Category Archives: Process Materials

PAUL STOCKMAN, Head of Market Development, Linde Electronics, Taipei, Taiwan

Nearly 60 years after Richard Feynman delivered his celebrated talk, which became the foundation for nanotechnology [1], many of the milestones he envisioned have been achieved and surpassed. In particular, he discussed computing devices with wires 10 to 100 atoms in width. Today we are reaching the smaller end of that range for high-volume FinFET and 10nm class DRAM chips, and device manufacturers are confidently laying the roadmaps for generations of conductors with single atom scales.

While device shrinkage has continued apace, it has not been without consequences. As chip circuit dimensions dip into the atomic range, bulk semiconductor properties which allowed for relatively simple scaling are breaking down and atomic-level physics are beginning to dominate. At this scale, every atom counts. And when different isotopes of the needed atoms have significantly different properties, the ability to create isotopically pure materials (IPMs) becomes essential.

In this paper, we begin by discussing several examples of IPMs used in current high-volume electronics manufacturing:  the physics at play and the materials selected. With the near future in focus, we then look at coming applications which may also require IPMs. Finally, we look at the current supply of IPM precursors, and how this needs to be developed in the future.

What is an isotope?

Isotopes are atoms of a particular element which have the same number of protons in their nucleus, but different numbers of neutrons (FIGURE 1). The number of protons determines which element the atom is:  hydrogen has one proton, helium has two protons, and so on in the ordering used for the periodic table.

FIGURE 1. The stable isotopes of hydrogen and helium. The number of protons determines the element, and the number of neutrons determines the isotope. The superscript number is the sum of the protons and the neutrons.

Different isotopes of the same element all have nearly exact chemical behavior – that is how they form and break molecular bonds in chemical reactions – but sometimes exhibit significantly different physical behavior. It is these differences in physical behavior which become important when electronics are made on the atomic scale.

For the purposes of engineering semiconductors, it is important to consider two different classes of isotopes.

  • Radioactive: The nuclei of these of these isotopes are unstable, break apart into different and lighter elements, and often emit radiation in the form of alpha particles or light. The rate at which this happens can be quite fast – much less than a nanosecond – to longer than a billion years for half of the material to undergo decay. It is this type of isotope which was first historically observed and which we often first learn about. The element uranium has five different isotopes which naturally occur on earth, but all of these are radioactive.
  • Stable: All other isotopes are termed stable. This means that they have not been observed to break apart, even once, when looking at bulk quantities of material with many billions of atoms. We do have evidence that some of these isotopes, termed primordial, are indeed stable over the span of knowable time, as we have evidence that they have formed and not decayed since the formation of the universe.

For the use of IPMs to engineer semiconductors, only stable isotopes are considered. Even for radioactive isotopes which decay slowly, the fact that current logic and memory chips have more than a billion transistors means that one or more circuits have the likelihood to be corrupted over the useful lifetime of the chip. In FIGURE 2, we show some common elements used in semiconductor manufacturing with their stable isotopes and naturally occurring abundances.

FIGURE 2. The natural abundance of stable isotopes for elements most relevant for electronics devices.

Current applications

There are already several electronics applications for IPMs, which have been used in high volume for more than a decade.

Deuterium (D2 = 2H2):  Deuterium (D) is the second stable isotope of hydrogen, with one proton and one neutron. As a material, it is most commonly used in electronics manufacturing as the IPM precursor gas D2. Chemically, deuterium can be substituted directly for any reaction using normally abundant hydrogen. Deuterium is made by the electrolysis of D2O, often called heavy water, which has been already enriched in the deuterium isotope.

Important physical property — mass:  For most elements, the difference in mass among their isotopes is only a few percent. However, for the lightest element hydrogen, there is a two-fold difference in mass. The chemical bond between a hydrogen atom and a heavier bulk material can be roughly approximated by the simple classical mechanics example of a weight at the end of the spring. When the weight is doubled, the force on the spring is also doubled (FIGURE 3).

FIGURE 3. According to Hooke’s Law, a spring will stretch twice as far when attached to a mass twice the original. When the balls and springs are bonds to naturally abundant hydrogen 1H and deuterium 2H, they vibrate at different frequencies.

At the atomic level, quantum mechanics applies, and only certain amounts of energy, or quanta, can excite the spring. When deuterium 2H is substituted for the much more abundant 1H, the amount of energy which can excite the spring changes by almost 50%.

