Category Archives: Process Materials

In electronics, the race for smaller is huge.

Physicists at the University of Cincinnati are working to harness the power of nanowires, microscopic wires that have the potential to improve solar cells or revolutionize fiber optics.

University of Cincinnati physicist Hans-Peter Wagner is exploring nanowire semiconductors to harness the power of light at the nano level. Credit: Andrew Higley/UC Creative Services

University of Cincinnati physicist Hans-Peter Wagner is exploring nanowire semiconductors to harness the power of light at the nano level. Credit: Andrew Higley/UC Creative Services

Nanotechnology has the potential to solve the bottleneck that occurs in storing or retrieving digital data – or could store data in a completely new way. UC professors and their graduate students presented their research at the March 13 conference of the American Physical Society in New Orleans, Louisiana.

Hans-Peter Wagner, associate professor of physics, and doctoral student Fatemesadat Mohammadi are looking at ways to transmit data with the speed of fiber optics but at a significantly smaller scale.

Wagner and lead author Mohammadi are studying this field, called plasmonics, with researchers from three other universities. For the novel experiment, they built nanowire semiconductors with organic material, fired laser pulses at the sample and measured the way light traveled across the metal; technically, the excitations of plasmon waves.

“So, if we succeed in getting a better understanding about the coupling between the excitations in semiconductor nanowires and metal films, it could open up a lot of new perspectives,” Wagner said.

The successful harnessing of this phenomenon — called plasmon waveguiding — could allow researchers to transmit data with light at the nano level.

Universities around the world are studying nanowires, which have ubiquitous applications from biomedical sensors to light-emitting diodes or LEDs. Four UC papers on the topic are among more than 150 others by nanowire researchers around the world to be presented at the March conference.

“You’re trying to optimize the physical structure on something approaching the atomic scale. You can make very high efficiency devices like lasers,” said Leigh Smith, head of UC’s Department of Physics. Smith and UC Physics Professor Howard Jackson also presented papers on nanowires at the conference. Virtually everyone benefits from this line of research, even if the quantum mechanics underlying the latest biosensors exceed a casual understanding. For example, home pregnancy tests use gold nanoparticles – the indicator that turns color. People use technologies all the time that they don’t understand,” Smith said.

Gordon Moore, co-founder of Intel Corp., observed that the number of transistors used in a microchip has roughly doubled every two years since the 1970s. This phenomenon, now called Moore’s Law suggests that computer processing power improves at a predictable rate.

Some computer scientists predicted the demise of Moore’s Law was inevitable with the advent of microprocessors. But nanotechnology is extending that concept’s lifespan, said Brian Markwalter, senior vice president of research and technology for the Consumer Technology Association. His trade group represents 2,200 members in the $287 billion U.S. tech industry.

“It’s not a race to be small just to be the smallest. There’s a progression of being able to do more on smaller chips. The effect for consumers is that every year they get better and better products for the same price or less,” he said.

Nanotechnology is opening a universe of new possibilities, Markwalter said.

“It’s almost magical. They get better, faster, cheaper and use less power,” he said.

Markwalter said UC professor Wagner’s research is exciting because it shows promise in using optical switches to address a bottleneck in data transmission that occurs whenever you try to store or remove data.

“It’s really a breakthrough area to merge the semiconductor world and the optical world,” Markwalter said. “[Wagner’s] working at the intersection of fiber optics and photonics.”

But even nanotechnology has its limits, Smith said.

“We’re running toward the limits of what’s physically possible with present technologies,” Smith said. “The challenges are pretty immense. In 10 or 20 years there has to be a fundamental paradigm shift in how we make structures. If we don’t we’ll be caught at the same place we are now.”

How one UC experiment works:

UC graduate student Fatemesadat Mohammadi and Associate Physics Professor Hans-Peter Wagner fire laser pulses at semiconductor nanowires to excite electrons (called excitons) that potentially serve as an energy pump to guide plasmon waves over a coated metal film just a few nanometers thick without losing power, a nettlesome physical property called resistivity

They measure the resulting luminescence of the nanowire to observe how light couples to the metal film. By sending light over a metal film, a process called plasmonic waveguiding, researchers one day could transmit data with light at the nano level.

