Confinement‐Induced Phonon Softening and Hardening in Sb₂Te₃ Thin Films

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 (Sb₂Te₃). 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_1g¹ 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_1g² 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 Sb₂Te₃ 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.