Vicinal SrTiO3 Research Articles

YBaCuO/PrBaCuO/YBaCuO trilayer junctions with the polished YBaCuO base electrodes

Superconducting wiring layer is indispensable to actualize digital circuits constructed with trilayer junctions. However, it is complicated to fabricate the junctions with the superconducting wiring layer. Thus, we have formed Josephson junctions on mechanical polished surface of YBaCuO films for introducing the simple device structure, such as a Pb junction, to the YBaCuO junction. The surface of YBaCuO film sputtered on vicinal SrTiO 3(0 0 1) substrate was polished by 12 nm depth. Peak–valley roughness of the surface decreased from 8 to 2 nm. After that YBaCuO film with 2 nm thickness for re-growth, PrBaCuO film with 13 nm thickness for barrier layer, and YBaCuO film for counter electrodes were consecutively deposited on the polished surface of the YBaCuO base electrodes. The junctions with the size of 10 μm × 10 μm showed resistively shunted junction like I– V curves with no excess current. The spreads of I c were 12% for 16 junctions on a chip. Applying mechanical polishing may improve the uniformity of the barrier thickness.

Uniaxial magnetic anisotropy and magnetic switching in La0.67Sr0.33MnO3 thin films grown on vicinal SrTiO3(100)

La 0.67 Sr 0.33 MnO 3 ultrathin films grown on vicinal SrTiO3(100) surface show an in-plane uniaxial magnetic anisotropy with easy axis along the substrate atomic steps generated by a 10° miscut off the (100) plane. Over a large angular range, the angular dependence of magnetic switching field is found to obey the 1/cos φ law, indicating that the magnetic reversal is completed by a 180° domain nucleation and sweeping along the easy axis. However, when the applied field is perpendicular to the hard axis (φ=90°), the magnetization reversal is found to be well described by the Stoner–Wohlfarth model, in which the magnetization coherently rotates from the easy axis to hard axis.

Time dependence of laser-induced thermoelectric voltages in La1−xCaxMnO3 and YBa2Cu3O7−δ thin films

The time dependence of laser-induced thermoelectric voltage (LITV) in La1−xCaxMnO3 (LCMO) or YBa2Cu3O7−δ (YBCO) thin films grown on a vicinal SrTiO3 substrate is derived. From the fitted curve of the measured time evolution of the LITV, several parameters of a LCMO thin film are deduced.

Synchrotron X-Ray Scattering Study of the Substrate Effects on Relaxor Pb(Mg1/3Nb2/3-δ)O3/SrTiO3 Thin Films

Perovskite Pb(Mg1/3Nb2/3-δ)O3 (PMN) thin films are grown epitaxially on on-axis SrTiO3 (001) (STO), vicinal SrTiO3 (001), and MgO (001) substrates by radio-frequency magnetron sputtering. The substrate effects on the structure of PMN thin films have been studied using synchrotron X-ray scattering technique. Initially PMN films grow in two-dimensional planar layers on the STO substrates, while they grow in the form of three-dimensional (3D) islands on the MgO substrate. The surface steps on the vicinal STO encourage the 3D island growth as the film thickness increases. We also found an unexpected monoclinic phase in PMN/STO and PMN/vicinal-STO films, which has been reported in bulk Pb(Mg1/3Nb2/3)O3-35%PbTiO3 solid solution. With increasing the film thickness, the unit cell structure of PMN films transforms from tetragonal to other structures as the strain relaxes.

YBaCuO/PrBaCuO/YBaCuO trilayer junctions with the etched YBaCuO base electrodes

Trilayer junctions are attractive for digital electronic applications, but superconducting wiring layers and contacts are required to fabricate the circuits. If the contact areas in this device structure are able to become Josephson junctions, YBaCuO films as counter electrodes can be used as the wiring layers; the fabrication process will become simple as that of ramp-edge type junctions. Thus, we tried to fabricate Josephson junctions on etched surface of YBaCuO films. Ar-ion milling was performed on YBaCuO film grown on a vicinal SrTiO 3(0 0 1) substrate by RF magnetron sputtering. After that, PrBaCuO film with 15 nm thickness and YBaCuO film were consecutively deposited on the etched surface of the YBaCuO film as base electrodes. The junctions with the size of 10 μm×10 μm showed resistively shunted junction-like I–V curves with the critical current of 0.12 mA.

