Yu. A. Boikov

Epitaxial YBa2Cu3O7−δ /BaxSr1−xTiO3 heterostructures on silicon‐on‐sapphire for tunable microwave components

Epitaxial trilayer heterostructures of YBa2Cu3O7−δ/BaxSr1−xTiO3/YBa2Cu3O7−δ were grown on silicon‐on‐sapphire buffered by a double layer of CeO2/Y–ZrO2. Such structures may be considered for tunable microwave filters. The top and bottom YBa2Cu3O7−δ films were well c‐axis oriented, free from microcracks and had superconducting transitions Tc’s in the range 86–90 K. A thin antidiffusion layer of SrTiO3 (d≊70 A) between YBa2Cu3O7−δ and BaxSr1−xTiO3 (x=0.25–0.9) promoted better crystallinity and higher Tc of the top superconducting film. An Ag/BaxSr1−xTiO3/YBa2Cu3O7−δ capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan δ of the BaxSr1−xTiO3 layer. Maximum values of the permittivity of the BaxSr1−xTiO3 layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1−xTiO3 (x=0.25–0.75) layers depended strongly (≊20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan δ was much hig...

High tunability of the permittivity of YBa2Cu3O7−∂/SrTiO3 heterostructures on sapphire substrates

The combination of a low loss, high-Tc superconductor and a polarizable dielectric looks interesting from the point of tunable, high Q filters for cellular communication. Epitaxial heterostructures of YBa2Cu3O7−∂/SrTiO3/YBa2Cu3O7−∂/CeO2 and SrTiO3/YBa2Cu3O7−∂/CeO2 were grown on sapphire substrates in this study. Superconducting transition temperatures of 88–90 K and a critical current density of about 2×106 A/cm2 at 77 K were determined for YBa2Cu3O7−∂ films on CeO2 buffered sapphire. An effective permittivity of 340 at 300 K was measured capacitively for the SrTiO3 layer between YBa2Cu3O7−∂ electrodes; it increased three times at 50 K. The dielectric constant of SrTiO3 was observed to decrease a factor of 2 as a dc bias voltage of ±2.5 V was applied between the electrodes. Much smaller nonlinearities and temperature dependences of the dielectric constant were noted if the top YBa2Cu3O7−∂ electrode was replaced by an Ag one.

Slow capacitance relaxation in (BaSr)TiO3 thin films due to the oxygen vacancy redistribution

Capacitance relaxation in Ba0.8Sr0.2TiO3 thin film structures has been investigated. Slow decrease of the capacitance after a bias voltage switching on is explained by suppressing the film dielectric permittivity by the field of a p-n junction originated as a result of the oxygen vacancy migration in the bias field. After the bias switching off, the p-n junction gradually disappears due to the vacancy diffusion, and the capacitance increases. The relation determining capacitance increase after the bias switching off has been obtained in agreement with experimental data. The proposed relaxation mechanism is considered as a certain type of the size effect when the p-n junction depleted region spreads over the film thickness.

Epitaxial growth and properties of YBa2Cu3Ox‐Pb(Zr0.6Ti0.4)O3‐YBa2Cu3Ox trilayer structure by laser ablation

We have grown YBa2Cu3Ox‐Pb(Zr0.6Ti0.4)O3‐YBa2Cu3Ox multilayer structure on SrTiO3 and Al2O3 substrates using laser ablation. The deposition conditions for the growth of trilayers and their properties are studied in this investigation. Scanning electron microscope images and x‐ray diffraction analyses indicate that all the constituent films in the trilayer grow epitaxially on SrTiO3 and were highly oriented on Al2O3. Transport measurements on these multilayers show that top YBa2Cu3Ox films have good superconducting properties.

Improved cationic stoichiometry and insulating behavior at the interface of LaAlO3/SrTiO3 formed at high oxygen pressure during pulsed-laser deposition

Medium-energy ion spectroscopy, MEIS, and scanning transmission electron microscopy, STEM, were used to correlate the atomic structure of LaAlO3/SrTiO3 interfaces with their electrical properties. Interfaces were prepared at high (5x10(-2) mbar) and low (10(-4) mbar) oxygen pressure by pulsed-laser deposition. The high-oxygen-pressure heterostructures were insulating for all thicknesses while the low-oxygen-pressure ones became metallic for thicknesses above 4 unit cells. MEIS data show enhancement of the Sr surface peak and suppression of the La one in interfaces prepared at low oxygen pressure, which is interpreted as a La-Sr intermixing. The effect was considerably smaller in high-oxygen-pressure samples. Analysis of high-angle annular-dark-field STEM images of the LAO films also indicates intermixing between La and Sr in low-oxygen-pressure samples, supporting MEIS data. Our results reveal the important role of oxygen pressure on the formation of the interface electron gas. Copyright (C) EPLA, 2011

Epitaxial growth and properties of YBa2Cu3O7−δ/NdGaO3/YBa2Cu3O7−δ trilayer structures

We have used laser deposition to make YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}/NdGaO{sub 3}/YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} trilayer structures. NdGaO{sub 3} films grow epitaxially on well lattice-matched substrates, like SrTiO{sub 3} (100), and on {ital c}-axis oriented YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films. Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films were grown on top of the NdGaO{sub 3} films, and there was no significant difference in critical temperature between top and bottom layers of the trilayer structures. The resistivity of a 300-nm-thick NdGaO{sub 3} interlayer was as high as 10{sup 8} {Omega} cm at 300 K.We have used laser deposition to make YBa2Cu3O7−δ/NdGaO3/YBa2Cu3O7−δ trilayer structures. NdGaO3 films grow epitaxially on well lattice‐matched substrates, like SrTiO3 (100), and on c‐axis oriented YBa2Cu3O7−δ films. Epitaxial YBa2Cu3O7−δ films were grown on top of the NdGaO3 films, and there was no significant difference in critical temperature between top and bottom layers of the trilayer structures. The resistivity of a 300‐nm‐thick NdGaO3 interlayer was as high as 108 Ω cm at 300 K.

