Step towards ‘holy grail’ of silicon photonics

Creation of first practical silicon-based laser has the potential to transform communications, healthcare and energy systems

A group of researchers from the UK, including academics from Cardiff University, has demonstrated the first practical laser that has been grown directly on a silicon substrate.

It is believed the breakthrough could lead to ultra-fast communication between computer chips and electronic systems and therefore transform a wide variety of sectors, from communications and healthcare to energy generation.

The EPSRC-funded UK group, led by Cardiff University and including researchers from UCL and the University of Sheffield, have presented their findings in the journal Nature Photonics.

Silicon is the most widely used material for the fabrication of electronic devices and is used to fabricate semiconductors, which are embedded into nearly every device and piece of technology that we use in our everyday lives, from smartphones and computers to satellite communications and GPS.

Electronic devices have continued to get quicker, more efficient and more complex, and have therefore placed an added demand on the underlining technology.

Researchers have found it increasingly difficult to meet these demands using conventional electrical interconnects between computer chips and systems, and have therefore turned to light as a potential ultra-fast connector.

Whilst it has been difficult to combine a semiconductor laser – the ideal source of light – with silicon, the UK group have now overcome these difficulties and successfully integrated a laser directly grown onto a silicon substrate for the very first time.

Professor Huiyun Liu, who led the growth activity, explained that the 1300 nm wavelength laser has been shown to operate at temperatures of up to 120°C and for up to 100,000 hours.

Professor Peter Smowton, from the School of Physics and Astronomy, said: “Realising electrically-pumped lasers based on Si substrates is a fundamental step towards silicon photonics.

“The precise outcomes of such a step are impossible to predict in their entirety, but it will clearly transform computing and the digital economy, revolutionise healthcare through patient monitoring, and provide a step-change in energy efficiency.

“Our breakthrough is perfectly timed as it forms the basis of one of the major strands of activity in Cardiff University’s Institute for Compound Semiconductors and the University’s joint venture with compound semiconductor specialists IQE.”

Professor Alwyn Seeds, Head of the Photonics Group at University College London, said: “The techniques that we have developed permit us to realise the Holy Grail of silicon photonics - an efficient and reliable electrically driven semiconductor laser directly integrated on a silicon substrate. Our future work will be aimed at integrating these lasers with waveguides and drive electronics leading to a comprehensive technology for the integration of photonics with silicon electronics"

Cardiff University

Darwin on a chip

UT researchers develop (r)evolutionary circuits

Researchers of the MESA+ Institute for Nanotechnology and the CTIT Institute for ICT Research at the University of Twente in The Netherlands have demonstrated working electronic circuits that have been produced in a radically new way, using methods that resemble Darwinian evolution. The size of these circuits is comparable to the size of their conventional counterparts, but they are much closer to natural networks like the human brain. The findings promise a new generation of powerful, energy-efficient electronics, and have been published in the leading British journal Nature Nanotechnology.

One of the greatest successes of the 20th century has been the development of digital computers. During the last decades these computers have become more and more powerful by integrating ever smaller components on silicon chips. However, it is becoming increasingly hard and extremely expensive to continue this miniaturisation. Current transistors consist of only a handful of atoms. It is a major challenge to produce chips in which the millions of transistors have the same characteristics, and thus to make the chips operate properly. Another drawback is that their energy consumption is reaching unacceptable levels. It is obvious that one has to look for alternative directions, and it is interesting to see what we can learn from nature. Natural evolution has led to powerful ‘computers’ like the human brain, which can solve complex problems in an energy-efficient way. Nature exploits complex networks that can execute many tasks in parallel.

Moving away from designed circuits

The approach of the researchers at the University of Twente is based on methods that resemble those found in Nature. They have used networks of gold nanoparticles for the execution of essential computational tasks. Contrary to conventional electronics, they have moved away from designed circuits. By using 'designless' systems, costly design mistakes are avoided. The computational power of their networks is enabled by applying artificial evolution. This evolution takes less than an hour, rather than millions of years. By applying electrical signals, one and the same network can be configured into 16 different logical gates. The evolutionary approach works around - or can even take advantage of - possible material defects that can be fatal in conventional electronics.

