DC-magnetron sputtering of YBa2Cu3O7−δ thin films for microwave applications

doi:10.1016/0921-4534(92)90218-2

Physica C: Superconductivity

Volume 198, Issues 3–4, 1 August 1992, Pages 389-396

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Abstract

High-quality YBa₂Cu₃O_7−δ thin films with optimized surface smoothens suitable for multilayer technology have been deposited by DC-magnetron sputtering. The films not only show high critical temperatures (T_c) up to 90 K and high critical current densities (j_c) up to 4.2×10⁶ A/cm² at 77 K, but also a smooth surface with only one outgrowth of a size of more than 200 nm within an area of 150×150 μm². The T_c and j_c values are slightly lower than our best values for rough films with optimized conditions for T_c and j_c, where we reach up to 6×10⁶ A/cm² at 77 K and a T_c offset up to 91.5 K on SrTiO₃ and La AlO₃. Additionally, the surface resistances (R_s) of our films were measured using coplanar resonators. We determined an R_s of 120 μΩ at 77 K and 60 μΩ at 9 K and at 6.2 GHz for our best film. The penetration depth was evaluated to be between 160 and 200 nm for this film. A dependence of R_s on the surface magnetic field showed a higher crystal quality of our films with increasing j_c measured with a remanent field technique.

