Category Archives: Packaging Materials

Scientists have developed the world’s best-performing pure spin current source[1] made of bismuth-antimony (BiSb) alloys, which they report as the best candidate for the first industrial application of topological insulators[2]. The achievement represents a big step forward in the development of spin-orbit torque magnetoresistive random-access memory (SOT-MRAM)[3] devices with the potential to replace existing memory technologies.

A research team led by Pham Nam Hai at the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology (Tokyo Tech), has developed thin films of BiSb for a topological insulator that simultaneously achieves a colossal spin Hall effect[4] and high electrical conductivity.

Table 1: θSH: spin Hall angle, σ: conductivity, σSH: spin Hall conductivity.
The figures in the bottom row are those achieved in the present study. Remarkably, the spin Hall conductivity, shown in the right-hand column, is two orders of magnitude greater than the previous record. Credit: Pham Nam Hai

Their study, published in Nature Materials, could accelerate the development of high-density, ultra-low power, and ultra-fast non-volatile memories for Internet of Things (IoT) and other applications now becoming increasingly in demand for industrial and home use.

The BiSb thin films achieve a colossal spin Hall angle of approximately 52, conductivity of 2.5 x 105 and spin Hall conductivity of 1.3×107 at room temperature. (See Table 1 for a performance summary, including all units.) Notably, the spin Hall conductivity is two orders of magnitude greater than that of bismuth selenide (Bi2Se3), reported in Nature in 2014.

Making SOT-MRAM a viable choice

Until now, the search for suitable spin Hall materials for next-generation SOT-MRAM devices has been faced with a dilemma: First, heavy metals such as platinum, tantalum and tungsten have high electrical conductivity but a small spin Hall effect. Second, topological insulators investigated to date have a large spin Hall effect but low electrical conductivity.

The BiSb thin films satisfy both requirements at room temperature. This raises the real possibility that BiSb-based SOT-MRAM could outperform the existing spin-transfer torque (STT) MRAM technology.

“As SOT-MRAM can be switched one order of magnitude faster than STT-MRAM, the switching energy can be reduced by at least two orders of magnitude,” says Pham. “Also, the writing speed could be increased 20 times and the bit density increased by a factor of ten.”

The viability of such energy-efficient SOT-MRAMs has recently been demonstrated in experiments, albeit using heavy metals, conducted by IMEC, the international R&D and innovation hub headquartered in Leuven, Belgium.

If scaled up successfully, BiSb-based SOT-MRAM could drastically improve upon its heavy metal-based counterparts and even become competitive with dynamic random access memory (DRAM), the dominant technology of today.

An attractive, overlooked material

BiSb has tended to be overlooked by the research community due to its small band gap[5] and complex surface states. However, Pham says: “From an electrical engineering perspective, BiSb is very attractive due to its high carrier mobility, which makes it easier to drive a current within the material.”

“We knew that BiSb has many topological surface states, meaning we could expect a much stronger spin Hall effect. That’s why we started studying this material about two years ago.”

The thin films were grown using a high-precision method called molecular beam epitaxy (MBE). The researchers discovered a particular surface orientation named BiSb(012), which is thought to be a key factor behind the large spin Hall effect. Pham points out that the number of Dirac cones[6]0 on the BiSb(012) surface is another important factor, which his team is now investigating.

Challenges ahead

Pham is currently collaborating with industry to test and scale up BiSb-based SOT-MRAM.

“The first step is to demonstrate manufacturability,” he says. “We aim to show it’s still possible to achieve a strong spin Hall effect, even when BiSb thin films are fabricated using industry-friendly technologies such as the sputtering method.”

“It’s been over ten years since the emergence of topological insulators, but it was not clear whether those materials could be used in realistic devices at room temperature. Our research brings topological insulators to a new level, where they hold great promise for ultra-low power SOT-MRAM.”

Excitons could revolutionize the way engineers approach electronics. A team of EPFL researchers has created a new type of transistor – one of the components of circuits – using these particles instead of electrons. What is remarkable is that their exciton-based transistor functions effectively at room temperature, a hitherto insurmountable obstacle. They achieved this by using two 2D materials as semiconductors. Their study, which was published today in Nature, has numerous implications in the field of excitonics, one of the most promising new areas of study alongside photonics and spintronics.

