Fabrication of Planar Sandwich Type Josephson Junctions Using ${\rm MgB}_{2}$ Thin Film With a Nb Counter Electrode

We have fabricated sandwich type MgB₂ Josephson junctions by using a Nb thin film as the counter electrode. The junction either contained a normal Au barrier layer or not. Superconducting MgB₂ films were prepared on sapphire substrates by either in-situ annealing of cosputtered Mg and B mixture films or ex-situ diffusion annealing of B precursor films in Mg vapor. Surface of the MgB₂ base film was mechanically dry-polished with 1/4 mum diamond powder to remove nonsuperconducting residues and subsequently cleaned by using argon ion beam. Junction was defined on an area of 4 mumtimes4 mum through a 150 nm thick SiO_x insulating layer by lift-off and completed by depositing a normal thin Au layer (0~10 nm) and finally a 150 nm thick Nb counter electrode. T_c0 was at least 30 K for the MgB₂ base electrode and 9 K for the Nb counter electrode. Current-voltage curves showed typical Josephson transition characteristics at critical currents. The junction critical current was modulated periodically in response to an applied magnetic field, in qualitative agreement with the theory of the Fraunhoffer diffraction.