Oxygen Gas Atmosphere Research Articles

Improvement of superconductivity in Bi-2212 single crystals grown by a vertical Bridgman method

Abstract We grew Bi2Sr2CaCu2Oy (Bi-2212) single crystals by the modified vertical Bridgman method using the starting materials of Bi:Sr:Ca:Cu=x:2:1:2 (x=2–3), and investigated the optimum annealing conditions for the improvement of the superconductivity. From the results, we found that the optimum composition of starting materials for the large and plate-like Bi-2212 single crystal with good superconductivity and crystallinity was Bi:Sr:Ca:Cu=2:2:1:2, and that the Tc, the full-width-half-maximum value of (0 0 1 0 ) X-ray diffraction peak and the size of the single crystal were 82 K, 0.05° and 18 × 5 mm2, respectively. In the single crystal annealed at 600 °C for 10 h in an atmosphere of oxygen gas, the Tc of the as-grown single crystals increased with annealing time and changed from 82 to 86.3 K. On the other hand, the ΔT of the single crystal annealed at 600 °C for 5 h tended to decrease with increasing annealing time and changed from 6.2 to 4.2 K.

Indium tin oxide films with low resistivity and low internal stress

Indium tin oxide (ITO) films prepared by dc magnetron sputtering were annealed in air, vacuum, and oxygen gas atmospheres. The electrical properties and internal stresses of these annealed ITO films were systematically investigated. It was found that, among the above postannealing treatments, oxygen gas annealing significantly reduced both the resistivity and the internal stress in ITO films at fairly low temperatures of 100–150 °C. Resistivities and internal stresses as low as 7×10−4 Ω cm and 38 MPa, respectively, were obtained by annealing in oxygen gas atmosphere at 100 °C. It was also revealed that the (111) crystal orientation becomes dominant and that whole grains grow dramatically as a result of postoxygen annealing, even at 100 °C.

Photo‐excited photonic characteristics of ZnO thin films deposited by laser ablation method

AbstractZinc oxide (ZnO), having a wide direct band gap of 3.37 eV at room temperature, may be a new potential candidate for optoelectronic devices such as ultraviolet (UV) laser, blue light‐emitting diode, and phosphorescent display. We have studied the ZnO thin films deposited by the pulsed laser ablation (PLA) method. In order to prepare high‐quality ZnO thin films, the spectroscopic properties of the plasma plume produced during film deposition were investigated to correlate the properties of the deposited film with plasma characteristics. Furthermore, we have deposited the ZnO thin films at various deposition conditions such as ambient oxygen gas pressure, substrate material, and substrate temperature. We have investigated the effect of deposition conditions on structural and optical properties of the ZnO thin films. X‐ray diffraction, atomic force microscopy, UV–visible photometry, and photoluminescence measurements were used to characterize the grown films. The ZnO thin films deposited at ambient oxygen gas pressure of 5 mTorr and substrate temperature of 550 °C showed strong c‐axis (002) orientation. The ZnO thin film deposited on Al2O3 (0001) at these deposition conditions provided the UV‐stimulated emission around 395 nm. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 144(3): 1–7, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10177

Valency states of Yb, Eu, Dy and Ti ions in Li2B4O7 glasses

Abstract Absorption and emission spectra of Eu and Dy, Yb and Ti ions in Li2B4O7 glasses grown in oxygen and hydrogen gas atmospheres were measured for valency states and lattice-sites analysis. For the Li2B4O7 glass doped with Eu2+, Eu3+ and Dy3+ ions which were grown in oxidizing and reducing atmospheres, absorption and emission bands due to these ions were investigated before and after γ-irradiation. For the Yb3+-doped Li2B4O7 glass, a weak, broad band was observed near the sharp 976.3 nm absorption band. The origin of this band is discussed in comparison with other glasses. Moreover, irradiation experiments using γ-rays were also performed in order to investigate the possibility of valency change of Yb ions. It was found that Ti4+ ions, which are produced under oxidizing atmosphere, change to Ti3+ ions after γ-irradiation with a dose of 105 Gy. An additional absorption band observed at about ∼500 nm is due to the Ti3+ ions accompanied by charge-compensating vacancy and does not give any emission.

