Study on phonon lifetime in bulk silicon–germanium through observation of acoustic phonon spectra broadening by inelastic x-ray scattering

We report on the behavior of an acoustic phonon spectral linewidth of bulk single-crystalline Si_1−xGe_x alloy with the x of 0.16, 0.32, and 0.45 in the phonon dispersion relation along the Γ–X ([00q]) direction. Broadening of both transverse acoustic (TA) and longitudinal acoustic (LA) modes of the bulk Si_1−xGe_x alloy was directly observed using inelastic x-ray scattering (IXS) with increasing momentum (from Γ to X points in the Brillouin zone), which cannot be observed in pure Si or pure Ge. The IXS spectral linewidth of the TA mode indicated Ge dependence, which suggests the overlapping of a low-energy local vibration mode (LVM) caused by Ge clusters surrounded by Si atoms around the X point. Although the behavior of the IXS spectral linewidth of the LA mode showed almost no dependence on Ge fraction, the IXS spectra of the LA mode indicated broadening after crossing with a low-energy LVM with increasing momentum. The results obtained by molecular dynamics showed almost the same behavior of the acoustic phonon spectral linewidth. These results suggest that a change in the acoustic phonon spectral linewidth between the Γ and X points indicates a reduction in the acoustic phonon lifetime caused by the appearance of a localized mode originated from a random atom position in the alloy structure, leading to suppression of the thermal transport in the SiGe alloy.