Zr-induced structural changes in Hf1−x Zr x O2 high-k thin films

The local symmetries around the Hf sites in thin films of Hf_1?x Zr _x O₂/Si(100) were probed using grazing incidence extended X-ray absorption fine-structure spectroscopy (EXAFS). The effects of the Zr incorporation on the local crystal symmetries were investigated using Hf L₃ EXAFS at the Beamline X23A2 of the Brookhaven National Laboratory. The Zr ratios in the various films were set to between 0.0 and 1.0. Significant changes in the local environment were observed for x = 0.25 or greater values. For x = 0.0, the film local structure around Hf sites remain in the equilibrium monoclinic phase as referenced from our previous studies on HfO₂ thin films on Si(100). When Zr is introduced, tetragonal symmetry around the Hf atom appears and becomes dominant at x = 0.63. Using the EXAFS theoretical simulations and non-linear least-square fit results, the fractions of the monoclinic versus tetragonal phases were identified in each film.     

Model of current and optical conductivity switching in YBa2Cu3O7−x films

A model is proposed for the thermal and electrical responses of films of the high-T _c superconducting material YBa₂Cu₃O7_7−x to current and optical pulses. Numerical calculations are compared with experimental data for current pulses of duration 100 μs and laser pulses of duration 0.1 ns; this yields improved data on the thermal conductivity of thin YBa₂Cu₃O7_7−x films (1.5–2 W/m·K) and thermal resistance of the film-substrate contact (5×10⁻⁸m²·K/W) in the neighborhood of the superconducting transition. This model can be used for optimizing the film structure parameters and control regimes for switching elements for pulses lasting longer than 0.1 ns. Zh. Tekh. Fiz. 69, 77–82 (October 1999)     

Infrared absorption of O−H in thin films of hydrogenated Bi2Sr2CaCu2O8

Thin films of hydrogenated Bi₂Sr₂CaCu₂O₈ show three O–H vibrational absorption bands that shift upon deuteration with frequency ratiosv _O-H/v _O-D1.3, as expected for local modes. The band positions indicate that hydrogen atoms may be located at two different sites, namely in CuO₂ layers and, for the most part, in BiO double layers. The temperature dependences of these absorption bands give no hint of any direct interactions between the O–H vibrational modes and the superconducting phase transition.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.     

Misfit stresses in YBa2Cu3O7−x films

Misfit and thermoelastic stresses in HTSC layers 1-2-3 grown on various substrates are, analyzed with the use of ultrasonic measurements of Young's moduli inYBa ₂ Cu ₃ O _7−x . It is shown that the misfit stress gives the main contribution to the formation of the HTSC layer strain. Military Academy of Strategic Rocket Forces. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 87–90, April, 2000.     

Growth of YBa2Cu3O7−x thin films on sapphire with a cerium-oxide sublayer

The possibility is demonstrated of preparing high-quality films of the high-temperature superconductor YBa₂Cu₃C_7-δ with thicknesses up to 2.6 μm by dc magnetron sputtering. It is found that inclusions consisting of CuO and YBa₂Cu₃O₈ coexist with the growing film and are “sinks” for defects, nonstoichiometric atoms, and mechanical stresses. Using x-ray diffraction and Rutherford backscattering, we find that the structural perfection of the films is improved by increasing the thickness when using the proposed fabrication technique. Zh. Tekh. Fiz. 69, 94–98 (January 1999)     

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.     

Effects of laser irradiation on optical properties of a-Se100−x Te x thin films

The effect of laser irradiation on the optical properties of thermally evaporated Se_100?x Te _x (x=8, 12, 16) chalcogenide thin films has been studied. The result shows that the irradiation causes a shift in the optical gap. The results have been analyzed on the basis of laser irradiation-induced defects in the film. The width of the tail of localized state in the band gap has been evaluated using the Urbach edge method. As the irradiation time increases, the values of the optical energy gap for all compositions decrease, while tail energy width increases. It is also observed that the optical energy gap decreases with increasing Te content in the alloy. These changes are a consequence of an increment in disorder produced by laser irradiation in the amorphous structure of thin film.     

