Category Archives: Process Materials

Gallium nitride (GaN) has emerged as one of the most important and widely used semiconducting materials. Its optoelectronic and mechanical properties make it ideal for a variety of applications, including light-emitting diodes (LEDs), high-temperature transistors, sensors and biocompatible electronic implants in humans.

In 2014, three Japanese scientists won the Nobel Prize in physics for discovering GaN’s critical role in generating blue LED light, which is required, in combination with red and green light, to produce white LED light sources.

Now, four Lehigh engineers have reported a previously unknown property for GaN: Its wear resistance approaches that of diamonds and promises to open up applications in touch screens, space vehicles and radio-frequency microelectromechanical systems (RF MEMS), all of which require high-speed, high-vibration technology.

The researchers reported their findings in August in Applied Physics Letters (APL) in an article titled “Ultralow wear of gallium nitride.” The article’s authors are Guosong Zeng, a Ph.D. candidate in mechanical engineering; Nelson Tansu, Daniel E. ’39 and Patricia M. Smith Endowed Chair Professor in the Electrical and Computer Engineering department, and Director of the Center for Photonics and Nanoelectronics (CPN); Brandon A. Krick, assistant professor of mechanical engineering and mechanics; and Chee-Keong Tan ’16 Ph.D., now assistant professor of electrical and computer engineering at Clarkson University.

GaN’s electronic and optical properties have been studied extensively for several decades, said Zeng, the lead author of the APL article, but virtually no studies have been done of its tribological properties, that is, its resistance to the mechanical wear imposed by reciprocated sliding.

“Our group is the first to investigate the wear performance of GaN,” said Zeng. “We have found that its wear rate approaches that of diamonds, the hardest material known.”

Wear rate is expressed in negative cubic millimeters of Newton meters (Nm). The rate for chalk, which has virtually no wear resistance, is on the order of 10 2 mm3/Nm, while that of diamonds is between 10-9 and 10-10, making diamonds eight orders of magnitude more wear resistant than chalk. The rate for GaN ranges from 10¬-7 to 10-9, approaching the wear resistance of diamonds and three to five orders of magnitude more wear resistant than silicon (10-4).

The Lehigh researchers measured the wear rate and friction coefficients of GaN using a custom microtribometer to perform dry sliding wear experiments. They were surprised by the results.

“When performing wear measurements of unknown materials,” they wrote in APL, “we typically slide for 1,000 cycles, then measure the wear scars; [these] experiments had to be increased to 30,000 reciprocating cycles to be measurable with our optical profilometer.

“The large range in wear rates (about two orders of magnitude)…can provide insight into the wear mechanisms of GaN.”

That range in wear resistance, the researchers said, is caused by several factors, including environment, crystallographic direction and, especially, humidity.

“The first time we observed the ultralow wear rate of GaN was in winter,” said Zeng. “These results could not be replicated in summer, when the material’s wear rate increased by two orders of magnitude.”

To determine how the higher summer humidity was affecting GaN’s wear performance, the researchers put their tribometer in a glove box that can be backfilled with either nitrogen or humid air.

“We observed that as we increased the humidity inside the glove box, we also increased the wear rate of GaN,” said Zeng.

Zeng gave a presentation about the Lehigh project in October at the International Workshop on Nitride Semiconductors (IWN 2016) in Orlando, Florida. The session at which he spoke was titled “Wear of Nitride Materials and Properties of GaN-based structures.” Zeng was one of seven presenters at the session and the only one to discuss the wear properties of GaN and other III-Nitride materials.

Tansu, who has studied GaN for more than a decade, and Krick, a tribology expert, became curious about GaN’s wear performance several years ago when they discussed their research projects after a Lehigh faculty meeting.

“Nelson asked me if anyone had ever investigated the friction and wear properties of gallium nitride,” said Krick, “and I said I didn’t know. We checked later and found a wide-open field.”

Tansu said the group’s discovery of GaN’s hardness and wear performance could have a dramatic effect on the electronic and digital device industries. In a device such as a smartphone, he said, the electronic components are housed underneath a protective coating of glass or sapphire. This poses potential compatibility problems which could be avoided by using GaN.

“The wear resistance of GaN,” said Tansu, “gives us the opportunity to replace the multiple layers in a typical semiconductor device with one layer made of a material that has excellent optical and electrical properties and is wear-resistant as well.

“Using GaN, you can build an entire device in a platform without multiple layers of technologies. You can integrate electronics, light sensors and light emitters and still have a mechanically robust device. This will open up a new paradigm for designing devices. And because GaN can be made very thin and still strong, it will accelerate the move to flexible electronics.”

In addition to its unexpectedly good wear performance, said Zeng, GaN also has a favorable radiation hardness, which is an important property for the solar cells that power space vehicles. In outer space, these solar cells encounter large quantities of very fine cosmic dust, along with x-rays and gamma rays, and thus require a wear-resistant coating, which in turn needs to be compatible with the cell’s electronic circuitry. GaN provides the necessary hardness without introducing compatibility issues with the circuitry.

The Lehigh group has begun collaborating with Bruce E. Koel, a surface chemistry expert and professor of chemical and biological engineering at Princeton University, to gain a better understanding of the interaction of GaN and water under contact. Koel was formerly a chemistry professor and vice president for research and graduate studies at Lehigh.