“The luminescence is our interest. So we coat them and see: How does the photoluminescence characteristic change?” Mohammadi said.

To make the semiconductor, they use a technique called high-vacuum organic molecular beam deposition (pictured above) to spread organic and metal layers on gallium-nitride nanorods.

The use of organic film is unique to the UC experiment, Wagner said. The film works as a spacer to control the energy flow between excitons in the nanowire and the oscillation of metal electrons called plasmons.

The organic material has the added benefit of also containing excitons that, arranged properly, could support the energy flow in a semiconductor, he said.

Coating the nanorods with gold significantly shortens the lifetime of the exciton emission resulting in what’s called a quenched photoluminescence. But by using organic spacers between the nanorod and the gold film, the researchers are able to extend the emission lifetime to nearly the equivalent of nanorods without a coating.

Once the gold-coated sample is prepared, they take it to an adjacent lab room and subject it to pulses of laser light.

Mohammadi said it took days of painstaking work to arrange the small city of mirrors and beam splitters bolted at precise angles to a workbench for the experiment (pictured above left).

The reactions in the nanowire take just 10 picoseconds (which is a trillionth of a second.) And the laser pulses are faster still — 20 femtoseconds (a figure that has 15 zeros following it or a quadrillionth of a second.)

The UC project used a gold coating so that experiments could be replicated at a later date without risk of oxidation. But traditional coatings such as silver, Mohammadi said, hold even more promise.

Versum Materials, Inc. (NYSE: VSM), a materials supplier to the semiconductor industry, announced today it received the “Excellent Performance Award” by the world’s largest dedicated semiconductor foundry, Taiwan Semiconductor Manufacturing Company Limited (TSMC). TSMC maintains a rigorous supply chain, and each year recognizes its most valuable suppliers who have made an outstanding contribution to the supply of advanced materials and equipment used in semiconductor manufacturing processes. On February 23, 2017, Versum Materials received the prestigious “Excellent Performance Award” from TSMC in the Taiwanese city of Hsinchu for its contributions to TSMC.

SMC recognized Versum Materials for developing a new formulated post etch residue remover, a sustainable alternative to the material previously supplied for cleaning aluminum interconnects and pads. The new product advances TSMC’s green manufacturing initiatives by removing undesirable organic solvents in the product, process and eventually the waste stream. Furthermore, the new product reduces the required number of post-cleaning steps and cuts energy use during the manufacturing process at TSMC.

TSMC also credited Versum Materials for the positive impact its increased local presence had around improving the development responsiveness and cycle time affiliated with the new product development in the advanced materials realm. Additionally, TSMC cited superior technical support and reliable supply for our precursor offerings in the Advanced Deposition Materials (ADM) platform.

“We thank TSMC for recognizing Versum Materials with this prestigious award. The development of this new product demonstrates the importance of collaborating with our customers to drive technology forward, while also creating differentiated value for the customer. By working closely together with TSMC, we helped to further reduce their products’ environmental footprints,” stated Edward Shober, Senior Vice President, Materials at Versum Materials. “This award reflects the commitment that the entire company provides to TSMC, including our marketing, commercial, technology, operations, supply chain, procurement, EH&S and engineering teams.”

Quantum dots are very small particles that exhibit luminescence and electronic properties different from those of their bulk materials. As a result, they are attractive for use in solar cells, optoelectronics, and quantum computing. Quantum computing involves applying a small voltage to quantum dots to regulate their electron spin state, thus encoding information. While traditional computing is based on a binary information system, electron spin states in quantum dots can display further degrees of freedom because of the possibility of superposition of both states at the same time. This feature could increase the density of encoded information.

Readout of the electron spin of quantum dots is necessary to realize quantum computing. Single-shot spin readout has been used to detect spin-dependent single-electron tunneling events in real time. The performance of quantum computing could be improved considerably by single-shot readout of multiple spin states.

