Effect of different oxidizing gases on the in-situ growth of YBa2Cu3O7−δ films by pulsed laser deposition

The effectiveness of oxygen (O₂), nitrous oxide (N₂O), and nitrogen dioxide (NO₂) as oxidizing agents during in-situ growth of YBa₂Cu₃O_7−δ (YBCO) films on (100) SrTiO₃ substrates by pulsed laser deposition has been studied as a function of deposition temperature (700–800°C), and laser wavelength (193,248 and 355 nm), for a wide range of oxidizer gas pressure (0.1–200 mTorr). In general, the superconducting transition temperature of the films has been found to increase with increasing oxidant pressure, with zero-resistance temperature ≈90 K only obtained in films prepared in a relatively high pressure (150–200 mTorr) of oxidizer gas. At lower pressures, the transition temperature while being depressed is quite sensitive to the nature of the oxidant, the laser wavelength and the deposition temperature. Nevertheless, independent of the oxygen source or other growth parameters, an almost linear decrease in transition temperature with a corresponding increase in the c-axis lattice parameter has been observed for all the film. YBCO films have also been deposited in a low pressure background (≤ 1 mTorr) using a combination of atomic oxygen and pulsed molecular oxygen. The results are discussed in terms of the oxygen requirement for kinetic and thermodynamic stability of YBCO during growth of the film by pulsed laser deposition.     

Bi-axial texture in Ca0.1Y0.9Ba2Cu4O8 composite wires made by metallic precursors

High filament count, silver-sheathed composite wires of Ca_0.1Y_0.9Ba₂Cu₄O₈ (Y–124) were prepared by a metallic precursor route. The ductility of the metallic precursor enabled one to manufacture tapes containing up to 962 407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. By using a thermal-mechanical treatment to texture the Y–124 grains, transport critical current densities in the oxide filaments of 69 500 A/cm² at 4.2 K in self-field were obtained. Moreover, in an applied field of 0.1 T, the samples retained 39% of their self-field critical current density. A TEM investigation revealed significant bi-axial crystallographic texture: in areas viewed, c-axis alignment of adjacent grains was within 10° and oriented perpendicular to the tape face; a-axis alignment of adjacent grains was within 15° and oriented parallel to the longitudinal direction of the filaments. Furthermore, c-axis texture alone did not adequately predict the performance of these Y−124 composite conductors. Instead, performance scaled with the degree of bi-axial texture. These wires exhibited among the best reported J_c for a polycrystalline, sintered wire of YBCO in an applied magnetic field.     

Preparation of tantalum oxide thin films by photo-assisted atomic layer deposition

Tantalum oxide thin films were prepared by photo-assisted atomic layer deposition (Photo-ALD) in the substrate temperature range of 170–400 °C using Ta(OC₂H₅)₅ and H₂O as precursors. The constant growth rates of 0.42 and 0.47 Å per cycle were achieved for the films grown by normal ALD and Photo-ALD, respectively. The increased growth rate in Photo-ALD is probably due to the reactive surface by photon energy and faster surface reaction. In Photo-ALD, however, the constant growth rate started at lower temperature of 30 °C and one cycle time shortened up to 5.7 s than that of normal ALD. The films grown by normal ALD and Photo-ALD were amorphous and very smooth (0.21–0.35 nm) as examined by X-ray diffractometer and atomic force microscopy, respectively. Also, the refractive index was found to be 2.12–2.16 at the substrate temperature of 190–300 °C, similar to that of the film grown by normal ALD. However, the remarkably low leakage current density of 0.6×10⁻⁶ A/cm² to 1×10⁻⁶ A/cm² at applied field of 1 MV/cm is several order of magnitude smaller than that of normal ALD, probably due to the presence of reactive atom species.     

