Category Archives: Process Materials

SEMI, the global industry association representing the electronics manufacturing supply chain, today announced that the global semiconductor materials market increased 2.4 percent in 2016 compared to 2015 while worldwide semiconductor revenues increased 1.1 percent.

According to the SEMI Material Market Data Subscription, total wafer fabrication materials and packaging materials were $24.7 billion and $19.6 billion, respectively. Comparable revenues for these segments in 2015 were $24.0 billion for wafer fabrication materials and $19.3 billion for packaging materials. The wafer fabrication materials segment increased 3.1 percent year-over-year, while the packaging materials segment increased 1.4 percent.

For the seventh consecutive year, Taiwan was the largest consumer of semiconductor materials due to its large foundry and advanced packaging base, totaling $9.8 billion. Korea and Japan maintained the second and third places, respectively, while China rose in the rankings to claim the fourth spot during the same time. Annual revenue growth was the strongest in the China, Taiwan, and Japan markets. The materials market in Europe, Rest of World (ROW) and South Korea experienced nominal growth, while the materials market in North America contracted. (The ROW region is defined as Singapore, Malaysia, Philippines, other areas of Southeast Asia and smaller global markets.)

2015 and 2016 Regional Semiconductor Materials Markets (US$ Billions)

Region 2015* 2016 % Change
Taiwan

9.42

9.79

3.9%
South Korea

7.09

7.11

0.2%
Japan

6.56

6.74

2.8%
China

6.08

6.53

7.3%
Rest of World

6.09

6.12

0.6%
North America

4.97

4.90

-1.4%
Europe

3.07

3.12

1.5%
Total

43.29

44.32

2.4%

Source: SEMI, April 2017 Note: Figures may not add due to rounding.
* 2015 data have been updated based on SEMI’s data collection programs

A new way to grow narrow ribbons of graphene, a lightweight and strong structure of single-atom-thick carbon atoms linked into hexagons, may address a shortcoming that has prevented the material from achieving its full potential in electronic applications. Graphene nanoribbons, mere billionths of a meter wide, exhibit different electronic properties than two-dimensional sheets of the material.

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

“Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at the Department of Energy’s Oak Ridge National Laboratory. Graphene in sheets is an excellent electrical conductor, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.

Previous efforts to make graphene nanoribbons employed a metal substrate that hindered the ribbons’ useful electronic properties.

Now, scientists at ORNL and North Carolina State University report in the journal Nature Communications that they are the first to grow graphene nanoribbons without a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor into a graphene nanoribbon. At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells.

“Graphene is wonderful, but it has limits,” said Li. “In wide sheets, it doesn’t have an energy gap–an energy range in a solid where no electronic states can exist. That means you cannot turn it on or off.”

When a voltage is applied to a sheet of graphene in a device, electrons flow freely as they do in metals, severely limiting graphene’s application in digital electronics.

“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.”

In very narrow graphene nanoribbons, with a width of a nanometer or even less, how structures terminate at the edge of the ribbon is important too. For example, cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor. Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.

To grow graphene nanoribbons with controlled width and edge structure from polymer precursors, previous researchers had used a metal substrate to catalyze a chemical reaction. However, the metal substrate suppresses useful edge states and shrinks the desired band gap.

Li and colleagues set out to get rid of this troublesome metal substrate. At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, they used the tip of a scanning tunneling microscope to inject either negative charge carriers (electrons) or positive charge carriers (“holes”) to try to trigger the key chemical reaction. They discovered that only holes triggered it. They were subsequently able to make a ribbon that was only seven carbon atoms wide–less than one nanometer wide–with edges in the armchair conformation.

“We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction,” Li said. Moving the tip along the polymer chain, the researchers could select where they triggered this reaction and convert one hexagon of the graphene lattice at a time.

Next, the researchers will make heterojunctions with different precursor molecules and explore functionalities. They are also eager to see how long electrons can travel in these ribbons before scattering, and will compare it with a graphene nanoribbon made another way and known to conduct electrons extremely well. Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.

