Raman Spectroscopy of SmB_x and YB_x thin films

Topological superconductivity may emerge at the interface between isostructural rare-earth hexaborides SmB₆ (a topological Kondo insulator) and YB₆ (an s-wave superconductor). We aim to synthesize thin films of SmB₆ and YB₆ using molecular beam epitaxy. A key challenge is to realize the cubic hexaboride phase, rather than the tetragonal tetraboride phase. Here we explore several substrates and growth parameters to optimize the rare-earth hexaboride stoichiometry. Our x-ray diffraction measurements show that across a wide range of boron to cation flux ratios, SmB_x films grown on Si and Ge crystallize in the cubic structure, while those grown on BaF₂ tend to crystallize in the tetragonal structure. Over the same range of growth conditions, YB_x films grown on Si and Ge tend to crystallize in the tetragonal structure. We discuss Raman spectroscopy as an additional rapid characterization tool. Distinct Raman peaks can differentiate between the hexaboride and tetraboride structures, enabling rapid feedback to tune growth parameters and achieve high-quality stoichiometric films and pristine interfaces.