“Our research showed that, by manipulating excitons, we had come upon a whole new approach to electronics,” says Andras Kis, who heads EPFL’s Laboratory of Nanoscale Electronics and Structures (LANES). “We are witnessing the emergence of a totally new field of study, the full scope of which we don’t yet know.”

This breakthrough sets the stage for optoelectronic devices that consume less energy and are both smaller and faster than current devices. In addition, it will be possible to integrate optical transmission and electronic data-processing systems into the same device, which will reduce the number of operations needed and make the systems more efficient.

Higher energy level

Excitons are actually quasiparticles, a term used to describe the interaction between the particles that make up a given substance rather than the substance itself. Excitons consist of an electron and an electron hole. The two are bound together when the electron absorbs a photon and achieves a higher level of energy; the “excited” electron leaves behind a hole in the previous level of energy, which, in band theory, is called a valence band. This hole, also a quasiparticle, is an indication of the missing electron in this band.

Since the electron is negatively charged and the hole is positively charged, the two particles remain bound by an electrostatic force. This bond between the electron and the hole is called Coulomb attraction. And it is in this state of tension and balance that they form an exciton. When the electron finally falls back into the hole, it emits a photon. And with that, the exciton ceases to exist. Put more simply, a photon goes in at one end of the circuit and comes out the other; while inside, it gives rise to an exciton that acts like a particle.

Double success

It is only recently that researchers have begun looking at the properties of excitons in the context of electronic circuits. The energy in excitons had always been considered too fragile and the excitons’ life span too short to be of any real interest in this domain. In addition, excitons could only be produced and controlled in circuits at extremely low temperatures (around -173 oC).

The breakthrough came when the EPFL researchers* discovered how to control the life span of the excitons and how to move them around. They did this by using two 2D materials: tungsten diselenide (WSe2) and molybdenum disulfide (MoS2). “The excitons in these materials exhibit a particularly strong electrostatic bond and, even more importantly, they are not quickly destroyed at room temperature,” explains Kis.

The researchers were also able to significantly lengthen the excitons’ life span by using the fact that the electrons always found their way to the MoS2 while the holes always ended up in the WSe2. And, working with two Japanese researchers**, they kept the excitons going even longer by protecting the semiconductor layers with boron nitride (BN).

“We created a special type of exciton, where the two sides are farther apart than in the conventional particle,” says the researcher. “This delays the process in which the electron returns to the hole and light is produced. It’s at this point, when the excitons remain in dipole form for slightly longer, that they can be controlled and moved around using an electric field.”

Growing a batch of carbon nanotubes that are all the same may not be as simple as researchers had hoped, according to Rice University scientists.

Rice materials theorist Boris Yakobson and his team bucked a theory that when growing nanotubes in a furnace, a catalyst with a specific atomic arrangement and symmetry would reliably make carbon nanotubes of like chirality, the angle of its carbon-atom lattice.

Rice University scientists have decoded the unusual growth characteristic of carbon nanotubes that start out as one chirality but switch to another, resulting in nearly homogenous batches of single-walled nanotubes. The nanotubes grow via chemical vapor deposition with a carbon-tungsten alloy catalyst. Credit: Evgeni Penev/Rice University

Instead, they found the catalyst in question starts nanotubes with a variety of chiral angles but redirects almost all of them toward a fast-growing variant known as (12,6). The cause appears to be a Janus-like interface that is composed of armchair and zigzag segments – and ultimately changes how nanotubes grow.

Because chirality determines a nanotube’s electrical properties, the ability to grow chiral-specific batches is a nanotechnology holy grail. It could lead to wires that, unlike copper or aluminum, transmit energy without loss. Nanotubes generally grow in random chiralities.

The Rice theoretical study detailed in the American Chemical Society journal Nano Letterscould be a step toward catalysts that produce homogenous batches of nanotubes, Yakobson said.

Yakobson and colleagues Evgeni Penev and Ksenia Bets and graduate student Nitant Gupta tackled a conundrum presented by other experimentalists at a 2013 workshop who used an alloy of cobalt and tungsten to catalyze single-walled nanotubes. In that lab’s batch, more than 90 percent of the nanotubes had a chirality of (12,6).

The numbers (12,6) are coordinates that refer to a nanotube’s chiral vector. Carbon nanotubes are rolled-up sheets of two-dimensional graphene. Graphene is highly conductive, but when it is rolled into a tube, its conductivity depends on the angle — or chirality — of its hexagonal lattice.

