The main problem here is to establish contacts between metal and semiconductor on a nanoscale in a reliable way. This is much more difficult with a high proportion of germanium than with silicon. The team at TU Wien, however, together with research teams from Linz and Thun (Switzerland), has now shown that this problem can be solved — with contacts made of crystalline aluminium of extremely high quality and a sophisticated silicon germanium layer system. This enables different interesting contact properties — especially for optoelectronic and quantum components.

The problem with oxygen

“Every semiconductor layer is automatically contaminated in conventional processes; this simply cannot be prevented at the atomic level,” says Masiar Sistani from the Institute for Solid State Electronics at TU Wien. First and foremost, it is oxygen atoms that accumulate very quickly on the surface of the materials — an oxide layer is formed.

With silicon, however, this is not a problem: silicon always forms exactly the same kind of oxide. “With germanium, however, things are much more complicated,” explains Masiar Sistani. “In this case, there is a whole range of different oxides that can form. But that means that different nanoelectronic devices can have very different surface compositions and therefore different electronic properties.”

If you now want to connect a metallic contact to these components, you have a problem: Even if you try very hard to produce all these components in exactly the same way, there are still inevitably massive differences — and that makes the material complex to handle for use in the semiconductor industry.

“Reproducibility is a big problem,” says Prof. Walter Weber, the head of the Institute for Solid State Electronics, TU Wien. “If you use germanium-rich silicon germanium, you can’t be sure that the electronic component, after you’ve put contacts on it, will really have the characteristics you need.” As a result, this material is only used to a limited extent in chip production.

That is a pity, because silicon germanium would have decisive advantages: “The charge carrier concentration is higher, especially positive charge carriers, the so-called “holes,” can move much more efficiently in this material than in silicon. The material would therefore allow much higher clock frequencies with increased energy efficiency than our current silicon chips,” says Lukas Wind, a doctoral student in Walter Weber’s research group.

The “perfect” interface

Now, however, the research team has been able to show how the problem can be solved: They found a method to create perfect interfaces between aluminium contacts and silicon germanium components on an atomic scale. In a first step, a layer system is produced with a thin silicon layer and the actual material from which the electronic components are to be made — the silicon-germanium.

By heating the structure in a controlled manner, a contact can now be created between the aluminium and the silicon: At around 500 degrees Celsius, distinctive diffusion occurs, the atoms can leave their place and start to migrate. Silicon and germanium atoms move into the aluminium contact relatively quickly, and aluminium fills the vacated space.

“The diffusion dynamics in the layer system used thus create an interface between aluminium and the silicon germanium with an extremely thin silicon layer in between,” explains Masiar Sistani. Through this manufacturing process, oxygen atoms never have the opportunity to get to this atomically sharp and highly pure interface.

“Our experiments show that these contact points can be produced in a reliable and easily reproducible way,” says Walter Weber. “The technological systems you need to do this are already being used in the chip industry today. So, this is not just a laboratory experiment, but a process that could be used relatively quickly in the chip industry.

The decisive advantage of the presented manufacturing process is that high-quality contacts can be produced regardless of the silicon-germanium composition. “We are convinced that the presented abrupt, robust and reliable metal-semiconductor contacts are highly interesting for a variety of new nanoelectronic, optoelectronic and quantum devices,” says Walter Weber.

One puzzle known as the “radius valley” refers to the rarity of exoplanets with a radius about 1.8 times that of Earth. NASA’s Kepler spacecraft observed planets of this size about 2-3 times less frequently than it observed super-Earths with radii about 1.4 times that of Earth and mini-Neptunes with radii about 2.5 times Earth’s. The second mystery, known as “peas in a pod,” refers to neighboring planets of similar size that have been found in hundreds of planetary systems. Those include TRAPPIST-1 and Kepler-223, which also feature planetary orbits of near-musical harmony.

“I believe we are the first to explain the radius valley using a model of planet formation and dynamical evolution that self-consistently accounts for multiple constraints of observations,” said Rice University’s André Izidoro, corresponding author of a study published this week in Astrophysical Journal Letters. “We’re also able to show that a planet-formation model incorporating giant impacts is consistent with the peas-in-a-pod feature of exoplanets.”

