Superconducting MgB2 films on MgO substrates

We report the preparation of homogeneous, polycrystalline MgB₂ films on single-crystalline MgO substrates by screen printing and airbrush methods. The superconducting material was obtained by the reaction of Mg vapor with boron precursor layers in special steel casings under argon/hydrogen atmosphere at 800 °C and ambient pressure. After polishing of the surface, films with a thickness of about 2 m were obtained. Electrolytic polishing improved the surface smoothness of the samples. Transition temperatures of about 33 K were measured with irreversibility fields of up to 6 T at 20 K. Transport critical current densities of 2×10⁹ Am^-2 were obtained at 4.2 K in self-field for a MgB₂ film on single-crystalline MgO. PACS 74.72.Yg; 74.76.Db; 74.60.Jg     

Production of YBa2Cu3O7−x superconducting thin films by pulsed pseudospark electron beam evaporation

Thin-film superconducting YBa₂Cu₃O_7–x layers have been produced in a single-step process by pulsed electron beam evaporation from a stoichiometric 1-2-3 target. The films were produced at the 100 surface of SrTiO₃ substrates heated to a temperature of approximately 1000 K in a pure oxygen atmosphere of about 10 Pa total pressure. After deposition the films were cooled in situ within 20 minutes to ambient temperature. At present, the films are polycrystalline and show a T_c,zero of 83 K with a transition width of 3–5 K. Critical current densities of 7·10⁴ A/cm² at 4.2 K and zero magnetic field have been achieved. The pulsed electron beams used in these experiments are produced by a pseudospark discharge; the estimated energy density deposited at the target surface by the electron beam is of the order of 4 J/cm².     

Epitaxial growth of YBa2Cu3O7−δ superconducting thin films by DC magnetron sputtering using a partially melted sintered target

In situ epitaxially grown YBa₂Cu₃O_7– (YBCO) thin films on (100)SrTiO₃ substrates with high critical current J _c=3.4 × 10⁶ A/cm² (at 77 K) and low microwave surface resistance R _s37 m (at 77 K, 50.9 GHz) are fabricated by dc magnetron sputtering using a large partially melted sintered planar target. A comparatively large homogeneous composition region can be obtained. The X-ray diffraction, X-ray double crystal diffraction, and high resolution transmission electron microscopy (HRTEM) analyses show that our deposited YBCO thin films are single crystalline containing mosaic blocks. The influence of substrate quality on the orientation behaviour of the films is also discussed. A method to improve the surface quality of the SrTiO₃ substrate, which improves the properties of the thin film, is presented.     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

Pulsed laser deposition of BiCuOSe thin films

Undoped and 10% Ca-doped BiCuOSe thin films are prepared by pulsed laser deposition without ex-situ processing. The influence of the preparation conditions on structure and properties of Bi_0.9Ca_0.1CuOSe thin films on amorphous silica substrates is studied. The highest achieved concentration and mobility of free holes (3.9×10²⁰ cm⁻³ and 3.5 cm²/Vs) was close to that measured in strongly c-axis oriented samples on SrTiO₃ substrates (4.0×10²⁰ cm⁻³ and 7.5 cm²/Vs). The Bi_0.9Ca_0.1CuOSe films on SrTiO₃ show almost temperature-independent Seebeck coefficient and their resistivity increases with increasing temperature. The Seebeck coefficient of undoped BiCuOSe films on SrTiO₃ increases below 150°K, and the resistivity shows a flat plateau centered at this temperature. Optical measurements suggest that BiCuOSe has an indirect bandgap of 0.8 eV and a strong absorption edge at 1.45 eV. Ab-initio calculations of the electronic band structure, effective masses and optical properties of BiCuOSe are also presented.     

Substrate temperature influenced structural, electrical and optical properties of dc magnetron sputtered MoO3 films

Molybdenum oxide (MoO₃) films were deposited on glass and (1 1 1) silicon substrates by sputtering of metallic molybdenum target in an oxygen partial pressure of 2 × 10⁻⁴ mbar and different substrate temperatures in the range 303-623 K using dc magnetron sputtering technique. X-ray photoelectron spectrum of the films formed at 303 K showed asymmetric Mo 3d_5/2 and Mo 3d_3/2 peaks due to the presence of mixed oxidation states of Mo⁵⁺ and Mo⁶⁺ while those deposited at substrate temperatures ≥473 K were in Mo⁶⁺ oxidation state of MoO₃. The films formed at substrate temperatures ≥473 K were polycrystalline in nature with orthorhombic α-phase MoO₃. Fourier transform infrared spectra of the films showed an absorption band at 1000 cm⁻¹ correspond to the stretching vibration of MoO, the characteristic of the α-MoO₃ phase. The electrical resistivity increased from 3.3 × 10³ to 8.3 × 10⁴ Ω cm with the increase of substrate temperature from 303 to 473 K respectively due to improvement in the crystallinity of the films. Optical band gap of the films increased from 3.03 to 3.22 eV with the increase of substrate temperature from 303 to 523 K.     

