A High-Resolution Brillouin Study of GeTe and Carbon-Doped GeTe Nanoscale Films: Implication for Thermoelectric and Memory Applications

In this work, we report the investigation of the acoustic guided modes in GeTe thin films with different microstructures and nanostructurations, interesting for thermoelectric and memory applications. Specifically, we have studied GeTe thin films where crystallization has been obtained through controlled annealing up to 400 and 450 °C, which induces different microstructures, and in a nanocomposite made of nanocrystals of GeTe surrounded by amorphous carbon, recently shown to exhibit glass-like thermal transport. We have performed a Brillouin scattering investigation on films of three different thicknesses per sample and on two different substrates for one of the samples. This has allowed us to estimate with good precision the two isotropic elastic constants and, from them, the bulk longitudinal and transverse sound velocities for the three samples. We observe that elastic constants are independent of the annealing temperature and microstructure in GeTe, while for the nanocomposite, a stiffening is found, in agreement with the estimation of the effective medium elastic moduli following the Reuss approximation. Finally, we extracted the intrinsic phonon attenuation for the Rayleigh mode, showing a much smaller attenuation in the nanocomposite. This has been interpreted as the consequence of a smaller roughness (finer microstructure, better interfaces) and a smaller anharmonicity in the nanocomposite. This latter finding confirms previous results and could be possibly related to a modification of the Peierls distortion in GeTe for nanometric grains, calling for further investigations.