In the quest for smaller, faster computer chips, researchers are increasingly turning to quantum mechanics — the exotic physics of the small.

The problem: the manufacturing techniques required to make quantum devices have been equally exotic.

That is, until now.

Researchers at Ohio State University have discovered a way to make quantum devices using technology common to the chip-making industry today.

This work might one day enable faster, low-power computer chips. It could also lead to high-resolution cameras for security and public safety, and cameras that provide clear vision through bad weather.

Paul Berger, professor of electrical and computer engineering and professor of physics at Ohio State University, and his colleagues report their findings in an upcoming issue of IEEE Electron Device Letters.

The team fabricated a device called a tunneling diode using the most common chip-making technique, called chemical vapor deposition.

“We wanted to do this using only the tools found in the typical chip-makers toolbox,” Berger said. “Here we have a technique that manufacturers could potentially use to fabricate quantum devices directly on a silicon chip, side-by-side with their regular circuits and switches.”

The quantum device in question is a resonant interband tunneling diode (RITD) — a device that enables large amounts of current to be regulated through a circuit, but at very low voltages. That means that such devices run on very little power.

RITDs have been difficult to manufacture because they contain dopants — chemical elements — that don’t easily fit within a silicon crystal.

Atoms of the RITD dopants antimony or phosphorus, for example, are large compared to atoms of silicon. Because they don’t fit into the natural openings inside a silicon crystal, the dopants tend to collect on the surface of a chip.

“It’s like when you’re playing Tetris and you have a big block raining down, and only a small square to fit it in. The block has to sit on top,” Berger said. “When you’re building up layers of silicon, these dopants don’t readily fit in. Eventually, they clump together on top of the chip.”

In the past, researchers have tried adding the dopants while growing the silicon wafer one crystal layer at a time — using a slow and expensive process called molecular beam epitaxy, a method which is challenging for high-volume manufacturing. That process also creates too many defects within the silicon.

Berger discovered that RITD dopants could be added during chemical vapor deposition, in which a gas carries the chemical elements to the surface of a wafer many layers at a time. The key was determining the right reactor conditions to deliver the dopants to the silicon, he found.

“One key is hydrogen,” he said. “It binds to the silicon surface and keeps the dopants from clumping. So you don’t have to grow chips at 320 degrees Celsius [approximately 600 degrees Fahrenheit] like you do when using molecular beam epitaxy. You can actually grow them at a higher temperature like 600 degrees Celsius [more than 1100 degrees Fahrenheit] at a lower cost, and with fewer crystal defects.”

Tunneling diodes are so named because they exploit a quantum mechanical effect known as tunneling, which lets electrons pass through thin barriers unhindered.

In theory, interband tunneling diodes could form very dense, very efficient micro-circuits in computer chips. A large amount of data could be stored in a small area on a chip with very little energy required.

Researchers judge the usefulness of tunneling diodes by the abrupt change in the current densities they carry, a characteristic known as “peak-to-valley ratio.” Different ratios are appropriate for different kinds of devices. Logic circuits such as those on a computer chip are best suited by a ratio of about 2.

The RITDs that Berger’s team fabricated had a ratio of 1.85.

“We’re close, and I’m sure we can do better,” he said.

He envisions his RITDs being used for ultra-low-power computer chips operating with small voltages and producing less wasted heat.

“Chip makers today are having a great difficulty boosting performance in each generation, so they pack chips with more and more circuitry, and end up generating a lot of heat,” Berger said. “That’s why a laptop computer is often too hot to actually sit atop your lap. Soon, their heat output will rival that of a nuclear reactor per unit volume.”

“That’s why moving to quantum devices will be a game-changer.”

RITDs could form high-resolution detectors for imaging devices called focal plane arrays. These arrays operate at wavelengths beyond the human eye and can permit detection of concealed weapons and improvised explosive devices. They can also provide vision through rain, snow, fog, and even mild dust storms, for improved airplane and automobile safety, Berger said. Medical imaging of cancerous tumors is another potential application.

His coauthors on the paper included Si-Young Park, and R. Anisha, both doctoral students in electrical engineering at Ohio State; and Roger Loo, Ngoc Duy Nguyen, Shotaro Takeuchi, and Matty Caymax, all of IMEC, an industrial research center in Belgium.