Preparation of ZnO thin films on various substrates by pulsed laser deposition

The structural and optical properties of ZnO thin films deposited on Si (1 0 0), silica glass, α-Al 2O 3 (0 0 0 1) and Corning 7059 glass substrates by pulsed laser deposition have been studied. We have investigated the dependence of film properties on deposition conditions such as an ambient oxygen gas pressure ( P O2) and a substrate temperature ( T s). XRD, AFM and photoluminescence (PL) measurements were used to characterize the grown films. The ZnO thin films deposited on various substrates under optimized condition of P O2=0.67 Pa and T s=550 °C were highly c-axis (0 0 2) orientated. The ZnO thin film deposited on α-Al 2O 3 (0 0 0 1) substrate under optimal condition emitted ultra-violet PL at approximately 395 nm by optically pumped excitation (355 nm) at a power of 466 kW/cm 2.

Pulsed excimer laser ablation deposition of YSZ and TiN/YSZ thin films on Si substrates

We report on the deposition of thin Yttria-stabilized zirconia (YSZ) films and TiN/YSZ bilayers on Si substrates at room temperature by pulsed KrF excimer laser ablation deposition (PLD). YSZ films were deposited onto (1 0 0) Si substrates by ablating an YSZ target in vacuum and in low-pressure oxygen atmosphere. TiN/YSZ/Si bilayers were deposited in two steps without exposure to the air using a multitarget system: first, an YSZ film was deposited onto a Si substrate by ablating an YSZ target in oxygen ambient gas, then a Ti target was ablated in nitrogen ambient gas. Morphological and structural characteristics were investigated by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray reflectivity (XRR) techniques.

Effects of Oxygen and Substrate Temperature on Properties of Amorphous Carbon Films Fabricated by Plasma-Assisted Pulsed Laser Deposition Method

Amorphous carbon (a-C) thin films were deposited on Si (111) substrates by an oxygen RF plasma-assisted pulsed laser deposition (PLD) method at an oxygen pressure of 53 mPa, as well as in vacuum and oxygen gas ambient at 53 mPa for comparison, at substrate temperatures (Tsub) between room temperature and 480°C. An X-ray photoelectron spectroscopy (XPS) analysis showed that the highest sp3 content of the film was 58% in oxygen plasma PLD at Tsub=410°C. Under this condition, the film surface morphology was shown to be quite smooth with a roughness of about 5 nm, by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Effects of the oxygen plasma and the substrate temperature on the film properties were examined.

Fabrication of Bi-doped YIG optical thin film for electric current sensor by pulsed laser deposition

Bi-doped yttrium iron garnet (Bi x Y 3− x Fe 5O 12, Bi:YIG) thin films, which can be used as electric current sensors, are grown on Gd 3Ga 5O 12 (GGG) substrates by pulsed laser deposition (PLD) using an ArF excimer laser. The growth condition for high quality epitaxial Bi:YIG thin films is investigated by varying the PLD process parameters, such as the substrate temperature and ambient oxygen gas pressure. The epitaxial film growth is attained at the substrate temperature of around 500 °C and at ambient oxygen pressure between 125 and 175 mTorr. The optical properties of epitaxial films are measured and the maximum magneto-optic sensitivity coefficient is observed to be 44.1°/T with a film thickness of about 0.7 μm at a wavelength of 500 nm. The results indicate that the PLD technique can be useful for realizing a miniature current sensing device with the Bi:YIG thin film.

無容器凝固によるY123およびNd123の直接成長の雰囲気依存性

Containerless solidification of YBa2Cu3O7−δ(Y123) and NdBa2Cu3O7−δ(Nd123), which are representative of stoichiometric compounds and compounds with solid solution, respectively, was performed in order to investigate the effects of the oxygen pressure on the direct growth behavior of Y123 and Nd123 from the undercooled melt. In the case of Nd123 samples, the dendritic morphology of the Nd123 phase indicates that the Nd123 phase solidified directly from the undercooled melt, which was independent of the oxygen partial pressure. On the other hand, the Y4Ba3O9(Y430) phase, the metastable high temperature phase, solidified as a primary phase at Ar and air gas atmosphere in the Y123 samples. Containerless solidification in oxygen gas flow allowed the direct growth of the Y123 phase, suppressing the primary solidification of the Y430 phase. The mechanism for the direct growth of the Y123 phase is mainly associated with the instability of the Y430 phase at oxygen gas atmosphere. Moreover, the difference in the direct growth behavior in the Y123 and Nd123 systems is understood by the difference in the high temperature phase: the Y430 phase for the Y123 system and the Nd2BaO4 phase for the Nd123 system.

