Tag Archives: letter-materials-tech

When it comes to putting technology in space, size and mass are prime considerations. High-power gallium nitride-based high electron mobility transistors (HEMTs) are appealing in this regard because they have the potential to replace bulkier, less efficient transistors, and are also more tolerant of the harsh radiation environment of space. Compared to similar aluminum gallium arsenide/gallium arsenide HEMTs, the gallium nitride-based HEMTs are ten times more tolerant of radiation-induced displacement damage.

Until recently, scientists could only guess why this phenomena occurred: Was the gallium nitride material system itself so inherently disordered that adding more defects had scant effect? Or did the strong binding of gallium and nitrogen atoms to their lattice sites render the atoms more difficult to displace?

The answer, according to scientists at the Naval Research Laboratory, is none of the above.

Examining radiation response

In a recent open access article published in the ECS Journal of Solid State Science and Technologyentitled, “On the Radiation Tolerance of AlGaN/GaN HEMTs,” the team of researchers from NRL state that by studying the effect of proton irradiation on gallium nitride-based HEMTs with a wide range of initial threading dislocation defectiveness, they found that the pre-irradiation material quality had no effect on radiation response.

Additionally, the team discovered that the order-of-magnitude difference in radiation tolerance between gallium arsenide- and gallium nitride-based HEMTs is much too large to be explained by differences in binding energy. Instead, they noticed that radiation-induced disorder causes the carrier mobility to decrease and the scattering rate to increase as expected, but the carrier concentration remains significantly less affected than it should be.

Applications in space exploration

Because of their relative radiation hardness, gallium arsenide- and gallium nitride-based HEMTs are desirable for space application. Take, for example, the Juno Spacecraft.

On July 4, the Juno Spacecraft successfully entered orbit around Jupiter – a planet scientists still know very little about, which generates extreme levels of radiation. Without the proper technology, the radiation levels of Jupiter could destroy the sensitive electronics in the satellite upon approaching the planet. Better understanding of why gallium arsenide- and gallium nitride-based HEMTs are more tolerant of radiation could ultimately accelerate innovative and bolster projects where radiation levels prove to be barriers.

Novel advancements in HEMTs

The paper was designated ECS Editors’ Choice due to its significance and expected impact on the solid state science and technology community.

“Editors’ Choice articles are elite publications because they are deemed by reviewers and journal editors to demonstrate a transformative advance, discovery, interpretation, or direction in a field,” says Dennis Hess, Editor of the ECS Journal of Solid State Science and Technology. “They represent very high quality science and engineering and hold the promise of altering current technology practices.”

Unexpected answers

The explanation for this novel discovery turns out to be rather elegant.

In gallium nitride-based HEMTs, a piezoelectric field forms at the aluminum gallium nitride/gallium nitride interface due to lattice strain. The field gives rise to two-dimensional electron gas by which carriers travel across the transistor from source to drain. It also provides an electrically attractive environment that causes carriers that are scattered out of the two-dimensional electron gas by radiation-induced defects to be reinjected. In this way, the scattering rate can increase and the mobility can decrease without greatly affecting the two-dimensional electron gas carrier density.

In other words, it is the internal structure itself that renders aluminum gallium nitride/gallium nitride HEMTs rad-hard.

“Gallium nitride is such a complicated system – not like gallium arsenide at all,” says Bradley Weaver, co-author of the study. “We struggled for four years to figure out why it’s so rad-hard, expecting a complicated solution. But the answer turned out to be really simple. Science does that sometimes.”

Materials researchers at North Carolina State University have fine-tuned a technique that enables them to apply precisely controlled silica coatings to quantum dot nanorods in a day – up to 21 times faster than previous methods. In addition to saving time, the advance means the quantum dots are less likely to degrade, preserving their advantageous optical properties.