Long-distance optical fibers:  Optical fibers, like semiconductors, rely on silicon oxide as a primary material. The fiber acts as a waveguide to contain and transmit bursts of near-infrared lasers along the length of the fiber. The surface of the silicon oxide fiber is covered with hydroxide (oxygen-hydrogen), which is formed during the manufacture of the fiber. Unfortunately, the hydroxide chemical bonds absorb small amounts of the laser light with every single reflection against the surface, which in turn diminishes the signal. By substituting deuterium 2H for normally abundant hydrogen 1H on the surface hydroxide, the hydroxide molecular spring no longer absorbs light at the frequencies used for communication.

Hot carrier effect between gate and channel:  As transistor sizes decrease, the local electric fields inside transistors increase. When the local field is high enough, it can generate free electrons with high kinetic energy, known as hot carriers. Gate oxides are often annealed in hydrogen to reduce the deleterious effects of these hot carriers. But the hydride bonds themselves can become points of failure because the hot carriers have just the right amount of energy to excite and even break the hydride bonds. Just like in the optical fiber application, substituting deuterium 2H for naturally abundant 1H changes the energy of the bond and protects it against hot carrier damage. The lifetimes of the devices are extended by a factor of 50 to 100.

11-Boron trifluoride (11BF3):  Boron has two naturally occurring stable isotopes 10B at 20% and 11B at 80%. Boron is used in electronics manufacturing as a dopant for silicon to modify its semiconducting properties, and is most commonly supplied as the gases boron trifluoride (BF3) or diborane (B2H6). 11BF3 is produced by the distillation of naturally abundant BF3, and can be converted to other boron compounds like B2H6.

Important physical property – neutron capture:  The earth is continuously bombarded by high-energy cosmic radiation, which is produced primarily by events distant from our solar system. We are shielded from most of this radiation because it reacts with the molecules in our outer atmosphere. A by-product of this shielding mechanism are neutrons which constantly shower the earth’s surface, but are not strong enough to pose any biological risk, usually passing through most materials without reaction. However, the nucleus of the 10B atom is more than 1 million times likely to react with background neutrons versus other isotopes, including 11B. This results in splitting the 10B atom into a 7Li (lithium) atom and an alpha particle (helium nucleus) (FIGURE 4).

FIGURE 4. 10B neutron capture. When a 10B atom captures a background neutron, it breaks into a smaller 7Li atom and emits alpha and gamma radiation.

The smallest semiconductor gates now contain fewer than 100 dopant boron atoms. If even one of these is transmuted into a lithium atom, it can change the gate voltage and therefore the function of the transistor. Furthermore, the energetic alpha particle can cause additional damage. By using BF3 which is depleted of 10B below 0.1%, semiconductor manufacturers can greatly reduce the risk for component failure.

Initially, IPMs D2 and 11BF3 were used for making chips with the most critical value, like high performance computing processors, or remote operating environments like satellites and space vehicles. Now, as chip dimensions shrink to single nanometers and transistors multiply into the billions, these IPMs are increasingly being adopted into more high volume manufacturing processes.

Developing and future applications

As devices continue to scale to atomic dimensions and new device structures are developed to continue the progression of electronics advance, IPMs will play a greater role in a future where every atom counts. We present in this section a few of the nearest and most promising applications.

Important physical property – thermal conductivity:  Thermal conductivity is the property of materials to transfer heat, which is especially important in semiconductor chips where localized transistor temperatures can exceed 150 C and can affect the chip performance. Materials like carbon (either diamond or graphene) and copper have relatively high thermal conductivity; silicon and silicon nitride medium; and oxides like silicon oxide and aluminum oxide are much lower. However, with all of the other design constraints and requirements, semiconductor engineers seldom choose materials to optimize thermal conductivity.

When viewed in classical mechanics, thermal conductivity is like a large set of balls and springs. In an atomically pure material like diamond or silicon, all the springs are the same, and all the balls are nearly the same – the differences being the different masses of the naturally occurring isotopes. When IPMs are used to make to make the material, all the balls are now exactly the same, there is less disorder, and heat is transferred through the matrix of balls and springs more efficiently. This has been demonstrated in graphene to improve the thermal conductivity at room temperature by 60% (Figure 5), and other studies have shown a similar magnitude improvement in silicon.