To determine the evolution of wear with GaN, the group has subjected GaN to stresses by running slide tests in which the slide distance and the corresponding number of cycles are varied. The group then uses an x-ray photoelectron spectrometer (XPS), which can identify the elemental composition of the first 12 nanometers of a surface, to scan the unworn surface of the GaN, the scar created by the slide machine, and the wear particles deposited by the slide machine on either side of the scar.

The group plans next to use aberration-corrected transmission electron microscopy to examine the lattice of atoms beneath the scar. Meanwhile, they will simulate a test in which the lattice is strained with water in order to observe the variations caused by deforming energy.

“This is a very new experiment,” said Zeng. “It will enable us to see dynamic surface chemistry by watching the chemical reaction that results when you apply shear, tensile or compressive pressure to the surface of GaN.”

Solid particles in the abatement exhaust must be properly managed, and in some cases, substantially reduced from the gas stream before it is released into the environment.

BY CHRIS JONES, Edwards Vacuum, Ltd., Clevedon, U.K.

Many semiconductor manufacturing processes create solid particles in the process exhaust. Like other exhaust contaminants, these must be properly managed, and in many cases, removed from the stream before it is released into the environment. The permitted release levels vary for particles of different sizes and compositions, depending on their toxicity or potential to damage the environment. Regulations governing particle releases are evolving rapidly. However, the management of particulate flows in process exhaust is also important due to its potential impact on the process itself. Left unmanaged, particulate accumulations can result in shut downs for unplanned maintenance, excessive and premature wear and costly repairs, all of which directly affect the profitability of the manufacturing operation.

Solids may be formed in the exhaust stream of a semiconductor manufacturing process from a number of sources. One important source, though not the focus of this discussion, is the condensation of process gases in vacuum pump exhausts. If not controlled with a thermal management system (e.g. Smart TMS, Edwards) that maintains the pipe surfaces at a sufficiently high temperature, this condensation can quickly accumulate and force a halt to the manufacturing process. This article will discuss issues further downstream in the abatement process, where toxic volatile compounds are converted to more benign forms, some of which form solid particles that must then be removed from the exhaust gases. Many of these solids are oxides formed when gases, such as tungsten hexaflu- oride, silane, organo- and halo- silanes and others, are exposed to heat, air, and water. The particles are typically amorphous, i.e. non-crystalline. Many abatement processes use combustion to supply the heat needed to decompose toxic compounds and chemically convert them to a more harmless form. The particles thus formed have varying sizes and may be hydrophilic (formed from halosilanes), hydrophobic (formed from organsilanes) or mixed (mixed chlorides or silicon, aluminum and boron, for example), depending on the species combusted and the nature of the combustion process. Particle sizes can range from tens of nanometers to tens of microns. As shown in FIGURE 1, the size of the particles depends on, among other factors, the length of the combustion flame. Longer flames maintain the components at high temperature for a longer periods and result in the formation of larger particles.

FIGURE 1. A longer flame maintains the combusting components at higher temperature for a longer time and results in the formation of larger particles.

FIGURE 1. A longer flame maintains the combusting components at higher temperature for a longer time and results in the formation of larger particles.

The behavior of particles once released into the environment varies depending on their sizes. Coarse particles, with diameters ranging from 2.5μm to 10μm, result largely from processes such as erosion, agriculture, or mining and include crustal dust, pollens, fungal spores, biological debris and sea salt. Because of their large size, these particles persist in the atmosphere for only a few hours or days. Fine particles, which range from 2.5μm to 0.1μm and include the particles of concern in semiconductor manufacturing exhaust, may be the direct result of a combustion processes or may also be formed by photochemical reactions between volatile organic compounds (VOC) and oxides in the presence of sunlight. Fine particles can stay suspended in ambient air for days to weeks. Ultrafine particles, less than 0.1μm, are generated by high temperature combustion or formed from the nucleation of atmospheric gases. Ultrafine particles are quickly removed from the atmosphere (minutes to hours) via diffusion to surfaces or coagulation, adsorption and condensing into fine particles.

Regulatory environment

Regulations governing the release of particles into the atmosphere are developing quickly worldwide as scientists expand their knowledge of the particles’ impacts on health and the environment. In addition to regulations governing emissions by particle size, there are specific regulations regarding especially harmful species, such as heavy metals, carcinogens and toxics. For example, the presence of an adsorbed species, like hydrofluoric acid (HF), on oxide particles increases the toxicity of the parent material.

In 2013 the United States Environmental Protection Agency specified an average daily limit of 150μg/m3 for coarse particles and 35μg/m3 for fine particles, and an average annual limit of 12μg/m3 for fine particles (down from 15μg/m3 in 2006). China, as of 2012, imposed limits based on both particle size and type, with permitted daily levels for coarse particles of 50μg/m3 and 150μg/m3 for type I and type II, respectively and 35μg/m3 and 75μg/m3, respectively for fine particles. China also limits annual averages for both sizes and types. The European Commission, the World Health Organization and the Australian National Environmental Council, among others, all specify their own limits. It is clearly incumbent on manufacturers to know and satisfy their local regulations. [1]

Health considerations

The health of employees in manufacturing facilities and people living near manufacturing operations is clearly a high priority for our industry. Epidemiological studies have provided plausible evidence that exposure to particulate material (PM) can impact health in a number of ways, including pulmonary and systemic inflammation, oxidative stress response, protein modification, stimulation of the autonomic nervous system, exaggerated allergic reactions, pro-coagulation activity, and suppression of immune response in the lungs.

