B. W. Tao

Dielectric properties and tunability of cubic pyrochlore Bi1.5MgNb1.5O7 thin films

The Bi1.5MgNb1.5O7 thin films with cubic pyrochlore structure were prepared onto Pt-coated sapphire substrates by rf magnetron sputtering deposition from a stoichiometric target. Dielectric measurements indicated that the Bi1.5MgNb1.5O7 thin films exhibited low dielectric loss of ∼0.0018–0.004, medium dielectric constant of ∼86, and superior tunable dielectric properties at room temperature. A bias field of 1.6 MV/cm resulted in the maximum voltage tunability of 39%. A brief discussion is given on the enhanced tunability compared to Bi1.5ZnNb1.5O7 thin films. The low loss and superior tunability make Bi1.5MgNb1.5O7 thin films promising for potential tunable capacitor applications.

Enhancement of flux pinning of TFA-MOD YBCO thin films by embedded nanoscale Y2O3

YBCO films with different levels of excess yttrium were prepared on single-crystal LaAlO3 with metal?organic deposition using trifluoroacetates (TFA-MOD). X-ray diffraction and transmission electron microscope measurements revealed excess yttrium in YBCO in the form of nanoscale Y2O3 with (400) preferred orientation. The field dependence of Jc demonstrated that YBCO films with Y2O3 doping had enhanced Jc in comparison with stoichiometric YBCO films in the magnetic fields. We think the reason for this is that the Y2O3 nanoparticles act as pinning centres. YBCO films with 60% yttrium excess display 43% increased Jc compared to stoichiometric YBCO films at a magnetic field of 1?T.

The effect of deposition rate on the microstructure of YBCO thin films prepared by inverted cylindrical magnetron sputtering

The dependence of YBCO thin film properties on the deposition rate was studied in this report. The thermodynamic growth parameters (substrate temperature, total pressure and oxygen partial pressure) were optimized at a fixed deposition rate first. Then keeping the thermodynamic growth parameters optimized values, YBCO thin films were deposited at different deposition rates. The effect of the deposition rate on the superconducting transition properties, c-axis lattice constant, concentration of lattice disorder and surface smoothness of YBCO thin films was studied. The measurements show that the optimized thermodynamic growth parameters are closely correlated to the deposition rate. At a fixed deposition rate of 0.5 A/s and its optimal thermodynamic growth parameters, YBCO thin films with superior superconducting transition temperature, the lowest concentration of lattice disorder, and the lowest surface roughness were prepared. By using the same thermodynamic growth parameters, changes in deposition rate were shown to degrade the superconducting transition temperature, increase the lattice disorder, and increase the roughness of the YBCO thin films.

The preparation of two inch double-sided YBCO thin films

The preparation of two inch double-sided YBCO thin films by simultaneous sputtering from a single target is reported. The lateral homogeneity of microwave surface resistance of the YBCO thin films, on both sides of the two inch wafer, is characterized by using a Fabry?Perot resonator at 145 GHz and 75 K. Values of microwave surface resistance Rs (75 K, 145 GHz, 0 T) below 55 m? were reached over the whole area of YBCO thin films on two inch LaAlO3 wafers. The majority of the wafer area has Rs (75 K, 145 GHz, 0 T) values in the range of 15 m? to 40 m?. The uniformity of Rs values in the whole two inch wafer is excellent and the properties of YBCO thin films were found to be very similar on both sides of the wafer.

The preparation of double-sided YBCO thin films by simultaneous sputtering from single target

Abstract The preparation of 1-inch double-sided YBCO thin films by simultaneous sputtering from a single target is reported. By rotating the substrate around the rod of the substrate holder and the normal of the substrate, YBCO thin films were simultaneously deposited on both sides of 1-inch (100)LaAlO 3 substrate from a single inverted cylindrical sputter gun. The T C0 values of the YBCO thin films on both sides of the wafer were 90.3 and 90.4 K, respectively. The transition width was 0.8 K. The microwave surface resistance, R s (77 K, 10 GHz), of the YBCO thin films on both sides of the wafer were 330 and 400 μΩ, respectively. The uniformity of R s values over the whole 1-inch wafer is good and the properties of YBCO thin films were found to be very similar on both sides of the wafer.