Powerful and energy-efficient

It is the first time that scientists have succeeded in this way in realizing robust electronics with dimensions that can compete with commercial technology. According to prof. Wilfred van der Wiel, the realized circuits currently still have limited computing power. “But with this research we have delivered proof of principle: demonstrated that our approach works in practice. By scaling up the system, real added value will be produced in the future. Take for example the efforts to recognize patterns, such as with face recognition. This is very difficult for a regular computer, while humans and possibly also our circuits can do this much better."  Another important advantage may be that this type of circuitry uses much less energy, both in the production, and during use. The researchers anticipate a wide range of applications, for example in portable electronics and in the medical world.

University of Twente

UCLA researchers create exceptionally strong and lightweight new metal

Magnesium infused with dense silicon carbide nanoparticles could be used for airplanes, cars, mobile electronics and more

A team led by researchers from the UCLA Henry Samueli School of Engineering and Applied Science has created a super-strong yet light structural metal with extremely high specific strength and modulus, or stiffness-to-weight ratio. The new metal is composed of magnesium infused with a dense and even dispersal of ceramic silicon carbide nanoparticles. It could be used to make lighter airplanes, spacecraft, and cars, helping to improve fuel efficiency, as well as in mobile electronics and biomedical devices.

To create the super-strong but lightweight metal, the team found a new way to disperse and stabilize nanoparticles in molten metals. They also developed a scalable manufacturing method that could pave the way for more high-performance lightweight metals. The research waspublished today in Nature. 

“It’s been proposed that nanoparticles could really enhance the strength of metals without damaging their plasticity, especially light metals like magnesium, but no groups have been able to disperse ceramic nanoparticles in molten metals until now,” said Xiaochun Li, the principal investigator on the research and Raytheon Chair in Manufacturing Engineering at UCLA. “With an infusion of physics and materials processing, our method paves a new way to enhance the performance of many different kinds of metals by evenly infusing dense nanoparticles to enhance the performance of metals to meet energy and sustainability challenges in today’s society.”

Structural metals are load-bearing metals; they are used in buildings and vehicles. Magnesium, at just two-thirds the density of aluminum, is the lightest structural metal. Silicon carbide is an ultra-hard ceramic commonly used in industrial cutting blades. The researchers’ technique of infusing a large number of silicon carbide particles smaller than 100 nanometers into magnesium added significant strength, stiffness, plasticity and durability under high temperatures.

The researchers’ new silicon carbide-infused magnesium demonstrated record levels of specific strength — how much weight a material can withstand before breaking — and specific modulus — the material’s stiffness-to-weight ratio. It also showed superior stability at high temperatures.

Ceramic particles have long been considered as a potential way to make metals stronger. However, with microscale ceramic particles, the infusion process results in a loss of plasticity.

Nanoscale particles, by contrast, can enhance strength while maintaining or even improving metals’ plasticity. But nanoscale ceramic particles tend to clump together rather than dispersing evenly, due to the tendency of small particles to attract one other.

To counteract this issue, researchers dispersed the particles into a molten magnesium zinc alloy. The newly discovered nanoparticle dispersion relies on the kinetic energy in the particles’ movement. This stabilizes the particles’ dispersion and prevents clumping.

To further enhance the new metal’s strength, the researchers used a technique called high-pressure torsion to compress it.

“The results we obtained so far are just scratching the surface of the hidden treasure for a new class of metals with revolutionary properties and functionalities,” Li said.

The new metal (more accurately called a metal nanocomposite) is about 14 percent silicon carbide nanoparticles and 86 percent magnesium. The researchers noted that magnesium is an abundant resource and that scaling up its use would not cause environmental damage.

The paper’s lead author is Lian-Yi Chen, who conducted the research as a postdoctoral scholar in Li’s Scifacturing Laboratory at UCLA. Chen is now an assistant professor of mechanical and aerospace engineering at Missouri University of Science and Technology.

The paper’s other authors from UCLA include Jia-Quan Xu, a graduate student in materials science and engineering; Marta Pozuelo, an assistant development engineer; and Jenn-Ming Yang, professor of materials science and engineering.

