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

Ultimate confinement of phonon propagation in silicon nano-crystalline structure