Effects of precursors on nucleation in atomic layer deposition of HfO2

Atomic layer deposition of hafnium dioxide (HfO₂) on silicon substrates was studied. It was revealed that due to low adsorption probability of HfCl₄ on silicon substrates at higher temperatures (450–600 °C) the growth was non-uniform and markedly hindered in the initial stage of the HfCl₄–H₂O process. In the HfI₄–H₂O and HfI₄–O₂ processes, uniform growth with acceptable rate was obtained from the beginning of deposition. As a result, the HfI₄–H₂O and HfI₄–O₂ processes allowed deposition of smoother, more homogeneous and denser films than the HfCl₄–H₂O process did. The crystal structure developed, however, faster at the beginning of the HfCl₄–H₂O process.     

Crystal structure of YBa2Cu3O7−x thin films prepared by pulsed laser deposition with substrate bias voltage

The crystal structure of YBa₂Cu₃O_7−x thin films has been investigated by cross-section transmission electron microscopy. The samples were deposited on MgO (100) substrates at 670°C with substrate bias voltages of ±300 V. For the unbiased case, c-axis, a-axis and (103) oriented domains normal to the substrate surface were observed. In this film, the c-axis oriented domains are dominant, but the crystal often exhibits a longer c-lattice constant than that of the YBa₂Cu₃O_7−x system, so extra cationic layers are inserted in the YBa₂Cu₃O_7−x intrinsic stacking sequence. For the case of −300 V, rotated domains were dominant in the entire film; however, c-axis oriented domains also grow from the substrate surface. Small-angle semicoherent grain boundaries between them were observed. In the case of +300 V, all the grains show c-axis oriented YBa₂Cu₃O_7−x. The degree of preferential orientation of the grains is reduced at negative bias voltage of −300 V and the structure defects are reduced by applying a positive bias of +300 V.     

Surface morphology and electrical properties of Bi---Sr---Ca---Cu---O thin films deposited by reactive sputtering from multiple targets

Smooth, superconducting films of Bi₂Sr₂CaCu₂O_8+° have been prepared by reactive sputtering from elemental targets in the presence of ozone. The influence of substrate temperature, deposition rate, and ozone pressure on the resulting films are discussed. Films deposited on SrTiO₃ substrates are c-axis oriented and featureless for substrate temperatures below 710°C. Above this temperature, small inclusions of CuO appear. Films on MgO exhibit mixed a-axis and c-axis orientation below about 710°C, and inclusions of CuO above this temperature. The temperature at which this transition occurs increases with increasing deposition rate. T_c increases and then decreases sharply with decreasing oxygen content. The oxygen partial pressure corresponding to the maximum T_c of 77 K is well above the thermodynamic stability limit for Bi₂Sr₂CaCu₂O_8+°, suggesting that an optimum carrier concentration has been achieved for these films.     

The dopant concentration and annealing temperature dependence of ferromagnetism in Co-doped ZnO thin films

A series of Zn_1−xCo_xO thin films with the atomic fraction, x, in the range of 0.03–0.10 were deposited on glass substrates at room temperature by magnetron co-sputtering technique and subsequently coupled with the post-annealing treatment for half hour at different temperatures (350 °C and 500 °C) under vacuum. A systematic study was done on the structural, optical and magnetic properties of Zn_1−xCo_xO thin films as a function of Co concentration and annealing temperature. X-ray diffraction and UV–vis spectroscopy results indicated that there are not any secondary phases and Co²⁺ substituted for Zn²⁺ of ZnO host. Magnetic hysteresis loops were observed at room temperature, indicating that both the as-deposited samples and the annealed ones exhibit the room temperature ferromagnetism. It was also found that the magnetic saturation moment per Co atom decreases with increasing Co concentration, while the post-annealing treatment can enhance the magnetic moment of the films effectively.     

High quality epitaxial YBCO(F) films directly deposited on sapphire

High-quality epitaxial YBa₂Cu₃O_7−δ (F) films were grown directly on r-plane (1 02) sapphire by a pulsed laser deposition technique using a multiphase fluorinated target. X-ray diffraction data showed that the film is highly c-axis oriented and in-plane aligned. The c-axis and in-plane mosaic spread are 1.1° and 2.4°, respectively. The microwave surface resistance measured by a con-focal resonator configuration is 56 mΩ at 94.1 GHz at 77 K which is the best reported value for a YBCO film on sapphire.