Electron microscopy of step-flow grown c-axis YBa2Cu3O7−δ thin films on vicinal substrates

Investigation by transmission electron microscopy and high-resolution transmission electron microscopy was carried out on YBa2Cu3O7−δ thin films grown on vicinal SrTiO3 substrates by pulsed laser deposition. The c-axis YBa2Cu3O7−δ films grown on 1°–3° miscut substrates displayed a step-flow crossover to three-dimensional island growths, and on 6°–15° miscut substrates displayed a complete step-flow growth. Perfect heteroepitaxial growth of Ba0.1Sr0.9TiO3/YBa2Cu3O7−δ (BST/YBCO) on 1.2° miscut (001) LaAlO3, has been achieved by modification of the deposition conditions. The capacitance–voltage properties of a BST/YBCO capacitor were measured under a dc bias of ±30 V and 1 MHz frequency at 77 K, and the dielectric constant, tunability and figure of merit were found to be 1200, 60% and 133, respectively. These values show that the BST films have potential application at liquid nitrogen temperature in electrically tunable microwave devices.

Initial growth stages of epitaxial BaTiO3 films on vicinal SrTiO3 (001) substrate surfaces

The initial growth mechanism of epitaxial BaTiO3 films is studied by combined application of atomic force microscopy, cross sectional high-resolution transmission electron microscopy, and x-ray diffraction. Epitaxial BaTiO3 thin films were grown by pulsed laser deposition on vicinal Nb-doped SrTiO3 (SrTiO3:Nb) (001) substrates with well-defined terraces. X-ray diffraction and cross sectional high-resolution transmission electron microscopy investigations revealed well-defined epitaxial films and a sharp interface between BaTiO3 films and SrTiO3:Nb substrates. The layer-then-island (Stranski–Krastanov mode) growth mechanism observed by analyzing the morphology of a sequence of films with increasing amount of deposited material has been confirmed by microstructure investigations.

Magnetic behavior of epitaxial SrRuO3 thin films under pressure up to 23 GPa

We report on measurements of electrical resistivity and determination of the Curie temperature of single-crystal epitaxial ferromagnetic SrRuO3 thin films under hydrostatic pressures up to 23 GPa. The SrRuO3 thin film was grown on vicinal (001) SrTiO3 substrate using 90° off-axis magnetron sputtering. At atmospheric pressure, the thin-film Curie temperature is about 150 K. To generate the high pressure, the Bridgman anvil technique was used. Up to 13 GPa, a linear decrease of the Curie temperature was observed at a rate of 5.9 K GPa−1. Above 13 GPa, a striking saturation of the magnetic transition temperature at 77 K was observed.

Perovskite phase stabilization in epitaxial Pb(Mg1/3Nb2/3)O3–PbTiO3 films by deposition onto vicinal (001) SrTiO3 substrates

We report the perovskite phase stabilization of 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (PMN-PT) epitaxial thin films by deposition onto miscut substrates. Films were grown on (001) SrTiO3 substrates with miscut angles from 0 to 8 degrees toward the [100] direction using 90° off-axis magnetron sputtering. Films on high miscut substrates (>4°) showed almost the pure perovskite phase in both x-ray diffraction and transmission electron microscopy image, and were nearly stoichiometric. In contrast, films on exact (001) SrTiO3 contained a high volume fraction of pyrochlore phases with Pb deficiency. Atomic force microscopy reveals that films on 8° miscut substrates have a much smoother surface morphology than those on exact (001) SrTiO3. Cross-sectional transmission electron microscopy images reveal that the pyrochlore phases nucleate on top of the perovskite phase, and that the amount of the pyrochlore phases increase with film thickness. A film on an 8° miscut substrate exhibits a polarization hysteresis loop with a remnent polarization of 20 μC/cm2 at room temperature.