Armchair nanotubes — so called because of the armchair-like shape of their edges — have identical chiral indices, like (9,9), and are highly desired for their perfect conductivity. They are unlike zigzag nanotubes, such as (16,0), which may be semiconductors. Turning a graphene sheet a mere 30 degrees will change the nanotube it forms from armchair to zigzag or vice versa.

Penev said the experimentalists explained their work “in a way which was puzzling from the very beginning. They said this catalyst has a specific symmetry that matches the (12,6) edge, so these nanotubes preferentially nucleate and grow. This was the emergence of the so-called symmetry matching idea of carbon nanotube selective growth.

“We read and digested that, but we still couldn’t wrap our minds around it,” he said.

Shortly after the 2013 conference, the Yakobson lab published its own theory of nanotube growth, which showed that the balance between two opposing forces — the energy of the catalyst-nanotube contact and the speed at which atoms attach themselves to the growing tube at the interface — are responsible for chirality.

Five years later, that turns out to be just as true in their new paper, though with a twist. The Rice calculations show that the alloy Co7W6 promotes the formation of the Janus-like interface that ensures the necessary kink at the edge and allows carbon atoms to attach themselves to the nanotube’s foundation. But the catalyst also forces the nanotube to incorporate defects that alter its initial chirality midstream.

“We uncovered two things,” Yakobson said. “One is that the carbon atom types at the base of the nanotube separate into armchair and zigzag segments. The second is the tendency for the formation of defects that drive the chirality, or helicity, change. That makes (12,6) a sort of transient attractor, at least during short experiments. If they were able to grow forever, (12,6) nanotubes would eventually switch to armchairs.”

The unusual growth pattern might have been diagnosed much earlier if it weren’t for an age-old typo that required some dogged detective work.

“The trouble was in a standard online database that gives the crystal structure of this cobalt-tungsten alloy,” said Bets, co-lead author of the paper with Penev. “One entry was wrong. That messed up the structure so badly that we couldn’t use it in our density functional theory calculations.”

Once they found the error, Bets and co-author Gupta went back to the 1938 German paper that was first to correctly detail the structure of Co7W6. Even with that in hand, the team’s calculations used every bit of computing power they could find to simulate the energetic connections between each atom in the catalyst and carbon feedstock.

“We figured out that if we had run the calculations in series instead of in parallel, they would have taken the equivalent of at least 2,000 years of computer time,” Bets said.

“This paper is remarkable in many aspects: in the timing, the amount of detail and the surprises we found,” Penev said. “We’ve never had a project like this. We don’t yet know how this will be applicable to other materials, but we’re working on it.”

“There are four or five experimental papers, pretty recent ones, that also show a change of chirality during growth,” Bets said. “In fact, because it’s a probabilistic process, it’s essentially unavoidable. But until now it’s never been considered in the theoretical investigation of growth.”

Solar cells need to slim down.

Solar cells are devices that absorb photons from sunlight and convert their energy to move electrons — enabling the production of clean energy and providing a dependable route to help combat climate change. But most solar cells used widely today are thick, fragile and stiff, which limits their application to flat surfaces and increases the cost to make the solar cell.

“Thin-film solar cells” could be 1/100th the thickness of a piece of paper and flexible enough to festoon surfaces ranging from an aerodynamically sleek car to clothing. To make thin-film solar cells, scientists are moving beyond the “classic” semiconductor compounds, such as gallium arsenide or silicon, and working instead with other light-harvesting compounds that have the potential to be cheaper and easier to mass produce. The compounds could be widely adopted if they could perform as well as today’s technology.

In a paper published online this spring in the journal Nature Photonics, scientists at the University of Washington report that a prototype semiconductor thin-film has performed even better than today’s best solar cell materials at emitting light.

“It may sound odd since solar cells absorb light and turn it into electricity, but the best solar cell materials are also great at emitting light,” said co-author and UW chemical engineering professor Hugh Hillhouse, who is also a faculty member with both the UW’s Clean Energy Institute and Molecular Engineering & Sciences Institute. “In fact, typically the more efficiently they emit light, the more voltage they generate.”

The UW team achieved a record performance in this material, known as a lead-halide perovskite, by chemically treating it through a process known as “surface passivation,” which treats imperfections and reduces the likelihood that the absorbed photons will end up wasted rather than converted to useful energy.