This work was partially supported by the National Science Foundation.

Recently, at Arizona State University’s Biodesign Institute, N.J. Tao and collaborators have found a way to make a key electronic component on a phenomenally tiny scale. Their single-molecule diode is described in this week’s online edition of Nature Chemistry.

This is a schematic for molecular diode. The symmetric molecule (top) allows for two-way current. The asymmetrical molecule (bottom) permits current in one direction only and acts as a single-molecule diode. (Credit: Biodesign Institute at Arizona State University)

In the electronics world, diodes are a versatile and ubiquitous component. Appearing in many shapes and sizes, they are used in an endless array of devices and are essential ingredients for the semiconductor industry. Making components including diodes smaller, cheaper, faster and more efficient has been the holy grail of an exploding electronics field, now probing the nanoscale realm.

Smaller size means cheaper cost and better performance for electronic devices. The first generation computer CPU used a few thousand transistors, Tao says noting the steep advance of silicon technology. “Now even simple, cheap computers use millions of transistors on a single chip.”

But lately, the task of miniaturization has gotten much harder, and the famous dictum known as Moore’s law—which states that the number of silicon-based transistors on a chip doubles every 18-24 months—will eventually reach its physical limits. “Transistor size is reaching a few tens of nanometers, only about 20 times larger than a molecule,” Tao says. “That’s one of the reasons people are excited about this idea of molecular electronics.”

Diodes are critical components for a broad array of applications, from power conversion equipment, to radios, logic gates, photodetectors and light-emitting devices. In each case, diodes are components that allow current to flow in one direction around an electrical circuit but not the other. For a molecule to perform this feat, Tao explains, it must be physically asymmetric, with one end capable of forming a covalent bond with the negatively charged anode and the other with the positive cathode terminal.

The new study compares a symmetric molecule with an asymmetric one, detailing the performance of each in terms of electron transport. “If you have a symmetric molecule, the current goes both ways, much like an ordinary resistor,” Tao observes. This is potentially useful, but the diode is a more important (and difficult) component to replicate (Fig 1).

The idea of surpassing silicon limits with a molecule-based electronic component has been around awhile. “Theoretical chemists Mark Ratner and Ari Aviram proposed the use of molecules for electronics like diodes back in 1974,” Tao says, adding “people around world have been trying to accomplish this for over 30 years.”

Most efforts to date have involved many molecules, Tao notes, referring to molecular thin films. Only very recently have serious attempts been made to surmount the obstacles to single-molecule designs. One of the challenges is to bridge a single molecule to at least two electrodes supplying current to it. Another challenge involves the proper orientation of the molecule in the device. “We are now able to do this—to build a single molecule device with a well defined orientation,” Tao says.

The technique developed by Tao’s group relies on a property known as AC modulation. “Basically, we apply a little periodically varying mechanical perturbation to the molecule. If there’s a molecule bridged across two electrodes, it responds in one way. If there’s no molecule, we can tell.”

The interdisciplinary project involved Professor Luping Yu, at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Professor Ivan Oleynik from the University of South Florida. The team used conjugated molecules, in which atoms are stuck together with alternating single and multiple bonds. Such molecules display large electrical conductivity and have asymmetrical ends capable of spontaneously forming covalent bonds with metal electrodes to create a closed circuit.

The project’s results raise the prospect of building single molecule diodes – the smallest devices one can ever build. “I think it’s exciting because we are able to look at a single molecule and play with it, ” Tao says. “We can apply a voltage, a mechanical force, or optical field, measure current and see the response. As quantum physics controls the behaviors of single molecules, this capability allows us to study properties distinct from those of conventional devices.”

Chemists, physicists, materials researchers, computational experts and engineers all play a central role in the emerging field of nanoelectronics, where a zoo of available molecules with different functions provide the raw material for innovation. Tao is also examining the mechanical properties of molecules, for example, their ability to oscillate. Binding properties between molecules make them attractive candidates for a new generation of chemical sensors. “Personally, I am interested in molecular electronics not because of their potential to duplicate today’s silicon applications, ” Tao says. Instead, molecular electronics will benefit from unique electronic, mechanical, optical and molecular binding properties that set them apart from conventional semiconductors. This may lead to applications complementing rather than replacing silicon devices.