Microstructure and dielectric parameters of epitaxial SrRuO3/BaTiO3/SrRuO3 heterostructures

Epitaxial films of ferroelectric barium titanate are desirable in a number of applications but their properties are inferior to those of bulk material. Relations between microstructure and dielectric properties may give better understanding of limitations. Trilayer heterostructures SrRuO3/BaTiO3/SrRuO3 were grown by laser ablation on (100)LaAlO3 and (100)MgO substrates. The BaTiO3 layer was granular in structure. When grown on (100)SrRuO3/(100)LaAlO3, it was preferentially a-axis oriented due to tensile mechanical stress. Using (100)MgO as a substrate, on the other hand, produced a mixture of about equal value of a-axis and c-axis oriented grains of BaTiO3. The dielectric permittivity, e, of the BaTiO3 layer was almost twice as large, at T>200 K and f=100 kHz, for the LaAlO3 substrate as compared to the MgO one. Its maximum value (e/e0≈6200) depended on temperature of growth, grain size, and electric field and compares well with optimal values commonly used for ceramic material. The maximum in the e(T) sh...

Structural aspect of YBa2Cu3O7−x films on Si with complex barrier layers

Abstract The microstructure of YBa 2 Cu 3 O 7− x (YBCO) thin films grown on Si with double (Y-stabilized ZrO 2 (YSZ) and CeO 2 or triple (MgO/CeO 2 /YSZ) buffer layers was characterized by means of transmission electron microscopy. The tetragonal distortion of the YSZ layer and the interaction between the MgO and CeO 2 layers which lead to the formation of the MgCeO 3 compound were found. The dislocation structure of the CeO 2 /YSZ interface consists mostly of 90° dislocations with a presence of 60° dislocation in the areas near the antiphase boundaries in YSZ. The YBCO films in both cases were oriented with the c -axis perpendicular to the substrate surface with a small density of a -axis oriented grains. The stacking sequence in the YBCO/CeO 2 interface is mostly CeO 2 BaOCuOBaOCuO 2 Y…. The presence of double CuO layers and other defects, due to atomic-height steps on the CeO 2 surface, was observed. The YBCO films on the CeO 2 /YSZ buffer were single crystalline and those on the MgO/CeO 2 /YSZ buffer were polycrystalline with low- and high-angle grain boundaries between the grains. Small yttria precipitates in two epitaxial orientations were observed in the YBCO films.

YBA2CU3O7-X FILMS ON YTTRIA-STABILIZED ZRO2 SUBSTRATES : INFLUENCE OF THE SUBSTRATE MORPHOLOGY

c-axis-oriented YBa2Cu3O7-x (YBCO) thin films were laser deposited on (001) yttria-stabilized ZrO2 (YSZ) substrates with different surface morphologies. The in-plane orientation of the films on smooth substrates was sensitive to the deposition conditions, often resulting in mixed orientations. However, a strongly dominating [110] YBCO//[110]YSZ orientation was obtained at a deposition temperature of 770°C. Films on substrates with surface steps, induced by depositing a homoepitaxial buffer layer or by thermally annealing the substrate, had a [110]YBCO//[100]YSZ orientation when deposited at the same temperature. It was concluded that the [110]YBCO//[100] YSZ orientation was promoted by a graphoepitaxial mechanism. Films prepared under identical conditions on smooth and stepped substrates grew with extended c axes on the former. It is proposed that the extension can be induced by disorder, invoked by a low oxygen pressure and a low density of adsorption sites. The disorder may be eliminated by either an increase of the oxygen pressure or an increase of the density of adsorption sites in the form of steps. The film microstructure influenced the microwave surface resistance, which was similar for films with one exclusive in-plane orientation and higher for films with mixed orientations. The films on the stepped surfaces had superior superconducting properties; inductive measurements gave a Tc onset of 88 K, a ΔT(90%-10%) c of 0.2 K, and the transport jc was 1.5×106 A/cm2 at 83 K, for films on substrates with homoepitaxial buffer layers.

Pseudogap and its temperature dependence in YBCO from the data of resistance measurements

The temperature dependence of the excess conductivity Δσ for Δσ = A(1 − T/T*)exp(Δ*/T) (YBCO) epitaxial films is analyzed. The excess conductivity is determined from the difference between the normal resistance extrapolated to the low-temperature range and the measured resistance. It is demonstrated that the temperature dependence of the excess conductivity is adequately described by the relationship Δσ = A(1 − T/T*)exp(Δ*/T). The pseudogap width and its temperature dependence are calculated under the assumption that the temperature behavior of the excess conductivity is associated with the formation of the pseudogap at temperatures well above the critical temperature Tc of superconductivity. The results obtained are compared with the available experimental and theoretical data. The crossover to fluctuation conductivity near the critical temperature Tc is discussed.