ICT2014 Abstracts

We investigate the control of thermal conduction in nanostructured Si, which can be applied to thermoelectrics, with both incoherent and coherent manner. Air-suspended Si nanowires with various widths between 60 and 150 nm and a one-dimensional (1D) phononic crystal nanostructure were fabricated on an SOI wafer by top-down approach using electron beam lithography, and their thermal conductivities were investigated. The thermal conductivity was obtained by measuring time-domain thermoreflectance signal from micro metal pad, which is attached to the nanostructures, and simulation by finite element method. Larger reduction in thermal conductivity, compared with that of a thin membrane (145 nm thick, 70 W/mK), was observed for thinner nanowires (60 nm wide, 47 W/mK) due to more frequent surface scattering of phonons. This result indicates that the phonon transport is ballistic in the investigated systems. The 1D phononic crystal showed smaller thermal conductivity compared with a nanowire with the same width. Theoretical investigation of this further reduction in the thermal conduction in 1D PnC clarified that phonons scattering is mixture of specular reflection and diffusive scattering with nearly the same ratio.

We also calculated the mean free path (MFP) of phonons in bulk Si at room temperature to discuss the possibility of coherent phonon transport by phononic crystal nanostructures. It is found that phonons of MFP = 100 nm to tens of micrometers mostly contribute to heat conduction. This results says a photonic crystal of hundred nanometer scale may affect heat transport by the coherent effect. Phononic band diagrams of various 1D phononic crystals with periods between 100 and 300 nm are calculated. The simulated phononic band gap of the cascaded architecture of these structures covers wide frequency area and ensures the proposed cascaded architecture enables the coherent control of heat conduction at cryogenic temperature.