Tag Archives: letter-wafer-tech

BY GUIDO GROESENEKEN, imec fellow

To be able to guarantee the reliability of transistors, we have been conducting research for some years now at imec to see what happens when transistors operate properly and when they fail. We’ve been doing this in terms of circuits, devices and materials – and sometimes right down to the level of atoms. The insights that we gather from this work help us to provide the right feedback to the process technol- ogists, who in turn are able to make the transistors more reliable. It is particularly interesting to note that in recent years the knowledge we have gained about these failure mechanisms can also be applied to other areas. These insights no longer only serve to solve problems, but are the basis for innovative and surprising solutions in very diverse domains.
Last year, imec spent a lot of time working on self- learning chips, data security codes, FinFET biosensors and computer systems that can correct themselves. These are innovations that draw on the knowledge present in imec’s reliability group.

Self-learning chips

For example, take the self-learning or neuromorphic chip that gave imec such extensive coverage in the media in 2017. The development of this chip is based, among other things, on our knowledge of “resistive RAM” or RRAM memories, which use the breakdown of an oxide to switch a memory bit on or off (0 or 1). This oxide breakdown – which was previously (and still is) a reliability problem – occurs because a conductive path is created through the oxide, known as a filament. However, the work conducted by imec’s reliability group has demonstrated that not only can you create a filament or make it disappear, but that there are intermediate levels as well, which means that the strength of the filament can be controlled. And that is precisely what happens in our brains: the connec- tions between neurons can become stronger or weaker according to the occurrence they are processing or the learning process they use, etc. This means that these RRAM filaments can be used in chips that work like our brains. It was this insight that provided us with the foundation for the development of imec’s neuromorphic chip, which – as has been demonstrated – can even compose music.

Data security

Since recently we are also working closely with COSIC, an imec research group at KU Leuven that specializes in computer security and cryptography. Also here we can draw on our knowledge of transistor breakdown mechanisms. These can be used to create and read out a fingerprint that is unique for each chip and that cannot be predicted, hence the name ‘physically unclonable functions’ (or PUFs). This unique fingerprint makes it possible to ascertain the identity of chips in data exchanges and thus to prevent hacking by means of rogue chips.

The phenomenon of ‘Random Telegraph Noise’, which has long been known in the area of transistor reliability, could also be used as a security fingerprint. Random telegraph noise is a name for sudden jumps in voltage or current levels as the result of the random trapping of charges in traps within the gate insulation of a transistor. This phenomenon is unpredictable and random, and hence it could also be perfectly usable as PUF. What was once a problem for us – the breakdown of oxides or the existence of random telegraph noise – is now at the base of major new solutions for computer security.

Biosensors

A third example of discipline-overlapping innovation brings us to the world of life sciences. FinFET transistors are essential for the current and future generations of computer chips. As a result of the research carried out in our group, we have now found out a great deal about the way the work, including their failure mechanisms, etc. So much so that we can now explore the possibility to use them as biosensors. What happens is that biomolecules have a certain charge and when that charge comes into the vicinity of a FinFET, the current in the FinFET will be influ- enced. As a result, there is the potential that the presence of a single biomolecule can be detected by such a FinFET.

Self-healing chips

And, finally, we are also working with system architects to produce reliable chips, even with transistors that are no longer reliable. Extremely small transistors with dimen- sions smaller than 5 nanometers can be very variable and the way they behave is unpredictable. For that reason we are working with system architects on solutions such as self- healing chips, based among other things on the existing models of the failure mechanisms that we provide them with. These self-healing chips will contain monitors that detect local errors. A smart controller then interprets this information and decides how to solve the problem, after which actuators are directed by the controller to carry out the task required.

What about scaling?

Numerous methods are currently being investigated to ensure that transistors can still be miniaturized and improved for as long as possible, as propounded in Moore’s Law. To do so, the classic transistor architecture has already been replaced by a FinFET architecture and in the future this will evolve even to nanosheets or nanowires. Materials other than silicon, with greater mobility, are also being looked at, such as III-V materials (germanium for pMOS and InGaAs for nMOS).

In the choice made for these future architecture, it is extremely important to also look right from the start to the failure mechanisms and reliability of the new solutions.

As an example, last year, our reliability team focused extensively on III-V transistors. Although these transistors score well in terms of mobility, their stability is still one of the main challenges remaining before we are able to take the next step and start manufacturing. The insulation layers in III-V transistors contain a lot of traps that cause this insta- bility in transistor characteristics. Understanding this phenomenon is essential if we are to find a solution for it. So, a breakthrough in this area is needed urgently and our results, which were published in a recent IEDM paper, are certainly a step in the right direction. In the invited paper by Jacopo Franco these instabilities are first analyzed in detail. Then, based on this analysis, practical guidelines are given for the development of III-V gate stacks that offer sufficient reliability.

