C-axis Of Film Research Articles

Direct high-resolution electron microscopy observations of sputtered a-axis oriented YBa2Cu3O7 films prepared by a self-template method on SrTiO3 substrates

A-axis oriented YBa2Cu3O7 films grown on a SrTiO3 substrate with and without the template layer have been studied by high-resolution electron microscopy. The self-template layer was found to be very effective to obtain highly crystalline a-axis oriented YBa2Cu3O7 film. A disordered cubic perovskite structure with a lattice parameter of 3.85 Å was found in the template layer which could be a good seed for further growth of a-axis film. In the film without the self-template layer, YBa4Cu3Ox cubic perovskite with lattice parameter 4.08 Å was observed to have grown at the interface as an island growth. Moreover, some faulted c-axis film at the very beginning of the template layer was observed even at the rather lower deposition temperature of 640 °C.

Microstructure of (001), (110) and (103) oriented thin films of YBa 2Cu 3O 7−x investigated with STM, SEM and HRTEM

Epitaxial films of (110) or (103) YBa 2Cu 3O 7− x grown on (110) SrTiO 3 substrates by laser ablation are compared to (001) films. Their microstructure is investigated with scanning tunneling microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. The growth of steps, terraces and spirals originating from screw dislocations commonly seen in c-axis films is inhibited in films grown on (110) SrTiO 3. In the case of (103) oriented films, the growth of 90° twisted domains produces a tooth-like structure with preferred orientation. The influence of vicinal substrates (a few degrees of misorientation) on the film growth, on the nature of defects and on the resistive behavior is also investigated.

Sign reversal of the flux-flow Hall effect in sputtered a-axis- and c-axis-oriented films of 1:2:3 superconducting cuprates.

Films of R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (R=Y, Ho, and Eu) have been obtained with c-axis orientation perpendicular to the film plane, by two dc sputtering methods, on different kinds of substrates. Also, pure a-axis-oriented films of ${\mathrm{EuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ have been grown by dc magnetron sputtering. The films have been characterized by x-ray diffraction, resistivity, and critical current. The Hall effect has been measured in the normal and in the mixed state using a magnetic field up to 9 T. The films show a strong anisotropy in the normal state, with n-type and p-type behaviors in a-axis and c-axis films, respectively. All the films show sign reversal of the Hall resistivity in the mixed state. The maximum positive value (a-axis films) and the minimum negative value (c-axis films) of the Hall-effect resistivity are sample dependent. The analysis of this result shows that the enhancement of the effect occurs when the samples are in the transition region between the dirty and clean regimes. Films with very different values of the mean free path l and coherence length ${\ensuremath{\xi}}_{0}$ show a vanishing small effect in comparison with films that present l\ensuremath{\simeq}${\ensuremath{\xi}}_{0}$.

Electrical noise in YBaCuO superconducting thin-films with differing axes of orientation

In this paper we present a comparison of dc and electrical noise characteristics of high-temperature superconductor thin-films with differing preferred axes of orientation. Four thin-film samples were studied, two of c-axis and two of a-axis orientation. All films were YBaCuO superconducting material on SrTiO/sub 3/ substrates. The R-T, I-V, and electrical noise properties were measured for each of the films and comparisons were made between them. The experimental data was compared with current hypotheses on conduction and crystal structure. Our results show that from the dc and electrical noise measurements that the c-axis films are much better than a-axis films in applications where sensitivity and sharp transition are important factors. These results also show that preliminary results fit a p-noise model for a 2-D normal conductor-superconductor percolation network.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Anisotropic transport properties of in-plane-aligned a-axis YBa2Cu3O7 films.

We have grown a-axis ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films aligned in the plane of the substrate (100) ${\mathrm{LaSrGaO}}_{4}$ by off-axis sputtering. X-ray and TEM analysis show a single predominant orientation of the b and c axes in these films. In-plane texture x-ray scans and Rutherford backscattering spectroscopy channeling reveal good crystalline quality. TEM analysis shows antiphase boundaries in the [010] direction and stacking fault boundaries in the [001] direction. We obtain an anisotropy ratio in resistivity of 8. The anisotropy in resistivity as well as in critical current density is less than that in single crystals. The reduction can be accounted for by the small percentage of misoriented grains in these films. We also investigate the resistive properties of antiphase boundaries and stacking fault boundaries. The field dependence of the critical current density in the [010] and [001] directions is similar to that of c-axis films, which is known not to be weak link limited.

