Confinement‐Induced Phonon Softening and Hardening in Sb 2 Te 3 Thin Films

• Published in: Advanced functional materials Vol. 34; no. 1
• Main Authors: Mertens, Julian, Kerres, Peter, Xu, Yazhi, Raghuwanshi, Mohit, Kim, Dasol, Schön, Carl‐Friedrich, Frank, Jonathan, Hoff, Felix, Zhou, Yiming, Mazzarello, Riccardo, Jalil, Abdur Rehman, Wuttig, Matthias
• Format: Journal Article
• Language: English
• Published: 01.01.2024
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202307681

Abstract Scaling effects in Sesqui‐chalcogenides are of major interest to understand and optimize their performance in heavily scaled applications, including topological insulators and phase‐change devices. A combined experimental and theoretical study is presented for molecular beam epitaxy‐grown films of antimony‐telluride  (Sb 2 Te 3 ). Structural,vibrational, optical, and bonding properties upon varying confinement are studied for thicknesses ranging from 1.3 to 56 nm. In ultrathin films, the low‐frequency coherent phonons of A 1g 1 symmetry are softened compared to the bulk (64.5 cm −1 at 1.3 nm compared to 68 cm −1 at 55.8 nm). A concomitant increase of the high‐frequency A 1g 2 Raman mode is seen. X‐ray diffraction analyses unravel an accompanying out of plane stretch by 5%, mainly stemming from an increase in the Te‐Te gap. This conclusion is supported by density functional theory slab models, which reveal a significant dependency of chemical bonding on film thickness. Changes in atomic arrangement, vibrational frequencies, and bonding extend over a thickness range much larger than observed for other material classes. The finding of these unexpectedly pronounced thickness‐dependent effects in quasi‐2D material Sb 2 Te 3 allows tuning of the film properties with thickness. The results are discussed in the context of a novel bond‐type, characterized by a competition between electron localization and delocalization.