The crystal perfection improvement of Y1Ba2Cu3O7-x thin films by afterglow plasma process

In this article, we report the growth of purely (001) oriented single crystal Y1Ba2Cu3O7−x thin films with excellent crystal perfection by afterglow plasma sputtering. Because of little energetic ion and electron bombardment, the crystal perfection of Y1Ba2Cu3O7−x thin films was significantly improved. The full width at half-maximum value of the rocking curve around the (005) diffraction peak of the films is 0.12°. The experiments demonstrate that the afterglow plasma process is very appropriate for the preparation of high-quality Y1Ba2Cu3O7−x thin films, which are very sensitive to ion bombardment effects.

The preparation of high-J c Gd0.5Y0.5Ba2Cu3O7−δ thin films by the MOCVD process

A home-designed metal organic chemical vapor deposition (MOCVD) system has been employed to prepare high critical current density (J c) Gd0.5Y0.5Ba2Cu3O7−δ (GdYBCO) thin films on LaMnO3/epitaxial MgO/ion beam assisted deposition (IBAD)-MgO/solution deposition planarization (SDP)-Y2O3-buffered Hastelloy tapes; the thin films were directly heated by the Joule effect after applying an heating current (I h ) through the Hastelloy tapes. The effect of the mole ratio of the metal organic sources has been systematically investigated. X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses indicated that the GdYBCO films crystallized better and became denser with the increasing of the Cu/Ba ratio from 1.0 to 1.1, yielding a J c at 77 K and 0 T of 200 nm GdYBCO film increasing from 2.5 MA cm−2 to 7 MA cm−2. In addition, SEM and energy dispersive spectrometer (EDS) characterizations revealed that more and more outgrowths appeared and the density of the film was reduced with an increase in the Cu/Ba ratio from 1.1 to 1.2. When the I h was 26.8 A and the mole ratio of Gd(tmhd)3, Y(tmhd)3, Ba(tmhd)2 and Cu(tmhd)2 in the precursor was 0.55:0.55:2:2.2, the critical current (I c) of the deposited 200 nm-thick GdYBCO film reached a 140 A cm−1 width (77 K, 0 T), corresponding to the J c 7 MA cm−2 (77 K, 0 T).

Disk Resonators as Nonlinear Surface Impedance Measurement Tools

The edge-current–free rotational symmetric disk resonator mode, TM010, was used as characterization tool for nonlinear surface impedance measurement of Y1Ba2Cu3O7−x (YBCO) thin films. The microwave surface resistance of high quality epitaxial YBCO thin films as a function of frequencies and rf surface magnetic field Hrf was measured using the circular disk resonator technique. Using this technique the properties degradation of the resonators by edge current crowding effect was avoided. The measured Rs values are divided into three regions. In the low field region (Hrf ≤ Hc1J), Rs is independent of Hrf. In the intermediate Hrf region, Rs can be well fitted to the formula of Rs(Hrf) = a(f) + b(f) Hrf2, and b(f) is proportional to f2. This form of microwave surface resistance quantitatively agrees with the modified coupled-grain model with the idea that the materials are composed of identical superconducting grains coupled together by identical intergranular regions that function as Josephson junctions. The experimental results show that the loss mechanisms are extrinsic properties resulting from defects in the films.

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y1Ba2Cu3O7 − x (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a Tconset of 85 K and a Δ Tc of 5 K. The resistance measurements gave a Tconset of 90 K and a Δ Tc of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.

A Comparative Study of YBCO Thin Film Growth by Inverted Cylindrical Magnetron Sputtering and Inverted Cylindrical Sputtering

AbstractA comparative study of Y1Ba2Cu3O7−x (YBCOrpar; thin film growth by inverted cylindrical magnetron sputtering and non-magnetron inverted cylindrical sputtering was done. At the total pressure of 40 Pa epitaxial YBCO thin films with good superconducting transition properties and smooth surface were prepared by the magnetron source. Inductive measurements gave