Light Helicity as a Probe for Thickness-Controlled Topological States in α-Sn/CdTe(110) Heterostructures

Although α-Sn exhibits a rich topological phase diagram, experimental techniques for both manipulation and unambiguous discrimination of its phases in the (110) orientation are still lacking. Here, we investigate the epitaxial growth of α-Sn thin films on CdTe(110) substrates and their thickness-dependent topological properties using helicity-dependent photocurrent (HDPC). High-quality α-Sn films were grown by molecular beam epitaxy (MBE) and characterized by reflection high-energy electron diffraction (RHEED), Raman spectroscopy, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM). The HDPC of the 5 nm α-Sn film shows an odd-function dependence on incident angle, whereas that of the 10 and 30 nm films exhibits an even-function dependence. The contributions of the circular photogalvanic effect (CPGE) and the circular photon drag effect (CPDE) to the HPDC are clearly identified. Combined with HDPC measurements under front and back illuminations, point-group symmetry analysis, and first-principles calculations, we reveal that a thickness-driven topological phase transition from a two-dimensional (2D) to a three-dimensional (3D) topological insulator occurs between 5 and 10 nm. This transition is attributed to the interplay of quantum tunneling-mediated coupling of surface states and quantum confinement effects under in-plane compressive strain. These results establish HDPC as a sensitive diagnostic for topological phase transitions and highlight the potential of α-Sn(110) films as a tunable platform for exploring topological phenomena, paving the way for advanced spin-based devices.