It’s very difficult to look ahead even further into the future, because as the end of Moore’s Law approaches, increasing numbers of different technologies and concepts are already on the radar (quantum computers, 2D materials, neuro- morphic computers, spinwave logic, etc.). However, none of these concepts has yet made a real breakthrough. But in my view 2017 was the year in which the industry began to take a strong interest in quantum computers, with major investments from important players such as Google and Intel. Imec also plans to play a major role in this field, with the launch of a new program on quantum computing, gathering the extensive expertise available. In the past, quantum computing has been considered more as a purely academic field of research – something of value for physi- cists at universities, but not for engineers and companies. So perhaps the breakthrough of industrial quantum computing will be the next milestone in the history of electronics. Or perhaps this milestone will come from a totally unexpected angle – by combining knowledge and people from entirely different disciplines, creating totally new ideas and concepts. Only the future will tell us!

UC Berkeley engineers have built a bright-light emitting device that is millimeters wide and fully transparent when turned off. The light emitting material in this device is a monolayer semiconductor, which is just three atoms thick.

The device opens the door to invisible displays on walls and windows – displays that would be bright when turned on but see-through when turned off — or in futuristic applications such as light-emitting tattoos, according to the researchers.

Gif of the device in action. Probes inject positive and negative charges in the light emitting device, which is transparent under the campanile outline, producing bright light. Credit: Javey lab.

Gif of the device in action. Probes inject positive and negative charges in the light emitting device, which is transparent under the campanile outline, producing bright light. Credit: Javey lab.

“The materials are so thin and flexible that the device can be made transparent and can conform to curved surfaces,” said Der-Hsien Lien, a postdoctoral fellow at UC Berkeley and a co-first author along with Matin Amani and Sujay Desai, both doctoral students in the Department of Electrical Engineering and Computer Sciences at Berkeley.

Their study was published March 26 in the journal Nature Communications. The work was funded by the National Science Foundation and the Department of Energy.

The device was developed in the laboratory of Ali Javey, professor of Electrical Engineering and Computer Sciences at Berkeley. In 2015, Javey’s lab published research in the journal Science showing that monolayer semiconductors are capable of emitting bright light, but stopped short of building a light-emitting device. The new work in Nature Communicationsovercame fundamental barriers in utilizing LED technology on monolayer semiconductors, allowing for such devices to be scaled from sizes smaller than the width of a human hair up to several millimeters. That means that researchers can keep the thickness small, but make the lateral dimensions (width and length) large, so that the light intensity can be high.

Commercial LEDs consist of a semiconductor material that is electrically injected with positive and negative charges, which produce light when they meet. Typically, two contact points are used in a semiconductor-based light emitting device; one for injecting negatively charged particles and one injecting positively charged particles. Making contacts that can efficiently inject these charges is a fundamental challenge for LEDs, and it is particularly challenging for monolayer semiconductors since there is so little material to work with.

The Berkeley research team engineered a way to circumvent this challenge by designing a new device that only requires one contact on the semiconductor. By laying the semiconductor monolayer on an insulator and placing electrodes on the monolayer and underneath the insulator, the researchers could apply an AC signal across the insulator. During the moment when the AC signal switches its polarity from positive to negative (and vice versa), both positive and negative charges are present at the same time in the semiconductor, creating light.

The researchers showed that this mechanism works in four different monolayer materials, all of which emit different colors of light.

This device is a proof-of-concept, and much research still remains, primarily to improve efficiency. Measuring this device’s efficiency is not straightforward, but the researchers think it’s about 1 percent efficient. Commercial LEDs have efficiencies of around 25 to 30 percent.

The concept may be applicable to other devices and other kinds of materials, the device could one day have applications in a number of fields where having invisible displays are warranted. That could be an atomically thin display that’s imprinted on a wall or even on human skin.

“A lot of work remains to be done and a number of challenges need to be overcome to further advance the technology for practical applications,” Javey said. “However, this is one step forward by presenting a device architecture for easy injection of both charges into monolayer semiconductors.”

Working up a sweat from carrying a heavy load? That is when the textile works at its best. Researchers at Chalmers University of Technology have developed a fabric that converts kinetic energy into electric power, in cooperation with the Swedish School of Textiles in Borås and the research institute Swerea IVF. The greater the load applied to the textile and the wetter it becomes the more electricity it generates. The results are now published in the Nature Partner journal Flexible Electronics.

Chalmers researchers Anja Lund and Christian Müller have developed a woven fabric that generates electricity when it is stretched or exposed to pressure. The fabric can currently generate enough power to light an LED, send wireless signals or drive small electric units such as a pocket calculator or a digital watch.