A Japanese research collaboration based at Osaka University has now achieved the first successful detection of multiple spin states through single-shot readout of three two-electron spin states of a single quantum dot. They reported their findings in Physical Review Letters.

To read out multiple spin states simultaneously, the researchers used a quantum point contact charge sensor positioned near a gallium arsenide quantum dot. The change in current of the charge sensor depended on the spin state of the quantum dot and was used to distinguish between singlet and two types of triplet spin states.

“We obtained single-shot ternary readout of two-electron spin states using edge-state spin filtering and the orbital effect,” study first author Haruki Kiyama says.

That is, the rate of tunneling between the quantum dot and electron reservoir depended on both the spin state of the electrons and the interaction between electron spin and the orbitals of the quantum dot. The team identified one ground state and two excited states in the quantum dot using their setup.

The researchers then used their ternary readout setup to investigate the spin relaxation behavior of the three detected spin states.

“To confirm the validity of our readout system, we measured the spin relaxation of two of the states,” Kiyama explains. “Measurement of the dynamics between the spin states in a quantum dot is an important application of the ternary spin readout setup.”

The spin relaxation times for the quantum dot measured using the ternary readout system agreed with those reported, providing evidence that the system yielded reliable measurements.

This ternary readout system can be extended to quantum dots composed of other materials, revealing a new approach to examine the spin dynamics of quantum dots and representing an advance in quantum information processing.

Entegris Inc. (NASDAQ: ENTG), a manufacturer of specialty chemicals and advanced materials handling solutions for the microelectronics industry, today announced it has signed an agreement with Spectrum Materials (Fujian) Co., Ltd. to expand its presence in China. According to the agreement, Spectrum Materials, a manufacturer and distributor of specialty chemicals, will manufacture Entegris specialty chemicals products at Spectrum Materials’ Quanzhou facility.

“We are excited about this partnership, as it will significantly improve our capabilities to meet growing demands for specialty chemicals in the industries we serve,” stated Entegris Senior Vice President of Specialty Chemicals and Engineered Materials, Stuart Tison. “Spectrum Materials is a well-established company in China that has experience supplying related high-purity chemicals and shares our expectations for quality and manufacturing standards. As we have done in other global regions, we continue to look for ways to better serve our customers and to add value with local collaboration, business processes and resources.”

Entegris currently manufactures specialty chemicals in both the U.S. and South Korea and has business operations in Beijing, Shanghai and Xi’an, China. The partnership with Spectrum Materials will expand its capability in China and shorten its supply chain for Chinese customers. This relationship is part of a broader strategic commitment by Entegris to support the growing semiconductor and related microelectronics industries in China.

“We are pleased to partner with Entegris in the manufacturing of its industry-leading specialty chemical products in China,” said President of Spectrum Materials, Guofu Chen. “Our new expansion, combined with Entegris manufacturing technology, establishes a world-class facility for the production of Entegris’ semiconductor grade specialty chemicals in China.”

Spectrum Materials will use a copy-exact manufacturing process to match existing Entegris processes and equipment and will implement the same quality control system in the manufacturing process.

Metamaterials don’t exist in nature, but their ability to make ultra-thin lenses and ultra-efficient cell phone antennas, bend light to keep satellites cooler and let photovoltaics absorb more energy mean they offer a world of possibilities.

Formed by nanostructures that act as “atoms,” arranged on a substrate to alter light’s path in ways no ordinary material can achieve, these surrogate substances can manipulate an incoming light beam to enable the creation of more efficient versions of ubiquitous, valuable devices — optical filters, lasers, frequency converters and devices that steer beams, for example.

But extensive commercial use of metamaterials has been restrained by the limitations imposed by the materials comprising them. Metal-based metamaterials are “lossy” (lose energy) at shorter wavelengths and can operate effectively only at low frequencies, such as the radio frequencies used by radar, before being overwhelmed by their own absorption. Silicon doesn’t emit light and can transmit it only in a limited wavelength range because of its narrow working range (bandgap). So neither class of material can create a metamaterial that will operate in the infrared and optical ranges, where most military and commercial applications would take place.