FIGURE 5. Thermal conductivity of graphene for different concentrations of 12C content. 99.99% 12C is achievable using commercial grade 12CH4 methane; 98.6% 12C is the value of naturally abundant carbon on earth; 50% 12C is an artificial mixture to demonstrate the trend. Source: Thermal conductivity of isotopically modified graphene. S Chen et al., Nature Materials volume 11, pages 203–207 (2012).

Silicon epilayers:  Epitaxially grown silicon is often the starting substrate for CMOS manufacturing. This may be especially beneficial for HD-SOI applications where trade-offs of substrate cost vs processing cost and device performance are already part of the value equation.

Sub 3nm 2D graphene FETs:  Graphene is a much-discussed material being considered for sub-3nm devices as the successor technology for logic circuits after the FinFET era. Isotopically pure graphene could reduce localized heating at the source.

Important physical property – nuclear spin:  Each atomic nucleus has a quantum mechanical property associated with it called nuclear spin. Because it is a quantum mechanical property, nuclear spin is measured in discrete amounts, and in this case half-integer numbers. Nuclear spin is determined by the number of protons and neutrons. Since different isotopes have different numbers of neutrons, they also have different nuclear spin. An atomically and isotopically pure material will have atoms with all the same spin.

Qubits and quantum computing:  Much research has been published recently about quantum computing as the successor to transistor-based processors. IBM and Intel, among others, have made demonstration devices, albeit not large enough for practical applications yet. The most promising near-term realization of quantum computing uses qubits – atomic two-state components—which store and transmit information via electron spin, which is a property analogous to nuclear spin, and can be affected by nuclear spin. Many of these early devices have been made with isotopically pure diamond (carbon) or silicon matrices to avoid disorder from having multiple values of nuclear spin in these devices.

12-Methane (12CH4):  Carbon is predominantly 12C, with about 1.1% 13C in natural abundance. A large demand already exists for 13C chemicals, primarily used as markers in studying chemical and biological reactions. 13C is produced primarily by the distillation of carbon monoxide (CO), and then chemically converted to other carbon-containing precursors like methane (CH4). At the same time 13C is produced, a large amount of 13C-depleted 12CO is produced, which serves as a less expensive feedstock for applications which require IPMs made from carbon.

28-Silane (28SiH4) and other silicon precursors:  There are no such large applications driving the production of isotopically pure silicon precursors yet. Currently, research quantities of silicon tetrafluoride (SiF4) are produced primarily by using gas centrifuges, and then converted into silicon precursors like silicon tetrachloride (SiCl4), trichlorosilane (SiHCl3), dichlorosilane (SiH2Cl2), and silane (SiH4). Distillation of one or more of these materials would be a less expensive option for larger-scale production.

Other IPM precursors already exist for oxygen and nitrogen compounds, and are relatively inexpensive because they are also by-products from the production of chemical markers for less abundant isotopes. Aluminum and phosphorous only have one stable isotope, and so all compounds produced with these are isotopically pure in these elements.

Production methods

Production of IPMs is challenging because of the limited differences in physical and chemical properties normally used to separate and purify materials, and because of the low concentration of some of the desired isotopes. A number of creative approaches have been applied, and often the methods are repeated to obtain the desired enrichment and purity of the IPM. We give a description here of the two methods used today in the production of IPMs relevant to the electronics industry. Importantly, all of these require gas-phase starting materials in order to enhance the physical differences that do exist among the isotopes.

Distillation:  The more familiar of the two is distillation, which relies on differing boiling points of materials to separate them into lower boiling point fractions called lights (which often are lighter in mass) and higher boiling point fractions which are called heavies. Because the boiling point differences are a fraction of a degree, very long distillation columns are used. Distillation is best used with lighter compounds, and is the preferred method for producing D2O, 11BF3, 12CO / 13CO, and isotopes of nitrogen and oxygen (FIGURE 6).

 

FIGURE 6. Isotope separation. Distillation. Gas is fed into the middle of the distillation column. By the process of condensation and boiling on many plates, the lighter isotope is separated as a gas leaving the top of the column, and the heavier isotope leaves as a liquid at the bottom.Centrifuge:  After a number of other methods were tried, gas centrifuges were the method ultimately chosen to scale the separation of the 235U uranium isotope in uranium hexafluoride (UF6) gas used for the first atomic devices during the Manhattan Project, and remains the preferred method of obtaining this useful isotope and other isotopes of heavier elements. The gas is spun at very high speeds – around 100,000 rpm – and the higher mass isotopes tend toward the outer regions of the centrifuge. Many gas centrifuges are linked in arrays to achieve the desired level of enrichment. Currently, useful quantities of of 28SiF4 and 30SiF4 are produced with this method (FIGURE 7).

