Controlling phase changes in solids

Rewritable CDs, DVDs and Blu-Ray discs owe their existence to phase-change materials, those materials that change their internal order when heated and whose structures can be switched back and forth between their crystalline and amorphous phases. Phase-change materials have even more exciting applications on the horizon, but our limited ability to precisely control their phase changes is a hurdle to the development of new technology. One of the most popular and useful phase-change materials is GST, which consists of germanium, antimony, and tellurium. This material is particularly useful because it alternates between its crystalline and amorphous phases more quickly than any other material yet studied. These phase changes result from changes in the bonds between atoms, which also modify the electronic and optical properties of GST as well as its lattice structure. Specifically, resonant bonds, in which electrons participate in several neighboring bonds, influence the material's electro-optical properties, while covalent bonds, in which electrons are shared between two atoms, influence its lattice structure. Most techniques that use GST simultaneously change both the electro-optical and structural properties. This is actually a considerable drawback since in the process of repeating structural transitions, such as heating and cooling the material, the lifetime of any device based on this material is drastically reduced. Schematic of the ultrafast transformation pathway Schematic of the ultrafast transformation pathway. (Image: ICFO/Fritz-Haber-Inst. MPG/SUTD) (click on image to enlarge) In a study recently published in ("Time-domain separation of optical properties from structural transitions in resonantly bonded materials"), researchers from the ICFO groups led by Prof. Simon Wall and ICREA Prof. at ICFO Valerio Pruneri, in collaboration with the Firtz-Haber-Institut der Max-Planck-Gesellschaft, have demonstrated how the material and electro-optical properties of GST change over fractions of a trillionth of a second as the phase of the material changes. Laser light was successfully used to alter the bonds controlling the electro-optical properties without meaningfully altering the bonds controlling the lattice. This new configuration allowed the rapid, reversible changes in the electro-optical properties that are important in device applications without reducing the lifetime of the device by changing its lattice structure. Moreover, the change in the electro-optical properties of GST measured in this study is more than ten times greater than that previously achieved by silicon materials used for the same purpose. This finding suggests that GST may be a good substitute for these commonly used silicon materials. The results of this study may be expected to have far-reaching implications for the development of new technologies, including flexible displays, logic circuits, optical circuits, and universal memory for data storage. These results also indicate the potential of GST for other applications requiring materials with large changes in optical properties that can be achieved rapidly and with high precision.
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Making the new silicon