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation -- something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation — something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

Optical metamaterials enter the arena

Sandia National Laboratories researchers are helping lead the way to the use of III-V semiconductors as the building blocks of metamaterials. (III-V refers to elements in those columns in the periodic table.) Sandia researchers have published technical papers, including three in the past year, on work featuring materials like gallium-arsenide and aluminum-arsenide, which are more efficient than metals for optical metamaterial applications, with wider bandgap ranges than silicon. The work is promising enough to have been featured on the covers of two technical journals.

“There is very little work worldwide on all-dielectric metamaterials using III-V semiconductors,” said Sandia researcher Igal Brener, who leads the Sandia work with researchers Mike Sinclair and Sheng Liu. “Our advantage is Sandia’s vast access to III-V technology, both in growth and processing, so we can move pretty fast.”

Shinier than gold

The new Sandia dielectric materials — a kind of electrical insulator — offer more than just efficiency. They lose little incoming energy and can even be fabricated in multiple layers to form complex, three-dimensional meta-atoms that reflect more light than shiny gold surfaces, usually considered the ultimate in infrared reflectivity. The III-V materials also emit photons when excited — something that silicon, which can reflect, transmit and absorb — can’t do.

Another advantage is their highly variable outputs, across the color spectrum so they might be used to extend the wavelength range of lasers or for generating “entangled photons” for quantum computing.

Sandia’s approach also is attractive for its relatively simple method of forming the artificial atoms, known as resonators, that are the guts of the metamaterial.

Created under the supervision of Liu, the meta-atoms are a few hundred nanometers in diameter and made of many actual atoms. One of Liu’s improvements was to oxidize these tiny groupings around their perimeters to create layered coatings with a low index of refraction, rather than use a more expensive, time-consuming “flip-chip” bonding process. The complexity of previous methods was an obstacle to cost- and time-efficiency. Other Sandia researchers had used a variant of his simplification previously to make lasers, but not metamaterials, he said.

The oxidized, low-index surface surrounds the high-index core “like in wintertime, you have a coat surrounding you,” Liu said. “To confine light, you need a high refractive-index contrast.” Put another way, interior light bumping into the low-indexed oxide surface is herded back by the refractive difference so it travels along the high-index core.

Liu’s Sandia colleague Gordon Keeler achieved controlled oxidation simply by putting III-V materials in a hot oven and flowing water vapor over the sample. “It will oxidize at a certain rate,” Liu says. “The more material, the longer it takes.”

The man-made meta-atoms are sculpted in place during a lithographic process that permits researchers to make any pattern they chose for the placement of the metamaterial components. “We use simulations to direct us,” Liu said. Spacing is determined to some extent by the size of the manmade atoms.

Fractured cubic nanostructures store unusually large amounts of energy

The researchers experimented with cylindrical and cubic nanostructures, reducing the symmetry of the latter to achieve even better properties.

“Cylinders are much easier to fabricate and typically can be used for conventional metasurfaces,” said Brener. “But broken-symmetry cubes are crucial to obtain very sharp resonances. That’s the key issue of the paper.”

The idea of intentionally reducing the symmetry of a cubic resonator nanostructure originated five or six years ago, said Sinclair, with a serendipitous design that happened to break the intentionally symmetrical shape of the meta-atoms when the team tried to mimic a particular manufacturing flaw.

“During a Laboratory Directed Research and Development [LDRD] Metamaterials Grand Challenge, when we were first fabricating cubic resonators in our effort to see if we could get beyond microwaves into infrared and optical metamaterials, we were playing with the shape of resonators to try to simulate the effect of lithography errors. In one simulation, we happened to cut a corner of the cube and all of a sudden very sharp reflection bands appeared,” Sinclair said.