Morphological control of the silica shell on CdSe/CdS core/shell quantum dot nanorods is reported, giving single or double lobes of silica or a uniform silica shell. Credit: Joe Tracy

Morphological control of the silica shell on CdSe/CdS core/shell quantum dot nanorods is reported, giving single or double lobes of silica or a uniform silica shell. Credit: Joe Tracy

Quantum dots are nanoscale semiconductor materials whose small size cause them to have electron energy levels that differ from larger-scale versions of the same material. By controlling the size of the quantum dots, researchers can control the relevant energy levels – and those energy levels give quantum dots novel optical properties. These characteristics make quantum dots promising for applications such as opto-electronics and display technologies.

But quantum dots are surrounded by ligands, which are organic molecules that are sensitive to heat. If the ligands are damaged, the optical properties of the quantum dots suffer.

“We wanted to coat the rod-shaped quantum dots with silica to preserve their chemical and optical properties,” says Bryan Anderson, a former Ph.D. student at NC State who is lead author of a paper on the work. “However, coating quantum dot nanorods in a precise way poses challenges of its own.”

Previous work by other research teams has used water and ammonia in solution to facilitate coating quantum dot nanorods with silica. However, those techniques did not independently control the amounts of water and ammonia used in the process.

By independently controlling the amounts of water and ammonia used, the NC State researchers were able to match or exceed the precision of silica coatings achieved by previous methods. In addition, using their approach, the NC State team was able to complete the entire silica-coating process in a single day – rather than up to one to three weeks needed for other processes.

“The process time is important, because the longer the process takes, the more likely it is that the quantum dot nanorods being coated will degrade,” says Joe Tracy, an associate professor of materials science and engineering at NC State and senior author on the paper. “The time factor may also be important when we think about scaling this process up for manufacturing processes.”

That said, researchers still have a problem.

The process of applying the silica coating etches the cadmium sulfide surface of the quantum dot nanorods, which shortens the length of the nanorods by as much as four or five nanometers. That shortening is indicative of etching, which reduces the brightness of the light emitted by the quantum dot nanorods.

“We think ammonia may be the culprit,” Tracy says. “We have some ideas that we’re pursuing, focused on how to substitute another catalyst for ammonia in order to minimize the etching and better preserve the quantum dot nanorod’s optical properties.”

The paper, “Silica Overcoating of CdSe/CdS Core/Shell Quantum Dot Nanorods with Controlled Morphologies,” is published online in the journal Chemistry of Materials. The paper was co-authored by Wei-Chen Wu, a former Ph.D. student in Tracy’s lab. The work was done with support from the National Science Foundation under grant number DMR-1056653.

Tracy has previously published related research in Chemistry of Materials on coating gold nanorods with silica shells.

Nano-electronics research center imec and Synopsys, Inc. (NASDAQ: SNPS) today announced an interconnect resistivity model to support the screening and selection of alternative interconnect metals and liner-barrier materials at the 7nm node and beyond. With the continued scaling of advanced process nodes, the impact of parasitic interconnect resistance on the switching delay of standard cells rises considerably. The new model developed through this collaboration enables the evaluation of interconnect material and process options through simulations in the early stages of technology development, when wafer data is not available, and in the process optimization and integration stages of technology development, where it reduces expensive and time-consuming wafer-based iterations.

“We have already released to our partners a number of sets of model parameters related to various liner/barrier systems for Cu metallization or to alternative metals, such as Ru and Co, which they will use to screen metallization options for next-generation interconnect technologies,” stated Dan Mocuta, director, Logic Device and Integration at imec.

To use the new resistivity model, customers simulate the fabrication of the interconnect structure in 3D using the Synopsys process emulation tool Process Explorer, and then simulate the wire and via resistance in Raphael, the Synopsys gold standard interconnect field solver. This simulation flow accounts for the impact of layout rules, multi-patterning flows, and process-induced 3D features on the resistance of any conductive net in a multilayer interconnect stack, thereby predicting the influence of material, process and patterning choices on the interconnect resistance at scaled dimensions.

Imec has calibrated the resistivity model to wafer data for Cu, W, Ru and Co interconnects.