An exotic material called gallium nitride (GaN) is poised to become the next semiconductor for power electronics, enabling much higher efficiency than silicon. laptop power adapter made by Cambridge Electronics using GaN transistors Shown here is a prototype laptop power adapter made by Cambridge Electronics using GaN transistors. At 1.5 cubic inches in diameter, this is the smallest laptop power adapter ever made. In 2013, the Department of Energy (DOE) dedicated approximately half of a $140 million research institute for power electronics to GaN research, citing its potential to reduce worldwide energy consumption. Now MIT spinout Cambridge Electronics Inc. (CEI) has announced a line of GaN transistors and power electronic circuits that promise to cut energy usage in data centers, electric cars, and consumer devices by 10 to 20 percent worldwide by 2025. Power electronics is a ubiquitous technology used to convert electricity to higher or lower voltages and different currents — such as in a laptop’s power adapter, or in electric substations that convert voltages and distribute electricity to consumers. Many of these power-electronics systems rely on silicon transistors that switch on and off to regulate voltage but, due to speed and resistance constraints, waste energy as heat. CEI’s GaN transistors have at least one-tenth the resistance of such silicon-based transistors, according to the company. This allows for much higher energy-efficiency, and orders-of-magnitude faster switching frequency — meaning power-electronics systems with these components can be made much smaller. CEI is using its transistors to enable power electronics that will make data centers less energy-intensive, electric cars cheaper and more powerful, and laptop power adapters one- third the size — or even small enough to fit inside the computer itself. “This is a once-in-a-lifetime opportunity to change electronics and to really make an impact on how energy is used in the world,” says CEI co-founder Tomás Palacios, an MIT associate professor of electrical engineering and computer science who co-invented the technology. Other co-founders and co-inventors are Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, now chair of CEI’s technical advisory board; alumnus Bin Lu SM ’07, PhD ’13, CEI’s vice president for device development; Ling Xia PhD’12, CEI’s director of operations; Mohamed Azize, CEI’s director of epitaxy; and Omair Saadat PhD ’14, CEI’s director of product reliability. Making GaN feasible While GaN transistors have several benefits over silicon, safety drawbacks and expensive manufacturing methods have largely kept them off the market. But Palacios, Lu, Saadat, and other MIT researchers managed to overcome these issues through design innovations made in the late 2000s. Power transistors are designed to flow high currents when on, and to block high voltages when off. Should the circuit break or fail, the transistors must default to the “off” position to cut the current to avoid short circuits and other issues — an important feature of silicon power transistors. But GaN transistors are typically “normally on” — meaning, by default, they’ll always allow a flow of current, which has historically been difficult to correct. Using resources in MIT’s Microsystems Technology Laboratory, the researchers — supported by Department of Defense and DOE grants — developed GaN transistors that were “normally off” by modifying the structure of the material. To make traditional GaN transistors, scientists grow a thin layer of GaN on top of a substrate. The MIT researchers layered different materials with disparate compositions in their GaN transistors. Finding the precise mix allowed a new kind of GaN transistors that go to the off position by default. “We always talk about GaN as gallium and nitrogen, but you can modify the basic GaN material, add impurities and other elements, to change its properties,” Palacios says. But GaN and other nonsilicon semiconductors are also manufactured in special processes, which are expensive. To drop costs, the MIT researchers — at the Institute and, later, with the company — developed new fabrication technologies, or “process recipes,” Lu says. This involved, among other things, switching out gold metals used in manufacturing GaN devices for metals that were compatible with silicon fabrication, and developing ways to deposit GaN on large wafers used by silicon foundries. “Basically, we are fabricating our advanced GaN transistors and circuits in conventional silicon foundries, at the cost of silicon. The cost is the same, but the performance of the new devices is 100 times better,” Lu says. Major applications CEI is currently using its advanced transistors to develop laptop power adaptors that are approximately 1.5 cubic inches in diameter — the smallest ever made.
Among the other feasible applications for the transistors, Palacios says, is better power electronics for data centers run by Google, Amazon, Facebook, and other companies, to power the cloud.

Currently, these data centers eat up about 2 percent of electricity in the United States. But GaN-based power electronics, Palacios says, could save a very significant fraction of that.

Another major future application, Palacios adds, will be replacing the silicon-based power electronics in electric cars. These are in the chargers that charge the battery, and the inverters that convert the battery power to drive the electric motors. The silicon transistors used today have a constrained power capability that limits how much power the car can handle. This is one of the main reasons why there are few large electric vehicles.

GaN-based power electronics, on the other hand, could boost power output for electric cars, while making them more energy-efficient and lighter — and, therefore, cheaper and capable of driving longer distances. “Electric vehicles are popular, but still a niche product. GaN power electronics will be key to make them mainstream,” Palacios says.