Ultraviolet Luminescence Emitted From ZnO Nanostructural Films by KrF Pulsed Laser Deposition

AbstractZinc Oxide (ZnO) thin films have been deposited on silicon (100), quartz glass, sapphire (0001), and glass substrates by pulsed laser deposition over a range of process conditions such as versus ambient oxygen gas pressure and substrate temperature. Photoluminescence measurements have been carried out to characterize ZnO thin films for optical device applications. We have also studied the influence of the process conditions on crystal structure and morphology of the grown films. Ultraviolet luminescence near the band gap at 3.25 eV was obtained from the optical pumped grown films. And ZnO thin films deposited on sapphire single crystal substrates exhibited strong stimulated emission around 3.12 eV with increasing pumping energy. ZnO thin films deposited at ambient oxygen gas pressure of 5 mTorr and a substrate temperature of 550 °C were predominantly c-axis oriented regardless of substrate material. The crystallite size of 22.7-49.8 nm was estimated using Scherrer's formula. AFM images of films grown on sapphire substrates display hexagonally shaped grains at various sizes.

レーザアブレーション法により作製されたZnO薄膜の光励起フォトニック特性

Zinc oxide (ZnO), having a wide direct band gap of 3.37 eV at room temperature, may be a new potential candidate for optoelectronic devices such as ultraviolet (UV) laser, blue light-emitting diode, and phosphorescent display. We have studied the ZnO thin films deposited by the pulsed laser ablation (PLA) method. In order to prepare high-quality ZnO thin films, the spectroscopic properties of the plasma plume produced during film deposition were investigated to correlate the properties of the deposited film with plasma characteristics. Furthermore, we have deposited the ZnO thin films at various deposition conditions such as ambient oxygen gas pressure, substrate material, and substrate temperature. We have investigated the effect of deposition conditions on structural and optical properties of the ZnO thin films. X-ray diffraction, atomic force microscopy, UV–visible photometry, and photoluminescence measurements were used to characterize the grown films. The ZnO thin films deposited at ambient oxygen gas pressure of 5 mTorr and substrate temperature of 550 °C showed strong c-axis (002) orientation. The ZnO thin film deposited on Al2O3 (0001) at these deposition conditions provided the UV-stimulated emission around 395 nm. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 144(3): 1–7, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10177

Valence-change- and defect-induced white luminescence of Eu 2O 3

Under irradiating with a CW UV laser light ( λ=325 nm) at room temperature in vacuum and oxygen gas atmosphere, the film, microcrystals and powder compacts of Eu 2O 3 exhibit a reversible photoluminescence (PL) spectral change between a red sharp-line structure and a white broad one. After stopping the UV irradiation, the ability of the white PL lasts for more than several months at room temperature under room light, in spite of changes of atmosphere. The reversible phenomena to be observed are interpreted as the results of both the valence-number change of europium ions (Eu 3+→Eu 2+) and the oxygen vacancy formation.

Superconducting transition temperature of NdBCO liquid phase epitaxy film on MgO substrate and effect of Mg diffusion

NdBCO thick films were grown by liquid phase epitaxy (LPE) for the electronic device substrates. To optimize the process for achieving high critical transition temperature ( T c), different oxygenation conditions were applied. At 250°C held for 200 h in the oxygen gas atmosphere, the high T co of NdBCO LPE film was achieved at 91 K with a sharp transition, which is lower than the reported maximum value of nearly 95 K for the NdBCO system. The contamination of Mg from the MgO substrate to the RE123 film plays a significant role in the T c reduction. The Mg diffusion coefficient D Mg in NdBCO and its relation with temperature were deduced.

Improvement of the microstructure of melt-processed Sm-based superconductors

We report on the effect of the atmosphere during the decomposition of the precursors of Sm–Ba–Cu–O bulk superconductors. It was found that the formation of voids was greatly suppressed by two methods, melting in an oxygen gas atmosphere or in vacuum. For both methods, the atmosphere during crystallization was switched to an Ar gas flowing one. The samples prepared by these methods exhibited high superconducting transition temperatures with sharp transition widths. However, the zero-field critical current density was lower than that of the reference sample, which was prepared by decomposing the precursor in an Ar gas flowing atmosphere.