“The new resistivity model developed through this collaboration with imec is an important component of our pre-wafer simulation solution to enable our mutual customers to perform early screening of interconnect technology options at advanced nodes,” said Dr. Howard Ko, senior vice president and general manager of the Silicon Engineering Group at Synopsys.

Imec’s research into advanced logic scaling is performed in cooperation with imec’s key partners in its core CMOS programs including GlobalFoundries, Intel, Micron, SK Hynix, Samsung, TSMC, Huawei, Qualcomm and Sony.

imec synopsys 1 imec synopsys 2

3D model of a multilayer interconnect stack (a) after process emulations using the Synopsys Sentaurus™ Process Explorer and 3D local resistivity profile (b) within wires and vias

3D-Micromac AG, a supplier of laser micromachining and roll-to-roll laser systems for the photovoltaic, medical device and electronics markets, announced that its microDICE laser micromachining system has been adopted by a major industrial manufacturer for volume production of high-power diodes. Leveraging 3D-Micromac’s proprietary TLS-Dicing technology, the microDICE system provides fast, clean and cost-effective dicing of wafers used for advanced semiconductors and power device applications. Its unique approach uses thermally induced mechanical stress to separate brittle semiconductor materials such as silicon, silicon carbide (SiC), germanium (Ge) and gallium arsenide (GaAs).

3d micromac

The microDICE laser micromachining system from 3D-Micromac supports volume production of high-power diodes.

TLS-Dicing is a contact- and residue-free process that provides significantly higher throughput, higher yields and greater functionality compared to traditional die-separation technologies. For example, throughput is up to 30X greater compared to saw dicing. The technology also provides lower cost of ownership than other approaches. A forceless and contactless machining process, TLS-Dicing eliminates tool wear and requires no expensive consumables for surface cleaning–resulting in cost savings of up to an order of magnitude or more.

“While significant time and resources are invested in the front-end of semiconductor manufacturing to produce a completed product wafer, back-end wafer processing has historically been viewed as a necessary evil,” stated Tino Petsch, CEO of 3D-Micromac. “That’s all changed with the adoption of new types of wafer substrates, thinner wafers and scaling to smaller dimensions, larger-size substrates, and new packaging technologies like 3D-stacking. Back-end process steps such as wafer dicing are evolving as critical value-add process steps that not only ensure, but also further enhance, device yields. Using our TLS-Dicing technology, the microDICE system provides superior wafer dicing performance over other approaches while considerably reducing the dicing cost per wafer. Our technology has been proven in the photovoltaic and other industrial markets, and we are pleased to bring the benefits of it to the semiconductor and power device manufacturing industry.”

3D-Micromac also announced today that it is expanding its global infrastructure with the opening of its new 3D-Micromac America headquarters in the heart of Silicon Valley, in San Jose, Calif. Serving as both an applications lab and sales and support facility, the office marks the company’s first major presence in North America and will enable 3D-Micromac to better meet rising customer demand for its laser micromachining products across all of its served markets, including solar, semiconductor, MEMS, display and smart glass.

According to Daniel Weber, sales and business development manager for 3D-Micromac America, “With our new regional headquarters and applications lab, 3D-Micromac can offer our North American-based customers a first-class network of sales and support services for our laser micromachining systems. Providing customer evaluations, applications development, and small-scale contract manufacturing is a unique offering among wafer dicing technology suppliers. We look forward to delivering all of these capabilities to our existing, new and potential customers.”

Graphene has emerged as one of the most promising two-dimensional crystals, but the future of electronics may include two other nanomaterials, according to a new study by researchers at the University of California, Riverside and the University of Georgia.

In research published Monday (July 4) in the journal Nature Nanotechnology, the researchers described the integration of three very different two-dimensional (2D) materials to yield a simple, compact, and fast voltage-controlled oscillator (VCO) device. A VCO is an electronic oscillator whose oscillation frequency is controlled by a voltage input.