Innovative ideas In launching CEI, the MIT founders turned to the Institute’s entrepreneurial programs, which contributed to the startup’s progress. “MIT's innovation and entrepreneurial ecosystem has been key to get things moving and to the point where we are now,” Palacios says. Palacios first earned a grant from the Deshpande Center for Technological Innovation to launch CEI. Afterward, he took his idea for GaN-based power electronics to Innovation Teams (i-Teams), which brings together MIT students from across disciplines to evaluate the commercial feasibility of new technologies. That program, he says, showed him the huge market pull for GaN power electronics, and helped CEI settle on its first products. “Many times, it’s the other way around: You come out with an amazing technology looking for an application. In this case, thanks to i-Teams, we found there were many applications looking for this technology,” Palacios says. For Lu, a key element for growing CEI was auditing Start6, a workshop hosted by the Department of Electrical Engineering and Computer Science, where entrepreneurial engineering students are guided through the startup process with group discussions and talks from seasoned entrepreneurs. Among other things, Lu gained perspective on dividing equity, funding, building a team, and other early startup challenges. “It’s a great class for a student who has an idea, but doesn’t know exactly what’s going on in business,” Lu says. “It’s kind of an overview of what the process is going to be like, so when you start your own company you are ready.”
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Graphene supercurrents go ballistic

Researchers with Europe's Graphene Flagship have demonstrated superconducting electric currents in the two-dimensional material graphene that bounce between sheet edges without scattering. This first direct observation of the ballistic mirroring of electron waves in a 2D system with supercurrents could lead to the use of graphene-based Josephson junctions in applications such as advanced digital logic circuits, ultrasensitive magnetometers and voltmeters. A Josephson junction is made by sandwiching a thin layer of non-superconducting material between two superconducting layers. Entwined pairs of superconducting electrons known as Cooper pairs are in certain circumstances able to travel without resistance through the insulating or partially-insulating middle layer. The resistance-free current occurs up to a critical current, above which a time-varying (alternating) voltage is set up across the junction. Detecting and measuring the change between current states is the basis of many applications that exploit Josephson junctions. Electronic logic circuits can be constructed from arrays of Josephson junctions, which are also used in superconducting quantum interference devices. SQUIDs are extremely sensitive to electromagnetic fields, and form the basis of magnetometers that can measure fields as low as a few attoteslas (10-18 T), and voltmeters responsive to potential differences of picovolts (10-12 V). Practical uses of such ultra-sensitive devices include the measurement of neurological currents in the brain or heart, and geophysical research. Military applications include remote submarine detection. Josephson junctions in edge-contacted graphene (Image: Technical University of Delft) In the latest issue of the journal ("Ballistic Josephson junctions in edge-contacted graphene"), an international team of physicists led by Graphene Flagship member Lieven Vandersypen, who is based at the Kavli Institute of Nanoscience in Delft, demonstrate unambiguous signatures of Josephson junctions in graphene, a two-dimensional allotrope of carbon atoms arranged in a hexagonal lattice. In the paper, the lead authors of which are Victor Calado and Srijit Goswami, the researchers look at ballistic supercurrents in graphene, with the electrons mirroring between one-dimensional edge contacts made of molybdenum-rhenium. The ultraclean graphene used in the experiment – required in order to preserve the material's unique electrical properties – is shielded from environmental contamination by being encapsulated between sheets of the insulating 2D material hexagonal boron nitride. This three-layer stack is then cut into the desired shape, and the graphene placed in contact with the superconducting alloy. Just as with light bouncing back and forth between two mirrors, leading to an interference pattern set up by the superposition of incident and reflected electromagnetic waves, electrons can reflect from the edges of a superconductor. The difference is that electron interference is only observed in ultraclean samples, in which it is possible for the charged particles to move in ballistic trajectories with minimal scattering from impurities in the material. This is what Calado, Goswami and colleagues observed in their setup, with a striking modulation of the supercurrent. In their paper, the researchers refer to the critical current oscillating as a result of phase-coherent interference of the electrons and electron holes that carry the current. This is caused by the formation of a resonant (Fabry-Pérot) cavity between the mirror points. Furthermore, relatively large supercurrents are seen, travelling over distances of up to 1.5 micrometres. The researchers believe this to be the first direct observation of the ballistic mirroring of supercurrents in graphene. "This work allows us to unravel new physics related to the interplay between superconductivity and the relativistic behaviour of electrons in graphene," said Goswami. "With this technology, we can study and exploit graphene Josephson junctions in a new, exciting regime."
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