Izidoro, a Welch Postdoctoral Fellow at Rice’s NASA-fundedCLEVER Planets project, and co-authors used a supercomputer to simulate the first 50 million years of the development of planetary systems using a planetary migration model. In the model, protoplanetary disks of gas and dust that give rise to young planets also interact with them, pulling them closer to their parent stars and locking them in resonant orbital chains. The chains are broken within a few million years, when the disappearance of the protoplanetary disk causes orbital instabilities that lead two or more planets to slam into one another.

Planetary migration models have been used to study planetary systems that have retained their resonant orbital chains. For example, Izidoro and CLEVER Planets colleagues used a migration model in 2021 to calculate the maximum amount of disruption TRAPPIST-1’s seven-planet system could have withstood during bombardment and still retained its harmonious orbital structure.

In the new study, Izidoro partnered with CLEVER Planets’ investigators Rajdeep Dasgupta and Andrea Isella, both of Rice, Hilke Schlichting of the University of California, Los Angeles, and Christian Zimmermann and Bertram Bitsch of the Max Planck Institute for Astronomy in Heidelberg, Germany.

“The migration of young planets towards their host stars creates overcrowding and frequently results in cataclysmic collisions that strip planets of their hydrogen-rich atmospheres,” Izidoro said. “That means giant impacts, like the one that formed our moon, are probably a generic outcome of planet formation.”

The research suggests planets come in two “flavors,” super-Earths that are dry, rocky and 50% larger than Earth, and mini-Neptunes that are rich in water ice and about 2.5 times larger than Earth. Izidoro said new observations seem to support the results, which conflict with the traditional view that both super-Earths and mini-Neptunes are exclusively dry and rocky worlds.

Based on their findings, the researchers made predictions that can be tested by NASA’s James Webb Space Telescope. They suggest, for instance, that a fraction of planets about twice Earth’s size will both retain their primordial hydrogen-rich atmosphere and be rich in water.

The research was funded by NASA (80NSSC18K0828), the Welch Foundation (C-2035-20200401) and the European Research Council (757448-PAMDORA).

Story Source:

Materials provided by Rice University. Original written by Jade Boyd. Note: Content may be edited for style and length.

The existence of such a brain-preserving state could explain why some patients wake up days or even weeks after they stop receiving ventilation, and it suggests that physicians should take these lengthy recovery times into account when determining a patient’s prognosis.

In a study published Nov. 7 in the Proceedings of the National Academy of Sciences, the investigators connect the pattern seen among those who have survived severe COVID-19 with similar delays known to occur in a small fraction of cardiac arrest patients.

“The delayed recoveries in COVID-19 patients are very much like the rare cases we’ve documented in previous research. In this new paper, we describe a mechanism to explain what we’re seeing in both types of patients,” said study co-senior author Dr. Nicholas D. Schiff, the Jerold B. Katz Professor of Neurology and Neuroscience in the Feil Family Brain and Mind Research Institute and co-director of the Consortium for the Advanced Study of Brain Injury (CASBI) at Weill Cornell Medicine.

He found evidence for this explanation — that patients’ brains are protecting themselves — in animals, most notably painted turtles, that can tolerate extended periods without oxygen.

More than a decade ago, Dr. Schiff and his colleagues first observed these delays among comatose cardiac arrest patients who received cooling therapy to reduce brain damage caused by a loss of blood flow. In one such case, a 71-year-old patient took 37 days to awaken, before ultimately making a near-complete recovery.

During the pandemic, Dr. Schiff, who is also a neurologist at NewYork-Presbyterian/Weill Cornell Medical Center, performed neurology consultations for COVID-19 patients, and he soon began seeing similar, delayed awakenings occurring when patients were taken off ventilators and stopped receiving the sedatives given to them to reduce their movement.

