YBa 2 Cu 3 O 7−δ step-edge Josephson junctions fabricated on sapphire substrates

Sapphire is an ideal substrate for cryogenic device technologies. We report here the fabrication of YBa2Cu3O7−δ step-edge junctions on the R-plane (11̄02) sapphire substrates. With an “overhang” shadow mask produced by a photolithography technique, a steep step edge was created on the CeO2 buffer layer by Ar+ ion milling with optimized parameters for minimum ion-beam divergence. The step angle was determined to be in excess of 80° by atomic force microscopy. Josephson junctions fabricated from those step edges exhibited resistively shunted junction like current–voltage characteristics. IcRn values in the 200–500 μV range were measured at 77 K. Shapiro steps were observed under microwave irradiation. The magnetic field dependence of the junction Ic indicates uniform current distribution.