Mesoscale structural studies of thin films of YBa 2Cu 3O 7

The surface morphology and growth mechanism of thin films of YBa 2Cu 3O 7 on a variety of single crystal substrates has been investigated using scanning tunneling microscopy. Over a wide range of deposition conditions c-axis films nucleate as islands and grow by a dislocation-mediated mechanism (spiral growth). a-axis films, on the other hand, grow by edge addition of vapor phase material to the component layers, giving a very different morphology. Many defect features associated with these growth patterns have been been observed and their dependence on deposition conditions and substrate material, as well as their importance for some superconducting properties is discussed.

Magnetism and epitaxy in Lu/Dy/Lu trilayers

Thin dysprosium c-axis films (40–400 Å) were grown coherently between 500-Å lutetium layers by molecular beam epitaxy. Bulk magnetization measurements show that these sandwich structures order magnetically at TN≂178 K (=TN of elemental Dy) and undergo ferromagnetic transitions at temperatures which range from 100 K (400 Å Dy) to 175 K (40 Å Dy), significantly enhanced from the bulk TC=85 K. The Dy basal plane lattice parameters in the films were determined by room-temperature x-ray diffraction. We observe a change in these values that correlates with the rise in TC, which suggests that this rise is due to epitaxial strain. The relatively small low-temperature magnetic susceptibility displayed by these samples indicates the presence of a large anisotropy in the basal plane. We address both the issues of the susceptibility and the high ferromagnetic transition temperature.

Ab-oriented YBa2Cu3O7 thin film grown on c-axis layer by radio frequency sputtering

YBa2Cu3O7 films can be grown on different substrates by the off-axis ac magnetron sputtering method. Films in the ab orientation tend to grow at a lower substrate temperature than c-axis films. However, as the temperature is lowered, all the substrates we have tested, except for SrTiO3, form amorphous phases that degrade the quality of the films being made. In this manuscript it is shown that if a thin layer of c-axis film is deposited on the substrate first, then it effectively reduces the formation of the amorphous phases and improves the film quality significantly.

Growth mechanisms and properties of 90 degrees grain boundaries in YBa2Cu3O7 thin films.

The transport properties of three types of 90\ifmmode^\circ\else\textdegree\fi{} grain boundaries of (103) oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (YBCO) thin films grown epitaxially on (101) ${\mathrm{SrTiO}}_{3}$ and (101) ${\mathrm{LaAlO}}_{3}$ substrates in situ by 90\ifmmode^\circ\else\textdegree\fi{} off-axis sputtering are compared. A simple description of the in-plane crystallographic film orientation is given by substrate [010]\ensuremath{\parallel}YBCO[010] and substrate [1\ifmmode\bar\else\textasciimacron\fi{}01]\ensuremath{\parallel}YBCO 〈301〉. A domain structure exists with the ${\mathrm{CuO}}_{2}$ planes oriented at \ifmmode\pm\else\textpm\fi{}45\ifmmode^\circ\else\textdegree\fi{} to the substrate surface (i.e., parallel to the substrate [010] direction). Therefore, specific sets of 90\ifmmode^\circ\else\textdegree\fi{} grain boundaries are observed in both principal in-plane directions. The normal-state conductivity and the critical current density of these films along the YBCO [010] direction are as high as the best quality c-axis films, which have no high-angle grain boundaries. The normal-state conductivity and critical current density along the 〈301〉 direction are much lower than in the [010] direction. However, the normalized magnetic-field dependence of ${\mathit{J}}_{\mathit{c}}$ for both those directions is similar and shows no evidence of weak link behavior. These results have important implications for understanding the behavior of step-edge Josephson junctions. The anisotropic transport behavior in the normal and superconducting state is explained by the microstructure and a simple transport model.