 

FIGURE 7. In this isotope separation centrifuge, gas is fed into the center of the centrifuge, which is spinning at a very high rate of 100,000 rpm. The heavier isotope is thrown to the sides, while the lighter isotope remains in the center.

Conclusion

In the hundred-year anniversary of Richard Feynman’s birth, we are still finding plenty of room at the bottom. But as we go further down, we must look more carefully at what is there. Increasingly, we are seeing that individual atoms hold the properties that are important today and which will support the developments of a not-too-distant tomorrow. IPMs are an important part of creating that reality.

Linde Electronics is the leader in the production of IPMs which are important to electronics today, and holds the technology to produce the IPMs which will support the development of tomorrow’s devices. Linde has made recent investments for the production of deuterium (D2) and 11BF3 to satisfy global electronics demand, and has a long history in the production, purification, and chemical synthesis of stable isotopes relevant to semiconductor manufacturing.

Reference

  1. Feynman, Richard P. (1960) There’s Plenty of Room at the Bottom. Engineering and Science, 23 (5). pp. 22-36.

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

Researchers at Tokyo Institute of Technology (Tokyo Tech) report a unipolar n-type transistor with a world-leading electron mobility performance of up to 7.16 cm2 V-1 s-1. This achievement heralds an exciting future for organic electronics, including the development of innovative flexible displays and wearable technologies.

Researchers worldwide are on the hunt for novel materials that can improve the performance of basic components required to develop organic electronics.

Now, a research team at Tokyo Tech’s Department of Materials Science and Engineering including Tsuyoshi Michinobu and Yang Wang report a way of increasing the electron mobility of semiconducting polymers, which have previously proven difficult to optimize. Their high-performance material achieves an electron mobility of 7.16 cm2 V-1 s-1, representing more than a 40 percent increase over previous comparable results.

In their study published in the Journal of the American Chemical Society, they focused on enhancing the performance of materials known as n-type semiconducting polymers. These n-type (negative) materials are electron dominant, in contrast to p-type (positive) materials that are hole dominant. “As negatively-charged radicals are intrinsically unstable compared to those that are positively charged, producing stable n-type semiconducting polymers has been a major challenge in organic electronics,” Michinobu explains.

The research therefore addresses both a fundamental challenge and a practical need. Wang notes that many organic solar cells, for example, are made from p-type semiconducting polymers and n-type fullerene derivatives. The drawback is that the latter are costly, difficult to synthesize and incompatible with flexible devices. “To overcome these disadvantages,” he says, “high-performance n-type semiconducting polymers are highly desired to advance research on all-polymer solar cells.”

The team’s method involved using a series of new poly(benzothiadiazole-naphthalenediimide) derivatives and fine-tuning the material’s backbone conformation. This was made possible by the introduction of vinylene bridges[1] capable of forming hydrogen bonds with neighboring fluorine and oxygen atoms. Introducing these vinylene bridges required a technical feat so as to optimize the reaction conditions.

Overall, the resultant material had an improved molecular packaging order and greater strength, which contributed to the increased electron mobility.

Using techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS), the researchers confirmed that they achieved an extremely short π-π stacking distance[2] of only 3.40 angstrom. “This value is among the shortest for high mobility organic semiconducting polymers,” says Michinobu.

There are several remaining challenges. “We need to further optimize the backbone structure,” he continues. “At the same time, side chain groups also play a significant role in determining the crystallinity and packing orientation of semiconducting polymers. We still have room for improvement.”

Wang points out that the lowest unoccupied molecular orbital (LUMO) levels were located at -3.8 to -3.9 eV for the reported polymers. “As deeper LUMO levels lead to faster and more stable electron transport, further designs that introduce sp2-N, fluorine and chlorine atoms, for example, could help achieve even deeper LUMO levels,” he says.

In future, the researchers will also aim to improve the air stability of n-channel transistors — a crucial issue for realizing practical applications that would include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectrics.

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

Future technologies based on the principles of quantum mechanics could revolutionize information technology. But to realize the devices of tomorrow, today’s physicists must develop precise and reliable platforms to trap and manipulate quantum-mechanical particles.