The improved film quality is attributed to the fluorine which is transported from the target to the growing surface of the film where it acts as an etching and/or catalytic agent to preferentially remove defects and second phase impurities and to promote better in-plane epitaxy.     

Enhanced magnetic properties of Nd–Fe–B thin films crystallized by heat treatment

Nd₂Fe₁₄B Φ phase crystallites were formed in Nd_16.7Fe_65.5B_17.8 thin films prepared by RF sputtering with subsequent heat treatment. The 2 μm-thick films were deposited onto 0.1 mm Mo sheets at an average substrate temperature (T_s) of 365°C. The enhanced magnetic properties of the magnetically anisotropic thin films were investigated using different heating rates (h_r) of 10°C, 20°C, 50°C and 100°C/min in an annealing experiment. Transformation from the amorphous phase to the crystalline phase is clearly manifested by the formation of fine crystallites embedded as a columnar matrix of Nd₂Fe₁₄B phase. High-resolution scanning electron microscope data of the cross-section of the annealed films show columnar stacking of Nd₂Fe₁₄B crystallites with sizes <500 nm. Transmission electron microscope observations revealed that the microstructure of these films having out-of-plane magnetization consists of uniformly distributed Φ phase with grain size around 400 nm together with small Nd rich particles. This grain size of Φ phase is comparable to the single domain particle diameter of Nd₂Fe₁₄B. Significant change in _iH_c, 4πM_r and 4πM_s with h_r was confirmed. Annealing conditions with a heating rate of 50°C/min to an annealing temperature (T_a) of 650°C for 30 min was consequently found to give optimum properties for the NdFeB thin films. The resulting magnetic properties, considered to be the effect of varying h_r were _iH_c= 1307–1357 kA/m, 4πM_r=0.78–1.06 T and 4πM_s=0.81–1.07 T.     

Synthesis and physical behaviour of In2S3 films

In₂S₃ layers have been grown by close-spaced evaporation of pre-synthesized In₂S₃ powder from its constituent elements. The layers were deposited on glass substrates at temperatures in the range, 200–350 °C. The effect of substrate temperature on composition, structure, morphology, electrical and optical properties of the as-grown indium sulfide films has been studied. The synthesized powder exhibited cubic structure with a grain size of 63.92 nm and S/In ratio of 1.01. The films grown at 200 °C were amorphous in nature while its crystallinity increased with the increase of substrate temperature to 300 °C. The films exhibited pure tetragonal β-In₂S₃ phase at the substrate temperature of 350 °C. The surface morphological analysis revealed that the films grown at 300 °C had an average roughness of 1.43 nm. These films showed a S/In ratio of 0.98 and a lower electrical resistivity of 1.28 × 10³ Ω cm. The optical band gap was found to be direct and the layers grown at 300 °C showed a higher optical transmittance of 78% and an energy band gap of 2.49 eV.     

H2O surface interaction of oxygen-free polyphenylacetylene films investigated by XPS

The interaction of water with the surface of polyphenylacetylene (PPA) films is dependent on the preparation and casting procedures. Films with an oxygen-free surface were obtained. Molecular adsorption of H₂O is indicated to be the major phenomenon by XPS. On annealing up to ≈ 250°C partial loss of adsorbed water occurs. The thickness of the reacted surface layer is estimated to be ≈ 6.0Å when the PPA film is exposed to water vapour at pressure p ≈ 760 Torr.     

Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films

The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa₂Cu₃O₇ (YBCO) films was systematically investigated. The MgO substrates were either “as received” or stored for some time, cleaned using different methods and lithographically prepared for our step-edge junction devices. The YBCO films were grown via reactive thermal co-evaporation by Theva, GmbH. The surface characterisation of MgO substrates was studied using X-ray photoelectron spectroscopy (XPS). The in-plane grain orientation of the YBCO films was studied by means of X-ray diffraction (XRD) φ-scan and the critical current density was measured for the XRD scanned samples. The surface condition of the MgO substrates was found to have a strong influence on the in-plane grain orientation and the critical current density of the YBCO films. The MgO substrates with a degraded or contaminated surface gave rise to 45° grain misorientation in YBCO films and reduced the critical current density. A final process step using a low energy Ar ion beam etching (IBE) of the MgO substrates prior to the YBCO film deposition was found effective in removing the in-plane grain misorientation and promoting the growth of perfectly aligned c-axis YBCO films.     

The effect of Ag or Au overlayers on underlying YBa2Cu3O7 thin films

Although Ag and Au are commonly used to provide low-resistance contacts to YBa₂Cu₃O₇ (YBCO), their effect on the electrical properties of the underlying YBCO has been largely neglected. Epitaxial YBCO thin films on LaAlO₃ substrates were used in this study. Thin (50 nm) and thick (1 μm) layers of Ag or Au were deposited as overlayers, and in some cases annealed in oxygen at 550–650°C. Compared to samples with no metal overlayers, for both Ag and Au the normal-state parameters changed (resistivity, its temperature coefficient, and the transition width), whereas the transition temperature and critical current density remained unaltered. These results are encouraging for the use of these metals as contacts and/or conducting overlayers on YBCO.     

The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

Chemically deposited copper oxide thin films: structural, optical and electrical characteristics

Thin films of copper oxide with thickness ranging from 0.05–0.45 μm were deposited on microscope glass slides by successively dipping them for 20 s each in a solution of 1 M NaOH and then in a solution of copper complex. Temperature of the NaOH solution was varied from 50–90°C, while that of the copper solution was maintained at room temperature. X-ray diffraction patterns showed that the films, as prepared, are of cuprite structure with composition Cu₂O. Annealing the films in air at 350°C converts these films to CuO. This conversion is accompanied by a shift in the optical band gap from 2.1 eV (direct) to 1.75 eV (direct). The films show p-type conductivity, 5×10⁻⁴ Ω⁻¹ cm⁻¹ for a film of thickness 0.15 μm. Electrical conductivity of this film increases by a factor of 3 when illuminated with 1 kW m⁻² tungsten halogen radiation. Annealing in a nitrogen atmosphere at temperatures up to 400°C does not change the composition of the films. However, the conductivity in the dark as well as the photoconductivity of the film increases by an order of magnitude. The electrical conductivity of the CuO thin films produced by air annealing at 400°C, is high, 7×10⁻³ Ω⁻¹ cm⁻¹. These films are also photoconductive.     

EXAFS analysis of Er sites in Er–O and Er–F co-doped crystalline Si

We present extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) analyses of Er–O and Er–F co-doped Si. Samples were prepared by multiple implants at 77 K of Er and co-dopant (O or F) ions resulting in the formation of a2 μm thick amorphous layer uniformly doped with 1×10¹⁹ Er/cm³ and 3×10¹⁹ O/cm³, 1×10²⁰ O/cm³ or 1×10²⁰ F/cm³. EXAFS measurements show that the local environment of the Er sites in the amorphous layers consists of 6 Si first neighbors. After epitaxial regrowth at 620°C for 3 h, Er is fully coordinated with 8 F ions in the Er–F samples, while Si and O ions are concomitantly present in the first shell of O co-doped samples. Post regrowth thermal treatments at 900°C leave the coordination unchanged in the Er+F, while the Er+O (ratio 1 : 10) doped samples present Er sites with a fully O coordinated shell with an average of 5 O atoms and 4 O atoms after 30 s and 12 h, respectively. We have also found that the fine structure and intensity of the high-resolution PL spectra are strongly dependent on the Er-impurity ratio and on thermal process parameters in the Er–O co-doped samples, while this is not observed for the F-doped samples. The most intense PL response at 15 K was obtained for the 1 : 3 E : O ratio, suggesting that an incomplete O shell around Er is particularly suitable for optical excitation.     

