Thermal acoustic excitations with atomic-scale wavelengths in amorphous silicon

Abstract

The vibrational properties of glasses remain a topic of intense interest due to several unresolved puzzles, including the origin of the Boson peak and the mechanisms of thermal transport. Inelastic scattering measurements have revealed that amorphous solids support collective acoustic excitations with low THz frequencies despite the atomic disorder, but these frequencies are well below most of the thermal vibrational spectrum. Here, we report the observation of acoustic excitations with frequencies up to 10 THz in amorphous silicon. The excitations have atomic-scale wavelengths as short as 6 Å and exist well into the thermal vibrational frequencies. Simulations indicate that these high-frequency waves are supported due to the high group velocity and monatomic composition of a-Si, suggesting that other glasses with these characteristics may also exhibit such excitations. Our findings demonstrate that a substantial portion of thermal vibrational modes in amorphous materials can still be described as a phonon gas despite the lack of atomic order.

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

“…For instance, the predicted temperature dependence of THz excitations in Ref. [33] is not observed experimentally using inelastic x-ray scattering [9]. In the hypersonic frequency band ∼100 GHz, the measured values of attenuation are lower than those predicted by anharmonic damping [37].…”

“…We used TG to measure the thermal diffusivity of ∼500-nm-thick, free-standing aSi thin films with variable grating period L from ∼0.75-15.7 μm and at several temperatures from 60 to 315 K. The samples were prepared using the method of Ref. [9]. Amorphous silicon was deposited on a silicon nitride handle wafer using plasma-enhanced chemical vapor deposition (PECVD) with silane gas diluted (5%) in argon gas at a deposition table temperature of 470 K. The expected hydrogen content is ∼20% [9], which was reported not to measurably affect the thermal transport [40] (see also Sec.…”

“…[9]. Amorphous silicon was deposited on a silicon nitride handle wafer using plasma-enhanced chemical vapor deposition (PECVD) with silane gas diluted (5%) in argon gas at a deposition table temperature of 470 K. The expected hydrogen content is ∼20% [9], which was reported not to measurably affect the thermal transport [40] (see also Sec. II of Ref.…”

“…The collective acoustic excitations of amorphous solids are of fundamental interest due to their anomalous properties compared to those of crystalline solids, including an excess heat capacity at cryogenic temperatures [1,2] and scattering of acoustic excitations by two-level systems [3][4][5][6][7]. The dispersion and scattering of acoustic excitations responsible for heat transport have been extensively explored in many glasses using experimental methods such as inelastic scattering [8][9][10][11][12][13], tunnel junction spectroscopy [14], Brillouin scattering [15][16][17], and picosecond acoustics [18,19], among others. These studies have generally found that excitations with well-defined frequency and wave vector are supported up to ∼1 THz.…”

“…Second, additional data on the acoustic dispersion and scattering are known for aSi. Specifically, the dispersion of acoustic excitations is isotropic and experimentally available [9], and the low and high frequency limits of the attenuation coefficient are known from picosecond acoustics [37] and inelastic x-ray scattering [9], respectively. Collectively, this information provides ample constraints for the reconstruction.…”