Some studies have provided good news as well, specifically, that the amorphous silica particles produced during the abatement of gases used in semiconductor manufacturing have much less impact on lung function than the crystalline silica particles more often encountered in mining and building industries. These studies looked specifically at the effects of pure silica particles, an important caveat. Silica and other dusts that may have acids, such as HF, adsorbed on the particle surface constitute substantially greater health risks than the simple oxide. Other particulate oxides also represent serious health challenges. These include oxides of antimony, arsenic, barium, chromium, cobalt, nickel, phosphorus, tellurium and selenium.

Abatement performance

Just as condensed material deposited in the vacuum lines can shut down the production process, the accumulation of combustion-generated particulates can degrade the performance of the whole facility. In a typical point-of-use (POU) abatement system, after combustion the exhaust gases pass through a series of operations designed to remove particulates and other by-products. In the example shown in FIGURE 2 these include a water weir, quench tanks, a packed-bed scrubber and an atomized spray. Atomizing spray systems, in particular, have been shown to improve solids removal performance from 50 to 75 percent. Blockages can occur at the damper, in duct spurs leading from the abatement to the main duct, in the main duct, before or within the scrubber. In addition to blockages, failure to remove particulate at the primary abatement unit can also lead to environmental discharges and visible plumes at stacks. Any blockage will result in a process shutdown for system maintenance, lasting from a few hours to an entire day.

FIGURE 2. The accumulation of combustion generated particulates can degrade abatement system performance.

FIGURE 2. The accumulation of combustion generated particulates can degrade abatement system performance.

Mitigation options

A number of approaches exist for removing particulates downstream of the abatement system. One solution does not fit all and it is important to pick the one that best addresses the specific challenges. FIGURE 3 shows performance characteristics for various technologies. For example, highly toxic particles may require much higher removal rates than less harmful particles.

FIGURE 3. Performance characteristics for various particle removal technologies downstream of the abatement system. Courtesy: Waste-to-Energy Research and Technology Council (greyed out area not relevant to solids).

FIGURE 3. Performance characteristics for various particle removal technologies downstream of the abatement system. Courtesy: Waste-to-Energy Research and Technology Council (greyed out area not relevant to solids).

Edwards’ standard solution (FIGURE 4) for POU removal of fine particles is a wet electrostatic precipitator (WESP). A WESP uses electrostatic forces to remove particles. It requires power, water and pneumatics and can remove up to 95 percent of silica particles at flow rates of 1m3/ min, 85% at 2m3/min. WESP technology can be scaled to handle an entire facility. In one example, Edwards partnered in the installation of a large scale dual WESP integrated with a packed-bed wet scrubber and designed it to meet the specific challenges of arsenic abatement. The system ultimately demonstrated a 99 percent removal rate to meet the stringent requirements of the Chinese government for this highly toxic substance.

FIGURE 4. POU WESP uses electrostatic forces to remove particulates from the exhaust stream. It can remove up to 95 percent of silica particles at a flow rate of 1m3/min.

FIGURE 4. POU WESP uses electrostatic forces to remove particulates from the exhaust stream. It can remove up to 95 percent of silica particles at a flow rate of 1m3/min.

Alternative technologies that may be appropriate, but have not been evaluated for use in the management of waste gases from semiconductor manufacturing, are the Rotoclone family (from AAF International). POU units handle flow rates of 30m3/min, removing >97 percent of 1μm particles and >99.8 percent of 10μm particles. Duct-based Rotoclones with flow rates up to 1250m3/ min remove as much as 86 percent of 1μm particles and 99 percent of 10μm particles. Rotoclones require power, water, pneumatics and a drain.

More conventionally, a Venturi scrubber can be configured for various flow and removal rates. As a rule, smaller units controlling a low concentration waste stream will be much more expensive per unit of volumetric flow than larger units cleaning high pollutant-load flows. Venturi scrubbers can handle mists and flammable or explosive dusts. They have relatively low maintenance requirements, are simple in design and easy to install. Their collection efficiency can be varied. They can cool hot gases and neutralize corrosive gases. They are susceptible to corrosion and must be protected from freezing. Treated gases may require reheating to avoid a visible water plume. The collected particulate material may be contaminated and not recyclable, requiring expensive disposal of the waste sludge.

Filtration is another alternative for particle removal. It is normally restricted to the management of dry dusts at flow rates of 5 to 250m3/min. Removal rates higher than 99.9 percent are achievable. We have seen a limited number of large filter installations for the removal of hydrophobic silica solids at relative humidities as high as 80 percent. It is not clear how the presence of hydrophilic powder might impact the performance of these facilities.