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Intrinsic Josephson junctions of Tl<inf>2</inf>Ba<inf>2</inf>CaCu<inf>2</inf>O<inf>x</inf> thin films for high frequency devices
1997, Physica C: Superconductivity and its Applications
We successfully observed intrinsic Josephson effects of Tl₂Ba₂CaCu₂O_x (Tl-2212) thin films by structuring step-edges or mesas and measuring the electrical transport properties along the c-axis. For a step-edge with the height of 350 nm, a hysteretic current-voltage (I–V) curve was observed up to 50 K. For a 5 μ m x 5μ m mesa structure with a height of 400 nm, a hysteretic I–V curve was also observed up to 80 K. Both show that series connected SIS-type junctions are formed. Comparing with the critical current density (J_c) of more than 10⁶ A/cm² parallel to the ab-plane, an anisotropic J_c of 1.4 × 10² A/cm² at 4.9 K was observed. The normal resistance (R_nk) of a unit SIS junction was estimated to be 580 Ω by fitting the Ambegaokar and Baratoff relation. The capacitance (C_k) of the unit SIS junction was estimated to be 3.6 × 10⁻¹⁰ F/cm² at 77 K by the calculation of McCumber parameter (β_c). Then, the cut-of frequency $1 2πR_{nk} C_{k}$ was estimated to be 3.1 THz at 77 K as a THz detector.
Ka-band performance of step-edge Josephson junctions using Tl<inf>2</inf>Ba<inf>2</inf>CaCu<inf>2</inf>O<inf>x</inf> films on MgO substrates
1996, Physica C: Superconductivity and its Applications
Tl₂Ba₂CaCu₂O_x (Tl-2212) step-edge Josephson junctions (SEJJs) on MgO substrates have been frabricated and tested for millimeter wave applications. Although the single SEJJs exhibited clear Shapiro steps under Ka-band (27 to 40 GHz) irradiation and a Fraunhofer-like pattern under a magnetic field, their responsivities as Ka-band detectors were too small to construct a mixing device. This is due to their small normal resistance (R_n) of 0.2 to 0.9 Ω, which determines the impedance matching for the microstrip signal lines. Therefore, series-connected six SEJJs (6-SEJJs) have also been fabricated to improve the impedance matching. Their responsivities are 2–3 orders of magnitude larger than those of single SEJJs. Furthermore, efficient IF (intermediate frequency) output of fundamental mixing with low LO (local oscillation) power of a few μW is observed up to 95 K.
High critical current density in epitaxial YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> thin films
1995, Physica B: Physics of Condensed Matter
Very large critical current densities are obtained reproducibly in epitaxial YBa₂Cu₃O_7−δ thin films prepared on (1 0 0) SrTiO₃ and MgO substrates by inverted cylindrical magnetron sputtering. The structure properties studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and Rutherford backscattering show a very high-quality epitaxial thin film growth. Critical current densities of the order of 10⁷ A/cm² at 77 K were measured directly and are in agreement with indirect AC-susceptibility measurements. A flux creep model gives a good agreement with the experiment data.
High temperature superconducting coplanar and epitaxially grown microstrip transmission lines studied by a half-wavelength resonator technique
1994, Applied Superconductivity
Coplanar YBa₂Cu₃O_7−x (YBCO) thin film and epitaxial YBCO/NdAlO₃/YBCO microstrip transmission lines were investigated using planar half wavelength resonators. The results were compared to theoretical calculations. Coplanar transmission lines in coplanar waveguide and coplanar stripline geometries exhibited unloaded quality factors up to 2600 and attenuation constants down to 0.6 dB/m at 77 K and at frequencies of 5.5 GHz. These results are two orders of magnitude below normal metal lines. Planar microstrip resonators which were epitaxially grown by combined d.c./r.f. sputtering showed attenuation constants down to 26 dB/m at 50 K and at 4.7 GHz. The results achieved with the microstrip lines require a further technological development in order to be improved while they are already significantly below normal metal based lines. These results are already promising with respect to a use as transmission lines in integrated circuits.
Long-time behaviour of the high-frequency properties of YBCO thin films
1994, Physica C: Superconductivity and its applications
The long-time behaviour of the high-frequency properties of c-axis oriented YBa₂Cu₃O_7-δ (YBCO) thin films was studied using photolithographically patterned coplanar waveguide resonators. The temperature and power dependence of the surface resistance (R_s), the transition temperature (T_c) and the critical current density (j_c) were monitored over time periods of more than one year while keeping the samples at ambient air. There was no evidence for a significant degradation for samples with j_c (77 K) as high as 4 × 10⁶ A/cm² as well as for films exhibiting a granular high-frequency behaviour with j_c (77 K) of 1 × 10⁶ A/ cm². The results are discussed with respect to the reactivity of YBCO thin-film surfaces to atmospheric components as observed by X-ray photoelectron spectroscopy (XPS).
Coplanar waveguide and epitaxial YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-x</inf>/NdAlO<inf>3</inf>/YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-x</inf>-trilayer transmission line resonators for microwave applications
1993, Applied Superconductivity
High quality YBa₂Cu₃O_7−x (YBCO) thin films were grown on LaAlO₃ by de-magnetron sputtering. The films showed critical temperatures of 91 K and critical current densities well beyond 10⁶ A/cm² at 77 K. From 200 nm thick films coplanar waveguide resonators were patterned with characteristic impedances between 40 and 50 Ω. At 77 K the highest unloaded quality factors reached values up to 1200 at 6.2 GHz measured at an input power of 10 dBm. This results in an attenuation constant of 1.8 dB/m of the transmission line. From these measurements the surface resistances R_s and the magnetic penetration depths λ_l were evaluated. The lowest r_s value was 105 μΩ at 77 K and at 6.2 GHz. The smallest magnetic penetration depth λ_l(0) was 160 nm. Additionally, for the first time trilayer structures with a dielectric thickness of 400 nm and 800 nm respectively were patterned into microstrip resonators. At 50 K we measured a quality factor of 85 at 4.7 GHz resulting in an attenuation of 26 dB/m and a surface resistance R_s of 340 μΩ. For thick dielectrics the upper YBCO films already contained significant portions of polycrystalline material.

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DC-magnetron sputtering of YBa2Cu3O7−δ thin films for microwave applications

Abstract

Physica C

Physica C

Physica C

Appl. Phys. Lett.

Fresenius J. Anal. Chem.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

DC-magnetron sputtering of YBa₂Cu₃O_7−δ thin films for microwave applications