Thermal waves observed in semiconductor materials

For the first time, and unexpectedly, it has been possible to observe thermal waves in germanium, a semiconductor material. This discovery could allow a significant improvement in the performance of our electronic devices in the not too distant future.

The study that has led to this pioneering observation is led by researchers from the Barcelona Institute of Materials Science (ICMAB), attached to the Spanish Higher Council for Scientific Research (CSIC), in collaboration with researchers from the Autonomous University of Barcelona (UAB). ), and the University of Cagliari in Italy.

Heat, as we know it, originates when atoms vibrate, and is transferred by diffusion, at room temperature. Unfortunately, heat is quite difficult to control, and strategies to manipulate it are quite ineffective. For example, it is common for large amounts of residual heat to accumulate in our computers, mobile phones and, in general, in most electronic devices.

However, if heat were transported through waves, like light, it would offer new alternatives to control it, especially thanks to the unique and intrinsic properties of waves.

Until now, thermal waves have only been observed in a few materials, such as solid helium or, more recently, graphite. Now, the new study presents the observation of thermal waves in solid germanium, a semiconductor material commonly used in electronics, similar to silicon, and at room temperature. “We did not expect to find these wave effects, known as second sound, in this type of material, nor in these conditions,” says Sebastián Reparaz, ICMAB researcher in the Group of Nanostructured Materials for Optoelectronics and Energy Capture (NANOPTO) and co-author of this study.

The discovery came from studying the thermal response of a germanium sample under the effect of a laser, producing a high-frequency oscillating thermal wave on its surface. The experiments showed that, contrary to what was believed until now, heat was not dissipated by diffusion, but rather spread through the material through thermal waves.

Set of devices used to study the existence of the second sound in germanium. Two different lasers are focused on the surface of the samples using a microscope objective. A fairly broad combination of optical elements allows the size and shape of the spot to be controlled and modified, as well as the power and harmonic modulation of lasers. Cold nitrogen gas is used for better visualization of the optical path of the laser beams. (Photo: ICMAB, CSIC)

Apart from the observation itself, the study reveals the approach to be able to observe thermal waves, possibly in any material system.

First observed in the 1960s in solid helium, thermal transport through waves, known as second sound, has been a recurring theme in the scientific community, which has repeatedly tried to prove its existence in other materials. Recent successful demonstrations of this phenomenon in graphite have revitalized his experimental study.

“The second sound is the thermal regime in which heat can propagate in the form of thermal waves, rather than the diffusive regime often observed. This type of wave-shaped thermal transport has many of the advantages that waves offer, such as interference and diffraction “, affirms the ICMAB researcher Sebastián Reparaz.

“These wave effects can be unlocked by introducing the system into a rapidly varying temperature field. In other words, a rapidly varying temperature field forces the propagation of heat in the wave regime”, explains Reparaz, adding: “The interesting The conclusion of our work is that these wave effects could potentially be observed in most materials at a sufficiently large frequency of modulation of the temperature field. Their observation is not limited to some specific materials, which is very interesting. “

“The possible applications of the second sound are unlimited”, affirms Sebastián Reparaz. However, it will be necessary to know in depth the ways to unblock and control this regime of thermal propagation in any material to be able to apply them. Being able to control the propagation of heat through the properties of waves opens up new ways to design the next generations of thermal devices, in a similar way to those already developed and established for light. “In particular, the thermal wave regime could be used to rethink how we treat waste heat,” he adds.

From a theoretical point of view, “these findings make it possible to unify the current theoretical model, which until now considered that the materials in which this type of wave behavior was observed (such as graphite) were very different from the semiconductor materials that are currently used. in the manufacture of electronic chips (such as silicon and germanium) “, affirms F. Xavier Álvarez, a researcher at the UAB. “Now all these materials can be described using the same equations. This observation establishes a new theoretical framework that will allow us, in the not too distant future, a significant improvement in the performance of our electronic devices,” adds Álvarez.

The study is titled “Observation of second sound in a rapidly varying temperature field in Ge”. And it has been published in the academic journal Science Advances. (Source: UAB)

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