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Superconducting YBa2Cu3O7−x (YBCO) thin films with a surface resistance of 0.86 mΩ at 10 GHz and 76 K have been grown on polycrystalline ferrite yttrium iron garnet (YIG) substrates. The chemical and structural mismatches between YBCO and YIG are solved by using a double buffer layer of biaxially oriented yttria-stabilized zirconia (YSZ) and CeO2, where YSZ is deposited by an ion-beam-assisted-deposition technique. The YBCO films are c axis oriented with an in-plane mosaic spread [full width at half maximum of an x-ray φ-scan on (103) reflection] of less than 8°. The films have a superconductive transition temperature above 88 K with a transition width less than 0.3 K, giving a critical current density above 106 A/cm2 in self field at 75 K. At 75 K in an external magnetic field of 1 T perpendicular to the film surface, the films maintain a critical current density over 2×105 A/cm2.

We have studied the microwave properties of 0.4 μm thick YBa2Cu3O7−x (YBCO) films on polycrystalline substrates with ion-beam-assisted-deposited yttria-stabilized zirconia buffer layers using a parallel-plate resonator technique at 10 GHz. The YBCO films with similar in-plane texture grown on both forsterite and Ni-based alloy substrates show similar microwave properties. We measure low-power surface resistance Rs values of about 0.5 mΩ at 76 K and 0.15 mΩ at 4 K for films with an in-plane mosaic spread of about 7°. Single-tone power-dependence measurements show that the surface resistance and the surface reactance increase linearly and by the same amount with increasing microwave field level. At intermediate power levels, the intermodulation measurements show odd-order intermodulation products that increase quadratically with two-tone input power. These results indicate a hysteretic vortex penetration mechanism in the weak links as the most plausible source of the observed nonlinearities in these films.

We present an experimental, analytical, and numerical investigation of the passage of electromagnetic signals through a device with voltage-dependent differential capacitance. This dependence gives rise to the device's nonlinear response, which can then be tuned by an externally applied static electric field. The system is modeled with a wave equation for the current and the charge density with continuity conditions at the boundaries between two linear regions and the nonlinear medium they sandwich. We derive asymptotic formulae for transmission and reflection coefficients of a monochromatic signal and its nonlinearity-induced second harmonics. Predictions based on this analysis are then compared with numerical and experimental results, across a range of parameter values, including those tuning the nonlinearity by means of an imposed voltage. The experiments are carried out at microwave frequencies using 1cm2 devices consisting of a superconducting thin film meandering waveguide on a nonlinear dielectric ...

We have used a parallel‐plate resonator technique to measure the microwave surface resist‐ ance Rs of YBa2Cu3O7−x (YBCO) films on buffered ceramic substrates at around 10 GHz, and studied the correlation between their Rs and materials properties. A 0.4‐μm‐thick YBCO film (with an in‐plane mosaic spread of 7°) grown on a polycrystalline alumina substrate with an ion‐beam‐assisted‐deposited yttria‐stabilized zirconia buffer layer showed an Rs of 1.89 mΩ at 76 K and 0.21 mΩ at 4 K. We have observed a strong correlation between the Rs of the samples and the in‐plane mosaic spread of the YBCO films. This correlation can be explained qualitatively in terms of a simple model in which the weak links between the grains of the YBCO film form an electrical network of Josephson junctions.

We have prepared an electrically tunable and adaptive 3‐pole half‐wave bandpass coplanar waveguide filter incorporating a 1.2‐μm‐thick paraelectric SrTiO3 bottom layer and a 0.4‐μm‐thick superconducting YBa2Cu3O7−x top electrode layer on a LaAlO3 substrate. By applying a separate bias voltage on each pole and also on each coupling capacitance of the device at 4 and 76 K, the filter response is not only fine tuned to achieve symmetric and optimized filter characteristics with less than 2% bandwidth centered around 2.5 GHz, but also broadband tuned to shift the passband by more than 15%.

Conductive RuO2 thin films have been heteroepitaxially grown by pulsed laser deposition on Si substrates with yttria‐stabilized zirconia (YSZ) buffer layers. The RuO2 thin films deposited under optimized processing conditions are a‐axis oriented normal to the Si substrate surface with a high degree of in‐plane alignment with the major axes of the (100) Si substrate. Cross‐sectional transmission electron microscopy analysis on the RuO2/YSZ/Si multilayer shows an atomically sharp interface between the RuO2 and the YSZ. Electrical measurements show that the crystalline RuO2 thin films are metallic over a temperature range from 4.2 to 300 K and are highly conductive with a room‐temperature resistivity of 37±2 μΩ cm. The residual resistance ratio (R300 K/R4.2 K) above 5 for our RuO2 thin films is the highest ever reported for such films on Si substrates.