Conduction centers in a Ta2O5−δ Fermi glass

We compare our photoemission spectra with the calculated energy spectra of oxygen vacancies in Ta oxide to identify the defects responsible for the formation of a band in the gap of a Ta oxide-based memristor. We have previously explained transitions between high and low resistance states in a memristor conducting channel under bias reversal by accumulation and depletion of oxygen in the channel. Oxidation leads to a higher resistance due to sparser conduction centers for the electrons to hop between. Here, we show that they are likely due to neutral oxygen vacancies located at ‘in-plane’ sites of the Ta–O polyhedra.     

Surface nanostructure effects on optical properties of Pb(Zr x Ti1−x )O3 thin films

Optical scattering properties of nanostructured matter have crucial impact on performance efficiency of various photonic components, such as waveguides, display elements, and solar cells. In this paper, diffuse transmission properties of nanocrystalline Pb(Zr _x Ti_1?x )O₃ thin films with a high refractive index of ~2.5 and optical transmittance are presented. Thin films with a thicknesses ranging from 50 to 500 nm were studied using integrating sphere technique and results were compared to simulations performed by a scalar scattering theory. Thin films were deposited by pulsed laser deposition at room temperature on MgO(100) substrates and post-annealed at a temperature of 800 °C. Structural phase evolution-induced surface effects, which introduced periodicity on the film surface, cause the definite diffuse elements in transmission spectra of the films. Low and evenly distributed scattering amplitudes in k-space were seen for highly tetragonal- or trigonal-oriented films with non-textured surfaces, which led to low diffuse transmission values (T _D ≈ 5 %), while confined and increased scattering amplitudes in k-space were seen for tetragonal–trigonal-oriented films, with phase co-existence, which led to microstructure-induced textured surfaces and increased diffuse transmission values (T _D ≈ 50 %). For highly textured surfaces, scattering amplitudes distributed in tilted ellipsoid shape in k-space was observed. Difference between modeled and measured values was 3.8 % in maximum.     

Hyperfine fields and magnetic anisotropy in Fe1000−x Gd x thin films and Fe80−x GdxB20(0≤x≤24) thin films and ribbons

The Mössbauer measurements performed on Fe_100–x Gd _x thin films and on Fe_80–x Gd _x B₂₀ both as thin films and ribbons show a dependence of the spins orientation and H_hyp versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and H_hyp decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.     

Various properties of sputter-deposited Ta–Ru thin films

This paper discusses several structural, electrical and oxidation characteristics of co-sputtered Ta–Ru alloy films on oxidized Si-substrates. From X-ray examination, the Ta₁Ru₁ phase has formed and dominates in the compositions exceeding 54 at.% Ru content. The resistivity of the Ta–Ru thin films can reach a maximum of ∼320 μΩ cm in the composition range between 35 and 54 at.% Ru. After thermal treatment in air (600°C, 1 h), Ru-rich samples show a less increase in resistivity than Ta-rich ones. The observed preferential oxidation of Ta in the Ta–Ru samples can be further interpreted by thermodynamic calculations. The Ta-rich surface oxide is believed to be responsible for the passivating ability of the Ru atom toward oxidation at high temperatures. This results in the Ru of the metallic state though the oxidation of Ta occurs.     

In-plane and out-of-plane anisotropic magnetoresistances in La1 − x Pb x MnO3 thin films

The temperature and field dependences of in-plane (IP) and out-of-plane (OP) anisotropic magnetoresistances (AMRs) have been measured in La_{1 ? x} Pb _x MnO₃ (LPMO) thin films having different microstructures, namely the single-crystal (SC), nanocrystalline (NC) and polycrystalline (PC) microstructures. The OP AMR, irrespective of the microstructure, is found to be considerably larger than the IP AMR. The magnetization data show that the larger OP AMR arises because magnetization is favoured in the film plane owing to strain anisotropy. In addition, the temperature and field dependences of both IP and OP AMRs are governed by the crystallinity of the films, indicating that the AMRs are strongly influenced by the magnetization process.     