“One large problem with perovskite solar cells is that too much absorbed sunlight was ending up as wasted heat, not useful electricity,” said co-author David Ginger, a UW professor of chemistry and chief scientist at the CEI. “We are hopeful that surface passivation strategies like this will help improve the performance and stability of perovskite solar cells.”

Ginger’s and Hillhouse’s teams worked together to demonstrate that surface passivation of perovskites sharply boosted performance to levels that would make this material among the best for thin-film solar cells. They experimented with a variety of chemicals for surface passivation before finding one, an organic compound known by its acronym TOPO, that boosted perovskite performance to levels approaching the best gallium arsenide semiconductors.

“Our team at the UW was one of the first to identify performance-limiting defects at the surfaces of perovskite materials, and now we are excited to have discovered an effective way to chemically engineer these surfaces with TOPO molecules,” said co-lead author Dane deQuilettes, a postdoctoral researcher at the Massachusetts Institute of Technology who conducted this research as a UW chemistry doctoral student. “At first, we were really surprised to find that the passivated materials seemed to be just as good as gallium arsenide, which holds the solar cell efficiency record. So to double-check our results, we devised a few different approaches to confirm the improvements in perovskite material quality.”

DeQuilettes and co-lead author Ian Braly, who conducted this research as a doctoral student in chemical engineering, showed that TOPO-treating a perovskite semiconductor significantly impacted both its internal and external photoluminescence quantum efficiencies — metrics used to determine how good a semiconducting material is at utilizing an absorbed photon’s energy rather than losing it as heat. TOPO-treating the perovskite increased the internal photoluminescence quantum efficiencies by tenfold — from 9.4 percent to nearly 92 percent.

“Our measurements observing the efficiency with which passivated hybrid perovskites absorb and emit light show that there are no inherent material flaws preventing further solar cell improvements,” said Braly. “Further, by fitting the emission spectra to a theoretical model, we showed that these materials could generate voltages 97 percent of the theoretical maximum, equal to the world record gallium arsenide solar cell and much higher than record silicon cells that only reach 84 percent.”

These improvements in material quality are theoretically predicted to enable the light-to-electricity power conversion efficiency to reach 27.9 percent under regular sunlight levels, which would push the perovskite-based photovoltaic record past the best silicon devices.

The next step for perovskites, the researchers said, is to demonstrate a similar chemical passivation that is compatible with easily manufactured electrodes — as well as to experiment with other types of surface passivation.

“Perovskites have already demonstrated unprecedented success in photovoltaic devices, but there is so much room for further improvement,” said deQuilettes. “Here we think we have provided a path forward for the community to better harness the sun’s energy.”

Semiconductor Research Corporation (SRC), today announced the release of $26 million in added research funding for its New Science Team (NST) Joint University Microelectronics Program (JUMP). JUMP will fund 24 additional research projects spanning 14 unique U.S. universities. The new projects will be integrated into JUMP’s six existing research centers. NST will continue to distribute funds over its five-year plan, and industrial sponsors are welcome to join to further accentuate those plans.

The awards have been given to 27 faculty and will enhance the program’s expertise in technical areas such as atomic layer deposition (ALD), novel ferroelectric and spintronic materials and devices, 3D and heterogeneous integration, thermal management solutions, architectures for machine learning and statistical computing, memory abstractions, reconfigurable RF frontends, and mmWave to THz arrays and systems for communications and sensing.

“The goal of the NST project is not only to extend the viability of Moore’s Law economics through 2030, but to also change the research paradigm to one of co-optimization across the design hierarchy stack through multi-disciplinary teams,” said Ken Hansen, President and CEO of Semiconductor Research Corporation. “Our strategic partnerships with industry, academia, and government agencies foster the environment needed to realize the next wave of semiconductor technology innovations.”

“A new wave of fundamental research is required to unlock the ultimate potential of autonomous vehicles, smart cities, and Artificial Intelligence (AI),” said Dr. Michael Mayberry, Senior Vice President and Chief Technology Officer of Intel and the elected Chairman of the NST Governing Council. “Such advances will be fueled by novel and far-reaching improvements in the materials, devices, circuits, architectures, and systems used for computing and communications.”

The JUMP program, a consortium consisting of 11 industrial participants and the Defense Advanced Research Projects Agency (DARPA), is one of two complementary research programs for the NST project—a 5-year, greater than $300 million SRC initiative launched this January. JUMP and its six thematic centers will advance a new wave of fundamental research focused on the high-performance, energy-efficient microelectronics for communications, computing, and storage needs for 2025 and beyond.