We have prepared electrically tunable microwave resonators incorporating superconducting YBa2Cu3O7−x (YBCO) and paraelectric SrTiO3 (STO) layers on LaAlO3 substrates. The top YBCO layer for each sample was patterned into a 8 mm long coplanar transmission line with a 40 μm gap and a 20 μm center line width. The microwave transmission through the coplanar transmission line exhibits resonances corresponding to standing microwaves along the coplanar transmission line. These resonances are modulated by applying a bias voltage between the center line and the ground planes. Samples with a 0.5 μm thick (2 μm thick) bottom STO layer show, for a resonance at around 8 GHz (5 GHz), a frequency modulation of about 4% (24%) and a quality factor Q of about 200 (50) under 100 V bias at 80 K.

We have investigated buffer layers of Pr/sub 0.55/Y/sub 0.45/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (PY)BCO for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO)/SrTiO/sub 3/ field effect devices. The transitions of single unit cell thick YBCO films in (PY)BCO/YBCO/(PY)BCO trilayers depend strongly on the buffer layer thickness, with optimum T/sub c//spl ap/40 K obtained at a buffer layer thickness of 5nm. SuFET devices deposited on such an optimized buffer layer exhibit substantial improvements in channel quality. For a 3.6 nm thick channel, transition temperature >55 K and critical current density /spl sim/10/sup 6/ A/cm/sup 2/ are obtained. The modulations observed in both the normal and superconducting states are consistent with those obtained in earlier work on non-weak-link dominated samples. >

A pulsed laser deposition (PLD) oscillating target scanning technique, in which the laser ablates the target from opposing directions as the deposition proceeds, is described and compared to the conventional technique of target rotation over a cumulative ablation exposure of ∼1500 shots/site. Using conventional rotation the plume direction is eventually displaced by 6° while both the number density and size of PLD associated particles increase. In contrast, the oscillating technique results in a spatially stable plume (displacement

We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.

Meander lines have been utilized for characterizing laser-deposited, thin-film, high-T/sub c/ superconducting material. The dispersion characteristics of the meander line have been modeled and the power dependence of high-T/sub c/ materials as a function of frequency and temperature has been investigated. Measurements showed that magnetic fields as low as 0.1 Oe will affect the superconducting material, decreasing the Q of the meander lines. The Q deterioration is temperature and frequency dependent and the latter is less pronounced at higher frequencies. >

In this work, we have used the Warren-Averbach method to analyze the x-ray diffraction patterns of several Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/ (BST) epitaxial thin films deposited via 90/spl deg/ off-axis rf-magnetron sputtering. This method allows for the determination of the coherent strains, incoherent strains, crystallite sizes, and defect densities in these films. Results indicate that for BST (60/40) films, the incoherent strain normal to the substrate is smaller on LAO compared to MgO for films 200 /spl Aring/ - 3000 /spl Aring/ thick. However, the opposite relationship holds for the coherent strains. The thickest films showed dielectric constants of 1350 and 925 on MgO and LAO, respectively (at 1 MHz). All showed dielectric loss on the order of 0.007 with the room temperature tunability being 3 times greater for the films on MgO. In-plane measurements of the coherent strain were also completed, showing good agreement with theoretical predictions of the effects of strain on the dielectric properties.

Thin films of Sr2AlTaO6 (SAT) and multilayers of YBa2Cu3O7 (YBCO)/SAT have been grown by pulsed laser deposition on [001] LaAlO3 substrates. X‐ray diffraction shows that SAT grows on [001] LaAlO3 with the c‐axis oriented normal to the substrate plane. X‐ray rocking curve and Rutherford backscattering channeling measurements on SAT films yield full width at half maximum of

A multilayer high Tc superconducting field‐effect transistor‐like structure was made from ultrathin YBa2Cu3O7−x films. An epitaxially grown dielectric SrTiO3 insulation layer, which had a forward bias breakdown voltage of about 20 V, allowed an electric field induced change in the channel layer of 1.25×1013 carrier/cm2 per volt of the gate voltage. A significant modulation of the normal state and superconducting properties was observed in samples with YBa2Cu3O7−x channel layers of a few unit cells thick. By applying gate voltage of different polarities, Tc was both suppressed and enhanced by ∼1 K. The resistance was modulated by as much as 20% in the normal state and by over 1500% near the zero resistance temperature.

