|
Many of the applications envisioned for the high temperature superconductors (HTSC) lie in the area of Josephson electronics, but realization of this technological promise requires a controllable multi-layer process which would admit the possibility of insulating layers, useful for applications from ground-plane isolation to tunnel-barrier formation, in combination with high quality superconducting or normal-conducting layers. We have begun investigating the growth and processing techniques necessary to realize these structures by studying a prototype multi-layer system based on Y Pr1-xBa2Cu307-x alloys. This class of materials is of great interest due to two facts: By varying the Pr concentration, x, the material's resistivity can be tuned. For x=0, we have pure YBa2Cu307-x (YBCO) which is a 90K superconductor; with increasing x, Tc decreases until at x =1 with pure PrBa2Cu307_ (PBCO) we find a divergent resistivity with decreasing T. Secondly, over the full alloy range, 0<x <1, the material remains orthorhombic in structure and is lattice matched to YBCO to a fraction of a percent. 1 Our major effort to date has been to study Josephson weak-link formation by using YBCO for electrodes and PBCO as a lattice-matched semiconducting barrier layer. 2 The first step is the growth of a heteroepitaxial four-layer structure of YBCO - PBCO YBCO - Au, using our standard laser deposition process. "All layers are grown during a single cycle of the vacuum system. We use Rutherford backscattering, both in the random and channeling modes, and X-ray scattering to verify layer stoichometry and heteroepitaxial growth of the orthorhombic perovskite structure, c-axis normal to the substrate. Both YBCO layers have sharp superconducting transitions above 80K upon removal from the vacuum system. Single devices with areas, A, from 2.5x10-5cm2 to 2x10-7crn2 are isolated with a four-step fabrication procedure involving standard photolithography and Ar ion milling. These devices show Current-Voltage characteristics similar to those observed for low temperature Superconductor /Normal metal /Superconductor (SNS) devices. We find good scaling of the critical currents. I. with A and scaling of the resistances, R7, with 1/A; the typical values of the /c.R7 product of 3.5rnV are consistent with traditional SNS behavior. Further, we observe Shapiro steps in response to 100GHz mm-wave radiation and oscillation of the d.c. supercurrent in a transverse magnetic field thus demonstrating that both the a.c. and d.c. Josephson effect occurs in these devices. These preliminary results suggest that our multi-layer process can be optimized for a variety of Josephson devices essential in producing electronic circuitry from the HTSC systems.
|
