H. Kinder

YBCO films on metal substrates with biaxially aligned MgO buffer layers

We successfully deposited high quality YBCO films on metal tapes with biaxially aligned MgO buffer layers. e-gun evaporation on inclined substrates with a very high deposition rate of up to 250 nm/min was used for the deposition of the buffer layers. The MgO layers are biaxially textured with an in-plane full width at half maximum of 80. The MgO [001]-axis is not parallel to the substrate normal but tilted significantly towards the deposition direction. YBCO films were deposited on ISD buffer layers by reactive thermal co-evaporation. The critical current density of the films was found to be anisotropic due to the tilt of the [001]-axis. Critical current densities up to 7.9/spl times/10/sup 5/ A/cm/sup 2/ at 77 K were achieved.

Low‐temperature preparation of superconducting YBa2Cu3O7−δ films on Si, MgO, and SrTiO3 by thermal coevaporation

We used simple thermal coevaporation of yttrium, barium, and copper in an oxygen atmosphere to produce thin films of the 90 K superconductor, YBa2Cu3O7−δ, on silicon, magnesium oxide, and strontium titanate. The films were deposited on 650 °C substrates and were superconducting when removed from the evaporator. A short post‐anneal at 480 °C produced films on bare Si with transition temperatures (R=0) of 85 K. Films as thin as 0.13 μm, superconducting at 91 K (on MgO) and 89 K (on SrTiO3), were obtained with no post‐anneal.

Homogeneous high quality YBa2Cu3O7 films on 3″ and 4″ substrates

Abstract The fabrication of homogeneous high quality YBa 2 Cu 3 O 7 films on large area substrates is the first essential step on the way to commercial high temperature superconductor devices. We have succeeded in growing YBa 2 Cu 3 O 7 films with excellent electronic properties and outstanding homogeneity in thickness and composition up to 4″ diameter. This was possible by employing reactive co-evaporation in combination with a special substrate heater design: a rotating disk holder separates oxidation and deposition enabling a reactive high oxygen pressure zone in a surrounding high vacuum background. Epitaxial YBa 2 Cu 3 O 7 films were grown either directly on MgO (30 × 30 mm 2 ) and LaAlO 3 (3″) or with intermediate buffer layers on sapphire (4″), silicon (4″) and GaAs (3″). For GaAs substrates we developed a chemical encapsulation to avoid contamination by volatile arsenic. On all of these substrates we obtained zero resistance temperatures around 86 K and critical current densities j c (77 K) ⩾ 2 × 10 6 A/cm 2 .

Epitaxial YBa2Cu3O7−δ films on silicon using combined YSZ/Y2O3 buffer layers: A comprehensive study

Abstract Epitaxial YBa 2 Cu 3 O 7−δ films with excellent structural and electronic properties were grown on silicon by reactive thermal coevaporation employing a combination of yttria-stabilized zirconia (YSZ) and Y 2 O 3 as a buffer layer. To provide optimum conditions, the YSZ growth was studied over a wide range of deposition parameter space. For the first time the effectiveness of the buffer has been derived quantitatively from SNMS profiles. Although the buffer layer can solve the problem of interdiffusion, the large differential thermal expansion between silicon and YBa 2 Cu 3 O 7−δ cannot be compensated in any way. In many cases this results in a fracture of films thicker than 70 nm and a fast degradation which imposes severe restrictions on the technical applications of silicon substrates. A thorough study of crack formation led to the conclusion that the theoretical limitation has not yet been reached and that there is still hope to overcome this problem.

YBCO film deposition on very large areas up to 20×20 cm2

Abstract In the last decade we have developed thermal reactive co-evaporation as a technique to produce high quality YBCO and other oxide films of very large size up to 9 inches in diameter. This was achieved by intermittent deposition and reaction with oxygen using a heater which rotates the substrate in and out of an oxygen pocket. Even larger substrates, e. g. coated conductors, cannot be rotated. Therefore we have recently developed a new setup where the substrate is held fixed, and the oxygen pocket is set in linear reciprocation. This technique allows simultaneous deposition on a square of 20×20 cm2. Moreover, we have developed an instant refill mechanism for the thermal boats, and stable rate control by atomic absorption spectroscopy (AAS), in order to obtain a continuous process suitable for small scale mass production.

