Origin of micrometer-scale propagation lengths of heat-carrying acoustic excitations in amorphous silicon

Abstract

The heat-carrying acoustic excitations of amorphous silicon are of interest because their mean free paths may approach micron scales at room temperature. Despite extensive investigation, the origin of the weak acoustic scattering in the heat-carrying frequencies remains a topic of debate. Here, we report measurements of the thermal conductivity mean free path accumulation function in amorphous silicon thin films from 60 to 315 K using transient grating spectroscopy. With additional picosecond acoustics measurements and considering the power-law frequency dependence of scattering mechanisms in glasses, we reconstruct the mean free paths from ∼0.1-3 THz. The mean free paths are independent of temperature and exhibit a Rayleigh scattering trend over most of this frequency range. The observed trend is inconsistent with the predictions of numerical studies based on normal mode analysis but agrees with diverse measurements on other glasses. The micron-scale MFPs in amorphous Si arise from the absence of anharmonic or thermally activated relaxation damping in the sub-THz frequencies, leading to heat-carrying acoustic excitations with room-temperature MFPs comparable to those of other glasses at cryogenic temperatures.

Strong phase correlation between diffusons governs heat conduction in amorphous silicon

“…Considering the atomic instability and the absence of lattice periodicity in amorphous materials, thermal vibrations in a-Si are treated as collective excitations as studied from the DSF [21,23], which also refer to acoustic collective excitations, where atoms are vibrating along the same polarization (See Eqs. ( 2) and ( 3)).…”

“…The similarity in the lattice aperiodicity and atomic instability makes the collective excitation a reasonable description of thermal vibrations in amorphous materials as discussed before in Refs. [21,23,43]. The calculation method of DSF is presented in the Methodology section.…”

“…1 can also provide the information of wavevector for different vibrations but in the frame of collective excitations as proposed in Refs. [21,23,43]. By implementing the wave-packet simulation, we further study the detailed transport properties of propagons and diffusons.…”

“…The utilized normal modes and group velocity, which are both ill-defined in amorphous solids [10], raise questions on the fundamentals of Allen-Feldman and QHGK descriptions. In addition, in analogy to liquids, the collective excitations are experimentally observed and applied to understand the thermal vibrations in amorphous materials [18][19][20][21][22][23]. Moon et al [20,21] found that the thermal transport in amorphous silicon (a-Si) is dominated by acoustic collective excitations rather than by normal modes and that the elastic scattering predominates the scattering processes.…”

“…A theoretical model that can assess the thermal transport in amorphous materials by incorporating the collective excitations, however, is still missing. In addition, the controversies regarding the dependencies of thermal conductivity (κ) on temperature and length remain topics of debate [23][24][25][26][27].…”