Thermal phonon transport in silicon nanowires and two-dimensional phononic crystal nanostructures

Abstract

Thermal phonon transport in silicon nanowires (Si NWs) and two-dimensional phononic crystal (2D PnC) nanostructures was investigated by measuring thermal conductivity using a micrometer-scale time-domain thermoreflectance. The impact of nanopatterning on thermal conductivity strongly depends on the geometry, specularity parameter, and thermal phonon mean free path (MFP) distribution. Thermal conductivities for 2D PnC nanostructures were found to be much lower than that for NWs with similar characteristic length and surface-to-volume ratio due to stronger phonon back scattering. In single-crystalline Si, PnC patterning has a stronger impact at 4 K than at room temperature due to a higher specularity parameter and a longer thermal phonon MFP. Nanowire patterning has a stronger impact in polycrystalline Si, where thermal phonon MFP distribution is biased longer by grain boundary scattering.

Heat transport in phononic-like membranes: Modeling and comparison with modulated nano-wires

“…This might be due to phononic effects (i.e. any effect related to a modification of phonon dispersion properties like band flattening, band gap ... or coherent scattering of phonon 50 due to the periodicity of the hole pattern), but authors mentioned that the "local angles between local and global heat flux" could be responsible of this phenomenon [1]. In other words, they point out the different impact of scattering surfaces on thermal transport according to their spatial orientation compared to the direction of the main thermal gradient.…”

“…Nomura et al compared the experimental TCs of membranes with periodic 45 cylindrical pores to those of nano-wires [1]. They found that thermal transport is weaker in membranes, despite their low scattering surface to volume ratio compared to nano-wires with similar characteristic size.…”

Crystal structure dependent thermal conductivity in two-dimensional phononic crystal nanostructures

“…The main aim of this study is to investigate how the crystal lattice type, i.e., the difference in the placement of the holes, is reflected in the thermal conductivity. We know that phonon transport occurs in the semi-ballistic regime in the investigated PnC nanostructures from our previous investigations in similar sizes 17 . Therefore, the placement of holes was expected to have some influence on thermal conductivity.…”

“…Many research studies have reported characteristic transport properties and large thermal conductivity reductions by shortening the effective mean free path (MFP) of thermal phonons in a variety of nanostructured semiconductors [1][2][3][4][5][6][7][8] . Singlecrystalline Si provides an ideal phonon transport system due to its long thermal phonon MFP, which is longer than 100 nm 9,10 at room temperature, and which enables a systematic investigation of thermal conductivity control by welldefined nanopatterning, such as phononic crystal (PnC) nanostructures formed by electron beam (EB) lithography [11][12][13][14][15][16][17][18] . To date, thermal conduction in PnCs has been mainly investigated with square lattices, but there have been few experiments for other lattice types so far.…”

Control of Phonon Transport by Phononic Crystals and Application to Thermoelectric Materials

“…In nanostructures, since the size of the system is equivalent to the mean free path (MFP) of the carrier, in order to more clearly understand the phenomenon of thermal conductivity we must consider the ballistic transport of phonons without describing their behavior as a diffusion process. Ballistic phonon transport, as well as characteristic phonon transport due to scattering caused by the structure, has been reported in various structures including atomic scale carbon nanotubes 1) , graphene 2) , semiconductor super lattices 3,4) , structures containing nano particles 5,6) , porous structures 7,8) , nanowires 9) and phononic crystal (PnC) structures [10][11][12][13][14][15][16] . Since the thermal conductivity in nanostructures is system-dependent, using certain materials it is possible to fabricate monolithic devices with various thermal conductivities.…”