The technology is based on the piezoelectric effect, which results in the generation of electricity from deformation of a piezoelectric material, such as when it is stretched. In the study the researchers created a textile by weaving a piezoelectric yarn together with an electrically conducting yarn, which is required to transport the generated electric current.

“The textile is flexible and soft and becomes even more efficient when moist or wet,” Lund says. “To demonstrate the results from our research we use a piece of the textile in the shoulder strap of a bag. The heavier the weight packed in the bag and the more of the bag that consists of our fabric, the more electric power we obtain. When our bag is loaded with 3 kilos of books, we produce a continuous output of 4 microwatts. That’s enough to intermittently light an LED. By making an entire bag from our textile, we could get enough energy to transmit wireless signals.”

The piezoelectric yarn is made up of twenty-four fibres, each as thin as a strand of hair. When the fibres are sufficiently moist they become enclosed in liquid and the yarn becomes more efficient, since this improves the electrical contact between the fibres. The technology is based on previous studies by the researchers in which they developed the piezoelectric fibres, to which they have now added a further dimension.

“The piezoelectric fibres consist of a piezoelectric shell around an electrically conducting core,” Lund says. “The piezoelectric yarn in combination with a commercial conducting yarn constitute an electric circuit connected in series.”

Previous work by the researchers on piezoelectric textiles has so far mainly focused on sensors and their ability to generate electric signals through pressure sensitivity. Using the energy to continuously drive electronic components is unique.

“Woven textiles from piezoelectric yarns makes the technology easily accessible and it could be useful in everyday life. It’s also possible to add more materials to the weave or to use it as a layer in a multi-layer product. It requires some modification, but it’s possible,” Lund says.

The researchers consider that the technology is, in principle, ready for larger scale production. It is now mainly up to industrial product developers to find out how to make use of the technology. Despite the advanced technology underlying the material, the cost is relatively low and is comparable with the price of Gore-Tex. Through their collaboration with the Swedish School of Textiles in Borås the researchers have been able to demonstrate that the yarn can be woven in industrial looms and is sufficiently wear-resistant to cope with the harsh conditions of mass production.

Magnolia Optical Technology, Inc. announced that it is working with the Defense Advanced Research Projects Agency (DARPA) under the Phase II SBIR Program for Development of High-Performance Thin-Film Solar Cells for Portable Power Applications (Contract No D15PC00222).

Photovoltaic devices can provide a portable source of electrical power for a wide variety of defense and commercial applications, including mobile power for dismounted soldiers, unmanned aerial vehicles, and remote sensors.

“The goal of the current program is to develop high-efficiency GaAs-based solar cells that maintain their performance over changing environmental conditions, and that are thinner and thus more cost-effective to produce,” said Dr. Roger Welser, Magnolia’s Chief Technical Officer. “By combining thin III-V absorbers with advanced light-trapping structures, single-junction GaAs-based devices provide a means to deliver high efficiency performance over a wide range of operating conditions at a fraction of the cost of the multi-junction structures typically employed for space power. In addition, the incorporation of nano-enhanced III-V absorbers provides a pathway to extend infrared absorption and increase the photovoltaic power conversion efficiency of cost-effective thin-film solar cells.”

Dr. Ashok Sood, President of Magnolia stated “changes in the solar spectrum can dramatically degrade the performance of traditional multi-junction devices – changes that occur naturally throughout the day, from season to season, and from location to location as sunlight passes through the earth’s atmosphere. Moreover, multi-junction III-V cells require thick, complex epitaxial layers and are therefore inherently expensive to manufacture. The technology under development as part of this DARPA-funded program addresses these key weaknesses in the established high-performance photovoltaic technology. The photovoltaic market is a rapidly growing segment of the energy industry with a wide range of commercial and defense applications.”

Magnolia specializes in developing optical technologies for defense and commercial applications. Based in Woburn, MA, Magnolia develops both thin film and nanostructure-based technologies that cover the ultraviolet, visible, and infrared part of the spectrum. These technologies are developed for use in advanced military sensors and other commercial applications including solar cells.

Researchers at RIT have found a more efficient fabricating process to produce semiconductors used in today’s electronic devices. They also confirmed that materials other than silicon can be used successfully in the development process that could increase performance of electronic devices. This fabrication process–the I-MacEtch, or inverse metal-assisted chemical etching method–can help meet the growing demand for more powerful and reliable nano-technologies needed for solar cells, smartphones, telecommunications grids and new applications in photonics and quantum computing.