In cases of highly toxic particles, high efficiency air particle (HEPA) filters can provide very high removal rates, higher than 99.999 percent. However, HEPA filters are appropriate only for very low contaminant concentrations. Edwards has been partnering with third-party suppliers regarding HEPA filtration for highly toxic dusts such as those generated during arsine management. These solutions are often used for highly toxic materials so they are often designed with bag-in-bag-out capability to eliminate potential exposure of maintenance personnel to the removed contaminants. Typically, these critical installations are also designed as dual systems with auto turnover to allow continuous operation of one system while the redundant system is serviced. HEPA technology can scale from POU to full facility.

Conclusion

All of these technologies are available now, but not all have been demonstrated in semicon- ductor manufacturing. Semiconductor manufacturers have long used POU WESPs and Venturi scrubbers and are very familiar with HEPA filtration systems, but primarily for particulate removal for air conditioning. Conventional filters are in operation on flat panel display exhausts (mainly on burner only dry abated CVD processes). Some of the technologies we have described, however, have not been proven in semiconductor applications, but are well developed and widely accepted in other industries. Rotoclone systems, for instance, are UL and CE certified, but have not been SEMI qualified. As semiconductor manufacturing processes continue to evolve, it will behove manufacturers to stay current on available technol- ogies and consider alternatives as performance and cost requirements dictate.

References

1. Review of the health impacts of emission sources, types and levels of particulate matter air pollution in ambient air in NSW; December, 2015; Produced for the NSW Environment Protection Authority and NSW Ministry of Health, Environmental Health Branch.

An electric current will not only heat a hybrid metamaterial, but will also trigger it to change state and fade into the background like a chameleon in what may be the proof-of-concept of the first controllable metamaterial device, or metadevice, according to a team of engineers.

“Previous metamaterials work focused mainly on cloaking objects so they were invisible in the radio frequency or other specific frequencies,” said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of electrical engineering, Penn State. “Here we are not trying to make something disappear, but to make it blend in with the background like a chameleon and we are working in optical wavelengths, specifically in the infrared.”

Metamaterials are synthetic, composite materials that possess qualities not seen in natural materials. These composites derive their functionality by their internal structure rather than by their chemical composition. Existing metamaterials have unusual electromagnetic or acoustic properties. Metadevices take metamaterials and do something of interest or value as any device does.

“The key to this metamaterial and metadevice is vanadium dioxide, a phase change crystal with a phase transition that is triggered by temperatures created by an electric current,” said Lei Kang, research associate in electrical engineering, Penn State.

The metamaterial is composed of a base layer of gold thick enough so that light cannot pass through it. A thin layer of aluminum dioxide separates the gold from the active vanadium dioxide layer. Another layer of aluminum dioxide separates the vanadium from a gold-patterned layer that is attached to an external electric source. The geometry of the patterned mesh screen controls the functional wavelength range. The amount of current flowing through the device controls the Joule heating effect, the heating due to resistance.

“The proposed metadevice integrated with novel transition materials represents a major step forward by providing a universal approach to creating self-sufficient and highly versatile nanophotonic systems,” the researchers said in today’s (Oct. 27) issue of Nature Communications.

As a proof of concept, the researchers created a .035 inch by .02 inch device and cut the letters PSU into the gold mesh layer so the vanadium dioxide showed through. The researchers photographed the device using an infrared camera at 2.67 microns. Without any current flowing through the device, the PSU stands out as highly reflective. With a current of 2.03 amps, the PSU fades into the background and becomes invisible, while at 2.20 amps, the PSU is clearly visible but the background has become highly reflective.

The response of the vanadium dioxide is tunable by altering the current flowing through the device. According to the researchers, vanadium dioxide can change state very rapidly and it is the device configuration that limits the tuning.

Researchers have developed a prototype of a next-generation lithium-sulphur battery which takes its inspiration in part from the cells lining the human intestine. The batteries, if commercially developed, would have five times the energy density of the lithium-ion batteries used in smartphones and other electronics.

This is a computer visualization of villi-like battery material. Credit: Teng Zhao

This is a computer visualization of villi-like battery material. Credit: Teng Zhao

The new design, by researchers from the University of Cambridge, overcomes one of the key technical problems hindering the commercial development of lithium-sulphur batteries, by preventing the degradation of the battery caused by the loss of material within it. The results are reported in the journal Advanced Functional Materials.

Working with collaborators at the Beijing Institute of Technology, the Cambridge researchers based in Dr Vasant Kumar’s team in the Department of Materials Science and Metallurgy developed and tested a lightweight nanostructured material which resembles villi, the finger-like protrusions which line the small intestine. In the human body, villi are used to absorb the products of digestion and increase the surface area over which this process can take place.

In the new lithium-sulphur battery, a layer of material with a villi-like structure, made from tiny zinc oxide wires, is placed on the surface of one of the battery’s electrodes. This can trap fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused.

“It’s a tiny thing, this layer, but it’s important,” said study co-author Dr Paul Coxon from Cambridge’s Department of Materials Science and Metallurgy. “This gets us a long way through the bottleneck which is preventing the development of better batteries.”

A typical lithium-ion battery is made of three separate components: an anode (negative electrode), a cathode (positive electrode) and an electrolyte in the middle. The most common materials for the anode and cathode are graphite and lithium cobalt oxide respectively, which both have layered structures. Positively-charged lithium ions move back and forth from the cathode, through the electrolyte and into the anode.