Improvement of interface-treated junctions using vicinal SrTiO3substrates

We have studied the improvement of vertically-stacked-structureJosephson junctions, employing both the interface-treated junction techniqueand flatYBa2Cu3O7-δ(YBCO) films usingvicinal SrTiO3 (STO) substrates.The obtained film exhibited root-mean-square roughnessof 2.3 nm, which was smallerthan that of the films on normal substrates.We fabricated two chips in order to examine the reproducibility. We measuredsevenjunctions on one chip.These junctionsshowed typical magnetic field modulation of the critical current density(Jc) of 83% at4.2 K.The averaged Jcand the averaged normalized resistance(RnA) were5.0×103 A cm-2with a 1σspread of 4.0% and3.1×10-7 Ω cm2with a 1σspread of 6.4%, respectively.Thus junctions of small excess current with small spread were obtained.To examine the reproducibility of the junctions, we compared thesejunctions with those on the other chip,where the average Jc were 3.7×103 A cm -2.The results showed the improvement of the reproducibilityby flat YBCO films using vicinal STO substrates.

Influence of tilting of SrTiO 3 substrates on the morphology and superconductivity of YBa 2Cu 3O 7− δ thin films

Morphology and superconductivity of YBa 2Cu 3O 7− δ thin films grown on vicinal SrTiO 3 substrates by pulsed laser deposition are studied. Ex situ atomic force microscopy and scanning tunneling microscopy are used to observe the film surface morphology. Novel anisotropic step-mediated island growth is found for the first time. The anisotropic evolution of the surface morphology is discussed in terms of the anisotropic surface diffusion. Relatively low deposition temperature (750°C) is found to be the optimum condition for ultra-smooth step-mediated growth of YBa 2Cu 3O 7− δ thin films. Tilting of the substrates is found favorable for the superconductivity of YBa 2Cu 3O 7−δ thin films grown on them. The critical temperature at zero resistance ( T c 0 ) of YBCO films is found to be a function of the vicinal angle of the SrTiO 3 substrates.

Precipitate-free epitaxy of YBa 2Cu 3O 7− δ by atomic control of step arrays on vicinal SrTiO 3 (1 0 0) substrates

We have developed a new method which enables us to atomically control the height and the periodicity of the step distribution on vicinal SrTiO 3 (1 0 0) substrates. This method also enables us to obtain precipitate-free epitaxy of oxides. Practically thick YBa 2Cu 3O 7− δ films grown on such terraced vicinal substrates exhibited a regular epitaxial growth, and smooth and precipitate-free surfaces of practical device sizes.

Josephson plasma observed in underdoped (110) YBa2Cu3O7−δ films

(110) oriented YBa2Cu3O7−δ thin films have been epitaxially grown on vicinal (110)SrTiO3 substrates by pulsed-laser deposition. Films with different oxygen deficiencies were obtained by annealing under various oxygen ambient. These films have very smooth surface and with their c axis aligned in one direction in the substrate plane. Electronic transport and polarized far-infrared reflectivity measurements reveal the unique features of the c-axis charge dynamics. A c-axis far-infrared reflectivity edge associated with the Josephson plasma in the superconducting state is observed. Our results demonstrate the potential usage of (110) thin films for the investigation of intrinsic interlayer charge dynamics as well as device application of high Tc superconductors.

Initial Growth Stages of Epitaxial BaTiO3 Films on Vicinal SrTiO3:Nb (001) Substrates

AbstractThe growth mechanism of epitaxial BaTiO3 films on vicinal Nb-doped SrTiO3 (SrTiO3:Nb) (001) substrate surfaces was studied in terms of surface morphology, crystalline orientation, microstructure, and film/substrate interface. Well-oriented BaTiO3 thin films were grown on SrTiO3 substrates with well-defined terraces by pulsed laser deposition. The regularly terraced TiO2-terminated surfaces of vicinal SrTiO3:Nb (001) substrates were prepared by a definite chemical and thermal treatment. Under our conditions, BaTiO3 seems to grow with a layer-then-island (Stranski-Krastanov) growth mechanism. In order to investigate the orientation and crystallinity of the BaTiO3 films, x-ray diffraction and high-resolution transmission electron microscopy were performed. Ferroelectricity of the BaTiO3 films was proved by electrical measurements performed on Pt/BaTiO3/SrTiO3:Nb heterostructures.

