Thermal Stability Enhancement in Epitaxial α-Sn Films by Strain Engineering

Abstract

Exploring new topological materials with large topological nontrivial bandgaps and simple composition is attractive for both theoretical investigation and experimental realization. Recently alpha tin (α-Sn) has been predicted to be such a candidate and it can be tuned to be either a topological insulator or a Dirac semimetal by applying appropriate strain. However, free-standing α-Sn is only stable below 13.2 °C. In this study, a series of high-quality α-Sn films with different thicknesses have been successfully grown on InSb substrates by molecular beam epitaxy (MBE). Confirmed by both X-ray diffraction (XRD) and reciprocal space mapping (RSM), all the films remained fully strained up to 4000 Å, proving the strain effect from the substrate. Remarkably, the single-crystalline α phase can persist up to 170 °C for the 200 Å thick sample. The critical temperature where the α phase disappears decreases as the film thickness increases, showing the thermal stabilization can be engineered by varying the α-Sn thickness. A plastic flow model taking the work hardening into account is introduced to explain this dependence, assuming the strain relaxation and the phase transition occur successively. This enhanced thermal stability is prerequisite for above room-temperature characterization and application of this material system.