CeO2 as insulation layer in high T c superconducting multilayer and crossover structures

Abstract

We present a study of the electrical properties of insulating CeO 2 layers in combination with superconducting ͑Y/Dy͒ Ba 2 Cu 3 O 7Ϫ␦ ͑RBCO͒ films over ramps and in crossover structures. CeO 2 is frequently used as a buffer layer, or template layer for biepitaxial grain boundary junctions, but can also be used as an insulating layer in ramp-type junctions and other multilayer structures. Epitaxial thin films of CeO 2 were deposited by pulsed laser ablation using SrTiO 3 substrates. We characterized the insulating performance of CeO 2 thin films in terms of breakdown field E bd and the relative dielectric constant ⑀ r . For 80 nm thick CeO 2 at 77 K we found E bd ϭ1ϫ10 6 V/cm, using a 1 nA/100 m 2 breakdown criterion, which gives us a specific resistivity of у10 9 ⍀ cm up to breakdown. From capacitance measurements on planar RBCO/CeO 2 /RBCO structures we obtained for the dielectric constant: ⑀ r Ϸ15. The texture of CeO 2 in combination with RBCO on ramped surfaces, simulated by SrTiO 3 ͑STO͒ ͑103͒ substrates having their normal tilted by 18°away from the STO͓001͔ direction, has been studied by x-ray diffraction. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02604-6͔High-quality heteroepitaxial multilayers of superconductors and insulators are required for high T c superconducting electronic devices, like ramp-type junctions and crossover structures in multiturn coils. For the use in highfrequency applications, an insulator having a low dielectric constant is preferred to reduce capacitive shunting. Materials often used as insulators in high T c superconducting devices are PrBa 2 Cu 3 O 7 ͑PBCO͒ 1 and SrTiO 3 ͑STO͒. 2 The use of PBCO has some disadvantages for its relatively low resistivity at 77 K and the anisotropic growth. The last property enhances the formation of granular films, and leaks are likely to occur at boundaries between a-and c-axis oriented grains. The main drawback of STO is its large dielectric constant ͓⑀ r Ϸ300 for thin films ͑Ref. 2͔͒, which severely limits its applicability in high-frequency devices. CeO 2 has been shown to be useful as a planar insulator. 3 It has a cubic crystal structure with lattice constant a 0 ϭ5.411 Å, and CeO 2 ͓100͔ is commensurate with YBCO͓110͔, corresponding to a relative rotation of 45°of the lattices. The very simple crystal structure, containing only one metal ion and oxygen, makes CeO 2 an easy material to grow by pulsed laser deposition, as no problems with the stoichiometry are to be expected. The use of CeO 2 in ramp-type junctions and crossover structures requires a nearly perfect heteroepitaxy with RBCO, both on top of flat surfaces and on artificially created ramped surfaces. The rotation of CeO 2 relative to RBCO, however, might induce grain boundaries or cracks when growing on a ramped surface. Reuvekamp et al. 4 showed that the preferential orientation of RBCO grown on YSZ is with the c-axis perpendicular to the local YSZ substrate surface. The difference in orientation in the upper YBCO layer on the ramp and on flat surfaces then caused cr...

Cerium dioxide buffer layers at low temperature by atomic layer deposition

“…8 In addition to the buffer layers, CeO 2 can be used as a compatible intermediate material in the fabrication of superconductorinsulator-superconductor (SIS) multilayer structures. 11 Further more, cerium is one of the constituents of the Nd-Ce-Cu-O superconductor. 12 Cerium dioxide thin films have been deposited by many different techniques.…”

Synthesis and Characterization of Volatile, Fluorine-Free β-Ketoiminate Lanthanide MOCVD Precursors and Their Implementation in Low-Temperature Growth of Epitaxial CeO2 Buffer Layers for Superconducting Electronics

“…The growth and properties of lanthanide-containing oxide films is of great current interest to the chemistry, materials science, and electronics communities. − Applications include multilayer device buffer layers (e.g., CeO 2 ), − high dielectric constant (high- k ) materials (e.g., Gd 2 O 3 ), , high- temperature superconductors (e.g., LnBa 2 Cu 3 O 7 - δ ), − phosphor dopants, − solid-state oxide fuel cells, and magnetic materials …”

Reducible Oxides as Ultrathin Epitaxial Films

“…The (100) and (110) surface orientation can be stabilized through epitaxy using substrates with a very small lattice mismatch with CeO 2 . These are typically non-metallic and include Si [22,23], YSZ [24] and SrTiO 3 [25,26] . However, most of the studies performed using non metallic substrates focus on thick films with several tens of nm thickness, possibly because of the difficulties in applying surface science techniques, necessary to obtain information at the ultrathin limit, on substrates with low conductivity.…”