Applied Materials, Inc. today announced it has been awarded a contract by the Defense Advanced Research Projects Agency (DARPA) to develop a new type of electronic switch for artificial intelligence that mimics the way the human brain works to enable dramatic improvements in performance and power efficiency. The project is being supported by DARPA’s Electronics Resurgence Initiative, a multi-year research effort intended to achieve far-reaching improvements in electronics performance well beyond the limits of traditional Moore’s Law scaling.

Applied is working with Arm and Symetrix to develop a new neuromorphic switch based on CeRAM memory that can allow data to be stored and processed in the same material. The goal of the project is to enable a major improvement in artificial intelligence compute performance and power efficiency with the use of analog signal processing as compared to current digital approaches.

“This project is a perfect example of how new materials and architectures can be developed to enable new ways to accelerate artificial intelligence applications as classic Moore’s Law scaling slows,” said Steve Ghanayem, senior vice president of New Markets and Alliances at Applied Materials. “Applied has the industry’s broadest portfolio in materials engineering capabilities and is excited to be part of a team enabling breakthroughs for artificial intelligence.”

Today’s announcement was part of DARPA’s first annual ERI Summit in San Francisco. Applied Materials’ president and CEO, Gary Dickerson, delivered a keynote speech at the event highlighting the need for materials innovation in the AI era and calling for a new level of industry connectivity to speed progress across materials engineering, design and manufacturing.

Announced in September 2017, the ERI Materials & Integration programs seek to answer this question: Can we use the integration of unconventional electronics materials to enhance conventional silicon circuits and continue the progress in performance traditionally associated with scaling?

The Applied Materials team is part of the ERI Foundations Required for Novel Compute (FRANC) program, which seeks innovations that go beyond von Neumann compute architectures. Central is the design of circuits that leverage the properties of new materials and integration schemes to process data in ways that eliminate or minimize data movement. The novel compute topologies that come out of this effort could allow processing to happen where the data is stored with structures that are radically different from conventional digital logic processors, ultimately allowing for significant gains in compute performance.

Applied Materials, Inc. (Nasdaq:AMAT) is a developer of materials engineering solutions used to produce virtually every new chip and advanced display in the world.

After a quiet period due to the saturation of the mobile handset industry, the GaAs wafer market wakes up.  The technical choice made by Apple creates a real and vast enthusiasm for GaAs solutions. 3D sensing in mobile phone as well as LiDAR’s applications are giving a new breath for GaAs substrates suppliers.

Under its new technology & market report “GaAs Wafer & Epiwafer Market: RF, Photonics, LED and PV applications”, Yole Développement (Yole) announces a 15% CAGR between 2017 and 2023 (in volume), with an impressive 37%, especially for photonics applications (1).

GaAs analysis from Yole proposes a comprehensive overview of the GaAs wafer and epi wafer industry. This report outlines Yole’s understanding of the industrial landscape, its evolution as well as the technical challenges. The analysts are offering a relevant technical description of GaAs wafer and epiwafer growth. Market size and forecasts are also delivered in four big applicative markets: RF, Photonics, LED, and PV. Photonics applications are driving the GaAs wafer and epiwafer market into a new era. Yole’s analysts invite you to discover the latest GaAs technology and market trends.

Figure 1

 As one of the most mature compound semiconductors, GaAs has been ubiquitous as the building block of power amplifiers in every mobile handset. In 2018, GaAs RF business represents more than 50% of the GaAs wafer market. However, market growth has slowed down in the past couple years due to the handset market’s gradual saturation and shrinking die size. “At Yole, we expect GaAs to remain the mainstream technology for sub-6 GHz instead of CMOS, owing to GaAs’ high power and linearity performance as required by carrier aggregation and MIMO technology,” explains Dr. Hong Ling, Technology and Market Analyst at Yole.

Since 2017, GaAs wafer has been particularly notable in photonics applications. When Apple introduced its new iPhone X with a 3D sensing function using GaAs-based lasers, it paved the way for a significant boost in the GaAs photonics market. GaAs wafers market segment for photonics applications should reach US$150 million by 2023.

“GaAs-based ROY and infrared LED applications have also caught our attention”, asserts Dr. Ezgi Dogmus, Technology & Market Analyst at Yole. “We estimate, 2017-2023 CAGR achieves 21% (in units) for the total GaAs LED market, surpassing more than half of GaAs wafer volume by 2023.”