Titled “An integrated Tantalum Sulfide–Boron Nitride–Graphene Oscillator: A Charge-Density-Wave Device Operating at Room Temperature,” the paper describes the development of the first useful device that exploits the potential of charge-density waves to modulate an electrical current through a 2D material. The new technology could become an ultralow power alternative to conventional silicon-based devices, which are used in thousands of applications from computers to clocks to radios. The thin, flexible nature of the device would make it ideal for use in wearable technologies.

Graphene, a single layer of carbon atoms that exhibits exceptional electrical and thermal conductivities, shows promise as a successor to silicon-based transistors. However, its application has been limited by its inability to function as a semiconductor, which is critical for the ‘on-off’ switching operations performed by electronic components.

To overcome this shortfall, the researchers turned to another 2D nanomaterial, Tantalum Sulfide (TaS2). They showed that voltage-induced changes in the atomic structure of the ‘1T prototype’ of TaS2 enable it to function as an electrical switch at room temperature–a requirement for practical applications.

“There are many charge-density wave materials that have interesting electrical switching properties. However, most of them reveal these properties at very low temperature only. The particular polytype of TaS2 that we used can have abrupt changes in resistance above room temperature. That made a crucial difference,” said Alexander Balandin, UC presidential chair professor of electrical and computer engineering in UCR’s Bourns College of Engineering, who led the research team.

To protect the TaS2 from environmental damage, the researchers coated it with another 2D material, hexagonal boron nitrate, to prevent oxidation. By pairing the boron nitride-capped TaS2 with graphene, the team constructed a three-layer VCO that could pave the way for post-silicon electronics. In the proposed design, graphene functions as an integrated tunable load resistor, which enables precise voltage control of the current and VCO frequency. The prototype UCR devices operated at MHz frequency used in radios, and the extremely fast physical processes that define the device functionality allow for the operation frequency to increase all the way to THz.

Balandin said the integrated system is the first example of a functional voltage-controlled oscillator device comprising 2D materials that operates at room temperature.

“It is difficult to compete with silicon, which has been used and improved for the past 50 years. However, we believe our device shows a unique integration of three very different 2D materials, which utilizes the intrinsic properties of each of these materials. The device can potentially become a low-power alternative to conventional silicon technologies in many different applications,” Balandin said.

The electronic function of graphene envisioned in the proposed 2D device overcomes the problem associated with the absence of the energy band gap, which so far prevented graphene’s use as the transistor channel material. The extremely high thermal conductivity of graphene comes as an additional benefit in the device structure, by facilitating heat removal. The unique heat conduction properties of graphene were experimentally discovered and theoretically explained in 2008 by Balandin’s group at UCR. The Materials Research Society recognized this groundbreaking achievement by awarding Balandin the MRS Medal in 2013.

The Balandin group also demonstrated the first integrated graphene heat spreaders for high-power transistors and light-emitting diodes. “In those applications, graphene was used exclusively as heat conducting material. Its thermal conductivity was the main property. In the present device, we utilize both electrical and thermal conductivity of graphene,” Balandin added.

The Strategic Materials Conference (SMC 2016), focusing on “Scaling Challenges: The Future of Materials and Packaging,” will be held September 20-21, at the Computer History Museum in Mountain View, Calif.  The annual two-day SEMI conference brings together key players from the semiconductor industry ecosystem to share insights on the latest developments in advanced materials. SMC 2016 will delve into what’s driving demand for new materials and packaging and discuss how suppliers are responding to problems posed by existing scaling limitations.

