Selective Area Growth of Josephson Junctions in Ladder-Like Structures of Bismuth Islands

A distinguishing characteristic of Josephson junctions (JJs) is their nonlinear current-voltage response, which fulfills the requirements for superconducting quantum computing. Achieving atomically sharp interfaces between superconductors and weak links in JJs can realize the superconductivity proximity effect, advancing the investigation of intrinsic properties in unconventional superconductors and their potential applications. Here, a contamination-free approach to fabricating planar JJs using molecular beam epitaxy (MBE) is presented. The self-assembled Bi buffer layer forms a ladder-like framework on Si (111) 7 × 7 reconstructions, consisting of a wetting layer of Bi interspersed with Bi crystalline islands. Upon depositing Pb on this Bi buffer layer, Pb atoms dominantly nucleate on the Bi wetting layer, bypassing the Bi islands to form arrayed JJs. The selective area growth of Pb thin films is attributed to the higher nucleation densities for Pb on the Bi wetting layer compared to Bi crystalline islands. In situ scanning tunneling microscopy (STM) measurements reveal the superconducting correlations within the interior of junctions. The study establishes an in-vacuo deposition method for fabricating pristine JJs, facilitating the potential investigation of emergent superconducting phenomena in designed heterostructures.