Category Archives: Process Materials

Researchers from Graduate School of Bio-Applications and Systems Engineering at Tokyo University of Agriculture and Technology (TUAT) have sped up the movement of electrons in organic semiconductor films by two to three orders of magnitude. The speedier electronics could lead to improved solar power and transistor use across the world, according to the scientists.

They published their results in the September issue of Macromolecular Chemistry and Physics, where the paper is featured on the cover.

Led by Kenji Ogino, a professor at Graduate School of Bio-Applications and Systems Engineering at TUAT, Japan, the team found that adding polystyrene, commonly known as Styrofoam in North America, could enhance the semiconducting polymer by allowing electrons to move from plane to plane quickly. The process, called hole mobility, is how electrons move through an electric field consisting of multiple layers. When a molecule is missing an electron, an electron from a different plane can jump or fall and take its place.

Through various imaging techniques, it’s fairly easy to follow the electron trail in the crystal-based structures. In many semiconducting polymers, however, the clean, defined lines of the crystalline skeleton intertwine with a much more difficult-to-define region. It’s actually called the amorphous domain.

“[Electrons] transport in both crystalline and amorphous domains. To improve the total electron mobility, it is necessary to control the nature of the amorphous domain,” Ogino said. “We found that hole mobility extraordinarily improved by the introduction of polystyrene block accompanied by the increase of the ratio of rigid amorphous domain.”

The researchers believe that the way the crystalline domain connects within itself occurs most effectively through the rigid amorphous domain. The addition of polystyrene introduced more amorphous domain, but contained by flexible chains of carbon and hydrogen atoms. Even though the chains are flexible, it provides rigidity, and some degree of control, to the amorphous domain.

Electrons moved two to three times quicker than normal.

“The introduction of a flexible chain in semicrystalline polymers is one of the promising strategies to improve the various functionalities of polymer films by altering the characteristics of the amorphous domain,” Ogino said. “We propose that the rigid amorphous domain plays an important role in the hole transporting process.”

Enhanced hole mobility is a critical factor in developing more efficient solar devices, according to Ogino. Next, Ogino and the researchers plan to examine how the enhanced hole mobility affected other parameters, such as the chemical composition and position of the structures within the polymer film.

When chemists from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw were starting work on yet another material designed for the efficient production of nanocrystalline zinc oxide, they didn’t expect any surprises. They were greatly astonished when the electrical properties of the changing material turned out to be extremely exotic.

The exotic transformations causes that one of the precursors of zinc oxide, initially an insulator, at approx. 300 degrees Celsius goes to a state with electrical properties typical of metals, and at ~400 degrees Celsius it becomes a semiconductor. Credit: IPC PAS

The single source precursor (SSP) approach is widely regarded as one of the most promising of the various strategies employed for the preparation of semiconductor nanocrystalline materials. However, one of the key obstacles hampering both the rational design of SSPs and their controlled transformation to the desired nanomaterials with highly controlled physicochemical properties is the scarcity of mechanistic insights during the transformation process. Scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Chemistry of Warsaw University of Technology (WUT) have revealed that in the thermal decomposition process of a pre-organized zinc alkoxide precursor the nucleation and growth of the semiconducting zinc oxide (ZnO) phase is preceded by cascade transformations involving the formation of previously unreported intermediate radical zinc oxo-alkoxide clusters with gapless electronic states. Up to now, these types of clusters have not been considered either as intermediate structures on the path to the semiconductor ZnO phase or as a potential species accounting for the various defect states of ZnO nanocrystals.

“We discovered that one of the groups of ZnO precursors that have been studied for decades, zinc alkoxide compounds, undergo previously unobserved physicochemical transformations upon thermal decomposition. Originally, the starting compound is an insulator, when heated it rapidly transforms into a material with conductor-like properties, and a further increase in temperature equally rapidly leads to its conversion into a semiconductor,” says Dr. Kamil Soko?owski (IPC PAS).

The design and preparation of well-defined nanomaterials in a controlled manner remains a tremendous challenge and is acknowledged to be the biggest obstacle for the exploitation of many nanoscale phenomena. Professor Lewiski’s (IPC PAS, PW) group has for many years been engaged in the development of effective methods of producing nanocrystalline forms of zinc oxide, a semiconductor with wide applications in electronics, industrial catalysis, photovoltaics and photocatalysis. One of the studied approaches is based on the single source precursors. The precursor molecules contain all components of the target material in their structure and only temperature is required to trigger the chemical transformation.