Inclined ZnO thin films produced by pulsed-laser deposition

ZnO thin films with a uniformly inclined structure are grown by pulsed-laser deposition on SrTiO₃. The c-axis of ZnO films is inclined by an angle =20±0.5° and =42±0.5° against the surface normal of 25° miscut (100) SrTiO₃ and (110) SrTiO₃ single crystal substrates, respectively. The inclined structure is due to epitaxial growth of hexagonal ZnO on cubic SrTiO₃ as evidenced by X-ray diffraction and high-resolution transmission electron microscopy investigations. The range of deposition parameters (substrate temperature, oxygen background pressure) to achieve epitaxial growth is determined. The inclined films are smooth with an rms surface roughness of 1.5 nm for layer thicknesses up to 700 nm. PACS 61.10.-i; 68.37.-d; 81.15.Fg     

Electrical properties of Sr<Subscript>3</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>6</Subscript>O<Subscript>21</Subscript> thin films

Highly c-axis-oriented Sr₃Bi₄Ti₆O₂₁ (SBTi) thin films were fabricated on Pt-coated Si substrates by pulsed laser deposition (PLD). The structures were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). No peaks of SrTiO₃ (STO) could be detected in the XRD pattern, indicating the existence of the SBTi single phase. Good ferroelectric hysteresis loops of the films with Pt electrodes were obtained. With an applied field of 400 kV/cm, the measured remanent polarization (P_r) and coercive field (E_c) values were 4.1 C/cm² and 75 kV/cm respectively. The films showed little fatigue after 2.22×10⁹ switching cycles: the nonvolatile polarizations decreased by less than 5% of the initial values. The dielectric constant and the loss tangent of the films were measured to be 363 and 0.04 at 100 kHz. These results might be advantageous for nonvolatile ferroelectric random access memory (NVFRAM) and dynamic random access memory (DRAM). PACS 77.84.Dy; 77.22.-d; 68.55.Jk     

Microscopic structure and structuring of perovskite surfaces and interfaces: SrTiO3, RBa2Cu3O7-δ

3 surfaces and bicrystal interfaces and the growth of YBa₂Cu₃O_7-δ thin films on such substrates using scanning tunneling microscopy and X-ray diffraction. Proper annealing of SrTiO₃ in oxygen and/or ultrahigh vacuum produces uniformly terminated, atomically flat and well-ordered surfaces. For vicinal SrTiO₃(001) surfaces the particular annealing sequence and miscut angle sensitively determines the resulting step structure and thus the microscopic surface morphology. Steps of SrTiO₃(001) surfaces can be adjusted to a height of one, two, or multiple times the unit-cell height (a_STO=0.3905 nm). The growth of YBa₂Cu₃O_7-δ films on these substrates by pulsed laser deposition was traced from the initial nucleation to a thickness of about 300 nm. The morphology, texture, and defect structure of the films is determined by the specific structure and morphology of the pristine substrate. Anisotropic, planar defects, originating from substrate step edges, strongly modify the electronic transport properties of the film leading to critical currents up to ≈9×10⁷ A/cm² at 4 K as well as pronounced transport anisotropies. Surfaces and interface energy terms are discussed, which also determine the observed structure of bicrystal boundaries. Received: 16 April 1998/Accepted: 21 August 1998     

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.     