“It’s a way to tailor physical properties for energy applications,” Li said. “This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level.” Plus, the process could be scaled up and automated.

Researchers at North Carolina State University have developed a technique for converting positively charged (p-type) reduced graphene oxide (rGO) into negatively charged (n-type) rGO, creating a layered material that can be used to develop rGO-based transistors for use in electronic devices.

“Graphene is extremely conductive, but is not a semiconductor; graphene oxide has a bandgap like a semiconductor, but does not conduct well at all — so we created rGO,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and corresponding author of a paper describing the work. “But rGO is p-type, and we needed to find a way to make n-type rGO. And now we have it for next-generation, two-dimensional electronic devices.”

Specifically, Narayan and Anagh Bhaumik — a Ph.D. student in his lab — demonstrated two things in this study. First, they were able to integrate rGO onto sapphire and silicon wafers — across the entire wafer.

Second, the researchers used high-powered laser pulses to disrupt chemical groups at regular intervals across the wafer. This disruption moved electrons from one group to another, effectively converting p-type rGO to n-type rGO. The entire process is done at room temperature and pressure using high-power nanosecond laser pulses, and is completed in less than one-fifth of a microsecond. The laser radiation annealing provides a high degree of spatial and depth control for creating the n-type regions needed to create p-n junction-based two-dimensional electronic devices.

The end result is a wafer with a layer of n-type rGO on the surface and a layer of p-type rGO underneath.

This is critical, because the p-n junction, where the two types meet, is what makes the material useful for transistor applications.

A new class of carbon nanotubes could be the next-generation clean-up crew for toxic sludge and contaminated water, say researchers at Rochester Institute of Technology.

Single-walled carbon nanotubes filter dirty water in experiments at RIT. Credit: John-David Rocha and Reginald Rogers

Single-walled carbon nanotubes filter dirty water in experiments at RIT. Credit: John-David Rocha and Reginald Rogers

Enhanced single-walled carbon nanotubes offer a more effective and sustainable approach to water treatment and remediation than the standard industry materials–silicon gels and activated carbon–according to a paper published in the March issue of Environmental Science Water: Research and Technology.

RIT researchers John-David Rocha and Reginald Rogers, authors of the study, demonstrate the potential of this emerging technology to clean polluted water. Their work applies carbon nanotubes to environmental problems in a specific new way that builds on a nearly two decades of nanomaterial research. Nanotubes are more commonly associated with fuel-cell research.

“This aspect is new–taking knowledge of carbon nanotubes and their properties and realizing, with new processing and characterization techniques, the advantages nanotubes can provide for removing contaminants for water,” said Rocha, assistant professor in the School of Chemistry and Materials Science in RIT’s College of Science.

Rocha and Rogers are advancing nanotube technology for environmental remediation and water filtration for home use.

“We have shown that we can regenerate these materials,” said Rogers, assistant professor of chemical engineering in RIT’s Kate Gleason College of Engineering. “In the future, when your water filter finally gets saturated, put it in the microwave for about five minutes and the impurities will get evaporated off.”

Carbon nanotubes are storage units measuring about 50,000 times smaller than the width of a human hair. Carbon reduced to the nanoscale defies the rules of physics and operates in a world of quantum mechanics in which small materials become mighty.

“We know carbon as graphite for our pencils, as diamonds, as soot,” Rocha said. “We can transform that soot or graphite into a nanometer-type material known as graphene.”

A single-walled carbon nanotube is created when a sheet of graphene is rolled up. The physical change alters the material’s chemical structure and determines how it behaves. The result is “one of the most heat conductive and electrically conductive materials in the world,” Rocha said. “These are properties that only come into play because they are at the nanometer scale.”

The RIT researchers created new techniques for manipulating the tiny materials. Rocha developed a method for isolating high-quality, single-walled carbon nanotubes and for sorting them according to their semiconductive or metallic properties. Rogers redistributed the pure carbon nanotubes into thin papers akin to carbon-copy paper.

“Once the papers are formed, now we have the adsorbent–what we use to pull the contaminants out of water,” Rogers said.