“We dealt with a group of chemical compounds with the general formula RZnOR, as single source pre-designed ZnO precursors. A common feature of their structure is the presence of the cubic [Zn4O4] core with alternating zinc and oxygen atoms terminated by organic groups R. When the precursor is heated, the organic parts are degraded, and the inorganic cores self-assemble, forming the final form of the nanomaterial,” explains Dr. Soko?owski.

The tested precursor had the properties of an insulator, with an energy gap of about five electronvolts. When heated, it eventually transformed into a semiconductor with an energy gap of approximately 3 eV.

“An exceptional result of our research was the discovery that at a temperature close to 300 degrees Celsius the compound suddenly transforms into almost gap-less electronic state, showing electrical properties rather more typical of metals. When the temperature rises to approximately 400 degrees, the energy gap suddenly expands to a width characteristic of semiconductor materials. Ultimately, thanks to the combination of advanced synchrotron experiments with quantum-chemical calculations, we have established all the details of these unique transformations,” says Dr. Adam Kubas (IPC PAS), who carried out the quantum-chemical calculations.

The spectroscopic measurements were carried out using methods developed by Dr. Jakub Szlachetko (Institute of Nuclear Physics PAS, Cracow) and Dr. Jacinto Sa (IPC PAS and Uppsala University) at the Swiss Light Source synchrotron facility at the Paul Scherrer Institute in Villigen, Switzerland. The material was heated in a reaction chamber, and then its electron structure was sampled using an X-ray synchrotron beam. The set-up allowed for real-time monitoring of the transformations taking place.

This detailed in situ study of the decomposition process of the zinc alkoxide precursor, supported by computer simulations, revealed that any nucleation or growth of a semiconducting ZnO phase is preceded by cascade transformations involving the formation of previously unreported intermediate radical zinc oxo-alkoxide clusters with gapless electronic states.

“In this process homolytic cleavage of the R-Zn bond is responsible for the initial thermal decomposition process. Computer simulations revealed that the intermediate radical clusters tend to dimerise though an uncommon bimetallic Zn-Zn-bond formation. The following homolytic O-R bond cleavage then leads to sub-nano ZnO clusters which further self-organise to the ZnO nanocrystalline phase,” says Dr. Kubas.

Up to now, the radical zinc oxo clusters formed have not been considered either as intermediate structures on the way to the semiconductor ZnO phase or as potential species accounting for various defect states of ZnO nanocrystals. In a broader context, a deeper understanding of the origin and character of the defects is crucial for structure-property relationships in semiconducting materials.

The research, funded by the National Science Centre and the TEAM grant of the Foundation for Polish Science co-financed by the European Union, will contribute to the development of more precise methods of controlling the properties of nanocrystalline zinc oxide. So far, with greater or lesser success, these properties have been explained with the help of various types of material defects. For obvious reasons, however, the analyses have not taken into account the possibility of forming the specific radical zinc-oxo clusters discovered by the Warsaw-based scientists in the material.

Since the 2003 discovery of the single-atom-thick carbon material known as graphene, there has been significant interest in other types of 2-D materials as well.

These materials could be stacked together like Lego bricks to form a range of devices with different functions, including operating as semiconductors. In this way, they could be used to create ultra-thin, flexible, transparent and wearable electronic devices.

However, separating a bulk crystal material into 2-D flakes for use in electronics has proven difficult to do on a commercial scale.

The existing process, in which individual flakes are split off from the bulk crystals by repeatedly stamping the crystals onto an adhesive tape, is unreliable and time-consuming, requiring many hours to harvest enough material and form a device.

Now researchers in the Department of Mechanical Engineering at MIT have developed a technique to harvest 2-inch diameter wafers of 2-D material within just a few minutes. They can then be stacked together to form an electronic device within an hour.

The technique, which they describe in a paper published in the journal Science, could open up the possibility of commercializing electronic devices based on a variety of 2-D materials, according to Jeehwan Kim, an associate professor in the Department of Mechanical Engineering, who led the research.

The paper’s co-first authors were Sanghoon Bae, who was involved in flexible device fabrication, and Jaewoo Shim, who worked on the stacking of the 2-D material monolayers. Both are postdocs in Kim’s group.

