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Friday, 15 March 2013

Measuring phonon lifetimes in silicon membranes

In Physical Review Letters, a team led by ICREA Prof Dr Clivia M Sotomayor Torres reports findings on the lifetimes of acoustic phonons in ultra-thin silicon membranes.

ICREA Research Professor Dr Clivia M Sotomayor Torres and colleagues from ICN, the Universitat Autononoma de Barcelona, Universitaet Konstanz, in Germany, and the Technical Research Centre of Finland (VTT) have just published an article in Physical Review Letters, in which they describe surprising findings on the behaviour of phonons in silicon nanostructures—namely, as regards the degree to which sample thickness and surface roughness each determine phonon lifetimes (“Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes”).

Although solid objects appear static, their atoms are constantly vibrating. In highly ordered solids such as crystals, the collective movements among the atoms are themselves highly ordered. Thus, when external energy (e.g. light, sound or heat) is applied to such a solid, it generates collective excitations or vibrations known as phonons. An acoustic phonon is a collective movement of all of the atoms in a crystal outside of their equilibrium positions in the same direction; despite their name, acoustic phonons can actually be generated by almost any type of energy (light, sound, heat, etc.).

Understanding the behaviour of phonons is paramount in Nanotechnology. Nanoscale devices such as chips, motors and lasers can be radically compromised by the slightest interference from unwanted acoustic vibrations or heat, whether generated externally (e.g. a poor vibrational control of the surroundings) or generated internally (e.g. a memory chip that begins to overheat due to inhibited heat dissipation). However, by controlling the frequency and intensity of phonons, researchers hope to find ways to make heat transport more efficient and to create new technologies for unprecedented optical, acoustic, electronic, mechanical and thermal applications: for example, selectively exciting a laser crystal with sound waves of different frequencies in order to generate light of different colours.

In this work, the researchers generated acoustic phonons ranging in frequency from about 10 to 500 GHz in free-standing, ultra-thin, crystalline silicon membranes ranging in thickness from about 8 to 200 nm, measured the lifetimes of the phonons and then compared their results to theoretical predictions.

The type of membrane they used is an ideal model system for studying phonons: firstly, they can be fabricated with a high level of homogeneity, ensuring regular performance and consistent measurements; and secondly, unlike other sample types, they do not require the use of any substrate or metal layer, which can interfere with the propagation of phonons in the test material.

To generate and detect the phonons, the team used a technique known as asynchronous optical sampling (ASOPS), in which light of one wavelength is used to illuminate a small area of the sample in order to generate a phonon, and then the reflected light at another wavelength is subsequently recorded over time to indirectly measure the decay of the phonon (this is enabled by phonon-photon coupling).

As they expected based on theoretical predictions, the team observed a positive correlation between sample thickness and phonon lifetime (i.e. the thicker the membrane, the longer the phonon propagated without interruption) and between phonon frequency and lifetime (i.e. the higher its frequency, the longer phonon propagated without interruption): they observed a decay time of 4.7 nanoseconds for the thickest sample (194 nm) and 5 picoseconds for the thinnest sample (7.7 nm)—nearly 1,000 times shorter. However, they observed a surprising trend: at around 100 GHz and below, the phonon lifetimes were orders of magnitude shorter than the predicted values. They rationalised this discrepancy by incorporating the effects of surface scattering on phonon propagation.

“Based on these results, we now feel confident predicting the lifetimes of phonons in the frequency range of around 100 GHz to 1 THz, in silicon membranes thicker than 30 nm,” explains Prof Dr Sotomayor Torres.


To access the article “Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes”, click here.