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Wednesday, 27 June 2012

Phonons in slow motion

Team led by ICN Group Leader Prof Dr Clivia Sotomayor-Torres reports changes in phonon propagation in ultra-thin (sub-10 nm) silicon membranes in Nano Letters.

Their findings have major implications for understanding the physics behind heat dissipation in nanoelectronics and for developing novel nanoscale thermoelectric materials.

In unprecedented experiments on phonon transport in silicon, an international team of researchers led by ICREA Research Professor and UAB Professor Dr Clivia M Sotomayor Torres, who heads ICN's Phononics and Photonics Nanostructures (P2N) Group, has investigated phonon propagation in ultra-thin silicon membranes (less than 10 nm thick). Their results have just been published in Nano Letters, in an article entitled "Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes".

Using a technique known as angle-resolved Brillouin scattering spectroscopy, the researchers were able to clock the speed at which confined phonons propagate through free-standing silicon membranes from 400 nm down to 7.8 nm thick. In this non-destructive technique, laser light of a known frequency is applied to the membrane surface; this light is then scattered from the natural vibrations or phonons moving through the sample, with a minute change in frequency. By measuring this tiny change in frequency, and associating it with the correct direction of scattering, the velocity of the phonons can be determined.

Testing thinner and thinner membranes, the team found that the fundamental flexural phonons moved more than ten times more slowly in the thinnest sample (7.8 nm thick) compared to bulk silicon (> 1 mm thick), which they attribute chiefly to greater and greater interference between phonon waves at the top and bottom surfaces of the membrane. The fact that phonon velocities are size-dependent means that different-sized samples of the same material will have different values for parameters such as specific heat capacity, thermal conductivity andelectrical conductivity.

The team, which included researchers from University College Cork (Ireland), Universitat Autonoma de Barcelona (Spain), VTT Technical Research Centre (Finland), IEMN (France), and Université Mohamed I (Morocco), believe that their membranes may have potential for applications of phonon engineering, such as phonon storage. However, they underline the needfor future research to fully understand the behaviour of phonons in ultra-thin (< 20 nm) films, and the consequences of phonon confinement.

Phonons are the modes of vibrational energy that propagate through all material at a finite temperature. Their quantum analogues in light and electricity are photons and electrons, respectively. Phononics is a fledgling field in which many of the approaches used to study photons and electrons are now being adapted to phonons. Understanding and controlling phonons has powerful implications for fields like nanoelectronics, where heat dissipation is a major factor in device performance and energy consumption. In the case of silicon, this is especially relevant to computer chip engineering, as chip features quickly approach the 20 nm mark.

The article "Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes" can be accessed here.