C-axis Of Film Research Articles

YBa 2Cu 3O 7 − δ oriented thin films and magnetically aligned ceramic: measurement of anisotropic thermopower

We report measurements of the anisotropic thermopower in the normal state of YBa 2Cu 3O 7 − δ thin films with three different orientations: a-axis perpendicular to the plane of the substrate; c-axis perpendicular; and coherently tilted on a (305) SrTiO 3 substrate respectively. We have also measured the thermopower of magnetically aligned YBa 2Cu 3O 7 − δ ceramic samples. The thermopower in the direction of the c-axis is found to be positive and increases as the temperature increases. The thermopower along the b-axis direction has only a small temperature dependence, while the averaged thermopower in the ab-plane in c-axis films decreases as temperature increases. These results are consistent with those made on single crystals.

Properties of thin and ultra-thin YBCO films grown by a Co-evaporation technique

An original system has been set up to grow epitaxially high quality YBCO thin films in a very reproducible way. This in-situ quasi- MBE technique allows us to prepare films whose cationic composition is kept close to 1:2:3 within 2%, and which therefore display very good crystallographic properties together with a very flat surface. Moreover, shadow masks are used to grow up to 4 films of different thicknesses in a single run and to make in-situ contact pads and caps. As a result, we routinely obtain YBCO c-axis films with a Tc(R=0) over 89.5 K (for 50 nm thick films on SrTiO 3 and MgO) and a resistivity at RT in the 250 μω cm range. The reproducibility is excellent, and the spread in resistivities for 2 films in a single run is less than 2%. Ultra-thin films (5 nm to 50 nm thick) whose electrical properties can hence be closely studied have been grown successfully in a single run.

Carbon dioxide controlled annealing method for preparation of YBa 2Cu 3O 7− y films by dipping-pyrolysis process

A three-step annealing method is proposed for the preparation of highly c-axis-oriented YBa 2Cu 3O 7− y (Y123) films by dipping-pyrolysis (DP) process when the heating rate is relatively low. This method comprises sequential heat treatment with a maximum temperature of 770°C in the following atmospheres: first, high p(CO 2); second, CO 2-free and low p(O 2); and, finally, CO 2-free and high p(O 2). With this annealing method, contamination of a-axis grains is substantially prevented and highly c-axis Y123 films are obtained, because the formation of Y123 grains from the precursor (Y 2O 3BaCO 3CuO) is markedly suppressed under the high- p(CO 2) environment during the heating ramp, at which temperatures a-axis grains tend to grow in the CO 2-free atmospheres.

Crystalline structures of the PbTiO 3 films prepared using the ECR PECVD method

The electron cyclotron resonance plasma-enhanced chemical vapor deposition method is used to prepare ferroelectric PbTiO 3 films. Single-phase perovskite PbTiO 3 films are successfully fabricated on Pt/Ti/SiO 2/Si and Pt/SiO 2/Si substrates at temperatures of 390–530 °C using metal-organic (MO) sources. When the deposition temperature is sufficiently high (above 500 °C), lead titanate film has a stoichiometric composition independently of the MO source supply ratio. Whereas the deposition temperature is low (below 450 °C), the composition and, in turn, the structure are depended on the source supply ratio. With adequate MO source ratio, stoichiometric perovskite PbTiO 3 film can be obtained at a temperature as low as 390 °C. The variations of preferred orientation, degree of c-axis orientation and film morphology with process temperature, MO source supply ratio and substrate are also examined.

Microstructural study on 90° basal-plane-faced tilt boundaries showing d-wave paring characteristics

Two kinds of needle-like a-axis grains in a c-axis YBa 2Cu 3O x thin film on a (001) MgO substrate are chosen to study the symmetry of the superconducting paring state. One kind of grain boundary (90 ° GB) is formed by (001) of the a-axis and (100) of the r-axis film while another one (45 ° GB) is formed by (001) of the a-axis grain and (110) of the c-axis film. d-Wave paring characteristics are observed for the 45 ° GB. Cross-sectional transmission electron microscopy (TEM) observation for 90 ° and 45 ° GBs shows that the a-axis grains start from the MgO substrate. Both grain boundaries are found to have atomically sharp interfaces and the distortion due to mismatch is localized only in about one atomic layer range. Further high-resolution electron microscopy (HREM) observation indicates that the (001) surface atomic layer of the a-axis grain is always a BaO layer irrespective of the orientation of the opposite grain. Moreover, these needle-like a-axis grains are found to result from small steps on MgO substrate. This offers us a chance to fabricate this kind of natural basal-plane-faced (BPF) grain boundary artificially.