In a paper published Feb. 25 in the journal Nature, a team of physicists from the University of Washington, the University of Hong Kong, the Oak Ridge National Laboratory and the University of Tennessee, report that they have developed a new system to trap individual excitons. These are bound pairs of electrons and their associated positive charges, known as holes, which can be produced when semiconductors absorb light. Excitons are promising candidates for developing new quantum technologies that could revolutionize the computation and communications fields.

The team, led by Xiaodong Xu, the UW’s Boeing Distinguished Professor of both physics and materials science and engineering, worked with two single-layered 2D semiconductors, molybdenum diselenide and tungsten diselenide, which have similar honeycomb-like arrangements of atoms in a single plane. When the researchers placed these 2D materials together, a small twist between the two layers created a “superlattice” structure known as a moiré pattern — a periodic geometric pattern when viewed from above. The researchers found that, at temperatures just a few degrees above absolute zero, this moiré pattern created a nanoscale-level textured landscape, similar to the dimples on the surface of a golf ball, which can trap excitons in place like eggs in an egg carton. Their system could form the basis of a novel experimental platform for monitoring excitons with precision and potentially developing new quantum technologies, said Xu, who is also a faculty researcher with the UW’s Clean Energy Institute.

Excitons are exciting candidates for communication and computer technologies because they interact with photons — single packets, or quanta, of light — in ways that change both exciton and photon properties. An exciton can be produced when a semiconductor absorbs a photon. The exciton also can later transform back into a photon. But when an exciton is first produced, it can inherit some specific properties from the individual photon, such as spin. These properties can then be manipulated by researchers, such as changing the spin direction with a magnetic field. When the exciton again becomes a photon, the photon retains information about how the exciton properties changed over its short life — typically, about a hundred nanoseconds for these excitons — in the semiconductor.

In order to utilize individual excitons’ “information-recording” properties in any technological application, researchers need a system to trap single excitons. The moiré pattern achieves this requirement. Without it, the tiny excitons, which are thought to be less than 2 nanometers in diameter, could diffuse anywhere in the sample — making it impossible to track individual excitons and the information they possess. While scientists had previously developed complex and sensitive approaches to trap several excitons close to one another, the moiré pattern developed by the UW-led team is essentially a naturally formed 2D array that can trap hundreds of excitons, if not more, with each acting as a quantum dot, a first in quantum physics.

A unique and groundbreaking feature of this system is that the properties of these traps, and thus the excitons, can be controlled by a twist. When the researchers changed the rotation angle between the two different 2D semiconductors, they observed different optical properties in excitons. For example, excitons in samples with twist angles of zero and 60 degrees displayed strikingly different magnetic moments, as well as different helicities of polarized light emission. After examining multiple samples, the researchers were able to identify these twist angle variations as “fingerprints” of excitons trapped in a moiré pattern.

In the future, the researchers hope to systematically study the effects of small twist angle variations, which can finely tune the spacing between the exciton traps — the egg carton dimples. Scientists could set the moiré pattern wavelength large enough to probe excitons in isolation or small enough that excitons are placed closely together and could “talk” to one another. This first-of-its-kind level of precision may let scientists probe the quantum-mechanical properties of excitons as they interact, which could foster the development of groundbreaking technologies, said Xu.

“In principle, these moiré potentials could function as arrays of homogenous quantum dots,” said Xu. “This artificial quantum platform is a very exciting system for exerting precision control over excitons — with engineered interaction effects and possible topological properties, which could lead to new types of devices based on the new physics.”

“The future is very rosy,” Xu added.

Researchers at The University of Manchester in the UK, led by Dr Artem Mishchenko, Prof Volodya Fal’ko and Prof Andre Geim, have discovered the quantum Hall effect in bulk graphite – a layered crystal consisting of stacked graphene layers. This is an unexpected result because the quantum Hall effect is possible only in so-called two-dimensional (2D) systems where electrons’ motion is restricted to a plane and must be disallowed in the perpendicular direction. They have also found that the material behaves differently depending on whether it contains odd or even number of graphene layers – even when the number of layers in the crystal exceeds hundreds. The work is an important step to the understanding of the fundamental properties of graphite, which have often been misunderstood, esepcially in recent years.

In their work, published in Nature Physics, Mishchenko and colleagues studied devices made from cleaved graphite crystals, which essentially contain no defects. The researchers preserved the high quality of the material also by encapsulating it in another high-quality layered material – hexagonal boron nitride. They shaped their devices in a Hall bar geometry, which allowed them to measure electron transport in the thin graphite.