Oxygen diffusion in c-axis-oriented YBa2Cu3Ox films

C-axis-oriented YBa2Cu3O7- delta films produced by electron beam and thermal evaporation are composed of mosaic blocks. The deoxygenated YBa2Cu3Ox (x=6.1-6.2) films were annealed in an oxygen atmosphere. The change of oxygen concentration in the films during the annealing process was detected by the diffraction peak site of the (0011) peak. Applying the theory of lattice diffusion to analyse the oxygen-in-diffusion in the films, the authors obtain the activation energies for oxygen diffusion in the tetragonal and orthorhombic phases, which are 1.15 and 1.23 eV respectively. The change of shape of (00 11) diffraction peaks indicates that there is a coexistent range of two phases near the transition point. The difference between single crystals and c-axis film is that there are many mosaic boundaries in the c-axis film. A mosaic boundary acts as a short-circuit for oxygen diffusion and makes the diffusion time, during which equilibrium oxygen concentration is attained in the films, decrease greatly.

Angular magnetoresistance provides texture information on high- Tc conductors

Angular magnetoresistance measurements are performed by rotating a superconductor to an angle θ in a fixed magnetic field, while monitoring the resistance R. It is argued theoretically that for fields well above the lower critical field, H⪢ H cl, the bulk resistivity of the conductor is independent of θ if the crystallites of which it is composed are randomly oriented. Non-random orientation (a key aspect of texture) is revealed, therefore, by variations of R with θ. Dips in R indicate that the field is parallel to the copper oxide planes in a significant fraction of the current-carrying crystallites. C-axis, a-axis, and other textured film conductors are used to illustrate the technique. The angular magnetoresistance is found to be an important supplement to conventional texture determinations by microscopy and X-ray diffraction.

Processing Effects on the Contact Resistance of Ion Beam Sputtered Silver on Highly Oriented C-Axis and A-Axis YBCO Films

ABSTRACTThis paper reports on the processes used to achieve low resistance silver contacts to YBCO thin films that have either c-axis or a-axis orientation. Characterization by x-ray diffraction and TEM verified that these films are highly oriented with either the a or the c axis oriented perpendicular to the substrate surface. TEM examination of some of the Ag/YBCO interfaces reveals the presence of an amorphous layer. We will describe the effects of ion beam etching and RTA alloying on the contact resistivity for both orientations.

Electron channeling patterns of a-axis and c-axis YBa2Cu3O7−δ thin films

We have studied the electron channeling patterns (ECPs) of both pure a-axis and pure c-axis epitaxial YBa2Cu3O7−δ thin films on various substrates. ECPs from three representative a-axis films [on PrBa2Cu3O7−δ buffered LaAlO3 (001), MgO (001), and SrLaAlO4 (001)] and three c-axis films [on LaAlO3 (001), NdGaO3 (001), and SrTiO3 (001)] are presented. The differences between both sets of ECPs are discussed and recorded as references for future identifications.

The atomic structure of growth interfaces in Y–Ba–Cu–O thin films

We have examined the atomic structure of growth interfaces in thin films of Y–Ba–Cu–O grown on [001] perovskite or cubic substrates. At substrate heater temperatures in the range of 780–820 °C c-axis oriented growth is observed on these substrates. On SrTiO3, the first layer appears to be either a BaO or a CuO2 plane while on LaAlO3 the first layer appears to be a CuO chain layer. The mismatch on the a-b plane is accommodated by the formation of interface dislocations. Defects on the substrate surface propagate as defects in the film. These defects are primarily translational boundaries and in some cases second phases. At lower substrate heater temperatures, i.e., 650–700 °C, a, b-axis growth dominates. Defects and steps on the substrate surface are more detrimental in the growth of a, b-axis oriented films, since they tend to favor the nucleation of c-axis oriented domains. This is ascribed to the ledge mechanism of c-axis film growth, for which the surface steps are good nucleation sites.