Electrical properties and conduction mechanisms in La2/3Ca1/3MnO3 thin films prepared by pulsed laser deposition on different substrates

Perovskite manganite La_2/3Ca_1/3MnO₃ thin films were directly grown on MgO(100), Si(100) and glass substrates by pulsed laser deposition. From the XRD patterns, the films are found to be polycrystalline, single-phase orthorhombic. The metal–insulator transition temperature is 209 K for LCMO/MgO, 266 K for LCMO/Si and 231 K for film deposited on the glass substrate. The conduction mechanism in these films is investigated in different temperature regimes. Low-temperature resistivity data below the phase transition temperature (T _P) have been fitted with the relation \( \rho = \rho_{0} + \rho_{2} T^{2} + \rho_{4.5} T^{4.5} \) , indicating that the electron–electron scattering affects the conduction of these materials. The high-temperature resistivity data (T > T _P) were explained using variable-range hopping (VRH) and small-polaron hopping (SPH) models. Debye temperature values are 548 K for LCMO/Cg, 568 K for LCMO/Si and 508 K for LCMO/MgO thin films. In all thin films, the best fitting in the range of VRH is found for 3D dimension. The density of states near the Fermi level N (E _F) for LCMO/MgO is lower due to the prominent role of the grain boundary in LCMO/MgO and increase in bending of Mn–O–Mn bond angle, which decreases the double exchange coupling of Mn³⁺–O²–Mn⁴⁺ and in turn makes the LCMO/MgO sample less conducting as compared to the other films.     

Indium tin oxide as an optical memory material

Indium Tin Oxide (ITO) films prepared by reactive rf sputtering show excellent properties for optical recording applications in a very narrow range of oxygen partial pressure (around 4×10^–5 Torr). This narrow range is at the edge of a plateau in the electrical conductivity of the films. A small increase in the oxygen partial pressure (P(O₂)5×10^–5 Torr) causes a large and abrupt change in the electrical conductivity as well as in the structural and optical properties of these films. In addition, irradiating films at the edge of the plateau (P(O₂)4×10^–5) with a low-power pulsed laser (25 mW) yields transparent films. These results suggest that the same mechanism may be responsible for the opaque to transparent transformations observed in these experiments.     

One-step preparation of N-doped strontium titanate films by pulsed laser deposition

Perovskite-type oxynitrides exhibit promising electrical and optical properties and can possibly be used in the future as functional materials for electrical, photo-, and electrochemical applications. Continuous heterovalent substitution of oxygen ions by nitrogen ions allows tuning of the desired optical and/or electronic properties to the application specifications. In the present work deposition of SrTiO₃:N films by pulsed reactive crossed beam laser ablation was studied in order to examine the influence of different deposition parameters on the film crystallinity and composition. The deposited films exhibit a perovskite-type crystal structure and reveals epitaxial growth on MgO(100) substrates. The unit cell parameters of the deposited SrTiO₃:N films range within , which is slightly larger than for polycrystalline SrTiO₃ (a=3.905). The studied films reveal an oxygen content in the range of (2.70-2.98)±0.15. The relative N content (vs. O) can be tuned within the range of 1.0–3.0% by adjusting the deposition parameters. The N:O concentration ratio increases with increasing laser fluence and target-to-substrate distances, while the substrate temperature has a more complex influence on the nitrogen concentration. In the range of 580–650 °C the [N]/[O] ratio increases while further heating results in a gradual decrease of the N content. PACS  81.15.Fg; 68.55.-a; 81.05.Zx     

Preparation and transport properties of Ba1−x Sr x CuO2+δ infinite layer films grown by molecular-beam epitaxy

Thin films of Ba_1–x Sr _x CuO₂₊ in the infinite layer structure were prepared by molecular beam epitaxy on SrTiO₃ substrates. Excellent in-plane order during growth was shown by RHEED. The lattice constant inc-direction was determined by x-ray diffraction. It changed from 0.404 nm to 0.345 nm whenx increased from 0 to 1. The film surfaces were smooth with some outgrowths as revealed by atomic force microscopy. Excellent crystal structure and epitaxy of the films was demonstrated by high resolution transmission electron microscopy. The room temperature dc resistivities of the films varied from 10^–3 cm to 10² cm, depending onx and on the oxidation conditions during growth. The resistivities of most films showed negative temperature coefficients and obeyed the conduction model of variable range hopping at low temperatures. In the composition rangex=0.5–0.8, however, an anomalous resistance dependence on temperature was observed in many samples. The resistivities started to deviate from the monotonic behaviour just below 200 K and in some cases dropped remarkably at temperatures below 140 K.     

