Electron microscopy of high-T{sub c} Josephson junctions formed in the epitaxiallayer ramp-edge geometry : YBCO/barrier/YBCO.
The microstructures of YBCO/N/YBCO ramp-edge junctions have been studied by transmission electron microscopy and related to supercurrent transport properties which depend on the barrier layer N, the interfacial structures and defect structures in the multilayer epitaxial devices. Three different types of junction materials were investigated: Metallic oxide barriers, CaRuO{sub 3} and SrRuO{sub 3}; barriers isostructural to YBCO, Co-doped YBCO and Co-doped PrBa{sub 2}Cu{sub 3}O{sub 7}; and 'interface engineered' barriers, formed by plasma treatment of the YBCO ramp-edge. Metallic oxide barriers are characterized by high steps and strong variations in local barrier width. The observed topologies are consistent with an island growth mode for both CaRuO{sub 3} and SrRuO{sub 3}. The metallic oxide barriers are associated with interfacial strain fields that are believed to be the cause for interfacial oxygen depletion in YBCO and the observed excess normal-state resistance. A number of structural defects and deviations from perfect epitaxy have been observed. Most disruptive to the integrity of the multilayer structures and transport properties is the nucleation of a-axis YBCO grains at steep barrier steps and within the YBCO layers. The barrier layers in isostructural junctions are well structured with a high degree of interfacial coherence and for the most part, Cu-O planes are continuous across the interfaces. In contrast to heterostructured metallic oxide barriers, isostructural junctions contain few extraneous defects, such as a-axis grains at the barrier layer, moreover, the second YBCO layer is of good quality due to the perfect epitaxy between the materials. Among the junctions investigated the interface engineered junctions have shown the best electromagnetic properties. Their structure is characterized by narrow (2-3 nm) barriers that are continuous and crystalline. No significant interfacial strains and structural disorder were observed. This and a constant barrier thickness appear responsible for obtaining good uniformity of electric transport properties. The narrow pinhole-free barrier, coupled with excellent epitaxy and few defects, yields good reproducibility and a range of properties suitable for practical applications.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- ER
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 942403
- Report Number(s):
- ANL/MSD/JA-31069; MICOB5; TRN: US0902826
- Journal Information:
- Micron, Vol. 30, Issue 5 ; Oct. 1999; ISSN 0047-7206
- Country of Publication:
- United States
- Language:
- ENGLISH
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