The paper’s co-authors also included students and postdocs from within Kim’s group, as well as collaborators at Georgia Tech, the University of Texas, Yonsei University in South Korea, and the University of Virginia. Sang-Hoon Bae, Jaewoo Shim, Wei Kong, and Doyoon Lee in Kim’s research group equally contributed to this work.

“We have shown that we can do monolayer-by-monolayer isolation of 2-D materials at the wafer scale,” Kim says. “Secondly, we have demonstrated a way to easily stack up these wafer-scale monolayers of 2-D material.”

The researchers first grew a thick stack of 2-D material on top of a sapphire wafer. They then applied a 600-nanometer-thick nickel film to the top of the stack.

Since 2-D materials adhere much more strongly to nickel than to sapphire, lifting off this film allowed the researchers to separate the entire stack from the wafer.

What’s more, the adhesion between the nickel and the individual layers of 2-D material is also greater than that between each of the layers themselves.

As a result, when a second nickel film was then added to the bottom of the stack, the researchers were able to peel off individual, single-atom thick monolayers of 2-D material.

That is because peeling off the first nickel film generates cracks in the material that propagate right through to the bottom of the stack, Kim says.

Once the first monolayer collected by the nickel film has been transferred to a substrate, the process can be repeated for each layer.

“We use very simple mechanics, and by using this controlled crack propagation concept we are able to isolate monolayer 2-D material at the wafer scale,” he says.

The universal technique can be used with a range of different 2-D materials, including hexagonal boron nitride, tungsten disulfide, and molybdenum disulfide.

In this way it can be used to produce different types of monolayer 2-D materials, such as semiconductors, metals, and insulators, which can then be stacked together to form the 2-D heterostructures needed for an electronic device.

“If you fabricate electronic and photonic devices using 2-D materials, the devices will be just a few monolayers thick,” Kim says. “They will be extremely flexible, and can be stamped on to anything,” he says.

The process is fast and low-cost, making it suitable for commercial operations, he adds.

The researchers have also demonstrated the technique by successfully fabricating arrays of field-effect transistors at the wafer scale, with a thickness of just a few atoms.

“The work has a lot of potential to bring 2-D materials and their heterostructures towards real-world applications,” says Philip Kim, a professor of physics at Harvard University, who was not involved in the research.

The researchers are now planning to apply the technique to develop a range of electronic devices, including a nonvolatile memory array and flexible devices that can be worn on the skin.

They are also interested in applying the technique to develop devices for use in the “internet of things,” Kim says.

“All you need to do is grow these thick 2-D materials, then isolate them in monolayers and stack them up. So it is extremely cheap — much cheaper than the existing semiconductor process. This means it will bring laboratory-level 2-D materials into manufacturing for commercialization,” Kim says.

“That makes it perfect for IoT networks, because if you were to use conventional semiconductors for the sensing systems it would be expensive.”

A paper published in Nature Communications by Sufei Shi, assistant professor of chemical and biological engineering at Rensselaer, increases our understanding of how light interacts with atomically thin semiconductors and creates unique excitonic complex particles, multiple electrons, and holes strongly bound together. These particles possess a new quantum degree of freedom, called “valley spin.” The “valley spin” is similar to the spin of electrons, which has been extensively used in information storage such as hard drives and is also a promising candidate for quantum computing.

Research on Light-Matter Interaction Could Lead to Improved Electronic and Optoelectronic Devices. Credit: Rensselaer Polytechnic Institute

The paper, titled “Revealing the biexciton and trion-exciton complexes in BN encapsulated WSe2,” was published in the Sept. 13, 2018, edition of Nature Communications. Results of this research could lead to novel applications in electronic and optoelectronic devices, such as solar energy harvesting, new types of lasers, and quantum sensing.

Shi’s research focuses on low dimensional quantum materials and their quantum effects, with a particular interest in materials with strong light-matter interactions. These materials include graphene, transitional metal dichacogenides (TMDs), such as tungsten diselenide (WSe2), and topological insulators.

TMDs represent a new class of atomically thin semiconductors with superior optical and optoelectronic properties. Optical excitation on the two-dimensional single-layer TMDs will generate a strongly bound electron-hole pair called an exciton, instead of freely moving electrons and holes as in traditional bulk semiconductors. This is due to the giant binding energy in monolayer TMDs, which is orders of magnitude larger than that of conventional semiconductors. As a result, the exciton can survive at room temperature and can thus be used for application of excitonic devices.