Comparison of local structure measurements from c-axis polarized XAFS between a film and a single crystal of YBa2Cu3O7- delta as a function of temperature.

We have performed fluorescence x-ray absorption fine-structure (XAFS) measurements from 20-200 K on a 5000 \AA, c-axis film of YBa_2Cu_3O_{7-\delta} (YBCO) on MgO (Tc=89 K) using photons polarized perpendicular to the film. These data are compared to YBCO data from a single crystal and from a film on LaAlO_3 with the same Tc. The main difference between the single crystal and the film data is that while the single crystal data is well described by a two-site axial oxygen [O(4)] distribution, we see no evidence for such a distribution in either thin film sample. We place an upper limit on the size ofan axial oxygen splitting for the film on MgO at < 0.09 \AA. Therefore, the magnitude of the splitting is not directly related to Tc. Fits to the temperature dependent data from the YBCO film on MgO indicate that all bonds show a smooth change of their broadening factor \sigma, except the Cu-O(4) bonds, which show an increase in \sigma in the vicinity of Tc, followed by a decrease of the same magnitude. Such a feature does not occur in diffraction measurements. Since XAFS measurements of \sigma include any correlation between the atoms in a given bond, we conclude that the O(4) position becomes less correlated with the Cu positions near Tc. Correlation measurements of these and several further near-neighbors are also reported.

Influence of Copper Dopants on the Resistivity of ZnO Films

Copper 2p3/2 x-ray photoemission spectroscopy (XPS) measurements and near-edge x-ray absorption spectroscopy (NEXAFS) studies of copper-doped ZnO thin films were carried out in order to understand the role of the dopants in increasing the film resistivity. c-Axis-oriented ZnO films (thickness 120 nm) were deposited by r.f. sputtering on Cr-coated SiO2/Si substrates and subsequently implanted with metallic copper (0.5 at.%). After implantation, XPS results indicated that the dopant consists of a mixture of Cu+ and Cu0. The c-axis film resistivity was found to decrease to 7×107 Ω · cm (prior to implantation: 5×1010 Ω · cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cu2+) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2×1011 Ω · cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu2+. The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cu2+ ions to Cu+, which demonstrated that the observed decrease in film resistivity from 2×1011–107 Ω · cm is due to the Cu+ oxidation state.

Influence of Copper Dopants on the Resistivity of ZnOFilms

Copper 2p 3/2 x-ray photoemission spectroscopy (XPS) measurements and near-edge x-ray absorption spectroscopy (NEXAFS) studies of copper-doped ZnO thin films were carried out in order to understand the role of the dopants in increasing the film resistivity. c-Axis-oriented ZnO films (thickness 120 nm) were deposited by r.f. sputtering on Cr-coated SiO 2 /Si substrates and subsequently implanted with metallic copper (0.5 at.%). After implantation, XPS results indicated that the dopant consists of a mixture of Cu + and Cu 0 . The c-axis film resistivity was found to decrease to 7 x 10 7 Ω. cm (prior to implantation : 5 x10 10 Ω.cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cu 2+ ) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2 x 10 11 Ω.cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu 2+ . The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cu 2+ ions to Cu + , which demonstrated that the observed decrease in film resistivity from 2 x 10 11 -10 7 Ω. cm is due to the Cu + oxidation state.

Double CeO 2/YSZ buffer layer for the epitaxial growth of YBa 2Cu 3O 7 − δ films on Si (001) substrates

Both single YSZ and double CeO 2/YSZ buffer layers were considered for the epitaxial growth of YBaCuO films on silicon (001) substrates. The paper reports on the influence of YSZ and CeO 2 layer thicknesses on in-plane epitaxial relationships and superconducting properties of YBaCuO films. All the films were grown by pulsed laser ablation. The thickness and interface quality was deduced from grazing X-ray reflectivity and optical spectroscopic ellipsometry measurements. The structural properties have been characterized by X-ray diffraction analysis (XRD). θ-2θ scan XRD spectra showed that, in appropriate conditions, both YSZ and CeO 2 films have (001) orientation on Si (001) and YSZ/Si (001) substrates respectively. C-axis textured YBaCuO films were obtained on each buffer layer. A single YSZ buffer layer was considered first. Even when critical temperatures T c (zero resistance) of up to 88 K were obtained on YSZ/Si, ϕ-scan XRD spectra indicated that two different in-plane orientations exist in the YBaCuO films. With the introduction of an intermediate 10 nm thick CeO 2 layer on a 70 nm thick YSZ layer, the total YBaCuO in-plane alignment was obtained. YSZ and CeO 2 films were cube on cube aligned, whereas YBaCuO films were totally 45° rotated, around the c-axis, with respect to the Si (001) substrate. Critical temperatures T c ( R = 0) of 88.5 K and critical current densities above 10 6 A/cm 2 at 77 K confirm the high quality of the 40 nm thick YBaCuO films. The films are stable upon aging and thermal cycling as proved by their successful use for the realization of sensitive suspended YBaCuO microbolometers.