The filtration process works because “carbon nanotubes dislike water,” he added. Only the organic contaminants in the water stick to the nanotube, not the water molecules.

“This type of application has not been done before,” Rogers said. “Nanotubes used in this respect is new.”

Hydrogen is both the simplest and the most-abundant element in the universe, so studying it can teach scientists about the essence of matter. And yet there are still many hydrogen secrets to unlock, including how best to force it into a superconductive, metallic state with no electrical resistance.

“Although theoretically ideal for energy transfer or storage, metallic hydrogen is extremely challenging to produce experimentally,” said Ho-kwang “Dave” Mao, who led a team of physicists in researching the effect of the noble gas argon on pressurized hydrogen.

It has long been proposed that introducing impurities into a sample of molecular hydrogen, H2, could help ease the transition to a metallic state. So Mao and his team set out to study the intermolecular interactions of hydrogen that’s weakly-bound, or “doped,” with argon, Ar(H2)2, under extreme pressures. The idea is that the impurity could change the nature of the bonds between the hydrogen molecules, reducing the pressure necessary to induce the nonmetal-to-metal transition. Previous research had indicated that Ar(H2)2 might be a good candidate.

Surprisingly, they discovered that the addition of argon did not facilitate the molecular changes needed to initiate a metallic state in hydrogen. Their findings are published by the Proceedings of the National Academy of Sciences.

The team brought the argon-doped hydrogen up to 3.5 million times normal atmospheric pressure–or 358 gigapascals–inside a diamond anvil cell and observed its structural changes using advanced spectroscopic tools.

What they found was that hydrogen stayed in its molecular form even up to the highest pressures, indicating that argon is not the facilitator many had hoped it would be.

“Counter to predictions, the addition of argon did not create a kind of ‘chemical pressure’ on the hydrogen, pushing its molecules closer together. Rather, it had the opposite effect,” said lead author Cheng Ji.

Transition metal oxides (TMO) are extensively studied, technologically important materials, due to their complex electronic interactions, resulting in a large variety of collective phenomena. Memory effects in TMO’s have garnered a huge amount of interest, being both of fundamental scientific interest and technological significance.

Dr. Amos Sharoni of Bar-Ilan University’s Department of Physics, and Institute of Nanotechnology and Advanced Materials (BINA), has now uncovered a new kind of memory effect, unrelated to memory effects previously reported.

Dr. Sharoni, together with his student Naor Vardi, and supported by theoretical modelling by Yonatan Dubi of Ben-Gurion University in the Negev, utilized a simple experimental design to study changes in the properties of two TMOs, VO2 and NdNiO3, which undergo a metal-insulator phase-transition. Their results, just published in the journal Advanced Materials, not only demonstrate a new phenomenon but, importantly, also provide an explanation of its origin.

Ramp reversal memory

Metal-insulator transitions are transitions from a metal (material with good electrical conductivity of electric charges) to an insulator (material where conductivity of charges is quickly suppressed). These transitions can be achieved by a small variation of external parameters such as pressure or temperature.

In Sharoni’s experiment, when heated the studied TMOs transit from one state to another, and their properties undergo a change, beginning in a small area where “islands” develop and then grow, and vice-versa during cooling, similar to the coexistence of ice and water during melting. Sharoni cooled his samples while transition was in process, and then examined what happened when they were reheated. He found that when the reheated metal-oxide reached the temperature point at which re-cooling had occurred, that is, in the phase coexistence state – an increase in resistance was measured. And this increase in resistance was observed at each different point at which cooling was initiated. This previously unknown and surprising phenomenon demonstrates the creation of a “memory”.

Sharoni explains: “When the temperature ramp is reversed, and the sample is cooled rather than heated, the direction change creates a “scar” wherever there is a phase-boundary between the conducting and insulating islands. The ramp reversal sequence “encrypts” in the TMO a “memory” of the reversal temperature, which is manifested as increased resistance”. Moreover, it is possible to create and store more than one “memory” in the same physical space.