In a separate analysis of a large cohort of Covid-19 patients from Weill Cornell Medicine and two other major U.S. medical centers, Dr. Schiff and his colleagues, including co-author of the current paper, Dr. Emery N. Brown, professor of anesthesia at Harvard Medical School, Edward Hood Taplin Professor of Medical Engineering and Computational Neuroscience in The Picower Institute for Learning and Memory at MIT and an anesthesiologist at Massachusetts General Hospital, found that a quarter of patients who survived ventilation took 10 days or longer to recover consciousness. The more oxygen deprivation they suffered while on the ventilator, the longer that delay.

In the prior study of cardiac patients, the researchers recorded a distinctive pattern in brain activity, one also seen in patients under deep anesthesia. (Recordings from COVID-19 patients are extremely limited.) Dr. Schiff read that a similar pattern had been seen in the brains of painted turtles, which can withstand up to five months without oxygen under ice in the winter. To do so, they activate the same inhibitory system within the brain targeted by anesthetics given to human cardiac and COVID-19 patients but in novel ways developed by evolutionary specializations.

Dr. Schiff and Dr. Brown propose that, by chance, the same protective response emerges in the patients.

“It is our theory that oxygen deprivation as well as practices in the ICU, including commonly used anesthetics, expose elements of strategies that animals use to survive in extreme conditions,” Dr. Schiff said.

“These observations may offer new insights into the mechanisms of how certain anesthetics produce unconsciousness and new approaches for ICU sedation and for fostering recovery from disorders of consciousness,” Dr. Brown added.

When patients fail to regain consciousness for an extended time, physicians may recommend withdrawing life-supporting care. This threshold is typically set at 14 days or less for cardiac patients, while no such guidelines exist for COVID-19.

In light of this new research, however, so long as they lack brain injuries, physicians should avoid making negative projections about these patients’ potential to recover, note the researchers.

Supercomputer simulations have helped scientists reveal in a bilayer moiré system a new species of an electronic phenomenon called an exciton, which is an electrically neutral quasiparticle, yet one that can carry energy and consists of an electron and electron ‘hole’ that can be created for example by light impinging certain semiconductors and other materials.

The newly discovered excitons were produced by moiré patterns from two-dimensional sheets of exotic semiconductors called transition metal dichalcogenides, with the electron bound to the hole but separated from each other by a characteristic distance in the sheet. This was named an intralayer charge-transfer exciton and was a surprise to the scientists because such excitons do not exist in the individual sheets. The research can be used in the development of new optical sensors and communication technology such as optical fibers and lasers.

Novel Exciton Discovered

“In this work we discovered a novel exciton of unforeseen intralayer charge-transfer characteristics in a moiré superlattice formed by two atomically thin layers of transition metal dichalcogenide materials,” said Steven G. Louie, a distinguished professor of physics at the University of California, Berkeley (UC Berkeley), and a senior faculty scientist at the Lawrence Berkeley National Laboratory (LBNL).

Louie is the corresponding author of research published August 2022 in the journal Nature. In it, the scientists developed computer models that go beyond the conventional parameterized models that have been used to describe moiré systems and moiré excitons. Instead, they performed ab initio calculations that only start with the identity and initial position of the 3,903 atoms of the moiré superlattice unit cell.

First Principles

“It is a powerful method to accurately predict material properties,” Louie added, “because there’s no empirical fitting involved.”

In particular, to calculate the exciton states and the optical property of the moiré systems, they used the state-of-the-art GW plus Bethe-Salpeter equation (GW-BSE) approach, considered one of the most accurate approaches to predicting the optical properties of materials.

What’s more, Louie and colleagues used their calculations to predict the different electronic and optical responses of the excitons in the system. They worked with experimental colleagues Emma C. Regan, Zuocheng Zhang and Professor Feng Wang at UC Berkeley. In a nutshell, Wang and colleagues shone light on the material and then analyzed the reflected light to see how the system responded to the impinging photons under different conditions.

Sure enough, they verified the theoretical predictions by finding distinct signatures of the intralayer charge-transfer exciton in their measurements.

