Doctor of Philosophy
Institute for Superconducting & Electronic Materials - Faculty of Engineering
Zhao, Yue, Fabrication and characterization of superconducting PLD MgB2 thin films, PhD thesis, Institute for Superconducting & Electronic Materials, University of Wollongong, 2005. http://ro.uow.edu.au/theses/816
The aim of this thesis was to study the thin film of magnesium diboride (MgB2) superconductor based on PLD synthesis technique. The MgB2 is a very simple binary compound with a number of surprising properties. The discovery of superconductivity with a critical temperature (Tc) of 39 K in bulk MgB2 in January 2001 has attracted a huge research interests worldwide in this material. In a hope to substitute MgB2 superconducting electronics for low temperature superconducting electronics and compete with high temperature superconductors, the preparation techniques of MgB2 thin films need to be advanced from a material engineering point of view. On the other hand, new studies regarding the different behaviours of MgB2 thin films and MgB2 bulks will assist the scientific community to better understand the physics in this superconductor. We began the study with in situ MgB2 film preparation using normal on-axis geometry. The in situ annealing conditions of pulsed laser deposited MgB2 films were studied. We found that the superconducting properties depend in a crucial way on the annealing conditions: temperature, heating rate and time. We tested the Tc dependence of the in situ annealed MgB2 films by changing various process parameters, including laser energy density, target-substrate distance, background gas, annealing temperatures, heating rates and dwell times. The film processing conditions were optimized and good quality in situ films were obtained routinely, with good reproducibility. The hysteresis loops of magnetic moment versus applied field at different temperatures indicate a weak field dependence in high fields. Magnetooptical imaging of the films showed quite homogeneous magnetic flux penetration, indicating structural homogeneity. The films without annealing show no superconductivity. Another method using ex situ annealing has also been tested for a better crystallization of the MgB2 film, as well as for reference purposes. In the ex situ annealing process, we first deposited boron precursor film on a sapphire substrate and then wrapped it in Ta foil and sealed it in a stainless steel tube together with pure Mg pellets, under protective Ar atmosphere. The tube was then annealed in tube furnace and kept at 900ºC for 30 min. With this method, we obtained ex-situ MgB2 films of comparable quality as those reported so far in the literature, and used them as a benchmark to test possible improvements in MgB2 thin film technology based on PLD. Significant differences in properties between the in situ films and ex situ films were found. The ex situ annealed MgB2 film has a Tc onset of 38.1K, while the in situ film has a suppressed Tc onset of 34.5K. The resistivity at 40K for the in situ film is larger than that of the ex situ film by a factor of 6. The residual resistivity ratios are 1.1 and 2.1 for the in situ and ex situ films respectively. The field dependence of the resistivity-temperature curves has been measured. A large slope of the Hc2-T curve was obtained for the in situ annealed film. The Jc-H curves of the in situ film show a much weaker field dependence than those of the ex situ film, attributable to stronger flux pinning in the in situ film. The microstructural differences between the two types of films are observed by AFM and TEM. The small-grain (<60nm) size and a high oxygen level detected in the in situ annealed MgB2 films may be decisive for the significant improvement of Jc and Hc2. In order to enhance the performance of the MgB2 films, various amounts of Si up to a level of 18wt% were added into MgB2 thin films fabricated by pulsed laser deposition. Si was introduced into the PLD MgB2 films by sequential ablation of a stoichiometric MgB2 target and a Si target. The Tc's of the Si added MgB2 thin films were tested. A Jc enhancement was observed in the Si added MgB2 films. For the ~3.5wt% Si addition, the best enhancement circumstance, the magnetic critical current density (Jc) of the film at 5K was increased by 50% as compared to the undoped film. The slope of Hirr(T) and Hc2(T) curves of the 3.5wt% Si added MgB2 film was slightly higher than that for the undoped film. For the application in superconducting electronics, the surface smoothness of the MgB2 thin film is of crucial importance. We pioneered an off-axis deposition geometry in the PLD MgB2 films preparation. Highly smooth and c-axis oriented superconducting MgB2 thin films were successfully achieved with a off-axis geometry. The films were deposited on Al2O3-C substrates, aligned perpendicular to a stoichiometric MgB2 target in a 120 mTorr high purity Ar background gas. An in situ annealing was carried out at 650℃ for 1 min in a 760 Torr Ar atmosphere. Despite the short annealing time, an x-ray _-2_ scan shows fairly good crystallization, according to the clear c-axis oriented peaks for the films. Both atomic force microscopy and the x-ray diffraction results indicated that the crystallite size is less than 50nm. The root mean square roughness of our off-axis film was ~4 nm in a 55 μm2 area. The zero resistance Tc value of the best off-axis film reached 32.2 K with a narrow transition width of 0.9 K. The films showed no anisotropy in Hc2-T curves when parallel and perpendicular fields were applied relative to the film surface. The slope of Hc2-T curves is ~1 T/K, which is still among the highest reported values. On the basis of successful preparation of smooth off-axis MgB2 films, we obtained MgB2/Mg2Si multilayer structure by sequentially switching a stoichiometric MgB2 target and a Si target during off-axis pulsed-laser deposition. The transmission electron microscope cross-sectional image of the resulting film exhibits a layered structure with each MgB2 layer being 40-50 nm thick and the Mg2Si inter-layers about 5 nm thick. A clear enhanced anisotropy in the irreversibility lines and the vortex activation energy was observed. Pinning and the flux flow activation energy for this type of film was significantly increased in parallel applied fields.
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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.