Prior to that discovery, dielectric resonator metamaterials only showed broad bands that didn’t trap much energy. The researchers found the new sharp resonances allowed greater energy storage — beneficial for efficient frequency conversion, and perhaps even for light emission and lasing.

Exploration of the crimped resonator had to wait for a later project, sponsored by the Department of Energy’s Office of Science. Salvatore Campione, building on previous work by Lorena Basilio, Larry Warne and William Langston — all of Sandia — used electromagnetic simulations to unravel precisely how the cubes trap light. Sandia’s Willie Luk measured the cubes’ reflective properties. Another LDRD grant currently supports research into metamaterial lasing.

“We feel we’ve created a pretty flexible platform for a lot of different kinds of devices,” Sinclair said.

The ongoing work is aided by Sandia’s John Reno, nationally known for growing extremely precise crystalline structures, who contributed the III-V wafers.

Three patents on aspects of the work have been submitted.

The field of metamaterials, an intersection of materials science, physics, nanotechnology and electrical engineering, aims to produce structures with unusual electromagnetic properties. Through the careful combination of multiple materials in a precise periodic arrangement, the resulting metamaterials exhibit properties that otherwise couldn’t exist, such as a negative index of refraction. Some metamaterials can even channel electromagnetic waves around their surfaces, rendering them invisible for certain wavelengths of light.

The precision needed for arranging a metamaterial’s constitutive parts, also known as inclusions, has been a challenging step in their development and application.

Now, University of Pennsylvania engineers have shown a way to make metamaterials with a single inclusion, providing easier fabrication, among other useful features.

Physical experiments showed that the location of the dielectric rod and the shape of the ENZ material did not effect the properties of the resulting metamaterial. Credit: University of Pennsylvania

Physical experiments showed that the location of the dielectric rod and the shape of the ENZ material did not effect the properties of the resulting metamaterial. Credit: University of Pennsylvania

Analogous to electronic “doping,” where adding a small amount of atomic impurities to a “pure” material gives it electronic properties necessary for many computational and sensing devices, this “photonic doping” would allow for new ways of sculpting and tailoring light-matter interactions, with future impact on optical technology, such as flexible photonics.

The study, published in the journal Science, was led by Nader Engheta, H. Nedwill Ramsey Professor of Electrical and Systems Engineering, together with members of his group, Iñigo Liberal, Ahmed M. Mahmoud, Yue Li and Brian Edwards.

“Just as in electronic doping, when adding a set of foreign atoms in an otherwise pure material can significantly alter the electronic and optical properties of the host,” Engheta said, “‘photonic doping’ means adding a foreign photonic object in a specialized photonic host structure can change the optical scattering of the original structure in a major way.”

The phenomenon works with a specific class of materials that have permittivity, a parameter that has to do with the electric response of the material, mathematically represented by the Greek letter epsilon, that is nearly zero.

The key quality of these epsilon-near-zero, or ENZ, materials is that the wave’s magnetic field is distributed uniformly throughout the two-dimensional ENZ hosts, regardless of their cross-sectional shape. Such ENZ materials occur either naturally or can be made by traditional metamaterial means.

Rather than engineer complicated periodic structures that significantly alter the optical and magnetic properties of such materials, Engheta and his group devised a way for a single inclusion in a 2-D ENZ structure to accomplish the same task: changing which wavelengths of light that will reflect or pass through, or altering the magnetic response of the structure

“If I want to change the way a piece of material interacts with light, I normally have to change all of it,” Engheta said, “Not here. If I place a single dielectric rod anywhere within this ENZ material, the entire structure will look different from the perspective of an external wave.”

The dielectric rod is a cylindrical structure made out of an insulating material that can be polarized. When inserted in a 2-D ENZ host, it can affect the magnetic field within this host and consequently can notably change the optical properties of the host ENZ material.

Because the wave’s magnetic field in the 2-D ENZ host has a uniform spatial distribution, the dielectric rod can be placed anywhere within the material. Incoming waves thus behave as if the host material has a significantly different set of optical properties. Since the rod does not need to be placed at a precise location, construction of such photonically doped structures may be achieved with relative ease.