Productivity gains, process improvements, and materials innovation are being combined with novel device packaging approaches. Presentations from representatives of NVIDIA, Samsung, Intel Corporation, ICMTIA China, Tokyo Electron America, GLOBALFOUNDRIES, Micron Technology, Bank of America, Qualcomm Research, STATS ChipPAC and many other industry leaders will discuss trends and opportunities. Professionals interested in industry innovation and business success are encouraged to attend. The sessions include:

  • Economic & Market Trends: Industry and Wall Street experts will present findings on trends in materials, equipment, production, and emerging applications.
  • Future of Moore’s Law and Encounters of a 3D Kind: Will 2D CMOS scaling pace Moore’s Law or will new 3D options be the pathway to keep on track?
  • Disruptive Trends and Opportunities for New Materials: Demand for PCs, tablets, and smartphones is slowing. How quickly will new market segments such as IoT, smart cars, and wearable devices drive demand for new materials?
  • Advanced Packaging: Advanced packaging extends the scaling of advanced devices. As a result, competitors are introducing an array of new proprietary packaging technologies and designs.  Innovation in packaging materials is working hard to keep pace.
  • Contamination and Metrology Challenges: Devices are increasingly sensitive to contaminants, making detection, identification, and control a focal point in fab operations and the supply chain. How will the industry address contamination control issues at advanced technology nodes?
  • Strategic Materials Challenges from the Fab Perspective: Facing flat growth, cost sensitivity, and consolidation, the industry must now focus on next materials needs, supply chain issues, and building collaborative, win-win relationships between manufacturers and suppliers.

The Strategic Materials Conference provides comprehensive in-depth content and unprecedented networking opportunities for professionals who share common strategic objectives for the extended electronics ecosystem.  To register for SMC 2016, click here.

A research team at Clarkson University reports an interesting conclusion that could have major impacts on the future of nano-manufacturing. Their analysis for a model of the process of random sequential adsorption (RSA) shows that even a small imprecision in the position of the lattice landing sites can dramatically affect the density of the permanently formed deposit.

With the advent of nanotechnology, not only can we deposit tiny particles, but the target surfaces or substrates can be tailored to control the resulting structures.

This article addresses the precision that must exist in the pattern of the target surface, in order to achieve high perfection and high coverage in the pattern of deposited particles. To do this, it compares RSA on three types of surfaces: a continuous (non-patterned) lattice, a precisely patterned surface, and a surface with small imprecisions in the pattern. The researchers find that very small imprecisions can make RSA proceed as if the surface is continuous. The consequence is that the deposition process is less efficient, and the ultimate coverage is much lower. In the process of RSA, a continuous surface is covered slowly with a larger fraction of the area remaining uncovered than a precisely lattice-patterned surface. In the past when surfaces on which microscopic particles were deposited were naturally flat (continuous) or had a lattice-structure, the importance of small imprecisions had not been recognized.

The researchers explain their analysis this week in the Journal of Chemical Physics, from AIP Publishing.

Vladimir Privman at Clarkson University has been involved in studying aspects of such systems since 2007; however this study, conducted with graduate student Han Yan, was the first to consider the imprecision in the surface lattice-site localization, rather than in the particle size uniformity.

Initially suggested by computer modeling, their results were later derived by analytical model considerations which are novel for the research field of RSA.

“The greatest difficulty was to understand and accept the initial numerical finding that suggested results that seemed counterintuitive,” Privman explained. “Once accepted, we could actually confirm the initial findings, as well as generalize and systematize them by analytical arguments.”

Pre-patterned substrates have been studied for applications ranging from electronics to optics, to sensors, and to directed crystal growth. The reported results suggest that efforts at precise fixed positioning and object-sizing in nano-manufacturing might be counterproductive if done as part of forming structures by RSA, under practically irreversible conditions. A certain degree of relaxation, to allow objects to “wiggle their way” into matching positions, may actually be more effective in improving both the density and rate of formation of the desired dense structures, Privman said.

This work has implications that the team is preparing to explore.

“Now that we have realized that not only particle non-uniformity, but also substrate-pattern imprecision have substantial effects on the dynamics of the RSA process, we will begin studying various systems and patterning geometries, expanding beyond our original model,” Privman said.

What do you use to handle thin wafers and thin reconstituted wafers?  Increasingly miniaturized electronic devices require decreased profile heights, reduced foot-prints and ultimately, the perpetual thinning of wafers.  Initially, working with thin wafers typically required temporary bonding of the wafer to a carrier and use of a temporary coating layer for wafer protection.