We report on ex situ fabrication of gold contacts onto patterned YBa2Cu3O7−x films. Gold seed layers were blanket sputtered onto photoresist defined over the patterned films. In a second photo process, openings were aligned to the original pads and 8‐μm‐thick contacts were electroplated. The two layers of photoresist and sputtered gold were removed leaving defined contacts. We obtained specific contact resistance of 2.6×10−7 Ω cm2 on post‐annealed samples at 79 K. Microwave filters with contacts showed 4.2 dB loss at 9 GHz and 77 K, while comparable filters in copper showed 18 dB loss.

Using in situ deposited thin‐film Y1Ba2Cu3O7−δ on LaAlO3 substrates, we have developed a pseudo‐interdigital three‐pole stripline filter with a minimum passband loss under 3.5 dB at 9.5 GHz and 77 K. The passband is very narrow, having a bandwidth of only 50 MHz (0.5%). The filter occupies a volume of less than 0.1 cm3. An equivalent copper filter produced a loss of 18.5 and 28 dB at 77 and 300 K, respectively. The input and output coupling is achieved by ex situ sputtered Au thin‐film contacts defined on the Y1Ba2Cu3O7−δ by a lift‐off technology and ultrasonically bonded to hermetic K connectors. Some of the major concerns in the realization of narrow band filters such as air gaps between high dielectric constant substrates and uniformity of the substrate’s dielectric constant are addressed.

We report the crystal structures and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3 (BST), films deposited on LaAlO3 substrates using pulsed laser deposition, where x = 0.1 to 0.9 at an interval of 0.1. We have found that when x < 0.4 the c-axis is parallel to the plane of the substrate but normal as x approaches 1. Temperature-dependent capacitance measurements at 1 MHz show that the capacitance has a peak and that the peak temperature decreases with increasing x. We have found that the peak temperatures of the films are about 70 °C higher than those of bulk BSTs when x < 0.4. From room-temperature capacitance (C) vs applied voltage (V) measurements, we have found that the C-V curves of the BST films exhibit hysteresis except for x = 0.9 and that the peak voltage at which the capacitance becomes maximum decreases with increasing x. At room temperature, the Ba0.6Sr0.4 TiO3 film exhibits the largest capacitance tunability (≈ 37%) with an applied electric field of 40 kV/cm.

We have used a coplanar waveguide structure to study broadband electrodynamic properties of single-crystal and thin-film strontium titanate (STO), and thin-film barium strontium titanate (BSTO). We have implemented low-frequency capacitance (100 Hz - 1 MHz), swept-frequency transmittance (45 MHz - 4 GHz), and time-domain transmittance (dc - several GHz) measurements to determine effective refractive index (or, dielectric constant), and dissipation factor (or, loss tangent) as a function of dc bias (up to 4×106 V/m) and temperature (20 - 300 K). The STO samples used superconducting electrodes and were designed to operate at cryogenic temperatures, whereas BSTO samples used normal conducting electrodes and exhibited optimal performance around room temperature. By using nearly identical electrode geometries for all devices, we were able to conduct a direct comparative study among them, and investigate not only single-crystal vs thin-film, but also cryogenic vs room-temperature applications.

The low frequency dielectric properties of epitaxial SrTiO3 thin films deposited on LaAlO3 are presented. The films were deposited using radio-frequency magnetron sputtering from stoichiometric targets in an Ar/O2 atmosphere. For the first time, the effects of in situ ozone annealing during the early stages of deposition were explored. X-ray diffraction results indicated that the ozone treatment resulted in more symmetric and sharper diffraction peaks (2 Θ- FWHM decreased from 0.17° to 0.10°). In addition, the peaks for the ozone treated samples were shifted in 2 Θ towards values approaching the bulk value. Rutherford backscattering measurements showed Sr/Ti ratios of 1:1 for these samples, indicating these peak shifts are not due to compositional variations. The dielectric constant of the ozone treated samples increased from 275 at room temperature to 1175 at 22 K (measured at 100 kHz). The effective loss tangent of the device remained between 1 × 10−4 and 1 × 10−3 down to 100 K, where it began to increase. The tunability was also measured. The ozone treated sample showed tunability of 46%, 43% and 38% at 22 K, 40 K and 60 K, respectively. Finally, similar measurements were completed at 1 MHz, indicating a minimal dependence of these properties on frequencies in this range.

