New silicon nanowires can really withstand heat

Silicon-28 nanowire transmission electron microscope image with a layer of silicon dioxide on the surface. Credits: Matthew R. Jones and Muhua Sun / Rice University

Scientists have shown a new material that transfers heat 150% more efficiently than conventional materials used in advanced chip technologies.

The device — an ultra-thin silicon nanowire — could enable smaller, faster microelectronics with heat transfer efficiencies surpassing current technologies. Electronic devices powered by microchips that efficiently dissipate heat will in turn consume less energy – an improvement that could mitigate the energy consumption produced by burning carbon-rich fossil fuels that have contributed to global warming.

“By overcoming the natural limitations of silicon in its ability to transfer heat, our discovery faces an obstacle to mechanical microchipping,” said Junqiao Wu, the scientist who led the Physical Review Letters study reporting the new device. Wu is a School Scientist in the Department of Materials Science and Professor of Materials Science and Engineering at UC Berkeley.

The slow flow of heat through silicon

Our electronics are relatively affordable because silicon – the material of choice for computer chips – is cheap and plentiful. But while silicon is a good conductor of electricity, it is not a good conductor of heat when reduced to very small sizes — and when it comes to fast computing, this is a big problem for tiny microchips.

Inside each microchip are tens of billions of silicon transistors that direct the flow of electrons in and out of memory cells, encoding data pieces as units and zeros, the binary language of computers. Electric currents run between these hard-working transistors and these currents inevitably generate heat.

Heat flows naturally from a hot object to a cold object. But heat flow becomes difficult in silicon.

In its physical form, silicon is made up of three different isotopes — forms of a chemical element that contain an equal number of protons but different numbers of neutrons (hence different masses) in their nuclei.

About 92% of silicon is made up of the isotope silicon-28, which has 14 protons and 14 neutrons. About 5% is silicon-29, which weighs 14 protons and 15 neutrons. and just 3% is silicon-30, a relatively heavy one with 14 protons and 16 neutrons, explained co-author Joel Ager, a senior fellow at the Berkeley Lab’s Department of Materials Science and an assistant professor of materials science and engineering at UC Berkeley.

As phonons, the atomic vibration waves that carry heat, travel their way through the crystalline structure of silicon, their direction changes when they collide with silicon-29 or silicon-30, whose different atomic masses “confuse” the phonons, slowing them down.

“Phonons eventually get the idea and find their way to the cold end to cool silicon material,” but this indirect path allows heat to be generated, which in turn also slows down your computer, Ager said. .

A big step towards faster, denser microelectronics

For many decades, researchers believed that pure silicon-28 chips would exceed the thermal conductivity limit of silicon and thus improve the processing speeds of smaller, denser microelectronics.

But refining silicon on a single isotope requires intense energy levels that few facilities can provide – and even fewer specializing in building ready-to-buy isotopes, Ager said.

Fortunately, an international project from the early 2000s allowed Ager and leading semiconductor specialist Eugene Haller to procure silicon tetrafluoride gas – the starting material for isotopically purified silicon – from a former Soviet-era isotope plant.

This led to a number of groundbreaking experiments, including a 2006 study published in Naturewhere Ager and Haller formulated silicon-28 into monocrystals, which they used to demonstrate the storage of quantum memory information as quantum bits or qubits, units of data stored simultaneously as one and a zero at the spin of an electron.

Subsequently, semiconductor thin films and monocrystals made of Ager and Haller silicon isotope materials have been shown to have 10% higher thermal conductivity than natural silicon — an improvement, but from a computer industry point of view, this is probably not enough to justify cost a thousand times more money to build a computer than isotopically pure silicon, Ager said.

But Ager knew that silicon isotope materials were of scientific significance beyond quantum computers. So he kept what was left in a safe place at the Berkeley Lab, in case other scientists needed it, because few people have the resources to make or even buy isotopically pure silicon, he thought.

