R. E. Muenchausen

Epitaxial CeO2 films as buffer layers for high-temperature superconducting thin films

We have prepared epitaxial (100)CeO2 thin films on LaAlO3, sapphire, and yttria‐stabilized zirconia using pulsed laser deposition. It is demonstrated in this letter that the CeO2 films are chemically and structurally compatible to the high‐temperature superconductor YBa2Cu3O7−δ (YBCO). Epitaxial YBCO films on CeO2/LaAlO3 had a zero resistance temperature and critical current density in a zero field of 90 K and 5.9×106 A/cm2 at 75 K, respectively. Furthermore, epitaxial multilayers of CeO2/YBCO were prepared. This work demonstrated that CeO2 is an excellent buffer layer material for the high‐temperature superconductors.

Crystal growth and optical characterization of cerium-doped Lu1.8Y0.2SiO5

Czochralski growth of cerium-doped Lu1.8Y0.2SiO5 (LYSO) from a 90/10 solution of Lu2SiO5 (LSO) and Y2SiO5 (YSO) is demonstrated. The alloyed scintillator retains the favorable growth properties of YSO and the desirable physical and optical scintillator properties of LSO. Radioluminescence, thermally stimulated luminescence, optical absorption, and lifetime measurements confirm the equivalence of LYSO and LSO optical properties. Advantages of LYSO Czochralski growth relative to LSO include reduced melting point, less propensity for formation of crystalline inclusions, lower cost of starting material, and easier incorporation of cerium into the host lattice. This material offers an attractive alternative to LSO for scintillator applications.

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Origin of surface roughness for c -axis oriented Y-Ba-Cu-O superconducting films

One problem with the growth of high quality c‐axis oriented YBa2Cu3O7−x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c‐axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c‐axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a‐axis films. Outgro...

Properties of epitaxial SrRuO3 thin films

SrRuO3 thin films were deposited on (100) LaAlO3 using pulsed laser deposition. The films were (001) oriented normal to the substrate surface with a high degree of in‐plane orientation with respect to the substrate’s major axes. An ion beam minimum yield of 2.5% was obtained for the films, indicating high crystallinity. The films exhibited metallic behavior with a room temperature resistivity of ∼200 mW cm. A kink in the resistivity, corresponding to a ferromagnetic phase transition, was observed at ∼160 K. It was found that SrRuO3 is structurally and chemically compatible with the YaB2Cu3O7−d (YBCO) superconductors. High quality YBCO films were obtained on SrRuO3 LaAlO3. Multilayers of YBCO/SrRuO3 were successfully fabricated.

a‐axis oriented YBa2Cu3O7−x thin films on Si with CeO2 buffer layers

We have grown a‐axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x‐ray diffraction, x‐ray pole‐figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical‐current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a‐axis oriented YBCO films.

Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Effect of deposition rate on properties of YBa2Cu3O7−δ superconducting thin films

Superconducting thin films of YBa2Cu3O7−δ on (100) SrTiO3 are prepared in situ by a pulsed laser deposition technique at deposition rates from 1 to 145 A/s. Crystallinity of the films is examined by Rutherford backscattering in the channeling mode using a 2.2 MeV He+ ion beam. The backscattering minimum yield (χmin) increases with the deposition rate. A χmin of 3% is observed in the films deposited at the lowest deposition rate. Even at a deposition rate of 145 A/s, the films show good crystallinity with χmin of 15%, indicating epitaxial growth. The x‐ray diffraction measurements show that the films have strong c‐axis orientation normal to the substrates. The films have metallic resistance versus temperature behavior with zero resistance temperatures of 90 K. The results indicate that the pulsed laser deposition technique could be used to deposit large‐area films efficiently with adequate substrate movement.

Y2O3:Bi nanophosphor: Solution combustion synthesis, structure, and luminescence

Photoluminescence (PL), radioluminescence (RL), and thermoluminescence (TL) investigation of Y2O3:Bi nanophosphors prepared by solution combustion synthesis using urea, glycine, and hexamethylenetetramine (HMT) as fuels was carried out. The as-prepared nanopowders have increasing crystallinity and average crystallite sizes for urea, glycine, and HMT, respectively. Luminescence is composed of two emission bands centered at 408 and 505 nm due to two nonequivalent Bi3+ sites with symmetry S6 and C2, respectively. The occupancy of these sites depends on the synthesis conditions, in agreement with theoretical predictions. Annealing at 1000 °C for 1 h improves PL and RL efficiency due to enhanced crystallinity of the nanopowders and activation of recombination centers (Bi3+ ions). No shift in the PL peak position was observed as a function of average crystallite size. The concentration quenching was experimentally determined to have a maximum emission of around 3 mol % of the dopant. TL spectra present several ...

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21∕c structure, rather than the B2∕b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼25–100nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21∕c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.