Regensburg 2022 – scientific programme

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DS: Fachverband Dünne Schichten

DS 4: Thin Film Properties: Structure, Morphology and Composition (XRD, TEM, XPS, SIMS, RBS, AFM, ...) 2

DS 4.1: Talk

Monday, September 5, 2022, 11:00–11:15, H14

Scaling and confinement in ultrathin chalcogenide films as exemplified by GeTe — •Peter Kerres — 1st Institute of Physics "new Materials", RWTH Aachen University, 52066 Aachen, Germany

Chalcogenides such as GeTe, PbTe, Sb2Te3, and Bi2Se3 are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo-electrics to phase change materials, topological insulators and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, i.e. the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, we demonstrate for crystalline films of GeTe pronounced changes of optical and vibrational properties, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses of 40 nm. x-ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. We expect a thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism .