Gustavo R. Paz-Pujalt

Superconducting thin films in the BiSrCaCuO system by the decomposition of metallo-organic precursors

Abstract Thin films in the BiSrCaCuO system have been produced using the metallo-organic decomposition (MOD) technique. Multiphase films having the stoichiometries BiSrCaCu 3 O 6.5+ y andBiSrCaCu 2 O 5.54 were fabricated on single crystal MgO substrates and were found to have superconducting resistance behaviors. The superconducting phase was identified to be Bi 2 Sr 2 CaCu 2 O 8+ x . Highly c -axis-oriented films of Bi 2 Sr 2 CaCu 2 O 8+ x were achieved on MgO and SrTiO 3 substrates. SEM, EDS, and XRD analysis of these films detected only a single phase. Superconducting onset temperatures above 100 K and zero resistivity states at 84 K were observed.

Characterization of proton exchange lithium niobate waveguides

Proton exchanged samples of LiNbO3 have been profiled by micro‐Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x‐ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro‐optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes.

The processes of formation and epitaxial alignment of SrTiO3 thin films prepared by metallo-organic decomposition

The processes of formation and crystallization of thin films of SrTiO3 prepared by the method of metallo‐organic decomposition have been studied with particular emphasis on the relationship between the thermal decomposition of the metallo‐organic precursors and the eventual epitaxial alignment of the crystallized films. The films are deposited by spin coating onto single‐crystalline silicon and SrTiO3 substrates, pyrolyzed on a hot plate at temperatures ranging from 200 to 450 °C, and subsequently heat treated in a quartz tube furnace at temperatures ranging from 300 to 1200 °C. Heat treatment at temperatures up to 450–500 °C results in the evaporation of solvents and other organic addenda, thermal decomposition of the metallo‐organic (primarily metal‐carboxylates) precursors, and formation of a carbonate species. This carbonate appears to be an intermediate phase in the reaction of SrCO3 and TiO2 to form SrTiO3. Relevant to this work is the fact that the carbonate species exhibits diffraction lines, indi...

Solid state reactions in the formation of Bi-Sr-Ca-Cu-oxide superconductor from metallo-organic precursors

Abstract The preparation of Bi-Sr-Ca-Cu-oxide superconductor powders from citrate and carboxylate precursors is discussed. Solid state reactions following the pyrolysis of the precursors were examined by means of thermal analysis and X-ray diffraction. The initial powder was identified as a homogeneous mixture of bismuth trioxide, copper oxide and the double carbonate of strontium and calcium. With heat treatment CuBi 2 O 4 was detected as early as 500°C, and Bi 2 Sr 2 CuO 6 after 700°C. The Bi-Sr-Cu phase was identified as an intermediate for the formation of Bi-Sr-Ca-Cu-oxide superconductors. Further heat treatment at higher temperatures resulted in the formation of the 2212 superconductor phase. Various phase transformations were identified through-out the process.

Solid‐phase epitaxial growth of lithium tantalate thin films deposited by spray‐metalorganic chemical vapor deposition

We demonstrate the solid‐phase epitaxial crystallization of thin films of lithium tantalate deposited on lithium niobate and sapphire substrates. An organometallic compound, formed by reaction of lithium dipivaloylmethanate and tantalum(V) ethoxide, is used as a single‐source precursor for the deposition of amorphous thin films of lithium tantalate using a spray‐metalorganic chemical vapor deposition process. Annealing of the amorphous films results in their epitaxial alignment with respect to the underlying LiNbO3 or Al2O3 substrates. X‐ray diffraction, ion channeling, and scanning electron microscopy are used to evaluate and compare the crystalline quality of the films produced by this solid‐phase epitaxial process to films that are crystalline as deposited.