]*?)href\s*=\s*['"](.*?)['"]([^>]*)>(.*?) Ending uga_filter:

Recently, at Arizona State University’s Biodesign Institute, N.J. Tao and collaborators have found a way to make a key electronic component on a phenomenally tiny scale. Their single-molecule diode is described in this week’s online edition of Nature Chemistry.

This is a schematic for molecular diode. The symmetric molecule (top) allows for two-way current. The asymmetrical molecule (bottom) permits current in one direction only and acts as a single-molecule diode. (Credit: Biodesign Institute at Arizona State University)

In the electronics world, diodes are a versatile and ubiquitous component. Appearing in many shapes and sizes, they are used in an endless array of devices and are essential ingredients for the semiconductor industry. Making components including diodes smaller, cheaper, faster and more efficient has been the holy grail of an exploding electronics field, now probing the nanoscale realm.

Smaller size means cheaper cost and better performance for electronic devices. The first generation computer CPU used a few thousand transistors, Tao says noting the steep advance of silicon technology. “Now even simple, cheap computers use millions of transistors on a single chip.”

But lately, the task of miniaturization has gotten much harder, and the famous dictum known as Moore’s law—which states that the number of silicon-based transistors on a chip doubles every 18-24 months—will eventually reach its physical limits. “Transistor size is reaching a few tens of nanometers, only about 20 times larger than a molecule,” Tao says. “That’s one of the reasons people are excited about this idea of molecular electronics.”

Diodes are critical components for a broad array of applications, from power conversion equipment, to radios, logic gates, photodetectors and light-emitting devices. In each case, diodes are components that allow current to flow in one direction around an electrical circuit but not the other. For a molecule to perform this feat, Tao explains, it must be physically asymmetric, with one end capable of forming a covalent bond with the negatively charged anode and the other with the positive cathode terminal.

The new study compares a symmetric molecule with an asymmetric one, detailing the performance of each in terms of electron transport. “If you have a symmetric molecule, the current goes both ways, much like an ordinary resistor,” Tao observes. This is potentially useful, but the diode is a more important (and difficult) component to replicate (Fig 1).

The idea of surpassing silicon limits with a molecule-based electronic component has been around awhile. “Theoretical chemists Mark Ratner and Ari Aviram proposed the use of molecules for electronics like diodes back in 1974,” Tao says, adding “people around world have been trying to accomplish this for over 30 years.”

Most efforts to date have involved many molecules, Tao notes, referring to molecular thin films. Only very recently have serious attempts been made to surmount the obstacles to single-molecule designs. One of the challenges is to bridge a single molecule to at least two electrodes supplying current to it. Another challenge involves the proper orientation of the molecule in the device. “We are now able to do this—to build a single molecule device with a well defined orientation,” Tao says.

The technique developed by Tao’s group relies on a property known as AC modulation. “Basically, we apply a little periodically varying mechanical perturbation to the molecule. If there’s a molecule bridged across two electrodes, it responds in one way. If there’s no molecule, we can tell.”

The interdisciplinary project involved Professor Luping Yu, at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Professor Ivan Oleynik from the University of South Florida. The team used conjugated molecules, in which atoms are stuck together with alternating single and multiple bonds. Such molecules display large electrical conductivity and have asymmetrical ends capable of spontaneously forming covalent bonds with metal electrodes to create a closed circuit.

The project’s results raise the prospect of building single molecule diodes – the smallest devices one can ever build. “I think it’s exciting because we are able to look at a single molecule and play with it, ” Tao says. “We can apply a voltage, a mechanical force, or optical field, measure current and see the response. As quantum physics controls the behaviors of single molecules, this capability allows us to study properties distinct from those of conventional devices.”

