SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

High quality epitaxial YBa₂Cu₃O_7-x thin films have been succcessfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO₃, LaAlO₃ and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a J_c (at 77 K, H=0) over 10⁶A/cm². It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by X-ray diffraction. Rutherford backscattering and channeling spectroscopy, X-ray double-crystal diffraction and transmission election microscopy. The experimental results demonstrated that the epitaxial YBa₂Cu₃O_7-x films had excellent superconducting properties and quite perfect structure.     

Growth stages of YSZ-buffer layers and YBa2Cu3O7−x thin films on silicon substrates studied by scanning probe microscopy

Scanning probe microscopy has been applied to study various growth stages of YSZ (yttria-stabilized zirconia) buffer layers on silicon and of YBa₂Cu₃O_7–x thin films on YSZ/Si. YSZ buffer layers of 75 nm thickness exhibit a remarkable smooth surface with a rms roughness of about 0.5 nm for a surface area of 5 m×5 m. The subsequent growth of YBa₂Cu₃O_7–x thin films was investigated from nucleation to the formation of growth hills. Screw dislocations were found only in very rare cases.     

Effect of La2Zr2O7-buffered YSZ substrate on YBa2Cu3Oy thin films by chemical solution deposition

Epitaxial YBa₂Cu₃O_y (YBCO) thin films have been fabricated by chemical solution deposition (CSD) on La₂Zr₂O₇-buffered YSZ single crystal substrate, where the buffer layer has three kinds of morphology – flat surface, rough surface and pore surface. The effect of LZO buffer layer’s roughness on the YBCO films was evaluated by X-ray diffraction, scanning electron microscopy and temperature-dependent resistivity measurements. The flat surface of LZO layer is beneficial to highly epitaxial YBCO films and high critical current density.     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

Observation of the early stages of growth and the formation of growth spirals in epitaxial YBa2Cu3O7-δ thin films by AFM

Ultrathin epitaxial films of YBa₂Cu₃O_7– on SrTiO₃ prepared by Direct Current (DC) sputtering and pulsed laser deposition were imaged by Atomic Force Microscopy (AFM) to follow the different stages of growth of the thin films. Series of films with thicknesses between 1.2 nm and 12 nm (1–10 monolayers of YBa₂Cu₃O_7–) were prepared under identical conditions, optimized with respect to electrical and structural properties, to obtain information on the mechanisms responsible for the formation of growth spirals which are commonly observed in films having a thickness of several 10 nm or more. It could be shown that few layers are formed by a layered growth mode where material is attached laterally to 2D islands which are only one c-axis unit cell in height. In a later stage of growth when about 8–10 layers have been formed, the growth process changes to a mode which is mediated by growth spirals. This could be directly monitored in the AFM images where different defect structures like vertically sheared growth fronts and dendrite-like terraces of stacked islands as well as the resulting growth spirals could be identified.     

The growth and domain structure of YBa2Cu3Ox films on neodymium gallate substrates with a deviation of the normal to the surface from the [110] direction in NdGaO3

The growth, crystal structure, and electrophysical properties of YBa₂Cu₃O_x (YBCO) epitaxial films grown with and without a CeO₂ epitaxial sublayer on NdGaO₃ (NGO) substrates with the normal to the surface deviating from the [110] axis by 5° to 26.6° around the [001] axis are investigated. It is shown that the orientation of YBCO epitaxial films grown on such substrates is determined by the existence of symmetry-equivalent directions in the substrate and in the CeO₂ layer, as well as by the rate of film deposition. For a high deposition rate, YBCO films grow on the CeO₂ sublayer in the [001] orientation irrespective of the orientation of the substrate and the sublayer. It was found that when the angle of deviation of the substrate plane is from the (110) NGO plane, twinning of one or both twinning complexes in YBCO may be suppressed.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Distinct correlation between CeO2 and YSZ in out-of-plane and in-plane mosaic dispersions of heteroepitaxial CeO2/YSZ/Si(001) films

The structural correlations including the lattice constants and the mosaic dispersions between CeO₂ and yttria-stabilized ZrO₂ (YSZ) in CeO₂/YSZ/Si(001) heteroepitaxial films have been investigated by out-of-plane and in-plane X-ray-diffraction techniques. The distinct linear correlations of the full width at half-maximum (FWHM) of the ω scan between CeO₂ and YSZ have been found in both directions. CeO₂ always has a 0.7° lower FWHM of the ω scan than YSZ in the out-of-plane direction, but has a 2.6° higher FWHM in the in-plane direction. A possible relationship between the out-of-plane and in-plane FWHMs of the ω scans has been demonstrated with a lattice-rotation model. Besides, the lattice constants of CeO₂ are dependent on the FWHMs of the YSZ ω scans: as the FWHM is below 3.5°, CeO₂ has a tetragonal distortion, and as the FWHM is higher than 3.5°, CeO₂ exhibits a cubic structure without distortion. The results are of great interest, both for the fundamental understanding of the film-growth mechanisms and for potential applications. Received: 11 September 2000 / Accepted: 5 June 2001 / Published online: 30 August 2001     

