The growth and properties of niobium based trilayered and stacked Josephson junctions.

摘要

We have studied Nb based trilayered and stacked-multilayered superconducting Josephson junctions grown on silicon and oxidized silicon substrates by d.c. magnetron sputtering. AlO{dollar}sb{lcub}rm x{rcub}{dollar} and SiN{dollar}sb{lcub}rm x{rcub}{dollar} have been used as the insulating barriers. The junction sizes range from 10x10 to 50x50 {dollar}mu{dollar}m{dollar}sp2{dollar} High resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) have been used to study the surface morphology. Both the HRTEM and AFM pictures show that the Al layer planarizes the underlying Nb surface and that the surface of a stacked junction is much smoother than a trilayered junction.; The I-V characteristics and the Fraunhofer interference patterns of both single and multilayered junctions have been measured at 4.2 and 2 K. The sum gap of a three junction device was 5.8 meV. Because the intermediate Nb layer is only 15 nm, the sum gap of the multilayered device is suppressed. In order to test our junction quality we also fabricated a Nb/Al-AlO{dollar}sb{lcub}rm x{rcub}{dollar}/Nb superconducting quantum interference device (SQUID). The amplitude of the voltage oscillations with magnetic field was 80 {dollar}mu{dollar}V, indicating a good quality device. The critical current of the junctions are not identical which results in the appearance of three steps in the I-V characteristics.; A stacked junction offers several advantages over a trilayered junction and an array of junctions. One promising application of these devices is as radiation sources and detectors. The feasibility of packaging a radiation source and a detector on the same chip was demonstrated with a pair of Nb/Al-AlO{dollar}sb{lcub}rm x{rcub}{dollar}/Nb junctions prepared by the selective niobium anodization process (SNAP). The coupling efficiency could be increased by forming a transmission line between the two junctions.; Due to the interactions of electromagnetic radiation with the Josephson and quasiparticle tunneling currents interesting phenomena are observed when the junctions are exposed to the external radiation. We have studied the I-V characteristics of a stacked Nb/(Al-AlO{dollar}sb{lcub}rm x{rcub}{dollar}/Nb){dollar}sb{lcub}rm n{rcub}{dollar} multilayered junction with n = 3 irradiated with microwave radiation at 1.4 K; the Shapiro steps were observed.