Quantitative Scanning Transmission Electron Microscopy–High-Angle-Annular Dark-Field Study of the Structure of Pseudo-2D Sb2Te3 Films Grown by (Quasi) Van der Waals Epitaxy

Scanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out-of-plane oriented Sb2Te3 thin films deposited by sputtering on SiO2 and Si substrates. Nanobeam precession electron diffraction, energy-dispersive X-ray spectroscopy, and high-angle-annular dark-field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb2Te3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb2Te3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo-2D Sb2Te3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low-frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo-2D materials, such as Sb2Te3/GeTe superlattices.