Chau-Yun Yang

Microstructure of electron-beam-evaporated epitaxial yttria-stabilized zirconia/CeO2 bilayers on biaxially textured Ni tape

Abstract Transmission and scanning electron microscopy, atomic force microscopy, X-ray pole figure analysis and Auger electron spectroscopy were used to characterize the microstructure and surface topography of epitaxial yttria-stabilized zirconia (YSZ) and CeO 2 thin films deposited by electron beam evaporation on rolling-assisted biaxially textured Ni substrates (RABiTS™). The as-deposited YSZ layer is composed of highly crystallographically aligned, slab-shaped columnar grains with sharply defined, rectangular cross sections and average dimensions of 10 nm by 50 nm by the film thickness. The faces of the YSZ slabs lie on the {110} planes that contain the surface normal. Their caps are roof-shaped with a peak-to-valley height of about 10 nm and a RMS roughness, measured by atomic force microscopy, of 1.3 nm. The resultant surface morphology is rough, but shows a regular, cross-hatched pattern on the length scale of about 10 nm. The length scale and crystallographic directionality of the YSZ microstructure is retained when YBa 2 Cu 3 O 7− δ is pulsed laser deposited on it, but the YSZ columns appear to have sintered into a less angular, more distinctly porous microstructure. The CeO 2 layer also is columnar, but appears to be denser, with a flatter, less directional surface topography. Auger sputtering-depth profiling experiments revealed that the compositions of both films are constant through the film thickness and that interdiffusion along the surface normal is not extensive.

Microstructure of a high Jc, laser-ablated YBa2Cu3O7−δ/sol–gel deposited NdGaO3 buffer layer/(001) SrTiO3 multi-layer structure

Abstract A YBa 2 Cu 3 O 7− δ (YBCO) film with a transport critical current density ( J c ) value of 1 mA/cm 2 (77 K, 0 T) was grown on a solution deposited NdGaO 3 (NGO) buffer layer on (100) SrTiO 3 (STO). The 25-nm thick NGO buffer layer was dip-coated onto the STO single crystal from a solution of metal methoxyethoxides in 2-methoxyethanol. Pulsed laser deposition (PLD) was used to grow a 250-nm-thick YBCO film on the NGO. The epitaxial relationships are cube-on-cube throughout the structure when the pseudo cubic and pseudo tetragonal unit cells are used to describe the NGO and YBCO crystal structures, respectively: (001) YBCO∥(001) NGO∥(001) STO and [100] YBCO∥[100] NGO∥[100] STO. High resolution scanning electron microscopy (SEM) of the bare NGO surface revealed ∼40 nm diameter pinholes with number density of ∼2×10 13 m −2 , corresponding to an area fraction coverage of 2.5%, in an otherwise featureless surface. Cross-sectional transmission electron microscopy (TEM) showed that these pinholes penetrate to the STO; otherwise the NGO layer was uniformly thick to within approximately ±5 nm and defect free. The X-ray diffraction φ- and ω-scans indicated that the YBCO film was highly oriented with a full-width-half maximum peak breadth of 1.14° for in-plane and 0.46° for out-of-plane alignment, respectively. The film contained sparse a -axis oriented grains, an appreciable density of (001) stacking faults and apparently insulating second phase precipitates of the type that typically litter the surface of PLD films. All of these defects are typical of YBCO thin films. High-resolution cross-sectional TEM images indicate that no chemical reaction occurs at the YBCO/NGO interface.

Preparation of textured YBCO films using all-iodide precursors

Abstract We developed an all-iodide precursor approach to grow epitaxial YBa 2 Cu 3 O 7− y (YBCO) films on single crystal substrates. The respective metal iodides were used as the starting materials. Initially, yttrium and barium iodides were dissolved in 2-methoxyethanol, whereas copper iodide was dissolved in ammonium iodide/dimethylformamide solution. The solutions were mixed together and concentrated to give a 2.5-M total cation precursor solution. The precursor solutions were spin coated on SrTiO 3 (100) substrates and heat-treated at 800–830°C in 300 ppm oxygen atmospheres. X-ray diffraction (XRD) results from the theta–2theta (θ–2θ) scan, omega (ω) and phi (φ) scans for the films revealed a (100) cubic texture. The full-width-at-half-maximum (FWHM) values for YBCO (105) and YBCO (005) were 1.3° (in-plane epitaxy, Δ φ ) and 0.42° (out-of-plane epitaxy, Δ ω ), respectively. The YBCO films had a T c of 90.3 K. The measured transport J c , critical current density was 1.3×10 5 A/cm 2 at 77 K and self-field.

