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

Exploring new topological materials with large topological nontrivial bandgaps and simple composition is attractive for both theoretical investigation and experimental realization. Recently, alpha tin (alpha-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, freestanding alpha-Sn is only stable below 13.2 degrees C. Herein, a series of high-quality alpha-Sn films with different thicknesses are 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 remain fully strained up to 400 nm, proving the strain effect from the substrate. Remarkably, the single-crystalline alpha phase can persist up to 170 degrees C for the 20 nm thick sample. The critical temperature where the alpha phase disappears decreases as the film thickness increases, showing that the thermal stabilization can be engineered by varying the alpha-Sn thickness. A plastic flow model considering work hardening is introduced to explain this dependence, assuming that the strain relaxation and the phase transition occur successively. This enhanced thermal stability is prerequisite for aforementioned room-temperature characterization and practical application of this material system.