Chemists, physicists, materials researchers, computational experts and engineers all play a central role in the emerging field of nanoelectronics, where a zoo of available molecules with different functions provide the raw material for innovation. Tao is also examining the mechanical properties of molecules, for example, their ability to oscillate. Binding properties between molecules make them attractive candidates for a new generation of chemical sensors. “Personally, I am interested in molecular electronics not because of their potential to duplicate today’s silicon applications, ” Tao says. Instead, molecular electronics will benefit from unique electronic, mechanical, optical and molecular binding properties that set them apart from conventional semiconductors. This may lead to applications complementing rather than replacing silicon devices.

Start uga_filter:

Rutgers researchers have discovered novel electronic properties in two-dimensional sheets of carbon atoms called graphene that could one day be the heart of speedy and powerful electronic devices.

Graphene sample with electrodes, fabricated using electron beam lithography

The new findings, previously considered possible by physicists but only now being seen in the laboratory, show that electrons in graphene can interact strongly with each other. The behavior is similar to superconductivity observed in some metals and complex materials, marked by the flow of electric current with no resistance and other unusual but potentially useful properties. In graphene, this behavior results in a new liquid-like phase of matter consisting of fractionally charged quasi-particles, in which charge is transported with no dissipation.

In a paper issued online by the journal Nature and slated for print publication in the coming weeks, physics professor Eva Andrei and her Rutgers colleagues note that the strong interaction between electrons, also called correlated behavior, had not been observed in graphene in spite of many attempts to coax it out. This led some scientists to question whether correlated behavior could even be possible in graphene, where the electrons are massless (ultra-relativistic) particles like photons and neutrinos. In most materials, electrons are particles that have mass.

“Our work demonstrated that earlier failures to observe correlated behavior were not due to the physical nature of graphene,” said Eva Andrei, physics professor in the Rutgers School of Arts and Sciences. “Rather, it was because of interference from the material which supported graphene samples and the type of electrical probes used to study it.”

This finding should encourage scientists to further pursue graphene and related materials for future electronic applications, including replacements for today’s silicon-based semiconductor materials. Industry experts expect silicon technology to reach fundamental performance limits in a little more than a decade.

The Rutgers physicists further describe how they observed the collective behavior of the ultra-relativistic charge carriers in graphene through a phenomenon known as the fractional quantum Hall effect (FQHE). The FQHE is seen when charge carriers are confined to moving in a two-dimensional plane and are subject to a perpendicular magnetic field. When interactions between these charge carriers are sufficiently strong they form new quasi-particles with a fraction of an electron’s elementary charge. The FHQE is the quintessential signature of strongly correlated behavior among charge-carrying particles in two dimensions.

The FHQE is known to exist in semiconductor-based, two-dimensional electron systems, where the electrons are massive particles that obey conventional dynamics versus the relativistic dynamics of massless particles. However, it was not obvious until now that ultra-relativistic electrons in graphene would be capable of exhibiting collective phenomena that give rise to the FHQE. The Rutgers physicists were surprised that the FHQE in graphene is even more robust than in standard semiconductors.

Scientists make graphene patches by rubbing graphite – the same material in ordinary pencil lead – onto a silicon wafer, which is a thin slice of silicon crystal used to make computer chips. Then they run electrical pathways to the graphene patches using ordinary integrated circuit fabrication techniques. While scientists were able to investigate many properties of the resulting graphene electronic device, they were not able to induce the sought-after fractional quantum Hall effect.

Andrei and her group proposed that impurities or irregularities in the thin layer of silicon dioxide underlying the graphene were preventing the scientists from achieving the exacting conditions they needed. Postdoctoral fellow Xu Du and undergraduate student Anthony Barker were able to show that etching out several layers of silicon dioxide below the graphene patches essentially leaves an intact graphene strip suspended in mid-air by the electrodes. This enabled the group to demonstrate that the carriers in suspended graphene essentially propagate ballistically without scattering from impurities. Another crucial step was to design and fabricate a probe geometry that did not interfere with measurements as Andrei suspected earlier ones were doing. These proved decisive steps to observing the correlated behavior in graphene.

In the past few months, other academic and corporate research groups have reported streamlined graphene production techniques, which will propel further research and potential applications.