Computational Challenges Overcome With TACC Support

“The GW-BSE method is very accurate, but it is also very computationally challenging,” said study co-author Mit Naik, a postdoctoral researcher working with Professor Louie at UC Berkeley and LBNL.

The challenge in simulating an exciton is that it requires solving difficult six-dimensional integrals, numerous times, and the integrals are massive in the moiré superlattice because of the large size of the moiré unit cell — composed of many unit cells of the individual layers, 25×25 for one layer and 26×26 for the other.

Typical GW-BSE calculations are performed on systems of just up to a hundred atoms in the unit cell. Here, the authors needed calculations for a whooping 3,903 atoms. “Initially, it seemed almost impossible,” Naik said.

Undaunted, the scientists hit on a new method that reduced the computational cost by a million times without loss of accuracy.

“We found a way to approximate each layer’s two thousand atom integral as a sum over many three atom integrals, which were computationally much simpler to compute,” Naik said.

The theoretical technique they developed, called the pristine unit-cell matrix projection (PUMP) method, can be generalized to study other material systems such as interlayer or hybrid moiré excitons in multi-layer moiré superlattices, shallow defects in materials, and more.

The Texas Advanced Computing Center (TACC) supported Steven Louie’s first principles study of cutting-edge materials through a large resource allocation on TACC’s Frontera supercomputer, the most powerful academic system in the U.S and funded by the National Science Foundation.

He also was awarded allocations on TACC’s Stampede2 supercomputer made through the NSF-funded Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS), formerly the Extreme Science and Engineering Discovery Environment (XSEDE).

In their bid to push the limit of the number of atoms in their calculations, the researchers initially faced some problems with running some of the large-scale moiré calculations on Frontera.

“We received support from the research associates at TACC, who helped us resolve these problems,” Naik recalled.

For instance, they were unable to run the large-scale simulations of ground-state properties on Frontera using a standard density functional theory (DFT) package called Quantum Espresso, because of the instability created by a new version of the Intel compiler used to for the DFT package.

The TACC consulting team found a way around the problem by installing an older, more stable version of the compiler. “This allowed us to compute the electronic structure of the WSe2 moiré superlattice that went into this work,” Naik said.

“Frontera and Stampede2 hardware efficiently catered to our requirements by providing state-of-the-art internode connectivity and sufficient memory on each node,” Naik added.

SCIENCE IMPACT

“The intralayer charge-transfer exciton we discovered through this artificial stacking of two monolayers has revealed an extraordinarily strong interplay of the atomic structure and the type of exciton possible in the moiré systems,” Louie said. “This is an important step in advancing the engineering of materials for desired properties.”

That’s because, once light creates charge-transfer excitons, they can be distorted or dissociated more easily with an external field or other perturbations than the standard Wannier-type excitons.

These considerations are important in many basic phenomena and potential applications, such as novel optical sensors and electro-optic modulation nanodevices. “That’s what we’re working on with our experimental colleagues right now,” Louie said.

“Excitonic properties of materials form the basic principles behind many of the optical and optoelectronic devices and sensors that we encounter in our daily lives,” said study co-author Felipe Jornada, an Assistant Professor in the Department of Materials Science and Engineering at Stanford University and a Principal Investigator at the SLAC National Accelerator Laboratory.

He gave the example of OLED TVs and displays, which use light emission from the recombination of excitons in organic semiconductors. Also, new smartwatches use advanced optical sensors to monitor the wearer’s blood oxygen levels.

“Beyond existing devices, the transition metal dichalcogenides and heterostructures of these materials that we’re working on are considered a promising material platform for post-silicon nanoscale optoelectronic devices. Moving forward, we believe that these are going to be a promising route towards novel sensors, displays, and technologies,” Jornada said.

The majority of the funding for the study was provided by the U.S. Department of Energy’s van der Waals heterostructure program (KCWF16). The theory and algorithm development for this work was carried out at the Center for Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM) at the Lawrence Berkeley National Lab. They also used the National Energy Research Science Computing Center (NERSC) at the Lawrence Berkeley National Laboratory to perform part of the calculations.