Superconducting MgB₂ nanoribbons grown on graphene on SiC via selective area epitaxy

Hybrid structures of superconductors with 2D and topological materials have attracted intensive interest in recent years due to the possible realization of topological or chiral superconductivity. On the other hand, superconducting nanowires have their own applications such as in single photon detections. MgB₂ is the highest Tc s-wave superconductor with a critical temperature T_c of 39 K and upper critical field H_c2 exceeding 50 T. Therefore, hybrid structures using MgB₂ as the superconductor are desirable for higher temperature operation of the potential devices than using conventional superconductors. We fabricated epitaxial MgB₂ nanoribbons via selective area epitaxial growth on graphene on SiC substrate. We discovered that c-axis MgB₂ thin films only grow on areas without graphene, but not on top of graphene. This technique allows for direct synthesis of MgB₂-graphene hybrid structures and devices with desired patterns over a large area. Nanowires of 300 nm and 500 nm in width and 60 nm thick are fabricated with patterned epitaxial graphene on SiC using e-beam lithography by hybrid physical-chemical vapor deposition (HPCVD). The superconducting properties of the MgB₂ nanoribbons retain a T_c about 38 K and a critical current density J_c(2 K) on the order of ~10⁷ A/cm². Due to the C-plane epitaxial growth of the nanoribbons, a large upper critical field anisotropy was observed in contract to nanowires grown using other techniques.