Microstructure of artificial 45° [001] tilt grain boundaries in YBCO films grown on (001) MgO

It is well known that high-angle grain boundaries in YBa2Cu3O7−x (YBCO) show weak-link effects and behave as Josephson junctions. This kind of grain boundary junction (GBJ) has potential applications in magnetic field measurement and electronic devices. This work studies the microstructure of artificially made GBJs in YBCO films on (001) MgO and the mechanism of the boundary formation, with the goal to improve the GBJ quality and obtain a better understanding of the junctions’ transport properties.Ion-sputter-induced epitaxy is used to form YBCO films with isolated 45° [001] tilt grain boundaries. Prior to YBCO film growth the (001) MgO substrate is selectively sputtered by a low energy Ar ion beam. A portion of the substrate remains non-sputtered by protecting the surface with a patterned photoresist mask. After removing the mask, a YBCO film is grown on the substrate using pulsed organometallic molecular beam epitaxy (POMBE). Under suitable conditions single crystal YBCO c-axis films can be reproducibly obtained in both the sputtered and non-sputtered regions. The orientation between the films and the substrate has been examined by both x-ray diffraction and electron diffraction. The in-plane orientation relation is [110]YBCO//[100]MgO on the non-sputtered epitaxially polished MgO and [100]YBCO//[100]MgO on the sputtered MgO. Thus, 45° tilt boundaries are formed in the film at the boundary between the sputtered and non-sputtered substrate regions.

Effect of beam voltage on the properties of aluminium nitride prepared by ion beam assisted deposition

The effects of nitrogen-beam voltage on the structure, stress, energy band gap and hardness of AIN thin films deposited on Si (111), Si (100) and sapphire (0001) by ion beam assisted deposition (IBAD) are reported. As the nitrogen-beam voltage was increased from 50 to 200 V, the stress and disorder in the AIN films increased as determined by X-ray diffraction, FTIR and Raman spectroscopy. The preferred orientation of the film's c-axis changed from completely normal to the film at 100 V, to a mixture of normal and in the plane of the film at 200 V. For AIN films deposited under the same conditions, the films were more highly oriented on sapphire (0001) than in Si (111). The hardness of the films increased from 18.2 to 23.7 GPa with the nitrogen-beam voltage, and possible reasons for this change in hardness are considered.

Grain growth and grain boundaries in in-plane aligned a-axis oriented YBa 2Cu 3O 7− x films on (100) LaSrGaO 4

We have grown in-plane aligned a-axis oriented YBa 2Cu 3O 7− x films on (100) LaSrGaO 4 crystalline substrates by pulsed laser deposition under various deposition conditions. We have found that sizes and shapes of the grains in the films depend critically on the deposition pressures and temperatures. Films with very smooth surfaces (mean roughness of 30–50 Å) and densely packed grains were achieved by utilizing low oxygen deposition pressure (40–60 mTorr). Critical current density ( J c) depends on the granular nature of the films and the transport across each grain boundary can be modeled as a collection of multiple parallel narrow conductive paths. Though limited by the grain boundaries, J c is still relatively high (> 10 6 A/cm 2 at 60 K, in the b-direction). Measurements in fields up to 6 T indicate that the behavior of J c versus B along the b- and c-directions in these in-plane oriented a-axis films is similar to that of standard c-axis films, with no significant deleterious effects from weak links.

A new technique for measuring the microwave penetration depth in high- Tc superconducting thin films

The electron spin resonance (ESR) technique has been used to obtain the microwave (MW) penetration depth of YBa 2Cu 3O 7 thin films. An ESR-signal-generating marker was placed in between two high-temperature superconducting (HTSC) films to probe the MW field penetrating through the films. Below the superconducting transition temperature ( T c) the HTSC film started to screen the marker inside the sandwich. A meaningfully diminishing ESR signal was reproducibly recorded for various samples with different markers such as paramagnetic diphenyl pierylhydrazy (DPPH), Mn ++ impurity in MgO and a ferromagnetic (FM) permalloy film. The temperature dependence of the ESR signal intensity above and below T c has been studied to deduce the penetration depth from the measured signal intensity. A very rapid change of λ just below T c, slowed down later at lower temperatures and became smoothly changing. The value λ ab = 1020 A ̊ has been measured for c-axis films at 77 K.

