Critical current characteristics of YBa2Cu3O7 thin films on (110) SrTiO3

The material and electrical characteristics of YBa₂Cu ₃O₇ (YBCO) thin films deposited by inverted cylindrical magnetron sputtering on (110) SrTiO₃ (STO) were investigated. X-ray diffractometry shows the grain orientations to be predominantly the YBCO (110) and (103) with no evidence of c-axis grains, Electron micrographs show the film surface to consist of coupled elongated grains parallel to the (110) STO edge. The films were patterned into small 2.5 mm squares parallel to the substrate edges for electrical characterization. Transport currents parallel and perpendicular to the (110) substrate edge showed a 945:1 anisotropy in film resistance and a factor of two in critical current density for temperatures below 60% of the transition temperature (T_c). The temperature dependence of the critical current near T_c was quadratic-like and strongly dependent on the value of T_c used in the analysis. For the two orientations, there was nearly a 6 K difference in T_c as determined by the point at which the critical current became zero. The response of the critical current to small magnetic fields was greater for transport current along the c-axis direction and was observable over a temperature interval nearly four times greater than for current along the basal plain. These YBCO thin films have good response to small magnetic fields and are suitable for vortex flow device development     

Critical current density of Na doped YBCO thin films

Superconducting Y-Ba-Cu-O (YBCO) and Y-Ba-Na-Cu-O thin films were deposited on MgO substrate using a resistive evaporation technique. BaF₂ and NaF powder were used as source materials for Ba and Na elements, respectively, and pure small grains for the Y and Cu elements. In situ heat treatment has been carried out at low oxygen partial pressure. The critical current density of Na doped YBCO film was measured to be significantly higher than that of pure YBCO film.     

Possible three-terminal device with YBCO angle grain boundary

The DC and AC Josephson effects were observed in YBa₂Cu ₃O_7-a (YBCO) bridges with metal-organic chemical vapor deposited (MOCVD) thin films on (100)MgO substrates. In TEM observations and X-ray analysis, the angle grain boundaries (AGBs) in these films were observed. Josephson junction resulting from the AGBs showed the semiconducting barrier junction property dominantly, and the AGBs consisted of YBCO with oxygen deficiency. Based on these results, the authors propose a three-terminal device with the Josephson junction. The Josephson junction can be prepared by depositing an epitaxial YBCO thin film with one AGB on a bicrystal (100)SrTiO₃ or (100)Si substrate, and the insulated gate for the application of electric field to the AGB is formed over the junction. The supercurrent in the Josephson junction is controlled by the electric field effect of the semiconducting AGB in the junction     

Preparation and characterization of YBaCuO high Tcsuperconducting thin films

For thin film metalorganic chemical vapor deposition (MOCVD), a stoichiometric gas phase can be obtained from a mixture of the precursors in the desired mole ratios, in spite of differences in the volatilities of the individual compounds. Proper film composition is obtained by controlling the velocity of a carriage containing the precursors through the heating zone of a vaporizer. YBaCuO (YBCO) superconducting films were prepared on yttrium stabilized zirconia (YSZ) (001) crystal substrates by single source MOCVD using a mixture of powders of Y(thd)₃, Ba(thd)₂ and Cu(thd)₂ as sources. Nondestructive measurements of resistivity, critical current, composition, and surface morphology were performed on these films. A single crystal film with T_co=89 K was made under optimum conditions. Its homogeneity was investigated by resistivity and T_co measurements and by atomic force microscopy (AFM) at several points of the surface along a diameter of this film. The roughness of the film is higher at the edge than at the center, and this appears to be correlated with the normal state resistivity. The c-axis and a-axis YBCO phases compete with each other at substrate temperatures ranging from 750 to 820°C. The c-axis YBCO phase shows spiral growth structure and the spiral step height is about 12     

Development of planar microwave filters from double sided YBCO thin films on MgO substrates

Planar microwave filters patterned from thin film HTS on low loss substrates can offer waveguide-like performance at a fraction of the component mass and volume. First results are presented for a microstrip filter at 6.2 GHz fabricated from YBCO thin films deposited onto both sides of an MgO substrate. The measured results are compared with identically patterned filters assembled from gold and YBCO single sided films.<>     

Increasing the current density of dielectric-base transistors withan MgO emitter-base barrier