Applying these metamaterial concepts via “photonic doping” has implications for information processing systems and applications within telecommunications.

“When we’re working with a wave, this photonic doping can be a new way for us to determine the path this wave takes from A to B within a device,” Engheta said. “With a relatively small change in the dielectric rod, we can make waves ‘go this way’ and ‘don’t go that way.’ That we only need to make a change to the rod, which is a tiny part of the host material, should help with the speed of the device, and, because the effect is the same for the ENZ host with arbitrary shape while keeping its cross-sectional area fixed, this property may be very useful for flexible photonics.”

Further research demonstrates more complicated ways of applying photonic doping to ENZ materials, such as adding multiple rods with different diameters.

“The dielectric property of the rod can be responsive to thermal, optical or electrical changes,” Engheta said. “That means we could use the host ENZ material as the read-out of a sensor, as it would transmit or reflect light due to changes in that rod. Adding more rods would allow for even finer tuning of the material’s response.”

Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.

Similar to carbon, silicon forms two dimensional networks that are only one atomic layer thick. Like graphene, for whose discovery Andre Geim and Konstantin Novoselov received the Nobel Prize in 2010, these layers possess extraordinary optoelectrical properties. Silicon nanosheets might thus find application in nanoelectronics, for example in flexible displays, field-effect transistors and photodetectors. With its ability to store lithium ions, it is also under consideration as an anode material in rechargeable lithium batteries.

“Silicon nanosheets are particularly interesting because today’s information technology builds on silicon and, unlike with graphene, the basic material does not need to be exchanged,” explains Tobias Helbich from the WACKER Chair for Macromolecular Chemistry at TUM. “However, the nanosheets themselves are very delicate and quickly disintegrate when exposed to UV light, which has significantly limited their application thus far.”

Polymer and nanosheets – the best of both worlds in one

Now Helbich, in collaboration with Professor Bernhard Rieger, Chair of Macromolecular Chemistry, has for the first time successfully embedded the silicon nanosheets into a polymer, protecting them from decay. At the same time, the nanosheets are protected against oxidation. This is the first nanocomposite based on silicon nanosheets.

“What makes our nanocomposite special is that it combines the positive properties of both of its components,” explains Tobias Helbich. “The polymer matrix absorbs light in the UV domain, stabilizes the nanosheets and gives the material the properties of the polymer, while at the same time maintaining the remarkable optoelectronic properties of the nanosheets.”

Long-term goal of nanoelectronics – In leaps and bounds to industrial application

Its flexibility and durability against external influences also makes the newly developed material amenable to standard polymer technology for industrial processing. This puts actual applications within an arm’s reach.

The composites are particularly well suited for application in the up and coming field of nanoelectronics. Here, “classical” electronic components like circuits and transistors are implemented on scales of less than 100 nanometers. This allows whole new technologies to be realized – for faster computer processors, for example.

Nanoelectronic photodetector

The first successful application of the nanocomposite constructed by Helbich was only recently presented in the context of the ATUMS Graduate Program (Alberta / TUM International Graduate School for Functional Hybrid Materials): Alina Lyuleeva and Prof. Paolo Lugli from the Institute of Nanoelectronics at TU Munich, in collaboration with Helbich and Rieger, succeeded in building a photodetector based on these silicon nanosheets.

To this end, they mounted the polymer embedded silicon nanosheets onto a silicon dioxide surface coated with gold contacts. Because of its Lilliputian dimensions, this kind of nanoelectronic detector saves a lot of both space and energy.

The research is part of the ATUMS Graduate Program (Alberta / TUM International Graduate School for Functional Hybrid Materials (ATUMS; IRTG 2022)) in which German and Canadian scientists in the fields of chemistry, electrical engineering and physics collaborate closely. Their goal is not only to create novel functions based on nanoparticles and polymer materials, but, at the same time, to develop first applications. The work is funded by the German Research Council (DFG) and the Natural Science and Engineering Research Council of Canada (NSERC).