Ferromagnetic resonance study of Y3−z Ca z Fe5−x−y Co x Ge y O12 films

Conclusions It has been found that the character of the magnetic anisotropy changes markedly when the Co + Ge YIG films are doped with calcium. For the induced uniaxial and in-plane anisotropy we observe an increasing departure from the predictions based on the two-parameter model and for the cubic anisotropy the constantK ₁ becomes dependent on the growth direction. As a source for the latter effect the octahedral or tetrahedral preference of Co ions depending on the growth direction may be considered. The measured linewidth proportional to frequency indicates the presence of a relaxation mechanism which is, probably, connected with octahedral Co³⁺ ions.The authors express their thanks to Dr. P. Görnert and M. Neviva for preparation of the LPE films and to Dr. P. Novák for valuable discussions.Dedicated to Jan Kaczér DrSc on the occasion of his 65th birthday.     

Coherent magnetic vortex motion in optically formed channels for easy flow in YBa2Cu3O7−x superconducting thin films

We report our results of investigation of electric and magnetic properties of partially oxygen-depleted channels for easy vortex motion in YBa₂Cu₃O_7?x (YBCO) superconducting, 50-μm-wide, and 100-μm-long microbridges at temperatures below the onset of the superconducting state critical temperature T _c ^on . The channels were produced by means of a laser-writing technique. The writing was performed using a 0.1–0.3 W power, continuous-wave laser radiation focused down to a ~ 5 μm spot on the surface of a superconducting film in a nitrogen gas atmosphere, and resulted in perpendicular stripes (channels) with partial (x ~ 0.2) reduction of the oxygen content in the YBCO stripe. The oxygen-depleted channels exhibit a depressed T _c and lower both the critical current density and the first critical magnetic field, as compared with the laser-untreated areas. The bias current applied to the bridge self-produced a magnetic flux that penetrated the channels in a form of Abrikosov magnetic vortices that, subsequently, moved coherently (a quasi-Josephson effect) along the channels in the narrow temperature range of 0.943 T _c ^on –0.98 T _c ^on and manifested themselves as steps on the current–voltage characteristics of our microbridges. Our results demonstrate that laser-induced formation of artificial channels of the flux flow can be used for a precise control of vortex nucleation and their coherent motion in pre-assigned regions of thin-film YBCO devices.     

Weak ferromagnetism and spin density distributions in thin films of Gd x Bi1 − x FeO3 solid solutions

Polycrystalline thin-film Gd _x Bi_{1 ? x} FeO₃ (x = 0, 0.05, 0.10, 0.15, or 0.20) samples are synthesized by means of thermal vacuum deposition. The concentrations, temperatures, and magnetic field dependences of specific magnetization are studied. Self-consistent calculations of the spin density distribution are performed for R3c BiFeO₃ and Pnma Gd_0.25Bi_0.75FeO₃ using the density functional theory in the LSDA approximation.     

Raman study of the 90° grain boundaries in YBa2Cu3O7−δ thin films

YBa₂Cu₃O_7−δ (YBCO) films containing two different types of 90° grain boundaries were fabricated on the same substrates. Raman spectra from several parts of the basal-plane-faced tilt (TL) grain boundaries were measured and compared with those from the twist (TW) grain boundaries. The Raman results show that I(500)/I(340), the relative intensity of the A_1g mode near 500 cm⁻¹ with respect to that of the B_1g mode near 340 cm⁻¹, is suppressed in the TL boundaries, relative to the TW boundaries. The results can be interpreted to mean that the stress is stronger in the TL boundaries than in the TW boundaries. This may offer an alternative explanation for the weak-link behavior of the step-edge Josephson junctions.     

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr_1−x La _x CuO₂ (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO₃ buffered (001) SrTiO₃ substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr_1−x La _x CuO₂ films. The RHEED data showed that the Sr_1−x La _x CuO₂ films grow on BaTiO₃/SrTiO₃ following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d _c) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d _c increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr_1−x La _x CuO₂ films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr_1−x La _x CuO₂ films.