“The measurements were quite simple.” explains Dr Jun Yin, the first author of the paper. “We passed a small current along the Hall bar, applied strong magnetic field perpendicular to the Hall bar plane and then measured voltages generated along and across the device to extract longitudinal resistivity and Hall resistance.

Dimensional reduction

Fal’ko who led the theory part said: “We were quite surprised when we saw the quantum Hall effect (QHE) – a sequence of quantized plateaux in the Hall resistance – accompanied by zero longitudinal resistivity in our samples. These are thick enough to behave just as a normal bulk semimetal in which QHE should be forbidden.”

The researchers say that the QHE comes from the fact that the applied magnetic field forces the electrons in graphite to move in a reduced dimension, with conductivity only allowed in the direction parallel to the field. In thin enough samples, however, this one-dimensional motion can become quantized thanks to the formation of standing electron waves. The material thus goes from being a 3D electron system to a 2D one with discrete energy levels.

Even/odd number of graphene layers is important

Another big surprise is that this QHE is very sensitive to even/odd number of graphene layers. The electrons in graphite are similar to those in graphene and come in two “flavours” (called valleys). The standing waves formed from electrons of two different flavours sit on either even – or odd – numbered layers in graphite. In films with even number of layers, the number of even and odd layers is the same, so the energies of the standing waves of different flavours coincide.

The situation is different in films with odd numbers of layers, however, because the number of even and odd layers is different, that is, there is always an extra odd layer. This results in the energy levels of the standing waves of different flavours shifting with respect to each other and means that these samples have reduced QHE energy gaps. The phenomenon even persists for graphite hundreds of layers thick.

Observations of the fractional QHE

The unexpected discoveries did not end there: the researchers say they also observed the fractional QHE in thin graphite below 0.5 K. The FQHE is different from normal QHE and is a result of strong interactions between electrons. These interactions, which can often lead to important collective phenomena such as superconductivity, magnetism and superfluidity, make the charge carriers in a FQHE material behave as quasiparticles with charge that is a fraction of that of an electron.

“Most of the results we have observed can be explained using a simple single-electron model but seeing the FQHE tells us that the picture is not so simple,” says Mishchenko. “There are plenty of electron-electron interactions in our graphite samples at high magnetic fields and low temperatures, which shows that many-body physics is important in this material.”

Coming back to graphite

Graphene has been in the limelight these last 15 years, and with reason, and graphite was pushed back a little by its one-layer-thick offspring, Mishchenko adds. “We have now come back to this old material. Knowledge gained from graphene research, improved experimental techniques (such as van der Waals assembly technology) and a better theoretical understanding (again from graphene physics), has already allowed us to discover this novel type of the QHE in graphite devices we made.

“Our work is a new stepping stone to further studies on this material, including many-body physics, like density waves, excitonic condensation or Wigner crystallization.”

The graphite studied here has natural (Bernal) stacking, but there is another stable allotrope of graphite – rhombohedral. There are no reported transport measurements on this material so far, only lots of theoretical predictions, including high-temperature superconductivity and ferromagnetism. The Manchester researchers say they thus now plan to explore this allotrope too.

“For decades graphite was used by researchers as a kind of ‘philosopher’s stone’ that can deliver all probable and improbable phenomena including room-temperature superconductivity,” Geim adds with a smile. “Our work shows what is, in principle, possible in this material, at least when it is in its purest form.”

The connection from fridge magnets to cutting edge materials science is shorter than what one might expect. The reason why a magnet sticks to your fridge is that electronic spins or magnetic moments in the magnetic material spontaneously align or order in one direction, which enables it to exert an attractive force to the steel door of your fridge and reminds you to buy milk.

Magnets are one type of materials with such built-in order. A ‘topological defect’ in such a material occurs as a discontinuity in this order, i.e. a boundary region where the order does not seamlessly transition from one area to another. These topological structures form naturally or can be highly engineered in advanced functional materials.

An article published this week in the leading journal Nature Materials by FLEET CI Prof Jan Seidel outlines emerging research into different types of ‘defective’ order, i.e. topological structures in materials, and their potential highly interesting applications in nanotechnology and nanoelectronics.

Seidel was invited by the journal editor to review current and discuss future research on domain walls and related topological structures.