“What is novel about our work is that for the first time we are looking at applying I-MacEtch processing to indium-gallium-phosphide materials. I-MacEtch is an alternative to two conventional approaches and is a technique that has been used in the field–but the materials that have been explored are fairly limited,” said Parsian Mohseni, assistant professor of microsystems engineering in RIT’s Kate Gleason College of Engineering. He is also director of the EINS Laboratory at the university.

Demands for improved computer processing power have led researchers to explore both new processes and other materials beyond silicon to produce electronic components, Mohseni explained. The I-MacEtch process combines the benefits of two traditional methods–wet etching and reactive ion etching, or REI. Indium-gallium-phosphide is one of several materials being tested to complement silicon as a means to improve current capacity of semiconductor processing.

“This is a very well-known material and has applications in the electronics and solar cell industries,” he said. “We are not re-inventing the wheel; we are establishing new protocols for treating the existing material that is more cost effective, and a more sustainable process.”

Semiconductor devices are created on wafers through a multi-step process to coat, remove or pattern conductive materials. Traditional processes are wet etch, where a sample with blocked aspects is immersed in an acid bath to remove substances, and reactive ion etching, where ions bombard exposed surfaces on the wafer to change its chemical properties and remove materials in those exposed regions. Both have been used to develop the intricate electronic patterns on circuits and use silicon as a foundation for this type of patterning. Improving patterning methods by I-MacEtch could mean reducing fabrication complexity of various photonic and electronic devices.

Researchers and semiconductor fabrication scientists have been using MacEtch extensively for processing silicon. At the same time, assessments of other materials in the III-V range of individual elements that may be conducive to this same type of fabrication with similar advantages are underway. In his research, Mohseni is also looking at different alloys of those III-V materials, namely the ternary alloys such as indium-gallium-phosphide (InGaP).

The research detailed in the upcoming issue of the American Chemical Society’s Applied Materials and Interfaces journal highlights how the nanofabrication methodology was applied to InGaP and how it can impact the processing of device applications and generation of high aspect ratio and nano-scale semiconductor features, said Thomas Wilhelm, a microsystems engineering doctoral student and first-author of the paper. The novel processing method can be significant in the development of ordered arrays of high aspect ratio structures such as nanowires.

For solar cells, the goal is to minimize the cost-to-power-produced ratio, and if it is possible to lower the cost of making the cell, and increasing the efficiency of it, this improves the device overall. Exploring new methods of fabricating the existing, relevant materials in a way that allows for faster, less expensive and more controlled processing by combining the benefits of wet etching and RIE has been the focus of Mohseni’s work. The improved process means avoiding expensive, bulky, hazardous processing methods.

“We are using a simple benchtop set up and we end up with very similar structures; in fact, one can argue that they are higher in quality than the structures that we can generate with RIE for a fraction of the cost and with less time, less steps throughout, without the higher temperature conditions or expensive instrumentation,” he said.

 

An unexpected phenomenon known as zero field switching (ZFS) could lead to smaller, lower-power memory and computing devices than presently possible. The image shows a layering of platinum (Pt), tungsten (W), and a cobalt-iron-boron magnet (CoFeB) sandwiched at the ends by gold (Au) electrodes on a silicon (Si) surface. The gray arrows depict the overall direction of electric current injected into the structure at the back of the gold (Au) contact and coming out the front gold contact pad.

This is an illustration of an unexpected phenomenon known as zero field switching (ZFS) that could lead to smaller, lower-power memory and computing devices than presently possible. The image shows a layering of platinum (Pt), tungsten (W), and a cobalt-iron-boron magnet (CoFeB) sandwiched at the ends by gold (Au) electrodes on a silicon (Si) surface. The gray arrows depict the overall direction of electric current injected into the structure at the back of the gold (Au) contact and coming out the front gold contact pad. The CoFeB layer is a nanometer-thick magnet that stores a bit of data. A "1" corresponds to the CoFeB magnetization pointing up (up arrow), and a "0" represents the magnetization pointing down (down arrow). Credit: Gopman/NIST

This is an illustration of an unexpected phenomenon known as zero field switching (ZFS) that could lead to smaller, lower-power memory and computing devices than presently possible. The image shows a layering of platinum (Pt), tungsten (W), and a cobalt-iron-boron magnet (CoFeB) sandwiched at the ends by gold (Au) electrodes on a silicon (Si) surface. The gray arrows depict the overall direction of electric current injected into the structure at the back of the gold (Au) contact and coming out the front gold contact pad. The CoFeB layer is a nanometer-thick magnet that stores a bit of data. A “1” corresponds to the CoFeB magnetization pointing up (up arrow), and a “0” represents the magnetization pointing down (down arrow). Credit: Gopman/NIST

The CoFeB layer is a nanometer-thick magnet that stores a bit of data. A “1” corresponds to the CoFeB magnetization pointing up (up arrow), and a “0” represents the magnetization pointing down (down arrow). The “0” or “1” can be read both electrically and optically, as the magnetization changes the reflectivity of light shining on the material through another phenomenon known as the magneto-optical Kerr effect (MOKE).

