Year

2002

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The aim of this thesis was to study buffer layers for YBCO superconducting thin films on single crystal substrates and coated conductors. The work was divided into two parts: Ce02 films on single crystal yttrium stabilized zirconia (YSZ) substrates; and CeO films and NiO films on cubic textured Ni substrates.

In the first part, the focus was on a deep understanding of the growth processes and mechanisms of Ce02 films in order to better control the film properties, such as crystallinity, orientation, and surface morphology. The systematic study of Ce02 films deposited by the pulsed laser deposition (PLD) on YSZ substrates has shown that the degree of a c-axis orientation is related to the morphology of the Ce02 grains; the more spherical the shape of the grains is, the better the c-axis orientation and the smoothness of the Ce02 film surface are. This correlation appears to be an intrinsic feature of the Ce02 film, independent of the type of substrate or the deposition method.

By studying the orientation and the surface morphology of Ce02 films deposited at different temperatures, the relationship between epitaxial quality and growth mode could be analysed. The optimum epitaxial growth of a Ce02 film is layer-by-layer growth from the beginning to the end. The lower quality epitaxial growth is layer-by-layer and then island growth, and the growth mode of an imperfectly c-axis oriented film is island growth throughout. As the islands grow larger and become large outgrowths, the outgrowths prefer to connect to each other to form a network on the surface. For a low quality Ce02 film which was far from epitaxial, the growth of the film was characterized by the typical terrace-ledge-kink growth mode of cubic crystal.

The effects of an Ar + 10% H2 atmosphere on the Ce02 films were investigated. The Ar + 10% H2 atmosphere used for deposition can enlarge the epitaxial temperature range, and furthermore, significantly reduce the epitaxial temperature. Pure c-axis oriented Ce02 films were obtained with substrate temperatures between 350°C and 775°C using an Ar+10%H2 atmosphere. At the same time, the surface roughness and the number of larger outgrowths were decreased compared to using an O2 atmosphere. There is a reason to believe that the mobility of the species during deposition of a Ce02 film smaller using Ar + 10% H2 than using 02.

In the second part, the research relates to Ce02 and NiO buffer layers which were employed in coated conductors. The Ar + 10% H2 reducing gas was chosen in the deposition of Ce02 films by PLD on the cubic textured Ni. The optimum deposition conditions are: deposition temperature 395°C; pulsed laser at 3Hz and 500mJ; Ar + 10% H2 pressure of 100mTorr. The substrate-target distance is also an important parameter for the epitaxial growth of Ce02 films. The optimum out-of-plane and in-plane textures as evaluated by the FWHMs of an XRD ω-scan and ϕ-scan of for optimum Ce02 film are 4.9° and 12° respectively. (|)-scans of the Ce02 film and the underlying substrate showed that the growth of the Ce02 film on the cubic textured Ni (100) substrate is in agreement with near-coincidence site lattice match modes, e.g.CeO2//Ni. Some modifications have been made to eliminate the microcracks in Ce02 films. The deposition procedure of first depositing Ce02 film up to 10nm at 3Hz, then depositing Ce02 film at 1Hz up to 100nm can reduce the density of cracks and make the cracks narrow.

Three atmospheres, air, flowing oxygen, and flowing Ar +8%02, were used to fabricate biaxially textured NiO films by the surface oxidation epitaxy(SOE). The optimum deposition process is in flowing oxygen (Po2~ 1 atm) at an oxidation temperature 1075°C for 10min. NiO films have strong c-axis and a-b plane orientations, and the degree of the c-axis orientation is about 98%. XRD pole figures and Three atmospheres, air, flowing oxygen, and flowing Ar +8%02, were used to fabricate biaxially textured NiO films by the surface oxidation epitaxy(SOE). The optimum deposition process is in flowing oxygen (Po2~ 1 arm) at an oxidation temperature 1075°C for 10min. NiO films have strong c-axis and a-b plane orientations, and the degree of the c-axis orientation is about 98%. XRD pole figures and ϕ-scans were to investigate the a-b plane texture, and the FWHM value of a ϕ-scan for the (111) of an optimum NiO film was 12°. The morphologies and microdefects of NiO films were systematically investigated. The more (100) oriented the grains are, the smoother the surface of the NiO films is. The high roughness of NiO film can also be caused clusters of relatively large NiO grains which fonn numerous circular spots on the surface of the NiO film. Decreasing the thickness of NiO films by shortening the oxidation time can reduce the density of these coarse spots on the surface of the films.

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