Growth and Characterization of (Bi_xSb_1-x)₂Te₃/MgB₂ heterojunctions

Topological insulator-superconductor heterostructure, which was predicted to be an ideal system to search Majorana Fermions, has been extensively studied in recent years. Zero bias conductance peak (ZBCP) in such system was reported as a signature of Majorana bound states by several groups. Here we report the growth of epitaxial (Bi_xSb_1-x)₂Te₃ (BST) thin film on MgB₂ and study the vertical Josephson junction based on BST/MgB₂. The interface of BST/MgB₂ was found to be chemically reactive. Substrate temperature has to be lowered than typical growth temperature of BST to inactivate the reaction. In addition, a 2.5nm thin Nb layer inserted in between BST and MgB₂ can significantly improve the interface quality with little reduction to the pairing gap. Finally, 50nm Nb thin film was deposited on top to form the Nb/BST/MgB₂ and Nb/BST/Nb/MgB₂ junction. The former one shows tunneling behavior due to the formation of insulating layer at the interface of BST/MgB₂, while the latter one behaves more like a Josephson junction. ZBCPs were observed in both the junctions, which survive even above 4.2K, much higher than previously reported. Our study provides a new design for searching Majorana Fermions which may exist at a higher temperature than before.