In the device, electric current can flip the data state between 0 and 1. Previous devices of this type have also required a magnetic field or other more complex measures to change the material’s magnetization. Those earlier devices are not very useful for building stable, non-volatile memory devices.

A breakthrough occurred in a research collaboration between The Johns Hopkins University and NIST. The team discovered that they could flip the CoFeB magnetization in a stable fashion between the 0 and 1 states by sending only electric current through the Pt and W metal layers adjacent to the CoFeB nanomagnet. They did not need a magnetic field. This ZFS (zero-field switching) effect was a surprise and had not been theoretically predicted.

In their work, the researchers created a special kind of electric current known as a “spin” current. The electrons that carry electric current possess a property known as spin which can be imagined as a bar magnet pointing in a specific direction through the electron. Increasingly exploited in the emerging field known as “spintronics,” spin current is simply electric current in which the spins of the electrons are pointing in the same direction. As an electron moves through the material, the interaction between its spin and its motion (called a spin-orbit torque, SOT) creates a spin current where electrons with one spin state move perpendicular to the current in one direction and electrons with the opposite spin state move in the opposite direction. The resulting spins that have moved adjacent to the CoFeB magnetic layer exert a torque on that layer, causing its magnetization to be flipped. Without the spin current the CoFeB magnetization is stable against any fluctuations in current and temperature. This unexpected ZFS effect poses new questions to theorists about the underlying mechanism of the observed SOT-induced switching phenomenon.

Details of the spin-orbit torque are illustrated in the diagram. The purple arrows show the spins of the electrons in each layer. The blue curved arrow shows the direction in which spins of that type are being diverted. (For example, in the W layer, electrons with spin to the left in the x-y plane are diverted to move upward toward the CoFeB and the electron spins to the right are diverted to move down toward the Pt.) Note the electron spins in the Pt with spin to the right (in the x-y plane), however, are diverted to move upward toward the W and the electron spins with spin to the left are diverted to move downward toward the Si. This is opposite to the direction the electron spins in the W are moving, and this is due to differences in the SOT experienced by electrons moving through Pt and those moving through W. In fact, it is this difference in the way the electrons move through each of these two conductors that may be important to enabling the unusual ZFS effect.

The research team, including NIST scientists Daniel Gopman, Robert Shull, and NIST guest researcher Yury Kabanov, and The Johns Hopkins University researchers Qinli Ma, Yufan Li and Professor Chia-Ling Chien, report their findings today in Physical Review Letters.

Ongoing investigations by the researchers seek to identify other prospective materials that enable zero-field-switching of a single perpendicular nanomagnet, as well as determining how the ZFS behavior changes for nanomagnets possessing smaller lateral sizes and developing the theoretical foundation for this unexpected switching phenomenon.

Some novel materials that sound too good to be true turn out to be true and good. An emergent class of semiconductors, which could affordably light up our future with nuanced colors emanating from lasers, lamps, and even window glass, could be the latest example.

These materials are very radiant, easy to process from solution, and energy-efficient. The nagging question of whether hybrid organic-inorganic perovskites (HOIPs) could really work just received a very affirmative answer in a new international study led by physical chemists at the Georgia Institute of Technology.

Laser light in the visible range is processed for use in the testing of quantum properties in materials in Carlos Silva's lab at Georgia Tech. Credit: Georgia Tech/Allison Carter

Laser light in the visible range is processed for use in the testing of quantum properties in materials in Carlos Silva’s lab at Georgia Tech. Credit: Georgia Tech/Allison Carter

The researchers observed in an HOIP a “richness” of semiconducting physics created by what could be described as electrons dancing on chemical underpinnings that wobble like a funhouse floor in an earthquake. That bucks conventional wisdom because established semiconductors rely upon rigidly stable chemical foundations, that is to say, quieter molecular frameworks, to produce the desired quantum properties.

“We don’t know yet how it works to have these stable quantum properties in this intense molecular motion,” said first author Felix Thouin, a graduate research assistant at Georgia Tech. “It defies physics models we have to try to explain it. It’s like we need some new physics.”

Quantum properties surprise

Their gyrating jumbles have made HOIPs challenging to examine, but the team of researchers from a total of five research institutes in four countries succeeded in measuring a prototypical HOIP and found its quantum properties on par with those of established, molecularly rigid semiconductors, many of which are graphene-based.