We have prepared electrically tunable and active microwave devices incorporating (superconducting YBa2Cu3O7-x)/(nonlinear dielectric SrTiO3) or (normal metal Au)/(nonlinear dielectric Sr0.5Ba0.5TiO3) bilayers. The dielectric layer thickness for these samples varied between 0.5 μm and 2 μm. The top electrode layer for each sample was patterned into a coplanar waveguide device structure. We have configured these devices as voltage-tunable resonators, voltage-tunable phase shifters, voltage-tunable mixers, and voltage-tunable filters. Under dc voltage bias, these prototype devices have exhibited up to 30% resonant frequency modulation, about 1°/mm-GHz phase shift, more than 40 dB change in mixed microwave power, and finetunable symmetric filter profile with less than 2% bandwidth and more than 15% adaptive range.

We present a brief review of results on the surface impedance of cuprate superconductors, focusing mainly on YBa2Cu3O7-(delta ) (YBCO) and evidence of d-wave superconductivity in that material. We then discuss our recent results on Ba-K-Bi-O thin films, and the effects of DC electric fields on the surface impedance of YBCO films. A summary of our data on high quality thin films and single crystals of the electron-doped Nd1.85Ce0.15CuO4-(delta ) (NCCO) cuprate superconductor follows. Surprisingly, the measurements on NCCO are consistent with the behavior of an s-wave BCS superconductor, in striking contrast to recent results on YBCO. Finally we discuss some of the interesting potential implications of d-wave superconductivity for microwave applications of the cuprates.

We have studied the feasibility of a d. c. voltage bias controlled superconducting microwave lumped-circuit capacitor as an active device. A series-LC circuit was designed and fabricated using YBa2Cu3O7/SrTiO3 films. When a d. c. bias of 35 V was applied across the STO film at ~20 K, the total series capacitance of the circuit, measured at 1 MHz by an LCR meter, decreased by ~3% and the microwave resonant frequency of ~14 GHz increased by ~1.4%, in consistence with the predictions of a simple series-LC resonator model.

A high-Ta field-effect transistor-like structure (SuFET) was made which consisted of an ultrathinYBa2Cu3O (YBCO) film, a dielectric SrTiO3 layer and a gold gate electrode. The use of ultrathin filmsof a few unit cells thickness and an epitaxially grown dielectric layer allowed a relative change in the arealcarrier density of YBCO in excess of 20%. A comparable amount of modulation was obtained in normal

Recent accomplishments such as the demonstration of low microwave surface resistance over a wide range of frequencies and low magnetic flux noise in high quality thin films of the 90 K superconductor YBa2Cu3O7-x (YBCO), have heightened the hope that the new high temperature superconductors (HTSC) can play an important technological role. With improvement in the quality of films, very exciting superconducting state properties which till now were masked by inhomogeneities introduced by the fabrication processes, are beginning to be uncovered. Progress in these areas has been easiest for those in the community who initially chose to focus on understanding the materials issues relating to the growth of high quality thin films and single crystals. Current techniques for studying these materials include measurements of the high frequency response of single layer YBCO films in order to reveal the intrinsic properties of the superconductors and gauge their suitability for technological applications. By alloying the superconductors in the cation sites, we can also extract information on the mechanisms of superconductivity in these systems, generate new families of lattice matched materials, such as the Y1-yPryBa2Cu3O7-x system which for y=0 to 1 possess a variety of electrical properties and can form building blocks for multilayer junctions or superlattices.

The authors report studies of the effect of grain alignment on interface trap density of thermally oxidized aligned-crystalline silicon (ACSi) films by means of capacitance-voltage (C-V) measurements. C-V curves were measured on metal-oxide-semiconductor (MOS) capacitors fabricated on ⟨001⟩-oriented ACSi films on polycrystalline substrates. From high-frequency C-V curves, the authors calculated a decrease of interface trap density from 2×1012to1×1011cm−2eV−1 as the grain mosaic spread in ACSi films improved from 13.7° to 6.5°. These results demonstrate the effectiveness of grain alignment as a process technique to achieve significantly enhanced performance in small-grained (⩽1μm) polycrystalline Si MOS-type devices.

We studied the dependence of carrier mobility on grain mosaic spread for ⟨001⟩-oriented, 200-to-400-nm-thick Si thin films grown on polycrystalline metal substrates. The Hall mobility increased from 1% to 23% of that in bulk single-crystal Si with decreasing grain mosaic spread from 14° to 2°. For the same range of parameters, a model combining intragrain and grain boundary scattering yielded a decrease of the energy barrier height from 0.1eV to less than 10−3eV and an accompanying decrease of trap density from 6×1011cm−2 to less than 3×1010cm−2. These results demonstrate that, for polycrystalline Si films, improving the intergrain alignment is an effective and practical alternative to increasing the grain size to achieve enhanced mobility.