Sharoni likens the creation of a “scar” to the motion of waves on the seashore. A wave rushes up the beach and as it recedes it leaves a small sandy mound at the furthest point that it reached. When the wave returns it slows and brakes as it reaches the mound obstacle in its path. However, if a strong wave follows, it rushes over the mound and destroys it. Similarly, Sharoni found that further heating the TMO enables it to complete transition and to cross the scarred boundaries, “healing” the scars and immediately erasing the memory. In contrast cooling does not erase them.

Technology and security

The results of Sharoni’s work will have important impact on additional research, both experimental and theoretical, and the simplicity of the experimental design will enable other groups studying relevant systems to perform similar measurements with ease.

The multi-state nature of the memory effect, whereby more than one piece of information can coexist in the same space, could be harnessed for memory technology. And while deleted computer data is not secure and can be recovered, at least partially, by talented hackers, the “erase-upon-reading” property of this system could make an invaluable contribution to security technologies.

Princeton researchers have discovered a new form of the simple compound GeSe that has surprisingly escaped detection until now. This so-called beta-GeSe compound has a ring type structure like graphene and its monolayer form could have similarly valuable properties for electronic applications, according to the study published in the Journal of the American Chemical Society.

Graphene has been hailed as a two-dimensional wonder material for electronics but its lack of a band gap has hindered its development for devices. As such, a closely related material, black phosphorus, has been receiving intense research attention because it has a small band gap and a high charge carrier mobility, and can easily be reduced to nanometer thicknesses. The researchers calculated that GeSe is highly analogous to black phosphorus and can be considered a pseudo-group-V element.

This is the building blocks of graphene, black phosphorus, α-GeSe, and β-GeSe. Credit: Cava lab

This is the building blocks of graphene, black phosphorus, α-GeSe, and β-GeSe. Credit: Cava lab

Under extreme pressure, black phosphorus is transformed into a simple cubic form, so the team wondered if the same could be done to GeSe and heated the abundant alpha-GeSe form of the compound to 1200 °C under 6 GPa of pressure or 60,000 times atmospheric pressure.

“What we found was not only a new kind of GeSe–which is already unconventional by itself in that you rarely find new binary compounds anymore–but that it has this uncommon ‘boat’ conformation that we were amazed by,” said first author of the study Fabian von Rohr, a postdoctoral researcher in the laboratory of Robert Cava, the Russell Wellman Moore Professor of Chemistry.

beta-GeSe’s rare “boat” form is likely stabilized by the slightly smaller distance between its layers, while black phosphorus and alpha-GeSe exist in standard “chair” conformations. The difference in structures gives rise to the compounds’ different electronic properties. The researchers found that beta-GeSe possesses a band gap size in between that of black phosphorus and alpha-GeSe, which could prove promising for future applications. GeSe is also an attractive material for electronics because it’s robust under ambient conditions while black phosphorus is reactive to both air and water.

This article originally appeared on SemiMD.com and was featured in the March 2017 issue of Solid State Technology.

By Ed Korczynski, Sr. Technical Editor

As detailed in Part 1 of this article published last month by SemiMD, the inaugural Critical Materials Council (CMC) Conference happened May 5-6 in Hillsboro, Oregon. Held just after the yearly private CMC meeting, the public CMC Conference provides a forum for the pre-competitive exchange of information to control the supply-chain of critical materials needed to run high-volume manufacturing (HVM) in IC fabs. The next CMC Conference will happen May 11-12 in Dallas, Texas.

At the end of the 2016 conference, a panel discussion moderated by Ed Korczynski was recorded and transcribed. The following is Part 2 of the conversation between the following industry experts:

  • Jean-Marc Girard, CTO and Director of R&D, Air Liquide Advanced Materials,
  • Jeff Hemphill, Staff Materials R&D Engineer, Intel Corporation,
  • Jonas Sundqvist, Sr. Scientist, Fraunhofer IKTS; and co-chair of ALD Conference, and
  • John Smythe, Distinguished Member of Technical Staff, Micron Technology.

