Confinement‐Induced Phonon Softening and Hardening in Sb2Te3 Thin Films

Abstract

AbstractScaling 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 A1g1 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 A1g2 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.