The other authors on the paper are Hongseok Choi, of Clemson University; Xiaolong Ma, of North Carolina State University; Sanjit Bhowmick of Hysitron, Inc. of Minneapolis; and Suveen Mathaudhu of UC Riverside.

UCLA

Researchers gauge quantum properties of nanotubes, essential for next-gen electronics

How do you get to know a material that you cannot see?

That is a question that researchers studying nanomaterials--objects with features at the sub-micrometer scales such as quantum dots, nanoparticles and nanotubes--are seeking to answer.

Though recent discoveries--including a super-resolution microscopy which won the Nobel Prize in 2014--have greatly enhanced scientists' capacity to use light to learn about these small-scale objects, the wavelength of the inspecting radiation is always much larger than the scale of the nano-objects being studied. For example, nanotubes and nanowires-the building blocks of next-generation electronic devices-have diameters that are hundreds of times smaller than the light could resolve. Researchers must find ways to circumvent this physical limitation in order to achieve sub-wavelength spatial resolution and explore the nature of these materials for future computers.

Today, a group of scientists--John A. Rogers, Eric Seabron, Scott MacLaren and Xu Xie from the University of Illinois at Urbana-Champaign; Slava V. Rotkin from Lehigh University; and,William L. Wilson from Harvard University--are reporting on the discovery of an important method for measuring the properties of nanotube materials using a microwave probe. Theirfindings have been published in ACS Nano in an article called: "Scanning Probe Microwave Reflectivity of Aligned Single-Walled Carbon Nanotubes: Imaging of Electronic Structure and Quantum Behavior at the Nanoscale."

The researchers studied single-walled carbon nanotubes. These are 1-dimensional, wire-like nanomaterials that have electronic properties that make them excellent candidates for next generation electronics technologies. In fact, the first prototype of a nanotube computer has already been built by researchers at Stanford University. The IBM T.J. Watson Research Center is currently developing nanotube transistors for commercial use.

For this study, scientists grew a series of parallel nanotube lines, similar to the way nanotubes will be used in computer chips. Each nanotube was about 1 nanometer wide--ten times smaller than expected for use in the next generation of electronics. To explore the material's properties, they then used microwave impedance microscopy (MIM) to image individual nanotubes.

"Although microwave near-field imaging offers an extremely versatile 'nondestructive' tool for characterizing materials, it is not an immediately obvious choice," explained Rotkin, a professor with a dual appointment in Lehigh's Department of Physics and Department of Materials Science and Engineering. "Indeed, the wavelength of the radiation used in the experiment was even longer than what is typically used in optical microscopy-about 12 inches, which is approximately 100,000,000 times larger than the nanotubes we measured."

He added: "The nanotube, in this case, is like a very bright needle in a very large haystack."

The imaging method they developed shows exactly where the nanotubes are on the silicon chip. More importantly, the information delivered by the microwave signal from individual nanotubes revealed which nanotubes were and were not able to conduct electric current. Unexpectedly, they were finally able to measure the nanotube quantum capacitance-a very unique property of an object from the nano-world-under these experimental conditions.

"We began our collaboration seeking to understand the images taken by the microwave microscopy and ended by unveiling the nanotube's quantum behavior, which can now be measured with atomistic resolution," said Rotkin.

As an inspection tool or metrology technique, this approach could have a tremendous impact on future technologies, allowing optimization of processing strategies including scalable enriched nanotube growth, post-growth purification, and fabrication of better device contacts. One can now distinguish, in one simple step, between semiconductor nanotubes that are useful for electronics and metallic ones that can cause a computer to failure. Moreover this set of imaging modes sheds light on the quantum properties of these 1D structures.

Lehigh University

2D Islands in Graphene Hold Promise for Future Device Fabrication

Berkeley Lab Scientists Discovery Could Help Improve Graphene Electronics

 

In what could prove to be a significant advance in the fabrication of graphene-based nanodevices, a team of Berkeley Lab researchers has discovered a new mechanism for assembling two-dimensional (2D) molecular “islands” that could be used to modify graphene at the nanometer scale. These 2D islands are comprised of F4TCNQ molecules that trap electrical charge in ways that are potentially useful for graphene-based electronics.