Electrical conductivity and defect structure of Mg-doped Cr2O3

The electrical conductivity of Cr₂O₃ nominally doped with 2 mol% MgO has been studied by the four point a.c. technique as a function of the oxygen activity (O₂ + Ar, CO + CO₂ and H₂ + H₂O) in the temperature range 400–1200 °C. It is concluded that Cr₂O₃ doped with MgO is an extrinsic conductor and that the dissolved Mg-dopant is compensated by the formation of electron holes at near atmospheric oxygen pressures and by oxygen vacancies (or possibly interstitial chromium ions) at highly reduced oxygen activities (in CO + CO₂ and H₂ + H₂O gas mixtures). In H₂ + H₂O mixtures Mg-doped chromia also dissolves hydrogen as protons and significantly affects the defect structure and electrical conductivity. The defect structure of the oxide under various conditions is discussed.     

Effects of substitution of Ti for Fe in BiFeO3 films prepared by sol–gel process

Ti substituted BiFe_1−xTi_xO_3+δ films have been prepared on indium–tin oxide (ITO)/glass substrates by the sol–gel process. The films with x=0.00–0.20 were prepared at an annealing temperature of 600 °C. X-ray diffraction patterns indicate that all films adopt R3m structure and the films with x=0 and 0.10 show pure perovskite phase. Cross-section scanning shows the thickness of the films is about 300 nm. Through 0.05 Ti substitution, the 2P_r increases to 8.30 μC/cm² from 2.12 μC/cm² of the un-substituted BiFeO₃ film and show enhanced ferroelectricity at room temperature. The 2P_r values are 2.63 and 0.44 μC/cm² for the films with x=0.01 and 0.2, respectively. Moreover, the films with x=0.05 and 0.10 show enhanced dielectric property since the permittivity increases near 150 at the same measuring frequency. Through the substitution of Ti, the leakage conduction is reduced for the films with x=0.05–0.20.     

Microstructure and transport properties of YBa2Cu3O7−δ films produced by laser ablation from a BaF2/Y2O3/CuO target

The effects of varying the temperature and duration of the post-deposition anneal in watersaturated oxygen were investigated for YBa₂Cu₃O_7−δ films of varying thickness. The films were produced by laser ablation from pressed powder targets consisting of BaF₂,Y₂O₃, and CuO mixtures. This technique produces superconducting films with a highly textured surface. The films were fabricated on SrTiO₃ substrates and were analyzed with X-ray diffraction, scanning electron microscopy, and temperature dependent resistivity. Critical current density (J_c) measurements were performed in magnetic fields up to 1 T. For film thickness on the order of 900 nm, completely c-axis oriented films were obtained with a 60 min anneal at 850°C. Thinner films required less annealing, either shorter times or lower temperatures, to achieve similar results, indicating that the optimal annealing conditions are dependent on film thickness.     

Atomic layer deposition of Cr2O3 thin films: Effect of crystallization on growth and properties

Atomic layer deposition of Cr₂O₃ thin films from CrO₂Cl₂ and CH₃OH on amorphous SiO₂ and crystalline Si(1 0 0) and -Al₂O₃() substrates was investigated, and properties of the films were ascertained. Self-limited growth with a rate of 0.05–0.1 nm/cycle was obtained at substrate temperatures of 330–420 °C. In this temperature range epitaxial eskolaite was formed on the -Al₂O₃() substrates. The predominant crystallographic orientation in the epitaxial films depended, however, on the growth temperature and film thickness. Sufficiently thick films grown on the SiO₂ and Si(1 0 0) substrates contained also the eskolaite phase, but thinner films deposited at 330–375 °C on these substrates were amorphous. The growth rate data of films with different phase composition allowed a conclusion that the crystalline phase grew markedly faster than the amorphous phase did. The amorphous, polycrystalline and epitaxial films had densities of 4.9, 5.1 and 5.1–5.3 g/cm³, respectively.