Growth of superconducting Bi2Sr2CaCu2O8+x films on alumina, silicon, and fused quartz

Interactions between superconducting Bi2Sr2CaCu2O8+x films and substrates were investigated by ion backscattering, x‐ray diffraction, and four‐point probe resistivity measurements. During annealing at temperatures above‐ 800 °C, Bi2Sr2CaCu2 oxide films rapidly reacted with alumina, Si, Si covered with SiO2, and quartz, resulting in catastrophic failure. Zr‐based barrier layers were used to minimize film‐substrate interactions. When a single ZrO2 layer was interposed between the superconducting oxide film and the underlying substrate, the Bi2Sr2CaCu2 oxide films showed a large‐grained polycrystalline microstructure and exhibited the orthorhombic structure. Films on sapphire showed transitions to the superconducting state beginning near 100 K with zero resistance achieved at 70 K. Films on Si and thermally grown SiO2 showed a similar drop in resistance around 95 K, whereas the transition was broad and the zero resistance state was not reached. For films on quartz, high thermal stress caused cracking of the ...

Microstructural evolution and epitaxial alignment of thin films of lithium niobate deposited onto sapphire by metallo-organic decomposition

Abstract LiNbO 3 films were prepared by spin coating and thermal decomposition of a solution of metallo-organic precursors on single-crystal 〈0001〉 oriented Al 2 O 3 substrates, followed by heat treatment in a quartz tube furnace at temperatures ranging from 500 to 1000 °C. X-ray diffraction and ion-channeling analyses reveal that the LiNbO 3 grows epitaxially aligned with respect to the A1 2 O 3 substrate. The degree of epitaxial alignment depends on the heat treatment temperature, with the best films obtained on heat treatment at 900 °C for 30 min to 1 h. The morphology of the films appears to depend on the kinetics of the decomposition process. Films pyrolyzed by slow heating from room temperature to about 500 °C, and subsequently heat treated at higher temperatures, tend to be non-uniform, breaking into a network of grains and partially uncovering the substrate. Interestingly, pole-figure analysis shows that all the grains share the same in-plane as well as out-of-plane epitaxial orientation. Films prepared by rapid thermal decomposition of the precursor solution at 500 °C, followed by heat treatment at 900 °C, are continuous, smooth, and epitaxially aligned.

Fabrication and evaluation of thin-film solid-state sensors for hydrogen sulfide detection

Abstract Thin-film tungsten oxide sensors were fabricated by using metal-organic deposition (MOD) processes. Suitable tungsten carboxylate compounds were synthesized and used as precursor materials for the thin-films of tungsten oxide. The sensors were incorporated into a vacuum line as a new method of evaluation. Operation of the sensors under these conditions proved to be a useful way of providing a very sensitive, reproducible and stable system for measurement of low-level hydrogen sulfide evolved from aqueous solutions.

Solid state reactions in the formation of YBa2Cu3O7−δ high Tc superconductor powders

Abstract The synthesis of high Tc superconducting YBa2Cu3O7−δ powders obtained by three different schemes has been i investigated. The decomposition of metal carboxylates, mixed metal oxalates, and the conventional ceramic method of mixed oxide/carbonate are contrasted. Thermal analysis (TGA, DTA) results as well as X-ray diffraction data are used to follow the progress of the decomposition and solid state reactions leading to the formation of the required perovskite. The effect of cation mixing at the molecular level in the carboxylates and oxalates is compared to the physical mixing of coarser particles in the conventional approach. In the case of the carboxylates and mixed oxalate it is suggested that Ba and Y form a double carbonate that decomposes into barium carbonate and yttrium oxide in the progress of the reaction.

Depth profiling of proton exchanged LiNbO3 waveguides by micro-Raman spectroscopy

Z‐cut LiNbO3 wafers were proton exchanged (PE) with pyrophosphoric acid. The polished edges provided a side view of the exchanged region. The region was probed by micro‐Raman spectroscopy by stepping a 488.0 nm laser at intervals as small as 0.2 μm across the edge face starting below the exchanged region and moving towards the wafer surface, thereby traversing the PE region. Profiling revealed significant changes, in the 125–800 cm−1 and 3200–3600 cm−1 frequency ranges. The PE region expanded after annealing at 300 °C indicating proton penetration into the wafer. Continued annealing resulted in the progressive recovery of spectral characteristics resembling unexchanged lithium niobate. Micro‐Raman profiles provide spectroscopic information regarding which vibrational modes are affected by the exchange and the annealing processes as a function of depth.