As the density of the exciton increases, more electrons and holes pair together, forming four-particle and even five-particle excitonic complexes. An understanding of the many-particle excitonic complexes not only gives rise to a fundamental understanding of the light-matter interaction in two dimensions, it also leads to novel applications, since the many-particle excitonic complexes maintain the “valley spin” properties better than the exciton. However, despite recent developments in the understanding of excitons and trions in TMDs, said Shi, an unambiguous measure of the biexciton-binding energy has remained elusive.

“Now, for the first time, we have revealed the true biexciton state, a unique four-particle complex responding to light,” said Shi. “We also revealed the nature of the charged biexciton, a five-particle complex.”

At Rensselaer, Shi’s team has developed a way to build an extremely clean sample to reveal this unique light-matter interaction. The device was built by stacking multiple atomically thin materials together, including graphene, boron nitride (BN), and WSe2, through van der Waals (vdW) interaction, representing the state-of-the-art fabrication technique of two-dimensional materials.

This work was performed in collaboration with the National High Magnetic Field Laboratory in Tallahasee, Florida, and researchers at the National Institute for Materials Science in Japan, as well as with Shengbai Zhang, the Kodosky Constellation Professor in the Department of Physics, Applied Physics, and Astronomy at Rensselaer, whose work played a critical role in developing a theoretical understanding of the biexciton.

The results of this research could potentially lead to robust many-particle optical physics, and illustrate possible novel applications based on 2D semiconductors, Shi said. Shi has received funding from the Air Force Office of Scientific Research. Zhang was supported by the Department of Energy, Office of Science.

The vast majority of computing devices today are made from silicon, the second most abundant element on Earth, after oxygen. Silicon can be found in various forms in rocks, clay, sand, and soil. And while it is not the best semiconducting material that exists on the planet, it is by far the most readily available. As such, silicon is the dominant material used in most electronic devices, including sensors, solar cells, and the integrated circuits within our computers and smartphones.

Now MIT engineers have developed a technique to fabricate ultrathin semiconducting films made from a host of exotic materials other than silicon. To demonstrate their technique, the researchers fabricated flexible films made from gallium arsenide, gallium nitride, and lithium fluoride — materials that exhibit better performance than silicon but until now have been prohibitively expensive to produce in functional devices.

MIT researchers have devised a way to grow single crystal GaN thin film on a GaN substrate through two-dimensional materials. The GaN thin film is then exfoliated by a flexible substrate, showing the rainbow color that comes from thin film interference. This technology will pave the way to flexible electronics and the reuse of the wafers. Credit: Wei Kong and Kuan Qiao

The new technique, researchers say, provides a cost-effective method to fabricate flexible electronics made from any combination of semiconducting elements, that could perform better than current silicon-based devices.

“We’ve opened up a way to make flexible electronics with so many different material systems, other than silicon,” says Jeehwan Kim, the Class of 1947 Career Development Associate Professor in the departments of Mechanical Engineering and Materials Science and Engineering. Kim envisions the technique can be used to manufacture low-cost, high-performance devices such as flexible solar cells, and wearable computers and sensors.

Details of the new technique are reported today in Nature Materials. In addition to Kim, the paper’s MIT co-authors include Wei Kong, Huashan Li, Kuan Qiao, Yunjo Kim, Kyusang Lee, Doyoon Lee, Tom Osadchy, Richard Molnar, Yang Yu, Sang-hoon Bae, Yang Shao-Horn, and Jeffrey Grossman, along with researchers from Sun Yat-Sen University, the University of Virginia, the University of Texas at Dallas, the U.S. Naval Research Laboratory, Ohio State University, and Georgia Tech.

Now you see it, now you don’t

In 2017, Kim and his colleagues devised a method to produce “copies” of expensive semiconducting materials using graphene — an atomically thin sheet of carbon atoms arranged in a hexagonal, chicken-wire pattern. They found that when they stacked graphene on top of a pure, expensive wafer of semiconducting material such as gallium arsenide, then flowed atoms of gallium and arsenide over the stack, the atoms appeared to interact in some way with the underlying atomic layer, as if the intermediate graphene were invisible or transparent. As a result, the atoms assembled into the precise, single-crystalline pattern of the underlying semiconducting wafer, forming an exact copy that could then easily be peeled away from the graphene layer.

The technique, which they call “remote epitaxy,” provided an affordable way to fabricate multiple films of gallium arsenide, using just one expensive underlying wafer.