The crystal structure of the electrode materials determines how much energy can be squeezed into the battery. For example, due to the atomic structure of carbon, each carbon atom can take on six lithium ions, limiting the maximum capacity of the battery.

Sulphur and lithium react differently, via a multi-electron transfer mechanism meaning that elemental sulphur can offer a much higher theoretical capacity, resulting in a lithium-sulphur battery with much higher energy density. However, when the battery discharges, the lithium and sulphur interact and the ring-like sulphur molecules transform into chain-like structures, known as a poly-sulphides. As the battery undergoes several charge-discharge cycles, bits of the poly-sulphide can go into the electrolyte, so that over time the battery gradually loses active material.

The Cambridge researchers have created a functional layer which lies on top of the cathode and fixes the active material to a conductive framework so the active material can be reused. The layer is made up of tiny, one-dimensional zinc oxide nanowires grown on a scaffold. The concept was trialled using commercially-available nickel foam for support. After successful results, the foam was replaced by a lightweight carbon fibre mat to reduce the battery’s overall weight.

“Changing from stiff nickel foam to flexible carbon fibre mat makes the layer mimic the way small intestine works even further,” said study co-author Dr Yingjun Liu.

This functional layer, like the intestinal villi it resembles, has a very high surface area. The material has a very strong chemical bond with the poly-sulphides, allowing the active material to be used for longer, greatly increasing the lifespan of the battery.

“This is the first time a chemically functional layer with a well-organised nano-architecture has been proposed to trap and reuse the dissolved active materials during battery charging and discharging,” said the study’s lead author Teng Zhao, a PhD student from the Department of Materials Science & Metallurgy. “By taking our inspiration from the natural world, we were able to come up with a solution that we hope will accelerate the development of next-generation batteries.”

For the time being, the device is a proof of principle, so commercially-available lithium-sulphur batteries are still some years away. Additionally, while the number of times the battery can be charged and discharged has been improved, it is still not able to go through as many charge cycles as a lithium-ion battery. However, since a lithium-sulphur battery does not need to be charged as often as a lithium-ion battery, it may be the case that the increase in energy density cancels out the lower total number of charge-discharge cycles.

“This is a way of getting around one of those awkward little problems that affects all of us,” said Coxon. “We’re all tied in to our electronic devices – ultimately, we’re just trying to make those devices work better, hopefully making our lives a little bit nicer.”

Thermoelectric materials, which can directly convert thermal energy into electrical energy (Seebeck effect), can be effectively used for the development of a clean and environmentally compatible power-generation technology.

Picture of the synthesized bulk CaMgSi thermoelectric material through the procedure developed in this study. CREDIT: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

Picture of the synthesized bulk CaMgSi thermoelectric material through the procedure developed in this study. CREDIT: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

However, these materials are not commonly used for practical applications as they mostly include toxic and/or expensive elements.

Recently, researchers at the Materials Function Control Laboratory at the Toyohashi University of Technology and the Nagoya Institute of Technology have successfully synthesized a new thermoelectric material, CaMgSi, which is an intermetallic compound. The key to this development was the synthesis procedure; bulk CaMgSi intermetallic compound was synthesized by combining mechanical ball-milling (MM) and pulse current sintering (PCS) processes.

“Appearance of thermoelectric property in the intermetallic compound, CaMgSi, has been predicted by both theoretical and experimental studies”, explain the researchers of this work, Nobufumi Miyazaki and Nozomu Adachi. ” However, the biggest issue in front of us was the synthesis of thermoelectric CaMgSi of optimal size “, they continued. In general, alloys are produced by mixing the constituent elements in their molten forms. However, when a temperature is raised up to the melting temperature of Si, Mg vapors; liquids of Ca, Mg, and Si cannot exists at same time.

Associate Professor Yoshikazu Todaka says “To overcome the aforementioned problem, we chose the mechanical ball milling process to mix the elements homogeneously, without melting, and then a chemical reaction between Ca, Mg, and Si was induced using the pulse current sintering process”.

Consequently, the intermetallic compound, CaMgSi, with sufficient size was synthesized. The thermoelectric property of the synthesized CaMgSi exhibited a performance comparable to that of the previously developed Mg-based thermoelectric materials. It is expected that an addition of a fourth element to CaMgSi renders it with superior thermoelectric properties. Interestingly, they found that the novel thermoelectric can exhibit both n- and p-type conductivity with a slight change in the composition of CaMgSi. Such a property for the material is very significant for its application in power-generation modules.

The new thermoelectric material synthesized in this study is composed of lightweight elements, and has a low density of 2.2 g/cm3. Therefore, one of the possible applications of the material is in automobiles to utilize waste heat emitted from engines. These findings could contribute to the development of green energy technology.

Glass fibres do everything from connecting us to the internet to enabling keyhole surgery by delivering light through medical devices such as endoscopes. But as versatile as today’s fiber optics are, scientists around the world have been working to expand their capabilities by adding semiconductor core materials to the glass fibers.

