions have found the explanation for why a class of nanocrystals that has been intensively studied in recent years shines in such incredibly bright colours. The nanocrystals contain caesium lead halide compounds that are arranged in a perovskite lattice structure.

Three years ago, Maksym Kovalenko, a professor at ETH Zurich and Empa, succeeded in creating nanocrystals – or quantum dots, as they are also known – from this semiconductor material. “These tiny crystals have proved to be extremely bright and fast emitting light sources, brighter and faster than any other type of quantum dot studied so far,” says Kovalenko. By varying the composition of the chemical elements and the size of the nanoparticles, he also succeeded in producing a variety of nanocrystals that light up in the colours of the whole visible spectrum. These quantum dots are thus also being treated as components for future light-emitting diodes and displays.

In a study published in the most recent edition of the scientific journal Nature, the international research team examined these nanocrystals individually and in great detail. The scientists were able to confirm that the nanocrystals emit light extremely quickly. Previously-studied quantum dots typically emit light around 20 nanoseconds after being excited when at room temperature, which is already very quick. “However, caesium lead halide quantum dots emit light at room temperature after just one nanosecond,” explains Michael Becker, first author of the study. He is a doctoral student at ETH Zurich and is carrying out his doctoral project at IBM Research.

A cesium lead bromide nanocrystal under the electron microscope (crystal width: 14 nanometer). Individual atoms are visible as points. Credit: ETH Zurich / Empa / Maksym Kovalenko

A cesium lead bromide nanocrystal under the electron microscope (crystal width: 14 nanometer). Individual atoms are visible as points. Credit: ETH Zurich / Empa / Maksym Kovalenko

Electron-hole pair in an excited energy state

Understanding why caesium lead halide quantum dots are not only fast but also very bright entails diving into the world of individual atoms, light particles (photons) and electrons. “You can use a photon to excite semiconductor nanocrystals so that an electron leaves its original place in the crystal lattice, leaving behind a hole,” explains David Norris, Professor of Materials Engineering at ETH Zurich. The result is an electron-hole pair in an excited energy state. If the electron-hole pair reverts to its energy ground state, light is emitted.

Under certain conditions, different excited energy states are possible; in many materials, the most likely of these states is called a dark one. “In such a dark state, the electron hole pair cannot revert to its energy ground state immediately and therefore the light emission is suppressed and occurs delayed. This limits the brightness”, says Rainer Mahrt, a scientist at IBM Research.

No dark state

The researchers were able to show that the caesium lead halide quantum dots differ from other quantum dots: their most likely excited energy state is not a dark state. Excited electron-hole pairs are much more likely to find themselves in a state in which they can emit light immediately. “This is the reason that they shine so brightly,” says Norris.

The researchers came to this conclusion using their new experimental data and with the help of theoretical work led by Alexander Efros, a theoretical physicist at the Naval Research Laboratory in Washington. He is a pioneer in quantum dot research and, 35 years ago, was among the first scientists to explain how traditional semiconductor quantum dots function.

Great news for data transmission

As the examined caesium lead halide quantum dots are not only bright but also inexpensive to produce they could be applied in television displays, with efforts being undertaken by several companies, in Switzerland and world-wide. “Also, as these quantum dots can rapidly emit photons, they are of particular interest for use in optical communication within data centres and supercomputers, where fast, small and efficient components are central,” says Mahrt. Another future application could be the optical simulation of quantum systems which is of great importance to fundamental research and materials science.

ETH professor Norris is also interested in using the new knowledge for the development of new materials. “As we now understand why these quantum dots are so bright, we can also think about engineering other materials with similar or even better properties,” he says.