We measured the current-voltage characteristics of YBa₂Cu₃O_7-x/oxide/n-SrTiO₃ diodes using NdGaO₃, LaAlO₃, CeO₂, and MgO as the oxide. MgO films had the highest current density. We then fabricated dielectric-base transistors with a YBa₂Cu₃ O_7-x(YBCO) emitter/collector on a SrTiO₃ dielectric base with an MgO barrier. The transistors had both voltage and current gains exceeding unity at 4.2 K. The emitter current density was about 4×10³ A/cm² at a collector-emitter voltage of 10 V and base-emitter voltage 10 V; this is 2 to 3 orders of magnitude larger than that of transistors with NdGaO₃ emitter-base barrier. We obtained a transconductance of around 0.4 mS at a collector-emitter voltage of 10 V for a device with a 6-μm-diameter emitter     

The coplanar resonator technique for determining the surfaceimpedance of YBa2Cu3O7-δ thinfilms

We describe how coplanar microwave resonators fabricated from patterned thin films of YBa₂Cu₃O₇(-δ) (YBCO) can he used to measure the ab-plane microwave surface impedance Z_s=R_s+jX_s of the films, in particular the absolute value and temperature dependence of the magnetic penetration depth λ. The current distribution of the resonator is calculated by modelling the resonator as a network of coupled transmission lines of rectangular cross-sections; this is then used to estimate the ab-plane λ(T) from the measurements of resonators of different geometries patterned onto the same film. We obtain values of λ(0) in the range 150-220 nm. The unloaded quality factors of the linear resonators at 7.95 GHz are around 45000 at 15 K and around 6500 at 77 K. We estimate the corresponding values of the intrinsic R_s at 7.95 GHz to be 23 μΩ and 110 μΩ at 15 K and 77 K, respectively. These values are comparable with those of other high quality unpatterned YBCO films reported in the literature. Z_s for the best optimised films appears to be insensitive to the effects of patterning     

Substrate and Device Pattern Dependence of the Thermal Crosstalk in Y Ba2Cu 3O7−δ Transition Edge Bolometer Arrays

Using YBa₂Cu₃O_7-delta (YBCO) thin films, pulsed laser deposited on 1-mm-thick LaAlO₃ or SrTiO₃ substrates, we made 4times1 pixel arrays of transition edge bolometers with separations between neighboring pixels ranging from 40 mum to 170 mum for testing purposes. We investigated the effects of the YBCO film thickness (200 and 400 nm), substrate material, and back-etching of the substrate, on the crosstalk between the pixels of the arrays. The investigation was based on the analysis of the voltage response of the dc current biased bolometers versus the modulation frequency of a near-infrared laser source. We observed that the bolometer arrays made of 400-nm-thick films had less interpixel thermal crosstalk than the 200-nm-thick films. The effect of substrate thickness on the response of the pixels was investigated by up to 500 mum back-etching of the substrates. The bolometers made on back-etched LaAlO₃ substrates had anomalous crosstalk response behavior, which was effective at higher modulation frequencies. In addition, we present an analytical thermal model for explaining the observed effects of the thermal crosstalk on the response characteristics of the pixels of the arrays. We report the measured response and the anticipated thermal crosstalk of the characterized bolometers'. We describe the responses based on the thermal models and discrepancies from the model's predictions     

High current pressure contacts to Ag pads on thin filmsuperconductors

High current, low resistance, nonmagnetic, and nondestructive pressure contacts to Ag pads on YBa₂Cu₃O_7-δ (YBCO) thin film superconductors were developed in this study. The contact resistance reported here includes the resistance of the current lead/Ag pad interface, the Ag pad/YBCO interface, and the bulk resistance of the contact material. This total contact resistance is the relevant parameter which determines power dissipation during critical-current measurements. It was found that regardless of the optimization of the Ag pad/YBCO interface through annealing, a pressure contact can yield a lower total resistance than a soldered contact. The lowest resistance obtained with pressure contacts was 3 μΩ (for a 2×4 mm ² contact). These contacts may be useful for many different high temperature superconductor (HTS) studies where high-current contacts with low heating are needed     

On the electrical resistivity and chemical etching of TiSi2 thin films sputtered from a composite target

The electrical resistivity of TiSi₂ thin films sputtered onto an oxidised Si substrate using a composite alloy target is studied. It is found that the as-deposited films show high resistivity. Annealing the films at an elevated temperature leads to a significant fall in the resistivity. An optimum sheet resistance of 2om tq⁻¹ is obtained after annealing at 800°C for 30 min in argon ambient. The effect of annealing temperature on resistivity is studied. The sheet resistance is also found to be affected by the magnitude of the substrate bias during film deposition. The data are given. The patterning of TiSi₂ thin films by wet chemical etching for device applications is described.     

