Thermal waves observed in a semiconductor material

European researchers have reported the unexpected observation of thermal waves in the semiconductor material germanium — a phenomenon that may allow a significant improvement in the performance of our electronic devices in the near future. Their study has been published in the journal Science Advances.

Heat originates from the vibration of atoms, and transfers by diffusion at ambient temperatures. Unfortunately, it is rather difficult to control, and leads to simple and inefficient strategies to manipulate it. This is why, for example, large amounts of residual heat can accumulate in our computers, mobile phones and other electronic devices. However, if heat was transported through waves, such as light, it would offer new alternatives to control it.

Thermal waves have been observed to date in only a few materials, such as solid helium or and graphite. Now, researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC), the Universitat Autònoma de Barcelona (UAB) and the University of Cagliari have reportedly observed thermal waves in solid germanium — a semiconductor material used typically in electronics, similar to silicon — at room temperature.

The observation occurred when studying the thermal response of a germanium sample under the effect of lasers, producing a high-frequency oscillating heating wave on its surface. The experiments showed that, contrarily to what was believed until now, heat did not dissipate by diffusion, but it propagated into the material through thermal waves.

“It was not expected to encounter these wave-like effects, known as second sound, on this type of material, and in these conditions,” said study leader Sebastián Reparaz, a researcher in ICMAB’s Nanostructured Materials for Optoelectronics and Energy Harvesting (NANOPTO) group.

First observed in the 1960s on solid helium, thermal transport through waves, known as second sound, has been a recurrent subject for researchers who have repeatedly tried to demonstrate its existence in other materials. Reparaz explained, “Second sound is the thermal regime where heat can propagate in the form of thermal waves, instead of the frequently observed diffusive regime. This type of wave-like thermal transport has many of the advantages offered by waves, including interference and diffraction.

“Wave-like effects can be unlocked by driving the system in a rapidly varying temperature field,” Reparaz continued. “In other words, a rapidly varying temperature field forces the propagation of heat in the wave-like regime.

“The interesting conclusion of our work is that these wave-like effects could be potentially observed by most materials at a sufficiently large modulation frequency of the temperature field. And, what is even more interesting, its observation is not restricted to some specific materials.”

According to Reparaz, “The possible applications of second sound are limitless”; achieving these applications, however, will require a deep understanding on the ways to unlock this thermal propagation regime on any given material. Being able to control heat propagation through the properties of waves opens new ways to design the upcoming generations of thermal devices, in a similar way to the already established developments for light.

“Specifically, the second sound thermal regime could be used to rethink how we deal with waste heat,” Reparaz said.

UAB researcher F Xavier Álvarez said the team’s findings help to unify the current theoretical model, “which until now considered that materials where this type of wave-like behaviour was observed (such as graphite) were very different from the semiconductor materials currently used in the manufacture of electronic chips (such as silicon and germanium)”.

“Now all these materials can be described using the same equations,” he said. “This observation establishes a new theoretical framework that may allow in the not-too-distant future a significant improvement in the performance of our electronic devices.”

Image caption: Amplified frequency-domain thermoreflectance set-up used to study the existence of second sound in germanium. Two different lasers are focused onto the surface of the samples using a microscope objective. A rather large combination of optical elements helps to control and modify the spot size and shape, as well as the power and harmonic modulation of the lasers. Cold nitrogen gas is used for better visualisation of the lasers’ optical path. Image credit: ICMAB-CSIC.

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