Regensburg 2022 – scientific programme

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DS: Fachverband Dünne Schichten

DS 20: Poster

DS 20.13: Poster

Wednesday, September 7, 2022, 16:00–18:00, P3

Confinement induced coherent phonon softening in Sb2Te3 thin films — •Jonathan Frank¹, Julian Mertens¹, Felix Hoff¹, Mohit Raghuwanshi², and Matthias Wuttig¹ — ¹1. Institute of Physics (IA), RWTH Aachen University, Aachen, Germany — ²Forschungszentrum Jülich, Jülich, Germany

Femtosecond reflection-type optical pump-probe experiments on a thickness-series of MBE-grown Sb2Te3 thin films were carried out to study confinement effects of coherent optical phonons. Therefore, an isotropic detection scheme was applied to investigate the ultrafast dynamics of the LO-phonon mode of A1g-symmetry in Sb2Te3 thin films ranging from 1.3 to 55 nm. The ultrafast response of each thin film contains a distinct coherent feature uncovered by damped harmonic oscillations in the transient reflectivity traces. It is shown that coherent optical phonons are efficiently photoexcited in thin films of only a few quintuple layers. This finding supports the hypothesis that electron-phonon coupling in ultrathin sesqui-chalcogenide films does not fundamentally differ from bulk which is essential for the performance in strongly scaled applications as for example in topological insulators. Furthermore, a slight decrease in phonon-frequency accompanied by a more pronounced decrease in phonon-lifetime is observed in films smaller than ten nanometers; both quantities nearly monotonously decrease with film thickness. We ascribe this phonon softening to a decreased interlayer coupling in films of a few quintuple layers compared to a more bulk-like behavior observed in the thicker films.