Step-like growth of Bi2Sr2CaCu2O8 films on off axis oriented (001) SrTiO3

High-quality Bi₂Sr₂CaCu₂O₈ (BSCCO) films were grown by means of pulsed-laser deposition on 10° off-axis oriented (001) SrTiO₃ substrates. The films exhibit a step-like morphology and they are properly oriented along the (001) direction. Their normal-state resistivity measured along the tilt (off-axis (100) direction of the substrate) was, typically, 260 times higher than that along the (010) direction. Such films reveal a transverse Seebeck-effect which significantly exceeds that observed with YBa₂Cu₃O₇ (YBCO).     

Connection of Residual Losses of a Microwave Energy in Superconducting State of YBa2Cu3O7?δ Films on Sapphire Substrates with Their Resistance in a Normal State

In the work, the results of measurements of temperature dependence of the surface resistance Rs(T) of the YBa₂Cu₃O_– epitaxial films deposited on sapphire substrates have been presented. The films were obtained using the method of impulse laser deposition. The measurements of surface impedance Rs(T) have been carried out at a frequency of 135 GHz. It is shown that Rs(T) agrees with existing model, considering heterogeneity in the YBa₂Cu₃O_7– epitaxial films as the normal spherical domains. Analysis has been carried out for the set of specimens which were obtained at different technological regimes and have different critical parameters and geometry.     

Structural transition of secondary phase oxide nanorods in epitaxial YBa2Cu3O7−δ films on vicinal substrates

A theoretical study of a structural transition of secondary phase oxide nanorods in epitaxial YBa₂Cu₃O_7?δ films on vicinal SrTiO₃ substrates is presented. Two possible types of film/substrate interface are considered, with one assuming complete coherence, while the other is defective as manifested by the presence of antiphase grain boundaries. Only in the former case does the increase of the vicinal angle of the substrate lead to a substantial change of the strain field in the film, resulting in a transition of the nanorod orientation from the normal to the in-plane direction of the film. The calculated threshold vicinal angle for the onset of the transition and lattice deformation of the YBa₂Cu₃O_7?δ film due to the inclusion of the nanorods is in very good agreement with experimental observations. This result sheds lights on the understanding of the role of the film/substrate lattice mismatch in controlling self-assembly of dopant nanostructures in matrix films.     

Strain evolution and morphological transition of magnetron-sputtered CeO2 thin films induced by deposition parameters

Cerium dioxide (CeO₂) films were fabricated on yttria-stabilized zirconia (YSZ) single crystals using unbalanced radio frequency (RF) magnetron sputtering. X-ray diffraction measurements revealed film strain discrepancies under different deposition parameters. Strain evolution was induced by varying sputtering pressure, RF power, and sputtering gas. A distinct morphological transition from a granular surface to an interwoven surface was also realized by varying the above parameters. On the basis of the “atomic peening” mechanism, the influence of different parameters on film strain was discussed. The film surface characteristics were revealed to be highly correlated with film strain. YBa₂Cu₃O_7−δ (YBCO) films were post-deposited on the as-grown CeO₂/YSZ(001) stack by using the trifluoroacetate metal-organic deposition (TFA-MOD) method. The superconducting property of the YBCO layer varied significantly with the morphology of the CeO₂ buffer films.     

Characterization of defect type and dislocation density in double oxide heteroepitaxial CeO2/YSZ/Si(001) films

In order to qualitatively and quantitatively analyze the structural defects including the defect types and their concentrations in oxide heteroepitaxial films, a new X-ray rocking-curve width-fitting method was used in the case of doubleCeO₂/YSZ/Si (YSZ=yttria-stabilized ZrO₂) films that were prepared by pulsed laser deposition. Two main defect types, angular rotation and oriented curvature, were found in both CeO₂ and YSZ. Dislocation densities of CeO₂ and YSZ, which were obtained from the angular rotations, are functions of the YSZ thickness. A distinct two-step correlation between dislocation densities of CeO₂ and YSZ was found that as the dislocation density of YSZ is higher than 2.4×10¹¹ cm^-2, the dislocation density of CeO₂ shows a high sensitivity with that of YSZ compared with the low relativity in lower dislocation density (<2.4×10¹¹ cm^-2). In addition, YSZ always has higher dislocation densities and oriented curvatures than CeO₂ in each specimen, which can be attributed to the smaller mosaic domain sizes in YSZ than in CeO₂ as observed by high-resolution transmission electron microscopy. Received: 12 August 2002 / Accepted: 14 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-3/5734-3369, E-mail: chun_hua_chen@hotmail.com     