Although known for a long time, domain walls as one type of topological structure have only been intensively studied in detail over recent years. It is only with recent developments in high-resolution electron microscopy (HREM) and scanning probe microscopy (SPM) that it has been shown that they can significantly affect macroscopic materials properties, and even more interestingly, that they can exhibit intrinsic properties of their own. Research in this field pioneered in part by Prof Seidel has grown extensively in the last few years and now has entire conferences dedicated to it, such as the annual International Workshop on Topological Structures in Ferroic Materials (TOPO), for which the first meeting was held in 2015 in Sydney.

Nanoelectronics based on topological structures was published in Nature Materials on 20 February 2019. Prof Seidel acknowledges funding support by the Australian Research Council (ARC) through Discovery Grants and the ARC Centre of Excellence in Future Low Energy Electronics Technologies (FLEET).

Prof Jan Seidel is a Professor at the School of Materials Science and Engineering at UNSW Sydney. Contact [email protected]

FLEET is an ARC-funded research centre bringing together over a hundred Australian and international experts to develop a new generation of ultra-low energy electronics, motivated by the need to reduce the energy consumed by computing.

Nanowires have the potential to revolutionize the technology around us. Measuring just 5-100 nanometers in diameter (a nanometer is a millionth of a millimeter), these tiny, needle-shaped crystalline structures can alter how electricity or light passes through them.

EPFL researchers have found a way to control and standardize the production of nanowires on silicon surfaces. This discovery could make it possible to grow nanowires on electronic platforms, with potential applications including the integration of nanolasers into electronic chips and improved energy conversion in solar panels. Credit: Jamani Caillet / EPFL

They can emit, concentrate and absorb light and could therefore be used to add optical functionalities to electronic chips. They could, for example, make it possible to generate lasers directly on silicon chips and to integrate single-photon emitters for coding purposes. They could even be applied in solar panels to improve how sunlight is converted into electrical energy.

Up until now, it was impossible to reproduce the process of growing nanowires on silicon semiconductors – there was no way to repeatedly produce homogeneous nanowires in specific positions. But researchers from EPFL’s Laboratory of Semiconductor Materials, run by Anna Fontcuberta i Morral, together with colleagues from MIT and the IOFFE Institute, have come up with a way of growing nanowire networks in a highly controlled and fully reproducible manner. The key was to understand what happens at the onset of nanowire growth, which goes against currently accepted theories. Their work has been published in Nature Communications.

“We think that this discovery will make it possible to realistically integrate a series of nanowires on silicon substrates,” says Fontcuberta i Morral. “Up to now, these nanowires had to be grown individually, and the process couldn’t be reproduced.”

Getting the right ratio

The standard process for producing nanowires is to make tiny holes in silicon monoxide and fill them with a nanodrop of liquid gallium. This substance then solidifies when it comes into contact with arsenic. But with this process, the substance tends to harden at the corners of the nanoholes, which means that the angle at which the nanowires will grow can’t be predicted. The search was on for a way to produce homogeneous nanowires and control their position.

Research aimed at controlling the production process has tended to focus on the diameter of the hole, but this approach has not paid off. Now EPFL researchers have shown that by altering the diameter-to-height ratio of the hole, they can perfectly control how the nanowires grow. At the right ratio, the substance will solidify in a ring around the edge of the hole, which prevents the nanowires from growing at a non-perpendicular angle. And the researchers’ process should work for all types of nanowires.

“It’s kind of like growing a plant. They need water and sunlight, but you have to get the quantities right,” says Fontcuberta i Morral.

This new production technique will be a boon for nanowire research, and further samples should soon be developed.

A team of Cambridge researchers have found a way to control the sea of nuclei in semiconductor quantum dots so they can operate as a quantum memory device.

Quantum dots are crystals made up of thousands of atoms, and each of these atoms interacts magnetically with the trapped electron. If left alone to its own devices, this interaction of the electron with the nuclear spins, limits the usefulness of the electron as a quantum bit – a qubit.

Led by Professor Mete Atatüre, a Fellow at St John’s College, University of Cambridge, the research group, located at the Cavendish Laboratory, exploit the laws of quantum physics and optics to investigate computing, sensing or communication applications.

Atatüre said: “Quantum dots offer an ideal interface, as mediated by light, to a system where the dynamics of individual interacting spins could be controlled and exploited. Because the nuclei randomly ‘steal’ information from the electron they have traditionally been an annoyance, but we have shown we can harness them as a resource.”