Buffer layers for high-quality epitaxial YBCO films on Si

Efforts aimed at producing device-quality YBa₂Cu₃ O_7-δ (YBCO) films on Si, which have resulted in films with properties comparable to what can be achieved with conventional oxide substrates such as SrTiO₃, are described. It is reported how epitaxial YBCO films were grown on Si(100) using an intermediate buffer layer of yttria-stabilized zirconia (YSZ). Both layers are grown with an entirely in situ process by pulsed laser deposition (PLD). Ion channeling revealed a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal state resistivity was 250-300 μΩ-cm at 300 K; the critical temperature, Tc (R=0), was 86-88 K, with a transition width of 1 K. Critical current densities of 2×10⁷ at 4.2 K and 2.2×10⁶ at 77 K have been achieved. Noise measurements indicate that these films are suitable for use in highly sensitive far-infrared bolometers. Applications of this technology to produce in situ reaction patterned microstrip lines are discussed     

Role of interdomain and interplatelet boundaries on magnetic fluxpenetration in melt grown YBCO

The microstructure of a melt processed YBCO sample reveals the presence of random domains with aligned platelets separated by microcracks inside the domains. The lower critical field (H_c1) and critical current density (J_c) of these boundaries will be less than that of the grains, and their role in determining the bulk properties cannot be neglected. We have employed a sensitive lock-in flat-band detection technique and recorded the low field ac M-H loops at 33 Hz and 77 K and derived the flux profiles. The results show an enhancement in H_c1 due to melt processing. Inclusion of Y₂BaCuO₅, and Ag enhances the H_c1 further. The flux profiles show distinct slope changes at certain fields which reflect different coupling strengths of weaklinks that become operative for flux entry at different applied fields. While the low field flux profiles are found to reflect the effect of residual weaklinks (like liquid phase and micropores) and domain boundaries, those at higher fields might reflect the effect of platelet boundaries     

Sensitivity of Rs-measurement of HTS thin films by threeprime resonant techniques: cavity resonator, dielectric resonator, andmicrostrip resonator

The microwave surface resistance, R_s measurement of YBa ₂Cu₃0_{7 -δ} (YBCO) thin film deposited on 10 mm × 10 mm LaAlO₃ substrate using three prime resonating techniques, namely, cavity end plate substitution technique (20 GHz), dielectric resonator technique (18 GHz), and microstrip resonator technique (5 GHz), is reported. In addition, theoretical analysis for each technique has been performed to calculate the relative percentage error in the measured R_s -value of the YBCO thin film as a function of temperature. It has been found that the shielded dielectric resonator provides far better sensitivity for R.-measurement of the YBCO thin film with minimum relative percentage error (<4%) in the temperature range from 20 K to transition temperature of YBCO thin film compared to the other two techniques     

Pt-based metallization of PMOS devices for a compatible monolithic integration of semiconducting/Yba2Cu3O7−δ superconducting devices on silicon

Mo, Pt, Pt/Mo and Pt/Ti thin films have been deposited onto Si and SiO₂ substrates by RF sputtering and annealed in the YBa₂Cu₃O_7−δ (YBCO) growth conditions. The effect of annealing on the sheet resistance of unpatterned layers was measured. A Pt-based multilayered metallization for the PMOS devices was proposed and tested for a compatible monolithic integration of semiconducting devices and YBCO sensors on the same silicon substrate. The best results were obtained with a Pt/Ti/Mo-silicide structure showing 0.472 Ω_□ interconnect sheet resistivity and 2×10⁻⁴ Ω cm² specific contact resistivity after annealing for 60 min at 700 °C in 0.5 mbar O₂ pressure.     

Improvement of sensor performance of high-TC thin filmplanar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y₁Ba₂Cu₃O_7-x (YBCO) dc SQUID gradiometers on 24° bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter β_L from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I_CR_N product with values between 300 and 400 μV at 150-nm film thickness changing to values near 150 μV at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4×8 mm² dc SQUID gradiometer the authors obtained a white noise level of 4.2 μΦ₀/√Hz corresponding to a field gradient sensitivity of 430 fT/cm√Hz at 77 K after the trimming process     

Voltage tunable capacitors using high temperature superconductorsand ferroelectrics

