Managing heat flows in semiconductor materials is a significant challenge when creating even smaller and faster computer chips, efficient solar panels and better lasers and biomedical devices. By optimising thermal management in microchips, they can be packed more densely.
For the first time, an international research group, that includes Aalto University researchers, has succeeded in controlling the energy spectrum of acoustic phonons by scaling the dimensions of the semiconductor structure down to the nanometre level. Acoustic phonons, i.e. lattice vibration quanta, are quasi-particles that participate in the transfer of heat in materials. These results will have significant effects on the heat management of electronic components.
The group utilised GaAs nanowires made in Finland, and Brillouin-Mandelstam scattering to study how phonons move in crystalline nanostructures.
‘We can precisely control the nanowire's dimensions at the nanoscale by combining electron-beam lithography and epitaxy. The nanowires utilised in the research were around 80 nm in diameter. Precise control of the dimensions enabled modifying the energy spectrum of phonons,’ notes Joona-Pekko Kakko, who is writing his doctoral dissertation at the Department of Micro- and Nanosciences.
Managing the dispersion of phonons is essential when improving the heat dissipation of components at the nanoscale, which has formed the greatest obstacle for shrinking components further. Dispersion managing can also be utilised when improving the efficiency of thermoelectric generation. The reduction of thermal conductivity with phonons will help thermoelectric devices that produce electricity by utilising the temperature differences in semiconductors.
‘Over the years, the only conceivable method for altering the thermal conductivity of nanostructures was to tailor the interfaces of nanostructures, which then leads to the scattering of acoustic phonons. Our experiments show that the confining acoustic phonons in nanowires alters their velocity, which then changes their interaction with electrons, among others, and their ability to conduct heat. Our results creates new possibilities for optimising the heat and electric conductivity properties of semiconductor materials,’ says Professor Alexander Balandin, who lead the research.
The research was conducted in cooperation between Professor Alexander Balandin from the University of California, Riverside and Aalto University Professor Harri Lipsanen, and the results were published in an article in Nature Communications on Thursday, 10 November. The title of the article is “Direct observation of confined acoustic phonon polarization branches in free-standing nanowires”.
Link to the article www.nature.com/articles/ncomms13400
The work was supported by project Moppi in Aalto University’s Energy Efficiency Programme.
Doctoral candidate Joona-Pekko Kakko
Professor Harri Lipsanen
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