For fan-out wafer-level packaging and 3D packaging, thin wafer handling is critical; the wafer must not warp, bend or shift during any wafer-processing steps.  These wafer processing steps may involve different temperature ranges and exposure to a variety of chemicals depending on the processing steps such as etching, metallization, CMP, PVD, RDL in embedded, fan-out, and 3D wafer-level packaging.

AI Technology, Inc. (AIT) manufactures a series of temporary bonding materials for processing temperatures up to 150 Cº. They are well accepted for grinding, dicing, etching, and deposition.  AIT customers prefer AIT bonding materials over conventional wax materials specifically because AIT’s products feature ease of use and quick removal, especially for very delicate compound wafers and photonics.

For higher temperature processing, AI Technology, Inc. (AIT) developed high temperature wafer processing adhesives (WPA) that can withstand processing temperature up to 330ºC. Also important is the chemical resistance of these WPA materials to acids and bases during the etching processes.  The thermal and chemical stability allows these adhesive to maintain its chemical integrity allowing the thin wafer be separated from the wafer handler/carrier by heat-sliding or by laser de-bonding equipment.  The WPA adhesive layer is designed to absorb UV breaking chemical bonds at the interface allowing for ease of separation.  After separation, the WPA adhesive layer can be removed by peeling with minimum stress or solvent cleaning.

Besides supplying these WPA products in spin coating liquid, AI Technology, Inc. (AIT) also provides WPA as a thin film. This unique and innovative WPA-film minimizes processing time and total waste produced compared to a typical spin-coating process allowing higher through-put.  In high volume manufacturing, some fan-out packaging involves reconstituted panels with larger dimensions compared to the traditional circular and small wafer size. For these high volume manufacturing panels, adhesive film in sheet format may provide the most efficient productivity.  Typically heat-laminated onto a wafer first and followed by vacuum lamination of the wafer onto the carrier, AIT’s WPA thin film processing conditions and debonding techniques resemble the spin coating process used in WPA products.

AI Technology, Inc. (AIT) understands that different types of wafers, Si, GaAs, GaN, InP, glass, and sapphire are used in different applications and, depending on wafer processing conditions, demand highly specialized tools and equipment.  AIT is committed to working closely with our customers and equipment suppliers to satisfy customer needs.

Physicists from the Technological Institute for Superhard and Novel Carbon Materials, the Moscow Institute of Physics and Technology, and the Siberian Federal University have mathematically modelled diamond-based microstructures for producing compact high sensitivity sensors.

The researchers’ study investigates the problem of selecting a useful acoustic signal taking into account the excitation of Lamb waves in promising microwave microresonators with substrates of synthetic diamonds. The scientists proposed a mathematical model and experimentally studied acoustic waves in the piezoelectric layered structure, described their dispersion and proposed a number of ways of decreasing the effects of spurious peaks. In the future, diamond crystal based structures may be able to be used as high sensitivity sensors to detect pressure, acceleration, temperature, the thickness of ultrathin films etc. The paper has been published in Applied Physics Letters.

“I think that the results we have obtained from a piezoelectric layered structure based on synthetic diamonds are ahead of world-class research in this field. Our microresonators were used to obtain resonances at record high microwave frequencies in a range of up to 20 GHz, with the quality factor remaining at several thousand. The behaviour of diamond as a substrate for the acoustic microresonator was very significant and I hope that using diamonds in acoustics and electronics will lead to more exciting discoveries,” said the corresponding author of the study, Boris Sorokin, in an interview with MIPT’s Communications Office.

The quality factor is a feature of an oscillating system. It describes how quickly oscillations die down in a system; the higher the quality factor, the smaller the energy loss.