KORCZYNSKI:  We heard from David Thompson [EDITOR’S NOTE:  Director of Process Chemistry, Applied Materials presented on “Agony in New Material Introductions –  Minimizing and Correlating Variabilities”] today on what we must control, and he gave an example of a so-called trace-contaminant that was essential for the process performance of a precursor, where the trace compound helped prevent particles from flaking off chamber walls. Do we need to specify our contaminants?

GIRARD:  Yes. To David’s point this morning, every molecule is different. Some are very tolerant due to the molecular process associated with it, and some are not. I’ll give you an example of a cobalt material that’s been talked about, where it can be run in production at perhaps 95% in terms of assay, provided that one specific contaminant is less than a couple of parts-per-million. So it’s a combination of both, it’s not assay OR a specification of impurities. It’s a matter of specifying the trace components that really matter when you reach the point that the data you gather gives you that understanding, and obviously an assay within control limits.

HEMPHILL:  Talking about whether we’re over-specifying or not, the emphasis is not about putting the right number on known parameters like assay that are obvious to measure, the emphasis is on identifying and understanding what makes up the rest of it and in a sense trying over-specify that. You identify through mass-spectrometry and other techniques that some fraction of a percent is primarily say five different species, it’s finding out how to individually monitor and track and control those as separate parameters. So from a specification point of view what we want is not necessarily the lowest possible numbers, but it’s expanding how many things we’re looking at so that we’re capturing everything that’s there.

KORCZYNSKI:  Is that something that you’re starting to push out to your suppliers?

HEMPHILL:  Yes. It depends on the application we’re talking about, but we go into it with the assumption that just assay will not be enough. Whether a single molecule or a blend of things is supposed to be there, we know that just having those be controlled by specification will not be sufficient. We go under the assumption that we are going to identify what makes up the remaining part of the profile, and those components are going to need to be controlled as well.

KORCZYNSKI:  Is that something that has changed by node? Back when things were simpler say at 45nm and larger, were these aspects of processing that we could safely ignore as ‘noise’ but are now important ‘signals’?

HEMPHILL:  Yes, we certainly didn’t pay as close attention just a couple of generations ago.

KORCZYNSKI:  That seems to lead us to questions about single-sources versus dual-sourcing. There are many good reasons to do both, but not simultaneously. However, it seems that because of all of the challenges we’re heard about over the last day-and-a-half of this conference it creates greater burden on the suppliers, and for critical materials the fabs are moving toward more single-sourcing over time.

SMYTHE:  I think that it comes down to more of a concern over geographic risk. I’ll buy from one entity if that entity has more than one geographic location for the supply, so that I’m not exposed to a single ‘Act of God’ or a ‘random statistical occurrence of global warming.’ So for example I  need to ask if a supplier has a place in the US and a place in France that makes the same thing, so that if something bad happens in one location it can still be sourced? Or do you have an alternate-supply agreement that if you can’t supply it you have an agreement with Company-X to supply it so that you still have control? You can’t come to a Micron and say we want to make sure that we get at minimum 25% no matter what, because what typically happens with second-sourcing is Company-A gets 75% of the business while Company-B gets 25%. There are a lot of reasons that that doesn’t work so well, so people may have an impression that there’s a movement toward single-source but it’s ‘single flexible-source.’

HEMPHILL:  There are a lot of benefits of dual- or multiple-sourcing. The commercial benefits of competition can be positive and we’re for it when it works. The risk is that as things are progressing and we’re getting more sensitive to differences in materials it’s getting harder to maintain that. We have seen situations where historically we were successful with dual-sourcing a raw material coming from two different suppliers or even a single supplier using two different manufacturing lines and everything was fine and qualified and we could alternate sources invisibly. However, as our sensitivity has grown over time we can start to detect differences.

So the concept of being ‘copy-exactly’ that we use in our factories, we really need production lines to do that, and if we’re talking about two different companies producing the same material then we’re not going to get them to be copy-exactly. When that results in enough of a variation in the material that we can detect it in the factory then we cannot rely upon two sources. Our preference would be one company that maintains multiple production sites that are designed to be exactly the same, then we have a high degree of confidence that they will be able to produce the same material.

