Laser trimming of Y-Ba-Cu-O step-edge Josephson junctions
We report our studies on the laser-induced permanent changes in electrical transport of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) step-edge Josephson junctions. The junctions were fabricated from 200-nm-thick YBCO films deposited by laser ablation onto 300-nm-high steps etched in LaAlO/sub 3/ substrates. Laser annealing was performed in-situ by illuminating the junctions at 50 K with a focused Ar-ion laser beam of the intensity up to 10/sup 5/ W/cm/sup 2/. Under optimal irradiation, we observed up to 50% increase of the junction critical current. The modified critical currents were stable and did not change with time or during the helium-to-room-temperature thermal cycling. The annealed junctions exhibited RSJ-like current-voltage characteristics and an enhancement of the I/sub c/R/sub n/ product of up to 50%. The I/sub c/(B) characteristics of the illuminated junctions showed up to 10% shift of the first minima, indicating changes in the junction-barrier area. The I/sub c/(B) simulations, based on the Yanson model of large junction-barrier fluctuations, demonstrated that laser treatment improved barrier homogeneity. We believe that upon laser irradiation, photo-assisted thermal oxygen reordering takes place in the vicinity of the junction barrier, consequently leading to grain boundaries with increased oxygen content and improved junction dc characteristics. An illumination power exceeding 10/sup 5/ W/cm/sup 2/ resulted in oxygen out-diffusion from the junction area and loss of the Josephson-like properties.