A piezoelectric layered structure is a “sandwich” of various different materials with a piezoelectric effect. This term means that under compression or tension an electric field occurs around the material – and when an electrical voltage is applied, the material itself changes shape. Non-scientists will have seen the piezoelectric effect in lighters (pressing the button compresses the piezoelectric, which provides enough voltage for a spark). However, aside from lighters, the effect is used in microphones, precise micromanipulators, and many kinds of sensors for pressure, humidity, temperature etc. Another very important application of piezoelectrics is in highly stable piezoelectric resonators, which enable quartz clocks to display time accurately, for example, or computers to run programs smoothly.

The effect of an electric field on a piezoelectric, in this case a thin film of aluminium nitride AlN, leads to deformation and causes elastic waves which pass to the substrate in the same way that an elastic wave falling on the piezoelectric film causes an electric field. When it reaches the edge of the substrate, the wave is reflected and within the layers of several materials a number of oscillations occur at the same time – this effect resembles an echo that can be heard when you shout in a tunnel or into a wide tube.

Diamonds and waves

Diamond substrates were not chosen by chance. Piezocrystals are ideal for such devices, as they have a combination of properties such as low acoustic absorption, a high electromechanical coupling coefficient, and a high speed of sound. Diamonds satisfy all these requirements except for one – there is no piezoelectric effect. This is why the devices needed the aluminium nitride film. Engineers are, of course, slightly apprehensive regarding the price, but synthetic diamonds are now becoming more affordable. The properties of synthetic diamonds are superior to those observed in natural diamonds, particularly in terms of their impurity profile and reproducibility, however large natural gem-quality diamonds are much more expensive. The authors of the study believe that synthetic single crystal diamonds are most promising for developing new acoustoelectric devices.

Voluminous waves excited in the layered structure are able to resonate, creating both the basic type (mode) of oscillations, and also generating additional modes. In the substrate and piezoelectric film, in addition to the useful longitudinal-type oscillations, Lamb waves also occur under certain conditions. The spectrum of these waves is in separate branches with the phase velocity dependent on the frequency.

Lamb waves are a complex combination of elastic oscillations occurring in thin layers of elastic media and were first described by the British physicist Horace Lamb. Interestingly, the particles in these waves follow an elliptical path. There are symmetric and antisymmetric (bending) Lamb waves. Phase velocity is the velocity at which a point moves from a predetermined phase – e.g. the crest of a wave; the phase velocity of waves in a particular medium often depends on their frequency and this effect is called dispersion.

In this case it is geometric dispersion of waves in two-dimensional acoustic waveguides. On the one hand, excitation of Lamb waves is not useful in terms of the quality factor of the acoustic resonator in the main (longitudinal) mode, however these types of waves themselves may be of special interest.

Using mathematical modelling, researchers studied in detail the spectrum of various acoustic modes occurring within the diamond structure, using a visualization of the areas of acoustic displacement. They paid particular attention to resonances that occur as a result of there being a whole spectrum of natural oscillation frequencies in the layered “sandwich”. In the simplest case, this frequency corresponds to the frequency at which an elastic system would oscillate in the absence of external influences. If, for example, you touch and release an ordinary pendulum, it will swing with a natural frequency and applying force with this frequency is most effective for its swing. Resonance is when the natural frequency and the excitation frequency coincide – the oscillation amplitude increases sharply.

Natural frequencies depend on the properties of the materials, as well as the geometry of the structure. This means that detectors can be made that are able to detect even individual bacteria that have become attached to their surface – the bacteria slightly increase the mass of the entire system and shift the resonant frequency.

One of the main results was that the researchers succeeded in selecting and identifying different types of waves and forming dispersion laws for them. The results obtained will be useful in the development of microwave acoustoelectronic devices.

Acoustoelectronics is a science combining solid-state physics, semiconductors, and radioelectronics that studies the principles of building devices to detect, convert, and process signals. Acoustic resonators are widely used in science and technology as sensing elements in various physical and chemical sensors and in medical devices. Cavity resonators are popular because of their miniature size and high quality factor, while resonating at high and ultra-high frequencies. The higher the operating frequencies, the smaller the cross-sectional dimensions of resonators are required (~100 microns for a frequency of ~10 GHz).