The Cambridge team found a way to exploit the interaction between the electron and the thousands of nuclei using lasers to ‘cool’ the nuclei to less than 1 milliKelvin, or a thousandth of a degree above the absolute zero temperature. They then showed they can control and manipulate the thousands of nuclei as if they form a single body in unison, like a second qubit. This proves the nuclei in the quantum dot can exchange information with the electron qubit and can be used to store quantum information as a memory device. The findings have been published in Science today.

Quantum computing aims to harness fundamental concepts of quantum physics, such as entanglement and superposition principle, to outperform current approaches to computing and could revolutionise technology, business and research. Just like classical computers, quantum computers need a processor, memory, and a bus to transport the information backwards and forwards. The processor is a qubit which can be an electron trapped in a quantum dot, the bus is a single photon that these quantum dots generate and are ideal for exchanging information. But the missing link for quantum dots is quantum memory.

Atatüre said: “Instead of talking to individual nuclear spins, we worked on accessing collective spin waves by lasers. This is like a stadium where you don’t need to worry about who raises their hands in the Mexican wave going round, as long as there is one collective wave because they all dance in unison.

“We then went on to show that these spin waves have quantum coherence. This was the missing piece of the jigsaw and we now have everything needed to build a dedicated quantum memory for every qubit.”

In quantum technologies, the photon, the qubit and the memory need to interact with each other in a controlled way. This is mostly realised by interfacing different physical systems to form a single hybrid unit which can be inefficient. The researchers have been able to show that in quantum dots, the memory element is automatically there with every single qubit.

Dr Dorian Gangloff, one of the first authors of the paper and a Fellow at St John’s, said the discovery will renew interest in these types of semiconductor quantum dots. Dr Gangloff explained: “This is a Holy Grail breakthrough for quantum dot research – both for quantum memory and fundamental research; we now have the tools to study dynamics of complex systems in the spirit of quantum simulation.”

The long term opportunities of this work could be seen in the field of quantum computing. Last month, IBM launched the world’s first commercial quantum computer, and the Chief Executive of Microsoft has said quantum computing has the potential to ‘radically reshape the world’.

Gangloff said: “The impact of the qubit could be half a century away but the power of disruptive technology is that it is hard to conceive of the problems we might open up – you can try to think of it as known unknowns but at some point you get into new territory. We don’t yet know the kind of problems it will help to solve which is very exciting.”

Switching magnetic domains in magnetic memories requires normally magnetic fields which are generated by electrical currents, hence requiring large amounts of electrical power. Now, teams from France, Spain and Germany have demonstrated the feasibility of another approach at the nanoscale: “We can induce magnetic order on a small region of our sample by employing a small electric field instead of using magnetic fields”, Dr. Sergio Valencia, HZB, points out.

The cones represents the magnetization of the nanoparticles. In the absence of electric field (strain-free state) the size and separation between particles leads to a random orientation of their magnetization, known as superparamagnetism. Credit: HZB

The samples consist of a wedge-shaped polycrystalline iron thin film deposited on top of a BaTiO3 substrate. BaTiO3 is a well-known ferroelectric and ferroelastic material: An electric field is able to distort the BaTiO3 lattice and induce mechanical strain. Analysis by electron microscopy revealed that the iron film consists of tiny nanograins (diameter 2,5 nm). At its thin end, the iron film is less than 0,5 nm thick, allowing for “low dimensionality” of the nanograins. Given their small size, the magnetic moments of the iron nanograins are disordered with respect to each other, this state is known as superparamagnetism.

At the X-PEEM-Beamline at BESSY II, the scientists analysed what happens with the magnetic order of this nanograins under a small electric field. “With X-PEEM we can map the magnetic order of the iron grains on a microscopic level and observe how their orientation changes while in-situ applying an electric field”, Dr. Ashima Arora explains, who did most of the experiments during her PhD Thesis. Their results show: the electrical field induced a strain on BaTiO3, this strain was transmitted to the iron nanograins on top of it and formerly superparamagnetic regions of the sample switched to a new state. In this new state the magnetic moments of the iron grains are all aligned along the same direction, i.e. a collective long-range ferromagnetic order known as superferromagnetism.

The experiments were performed at a temperature slightly above room temperature. “This lets us hope that the phenomenon can be used for the design of new composite materials (consisting of ferroelectric and magnetic nanoparticles) for low-power spin-based storage and logic architectures operating at ambient conditions”, Valencia says.

Controlling nanoscale magnetic bits in magnetic random access memory devices by electric field induced strain alone, is known also as straintronics. It could offer a new, scalable, fast and energy efficient alternative to nowadays magnetic memories.