A chance observation of crystals forming a mark that resembled the stain of a coffee cup left on a table has led to the growth of customized polycrystals with implications for faster and more versatile semiconductors.

Thin-film semiconductors are the foundation of a vast array of electronic and optoelectronic devices. They are generally fabricated by crystallization processes that yield polycrystals with a chaotic mix of individual crystals of different orientations and sizes.

Significant advances in controlling crystallization has been made by a team led by Professor Aram Amassian of Material Science and Engineering at KAUST. The group included individuals from the KAUST Solar Center and others from the University’s Physical Science and Engineering Division in collaboration with Cornell University. Amassian said, “There is no longer a need to settle for random and incoherent crystallization.”

Crystallization behavior can be controlled locally, creating regions with different crystal patterns. Credit: KAUST 2017

Crystallization behavior can be controlled locally, creating regions with different crystal patterns. Credit: KAUST 2017

The team’s recent discovery began when Dr. Liyang Yu of the KAUST team noticed that a droplet of liquid semiconductor material dried to form an outer coffee-ring shape that was much thicker than the material at the center. When he induced the material to crystallize, the outer ring crystallized first.

“This hinted that local thickness matters for initiating crystallization,” said Amassian, which went against the prevailing understanding of how polycrystal films form.

This anomaly led the researchers to delve deeper. They found that the thickness of the crystallizing film could be used to manipulate the crystallization of many materials (see top image). Most crucially, tinkering with the thickness also allowed fine control over the position and orientation of the crystals in different regions of a semiconductor.

“We discovered how to achieve excellent semiconductor properties everywhere in a polycrystal film,” said Amassian. He explained that seeding different patterns of crystallization at different locations also allowed the researchers to create bespoke arrays that can now be used in electronic circuits (see bottom image).

This is a huge improvement to the conventional practice of making do with materials whose good properties are not sustained throughout the entire polycrystal nor whose functions at different regions can be controlled.

“We can now make customized polycrystals on demand,” Amassian said.

Amassian hopes that this development will lead to high-quality, tailored polycrystal semiconductors to promote advances in optoelectronics, photovoltaics and printed electronic components. The method has the potential to bring more efficient consumer electronic devices, some with flexible and lightweight parts, new solar power generating systems and advances in medical electronics. And all thanks to the chance observation of an odd pattern in a semiconductor droplet.

The team will now explore ways to move their work beyond the laboratory through industry partnerships and research collaborations.

Technavio analysts forecast the global carbon nanotube (CNT) market to grow at a staggering CAGR of almost 22% during the forecast period, according to their latest report.

The research study covers the present scenario and growth prospects of the global CNT market for 2017-2021. To calculate the market size, the report considers the revenue generated from the sales of CNTs worldwide.

The production capacities of CNTs will expand due to their growing demand. Factors such as the need to enhance the efficiency of electronic and semiconductor products, high use of CNTs in the aerospace and defense sectors, and the need to increase the efficiency of energy-sector-related devices are driving the market.

Technavio’s sample reports are free of charge and contain multiple sections of the report including the market size and forecast, drivers, challenges, trends, and more.

Technavio hardware and semiconductor analysts highlight the following three factors that are contributing to the growth of the global CNT market:

  • Advantages due to physical properties
  • Potential to replace other materials
  • Rise in production capacities

Advantages due to physical properties

The structure of CNTs is closely related to graphite, which is traditionally made by stacking sheets of carbon on top of another. These sheets can easily slide over each other. CNTs are made by rolling these sheets into a cylinder, with their edges joined. This structure offers extraordinary electrical, mechanical, optical, thermal, and chemical properties to CNTs.

Sunil Kumar Singh, a lead embedded systems analyst at Technavio, says, “Being a carbon-based product, CNTs are not vulnerable to environmental or physical degradation issues. Due to this advantage, CNTs are in high demand and are used in multiple applications such as medicine, aerospace and defense, electronics, automotive, energy, construction, and sports.”