“The properties were at least as good as in those materials and may be even better,” said Carlos Silva, a professor in Georgia Tech’s School of Chemistry and Biochemistry. Not all semiconductors also absorb and emit light well, but HOIPs do, making them optoelectronic and thus potentially useful in lasers, LEDs, other lighting applications, and also in photovoltaics.

The lack of molecular-level rigidity in HOIPs also plays into them being more flexibly produced and applied.

Silva co-led the study with physicist Ajay Ram Srimath Kandada. Their team published the results of their study on two-dimensional HOIPs on March 8, 2018, in the journal Physical Review Materials. Their research was funded by EU Horizon 2020, the Natural Sciences and Engineering Research Council of Canada, the Fond Québécois pour la Recherche, the Research Council of Canada, and the National Research Foundation of Singapore.

The ‘solution solution’

Commonly, semiconducting properties arise from static crystalline lattices of neatly interconnected atoms. In silicon, for example, which is used in most commercial solar cells, they are interconnected silicon atoms. The same principle applies to graphene-like semiconductors.

“These lattices are structurally not very complex,” Silva said. “They’re only one atom thin, and they have strict two-dimensional properties, so they’re much more rigid.”

“You forcefully limit these systems to two dimensions,” said Srimath Kandada, who is a Marie Curie International Fellow at Georgia Tech and the Italian Institute of Technology. “The atoms are arranged in infinitely expansive, flat sheets, and then these very interesting and desirable optoelectronic properties emerge.”

These proven materials impress. So, why pursue HOIPs, except to explore their baffling physics? Because they may be more practical in important ways.

“One of the compelling advantages is that they’re all made using low-temperature processing from solutions,” Silva said. “It takes much less energy to make them.”

By contrast, graphene-based materials are produced at high temperatures in small amounts that can be tedious to work with. “With this stuff (HOIPs), you can make big batches in solution and coat a whole window with it if you want to,” Silva said.

Funhouse in an earthquake

For all an HOIP’s wobbling, it’s also a very ordered lattice with its own kind of rigidity, though less limiting than in the customary two-dimensional materials.

“It’s not just a single layer,” Srimath Kandada said. “There is a very specific perovskite-like geometry.” Perovskite refers to the shape of an HOIPs crystal lattice, which is a layered scaffolding.

“The lattice self-assembles,” Srimath Kandada said, “and it does so in a three-dimensional stack made of layers of two-dimensional sheets. But HOIPs still preserve those desirable 2D quantum properties.”

Those sheets are held together by interspersed layers of another molecular structure that is a bit like a sheet of rubber bands. That makes the scaffolding wiggle like a funhouse floor.

“At room temperature, the molecules wiggle all over the place. That disrupts the lattice, which is where the electrons live. It’s really intense,” Silva said. “But surprisingly, the quantum properties are still really stable.”

Having quantum properties work at room temperature without requiring ultra-cooling is important for practical use as a semiconductor.

Going back to what HOIP stands for — hybrid organic-inorganic perovskites – this is how the experimental material fit into the HOIP chemical class: It was a hybrid of inorganic layers of a lead iodide (the rigid part) separated by organic layers (the rubber band-like parts) of phenylethylammonium (chemical formula (PEA)2PbI4).

The lead in this prototypical material could be swapped out for a metal safer for humans to handle before the development of an applicable material.

Electron choreography

HOIPs are great semiconductors because their electrons do an acrobatic square dance.

Usually, electrons live in an orbit around the nucleus of an atom or are shared by atoms in a chemical bond. But HOIP chemical lattices, like all semiconductors, are configured to share electrons more broadly.

Energy levels in a system can free the electrons to run around and participate in things like the flow of electricity and heat. The orbits, which are then empty, are called electron holes, and they want the electrons back.

“The hole is thought of as a positive charge, and of course, the electron has a negative charge,” Silva said. “So, hole and electron attract each other.”

The electrons and holes race around each other like dance partners pairing up to what physicists call an “exciton.” Excitons act and look a lot like particles themselves, though they’re not really particles.

Hopping biexciton light

In semiconductors, millions of excitons are correlated, or choreographed, with each other, which makes for desirable properties, when an energy source like electricity or laser light is applied. Additionally, excitons can pair up to form biexcitons, boosting the semiconductor’s energetic properties.

“In this material, we found that the biexciton binding energies were high,” Silva said. “That’s why we want to put this into lasers because the energy you input ends up to 80 or 90 percent as biexcitons.”

Biexcitons bump up energetically to absorb input energy. Then they contract energetically and pump out light. That would work not only in lasers but also in LEDs or other surfaces using the optoelectronic material.

“You can adjust the chemistry (of HOIPs) to control the width between biexciton states, and that controls the wavelength of the light given off,” Silva said. “And the adjustment can be very fine to give you any wavelength of light.”