In this paper we report the construction of both slot capacitors on bulk substrate and thin film interdigital capacitors using YBa₂ Cu₃O_7-δ (YBCO) and Ba_xSr _1-xTiO₃. Slot capacitors made on bulk Ba_x Sr_1-xTiO₃ (BST) yielded variations in capacitance of more than 6 to 1 at 86 K with a peak electric field strength of 25 kV/cm. With a metal organic deposition (MOD) grown Ba_xSr_1-xTiO₃ 300-nm overcoat on a YBCO thin film, an interdigital capacitor on LaAlO₃ substrate yielded an approximate tuning range of 10% for peak field of 66 kV/cm over temperatures ranging from 50 K to 120 K     

DC characteristics of patternedYBa2Cu3O7-x superconducting thin-filmbolometers: artifacts related to Joule heating, ambient pressure, andmicrostructure

Joule heating due to the bias current and resistance of the material in patterned YBa₂Cu₃O_7-x, superconducting films on 250-500-μm-thick MgO, LaAlO₃, and SrTiO₃ crystalline substrates, results in a number of effects: (1) a temperature rise in the film with respect to the measured temperature at the bottom of the substrate; (2) a possible thermal runaway, which may be local or uniformly distributed in the film, depending upon the dimensions of the superconducting pattern relative to that of the substrate; (3) an apparently sharper normal-to-superconducting transition in the measure R versus T curve; and (4) decrease of T_c to 60 K (ΔT_x>20 K) after being subjected to high-bias currents j~10⁵ A/cm² under vacuum, with recovery of T_c after exposure to room atmosphere. The magnitude of R at T_c*onset is found to be dependent on bias current in granular samples, with a lower R at currents higher than some on-set value. The slope of R versus T in the transition region in our granular samples is found to be lower at higher bias currents, since the widening of the transition overcomes the shift caused by the Joule heating. These various phenomena impact the responsivity of bolometers made from these films, as well as the predictions of possible attainable responsivity and speculations of mechanisms occurring in the films. In particular, misinterpretation of the Joule heating sharpening of the R versus T curve has led to predictions of responsivities over one order of magnitude higher than are justified, and shifts in properties of the films due to heating have been misinterpreted as nonequilibrium responses of the films     

High-temperature superconductor resonators and phase shifters

High-T_c resonators and hybrid digital phase shifters have been designed, fabricated, and tested. The YBa₂Cu₃O_7-δ (YBCO) films used were off-axis sputtered onto 0.5-mm-thick [100] LaAlO₃ substrates and have surface impedances at 10 GHz as low as 20 μΩ at 4.2 K and 300 μΩ at 77 K. The dielectric constant of the LaAlO₃ substrates was measured using straight-line and ring resonator techniques. The superconductor straight-line resonator, which uses silver as its ground plane, has a moderately high Q factor and has an electromagnetic feedthrough level below -65 dB up to 10 GHz. The authors also report the first demonstration of a semiconductor/superconductor microwave digital phase shifter. YBCO film was used to form the circuit, with semiconductor p-i-n diodes serving as switches. A 4-b superconductor phase-shifter design is also presented along with simulation results that indicate maximum total insertion loss (which occurs with all bits forward-biased) at 77 K to be 1.1 dB at 10 GHz     

Microstrip ring resonator technique for measuring microwaveattenuation in high-Tc superconducting thin films

Microwave attenuation of high-T_c superconducting (HTS) films sputtered on MgO and ZrO₂ were measured using a microstrip ring resonator circuit. The results of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O resonators were compared to those for gold-plated resonators of identical design. The losses of superconducting and gold-plated films were determined from unloaded Q-factor measurements. The attenuation of Y-Ba-Cu-O film on an MgO substrate is approximately 31% lower than that of gold films at 6.6 GHz and 33% lower at 19.2 GHz for temperatures below 50 K. The approach of using microstrips to characterize microwave losses shows the usefulness of HTS films in integrated circuit technology     

Temporal stability of circulating current in thin film Tl2Ba2CaCu2O8 and YBa2Cu3O7

The temporal stability of trapped transport current in annular thin film Tl₂Ba₂CaCu₂O₈ (TBCCO) and YBa₂Cu₃O₇ (YBCO) wafers has been accurately measured and has been found to be of suitable quality for the stringent requirements of nuclear magnetic resonance (NMR) magnets. No detectable decay, to the limit of the experimental apparatus (2*10^-14 Ω), was detected in those wafers with transport current at or below the critical current density J_c. The critical current density, as previously determined from 12 μm meander lines, was confirmed in a wafer with a width of 1.9 cm. The profile of trapped magnetic field resulting from induced current was modeled in order to assess its effect on the uniformity of an NMR magnet