Effect of CeO2 buffer layer thickness on the structures and properties of YBCO coated conductors

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on inclined-substrate-deposited (ISD) MgO-textured metal substrates by pulsed laser deposition. CeO₂ was deposited as a buffer layer prior to YBCO growth. CeO₂ layers of different thickness were prepared to evaluate the thickness dependence of the YBCO films. The biaxial alignment features of the films were examined by X-ray diffraction 2θ-scans, pole-figure, ?-scans and rocking curves of Ω angles. The significant influence of the CeO₂ thickness on the structure and properties of the YBCO films were demonstrated and the optimal thickness was found to be about 10 nm. High values of T_c = 91 K and J_c = 5.5 × 10⁵ A/cm² were obtained on YBCO films with optimal CeO₂ thickness at 77 K in zero field. The possible mechanisms responsible for the dependence of the structure and the properties of the YBCO films on the thickness of the CeO₂ buffer layers are discussed.     

Epitaxial growth of InN on nearly lattice-matched (Mn,Zn)Fe2O4

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe₂O₄ (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe₂O₄, which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe₂O₄, which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe₂O₄ (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.     

Dielectric and ferroelectric properties of pulsed-laser deposited BaTiO3 films

3 films were produced by KrF excimer-laser ablation. Films deposited on fused silica substrates were polycrystalline without preferential orientation and had cubic rather than tetragonal structure. BaTiO₃/Au/Ti/fused silica films showed a large dielectric constant, which increased with the thickness of the film, but poor ferroelectric properties. This behavior seems to be related to the small size of grains. On (100)MgO substrates oriented films were obtained. BaTiO₃/YBa₂Cu₃O_7-δ/(100)MgO films showed a large dielectric constant also and improved ferroelectric properties. Although this indicates a larger degree of tetragonality, the tetragonal structure of single crystal BaTiO₃ has not yet developed. Localized reduction and metallization of BaTiO₃/(100)MgO films by means of Ar⁺-laser radiation was demonstrated. This technique allows to produce conducting patterns in a single-s tep process. Received: 6 January 1997/Accepted: 21 April 1997     

Epitaxial growth of YSZ buffer layer for YBa2Cu3O7−δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO₂ buffered biaxially textured Ni-W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa₂Cu₃O_7−δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO₂/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3-4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.     

The orientation-selective growth of LaNiO3 films on Si(100) by pulsed laser deposition using a MgO buffer

Highly (100)-oriented, (110)-oriented and polycrystalline LaNiO₃ (LNO) films were successfully prepared on Si(100) using an oriented MgO film as a buffer. It was somewhat surprising to find that that the orientation relation between the LNO film and the corresponding MgO buffer was: LNO(100)\MgO(110), LNO(110)\MgO(111) and LNO(polycrystalline)\MgO(100). The crystalline quality of the LNO films was shown to be sensitive to the preparation conditions of the MgO buffer. The film surface was very smooth, without micrometer-sized droplets being observed. All LNO films were of metallic conductivity, with a room-temperature resistivities of approximately 250, 280 and 420 μΩ cm for the (110)-oriented, (100)-oriented and polycrystalline LNO, respectively. Received: 2 April 2001 / Accepted: 23 October 2001 / Published online: 3 June 2002     

Studies of resistance switching effects in metal/YBa2Cu3O7−x interface junctions

Current-voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa₂Cu₃O_7−x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa₂Cu₃O_7−x thin film-PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa₂Cu₃O_7−x films in the nano-scale vicinity of the junction interface under applied electrical fields.     

c-Axis correlated extended defects and critical current in YBa2Cu3Ox films grown on Au and Ag-nano dot decorated substrates

We report measurements of critical current in YBa₂Cu₃O_x films deposited on SrTiO₃ substrates decorated with silver and gold nanodots. An increase in critical current in these films, in comparison with the films deposited on non-decorated substrates, has been achieved. We argue that this increase comes from the c-axis correlated extended defects formed in the films and originated from the nanodots. Additionally to creating extended defects, the nanodots pin them and prevent their exit from the sample during the film growth, thus keeping a high density of defects and providing a lower rate of decrease of the critical current with the thickness of the films. The best pinning is achieved in the samples with silver nanodots by optimising their deposition temperature. The nanodots grown at a temperature of a few hundred °C have a small diameter of a few nanometres and a high surface density of 10¹¹–10¹² particles/cm². We give evidence of c-axis correlated extended defects in YBa₂Cu₃O_x films by planar and cross-sectional atomic force microscopy, transmission electron microscopy and angle-dependent transport measurements of critical current.