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a glass fiber with a semiconductor core. Rapid heating and cooling of this kind of fiber allows the researchers to make functional materials with applications beyond traditional fiber optics. Credit: Nancy Bazilchuk

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a glass fiber with a semiconductor core. Rapid heating and cooling of this kind of fiber allows the researchers to make functional materials with applications beyond traditional fiber optics. Credit: Nancy Bazilchuk

Now, a team of researchers has created glass fibers with single-crystal silicon-germanium cores. The process used to make these could assist in the development of high-speed semiconductor devices and expand the capabilities of endoscopes says Ursula Gibson, a physics professor at the Norwegian University of Science and Technology and senior author of the paper.

“This paper lays the groundwork for future devices in several areas,” Gibson said, because the germanium in the silicon core allows researchers to locally alter its physical attributes.

The article, “Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres,” was published in Nature Communications on October 24.

Melting and recrystallizing

To understand what the researchers did, you need to recognize that silicon and germanium have different melting points. When the two substances are combined in a glass fiber, flecks of germanium-rich material are scattered throughout the fiber in a disorderly way because the silicon has a higher melting point and solidifies, or “freezes” first. These germanium flecks limit the fiber’s ability to transmit light or information. “When they are first made, these fibers don’t look very good,” Gibson said.

But rapidly heating the fiber by moving it through a laser beam allowed the researchers to melt the semiconductors in the core in a controlled fashion. Using the difference in the solidification behavior, the researchers were able to control the local concentration of the germanium inside the fiber depending upon where they focused the laser beam and for how long.

“If we take a fibre and melt the core without moving it, we can accumulate small germanium-rich droplets into a melt zone, which is then the last thing to crystalize when we remove the laser slowly,” Gibson said. “We can make stripes, dots… you could use this to make a series of structures that would allow you to detect and manipulate light.”

An interesting structure was produced when the researchers periodically interrupted the laser beam as it moved along their silicon-germanium fibre. This created a series of germanium-rich stripes across the width of the 150-micrometer diameter core. That kind of pattern creates something called a Bragg grating, which could help expand the capability of long wavelength light-guiding devices. “That is of interest to the medical imaging industry,” Gibson said.

Rapid heating, cooling key

Another key aspect of the geometry and laser heating of the silicon-germanium fibre is that once the fibre is heated, it can also be cooled very quickly as the fibre is carried away from the laser on a moving stage.

Controlled rapid cooling allows the mixture to solidify into a single uniform crystal the length of the fibre — which makes it ideal for optical transmission.

Previously, people working with bulk silicon-germanium alloys have had problems creating a uniform crystal that is a perfect mix, because they have not had sufficient control of the temperature profile of the sample.

“When you perform overall heating and cooling, you get uneven composition through the structure, because the last part to freeze concentrates excess germanium,” Gibson said. “We have shown we can create single crystalline silicon-germanium at high production rates when we have a large temperature gradient and a controlled growth direction.”

Transistors that switch faster

Gibson says the laser heating process could also be used to simplify the incorporation of silicon-germanium alloys into transistor circuits.

“You could adapt the laser treatment to thin films of the alloy in integrated circuits,” she said.

Traditionally, Gibson said, electronics researchers have looked at other materials, such as gallium arsenide, in their quest to build ever-faster transistors. However, the mix of silicon and germanium, often called SiGe, allows electrons to move through the material more quickly than they move through pure silicon, and is compatible with standard integrated circuit processing.

“SiGe allows you to make transistors that switch faster” than today’s silicon-based transistors, she said, “and our results could impact their production.”

A new design for solar cells that uses inexpensive, commonly available materials could rival and even outperform conventional cells made of silicon.

A tandem perovskite solar cell boosts efficiency by absorbing high- and low-energy photons from the sun. Credit: Rongrong Cheacharoen/Stanford University

A tandem perovskite solar cell boosts efficiency by absorbing high- and low-energy photons from the sun. Credit: Rongrong Cheacharoen/Stanford University

Writing in the Oct. 21 edition of Science, researchers from Stanford and Oxford describe using tin and other abundant elements to create novel forms of perovskite – a photovoltaic crystalline material that’s thinner, more flexible and easier to manufacture than silicon crystals.

“Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost,” said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. “We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent, a rate comparable to silicon solar cells on the market today.”

The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic, McGehee added.

“The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30 percent efficiency,” said co-author Henry Snaith, a professor of physics at Oxford. “This is just the beginning.”

Tandem technology

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build, the authors said.

“A silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3,000 degrees Fahrenheit (1,600 degrees Celsius),” said co-lead author Tomas Leijtens, a postdoctoral scholar at Stanford. “Perovskite cells can be processed in a laboratory from common materials like lead, tin and bromine, then printed on glass at room temperature.”

But building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an “energy gap” and create an electric current.

A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

An efficient tandem device would consist of two ideally matched cells, said co-lead author Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington.

“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” Eperon said. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”

The smaller gap has proven to be the bigger challenge for scientists. Working together, Eperon and Leijtens used a unique combination of tin, lead, cesium, iodine and organic materials to create an efficient cell with a small energy gap.

“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8 percent conversion efficiency,” Eperon said. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”

The result: A tandem device consisting of two perovskite cells with a combined efficiency of 20.3 percent.