Fault current limiting properties of YBCO-films on sapphire substrates

We have studied the switching of YBCO thin film resistive fault current limiting devices. Films of 300 nm thickness were deposited on 2 inch and 4 inch sapphire substrates by thermal co-evaporation. Bridges 10 mm wide and 22 mm long (2 inch) or 42 mm long (4 inch) were structured by standard photolithography. Contacts were made by in-situ gold overlayers and soft solder. The gold film was removed from the switching area so that the YBCO film was not shunted. The films were tested by 30 /spl mu/s DC pulses and 50 Hz AC pulses for 50 ms. We find evidence that at the AC tests heat propagates over several cm under these conditions in sapphire so that hot spots can be avoided even without shunt layer with the prospect of higher switching power. The highest destruction free switching power-the RMS critical current times RMS voltage after switching-was 57 kVA. The highest switching power density achieved was 2.5 kVA/cm/sup 2/. To our knowledge, these are the highest switching power densities observed so far.

Third-order nonlinear microwave response of YBa2Cu3O7−δ thin films and single crystals

The nonlinear microwave response of superconducting YBa2Cu3O7−δ thin films and crystals was studied quantitatively by measuring the third-order harmonic amplitude E3ω (ω: 35 GHz, jRF⩽3×104 A/cm2). We find E3ω to be dominated by two extrinsic effects: (1) the interaction with flux lines in weak links, leading to an increased nonlinear response at low temperatures, and (2) rf related flux line creation at film edges, yielding a high E3ω-signal close to Tc. Both effects together lead to a minimum in E3ω around 40–60 K, which makes this temperature interval interesting for rf applications. Intrinsic nonlinearities are smaller by orders of magnitude and could not be detected.

Large area deposition of YBa2Cu3O7 films by thermal co-evaporation

Abstract Reactive evaporation techniques cannot be simply scaled up for larger YBCO films because the zone of enhanced O2 pressure in front of the substrate will also increase in height and is impenetrable for the metal vapours. We have circumvented this problem by separating the evaporation and oxidation zones. Smooth films without any precipitates on a 10×10 μm2 scale could be fabricated on 30×30 mm2 MgO substrates. The homogeneity of the stoichiometry and of the thickness was 2%. Good normal state electrical properties have been achieved: ρ(300 K )/ρ(100 K ) = 3.0 and ρ(100 K ) = 40 μΩ cm . The superconducting transition temperature was typically 87 K with a variation of ΔTc ≤ 0.5 K over the sample area while Jc at 77 K varied between 1 and 6·106 A/cm2. The microwave surface resistance at 77 K was found to be R eff = 60 mΩ at 87 GHz . In addition, we deposited YBCO films on 3 inch LaAlO3 wafers with Tc = 86 K ± 1 K and tested the feasibility of deposition of YBCO films on 4 inch silicon wafers with a YSZ buffer.

Commercial coated conductor fabrication based on inclined substrate deposition

Due to the complex layered architecture the fabrication of coated conductors consists of a multitude of processing steps. We present a viable and economic route to produce second generation HTS tape from the initial substrate preparation to the final metal coating. The most important and technically challenging steps are the deposition of an oriented buffer layer and the superconductor film in a reel to reel configuration. New evaporation techniques have been developed to enable reliable, high rate tape coating. Highly oriented MgO-buffer layers are realized by inclined substrate deposition (ISD) yielding an in-plane orientation of 7/spl deg/-8/spl deg/ FWHM and critical current densities up to 2.5 MA/cm/sup 2/. Buffered tape has been fabricated up to 40 m length. The subsequent HTS deposition has been performed up to 12 m and reaches current levels of 200 A/cm. High resolution critical current scans help to reveal defects and to track problems in the manufacturing process.

Epitaxial growth of YBa2Cu3O7 films on GaAs with MgO buffer layers

We have grown epitaxial YBa2Cu3O7 films on GaAs using MgO buffer layers similar to Fork, Nashimoto, and Geballe [Appl. Phys. Lett. 60, 1621 (1992)]. However, the YBa2Cu3O7 was deposited by thermal coevaporation at an even lower substrate temperature of 620 °C. We obtained substantially improved quality [Tc=86.8 K, jc(77 K)=1.2×106 A/cm2, ρ(100 K)≤100 μΩ cm] and smooth surfaces free of outgrowth. An important aspect is preventing As contamination from the gas phase. This was achieved by encapsulating the free sample surface.