“We’re reporting a scanning tunneling microscopy and non-contact atomic force microscopy study of F4TCNQ molecules at the surface of graphene in which the molecules coalesce into 2D close-packed islands,” says Michael Crommie, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s Physics Department. “The resulting islands could be used to control the charge-carrier density in graphene substrates, as well as to modify how electrons move through graphene-based devices. They might also be used to form precise nanoscale patterns that exhibit atomic-scale structural perfection unmatched by conventional fabrication techniques.”

Crommie is one of four corresponding authors of a paper describing this research published byACS Nano. The paper is titled “Molecular Self-Assembly in a Poorly Screened Environment: F4TCNQ on Graphene/BN.” The other corresponding authors are Steven Louie and Marvin Cohen, both with Berkeley Lab and UC Berkeley, and Jiong Lu of the National University of Singapore.

Graphene is a sheet of pure carbon just one atom thick through which electrons speed 100 times faster than they move through silicon. Graphene is also slimmer and stronger than silicon, making it a potential superstar material for the electronics industry. However, graphene must be electrically doped to tune the number of charge carriers it contains in order to be useful in devices, and F4TCNQ has proven to be an effective dopant for transforming graphene into a “p-type” semiconductor.

“F4TCNQ is known to extract electrons from a substrate, thus changing the substrate charge-carrier density,” Crommie says. “Previous studies looked at F4TCNQ adsorbed on graphene supported by a metal substrate, which creates a highly screened environment. F4TCNQ adsorbed on graphene supported by the insulator boron nitride (BN) creates a poorly screened environment. We found that, unlike with metals, F4TCNQ molecules on graphene/BN form 2D islands by a unique self-assembly mechanism that is driven by the long-range Coulomb interactions between the charged molecules. Negatively-charged molecules coalesce into an island, increasing the local work function above the island and causing additional electrons to flow into the island. These additional electrons cause the total energy of the graphene layer to decrease, resulting in island cohesion.”

Crommie and his co-authors believe that this 2D island formation mechanism should also apply to other molecular adsorbate systems that exhibit charge transfer in poorly screened environments, thereby opening the door to tuning the properties of graphene layers for device applications.

In addition to Crommie, Louie,Cohen and Lu, other co-authors of  ACS Nano paper were Hsin-Zon Tsai, Arash Omrani, Sinisa Coh, Hyungju, Sebastian Wickenburg, Young-Woo Son, Dillon Wong, Alexander Riss, Han Sae Jung, Giang Nguyen, Griffin Rodgers, Andrew Aikawa, Takashi Taniguchi, Kenji Watanabe and Alex Zettl.

Berkeley Lab

A step towards quantum electronics

Work of physicists at the University of Geneva (UNIGE), Switzerland, and the Swiss Federal Institute of Technology in Zurich (ETH Zurich), in which they connected two materials with unusual quantum-mechanical properties through a quantum constriction, could open up a novel path towards both a deeper understanding of physics and future electronic devices. Their results have just been published in the journal Science.

The researchers work with atoms that are trapped in laser beams and thus isolated from any external disturbance. Lasers are also used to cool the atoms to temperatures lower than those found anywhere else in the entire Universe. These 'ultracold' temperatures then enable creating clean materials that possess intriguing quantum-mechanical properties, such as unusual superconductivity. Thierry Giamarchi, professor at the UNIGE and responsible for the theoretical part of the study, explains: "In a cold-atom superconductor, the particles interact very strongly, whereas the interaction is usually very weak. This brings out strong-interaction effects through cooling could be compared to freezing water: the basic system is the same, but the result after cooling is very different."

The experimental team in Zurich, led by Tilman Esslinger and Jean-Philippe Brantut, has now overcome the challenges to efficiently transport ultracold atoms between two quantum superconductors with strong interactions through a single quantum point, a so-called quantum point contact. "With this new quantum connection, we can now reveal new effects in these superconducting quantum systems. It is a fundamental breakthrough in the way we can use quantum physics with cold atoms", says Giamarchi, from UNIGE's Faculty of Science.