That translates into any color of light the heart desires.

A research team from Tokyo Institute of Technology (Tokyo Tech) and Waseda University have successfully produced high-quality thin film monocrystalline silicon with a reduced crystal defect density down to the silicon wafer level at a growth rate that is more than 10 times higher than before. In principle, this method can improve the raw material yield to nearly 100%. Therefore, it can be expected that this technology will make it possible to drastically reduce manufacturing costs while maintaining the power generation efficiency of monocrystalline silicon solar cells, which are used in most high efficient solar cells.

This is the monocrystalline Si thin film peeled off using adhesive tape. Credit: CrystEngComm

This is the monocrystalline Si thin film peeled off using adhesive tape. Credit: CrystEngComm

Background

Solar power generation is a method of generating power where solar light energy is converted directly into electricity using a device called a “solar cell.” Efficiently converting the solar energy that is constantly striking the earth to generate electricity is an effective solution to the problem of global warming related to CO2emissions. By making the monocrystalline Si solar cells that are at the core of solar power generation systems thinner, it is possible to greatly reduce raw material costs, which account for about 40% of the current module, and by making them flexible and lighter, usage can be expected to expand and installation costs can be expected to decrease.

In addition, as a method of reducing manufacturing cost, thin-film monocrystalline Si solar cells that use porous silicon (Double Porous Silicon Layer: DPSL) via lift-off are attracting attention as having a competitive edge in the future.

Among the technical challenges related to monocrystalline Si solar cells using lift-off are 1) the formation of a high-quality thin film Si at the Si wafer level, 2) achieving a porous structure that can easily be lifted off (peeled off), 3) improving the growth rate and Si raw material yield (necessary equipment costs are determined by the growth rate), and 4) being able to use the substrate after lift-off without any waste.

In order to overcome challenge 1), it was necessary to clarify the main factors that determine the quality of thin film crystals grown on porous silicon, and to develop a technique for controlling these.

Overview of research achievement

A joint research team consisting of Professor Manabu Ihara and Assistant Professor Kei Hasegawa of the Tokyo Tech, and Professor Suguru Noda of Waseda University has developed a high-quality thin film monocrystalline silicon with a thickness of about 10 μm and a reduced crystal defect density down to the silicon wafer level at a growth rate that is more than 10 times higher than before. First, double-layer nano-order porous silicon is generated on the surface of a monocrystalline wafer using an electrochemical technique. Next, the surface was smoothed to a roughness of 0.2 to 0.3 nm via a unique zone heating recrystallization method (ZHR method), and this substrate was used for high-speed growth to obtain a moonocrystalline thin film with high crystal quality. The grown film can easily be peeled off using the double-layer porous Si layer, and the substrate can be reused or used as an evaporation source for thin film growth, which greatly reduces material loss. When the surface roughness of the underlying substrate is reduced by changing the ZHR method conditions, the defect density of the thin film crystal that was grown decreased, and the team eventually succeeded in reducing it to the Si wafer level of about 1/10th. This quantitatively shows that a surface roughness in the range of only 0.1-0.2 nm (level of atoms to several tens of layers) has an important impact on the formation of crystal defects, which is also of interest as a crystal growth mechanism.

The film formation rate and the conversion rate of the Si source to the thin film Si are bottlenecks in the production of thin-film monocrystalline Si. With chemical vapor deposition (CVD), which is mainly used for epitaxy, the maximum film forming rate is a few μm/h and the yield is about 10%. At the Noda Laboratory of Waseda University, instead of the regular physical vapor deposition (PVD) where raw Si is vaporized at around its melting point of 1414 ?C, by vaporizing the raw Si at much higher temperature of >2000 ?C, a rapid evaporation method (RVD) was developed with a high Si vapor pressure capable of depositing Si at 10 μm/min.

It was found that the ZHR technology developed this time can resolves technical problems and drastically reduce the manufacturing cost of the lift-off process.

Future development

Based on the results of this study, not only did the team discover the main factors for improving the quality of crystals during rapid growth on porous silicon used for the lift-off process, they succeeded in controlling these. In the future, measurement of the carrier lifetime of the thin film, which is directly connected to the performances of solar cells, and fabrication of solar cells will be carried out with the goal of putting the technology into practical use. The use of this Si thin films as low cost bottom cells in tandem type solar cells with an efficiency of over 30% will also be considered.

The results are published in the Royal Society of Chemistry (RSC) journal CrystEngComm and will be featured on the inside front cover of the issue.

One of the problems for Javier Vela and the chemists in his Iowa State University research group was that a toxic material worked so well in solar cells.