Fir josephson photoresponse of tilted c-axis epitaxial YBa2Cu3O7-δ

YBa2Cu3O7-δ thin films may be epitaxially grown, on suitably prepared substrates, with a non-zero tilt angle between the film's c-axis and its surface normal. These films are fast, room-temperature, thermoelectric detectors with a response extending from UV to FIR wavelengths. We have found that tilted c-axis films, when cooled below Tc, also exhibit a fast and sensitive Josphson photoresponse to 250 µm and 496 µm radiation, in contrast to the nil response of c-axis normal films. It is believed that the non-zero component of the radiation electric field along the c-axis of tilted films modulates relatively weak Josephson critical currents in this direction, generating a photoresponse from current biased samples.

Growth optimization and characterization of a-axis oriented Y-Ba-Cu-O thin films on [100] LaSrGaO/sub 4/ substrates

We have grown high quality a-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-y/ films on [100] LaSrGaO/sub 4/ substrates. We have optimized the crystalline orientation and the superconducting properties of our films. We have performed annealing studies on a- and c-axis films in vacuum and oxygen to compare the oxygen depletion of the films and found that T/sub c/ degradation of a-axis films is comparable to that of c-axis films. To further optimize the a-axis films for trilayer junction fabrication, we significantly reduced the surface roughness of the films by a gradual lowering of oxygen pressure during the deposition. >

Activation energy and critical current in UPd 2Al 3 films

First results concerning the E( j) characteristics of thin epitaxial UPd 2Al 3 films as a function of the magnetic field oriented parallel to the film's c-axis are presented. The activation energy for flux creep E A was extracted from resistive transitions in applied magnetic fields. E A is found to be temperature independent, but depends linearly on the magnetic field B for B ⩾ 1 T. The critical current density reaches 6500 A/cm 2 at B = 1 T and T = 0.32 K.

Morphological and structural properties of high quality YBCO thin films

High-quality YBCO thin films have been grown by Inverted Cylindrical Magnetron Sputtering (ICMS) on LaAlO3(100), SrTiO3(100), SrTiO3(110), and MgO(100) substrates. Transition temperatures of c-axis films exceed 90 K, and transition widths are within 1 K. Critical currents range up to 5 × 106 A/cm2 at 77 K. Structural and morphological features analyzed by x-ray diffraction and scanning electron microscopy, respectively, are found to be strongly dependent on film orientation and deposition temperature. In order to understand such dependence, a simple interpretation is proposed in terms of Gibbs energies and growth dynamics of the nucleation process.

Hall effect in the mixed state of a-axis oriented EuBa 2Cu 3O 7 films

EuBa 2Cu 3O 7 films of a-axis orientation were grown by DC magnetron sputtering on (100) SrTiO 3 substrates. The films were characterized by X-ray diffraction, ϱ(T) and J c(T) measurements. Hall effect has been measured in the mixed state with applied field up to 90 kOe. The Hall voltage is negative, showing just the opposite sign that appears in c-axis films. They also present sign reversal with a positive maximum. The apparition of this anomaly is clearly related to the value of the normal state resistivity. The differences with single crystal data are discussed in terms of sample microstructure.

Critical scaling and vortex glass transition in a-axis oriented EuBa 2Cu 3O 7 thin films

The I–V characteristics and the magnetoresistive transition of a-axis oriented EuBa 2Cu 3O 7 thin films grown by DC magnetron sputtering have been measured as a function of temperature. Evidence is found of a second order transition with critical exponents v=1.1 and z=8.5, greater than those reported in literature for c-axis films. The temperature dependence of the magnetoresistivity slightly above T g can be fitted to a scaling behavior ϱ ~ ( T− T g ) s with s≈8. These high values of the critical exponents may be attributed to a softening of the vortex lattice due to sample microstructure.

Film Degradation in AlN Preparation by Facing Target System

In AlN films prepared by the facing target system, a decrease in degree of c-axis orientation, film coloring and film cracking or peeling from the substrate were observed. This film degradation is thought to be related to insulative films in general. In order to clarify how these phenomena are induced, we prepared samples under various sputtering conditions; i.e., by changing target holders, placing a ground mesh in front of the substrate and using a positively biased electrode. It is confirmed that plasma exposure severely influences c-axis orientation of AlN films. To decrease this influence, it is important that electrons flow adequately into the anode.