A collaboration serving innovation

In general, it is difficult to produce a clean junction between quantum materials. Thanks to the collaboration between the teams in Geneva and Zurich, an important step has now been taken towards developing efficient junctions. For their ultracold atoms, the researchers produced junctions with a transparency close to 100 %. This advance is a crucial step towards understanding quantum transport in ultracold atoms and will enable fundamental studies of superconductors and other quantum materials. But interconnecting quantum materials such as superconductors might bring also new possibilities for more efficient information processing, beyond what is possible with techniques currently available for connecting, in computers and electronic devices, active elements such as transistors to form electronic circuits.

Now that junctions between quantum materials with strong interactions can be produced, scientists might eventually create novel materials that can be used in everyday applications. The unconventional approach developed in Geneva and Zurich therefore establishes the first basis for new technologies and opens up a new research direction that might lead to creating ultrafast and robust electronic networks -- a dream that many physicists share.

University of Geneva

Nanotechnology World Association

Spintronics, low-energy electricity take a step closer

Topological insulators are materials that let electric current flow across their surface while keeping it from passing it through their bulk. This exotic property makes topological insulators very promising for electricity with less energy loss, spintronics, and perhaps even quantum computing.

EPFL scientists have now identified a new class of topological insulators, and have discovered its first representative material, which could propel topological insulators into applications. The work, which was carried out within the framework of the EPFL-led NCCR Marvel project, is published in Nature Materials.

The technological promise of topological insulators has led to an intense search for optimal natural and man-made materials with such properties. Such research combines theoretical work that predicts what properties the structure of a particular material would have. The "candidate" materials that are identified with computer simulations are then passed for experimental examination to see if their topological insulating properties match the theoretical predictions.

This is what the lab of Oleg Yazyev at EPFL's Institute of Theoretical Physics has accomplished, working with experimentalist colleagues from around the world. By theoretically testing potential candidates from the database of previously described materials, the team has identified a material, described as a "crystalline phase" of bismuth iodide, as the first of a new class of topological insulators.

What makes this material particularly exciting is the fact that its atomic structure does not resemble any other topological insulator known to date, which makes its properties very different as well.

One clear advantage of bismuth iodide is that its structure is more ordered than that of previously known topological insulators, and with fewer natural defects. In order to have an insulating interior, a material must have as few defects in its structure as possible.

"What we want is to pass current across the surface but not the interior," explains Oleg Yazyev. "In theory, this sounds like an easy task, but in practice you'll always have defects. So you need to find a new material with as few of them as possible." The study shows that even these early samples of bismuth iodide appear to be very clean with very small concentration of structural imperfections.

After characterizing bismuth iodide with theoretical tools, the scientists tested it experimentally with an array of methods. The main evidence came from a direct experimental technique called "angle-resolved photoemission spectroscopy". This method allows researchers to "see" electronic states on the surface of a solid material, and has become a key technique for proving the topological nature of electronic states at the surface.

The measurements, carried out at the Lawrence Berkeley National Lab, proved to be fully consistent with the theoretical predictions made by Gabriel Autès, a postdoc at Yazyev's lab and lead author of the study. The actual electron structure calculations were performed at the Swiss National Supercomputing Centre, while data analysis included a number of scientists from EPFL and other institutions.

"This study began as theory and went through the entire chain of experimental verification," says Yazyev. "For us is a very important collaborative effort." His lab is now exploring further the properties of bismuth iodide, as well materials with similar structures. Meanwhile, other labs are joining the effort to support the theory behind the new class of topological insulators and propagate the experimental efforts.

This study was carried out within the framework of NCCR Marvel, a research effort on Computational Design and Discovery of Novel Materials, created by the Swiss National Science Foundation and led by EPFL. It currently includes 33 labs across 11 Swiss institutions. The work presented here involved a collaboration of EPFL's Institute of Theoretical Physics and Institute of Condensed Matter Physics with TU Dresden; the Lawrence Berkeley National Laboratory; the University of California, Berkeley; Lomonosov Moscow State University; Ulm University; Yonsei University; Pohang University of Science and Technology; and the Institute for Basic Science, Pohang. The study was funded by the Swiss National Science Foundation, the ERC, NCCR-MARVEL, the Deutsche Forschungsgemeinschaft, the U.S. Department of Energy, and the Carl-Zeiss Foundation.

EPFL