Growth conditions and microstructure of Y2O3 buffer layers on cube-textured Ni

Abstract Y 2 O 3 films were deposited on cube-textured Ni substrates by electron-beam evaporation as part of a buffer layer study for YBa 2 Cu 3 O 7− δ (YBCO) coated conductors. Their microstructure was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and their crystallographic texture and surface roughness evaluated by X-ray diffraction (XRD) and atomic force microscopy (AFM). Previous studies showed the Y 2 O 3 surface normal changed from {100} to {111} on decreasing the deposition chamber pressure from 10 −5 to 10 −6 Torr. In this work, 300 nm thick Y 2 O 3 buffer layers were deposited at two different deposition chamber pressures, ∼5×10 −4 Torr N 2 and ∼2×10 −5 Torr air, so as to make the {100} orientation dominant. However, the morphology of the Y 2 O 3 buffer layers was significantly affected by changing the deposition chamber pressure, the lower pressure buffer layer being denser and smoother than that made at the higher pressure. The Y 2 O 3 grains in a 600 nm thick Y 2 O 3 buffer layer grown under 2×10 −5 Torr air became larger and more uniformly square as the film grew thicker. The Y 2 O 3 grown on thermally grooved Ni deposited at 2×10 −5 Torr air, showed 〈111〉-oriented grains near the grooves. This local imperfection in the texture could lead to significant barriers to supercurrent flow in the YBCO overlayer.

Microstructural homogeneity and electromagnetic connectivity of YBa2Cu3O7−δ grown on rolling-assisted biaxially textured coated conductor substrates

Abstract The electromagnetic connectivity and microstructure of three YBa 2 Cu 3 O 7− δ (YBCO) films grown on biaxially textured substrates were investigated by magneto optic (MO) imaging and scanning electron microscopy (SEM). The films were deposited by pulsed laser deposition (PLD) on yttria-stabilized zirconia (YSZ) and CeO 2 -buffered, biaxially textured Ni tapes. The transport critical current density ( J c ) values of the films were 0.3, 0.6 and 0.7 MA/cm 2 (77 K, 0 T). MO imaging revealed clearly granular electromagnetic behavior in the lowest J c and one of the higher J c samples, but considerably better connectivity in the sample with a J c value of 0.6 MA/cm 2 . High resolution SEM showed a dense and rather featureless microstructure in the YBCO of the most highly electromagnetically connected sample, whereas pores and/or second phase particles cluttered the YBCO layers of the granular samples. Thus, the granular behavior in these samples appears to be caused by pores and second phase particles that locally obstruct the superconducting current in the YBCO layer. Control of these types of defects clearly is important for raising the J c value.

Studies of the improvement in microstructure of Y2O3 buffer layers and its effect on YBa2Cu3O7-x film growth

Two kinds of Y2O3 buffer layers have been directly deposited on cube-textured Ni substrates by electron beam evaporation under different gas pressures. One is deposited at 10-5 Torr air (a single-layer buffer). In order to reveal the improvement of the microstructure, the other is formed at first under 5×10-4 Torr and then 10-6 Torr of N2 (a double-layer buffer). As a result, the double-buffer layer obtained is more than 97% {100} orientation and is a relatively denser structure. YBa2Cu3O7-x overlayers are deposited by pulsed-laser deposition on these buffer layers. Large YBa2Cu3O7-x pop-off areas are seen for the single-buffer samples. These areas are eliminated on the double-layer buffer. However, the critical current does not increase appreciably for the YBa2Cu3O7-x film on the double-layer buffer, even though the microstructure of the buffer is improved. Other defects probably yielded percolative current flow and a low critical current density in the YBa2Cu3O7-x layers.

Preparation and characterization of Y/sub 2/O/sub 3/ buffer layers and YBCO films on textured Ni tape

The direct deposition of Y/sub 2/O/sub 3/ buffer layers on cube-textured nickel tape was successfully performed by electron beam deposition using Y metal which oxidized during deposition. The Y/sub 2/O/sub 3/ layer exhibited excellent out-of-plane alignment of FWHM of 2.3/spl sim/4/spl deg/ and good in-plane alignment with /spl sim/11/spl deg/ FWHM. Surface morphology, crystal orientation and grain size proved to be quite sensitive to the deposition pressure. The surface roughness and the grain size increased with increasing deposition pressure, and the crystal orientation changed from [111]Y/sub 2/O/sub 3/ to [100]Y/sub 2/O/sub 3/. Subsequently, YBCO superconducting films were deposited on [100]Y/sub 2/O/sub 3/ buffer layers by co-evaporation deposition and pulsed-laser deposition (PLD). Though a good in-plane alignment, as measured by X-ray /spl phi/-scan, was achieved in the YBCO films, their superconducting characteristics were not so good. The T/sub c/ onset was about 84 K for the [001]YBCO by PLD. The crystal alignment and the microstructure of YBCO superconducting films deposited by the two deposition techniques on cube-textured Ni tapes with Y/sub 2/O/sub 3/ buffer layers are compared.