In terms of the wafer and epiwafer businesses, each application requires a different size and quality when determining wafer and epiwafer prices. As a new entrant, photonics applications will impose new specification requirements compared to the well-established RF and LED wafer and epiwafers, creating significant ASP diversity.

From a value chain point of view, the GaAs photonics market’s remarkable growth potential will offer plenty of opportunities for wafer, epiwafer, and MOCVD equipment suppliers, as well as for investors.
GaAs wafer supply: Sumitomo Electric, Freiberger Compound Materials, and AXT, involved in GaAs wafer supply, lead the market with about 95% of market share collectively. And since new laser applications have very high specification requirements for GaAs wafer that are constantly evolving, Yole analysts’ expect the top players to maintain their technical advantage for at least another 3 – 5 years.

Regarding GaAs epiwafer production, Yole’s analysts identified different business models. The GaAs LED market is principally vertically integrated, with very well-established IDMs like Osram, San’an, Epistar, and Changelight. In parallel, GaAs RF businesses outsource significantly from well-established epihouses.

Within the GaAs photonics market, the epi business is still applications-dependent. GaAs datacom market segment is mostly epi-integrated, with dominant IDMs like Finisar, Avago, and II-VI. For 3D sensing in smartphones, epi outsourcing is significant.

In 2017, Apple’s supplier Lumentum used IQE as its VCSEL epi supplier. This resulted in an almost 10x increase in IQE’s stock price. Other leading GaAs epihouses are in qualification or ramping up. Yole expects the photonic epiwafer market to behave similar to the GaAs RF epiwafer market.

By Cherry Sun

Storage and memory chipmaker and SEMI China member Tsinghua Unigroup is gearing up to meet burgeoning product demand with huge investments in its manufacturing plants. But the high-tech enterprise under Tsinghua University is eyeing a much bigger prize – growth of the region’s semiconductor industry and the realization of its ambition to become a more prominent force on the global stage.

Inspired by the national strategy, the Tsinghua Unigroup’s big spends include USD 24 billion in Wuhan (Yangtze Memory Technologies Co., Ltd.,) USD 30 billion in Chengdu, USD 30 billion in Nanjing and USD 100 billion in Chongqing, said Liu Hongyu, senior vice president of Tsinghua Unigroup, speaking at the SEMI China Equipment and Materials Committee meeting last month.

Advanced packaging is another rich vein of opportunity the region is tapping for expansion, said Liu Hongjun, vice president of China Wafer Level CSP Co., Ltd., another SEMI China member attending the event, hosted by NAURA in Beijing. Hongjun sees strong growth for Fan-in, Fan-out, FCBGA, 2.5D and 3DIC, with Fan-out out front.

Liang Sheng, administrative commission director at BDA, a business advisory firm supporting high-technology manufacturing in the E-Town economic development zone, pointed to 5G chips and smart, networked electric automobiles as drivers of the next growth phase of Beijing’s integrated circuit (IC) industry.

Global tailwinds are lifting China’s semiconductor industry and the region’s hopes, with SEMI and major industry analysts raising their semiconductor industry growth projects for 2018 to between 9 percent and 16 percent. According to SEMI’s latest market report, global semiconductor industry manufacturing equipment revenue reached USD 17 billion in the first quarter of 2018, logging all-time highs after jumping 12 percent from the previous quarter and 30 percent year-over-year. Korea was the top-performing region at USD 6.26 billion, followed by China at USD 2.64 billion.

Tighter integration with the rest of the global semiconductor industry is critical to the growth of China’s chip sector, and SEMI China is squarely focused on this assimilation, said SEMI China president Lung Chu. The spearhead of this effort is the SEMI Innovation Investment Platform (SIIP) China, established by SEMI China last year to help grow China’s pool of skilled workers, promote advanced technology, generate industry capital, and expand China’s semiconductor industry while developing stronger connections with chip sectors in other regions.

To strengthen ties with other regions, SIIP China will stage a number of innovation and investment forums this year including Chinese Night at SEMICON West (July 10-12) and a SIIP China Forum in Silicon Valley (July 15). In August, representatives from the Korea chip industry will visit counterparts in China (August), and a China delegation will travel to Japan for meetings (October). SIIP China is also strengthening the region’s links with Germany and Israel as SEMI serves as a crucial bridge between China’s semiconductor sector and the global industry.

