Observation of Barrier Recrystallization Process and Properties of Ramp-Edge Josephson Junctions with Interface-Modified Barrier

Abstract

We have studied the recrystallization process and the microstructure of the interface-modified barrier, and the electrical properties of Josephson junctions with the barrier. Electron cyclotron resonance (ECR) ion bombardment was used for making an amorphous layer at the YBa2Cu3O7-δ (YBCO) film surfaces. Then, the samples were annealed to form an interface-modified barrier from the amorphous. A halo pattern showing the existence of an amorphous was changed to three types of patterns by changing the ECR and annealing conditions. The YBCO surface damaged at an ion acceleration voltage of 700 V showed clear recrystallization to YBCO. The surface damaged at lower voltages showed reflection high energy electron diffraction (RHEED) patterns different from that of YBCO. In these cases, regions with a cubic or pseudo-cubic structures were observed at the interface by transmission electron microscopy (TEM). Ramp-edge Josephson junctions with an interface-modified barrier have been fabricated. The junction properties strongly depend on the distance between the sample and the laser plume when upper layer YBCO is deposited. The junctions fabricated under optimum conditions exhibited resistively and capacitively shunted junction (RCSJ)-like I–V curves with a typical IcRn product at 4.2 K of 1.0–3.2 mV. We show that the recrystallization of an interface-modified barrier from an amorphous layer strongly depends on the kinetic energy from laser plume plasma as well as the thermal energy from substrate heating, and the Ic values of the junctions strongly depend on the actual bombardment energy of ions incident on the ramp surface. The difference of Ic may be explained in terms of the difference of the barrier microstructure which depends on the ion bombardment energy.