“There are thousands of possible compounds for perovskites,” Leijtens added, “but this one works very well, quite a bit better than anything before it.”

Seeking stability

One concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days.

“Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors wrote.

“The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon,” McGehee said. “Those who see the potential realize that these results are amazing.”

The next step is to optimize the composition of the materials to absorb more light and generate an even higher current, Snaith said.

“The versatility of perovskites, the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proven,” he said.

Researchers have found an unexpected way to control the thermal conductivity of two-dimensional (2-D) materials, which will allow electronics designers to dissipate heat in electronic devices that use these materials.

2-D materials have a layered structure, with each layer having strong bonds horizontally, or “in plane,” and weak bonds between the layers, or “out of plane.” These materials have unique electronic and chemical properties, and hold promise for use in creating flexible, thin, lightweight electronic devices.

For many of these potential applications, it’s important to be able to dissipate heat efficiently. And this can be tricky. In 2-D materials, heat is conducted differently in plane than it is out of plane.

For example, in one class of 2-D materials, called TMDs, heat is conducted at 100 watts per meter per Kelvin (W/mK) in plane, but at only 2 W/mK out of plane. That gives it a “thermal anisotropy ratio” of about 50.

To better understand the thermal conduction properties of 2-D materials, a team of researchers from North Carolina State University, the University of Illinois at Urbana-Champaign (UI) and the Toyota Research Institute of North America (TRINA) began experimenting with molybdenum disulfide (MoS2), which is a TMD.

The researchers found that, by introducing disorder to the MoS2, they could significantly alter the thermal anisotropy ratio.

The researchers created this disorder by introducing lithium ions between the layers of MoS2. The presence of the lithium ions does two things simultaneously: it puts the layers of the 2-D material out of alignment with each other, and it forces the MoS2 to rearrange the structure of its component atoms.

When the ratio of lithium ions to MoS2 reached 0.34, the in-plane thermal conductivity was 45 W/mK, and the out-of-plane thermal conductivity dropped to 0.4 W/mK- increasing the material’s thermal anisotropy ratio from 50 to more than 100. In other words, heat became more than twice as likely to travel in plane — along the layer, rather than between the layers.

And that was as good as it got. Adding fewer lithium ions made the thermal anisotropy ratio lower. Adding more ions also made it lower. But in both cases, the ratio was affected in a predictable way, meaning that the researchers could tune the material’s thermal conductivity and thermal anisotropy ratio.

“This finding was very counter-intuitive,” says Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State and co-corresponding author of a paper describing the work. “The conventional wisdom has been that introducing disorder to any material would decrease the thermal anisotropy ratio.

“But based on our observations, we feel that this approach to controlling thermal conductivity would apply not only to other TMDs, but to 2-D materials more broadly,” Liu says.

“We set out to advance our fundamental understanding of 2-D materials, and we have,” Liu adds. “But we also learned something that is likely to be of practical use for the development of technologies that make use of 2-D materials.”

University of Alabama at Birmingham researchers will use pressures greater than those found at the center of the Earth to potentially create as yet unknown new materials. In the natural world, such immense forces deep underground can turn carbon into diamonds, or volcanic ash into slate.

Credit: UAB

Credit: UAB

The ability to produce these pressures depends on tiny nanocrystalline-diamond anvils built in a UAB clean room manufacturing facility. Each anvil head is just half the width of an average human hair. The limits of their pressure have not yet been reached as the first 27 prototypes are being tested.

“We have achieved 75 percent of the pressure found at the center of the Earth, or 264 gigapascals, using lab-grown nanocrystalline-diamond micro-anvil,” said Yogesh Vohra, Ph.D., a professor and university scholar of physics in the UAB College of Arts and Sciences. “But the goal is one terapascal, which is the pressure close to the center of Saturn. We are one-quarter of the way there.”

One terapascal, a scientific measure of pressure, is equal to 147 million pounds per square inch.

One key to high pressure is to make the point of the anvil, where the pressure is applied, very narrow. This magnifies the pressure applied by a piston above the micro-anvil, much like the difference of being stepped on by a spiked high heel rather than a loafer.

A more difficult task is how to make an anvil that is able to survive this ultra-high pressure. The solution for the Vohra team is to grow a nanocrystalline pillar of diamond — 30 micrometers wide and 15 micrometers tall — on the culet of a gem diamond. The culet is the flat surface at the bottom of a gemstone.

“We didn’t know that we could grow nanocrystalline diamonds on a diamond base,” Vohra said. “This has never been done before.”

In the 264-gigapascal pressure test at Argonne National Laboratory in Lemont, Illinois, the nanocrystalline diamond showed no sign of deformation. Vohra and colleagues recently reported this result in the American Institute of Physics journal AIP Advances.

“The structure did not collapse when we applied pressure,” Vohra said. “Nanocrystalline diamond has better mechanical properties than gem diamonds. The very small-sized grain structure makes it really tough.”

As more micro-anvils are tested and improved, they will be used to study how transition metals, alloys and rare earth metals behave under extreme conditions. Just as graphitic carbon that is subjected to high pressure and temperature can turn into diamond, some materials squeezed by the micro-anvils may gain novel crystal modifications with enhanced physical and mechanical properties — modifications that are retained when the pressure is released. Such new materials have potential applications in the aerospace, biomedical and nuclear industries.