A NOVEL METHOD TO MEASURE THE THICKNESS OF YBCO THIN FILM

The laser induced voltage effect had been found in HTSC YBCO thin film grown on vicinal cut SrTiO3 substrate. The relationship between the decay time constant of response signal and the thickness of YBCO thin film was studied. It was found that the relationship between the decay time constant and the film thickness can be fitted by a fitting curve. The film thickness can be obtained from the fitting. We discussed the error induced to the fitting.

Ferroelectric/superconductor heterostructures

The reoccurrence of cracks have been found on the surfaces of epitaxial c-axis YBa2Cu3O7-δ (YBCO) films grown on the vicinal SrTiO3 substrates by pulsed laser deposition. At an optimum oxygen annealing condition, there are always cracks on the step-terrace surface of the YBCO films. Crack formation is attributed to the confinement of {110} mirror twinning induced by the anisotropic vicinal interfacial strain and the stress induced by the tetragonal-to-orthorhombic(T-O) phase transition of YBCO film. By use of vicinal SrTiO3 substrates of with small tilting angle(1°−3°) from (001) toward [010] and reduction of the oxidation pressure to 80Pa for T-O phase transition YBCO films with smooth surface was formed, but the films are superconducting at 60K.

Pits on the Surfaces of Epitaxial c-AxisYBa2Cu3O7-δ Thin Films Grown on Vicinal SrTiO3Substrates

The reoccurrence of pits has been found on the surfaces of c-axisYBa2Cu3O7-δ (YBCO) films grown epitaxially on10°vicinal SrTiO3 substrates by pulsed laser deposition. They are alwaysassociated with films with a step-terrace structure, whilepolycrystalline films have no pits. Pit formation is attributed to thelarge interfacial stress induced by vicinal substrate and oxygendeficiency of the YBCO film during the tetragonal-to-orthorhombic(T-O) phase transition, which cannot be released by forming twins,and leads to rupturing of the film surface. By reducing the oxidationpartial pressure for the T-O phase transition to 80 Pa, no pits were formed,and the YBCO film is superconducting at 60 K. PACS: 62. 20. Hk, 74. 76. Bz, 81. 15.

Structural characterization of epitaxial Bi 2Sr 2CuO 6+δ thin films deposited on SrTiO 3 substrates using inverted cylindrical magnetron sputtering

High quality c-axis oriented epitaxial Bi 2Sr 2CuO 6+δ thin films were deposited on 0° and 6° vicinal SrTiO 3 (100) substrates using an inverted cylindrical magnetron sputtering method. Their structural properties, regarding the orientation, epitaxy, surface morphology, composition, and surface microstructure, were studied by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The influences of the deposition parameters and substrate surface structure on the structural properties of these epitaxial Bi 2Sr 2CuO 6+δ thin films are studied. A metal-insulator transition is observed with a variation of the Sr deficiency. It is demonstrated that films on 0° and 6° vicinal substrates grow in the two-dimensional and step-flow modes, respectively. Optimal deposition conditions were obtained leading to an extremely smooth film surface and high epitaxial quality.

Resistive upper critical magnetic field of a superconducting Bi-2201 thin film grown on a vicinal substrate

Transport properties have been measured for a superconducting Bi 2Sr 2−xLa xCuO 6+σ (x ≈ 0.4) thin film grown on a vicinal SrTiO 3 single crystal substrate in magnetic field up to 23.5 T. The thin film was patterned into crossed bridges for studying the upper critical magnetic field H c2 (T). Numerical analyses of our data and previously published data lead to the conclusion that upward curvature of H c2 (T) does not imply a divergent H c2(T) value at T = 0K.

Detwinning in YBa 2Cu 3O 7−δ films on vicinal SrTiO 3 (0 0 1) due to anisotropic strain at the interface

The detwinning in YBa 2Cu 3O 7− δ films grown on vicinal SrTiO 3 (0 0 1)-substrates has been studied as a function of the vicinal miscut angle by X-ray diffraction using the two-dimensional q-scan technique. The investigated anisotropic strain at the interface between substrate and film is strongly correlated with the detwinning in the film. This observation is in good agreement with the here presented strain-minimizing model.