J.J. Chu

Epitaxial growth of CoSi2 on (111)Si inside miniature‐size oxide openings by rapid thermal annealing

Epitaxial growth of CoSi2 on (111)Si inside two‐dimensional and linear oxide openings by rapid thermal annealing has been investigated by transmission electron microscopy. Both annealing temperature and time were found to be critical in obtaining 100% epitaxy. The size of oxide openings and annealing temperature were found to exert strong influences on the morphology of epitaxial CoSi2 on silicon. The faceting of CoSi2 was found to occur at a lower temperature inside oxide openings of smaller size. The change in morphology of epitaxial CoSi2 with the size of oxide openings in the present study indicated that interfacial energy and/or stress, in addition to the surface energy, are important in determining the morphology of epitaxial CoSi2 on (111)Si.

Epitaxial growth of high Tc superconducting Y‐Ba‐Cu‐O thin films on (001)MgO by a chemical spray pyrolysis method

A chemical spray pyrolysis method has been applied to grow epitaxial films of a high Tc Y‐Ba‐Cu‐O compound (YBCO) on (001)MgO. Films as thin as 0.6 μm in thickness was found to exhibit excellent superconducting transition behavior. For films up to 2 μm in thickness, typical values of Tc onset, Tc zero and transition width (90%–10%) were measured to be 82, 76, and 1.5 K, respectively. Both plan‐view and cross‐sectional transmission electron microscopy revealed that the orientation relationships between the epitaxial films and the substrate are [001]YBCO//[001]MgO and (110)YBCO//(200)MgO. Twins, which may be perceived as domains that are rotated 90° along the c axis of the thin films with respect to the substrate, were found to be copiously present. The influences of the configuration of the oriented growth of overlayer thin films on the superconducting properties are addressed. The advantages of the chemical spray pyrolysis in producing superconducting thin films are outlined.

Epitaxial Growth of High-Tc Bi-Ca-Sr-Cu-O Superconducting Layer by LPE Process

A Bi-Ca-Sr-Cu-O epitaxy layer has been successfully synthesized by the liquid-phase epitaxial (LPE) process on the MgO(001) substrate. X-ray diffraction patterns and SEM surface morphology analysis indicated a highly preferred orientation. This film showed superconductivity with onset and zero resistivity temperature of 110 K and 81 K, respectively.

The Growth and Characterization of Pb-DOPED Bi-Sr-Ca-Cu-O Thin Films

The Bi-Pb-Sr-Ca-Cu-O thin films have been prepared on single crystal (001) MgO substrate by RF magnetron sputtering with a single target. With suitable amount of Pb doping and post-annealing condition, the formation of 110 K superconducting phase in the film could be ameliorated. X-ray diffraction patterns revealed that the film was dominated by the well-known 2212 and 2223 phases. The proper doping of Pb could not only accelerate the formation of high Tc phase but also improve the connetivity of high-Tc grain.

The effects of target composition on the Microstructure characteristic of epitaxial YBCO thin films by in-situ rf magnetron sputtering with single compound targets

Abstract The polycrystalline structure of epitaxial superconducting YBa 2 Cu 3 O x (YBCO) thin films was studied using STEM/EDS and HREM. Two kinds of YBCO films with significant difference in microstructure characteristic were found, which were prepared by rf magnetron sputtering with targets of different composition, although both of samples showed with zero resistivity about 83 K. These films were epitaxially grown on (001) MgO substrate and had the c-axis oriented. Impurity phases including CuO, Y 2 O 3 , Y 2 BaCuO 5 (211) and very tiny precipitates were observed. The formation of impurity phase were dependent on the initial target composition. The precipitate of Y 2 O 3 or 211 phase in films was found to have a special crystallographic relationship with YBCO matrix. The very tiny precipitates within YBCO matrix were believed to be responsible for the pinning centers.

In situ formation of Y‐Ba‐Cu‐O epitaxial films by 40.68‐MHz rf magnetron sputtering

The in situ epitaxial growth of the Y‐Ba‐Cu‐O films on MgO (001) was carried out by 40.68‐ MHz rf magnetron sputtering with an on‐axis single target. It appeared that the rf discharge strongly affected film composition, crystallinity, superconductivity, and reproducibility. Highly orientated superconducting films with the c‐axis perpendicular to the substrate surfaces and having zero resistance transition temperature of 78.2 K were obtained without a post‐annealing.

Impurities and defects of high‐Jc YBa2Cu3Ox superconductor with flux jumps

The microstructures of a highly textured bulk YBa2Cu3 (123) superconductor, with the high‐transport Jc (≥ (R18)3.7×104 A/cm2 at 77 K) and flux jumps (T<15 K), were characterized by x‐ray diffraction, polarized optical microscopy, scanning electron microscopy, and scanning transmission electron microscopy. It was found that the grains of the 123 superconductor are preferentially oriented along the a‐b plane of the 123 phase. The Y2BaCuO5 and CuO particles are dispersed uniformly in the 123 matrix, and the highly dense twins run through the matrix everywhere. Some irregular‐shaped BaCuO2‐CuO mixtures were observed at the 123 grain boundaries. The aligned 123 crystals and microstructural defects in the 123 matrix are considered to be responsible for the high‐Jc value and flux jumps.

Effects of substrate bias on crystal structures and magnetic properties of CoCrTa/Cr thin films

Abstract Effects of negative d.c. substrate bias on the film properties of CoCrTa/Cr magnetic recording media have been investigated. When the substrate bias was increased from zero to −250 V, the Ta content was found to increase from 3.9 to 15.0 at.%, whereas the Cr content decreased and the deposition rate was reduced. X-ray diffraction (XRD) analysis revealed the replicate growth of Co (112¯0) on the Cr (002) plane. The peak shift and line broadening of Co (112¯0) diffraction are attributed to the compositional change and presence of residual stress in the deposited film. The occurrence of maximum coercivity at −150 V bias is correlated to the formation of structural defects. However, too high a bias (−250 V) induces formation of an excessive amount of weak magnetic phase, which causes the magnetic properties to deteriorate. The enhancement of the coercivity due to substrate bias is primarily governed by hindrance of domain wall motion by structure defects and/or residual stress.

Analysis of the hysteresis curves of a laser-ablated YBCO thin film via an exponential critical state model

Abstract Our experimental data show that the magnetization of a laser-ablated YBCO thin film can be properly devided into reversible and irreversible parts. The time dependent irreversible magnetization follows flux-creep theory. By using the exponential critical state model, the irreversible magnetization of the hysteresis curves are perfectly quantitatively analyzed.

Flux creep characteristics after the increase in temperature or external field

Abstract The parameters of power-law magnetic relaxation; M s , R and β, have been studied on a laser-ablated YBCO thin film. It was found that at given T and H, M s was almost independent on measuring process. R and β obtained from fixed H and step-increased T process were respectively larger than those from fixed T and step-increased H process. These demonstrated that the rate of flux creep is more influenced by Lorentz force than by thermal activation factor.