The micro-anvils are made in a Class 7000 clean room in the UAB Diamond Microfabrication Lab, using maskless lithography and microwave plasma chemical vapor deposition.

Vohra says his research team wants to generate smaller grain sizes in the nanocrystalline diamond, which may make it even stronger; understand how the nanocrystalline diamond is bonded to the gem diamond; and use ion beams to machine the top of the micro-anvil to a hemispherical shape. That shape will mean an even narrower contact point, thus increasing the pressure.

Testing is done at Argonne because it has a very bright synchrotron X-ray source that can probe crystal structure of micron-sized materials under pressure. Vohra and two graduate students travel to Argonne about four times a year.

Researchers at the Nanoscale Transport Physics Laboratory from the School of Physics at the University of the Witwatersrand have found a technique to improve carbon superlattices for quantum electronic device applications. Superlattices are made up of alternating layers of very thin semiconductors, just a few nanometers thick. These layers are so thin that the physics of these devices is governed by quantum mechanics, where electrons behave like waves. In a paradigm shift from conventional electronic devices, exploiting the quantum properties of superlattices holds the promise of developing new technologies.

A schematic atomic diagram of a quantum well made from amorphous carbon layers. The blue atoms represent amorphous carbon with a high percentage of diamond-like carbon. The maroon atoms represent amorphous carbon which is graphite-like. The diamond-like regions have a high potential (diamond is insulating) while the graphite-like regions are more metallic. This creates a quantum well as electrons are confined within the graphite-like region due to the relatively high potential in the diamond-like regions. Superlattices are made up of a series of quantum wells. Credit: Wits University

A schematic atomic diagram of a quantum well made from amorphous carbon layers. The blue atoms represent amorphous carbon with a high percentage of diamond-like carbon. The maroon atoms represent amorphous carbon which is graphite-like. The diamond-like regions have a high potential (diamond is insulating) while the graphite-like regions are more metallic. This creates a quantum well as electrons are confined within the graphite-like region due to the relatively high potential in the diamond-like regions. Superlattices are made up of a series of quantum wells. Credit: Wits University

The group, headed by Professor Somnath Bhattacharyya has been working for the past 10 years on developing carbon-based nano-electronic devices.

“Carbon is the future in the electronics field and it soon will be challenging many other semiconductors, including silicon,” says Bhattacharyya.

The physics of carbon superlattices is more complex than that of crystalline superlattices (such as gallium arsenide), since the material is amorphous and carbon atoms tend to form chains and networks. The Wits group, in association with researchers at the University of Surrey in the UK, has developed a detailed theoretical approach to understand the experimental data obtained from carbon devices. The paper has been published in Scientific Reports (Nature Publishing Group) on 19 October.

“This work provides an understanding of the fundamental quantum properties of carbon superlattices, which we can now use to design quantum devices for specific applications,” says lead author, Wits PhD student, Ross McIntosh. “Our work provides strong impetus for future studies of the high-frequency electronic and optoelectronic properties of carbon superlattices”.

Through their work, the group reported one of the first theoretical models that can explain the fundamental electronic transport properties in disordered carbon superlattices.

Bhattacharyya started looking at the use of carbon for semiconductor applications almost 10 years ago, before he joined Wits University, when he and co-authors from the University of Surrey developed and demonstrated negative differential resistance and excellent high-frequency properties of a quantum device made up of amorphous carbon layers. This work was published in Nature Materials in 2006.

McIntosh undertook the opportunity at honours level to measure the electrical properties of carbon superlattice devices. Now, as a PhD student and having worked extensively with theoretician Dr. Mikhail V. Katkov, he has extended the theoretical framework and developed a technique to calculate the transport properties of these devices.

Bhattacharyya believes this work will have immense importance in developing Carbon-based high-frequency devices.

“It will open not only fundamental studies in Carbon materials, but it will also have industrial applications in the electronic and optoelectronic device sector,” he says.

Superlattices are currently used as state of the art high frequency oscillators and amplifiers and are beginning to find use in optoelectronics as detectors and emitters in the terahertz regime. While the high frequency electrical and optoelectronic properties of conventional semiconductors are limited by the dopants used to modify their electronic properties, the properties of superlattices can be tuned over a much wider range to create devices which operate in regimes where conventional devices cannot.

Superlattice electronic devices can operate at higher frequencies and optoelectronic devices can operate at lower frequencies than their conventional counterparts. The lack of terahertz emitters and detectors has resulted in a gap in that region of the electromagnetic spectrum (known as the “terahertz gap”), which is a significant limitation, as many biological molecules are active in this regime. This also limits terahertz radio astronomy.

Amorphous Carbon devices are extremely strong, can operate at high voltages and can be developed in most laboratories in the world, without sophisticated nano-fabrication facilities. New Carbon-based devices could find application in biology, space technology, science infrastructure such as the Square Kilometre Array (SKA) telescope in South Africa, and new microwave detectors.

“What was lacking earlier was an understanding of device modelling. If we have a model, we can improve the device quality, and that is what we now have,” says Bhattacharyya.