Reversible Polymorph Switching in IV–VI Thin Films with Epitaxial Control and Birefringence Contrast

Reversible polymorphic transformations provide a powerful route to tune the functionality in semiconductors. We demonstrate a cyclable phase transformation between the rocksalt cubic and layered orthorhombic structures in epitaxial PbSnSe thin films on GaAs(001). This thermally driven transformation occurs between -60 and 150 °C, with the transformation temperature depending on alloy composition. Subtle atomic displacements between polymorphs allow the transformation while retaining epitaxial registry for both states, while the dramatic change in symmetry drives a transition from an optically isotropic to birefringent state, with an index change exceeding Δ<i>n</i> = 1. Microstructural analysis reveals a transformation involving low-energy interfaces that coherently accommodates a 3% out-of-plane lattice mismatch. Thermal cycling shows limited endurance at present, tentatively attributed to defect accumulation at phase boundaries, but annealing at increased temperatures indicates that the transformation remains fundamentally reversible. These findings position PbSnSe among a small set of ultratunable semiconductors compatible with epitaxial integration for optoelectronic applications.