The acoustic properties of these sensitive elements are developed and studied at MIPT’s Department of Physics and Chemistry of Nanostructures, which is based at the Technological Institute for Superhard and Novel Carbon Materials. It was at this institute where scientists from a number of Russian organisations worked together to develop a method of creating a material harder than diamond; it was also the place where the secret of the abnormal stiffness of polycrystalline diamonds was uncovered – it was found that they are more rigid than single crystals.

Nowadays, our world is in search of cleaner energy sources to power our increasing industrial and economical needs. Solar energy is becoming an alternative source to fossil fuels, however, due to the accelerating pace at which we are consuming energy, we need to develop ubiquitous PV technologies that can be employed everywhere: on buildings, clothes, consumer electronics and wearables. This necessitates ultra-thin film, low-cost and ideally flexible solar cells without compromising the environment during production, use, or disposal.

Most of us know that the most common inorganic solar cells, displayed over roof tops and in solar farms, are made of silicon. However, the production of silicon solar cells can be expensive and energy demanding and the final modules are heavy and bulky. Many lower-cost thin film solar cells, alternative to silicon, are composed of toxic elements such as lead or cadmium, or contain scarce elements such as indium or tellurium.

Now ICFO researchers Dr. Maria Bernechea, Dr. Nicky Miller, Guillem Xercavins, David So, and Dr. Alexandros Stavrinadis, led by ICREA Prof. at ICFO Gerasimos Konstantatos have found a solution to this increasing problem. They have fabricated a solution-processed, semi-transparent solar cell based on AgBiS2 nanocrystals, a material that consists of non-toxic, earth-abundant elements, produced in ambient conditions at low temperatures. These crystals have shown to be very strong panchromatic absorbers of light and have been further engineered to act as effective charge-transporting medium for solution-processed solar cells.

This image shows a semi-transparent solar cell based on AgBiS2 nanocrystal. Credit: ICFO

This image shows a semi-transparent solar cell based on AgBiS2 nanocrystal. Credit: ICFO

What is special about these cells? As researcher Dr. Maria Bernechea comments, “They contain AgBiS2 nanocrystals, a novel material based on non-toxic elements. The chemical synthesis of the nanocrystals allows exquisite control of their properties through engineering at the nanoscale and enables their dissolution in colloidal solutions. The material is synthesized at very low temperatures (100ºC), an order of magnitude lower than the ones required for silicon based solar cells.´´

The team of researchers at ICFO developed these cells through a low temperature hot-injection synthetic procedure. They first dispersed the nanocrystals into organic solvents, where the solutions showed to be stable for months without any losses in the device performance. Then, the nanocrystals were deposited onto a thin film of ZnO and ITO, the most commonly used transparent conductive oxide, through a layer-by-layer deposition process until a thickness of approximately 35nm was achieved.

“A very interesting feature of AgBiS2 solar cells is that they can be made in air at low temperatures using low-cost solution processing techniques without the need for the sophisticated and expensive equipment required to fabricate many other solar cells. These features give AgBiS2 solar cells significant potential as a low-cost alternative to traditional solar cells.” as Dr. Nicky Miller states.

These cells, in this first report, have already achieved power conversion efficiencies of 6.3%, which is on par with the early reported efficiencies of currently high performance thin film PV technologies. This highlights the potential of AgBiS2 as a solar-cell material that in the near future can compete with current thin film technologies that rely on vacuum-based, high-temperature manufacturing processes.

As ICREA Prof at ICFO Gerasimos Konstantatos concludes, “This is the first efficient inorganic nanocrystal solid-state solar cell material that simultaneously meets demands for non-toxicity, abundance and low-temperature solution processing. These first results are very encouraging, yet this is still the beginning and we are currently working on our next milestone towards efficiencies > 12%”.

The results obtained from this study, which was financially supported by European Commission within the NANOMATCELL project, signifies a turning point in the concept and production of solar cells, moving from silicon cells to low-cost environmentally friendly solar cells that will definitely imply a safer and more sustainable world for the future.