Fabrications of cerium-substituted YIG thin films for magnetic field sensor by pulsed-laser deposition

We described the pulsed-laser deposition (PLD) and characterization of cerium doped yttrium iron garnet (Ce:YIG, Ce/sub 0.6/Y/sub 2.4/Fe/sub 5/O/sub 12/) thin films for magnetic field sensor applications. It was found that crystallized Ce:YIG films were obtained by the deposition at 900/spl deg/C substrate temperature and in 10 mtorr argon or oxygen ambient gas. The Faraday rotation angle was very large; 4.2 deg//spl mu/m at 430 nm wavelengths. The charge state control of cerium was important for the crystallinity and high Faraday rotation. The lower temperature or higher ambient gas pressure controlled the cerium charge state Ce/sup 4+/, because the oxygen and cerium released from an ablated target accelerated to form cerium oxide, the thin film was noncrystalline and no Faraday rotation occurred.

Reduction of interlayer coupling in bottom synthetic spin valves through a gas exposure process

The magnetoresistance (MR) ratio of spin valves can be improved by reducing the thickness of the nonmagnetic interlayer, such as Cu, due to not only reducing the shunt, but also increasing the probability of electrons scattered through the Cu. However, at small thickness, interlayer coupling between the free and pinned layer is increased, which makes it difficult to control the bias point. The minimum thickness of the interlayer Cu was thereby limited to around 3 nm. On the other hand, it is reported that the ferromagnetic interlayer coupling that arises from the film roughness can be reduced by controlling the residual gas inside the deposition chamber. The same effect can also be achieved by exposing the wafer into an oxygen gas atmosphere right after the deposition of the high conductive Cu interlayer. By this method the thickness of the interlayer Cu was reduced successfully from 3.0 to 2.0 nm without increasing the interlayer coupling. As a result, the MR ratio and dRsq was improved by 12% and 48%, and showed 10% and 1.82 Ω, respectively. Furthermore, by inserting a Cu backlayer to form a spin filter spin valve structure an antiferromagnetic interlayer coupling was also observed.

Preparation of perovskite, Pb(Zr, Ti)O3 thin-films on YSZ(111)/Si(111) substrates by post-deposition annealing

Perovskite Pb(Zr, Ti)O3 [PZT] thin-films on yttria-stabilized zirconia [YSZ] (111)/Si(111) substrates were prepared at various growth temperatures by quasi-metallic mode reactive sputtering (QMMRS) system using a (ZrTi+x%PbO) composite target. The surface of the PZT film deposited on a YSZ(111)/Si(111) substrate, however, revealed many holes (no growth region of PZT film). These holes could be eliminated by using a post-deposition annealing process. Perovskite Pb(Zr, Ti)O3 thin films were obtained by an annealing process at temperatures of approximately 600°C in an oxygen gas atmosphere for 1 h. Moreover, ferroelectric properties were observed on Pb(Zr, Ti)O3 thin films/YSZ(111)/Si(111) structure.

Bulk and surface phases of iron oxides in an oxygen and water atmosphere at low pressure

Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p⩽1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe–H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4–Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5 × 10−5 mbar oxygen at 920–1000 K results in a new γ-Fe2O3(111)-like intermediate surface phase during the oxidation to α-Fe2O3(0001). A model is suggested for the growth of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.

Ultrathin Ta 2O 5 films produced by large-area pulsed laser deposition

Tantalum pentoxide (Ta 2O 5) is an important material for the next generation of microelectronics devices. A large-area pulsed laser deposition (PLD) technique was used to uniformly grow ultrathin (5–20-nm-thick) Ta 2O 5 films on silicon wafers of 75 mm diameter, heated to a temperature of 400°C. A KrF excimer laser beam was used to induce the ablation of a target in oxygen gas ambient, at a pressure of 30 mTorr. The large-area coverage was obtained by rastering the laser beam over the radius of the rotating target, while the substrate was rotated simultaneously. The deposition rate was calculated to be 4.8 nm/min. The Ta 2O 5 films were fully dense and amorphous, and excellent uniformity was achieved in terms of structure, composition, thickness and optical properties across every wafer. An investigation of the Ta 2O 5/silicon interface revealed that the growth of the films was accompanied by the formation of a silicon oxide interlayer. This is the first time the PLD technique is demonstrated to produce uniform ultrathin films over large areas (75 mm diameter).

Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.