Soon after they reported their first results, the team wondered whether their technique could be used to copy other semiconducting materials. They tried applying remote epitaxy to silicon, and also germanium — two inexpensive semiconductors — but found that when they flowed these atoms over graphene they failed to interact with their respective underlying layers. It was as if graphene, previously transparent, became suddenly opaque, preventing atoms of silicon and germanium from “seeing” the atoms on the other side.

As it happens, silicon and germanium are two elements that exist within the same group of the periodic table of elements. Specifically, the two elements belong in group four, a class of materials that are ionically neutral, meaning they have no polarity.

“This gave us a hint,” says Kim.

Perhaps, the team reasoned, atoms can only interact with each other through graphene if they have some ionic charge. For instance, in the case of gallium arsenide, gallium has a negative charge at the interface, compared with arsenic’s positive charge. This charge difference, or polarity, may have helped the atoms to interact through graphene as if it were transparent, and to copy the underlying atomic pattern.

“We found that the interaction through graphene is determined by the polarity of the atoms. For the strongest ionically bonded materials, they interact even through three layers of graphene,” Kim says. “It’s similar to the way two magnets can attract, even through a thin sheet of paper.”

Opposites attract

The researchers tested their hypothesis by using remote epitaxy to copy semiconducting materials with various degrees of polarity, from neutral silicon and germanium, to slightly polarized gallium arsenide, and finally, highly polarized lithium fluoride — a better, more expensive semiconductor than silicon.

They found that the greater the degree of polarity, the stronger the atomic interaction, even, in some cases, through multiple sheets of graphene. Each film they were able to produce was flexible and merely tens to hundreds of nanometers thick.

The material through which the atoms interact also matters, the team found. In addition to graphene, they experimented with an intermediate layer of hexagonal boron nitride (hBN), a material that resembles graphene’s atomic pattern and has a similar Teflon-like quality, enabling overlying materials to easily peel off once they are copied.

However, hBN is made of oppositely charged boron and nitrogen atoms, which generate a polarity within the material itself. In their experiments, the researchers found that any atoms flowing over hBN, even if they were highly polarized themselves, were unable to interact with their underlying wafers completely, suggesting that the polarity of both the atoms of interest and the intermediate material determines whether the atoms will interact and form a copy of the original semiconducting wafer.

“Now we really understand there are rules of atomic interaction through graphene,” Kim says.

With this new understanding, he says, researchers can now simply look at the periodic table and pick two elements of opposite charge. Once they acquire or fabricate a main wafer made from the same elements, they can then apply the team’s remote epitaxy techniques to fabricate multiple, exact copies of the original wafer.

“People have mostly used silicon wafers because they’re cheap,” Kim says. “Now our method opens up a way to use higher-performing, nonsilicon materials. You can just purchase one expensive wafer and copy it over and over again, and keep reusing the wafer. And now the material library for this technique is totally expanded.”

Kim envisions that remote epitaxy can now be used to fabricate ultrathin, flexible films from a wide variety of previously exotic, semiconducting materials — as long as the materials are made from atoms with a degree of polarity. Such ultrathin films could potentially be stacked, one on top of the other, to produce tiny, flexible, multifunctional devices, such as wearable sensors, flexible solar cells, and even, in the distant future, “cellphones that attach to your skin.”

“In smart cities, where we might want to put small computers everywhere, we would need low power, highly sensitive computing and sensing devices, made from better materials,” Kim says. “This [study] unlocks the pathway to those devices.”

A team of semiconductor researchers based in France has used a boron nitride separation layer to grow indium gallium nitride (InGaN) solar cells that were then lifted off their original sapphire substrate and placed onto a glass substrate.

Ph.D. Student Taha Ayari measures the photovoltaic performance of the InGaN solar cells with a solar simulator. (Credit: Ougazzaden laboratory)

By combining the InGaN cells with photovoltaic (PV) cells made from materials such as silicon or gallium arsenide, the new lift-off technique could facilitate fabrication of higher efficiency hybrid PV devices able to capture a broader spectrum of light. Such hybrid structures could theoretically boost solar cell efficiency as high as 30 percent for an InGaN/Si tandem device.

The technique is the third major application for the hexagonal boron nitride lift-off technique, which was developed by a team of researchers from the Georgia Institute of Technology, the French National Center for Scientific Research (CNRS), and Institut Lafayette in Metz, France. Earlier applications targeted sensors and light-emitting diodes (LEDs).