At the invitation of Shanghai authorities and the Ministry of Commerce of the People’s Republic of China, SEMI China in November will join the China International Export & Import Exposition in Shanghai, an event that will underscore China’s commitment to the openness and cooperation of its semiconductor industry with the international chip community. As part of the exposition, SEMI will work with the Ministry of Commerce and domestic chip manufacturers to begin development of a special integrated circuit (IC) zone. SEMI China members are welcome to participate.

With workforce development no less vital to the future of China’s semiconductor industry, the Equipment & Materials Committee offered potential solutions to the industry’s talent gap. Measures included targeting university students and engineers with industry lectures and courses in key cities, campus recruiting, talent training that members said they are willing to help SEMI coordinate and stage and, much like the push to better integrate China with the global semiconductor industry, mobilizing member resources around a campaign to polish the image of the industry to make it more attractive to students and young workers.

Storage and memory chipmaker and SEMI China member Tsinghua Unigroup is gearing up to meet burgeoning product demand with huge investments in its manufacturing plants.

Cherry Sun is a marketing manager at SEMI China. 

Originally published on the SEMI blog.

Taking a multiband approach explains ‘electron-hole reverse drag’ and exciton formation

Mystifying experimental results obtained independently by two research groups in the USA seemed to show coupled holes and electrons moving in the opposite direction to theory.

Now, a new theoretical study has explained the previously mysterious result, by showing that this apparently contradictory phenomenon is associated with the bandgap in dual-layer graphene structures, a bandgap which is very much smaller than in conventional semiconductors.

The study authors, which included FLEET collaborator David Neilson at the University of Camerino and FLEET CI Alex Hamilton at the University of New South Wales, found that the new multiband theory fully explained the previously inexplicable experimental results.

Excitons travel across an ultra-low energy transistor without wasted dissipation of energy. Credit: FLEET: ARC Centre of Excellence in Future Low Energy Electronics Technologies

Exciton transport

Exciton transport offers great promise to researchers, including the potential for ultra-low dissipation future electronics.

An exciton is a composite particle: an electron and a ‘hole’ (a positively charged ‘quasiparticle’ caused by the absence of an electron) bound together by their opposite electrical charges.

In an indirect exciton, free electrons in one 2D sheet can be electrostatically bound to holes that are free to travel in the neighbouring 2D sheet.

Because the electrons and holes are each confined to their own 2D sheets, they cannot recombine, but they can electrically bind together if the two 2D sheets are very close (a few nanometres).

If electrons in the top (‘drive’) sheet are accelerated by an applied voltage, then each partnering hole in the lower (‘drag’) sheet can be ‘dragged’ by its electron.

This ‘drag’ on the hole can be measured as an induced voltage across the drag sheet, and is referred to as Coulomb drag.

A goal in such a mechanism is for the exciton to remain bound, and to travel as a superfluid, a quantum state with zero viscosity, and thus without wasted dissipation of energy.

To achieve this superfluid state, precisely engineered 2D materials must be kept only a few nanometres apart, such that the bound electron and hole are much closer to each other than they are to their neighbours in the same sheet.

In the device studied, a sheet of hexagonal-boron-nitride (hBN) separates two sheets of atomically-thin (2D) bilayer graphene, with the insulating hBN preventing recombination of electrons and holes.

Passing a current through one sheet and measuring the drag signal in the other sheet allows experimenters to measure the interactions between electrons in one sheet and holes in the other, and to ultimately detect a clear signature of superfluid formation.

Only recently, new, 2D heterostructures with sufficiently thin insulating barriers have been developed that allow us to observe features brought by strong electron-hole interactions.

Explaining the inexplicable: negative drag

However, experiments published in 2016 showed extremely puzzling results. Under certain experimental conditions, the Coulomb drag was found to be negative – i.e. moving an electron in one direction caused the hole in the other sheet to move in the opposite direction!

These results could not be explained by existing theories.

In this new study, these puzzling results are explained using crucial multi-band processes that had not previously been considered in theoretical models.

Previous experimental studies of Coulomb drag had been performed in conventional semiconductor systems, which have much larger bandgaps.

However bilayer graphene has a very small bandgap, and it can be changed by the perpendicular electric fields from the metal gates positioned above and below the sample.

The calculation of transport in both conduction and valence bands in each of the graphene bilayers was the ‘missing link’ that marries theory to experimental results. The strange negative drag happens when the thermal energy approaches the bandgap energy.