Preparation and electrical properties of Bi<Subscript>3.25</Subscript>Pr<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films

Bi_3.25Pr_0.75Ti₃O₁₂ (BPT) ferroelectric thin films have been prepared by chemical solution deposition on platinized Si substrates. Well-crystallized BPT films can be achieved by 600 °C rapid thermal annealing. The film surface is smooth and crack-free, composed of uniform spherical grains around 90–100 nm in diameter. The electrical properties of Pt/BPT/Pt thin film capacitors were characterized by hysteresis and impedance measurements. The remanent polarization of 700 °C annealed BPT films is around 20 C/cm² at 120-kV/cm stimulus field. The dielectric constant is around 380 at 10 kHz, 100-mV amplitude. The remanent polarization of BPT film showed a slight reduction, 10% of its original value, after 2.8×10⁹ cycles, while a 30% reduction of non-volatile polarization was observed. PACS 81.15.-z; 77.55.+f; 77.22.Gm     

Effect of deposition conditions on YBa2Cu3O7−δ thin films by inverted cylindrical magnetron sputtering and substrate effects

The dependence of YBCO thin film properties on the deposition conditions was studied for different substrates. The deposition conditions were optimized for the epitaxial growth of high quality YBCO thin films of 1500 Å thickness onto single crystal (100-oriented) SrTiO₃ (STO), MgO and LaAlO₃ (LAO) substrates by DC Inverted Cylindrical Magnetron Sputtering (ICMS). The samples were investigated in detail by means of X-ray diffraction analysis (XRD), EDX, AFM, ρ-T, magnetic susceptibility and current-voltage (I-V) characterizations. The samples show strong diamagnetic behavior and sharp transition temperatures of 89-91 K with ΔT<0.5 K. XRD of the samples exhibited highly c-axis orientation. The full width at half maximum (FWHM) values of the rocking curves were ranging typically from 0.22 to 0.28°. The samples have smooth surfaces as shown from AFM micrographs. The surface roughness, R_a, changed between 5-7 nm. I-V characteristics were obtained from the 20 μm-wide microbridges, which were patterned by a laser writing technique. The critical current densities (J_c, 1.06×10⁶ for LAO-based YBCO, 1.39×10⁶ for MgO-based YBCO, 1.67×10⁶ A/cm² for STO based YBCO) of the microbridges were evaluated from I-V curves at 77 K.     

In situ preparation of Bi-Sr-Ca-Cu-O superconducting films by laser sputtering

Superconducting films of Bi-Sr-Ca-Cu-O on (100) MgO substrates have been fabricated by XeCl-excimer-laser sputtering from ceramic targets of Bi_2.5Sr₂Ca₂Cu₃O_y in O₂ atmosphere. The films were polycrystalline with the c-axis (30.80±0.02Å) preferably oriented normal to the substrate surface. Without post-annealing the films showed metallic resistance behavior with zero resistance temperatures of up to T_c(0) }- 79 K. The critical current density of the films had values of up to j_c(50K)}- 10⁴ A/cm².     

Preparation of Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox films

Preparation of superconducting Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba₂Ca₂Cu₃O_x thin films is reported. From a sintered Ba₂Ca₂Cu₃O_x bulk sample we prepared using a pulsed Nd:YAG laser, Ba-Ca-Cu-O thin films on sapphire and SrTiO₃ single-crystal substrates. Subsequently the films were loaded with Tl by simultaneously annealing the films together with a sintered Tl₂Ba₂Ca₂Cu₃O₁₀ sample both enclosed in a small stainless steel box; in our procedure Tl contamination was reduced to a minimum. T_c values near 100 K and critical currents of 5·10³ A/cm² at 77 K were obtained.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

Laser deposited YBaCuO thin films

YBa₂Cu₃O_7-x thin films were deposited by laser ablation using KrF excimer laser. In deposition on polycrystalline ZrO₂/(1–102) sapphire substrates influence of deposition conditions on film properties were studied. Zero resistance temperatureT _z of 83 K of epitaxially grown YBaCuO films on poly-ZrO₂/sapphire substrates was reached. Epitaxially grown yttria stabilized zirconia (YSZ) buffer layers were deposited by laser ablation on (1–102) sapphire substrates. On YSZ/sapphire, SrTiO₃ and NdGaO₃ substrates, temperatures T_z between 89–90 K were measured. Results of X-ray diffraction and SEM are also presented.We would like to express our thanks to L. Cibulka, L. and L. Rouek, and J. Stránský for their efficient help during the solution of technological problems connected with the construction of experimental apparatus.