“By putting these structures together with photovoltaic cells made of silicon or a III-V material, we can cover the visible spectrum with the silicon and utilize the blue and UV light with indium gallium nitride to gather light more efficiently,” said Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE). “The boron nitride layer doesn’t impact the quality of the indium gallium nitride grown on it, and we were able to lift off the InGaN solar cells without cracking them.”

The research was published August 15 in the journal ACS Photonics. It was supported by the French National Research Agency under the GANEX Laboratory of Excellence project and the French PIA project “Lorraine Université d’Excellence.”

The technique could lead to production of solar cells with improved efficiency and lower cost for a broad range of terrestrial and space applications. “This demonstration of transferred InGaN-based solar cells on foreign substrates while increasing performance represents a major advance toward lightweight, low cost, and high efficiency photovoltaic applications,” the researchers wrote in their paper.

“Using this technique, we can process InGaN solar cells and put a dielectric layer on the bottom that will collect only the short wavelengths,” Ougazzaden explained. “The longer wavelengths can pass through it into the bottom cell. By using this approach we can optimize each surface separately.”

The researchers began the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections.

The InGaN attaches to the boron nitride with weak van der Waals forces, allowing the solar cells to be grown across the wafer and removed without damage. So far, the cells have been removed from the sapphire manually, but Ougazzaden believes the transfer process could be automated to drive down the cost of the hybrid cells. “We can certainly do this on a large scale,” he said.

The InGaN structures are then placed onto the glass substrate with a backside reflector and enhanced performance is obtained. Beyond demonstrating placement atop an existing PV structure, the researchers hope to increase the amount of indium in their lift-off devices to boost light absorption and increase the number of quantum wells from five to 40 or 50.

“We have now demonstrated all the building blocks, but now we need to grow a real structure with more quantum wells,” Ougazzaden said. “We are just at the beginning of this new technology application, but it is very exciting.”

In addition to Ougazzaden, the research team includes Georgia Tech Ph.D. students Taha Ayari, Matthew Jordan, Xin Li and Saiful Alam; Chris Bishop and Simon Gautier from Institut Lafayette; Suresh Sundaram, a researcher at Georgia Tech Lorraine; Walid El Huni and Yacine Halfaya from CNRS; Paul Voss, an associate professor in the Georgia Tech School of ECE; and Jean Paul Salvestrini, a professor at Georgia Tech Lorraine and adjunct professor in the Georgia Tech School of ECE.

CITATION: Taha Ayari, et al., “Heterogeneous Integration of Thin-Film InGaN-Based Solar Cells on Foreign Substrates with Enhanced Performance,” (ACS Photonics 2018) https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00663

In its September Update to The 2018 McClean Report, IC Insights discloses that over the past two years, DRAM manufacturers have been operating their memory fabs at nearly full capacity, which has resulted in steadily increasing DRAM prices and sizable profits for suppliers along the way.  Figure 1 shows that the DRAM average selling price (ASP) reached $6.79 in August 2018, a 165% increase from two years earlier in August of 2016. Although the DRAM ASP growth rate has slowed this year compared to last, it has remained on a solid upward trajectory through the first eight months of 2018.

Figure 1

The DRAM market is known for being very cyclical and after experiencing strong gains for two years, historical precedence now strongly suggests that the DRAM ASP (and market) will soon begin trending downward.  One indicator suggesting that the DRAM ASP is on the verge of decline is back-to-back years of huge increases in DRAM capital spending to expand or add new fab capacity (Figure 2). DRAM capital spending jumped 81% to $16.3 billion in 2017 and is expected to climb another 40% to $22.9 billion this year. Capex spending at these levels would normally lead to an overwhelming flood of new capacity and a subsequent rapid decline in prices.

Figure 2

However, what is slightly different this time around is that big productivity gains normally associated with significant spending upgrades are much less at the sub-20nm process node now being used by the top DRAM suppliers as compared to the gains seen in previous generations.

At its Analyst Day event held earlier this year, Micron presented figures showing that manufacturing DRAM at the sub-20nm node required a 35% increase in the number of mask levels, a 110% increase in the number of non-lithography steps per critical mask level, and 80% more cleanroom space per wafer out since more equipment—each piece with a larger footprint than its previous generation—is required to fabricate ≤20nm devices. Bit volume increases that previously averaged around 50% following the transition to a smaller technology node, are a fraction of that amount at the ≤20nm node.  The net result is suppliers must invest much more money for a smaller increase in bit volume output.  So, the recent uptick in capital spending, while extraordinary, may not result in a similar amount of excess capacity, as has been the case in the past.

