Ultimate confinement of phonon propagation in silicon nano-crystalline structure

Abstract

Temperature-dependent thermal conductivity of epitaxial silicon nano-crystalline (SiNC) structures composed of nanometer-sized grains separated by ultra-thin silicon-oxide (SiO2) films is measured by the time domain thermoreflectance technique in the range from 50 to 300 K. Thermal conductivity of SiNC structures with grain size of 3 nm and 5 nm is anomalously low at the entire temperature range, significantly below the values of bulk amorphous Si and SiO2. Phonon gas kinetics model, with intrinsic transport properties obtained by first-principles-based anharmonic lattice dynamics and phonon transmittance across ultra-thin SiO2 films obtained by atomistic Green's function, reproduces the measured thermal conductivity without any fitting parameters. The analysis reveals that mean free paths of acoustic phonons in the SiNC structures are equivalent or even below half the phonon wavelength, i.e. the minimum thermal conductivity scenario. The result demonstrates that the nanostructures with extremely small length scales and controlled interface can give rise to ultimate classical confinement of thermal phonon propagation.

arXiv comment15 pages, 4 figures, in press in Phys. Rev. Lett. (2018)

References in corpus (4)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Heat transport in silicon from first principles calculations
• Granular Electronic Systems
• Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium thermoelectrics

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