Andrei’s collaborators were Xu Du, now on faculty at Stony Brook University; Ivan Skachko, a post-doctoral fellow; Fabian Duerr, a master’s student; and Adina Luican, a doctoral student. The research was supported by the Department of Energy, the National Science Foundation, the Institute for Complex Adaptive Matter and Alcatel-Lucent.

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Rutgers researchers have discovered novel electronic properties in two-dimensional sheets of carbon atoms called graphene that could one day be the heart of speedy and powerful electronic devices.

Graphene sample with electrodes, fabricated using electron beam lithography

The new findings, previously considered possible by physicists but only now being seen in the laboratory, show that electrons in graphene can interact strongly with each other. The behavior is similar to superconductivity observed in some metals and complex materials, marked by the flow of electric current with no resistance and other unusual but potentially useful properties. In graphene, this behavior results in a new liquid-like phase of matter consisting of fractionally charged quasi-particles, in which charge is transported with no dissipation.

In a paper issued online by the journal Nature and slated for print publication in the coming weeks, physics professor Eva Andrei and her Rutgers colleagues note that the strong interaction between electrons, also called correlated behavior, had not been observed in graphene in spite of many attempts to coax it out. This led some scientists to question whether correlated behavior could even be possible in graphene, where the electrons are massless (ultra-relativistic) particles like photons and neutrinos. In most materials, electrons are particles that have mass.

“Our work demonstrated that earlier failures to observe correlated behavior were not due to the physical nature of graphene,” said Eva Andrei, physics professor in the Rutgers School of Arts and Sciences. “Rather, it was because of interference from the material which supported graphene samples and the type of electrical probes used to study it.”

This finding should encourage scientists to further pursue graphene and related materials for future electronic applications, including replacements for today’s silicon-based semiconductor materials. Industry experts expect silicon technology to reach fundamental performance limits in a little more than a decade.

The Rutgers physicists further describe how they observed the collective behavior of the ultra-relativistic charge carriers in graphene through a phenomenon known as the fractional quantum Hall effect (FQHE). The FQHE is seen when charge carriers are confined to moving in a two-dimensional plane and are subject to a perpendicular magnetic field. When interactions between these charge carriers are sufficiently strong they form new quasi-particles with a fraction of an electron’s elementary charge. The FHQE is the quintessential signature of strongly correlated behavior among charge-carrying particles in two dimensions.

The FHQE is known to exist in semiconductor-based, two-dimensional electron systems, where the electrons are massive particles that obey conventional dynamics versus the relativistic dynamics of massless particles. However, it was not obvious until now that ultra-relativistic electrons in graphene would be capable of exhibiting collective phenomena that give rise to the FHQE. The Rutgers physicists were surprised that the FHQE in graphene is even more robust than in standard semiconductors.

Scientists make graphene patches by rubbing graphite – the same material in ordinary pencil lead – onto a silicon wafer, which is a thin slice of silicon crystal used to make computer chips. Then they run electrical pathways to the graphene patches using ordinary integrated circuit fabrication techniques. While scientists were able to investigate many properties of the resulting graphene electronic device, they were not able to induce the sought-after fractional quantum Hall effect.

Andrei and her group proposed that impurities or irregularities in the thin layer of silicon dioxide underlying the graphene were preventing the scientists from achieving the exacting conditions they needed. Postdoctoral fellow Xu Du and undergraduate student Anthony Barker were able to show that etching out several layers of silicon dioxide below the graphene patches essentially leaves an intact graphene strip suspended in mid-air by the electrodes. This enabled the group to demonstrate that the carriers in suspended graphene essentially propagate ballistically without scattering from impurities. Another crucial step was to design and fabricate a probe geometry that did not interfere with measurements as Andrei suspected earlier ones were doing. These proved decisive steps to observing the correlated behavior in graphene.

In the past few months, other academic and corporate research groups have reported streamlined graphene production techniques, which will propel further research and potential applications.

Andrei’s collaborators were Xu Du, now on faculty at Stony Brook University; Ivan Skachko, a post-doctoral fellow; Fabian Duerr, a master’s student; and Adina Luican, a doctoral student. The research was supported by the Department of Energy, the National Science Foundation, the Institute for Complex Adaptive Matter and Alcatel-Lucent.

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