And so any substitute for the lead-containing perovskites used in some solar cells would have to really perform. But what could they find to replace the perovskite semiconductors that have been so promising and so efficient at converting sunlight into electricity?

What materials could produce semiconductors that worked just as well, but were safe and abundant and inexpensive to manufacture?

“Semiconductors are everywhere, right?” Vela said. “They’re in our computers and our cell phones. They’re usually in high-end, high-value products. While semiconductors may not contain rare materials, many are toxic or very expensive.”

Vela, an Iowa State associate professor of chemistry and an associate of the U.S. Department of Energy’s Ames Laboratory, directs a lab that specializes in developing new, nanostructured materials. While thinking about the problem of lead in solar cells, he found a conference presentation by Massachusetts Institute of Technology researchers that suggested possible substitutes for perovskites in semiconductors.

Vela and Iowa State graduate students Bryan Rosales and Miles White decided to focus on sodium-based alternatives and started an 18-month search for a new kind of semiconductor. The work was supported by Vela’s five-year, $786,017 CAREER grant from the National Science Foundation. CAREER grants are the foundation’s most prestigious awards for early career faculty.

They came up with a compound that features sodium, which is cheap and abundant; bismuth, which is relatively scarce but is overproduced during the mining of other metals and is cheap; and sulfur, the fifth most common element on Earth. The researchers report their discovery in a paper recently published online by the Journal of the American Chemical Society.

The paper’s subtitle is a good summary of their work: “Toward Earth-Abundant, Biocompatible Semiconductors.”

“Our synthesis unlocks a new class of low-cost and environmentally friendly ternary (three-part) semiconductors that show properties of interest for applications in energy conversion,” the chemists wrote in their paper.

In fact, Rosales is working to create solar cells that use the new semiconducting material.

Vela said variations in synthesis – changing reaction temperature and time, choice of metal ion precursors, adding certain ligands – allows the chemists to control the material’s structure and the size of its nanocrystals. And that allows researchers to change and fine tune the material’s properties.

Several of the material’s properties are already ideal for solar cells: The material’s band gap – the amount of energy required for a light particle to knock an electron loose – is ideal for solar cells. The material, unlike other materials used in solar cells, is also stable when exposed to air and water.

So, the chemists think they have a material that will work well in solar cells, but without the toxicity, scarcity or costs.

“We believe the experimental and computational results reported here,” they wrote in their paper, “will help advance the fundamental study and exploration of these and similar materials for energy conversion devices.”

Imec, a research and innovation hub in nanoelectronics and digital technologies, today presented its annual Lifetime of Innovation Award to Dr. Irwin Jacobs, Founding Chairman and CEO Emeritus of Qualcomm. The annual industry honor is presented to the individual who has significantly advanced the field of semiconductor technology.  The formal presentation will be made at the global Imec Technology Forum (ITF) in May in Belgium.

In making the announcement, Luc Van den hove, president and CEO of imec, said: “Irwin Jacobs’ many technological contributions laid the groundwork for creating the mobile industry and markets that we know today. Under his leadership, Qualcomm developed two-way mobile satellite communications and tracking systems deemed the most advanced in the world. He pioneered spread-spectrum technology and systems using CDMA (code division multiple access), which became a digital standard for cellular phone communications. Together, these technologies opened mobile communications to the global consumer market.”

Irwin Jacobs began his career first as an assistant and then associate professor of electrical engineering at MIT and, later, as professor of computer science and engineering at the University of California in San Diego. While at MIT, he co-authored Principles of Communication Engineering, a textbook still in use. He began his corporate life as a cofounder of Linkabit, which developed satellite encryption devices.  In 1985, he co-founded Qualcomm, serving as CEO until 2005 and chairman through 2009.  His numerous awards include the National Medal of Technology, the Marconi Prize, and the Carnegie Medal of Philanthropy.  His honors include nine honorary degrees including doctor of engineering from the National Tsing Hua University, Taiwan.

Imec initiated the Lifetime of Innovation Award in 2015 at their annual global forum known as ITF (Imec Technology Forum).  The award marks milestones that have transformed the semiconductor industry.  The first recipient was Dr. Morris Chang, whose foundry model launched the fabless semiconductor industry, spurring creation of new innovative companies.  In 2016, Gordon Moore was honored, creator of the famous Moore’s law theory and co-founder of Intel.  Dr. Kinam Kim was honored in 2017 for his contributions in memory technologies and his visionary leadership at Samsung.

Luc Van den hove concluded, saying: “Our mission is to create innovation through collaboration. By gathering global technology leaders at the ITF, imec provides an open forum to share issues and trends challenging the semiconductor industry. In this international exchange, imec and participants outline ways to collaborate in bringing innovative solutions to market.”