Potential to replace other materials

CNTs have the potential to replace the key materials in some industries such as semiconductor and energy. Research centers are developing CNTs that can be used in solar cells as an alternative to silicon, which is the key material used in producing electricity from solar energy. By using CNTs instead of silicon, the conversion efficiency of solar cells can be enhanced.

“CNTs have the potential to replace indium-tin-coated films, which are fragile and expensive. These films are used in liquid crystal displays, solar cells, organic light-emitting diodes, touchscreens, and high-strength materials like bulletproof vests and hydrogen fuel cells used to power cars,” adds Sunil.

Rise in production capacities

Production capacity for CNTS for 2015 was 4,567 metric tons globally. MWCNT dominates this market space due to its low production cost and high-scalability. Whereas, SWCNT still has issues with scaling up the volume produced and reduced the cost. Techniques available for CNT production such as substrate-free growth and substrate-bound growth while deploying vapor-solid-solid (VSS) and vapor-liquid-solid (VLS) are widely adopted for catalyst-based synthesis.

Many CNT vendors are investing heavily in new production facilities to meet the growing demand from sectors such as consumer goods, electrical and electronics, energy, healthcare, automobile, and aerospace and defense. Among countries, China has increased the production of CNTs backed by high government funding for nanomaterials.

In cooperation with Okmetic Oy and the Polish ITME, researchers at Aalto University have studied the application of SOI (Silicon On Insulator) wafers, which are used as a platform for manufacturing different microelectronics components, as a substrate for producing gallium nitride crystals. The researchers compared the characteristics of gallium nitride (GaN) layers grown on SOI wafers to those grown on silicon substrates more commonly used for the process. In addition to high-performance silicon wafers, Okmetic also manufactures SOI wafers, in which a layer of silicon dioxide insulator is sandwiched between two silicon layers. The objective of the SOI technology is to improve the capacitive and insulating characteristics of the wafer.

The researchers used Micronova's cleanrooms and, in particular, a reactor designed for gallium nitride manufacturing. The image shows a six-inch substrate in the MOVPE reactor before manufacturing. Credit: Aalto University / Jori Lemettinen

The researchers used Micronova’s cleanrooms and, in particular, a reactor designed for gallium nitride manufacturing. The image shows a six-inch substrate in the MOVPE reactor before manufacturing. Credit: Aalto University / Jori Lemettinen

“We used a standardised manufacturing process for comparing the wafer characteristics. GaN growth on SOI wafers produced a higher crystalline quality layer than on silicon wafers. In addition, the insulating layer in the SOI wafer improves breakdown characteristics, enabling the use of clearly higher voltages in power electronics. Similarly, in high frequency applications, the losses and crosstalk can be reduced”, explains Jori Lemettinen, a doctoral candidate from the Department of Electronics and Nanoengineering.

‘GaN based components are becoming more common in power electronics and radio applications. The performance of GaN based devices can be improved by using a SOI wafer as the substrate’, adds Academy Research Fellow Sami Suihkonen.

SOI wafers reduce the challenges of crystal growth

Growth of GaN on a silicon substrate is challenging. GaN layers and devices can be grown on substrate material using metalorganic vapor phase epitaxy (MOVPE). When using silicon as a substrate the grown compound semiconductor materials have different coefficients of thermal expansion and lattice constants than a silicon wafer. These differences in their characteristics limit the crystalline quality that can be achieved and the maximum possible thickness of the produced layer.

‘The research showed that the layered structure of an SOI wafer can act as a compliant substrate during gallium nitride layer growth and thus reduce defects and strain in the grown layers”, Lemettinen notes. GaN based components are commonly used in blue and white LEDs. In power electronics applications, GaN diodes and transistors, in particular, have received interest, for example in frequency converters or electric cars. It is believed that in radio applications, 5G network base stations will use GaN based power amplifiers in the future. In electronics applications, a GaN transistor offers low resistance and enables high frequencies and power densities.