As seen in Figure 2, the DRAM ASP is forecast to rise 38% in 2018 to $6.65, but IC Insights forecasts that DRAM market growth will cool as additional capacity is brought online and supply constraints begin to ease. (It is worth mentioning that Samsung and SK Hynix in 3Q18 reportedly deferred some of their expansion plans in light of expected softening in customer demand.)

Of course, a wildcard in the DRAM market is the role and impact that the startup Chinese companies will have over the next few years.  It is estimated that China accounts for approximately 40% of the DRAM market and approximately 35% of the flash memory market.

At least two Chinese IC suppliers, Innotron and JHICC, are set to participate in this year’s DRAM market. Although China’s capacity and manufacturing processes will not initially rival those from Samsung, SK Hynix, or Micron, it will be interesting to see how well the country’s startup companies perform and whether they will exist to serve China’s national interests only or if they will expand to serve global needs.

 

Transition metal dichalcogenides (TMDCs) possess optical properties that could be used to make computers run a million times faster and store information a million times more energy-efficiently, according to a study led by Georgia State University.

This is Dr. Mark Stockman, director of the Center for Nano-Optics and a Regents’ Professor in the Department of Physics and Astronomy at Georgia State University. Credit: Georgia State University

Computers operate on the time scale of a fraction of a nanosecond, but the researchers suggest constructing computers on the basis of TMDCs, atomically thin semiconductors, could make them run on the femtosecond time scale, a million times faster. This would also increase computer memory speed by a millionfold.

“There is nothing faster, except light,” said Dr. Mark Stockman, lead author of the study and director of the Center for Nano-Optics and a Regents’ Professor in the Department of Physics and Astronomy at Georgia State. “The only way to build much faster computers is to use optics, not electronics. Electronics, which is used by current computers, can’t go any faster, which is why engineers have been increasing the number of processors. We propose the TMDCs to make computers a million times more efficient. This is a fundamentally different approach to information technology.”

The researchers propose a theory that TMDCs have the potential to process information within a couple of femtoseconds. A femtosecond is one millionth of one billionth of a second. A TMDC has a hexagonal lattice structure that consists of a layer of transition metal atoms sandwiched between two layers of chalcogen atoms. This hexagonal structure aids in the computer processor speed and also enables more efficient information storage. The findings are published in the journal Physical Review B in the prestigious Rapid Communications section.

The TMDCs have a number of positive qualities, including being stable, non-toxic, thin, light and mechanically strong. Examples include molybdenum disulfide (MOS2) and tungsten diselenide (WSe2). TMDCs are part of a large family called 2D materials, which is named after their extraordinary thinness of one or a few atoms. In this study, the researchers also established the optical properties of the TMDCs, which allow them to be ultrafast.

In the hexagonal lattice structure of TMDCs, electrons rotate in circles in different states, with some electrons spinning to the left and others turning to the right depending on their position on the hexagon. This motion causes a new effect that is called topological resonance. Such an effect allows one to read, write or process a bit of information in only a few femtoseconds.

There are numerous examples of TMDCs, so in the future, the researchers would like to determine the best one to use for computer technology.

Picosun Group, a global provider of ALD (Atomic Layer Deposition) thin film coating solutions, and Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) report excellent quality titanium nitride (TiN) deposited with Picosun’s plasma-ALD technology.

In microelectronic component manufacturing, the ohmic contact between metallic and semiconducting material layers is critical regarding the component functionality and lifetime. Typically, pure metals such as titanium have been employed as the metallic material, but they have certain drawbacks which is why titanium nitride has been proposed as the substitute. TiN is metallic as well, and its conductivity and thermal stability are better than those of pure titanium metal, but to obtain high quality TiN films, the manufacturing method and conditions are critical.

