Micro-Fabrication Method of Josephson Junctions without Etching Process

摘要

Intrinsic Josephson junctions (IJJs) were fabricated without etching process by introduction of an oxygen-depleted layer to the surface of the Bi_2Sr_2CaCu_2O_(8+δ) (Bi-2212) single crystal changing the direction of current flow from in-plane (// ab) to out-of-plane (// c). This could be achieved by reducing the surface of the Bi-2212 single crystal. We took two methods to introduce the oxygen-depleted layer into the surface of Bi-2212 single crystal. One method is to introduce the oxygen-depleted layer by annealing the Bi-2212 single crystal in reductive atmosphere of H_2 gas. The Pt or Au electrodes for four-terminal measurement were deposited side by side on the surface of the Bi-2212 single crystal, and the Bi-2212 with electrodes was annealed in Ar+H_2 atmosphere. The resistivity-temperature (ρ-T) characteristics of the Bi-2212 single crystal changed from metallic (dρ/dT > 0) to semiconducting (dρ/dT < 0) behavior by annealing in Ar+H_2 atmosphere. In addition, hysteresis loop and voltage-jump, which are peculiar to IJJs, were observed in the current-voltage (I-V) characteristic of the sample annealed in Ar+H_2 atmosphere. It is known that Bi-2212 single crystals are reduced by H_2 annealing and the superconductivity of the reduced area is suppressed. Therefore, the surface of the Bi-2212 single crystal except for the masked area with the electrodes was reduced by Ar+H_2 annealing. As a result, the change in the direction of the current was caused due to the detour avoiding the reduced area, and IJJs were fabricated. Another method to introduce the oxygen-depleted layer is to deposit metals with low Gibbs free energy and raising the temperature to exceed the activation energy for oxygen ions to move from Bi-2212 single crystal to the metal. We prepared the sample on which Al with low Gibbs free energy was deposited between the voltage terminals. Here, this sample is referred to as Al/Bi-2212 structures. The ρ-T characteristics of the as-prepared Al/Bi-2212 structure showed metallic behavior, whereas the ρ-T characteristics of the Al/Bi-2212 structure showed semiconducting behavior by annealing in Ar atmosphere. Moreover, hysteresis loop and voltage-jump were observed in the I-V characteristic of the annealed Al/Bi-2212 structure in Ar atmosphere. These results show that the direction of current flow is changed from in-plane to out-of-plane by the selective reduction of the Bi-2212 single crystal due to the oxidation of the Al and that IJJs can be fabricated without etching process. The latter method is more advantageous to integration of superconducting circuit due to the high controllability of selective reduction.