The strong multiband effects also affect the formation of exciton superfluids in bilayer graphene, so this work opens up new possibilities for exploration in exciton superfluids.

The study Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures by M. Zarenia, A.R. Hamilton, F.M. Peeters and D. Neilson was published in Physical Review Letters in July 2018.

Acknowledgement: The study was led by David Neilson, and by Mohammad Zarenia while at the University of Antwerp, Belgium. The authors of the theoretical study worked with data provided by experimentalists from the two US groups: Cory Dean (Columbia University) and Emanuel Tutuc (University of Texas at Austin) who discovered the original puzzling results. The research was supported by the Flemish government (Belgium), the University of New South Wales, the University of Camerino and by the Australian Research Council via FLEET.

Superfluids and FLEET

Exciton superfluids are studied within FLEET’s Research theme 2 for their potential to carry zero-dissipation electronic current, and thus allow the design of ultra-low energy exciton transistors.

The use of twin atomically-thin (2D) sheets to carry the excitons will allow for room-temperature superfluid flow, which is key if the new technology is to become a viable ‘beyond CMOS’ technology. A bilayer-exciton transistor would be a dissipationless switch for information processing.

In a superfluid, scattering is prohibited by quantum statistics, which means that electrons and holes can flow without resistance.

In this single, pure quantum state, all particles flow with the same momentum, so that no energy can be lost through dissipation.

FLEET (the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies) brings together over a hundred Australian and international experts, with the shared mission to develop a new generation of ultra-low energy electronics.

The impetus behind such work is the increasing challenge of energy used in computation, which uses 5-8% of global electricity and is doubling every decade.

A key challenge of such ultra-miniature devices is overheating – their ultra-small surfaces seriously limit the ways for the heat from electrical currents to escape.

Working to address “hotspots” in computer chips that degrade their performance, UCLA engineers have developed a new semiconductor material, defect-free boron arsenide, that is more effective at drawing and dissipating waste heat than any other known semiconductor or metal materials.

This could potentially revolutionize thermal management designs for computer processors and other electronics, or for light-based devices like LEDs.

Illustration showing a schematic of a computer chip with a hotspot (bottom); an electron microscope image of defect-free boron arsenide (middle); and an image showing electron diffraction patterns in boron arsenide. Credit: Hu Research Lab / UCLA Samueli

The study was recently published in Science and was led by Yongjie Hu, UCLA assistant professor of mechanical and aerospace engineering.

Computer processors have continued to shrink down to nanometer sizes where today there can be billions of transistors on a single chip. This phenomenon is described under Moore’s Law, which predicts that the number of transistors on a chip will double about every two years. Each smaller generation of chips helps make computers faster, more powerful and able to do more work. But doing more work also means they’re generating more heat.

Managing heat in electronics has increasingly become one of the biggest challenges in optimizing performance. High heat is an issue for two reasons. First, as transistors shrink in size, more heat is generated within the same footprint. This high heat slows down processor speeds, in particular at “hotspots” on chips where heat concentrates and temperatures soar. Second, a lot of energy is used to keep those processors cool. If CPUs did not get as hot in the first place, then they could work faster and much less energy would be needed to keep them cool.

The UCLA study was the culmination of several years of research by Hu and his students that included designing and making the materials, predictive modeling, and precision measurements of temperatures.

The defect-free boron arsenide, which was made for the first time by the UCLA team, has a record-high thermal conductivity, more than three-times faster at conducting heat than currently used materials, such as silicon carbide and copper, so that heat that would otherwise concentrate in hotspots is quickly flushed away.

“This material could help greatly improve performance and reduce energy demand in all kinds of electronics, from small devices to the most advanced computer data center equipment,” Hu said. “It has excellent potential to be integrated into current manufacturing processes because of its semiconductor properties and the demonstrated capability to scale-up this technology. It could replace current state-of-the-art semiconductor materials for computers and revolutionize the electronics industry.”

The study’s other authors are UCLA graduate students in Hu’s research group: Joonsang Kang, Man Li, Huan Wu, and Huuduy Nguyen.

In addition to the impact for electronic and photonics devices, the study also revealed new fundamental insights into the physics of how heat flows through a material.

“This success exemplifies the power of combining experiments and theory in new materials discovery, and I believe this approach will continue to push the scientific frontiers in many areas, including energy, electronics, and photonics applications,” Hu said.