GIRARD:  I can give you a supplier perspective on that. We are seeing very different policies from different customers, to the point that we’re seeing an increase in the number of customers doing single-sourcing with us, provided we can show the ability to maintain business continuity in case of a problem. I think that the industry became mature after the tragic earthquake and tsunami in Japan in 2011 with greater understanding of what business continuity means. We have the same discussions with our own suppliers, who may say that they have a dedicated reactor for a certain product with another backup reactor with a certain capacity on the same site, and we ask what happens if the plant goes on strike or there’s a fire there?

A situation where you might think the supply was stable involved silane in the United States. There are two large silane plants in the United States that are very far apart from each other and many Asian manufacturers dependent upon them. When the U.S. harbors went on strike for a long time there was no way that material could ship out of the U.S. customers. So, yes there were two plants but in such an event you wouldn’t have global supply. So there is no one way to manage our supply lines and we need to have conversations with our customers to discuss the risks. How much time would it take to rebuild a supply-chain source with someone else? If you can get that sort of constructive discussion going then customers are usually open to single-sourcing. One regional aspect is that Asian customers tend to favor dual-sourcing more, but that can lead to IP problems.

[DISCLOSURE:  Ed Korczynski is co-chair of the CMC Conference, and Marketing Director of TECHCET CA the advisory services firm that administers the Critical Materials Council (CMC).]

—E.K.

For several years, a team of researchers at The University of Texas at Dallas has investigated various materials in search of those whose electrical properties might make them suitable for small, energy-efficient transistors to power next-generation electronic devices.

They recently found one such material, but it was nothing anyone expected.

In an article published online March 10 in the journal Advanced Materials, Dr. Moon Kim and his colleagues describe a material that, when heated to about 450 degrees Celsius, transforms from an atomically thin, two-dimensional sheet into an array of one-dimensional nanowires, each just a few atoms wide.

An image caught in mid-transformation looks like a tiny United States flag, and with false colors added, is arguably the world’s smallest image of Old Glory, Kim said.

This tiny US flag -- just a few nanometers wide and invisible to the naked eye -- is arguably the world's smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the "stripe" material -- molybdenum ditelluride -- were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures -- the 'stars' in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

This tiny US flag — just a few nanometers wide and invisible to the naked eye — is arguably the world’s smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the “stripe” material — molybdenum ditelluride — were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures — the ‘stars’ in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

“The phase transition we observed, this new structure, was not predicted by theory,” said Kim, the Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas.

Because the nanowires are semiconductors, they might be used as switching devices, just as silicon is used in today’s transistors to turn electric current on and off in electronic devices.

“These nanowires are about 10 times smaller than the smallest silicon wires, and, if used in future technology, would result in powerful energy-efficient devices,” Kim said. The lead authors of the study are Hui Zhu and Qingxiao Wang, graduate students in materials science and engineering in the Erik Jonsson School of Engineering and Computer Science.

Just a Phase?

When certain materials are subjected to changes in external conditions, such as temperature or pressure, they can undergo a phase transition. A familiar example is when liquid water is cooled to form a solid (ice), or heated to form a gas (steam).

For many materials, however, a phase transition means something a little different. As external temperature and pressure change, these materials’ atoms rearrange and redistribute to make a material with a different structure and composition. These changes can affect the new material’s properties, such as how electrons move through it. For scientists interested in new applications for materials, understanding such transitions is paramount.

In most cases, a type of graphic called a phase diagram helps researchers predict structural and property changes in a material when it undergoes a phase transition.

But nothing predicted what Kim’s team observed as it conducted experiments on a material called molybdenum ditelluride.

Nanoflags and Nanoflowers

Using a transmission electron microscope, the researchers started with atomically thin, two-dimensional sheets of molybdenum ditelluride, a material made up of one layer of molybdenum atoms and two layers of tellurium atoms. The material belongs to a class called transition metal dichalcogenides (TMDs), which show promise in replacing silicon in transistors.