AlN Sputtered Film Properties Prepared at Low Gas Pressures by Facing Target System

Several properties of sputtered AlN films prepared in N2 gas by planar magnetron sputtering with facing targets were investigated. Highly c-axis-oriented films with columnar structure were obtained at the pressure range of 10-3 Torr of N2 gas. Film degradations such as the decrease of the degree of c-axis orientation, film coloring, and film peeling were also observed at lower gas pressures below 1×10-3 Torr. From the results of optical transmittance and film composition estimation by energy-dispersion X-ray spectroscopy, these film degradation were related to the excess of N atoms in the film growth process.

Effects of deposition rate and substrate temperature on the orientation and the growth of YBa/sub 2/Cu/sub 3/O/sub x/ thin films

The authors report in situ studies of film growth by sputtering using synchrotron X-rays. The structure and growth habit of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) thin films deposited on [100] SrTiO/sub 3/ in a miniature, faced-magnetron sputtering system have been investigated. A combination of the substrate temperature and the deposition rate determines whether the film grows along the a, c, or multiple axes. At low substrate temperatures and low deposition rates, the films grow preferentially along the a-axis, In contrast, higher substrate temperatures and high deposition rates favor c-axis-oriented film growth with some admixture of [220]. The X-ray diffraction peaks were monitored in real time, revealing that a-axis- and c-axis-oriented grains nucleated on the surface of the [100] SrTiO/sub 3/ between 625 degrees and 765 degrees C, although the volume fraction of each orientation was temperature dependent. The structural quality of the a-axis films is superior to that of the c-axis films. The best a-axis films (deposited at 685 degrees C) had a rocking curve width of 0.08 degrees , which is 10 times smaller than that for the c-axis films (deposited at 800 degrees C). A shift of the [007] and [200,] peak positions during deposition was observed. The shift in the [007] peak is larger than that for the [200] peak. Defects in the c-axis films were observed, not only at the interface between the film and the substrate, but also on the upper surface of the (final) film; the latter seems to be inherent and is more severe in the films considered.

Preparation of Optically Smooth Surfaces of High TcSuperconducting Films

ABSTRACTWe report optical studies of predominantly c-axis oriented high-Tc superconducting films before and after ion milling. A significant increase in midinfrared reflectivity resulted from ion milling of a few thousand Å Features associated with α-axis texture vanished after ion milling, resulting in pure c-axis films. This suggests that α-axis grains reside mostly near the surface in post-annealed high-Tc superconducting films. Therefore, even after annealing (for the purpose of raising Tc), highly oriented c-axis films can be retained by ion milling. This may be important for device applications where optically smooth surfaces and interfaces are to be routinely prepared.

Origin of surface roughness for c-axis oriented Y-Ba-Cu-O superconducting films

One problem with the growth of high quality c-axis oriented YBa2Cu3O7−x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c-axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c-axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a-axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.

As-grown epitaxial YBaCuO films prepared by low energy oxygen ion assisting ion beam sputtering

The as-grown epitaxial YBaCuO films on (100) and (110)SrTiO 3 substrates were prepared by ion beam sputtering method with low accelerating oxygen ion assist. Oxygen ions were produced by the Kaufman type single grid ion source with the filament protection to oxygen. The orientation of epitaxial films on(100)SrTiO 3 substrate were varied by both the assisting ion current density and substrate temperature; the c-axis of film tended to be normal at higher substrate temperature and lower oxygen ion current, while the c-axis of the films tended to lie on the substrate at lower substrate temperature and higher oxygen ion current. The same trend was observed on a (110)SrTiO 3 substrate; the 〈110〉 orientation formed at lower substrate temperature and higher oxygen ion current, in contrast 〈103〉 formed at higher substrate temperature and lower oxygen ion current.

Feasibility of the high T c superconducting bolometer

A design analysis is given for a bolometric infrared detector that uses the resistive transition of a high-temperature superconductor as the temperature sensing element, and liquid nitrogen (LN) as the coolant. It is shown that for highly oriented c-axis films, the measured low-frequency noise causes little or no degradation of the performance. With the incoming radiation chopped at 10 Hz, noise equivalent powers (NEP) in the range (1–20)×10−12 W Hz−1/2 should be achievable. These values compare favorably with the NEP of other detectors operating at or above LN temperatures for wavelengths greater than 20 μm.