Inelastic X-ray Scattering Measurement on Single-Crystalline GeSn Thin Film

Thermoelectric conversion devices based on group IV semiconductor elements can improve conversion efficiency by reducing the thermal conductivity of the material. In particular, it is known that introducing Sn into the system can dramatically reduce the conductivity. It has been experimentally shown that the thermal conductivity of polycrystalline Ge and polycrystalline Si1-xGex can be reduced by introduction of Sn. However, there is no experimental report on the effect of Sn atoms on the phonons responsible for thermal conduction. In this study, we investigated the mechanism of thermal conductivity reduction due to the introduction of Sn by inelastic x-ray scattering measurements on a Ge1-xSnx single-crystalline thin film with 9% Sn composition. The phonon dispersion of Ge1-xSnx was obtained as a result of the measurements, and the slope of the acoustic mode in the phonon dispersion curve of Ge1-xSnx was smaller than that of Ge. The phonon group velocity is expressed as the slope of the dispersion curves of the acoustic mode. Therefore, it was suggested that the reduction of the phonon group velocity by the introduction of Sn partly contributes to the reduction of the thermal conductivity of Ge1-xSnx. Local vibration mode (LVM), which was independent of wavenumber, was also observed in the low-energy region, and we attribute the origin to the local structure of Sn-Sn pairs formed in Ge1-xSnx. Such LVM has also been reported in bulk single- and polycrystalline Si1-xGex and polycrystalline Si1-x-yGexSny and is considered to influence the thermal conductivity reduction.