This is where Picosun’s remote plasma ALD (RPEALD) technology shows its strength. In Picosun’s approach, the plasma source is located on a high enough distance from the substrate, so that instead of aggressive ion bombardment, highly reactive radicals react at the substrate surface. This allows low process temperatures without thermal stress or physical ion damage to the substrate and enables deposition of also conductive materials without the risk of short-circuiting, or gas back-diffusion into the plasma source. The right selection of precursor chemicals and plasma gases guarantees high purity TiN films with very low oxygen content and work function, low sheet resistivity, exact stoichiometry, and high uniformity (*). Furthermore, the process window is wide regarding the process parameters and temperature, enabling the process to be introduced on a large variety of substrate materials.

“We are happy to report these excellent TiN results to our customers in micro- and optoelectronic industries. TiN is a central material in their applications, especially in components manufactured on GaN and on small, up to 200 mm diameter Si wafers. Picosun is specially dedicated to providing cost-efficient, turn-key production solutions for up to 200 mm wafer markets. We would like to welcome you all to meet us at the 4th China ALD conference which takes place 14-17 October 2018 in the city of Shenzhen, and where we are again the platinum sponsors, to discuss further how our ALD technology could improve your products and enable new breakthroughs in your industry,” say Mr. Edwin Wu, CEO of Picosun Asia Pte. Ltd. and Mr. Jurgen Yeh, CTO of Picosun China Co. Ltd.

“It is always a pleasure to work with Picosun. The quality of their ALD equipment is outstanding and enables us to develop cutting-edge ALD processes to be introduced to our other collaboration partners in the industries. An immensely important benefit in using PICOSUN™ ALD tools is also the smooth scalability of the processes to production scale, as all PICOSUN™ ALD systems, from R&D units to full-scale industrial production platforms share the same core design and operating principles,” continues Prof. Sunan Ding from the Nano-X lab of SINANO.

SINANO and Picosun have been collaborating since the beginning of 2017. The goal of the collaboration is to develop advanced micro- and optoelectronic components such as HEMTs (high-electron mobility transistors) and laser diodes, and lithium ion batteries utilizing ALD in their joint lab in Suzhou, one of China’s most prominent hubs for electronics and other high-tech products manufacturing. The lab is equipped with several state-of-the-art PICOSUN™ ALD systems. The collaboration is further supported by Picosun’s local subsidiary, Picosun China Co. Ltd. also located in Suzhou.

The prevalence of electronic devices has transformed life in the 21st century. At the heart of these devices is the movement of electrons across materials. Scientists today continue to discover new ways to manipulate and move electrons in a quest for making faster and better functioning devices.

Scientists from the Femtosecond Spectroscopy Unit led by Prof. Keshav Dani at the Okinawa Institute of Science and Technology Graduate University (OIST) have demonstrated a new mechanism that can potentially allow the control of electrons on the nanometer (10-9 of a meter) spatial scale and femtosecond (10-15 of a second) temporal scales using light. The study has been published in the journal Science Advances.

When a voltage is applied across semiconducting materials, an electric field is generated that directs the flow of electrons through the materials. Dr. E Laine Wong, a recent PhD graduate at OIST, and her colleagues have used a physical phenomenon called surface photovoltage effect, to induce electric fields on the material surface allowing them to. Surface photovoltage effect is an effect where the surface potential of the materials can be varied by changing the light intensity. “By making use of the nonuniform intensity profile of a laser beam, we manipulate the local surface potentials to create a spatially varying electric field within the photoexcitation spot. This allows us to control electron flow within the optical spot,” says E Laine.

Using a combination of femtosecond spectroscopy with electron microscopy techniques, E Laine and her colleagues made a movie of the flow of electrons on femtosecond timescales. Typically, in femtosecond spectroscopy, an ultrafast laser beam known as the ‘pump’ is first used to excite the electrons in the sample. A second ultrafast laser beam known as the ‘probe’ is then shone upon the sample to track the evolution of the excited electrons. This technique, also known as pump-probe spectroscopy, has allowed the scientists to study the dynamics of the excited electrons at a very short time scale. The combination of an electron microscope then further provides the scientists with the spatial resolution required to directly image the movement of the excited electrons even within the small area of the laser beam spot. “The combination of these two methods with both high spatial and temporal resolutions has allowed us to record a movie of the electrons being directed to flow in opposite directions,” says E Laine.

The findings of the study are also promising to control the movement of electrons beyond the resolution limit of light by utilizing the spatial intensity variations of the laser beam within the focal spot. The mechanism could therefore be potentially used to operate nanoscale electronic circuits. Prof. Dani and his team are now working towards building a functional nanoscale ultrafast device based on this newfound mechanism.