Two-Step Growth Method for High Quality (Bi_0.5Sb_0.5)₂Te₃ and Cr doped (Bi_0.5Sb_0.5)₂Te₃ on Nb surfaces for Topological Josephson Junctions

It has been theoretically predicted that Majorana bound state can appear at the interface between topological insulators and superconductors. To obtain high quality (Bi_0.5Sb_0.5)₂Te₃ films on superconducting Nb surface, a two-step growth method was developed to promote proper (Bi_0.5Sb_0.5)₂Te₃ film nucleation on Nb surfaces in the early growth stage, where Bi, Sb and Te clusters were firstly evaporated at a relatively low temperature and then annealed to form a crystalized passivation layer, and a standard (Bi_0.5Sb_0.5)₂Te₃ film was grown under the normal deposition temperature secondly. Based on the two-step method, we also grew high quality Cr doped Bi-Sb-Te films on Nb surface with a similar procedure. Reflection high-energy electron diffraction (RHEED), high resolution transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterize the quality of the films. Finally, a top Nb film was deposited by magnetron sputtering at the room temperature. The Hetero-Nb/epitaxial (Bi_0.5Sb_0.5)₂Te₃ (with and without Cr doped)/ Nb stacks were further fabricated into micro Josephson junctions and the results of transport measurement also demonstrate a good quality of the multi-layer stacks.