A path to cooler technology with silicon-28

About three years ago, Wu and his graduate student Penghong Ci were trying to find new ways to improve the heat transfer rate of silicon chips.

One strategy for making more efficient transistors involves using a type of nanowire called the Gate-All-Around Field Effect Transistor. In these devices, silicon nanowires are stacked to carry electricity while generating heat, Wu explained. “And if the heat generated is not extracted quickly, the device would stop working, like a fire alarm ringing in a tall building without an evacuation map,” he said.

But the heat transfer is even worse on silicon nanowires because their rough surfaces – signs of chemical treatment – scatter or “confuse” the phonons even more, he explained.

“And then one day we wondered, ‘What if we were building a nanowire from isotopically pure silicon-28?'” Wu said.

Silicon isotopes are not something that can be easily bought on the open market, and Ager still had some silicon isotope crystals in storage at Berkeley Lab – not much, but enough to share “if one has a good idea of ​​how to use it, “said Agger.” And Junqiao’s new study was one such case. “

An amazing big revelation with nanotests

“We are really fortunate that Joel happened to have the isotopically enriched silicon material ready to be used for the study,” Wu said.

Using Ager silicon isotope materials, Wu’s team tested the thermal conductivity of large 1-millimeter-sized silicon crystals against natural silicon – again, their experiment confirmed what Ager and his colleagues discovered years ago – that Bulk silicon-28 transfers heat only 10% better than natural silicon.

Now for the nano test. Using a technique called electrolytic etching, Ci made natural silicon and silicon-28 nanowires just 90 nanometers (billionths of a meter) in diameter — about a thousand times thinner than a single strand of human hair.

To measure the thermal conductivity, Ci hung each nanowire between two microwave cushions equipped with electrodes and platinum thermometers and then applied electricity to the electrode to generate heat on one cushion flowing to the other cushion via the nanowire.

“We expected to see only an incremental benefit – something like 20% – from using isotopically pure material to transfer heat by nanowires,” Wu said.

But Ci’s measurements surprised everyone. Si-28 nanowires generated heat not 10% or even 20%, but 150% better than natural silicon nanowires with the same diameter and surface roughness.

That defied everything they expected to see, Wu said. The rough surface of a nanowire usually slows down the phonons. So what happened?

High-resolution TEM (transmission electron microscopy) images of the material recorded by Matthew R. Jones and Muhua Sun at Rice University revealed the first indication: a glass-like layer of silica on the surface of silicon-28 nanowire.

Computer simulation experiments at the University of Massachusetts Amherst, led by Zlatan Aksamija, a leading expert in the thermal conductivity of nanowires, found that the absence of isotope “defects” – silicon-29 and silicon-30 – prevented phonon from escaping to the surface. where the silicon layer would dramatically slow down the phonons. This in turn kept the phonons in orbit along the direction of heat flow – and therefore less “tangled” – inside the “core” of silicon-28 nanowire. (Aksamija is currently an Associate Professor of Materials Science and Engineering at the University of Utah.)

“It was really unexpected. To discover that two separate phonon blocking mechanisms – the surface versus isotopes, which were once thought to be independent of each other – now work synergistically in our favor of heat conductivity is very surprising but also very enjoyable,” he said. Wu.

“Junqiao and the team have discovered a new natural phenomenon,” Ager said. “This is a real triumph for curiosity-based science. It’s quite exciting.”

Wu said the team then plans to take its discovery to the next step: exploring how to “control, instead of just measuring, the thermal conductivity of these materials”.

Silicon thermoelectric material achieves a record of low thermal conductivity

More information:
Penghong Ci et al, Giant Isotope Effect of Thermal Conductivity in Silicon Nanowires, Physical Review Letters (2022). DOI: 10.1103 / PhysRevLett.128.085901

Provided by Lawrence Berkeley National Laboratory

Reference: New silicon nanowires can really withstand the heat (2022, May 17) retrieved on May 18, 2022 from

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