“We wanted to understand the thermal stability of this particular material,” Kim said. “We thought it was a good candidate for next-generation nanoelectronics. Out of curiosity, we set out to see whether it would be stable above room temperature.”

When they increased the temperature to above 450 degrees Celsius, two things happened.

“First, we saw a new pattern begin to emerge that was aesthetically pleasing to the eye,” Kim said. Across the surface of the sample, the repeating rows, or stripes, of molybdenum ditelluride layers began to transform into shapes that looked like tiny six-pointed stars, or flowers with six petals.

The material was transitioning into hexa-molybdenum hexa-telluride, a one-dimensional wire-like structure. The cross section of the new material is a structure consisting of six central atoms of molybdenum surrounded by six atoms of tellurium.

As the phase transition progressed, part of the sample was still “stripes” and part had become “stars.” The team thought the pattern looked like a United States flag. They made a false-color version with a blue field behind the stars and half of the stripes colored red, to make a “nanoflag.”

Not in the textbooks

“Then, when we examined the material more closely, we found that the transition we were seeing from ‘stripes’ to ‘stars’ was not in any of the phase diagrams,” Kim said. “Normally, when you heat up particular materials, you expect to see a different kind of material emerge as predicted by a phase diagram. But in this case, something unusual happened — it formed a whole new phase.”

Each individual nanowire is a semiconductor, which means that electric current moving through the wire can be switched on and off, Kim said. When many of the individual nanowires are grouped together in bulk they behave more like a metal, which easily conducts current.

“We would want to use the nanowires one at a time because we are pushing the size of a transistor as small as possible,” Kim said. “Currently, the smallest transistor size is about 10 times larger than our nanowire. Each of ours is smaller than 1 nanometer in diameter, which is essentially an atomic-scale wire.

“Before we can put this discovery to use and make an actual device, we have many more studies to do, including determining how to separate out the individual nanowires, and overcoming technical challenges to manufacturing and mass production,” Kim said. “But this is a start.”

To realize the next generation of devices for information processing based on new phenomena such as spintronics, multiferroics, magnetooptics, and magnonics, their constituent materials need to be developed. Recent rapid progress in nanotechnology allows us to fabricate nanostructures that are impossible to obtain in nature.

However, complex magnetic oxides are one of the most complicated material systems in terms of development and analysis. In addition, the detailed mechanism is unknown by which changes in atomic composition that do not affect overall structure lead to drastic changes in material characteristics even though the material structure is similar.

Now, researchers at Spin Electronics Group at Toyohashi Tech and at Myongji University, Harbin Institute of Technology, Massachusetts Institute of Technology, Universidad Técnica Federico Santa María, University of California, San Diego, and Trinity College Dublin found that nanoscale pillar-shaped distribution of iron in strontium titanate (STF) changes its magnetic and magnetooptical response drastically. Surprisingly, the polycrystalline sample showed stronger magnetism than single crystalline film.

Image of nanopillar-like poly-crystalline STF film obtained by transmission electron microscopy. Credit: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

Image of nanopillar-like poly-crystalline STF film obtained by transmission electron microscopy.
Credit: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

“In usual oxide systems, magnetic and magnetooptical effects are stronger in highly ordered structures. In other words, single crystalline material is better for obtaining better magnetic properties,” explains Assistant Professor Taichi Goto, “However, iron-substituted strontium titanate deposited at certain oxygen pressure is different.”

The STF films were prepared by pulsed laser deposition at various pressures directly on silicon substrate, and crystalline structure and magnetic properties were characterized systematically. A sample deposited at a certain pressure showed significantly stronger magnetism and larger Faraday rotation angle (magnetooptical effects) at room temperature. Several tests analyzing the oxygen stoichiometry and the corresponding Fe valence states, the structure and strain state, and the presence of small-volume fractions of iron revealed that the nanostructure and clustering of the elements enhanced magnetism.

These results show the broad possibility of polycrystalline films being used in silicon-based devices. In this paper, the integration of STF film with 0.1 mm scale optical resonator was demonstrated. Further, the integration of such novel oxides with conventional device concepts would pave a way for interesting systems in the future.